Harrell

G. Chotas,

Small

MS

#{149} Richard

Object

L. Van

Lung,

Mediastinum, phy, digital, 60.1215 Radiology

radiography,

C

radiography

HEST

plays

an essen-

of exposure

From

States.

a chest

contrast

detail

image

that

has

in all regions.

an

image

x-ray

exposure

tech-

obtained

high

penetrate

with

enough the

subdiaphragm

good With

radiographic

niques,

and

Unfortunately,

differences in x-ray that exist between chest it extremely difficult to

acquire

an

to ade-

mediastinum

areas

results in overexposure hyperlucent lung areas.

normally of the more The use of

wide-latitude films to better span the exposure range reduces this problem somewhat, but object contrast regions.

at the in one

Equalization the that

expense or more

radiography

dynamic reaches

of small image

enables

range of x-ray exposure the image recorder to be

reduced by altering the x-ray beam profile incident on the chest so that the mediastinal and subdiaphragmatic

areas

more The

receive

radiation

ization (Optical

comparatively

than

advanced

the

radiography Industries

Netherlands)

search

Building,

Department

is the

first

AMBER

images

have

Duke

C.E.R.) and Eastman

University

Kodak

Medical

Company,

Thoracic

Imaging

Center,

Health

on image

Box 3302,

Sciences

Division,

Durham,

Division,

with

the by expoobject image

ity) and may be further affected by the presence of cross-scattered radiation between chest regions. With this subject contrast information, a film

with

the appropriate

latitude

and

characteristic curve shape can be selected to achieve the desired level of display contrast in each image region.

The acterize

goal of this study was to charthe effects of exposure equalon small object contrast in two (a) by using a linear exposure

ization ways: detector to quantify is available to the (subject contrast),

selected

the contrast that image recorder and (b) by using a

screen-film

the nonlinear of the image trast).

sys-

studies

of dramatic of ana(1,2).

of the effects

Rm 139, Bryan

NC 27710

Rochester,

to be used

been comparof a screen-

recorder. Subject contrast may be related to patient size, body habitus, and clinical condition (eg, lung clar-

AMBER

Several

in visualization in the mediastinum

Research

system

have Selection

image

tionally

to observe effects final

the

(radiographic

Contrast

compared

system

transformation recorder on

con-

in AMBER

with

that

acquired

images

found

was

in conven-

images.

commercially

noted

evaluations

film

equal-

equalization

chest radiography. performance

Quantitative

of Radiology,

equalization

however, few (3,4).

region.

(AMBER) system Oldelft, Delft, The

scanning

tem for observer

lung

multiple-beam

improvements tomic detail

the

MD

AMBER system can be facilitated understanding the effects that sure equalization has on small contrast that is available to the

United

MATERIALS AMBER The sure raphy

(H.G.C.,

NY (R.L.V.M.).

Re-

G.A.J.,

Received

January 14, 1991; revision requested March 8; revision received April 1; accepted May 9. Address reprint requests to H.G.C. Dr Richard Van Metter is an employee of the Eastman Kodak Company. Eastman Kodak manufactures the radiographic intensifying screens and the ifims used in this study. Eastman Kodak has joint marketing agreements with Delft Instruments Medical Imaging, which manufactures the AMBER system used in this study. C RSNA, 1991

AND

System AMBER

equalization and has

METHODS

Description

system

is a dedicated

system for been described

expo-

chest radiogin detail

elsewhere (3). Briefly, the AMBER system (Fig 1) projects a horizontal fan beam that scans

I

E. Ravin,

in the

quately

180:853-859

#{149} Carl

content, atively

conventional

60.1215

PhD

tial role in the diagnosis of thoracic disease and is the most frequently performed x-ray examination the very large transmittance regions makes

radiography, 67.1215 . Radiogra60.1215 #{149} Thorax, radiography,

1991;

Johnson,

Radiography’

available terms:

Allan

in AMBER

Chest

The ability of a commercially available scanning equalization system for chest radiography to render small object contrast in the lung-, mediastinum-, and subdiaphragm-equivalent regions of an acrylic chest phantom was quantitatively evaluated. Images from nine chest phantoms that represented a wide range of patient sizes and dynamic ranges of x-ray transmittance were analyzed. Subject contrast was measured with a photostimulable phosphor detector, and images were acquired in both equalized and nonequalized (conventional) imaging modes. Available subject contrast in the lung-equivalent region was 8%15% lower in the equalized images compared with the nonequalized finages in all phantoms (patient types); contrast in the mediastinum-, retrocardiac-, and subdiaphragm-equivalent regions was 11%-63% higher in the equalized images, with the degree of improvement increasing as patient size and dynamic range increased. Images of each phantom were also acquired with the screenfilm systems currently in use at the authors’ institution, permitting an assessment of the relative performance (in terms of radiographic contrast) of these imagers with and without use of equalization. Index

#{149} G.

