.,

:

:

AAPM :: Tutorial

Stephen

.

Mammography Lincoln

author

introduces

first

of this

series

physics

in general.

on

principles

Annual the

Symposium

physics

applied

details

is used

and detection

clinical objects that parameters, and the

basic

diographic fit versus

to obtain

clinical

those

differing

of general

about

and the choices

that

need

emphasis is on image clinical application

are also

radiography

by the

exist

to be made

are to

demands.

patients

formation. of the

ma-

equipment

clinical

Interrelationships

In the basic

is intended and radiography

to be visualized, the patient’s anatomy, receptor. In this tutorial, breast imaging

imaging. The risk considerations,

cal development

imaging

information

images.

Physics. the

The analysis in particular

from

of the

x-ray

need image

principles

differ

because

of transmission

on Basic

of mammography,

in radiographic

systems

important

INTRODUCTION Radiography

trate

1 5th

Mammographic

in several

‘

the of tutorials

in terms of a mammographic system. in the understanding of mammography

assist

System1

PhD

The

discussed

PhD

as a Radiographic

B. Hubbard,

diologic

Balter,

the

formation

between

the

x-ray is used

beam to illus-

to obtain

optimal

In addition, technique,

dose,

ra-

beneits histoni-

and

reviewed.

The formation of a radiograph begins with the formation of a shadow image; this is shown schematically in Figure i A uniform x-ray beam is directed toward the patient. As it passes through the patient, it is attenuated. There will be a differential transmission of radiation if the attenuation of an object differs from that of its surroundings. This differentially transmitted beam carries the diagnostic information. After this beam exits the patient, it is detected and captured by an image me.

S

ceptor.

The

captured

patient

and

object.

dered. Our

The image concentration

tance only

makes

sense

the handling In particular,

Index

terms:

I

From

1990;

the

radiologic of the

©RSNA,

Department

IL 605

read,

in the

context

radiography,

quality

of the

assurance

a visible and

representation

a diagnostic

.

Breast

that report. widen

lead from the In addition, clinical

referral, diagnosis

radiography,

of the

impression

for future reference. is not intended to minimize

activities diagnostic

the

men-

impon-

positioning radiologic

process,

of the diagno-

which

includes

treatment, and follow-up. does not stand by itself; it is

radiation

dose

#{149} Images,

quality

Hubbard,

and

#{149} Physics

10:103-113 of Medical

Physics,

Rush

mc, Glencoc, Ill. From the 15th Annual Symposium vember 7, 1989; accepted November 8. Address Grove,

to produce

is ultimately

of other diagnostic information, the ability to obtain a correct

Breast

RadloGraphics

is processed

image

may then be archived on image creation

of the many other to the completion

patient sis

image The

University,

Chicago,

on Basic Physics reprint requests

and

Fields,

Griffith,

at the 1988 RSNA annual to the author, 4 1 1 3 West

meeting. End Rd.

Broadbent, Received Downers

No-

1 5-2307.

1990

103

X-RAY SOURCE

----

Figure

2.

Image

iions

of the

tient.

Variations

sult

formation

x-ray

in a range

beam

requires

por-

to penetrate

in patient

the

thickness

of radiation

pa-

will

intensities

re-

reach-

ing different points on the image receptor (eg, A, B, C, D). The dynamic range (latitude) of the image receptor must be sufficient

to record

these

varied

intensities.

INCIDENT BEAM

dx

OBJECT PATIENT

Figure

3.

Subject

differential

different

IMACER

Figure 1. Projection radiographic systems provide information about structures within patients by means of a shadowgraph of the area of interest produced with a penetrating x-ray beam emanating from a small source.

certainly not to be confused of any radiologic procedure social environments. SYSTEM

REQUIREMENTS

104

U

RadioGraphics

A radiographic

imaging

with will

system

a cure. change

must

contrast

transmission

structures

is formed of x rays

in the patient.

by the

through

In a uni-

form slab of tissue, the subject contrast of a dense object will depend on the thickness of the object (dx) and the difference be-

tween ject

the attenuation and

As time passes, both in response to changes

produce

coefficients

its surrounding

a spatially

uniform

of the ob-

tissue.

