AAPM

T utorial

Stephen

Balter,

Physics Recording

of Mammography: Process1

MartinJ.

PhD

The

Yaffe,

many

physical

factors

ty in mammography are are

at work discussed

screen-film magnification

are

influence

reviewed

in this

of screen-film author reviews

mammography mammography.

and

is one

multaneously

of the

excellent

and good contrast the image-recording over a wide range

the

most

spatial

physical radiation,

quality

and

processing The

for

demanding

image

quali-

principles

that

image receptors of quality control for

xeromammography

madiobogic to allow

and physics

mammographic methods

sensitivity to allow detection system must be adequate of intensities, comnesponding

image

Many scattered

image tutorial.

principles

resolution

where theme is little attenuation, wall. Finally, because the breast the examination must be done

the required

Image

in the function in detail. The

INTRODUCTION Mammography

skin, chest body,

that

PhD

and

techniques,

visibility

requiring

si-

of micmocalcifications

of subtle tumors. The latitude of to record information faithfully to regions in the breast near the

and to the dense, thicker regions near is one of the more radiosensitive sites at the lowest radiation dose compatible

the in the with

quality. factors affect noise, and

radiation

dose

the quality compression

of the mammographic image, including of the breast. In addition, both image

in mammography

depend

critically

on

image

process-

ing. This tutorial reviews these factors and describes the image receptors used for mammography and how they are optimized for that purpose. Methods for quality control and future techniques that may further enhance the value of mammography

are

also

PHYSICAL The subject

presented.

FACTORS on attenuation

may transmission

AFFECTING contrast

through

two

age of the two values. In the effects of scattered radiation which contrast is computed Abbreviation: Index

MTF

terms:

Breast

RadioGraphics I

From

1990;

the

Departments

Ont, Canada ceivedjune RSNA,

modulation

#{149} Images,

of the

QUALITY be defined

breast

divided

as the by

difference

either

the

in x-

sum

or

aver-

is used, and the signal N from transmitted ma-

function. processing

#{149} Images.

quality

#{149} Imaging

plate

#{149} Physics

of Toronto,

500

Sherbourne

1-363

of Medical

M4X 1K9 and the 23, 1989; accepted

IMAGE 1) can

example shown in Figure 1 , the sum are ignored. Thus, the total radiation is also equal to the primary or directly

transfer

radiography 10:34

parts

(Fig

Biophysics

and

Radiology,

Physics Division, Ontario December 5. Address

Cancer reprint

University Institute. requests

From the 1988 to the author.

RSNA

annual

St. Toronto. meeting.

Re.

1990

341

No

No

$

$

N1=P1

N2-P2

N N

N1 N2 N1 N2

S

Position

Position

(P1

N2-N2 N1 + N2

Cs

Figure

Cs-

12 #{149} P1

+

Figure

1.

radiation

S) - (P2 +S)+(P2+S) 4’

2.

toward

3.

Diagram

U

Yaffe

depicts

the bottom.

(left)

of a contrast-detail

of disks of varying diameter, and of varying contrast, which

Graph

(right)

depicts

of these disks. Objects with contrast diameters to the right of it are perceptible bility

RadioGrapbics

Pi

P2

-

P1 +P2+2S

determination

of subject contrast (C,) in the presence of scatter radiation. N total radiation signal, No incident radiation, P primary radiaiion signal, S uniform scatter intensity.

Radiograph

phantom consists size to the right,

U

S)

signal.

Figure

342

+

P2

Diagram and graph depict how subject contrast (Cs) in mammography is determined. Effects of noise and scatter are excluded in this example. N total radiation signal, No = incident radiation, P pnimary

(P1

phantom.

The

which decrease in decrease in degree

the threshold above in the

of pencepti-

the curve radiograph.

Volume

10

and

with

Number

2

diation signal P. It is the relative difference that allows detection of the lesion. Figure 2 graphically demonstrates how perceptibility

of structures.

visible; those below dines in quality for attenuation between

be visualized. tmast

and

subject trast.

The

display

Phantom

the curve fine detail; the disks

contrast gradient

contrast

below

Increasing

the

lesion

factors

of subject

and

such

background

as contrast

contrast

above

affect

the

curve

are

are not. The performance of the image receptor thus, for smaller disks (on lesions) , differences and background must be greater if the disks are

presented image

curve

are

gradient

various

disks

of the

the

in P between

of the

to the viewer receptor.

not

seen

image

is a result The

because

receptor

of x-ray

larger

disks

subject

con-

corresponding

of inadequate would

dein to

render

to

overall them

con-

visible.

Objects to the left of the curve are not reliably detectible, even though their size is within the resolution limit of the imaging system. Such limits, however, apply to objects of high contrast imaged under noise-free conditions. Small-diameter breast lesions such as microcalcifications may not be perceived because of noise. Scattered kibovoltages

radiation has an important influence on image quality, even at the low used in mammography. When scattered radiation reaches the image receptor, it can often be considered to contribute a uniform intensity S to all points on the image (Fig 3) . This scattered radiation forms a haze that reduces contrast, as shown in the contrast equation in Figure 3 . The presence of scatter cancels in the numerator of the equation but adds in the denominator, thus reducing the effective contrast in the image. In mammography when no antiscattem de-

vice is used, age receptor

the ratio of the scattered-to-primary (ie, S/P ) can be 1 .0 or larger,

breast (1) . This can result Although the major effect also contributes to the noise structures. Recently, grids The use of a grid necessitates (2) . Theme are two reasons interspace material attenuate an increase in the incident radiation does not provide and,

when

ue of using strates

eliminated,

a grid

significantly

must

is illustrated improved

in a of or have a

Scattered Radiation

radiation signals reaching the imdepending on the thickness of the

loss of subject contrast of 50% or more. scatter is reduction of contrast, scattered radiation mottle, which reduces perceptibility of subtle become available to reduce scattered radiation. two to three and one-half times increase in dose

for this.

First,

both

the metal

septa

of the grid

and

the

primary radiation, and this must be compensated radiation to the breast. Second, although the scattered useful contrast, it does contribute to film darkening be replaced

in Figure contrast

by additional

primary

radiation.

