Optical coherence tomographic elastography techniq

文件名称: Optical coherence tomographic elastography technique

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详细说明：光学相干弹性成像的一种基础算法，从图像匹配到弹性图的获取，整个模型的建立很完整，特别适合对光学相干弹性成像初步的探索。ka patel. fu placement cale u 5 relation ale C 0.0 Strain scale 99 Axis Lateral Maximum Axial strain displacement displacement cross-correlation maps Figure 3 Axial and lateral displacements, maximum cross correlation, and strain maps calculated for phantom images on fig 2 by cross correlation th kernels of (A)21 x 21, B)31 31,(41×41,(D51×51,and(E61×61 pixel W com Optical coherence tomographic elastography 559 Calculation of strain maps Our goal is to calculate the local values of elastic modulus If tissues are assumed to be uniform, isotropic, and in the phantoms. If we assume that the phantoms are incompressible and if the stress is applied along one axis uniform, isotropic, and incompressible and the stress is only,the Young's modulus (or clastic modulus)E is described applicd uniformly in the axial direction, then the local stress b is cqual to the applied stress constant throughout the sample) and only the local strain is needed for calculation of E Young’ s modulus Local strain e is determined from an estimate of tissue axial displacement where o is the axial (or normal)stress (dcfincd as the forcc perpendicular to the cross sectional area, divided by the cross sectional area; units are lb/in or N/m+) F (units are lb/in2 or Nim) wnere d is the displacenent estimate at a dis lance z roiL A the top of the phantom and d2 is the displacement estimate at a distance z Az from the top of the sample (the displace and s is the axial strain (defined as the fractional change in ment values are calculated by the cross correlation techni- ength; strain has no units) que) The local strain can be shown as an image or map. A map △L of local strain is therefore an inverse map of elastic modulus L (Strain has no units) this is the way an elastogram is usually presented 6间bpmp)21023184x05 / hearting Rogowska, Patel, Fujimoto, et al Table 1 Axial displacement measurements of phantoms with varying cross correlation ernel size Kernel size Calculated mean axial Measured mean axial Percentage (pixels) displacement (ym) displacement (um) 21×2 63.53 94.5 32.77% 31×31 82.9 94.5 12.26% 945 1.88% 51×51 1060 945 12.17 61×6 10896 94.5 15.30% igure 5 (A) Original and (B) displaced aorta images All image processing techniques were implemented Inlaxilllulll cross correlation and straill Illaps calculated b Matlab(MathWorks, Natick, Massachusetts, USA) cross correlation with kernels of2l×2l,3l×3l,4l×41, 51 x 51, and 61 x 61 pixels. Figure 4 shows the correspond RESULTS ing displacement vectors obtained by cross correlation with The phantom images were processed by cross correlation kernels of2l×21,31×31,4l×4l,5l×5l,and6l×6 (equalion 1)with several kernel sizes varying Iron 21 x 21 pixels. Table I shows mean displacement values calculated to 61 x 6I pixels. Larger kernel sizes were not able to track for different kernel sizes and the percentage errors between the small charcoal particles and were not used in this stud calculated and measured displacements Figure 2 presents original and displaced phantom images Fig 5 shows the original and displaced aorta images Figure 3 shows the axial and lateral displacements, as well as Figures 6,7, and 8 depict the results from the correlation Displacement A 1-300 起需 Cross-correlation scale 0.5 D 0.0 Lateral aXImum displacement displacement cross-correlation Figure 6 Axial, lateral, and maximum cross correlation maps calculated for aorta images in fig 5 with the following kernels: (A)21 x 21 (B)31×31,(C41 and(D)61×61 pixels www.heartinl.com Optical coherence tomographic elastography 561 technique applied to the aorta images. Fig 6 presents the axial and lateral displaceillell inla ps and IllaxiInuIll cross correla- tion maps. Figure 7 shows