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467

Detection Disease: Digital

Kathleen M. Buckley1 Cornelia M. Schaefer1’2

Reginald

Greene1

Steve

Agatston1 Jane Fay1 Henry J. Llewellyn1 Helen E. Mrose1 Jami R. Rubens1

of Bullous Lung Conventional Radiography vs

Storage

The detection of radiologic diagnosis contrast resolution five postprocessing

Phosphor

Radiography

fine linear contours and altered aeration are requirements for the of bullous lung disease and are severe measures of the spatial and of chest imaging systems. We compared plain film radiography with algorithms of storage phosphor digital radiography (2144 x 1744 x

10 bit matrix with 0.2-mm pixel size) in the detection of CT-proved bullous lung disease (35 patients and 25 normal control subjects). Receiver-operating-characteristic analysis of 2160 observations by six interpreters was done to evaluate the observers’ performance. By analysis of variance (p < .05), we found that the default digital algorithm and the three edge-enhancing algorithms of high and medium frequencies performed less well than plain films did, but the differences fell short of statistical significance. Gray-

scale reversal

was the only digital algorithm

that

performed

significantly

less

well

than

plain films did.

We conclude that any differences between digital algorithms and plain films in the detection of bullous lung disease were too small to be detected in this study. Any difference between the two methods in providing clinically important, diagnostic information AJR

is likely to be insignificant.

March 1991

156:467-470,

Storage phosphor digital radiography has a number of technical advantages (e.g., high dynamic range, flexible adjustment of contrast and density scale, and other postprocessing capabilities) that may offset its lower spatial resolution relative to plain film radiography [i -6]. The detection of fine linear contours and altered aeration that is necessary for the diagnosis of bullous lung disease provides a unique test of the spatial and contrast resolution of an imaging system. Previous experimental and clinical studies have reported discordant results in the detection of fine linear structures with digital radiography and have suggested the important role of edge-enhancement postprocessing [2-4, 7, 8]. We compared plain film radiography with five postprocessing algorithms of storage phosphor radiography in the detection of bullous lung disease. Materials Received sion October

March 29, 1990; accepted 10, 1990.

after

revi-

Department of Radiology, Massachusetts General Hospital and Harvard Medical School, 32 Fruit St., Boston, MA 02114. Address reprint requests to R. Greene. 2Presont address: Department of Radiology, 1

Medizinische Germany.

Hochschule

Hannover,

0361-803x/91/1563-0467 © American Roentgen Ray Society

Hannover,

and Methods

We studied a total of 60 patients who had undergone CT of the chest for clinical indications. Thirty-five patients had a diagnosis of bullous lung disease. Twenty-five persons served control subjects. There were 40 men and 20 women from 25 to 88 years of age (mean standard error = 60 ± 1 .5 years). Each patient in the study group had posteroanterior radiographs

of the

chest

obtained

with

storage

phosphor

and

conventional

technique

as ±

within

1 week of the CT examination ofthe thorax. The CT examinations (GE 9800, General Electric Medical Systems, Milwaukee, WI) consisted of multislice scans (iO mm thick at i 0-mm intervals) through the chest with selected thin sections (1.5 mm thick with high-resolution algorithm) as warranted by clinical circumstances. The official interpretation of the CT findings served as the basis for the expected findings on the radiographs. Criteria for the diagnosis of bullous

lung

disease

included

the identification

of discrete,

localized

areas

of low attenuation

BUCKLEY

468

surrounded pleura

by well-defined,

or compressed

bullae

were

they

were

surrounded located

of localized

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were

classes

diameter

(n

diffuse

to size:

and

eight

2-4

cm

subjects

in most

were

than

had

and

with

with

factor,

same

and

exposure

nominal

and

1 2:1

Erlangen,

radiographs

1 40

kVp,

oscillating

phototimed

Systems,

digital

factors:

focus,

tance,

average

skin

entrance

with

both

techniques

with

Lanex

medium

with

standard

The OC

(Kodak

digitized

America,

with

Tustin,

a helium-neon CA).

