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Published in final edited form as: J Magn Reson Imaging. 2009 May ; 29(5): 1116–1124. doi:10.1002/jmri.21700.
In Vivo Quantification of Femoral-Popliteal Compression during Isometric Thigh Contraction: Assessment using MR Angiography Ryan Brown, PhD1,2, Thanh D. Nguyen, PhD1, Pascal Spincemaille, PhD1, Martin R. Prince, MD, PhD1, and Yi Wang, PhD1,2,3 1Department of Radiology, Weill Medical College of Cornell University, 416 E 55 St., New York, NY 10022 2Department of Physiology, Biophysics and Systems Biology, Weill Graduate School of Medical Sciences of Cornell University, 1300 York Ave, New York, NY 10021 3Department of Biomedical Engineering, Cornell University, Ithaca, NY 14853
Abstract NIH-PA Author Manuscript
Purpose—To quantify femoral-popliteal vessel deformation during thigh contraction. Materials and Methods—Eleven subjects underwent an MR examination of the femoral-popliteal vasculature on a 1.5 T system. A custom 3D balanced steady state free precession (SSFP) sequence was implemented to image a 15–20 cm segment of the vasculature during relaxation and voluntary isometric thigh contraction. The arterial and venous lumina were outlined using a semi-automated method. For the artery, this outline was fit to an ellipse whose aspect ratio was used to describe arterial deformation, while venous deformation was characterized by its cross-sectional area. Results—Focal compression of the femoral-popliteal artery during contraction was observed 94 to 143 mm superior to the condyle that corresponds to the distal adductor canal (AC) immediately superior to the adductor hiatus. This was illustrated by a significant reduction (p ≤ 0.05) in aspect ratio from 0.88 ± 0.06 during relaxation to 0.77 ± 0.09 during contraction. A negligible change in arterial aspect ratio was observed inferior to the AC and in the proximal AC. Similarly, venous area was dramatically reduced in the distal AC region during contraction.
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Conclusion—Rapid 3D SSFP MR angiography of the femoral-popliteal vasculature during thigh contraction demonstrated focal compression of the artery in the distal AC region. This may help explain the high stent failure rate and the high likelihood of atherosclerotic disease in the AC. Keywords peripheral vascular disease; magnetic resonance imaging; adductor canal; atherosclerosis; vessel deformation
INTRODUCTION Atherosclerosis has been linked to multiple risk factors including hypertension, elevated cholesterol, smoking, diabetes, and obesity. Holding these systemic factors constant, atherosclerosis does not occur randomly in the vasculature system, but is most likely to be found at sites with a particular vessel geometry, certain geometry-induced flow patterns, and in the presence of endothelial injury. Atherosclerosis is typically associated with vessel branching, curvature, low wall shear stress (WSS) (1-5), or external mechanical factors Corresponding author Yi Wang, Ph.D., Weill Medical College of Cornell University, 416 East 55 St, New York, NY 10022, Email: Email:
[email protected], Tel: (212) 746-6880, Fax: (212) 752-8908.
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(6-10). The adductor canal (AC) is the region in which the femoral artery passes from the anterior thigh at the femoral triangle, between the vastus and adductor muscles, to the posterior thigh at the adductor hiatus where it exits the canal and becomes the popliteal artery (11). This region of the artery is relatively straight and free of major branches, yet is a prevalent site for disease (6,12-16), suggesting that mechanical factors are especially important here. The femoral-popliteal artery has been shown to be particularly vulnerable to such factors, as intimate contact with the adductor magnus tendon can cause acute arterial occlusion (8-10). Similarly, vessel trauma from intermittent compression is thought to initiate and accelerate chronic stenosis and occlusion (6,7). Furthermore, percutaneous transluminal angioplasty and stent deployment have become established techniques for lesion treatment in the femoralpopliteal artery. However, rates of stent failure, reocclusion and restenosis in the AC are unusually high (17,18), and mechanical trauma has been suggested as a primary cause (19). Although related studies have described artery deformation during hip and knee flexion (20-22), the effect of muscle contraction on the cross-sectional geometry of the artery has not been reported.
