DELAUNAY TRIANGULATION BASED IMAGE ENHANCEMENT FOR ECHOCARDIOGRAPHY IMAGES

Transcription

1 17th European Sgnal Processng Conference (EUSIPCO 9) Glasgow, Scotland, August 4-8, 9 DELAUNAY TRIANGULATION BASED IMAGE ENHANCEMENT FOR ECHOCARDIOGRAPHY IMAGES V Ahanathaplla 1, J. J. Soraghan 1, P. Soneck 1 Department of Electroncs and Electrcal Engneerng, Unversty of Strathclyde, 4 George Street, Glasgow - G1 1XW Department of Cardology, Western Infrmary, Glasgow - G11 6NT emal: vj@eee.strath.ac.uk, ABSTRACT A novel mage enhancement approach for automatc echocardography mage processng s proposed. The man steps nclude undecmated wavelet based speckle nose reducton, edge detecton, followed by a regonal enhancement process that employs Delaunay trangulaton based thresholdng. The edge detecton s performed usng a fuzzy logc based center pont detecton and a subsequent radal search based fuzzy multscale edge detecton. The edges obtaned are used as the vertces for Delaunay trangulaton for enhancement purposes. Ths method enhances the heart wall regon n the echo mage. Ths technque s appled to both synthetc and real mage sets that were obtaned from a local hosptal. 1. INTRODUCTION Echocardography (or Echo) s one of the most wdely used medcal magng technques for the dagnoss of heart condtons, such as wall moton abnormalty, and help dagnose the possble presence of damaged tssue n the heart wall. Echo scan s a move captured over tme, to cover a complete cardac cycle. Although Echo scans are low qualty mages, these scans are preferred to that of the other cardac magng technques (such as MRI, CT). Ths s due to the fact that they are: relable; harmless; panless; cost effectve and nonnvasve. Addtonally, the echo mages are sutable for determnng left ventrcle (LV) wall thckness, and regonal wall moton abnormaltes. Despte advancements n the ultrasonc magng technque, the captured mages may stll contan varous magng artefacts, such as, large amount of nose, mssng structures (those parallel to the ultrasound beam), false echoes, shadowng, and restrcted echo wndows [1]. Also, the qualty of the echo mages s both patent dependent (the mages taken n obese patents have more nose and lower contrast) and clncan dependent. Manual nterpretaton of echo mages by an expert s a laborous process due to the poor contrast resoluton, a hgh level of nose and other artefacts. Due to the nature of these mages, there s sgnfcant nter-observer and ntra-observer varablty n the dagnoss of heart condtons. Ths results n the need for research n the feld of echocardography mage enhancement technques. A new algorthm to enhance the heart wall regon n low qualty echocardograph scans s presented n ths paper. A detaled descrpton of the varous stages nvolved s descrbed n secton. The expermental results are presented n secton 3, and secton 4 contans concludng remarks and suggestons for future work.. METHODOLOGY A hgh level overvew of the proposed enhancement algorthm s presented n Fg.1. The proposed algorthm reduces speckle nose and enhances the features of dagnostc mportance, such as the heart wall n short axs (SA) and long axs (LA) vews. Undecmated wavelets are known to smooth sgnal-ndependent nose [, 3] and are used to reduce speckle nose n the echo scans. Varous mage processng technques have been appled to segment the heart wall boundares to assst the nterpretaton of echo mages [4]. The segmentaton of the heart wall n the echo mage s strongly nfluenced by the qualty of the mage and the presence of speckle nose (a characterstc of coherent mages lke ultrasound mages whch reduce the contrast of the mage). Enhancement of features n echocardography mages, performed n [5] nvolves phase congruency based feature detecton, followed by nonlnear post processng. D feature asymmetry used here to detect the features, does not yeld clean features, n nosy mages, f the sgnal to nose rato s low. Contrast enhancement of the echocardograph mages by multscale wavelet analyss along wth wavelet shrnkage technque s performed n [6]. EURASIP,

2 Fg. Orgnal Image (left) and mage after speckle nose reducton (rght) s appled to all rows of the sub-bands. Then the mage s reconstructed usng all the modfed sub bands to get the nose reduced mage as shown n Fg.. Fg. 1 Enhancement Algorthm Overvew In our work, pror knowledge of the mages s taken nto account for enhancng the echo scan. To determne the regon of nterest, the center pont and wall boundares are detected by performng fuzzy center pont detecton [7] and fuzzy multscale edge detecton [8]. Then, a trangular meshng technque called Delaunay trangulaton [9] s used to dvde the regon of nterest nto smaller regons usng the edges obtaned n the prevous step. Ths trangulaton technque dvdes a surface nto regons wth common characterstcs that are partcularly well-suted for mage processng applcatons[9]. Delaunay has