Christensen’s Physics of Diagnostic Radiology

168 FLUOROSCOPIC IMAGING

Table 12-1. Atomic Number and electrons produced are proportional to the
K-Absorption Edge
intensity of the light (Fig. 12-3A).
ELEMENT ATOMIC K·ABSORPTION
NUMBER Now we have to get the electrons to the
Sulfur EDGE (keV) other end of the image intensifier tube and
Zinc 16 make them maintain their relative position.
Cadmium 30 2.5 We do this using an electrostatic focusing
Iodine 48 9.7 lens and an accelerating anode.
Cesium 53 26.7
55 33.2 Electrostratic Focusing Lens
36.0
The lens is made up of a series of posi­
phors. The energy of the K edge of cad­ tively charged electrodes that are usually
plated onto the inside surface of the glass
mium (26.7 keV) is quite good, but its envelope. These electrodes focus the elec­
chemical mates, zinc (9.7 keV) and sulfur tron beam as it flows from the photocath­
(2.5 keV), are far from ideal. The Kedges ode toward the output phosphor. Electron
of cesium (36 keV) and iodine (33.2 keV) focusing inverts and reverses the image.
This is called a point inversion because all
are almost perfect. The more appropriate the electrons pass through a common focal
atomic numbers of cesium and iodine give point on their way to the output phosphor
these screens a substantial advantage over
those made of zinc-cadmium sulfide. Ce­ (Fig. 12-2). Each point on the input phos­
sium iodide input screens absorb approx­
imately two thirds of the incident beam as phor is focused to a specific point on the
opposed to less than one third for zinc­ opposite side of the output phosphor. For
cadmium sulfide, even though the cesium undistorted focusing, all photoelectrons
iodide screen is only one third as thick. must travel the same distance. The input
phosphor is curved to ensure that electrons
The photocathode is a photoemissive emitted at the peripheral regions of the
photocathode travel the same distance as
metal (commonly a combination of anti­ those emitted from the central region. The
mony and cesium compounds). When light image on the output phosphor is reduced
from the fluorescent screen strikes the pho­ in size, which is one of the principal reasons
tocathode, photoelectrons are emitted in
numbers proportional to the brightness of 1+-- Anode
the screen. The photocathode is applied
directly to the Csl input phosphor. Light Evacuated Glass
from the Csl passes directly into the pho­ Envelope
tocathode. Older tubes had a thin light
transparent barrier between the input Focal Point
phosphor and the photocathode. Light dif­
fusion in this barrier reduced resolution. Electrostatic Lens

Let us review the process to this point. Photocathode ond
Input Fluorescent
A uniform beam of x rays has passed Screen

through a patient and been attenuated by Figure 12-3 X-ray image intensifier tube
the patient. This attenuated beam ofx rays
passes through the glass front of the image
intensifier tube and the thin aluminum
substrate of the input phosphor layer (Csl).
The Csl crystals produce light in propor­
tion to the intensity of the incident x-ray

beam. The light photons react with the
photocathode. Photoelectrons are emitted
from the photocathode. The numbers of

FLUOROSCOPIC IMAGING 169

why it is brighter, a point to which we will /
return later. Output
Phosphor
Accelerating Anode. The anode is lo­
Figure 12-4 Mirror optical system of an
cated in the neck of the image tube (Fig. image intensifier
12-2). Its function is to accelerate electrons
emitted from the photocathode toward the limited by the mirror, making it difficult,
output screen. The anode has a positive if not impossible, to palpate the patient.
potential of 25 to 35 kV relative to the pho­ Also, only one observer can view the image,
tocathode, so it accelerates electrons to a which is a serious disadvantage in training
tremendous velocity. beginning fluoroscopists.

Output Phosphor. The output fluoro­ Viewing the output of an image inten­
sifier (II) is done via a closed circuit tele­
rescent screen of image intensifiers is sil­ vision chain in modern systems. But we
ver-activated zinc-cadmium sulfide, the would also like to be able to record the
same material used in first-generation in­ image on photospot or cine film when ap­
put phosphors. Crystal size and layer thick­ propriate. We could record the film image
ness are reduced to maintain resolution in from the TV camera signal, but the re­
the minified image. The diameter of most sulting image would be degraded by the
television chain. It is better to expose the
output screens ranges from about \{ to 1 film directly to the output phosphor of the
image intensifier tube. To maintain contin­
in. Because the electrons are greatly accel­ uous TV viewing while exposing the film
erated, they emit more light photons from means that we must split the light from the
the output screen than were originally II output into two paths at the time of film
present at the input screen. The number exposure. During routine fluoroscopy, all
of light photons is increased approximately the light output of the II is directed to the
50-fold. TV camera. When film mode is selected, a
semitransparent (often termed "partially
A thin layer of aluminum is plated onto silvered") mirror is positioned in the light
the fluorescent screen (Fig. 12-3B) to pre­ beam, as shown in Figure 12-5. With this
vent light from moving retrograde arrangement, most of the light (about 90%)
through the tube and activating the pho­ goes to the film camera, but enough can
tocathode. The aluminum layer is very pass through the mirror to form a TV pic-
thin, and high-energy photoelectrons eas­
ily pass through it en route to the output
screen.

The glass tube of the image intensifier is
about 2 to 4 mm thick, and is enclosed in
a lead-lined metal container. The lead lin­
ing protects the operator from stray radi­
ation.

The output phosphor image is viewed
either directly through a series of lenses
and mirrors, or indirectly through closed­
circuit television. A mirror optical system
is shown in Figure 12-4. As you can see,
light travels a long distance, and it is re­
flected and focused several times. This can
be done with only minimal loss of bright­
ness. The mirror image is only visible in a
small viewing angle. If the operator moves
his head a few degrees to one side, the
image is lost. His freedom of movement is

170 FLUOROSCOPIC IMAGING

TV
camera

'

'

'

'

'

l �
' '
Lead lined ' '
' '
intensifier tube \ '

housing \, /1

v

Film

camera

Figure 12-5 Optical coupling between an image intensifier and viewing system

ture. Since the exposure level has been in­ The brightness gain is the ratio of the two
creased, 10% of the available light will still illuminations:
produce a satisfactory TV image. In the
filming mode, the exposure is not contin­ intensifier luminance
uous so the TV picture comes on only dur­ Brightness gain = ---­
ing the exposure time and shows the actual
image being recorded by the film. We will Patterson B-2 luminance
return to the topic of photospot and cine
cameras in the next chapter. If the image intensifier is 6000 times
brighter, the brightness gain is 6000. The
In some systems, the image is coupled to concept is easy to understand, and was
the TV camera by a fiberoptic bundle (fiber readily accepted by radiologists. Patterson­
face plate). Fiberoptic coupling precludes type B-2 fluoroscopic screens, however,
filming directly from the output phosphor vary from one batch to another, and de­
of the image intensifier tube. A fiber face teriorate at an unpredictable rate with time
plate is a bundle of fine optically shielded so brightness gain measurements are not
glass fibers (several thousand per mm2) that reproducible. Because of this lack of re­
is a few millimeters thick. In the future it producibility, the International Commis­
may be possible to obtain films that are ac­ sion on Radiologic Units and Measure­
ceptable from the TV signal, and we pre­ ments (ICRU) has recommended a second
dict that all image tubes will be coupled method of evaluation, called the "conver­
directly to TV cameras. sion factor," to supersede the older bright­
ness gain method. The conversion factor
is a ratio of the luminance of the output
phosphor to the input exposure rate:

BRIGHTNESS GAIN . cd/m2
Convers1on factor = -­

mRisec

Two methods are used to evaluate the Output screen luminance is measured m
brightness gain of image intensifiers. The candelas (abbreviated cd, and defined as
first compares the luminance of an inten­ the luminous intensity, in the perpendic­
sifier output screen to that of a Patterson­ ular direction, of a surface of a l/600,000
type B-2 fluoroscopy screen when both are m2 of a black body at the temperature of
exposed to the same quantity of radiation. freezing platinum under a pressure ·of

FLUOROSCOPIC IMAGING 171

101,325 Nt/m2 . .. ridiculous to remem­ output screen approximately 1 in. in di­
ber). Radiation quality and output lumi­ ameter. Image intensifiers with 12- to 16-
nance are explicitly defined, so the method in. diameter input screens are available. We
is accurate and reproducible. will briefly consider these big tubes shortly.
With a l-in. output screen, the minification
The brightness gain, often called "inten­ gain is simply the square of the diameter
sification factor," of modern image inten­ of the input screen; that is, a 9-in. inten­
sifiers can easily reach 10,000 (values of sifier has a gain of 81.
20,000 are possible). The conversion factor
usually equals about 1% of the brightness The brightness gain from minification
gain, so a conversion factor of 100 is about does not improve the statistical quality of
the same as brightness gain of 10,000. Most the fluoroscopic image.The same number
specifications for image intensifiers will of light photons make up the image re­
quote the conversion factor. gardless of the size of the output screen.
For example, a 6-in.intensifier with a 2-in.
The brightness gain tends to deteriorate output phosphor has a minification gain of
as an image intensifier ages. This means 9 (62 -;- 22 = 36 -;- 4 = 9), whereas the
that the patient dose with an old image in­ same intensifier with a l-in.output phos­
tensifier tends to be higher than that with phor has a gain of 36.The total light output
a new intensifier of the same type.Because of both units is exactly the same, however,
the deterioration can proceed at a rate of so the same number of photons make up
about 10% per year, a periodic check of both images.The photons are compressed
image intensifier brightness can be valua­ together on the smaller screen and the
ble. Unfortunately, an accurate check is image is brighter, but its statistical quality
somewhat complicated. Some indication of is not improved.
image intensifier aging can be obtained by
comparing the input dose level required Theoretically, brightness can be in­
for automatic brightness control operation creased indefinitely by minification. A 9-in .
with the dose level under the same condi­ intensifier with a 1/win. output screen
tions as when the intensifier was new. would have a brightness gain from mini­
fication alone of over 20,000. Excess mini­
The brightness gain of an image inten­ fication produces a very small image,
sifier comes from two completely unrelated though, which has a definite disadvantage.
sources, called "minification gain" and Before the image can be viewed it must be
"flux gain." We will discuss them separately. greatly magnified, which not only reduces
the brightness but also magnifies the fluor­
Minification Gain oscopic crystals in the output screen, re­
sulting in a precipitous drop in resolution.
The brightness gain from minification is
produced by a reduction in image size. The Flux Gain
quantity of the gain depends on the relative
areas of the input and output screens. Be­ Flux gain increases the brightness of the
cause the size of an intensifier is usually fluoroscopic image by a factor of approx­
indicated by its diameter, it is more con­ imately 50. For each light photon from the
venient to express minification gam m input screen, 50 light photons are emitted
terms of diameter: by the output screen. In simplified terms,
you may think of one light photon from
(�od1)2Minification gain = the input screen as ejecting one electron
from the photocathode.The electron is ac­
where d1 is the diameter of input screen, celerated to the opposite end of the tube,
and d0 is the diameter of output screen. gaining enough energy to produce 50 light
Most x-ray image intensifiers have an input photons at the output screen.
screen from 5 to 9 in. in diameter and an

172 FLUOROSCOPIC IMAGING

The total brightness gain of an image fluoroscopic image. Some light photons
intensifier is the product of the minification penetrate through the aluminum, pass
and flux gains: back through the image tube, and activate
the photocathode. The cathode emits pho­
Brightness gain = minification gain x flux gain toelectrons, and their distribution bears no
relationship to the principal image. These
For example, with a flux gain of 50 and a electrons produce "fog" and further re­
minification gain of 81 (9-in. intensifier duce image contrast. Contrast tends to de­
with a l-in. output screen), the total bright­ teriorate as an image intensifier ages.
ness gain is 4050 (50 x 81).

IMAGING CHARACTERISTICS Lag

Contrast Applied to image intensifiers, lag is de­
fined as persistence of luminescence after
Contrast can be determined in vanous x-ray stimulation has been terminated.
ways, and there is no universal agreement With older image tubes, lag times were 30
as to which is best. The following is a de­ to 40 ms. With Csl tubes, lag times are
scription of the simplest method. A Y:;.-in. about l ms. The lag associated with vidicon
thick lead disc is placed over the center of television tubes is now more of a concern
the input screen. Disc size is selected to than image intensifier lag times.
cover 10% of the screen; that is, a 0.9-in.
diameter disc is used for a 9-in. image in­ Distortion
tensifier. The input phosphor, with the disc
in place, is exposed to a specified quantity The electric fields that accurately control
of radiation, and brightness is measured at electrons in the center of the image are not
the output phosphor. Contrast is the capable of the same degree of control for
brightness ratio of the periphery to the cen­ peripheral electrons. Peripheral electrons
ter of the output screen. Defined in this do not strike the output phosphor where
way, contrast ratios range from approxi­ they ideally should, nor are they focused
mately l0: l to better than 20: l, depending as well. Peripheral electrons tend to flare
on the manufacturer and intended use. out from an ideal course. The result is un­
equal magnification, which produces pe­
Two factors tend to diminish contrast in ripheral distortion. The amount of distor­
image intensifiers. First, the input screen tion is always greater with large intensifiers
does not absorb all the photons in the x­ because the further .an electron is from the
ray beam. Some are transmitted through center of the intensifier, the more difficult
the intensifier tube, and a few are even­ it is to control. Figure 12-6 shows a cine
tually absorbed by the output screen. image of a coarse wire screen taken with a
These transmitted photons contribute to 9-in. intensifier. As you can see, the wires
the illumination of the output phosphor curve out at the periphery; this effect is
but not to image formation. They produce most noticeable at the corners. This same
a background of fog that reduces image effect has been observed in optical lenses
contrast in the same way that scattered and termed the "pincushion effect," and
x-ray photons produce fog and reduce con­ the term is carried over to image tubes. The
trast in a radiographic image. The second distortion looked like a pincushion to the
reason for reduced contrast in an image guy who named it. Generally this distortion
intensifier is retrograde light flow from the does not hamper routine fluoroscopy, but
output screen. Most retrograde light flow it may make it difficult to evaluate straight
is blocked by a thin layer of aluminum on lines (for example, in the reduction of a
the back of the screen. The aluminum layer fracture).
must be extremely thin, however, or it
would absorb the electrons that convey the Unequal magnification also causes un-