4 . The

obtained

with

compared

image with

that

of the

The

a grid

nongnid

Figure

vab-

demonimage.

4.

grams

by

Mammo-

obtained

with

(a) and without (b) a grid demonstraw

increase

contrast tesy

of Laszbo

MD, Falun, den.)

March

1990

in

in a. (Coun-

Yaffe

Tabar,

Swe-

U

Ra4ioGrapbics

U

343

No

Position

N1

Figure

5.

Diagram

visualization by quantum tion signal,

Noise

Another

and graph

of the breast noise (cf Fig No

=

incident

important

depict

lesion 1) . N

is affected total radia-

E.g.

radiation.

factor

influencing

N2

how

image

quality

N

=

1 00

N

=

i.ooo,ooo

is noise

N

10

iooo

or mottle.

Noise

me-

sults from statistical fluctuation of the number of x rays absorbed by the screen and used to form the image. This fluctuation makes detection of subtle lesions less reliable. Quantum noise, expressed as the fractional standard deviation in the number of absorbed x rays, decreases as the average number of quanta increases. If in any megion of the image the mean number of x rays absorbed by the screen or image meceptor is N, the standard deviation in this number is N N”2. For example, if, on the average, 1 00 quanta are absorbed in a given area, the standard deviation will

be 1 0 quanta,

which

is 1 0% of the average.

Under

such

conditions,

differences

in

x-ray transmission on the order of 1 0% due to a lesion cannot be reliably distinguished from such random variation. If 1 million quanta are absorbed, the standard deviation of 1 ,000 is greaten, but, relative to the average value, it is only 0. 1 %. Figure 5 shows the effect of quantum noise graphically. Quantum noise can be reduced by increasing the number of x rays used to make the image. X rays can be increased in two ways: (a) use of an image receptor with high quantum efficiency, which maximizes the fraction of incident x nays that it absorbs and (F) use of a relatively slow image receptor that needs many quanta to make the image. Thus, a low-noise imaging system for mammography would have high quantum efficiency but might employ a technique (eg, the addition of a dye) that would slow the system down and would require more quanta to make an im-

age. means

344

U

RadioGrapbks

U

Figure

6 shows

of increasing

Yaffe

how the

detail

perceptibility

is improved

after

noise

is reduced

by

quanta.

Volume

10

Number

2

Figure 6. Images ofa phantom produced under similar conditions. However, the image on the right was obtained with 50 times more quanta than those used

to produce

image

the

on the left.

Figure 7. Mammograms obtained with (a) and without (b) firm compression demonstrate dramatic

increase

contrast heads

lignant printed, mission,

indicate malesion. (Rewith perfrom

reference

Compression

and

of the

image

quality

mammography

At least spreads thereby

breast has an important Firm compression 7).

(3) (Fig

six advantages out the anatomic improving the

.

of compression structures, perceptibility

proved, since with the thinner of the image receptor. Because tion,

March

the

1990

range

of x-ray

intensities

influence is absolutely

on both essential

can be identified. minimizing the of the structures.

(latitude)

that

must

4.)

Compression

radiation exposure for high-quality

Firm

superposition Contrast

breast, less scattered radiation the compressed breast is more

in

in a. Arrow-

compression of shadows is further im-

reaches uniform

be recorded

and

a given area in attenua-

by the

Yaffe

imaging

U

RadioGraphics

U

345

z 0 10 0

8 >.

Figure

8. The scatter-to-primary radiation ratio for two different phantom thicknesses is plotted versus the diameter of the radiaiion field. As can be seen, the ratio increases rapidly with phantom thickness and field sizes up to 8 cm. For larger field sizes, the ratio increases very slowly. (Reprinted, with permission, from reference 1.)

6 #{176}

4

2

2

4

6

(“

8

DIAMETER RADIATION

CIRCULAR

10

12

OF FIELD

(cm)

(..

:

9Figures

9, 10.

positioning.

with

that

Radiation sitioned markings control.

The

achieved

(9)

Diagrams

flat

plate

with

show

on the

the curved

the effect

right

plate

provides

of the shape more

on the left.

of the compression

uniform

plate

compression

(Courtesy

on

compared

of National

Council

on

Protection and Measurements.) (10) Photograph shows patient with breast pofor a craniocaudal view under a compression plate with a straight edge. The on the plate indicate selectable positions of the sensor for automatic exposure The sensor itself is beneath the cassette. (Courtesy of Toronto Institute of Medi-

cal Technology.)

system is reduced. Higher contrast film can then be used to record and display the image. X-ray transmission through the thinner breast is higher; therefore, less cxposure is needed to obtain adequate image density. Compression also affects the sharpness of the image. With the resulting smaller distance between all parts of the breast and the imaging system (ie, a lower magnification factor) , the blurring effects of focal spot or geometric unsharpness are reduced. Finally, the clamping action of compression reduces anatomic motion. Compressing the breast does not change the total volume of tissue irradiated; thus, one might ask why compression results in less scattered radiation reaching

the image the effect

receptor. A detailed explanation is complicated, of compression on the ratio of scatter-to-primary

seen by the graph, nificantly greater

the reduction in breast thickness due effect in reducing the scatter-to-primary

but Figure 8 illustrates radiation. As can be to compression radiation

does the increase in field size on increasing that ratio. When compression is applied, it is important that it is done rnizes the benefits already described. Use more uniform compression of the breast ally displayed with good contrast on the shown under compression in Figure 10. device with a straight edge that is placed effective compression (Fig 10).

346

U

RadioGraphics

U

Yaffe

ratio

in a way

has a sig. than that

maxi-

of a flat compression plate provides (Fig 9) . Thus, the amount of tissue actumammograrn is maximized. The breast is Many radiologists find that a compression along the chest wall provides the most

Volume

10

Number

2

0

S U-

S

. Mammography

C

Cs

IC 0

Cs 1

V 0

Radography

Figure 11. Graph of MTF5 for mammographic and conventional radiographic screen-film systems. (Reprinted, with pen-

.04

Spatial

Three

types

sure

film,

Frequency

of image

(cycles

mission,

I mm)

receptors

a screen-film

have

been

combination,

used

and

the

from

reference

4.)

for mammography:

dmrect-expo-

xcroradiographic

plate.