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To gain insight into muscle-induced trauma in the femoral-popliteal vasculature, we measured its cross-sectional deformation during maximal voluntary isometric thigh contraction. Isometric thigh contraction exercises the vastus muscles in the anterior thigh which are responsible for leg extension and stabilization required for everyday activities such as walking. A previous work, which utilized 2D spoiled gradient echo (SPGR) MRI to image two discrete femoral-popliteal artery locations, demonstrated that isometric thigh contraction compressed the artery in the distal AC region, while neighboring adipose tissue alleviated arterial compression in the popliteal fossa (23). However, this imaging technique required two separate thigh contractions, potentially leading to less accurate results due to slice misregistration and inconsistently applied contraction forces. To overcome these limitations and to minimize motion artifacts that may occur during prolonged muscle contraction, a rapid 3D steady-state free precession (SSFP) sequence was developed in this work to image a long, continuous section (15-20 cm) of the vasculature in a single short scan. The 3D SSFP acquisition was chosen over SPGR due to superior image quality, efficiency, and volumetric coverage (24, 25). In addition, the SSFP acquisition allowed the femoral-popliteal artery and vein to be imaged simultaneously. SSFP images were then processed offline to extract metrics describing artery and vein cross-sectional geometry to compare the vasculature during thigh contraction and relaxation. Finally, hemodynamics also plays an important role in atherogenesis, and 2D cine phase contrast (PC) imaging was performed to measure blood flow in the vasculature during relaxation and contraction.
MATERIALS AND METHODS NIH-PA Author Manuscript
Subjects and MRI Protocol A total of eleven volunteers (five male, six female, mean age of 27 ± 4 years, age range of 24 to 36 years) were imaged on a 1.5 T commercial scanner (maximum gradient amplitude 33 mT/m, slew-rate 120 T/m/s; Excite 14M4 software version; GE Healthcare Technologies, Waukesha, WI, USA). This study was approved by our local institutional review board and written informed consent was obtained from each subject prior to imaging. Signal reception was provided by a flexible four channel phased-array coil (two anterior and two posterior 14 cm (L/R) × 20 cm (S/I) rectangular elements) wrapped around the subject’s right leg which covered a region from the knee to the mid-thigh. Peripheral gating was used for cardiac synchronization. Following a localizer sequence, a reference scan was performed during thigh muscle relaxation using a custom cardiac-triggered 3D SSFP sequence with the following typical imaging parameters: axial slices, TE = 1.4 ms, TR = 3.9 ms, flip angle = 60°, FOV = 20 cm, acquisition matrix = 256 × 256, number of excitations (NEX) = 0.5, bandwidth = 488
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Hz per pixel, slice thickness = 4 mm, parallel imaging reduction factor = 1.5, fat saturation pulse for perivascular fat suppression, 6 Kaiser-Bessel ramp for SSFP magnetization preparation (26), centric view order, minimum trigger delay, and 160 to 256 echoes per trigger. Since the artery cannot be assumed to be perfectly straight within the thick (15-20 cm) imaging volume, the selection of an orthogonal 3D imaging plane was not possible. Here an axial imaging plane was chosen to simplify scan prescription. A non-selective (“hard”) RF pulse was used to provide a uniform excitation profile across the region of interest covered by the coil. Acquisition time was approximately 22 sec for a nominal heart rate of 60 beats per minute. This scan was then repeated during maximal voluntary isometric thigh contraction. Volunteers were instructed to inform the operator if they were unable to maintain thigh contraction for the duration of the scan. Images were also visually inspected for motion related artifacts indicating the contraction was not maintained.
