Effects of Grayscale Window/Level on Breast Lesion DetectabilityJeffrey Johnson, PhD aJohn Nafziger, PhD a Elizabeth Krupinski, PhD bHans Roehrig, PhD bbaSupported by U. S. Army Medical Research and Materiel Command, grant DAMD-17-01-1-0621

RationaleNearly 50% of breast lesions missed at initial screening are visible retrospectivelyDigital mammography could reduce perceptual errors by enhancing lesion conspicuity with image processingPerceptual models could be useful tools for automating and optimizing techniques for image enhancement

OverviewThis 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 lesionsCompared model and observer performance in two experiments:2AFC detection thresholds with simulated mammograms and nonmedical observersROC observer performance study with radiologists and digitized mammograms

Methods: Simulated MammogramsBackgroundsFiltered noise, 1/f3 noise power spectrumTwo groups: Bright and Dark central regionsLesion signalsMass: 2D Gaussian (d=50 arcmin)Microcalcification cluster: six blurred disks or specks (disk d=8 arcmin, cluster d=40 arcmin)

Methods: W/L ConditionsP-value transformations:Fully stretchedUnderstretched (-25%)Overstretched (25%)Bright shifted (+25%)Dark shifted (-25%)Applied to full 512x512 pixel image or 170x170 pixel central region of interest containing lesion

Example Test ImagesFullystretched(FS) Understretched(US) Overstretched(OS) GaussianFull W/LBright CenterGaussianCentral W/LBright CenterSpecksFull W/LDark CenterSpecksCentral W/LDark Center

Example Test ImagesBright shifted(BS) Dark shifted(DS) GaussianFull W/LBright CenterGaussianCentral W/LBright CenterSpecksFull W/LDark CenterSpecksCentral W/LDark Center

2AFC Threshold DetectionSide-by-side presentation of same background with/without signalSignal amplitude varied in 1-up/3-down staircase procedure; detection threshold at ~80% correctFive W/L conditions interleaved in same sessionSeparate sessions for two signal and two background types

Test ConditionsSiemens 5M-pixel CRT monitor (P45)Luminance range = 0.3 to 290 cd/m2Barco 10-bit display controllerDICOM-14 grayscale display functionThree nonmedical observersViewing distance = 52 cm; chin restAmbient lights off

Results: Detection Thresholds for Gaussian SignalsError bars show 95% confidence intervalsBright BackgroundsDark Backgrounds

Results: Detection Thresholds for Speck ClustersError bars show 95% confidence intervalsBright BackgroundsDark Backgrounds

Experimental Detection ThresholdsSignificant variations across W/L conditionsGenerally lower for central vs. full W/Ldue to local contrast enhancement- fully stretched not always optimalFull W/L: Lowest thresholds for fully stretched, understretched (specks only)dark shifted on bright, bright shifted on darkCentral W/L: Lowest thresholds for overstretched for Gaussians and specks on darkdark shifted on bright, bright shifted on dark

Visual Discrimination ModelingSimulates physiological response of human visual system to visual stimuli: luminance patterns from images & videoOutput is a deterministic prediction of feature or image discriminability as function of spatial location, spatial frequency, and timeDiscriminability measured in units of Just Noticeable Differences (JND)

VDM ArchitectureJND scalarSpatial frequencybandsSpatial orientation responsesDisplay& OcularProcessingOpticsCrossband MaskingJND mapJND DistanceCombin. RuleDisplay luminancePair of input imagesProbabilityContrast Pyramid (visual cortex)Within-band MaskingContrast Pyramid

VDM vs. Experimental Thresholds for Gaussians on Bright BackgroundsError bars show 95% confidence intervalsFull W/LCentral W/L

VDM vs. Experimental Thresholds for Gaussians on Dark BackgroundsError bars show 95% confidence intervalsFull W/LCentral W/L

VDM vs. Experimental Thresholds for Specks on Bright BackgroundsError bars show 95% confidence intervalsFull W/LCentral W/L

VDM vs. Experimental Thresholds for Specks on Dark BackgroundsError bars show 95% confidence intervalsCentral W/LFull W/L

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

ROC Observer StudyDetermine effects of W/L functions and size on detection of microcalcification clusters by mammographersEvaluate utility of localized ROI contrast enhancement (central vs. full W/L)

ROC Observer Study: Image PreparationDigitized mammograms (n=15) from Digital Database for Screening MammographyExtracted 512x512-pixel sections with single, centered microcalcification clusterRemoved calcifications by median filteringGenerated 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

ROC Observer Study: Test Conditions6 radiologists at Univ. of Arizona225 images/session2 reading sessions ~2 weeks apartDecision confidence on 6-point scaleNo image processing, no time limits, ambient lights off; viewed at ~25 cmSiemens 5M-pixel CRT monitor (P45)Luminance = 0.8 to 500 cd/m2DICOM-14 grayscale display function

Examples of Test ImagesFully stretched(FS, 0-4095) Understretched(US, 15%) Overstretched(OS, 15%) FullW/LCentralW/L

ROC Observer Study: ResultsCompared central vs. full W/L across all W/L functions, all lesion contrastsObserver performance statistically better (p

ROC Observer Study: ResultsNo 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)

ROC Observer Study: AnalysisCentral W/L enhanced lesion contrast but changed appearance of parenchymal tissue relative to surrounding areasDecision confidence lowered by nonuniform appearance of background tissue characteristicsConclusion: Calcifications may be easier to perceive (due to higher contrast) but more difficult to interpret (due to cognitive factors, past experience)

ConclusionsFor simulated lesions and backgrounds, VDM was generally a reliable predictor of W/L conditions for optimal detectabilityResults with simulated images suggested benefits of localized contrast enhancementDecision confidence and performance of mammographers actually lower with localized W/L, probably due to nonuniform tissue appearance

Future DirectionsAllow 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/calcificationsinclude effects of background noise via statistical observer model