IMAGE

At one

time,

RECEPTORS

direct-exposure film was commonly used for mammography because it was the only method of obtaining the resolution required to image microcalcifications. The other two techniques are now capable of providing excellent spatial resobution at a much lower dose than that required to achieve comparable resolution with direct-exposure film. Thus, the latter receptor is no longer considered acceptable because of its poor sensitivity. The same ments used.

physics

principles

of the screen-film

as those used for of mammography,

The

modulation

system

quality.

transfer

screen-film at each

function

used

The a much

in mammography spatial higher

MAIMOGRAPHY

arc the

resolution resolution

requiresystem

Screen-Film Systems be

(MiT) may be used as a measure of imaging for both mammographic and conventional arc compared. The MTF is simply a graph that

MTFs

systems

spatial

system

radiography. dictate that

1 1 , the

In Figure

radiographic scribes,

conventional however,

IN

SCREEN-HLM

frequency,

the

ability

of the

imaging

system

to record

de-

and

display information. The MTF is analogous to the frequency response of an audio amplifier, except that frequency is expressed in cycles pen millimeter rather than hertz or cycles per second. High spatial frequency represents fine detail. As the MTF decreases, for the same subject contrast, fine-detailed structures will be meproduced at lower contrast than coarser objects. The mammographic image receptom does an excellent job of preserving the incident subject contrast, since its MTF is higher

than

quencies. tion

that

Good

of the

for

conventional

at high

microcalcifications

Another,

less

a limiting

way

bar patterns. the bar-pattern

resolution

radiographic

receptor

frequency

is essential

spatial

and

satisfactory

of imaging resolution single number. With shows

the

response

the

edges

of measuring

of fine

at all spatial

to allow

fre-

visualiza-

structures.

frequency

performance

is by

means

In this method, resolution is characterized as a test, the conventional radiographic system

of approximately

5-6

line

pairs

per

millimeter,

whereas the mammography system might well resolve greater than 1 5 line pairs per millimeter. Generally, high resolution in radiographic systems is achieved only at the cxpense of a reduction in sensitivity to radiation or speed of the system. Speed can be conveniently defined as speed l/Erec, where Erec is the exposure required at the image receptor to obtain a film of some specified optical density (eg, a net op. tical density of 1 .0 above base + fog) With this definition, the smaller the amount of radiation required to obtain the specified density, the more sensitive the image receptor and the greater the speed value. Typically, mammographic systems are 1 0-20 times less sensitive than those used for general radiography. The speed of the imaging system depends on several factors. The image receptor .

must

be able

portant

March

1990

factor

to absorb

the

is the fraction

incident

radiation

of incident

to produce

radiation

absorbed

an image.

Thus,

one

or the quantum

Yaffe

im-

effi-

U

RadioGrapbics

U

347

Sp..d

Low resolution High Speed

A Figure screen

12. Schematics illustrate effect thickness on radiation sensitivity

spatial

resolution

of image

the thick screen (schematic left), the light from x-ray

receptor.

of and

With

and graph interactions

iion

spreads laterally and results tion images. With the thinner

matic

and graph

interactions

there

(*)

in lower-nesoluscreen (sche, the most distant to the film, so

on night) are

closer

is less spread

and greaten

Resolution Speed

Figure 13. Schematics show the effect of dye on screen resolution and sensitivity. Dye added to the screen (schematic and graph on night) allows the light quanta that have spread out the least from the x-ray interaction (*) ty of reaching

on (*)

High Low

to have the greatest probabilithe film; this increases resolu-

but at the expense

of reduced

sensitiv-

ity.

resolution.

X-ray beam

JI

____..///

emulsion //7.lntensifying

\_Card

layer screen layer

support polyethylene

Figure

14.

Schematic

bag

of a single-screen

mammographic receptor, in this case conrained in a light-tight polyethylene bag. Figure graphic show

ciency of the intensifying contribution to the overall

depends

ciency

made

out

oxysubfide)

of the with

screen. image

on both

the type

recently

introduced

high-attenuation

15. Photograph of rigid mammocassette, with the film bent back emulsion on its lower side.

The film itself absorbs some density is generally negligible.

of phosphor rare-earth coefficients

and

thickness

phosphors can

be

x nays; however, its Quantum effi-

of the screen. (typically,

relatively

to

thin

Screens

gadolinium and

still

pro-

vide reasonable quantum efficiency. Once the screen has absorbed the radiation, it must efficiently convert the x-ray energy into light and allows this bight to reach the film. The conversion factor depends on the type and formulation of the phosphor used in the screen. The transfer of light to the film can be modified by the use of dyes and other non-bight-producing components incorporated in the screen.

348

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RadioGrapbics

U

Yaffe

Volume

10

Number

2

The

Speed of the image spatial resolution

raphy;

however,

larity

noise

small

objects.

system of the

the

faster

or mottle

can

is also controlled film itself is not

film

products

by selection of the film generally a limiting factor

tend

be a disturbing

to use

feature

larger

that

sensitivity. in madiog-

grains,

and

the

dctectibility

masks

film

granu-

of

For a given choice of film, both speed and gradient are affected by the processing, with its major variables being time of immersion within the developer bath, temperature of the developer solution, and degree of replenishment. Each vanable must be optimized to obtain the best contrast in the final image and at the lowest consistent dose to the breast. Several physics principles to maximize resolution

age formation

with

and

thick

action has an exponential near the top of a screen phor material, a thicker ciency) ; therefore, the

greater

range

must be considered sensitivity. Figure

and

thin

in the design 1 2 illustrates

intensifying

screens.

dependence on depth than at the exit side near screen will have greaten

sensitivity between

of distances

of intensifying the difference

screens in im-

In any material,

x-ray

Screen

Design

inter-

so that more x rays are absorbed the bottom. For the same phosx-ray attenuation (quantum effi-

will be higher. In the thicker screen, the point where the x ray interacts

theme is a and the

film. The energy of the increasing x ray is converted into light, and it can be assumed that the light is emitted isotnopicably and spreads out from the point of its creation. Light from interactions distant from the film has a greater opportunity to spread laterally before the film is encountered and, therefore, gives rise to broader intensity distributions on the film (ie, blurring) than does bight from x-ray interactions closer to the film. Therefore, with the thinner screen, since the most distant interactions are closer to the film, there will be less spread, on the average. Thus,

the

image

will

proved MTF. Resolution

be sharper, and

which

sensitivity

can

backings in the screen (Fig very good chance of reaching

coating

can be applied

results also

in greater

be affected

reaching those spread

of dyes

and

in the of light,

screen has a reflective

to the

the film.

