Christensen’s Physics of Diagnostic Radiology

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 189

same exposures are recommended for 35 mm FILM
both 70 mm and 105 mm films.
16 mm FILM CJ CJ
Cinefluorography D CJ
g D CJ
Cinefluorography is the process of re­
cording fluoroscopic images on movie 7.5x103 CJ CJ
(cine) film. The cine camera records from
the output phosphor of the image inten­ mm 18 mm x 24 mm
sifier through a series of lenses and mir­
rors. In this edition we will eliminate the Figure 13-11 Film formats as defined by the
detailed discussion of lens systems found apertures of 16 mm and 35 mm cameras
in previous editions. A beam splitting mir­
ror allows simultaneous cine recording and of 160° are usually employed for cinefluo­
television viewing, just as with spot film rography. While the shutter is closed, the
camera recording. pulldown arm advances the film to the cor­
rect position for the next exposure (Fig .
Two film sizes are currently being used 13-13). The pressure plate holds the film
for cinefluorography, 16 mm and 35 mm. against the camera aperture so that it is
In the United States, 98% of all cine is done located in the proper image plane.
on 35 mm film, and 95% of all cine studies
involve the heart. Therefore, our major An electric drive motor advances the film
emphasis will be on 35 mm cardiac cine­ from the supply reel past the aperture to
fluorography. the takeup reel. A meter attached to the
supply reel indicates the amount of unex­
Cine Camera. Cinefluorography is mar­ posed film in the camera . The x-ray pulses
and shutter opening are synchronized by
ried to the motion picture industry. We are an electrical signal from the drive motor.
stuck with their horizontal rectangular for­ The framing frequency, or number of
mat, even though it is not well suited to our frames per second, is usually 60, divided
needs. All cine cameras are commercial or multiplied by a whole number (e.g., 7�,
movie cameras with a few minor modifi­ 15, 30, 60, or 120) .
cations. Two film sizes are available, 16 mm
and 35 mm. The basic components of the APERTURE
camera are a lens, iris diaphragm, shutter,
aperture, pressure plate, pulldown arm, �
and film transport mechanism.
0�
Light enters the camera through the lens
and is restricted by the aperture, a rectan­ --SHUTTER
gular opening in the front of the camera.
The size and shape of the aperture define Figure 13-12 Cine camera shutter
the configuration of the image reaching the
film. Figure 13-11 shows the image sizes
as defined by the apertures of 16-mm and
35-mm cameras. Apertures for 35 mm film
are usually 18 x 24 mm.

The shutter is a rotating disc with a sec­
tor cut out of its periphery (Fig. 13-12). It
is located in front of the aperture. As the
shutter rotates, it interrupts light flow into
the camera. The size of the shutter opening
is expressed as the number of degrees in
the cutout portion of the sector. Openings

190 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

PRESSURE
PLATE--'Tr·

Figure 13-13 Cine camera

The combination of the framing fre­ ing means that some of the output image
is not recorded. Extremes of overframing
quency and shutter opening determines are generally avoided because patient ex­
posure in areas not recorded is undesira­
the amount of time available for both the ble. Framing characteristics are established
when the cine system is installed.
exposure and pulldown. For example, with
X-Ray Exposure. The timing and inten­
a 180° shutter opening and 60 frames per
sity of the x-ray exposure are controlled
second, the time available for both the ex­ during cinefluorography by two electrical
signals that originate from within the cine
posure and pulldown is 11120 sec. At slower system. One signal coordinates the x-ray
exposure with the open time of the camera
framing frequencies, both are longer. With shutter (synchronization), and the other
maintains a constant level of intensifier il­
a smaller shutter opening, the available ex­ lumination by varying the exposure factors
for areas of different thickness or density
posure time is shorter than the pulldown (automatic brightness control).

time . Synchronization. When cinefluoro­

Framing. The concept of framing is pre­ graphic equipment was in the embryonic
stages of its development, x rays were gen­
sented in the spot film camera section of erated continuously throughout a filming
sequence, and the patient was needlessly
this chapter. The term overframing is im­ irradiated when the camera shutter was
closed. These continuous exposures had
portant to understand. Framing is con­

trolled by the lens of the cine camera. Exact

framing means that the entire image (the

output phosphor of the II) just fits on the
cine film (refer back to Fig. 13-10). Over­