FLUOROSCOPIC IMAGING 173

911 MODE FOCAL POINTS

Figure 12-6 Test film of a wire screen (35 mm 611MODE
cine frame) from a 9-in. image intensifier

equal illumination. The center of the out­ Figure 12-7 Dual-field image intensifier
put screen is brighter than the periphery
(Fig. 12-6). The peripheral image is dis­ shows this principle applied to a dual-field
played over a larger area of the output image intensifier. In the 9-in. mode, the
screen, and thus its brightness gain from electrostatic focusing voltage is decreased.
minification is less than that in the center. The electrons focus to a point, or cross,
A fall-off in brightness at the periphery of close to the output phosphor, and the final
an image is called vignetting. Unequal fo­ image is actually smaller than the phos­
cusing has another effect on image quality; phor. In the 6-in. mode the electrostatic
that is, resolution is better in the center of focusing voltage is increased, and the elec­
the screen. trons focus farther away from the output
phosphor. After the electrons cross, they
In summary, the center of the image in­ diverge, so the image on the output phos­
tensifier screen has better resolution, a phor is larger than in the 9-in. mode. The
brighter image, and less geometric distor­ optical system is preset to cover only the
tion. format, or size, of the smaller image of the
9-in. mode. In the 6-in. mode, the optical
MULTIPLE-FIELD IMAGE system "sees" only the central portion of
INTENSIFIERS the image, the part derived from the cen­
tral 6 in. of the input phosphor. Because
Dual-field or triple-field image intensi­ this image is less minified, it appears to be
fiers attempt to resolve the conflicts be­ magnified when viewed through a televi­
tween image size and quality. They can be sion monitor. The physical size of the input
operated in several modes, including a 4.5- and output screens is the same in both
in., a 6-in., or 9-in. mode. The 9-in. mode modes; the only thing that changes is the
is used when it is necessary to view large size of the output image. Obviously, the 6-
anatomic areas. When size is unimportant, and 9-in. modes have different minifica­
the 4.5- or 6-in. mode is used because of tion gains. Exposure factors are automat­
better resultant image quality. Larger ically increased when the unit is used in the
image intensifiers (12- to 16-in.) frequently 6-in. mode to compensate for the de­
have triple-field capability. creased brightness from minification.

Field size is changed by applying a simple While we are discussing intensifier size,
electronic principle: the higher the voltage there is another point to consider. A 9-in.
on the electrostatic focusing lens, the more image intensifier does not encompass a
the electron beam is focused. Figure 12-7

174 FLUOROSCOPIC IMAGING

I procedures. Remember that there is �n au­
tomatic increase in exposure rate m the
1.0
magnified viewing mode. Contrast and res­
1 olution will be improved when only the
central area of these large tubes is used,
Figure 12-8 Reduction of fluoroscopic field and distortion will be minimized. These
size by an image intensifier tubes are large, somewhat bulky to use, and
very expenstve.
9-in. field in the patient. The x-ray image
is magnified by divergence of the beam SUMMARY
(Fig. 12-8). The intensifier sees a much
smaller field than its size would imply, an Early fluoroscopy was accomplished by
important point to consider when ordering radiologists looking directly at a fluoro­
a unit to perform a particular function. scopic screen. The image on the screen was
only .0001 as bright as the image of a rou­
Large Field of View Image Intensifier tinely viewed radiograph, so dark adapta­
Tubes tion of the eyes was required.

The development of digital angiography In the 1950s the image intensifier alle­
was associated with a need for large image viated this situation by producing an image
intensifier tubes that could image a large bright enough to be viewed with cone vi­
area of the patient. This is especially true sion . The input phosphor of modern
in abdominal angiography. Image intensi­ image intensifiers is cesium iodide; the out­
fier tubes with diameters of 12, 14, and 16 put phosphor is zinc cadmium sulfide
in. are now available to meet this need. (green light). Brightness gain is the pr�d­
uct of minification gain and flux gam.
Conventionally sized image intensifier Imaging characteristics important in the
tubes (up to about 10 in.) are made with a evaluation of image intensified fluoroscopy
glass envelope with a glass convex input include contrast, lag, and distortion. Large
window. Use of a glass input window on field of view image intensifier tubes are
the new large tubes became impractical be­ available to fill special needs, such as digital
cause the glass would be so thick it would and spot-film angiography. Most image in­
absorb a significant fraction of the x-ray tensifiers allow dual-field or triple-field
beam (remember that these tubes are un­ imaging.
der a high vacuum) . These larger tubes
REFERENCES
? �may have an all-metal envelope wit a ligh ­
l. Glasser, 0.: D r. W.C. Rontgen. Springfield, IL,
weight, non-magnetic metal (alummum, ti­
tanium, or stainless steel) input window. Charles C Thomas, 1945.
These big tubes usually allow triple-field
imaging, such as a 6-in., 9-in., and 14-in. 2. Medical X-Ray and Gamma-Ray Protection for
mode with a 14-in. diameter tube. The
magnification factor with the small field Energies up to 10 MeV. W ashing.ton, DC, Na­
can be helpful when fluoroscopy is used to tional Council on Rad1atwn Protection and Meas­
guide delicate invasive procedures such as
percutaneous biliary and nephrostomy urements, 1968, Report No. 33.

3. Chamberlain, W. E.: Fluoroscopes and fluoros-

4. copy. Radiology, 38:383, 1942. ..
Sturm, R.E., and Morgan, R.H.: Screen mtensl­

fication systems and their limitations. Am. J.

�Roentgenol. , 62:613, 1949.

5. Thompson, T.T.: A Practical Approach to od­

ern Imaging Equipment. Boston, MA, Little,

Brown and Co., 1985, pp. 81-126.

CHAPTER Viewing and Recording
the Fluoroscopic Image
13

Before the invention of the x-ray image video camera, where it is converted into a
intensifier, attempts at displaying the fluor­
oscopic image on television were only par­ series of electrical pulses called the video
tially successful. The large fluoroscopic signal. This signal is transmitted through
screen required an elaborate optical sys­
tem, and suboptimal screen brightness pro­ a cable to the camera control unit, where
duced a weak video signal. The develop­ it is amplified and then forwarded through
ment of the image intensifier solved both another cable to the television monitor.
these problems. Its small output phosphor The monitor converts the video signal back
simplifies optical coupling, and its bright into the original image for direct viewing.
image produces a strong video signal.
Before discussing the individual com­
CLOSED-CIRCUIT TELEVISION ponents of a television system, we will move
a little ahead of ourselves and describe the
The components of a television system nature of the video picture. An apprecia­
are a camera, camera control unit, and
tion of this will make the design of both
monitor (Fig. 13-1). To avoid confusion in the camera and monitor easier to under­
stand. The television image is similar to the
nomenclature, we will use the terms "tel­
evision" and "video" interchangeably. screened print shown in Figure 13-2. It is
Fluoroscopic television systems are always
closed-circuit systems; that is, the video sig­ made up of a mosaic of hundreds of
nal is transmitted from one component to thousands of tiny dots of different bright­
the next through cables rather than ness, each contributing a minute bit to the
through the air, as in broadcast television. total picture. When viewed from a distance
A lens system or a fiberoptic system conveys the individual dots disappear, but at close
the fluoroscopic image from the output range, or with magnification, they are
phosphor of the image intensifier to the clearly visible. The dot distribution is not
random or haphazard in a television pic­
Camero Camero �4mtiJ ture. Instead, the dots are arranged in a
Control Monitor specific pattern along horizontal lines,