the

risk in the

and benefit medical and

primary

x-ray

beam that is simultaneously capable of penetrating the patient and is sensitive to attenuation differences within the patient. Such a beam will develop contrast in passing through the patient. The x-ray beam must also have suitable geometric properties. Spatial resolution is directly influenced by the focal spot size. The source-image distance and anode angulation determine the radiation field size. Motion unsharpness is influenced by exposure time. This in turn is a function of tube loadability and therefore partially anode angulation. The source-image distance and patient position determine the magnification of the image. Magnification geometry reduces the effects of scatter but usually requires smaller x-ray focal spots. Other considerations, for any imaging system and geometry, include patient dose and scattered radiation control. It can be seen that no general solution can be obtained for all radiography; appropriately optimized, dedicated systems are needed for different examinations.

U

Hubbard

Volume

10

Number

1

The choice of an appropriate imaging system is complicated by the need to balance radiographic contrast, noise, and patient dose. In most imaging systems, contrast and dose are both reduced as the voltage applied across the x-ray tube increases. If receptor sensitivity is increased, fewer x-ray quanta are needed to form the image. Quantum noise is therefore increased. Sensitive receptors tend to have an inherent noisy structure. This further increases the total image noise. The pres-

ence there

of noise are real

in the upper

image limits

obscures on tube

the visibility voltages and

of low-contrast detail. receptor sensitivities.

Thus,

The portion of the patient containing the object of interest has a major influence on imaging technique. The thickness of the part sets a lower limit on beam penetrability. A sufficient number of photons must be transmitted through the part so as to form a usable image. This is seen in Figure 2 The differences in transmitted intensity across the image must be limited so as to fit within the dynamic range of the image receptor (too much contrast is not helpful) However, the difference in transmission through a region in the object of interest relative to adjacent tissue must produce enough radiographic contrast so that the object can be distinguished from noise fluctuations (too little contrast is not helpful either) The quality, or penetrating ability, of an x-ray beam is customarily stated in terms of its half-value layer. .

.

.

The formation of contrast in the x-ray beam transmitted through the patient occurs as a result of differential attenuation along different rays in the beam. This is illustrated in Figure 3 In the absence of scatter, the local transmission of a ray passing a distance dx through an object with a linear attenuation coefficient u1 (where

CONTRAST

.

‘0

=

incident

intensity)

is given

by I

dx

11’0e

A nearby uation

nay passing

coefficient

through

u2) will

an identical

have

a local

thickness

transmission

of surrounding given

tissue

(atten-

by

-u,dx

‘2’0e The overall transmission sion and the transmission remainder of the patient

the two

of each through is identical

ray will be the product of this local transmisthe remainder of the patient. Assuming that the for both nays, one can calculate the ratio of

transmissions: u2)dxl

(-(U,

exp

‘1/’2

which

can

be approximated

by 1

‘l/’2

-

[(u1

u2)dx].

-

The term in brackets describes the scatter-free contrast in the transmitted x-ray beam. Contrast depends on the object thickness (dx) and the difference between the two attenuation coefficients (u1 and u2) Contrast will increase linearly with .

the

thickness

coefficients

of the is more

object. complex.

The

influence This

difference

of the

difference

decreases

between as the

quality

attenuation of the

x-ray

beam increases; it increases as the atomic number (Z ) of the object increases, and it increases as the density (p) of the object increases. Thick, dense objects with high atomic numbers are easily seen with a soft x-ray beam. Conversely, thin tissue-density objects with lower atomic numbers are extremely difficult to visualize with a hard x-ray beam.

January

1990

Hubbard

U

Ra4ioGrapbics

U

105

0.05

ATTENUATION

X-RAY

SPECTRA

WATER

.