PhD

Contrast

Conventional

and

Melter,

the

patient

in a vertical

direction,

exposing the entire chest in 0.8 seconds. The chest image is recorded on an area detector such as film or a photostimulable phosphor (PSP) imaging plate. The fan

Abbreviations: beam equalization

stimulable

AMBER = advanced radiography, PSP

=

multiple photo-

phosphor.

853

Film cassette

Lead strip

Patient (top)

Aft sit with detector Lead strip (bottom)

array

Projected beam

Computer

Beam

Beam

modulators

Fore

(Electronic

feedback

fan

intensity

modulators

control)

slit X-ray

tube

2. Figures

1, 2.

(1) Illustration

of AMBER

system

components.

A horizontal and film tracks

x-ray

fan beam

scans

the chest

vertically,

exposing

an area

detector

(eg, screen-film cassette). A detector array between patient the moving fan beam, and associated exposure modulators dynamically alter the x-ray intensity of each of 20 beam segments during the scan. A 12:1 antiscatter grid (not shown) and narrow lead strips above and below the detector array help reduce scattered radiation in the final image. (2) Illustration of exposure modulators shows the AMBER modulator positions and resultant beam intensity profile when the x-ray fan beam simultaneously crosses the lung and mediastinum.

beam

consists

ments,

each

of 20 adjacent of which

beam

can

be

dently controlled by an intensity tor (located in front of the x-ray

a corresponding

exposure

seg-

tamed with conventional including the distribution

indepenmodulatube) and

detector

diation

back

centimeter) array, and

mechanism

modulators

is measured an electronic

adjusts

each

to partially

and

with feedthe

are

subdiaphragm, fully open (100%)

effects of a small, medium, and large dynamic range of x-ray transmittance on image content. Each phantom was made of acrylic plates (approximately 2.2 cm thick)

x-ray

local

cent lators

regions such as the lungs, the moduare closed in relation to the amount

exposure

flux;

in the

reaching the open), thereby

in these

source-to-image inches. All images

areas

more

plates.

acquired

(35

fixed with

in imaging

ter grids, an

beam

evaluation

equalization ages, the

geometry,

energy, of the alone.

degree

etc)

the

effects

of exposure

the ISO scribed

400 speed in Image

ventional nism was obtained intensity

region.

with

system For the

the equalization and radiographs

a scanning

beam

integrated over the Previous investigations

onstrated used with radiographs

854

images, disabled,

screen-film Acquisition.

decon-

mechawere of uniform

entire chest have dem-

that the AMBER system, when equalization disabled, produces comparable with those ob-

#{149} Radiology

Full chest-equivalent

x 43 cm)

torn

the manufacturer’s “normal” setting, and the system speed (relative base density) was set to 1.26. This system setup provides excellent clinical images when used with

and 1 .2-cm-thick

were

and

acrylic

acrylic

interleaved

plates

with

medi-

subdiaphragm-equivalent

lent plates were made large enough to model both the mediastinum and the heart, and subsequent references to the mediastinal area also pertain to the retrocardiac area. Scatter fractions in the phan-

to permit

For the AMBER imof equalization was set

the medifull chest (in-

plates to provide a balanced distribution of scattering material at all depths within the phantoms. The mediastinum-equiva-

antiscat-

and

0.6-cm-

astinum-

at 72

AMBER system, including both equalized (AMBER) and nonequalized (conventional) images. This was done to eliminate unnecessary experimental variables (eg, differences

with

sary

(minifilm

(Fig 2). The was

shapes to represent subdiaphragm, and

cluding lung) regions. When fully assembled, each phantom resembled the object shown in Figure 3. Fine adjustments in phantom thickness were made as neces-

hyperlu-

detectors reducing

distance were

of varying astinum,

the to maxi-

mize

of radiation mum, 10%

x-ray

ra-

(4).