been used to represent shapes n mages and used for mage segmentaton [10] and also used to form hgh resoluton mage from low resoluton frames [11]..1 Speckle nose reducton Speckle nose caused by backscatterng, s a random, nterference pattern n coherent mages such as ultrasound mages. The texture of the observed speckle pattern does not correspond to the underlyng anatomcal structure of the heart. In ths algorthm, undecmated wavelets are used to decompose the mage (up to 4 levels) nto approxmaton and detals. An average flterng s appled to the approxmaton and mean based smoothng s appled to the detals, whch s defned mathematcally for an mage I( as I( I = N f f I( N I( > N ( (1) where N s the mean value of row of the pxel I( and y are the pxel coordnates. The above representaton can be explaned as follows. Frstly the mean value (N) of each row s computed. If the absolute value of a coeffcent n the row exceeds the mean value then t s replaced by the mean. Ths operaton s used to smooth the sudden changes of coeffcent values. The same operaton. Edge detecton The multresoluton-based algorthm proposed by [7] s used n ths work for cardac left ventrcular (LV) epcardal and endocardal boundary detecton. It s a center-based approach, where the edges are searched along the radal lnes startng from the LV center. The advantage of usng ths approach s that the processng tme s reduced as the boundary search problem s transformed from two dmensons to one dmenson. Ths fuzzy multscale edge detecton technque comprses of two man stages, the automatc LV center pont detecton for all the echo frames, LV boundary detecton on radal lnes and spatal/temporal processng of the extracted boundares and Cubc B-Splne LV boundary approxmaton [8]. These stages are descrbed n the followng sectons...1 Left Ventrcle Centre Pont (LVCP) detecton: The automatc LVCP detecton s performed usng the knowledge that the LV s close to the center of the mage plane and the grey level of LV cavty s dark wth characterstc dark neghbourhood pattern. The local and global knowledge of the echo mages are represented by fuzzy membershp functons and the canddate LVCP pxels are obtaned by usng fuzzy logc operators. In other words, fuzzy membershp functons are used to represent Spatal nformaton (LVCP s n the center part of the mage), Morphologcal nformaton (LVCP s close to the vertcal dameter of the LV passng through the lowest pont of the posteror epcardal boundar, and Intensty nformaton (LVCP s a dark pxel). These membershp functons are combned by applyng fuzzy operator for ntersecton to select the hgher membershp degree at the output. After obtanng most probable canddates for the LVCP, a template matchng s performed for these canddate pxels to obtan the most lkely LVCP. Template matchng s used here to search for specfc patterns (dark neghbourhood pattern) wthn the LV cavty [7]... Left Ventrcle Boundary detecton: The reason for usng the center-based and radal search based approach, proposed by [8] s the observaton of radal or- 1879

3 ented moton of the LV boundary n the echo move. In ths approach, the edges are searched along the radal lnes startng from the LV center. The radal lnes through the anteror wall are combned to determne the epcardal, endocardal edges and ther dfference to get the LV wall thckness. Usng these edges along wth the LVCP, a regon of nterest (ROI) s defned for both endocardal and epcardal boundares. A rough estmate of the epcardum s estmated. Other than the frst radal lne, a ROI s modelled by ncludng a regon on ether sde of the detected edge n the prevous radal lne. Smlar ROI s modelled for the endocardal edges. The endocardal and epcardal edge ponts are extracted from these radal lnes n the ROI usng Fuzzy Multscale Edge Detecton (FMED) [8]. The wavelet transform (WT) of the radal lne profle s performed. Local maxmum at a gven WT scale represents the postve step edge n the sgnal. Fuzzy membershp functons are used to represent the WT scales. Approprate membershp value s assgned to all the ponts concdent wth the local maxma. The dfferent scales are combned by applyng fuzzy operator for ntersecton to extract the edge n the ROI. The extracted endocardal and epcardal boundares are refned by applyng cubc B-cplne approxmaton to get a contnuous and smooth boundary..3 Delaunay trangulaton The tssue or wall regon n the echo mages s represented as brght regons due to strong reflecton, whle the left ventrcle cavty or the blood regon s a dark regon. The contrast enhancement s acheved by brghtenng the wall regon and darkenng the blood regon, by dvdng the echo mage nto smaller regons and performng non-lnear processng n those regons. Dvdng the entre mage nto unform regons does not work as dfferent regons have to be processed dfferently. It s also computatonally expensve and unnecessary. Delaunay trangulaton, ntroduced by Bors