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2D cine PC imaging was used to measure blood velocity in the femoral-popliteal artery and vein. Imaging parameters were: single axial slice, TE = 3.5 ms, TR = 7.9 ms, flip angle = 25°, FOV = 24 cm × 19 cm, acquisition matrix = 256 × 128, NEX = 1, slice thickness = 8 mm, bandwidth = 244 Hz per pixel, 20 reconstructed cardiac phases, velocity encoding = 100 cm/ s. Acquisition time was approximately 23 sec for a heart rate of 60 beats per minute. This scan was performed during relaxation and repeated during thigh contraction. Standard processing of PC data was performed using flow quantification software (Flow v.3.2; Medis medical imaging systems, Leiden, The Netherlands) to obtain blood velocity and stroke volume measurements. Quantification of Vessel Compression SSFP images were processed off-line using MATLAB software (The Math Works; Natick, MA, USA). In each slice, the arterial region was manually cropped and the lumen was automatically outlined using the Canny edge detection algorithm (27). In a minority of images, for which it was difficult to distinguish the vein from the artery, the automatic algorithm failed to segment the latter, and the arterial lumen contour was instead determined manually by an experienced reader.
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Since the main artery axis was not straight throughout the imaging slab and was not strictly orientated along B0, the axial imaging plane potentially made a double oblique cut through the artery, requiring correction for apparent stretching or skewing of the artery cross-section (see Figure 1). Initially, an ellipse was fit to the previously identified contour pixels using the method of least squares (28) from which the artery center was determined. The local 3D orientation of the artery at a given slice was estimated by the line connecting the artery centers of the two immediately adjacent slices (for the first and final slices in the acquisition, the 3D orientation of the artery was estimated by the line connecting the artery centers of the slice in question to that of its next-nearest neighbor). Contour pixel coordinates were then projected and transformed (29) from the imaging plane to a plane normal to the local artery orientation vector (see Appendix). Based on the transformed contour coordinates, a new best-fit ellipse with semimajor and semiminor axes, a and b, was calculated to characterize artery geometry by its aspect ratio (b / a), area A = π a b and circumference . Further, to characterize the femoral-popliteal vein, the venous region was manually cropped and the lumen was outlined using the automatic Canny method in the majority of images in which there was a clear division between the vein and surrounding structures; otherwise the venous lumen was outlined manually. Unlike the artery, the vein frequently assumed an irregular cross-sectional shape that was not well-characterized by an ellipse. Accordingly, the vein center point in a given slice was defined by the mean value of the contour pixel coordinates.
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As described above, the local 3D orientation of the vein was estimated from the center coordinates, which was used to project and transform the contour pixels from the imaging coordinate system into the corrected coordinate system with axes normal to the vein. Finally, the area within the polygon specified by the corrected contour pixel coordinates was calculated to characterize the venous cross-section. To verify projection and coordinate system transformation calculations, images of a cylindrical phantom with a circular cross-section were acquired. The phantom was arbitrarily orientated in the magnet such that axial imaging slices resulted in double oblique cuts through the phantom. Three contiguous images were acquired using an SSFP sequence with the following parameters: TE = 2.9 ms, TR = 10.5 ms, flip angle = 60°, FOV = 13 cm, acquisition matrix = 256 × 256, NEX = 1, bandwidth = 488 Hz per pixel, and slice thickness = 4 mm. Processing identical to that described above was performed to characterize the cross-sectional geometry of the phantom with and without coordinate correction. To generate a baseline measurement of the phantom area, the scan and processing steps were repeated with the phantom main axis orientated parallel to B0. Statistical Analysis
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Given variable thigh length among subjects, a normalized system was implemented to provide meaningful inter-subject evaluation of measured characteristics at given locations along the thigh. In each subject, localizer images were used to note the inferior edge of the femoral condyle, the adductor hiatus, and the superior edge of the femoral head. Thigh length was normalized such that the inferior edge of the femoral condyle corresponded to distance d = 0 and the superior edge of the femoral head corresponded to d = 1. Linear interpolation was used to convert absolute location (distance in mm from the condyle) to normalized location at 0.01 unitless intervals. At each normalized location, the two-tailed paired-sample t-test was used to determine the significance level of the change in arterial aspect ratio and venous area. P values less than or equal to 0.05 were considered statistically significant. All values were expressed as mean ± standard deviation.
RESULTS Comparison of axial and oblique images of the cylindrical phantom and corresponding elliptical contours show that the oblique orientation resulted in an underestimated aspect ratio and inflated area (Figure 2, columns 1 and 2). These metrics were accurately corrected by transforming contour coordinates from the imaging system to a new system normal to the main axis of the phantom (Figure 2, column 3).
