Frequency diversity speckle processing

ABSTRACTS,

ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION

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IN MEDICAL IMAGING, & Technology, Center for Rockville, MD 20857. accuracy suffers from two THEORY

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from various test targets autocorrel at i on functions. be presented. EYin part by NIH grants

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it depends on the the disease or abnormal i ty being tested for. up is to specify the true positive f t-action (sensitivity) and true negative fraction (specificity), but these measures do not control for the implicit variable, namely, the criterion or threshold value that is used in the d i agnost i c test to make the cut between abnormals and normals. The receiver operating characteristic curve (ROC curve) allows for the separation effect of the threshold criterion of the (or mind set of the diagnostician) from the actual intrinsic separability of the normal from the abnormal populations using the test under study. This tutorial presentation will motivate the ROC curve approach to diagnostic system assessment, describe the design of such a study, and examine the links between objective image system assessment (using signal-to-noise ratio analysis) and the outcome of ROC curve analysis. Finally, the advantages and disadvantages of tissue substitute phantom studies and contrast/detail analysis will be discussed within the context of signal detection theory and ROC curve analysis. that are prevalence The next

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SESSION FREGKJENCY F. Graham

DIVERSITY Sommer=, CA 95054

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SPECKLE PROCESSING,

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REDUCTION Steve

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Scott Radiology,

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Santa

Clara, and =Department of Stanford University Medical Center, Stanford, CA 94305. Several images of targets from a contrast-detail phantom were acquired by digitizing the rf from a mechanicallytranslated disk transducer, and stored for later processing on IBM PC/AT compatible floppy disks. The images were displayed using a Vectrix board with image display digital filtering techniques used for envelope detection and speckle processing. Frequency diversity techniques were employed to reduce the speckle noise component, at the expense of i mage resolution, by breaking the spectrum into several overlapping bands. Each processed image was computed as the incoherent aver age of the individual narrowband images. The filters were designed to cover the original transducer spectrum, and the spacing selected to yield 50 percent correlation based on theoretical considerations. The amplitude signal -to-noise ratio (SNR) of the processed images was improved by N, where N is the number of filters employed, verifying the theoretical correlation calculation. A 51 ight degradation in spatial resolution resulted, with a significant increase in scale 9-y information. The preand post-processed images are shown for various numbers of filters, and various contrast targets from hypoechoic to hyperechoic.

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ABSTRACTS, ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM

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Contract

# 1 N43CM57808-00. SPECKLE SNR VS. LATERAL WHAT * S THE TRADE-OFF?, von Ramm”, %Center for Rockvi 1 le, MD 29857 and We shall examine

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t rasound scan or should a compound image several partially-overlapping subapertures? should a 40 mm array be broken into several 20 for spatial compounding’? It is nece5sar y t increase in speckle SNR vs. the factor desirable To answer lateral resolution. this question for detecti on in speckle by ideal observers, lesion previously-reported model which predicts ul

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d is the lesion diameter, N is image samples in a compound scan and average lateral and axial resolution cell respectively. We also use measurements of speckle sizes, decorrelation rate5 reported 1 ast year showing N = 3.2 independent samples available for spatial compounding when a moved over transducer subaperture is its length. The model scan SNR increase of (3.2/2) +‘= = 26 predicts a compound Our experimental percent over the large aperture simple scan. measurements on lesions in the Echobloc phantom (characterized by Rayleigh statistics) yielded an average SNR increase of 25 confirming these predictions. percent, where the sex

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SPATIAL COMPOUNDING FREQUENCY AND REDUCTION, G.E. Trahey*, 3-W. AllisonL, von Ramm% , ‘Duke University, Durham, for Devices and Radiological Health,

FOR INCREASED S.W. Smith'*", NC 27706 and FDA, Rockvi lle,

spatial or frequency compounding in an emp 1 oymen t of i magi ng system necessitates a reduction in its ultrasonic lateral or axial resol ut i on, respectively. In optimal the speckle reduction achievable by either of these addition, is limited available acoustical two techniques alone by the window and bandwidth. investigating the possibility of increasing We have been reduction while reducing the related resolution loss speckle use of spatial and frequency through the si mu1 taneous St heme relies on the compounding. The success of such a of the speckle decorrelation achieved by spatial independence compounding from that achieved by frequency compounding . We experimentally determined the interdependence of speck1 e have decorrelation achieved by these two methods. Our results indicate that their si mu1 taneous use will considerably reduction achievable over that of either increase the speckle method alone. MAXIMUM

J.D.

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Kretztechnik, signals source

GUIDE TO OPTIMAL ULTRASOUND IWING, 4871 Zipf, Clustria. conventional imaging devi ces are from of information for describing tissue

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68