E >I-

I-

U)

z

z

LU

LU I-

U

z

U. U.. 0. LU

LU

50 KVP

>

unfiltered

I-

10

0 U

z 0

R

LU

50

filtered b; 1.5 mm, Al -‘

z

LU

,

10

I-

I-

20

4.

produced

The when

ENERGY

shape

target

In

(keV)

of the x-ray

a thick

(I)

50

140

30

PHOTON

Figure

spectrum

is struck

0.01

by an

electron beam can be approximated by a tnangular shape. In this illustration, 50 and 60 kVp spectra are represented by heavy and light solid lines. Filtering of the x-ray beam will remove many of the low-energy x-ray photons. The 50.kVp spectrum, filtered by 1 .5 mm of aluminum, is shown by the

dashed

0.05

KVP

line.

ic

20

110

PHOTON

60

ENERGY

100

150

(keV)

Figure 5. The attenuation of x rays between 1 0 and 1 50 keY is the result of three independent processes: the photoelectric process, coherent scattering, and the Compton process. The total attenuation coefficient of the beam at any energy is the sum of the three individual attenuation coefficients.

X-RAY SPECTRA

The previous analysis, nect for a monoenengetic lion have trates the

The

first

a spectrum behavior

which used a single beam of radiation. of photon of an x-ray

approximation

energies. The system in which

of the spectrum

angular distribution of intensity ure 4. The initial spectrum (50 the tube voltage can be changed. initial spectrum produced at 60 number of photons (intensity)

.

kVp

the

line

relative

to that

under

attenuation Almost

coefficient all practical

u, is only

sources

con-

of radia-

following simplified model illusthe x rays have a range of energies.

produced

by a thick

x-ray

target

is a tn-

versus energy. This is shown schematically in FigkVp) can be modified in two principal ways. First, In Figure 4 , the lighter straight line shows the kVp. Increasing the voltage increased the total This is seen by the greater total area under the 60-

50-kVp

line.

The

average

photon

energy

has

also been shifted toward a higher value as the voltage was increased. Second, the shape of the spectrum can be modified by allowing the beam to pass through an absorber on filter on its way to the patient. Filtration usually removes more of the lower-energy photons than the higher-enengy photons. The result of filtration en with an increased average beam some of the filtration is accomplished

lope

of the x-ray

tube

or portions

is a reduction energy (beam by means

of the

light

of x-ray beam intensity togethquality) In an imaging situation, of structures, such as the enve-

localizer.

.

Additional

aluminum

fil-

tration is usually added to bring the beam quality to an acceptable value. The genenal appearance of the filtered spectrum shown in Figure 4 is typical of most diagnostic beams. The actual beam parameters are close to those used in clinical xeromammography.

106

U

RadioGraphics

U

Hubbard

Volume

10

Number

1

PHOTON U. LU

TRANSMISSION

IN

0

WATER

U

z

LU

I-. IU) U)

.c z 0 U) U) U)

Figure

6.

thickness

properties

and

x rays properties change rapidly with energy, transmission will also vary considerably. Since x-ray photons of very low energy

-I

2

5 U.

The attenuation

of tissue determine the fraction that is transmitted. Since attenuation

negligible to the transmissibility, tribute information

0 01

PHOTON

(keV)

ENERGY

X-ray beam filtration used for screen-film complicated than that previously described tration is done properly, the beam quality should be noted here that mammographic mography

and

Two

for

reference

xenomammography

energies

are

are shown

of

have

they do ofnotthe concontent ra-

diograph.

Most of the x-ray

that

is not

transmitted

utes

to the patient’s

beam

energy

eventually

Integral

contnib-

dose.

mammography is considerably more and will not be discussed here. If filis lowered for this special situation. It beams optimized for screen-film mamnot

interchangeable.

on the horizontal

axis of Figure 4 They are shown because most of the photons that are useful for breast imaging lie between these two energies. The physical explanation for these particular values follows. For the purpose of discussing x-ray interactions, one may simplify the descniption of the patient to that of a uniform cient for water as a function of energy tions of x rays with matter at diagnostic ton interactions are the most important. attenuations

caused

by

each

.

thickness of water. The attenuation coeffiis shown in Figure 5 Of the three interacenergies, the photoelectric and the CompTotal attenuation is the sum of the

individual

.

mechanism.

