Breath-hold MR measurements of blood flow velocity in internal mammary arteries and coronary artery bypass grafts

Breath-hold velocity-encoded cine MR (VENC-MR) imaging is a feasible method for measuring phasic blood flow velocity in small vessels that move during respiration. The purposes of the current study are to compare breathhold VENC-MR measurements of flow velocities in the internal mammary arteries (IMA) with nonbreath-hold measurements and to characterize the systolic and diastolic flow velocity curves in a cardiac cycle in native IMA and IMA grafts. Flow velocity in 30 native IMA and 8 IMA grafts were evaluated with a breath-hold VENC-MR sequence with K-space segmentation and view-sharing reconstruction(TR/TE=16/9 msec, VENC=100 cm/s). In 10 native IMA, nonbreathhold VENC-MR images were acquired as well for comparison. Breath-hold VENC-MR imaging showed significantly higher systolic and diastolic peak velocities in native IMA (43.1 cm/second ± 15.0 and 10.0 cm/second ± 4.8), in comparison to those of nonbreath-hold VENC-MR imaging (27.6 cm/second ± 10.2 and 7.3 cm/second ± 3.9, P<.05). The diastolic/systolic peak velocity ratio in the IMA grafts (.88 ± .41) was significantly higher than that in native IMA (.24 ± .08, P<.01). Interobserver variability in the flow velocity measurement was less than 4%. Breath-hold VENC-MR imaging demonstrated higher peak flow velocity in the IMA than nonbreath-hold VENC-MR imaging. This technique is a rapid and effective method for the noninvasive assessment of blood flow velocity in IMA grafts.     

Blood flow in coronary artery bypass vein grafts: volume versus velocity at cardiovascular MR imaging

Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.     

冠状动脉搭桥术后磁共振桥血管成像的初步探讨

目的 以ＭＲ成像评价桥血管开通及功能,探讨适用于国人冠状动脉搭桥（ＣＡＢＧ）术后的影像学随访手段,方法 用１．５ＴＭＲ机检查２７例ＣＡＢＧ术后患者的７４支桥血管,行屏气快速场回波（ＦＦＥ）序列扫描观察桥血管的开通。其中１６例４２支桥血管行相位对比磁共振血流成像（ＰＳＭＲＡｆｌｏｗ）,得出血流速度和流量曲线,对桥血管的开通进行再评价。结果 本组桥血管ＦＦＥ成像开通率为８９．２％（６６／７４）。以ＰＳ     

Assessment of coronary artery bypass grafts: value of different breath-hold MR imaging techniques

OBJECTIVE: Our aim was to evaluate the patency of coronary artery bypass grafts and to detect graft stenosis using different breath-hold MR imaging techniques. SUBJECTS AND METHODS: Twenty-two patients with 59 grafts (14 internal mammary artery grafts and 45 saphenous vein grafts) and 76 distal anastomoses (singular and sequential grafts) were studied using a 1.5-T scanner. A two-dimensional T2-weighted breath-hold half-Fourier acquisition single-shot turbo spin echo (HASTE) sequence and a three-dimensional breath-hold contrast-enhanced MR angiography sequence (fast imaging with steady-state free precession) were performed. For MR angiography, a bolus of 20 ml of gadopentetate dimeglumine was used. Time delay for contrast injection was calculated by a test bolus. The gold standard was coronary angiography performed within 14 days of MR imaging. All images were evaluated independently by two radiologists. RESULTS: With the HASTE sequence, 95% of the patent grafts were recognized (42/44); specificity was 93% (14/15). MR angiography had both a sensitivity (41/44) and specificity (14/15) of 93%. Interobserver agreement for both sequences was good (Cohen's kappa = 87%; McNemar test, p = 56%). Forty-nine (83%) of 59 patent distal graft anastomoses were revealed with the HASTE sequence; 38 (64%) of 59 were seen on contrast-enhanced angiography. With HASTE imaging, only two of eight hemodynamically significant graft stenoses were detected. MR angiography revealed only four of eight significant graft stenoses. CONCLUSION: The HASTE sequence and three-dimensional MR angiography proved to be useful MR techniques when evaluating the patency of coronary artery bypass grafts. However, reliable detection of graft stenosis does not yet seem possible with these imaging techniques.     

Multidetector CT and coronary artery bypass grafts

The aim of this review is to provide--starting from anatomical, surgical and pathophysiological data--elements for evaluating the status of coronary artery bypass grafts with multidetector computed tomography (CT), taking into consideration the most common conduits used (left and right internal mammary arteries, saphenous vein, radial artery, gastroepiploic artery) and early and late complications (stenosis or obstruction, vasospasm, aneurysms and pseudoaneurysms, malposition). Some of the major problems regarding the examination technique and image analysis are also discussed. Finally, we offer general guidelines for reporting the examination results.     