Unit called horizontal scan lines. The number

Figure 13-1 Components of a television sys­ of lines varies from one television system
tem to another but, in the United States, most
fluoroscopy and all commercial television

systems use 525 scan lines. To avoid con­

fusion we must clarify the meaning of tel­
evision lines. When a radiologist thinks of
lines, it is usually in terms of lines per unit

length. For example, if a grid has 80 lines,

the unit is "lines per inch." Television lines

175

176 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

Figure 13-2 Dot picture and enlargement of a screened print

have only the unit of "lines," and no unit the face plate on its way to the target. The
signal plate is a thin transparent film of
of length. The 525 lines in most television graphite located on the inner surface of the
face plate. It is an electrical conductor with
systems represent the total number in the
entire picture, regardless of its size. The a positive potential of approximately 25 V.
lines are close together in a small picture
tube and spread apart in a large tube, but The vidicon target is functionally the
in both the total number is the same. most important element in the tube. It is a
thin film of photoconductive material, usu­
Television Camera ally antimony sulfide (Sb2S3) suspended as
globules in a mica matrix. In a plumbicon
The vidicon camera is the one usually the photoconductive material is lead mon­
employed for fluoroscopy, and is the only
one that we will discuss in any detail. There oxide (PbO). Each globule is about 0.00 1
are several types of vidicons; one is the
plumbicon, which we will mention. The in. in diameter and is insulated from its
vidicon camera is a relatively inexpensive, neighbors and from the signal plate by the
compact unit. The essential parts of a vid­ mica matrix. The function of the globules
is complex, but they behave like tiny ca­
icon camera are shown in Figure 13-3. The pacitors. After reviewing the other ele­
ments in the camera, we will return to these
most important part is the vidicon tube, a globules and discuss them in more detail.
small electronic vacuum tube that meas­
The cathode is located at the opposite
ures only 1 in. in diameter and 6 in. in end of the vidicon tube from the target and
is heated indirectly by an internal electric
length (larger tubes are sometimes used). coil. The heating coil boils electrons from
The tube is surrounded by coils, an elec­ the cathode (thermionic emission), creating
tromagnetic focusing coil, and two pairs of an electron cloud. These electrons are
electrostatic deflecting coils. immediately formed into a beam by the
control grid, which also initiates their ac­
The fluoroscopic image from the image celeration toward the target. The cathode­
intensifier is focused onto the target assem­ heating coil assembly with the control grid
is called an "electron gun" because it shoots
bly, which consists of three layers: ( 1) a electrons out of the end of the control grid.
glass face plate; (2) a signal plate; and (3) As the electron beam progresses down the

a target. The only function of the glass face
plate is to maintain the vacuum in the tube
(remember, an electron beam must travel
in a vacuum). Light merely passes through

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Video Signal OSypstticeaml

Figure 13-3 Vidicon camera

tube, it moves beyond the influence of the This coil extends almost the entire length
control grid and into the electrostatic field of the tube and creates a constant magnetic
of the anode. The anode has a positive po­ field parallel to the long axis of the tube;
tential of approximately 250 V with respect this field keeps the beam of electrons in a
to the cathode. The electrons are acceler­ narrow bundle. The electrons progress
ated to a relatively high velocity, but they down the tube in a series of oscillating spi­
are still low energy electrons (about 250 rals, and strike the target as a finely focused
eV). The anode extends across the target
end of the tube as a fine wire mesh. The beam (Fig. 13-4A).
wire mesh and signal plate form a uniform
decelerating field adjacent to the target. The electron beam is steered by variable
The signal plate ( + 25 V) has a potential electrostatic fields produced by two pairs
of 225 V less than that of the wire mesh of deflecting coils that wrap around the vid­
( + 250 V), so electrons should flow from icon tube. Vertical deflecting coils are
the signal plate to the wire mesh. The elec­
trons from the cathode are accelerated to A
relatively high velocities, however, and
they coast through the decelerating field 1··· ··· · ··· · · · · · ·· 1......... . ... . . . . . . .............................................................................................................................................. Spiral
like a roller coaster going uphill. By the Path
time they reach the target, they have been ELECTRON BEAM"'-..
slowed to a near standstill (they are now
25-eV electrons). The decelerating field FOCUSING COIL/
also performs a second function: it
straightens the final path o f the electron B
beam so that it strikes the target perpen­
dicularly. 10.{07#$#�+ 1ELECTRON BEAM===:'==F-_ie-_ld_-_-_-_,_..

Because the electron beam scans a fine /Electric
mosaic of photoconductive globules, it is
critical that the electron beam not spread �DEFLEACTINGICOIL ]
out as it goes through the tube. This is ac­
complished by an electromagnetic focusing Figure 13-4 Focusing and deflecting coils
coil that wraps around the vidicon tube.

178 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

shown in Figure 13-4B. By alternating the When a globule absorbs light, photoelec­

voltage on the coils, the focused electron trons are emitted (Fig. 13-SB). The elec­
beam is moved up and down to scan the
target. The other pair of coils moves the trons are immediately attracted to the an­
beam from side to side along a horizontal ode and removed from the tube. The
line. All four coils, working together, move globule, having lost electrons, becomes
the electron beam over the target in a re­ positively charged. Since the globule is in­
petitive scanning motion. sulated from its surroundings it behaves
like half of a tiny capacitor, and draws a
Video Signal current onto the conductive signal plate.
The current that flows onto the signal plate
Now that we have discussed the physical is ignored, or clipped, and is not recorded
makeup of the camera, we can return to
the critical target end of the tube and the (Fig. 13-5C). Similar events occur over the

formation of the video signal (Fig. 13-SA). entire surface of the target. A brighter area
in the light image emits more photoelec­
Signal Plate trons than a dim area, and produces a
stronger charge on the tiny capacitors. The
Globule Glass result is a mosaic of charged globules that
Mica Face Plate store an electrical image that is an exact
replica of the light image focused onto the
Photo­ Lioht target.
Electrons
The electron beam scans the electrical
Electrical ' image stored on the target and fills in the
Image holes left by the emitted photoelectrons,
o- " thus discharging the tiny globule capaci­
-= ' tors. After the capacitors are fully dis­
' charged (no more positive charges are left),
no additional electrons can be deposited in
{c the globules. It was indicated earlier that
Clipped Sional the electrons in the electron beam were re­
I duced to low energy electrons before they
entered the target. There are two reasons
Electron ee: · for this. Reason one is that we want no
electrons to enter the target after the pos­
Beam 8- , itive charge has been neutralized. The sec­
ond reason is that the electrons should not
Signal Plate have sufficient energy to produce second­
ary electrons when they do enter the glob­
Target - ,.·.. ules. Of course, high energy secondary
electrons would be able to neutralize the
Globule -TI@B'ilillliLJ-.:J: Glass positive charge in other globules and de­
Mica grade the image. Excess electrons from the
Face Plate scanning beam drift back to the anode and
are removed from the tube. When the elec­
Figure 13-5 Formation of the video signal trons in the scan beam neutralize the pos­
itive charge in the globules, the electrons

on the signal plate (Fig. 13-5D) no longer

have an electrostatic force to hold them on
the plate. They will leave the plate via the
resistor. These moving electrons form a