Nine geometric chest phantoms representing small, medium, and large patient sizes (three phantoms in each size) were constructed to independently assess the

exposure that reaches the image recorder from each beam segment. In the relatively opaque regions of the chest such as the mediastinum modulators

image

Phantoms

of the beam

equalize

recorded

(be-

tween the patient and the image recorder). As the fan beam scans the patient, the local exposure that is transmitted by the patient and the antiscatter grid (12:1, 36 lines per the detector

in the

radiography, of scattered

to

images

were

measured

with

beam stop technique described and Vogelstein (5), and it was that

the

scatter

fractions

the

lead

the

by Plewes

confirmed in images of the

acrylic phantoms (24%-36% in the lungequivalent areas and 48%-81% in the mediastinumand subdiaphragm-equivalent areas) sured ages

were

consistent

with

in conventional obtained with

an

scatter grid Thicknesses

by Niklason

each phantom thicknesses

are

were thicknesses

acrylic transmission those

of acrylic

of the

shown

those

human equivalent

in Table

properties

lung

thickness

phantoms

to

medium,

was

and

range

values,

tected

exposure

held The

computed

in the

20:1

desired

for

the

small,

that they differences population,

consistent

with

dynamic ratio

were

of de-

5:1,

medium,

respectively. were

small,

regions

of equalization,

ranges

all

(ie,

lung-equivalent

use

ranges,

namic

for

size

as the

pa-

hospital.

constant

patient

large).

largest

at our

subdiaphragm-equivalent

dynamic

tient in

and

clinically

of a given

believe mission

the x-ray regions

Lung

and of

average-size,

examined

without

1. Lung

equivalent

smallest,

tients

and

imanti-

et al (6). in each region

set by establishing that had gross

unobstructed

mea-

chest

Figure 3. Acrylic chest phantom. Nine geometric chest phantoms were constructed of 2.2-cm-thick acrylic plates, with full-area “lung” plates interleaved with smaller plates representing the mediastinal and subdiaphragm regions of the chest. Small aluminum cylinders were included in each phantorn to create test signals for quantitative analysis.

and These

selected

because

10:1, large dywe

span the range of transobserved in our paand those

because

they

are

obtained

by

Lem-

September

1991

Table

sure latitude of the screen-film was the case in this investigation. Four images were obtained

1 Phantom

Acrylic

Thicknesses

Phantom Size

Dynamic

Lung

Mediastinum

Small

6.7

14.5

Medium

6.7

16.2

Medium

Large

6.7

18.0

Small

11.1 11.1 11.1 15.3 15.3 15.3

Medium Large Large

Small

Medium Large The dynamic spectively.

range values

*

Table

were

exiting

2 Settings

Exposure

E#{231}posure Setfings(mA)

Conven-

Phantom

Dynamic

Size

Range

Small Medium Large

All All All

use

AMBER

of the

same

Adjustments within each altering the

tional

65

21

130

52 130

330

et al (7) in a clinical

mers

population

measurement

with

by attaching a 3 x 4 0.6-cm-thick alu-

were

positioned

projected region (three

lung-equivalent

so

into the in each

lung), three in the mediastinum-equivalent region, and three in the subcliaphragm-equivalent region. This “signals plate” was positioned midway between the front and back surfaces of each phantom. Measurements sion and AMBER

region

of each

of local system

phantom

x-ray transmisresponse in each

were

made

by

means of the x-ray detection and readout capability of the AMBER system. The AMBER system contains an on-board com-

puter system two-dimensional sure)

and

232

serial

that measures and stores arrays of detector (expo-

modulator

data used

during

(feedback

the procedure.

communications

line

control)

An RSand

com-

puter terminal permits access to this information in tabular form. The AMBER system’s

detector

values

23.7

23.4

26.0 28.6

26.8 30.3 and

chest

each

large

region

tional

medium

phantoms,

after

Image

re-

conven-

as two

used

Conn)

screen-ifim

as image

has

been

are linear

systems

detectors.

The PSP

described

in detail

else-

When

the

imaging

to x rays, the incident

stored

as a latent

readout

system

image.

plate

radiation

is

An automated

uses a scanning

laser beam

to stimulate the stored energy in the plate and to extract the image information, resuiting in a digital image that is immedi-

ately

available

for display monitors.

for quantitative

analysis

or

on laser-printed film or video In this study, only the digital

data from the PSP system were anathe laser-printed ifims were not

image

lyzed; used.