Quantitative measurements of cerebral blood flow in patients with unilateral carotid artery occlusion: a PET and MR study.

Although it has been demonstrated that quantitative measures of cerebral blood flow (CBF) can be obtained with the singular value decomposition (SVD) algorithm, the extent to which quantitative CBF measurements can be utilized under pathophysiological conditions has not been systematically studied. A total of five healthy volunteers and five patients with unilateral carotid artery occlusion were studied. Only magnetic resonance (MR) images were acquired for the volunteer group while both MR and positron emission tomography (PET) images were acquired for the patient group. Assessments of CBF from normal volunteers compared favorably with values reported in the literature. However, while a linear relationship was observed for each patient when MR measured CBF was compared to that obtained from PET, this linear relationship diminished when all patients were analyzed as a group (r = 0.41). A correction factor (CF) was proposed that was equal to the ratio of the area of the venous output function (VOF) in each patient to the mean VOF obtained from the volunteer group. After globally scaling the CBF of each patient based on the experimentally derived CF, a substantial improvement was observed (a slope of 1.02 and r = 0.8 for the linear regression line) in the relationship between MR estimated CBF and those obtained from PET.     

A study of coronary artery bypass graft patency using MR imaging

This is a prospective study comparing selective angiography and magnetic resonance (MR) imaging in the evaluation of the patency of coronary artery bypass grafts (CABGs). Twenty-eight patients with 52 grafts were studied (10 internal mammary artery grafts, 19 saphenous vein grafts including 2 sequential grafts, and 15 right coronary artery saphenous vein grafts). The mean interval between MR imaging and the surgical procedure was 13.2 months. Results obtained with angiography were as follows: 39 patent grafts; 4 patent but stenotic (greater than 50%); 9 occluded CABGs. Magnetic resonance imaging was performed with axial imaging, spin echo images, and electrocardiographic gating. Grafts that were normal on angiographic examination appeared without a signal on both the first and the second echoes in 38 of 39 cases. Grafts that were patent but presented significant stenosis as evidenced by angiography presented the same appearance. In one case, there was an aspect corresponding to a decreased blood flow (presence of a signal on the first echo that became stronger on the second echo). The CABG occlusion was determined in seven of nine cases. Occluded grafts twice presented a signal variation corresponding to an old thrombus formation. In two cases, only the origin of the graft was visible without any visualization of its distal portion. In three other cases, neither the proximal nor the distal portions of the graft were identified. Among six CABGs that could not be identified, three were patent. This study demonstrated that MR imaging makes it possible to correctly identify patent CABGs, but this modality has significant limitations because patent but stenotic CABGs may present in the same way as patent CABGs without stenosis. On the other hand, the correct identification of occluded grafts may be accurately performed using MR imaging.     

Evaluation of coronary artery bypass grafts by magnetic resonance imaging.

Magnetic resonance (MR) angiography and flow mapping have the potential to become a major noninvasive diagnostic tool for the assessment of coronary artery bypass graft morphology and function. Several MR sequences, such as conventional non-respiratory compensated methods, and phase contrast cine flow sequences have been reported for the evaluation of bypass graft patency. However the visualization of different graft segments and the detection of graft stenosis remains difficult. Recent advances in MR coronary angiography and flow mapping are volume coronary angiongraphy with targeted scans, navigator gated angiography, contrast-enhanced angiography, and breath-hold or navigator gated flow sequences. Future approaches, such as navigator gated fast MR techniques resulting in high-resolution angiography in combination with breath-hold MR flow mapping with high temporal resolution, might allow a comprehensive evaluation of bypass graft stenosis and function. This review article will address the major issues concerning the MR evaluation of bypass grafts.     

A validation of a flow quantification by MR phase mapping software

Aim: We evaluated a Siemens software of flow quantification (FQ) by MR phase mapping, in the framework of a common practical use. Methods: Experiments with a laminar flow phantom and in vivo pulsatile flow were performed. In particular, FQ in ascending aorta was investigated in healthy volunteers. Results and conclusion: Flow phantom experiments reveal that the FQ slightly underestimates (8% on the average) actual velocities (mean velocities over a vessel area), and also that velocity uncertainties are related to the encoding velocity value, whatever the measured velocity. Furthermore, using well characterized working criteria, we found low intraobserver variability and negligible interobserver variability in ascending aorta FQs. The role played by the choice of reference area in FQ accuracy is emphasized. When recording several cardiac cycles during the same acquisition, it is shown that the FQ software may provide erroneous results. Several comments for FQ software use in the ascending aorta are added.     

Use of a Doppler guide wire for intravascular blood flow measurements: a validation study for potential neurologic endovascular applications.