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 179

current flowing through a resistor, and one line at a time, so here, too, we have
therefore a voltage appears across the re­ scan lines.
sistor. This voltage, when collected for each
neutralized globule, constitutes the video The readout process reminds us of a fire
signal (Fig. 13-5£). The globules are not brigade with water buckets. The brigade
all discharged at the same time. Only a passes the buckets along a line of men until
small cluster, a dot, is discharged each in­ the water reaches the end of the line. This
stant in time. Then the electron beam comparison is pretty good, but we would
moves on to the next dot in an orderly se­ have to make one small change. Instead of
quence, discharging all the globules on the the buckets being passed along, we would
target. The result is a series of video pulses, have to dump the water from one bucket
all originating from the same signal plate into the next. A problem comes up because
but separated in time. Each pulse corre­ there is already water in all the buckets. In
sponds to an exact location on the target. the fire brigade then, we would need empty
Reassembling these pulses back into a vis­ buckets between each bucket containing
ible image is done by the camera control water. So in the CCD, there are empty
unit and the television monitor. spaces into which the charge may move.
The motion of the charge is controlled by
Charge-Coupled Device TV Camera changing the depth of the charge buckets.
To collect charge, the buckets must have a
A charge-coupled device is usually writ­ deeper potential well (bucket), than the
ten and spoken of as a CCD. A CCD is a next well position. The charge may be
semiconductor device that can store charge passed to the next well position by making
in local areas and, on an appropriate signal that position have a deeper potential well
from the outside, transfer that charge to a than the bucket containing the charge. It
readout point. In conjunction with a pho­ is important to realize that electrons from
toelectric cathode, the CCD makes a very one bucket can never mix with electrons
nice TV camera (or video camera). The from another bucket.
CCD camera forms a picture in much the
same way as a vidicon: the light photons CCD cameras need no readout electron
from the scene to be imaged are focused beam (or controlling coils), and can be
on the photoelectric cathode where elec­ made shorter than the vidicon tube. In ad­
trons are liberated in proportion to the dition to small size, the readout process is
light intensity. These electrons are cap­ amenable to digital systems, so the CCD
tured in little charge buckets (potential camera would fit nicely in a digital imaging
wells) built into the CCD device. So the dis­ system (enhanced by digital control of the
tribution of captured (stored) electrons in device readout). However, resolution is still
the charge buckets represents the stored controlled by the same things that control
image. This is exactly the same as the resolution from a vidicon. We might expect
stored image in the vidicon, except the vid­ the use of CCD cameras to increase in ra­
icon stores positive charge whereas the diology because it is an emerging technol­
CCD stores negative charge. The differ­ ogy, not because of improved resolution.
ence between the vidicon and the CCD The CCD finds application in information
camera is in the readout process. The vid­ processing as well as imaging, and may be
icon is read out by an electronic beam. The more important in memory applications
CCD is read out by the charge in the charge than in imaging application. We will give
buckets being moved from one bucket to you the information from a Fairchild pub­
the next until the charge reaches the edge lication.2 A Fairchild CCD 211 is a 244 X
of the CCD where it forms an electrical 190 element array, which is a 4 to 3 ratio
signal. The total readout is accomplished for TV presentation. This device dissipates
100 mW at a 7 MHz data rate. It operates

180 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

at voltages from 12 to 15 V (note that there cent screen. It carries a much higher pos­
are no extremely high voltages, as in vidi­ itive potential (10,000 V) than the anode
con). All this on a chip that is .245 by .245 of the camera tube (250 V), so it accelerates
in. The TV camera you buy for home use the electron beam to a much higher veloc­
today will be a CCD camera. ity. The electrons strike the fluorescent
screen at the flared end of the tube, which
Television Monitor makes the screen emit a large number of
light photons. The generation of light pho­
The last link in the television chain is the tons over the entire surface of the tube is
monitor. It contains the picture tube and the visible television image . Many second­
the controls for regulating brightness and ary electrons are set free by the impact of
contrast. the electron beam with the screen, and they
are attracted to the anode and conducted
A picture tube is similar to a vidicon cam­ out of the picture tube.
era tube (Fig. 13-6) . Both are vacuum
tubes and both contain an electron gun, A large tube, like a television picture
control grid, anode, focusing coil, and de­ tube, can never be completely evacuated.
flecting coils. A picture tube, however, is Some residual gas is always present, and
much larger. The focusing and deflecting "outgassing" (release of absorbed gas) from
coils are wrapped around the neck of the the components of the monitor adds to the
tube, and they control the electron beam problem. These gas molecules are even­
in exact synchrony with the camera tube. tually ionized and removed from the tube
The brightness of the individual dots in the by an ion trap located in the end of the
picture is regulated by the control grid . monitor (Fig. 13-6).
The control grid receives the video signal
from the camera control unit, and uses this Color monitors are slightly different in
signal to regulate the number of electrons two respects. First, three electron guns are
in the electron beam. To produce a bright required, one for each of the colors red,
area in the television picture, the grid al­ blue, and yellow. The second difference is
lows a large number of electrons to reach that the fluorescent screen must be com­
the fluorescent screen. To produce a dark posed of three different fluorescent ma­
area, the grid cuts off the electron flow al­ terials, one for each of the three colors.
most completely. The screen is not made up of continuous
fluorescent material, but rather is made up
The anode is plated onto the inside sur­ of alternating dots of the three colors. The
face of the picture tube near the fluores- controls must allow the yellow electron gun
to send electrons only to the yellow phos­
Electron Gun ' phor. Of course, the same is true for the
' red and blue electron guns. With this ar­
' rangement, a red image can be obtained
' by only the red electron gun emitting elec­
I trons, but orange requires both the red gun
and the yellow gun to emit electrons. With
Fluorescent -- I appropriate combination of the three col­
ors, the entire color spectrum from black
Screen to white can be covered. In radiology, we
do not need color TV cameras because the
Figure 13-6 Television monitor images to be recorded are produced by a
single color phosphor. But if we were to
use color cameras, we would find that they
must have photoconducting targets for

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 181

each of the three colors and some filtering numbered lines are scanned during the
system to assure that the right color inter­ second half (Fig. 13-7). Each pass of the
acts with the right phosphor. Today, color electron beam over the video target is
television pictures that appear in various called a field, and consists of 262� lines.
radiologic procedures are really nothing Although only 30 frames are displayed
more than computer-generated color each second, they are displayed in 60
images (e.g., color Doppler, SPECT, and flashes of light (fields), and flicker disap­
nuclear medicine). pears.

