Three-dimensional MRI of coronary arteries using an intravascular contrast agent

To assess the effectiveness of an intravascular contrast agent, MS-325, for enhancing the vascular signal in coronary MR angiograms, six minipigs were studied using a three-dimensional, gradient-echo sequence with retrospective respiratory gating. To suppress the myocardial signal, preparatory RF pulses were applied before data acquisition. With the administration of MS-325, the blood signal-to-noise ratio increased by 97-276%, depending on the region of interest in which the blood signal was measured and the precontrast imaging sequence structures. The blood/myocardium contrast-to-noise ratio also significantly increased. High-resolution images (0.58 × 0.58 × 1 mm³) obtained from postmortem pig hearts demonstrated the potential delineation of coronary arteries with MS-325. In conclusion, this study supports further evaluation of the utility of MS-325 in improving coronary MR angiography in humans.     

Proximal coronary artery stenosis: Three-dimensional MRI with fat saturation and navigator echo

The purpose of this study was to compare the diagnostic value of MR angiography (MRA) with conventional contrast angiography in coronary artery disease. Thirty-five patients underwent MRA and coronary angiography within 4 hours. Of these, three patients were investigated twice: once before and once after balloon angioplasty. The pulse sequence was a cardiac-triggered, single-slab, three-dimensional gradient-echo sequence, employing a spin-echo navigator echo measurement to track the variation of the diaphragm during the scan. The following segments of the coronary arteries were included in this prospective study: left main coronary artery, proximal and middle left anterior descending, proximal and middle left circumflex, proximal and middle right coronary artery, and intermediate branch, if present. In total, 176 segments were classified as normal or having a stenosis of less than 50% and as having a stenosis of more than 50%. Five patients were excluded because of lack of cooperation. Over all, 45 of 54 stenoses were detected and interpretable by MRA. Sensitivity, specificity, and positive and negative predictive values of MRA for detecting significant stenoses were 83%, 94%, 87%, and 93%, respectively. MRA identified significant stenoses within the major coronary arteries with a high degree of accuracy. Sensitivity and specificity are higher compared with exercise tests or scintigraphy or top of the precise localization.     

Recurrent coronary artery stenosis: assessment with three-dimensional MR imaging

PURPOSE: To assess the diagnostic value of three-dimensional coronary magnetic resonance (MR) angiography with fat saturation and navigator echo in the setting of restenosis after percutaneous transluminal coronary angioplasty (PTCA). MATERIALS AND METHODS: Thirty consecutive patients who had PTCA and were referred for elective coronary reangiography underwent MR imaging and coronary angiography. The pulse sequence was a cardiac triggered, single-slab, three-dimensional, gradient-echo sequence, employing a spin-echo navigator echo measurement to track the variation of the diaphragm during the scan. The following segments of the coronary arteries were included in this prospective study: left main coronary artery, proximal and middle left anterior descending, proximal and middle left circumflex, proximal and middle right coronary artery, and intermediate branch, if present. The quality of the MR images was graded from 0 to 5. RESULTS: In total, 221 coronary artery segments could be identified. Mean image quality was 3.3. Overall accuracy for segments with an image quality of grade 2 or more was 90%. To achieve a positive predictive value >70% for a significant stenosis/restenosis, only segments with quality >/=3 could be assessed, whereas an acceptable negative predictive value could be achieved for nearly all segments. CONCLUSION: Our preliminary data suggest that MR coronary angiography may be most helpful as a screening test in selected patients to exclude clinically relevant stenoses or to assess restenoses after PTCA or in patients in whose coronary angiography is relatively contraindicated.     

Coronary MR angiography during peak-systole: Work in Progress

The purpose of this study is to compare the quality of images of coronary arteries obtained with two-dimensional breath-hold coronary MR angiography during peak systole and mid diastole. Two-dimensional coronary MR angiography was performed in eight normal volunteers at peak systole and in mid diastole with a commercial 1.5-T MR imager. An ultrafast gradient-echo sequence with incremented flip angle series and k-space segmentation was used. The image quality grade, length, and proximal diameter of each visualized coronary artery were measured. The highest quality images in systole and diastole were compared. Coronary MR angiography provided high quality images in systole and diastole in 14 of 16 coronary vessels (87.5%). In 8 of 14 vessels (57%), there was no visible coronary MR angiogram image degradation when comparing peak systolic with mid-diastolic images. In 4 of 14 vessels (29%), there was mild MR image degradation. There was significant MR image degradation in only one case (7%). And in one case (7%), there was mild image improvement during systole. The width and length of the visualized coronary vessels did not change significantly from diastole to systole. Existing two-dimensional breath-hold coronary MR angiography provides MR images during peak systole and mid-diastole with little or no perceptible difference in quality.     