framing means that only a portion of the

image is recorded on the film. The film

image with overframing is larger than the

film image for exact framing, and this in­

creased size is usually desirable. Overfram-

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 191

two serious disadvantages: large patient ex­ one or several television monitors. Both
posures and decreased life expectancy of these transmissions are conducted through
the x-ray tube. In all modern cinefluoro­ cables, so it is a closed-circuit television sys­
graphic systems the x-ray output is inter­ tem. The image is stored on Y:;-, %-, 1-, or
mittent, and the exposure is synchronized 2-in. wide polyester base tape, coated on
with the open time of the camera shutter. one side with a magnetic film. Two-inch
The shutter of the camera is timed by 60 video tape records a better quality image.
Hz power and permits shutter speeds that Both open-reel (reel-to-reel) and cassette
are fractions or multiples of the number tapes are available.
60 (e.g., 7Y:;, 15, 30, 60, and 120 frames
exposed per second). The x-ray beam is A schematic presentation of a tape re­
usually synchronized with shutter opening corder is shown in Figure 13-14. The three
either by switching in the secondary circuit essential components, besides the elec­
of a constant potential generator or by use tronic circuitry, are a magnetic tape, a writ­
of a grid-controlled x-ray tube. We have ing head, and a tape transport system.
already discussed the topic of automatic These requirements are the same for either
brightness control with image intensified open-reel or cassette tape recorders. Re­
fluoroscopy. ally, the only difference in the two types is
the format in which the tape is stored and
To conduct a cine examination, the ra­ handled. The same type of head is used for
diologist must be able to monitor the cine both recording and playback, and we will
image. The beam-splitting mirror reflects call it a "writing head." The writing head
approximately 10% of the light from the converts an electrical signal into a fluctu­
image intensifier for monitoring purposes. ating magnetic field for recording, and
Although this percentage is small, it must converts a magnetic signal into an electrical
be remembered that the exposure factors signal for replay. Some recorders have sep­
and brightness of the intensifier phosphor arate recording and playback heads,
are greatly increased during cinefluorog­ whereas in others one head performs both
raphy, and that the monitoring image functions. The drive spindle moves the
brightness is not much different from that tape past the writing head at a constant
used for routine fluoroscopy. At low fram­ velocity. The tape is kept in physical contact
ing frequencies the image flickers because with the writing head at all times during
the x rays are pulsed, but flicker does not both recording and playback.
interfere with monitoring.
The writing head of a tape recorder is
TV IMAGE RECORDERS shown in Figure 13-15. The head is similar
to a transformer in that it consists of a mag­
We have just discussed methods of re­ netic core, such as an iron-nickel alloy
cording the image from the light output of wrapped with two coils of wire, but the
the image intensifier. The second method writing head is different in two ways. First,
of recording the fluoroscopic image in­ a narrow segment, or gap, is cut from the
volves recording the electrical signal from core, as shown in Figure 13-15. Second,
the TV camera. We will discuss magnetic the two coils are wired together so that
tape, magnetic disc, and optional disc re­ their magnetic fields reinforce each other.
corders. As a changing electric current moves
through the coils, a changing magnetic
Tape Recorders field is produced in the gap. When the cur­
rent in the illustration moves from left to
A tape recorder is used for both record­ right, the magnetic field is in the direction
ing and playback. As a recorder, it receives of the arrows. The field reverses when the
a video signal from the camera control unit current changes direction. The magnetic
and, for playback, transmits the signal to

192 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

INPUT RECORD
IMAGE AMPLIFIER

WRITING []'
.
HEAD
TV
� MONITOR

/

Tape

Figure 13-14 Components of a tape recorder

field extends out beyond the gap in the point. The actual alignment is proportional
to the strength of the magnetic field. Com­
writing head (Fig. 13-16). This extended plete alignment is prevented by the neigh­
boring molecules, which offer some resist­
magnetic field is the critical portion that ance to dipole movement, and by the
interacts with the magnetic tape. inertia of the dipoles themselves. Once the
tiny magnetic particles leave the magnetic
The magnetic layer of tape is composed field in the gap, they retain their orienta­
of oxides of magnetic materials. The mol­ tion until some other magnetic force causes
ecules of this material behave like tiny bar them to change.
magnets, called "dipoles." Each dipole
aligns itself in a magnetic field like the nee­ Playback is exactly the reverse of the re­
dle of a compass. They are randomly ar­ cording process, except that the magnetic
dipoles are not unaligned in playback. The
ranged on unrecorded tape (Fig. 13-16A). partially aligned magnetic dipoles have a
magnetic field of their own. As this field
As the tape moves past the writing head moves past the gap in the writing head, it
gap, the alignment of the dipoles is induces a magnetic field in the core, which
changed to coincide with the magnetic field in turn induces an electrical signal in the
impressed on them while they are passing wire coils. This is the video signal that is
forwarded to the display monitor.
the gap (Fig. 13-16B). When the video sig­
nal (sine curve in Fig. 13-16) is negative, Every component of the video signal
must be recorded on a different portion of
the dipoles are aligned toward the left; the magnetic tape. The transport system
when the signal is positive, alignment is in must move fast enough to keep a fresh sup­
the opposite direction. At the zero points ply of tape at the writing heads. If the tape
in the video signal, no magnetic field exists moves too slowly, the dipole alignment of
in the gap, and dipole alignment is ran­ one cycle is unaligned by the magnetic field
dom. The degree of alignment is exagger­ of the next cycle. To record a frequency of
ated in the illustration to demonstrate the several million cycles per second, the tape
must move at a very high velocity. This is
WRITING HEAD accomplished by moving both the tape and