Contrast

in mammography

is

almost totally attributable to the photoelectric interaction because it is a function of the atomic number Z. The quantity of Compton attenuation is mainly a function of density; this varies slightly in the breast. The upper reference energy in Figure is that at which less than 25% of the total interactions are due to the photoelectric effect. Above this energy, most of the x-ray interactions in the patient will create radiation dose without creating much tissue contrast. At a given energy, if scatter is disregarded, the attenuation coefficient and the thickness

determine

the

fraction

of the

incident

energy

that

will

5

be transmitted

6 shows two transmission curves: one for 5 cm of water (an approximation ofthe breast) and one for 20 cm ofwater (an approximation of the abdomen) The lower reference energy in Figure 6 corresponds to a i % transmission through 5 cm of water. Photons with lower energy are almost totally absorbed in tissue. They contribute to patient dose without yielding diagnostic inforthrough

an

object.

Figure

.

mation.

These

photons

should

be excluded

from

incident

to produce

in thick

obtained

January

As shown in Figure 6, the low-energy higher. Therefore, it is not possible patient parts that is comparable to that

the

as possible. ten is much

1990

reference

x-ray

point

beam

radiographic

in breast

as much

for 20 cm of wacontrast

images.

Hubbard

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107

>.

0.14

SPECTRAL THROUGH

I-

Figure

7.

The filtered

50-kVp

x-ray

trum

shown in Figure 4 is further by its transmission through 5 cm

spec-

the effect

.

a “filtered

WATER

transmitted

?

beam”

0.2

o.

“

i

0

10

20

(50

medium,

the

through

kVp

the

+ i-mm

30

40

ENERGY

PHOTON

beam is incident on an absorbing are modified by its transmission

of passing

5 cm

transmissIon factor

modified ofwater.

The high-energy portion of the transmitted spectrum is mainly determined by the shape of the corresponding portion of the incident spectrum. The shape of the low-energy portion of the transmitted spectrum is mainly determined by the attenuation properties of 5 cm of water.

If an x-ray its spectrum

TRANSMISSION

50

(keV)

intensity

and

medium.

shape of 7 shows

Figure

aluminum

filtration)

through 5 cm ofwater. The incident spectrum has a peaked shape. The transmitted on exit spectrum has a narrower peaked shape. The intensity of the transmitted spectrum is normalized to match the intensity of the incident spectrum. This normalization allows us to compare the shapes of the two spectra. It may be seen that the lower-energy portion of the transmitted spectrum is primarily shaped by the transmission characteristics of water, while the high-energy sides of the two spectra are similar.

The filtered

shapes of the spectra of the transmitted beams, 50-kVp beams are incident on 5 cm of water,

when

both

filtered

and

are shown in Figure intensity of the filtered

un-

8. The beam

two beams have been normalized by increasing the by approximately 50%. The spectra of these two exit beams are strikingly similar. The principal difference is that the filtered input beam produces a spectrum that is shifted

slightly

toward

higher

energy.

Thus,

there

will

be

a small

reduction

in

contrast in the images produced by the filtered beam. Figure 9 shows the spectra of the corresponding incident beam. The total input intensity of the unfiltered beam is much higher than that of the filtered beam. Thus, the filtered beam will produce much less patient dose. This is true even though it was necessary to increase the quantity of x-ray production by about 50% for the case of the filtered beam. This latter requirement increases heat loading on the x-ray tube and may mequine an increase in exposure time.

BREAST IMAGING

Requirements anatomy and signs

of cancer.

cause tween

contrast fat and

is generally soft tissue.

erable

contrast.

Since

is usually

tial

108

U

Ra4ioGrapbks

for breast imaging are dominated by the of objects such as masses and calcifications,

modest.

resolution

U

Both

Hubbard

The

as well

normal

and

abnormal

structures

nature are

created by the small differences Small amounts of calcium in the

the typical visualization

as adequate

breast

calcification

of small

calcifications

of normal

which

are

difficult

to image

in atomic breast can

is small,

breast

diagnostic

the absolute

requires

be-

number beyield consid-

contrast

excellent

spa-

contrast.

Volume

10

Number

1

c\

>..

INCIDENT

X-RAY

I-. U)

TRANSMITTED

z

SPECTRA

SPECTRA

LU

I-.

I-

filtered

z

U)

z

LU I-

unfiltered

LU

z

>

unfiltered

-

LU

I-

>

I-

-I LU

LU

0

10

20

30

PHOTON

40

50

ENERGY

0

(keV)

8, 9.