PURPOSETo conduct a validation study of a Doppler guide wire for potential neuroendovascular applications.METHODSA 12-MHz, 0.018-inch Doppler guide wire was evaluated in eight swine under various blood flow conditions using two types of in vivo cerebrovascular models (physiologic and arteriovenous shunting). Flow conditions were mechanically and pharmacologically altered. Doppler average peak velocity was compared with volumetric blood flow, and flow profile corrections were calculated and analyzed. Qualitative aspects of the Doppler guide wire spectra were also assessed.RESULTSPlots of average peak velocity versus volumetric blood flow showed excellent linear relationships (r2 > 0.94), which were maintained at high flow conditions (average peak velocity, 99 to 236 cm/sec; volumetric blood flow, 392 to 889 mL/min). Values of flow profile correction varied from 0.43 to 0.94 and showed no consistent relationship to changes in volumetric blood flow.CONCLUSIONSThe excellent correlation between average peak velocity and volumetric blood flow over a wide range of blood flow conditions and the additional qualitative information of the Doppler guide wire spectra establish a foundation for clinical implementation. The unpredictable variations of flow profile corrections remain obstacles for calculating volumetric blood flow based on Doppler guide wire average peak velocity.     

Evaluation of coronary artery bypass grafts using multidetector-row CT with Japanese patients

Purpose To evaluate the patency of coronary artery bypass grafting (CABG) with multidetectorrow computed tomography (MDCT) in Japanese patients, who have narrower coronary arteries than Caucasians. Materials and Methods Nineteen patients (12 men and 7 women, mean age: 63±12 years) with 33 coronary bypass grafts were examined with three-dimensional volume rendering (3D-VR) and curved multiplanar reconstruction (MPR) of MDCT. All grafts were compared with those obtained with selective graft angiography. Results For the 19 patients with 33 grafts, the overall sensitivity and specificity, compared with those for selective bypass angiography, were 96.7% and 100%, respectively. Conclusion The patency of CABG in Japanese patients can be effectively evaluated with 3D-VR and curved MPR of MDCT.     

双源CT在冠状动脉搭桥术后随访中的临床应用

目的 评价双源CT冠状动脉造影对冠状动脉搭桥术后桥血管病变随访的应用价值.方法 50例冠状动脉搭桥患者,术后6～20个月行双源CT冠状动脉造影检查;对原始数据行VR、CPR、MIP多种图像后处理技术进行重建.由2位有经验的放射科医生对重建图像进行观察,对桥血管通畅性进行分级诊断.结果 50例患者总计140支桥血管,134支桥血管显示良好,其中通畅或狭窄＜50％的桥血管共127支（91％）,狭窄≥50％的桥血管7支（5％）,闭塞未显示的桥血管共6支（4％）.结论 DSCT冠状动脉造影能够清晰显示并评价冠状动脉桥血管病变,尤其是重度狭窄和闭塞的桥血管,可作为冠状动脉搭桥术后随访的有效评价手段.     

Computed tomographic assessment of coronary artery bypass grafts with fast dynamic scanning

28 coronary grafts in 12 symptomatic patients were studied at the time of graft angiography at Prince Charles Hospital. The grafts were studied using the Toshiba 900S scanner with bolus injections of contrast agent into an anticubital vein. The assessment was carried out using fast incremental and continuous acquisition scanning with stepped reconstructions in the last 5 patients. Of the 27 grafts, 16 were shown at angiography to be patent and of good calibre. 14 of these were similarly assessed at CT. 11 were either diseased or blocked at angiography, and these were graded also as small, stenosed or blocked at CT. This early study would support previous reports that CT does offer a reliable method of diagnosing good patent grafts. The study also suggests that with conventional fast dynamic scanning some information regarding graft quality can be obtained.     

三维增强MR血管成像对冠状动脉桥血管的评价

目的：探讨三维增强MR血管成像（3D CEMRA）评价冠状动脉搭桥术（CABG）桥血管开通状况的临床价值。方法：20例CABG患者共47支桥血管，用3D CEMRA方法进行了研究。MR检查距手术15d至16年。20例患者中14例CABG术后5个月至16年有胸痛症状再发，2例术后临床疑桥血管急性闭塞。对3D CEMRA资料进行评价，确认桥血管开通状况。对7例15支桥血管同时有3D CEMRA和X线血管造影者进行了双盲分析。结果：47支桥血管，3D CEMRA显示通畅25支，闭塞22支，闭塞率为46.8％。7例同时有3D CEMRA和X线血管造影者共15支桥血管，通畅8支，闭塞7支，二者显示完全一致。结论：3D CEMRA对桥血管的评价是可靠和准确的，可作为CABG术后患者一种无创性影像检查方法。