A Preliminary Study On A Nonsubtraction Digital Angiogram Processing Technique

A Preliminary Study On A Nonsubtraction Digital Angiogram Processing TechniqueYen Wang, M.D.,D.Sc.(Med.)Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, PA 19107C.C. Li, Ph.D.Zhixiang Xu, B.S.*Liben Huang, B.S. **Department of Electrical Engineering, University of Pittsburgh, Pittsburgh, PA 15261AbstractA contrast enhancement digital filtering technique has been explored for improving thevisibility of vasculature in nonsubtraction digital angiograms. Experimental results onsample images showed that the average contrast between the vasculature and its surroundingtissue was increased by sixfolds and the signal -to -noise ratio was increased by twofolds.This suggests a great potential of this enhancement filtering technique for digital non -subtraction angiography, bypassing the problem of patient body motion which causesdifficulty in image registration in digital subtraction angiography.IntroductionDigital radiography has been under rapid development during the past several years.The major effort has been focused upon the systems in which X -ray signals are detectedelectronically, converted into digital form, and processed as required prior to beingrecorded and displayed. Among these systems, the digital subtraction angiogram system isthe most developed one where temporal subtraction of images, obtained before and afterintravenous injection of an iodinated contrast medium, is performed to enhance the visi-bility of blood vessels for diagnosis of atherosclerosis.) The matched filtering on aset of images acquired during the flow of contrast material through the area of interesthas also been studied as an alternative to digital subtraction.2 However, the presenceof patient body motion artifacts is a major limitation to these techniques of digitalangiography. Recently, a method of the.unsharp mask filtering for digital radiographshas shown excellent results of enhancing the image clarity.3 This paper presents apreliminary study on a digital image enhancement technique for angiograms where registra-tion and subtraction of images is not required.Contrast Enhancement Digital FilteringThe notch filtering4,5'6 is cabable of enhancing the contrast between vasculature andits surrounding tissue in a nonsubtraction intravenously injected digital angiogram.Let G(i,j) be the gray value of the angiogram at pixel (i,j), and -U(i,j) be the gray valueof the averaged image at (i,j) where the averaging operation takes place in a 3x3 pro-cessing window centered at (i,j) and is iterated k times. For a region containing smallvessels with low contrast from the background, the iterative averaging gives a muchsmoothed and defocused image which is to be subtracted from its original image; thedifference is then amplified and superimposed onto the original image or an attenuatedversion of the original image. The notch filtering provides an output image {Gz(i,j)}given byGz(i,j) = k1G(i,j) + k2 [G(i,j) - G(i,j)]where k is an enhancement factor much greater than one, and Kl is an attenuationcoefficient less than or equal to one. The second term in the above equation has theeffect of subtracting the background and enhancing the contrast of the vasculature.For structures of high contrast in the original image, the notch filtering may producean undesirable ringing artifact near tide sharp transition in gray values. To reduce suchan artifact, the zonal notch filtering can be used where -(i,j) becomes the zonal averagegray value at pixel (i,j), which is the average gray value of all pixels (m,n) in anaveraging neighborhood Nay within the processing window where IG(m,n) - G(i,j)1 < T. Thethreshold value T should be chosen such that it is slightly greater than the weak contrast*Prof. Z. Xu is a visiting scholar from Dept. of Radio Electronics, Shanghai University ofScience & Technology, Shanghai, China. * *Prof. L. Huang is a visiting scholar from HefeiResearch Institute of Intelligent Machines, Academia Sinica, Hefe, Anhei, China