Feasibility of in vivo measurement of carotid wall shear rate using spiral fourier velocity encoded MRI

Arterial wall shear stress is widely believed to influence the formation and growth of atherosclerotic plaque; however, there is currently no gold standard for its in vivo measurement. The use of phase contrast MRI has proved to be challenging due to partial‐volume effects and inadequate signal‐to‐noise ratio at the high spatial resolutions that are required. This work evaluates the use of spiral Fourier velocity encoded MRI as a rapid method for assessing wall shear rate in the carotid arteries. Wall shear rate is calculated from velocity histograms in voxels spanning the blood/vessel wall interface, using a method developed by Frayne and Rutt (Magn Reson Med 1995;34:378–387). This study (i) demonstrates the accuracy of the velocity histograms measured by spiral Fourier velocity encoding in a pulsatile carotid flow phantom compared with high‐resolution two‐dimensional Fourier transform phase contrast, (ii) demonstrates the accuracy of Fourier velocity encoding–based shear rate measurements in a numerical phantom designed using a computational fluid dynamics simulation of carotid flow, and (iii) demonstrates in vivo measurement of regional wall shear rate and oscillatory shear index in the carotid arteries of healthy volunteers at 3 T. Magn Reson Med 63:1537–1547, 2010. © 2010 Wiley‐Liss, Inc.     

老年下肢动脉硬化症肱动脉及股浅动脉舒张功能的对比研究

目的　了解老年下肢动脉硬化症(LEASD)患者肱动脉及股浅动脉内皮依赖性舒张功能 (血流介导的血管扩张功能 ,FMD)和硝酸甘油介导的非内皮依赖性舒张功能 (NMD) ,并评价二者的相关性及特点。方法　采用高分辨率超声诊断系统检测 33例老年LEASD患者 (Ⅰ组 )肱动脉及股浅动脉的FMD及NMD ,并分别与 4 0例健康老人 (Ⅱ组 )及 30例具有心血管危险因素的老年非LEASD患者(Ⅲ组 )进行对照研究。结果　老年LEASD患者肱动脉的FMD(3 5 6 %±1 92 % )及NMD(8 5 4 %± 4 72 % )与股浅动脉的FMD(2 16 %±1 2 2 % )和NMD(5 2 7%± 3 6 9% )均显著低于Ⅱ组和Ⅲ组 (P <0 0 5 )。老年LEASD患者股浅动脉的FMD显著低于肱动脉的FMD(P =0 0 0 0 8) ;股浅动脉的NMD亦显著低于肱动脉的NMD(P =0 0 0 2 6 ) ;而股浅动脉的FMD与肱动脉的FMD相关性好 (r=0 9314 ,P <0 0 1) ,两者的NMD亦有较好的相关性(r=0 9191,P<0 0 1)。结论　老年LEASD患者肱动脉及股浅动脉的FMD及NMD均受损 ;对于老年LEASD患者FMD及NMD的检测 ,股浅动脉与肱动脉相关性好 ,股浅动脉的FMD及NMD能更敏感、更直接地反映老年LEASD患者的血管反应性     

Reconstruction of carotid bifurcation hemodynamics and wall thickness using computational fluid dynamics and MRI.

A thorough understanding of the relationship between local hemodynamics and plaque progression has been hindered by an inability to prospectively monitor these factors in vivo in humans. In this study a novel approach for noninvasively reconstructing artery wall thickness and local hemodynamics at the human carotid bifurcation is presented. Three-dimensional (3D) models of the lumen and wall boundaries, from which wall thickness can be measured, were reconstructed from black-blood magnetic resonance imaging (MRI). Along with time-varying inlet/outlet flow rates measured via phase contrast (PC) MRI, the lumen boundary was used as input for computational fluid dynamic (CFD) simulation of the subject-specific flow patterns and wall shear stresses (WSSs). Results from a 59-year-old subject with early, asymptomatic carotid artery disease show good agreement between simulated and measured velocities, and demonstrate a correspondence between wall thickening and low and oscillating shear at the carotid bulb. High shear at the distal internal carotid artery (ICA) was also colocalized with higher WSS; however, a quantitative general relationship between WSS and wall thickness was not found. Similar results were obtained from a 23-year-old normal subject. These findings represent the first direct comparison of hemodynamic variables and wall thickness at the carotid bifurcation of human subjects. The noninvasive nature of this image-based modeling approach makes it ideal for carrying out future prospective studies of hemodynamics and plaque development or progression in otherwise healthy subjects.     