Delaunay n 1934 s used n ths work to dvde a surface nto trangular regons wth common characterstcs that are partcularly well-suted for mage processng applcatons. Ths trangulaton s performed by usng the ponts or edges obtaned from the mage. For a set of ponts, V n a plane, Delaunay trangulaton s performed n such a way that there s no pont n the crcumcrcle of the trangles (that s, no pont s wthn the crcle passng through three vertces of trangle) n the network. Delaunay trangulaton s advantageous over other meshng based technques as t maxmzes the mnmum vertex angles of the trangles n the network, so that the trangles formed tend towards equangular trangles, avodng sharp and stretched trangles. Ths trangulaton s sutable for nterpolaton as the pxel values wthn the regon n the fnal mage s drectly related to the trangle regon n the network. Delaunay trangulaton s performed n the heart wall regon between the endocardum and the epcardum to dvde t nto trangular regons and these regons are processed by applyng a thresholdng technque. Ths s followed by a processng the blood regon on a smlar fashon..3.1 Processng tssue or wall regon: To select the wall regon for processng, the endocardal and epcardal boundary edges along wth other set of edges formed, 3 pxels and 6 pxels away from both the edges are consdered as vertces for trangulaton. The wall regon closer to the edges s dvded nto smaller regons by formng bands wth 3 pxels wdth from the nner and outer wall and the regon n-between them s dvded nto a comparatvely larger trangles. The trangulaton performed n the wall regon can be seen n Fg. 3(b). The trangles formed n the blood regon are not consdered for processng n ths stage. After performng the trangulaton, a thresholdng s appled wthn the trangular regon. In the tssue regon, a mean based thresholdng s appled, whch s defned mathematcally for a trangular regon ( x I M I + = ( 5 I ( I y and I, as f I ( M f I ( > M ( () where ( ) M are the pxel value and the mean of the pxels wthn the trangular tssue regon, x and y are the pxel coordnates. The above representaton can be explaned as follows. Each pxel wthn the trangular regon s compared wth mean of the pxels wthn the regon. If the actual pxel value s less than or equal to the mean, then the pxel value s ncremented by 1/5th of the mean, otherwse the pxel s retaned..3. Processng LV cavty or blood regon: The blood regon s dvded nto trangular regons by consderng the center of the LV cavty and the endocardal boundary as vertces for trangulaton, followed by thresholdng whch s defned mathematcally for a trangular regon ( x I, as I( 5 f I( M M ( = I ( f M I( M+ I( f I( > M+ I (3) where ( x I, and M are the pxel value and the mean of the pxels wthn the trangular blood regon, x and y are the pxel coordnates. In other words, f the actual pxel value s less than or equal to the mean, then the pxel value s decremented by 5. If the pxel les between the mean and M +, then the pxel value s decremented by half of the mean. If the pxel s greater than M +, the pxel s retaned so that the heart wall tssue structure f encountered whle processng the blood regon, wll not be affected. The contrast mprovement s performed based on the local nformaton so that weak regons (lateral wall regon) of the mage are enhanced more than strong regons (top and bottom wall regon). M 18

4 The new modfed mage s reconstructed as follows: I fed x y = ( P y + P y ) (4) mod ( ) ( ) ( ), 1 mages. The pxel value n the mage vares from 0 and 18 whch s defned usng Raylegh dstrbuton wth dfferent reflectvty factor for the reflected ultrasound sgnal due to (a) LV Center and Boundary (a) (b) Trangulaton - Endocardal edge + Epcardal edge (b) s the mage after processng the wall re- where P ( gon and P ( (c) Trangulaton - Endocardal edge + Center 1 Fg. 3 Shows the dfferent stages n enhancement step for the synthetc Short Axs heart mage. (a) Orgnal mage wth extracted LVCP and wall boundary, (b) Delaunay trangulaton for only the wall regon (usng the extracted wall boundares) and (c)trangulaton for the blood regon regon (usng the extracted nner wall and center) s the mage after processng the blood regon n the orgnal mage. Averagng the processed mages s performed to reduce the effect of ncorrect modfcatons to the anatomcal structures f an ncorrect boundary s detected. 