Unlike

film, the sensitivity

of the PSP

imaging plate to x rays is linear (with respect to the logarithm of exposure) over a very

wide

[2.58

x

been

shown

exposure

106_2.58

range x

i0_2

to be well

(0.01-100 mCfkgJ)

suited

mR

and

for quantita-

tive radiographic exposure measurement (10). PSP images in this investigation were processed with the PSP system at a fixed sensitivity of 200, fixed latitude cades), and fixed time between

Volume

180

the #{149} Number

detected 3

exposure

sure

intensity.

When

used

(4.0 deexposure

in this

way,

the

PSP system is superior to film as a quantitative image detector in measuring x-ray that

may

fall outside

Kodak),

C

an ISO

screen-film because

currently

they

in use in

ting)

was

current

adjusted

(milliampere

set-

independently

for each

phantom, and these settings are listed in Table 2. Exposure settings were selected

scanning

so that the modu25% open

the lung-equivalent by

the

“modulator

region data”

ar-

and lighter

than

the average

lung

optical density; the estimated average optical density was used in this study to represent the middle optical density of a wellexposed lung and to account for the variety of optical densities in that region.

Image

Analysis

Contrast was measured the PSP and film images test

signal

locations

separately in at each of the 12

in each

image,

and

the

mean contrast in each chest-equivalent region of each image was computed. In the PSP images, the subject contrast detected in the image plane was computed as (E0 , where E,, was the ob-

has

exposures

observation

recorders

The generator

darker

is

set by examining

direct

x-ray

exposed

(Eastman

a

ray of the AMBER system). Images obtamed at all exposure settings had average optical densities in the lung-equivalent regions comparable with those of clinical images in each film type (1.33 ± .07 for AMBER and 0.99 ± .03 for conventional radiographs). Every clinical chest image, of course, contains lung regions that are

as well

(8,9). Briefly, the PSP system is on a reusable storage phosphor imaging plate that serves as an area detector and resides in a cassette that is similar to (and the same size as) a traditional screenfilm cassette.

Ortho Lanex

our hospital for clinical images obtained with the AMBER system and in our dedicated chest examination rooms, respectively. Film was developed in an RP X-OMAT processor (Eastman Kodak), and periodic sensitometry was performed throughout the procedure to verify processing stability. Film was processed 5 minutes after x-ray exposure. All images were obtained at 140 kV with 4.3-mm aluminum-equivalent filtration.

(as indicated

(PCR 901; Philips

Shelton,

traditional

were

with

system. The two were selected

are the image

when

Systems,

and two

one was obtained

for each AMBER image lators were approximately

system

of each

images

and the other with gradient, 1.8) with

screens

250-speed combinations

Acquisition

Medical

images,

detector film (average PSP

and readout (20 minutes) to permit the conversion of digital pixel values to expo-

permitting

to detected

25.6

exposure, of relative x-ray flux exiting each location in the chest phantom. The desired dynamic range of x-ray transmittance for each phantom was with

respect

data

23.1

where based

rectangular)

plates. The cylinders that six signals were

19.3 22.3

tional exposures and then adjusting acrylic thicknesses in the chest phantoms. Similarly, the degree to which each modulator was closed or left open at each scanning position was determined by examin-

system

to one of the full chestphantom

cylinders (large

19.6 21.2

A PSP imaging

technique.

mediastinumand subdiaphragm-equivalent areas. Image test signals used for contrast

minum equivalent

14.2 17.2 20.6

ing the modulator arrays after an equalized exposure was made. This information was used to establish the appropriate x-ray exposure level for each phantom, as described below.

in dynamic range values phantom size were made by acrylic thicknesses only in the

analysis were created array of 0.6-cm-diameter,

Subdiaphragm

5:1, 10:1, and 20:1 for the small, medium,

two AMBER

as

conventional images. Of the AMBER images, one was obtained with a PSP detector and the other with T-MAT L film (average gradient, 2.2) with Lanex regular screens (Eastman Kodak, Rochester, NY), an ISO 400-speed system. Of the conven-

f

Tr

Range*

Small

phantom:

Thidmess (cm)

.

system,

the expo-

served mean x-ray exposure in the circular shadow of the aluminum test signal and was the mean exposure in the area immediately surrounding that shadow. Exposure measurements (in milliroentgens) used in this equation were cornputed from mean PSP pixel values in each small

area

the mean

as 121.29*

PSP pixel

where

P

is

value.