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No motion artifacts were evident in images acquired during thigh contraction and all volunteers confirmed that thigh contraction was maintained throughout the acquisition. Figure 3 illustrates the overall effect of the thigh muscles on the artery and vein during contraction in three regions: inferior to the AC, the distal AC region, and the proximal AC region. In the distal AC, vessels were compressed by surrounding muscles which significantly modified arterial cross-sectional geometry and reduced venous area (Figure 3 d). In comparison, less compression was observed inferior to the AC and in the proximal AC (Figures 3 b, f). Similarly, the most substantial reduction in venous area occurred in the distal AC. The mean distance from condyle to femoral head was 448 ± 36 mm, while adductor hiatus, or distal end of the AC was 99 ± 19 mm superior to the condyle. For the normalized thigh location system, due to variable coil placement, variable leg length, limited coil coverage, and limited slab thickness, data points corresponding to d < 0.09 and d > 0.42 were only available for three or fewer subjects and were excluded from the analysis.
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The artery aspect ratio and corresponding p-values as a function of normalized distance d are shown in Figure 4. A statistically significant (p ≤ 0.05) decrease in aspect ratio was found during contraction in the range d = 0.21 to 0.32, or 94 to 143 mm superior to the condyle (assuming 448 mm from the condyle to femur head) which corresponds to the distal AC region immediately superior to the adductor hiatus. In this range, the aspect ratio was 0.88 ± 0.06 during relaxation and 0.77 ± 0.09 during contraction. Outside of this range (0.09 ≤ d ≤ 0.20 and 0.33 ≤ d ≤ 0.42), a negligible change (p > 0.05) in aspect ratio was found; the aspect ratio was 0.87 ± 0.07 during relaxation and 0.83 ± 0.09 during contraction. Artery area (Figure 5) and circumference were slightly greater in the contracted state, although these trends were not statistically significant and were not associated with a particular region of the femoral-popliteal artery. Averaged over the entire range reported here (0.09 ≤ d ≤ 0.42, or 40 to 188 mm superior to the condyle), artery area was 37.8 ± 9.5 mm2 and 38.8 ± 10.5 mm2 during relaxation and contraction, respectively, while artery circumference was 21.7 ± 2.8 mm and 22.1 ± 3.0 mm.
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Reduction in venous area during contraction was statistically significant over most of the range examined here (Figure 5). The greatest level of significance occurred in the distal AC, where an 82% reduction in area was found during contraction in the range d = 0.21 to 0.32. In comparison, vein area was reduced by 37% inferior to the AC (0.09 ≤ d ≤ 0.20) and 34% in the range 0.33 ≤ d ≤ 0.42. Flow data was acquired in a single slice at d = 0.46±0.06 (207±25 mm superior to the condyle). A negligible change in peak velocity was observed during contraction in the femoral artery and vein (Table 1). For the artery, a significant increase in flow volume was observed during contraction, while venous flow volume was unchanged. Flow velocity curves in Figure 6 shows that mean arterial blood velocity increased from 5.1 ± 3.1 cm/s during relaxation to 7.0 ± 3.0 cm/s during contraction (p = 0.06), corresponding to the increase in arterial flow volume. Change in mean venous velocity was negligible; -3.8 ± 1.5 cm/s during relaxation to -3.6 ± 2.0 cm/s during contraction (p = 0.82).