Core writing heads (Fig. 13-17). The tape

Magnetic Loyer moves diagonally past paired writing heads
mounted on either side of a rapidly re­
I volving drum. The tape moves in one di-

Figure 13-15 Writing head

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 193
MAGNETIC FIELD

A. B.

Figure 13-16 The magnetic signal on tape

rection and the writing heads move in the ing in popularity. They have only one ad­
opposite direction. While one head is re­ vantage over other systems, ease of loading
cording the other is away from the tape, so and unloading the tape. Otherwise cassette
only one writing head is recording at any recorders are exactly the same as other tape
particular time. Each writing head records recorders.
one video frame as it passes diagonally
across the tape. The signal is laid down in Magnetic tape is low in cost and widely
used. But it has significant limitations. The
separate tracks (Fig. 13-18). The tape rate at which data can be recorded is lim­
ited by the speed of tape movement. Re­
moves just fast enough to separate the trieving a stored image from a tape can
tracks of the two heads. Consecutive lines lead to long access times if the exam is
are always written by different heads. stored far from the start of the tape. Mag­
Video tracks are separated by a narrow netic tape is not a good permanent record­
guard band. ing medium because the direct contact be­
tween the head and the tape causes tape
Cassette tape recorders are now increas- wear and degradation of the recorded

Writing Head Toke-up Reel --H- EAD DRUM (1,000 in/sec)
\

VIDEO
TAPE

Figure 13-17 Tape recorder Figure 13-18 Video tracks on tape

194 VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE

data. The polyester base of the tape must 90%) may be obtained. Wear is reduced
be properly stored or distortion will result. because the writing head does not touch
Shelf life of magnetic tape is about 2 years. the disc. In a clinical setting, disc recorders
may be used as a substitute for spot films,
Magnetic Disc Recorders and the x-ray exposure factors are fre­
quently increased, just as they would be for
Magnetic disc and tape recorders oper­ spot films. This exposure increase im­
ate on similar physical principles, but they proves the quality of the disc image by de­
have different functions. Tape recorders creasing quantum mottle. Primarily be­
are designed to show motion, comparable cause of this exposure increase, image
to a movie camera. Disc recorders are de­ quality is generally better with discs than
signed to show stationary images, more like with tapes. Resolution is best with the small­
a spot film camera. A tape recorder can be est image intensifier mode consistent with
used to show a single frame by stopping the clinical situation.
the tape against the revolving drum, but
this subjects the tape to considerable phys­ Magnetic disc or tape may be used to
ical wear. Disc recorders are designed to record either digital or analog signals. The
stop action. The discs themselves look like images recorded from a TV camera are
phonograph records.The video tracks are normally recorded as analog signals. With
laid down in preset grooves. Each groove the increasing interest in digital imaging,
in a disc is a completely separate track. The more images undoubtedly will be stored in
tracks are not continuous from groove to the digital format. Digital interest is a prod­
groove. One picture frame is recorded in uct of computer technology, since com­
each track. When a particular frame is se­ puters use the digital format. Nearly every
lected for replay, the same picture is shown computer in the world has a magnetic disc
over and over 30 times a second for as long recorder associated with it.
as the operator desires.
Magnetic disc recorders used in radiol­
Discs have several advantages over tape. ogy now have a bandpass limitation of
The most important one is random access. about 5 MHz. However, enormous infor­
The video grooves are numbered, and the mation storage capabilities are available.
recorder can go directly to any desired Information can now be stored at a rate of
number, without playing the intervening 2.5 megabytes per second, and it is antici­
numbers. This instantaneous access (as pated that storage of 1.8 gigabytes (a gi­
short as 30 ms) is considerably different gabyte = 109 bytes) on an 8-in. disc will
than a tape recorder, which may take sev­ soon be available. Magnetic discs find wide
eral minutes to reach a specific frame, even application in computer science.
with a rapid wind mode. Another advan­
tage of discs is that they are not subjected Optical Discs
to physical wear in the stop action mode.
Actually, they are designed to operate in It is anticipated that laser optical discs
this mode. One use of this "stop action" or will begin to replace magnetic hard discs.
"freeze frame" mode is to record a fluor­ Current technology centers on the optical
oscopic image, then review that image WORM (Write Once Read Many Times)
while planning the proper approach for a discs, but erasable optical discs are now be­
variety of invasive procedures (rather than coming commercially available. WORM
use continuous fluoroscopic viewing). Typ­ discs begin with a rigid substrate such as
ical clinical applications include angio­ glass, plastic, or aluminum. A photoactive
plasty, arterial embolizations, and hip pin­ recording medium is coated on the sub­
nings. Dose reductions to patient and strate. Recording is accomplished by mark­
physician greater than 50% (as high as ing or burning an indentation on the pho­
toactive layer using a laser source. Readout