TELEVISION SCANNING We have used a 525-line system to illus­
trate scanning. Systems with a larger (two
The television image is stored as an elec­ or four times) number of vertical lines are
trical image on the target of the vidicon available and desirable, but are not yet ec­
tube, and it is scanned along 525 lines by onomically practical for routine use.
a narrow electron beam 30 times per sec­
ond. Each scan of the entire target is called Video Signal Frequency (Bandpass)
a "frame." The electron beam scans the tar­
get in much the same manner that we read Bandpass, also called "bandwidth," is the
a page in a book. Beginning at the top left frequency range that the electronic com­
corner, it drifts across a line, sending out ponents of the video system must be de­
a video signal as it moves, and then rapidly signed to transmit. An analogy with sound
returns to the left margin and repeats the will simplify the explanation. Sound is au­
process over and over until all 525 lines dible at frequencies from about 16 Hz to
have been read. When we reach the last 30,000 Hz. Sound equipment (e.g., record­
line of a page, we move on to the top line ers, amplifiers, and speakers) is designed
of the next page and continue reading. The to transmit this range of frequencies. The
electron beam does exactly the same thing, range from the lowest to the highest fre­
only it does not have to turn pages. Instead, quency is called the bandpass (Fig. 13-8).
as the beam reads, it also erases. As the The electronic components are engineered
electron beam discharges the globule ca­ to transmit all the frequencies in this range
pacitors, it erases their image. As soon as as accurately as possible. The cutoffs are
a line is read and erased, it is ready to rec­ not sharply defined at the two frequency
ord a new image, and it begins immediately. extremes, and not all frequencies are trans­
When the electron beam returns it sees a mitted with the same quality. Thinking in
different image than it saw the time before. terms of music, bandpass is the frequency
Because the electron beam scans the target range that the electronic components must
30 times each second, the change in the "pass" from the "band" to the listener. A
image from one scan to the next is slight, video signal is exactly the same as an audio
and our eyes perceive a continuous motion signal, except that the video signal covers
in exactly the same way that we see motion a wider range of frequencies.
in a cine film.
As the electron beam moves along a hor­
The eye can detect individual flashes of izontal scan line, a video signal is trans­
light, or flicker, up to 50 pulses per second. mitted to the camera control unit and then
A television monitor only displays 30 forwarded to the TV monitor. The fre­
frames per second, so an electronic trick, quency of the video signal will fluctuate
called interlaced horizontal scanning, is from moment to moment, depending on
employed to avoid flicker. Instead of scan­ the nature of the television image. Figure
ning all 525 lines consequently, only the 13-9 shows one scan line of an image con­
even-numbered lines are scanned in the taining four equally spaced black-and­
first half of the frame, and only the odd- white lines, or four line pairs (4lp). As the
electron beam moves across the image, the
video signal increases and decreases in volt-

182 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

FIELD I FIELD li FRAME

I2 - 525 Lines

34 I

5+ 3o Sec.

6
7

8

262 + Lines 262 + Lines

I I

60 Sec. 6o Sec.

Figure 13-7 Interlaced h or izontal scanning

age in a cyclic fashion. Like alternating cur­ signal occurs with an image of one line pair
rent, one cycle (the shaded area in the il­ (i.e., a screen that is half black and half
lustration) includes one up-limb and one white). The highest frequency signal is de­
down-limb and represents two lines, or one termined by the amount of money that one
line pair. The scanning process is repeated is willing to pay for electronic equipment.

over and over, 525 times per frame and 30 Vertical resolution is determined only

frames per second. We can calculate the by the number of vertical scan lines (525
frequency, or number of cycles per second, in our illustration). Suppose we would like
of the video signal generated by the four­
line-pair image by multiplying the number to image a test object that has 2000 black­
of cycles per scan line (four in this case) by and-white line pairs (4000 lines total). What
the number of scan lines per frame by the
number of frames per second: would we see with our 525-line camera?

cycles x scan lines x frames = c cles/sec The 2000 line pairs might be recorded in
scan line frame seconds Y
the photocathode, but our scan electron
4 X 525 X 30 = 63,000 beam, which covers the full area of the TV
monitor in 525 lines, would see on each
When the number of line pairs in the line scan four black lines and four white
image changes, the frequency of the video lines. The output video for that scan line
signal also changes. The lowest frequency would be an average of the four line pairs,
or a gray line. The next scan would see the
LOW HIGH same. The total output would be a nice uni­
FREQUENCY FREQUENCY form gray image (no line-pair resolution).
{30,000 Herz) What we need to image line pairs is a dark
(30 Herz\ line for one scan line and a light line for
the next scan line. This is the most that we
L_--�L-� can do. For a 525-vertical-line system, the
10 tOO t,OOO 10,000 100.000 maximum line-pair structure that can be
resolved on the TV monitor is 2621� line
FREQUENCY OF SOUND pairs. Notice that there are no units of line
pairs (not 262� lines per inch or meter or
Figure 13-8 Frequency range of audible mile), just 2621� line pairs per TV monitor
sound and the bandpass required for accurate image. These 2621� line pairs of a 525-line
t ransmission system displayed on a 6-in. monitor will
look better than the same 525 lines dis­

played on a 13- or 19-in. monitor. A system

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 183

TV CAMERA

VIDEO
SIGNAL

Figure 13-9 The video signal from one scan line of a line pair phantom

with more scan lines would give better ver­ The frequency of the video signal will fluc­
tical resolution. Our task now is to match tuate between a minimum of 15,750 Hz
horizontal resolution to vertical resolution, and a maximum of 4,130,000 Hz. To trans­
a more difficult concept to explain, since mit the signal accurately, the electronic
horizontal resolution is not determined by components should have a bandpass of
the number of scan lines. 4.13 to 0.016 MHz, or approximately 4.1
MHz. Actually, a somewhat higher band­
A reasonable compromise is a system pass is required. About 10% of the scan
with equal vertical and horizontal resolu­ time is lost in retracing from one line to
tion. For the horizontal resolution to be another. This additional 10% increases the
equal to the vertical resolution in a 525-line required bandpass to approximately 4.5
system, the horizontal resolution must be MHz for a 525-line system. At this band­
262� line pairs . The horizontal resolution pass, vertical and horizontal resolution are
is determined by the bandpass of the TV equal.
system. The bandpass must be able to
transmit sufficient information to produce Remember, vertical resolution depends
this resolution. For a 525-line system, the on the number of vertical lines (such as
lowest and highest frequency signals will 525), whereas horizontal resolution is de­
be as follows: termined by the bandpass. Most x-ray tel­
evision systems today have a bandpass (also
�c'y -c_le_s_ x scan lines x s-frea-cmo- ne-sd = eye1es1sec called bandwidth) of 5 MHz with a 525-line
scan line frame system. Higher vertical-line systems are
available, such as 875 and 1024 lines with
Minimum: 525 X 30 = 15,750 a bandpass of up to 20 MHz. These systems
1X are expensive and not yet used routinely.

Maximum: 525 X 30 = 4,130,000 For the purpose of illustrating concepts
262% X our discussion thus far has assumed that a
525-line TV system actually uses all 525