DISCUSSION
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Predicting sites of atherosclerosis requires an understanding of vessel geometry, flow patterns, and external mechanical factors. Although there are many approaches that study flow, WSS and lumen geometry (1-5,30), evaluation of mechanical factors in vivo has been more challenging. These data demonstrate a simple non-invasive approach to quantify compression of the femoral-popliteal vessels using fast non-contrast MR angiography. Changes in lumen geometry are characterized semi-automatically by edge detection for the artery and vein, and by ellipsoidal fitting for the artery. The technique developed here may be useful for identification of other arteries that are subject to repetitive compression or other trauma, such as the anterior tibial, the left subclavian, the coronaries, and the renal artery origins. Furthermore, since two scans are required to characterize changes in vessel geometry with muscle contraction, MR or ultrasound may be better suited than x-ray or CT angiography which would call for double exposure to ionizing radiation. The maximum femoral-popliteal artery compression identified in eleven subjects corresponds to the distal AC, a site commonly affected by atherosclerosis (6,12-16). In the distal AC, the vasculature was located in a relatively narrow space between the vastus medialis and adductor muscles (Figure 3 c). During isometric thigh contraction, the vessels were pinched by the posterior expansion of the vastus medialis muscle (Figure 3 d). This resulted in vessel compression and significantly modified arterial cross-sectional geometry and reduced venous area. In contrast, reduced compression was observed toward the knee due to a larger presence
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of adipose tissue and to the increased distance between the vessels and the vastus medialis muscle expansion path (Figure 3 a, b). Similarly, toward the mid-thigh, the vessels were again located lateral to the vastus medialis expansion path where contraction was less likely to compress the vessels (Figure 3 e, f). While atherogenesis is not yet fully understood, it is generally viewed as a chronic inflammatory disease that progresses with injury to the arterial wall. These data on artery compression are in agreement with this principle, as 60% to 80% of occlusions in the femoral-popliteal artery occur in the AC (6,15,16), where compression is likely to accelerate the endothelial response-to-injury process. Stent failure is particularly high in the AC (17,18). Mechanical fatigue from stent shortening corresponding to knee flexion is considered an important contributor to the high failure rate. However, the data reported here suggest that compression due to thigh muscle contraction also contributes to AC stent failure. While stent fatigue tests typically focus on elongation and shortening (31), increased tolerance to compressive fatigue may be essential for the design of improved femoral-popliteal stents. For example, femoral-popliteal stent fracture is more frequent in those who walked more than 5000 steps per day than in sedentary patients (32). Correspondingly, stent fatigue tests should mimic action generated during everyday activities where the stent is subject to opposing forces from the adductor and vastus medialis muscles (Figure 3d).
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Hemodynamics is considered an important factor for atherosclerotic predilection as lesions tend to develop where WSS is low (1-5). Thigh contraction likely modifies WSS characteristics in the femoral-popliteal artery, as WSS is a function of vessel geometry and blood flow patterns. In this preliminary work, we observed only a nominal change in blood velocity during thigh contraction (Figure 6 and Table 1). Nevertheless, it may be compelling to perform a more detailed analysis on the effect of muscle contraction on femoral-popliteal blood flow and WSS. It is interesting to note the collapse of the femoral-popliteal vein in the distal AC during contraction. Images in Figure 3 and data in Figures 4 and 5 indicate that the vein was more susceptible to compression than the artery, which is expected, as veins are more compliant and have lower internal pressure than arteries. Muscular pressure exerted on the veins indeed helps move blood through the unidirectional valves and back to the heart. Figure 3 also illustrates that the venous cross-section can be highly variable; from somewhat circular (Figure 3a), to elliptical (Figure 3c), to triangular (Figures 3d,f), to nearly completely collapsed (Figure 3d) in the same volunteer, making it inappropriate to characterize the venous cross-sectional geometry using ellipse fitting. Instead, we measured its area to quantify deformation. 3D SSFP Imaging
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The 3D SSFP pulse sequence developed for this study was chosen because it provided adequate contrast between the lumen and background tissue in a short acquisition time. Image data acquisition was performed over nearly the entire cardiac cycle to reduce scan time and muscle fatigue. One disadvantage of this approach is blurring due to vessel pulsation. This was addressed by using centric view ordering which can reduce blurring by sampling central kspace points over a shorter time interval compared to standard view ordering. Furthermore, the results presented here focus on comparative differences between relaxation and contraction, indicating that blurring due to pulsation may be inconsequential if the associated effects are similar during both relaxation and contraction. SSFP imaging provided both arterial and venous lumen signal, allowing both to be characterized from a single scan. On the other hand, a minor drawback of the SSFP acquisition was that the arterial and venous lumens were difficult to distinguish in some images. To exclusively image the artery during muscle contraction, a SPGR sequence has been utilized with some success (23), although acquisition time for a 15 cm artery segment was too long for J Magn Reson Imaging. Author manuscript; available in PMC 2009 May 8.