VIEWING AND RECORDING THE FLUOROSCOPIC IMAGE 195

depends on reflection or scattering of light (number of vertical lines), whereas hori­
by these defects in the photoactive layer. zontal resolution is a function of the band­
The storage capability of the disc is deter­ pass. Display formats using more vertical
mined by the size of the focused laser beam lines and a higher bandpass are available
and the ability to space adjacent tracks close and desirable, but currently limited in clin­
to each other. Storage capabilities of optical ical use because of higher cost.
discs range from 10 to 50 times that of a
comparably sized magnetic disc. Standard­ It is often necessary to record the fluor­
ization of optical discs is not yet developed. oscopic image. Either the light image from
Currently, disc sizes vary from 3.5 to 14 the II output phosphor or the electronic
inches in diameter, with storage capabilities signal from the TV camera may be re­
ranging from 50 megabytes to 6.8 giga­ corded. The light image is recorded by a
bytes. The shelf life of an optical disc is at photospot camera or a cine camera. We
least 10 years, and probably much longer. have included routine spot films in this cat­
It is anticipated that optical discs will re­ egory because we did not know where else
place magnetic discs, both for use with to put them. The electronic signal from the
image intensified fluoroscopy and for dig­ TV camera may be recorded in analog or
ital archiving (data storage) applications. digital format on magnetic tape, magnetic
disc, or optical disc.

SUMMARY REFERENCES

Once the image is formed by the x-ray l. International Commission on Radiation Units
beam it is necessary to get the information and Measurements: Cameras for image intensi­
to the radiologist. With fluoroscopy, this is fier fluoroscopy. Washington, DC, International
done by displaying the image on a TV Commission on Radiation Units and Measure­
monitor. To make such a display, the image ments, 1969, Report 15.
is processed by a television camera tube.
The tube may be a standard vidicon, a 2. Fairchild Weston. The solid state imaging tech­
plumbicon, or a CCD . The image is dis­ nology. CCD imaging division. 810 West Maude
played on a TV monitor. Standard x-ray Ave., Sunnyvale, CA 94086.
closed-circuit television uses a 525 X 525
format with a 5-MHz bandpass . Vertical 3. Jost, R.G., and Mankovich, N.J. Digital archiving
resolution is limited by the scan line format requirements and technology. Investigative Ra­
diology , 23:803, 1988.

4. Judkins, M.P., Abrams, H.L., Bristow, J.D., et a!.:
Report of the Inter-Society Commission for
Heart Disease Resources. Circulation, 53:No. 2,
February , 1976.

5. Thompson, T.T., editor. A Practical Approach to
Modern Imaging Equipment. Little, Brown and
Co., Bostonfroronto, 1985.

CHAPTER The Radiographic Image

14

The basic tool of diagnostic radiology is Radiographic contrast depends on three
the radiographic image. The radiologist factors:
must be thoroughly familiar with the fac­
tors that govern the information content of 1. subject contrast
these images. R. E. Wayrynen* has sug­
2. film contrast
gested that the term image clarity be used 3. fog and scatter

to describe the visibility of diagnostically Subject Contrast
important detail in the radiograph.21 Two
basic factors determine the clarity of the Subject contrast, sometimes called radi­
radiographic image: contrast and image ation contrast, is the difference in x-ray
quality.
intensity transmitted through one part of
Image clarity the subject as compared to that transmitted
through another part. In Figure 14-1, as­
,/ "' sume that a uniform beam·of x rays strikes
an object made up of a block of muscle (A)
Contrast Image quality
and a block of bone (B) of equal thickness.
A discussion of image clarity is difficult
because there are so many subjective fac­ Few x rays are transmitted through the
tors involved. Many problems concerning bone, but most go through the muscle. The
image visibility are the result of the phys­ attenuated x-ray beam now contains many
iologic and psychologic reactions of the ob­ x rays in the area beneath muscle and few
server rather than of the physical proper­ beneath bone; this difference of intensity
ties of the image. There is no well defined in the beam caused by the object is subject
relationship between the amount of infor­ contrast.
mation on a film and the accuracy of in­
terpretation of the film. Many overlooked .. . . .
lesions are large and easy to see in retro­
spect. In this chapter the physical prop­ e. A . . 8 ......... :......•..·....· ...·.0·..ee. ... . .
erties of contrast and image quality will be . .. :·e·=:.·e:.::I.····.:··.:.:e.·.:.....·..e®.·.·..· e··..e·...·e...·....::.··:...•..:·..:.·.·e..-:..·.
considered. . . ..
. :. .·.. ·...·
CONTRAST
.·
The term radiographic contrast refers
Figure 14-1 Equal thickness of muscle (A)
to the difference in density between areas and bone (B) do not equally attenuate an x-ray
in the radiograph. Differing degrees of beam
grayness, or contrast, allow us to "see" the
information contained in the x-ray image.