the strain images. Figure 8 shows the corresponding displacement vectors obtaincd by cross correlation with kernels of 21 x21,31x31, 41 x41, and 6l×6 I pixels DISCUSSION Phantoms and in vitro aorta wcre cxamincd to asscss spccklc modulation and measure the displacements and strain maps y using phantoms, we investigated the influence of kernel size on the accuracy of the displacement measurements. In terms of a percentage error between calculated and measured displacements, the best results for phantoms were obtained with a 41 x 41 kernel(1.88% error). For both phantom and aorta images we found that, with the increasing size of cross corrclation kernel, thc axial and lateral displacement maps are less noisy and the displacement vectors are more clearly defined. However, the large kernels tend to average out the differences in displacements of small particles in phantoms and decrease the ability of speckle tracking to make microstructural assessments. Therefore, it is important to select kernel size carefully, based on the image features The calculation of displacement maps can also be improved. Since cross corrclation is a very time consuming process, it can be modified by either a"coarse to fine"o 'feature selection"technique. In the first approach, an image is divided into large blocks (perhaps overlapping)and the correlation is calculated between them( this takes just a few calculations). Only blocks with maximum cross correlation are further analysed (by cross correlation). In the feature selection technique, some important features or areas of igure 8 Displacement vectors for aorta images in fig 5 calculated by A (C)41×41,and{D)61×pixe"es cross correlation with the following ker (A)21×21,(B)31×31, scale terest are sclccted first (for cxample, charcoal particles or latex spheres on phantom images or plaque areas on arterial images) and the cross correlation is calculated for onl selected features Features can be preselected either manually or by automated image processing segmentation techniques In summary, we showed that preselected analysis criteria are critical for the correct interpretation of oCT elastography resulis. Future work is needed to understand how oct call be best applied to assessing risk associated with vulnerable ACKNOWLEDGEMENTS ch is supported in part by the United States na InsLilules of Health (Contracts NIH-ROl-AR448 12, NIH ROI AR46996, NiH ROI-HL63953, NIH-1-RO1-HL55686, and Nih Ro1 EB000419)and the Whitaker Foundation (Contract No. 96-0205) authors affiliations J Rogowska, N A Patel, M E Brezinski, Orthopedics Departmen Brigham and Women's Hospital/Harvard Medical School, 75 Francis Street, Boston Massachusetts 02115. USA G Fujil of Electrical e ing and Comp Science, Massachusetts Institute of Technology, Cambridge, assachusetts 02139 USA REFERENCES Figure7 Axial strain maps for aorta images in fig 5 calculated with the following kernels::(A)21×21,(B31×31,(C)41×41,and 1 Brezinski ME, Tearney GJ, Bouma BE, et al. Optical coherence tomography for optical biopsy: properties and demonstration of vascular pathology D)61×6 I pixels Circulation 1996: 93: 1206-13 www.heartinl.com 562 Rogowska, Patel, Fujimoto, et al 2 Brezinski ME, Tearney GJ, Bouma BE, et al. Imaging of coronary artery 12 Ophir J, Cespedes I, Ponnekanti H, et al. Elastography: a quantitative method microstructure with optical coherence tomography. Am J Cardiol for imaging the elasticity of biological tissues. Ultrason Imaging 1996:77:92-3 1991:13:111-34 3 Brezinski ME, Tearney GJ, Weissman NJ, et al. Assessing atherosclerotic 13 Shapo BM, Crowe JR, Erkamp RQ, et al. Strain imaging of coronary arteries plaque morphology: comparison of optical coherence tomography and hi with intraluminal ultrasound experiments on an inhomogeneous phantol frequency ultrasound. Heart 1997: 77: 397-403 Ultrason Imaging 1996: 18: 173-91 4 Tearney G), Brezinski ME, Boppart SA, et al. Catheter based optical imaging 14 Heers G, Jenkyn T, Dresner MA, et a/. Measurement of muscle activity with of a human coronary artery. Circulation 1996; 94: 3013 magnetic resonance elastography. Clin Biomech risto with optical coherence tomography and high frequency ultrasoun yague 5 Patwari P, Weissman NJ, Boppart SA, et al. Assessment of corond 2003;18:537-42. 