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continuous thigh contraction. A conventional spatial saturation pulse could be applied inferior to the imaging volume to suppress vein conspicuity. In our experience, however, this technique yielded little benefit, likely because venous blood spins traveling through the thick imaging volume had sufficient time for magnetization recovery. Finally, contrast enhanced MRA using digital subtraction may provide superior delineation of the artery, but at the expense of protocol complexity due to timing issues. Cylindrical Tubes Under Pressure
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Reports on the subject of cylindrical tubes subjected to external pressure emphasize the relationship between external pressure and cross-sectional area, and generally conclude that applied pressure initially (prior to the buckling pressure) yields a cylinder with slightly reduced cross-sectional area and roughly unchanged shape (33-39). Whereas we found that an initial increase in external pressure significantly deformed the arterial shape (Figure 4) while its area remained constant (Figure 5). This distinction may be attributed to the condition that, in the cited works, the cylinder was subjected to uniform pressure. This is an appropriate assumption for many situations such as pressure change brought about by a change in physiological arterial pressure or by a blood pressure cuff (37,38). However, in the experiments performed here, external pressure was generated by expansion of the vastus medialis muscle onto the adductor muscles, implying acute forces and non-uniform pressure distribution. Therefore, a parameter describing artery compression such as the aspect ratio reported here may be closely related to external pressure and more informative than the area parameter alone. Study Limitations This study included a small sample of young subjects with no indication of peripheral vascular disease. Vessel compression during thigh contraction may be reduced in older diseased patients with increased artery stiffness and adipose tissue. Still, these data agree with the predilection of arterial disease in the AC, as disease results from an accumulation of endothelial damage over an extended period of time. A second limitation was that the external pressure generated by isometric thigh contraction was not quantified and that an absolute relationship between muscle contraction and vessel geometry cannot be introduced. Electromyography (EMG) is the standard technique for muscle contraction measurement in vivo, but cannot be readily performed in a MRI environment due to interference from the strong magnetic field and gradient pulses. Alternatively, it is possible to measure muscle strength from its physiological cross-sectional area which can be determined directly from MR images (40).
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Ghosting artifacts in Figure 3 can be attributed mainly to imperfect parallel imaging reconstruction and associated noise amplification; a four-channel coil was employed in conjunction with the array spatial sensitivity encoding technique (ASSET), a standard feature on our clinical scanner. Typically these artifacts appeared toward the leg periphery, where they did not interfere with the centrally-located vasculature. In conclusion, we have utilized MRI to visualize the femoral-popliteal vasculature during maximal isometric thigh contraction. To the best of our knowledge, this is the first report to illustrate in vivo cross-sectional deformation of the femoral-popliteal vasculature during muscle contraction. Thigh contraction caused a reduction in venous area throughout the imaged region, while the artery suffers from locally intense compression in the distal AC immediately superior to the adductor hiatus that may coincide with the high likelihood of atherosclerotic disease.
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APPENDIX NIH-PA Author Manuscript
Projection and coordinate system transformation (29) was performed to correct for axial imaging slices that were oblique with respect to the vessel. The vessel was assumed straight within a segment equal to twice the slice thickness, yielding the unit vector of the local vessel axis in the imaging coordinate system:
[1]
where δx and δy correspond to the change in center point coordinates in slices immediately adjacent to the slice in question, t is the slice thickness, and î, ĵ, and k̂ are the unit vectors of the imaging axes (Figure 1). First, projection of the imaged contour pixels from the axial imaging plane (x,y) onto the new corrected plane (x’,y’) with normal vector n⃑ was performed. Next, coordinate system transformation (from (x,y) to (x’,y’)) was applied to the output of the previous step. Note that (x’,y’) can be obtained from (x,y) by a rotation where the axis of rotation (denoted as c⃗ in Fig.1) is orthogonal to the normal unit vectors k̂ and n⃑. Correspondingly, c⃗ is given by their cross product:
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[2]
and is used to determine the skew-symmetric matrix
[3]
Coordinates (α, β, γ) in the imaging plane are projected and transformed to the corrected coordinate system using the Euler-Rodrigues formula
[4]
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where î′, ĵ′, and k̂′ are the unit vectors of the new coordinate system, I is the 3×3 identity matrix, is the projected coordinates before coordinate system is the rotation angle between the two coordinate transformation, systems, and γ′ = 0 by virtue of the definition of c⃗.