This equation is of the form F = a*10P which has been reported previously (10), with coefficients a and 13 derived by fitting a curve to data from a calibrated exposure series and the

Radiology

#{149} 855

resultant

PSP

perirnental

pixel

Subject

contrast

ages

values

conditions

can

be

formation

measured

necessarily

the

as the

is available

captured

ex-

earlier.

in the

interpreted

that

image

under

described

PSP

im-

contrast

in-

to (though

by)

the

C

a

a

0

not

C 0

C.)

screen-film

a 0

recorder.

In the film images,

contrast

was

sured as the difference densities inside and

in observed surrounding

signal

quantity

location.

This

meaoptical test

s/s

is referred

s/U

S/L

Phantom

each

U/S

N/N

M/L

(Size/Dynamic

L/S

L/N

L/L

the was

outside mean used

tical

each

value in the

densities

test

of these difference

were

Pa),

with

It should

be

area,

±0.01

e C 0

a

C.)

0

Op-

with

(Tobias

a TA-

Associates,

s/s

Ivy-

s/s

that

S/N

S/L

Phantom

precision.

noted

N/S

U/N

Mu

(Size/Dynamic

L/S

L/U

Lu

reason, in the

used

and and

the

image

computed

plane

detected

was

exposure

AMBER Dynamic

aphragm-equivalent with age,

lung-

C 0

range for

C.)

ratio

and

of

subdi-

as measured

pixel values to exposure

in the PSP values.

im-

RESULTS Figure trast

4 illustrates

measured

the

in the

subject

PSP

con-

images

s/s

S/M

S/L

Phantom

M/S

tAIL

M/M

(Size/Dynamic

1/S

L/M

L/L

in the

AMBER

and

Figure 4. Bar graphs illustrate the subject contrast of test signals measured in the image plane with the PSP image recorder. Separate plots for the observed contrast in lung-, mediastinum-, and subdiaphragm-equivalent regions are shown as a function of phantom size and dynamic range. Black bars = AMBER images, striped bars = conventional images. S = small, M = medium, L = large.

with

conven-

size

thickness)

(lung

increasing

num

and

Subject

lent

by

regions on

in the

of the

average,

AMBER

conventional

same

phantom

C 0

the mediastinumphragm-equivalent subject

was

images

tional

images

with

increasing

increasing

that

of the

8%-15%).

higher

than

In

and

in the

in the

conven-

decreased

phantom

dynamic

sure equalization contrast relative

increased subject to the conventional

11%-63%).

In the

regions,

generally centage centages

856

of 31%

increased by with equalization. are summarized

#{149} Radiology

Medastthum

Phantom

6.

Bar graphs

show

of

2(

14

Subdiaphrgm

Region

5. Bar graph shows ratios of subject contrast in AMBER and conventional images for each region of interest (lung-, mediastinum-, and subdiaphragm-equivalent regions) and for each phantom (size and dynamic range indicated). Ratios greater than 1.0 indicate higher subject contrast in the AMBER image; ratios less than 1.0 indicate AMBER contrast is lower than in the conventional image. S = small, M = medium, L = large.

contrast

Figure

me-

expo-

(range,

subdiaphragm-

the

1/1

-

s/s

MIS 1/S 5/M SlIMIJM

S/L

M/t,

IA

Phantom

Figure 7. Bar graph illustrates the dynamic range (DR) of each phantom in an unequalized exposure as determined by means of two independent measurement techniques. Differences in measured values for a given phantom may be attributable to scattered radiation between regions and systematic differences in the measurement techniques. S = small, M = medium, L = large.

and

In the

regions,

equivalent

L/U

both

size

range.

by an average

Liip

Figure

than

diastinum-equivalent

image

0

and the subdiaregions, however,

contrast

AMBER

1/S

Range)

1/

z

images

images

U/N

U/S

(Size/Dynamic

0 0

in mediasti-

lower

(range,

comparison

S/I

radiographic contrast measured in AMBER and conventional images. Separate plots for the observed contrast in the lung-, mediastinum-, and subdiaphragm-equivalent regions are shown as a function of phantom size and dynamic range. Black bars = AMBER images, striped bars = conventional images. S = small, M = medium, L = large.