light out

of contributing age. The greatest

lower

surface

of the screen

The

x-ray

image

technique

than

absorption

to deflect

reflective

some

a

of these

This technique yields greater sensitivity but of resolution. This approach is often taken in nonspeed is paramount and resolution requirecan be added to the phosphor those that have some distance

reduces

quanta that have traveled the least from the point

to the

or an im-

use

.

ments arc not so great. Alternatively, a dye that will attenuate bight quanta, especially gives have

by the

resolution

1 3) Light produced at any point the film. To improve capture

light rays upward toward the film. course greater spread and reduced mammogmaphic applications when

before

spatial

the sensitivity

of the system

but

the shortest distance (ie, those that of x-ray interaction) a higher probability

those

traveling

occurs

screen to travel

farther.

on the side

This

where

yields

a sharper

the x nays enter

im-

the

screen. Thus, to optimize resolution and speed when a single screen is used, the film emulsion is placed in contact with the top of the screen. Thus, the majority of light quanta collected by the film have traveled only short distances from their points of creation. A typical configuration for mammography is shown in Figure 1 4 . The x-ray beam passes through a cover ofa bight-tight envelope, the base, and then the emubsion of the film to interact with the phosphor of the intensifying screen. The bight

produced

travels

spatial

resolution,

required.

In this

sealing

room

it by means

and

the film

upward

to be recorded

intimate

case,

contact

There

of heat.

March

1990

is a wide

selection

to obtain

a range

on the film

between

is assured After

is processed.

mography (Fig 1 5) , which Figure 1 4 . A foam backing screen and film. bination

contact the

There

are

acts

the

exposure,

as a spring

available

bag

special similar

and

properties.

is cut

to the

screens

film

high

emulsion

the plastic open

cassettes

to maintain

of films

the

air from

the

are also

To maintain

and

by removing

schematically

of sensitornetnic

emulsion.

screen

in the

dark-

designed

for marn-

configuration

good

contact that

Systems

is

bag and

shown

between

can be used currently

in

the in corn-

available

Yaffe

U

RadioGrapbics

U

349

.B ‘5

U) C

1.00

10.0

100.0

Log Exposure

1000.0

(mA)

Figure

16. Characteristic curves provided with different mammographic screen-film combinations. Curves lying to the left indicate systems requiring less radiation to produce a given optical density. Note variation in slope or gradient of the characteristic curve. The gradient fers.

describes

In other

the display tnast such

of contrast

a given

characteristics

subject

amplification or x-ray

of the film

that

contrast

to provide

‘

of the curve

at low exposures

and the

‘

the film

of-

is enhanced

greater

to the viewer. The shape of the characteristic contrast to a limited range of x-ray intensity,

‘toe’

‘

degree

words,

overall

curve defined

‘shoulder’

by

con-

restricts by the ‘

at high

cx-

posunes. In these regions, the gradient is low, and the incident subject contrast is actually attenuated by the film. For a given screen

product, working

this range can be increased by an increase in the maximum density of the film. This is seen by comparing the two combinations in which screen B is used. The use of film D with the screen provides a much higher maximum optical density; therefore, the range over which good contrast is available is extended considcrabby over film B.

Figure 17. Noisepower spectrum of a mammographic screen-film receptor

6

C

--

shows the different sources of noise. Note that at high spatial frequency,

,

the noise is dominated by film granularity.

i

,

t

Total Film

.-.

-

-

C

noise power noise power

Quantum

noise

power

-6

10

10

0.1

0.2

0.5

.0 Spotiol

for mammography

offer

about

a fourfold

range

2 frequency

in speed

film combination. Figure 1 6 illustrates graphically the nations of films and screens have on sensitivity, contrast, sity. The careful selection of a screen-film combination sensitivity, contrast, and latitude.

350

U

Ra4ioGrapbks

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Yaffe

5

0

20

40

(cycles/mm)

by selection

of the screen-

effect that different combiand range of optical denallows optimization of

Volume

10

Number

2

Front

Screen

Front

Emulsion

Film

Base

Rear

Emulsion

Rear

Screen

Number of x-ray photons front

screeni

rear

Figure 18. double-emulsion

screen

: -

Depth

The choice the maximum

of sensitivity acceptable

treated tween

of a screen-film level of quantum

Schematic

film

to prevent

of a double-screen,

receptor.

crossover

The film of light

be-

emulsions.

combination is partly determined noise. Another important source

by of

noise is film granularity (ie, the random deposition of grains in the film emubsion) Faster imaging systems not only require fewer quanta to form an image also generally use coarser-gramned (noisier) film. Therefore, efforts to reduce quantum noise by using a slower film will generally also reduce film granularity noise.

but

.

The

transfer

of useful

information

by the

image

is

receptor

is described

by the

which was previously discussed. As shown in Figure 1 1 , information is not transferred as effectively at higher spatial frequencies as it is at lower spatial frequencies. The noise properties can also be described versus spatial frequency as the noise power or Wiener spectrum (Fig 1 7) . Standard deviation tells us something about the total random fluctuation in image signal; the noise power spectrum tells us how that fluctuation depends on the size of the details being imaged. The upper curve represents total noise; the other two curves represent its compoMTF,

nents: quantum noise power and noise power due to film granularity. While turn noise decreases with spatial frequency in a manner similar to that of the film granularity noise tends to persist and become the dominant noise source high spatial frequency (5) . It is this effect that makes reliable perception of details such as microcalcifications particularly difficult. Thus, in choosing a screen-film combination, it is important to select a film in which granularity is as small

the rear

with double-coated As seen in Figure The front screen

screen,

in which

ems, is thicker. Double-screen pared with that of single-screen

To maintain crossover,

and

exposing

light through application,

March

1990

noise

as possible.

