Influence of Monitor Luminance & Tone Scale on Observers’ Search & Dwell Patterns

Elizabeth Krupinski, PhD & Hans Roehrig, PhD

University of Arizona

This Work Was Supported in Part by:

Toshiba Medical Imaging Tokyo, Japan DataRay Corp. Westminster, CO

Rationale Full-field digital mammography

systems will soon replace traditional film acquisition systems. To maximize the benefits of digital mammography (e.g., image processing, CAD), we need to understand and maximize the properties of CRT monitors used to view these digital images.

Purpose The goal of this study was to

measure the influence of display luminance and tone scale choice on visual search behaviors. The results of this study should be helpful in establishing minimally acceptable viewing conditions for viewing radiographs on CRT monitor displays.

Images

50 Pairs Of Mammograms

- CC & MLO views - right or left breast- 18 mass- 18 microcalcification

cluster- 14 lesion-free- All lesions malignant- Digitized 80 microns, 12

bits

Display Monitors

2 DataRay DR110 monitors- Portrait Mode- Size: 21” FS- Deflection Angle: 90 deg- Active Area: 11.5” x 15.5”- Phosphor: P-45- Bulb Transmission: 52%- Panel: 92% and 62%- Resolution: 1728 x 2304- Refresh rate: 70 Hz

80 ftL 140 ftL

Characteristic Curves

0.00 100.00 200.00 300.00Command Level (ADU)

0.00

40.00

80.00

120.00

Lum

inan

ce (f

t - L

)

Monitor 1 (140 ftL)

Default - - -Barten ------

Monitor 2 (80 ftL)

- - - Default------ Barten

Two characteristiccurves were selectedfor the study.

1) The Barten curve is a perceptually linearized tone scale.

2) The Default curve isa non-perceptuallylinearized tone scale(set using SMPTE pattern).

Display Luminance

100.00 200.00 300.00 400.00

Beam Current [micro-A]

0.00

40.00

80.00

120.00

160.00

Out

put L

umin

ance

[ft-

L] Monitor 1 (140 ftL)

Monitor 2 (80 ftL)

Output Luminance versus Beam Current forData Ray Monitor DR 110 with P45 Phosphorand 92 % Panel (Monitor 1) and 62 % Panel(Monitor 2)The two monitors were

identical in every wayexcept for the frontpanel.

1) Monitor 1 had a baseluminance of 140 ftL.

2) Monitor 2 had a baseluminance of 80 ftL.

Procedure 2 studies

- changing luminance (140 ftL vs 80 ftL) - changing tone scale (Barten vs Default) 6 radiologists in each study Counterbalanced randomized design - Each image seen twice - Once in each condition of the study - Minimal 2 week separation between

sessions Unlimited viewing time

Observers’ Task

Determine if a lesion is present or absent Indicate if lesion is mass /

microcalcification Rate confidence in decision on 6-point

scale

1 = no lesion, definite

6 = lesion present, definite Indicate lesion location: could indicate

more than one location Detection only, no classification

Eye-Position Recording Eye-position was recorded for each case ASL 4000SU Eye-Tracker was used Records head movement so observers can

move while viewing images

Accuracy ~ 1 deg Data provides:

- Decision dwell times

- Number of fixations or clusters

of fixations landing on image areas

- Scan patterns

Typical Search Pattern

Performance Analyses The confidence data were

submitted to Alternative Free Response Receiver Operating Characteristic Analysis (AFROC) and Area Under the Curve (A1) values were calculated

These data were reported in full at the 1998 SPIE Medical Imaging Conference

Search Analyses

Eye-position data were analyzed to determine:- Total viewing times- Median decision dwell times- Number of fixation clusters generated on the mass, microcalcification and normal (lesion-free) cases

Detection Performance*

Barten curve = .9720 Default curve = .9511

t = 5.423, df = 5, p = .0029

80 ftL monitor = .9594 140 ftL monitor = .9695

t = 1.685, df = 5, p = .1528

* Average area under the AFROC curve (A1)

Total View Times - Tone Scale

0

10

20

30

40

50

60

70

80V

iew

ing

Tim

e (s

ec)

Barten Default

MassMicrocalcificationLesion-Free

Total View Times - Luminance

0

10

20

30

40

50

60

70

80

90V

iew

ing

Tim

e (s

ec)

140 ftL 80 ftL

MassMicrocalcificationLesion-Free*

* Lesion-free significantly longer than mass or microcalcification

Decision Dwells - Tone Scale

0

500

1000

1500

2000

2500

3000

3500M

edia

n D

wel

ls (

mse

c)

TP FN FP TN

BartenDefault

Decision Dwells - Luminance

0

500

1000

1500

2000

2500

3000

3500M

edia

n D

wel

ls (

mse

c)

TP FN FP TN*

140 ftL80 ftL

* 2 = 4.08 df = 1 p < 0.05

Number Of Fixation Clusters Per Case - Tone Scale

0

2

4

6

8

10

12

14

16M

ean

# of

Clu

ster

s

Mass Calc Normal

BartenDefault

Number Of Fixation Clusters Per Case - Luminance

0

2

4

6

8

10

12

14

16

18M

ean

# of

Clu

ster

s

Mass Calc Normal

140 ftL80 ftL

*

* t = 2.83 df = 166 p < 0.01

Conclusions A perceptually linearized display

(e.g., using the Barten curve) yields significantly better detection performance than a non-perceptually linearized display

A 140 ftL display yields marginally better detection performance than an 80 ftL display

Conclusions Visual search is more efficient

with the perceptually linearized and higher luminance displays

Total view and decision dwells were consistently shorter and fewer clusters were generated on all types of cases with these displays - especially the true-negative decisions and lesion-free images

Possible Explanation Lower luminance and non-linearized displays seem to

affect most the search and evaluation of normal image areas - TN dwell times are longer and more clusters are generated on lesion-free cases.

These parameters reflect an increase in information processing operations, even though the same decisions are reached as with the higher luminance and linearized displays.

Recognizing normal variations in images may be more difficult with non-optimal displays, so radiologists must alter their search behaviors to compensate for display short-comings.

Conclusions & Recommendations

CRT monitor luminance and tone scale can affect diagnostic accuracy and visual search efficiency in significant ways.

For the display of digital mammograms on CRT monitors, a relatively high luminance monitor should be used and the display should be perceptually linearized.