.

lung-equiva-

11%

Figure

S/N

Phantom

a

thickness).

in the

U/I

s/s

rela-

variations range (ie,

subdiaphragm contrast

was,

was

I JUL.,.

2/

phantom

but

lively unaffected phantom dynamic

___________________

:2:

in

tional images is shown. In the lungequivalent region, subject contrast decreased

J

Range)

each region of each chest phantom. Data from each of the three regions are plotted separately, and the subject contrast

Ill.

1./N

a

detector arrange in the

areas

the mean transformed

f

between For this

as the

in the

L/S

Range)

of

of dynamic were also made

comparison with ray measurements.

N/L

_._Subdiaphragm

O.4O’j

measurements

the distance the detector.

measurements image plane

S/PA S/L M/S N/N Phantom (Size/Dynamic

Range)

x-ray transmission through the chest can be strongly influenced by the detection technique the phantom

Range)

0

and

measurements calculation.

measured

TBX densitometer land,

signal

(Size/Dynamic

to

as radiographic contrast and is a function of both subject contrast and film gamma. Four optical density measurements were

made

Phantom

Range)

was

a larger perThese perin Figure 5.

6 shows

the

radiographic

contrast measured in each region of each phantom. In every case, radiographic contrast in the AMBER im-

exceeded conventional ages

phantom. gions, the AMBER

sured

that image

measured in the of the same

In the lung-equivalent reradiographic contrast in the images exceeded that meain the conventional images by

an average num-equivalent

exceeded

of 57%

.

In the

regions,

that

measured

mediastithe

contrast

in the

ventional images by an average 382% . In the subdiaphragm-equiva-

lent

region,

this

contrast

increase

even greater. In two cases, the intensity of the subdiaphragm-equivalent regions was not visually able in the conventional images

September

conof

was signal detectbut

1991

Log X-Ray

Transmittance

Log X-Ray

(Relative)

Transmittance

(Relative)

b.

a. Figure

trate

8. Graphs illustrate the effective small-area contrast)

was

still

on the

measurable

AMBER

Dynamic

and

response and

of two film (b) film contrast

and

visualized

images.

range

measurements

of

within 10% of the target ratios 10:1, and 20:1) in each dynamic

(5:1, range

group. Dynamic range values computed from the PSP exposure data, however, were consistently smaller than the AMBER-derived values in the medium and large dynamic range phantom groups, and within both of those groups, the discrepancy creased as the phantom size thickness) increased.

in(lung

DISCUSSION Contrast

The primary purpose of the AMBER system is to modify the x-ray exposure intensities entering the chest

to substantially

posure aphragm

increase

of the mediastinum while maintaining

the

ex-

and subdiproper

x-ray fluence through the lungs for optimum film exposure. One expected result of exposure equalization is a change

in

the

distribution

of

tered

radiation

which,

in turn,

ences ages,

the scatter measured

fractions in the as the ratio of

scat-

influim-

scattered and total (primary plus scattered) radiation. This effect has been measured in both an acrylic step wedge and in an anthropomorphic chest phantom by Chotas et al (4). In both cases, exposure equalization increased

the

scatter

fraction

thinner, lung-equivalent alive to the nonequalized Volume

180

#{149} Number

in the

regions (relexposure) 3

in this

significantly

fractions

each phantom without use of equalization are shown in Figure 7. Dynamic range values computed from the AMBER detector array data were

Subject

types used (gamma).

study

in regard

decreased

in the

to (a) film

the

mediastinum-

scatter and

subdiaphragm-equivalent regions. The changes in image scatter fractions were attributed to changes in relative amounts of cross-scatter between chest regions. In this study, we demonstrate that AMBER exposure equalization decreases the subject contrast of small test signals in the lung field and increases the subject contrast in the mediastinum and subdiaphragm. This finding is consistent with the altered scatter fractions reported by Chotas et al, but cannot be compared quantitatively because of the difference in the phantoms used in the two studies. It is interesting to note that the magnitudes of the observed changes in subject contrast (AMBER vs conventional) were very similar in all phantoms and in all simulated chest regions (Fig 4), with differences ranging only from 0.03 to 0.06. This is rather surprising given the large variations that existed in phantom dynamic range and “lung” thickness. This

fields sents trast

contrast

reduction

of the AMBER

in the

images

lung

repre-

only a small fraction of the conthat is available in the conven-

tional image (ie, causes minor subject contrast degradation), but in the more opaque chest regions, this same contrast

change

is large

compared

that in the conventional significantly increases subject contrast).