3. EXPERMENTAL RESULTS Ths secton presents expermental results of the system for automated dagnoss of abnormal wall moton. Echo mages can be recorded n dfferent vews, each of whch s mportant to dentfy crtcal parameters to evaluate dfferent heart condtons. Here, short axs vew (SA) and long axs (LA) vews are consdered. The echo moves are captured over a tme perod; to cover a complete cardac cycle conssts of an average of 5 frames. The number of the radal lnes used for edge detecton n one frame s emprcally selected as. Synthetc mages models and real mage data are used for the experment. Synthetc mages are generated usng the method used n [8] and conssts of 5 frames of 0 x 0 blood, heart wall and tssue other than heart. A Raylegh dstrbuton wth reflectvty factor σ s gven as n ( n) = σ 1 1 n exp σ R (5) In the frst synthetc mage, reflectvty factors defned n the left and rght ventrcular regons representng blood wth σ 1 =, heart wall regon wth σ =, tssue outsde the heart n the two lateral regons wth σ 3 = and the lung tssue outsde the posteror wall regon wth σ 4 = 1. The relatve contrast between the regons s σ 1 / σ = σ / σ 3 = 0.5 and σ / σ 4 = For the second synthetc mage, the relatve contrast between the regons s σ 1 / σ = σ / σ 3 = 0.33 and σ / σ 4 = 0.5. The enhanced mage results for the synthetc mages are shown n Fg. 4 and the results for the real mages (both SA and LA vews) are shown n Fg. 5. The mages on the left are the orgnal mages and the mages on the rght are the enhanced mages. It can be seen that the contrast between the tssue and the blood regon s mproved and the wall regon (whch s of dagnostc mportance) s well defned n the enhanced mages. The contrast resoluton (CR) between the tssue regon, S T and the blood regon, S B s measure by [1] CR = (S T -S B )/(S T +S B ) 1 Fg. 4 Synthetc Short Axs mage (left) and enhanced mage showng the contrast mprovement (rght). (a) σ 1 / σ = σ / σ 3 = 0.33 and σ / σ 4 = 0.5. (b) σ 1 / σ = σ / σ 3 = 0.5 and σ / σ 4 = where S T s the pxel average n a small secton of tssue regon and S B s the pxel average n a small secton of the blood regon. 1881

5 The CR n a secton of the mage s measured to gve a quanttatve measure of the mage enhancement, as well as the subjectve measure. Table. 1 shows the contrast resoluton for sample orgnal mages, after applyng Medan flter, Wener flterng and the proposed Delaunay trangulaton based Enhancement algorthm. From the table, t can be seen that the contrast resoluton s mproved for the proposed method when compared to the orgnal mage, Medan and Wener flter. (a) Table 1: Comparson of Contrast Resoluton measures Image Orgnal Medan Wener Proposed (b) CONCLUSION AND REMARKS In ths paper, a novel echocardography mage enhancement algorthm whch combnes undecmated wavelet based speckle nose reducton, edge detecton, followed by a regonal enhancement process that employs Delaunay trangulaton based thresholdng s presented. The algorthm s appled to both synthetc and real mage data sets of short axs and long axs vew echocardography sequence. The ntal results are encouragng and suggest that ths regon based enhancement mproves contrast near the edges, whch are of dagnostc mportance. The contrast mprovement s performed based on local nformaton so that weak regons of the mage are enhanced more than strong regons. Ths could be benefcal to experts when manually defnng the edges for dagnosng purposes and also as a pre-processng stage for the automated analyss of heart functon. ACKNOWLEDGMENT The authors would lke to thank the Cardology Department of the Western Infrmary n Glasgow for provdng the echo mages. REFERENCES [1] S. Kaddoura, Echo Made Easy, Churchll Lvngstone, (1). [] D. Gnanadura, V. Sadasvam, Undecmated wavelet based speckle reducton for SAR mages, Pattern Recognton Letters, 6, pp (5) [3] F. Argent, G. Torrcell, Speckle Suppresson n Ultrasonc Images Based on Undecmated Wavelets, EURASIP Journal on Appled Sgnal Processng, 5, (3) [4] J. A. Noble, D. Boukerrou Ultrasound Image Segmentaton: A Survey, IEEE Transacton on Medcal Imagng, vol. 5(8), pp (6) [5] D. Boukerrou, J. A. Noble, M. Brady, Feature enhancement n low qualty mages wth applcaton to echocardography, Lecture Notes n Computer Scence; Vol (c) Fg. 5. Sample real mages (left) and enhanced mage (rght) showng the contrast mprovement. (a) and (b) are sample short axs vew echo mages and (c) s a sample long axs vew echo mage. 8, Proceedngs of the 17th Internatonal Conference on Informaton Processng n Medcal Imagng, pp (1) [6] X. Zong, A. F. Lane, E. A. Geser, Speckle reducton and contrast enhancement of echocardograms va mult scale nonlnear processng, IEEE Transacton on Medcal Imagng, vol. 17 (4), pp (1998) [7] S. K. Setarehdan, J. J. Soraghan, Automatc left ventrcular Center pont extracton n echocardographc mages, Sgnal Processng, 61, pp , (1997). [8] S. K. Setarehdan, J. J. Soraghan, Automatc left ventrcular feature extracton and vsualsatonfrom echocardographc mages, Computersn Cardology,3,pp.9-1, (1996) [9] J. R. Shewchuk, Lectures Notes on Delaunay Mesh Generaton, Unversty of Calforna at Berkeley, [10] L. Prasad, A. N. Skourkhne, Vectorzed mage segmentaton va trxel agglomeraton, Pattern Recognton, 39, pp ,(6) [11] S. Lertrattanapanch, N. K. Bose Hgh resoluton mage formaton from low resoluton frames usng Delaunay Trangulaton IEEE Transacton on Image processng, 11(1), pp , (). [1] 188