Acknowledgements FUNDING - This work was funded by NIH grant R01HL060879.
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23. Brown, R.; Zhang, HL.; Sant’anna, C.; Zhang, X.; Prince, MR.; Wang, Y. Femoral artery stress in the adductor canal due to leg muscle contraction. Proceedings of the 15th Annual Meeting of ISMRM; Berlin. 2007; p. 95 24. Spuentrup E, Buecker A, Stuber M, et al. Navigator-gated coronary magnetic resonance angiography using steady-state-free-precession: comparison to standard T2-prepared gradient-echo and spiral imaging. Invest Radiol 2003;38:263–268. [PubMed: 12750615] 25. Weber OM, Pujadas S, Martin AJ, Higgins CB. Free-breathing, three-dimensional coronary artery magnetic resonance angiography: comparison of sequences. J Magn Reson Imaging 2004;20:395– 402. [PubMed: 15332246] 26. Nguyen TD, Spincemaille P, Prince MR, Wang Y. Cardiac fat navigator-gated steady-state free precession 3D magnetic resonance angiography of coronary arteries. Magn Res Med 2006;56:210– 215. 27. Canny J. A computational approach to edge detection. IEEE Transactions on Pattern Analysis and Machine Intelligence 1986;PAMI-8:679–698. 28. Halif R, Flusser J. Numerically stable direct least squares fitting of ellipses. Proceedings of the 6th Int’l Conf of Computer Graphics and Visualization 1998:125–132. 29. Litvin, FL.; Fuentes, A. Gear geometry and applied theory. Cambridge: Cambridge University Press; 2004. 30. Giddens DP, Zarins CK, Glagov S. The role of fluid mechanics in the localization and detection of atherosclerosis. J Biomech Eng 1993;115:588–594. [PubMed: 8302046] 31. Nikanorov A, Smouse HB, Osman K, Bialas M, Shrivastava S, Schwartz LB. Fracture of selfexpanding nitinol stents stressed in vitro under simulated intravascular conditions. J Vasc Surg 2008;48:435–440. [PubMed: 18486426] 32. Iida O, Nanto S, Uematsu M, et al. Effect of exercise on frequency of stent fracture in the superficial femoral artery. Am J Cardiol 2006;98:272–274. [PubMed: 16828607] 33. Kresch E, Noordergraaf A. Cross-sectional shape of collapsible tubes. Biophys J 1972;12:274–294. [PubMed: 5016113] 34. Kresch E. Cross-sectional area of flexible tubes. Bull Math Biol 1979;41:39–52. [PubMed: 420957] 35. Moreno AH, Katz AI, Gold LD, Reddy RV. Mechanics of distension of dog veins and other very thin-walled tubular structures. Circ Res 1970;27:1069–1080. [PubMed: 5494860] 36. Flaherty JE, Keller JB, Rubinow SI. Post buckling behavior of elastic tubes and rings with opposite sides in contact. SIAM J Appl Math 1972;23:446–455. 37. Partsch B, Partsch H. Calf compression pressure required to achieve venous closure from supine to standing positions. J Vasc Surg 2005;42:734–738. [PubMed: 16242562] 38. Foran TG, Sheahan NF. Compression of the brachial artery in vivo. Physiol Meas 2004;25:553–564. [PubMed: 15132318] 39. Fung, Y. Biomechanics: circulation. New York: Springer; 1997. 40. Narici MV, Landoni L, Minetti AE. Assessment of human knee extensor muscles stress from in vivo physiological cross-sectional area and strength measurements. Eur J Appl Physiol 1992;65:438–444.