(8)

ted through

The

final

spectra

5 cm of water

low-energy

been

absorbed

scale)

that

are of similar

photons

from

the

by the aluminum.

would

yield

of the

the

The normal shape ties if compression used to accommodate ceptable.

makes

or unfiltered

and intensity.

unfiltered

spectrum,

These

are the incident

(9)

spectra

shown

entire

was

ENERGY

50

(keV)

of the possible

such

breast the

as ultrasonography

requirements system.

recognized

early

which

would

spectra

have

(plotted

special

need

in the history

thickness

for

low

almost otherwise

on the same photon much

results

meet

equalizes

the

demanding

in breast

x-ray

for

of mammography.

is

trans-

image

These imaging

me-

and

res-

are specialtechnol-

imaging.

led to progressive

kilovoltage

intensidetector be unac-

contrast

mammography. systems. Other

enless

in a much

(high-contrast)

roles

have

transmit-

variation in transmitted If a wide-latitude loss of contrast may

of narrow-latitude

have The

beams

absorbs

8. At higher energies, it has

to a uniform

use

of mammography

imaging

50-kVp

The water

more intensity; at lower intensity of the filtered spectrum

ceptoms. Theme are two image receptor systems that olution requirements of general and screening ized screen-film systems and xeroradiographic The

110

in Figure

of the breast produces a wide is not used during mammography. the beam, the corresponding

Compression and

filtered

shape

transmitted

ergies, the filtered beam has a little intensity. The markedly lower total lower dose to the patient.

ogies

30

9.

Figures

mission

20

PHOTON

8#{149}

all the

10

specialization

adequate

Direct

of the

subject

exposure

contrast

of industri-

al x-ray film was shown to produce usable images at doses of about i 00 mGy. Today, the use of specialized screens and films has reduced dose to around i mGy, while producing diagnostically useful images. Some of this dose reduction is ob-

tamed filter,

by use of a dedicated as well

as with

In xeroradiography,

x-ray

an appropriate

a charged

unit

equipped compression

selenium

plate

with

a molybdenum

target

and

device.

is used

as the detector,

along

with

a unique processing mechanism. It overcomes many of the problems associated with varying tissue thickness. The sensitivity of the plate is not as great as that of current screen-film systems. Even with separately optimized target and filter combinations (molybdenum-aluminum or tungsten-aluminum), xeromadiography presently requires doses of about 3 mGy.

January

1990

Hubbard

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109

0.7

-

BREAST CANCER INCIDENCE MASSACHUSETTS

0 6 -

>-

.

0 5

LOW

DOSE

BREAST CANCER INCIDENCE

FLUOROSCOPY

-

.-

-

I-

&o.i

-

0.4

0.2-

-

0. 3

r15

-

.‘

Exams

‘-NaturaI

--.-#{149}-.--

J

__

FIT

I

IncIdence

05

BREAST

DOSE,

Grays

BREAST

Grays

DOSE.

10. 11. Figures 10, 11. (10) High doses of radiation have been correlated with an increased incidence of cancer. In this illustration, high fluoroscopic doses correlate with an increased incidence of breast cancer. The large errors (vertical bars) make it impossible to define fully the functional form of the relationship between dose and risk. The linear, no-threshold hypothesis is shown by the dotted line. (11) The graph is an expansion of the low-dose portion of Figure 1 0 so as to analyze the dose delivered in modern mammography. Under the linear no-threshold hypothesis, the total incidence of breast cancer in a population

be seen grams

DOSE

that were

of women

this is very

undergoing

close

1 5 mammographic

to the incidence

examinations

in the population

is shown.

in which

It can

no mammo-

obtained.

The dose delivered to a patient during irradiation has an associated risk. Figure iO demonstrates the risk of breast cancer induction due to high radiation doses. The uncertainties, as shown by the error bars, are consistent with either the usual linear dose-risk fit on other functional models. However, the doses required for modcnn mammography are much less than those used in the study upon which Figure i 0 is based. Figure i 1 , an enlarged detail from Figure 1 0, illustrates the risk associated with modern screen-film mammographic examinations. If the linear doserisk hypotheses are correct, the total risk associated with i 5 mammograms is the value plotted in Figure i i This is a small value in comparison to the natural risk of breast cancer. Even though the risk may be small for an individual patient, it .

could

be considered

significant

when

millions

of patients

are irradiated.