Double-screen receptors for use in mammography. screen-film combinations.

ed, while

quanMTF, at fine

spatial

that

the opposite

of the signal

systems generally systems.

resolution,

is, light

most

films have recently been introduced 1 8, these are similar to conventional is thin so that resolution is not degrad-

produced

emulsion

double-screen from

one

screen

of the film.

comes

from

provide

increased

systems

attempt

passing

Because

the film base, crossover results in image the base is specially treated to minimize

the upper speed

Double-Screen Receptors

lay-

com-

to minimize

through

the

film

of the diffusion

blur. In films crossover.

base

of the

used

for

Yaffe

this

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IMAGE

FORMATION;

Front

Screen

Front

Emulsion

Rear

Emulsion

Rear

Screen

IMAGE

Figure 19. Diagram illustrates effect that might develop during ble-emulsion systems. Schematic set of bars

being

imaged

with

I

Ak

VIEWING

!I\

the parallax use of doushows a

Il Front

a source-to-

image distance (SID) of 60 cm (typical tance for modern equipment) . Because the displacement of the two emulsions,

viewed

20-25 sions

cm, would

causing

at a shorten

reduction

a parallax

Most

disof two

\

Base

I I

Rear

GENERA1’ION PROCESSING

radiation

AND

cated Automatic Exposure Control

through

erence

side

It is very

machines

much

it. As shown

exposure

ty-even

control

important

receptor,

U

RadioGrapbics

Double-screen,

dose receptor

in magnification is less critical.

an automatic

thickness

equipment

or

may

during

the control exposure

composition

wall. the

terminates may

be operated

maintains a range

ref.

exposure.

image

densi-

of kibovoltages

mode. Furthermore, the automatic exposure control must compensate for efof reciprocity law failure of the film during very long exposures. This situaoccurs when a very dense breast must be imaged. In such a case, long expo-

Yaffe

photographic to the same

grid,

the

a preset

be used;

U

in nongnid,

generally

from

until

bo-

trans-

can

signal the x-ray

vary;

of

of a sensor,

sensor

The

exposure

control

exposure

the radiation

of the

chest

usefulmay

the quantity

consists

measures

location

and

automatic

control

which

is accumulated

time

with

in determining The

1 0, the

surcs at low intensity are used. The resultant generally be bess than that for a film exposed at a higher intensity and shorter time.

352

resolution.

equipped film.

their system

may

lion

the

exposed

limit

iion fects

and

are

the nipple

at which that

breast

in reduced

of the guesswork

in Figure

between

is reached,

though

result

of the image

in a position value

have another problem that may in Figure 19, the double-emulsion

for a properly

on the exit

automatic

Signal

may have a role in controlling the resolution of the image

eliminates

required

be varied

-

I

way,

that would

mammography

which

mitted

\

-I-

Emulsion

I

such as emul-

systems As shown

systems in which

modern

control,

film

effect

double-emulsion mammography, IMAGE

distance

in resolution.

Double-emulsion ness in mammography.

encur

distance,

the images from the two not line up in the same

viewing

\

Emulsion

sharp but slightly different images are formed, one on each emulsion. If the image

were

20 cm

/l \ /l \

or magnifica-

effect on the film will total amount of radiation

Volume

10

Number

2

2.0

J28kVpVp

-

f28,30,31 kVp After Modification

Figure

Cl)

20.

Graph

illustrates

1.0-

ance of an automatic exposure control in

LNNN

0

attempting

constant optical density of the processed film as breast thickness varies. Resuits are shown for different kilovoltages.

I

I

I

2

4

6

8

Thickness of Lucite (cm)

+40

Figure

+20

illustrates

SPEED (% Change)

to main-

tam

upT1

0-

perform-

21.

Graph effect

0

-20

age gradient, 3.20

CONTRAST

3.00

(Average Gradient)

2.80

of

film developer temperature on speed (sensitivity) , averfilm line

and

fog. Vertical indicates manu-

facturer’s

recom-

mendations. (Reprinted, with permission, from

2.60 .19

reference

4.)

.18

FOG

.17

91F

95F

99F

103F

33#{149}C

35CC

37CC

39CC

Developer

Temperature

Clearly, the demands on the automatic a need for a more sophisticated system in Figure

quirements

20,

commercial

of maintaining

automatic

constant

exposure has become

exposure

density

control evident

controls

do

for all breast

are fairly in recent not

always

thicknesses

rigorous, and years. As seen meet

ne-

(6,7).

Once the latent image is formed on the film, it is essential that the film be processed to bring out the information optimally. The dependence of processing speed, gradient, and fog levels on development temperature is shown in Figure 2 1 Two points can be inferred. First, if processing temperature is reduced, both speed and gradient will decrease, and increased radiation will be required to ob-

Processing

.

tam

a film

of acceptable

optical

density.

In addition,

the

resultant

image

will

have

reduced contrast. Second, it may be possible to improve imaging performance if one carefully exceeds the manufacturers’ normally recommended processing ternperature (8) The upper limit on increased temperature during processing is reached when the fog density increases to the point at which image quality is degraded. If manufacturers’ temperature recommendations are exceeded, it is even more important to monitor imaging performance closely as part of the quality control program. .

March

1990

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4.0

3.5

3.0

2.5

C

!

2.0

a 1.5

1.0

.5

Figure 22 Graph shows effect of development temperature on the characteristic curve. (Courtesy of E.I. Dupont de Nemours.)

Figure 23. Graph of sensitometnic curveobtainedby exposing film to a set of different intensities with

NetOD

Index

p

NetOD I

I.U

-

.4

/

light

effect

Contrast

a-/I--a-I

reaches the film) and speed and contrast indexes of the processed film.

The

/

2.0

2.0

measured with a densitometer and then plotted can be used to calculate base-plug-fog (measuned where no sen-

Figure greater possible equally

Exposur.

V

light use

densities

sitometnic

Log R&atlv.