*Technical Manager, X-ray Markets, E.l. du Pont de
Nemours & Company, now retired.

196

THE RADIOGRAPHIC IMAGE 197

Obviously, the effect of subject contrast I
on the x-ray beam cannot be seen directly.
Because the x-ray beam exposes the film,
however, anything that attenuates the
x-ray beam will similarly affect the radio­
graphic image on the film. As has been pre­
viously discussed, variations in the intensity
of the x-ray beam caused by subject con­
trast are greatly amplified by the film. Sub­
ject contrast is the result of the attenuation
of the x-ray beam by the patient, and at­
tenuation has been discussed in detail in

Chapter 5. A brief review will emphasize

the pertinent points.
Subject contrast depends on

l. thickness difference

2. density difference
3. atomic number difference

4. radiation quality (kVp)

Thickness Difference. If an x-ray beam SUBJECT CONTRAST =

is directed at two different thicknesses of Figure 14-2 Subject contrast
the same material, the number of x rays
transmitted through the thin part will be Density Difference. The difference in
greater than the number transmitted
through the thick part. This is a relatively density between body tissues is one of the
simple but important factor contributing to most important factors in causing subject
subject contrast. If 15 is the intensity of the contrast. The greater the density (i.e., mass
x-ray beam (I) transmitted through the per unit volume) of a tissue, the greater is
thin (small) segment, and IL is the intensity its ability to attenuate x rays. Consider ice
transmitted through the thick (large) seg­ and water. Because ice floats in water, it is
ment, subject contrast may be defined m
less dense than water. Water is about 9%
the following way (Fig. 14-2):
more dense than ice. Equal thicknesses of
�Subject contrast = ice and water will demonstrate subject con­
trast to an x-ray beam because the water
Another less obvious cause of difference
in tissue thickness is the presence of a gas­ attenuates 9% more of the beam than does
filled cavity. Because gas attenuates almost
no x rays, the presence of a pocket of gas the ice.
in a soft tissue mass has the same effect on
the x-ray beam as decreasing the thickness Atomic Number Difference. Subject
of the soft tissue mass. In ventriculography
the brain ventricles are visible because they contrast depends on the relative difference
are filled with air. The x-ray beam then in attenuation of the x-ray beam by differ­
"sees" the cross section of brain containing ent tissues in the body. In diagnostic radi­
a gas-filled ventricle as being "thinner" ology, attenuation of the x-ray beam by the
than that of the adjacent solid brain sub­ photoelectric effect makes the most im­
stance. portant contribution to subject contrast.
Photoelectric absorption is increased in
substances with high atomic numbers, es­
pecially when low-kVp x rays are used. The

198 THE RADIOGRAPHIC IMAGE

effective atomic numbers of bone, muscle by a simple example. In Figure 14-2, sub­
(water), and fat are
ject contrast was defined as Is. Assume that
IL

Bone 13.8 100 x rays of low kVp (such as 50 kVp)
Muscle 7.4 strike the object (Fig. 14-3A). Most of these
Fat 5.9
low-energy x rays are attenuated by the

thick part, but quite a few can penetrate

Bone will attenuate many more x rays the thin part. Assume the numbers to be
than muscle or fat, assuming equal thick­
nesses. Subject contrast between bone and 25 (thick) and 40 (thin). By the definition
muscle is high. Muscle and fat, with little
difference in atomic number, show little of subject contrast,
difference in their ability to attenuate
x rays by the photoelectric absorption = =. 40
process, and less difference by Compton Subject contrast 25 1.60,

reactions. Use of low-kVp (below 30) x rays which states that the thin part transmits