A laser

mR

and

processor,

laser printer

x

106

were

automatic

film

bit

(ST,

Fuji,

readout

unit

NY) size)

(TCR

201

area

phosphor

digital were

radiographs

with

images

were

processed

enhanced

with

an unsharp

to the

A + f (A

image,

unsharp

=

following

formula:

B)

-

produced

by

the final

The

moderate,

The

4) and five

Mode

at

ance

were

degree

Toshiba

enhancement

17 x 21 cm hardcopies

enhanced

Hurter

of

the

f

mask

enhancement

=

(B) was

produced

(cycles/mm)

edge-enhanced edge

image

enhancement

effect

(A

was

(mode

and high

of gray-scale

pronounced

that

was

image

was

B) to the

-

determined

by

were

spatial range

(mode

of high

2) and

(modes

1,

as follows:

algorithm

Driffield

reversal

enhancement

5) frequencies.

conventional

of

This

of edge

the

default

frequency

image,

frequency

(A*).

algorithms

The

original unsharp

factor (f), which could be chosen on a scale of

medium

of an optimal

contrast

spatial

addition

and

digital

1:

=

The

image

weighted

image.

A

mask.

pixel values across a certain area. The size of

determined in the

3, and

Japan) ,

B

enhanced

subtle,

processing

Kanagawa,

according

the flexible weighting 0 to 1 6. We tested

obtained

matrix

Edges

technique

edge-enhanced and

original

C/kg)

Rochester,

x 10

produced

dis-

measurements, (5.16

images

film,

plates

phosphor

1 .2-mm

source-to-image

M7B

acquired

the

2 MP, Siemens Medical

conventional

x 2144

storage

20

with

filter,

to previous

was

36#{176}C. The digital images (1744 with

71

According

screens,

chemicals

1 83-cm

storage

algorithms.

by locally averaging

of diffuse

obtained

aluminum

(Tridoros

exposure

[9].

were

3-mm

grid,

exposure Germany).

the

and

A*

this

conventional

to five

subtraction

A*

emphysema. The

of the

at 36.5#{176}C. The

1991

axis

patients

a CT diagnosis

according mask

Japan)

chemicals

March

or

22);

=

Kanagawa,

into

2 cm in short (n

AJA:156,

standard

causes

divided

7). Seven

=

the cases

Other

in diameter

(n

cases

bronchiectasis,

bullae less

in diameter

control

lung,

The

) small,

6); (2) intermediate,

=

bullae

(1

but

(Fugi,

of inverted

In some

parenchyma.

honeycomb

excluded.

(3) large, greater than 4 cm large

normal

(e.g.,

specifically

consisting

or both. emphysema,

otherwise

according

margins

parenchyma

by

within

hyperinflation

oligemia) three

paper-thin

lung

ET AL.

(SR-i) radiograph.

(H&D)

curve

frequencies centered

approximated It used with

slight

(enhancement

the

appear-

a sigmoid,

long-

nonlinear

edge

factor,

0.5;

at 1 .4 cycles/mm).

Fig. i.-A-G, cT scan (A), plain film (B), and five different algorithms (C-G) of a storage phosphor digital radiograph show a left-sided intraparenchymal cyst (arrows). Five algorithms are default mode (C), gray-scale reversed mode (D), moderately edge-enhanced mode of high frequencies (E), markedly

edge-enhanced

mode of high fre-

quencies (F), and moderately edge-enhanced mode of medium frequencies (G). Notice that edge enhancement of high and medium frequencies (E, F, and G) amplifies surrounding fine linear rim as compared with plain film (B) and default digital mode (C).

A

D

B

E

C

F

G

AJA:i56,

March

BULLOUS

1991

Mode 2: The gray-scale reversed H&D curve as the default algorithm

algorithm

(SR-2)

used

LUNG

an identical

except for reversal of gray-scale

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polarity.