Comparison of 15 different stents in superficial femoral arteries by high resolution MRI ex vivo and in vivo

PURPOSE: To evaluate the MRI compatibility of 15 different commercially available, new generation, U.S. Food and Drug Administration (FDA)-approved stents suitable for deployment in superficial femoral arteries (SFAs), and to identify the ones that permit MRI to visualize the wall and lumen of stented arteries with sufficient spatial and contrast resolution to quantify restenosis after stent placement. MATERIALS AND METHODS: A total of 13 nitinol stents and two stainless-steel stents were placed in excised cadaveric SFAs and imaged by MRI at 1.5 T ex vivo. The images were evaluated qualitatively for the presence of artifacts and for the effects of the stent on image contrast, and quantitatively for the effect on signal-to-noise ratio (SNR) of the lumen of the artery inside the stent compared to the SNR of the fluid outside the artery. A nitinol stent was placed in the SFA of a 60-year-old man and imaged at 1.5 T in vivo. RESULTS: Both the vessel wall and the lumen could be visualized in cadaveric SFAs containing either the Absolute nitinol stent, the Dynalink nitinol stent, or the aSpire nitinol-covered stent. Their inside stent/outside stent SNR was 0.7, 0.8, and 0.8, respectively. The other 10 nitinol stents tested obscured the lumen but did not cause major image shape artifacts. Both stainless-steel stents tested, the WallGraft and WallStent, completely obscured the lumen and caused significant distortion of the image shapes. When the Dynalink stent was inserted into a highly stenosed SFA in vivo, the image showed a dark expanded eccentric lumen, circumscribed by a medium intensity band containing the stent. CONCLUSION: MRI can be used to visualize both the lumen and wall of SFAs containing selected nitinol stents ex vivo and in vivo. These results suggest that MRI can be used to monitor restenosis in stents placed in the femoral arterial bed.     

Scan‐rescan reproducibility of carotid atherosclerotic plaque morphology and tissue composition measurements using multicontrast MRI at 3T

Purpose:

To evaluate interscan reproducibility of both vessel morphology and tissue composition measurements of carotid atherosclerosis using a fast, optimized, 3T multicontrast protocol.

Materials and Methods:

A total of 20 patients with carotid stenosis >15% identified by duplex ultrasound were recruited for two independent 3T MRI (Philips) scans within one month. A multicontrast protocol including five MR sequences was applied: TOF, T1‐/T2‐/PD‐weighted and magnetization‐prepared rapid acquisition gradient‐echo (MP‐RAGE). Carotid artery morphology (wall volume, lumen volume, total vessel volume, normalized wall index, and mean/maximum wall thickness) and plaque component size (lipid rich/necrotic core, calcification, and hemorrhage) were measured over two time points.

Results:

After exclusion of images with poor image quality, 257 matched locations from 18 subjects were available for analysis. For the quantitative carotid morphology measurements, coefficient of variation (CV) ranged from 2% to 15% and intraclass correlation coefficient (ICC) ranged from 0.87 to 0.99. Except for maximum wall thickness (ICC = 0.87), all ICC were larger than 0.90. For the quantitative plaque composition measurements, the ICC of the volume and relative content of lipid rich/necrotic core and calcification were larger than 0.90 with CV ranging from 22% to 32%.

Conclusion:

The results from the multicontrast high‐resolution 3T MR study show high reliability for carotid morphology and plaque component measurements. 3T MRI is a reliable tool for longitudinal clinical trials, with shorter scan time compared to 1.5T. J. Magn. Reson. Imaging 2010;31:168–176. © 2009 Wiley‐Liss, Inc.     