produces the greatest possible difference 60% more x rays than the thick part at 50
in photoelectric x-ray absorption between
muscle and fat. Soft-tissue radiography, kVp. If the kVp is then increased to 80
such as mammography, requires the use of
low-kVp x rays because the small differ­ (Fig. l4-3B), more x rays will get through
ences in atomic number between breast tis­
sues produce no subject contrast unless both the thick and thin parts. Both Is and
maximum photoelectric effect is used.
IL will increase, but Is will increase more
Contrast Media. The use of contrast ma­
Is
terials with high atomic numbers (53 for than lu so IL becomes smaller, and subject
iodine and 56 for barium) gives high sub­
contrast is decreased. Figure 14-3B as­
ject contrast. Photoelectric absorption of x
rays in barium and iodine will be propor­ = =sumes values of Is 80 and IL 60, giving
tionally much greater than that in bone and :� =subject contrast of
tissue because of the large differences in 1.33, or only 33%
atomic number.
difference in transmitted radiation inten­
Radiation Quality. The ability of an x­ sity.
ray photon to penetrate tissue depends on
its energy; high-kVp x rays have greater As a general rule, low kVp gives high
energy. Selecting the proper kVp is one of subject contrast. This is often called short­
the most important matters to consider in scale contrast because everything is black
choosing the proper exposure technique. or white on the film, with fewer shades of
If the kVp is too low, almost all the x rays gray in between. High kVp gives lower sub­
are attenuated in the patient and never ject contrast, called long-scale contrast, be­
reach the film. cause there is a long scale of shades of gray
between the lightest and darkest portions
The kilovoltage selected has a great ef­ of the image.
fect on subject contrast. Low kVp will pro­
duce high subject contrast, provided the Exposure Latitude. A low-contrast film
kVp is high enough to penetrate the part (shallow slope of the characteristic curve)
being examined adequately. has greater exposure latitude. That is, a
wider range of mAs settings will produce
In general terms, the reason low kVp proper film density if the kVp is satisfac­
produces greater subject (or radiation) tory. Similarly, kVp will also have an effect
contrast than high kVp can be explained on exposure latitude. High-kVp tech­
niques will allow a wider range of mAs set­
tings (wide exposure latitude) but will re­
sult in relatively less contrast. Low-kVp
techniques produce high subject contrast
because there is a large variation in the in-

THE RADIOGRAPHIC IMAGE 199

® ®
kVp =50 kVp = 80

I=IOO I= 100

��SUBJECT CONTRAST= = 1.60 :�SUBJECT CONTRAST = = 1.33

Figure 14-3 Subject contrast varies with the kVp of the x-ray beam

tensity of the transmitted x-ray beam in Figure 14-4A shows that if the exposure
different parts of the patient. The x-ray
film must then "decode" a range of expo­ (mAs) is only a little too low, the low-ex­
sure from low to high. This wide range of posure areas (i.e., under bone) will produce
exposure (log relative exposure) must fall exposures in the toe area of the curve.
within the steep portion of the character­ Likewise , excessive mAs will rapidly move
high-density exposures onto the shoulder
istic curve (Fig. l4-4A). When a low kVp of the curve. Both these mistakes will de­
crease film contrast. High-kVp exposures
is used, the mAs must be carefully selected.

Low kVp High kVp

I. High contrast I. Low contrast

2. Small ex­ 2. Large ex­

posure posure

>- latitude >- latitude

!:: t:
en
en

z z
UJ UJ

0 0

AB

LOG RELATIYE EXPOSURE E2EI

LOG RELATIVE EXPOSURE

Figure 14-4 Exposure latitude varies with the kVp of the x-ray beam

200 THE RADIOGRAPHIC IMAGE

produce less difference in intensity be­ kVp is increased from 50 to 60 the change
tween areas of the attenuated x-ray beam; in film density (if mAs remains constant)
this is why high kVp gives less subject con­ will be about
trast. With a high-kVp technique, the film
has to "decode" a smaller difference in log 604
relative exposure between the low- and - = 2.07
high-exposure areas. so•

Figure 14-4B diagrams how a high-kVp Because kVp has doubled film density,
technique (using the same object and film­ mAs would have to be cut in half. An old
screen system) will "use up" a much shorter rule says: "If you add 10 kVp, cut mAs in
portion of the steep portion of the film's half." This holds true around 50 to 60 kVp,
characteristic curve. Using high kVp, the but kVp change has less effect on film den­
technologist has considerable room for er­ sity at higher ranges. For example, at 85
ror in the choice of mAs, because the ex­ kVp, an increase of 15 kVp is required to
posure range can move up or down on the double the film blackening power of the
curve and still fall within the steep portion beam.
of the curve.
Subject contrast thus is seen to vary with
To review, kVp influences subject con­ the makeup of the subject (thickness, den­
trast (exposure differences) and exposure sity, atomic number), the use of contrast
latitude; mAs controls film blackening material, and the kVp of the x-ray beam.
(density). Consider a specific example. A
Film Contrast
chest film (par speed film-screen combi­
nation, 6-ft distance, no grid, exposed with Film contrast has been discussed as a
factors of 70 kVp and 6.6 mAs [400 rnA, photographic property of x-ray film (see
l/60 sec]) would result in a radiograph with Chap. 11). X-ray film will significantly
high contrast. The exposure of 6.6 mAs, amplify subject contrast provided the ex­
however, is critical at 70 kVp. An error of posure (mAs) is correct (keep away from
±50% would probably produce a radio­ the toe and shoulder of the characteristic
graph with a great deal of its density on curve). Under good viewing conditions, a
the toe or shoulder of the film curve, re­ density difference of about 0.04 (differ­
sulting in an unacceptable loss of contrast. ence in light transmission of 10%) can be
Another chest examination exposed at 100 seen.