Many different doses could be defined. For example, one might consider the dose at the point on the patient’s skin at which the beam enters (skin dose) , the dose at the midline of the breast, or the average dose to the glandular tissue of the breast. These three kinds of doses vary in their accuracy, utility, and ease of measumement.

Figure three that ical model backs skin

i 2 schematically

associated dose

shows

these

can be measured directly. The on physical phantom in order with

is delivered

skin

dose.

to different

As shown points

concepts.

Skin

dose

other two require to be evaluated. in Figure in the

patient.

is the only

one

of the

some sort of mathematThere are several draw-

i 3 , for Skin

different dose

views, is not

the

directly

additive to obtain a sensible total dose for a multiview study. Patient dose should be minimized to a value as low as is consistent with clinically adequate imaging. This can be accomplished by a variety of means. We have seen how the selection of an appropriate beam quality and filter is of major importance. The choice of image receptor and processing will also effect the dose. For

110

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Volume

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Number

1

SKIN

or

ENTRANCE

AVERAGE

()

GLANDULAR

MIDLINE

12. Figures central at which

the

defined

central

shown

have

doses

example,

(13)

skin

for

maximum

fraction

of the

We must

If two

beams

enter

by different

film

the

practical

can

processing.

compression,

beam

from

will

dose

is not

directions,

together.

each

Average

the dose

by 1 5%-30%

examination

reduce

dose,

relative

to

technique,

since

the

effective

tissue

thickness

benefits

of mammography.

such

transmitted

is reduced.

PUBLIC HEALTH

death

from

breast

cancer

can

be

and

different

values

Proper

as the

the meal risks

breast

glandular

to all of the glandular

risk

against How tribute

identify

increases

average

delivered

be added.

can reduce

film

The

to add these

views

processing

breast.

of the dose

at which the as the point

of mammography is the induction of cancer due to the examining x-ray beam. The primary benefit is the detection of cancers at stages for which there will be increased mates of survival and cure compared with those for unexamined populalions. It is to be noted that epidemiologic studies of benefit defined in this way automatically include the risks of irradiation. In addition, real patient benefits are often associated with true-negative examinations. Efforts are also made to reduce benefits and risks to monetary terms. Assuming that the cost to society of an “early”

clearly

the

It is inappropriate

1 .5-minute

x-ray

through

It is the average

dose.

produced

3-minute

required

as the

point.

in gray.

its own

glandular

that

ray is halfway

at any given

tissue

will

13. The skin or entrance dose is measured at the point beam enters the breast. The midline dose is defined

12, 13. (12) ray of the x-ray

obtained,

this

information

the cost of a mammogmaphic screening program. can the benefits of mammography be improved? by developing techniques for improving image

Medical quality

can

The

be

real

balanced

physicists and reducing

conpa-

tient dose. Quality assurance programs can optimize and stabilize the quality of existing systems. The accreditation of facilities that meet certain operating and imaging standards becomes one way in which the public is told where quality examinations can be obtained. Quality assurance methods used for mammography are similar to those applicable to general radiography. These include control of the mechanical aspects of the equipment (including patient positioning devices) , x-ray beam calibration, the evaluation of focal spot characteristics, and the performance of automatic exposure control systems. The quality of image processing is important for all of radiography but is critical for mammography. Retake and artifact analysis are tools that often give insight into overall system problems. Because mammography makes special demands on the imaging system, it mequines

January

a specialized

1990

quality

assurance

program.

For example,

a 2#{176} star

pattern

Hubbard

is

U

RadioGrapbics

U

111

-.--.---S

-

14. Figures 14, size on many

15.

(14)

When

mammographic

ter to be seen. This pattern (15) For many mammographic measure the effective focal

15. a 2#{176} star pattern is used

to measure

machines,

blur

the

is, however, systems, spot size.

resulting

pattern

the effective is too

close

focal

spot

to the

cen-

adequate for most general radiographic systems. the 0.5#{176} star pattern can be used to successfully

17.