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NIH-PA Author Manuscript Figure 1.
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a) Schematic diagram shows that an axial imaging plane that is oblique with respect to the main vessel axis results in a skewed vessel cross-section. To correct for this effect, vessel edge coordinates (γ, β, α) in the imaging coordinate system were transformed into a new coordinate system given by the plane normal to the vessel. The normal vector n⃗ was based on artery center coordinates in images immediately preceding (0, 0, 0) and subsequent (δx, δy, 2t) to a given slice. c⃗ and ϕ are the axis and angle of rotation, respectively, that transforms the axial imaging plane (x, y) to the corrected plane (x’, y’).b) Transformed edge coordinates (γ′, β′) were then fit to an ellipse characterized by its semimajor and semiminor axes, a and b, respectively.
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Figure 2.
Images of a circular-cylindrical phantom and corresponding uncorrected and corrected bestfit ellipse contours for axial (top row) and double oblique (bottom row) phantom orientations. Aspect ratio and area measurements for the uncorrected ellipse in the double oblique plane (bottom row, middle column) do not properly represent the phantom cross-section. Contour coordinates of the uncorrected ellipse were transformed into a new coordinate system such that the uncorrected axial ellipse and corrected double oblique ellipse were well-matched.
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Figure 3.
Representative 3D SSFP images of the thigh during relaxation and contraction at three locations: (a, b) inferior to the AC, (c, d), distal AC, and (e, f) and proximal AC. The thigh is relaxed in the left column and contracted in the right. Note the muscle action in the distal AC (d), where the vastus medialis muscle compresses the femoral-popliteal artery and vein. Zoomed images (5 × 5 cm2) are inset to show greater detail. Solid markers indicate the femoralpopliteal artery and open markers the vein. AC = adductor canal, AM = adductor magnus muscle, BFL = long head of biceps femoris muscle, BFS = short head of biceps femoris muscle, F = femur, G = gracilis muscle, RF = rectus femoris muscle, S = sartorius muscle, SM = semimembranosus muscle, ST = semitendinosus muscle, VI = vastus intermedius muscle, VL= vastus lateralis muscle, and VM = vastus medialis muscle.
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NIH-PA Author Manuscript NIH-PA Author Manuscript Figure 4.
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Mean artery aspect ratio (a) and p-values corresponding to aspect ratio change during contraction (b) (n = 11). A statistically significant change in aspect ratio was seen in the distal AC (p ≤ 0.05), while negligible compression was seen inferior to the AC or in the proximal AC. Note that d = 0 corresponds to the condyle, d = 0.22 to the adductor hiatus, and d = 1 to the femoral head. The p = 0.05 significance level is shown in (b) for reference (dashed line).
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Figure 5.
Mean artery and vein area (a) and p-values corresponding to area change during contraction (b) (n = 11). In general, the change in artery area was negligible. The reduction in vein area was found to be most statistically significant in the distal AC, while negligible change was observed near the mid-thigh and marginal significance was observed near the condyle. Note that d = 0 corresponds to the inferior condyle, d = 0.22 to the adductor hiatus, and d = 1 to the femoral head. The p = 0.05 significance level is shown in (b) for reference (dashed line).
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Figure 6.
Mean blood velocity in the femoral-popliteal artery and vein (n=11). A pulse oximeter on the index finger was used for cardiac gating.
NIH-PA Author Manuscript J Magn Reson Imaging. Author manuscript; available in PMC 2009 May 8.
NIH-PA Author Manuscript 34.4 ± 7.4 −6.2 ± 3.0
31.3 ±7.4 −5.4 ± 1.6
Artery
Vein
Contracted
Relaxed
Vessel
Peak Velocity (cm/s)
NIH-PA Author Manuscript 0.26
0.19
P-value
Table 1
−103 ± 41
184 ± 95
Relaxed
−93 ± 62
256 ± 113
Contracted
Flow Volume (mL/min)
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Blood flow characteristics.
0.45
0.03
P-value
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