kVp and about 3 mAs would result in a Fog and Scatter

low-contrast radiograph, but changing the The effect of fog and scatter is to reduce
radiographic contrast.
exposure (3 mAs) by a factor of ± 100%
Scattered radiation is produced mainly
would probably not reduce radiographic as a result of Compton scattering. The
contrast. Obviously, fewer mAs or more amount of scatter radiation increases with
mAs produces a lighter (less density) or increasing part thickness, field size, and en­
darker (more density) radiograph, but the ergy of the x-ray beam (higher kVp). Scat­
density range still falls on the steep portion ter is minimized by collimation of the x-ray
of the film curve. At 100 kVp the exposure beam (use as small a field size as possible)
can be varied considerably; this is termed and the use of grids or air gaps. Scatter
large exposure latitude. At 70 kVp, the radiation that reaches the x-ray film or
exposure must fall within a narrow range film-screen combination produces un­
(i.e., there is small exposure latitude). wanted density.

To make matters more complicated, kVp Fog is strictly defined as those silver hal­
does have an effect on film blackening. ide grains in the film emulsion that are de­
This effect is approximately equal to the veloped even though they were not ex-
fourth power of the kVp. For example, if

THE RADIOGRAPHIC IMAGE 201

posed by light or x rays. The amount of 3.5
fog in an unexposed x-ray film can be dem­
onstrated easily. Cut the unexposed film in 3.0
half. Develop (fully process) one-half of the
film and clear (fix, wash, and dry only) the 2.5
other. The density difference between the
two film halves represents the amount of 2.0
fog present. Fog produces unwanted film
density, which lowers radiographic con­ C/}
trast.
z
Another type of unwanted film density u.J 1.5
may result from accidental exposure of 0
film to light or x rays . This is usually also
called "fog" or "exposure fog" and, al­ 0.9 ----------------------- �Gamma= 2.4
though the term is not absolutely correct,
it has established itself by common usage. Addilion of fog--
These two types of "fog" are different in 0.5 and scalier
origin, but both lower film contrast in the
same manner. LOG RELATIVE EXPOSURE

True fog is increased by the following Figure 14-5 Fog and scatter decrease radio­
conditions: graphic contrast. (Courtesy ofR. E. Wayrynen2')

l. Improper film storage (high temper­ To repeat, fog and scatter are undesir­
ature or humidity) able because they decrease radiographic
contrast by decreasing film contrast.
2. Contaminated or exhausted devel­
oper solution Image Quality

3. Excessive time or temperature of de­ The second basic factor determining
velopment image clarity is image quality. The quality
of the radiographic image may be defined
4. Use of high-speed film (highly sensi­ as the ability of the film to record each
tized grains) point in the object as a point on the film.
In radiology this point-for-point repro­
Fog, "exposure fog," and scatter add duction is never perfect, largely because of
density to the film. By knowing the mag­ the diffusion of light by intensifying
nitude of this density and the characteristic screens. There is no general agreement as
curve of the film, it is possible to calculate to what should be included in a discussion
the effect of the added density on radio­ of image quality. Our approach will involve
graphic contrast accurately.. Figure 14-5 almost no mathematics .
shows how fog and scatter change the slope
of the characteristic curve of a film. Note Image quality is influenced by
that the slope of the curve is decreased
(contrast is decreased) most at lower levels l. radiographic mottle
of den.sity. These are the densities used 2. sharpness
most frequently in diagnostic radiology. In 3. resolution
Figure 14-5 the gamma of the normal film,
at a density of 0.9, is 2.4. When fog and Radiographic Mottle
scatter are added, the gamma drops to 1.8. If an x-ray film is mounted between in­
At a density of 2.5, fog and scatter have no
significant effect on the shape of the curve, tensifying screens and exposed to a uni-
but densities as high as 2.5 are seldom used
in diagnostic radiology.