Flow velocity quantification in human coronary arteries with fast,breath-hold MR angiography

Measurement of coronary artery flow velocities has, until now, largely required the use of invasive technologies. The authors have implemented a breath-hold magnetic resonance (MR) angiography technique for depicting the coronary arteries and for quantifying flow velocities. The method was tested in flow phantoms and then applied to a series of subjects: 11 subjects were studied at rest, and four were studied before and during pharmacologic stress induced by intravenous adenosine. Flow velocities at rest in the midportion of the right coronary artery were 9.9 cm/sec ± 3.5 (n = 12); in the proximal left anterior descending coronary artery, they were significantly higher, measuring 20.5 cm/sec ± 5.2 (n = 6). With adenosine, flow velocities typically increased at least fourfold. The authors conclude that noninvasive measurement of coronary artery flow velocities is feasible with MR angiography; this method may prove useful for determining the physiologic significance of coronary artery stenosis.     

Coronary arteries: breath-hold MR angiography

The authors describe a method for performance of ultrafast magnetic resonance (MR) angiography of coronary arteries with a standard clinical MR system and a body coil. Each image was obtained within a single breath hold by using an electrocardiography-gated, segmented, ultrafast, gradient-echo pulse sequence with an incremental excitation flip angle for the eight phase-encoding steps acquired per segment. By using overlapping 4-mm-thick sections, the coronary arteries were routinely depicted from the coronary ostia distally at MR in healthy subjects. Ultrafast MR angiography of the coronary arteries is feasible with use of a standard body coil. This technique offers considerable potential as an investigational tool and, with further development, may become a clinically useful imaging application.     

MR angiography of the thoracic aorta with an electrocardiographically triggered breath-hold contrast-enhanced sequence.

An electrocardiographically (ECG) triggered breath-hold contrast material-enhanced magnetic resonance (MR) angiography sequence has been developed for imaging the thoracic aorta. A three-dimensional (3D) gradient-echo sequence is used with a contrast material bolus. Forty-nine patients with various aortic abnormalities and five healthy volunteers underwent imaging with the sequence. All studies were performed in a single breath hold. ECG-triggered breath-hold contrast-enhanced MR angiography was tolerated in 48 of the 49 patients. The images demonstrated no respiratory motion artifacts and diminished pulsation artifacts. The cardiac chambers, aortic root, ascending and descending aorta, aortic arch, proximal arch vessels, and proximal coronary arteries were clearly demonstrated and not obscured by ghost artifacts. The 3D data set allowed excellent multiplanar reformation, permitting orthogonal or oblique views of the vascular anatomy. A variety of congenital and acquired abnormalities were clearly identified. When this sequence is used, it is important to evaluate both the maximum-intensity projection and source images. Delayed imaging should be performed to detect late filling. In conjunction with cine MR and T1-weighted spin-echo imaging, ECG-triggered breath-hold contrast-enhanced MR angiography should be considered the technique of choice for imaging the thoracic aorta.     

Three-dimensional MR imaging of the coronary arteries: Preliminary clinical experience

A three-dimensional (3D) magnetization-prepared (MP) rapid gradient-echo (RAGE) and 3D RAGE technique was used to image the coronary arteries in healthy volunteers and patients with known disease. Each sequence produced images of volumes partitioned into 16 thin sections with differing blood-fat-myocardium contrast. The two types of images were subtracted to null fat signal, thus producing a third image set that showed flowing blood. Total imaging time was about 17 minutes. In the volunteers, the 3D MP-RAGE and subtraction images consistently showed the morphology of the right coronary artery. The left main and left anterior descending arteries were also well seen. The circumflex artery was less consistently identified. Of the 17 diseased coronary artery segments identified at catheterization, 16 had altered signal intensity (narrowing, occlusion, reduced contrast-to-noise ratio, irregularity) on the subtraction images, while 13 had altered signal intensity on the 3D MP-RAGE images. The results indicate that this 3D MP-RAGE and 3D RAGE technique has potential utility as a screening method for coronary heart disease.     

Efficacy of cardiac MRI in the evaluation of ischemic heart disease.

