Effects of Grayscale Window/Level on Breast Lesion Detectability Jeffrey Johnson, PhD a John Nafziger, PhD a Elizabeth Krupinski, PhD b Hans Roehrig, PhD b ba Supported by U. S. Army Medical Research and Materiel Command, grant DAMD

2 Rationale  Nearly 50% of breast lesions missed at initial screening are visible retrospectively  Digital mammography could reduce perceptual errors by enhancing lesion conspicuity with image processing  Perceptual models could be useful tools for automating and optimizing techniques for image enhancement

3 Overview  This study evaluated the use of a visual discrimination model (VDM) for predicting effects of one type of image enhancement - grayscale window width and level (W/L) - on the detectability of breast lesions  Compared model and observer performance in two experiments: –2AFC detection thresholds with simulated mammograms and nonmedical observers –ROC observer performance study with radiologists and digitized mammograms

4 Methods: Simulated Mammograms  Backgrounds –Filtered noise, 1/f 3 noise power spectrum –Two groups: Bright and Dark central regions  Lesion signals –Mass: 2D Gaussian (d=50 arcmin) –Microcalcification cluster: six blurred disks or “specks” (disk d=8 arcmin, cluster d=40 arcmin)

5 Methods: W/L Conditions  P-value transformations: –Fully stretched –Understretched (-25%) –Overstretched (±25%) –Bright shifted (+25%) –Dark shifted (-25%)  Applied to full 512x512 pixel image or 170x170 pixel central region of interest containing lesion

6 Example Test Images Fully stretched (FS) Under stretched (US) Over stretched (OS) Gaussian Full W/L Bright Center Gaussian Central W/L Bright Center Specks Full W/L Dark Center Specks Central W/L Dark Center

7 Example Test Images Bright shifted (BS) Dark shifted (DS) Gaussian Full W/L Bright Center Gaussian Central W/L Bright Center Specks Full W/L Dark Center Specks Central W/L Dark Center

8 2AFC Threshold Detection  Side-by-side presentation of same background with/without signal  Signal amplitude varied in 1-up/3-down staircase procedure; detection threshold at ~80% correct  Five W/L conditions interleaved in same session  Separate sessions for two signal and two background types

9 Test Conditions  Siemens 5M-pixel CRT monitor (P45)  Luminance range = 0.3 to 290 cd/m 2  Barco 10-bit display controller  DICOM-14 grayscale display function  Three nonmedical observers  Viewing distance = 52 cm; chin rest  Ambient lights off

10 Results: Detection Thresholds for Gaussian Signals Error bars show 95% confidence intervals Bright BackgroundsDark Backgrounds

11 Results: Detection Thresholds for Speck Clusters Error bars show 95% confidence intervals Bright BackgroundsDark Backgrounds

12 Experimental Detection Thresholds  Significant variations across W/L conditions  Generally lower for central vs. full W/L –due to local contrast enhancement - fully stretched not always optimal  Full W/L: Lowest thresholds for … –fully stretched, understretched (specks only) –dark shifted on bright, bright shifted on dark  Central W/L: Lowest thresholds for … –overstretched for Gaussians and specks on dark –dark shifted on bright, bright shifted on dark

13 Visual Discrimination Modeling  Simulates physiological response of human visual system to visual stimuli: luminance patterns from images & video  Output is a deterministic prediction of feature or image discriminability as function of spatial location, spatial frequency, and time  Discriminability measured in units of Just Noticeable Differences (JND)

14 VDM Architecture … JND scalar Spatial frequency bands Spatial orientation responses Display & Ocular Processing Optics Crossband Masking JND map JND Distance Combin. Rule Display luminance Pair of input images Probability Contrast Pyramid (visual cortex) Within-band Masking Contrast Pyramid …

15 VDM vs. Experimental Thresholds for Gaussians on Bright Backgrounds Error bars show 95% confidence intervals Full W/L Central W/L

16 VDM vs. Experimental Thresholds for Gaussians on Dark Backgrounds Error bars show 95% confidence intervals Full W/L Central W/L

17 VDM vs. Experimental Thresholds for Specks on Bright Backgrounds Error bars show 95% confidence intervals Full W/L Central W/L

18 VDM vs. Experimental Thresholds for Specks on Dark Backgrounds Error bars show 95% confidence intervals Central W/L Full W/L

19 VDM vs. Experimental Thresholds: Simulated Lesions & Backgrounds  Generally good agreement between model and experimental detection thresholds and variations across W/L conditions  Consistently reduced thresholds with central (local ROI) vs. full-image W/L  Largest modeling discrepancies for specks, especially on dark backgrounds

20 ROC Observer Study  Determine effects of W/L functions and size on detection of microcalcification clusters by mammographers  Evaluate utility of localized ROI contrast enhancement (central vs. full W/L)

21 ROC Observer Study: Image Preparation  Digitized mammograms (n=15) from Digital Database for Screening Mammography  Extracted 512x512-pixel sections with single, centered microcalcification cluster  Removed calcifications by median filtering  Generated five lesion-contrast levels: 0, 25, 50, 75, and 100%  Applied three W/L functions: Fully stretched, under and over stretched by 15%  Full and Central W/L sizes

22 ROC Observer Study: Test Conditions  6 radiologists at Univ. of Arizona  225 images/session  2 reading sessions ~2 weeks apart  Decision confidence on 6-point scale  No image processing, no time limits, ambient lights off; viewed at ~25 cm  Siemens 5M-pixel CRT monitor (P45)  Luminance = 0.8 to 500 cd/m 2  DICOM-14 grayscale display function

23 Examples of Test Images Understretched (US, 15%) Overstretched (OS, 15%) Fully stretched (FS, ) Full W/L Central W/L

24 ROC Observer Study: Results  Compared central vs. full W/L across all W/L functions, all lesion contrasts  Observer performance statistically better (p<0.05) for FULL W/L size A z Values

25 ROC Observer Study: Results  No statistically significant variations: –between central and full W/L sizes for a single W/L function (all lesion contrasts) –between central and full W/L sizes for a single combination of W/L function and lesion contrast (except FS, 50%) –across W/L functions in central and full W/L sizes considered separately (all lesion contrasts)

26 ROC Observer Study: Analysis  Central W/L enhanced lesion contrast but changed appearance of parenchymal tissue relative to surrounding areas  Decision confidence lowered by nonuniform appearance of background tissue characteristics  Conclusion: Calcifications may be easier to perceive (due to higher contrast) but more difficult to interpret (due to cognitive factors, past experience)

27 Conclusions  For simulated lesions and backgrounds, VDM was generally a reliable predictor of W/L conditions for optimal detectability  Results with simulated images suggested benefits of localized contrast enhancement  Decision confidence and performance of mammographers actually lower with localized W/L, probably due to nonuniform tissue appearance

28 Future Directions  Allow toggling between full and local W/L modes (combine uniform contextual data with local contrast enhancement)  Evaluate effects of W/L on detection of very subtle lesions (low contrast, near threshold)  Model refinements: –improved crossband masking for higher frequency signals: specks/calcifications –include effects of background noise via statistical observer model