Multicontrast black-blood MRI of carotid arteries: comparison between 1.5 and 3 tesla magnetic field strengths

PURPOSE: To compare black-blood multicontrast carotid imaging at 3T and 1.5T and assess compatibility between morphological measurements of carotid arteries at 1.5T and 3T. MATERIALS AND METHODS: Five healthy subjects and two atherosclerosis patients were scanned in 1.5T and 3T scanners with a similar protocol providing transverse T1-, T2-, and proton density (PD)-weighted black-blood images using a fast spin-echo sequence with single- (T1-weighted) or multislice (PD-/T2-weighted) double inversion recovery (DIR) preparation. Wall and lumen signal-to-noise ratio (SNR) and wall/lumen contrast-to-noise ratio (CNR) were compared in 44 artery cross-sections by paired t-test. Interscanner variability of the lumen area (LA), wall area (WA), and mean wall thickness (MWT) was assessed using Bland-Altman analysis. RESULTS: Wall SNR and lumen/wall CNR significantly increased (P < 0.0001) at 3T with a 1.5-fold gain for T1-weighted images and a 1.7/1.8-fold gain for PD-/T2-weighted images. Lumen SNR did not differ for single-slice DIR T1-weighted images (P = 0.2), but was larger at 3T for multislice DIR PD-/T2-weighted images (P = 0.01/0.03). The LA, WA, and MWT demonstrated good agreement with no significant bias (P 0.5), a coefficient of variation (CV) of < 10%, and intraclass correlation coefficient (ICC) of > 0.95. CONCLUSION: This study demonstrated significant improvement in SNR, CNR, and image quality for high- resolution black-blood imaging of carotid arteries at 3T. Morphologic measurements are compatible between 1.5T and 3T.     

MRI measurement of time-resolved wall shear stress vectors in a carotid bifurcation model,and comparison with CFD predictions

PURPOSE: To study pulsatile fluid flow in a physiologically realistic model of the human carotid bifurcation, and to derive wall shear stress (WSS) vectors. MATERIALS AND METHODS: WSS vectors were calculated from time-resolved 3D phase-contrast (PC) MRI measurements of the velocity field. The technique was first validated with sinusoidal flow in a straight tube, and then used in a model of a healthy human carotid bifurcation. Velocity measurements in the inflow and outflow regions were also used as boundary conditions for computational fluid dynamics (CFD) calculations of WSS, which were compared with those derived from MRI alone. RESULTS: The straight tube measurements gave WSS results that were within 15% of the theoretical value. WSS results for the phantom showed the main features expected from fluid dynamics, notably the low values in the bulb region of the internal carotid artery, with a return to ordered flow further downstream. MRI was not able to detect the high WSS values along the divider wall that were predicted by the CFD model. Otherwise, there was good general agreement between MRI and CFD. CONCLUSION: This is the first report of time-resolved WSS vectors estimated from 3D-MRI data. The technique worked well except in regions of disturbed flow, where the combination with CFD modeling is clearly advantageous.     

Swallowing, arterial pulsation, and breathing induce motion artifacts in carotid artery MRI

PURPOSE: To identify and quantify the potential sources of motion in carotid artery imaging. MATERIALS AND METHODS: Two healthy volunteers and 12 patients (20-75 years old) with atherosclerotic disease were scanned on a Philips Intera 1.5T system. A single-shot balanced-fast field echo (bFFE) sequence was used to acquire real-time axial views of the carotid artery wall (three images per second). A three-step acquisition protocol was performed to analyze the three types of motion (arterial pulsation, breathing, and swallowing) separately. The isocenter carotid artery motion amplitude in either the x or y direction was measured. Radial variation in the carotid lumen between the systolic and diastolic phases was analyzed. Motion frequency was reported for each patient. RESULTS: Significant motion related to arterial pulsation (amplitude = 0.27-0.93 mm, mean = 0.6, SD = 0.19), breathing (amplitude = 0.5-3.6 mm, mean = 1.56, SD = 0.99)), and swallowing (amplitude = 1.4-9.2 mm, mean = 4.7, SD = 2.4) were visualized. CONCLUSION: Pulsation, breathing, and swallowing are sources of significant motion in the carotid artery wall. Such motion should be considered in the future to improve carotid artery image quality.     