184 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

lines to form an image. In fact, significantly system) depends on the size of the input
fewer than 525 lines are available for image image. If the TV monitor resolution of 185
formation. This is a technical problem that lp is used to show a large picture, resolution
we will not discuss in depth. will be poor. For example, suppose we use
television to display the image from a 9-in.
Synchronization image intensifier. Converting to millime­
ters, a 9-in. image intensifier is 229 mm in
It is necessary to synchronize or coor­ diameter (9 in. X 25.4 mm/in.). The system
dinate the video signal between the camera resolution in lp/mm is as follows:
and monitor; that is, to keep them in phase
with each other. The camera control unit 185 lp
adds synchronization pulses to the video = 0.8 lp/mm
signal at the end of each scan line and scan
field. Appropriately, they are called "hor­ 229 mm
izontal and vertical synchronization
pulses." They are generated during the re­ Table 13-1 shows resolutions for three
trace time of the electron beam, while no different-sized image intensifiers. The
video signal is being transmitted. First, the same numbers would apply to a triple­
picture screen is blackened by a blanking model (4.5-, 6-, and 9-in.) image intensifier.
pulse, and the synchronization signal is The television monitor always displays the
added to the blanking pulse. If the syn­ same 185 lp but, when supplied with the
chronization pulses were added to the image from a 4.5-in. image intensifier, this
video signal while the screen was white, 185 lp represents a resolution of 1.6 lp/mm,
they would generate noise in the form of twice as good as the 0.8 lp/mm of the 9-in.
white streaks, but no visible noise is pro­ image intensifier. Even though a resolution
duced by synchronization on a black of 1.6 lp/mm is a considerable improve­
screen. ment, it falls far short of displaying the
entire resolution of cesium iodide image
TELEVISION IMAGE QUALITY intensifiers. These tubes have a resolution
of up to 1 lp/mm, and some are available
Resolution in 12- to 16-in. sizes. The only way that this
resolution can be adequately displayed at
As we have stated, the number of scan the present time is with a film system such
lines available for image formation in a as a 35 mm cine, or spot film cameras.
525-line TV camera is significantly less
than 525 lines. Some lines are lost to pre­ The 370 vertical lines must be displayed
vent the retrace and blanking signals from as 370 lines on the TV monitor. A 10-in.
showing on the TV screen. We have been (diagonal diameter) TV monitor has a ver­
discussing absolute maximum resolution. tical height of about 6 in., and 370 lines
In no system do we ever attain maximum distributed over 6 in. will produce a good
values, and in the final analysis the reso­ image. If the same 370 lines are displayed
lution of a system must be measured. Usu­ on a larger TV monitor, individual lines
ally this determination is made by the might become visible. At the same normal
manufacturer, and may appear in the spec­
ifications as vertical lines and bandpass. Table 13-1. Resolution of a TV Imaging
Resolution of 370 lines (185 line pairs) may System for Various-Sized Image Intensifiers
be typical in a 525-line system, and this is
the value we will use in the following dis- SIZE OF TELEVISION
cussion. IMAGE INTENSIFIER
RESOLUTION
The overall resolution of the imaging in. mm
system (i.e., the image-intensifier/lines/TV (lplmm)

4.5 114 1.6
6 152 1.2
9 229 0.8

*Based on a 525-line TV system with a total res­
olution of 185 lp.

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 185

viewing distance, the image on a small TV image generated by a plumbicon tube will
screen will look better than that on a large show more quantum mottle than a stand­
screen. ard vidicon image.

Contrast Automatic Brightness Control

Both the camera and monitor affect the The brightness of the image changes
contrast of a television image. A vidicon constantly in all fluoroscopic systems as the
camera reduces contrast by a factor of ap­ fluoroscope is moved from one area of the
proximately 0.8, and the monitor enhances patient to another. Usually this does not
contrast by a factor of 2. The lead mon­ cause any serious problem, and both the
oxide target vidicon (plumbicon) tube does chest and abdomen can be examined with
not cause any decrease in image contrast. preset exposure factors. With television
The net result is a definite improvement in fluoroscopy, however, changes in image
contrast beyond that of the image intensi­ brightness seriously affect image quality.
fier alone. Furthermore, both the bright­ As the fluoroscope is moved from the ab­
ness and contrast levels can be regulated domen to the chest a sudden surge of
with the monitor, so the optimum combi­ brightness floods the system, the image be­
nation can be selected to best show a point comes chalky, and all detail is lost. There­
of interest. There is a correct way to adjust fore, the brightness level of the television
contrast and brightness of the television monitor must be controlled within rather
monitor. Turn up the contrast control knob narrow limits. This may be accomplished
until background noise becomes visible, by varying the x-ray exposure factors or
then reduce the contrast level slightly. varying the sensitivity of the television sys­
Then turn down the brightness control tem. A photocell located between the image
level until the darkest part of the picture intensifier and television camera measures
is black. Increasing the brightness level the brightness of the fluoroscopic image,
alone will decrease contrast, so the bright­ and it transmits a signal back to the x-ray
ness and contrast control knobs should al­ control unit to adjust the exposure factors.
ways be adjusted together. If brightness is controlled by varying sen­
sitivity of the TV system the term auto­
Lag matic gain control is sometimes used. Au­
tomatic brightness control refers to
An undesirable property of most vidicon control of x-ray exposure levels.
tubes is lag, or stickiness, which becomes
apparent when the camera is moved rap­ AUTOMATIC GAIN CONTROL
idly during fluoroscopy (i.e., the image
blurs). Lag occurs because it takes a certain One way to control brightness of the
amount of time for the image to build up image on the TV monitor is to vary the
and decay on the vidicon target. In one sensitivity of the television system by vary­
respect a certain amount of lag is actually ing the sensitivity of the television cameras
advantageous. It averages out the statistical or varying the gain of the television am­
fluctuations that normally occur with low­ plification system. Automatic gain control
dose fluoroscopy, and minimizes the an­ is a fairly simple and inexpensive way to
noying effects of quantum mottle. Plum­ control image brightness. However, it does
bicon (lead monoxide) television camera not change the x-ray dose rate to the pa­
tubes demonstrate significantly less lag tient. This can result in unnecessary patient
than do standard vidicon tubes. The lag of exposure, because brightness may be re­
a vidicon is usually not a problem with rou­ duced by reducing TV system sensitivity
tine fluoroscopy, but may become a prob­ rather than decreasing patient exposure.
lem in cardiovascular fluoroscopy. The Also, increasing sensitivity of the system

186 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

without increasing radiation exposure will the advantage of very fast response times
not improve quantum mottle and will in­ and a very broad dynamic range, and al­
crease electronic noise. lows the operator to choose the rnA and
kVp levels best suited for the examination.
Automatic Brightness Control. There One obvious disadvantage is the complex­
are three ways to change the radiation in­ ity and expense associated with switching
put to the image intensifier input phos­ in the secondary circuit.
phor:
FLUOROSCOPIC IMAGE
kVp variability RECORDERS
rnA variability
pulse width variability There are two modes of recording the
kVp Variability. For kVp variability, kVp fluoroscopic image. First, the light image
will vary while rnA stays constant. This from the output phosphor of the image
method offers fast response times, as op­ intensifier may be recorded on film with a
posed to variable rnA techniques. Also, var­ photospot camera or cine camera. Routine
iable kVp units produce satisfactory images spot films are made directly with x rays,
over a wide range of viewing conditions (a but since they are recorded on film we dis­
wide dynamic range). cuss them with cine and photospot cam­
mA Variability. An rnA-variable bright­ eras. The second group of recorders makes
ness control allows the operator to choose use of the electrical signal generated by the
a fixed kVp. The automatic brightness con­ TV camera. This group of recorders in­
trol varies the rnA as needed. The response cludes magnetic tape, magnetic discs, and
time of this type of control is slow since optical discs. The three recorders may em­
changes in rnA require that the tempera­ ploy either analog or digital signals.
ture of the x-ray tube filament change in
order to change rnA. This method is rel­ Light Image Recorders
atively simple and inexpensive, and offers
the operator control of kVp. The dynamic Spot Film Recorder. Spot film devices
range of this control is less than the kVp­ are old friends of radiologists and tech­
variable units, so the operator must choose nologists of all ages. These devices inter­
a kVp appropriate for the examination be­ pose an x-ray film cassette between the x­
ing done. It has been suggested than an ray beam and the image intensifier tube.
rnA-variable system, with a convenient kVp The standard 9�-in. square cassette may
control, is the best general purpose auto­ now be replaced with cassettes of several
matic brightness control system. sizes in some fluoroscopic units. The tech­
Combined Control. A number of systems nique of using a spot film device should be
vary both kVp and rnA to control bright­ familiar to anyone reading this text.
ness. This technique offers a wide dynamic
range. Many of these units offer the option During fluoroscopy, the radiologist may
of manual selection of either kVp or rnA, at any time elect to move the cassette from
making them in effect either a kVp- or rnA­ its park position (where it is shielded by
variable unit. lead) to a position ready for exposure.
Pulse Width Variability. This type of There is a delay of about% to 1 sec before
brightness control system is used with cine a film can be moved into position and an
cameras, or with video disc storage systems. exposure made. Several factors make this
Both kVp and rnA are constant for one delay necessary. First, the cassette is rather
examination. The length of each exposure heavy, and some time is required to move
is controlled with a grid-controlled x-ray it into position and bring it to a completely
tube or a constant potential generator with motionless position prior to exposure.
secondary switching. This method offers Some changes at the x-ray tube are also
required. Fluoroscopy is conducted at