3.0

of a sensitometer. Optical (OD)

.0

,

S5d

Point

#{149}1 I4mR

I

I.O

10.0

00.0

000.0

1a

of development

temperature

on the characteristic

curve

is shown

in

22. As temperature is increased, the curve shifts to the left, indicating speed, and becomes steepen, indicating higher gradient. Similar results arc if developing time rather than temperature is increased. Not all films are sensitive to these changes, and a film must be tested before changes are

implemented. Quality

The

Control

mal form. A routine quality control program must graphic facility. Factors causing variation in image

related

354

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best

equipment

will

to the image-recording

Yaffe

not

live

up to its potential

process

include

unless

it is maintained

be in effect in the quality on patient

choice

of image

in opti-

rnammodose that

receptor;

Volume

are

process-

10

Number

2

Table 1 Quality Control Test

Procedure

s for Screen-Film

or Procedure

Processing

quality

Frequency

control

(sensitometry,

Mammography

Every

Performed

operational

day

by

Technologist

tempera-

tune) Uniformity and with phantoms

contrast

test

Patient entrance exposure and tube output Phototiming parameters cluding operation of back-up timer Collimation

Weekly

ing speed, tion

contrast,

contrast

of a grid; ty must

Table raphy,

mo and when mais altered or serviced mo and when mais altered or serviced

Every 6 chine Every 6 chine Weekly

mo and when mais altered or serviced mo and when mais altered on serviced

and

fog;

careful

1 lists

recommended

average distance.

the

film

to ensure

quality

frequency

to a graded

scale

A step

a densitometer,

of light

is selected

that

which

and

those

x-ray

intensities.

The

quali-

of equipment.

for screen-film

procedures

yields

the optical

use

image

mammog-

should

(9). to maintain image quality be done with a sensitometer, gives

attcnua-

composition;

affecting

performance

procedures

which

filter

all factors

peak

control

with

of the image;

processed

be carried

and film

must which

be cx-

is then

at points in the film under each step of the sensitometric exposures (9, 1 0) . Figure 23 shows a plot of the optical density values on the processed film. From these data, baseplus-fog density, speed index, and gradient or contrast index can be determined. The base-plus-fog density is measured in a region where the film is not exposed to light.

with

density waveform,

However,

and who should perform the activity Monitoring film processing is essential done daily. Processor monitoring should measured

Technologist

optical

out

poses

Physicist

6 mo

attention

the

Physicist

Technologist

by kibovobtage,

be given both

Physicist

Technologist

Every

source-to-image

giving

Physicist

Monthly

as influenced

and

Every 6 chine Every 6 chine

in-

Half-value layer, kilovoltage Screens Cleaning Uniformity test with phantom Contact

Technologist

a processed

density

density

of approximately

1 .0 above

base-plus-fog density. The optical density under this step is permanently the measurement for the speed index. Another step on the sensitometric is selected,

in this

case

one

that

The difference in optical density the speed index yields an index These three parameters should cessor.

In this

way,

changes

rective

action

can

be taken

necessary

example, fecting

March

1990

to perform

whether processing

such

changes quality.

yields

a net

density

of approximately

between this step and the one of the gradient. be plotted for every operational

in processing

as soon tests

optical

as possible.

at several

in ambient

performance times

water

When during

temperature

can

used

2.0.

to determine

day of the pro.

be identified,

problems the

used as exposure

day

and conit may be to determine, for

occur,

or pressure

may be af-

Yaffe

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355

im: Figure 24. Radiograph of uniform plastic phantom used for verifying performance of overall system including x-ray unit and film processor. A thin, shown at the side

tency

of overall

Interface

Figure 25. Images obtained with an cxpenimental anthropomorphic phantom provide realistic detail for subjective assessment of image quality.

aluminum step wedge is used to monitor consis-

contrast.

layer oxide

Aluminum

-Insulator

-Radiation

Selenium -Amorphous-.-not Overcoatl

crystals -

ng -For

26. Figures

creates:

Electron-hole Sensitivity

depends

-a-Thickness -kVp

protection

pairs on:

of Se

27.

26, 27.

and aluminum discharge

(26)

(AL)

of the

.

charged

Schematic

(Courtesy

of xeronadiographic

of Xerox.)

selenium

image

(27)

plate

composed

Drawing

depicts

upon

exposure

receptor

of selenium

(Se)

the photoconductive to x rays.

(Courtesy

of

Xerox.)

The

uniformity

portant rendition

The

and consistency

of the overall

imaging

and should be tested weekly. Reproducibility of the contrast scale can be conveniently

phantom

or a series

must of slabs

be uniform of material

in construction, with

attenuation

system

are also

very

im-

of optical density and the tested by imaging a phantom.

consisting values

of either similar

to that

a single of breast

block tis-

sue (Fig 24) . The total thickness should be in a range of 4-6 cm. If the phantom is composed of slabs of material, it can also be used to verify that the automatic cxposure control yields constant density independent of breast thickness. The mammogmaphic step wedge is used to obtain a contrast measurement. With the use of these test objects, the overall indexes of speed, contrast, and fog should be determined and plotted.

356

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Volume

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Number

2

‘3

‘3 ‘3 ‘3

‘333T’

CHARGING

PLATE

X-RAY

EXPOSUI1E

©

©

DEVELOPMENT

Figure 1

IMAGE

2&

Diagram

initial

=

charging

absorption cessing

of the

of radiation, of the

FIXING

TRANSFER

summarizes

image

CLEANING

the xeroradiographic

process. 2 discharge of the plate due to the resultant latent image; 3 pro-

plate;

with by spraying

the

plate

with

a cloud

of charged

toner particles; 4 transfer of the image to paper by means of an electrostatic technique; 5 fixing the toner particles to the paper; 6 = cleaning the plate in preparation for reuse. (Courtesy of Xe-

rox.)

To prevent of the quality should

be

artifacts, control

all screens must program. Screens

be inspected with scratches

regularly and cleaned and other permanent

as part marks

discarded.

In addition to quantitative to have phantoms that yield

evaluations of imaging performance, images similar in appearance to those

it may

be useful

of an actual

breast. Such anthropomorphic phantoms (Fig 25) may be used by the radiologist in optimizing imaging performance of a given system, comparing systems for potential purchase, or attempting to standardize operations among different facilities (1 1) These phantoms can also be used to compare results achieved with and without grids. .