Assessment of the accuracy of MRI wall shear stress estimation using numerical simulations

Purpose:

To investigate the accuracy of wall shear stress (WSS) estimation using MRI. Specifically, to investigate the impact of different parameters and if MRI WSS estimates are monotonically related to actual WSS.

Materials and Methods:

The accuracy of WSS estimation using methods based on phase‐contrast (PC) MRI velocity mapping, Fourier velocity encoding (FVE) and intravoxel velocity standard deviation mapping were studied using numerical simulations. The influence of spatial resolution, velocity encoding, wall segmentation, and voxel location were investigated over a range of WSS values.

Results:

WSS estimates were found to be sensitive to parameter settings in general and spatial resolution in particular. All methods underestimated WSS, except for the FVE‐based method, which instead was extremely sensitive to voxel position relative to the wall. Methods using PC‐based WSS estimation with wall segmentation showed to be accurate for low WSS, but were sensitive to segmentation errors.

Conclusion:

Even in the absence of noise and for relatively simple velocity profiles, MRI WSS estimates cannot always be assumed to be linearly or even monotonically related to actual WSS. High WSS values cannot be resolved and the estimates depend on parameter setting. Nevertheless, distinguishing areas of low and moderate WSS may be feasible. J. Magn. Reson. Imaging 2012;36:128–138. © 2012 Wiley Periodicals, Inc.     

Atherosclerotic disease distribution in carotid and vertebrobasilar arteries: clinical experience in 100 patients undergoing fluoro-triggered 3D Gd-MRA

PURPOSE: To determine how often three-dimensional gadolinium-enhanced magnetic resonance angiography (3D Gd-MRA) shows disease outside of the standard two-dimensional time-of-flight magnetic resonance angiography (2D TOF-MRA) imaging volume. MATERIALS AND METHODS: One hundred consecutive patients with known or suspected cerebrovascular disease were examined using fluoro-triggered 3D Gd-MRA with a 1.5 Tesla scanner. All vessel segments from the aortic arch to the circle of Willis were independently evaluated for the presence of occlusive disease, ulceration, aneurysm, and anatomic variations by two radiologists blinded to clinical information. Branch vessel and circle of Willis visibility were also assessed. Kappa statistics were calculated to determine interobserver agreement. RESULTS: Interobserver agreement for the degree of stenosis was good to excellent (kappa = 0.83-1.0). Disagreement was more frequent in the cases of mild stenoses and was less frequent for stenoses > or = 50%. In 38% of the patients, 3D Gd-MRA demonstrated either ulcerated plaques in carotid arteries (N = 14) or stenosis > or = 50% in great vessel (N = 20) and/or vertebral artery (N = 38) origins, none of which can be readily evaluated on standard TOF-MRA. CONCLUSION: Fluoro-triggered 3D Gd-MRA provides a comprehensive evaluation of the carotid arteries including all vessel segments from the aortic arch to the circle of Willis.     

Comparison of 2D and multislab 3D magnetic resonance techniques for measuring carotid wall volumes

Purpose

To compare a multislab three‐dimensional volume‐selective fast spin‐echo (FSE) magnetic resonance (MR) sequence with a routine two‐dimensional FSE sequence for quantification of carotid wall volume.

Materials and Methods

One hundred normal subjects (50 men, mean age 44.6 years) underwent carotid vessel wall MR using 2D and 3D techniques. Carotid artery total vessel volume, lumen volume, wall volume, and wall/outer wall (W/OW) ratio were measured over 20 contiguous slices. Two‐ (2D) and three‐dimensional (3D) results were compared.