Mode 3: The edge-enhanced algorithm (SR-3) performed a moderate enhancement of high spatial frequencies and was identical to the default algorithm except for a larger enhancement factor of 3. Mode 4: The edge-enhanced algorithm (SR-4) performed a marked enhancement

of

high

spatial

frequencies

and

was

identical

to

the

default mode 1 except for a larger enhancement factor of 5. Mode 5: The edge-enhanced algorithm (SR-5) performed a moderate enhancement of medium spatial frequencies and was identical to mode 3 except for the enhanced frequency range (enhancement factor 3, enhanced frequency range centered at 0.71 cycles/mm, Figs. iA-iG). The conventional and digital images were intermingled in random order and divided into three subsets. Six board-certified radiologists interpreted the subsets in differing order to avoid interpreter bias. Three separate sessions per interpreter extended over a 4-week period.

The

definitions

and

exclusions

in this study were reviewed not

know

the

of bullous

lung

disease

of

normal

to

abnormal

cases.

They

did were

asked to indicate the presence of bullae according to a five-level scale of confidence (1 definitely negative, 2 probably negative, 3 indeterminate, 4 probably positive, 5 definitely positive). =

=

=

=

Interpretation

time

was

(ROC)

interpreter obtained

data by

for

digital algorithm that performed significantly worse than plain films did (Fig. 2 and Table 1). The algorithm with moderate enhancement of high spatial frequencies (mode 3) performed best among the digital modes. Five of the six interpreters had higher ROC areas with the plain films than with the best digital algorithm. However, the differences were not statistically significant (Table 2). Analysis of performance according to the size of bullae indicated that the gray-scale reversed mode was inferior to plain films in the detection of small and intermediate-sized bullae. Plain films and the best digital algorithm with moderate enhancement of high spatial frequencies (mode 3) were equivalent in the detection of bullae of all sizes (Table 3).

10

z 08

0 0

=

not

Observer performance characteristic

limited.

was tested according

analysis a total

of

of 21

pooled,

60 observations.

a maximum-likelihood

w >

to receiver-operating-

averaged,

and

individual

algorithm,

and

06

U)

ROC curves were

curve-fitting

U-

per-

0 0

w 04

ceptual accuracy was measured by the area under the ROC curve 110-13]. ROC curves were produced by fitting the pooled interpreter data from

all observers

and

ically [1 0]. Averaged by

individually

fitting

were

used

to demonstrate

ROC areas (A7 the

performance

± standard

confidence

error)

ratings

for

were

each

graph02

produced

observer

and

averaging the estimated areas across the observers. These areas were used to measure performance of each imaging mode [i 1-13]. The significance of differences between the A of the six imaging modes was tested with a one-factor analysis of variance for repeated measures. Paired A values were compared with Scheff#{233}’s F-test at a two-tailed p less than .05 [i4]. A preliminary arc-sine transformation was

performed

homogeneity

to stabilize

the

normal

distribution

and

data

and

to meet

of variances

the

assumptions

needed

of

for analysis

of variance Ii 4]. The arc-sine transformation was considered the appropriate procedure because it applies to variables that express proportions. Performance with respect to the three subclasses of bullae (small, intermediate, and large) was tested with pooled and individual data for

plain

film

performance.

and

for

the

Significance

digital of

algorithms differences

with for

the the

best pooled

and

worst

data

that

incorporated

estimations

of variances

and

00 0.0 Fig.

02

06

04

2.-Receiver-operating-characteristic

covariances

of the ROC parameters into a univariate z-score test (p < .05) for the true-positive fraction at a constant false-positive fraction of 0.2 [1 6-i 8].

high spatial

TABLE

curves

Performance with all digital algorithms (averaged ROC area ± standard error) was lower than that with plain films. However, no statistically significant difference was found between plain films and four of the five digital algorithms at a p level of .05. The gray-scale reversed algorithm was the only

10

derived

from

pooled

frequencies.

1: Average

Performance

Operating-Characteristic

and Range

(ROC)

Areas

Plain

of Receiver-

by Six Readers

ROC Area

Technique/

Postprocessing

Algorithm

film

Mean 0.76

SR-i, Default: slight edge enhancement (high

± SEM

Range

Significance: VS Plain

SR

Film

± 0.04 ± 0.02

0.64-0.87 0.67-0.75

p < .19

0.72

± 0.05 ± 0.02

0.37-0.72 0.67-0.80

p < .05 p < .31

0.71

±

0.02

0.60-0.80

p