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 187

about 80-90 kVp (sometimes higher) and more convenient than conventional spot
1 to 3 rnA of tube current. Exposure of the films. The film does not have to be changed
spot film uses fluoroscopic kVp, but re­ between exposures, and the delay between
quires much higher rnA settings in the 300 initiation and completion of an exposure is
to 400 rnA range. Some time is required to shorter, because no cassette has to be
increase x-ray tube filament heating, and moved into position for a spot film camera.
the x-ray tube anode rotation speed is usu­ Exposure times are shorter (in the 50-ms
ally increased. A phototimer controls the range), so motion is less likely to be a prob­
length of exposure. lem. In addition, films can be taken more
rapidly. Most commercial units are de­
Spot Film Cameras signed to function at a maximum framing
frequency of 12 frames/second. Since the
A spot film (often called "photospot") camera is recording directly from the out­
camera is a camera that records the image put phosphor of the image intensifier tube,
output of an image intensifier on a film. it is possible to record and view the image
Typically, the film is roll film of size 70 or at the same time. The resolution of the re­
105 mm, or cut film 100 mm in size. sulting films is that of the image intensifier,
about 4 line pairs per mm (a range of 3 to
Recall that the light from the output 5 lp/mm). Resolution of routine spot films
phosphor of the image intensifier is con­ is theoretically greater than that of spot
verted into a parallel beam image by a lens film camera films, but longer exposure
placed close to the output phosphor (refer times required for spot films may degrade
back to Fig. 12-5). A semitransparent mir­
ror placed in this parallel beam will allow -
about 15% of the light to travel on to the the image because of motion unsharpness.
TV camera, and reflect the remainder to a
photospot camera or a cine camera. Orig­ Many observers feel that angiography
inally confined to gastrointestinal fluoros­ recorded on 100- or 105-mm spot film cam­
copy, spot film cameras are now used to eras is the equal of that recorded on stand­
record all types of fluoroscopic images, in­ ard x-ray film exposed in rapid film chang­
cluding angiography. This has been made ers. This is largely a reflection of the very
possible by the improved resolution of ce­ short exposure times with spot film cam­
sium iodide image intensifier tubes. Large eras. Other advantages of spot film cam­
field of view image intensifier tubes with eras include reduction in dose to only 30%
input phosphor diameters of up to 16 in. or less of that of routine filming, and a
allow large areas of the body to be imaged savings in film costs of greater than 80%.
without moving the image intensifier. One must admit that the little films are a
Therefore, we can take a single photospot nuisance to process and store, and it takes
film that covers almost the entire abdomen some practice to feel comfortable looking
(this could not be done with a 9-in. tube). at angiograms on such a small format.
A significant advantage of spot film cam­
eras over any direct filming methods is a Framing with Spot Film Cameras. The
substantial reduction in patient exposure. output phosphor of the image intensifier
The recommended per-frame exposure tube is round, but the shape of the film
for photospot film cameras is 100 micro­ used in spot film cameras is square. Match­
Roentgens (f.LR). The exposure for a com­ ing the output to the film means that either
parable spot film is 300 f,LR, three times as part of the image or part of the film cannot
much. The disadvantages and other ad­ be used. The term "framing" refers to the
vantages of spot film cameras relate to con­ utilization of the available area on the film.
venience, economics, and the clinical set­ Four framing formats may be used with
ting. For gastroenterology, cameras are spot film cameras. These are illustrated in
Figure 13-10, and we will briefly describe
each.

188 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

FRAMING EXACT EQUAL AREA MEAN DIAMETER TOTAL
MODE FRAMING OVERFRAMING
•FRAMING •FRAMING
ILLUSTRATION ��

FILM 79% 91% 96% 100 %
AREA USED 0% 9 °/o 16% 36%
1.0 1.21 1.41
IMAGE 1.13
AREA UNUSED

RELATIVE
IMAGE SIZE

Figure 13-10 Framing formats for spot film cameras

Exact Framing. The entire circular in­ larges as the diameter of the framing
tensifier image is included in the useable
format increases (Fig. 13-10). Note in the
square film frame. The word "useable" re­
fers to the fact that part of the film is lost illustration that the arrow increases in size
to the transport system (i.e., perforations as the framing format increases. For this
along the edge of the film). The exact reason, more overframing is recom­
amount lost depends on the film size and
design of the transport system, and varies mended for small films (70 mm) than for
somewhat from one manufacturer to an­ larger films (105 mm). In general, exact
other. With exact framing, no part of the
framing is not recommended for any clin­
image is lost but 21% of the film is wasted. ical application. It is especially undesirable

Equal Area Framing. The area of the in­ for 70 mm film, because the final image is
tensifier image is equal to the useable
too small. Total overframing is not rec­
square film area. Approximately 9% of
ommended, except possibly for 70 mm film
both the image and film are lost.
Mean Diameter Framing. The diameter in which maximum magnification is desir­
able. Either equal area or mean diameter
of the intensifier image is equal to the mean framing is recommended for most clinical
of the transverse and diagonal dimensions situations. Total overframing is impractical

of the useful square film area; 16% of the with larger film sizes ( 100 mm and 105
image is lost but only 4% of the film is un­
mm).
used. The film for spot film cameras should
Total Overframing. The diameter of the
have a relatively low contrast. An average
intensifier image is equal to the diagonal
dimensions of the useful square film area. gradient between 1.0 and 1.6 has been rec­

The entire film is used, but 36% of the ommended by the Inter-Society Commis­

image is unused. sion for Heart Disease Resources.4 A min­
The optimum frame format depends on
imum exposure of 100 j.LR/film is required
several factors: (1) the size of the image
intensifier, (2) the size of the film, and (3) to minimize quantum mottle. This is the
exposure at the input phosphor of the
the clinical application for which the imag­ image tube after the x-ray beam has tra­
ing system is intended. For any given sys­
tem, the size of the recorded image en- versed the patient and grid. Remember

that quantum mottle originates in the flu­
orescent screen, so it is not affected by the
size of the recorded image. Therefore, the