Results

The mean difference between 2D and 3D datasets (as a percentage of the mean absolute value) was 1.7% for vessel volume, 4.9% for lumen volume, 4.7% for wall volume, and 5.8% for W/OW ratio. There was good correlation between 2D and 3D models for total vessel volume (R² = 0.93, P < 0.001), lumen area (R² = 0.92, P < 0.001), and wall volume (R² = 0.77, P < 0.001). The correlation for the W/OW ratio was weaker (R² = 0.30; P < 0.001). The signal‐to‐noise ratio (SNR) for the 3D technique was 2.1‐fold greater than for the 2D technique (P < 0.001). When using the 3D sequence, scan time was reduced by 63%.

Conclusion

Multislab volume selective 3D FSE carotid arterial wall imaging performs similarly to a conventional 2D technique, but with over twice the SNR and substantially reduced scan time. J. Magn. Reson. Imaging 2008;28:1476–1482. © 2008 Wiley‐Liss, Inc.     

颈动脉超声与头颅MRI筛查老年无症状脑梗死的临床意义

目的探讨颈动脉超声与头颅MRI在老年无症状性脑梗死(SBI)检查中的价值。方法选取我院门诊部体检中心体检者88例,均进行颈动脉超声、头颅MRI及血脂的测定。结果 88例体检者中,采用头颅MRI共检出脑梗死病变50例,占总例数56.82%。颈动脉超声检出一侧以上颈动脉内膜中层增厚(IMT)或斑块形成者72例,其中45例同时合并脑梗死,占颈动脉超声异常的62.50%。头颅MRI诊断脑梗死阳性与阴性之间进行比较,颈动脉IMT差异有统计学意义(P<0.05)。结论头颅MRI是一种早期发现无症状脑梗死无创、可靠的检查和诊断方法,凡颈动脉超声提示异常者,无论血脂异常与否,均有必要行头颅MRI检查进一步明确诊断。     

颅内VW-MRI豆纹动脉成像：大脑中动脉粥样硬化斑块与梗死类型的关系

目的 基于高分辨MR血管壁成像（VW-MRI）技术探讨不同类型脑梗死病人的大脑中动脉（MCA）斑块体积和豆纹动脉（LSA）数量、长度间的关系。方法 回顾性纳入因可疑脑卒中或其他脑血管病行头颅MRI检查的病人58例,均经VW-MRI检查明确诊断。其中,单侧基底节区急性脑梗死病人39例[男28例,女11例,年龄45~75岁,平均（60.92±12.21）岁],根据其MR影像分为大动脉粥样硬化型[较大皮质下梗死（LSI）]组21例和小动脉闭塞型[较小腔隙性梗死（SLI）]组18例;其余19例无急性脑梗死灶且单侧MCA无斑块的病人作为对照组[男8例,女11例,年龄42~81岁,平均（63.84±12.05）岁]。由2名放射科医生统计3组临床资料并测量LSA长度、数量,利用斑块软件测量LSI组和SLI组的MCA M1-2段斑块体积。2组间比较采用独立样本t检验,3组间比较采用单因素方差分析或χ²检验。采用无序多分类logistic回归分析不同梗死类型的影响因素。采用组内相关系数（ICC）评价2名医师测量数据的一致性。结果 3组病人临床资料差异均无统计学意义（均P>0.05）。LSI、SLI和对照组的LSA平均分支长度和分支数量分别为13.48±2.99、15.8±2.71、（16.4±2.69） mm和2.00±0.77、2.78±0.94、（3.11±1.15）支,组间差异均有统计学意义（均P<0.05）。其中,LSI组的分支长度、数量最小（P<0.05）,SLI组和对照组间差异均无统计学意义（均P>0.05）。LSI组的斑块体积大于SLI组[分别为（87.5±17.7） mm³、（75.5±9.4） mm³,t=2.579,P=0.014]。LSA长度越短和分支数越少则LSI型的风险增加,OR（95%CI）值分别为0.665（0.497~0.890）、0.253（0.106~0.604）。2名医师测量斑块体积和长度的一致性较好,ICC分别为0.78和0.84。结论 采用VW-MRI对MCA及LSA成像有助于揭示深穿支梗死类型的发生机制,其中MCA斑块体积及LSA数量和长度是影响脑梗死分型的关键因素。