-

Figures

16, 17 (16) The processing of a charged, unexposed xeroradiographic plate is called dark dusting. In the resulting image, the numerous white spots are indicative of a plate that has exceeded its useful life. The dank spots demonstrate that toner powder is not being dispersed properly by the processor. (17) The phantom used as part of the American College of Radiology mammography accreditation program contains fibers, specks, and circular disks. This radiograph shows some of the most prominent objects in the phantom as well as some system artifacts.

suitable for evaluation of the effective image shown in Figure 14 was made small focal spot and imaging geometry

focal spot size for general radiography. The with such a star on a mammographic unit; the results in a blur pattern too close to the

center for evaluation. Figure i 5 shows the the same machine. Even though this image pattern is none too Quality assurance

mographic

units

large. methods

as well

for

image

as to screen-film

image

covers

processing

systems.

of a 0.5#{176}star

pattern

an 8 X i 0-inch should

Figure

be applied

U

RadioGrapbics

U

Hubbard

Volume

on

the blur

to xeromam-

i 6 is an example

“dark dusting,” a useful test of the xeromadiographic system. A prepared has not been exposed to radiation is processed, and the resulting image clues to the proper performance of the plate and processor.

112

made

cassette,

of plate provides

10

that

Number

1

Table 1 Typical Parameters

for Two

Radiographic Chest Radiography

Parameter Kilovoltage (kVp) Fi!ter Skin dose (mGy)

Focal

Procedures Mammography

140 0.1 Cu + 1 Al 0. 1

30 0.03 Mo 2

1.0

0.5

1 83 22

65 4.5

spot

size (mm) Source-image distance (cm) Thickness (cm)

Note-Al

aluminum,

Cu

copper,

Mo

molybdenum.

Image quality is one of the major components of current accreditation programs for mammogmaphic facilities. A resolution “standard” developed by Dr Harold Lasky (i) is used to test the performance of mammognaphic systems in accreditation programs such as that of the Illinois Chapter of the American Cancer Society. This standard consists of several small specks of zinc tellunide and rock salt, which

are placed

on a patient’s

breast

and

imaged

during

mammography.

The visualiza-

tion of the standard is a major criterion for approval in this accreditation program. The national mammography accreditation program of the American College of Radiology includes testing the performance of the imaging system on a phantom containing fibers, specks, and circular “masses.” A radiograph of this phantom is shown in Figure 1 7. The standard requires visualization of an adequate number of each of these objects.

The

materials

lion

radiography.

require with

discussed

different those

in this

However,

the

optimizations.

article

can

special

Table

i compares

of screen-film

mammography. tween the requirements for these two 1 The chest has a greater thickness; quined for chest radiography. 2.

Objects

soft

large that radiographs. with that

in the

tissue

have

bone

much

and

greater

the difference

a much lower radiographic contrast Film with lower contrast is used used in mammography. In addition,

kVp are used 3

chest

and

to minimize

subject

to other of other

chest

There are examinations. thus, a more

.

tween

be extended

requirements

forms

main

contrast.

x-ray The

soft tissue

OThER RADIOGRAPHIC

will

parameters

differences

penetrating

between

tasks

radiographic

three

inherent

of projec-

imaging

beam

is me-

difference

and

SYSTEMS

be-

lung

be-

are so

is required to obtain optimal chest for chest radiography compared x-ray tube voltages in excess of 100

contrast.

The spatial resolution that is adequate for chest radiography is lower than that required for mammography. Since the sensitivity of radiographic detectors and their spatial resolutions are trade offs with each other, this permits the use of a much more sensitive screen-film system for chest radiography. Among other things, this reduces the exposure time, which in turn minimizes motion unsharpness. The use of a more sensitive image receptor and a higher kilovoltage peak in chest radiography results in lower patient dose than that required for mammography. As shown in Table i , the use of a composite copper-aluminum filter can me.

duce the dose even general radiography.

1

January

.

Lasky

HJ.

Quality

bly

and

ber

30-December

1990

Annual

further

than

standard Meeting

that

needed

for mammograms. of the

Radiological

with

the all-aluminum

Presented Society

at the 72nd

of North

America,

filter

Scientific Chicago,

used

for

Assem-

REFERENCE

Novem-

5, 1986.

Hubbard

U

RadioGrapbks

U

113