With the development of fast scan techniques and technical advances in software, cardiac MRI can now be used for morphological and functional evaluation of the heart with good reliability and high spatial and temporal resolution. Cardiac MRI is employed at many institutions, mainly for assessing ischemic heart disease. Cardiac MRI can be used to identify coronary artery stenosis, evaluate myocardial viability, assess left ventricular wall motion and function, measure coronary blood flow and flow reserve, and obtain other useful information for the diagnosis of ischemic heart disease in a single examination, serving as a true comprehensive cardiac study. With regard to the evaluation of coronary artery stenosis, new techniques, such as whole-heart coronary MRA, permit visualization of the coronary arteries to their peripheral branches without contrast agent. Good results have been reported for whole-heart MRA as compared with X-ray coronary angiography (CAG). Attempts to evaluate plaque characteristics by visualizing the walls of the coronary arteries have also been reported recently. Technical improvements have been made in myocardial perfusion MRI to detect myocardial ischemia and in delayed contrast-enhanced MRI to assess myocardial viability, and some researchers have recently reported that the diagnostic capabilities of these techniques match or surpass those of cardiac nuclear medicine studies. We outline the features of the latest MR imaging techniques for the diagnosis of ischemic heart disease, discuss their practical applications, and compare them with other imaging modalities.     

MR evaluation of coronary stents with navigator echo and breath-hold cine gradient-echo techniques

The aim of this study was to evaluate coronary artery stents with MR. Thirty-eight patients underwent MR imaging 48.1 +/- 6.6 days (range 38-60 days) after placement of 47 coronary stents of 11 different types, using navigator echo (NE) and cine gradient-echo (GE) techniques. For both sequences the low signal artifact was used to localize the stent, whereas the flow-related high signal before and distal to the stent was considered as a patency sign. Exercise electrocardiographic test (EET) had been performed 1-7 days before MR. No adverse event with possible relation to the MR examination was observed. All the stents were recognized as signal void with GE, and all but one with NE. Of the 2 patients with positive EET, the first one, with a stent on the left anterior descending coronary artery, presented low signal distal to the stent at both MR sequences, suggesting dysfunction [60% stenosis at conventional coronary angiography (CCA)]; the second one, with two sequential stents on the right coronary artery, presented lack of signal distal to the stents at both MR sequences, suggesting occlusion (97% stenosis at CCA). For the 44 remaining stents in 36 patients with negative EET, MR high signal before and distal to the stent suggested patency at both sequences. MR seems to be a safe and promising technique for non-invasive evaluation of coronary stents.     

Assessment of coronary artery patency after stent placement using magnetic resonance angiography

The ability to noninvasively assess the patency of coronary stents would represent a significant advance. We evaluated the safety and ability of two-dimensional coronary MR angiography In imaging stents and suggesting patency. Coronary MR angiography of 26 coronary stents (Palmaz-Schatz) was performed in 16 patients 39 to 73 years of age. Studies were performed between 2 and 4 months after stent placement. All patients were symptom free at the time of imaging. Coronary MR angiography was performed with a commercial 1.5-T MR imager using an electrocardio-graphically gated pulse sequence with breath-holding. Images were obtained in mid-diastole with and without fat suppression. Image artifacts caused by the metal in the stents were clearly visualized in all 26 stents (100% sensitivity for stent detection). Arterial flow signal was seen in the coronary artery or graft distal to the stent in 25 of 26 cases (96%). All patients, except for the one in which distal flow could not be seen, remained symptom free for >2 years. The distribution of stent locations was as follows: 10 in the right coronary artery (RCA), 10 in the left anterior descending coronary artery (LAD), 2 in the left circumflex coronary artery, and 4 in saphenous vein grafts (SVGs) to RCA. One patient had 2 RCA and 2 LAD stents, one had 3 RCA and 1 LAD stents, one had 3 SVG stents, and two had double RCA stents. Coronary MR angiography is safe for noninvasive imaging of coronary stents, and in the proper clinical setting, it can be used to help suggest patency.     

MR characterization of blood flow in native and grafted internal mammary arteries