The

other

major

image-receptor

technique

used

for mammography

is xcroradiog-

raphy (1 2) The principle of xcroradiography is very similar to that used in the photocopy machine and is called photoconductivity. The xeroradiographic plate consists of a layer of selenium deposited on a conductive aluminum backing (Fig 26) . (The thickness of the selenium layer originally was 1 40 tim; however, it has recently been increased by the manufacturer to about 320 tm.) In the dark, selenium is an excellent insulator; when it is exposed to either bight or x rays, it be.

OTHER MAMMOGRAPIUC TECHNIQUES Xeroradiography

comes a conductor. In a charged xeroradiographic plate, a uniform layer of posilive charge is deposited on the surface of the selenium. When the plate is exposed to x rays, it discharges, the amount of discharge being related to the local intensity of x rays interacting with the plate (Fig 27) . The x rays interacting with the seleniurn liberate electron-hole carriers, which neutralize the surface charge on the selenium. The sensitivity of the plate depends on its quantum efficiency, which decreases

with

kilovoltage,

which increases with ness of the selenium electrical charge on The latent image with charged, colored pended in a liquid. lines of force set up surface. The overall is briefly

March

1990

summarized

and

kilovoltage. plate. The

on

the

conversion

Quantum result at this

of x-ray

energy

to charge

efficiency also depends point is a latent image

carriers,

on the thickin the form of

the surface of the selenium. is made visible in the processing stage by spraying the plate toner particles or exposing it to charged toner particles susThese particles fall toward the plate and arc directed along the by the charge on the plate to stick at specific locations on its process, including the transfer of the charged image to paper, in Figure

28.

Yaffe

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357

) I

Figure

29

Dia-

gram shows distorlion of electrical lines of force at the boundary between areas of different charge

correspond-

ing to a change of attenuation in the breast. The “robbing’ of toner from one area and pile-up

I

‘

in an adjacent

creates hancement (Courtesy

Lpietion region

a

area

an edge eneffect. of Xe-

Pile up region

Positively )jj

charged

half plane

toll

\\\\\\I.\\\,\\\

0

\

IOOjj

rox.)

Figure 30. Diagrams depict the development 5ccnanios for positive-mode (upper left) and negativemode (upper right) xeronadiography. The edge enhancement effects are shown for the positive (lower left) and negative (lower right) modes. H high and L = low charge density on latent image. (Courtesy

Pesstxe

back

bas

LH

Negatjve

I

++++++++

4

oee,

+

+

@e

e

e

Postue

cloud

e

tone

cloud

of Xerox.)

POSITIVE

Figure

bjas

EH

+

e e o e

Negatw

back

Sensitometnic curves of xeroraand screen-film mammography indicate that the former has greater exposure latitude. (Reprinted, with permission, from reference 13.)

NEGATIVE

31.

3.0

diography

2.5

>-

2.0

(1)

z

w 0

SCREEN/FILM

.

1.5

COMBINATION

1.0

XERORADIOGRAPHY

.

.5

0

/

I

I

.5

1.0

LOG

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RELATIVE

I

I

2.0

2.5

EXPOSURE

Volume

10

Number

2

32.

33.

Figures 32, 33. part of the wedge

(32) A thin aluminum step wedge (seen on the left, with the thickest at the bottom) was imaged with xeroradiography and screen-film mammography. In the screen-film image (right), latitude is limited because of the toe and shoulder of the characteristic curve. The xeroradiograph (center) does not vary significantly in intensity from step to step, but edges of each step are clearly defined. (33) In this pair of images obtained of a phantom containing objects with smooth edges of different sizes and contrast, the screen-film image (right) clearly shows the objects, and the xeroradiograph (left) is much less effective because there is no ‘ ‘edge’ ‘ to cause a sharp transition in x-ray intensity.

An effect unique to xeroradiography is that of edge enhancement. This occurs when there is a discontinuity or edge in the charge pattern on the plate. The dcctrical lines of force then bend in such a way as to pull powder over from areas that have been discharged by x-ray exposure (Fig 29) . Theme is, therefore, a pile-up of powder at the edge of the charge distribution and a depletion of powder just outside that edge where powder has been mobbed and moved over to the pile-up megion. There are two modes of xeroradiographic development. In both cases, the plate is initially positively charged. However, depending on the selection of the polaniof the toner particles used in development, a positive or negative image can be produced. Figure 30 illustrates the two development modes. In positive mode development, negatively charged toner particles are used, and these produce a deposition of toner in regions where there is residual charge on the plate after exposure, that is, where tissue attenuation is greater. More heavily discharged regions, where many x rays reach the receptor, appear white in xeroradiographs. Just inside the areas where the negatively charged particles have collected, theme will be a colored line where the toner has piled up and, just outside this line, a white depletion region. In negative mode development, positively charged powder particles are selected and tend to be attracted toward the most heavily discharged areas of the plate. Thus, these areas appear dark, and dense breast tissue and more attenuating calcifications appear white in negative-mode xeroradiographs. Xeroradiography has a very different characteristic curve from that of screenfilm imaging (Fig 3 1) The differences in capabilities between xeroradiogmaphy and screen-film radiography can be seen more readily in images obtained with phantoms (Figs 32, 33). In the screen-film radiograph of a step wedge (Fig 32), the variation in density due to varying amounts of radiation reaching the image meceptor is clearly seen. On the other hand, the density of the xemoradiograph does ty

.

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1990

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Figure

34.

Conventional

craniocaudal

mammognam (top) shows two abnormalities in the lower inner quadrant of the breast, one ofwhich is ill defined. The magnified view (bottom) clearly shows the classic spiculated appearance of a malignant lesion. (Courtesy of Robert Schmidt, MD, University of Chicago.)

35. Graph shows the effect of magnification (Mag) on signal-to-noise ratio (S/N). With geometric magnification, fine details can be imaged at lower spatial frequencies, at which the performance of the image receptor is improved. Figure

0

.

Mag.

=

2

Spatial Frequency (cyc/mm) (in Plane of Receptor)

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Volume

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Number

2

not

vary

much

with

x-ray

intensity

except

at the

transitional

areas

or edges.