Radiographic the

with

image (ie, mediastinal

Contrast

Increased radiographic test signal areas was

contrast observed

of in

optical

density

(dotted

line

segments

illus-

all regions of all the AMBER images when compared with that in the conventional film radiographs. These observed changes in radiographic contrast

arise

from

several

factors,

as

follows: 1. Equalization compresses the dynamic range of transmitted exposure, which permits a higher contrast film to be used. In this investigation, Ortho

C film,

which

is routinely

used

for

conventional upright chest examinations, has been replaced by T-MAT L film. With film optical density between 1.0 and 1.5, this change in film causes

a contrast

increase

of approxi-

mately

35% at any particular density. 2. The average optical density of the lungs, mediastinum, and subdiaphragm regions is higher on the AMBER images than on the conventional images. Because film contrast increases with optical density up to 2.0 in these

selected

films,

the

AMBER

images have higher small object contrast in all chest regions. 3. As shown in Figure 4, AMBER changes the subject contrast, with contrast reduced in the lungs and increased in the mediastinum and subdiaphragm. Predictions of radiographic contrast on the basis of subject contrast (from the quantitative PSP image data) and the sensitometry curves for the two selected films agree well with the observed radiographic contrast values. This agreement increases our confidence in the use of the PSP system as a tool for quantitative exposure measurements. Figure 8 shows predictions of film optical density and film contrast for the two film types used in this study. Radiology

#{149} 857

The two represent

thickest lines the response

in each of the

plot two film

types as they were used in this investigation, and the thinnest line represents the predicted response of the

higher-contrast film (Kodak T-MAT without AMBER exposure equalization. In the optical density plot (Fig 8a),

the

inflection

points

in the

L)

higher

contrast film curve show the range over which AMBER exposure equalization occurred. Within this range, local film response (ie, in areas smaller than the AMBER system equalizes) is indicated by the dashed line segments.

Predicted

local

film

contrast

is

presented in Figure 8b. This figure illustrates that the higher contrast film, when used with AMBER, should yield a higher small area contrast at all image locations and span a larger effective x-ray transmittance range relative

to the

wider

latitude

film

the conventional ment. This agrees our study.

imaging environwith the results

Dynamic

Measurements

Range

A comparison values

by

of dynamic

means

ment

techniques

array

vs PSP

of the

b.

in

of

range

two

(AMBER exposure

a.

measure-

detector

data)

revealed

significant differences in the dynamic range of each phantom in the medium and large dynamic range phantom groups (Fig 7). This difference may

result

from

exposure during whereas

a point

plate

measures

sure

accumulated

procedure radiation regions).

the

fact

that

the

detector array measures at a distinct moment the imaging procedure,

AMBER

on

the

the

PSP

integrated

x-ray in time

expo-

throughout

the

(ie, including both primary and scatter from adjacent The

AMBER-based

Observations

The potential clinical findings is illustrated ventional and AMBER our

graphs

in Figure

importance of in the conchest radio-

9. In the

postero-

anterior AMBER images, dramatic improvement in visualization of mediastinal, retrocardiac, and subdiaphragmatic areas is due to the increased radiographic contrast in these areas. Normal anatomic landmarks

such as the tracheal carina, azygoesophageal recess, and posterior costophrenic gutters are routinely and easily visualized. In the lateral 858

#{149} Radiology

Figure

9.

Conventional

and

AMBER

chest

radiographs.

image obtained with Ortho C film with Lanex image obtained with T-MAT L film with Lanex obtained with Ortho C film with Lanex medium with T-MAT L film with Lanex regular screens.

medium regular screens.

(a) Conventional

posteroanterior

screens. (b) AMBER posteroanterior screens. (c) Conventional lateral image (d) AMBER lateral image obtained

exposure

data, therefore, are likely to be less contaminated by scattered radiation and may represent the true dynamic range of the phantom better. Clinical

d.