In the postoperative patient with anginal symptoms, differentiation between bypass graft compromise and nonischemic causes has until now been accomplished only by means of x-ray angiography. A noninvasive test is clearly desirable. The authors used a cine phase-contrast (PC) magnetic resonance (MR) imaging technique to characterize blood flow in native and grafted internal mammary arteries (IMAs). Ten volunteers and 15 patients who had recently undergone IMA coronary artery bypass grafting were imaged. Cine PC MR imaging was performed in the transaxial plane at the level of the pulmonary artery bifurcation. Flow in both IMAs was quantified and expressed as a percentage of cardiac output measured in the ascending aorta. In the 15 patients, flow analysis was performed in both the native and grafted IMAs. In the volunteers, IMA blood flow ranged from 2.1% to 4.3% of cardiac output on the left (mean, 3.5%) and 2.1% to 5.1% (mean, 3.5%) on the right. There was considerable intersubject variability, with coefficients of variation of 10.7% for the left and 12.3% for the right IMA. Intrasubject variability was limited, with estimated common standard deviations of 0.45% of cardiac output (range, 0.2%–1.1%) for the left and 0.39% (range, 0.1%–0.6%) for the right IMA. Flow in grafted IMAs was identified in 13 of 15 patients. In one of two patients without demonstrable IMA graft flow, cardiac catheterization confirmed lack of flow. IMA graft flow varied from 28 to 164 mL/min (mean, 80.3 mL/min). This study shows the feasibility of using cine PC MR imaging as a quantitative method of evaluating blood flow in IMA coronary artery bypass grafts.     

Coronary MR angiography

MR angiography of the coronary arteries became possible in 1991 with the development of a new group of fast MR imaging sequences. Although the role of coronary MR angiography in screening for coronary artery lesions has not yet been established, coronary MR angiography already has been very successful in the detection of coronary artery variants and the imaging of coronary stents and bypass grafts. Variants of these new MR imaging techniques also can quantitate velocity in native coronary arteries. Several generations of coronary MR angiographic techniques exist; all techniques use EKG-triggering. The use of MR contrast agents appears to further improve all techniques. Technical progress and changes in this subfield of cardiac MR imaging have been so fast that large-scale preclinical trials have not been conducted with the majority of the first and second generation coronary MR angiographic pulse sequences as known today. This article reviews the development of these new cardiac MR imaging techniques and the initial successes with clinical application using commercial MR scanners.     

Dynamic coronary MR angiography and first-pass perfusion with intracoronary administration of contrast agent

PURPOSE: To evaluate whether dynamic imaging of the coronary arteries can be performed with intracoronary infusion of low-dose gadolinium (Gd)-based contrast agent and assess the effect of long duration and multiple infusions on the image signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). MATERIALS AND METHODS: Dynamic coronary magnetic resonance (MR) imaging (130 msec/image) and contrast agent first pass myocardial perfusion studies were performed with intracoronary infusions of low-dose Gd-based MR contrast agent on dogs (N = 4) using a fast multislice gradient recalled echo (GRE) sequence. RESULTS: Contrast-enhanced coronary arteries were clearly imaged during infusion periods as long as 2.3 minutes. The SNR and CNR of the contrast-enhanced coronary arteries remained essentially unchanged over multiple consecutive angiographic sessions. In addition, we demonstrated that first pass studies performed with intracoronary injection of MR contrast agent can be used as a means of assessing regional myocardial perfusion. CONCLUSION: These studies demonstrated that, using intracoronary infusion of Gd, coronary magnetic resonance angiography (MRA) can be performed with high temporal resolution, and multiple low-dose slow infusions of Gd-based MR contrast agent can be performed without compromise of the vessel SNR and CNR.     

Free-breathing 3D steady-state free precession coronary MR angiography with radial k-space sampling: comparison with cartesian k-space sampling and cartesian gradient-echo coronary MR angiography--pilot study

The authors compared radial steady-state free precession (SSFP) coronary magnetic resonance (MR) angiography, cartesian k-space sampling SSFP coronary MR angiography, and gradient-echo coronary MR angiography in 16 healthy adults and four pilot study patients. Standard gradient-echo MR imaging with a T2 preparatory pulse and cartesian k-space sampling was the reference technique. Image quality was compared by using subjective motion artifact level and objective contrast-to-noise ratio and vessel sharpness. Radial SSFP, compared with cartesian SSFP and gradient-echo MR angiography, resulted in reduced motion artifacts and superior vessel sharpness. Cartesian SSFP resulted in increased motion artifacts (P <.05). Contrast-to-noise ratio with radial SSFP was lower than that with cartesian SSFP and similar to that with the reference technique. Radial SSFP coronary MR angiography appears preferable because of improved definition of vessel borders.     

Contrast-enhanced magnetic resonance imaging of hypoperfused myocardium.