15 Manduca A, Oliphant TE, Dresner MA, et al. Magnetic resonance Amj cardiol 2000 85: 641-4 elastography: non- invasive mapping of tissue elasticity. Med Image Anal oppart SA, Tearney GJ. High resolution in vivo intra-arterial maging with opical coherence tomography. Heart 1999:82:128-33 16 Bishop J, Samani A, Sciarretta J, ef al. Two-dimensional MR elastography 7 Bouma BE, Tearney GI, Yabushita H, et al. Evaluation of intracoronary with linear inversion reconstruction: methodology and noise analysis. phy stenting by intravascular optical coherence tomography. Heart Med Bio|2000:45:2081-91 200389:317-20 8 Jang IK, Bouma BE, Kang DH, ct al. Visualization of coronary atherosclcrotic intravascular ultrasound. J Am Coll Cardiol 2002: 39: 604-pparison with plaques in patients using optical coherence tomography: con 2000:45:157990 18 Muthupillai R, Ehman RL. Magnetic resonance elastography. Nat Me 9 Erkamp RQ, Wiggins P, Skovoroda AR, et al. Measuring the elastic modulus 19%6201-3 of small tissue samples. Ultrason Imaging 1998: 20: 17-28 19 Schmitt JM. OCT elastography: imaging microscopic deformation and strain 10 De Korte CL, Cespedes l, van der Steen AFW, et al. Intravascular elasticity in tissue. Opt Express 1998; 3: 199-211 imaging using ultrasound. Ultrasound Med Bio/ 1997: 23: 725-46 20 Rogowska J, Patel N, Fujimoto JG, et al. OCT elastography of vascular tissue 11 De Korte CL, van der Steen AFW, Cespedes I, et al. Intravascular ultrasound importance of cross-correlation kernel size. Proceedings of the OSA elastography in human arteries: initial experience in vitro Ultrasound Med Biomedical Topical Meetings, Advances in Optical Imaging and Photon Bo199824401-9 Migration, Miami, 2002: PD20-1-PD20-3 IMAGES IN CARDIOLOGY li:10.136/hrt.2003025494 Embolism of thrombus in the right coronary artery to the left anterior descending artery in a woman with a single coronary artery 64 year old woman with chest pain and ECG consistent with an inferior myo A B cardial infarction received thromboly sis. There was no evidence of reperfusion and shc underwent rescue angioplasty. Shc was found to have a single coronary artery arising from the right sinus of Valsalva. The right RCA coronary artery (RCA) was occluded by hrombus(panel A). During the first contrast injcction a portion of the thrombus was dislodged and travelled across the left main stell (panel B)into the left anterior des cending artery(LAd) causing a distal occlu ion(panel C). The proximal occlusion of the RCA was successfully treated by thrombect omy(X-sizer, Plymouth, Minnesota, USA and stelling(Sonic, Velocily 4.0 x 28 I11Inl) with an excellent final angiographic result (panel D), although unfortunately there The patient required an intra-aortic balloon c D pump and inotropic support for 24 hours The peak creatine kinase concentration was 5500u/. Single coronary arteries are a recognised but rare anomaly with an incidence of around 0.02%. They may be clinically sig. nificant when a major branch passes between the aorta and the right ventricular LAD outflow tract as was found in this case. This particular anomaly is associated with sudden RCA cardiac death possibly due to compression between the major vessels. In our middle LAD aged patient it is likely that the thrombus in the proximal RCa was a consequence of underlying atherosclerosis. Unfortunately cclusion of the distal lad resulted from embolised thrombus dislodged from the rCa by contrast injection-an extremely rare event s Leslie IR Starkey i leslie ed ac uk www.heartinl.com Heart

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