202 THE RADIOGRAPHIC IMAGE

form x-ray beam to produce a density of beam may be thought of as containing a
certain number of x-ray photons, or an
about 1.0, the resulting film will not show equivalent number of quanta.

uniform density but will have an irregular By showing a pattern of mottle, or non­
mottled appearance. This mottled appear­ uniform density, the x-ray film is telling us
ance (caused by small density differences) that it has "seen" a nonuniform pattern of
is easily detected by the unaided eye, and light on the surface of the intensifying
may be seen, if looked for, in any area of screen. The nonuniform pattern of light
"uniform" density in a radiograph exposed on the screen is caused by fluctuations in
with screens (e.g., the area of a chest film the number of photons (or quanta) per
not covered by the patient). This uneven square millimeter in the beam that arrived
texture, or mottle, seen on a film that at the screen. What this means is that a
"should" show perfectly uniform density is "uniform" beam of x rays is not uniform
called radiographic mottle. Radiographic at all. Suppose a "uniform" x-ray beam
mottle has three components: could be frozen in space and cut into cross
sections. If the number of x-ray photons
Radiographic mottle per square millimeter were counted, it
would be unlikely that any two square mm
r { \s eomo«l would contain exactly the same number of
photons. The "uniform" beam is not uni­
Structure Quantum Film graininess form.
mottle mottle
The actual number of x-ray photons per
Only quantum mottle is of any importance mm2 obeys the law of probability, because
in diagnostic radiology. the emission of x rays by the x-ray tube is
a random event. The average number of
Film Graininess. Film graininess makes photons per mm2 can be calculated by add­
no contribution to the radiographic mottle ing the number in each mm2 and dividing
observed in clinical radiology. Film grain­ by the number of squares. It will then be
found that the actual number in any square
iness can be seen when the film is examined will almost never be the average value, but
that all numbers will fall within a certain
with a lens producing magnification of 5 to range (percent fluctuation) of the average.
10 x . With this magnification, the image The law of probability says that the mag­
nitude of this fluctuation is plus or minus
is seen to be made up of a nonhomoge­ the square root of the average number of
neous arrangement of silver grains in gel­ photons per mm2• (The square root of the
average number of photons is usually re­
atin. Because a radiograph is almost never ferred to as the standard deviation.) For
example, if an x-ray beam contains an
viewed at an enlargement of 5 x, film
average of 10,000 photons per mm2, the
graininess is not seen.
Screen Mottle. Screen mottle has two number in any one square mm will fall in
the range
components, structure mottle (unimpor­
tant) and quantum mottle (important). 1 0,000 ± v'1 0,000 = 1 0,000 ± 1 00
Structure mottle is caused by defects in the
intensifying screen, such as varying thick­ or any mm2 may be expected to contain

ness or physical imperfections in the phos­ between 9900 and 10,100 photons. In some
of the squares (32%) the variation will be
phor layer. Such screen in�egularities can
occur, but the quality control used in screen even greater than this.
manufacturing is so good that structure

mottle may be dismissed as making no con­
tribution to radiographic mottle.

Quantum mottle is the only important
cause of radiographic mottle. Quantum re­

fers to a discrete unit of energy and, in this
discussion, it may be considered as the en­
ergy carried by one x-ray photon. An x-ray

THE RADIOGRAPHIC IMAGE 203

The percent fluctuation in the actual of the cardboard top of a hatbox; small
cardboard figures in the shape of a circle,
number of photons per mm2 becomes square, and triangle placed in the hatbox
top; and 1000 pennies. The pennies rep­
greater as the average number becomes resent x-ray quanta, and were dropped
into the hatbox top in a completely random
smaller. If 100 photons are used, fluctua­ manner. Radiographs of the model were
made after 10, 100, 500, and 1000 pennies
tions will be ± y!T{)O, or ± 10, giving a had been dropped into the hatbox top. No­
tice that the radiograph of 10 pennies
percent fluctuation of 1° 10%. Using shows no information about the makeup of
, or the model; 100 pennies show that the hat­
100 box top is probably round; 500 pennies
show three filling defects in the hatbox top,
10,000 photons, fluctuation is ± 100, or a which is definitely round; and all 1000 pen­
nies clearly define the shape of each filling
percent fluctuau. on of 100 or 1 Of defect. In other words, a lot of pennies
lO,OOO ;o . (quanta) provided a lot of information
about the model, fewer pennies provided
Quantum mottle is caused by the statis­ less information, and too few pennies pro­
vided no useful information.
tical fluctuation in the number of quanta
Quantum mottle is difficult to see unless
per unit area absorbed by the intensifying a high-quality radiograph is produced. A
film of high contrast and good quality will
screen. The fewer quanta (x-ray photons) show visible quantum mottle. If film con-

used, the greater will be the quantum mot­

tle (more statistical fluctuation). Intensify­

ing screens are used because they decrease

the x-ray exposure (number of photons)

needed to expose the x-ray film. Quantum

mottle is seen when intensifying screens are

used. Quantum mottle will be greater with

high-kVp x rays because they produce a

higher screen intensification factor.

The concept of quantum mottle is illus­

trated in Figure 14-6. The model consists

Figure 14-6 The "pennies-in-the-hatbox" model iilustrating the concept of quantum mottle