Contrast-enhanced magnetic resonance (MR) imaging can define myocardial perfusion defects due to acute coronary occlusion. However, since most clinically important diagnostic examinations involve coronary arteries with subtotal stenoses, we investigated the ability of MR imaging with a manganese contrast agent to detect perfusion abnormalities in a canine model of partial coronary artery stenosis. The contrast agent was administered after the creation of a partial coronary artery stenosis with the addition of the coronary vasodilator dipyridamole in six of 12 animals. The hearts were imaged ex situ using gradient reversal and spin-echo sequences, and images were analyzed to determine differences in signal intensity between hypoperfused and normally perfused myocardium. Comparison of MR images with regional blood flow and thallium-201 measurements showed good concordance of hypoperfused segments in those animals given dipyridamole, with 75% of the abnormal segments correctly identified. In those animals not given dipyridamole, 48% of segments were correctly identified. Thus, ex vivo MR imaging with a paramagnetic contrast enhancement can be used to detect acute regional myocardial perfusion abnormalities due to severe partial coronary artery stenoses.     

Appearance of the Normal Pericardium on Coronary MR Angiograms

We evaluated the appearance of the normal pericardium on breath-hold MR images used to visualize coronary arteries. A coronary MR angiogram was obtained in 23 subjects (17 healthy volunteers and six patients with no known pericardia! disease) using a breath-hold K-space segmented gradient-recalled echo sequence with fat suppression. Each coronary MR angiographic study included imaging planes equivalent to the following echocardiographic planes: four-chamber view, vertical two-chamber view, and two short-axis views (at base and mid ventricular level). The average pericardial thickness was 1.7 mm (range, 1.5–2.0 mm), and an average length of 60 mm (range, 20–110 mm) of pericardium was visualized. A significantly longer portion of the pericardium was seen in the vertical two-chamber view and the basal short-axis view than in the two other views (P <.001). Normal anatomic variations and overlapping structures and image artifacts can alter the appearance of the pericardium. Breath-hold MR imaging techniques used for coronary MR angiography allow routine, time-efficient evaluation of large portions of the pericardium.     

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     

Advanced cardiac MR imaging of ischemic heart disease.

Important advances in rapid magnetic resonance (MR) imaging technology and its application to cardiovascular imaging have been made during the past decade. High-field-strength clinical magnets, high-performance gradient hardware, and ultrafast pulse sequence technology are rapidly making the vision of a comprehensive "one-stop shop" cardiac MR imaging examination a reality. This examination is poised to have a significant effect on the management of coronary artery disease by means of assessment of wall motion with tagging and pharmacologic stress testing, evaluation of the coronary microvasculature with perfusion imaging, and direct visualization of the coronary arteries with MR coronary angiography. This article reviews current state-of-the-art pulse sequence technology and its application to the evaluation of ischemic heart disease by means of MR tagging with dobutamine stress testing, MR perfusion imaging, and MR coronary angiography. Cutting edge areas of research in coil design and exciting new areas of metabolic and oxygen level-dependent imaging are also explored.     

Gadolinium-enhanced breath-hold three-dimensional time-of-flight MR angiography of the abdominal and pelvic vessels: The value of ultrafast MP-RAGE sequences

MR angiography (MRA) was performed in 50 consecutive subjects (mean age, 59 years), who had been referred for abdominal MRA, on a 1.5-T superconductive unit that used a body phased-array coil. Three breath-hold three-dimensional sequences were evaluated both in phantom and clinical studies: (a) standard fast three-dimensional gradient-echo sequence (TR = 15, TE = 6; imaging time, 32 seconds), (b) ultrafast three-dimensional gradient-echo sequence (TR = 8.2, TE = 3; imaging time, 18 seconds), and (c) ultrafast magnetization-prepared (MP) rapid acquisition gradient echo (RAGE) (TR = 5.8, TE = 2.9, inversion time [TI] = 20; imaging time, 15 seconds). The initial 30 patients were randomized into three groups by three separate sequences. For the remaining 20 patients, ultrafast-gradient-echo and ultrafast MP-RAGE sequences were performed. Conventional angiography was performed on 36 patients. Signal measurements of the phantom and clinical images of the aorta, visceral branches of the aorta, iliac arteries, inferior vena cavae, and portal veins were performed. The overall image quality and background fatty tissue contrast of the vessels were rated subjectively. Comparison of images between MRA and conventional angiography also was performed. The contrast between the vessels and background fatty tissue was significantly higher in the ultrafast MP-RAGE sequence in both quantitative and qualitative analysis, and image-quality ultrafast MP-RAGE was superior to the other two sequences (P < .01). The aorta and iliac arteries could be visualized in all pulse sequences, and abnormalities of these vessels were diagnosed correctly. The renal artery was visualized more clearly with the two ultrafast sequences.