C.

imaging

AMBER

sualization

image, there is improved viof the apical areas of the

lung. Accordingly, abnormalities in the mediastinum, retrocardiac, subdiaphragmatic, and apical lung areas are potentially more easily recog-

nized.

without

might of the artifacts

Considerable given to the

attention

choice

has

been

of film used

with

the AMBER system in our hospital. In choosing a film, we attempted to satisfy two basic requirements. First, the

film must that spans

have an exposure latitude the dynamic range of

transmitted examinations. ily met with

x-ray

and difficult requirement film must provide “good” contrast detail in all image

films

compression provides.

exposures in clinical chest This requirement is easa variety of commercial

due

to the

that the The second,

dynamic AMBER more

range system subjective

is that the anatomic regions excessive image contrast that decrease the diagnostic utility image by amplifying system or

rendering

an

image

with

so much contrast that the normal “gestalt” of the image is lost. Plewes et al (11) have shown that the maximum film contrast that can be used effectively in scanning equalization systems varies inversely with the area of the beam. However, some equalization artifacts such as edge enhancement at the interface between regions of large transmission differences (eg, at the diaphragm or the heart-lung border) can become distracting as film contrast increases. Other AMBER arti-

September

1991

facts (such as vertical stripes in the image that can result when the modulators are driven to their limit) are made more apparent with higher film contrast. Extreme contrast enhancement of normal anatomy can also be problematic in a diagnostic task, particularly when AMBER images are viewed in an environment that includes other (conventional) chest radiographs. Since the clinical evaluation of radiographic images requires both pattern recognition prominence

and estimations of the of various structures,

large trast

differences in radiographic conamong chest images in a radiology department can be disadvantageous in the detection and diagnosis of disease. In preliminary studies, we have determined

that

a satisfactory

balance

of these requirements can be achieved by using a film (T-MAT L) that provides approximately 20% greater contrast than the film used in our hospital for conventional chest radiography. The reduction in subject contrast in the lungs that results from exposure equalization (due to increased scatter fractions) with AMBER

is slightly more than offset by this increase in ifim contrast. Higher contrast films were not acceptable because they were judged to yield images that were too different from the conventional chest films and that were more likely to highlight image artifacts. It is important to note, however, that in our institution, the standard film for conventional chest radiography is a wide latitude film. In other institutions in which a higher contrast film is used for conventional chest radiography (and is therefore already familiar to the radiologists), an alternative film selection might prove acceptable for use with An important advantage of AMBER is that more film types can become eligible for use in chest radiography (due to the reduced dynamic range requirements), and, therefore, the radiologist has more options when selecting the screen-film system that is most visually acceptable. #{149}

180

#{149} Number

3

Conf. Madison: 3.

4.

5.

Schultze

H, et aL

Kool

U, Busscher

Advanced

radiography simulated nodule ogy 1988; 169:35-39. Ravin CE. Advanced equalization radiography ization

DLT,

Vlasbloem

multiple-beam equalin chest radiology: a detection study. Radiol-

Chest Imaging

Proc

Medical

Physics,

6.

7.

1987;

60-

H, Schultze multiple-beam

Kool U. AMBER: a equalization sysradiography. Radiology 1988;

169:29-34. Chotas HG, Floyd CE, Dobbins JT, Lo JY, Ravin CE. Scatter fractions in AMBER imaging. Radiology 1990; 177:879-880. Plewes DB, Vogelstein E. A scanning sys-

tem for chest radiography exposure control: practical tion. Med Phys 1983; Niklason LT, Sorenson

Scattered Med Phys Lemmers Elburg H, transmission population:

imaging

with regional implementa-

10:655-663. JA, Nelson

JA.

radiation in chest radiography. 1981; 8:677-681. HESAJ, Schultze Kool U, Van Van Metier R. Low frequency of the chest in an out-patient implications Proc

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SPIE

for the AMBER 1990; 1231:437-

441.

CRB, Matthews CC, Scheinhorn D, Balter S. Digital imaging of the chest. J Thorac Imaging 1985; 1:1-13.

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Merritt

9.

Sonoda

M, Takano

Computed laser

stimulated

J, Kato

M, Miyahara

radiography

utilizing

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References 1.

63. Vlasbloem scanning

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2.

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Plewes

DB, McFaulJ,

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M.

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