Xemo-

radiography has considerably greater exposure latitude in this respect. However, the results are quite different when the objects imaged have diffuse boundaries (Fig 33) Under these conditions, the edge enhancement feature of xeroradiography is not effective. For this reason, lesions that may not have well-defined edges may not be depicted as well with xcroradiography as with screen-film mammography. The result is that screen-film mammography tends to perform better at revealing broad-area contrast, while xenoradiography is excellent in areas of the image depicting edges, such as microcalcifications. .

Magnification

is used

in mammography

to verify

or refine

the

diagnosis

of a lesion;

the technique improves the visibility of detail considerably (Fig 34). Magnification is accomplished simply by inserting a spacer to separate the breast from the image receptor. The geometric magnification factor for a particular plane within the breast is given by the ratio of the source-to-image distance to the distance between the source and that plane. Because the breast is closer to the focal spot when magnification technique is used, the inverse square effect will cause the madiation exposure to increase. When magnification is used, a smaller focal spot must be employed to minimize geometric unsharpncss (or penumbra) Generally, since the tube currents used with smaller focal spots cannot be as high as those used with larger spots, the exposure time must be increased when magnification is employed. One must then be concerned about the effects of reciprocity law failune and patient motion. What is the advantage of this type of geometric magnification as opposed to simply viewing the film with a magnifying lens? The key is in the signal-to-noise ratio of the image receptor versus spatial frequency (Fig 35) . We saw previously that the MTF decreases at higher spatial frequencies, that is, for fine details, and since it declines faster than does the noise-power spectrum, information at high spatial frequencies is not transferred effectively to the viewer. Much of this effect is due to the film granularity noise, which persists at high spatial frequencies. Magnificaiion reduces the effective spatial frequency of the information presented to the image receptor, allowing it to operate in a region of improved response, where the transfer of signal is higher both in an absolute sense and relative to film granulaity noise.

Magnification Mammography

.

March

1990

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LOG

RELATiVE

EXPOSURE

36.

37(36) A characteristic curve for mammographic film illustrates the compromise between contrast and range of exposure. (Reprinted, with permission, from reference 1 4 .) (37) Mammogram in which the region of interest has been digitally enhanced to improve contrast and visualization of edges. The digital image has been displayed on a high-resolution monitor. Figures

THE

FUTURE

36,

37.

The future of mammography may involve other imaging methods that will further enhance its performance. Aside from film granularity, one of the main drawbacks in existing mammography is the characteristic curve (Fig 36), for which the gradient or display contrast is significant over only a limited latitude or dynamic range. Digital mammography or digital enhancement of mammograms may be used to overcome such limitations (Fig 37) . In a digitized image, processing can be used to enhance contrast and resolution, either locally as shown in Figure 37 or oven the entire image. Another important possibility that can be accomplished with either conventiona! imaging or a specially designed digital radiography detector is equalization. Equalization techniques can result in substantially better perceptibility of, for cxample, calcifications over the complete range of overall attenuation provided by the breast (Fig 38). In addition to digital mammography, we must also watch carefully the developments in other techniques that do not necessarily use x rays. Examples are improvement of ultrasonography as an auxiliary test to help distinguish solid from cystic lesions and reduce the number of unnecessary biopsies and the development of magnetic resonance imaging and spectroscopy as potential methods for diagnosis of breast cancen. Acknowledgments:

the article

362

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Yaffe

and Evelyn

I am grateful to Gordon Mawdsley, Chatarpaul for typing the manuscript.

BSc,

for assistance

Volume

in preparing

10

Number

2

Igure 38. (a) Latitude phantom ains three rows of different-sized

that concalcificaions in a background of varying thickness. b) Conventional screen-film image of the itom demonstrates the limited exposure nge over which contrast is adequate to reieal calcifications. This limit is due pnimani-r to display characteristics of the film. ) Digital image of the phantom in which qualization was applied to eliminate the ffect of background thickness and reveal alcifications throughout the entire phan)m. -

-

.

C.

1.

Barnes

GT, Brezovich

of scattered 2.

3.

radiation

5.

6.

7.

8.

intensity

In: Logan WW, Muntz dose mammography.

New York: Masson, 1979. Haus AG. Recent advances film mammography. Radiol Am 1987; 25:913-928.

Nishikawa

RM, Yaffe

MJ.

in screenClin North

Taban

L, Haus AG.

173:65-

1990

films:

9.

Processing

technical Radiology

of mamand

clinical

American

Association

Medicine.

10.

of Physicists

Equipment

REFERENCES and

quality control for mammography. In: Report of diagnostic x-ray imaging task group 7, 1990. New York: American Institute of Physics (in press). GrayjE, Winkler NT, StearsJ, Frank ED. Quality control in diagnostic imaging. Baltimore: University Park, 1982.

Yaffe MJ Johns PC, Nishikawa Mawdsley GE, Caldwell CB. morphic radiologic phantoms.

1 2.

DeWerd

1986;

LA.

1984;

Electroradiography equipment In: Waggener

JG, Shalek physics.

RM, AnthropoRadiology

158:550-552.

radiography): principles.

13 .

in

requirements

1 1.

Signal-to-noise

properties of mammographic film-screen systems. Med Phys 1985; 12:32-39. Niklason LT, Barnes GT, Rubin E. Mammognaphy phototimer technique chart. Radiology 1985; 157:539-540. LaFrance R, Gelskey DE, Barnes GT. A circuit modification that improves mammographic phototimer performance . Radiology 1988; 166:773-776. mognaphic considerations.

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The

Radiology 1978; 126:243-247. Sickles EA, Weber WN. High contrast mammography with a moving grid: assessment ofclinical utility. AJR 1986; 146:1137-1139. Logan WW, Norland AW. Screen-film mammography technique: comparison

and other factors. EP, eds. Reduced 4.

IA.

in mammography.

RJ, eds. Handbook Vol

2. Boca

(xero-

and physical RR, Kereiakas

Raton,

of medical Fla: CRC,

123.

Haus AG. Physical principles tion dose in mammography.

and radiaIn: Seig SA,

McLelland R, eds. Breast carcinoma: current diagnoses and treatment. New York; 1 4.

Masson, 1983; 99-114. Lawrence DJ. A simple method of processor control. Med Radiogn Photogn 1973; 49:2-6.

1989;

69.

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