Fast MR cardiac profiling with two-dimensional selective pulses

A rapid-profiling NMR pulse sequence has been designed to provide an interactive, real-time cardiac probe analogous to M-mode ultrasound. The pulse sequence employs a two-dimensional (2D) selective NMR pulse to excite a narrow (nominally 1-cm-diameter) cylinder of magnetization intersecting the heart. This procedure is followed by a readout gradient applied along the length of the cylinder, or "beam," to yield an M-mode type profile with a one-dimensional Fourier transform reconstruction. k-space techniques were used to design 2D pulses which excite cylinders characterized by either Gaussian or square radial excitation profiles. Images of phantoms acquired at 1.5 T confirm the predictions of the k-space analysis. The cylinder can be displaced interactively by modulating the rf excitation and the beam axis can be reoriented to any oblique direction by changing the relative mixing of the gradient waveforms. Flow compensation using bipolar gradient waveforms inverts the contrast of flowing blood and suppresses flow artifacts. A gated cardiac image is acquired as a reference to locate the excitation axis. A series of cardiac experiments was performed on several healthy volunteers. As the beam is moved and rotated to probe the myocardium, the profile plots resemble an M-mode echocardiogram. Unlike in M-mode echocardiography, however, the axis of interrogation is not limited to specific windows, and there is distinct flexibility of contrast. However, the temporal resolution is currently less than that achieved by ultrasound. NMR M-mode profiling provides a direct, fast method of measuring heart motion to assess cardiac function as part of an MR cardiac exam.     

Quantitation in echocardiography

Imaging of the heart is the predominant approach to cardiovascular diagnosis in current practice. Of the wide variety of cardiac imaging techniques available, echocardiography is one of the most widely used. Standard methods of quantitation of M-mode and two-dimensional echocardiograms yield reproducible, accurate measurements of cardiac chamber, wall, and great vessel dimensions. Qualitative analysis of valvular appearance and motion permits the diagnosis of a wide variety of valvular disorders. Doppler echocardiography yields information on blood flow velocity and pattern in the heart and great vessels. Evolving methods of quantitation in echocardiography include computerized image enhancement, computer-assisted border detection, analysis of regional left ventricular contraction, three-dimensional reconstruction, contrast-enhanced echocardiography, ultrasound myocardial tissue characterization, and intraoperative echocardiography. Echocardiography is a dynamic, evolving discipline with the potential of defining cardiac structure, function, blood flow dynamics, myocardial perfusion, and tissue characteristics. Thus, ultrasonography will continue to be of major importance in the diagnosis of cardiac disease.     

Rapid NMR cardiography with a half-echo M-mode method

A real-time NMR cardiac profiling pulse sequence has been developed that incorporates two-dimensional (2D) selective excitation and a half-echo readout. The time resolution has been improved by a factor of two relative to the previous flow-compensated, full-echo version. The technique produces a 2D plot of "beam"-axis position versus time, analogous to M-mode echocardiography. In human subjects, details of valve leaflet motion, intracardiac flow, wall motion, and wall thickening may be observed along optimal lines of sight selected interactively. The pulse sequence uses a low-tip-angle 2D selective-excitation pulse derived from a spiral k-space trajectory to excite a narrow cylinder of magnetization, followed by a half-echo readout gradient oriented along the axis of the cylinder. One-dimensional Fourier transformation of the acquired signal results in a magnetization profile along the length of the cylinder, or beam. The pulse sequence is effectively flow compensated without any additional gradient lobes, because the rapid oscillation in the gradient wave forms of the 2D excitation pulse produces relatively small net gradient moments, and the shortened readout gradient has minimal first-order moment relative to center echo. The signal from moving blood can alternatively be velocity encoded by the addition of bipolar gradients along any of the three axes, producing Doppler-like traces of intracardiac blood flow.     

应用全方向M型超声研究正常短轴方向心肌速度梯度

目的:探讨采用全方向M型超声心动图评价正常人室壁短轴方向心肌速度梯度的价值.材料和方法:应用全方向M型超声心动图技术分别于左室三个水平节段测量40例正常人室间隔、左室各壁收缩与舒张期内外膜速度及厚度,计算内外膜心肌速度梯度,并进行对比分析.结果:短轴方向上,从二尖瓣水平到心尖水平,速度梯度逐渐降低,左室游离壁速度梯度大于室间隔.结论:正常人室壁运动速度梯度并不完全一致.全方向M型超声心动图可对局部室壁短轴方向的心肌速度梯度做定量分析.     

Keyhole method for high-speed human cardiac cine MR imaging.

Although electrocardiographic (ECG)-gated magnetic resonance (MR) imaging is widely used for cardiac imaging, it has several disadvantages, such as long imaging time, respiratory artifacts, and motion artifacts induced by arrhythmia. An MR image can be acquired within about 0.3 seconds by using a fast gradient-echo imaging method. When this method is continuously applied, only two to three images can be obtained during a single cardiac cycle. The goal of this study is to obtain cine MR images in a single cardiac cycle using fast gradient-echo imaging combined with the "keyhole" method. The optimal conditions for the keyhole method for cardiac cine imaging were obtained by computer simulation based on a simplified cardiac model. When the read-out direction was set parallel to the cardiac short axis, left ventricular motion was almost correctly reproduced by the keyhole method with acquisition time reduced to one-fourth. J. Magn. Reson. Imaging 1999;10:778-783.     

心肌灌注显像中位移伪影的辨析

目的 探讨心肌灌注显像时位移伪影的影像学特征、不同轴向、发生位移起始点和帧数与伪影的相关性。方法 在心肌显像过程中依次沿x,y和z轴方向,分别在不同起始点,对不同帧数作一定距离的位移。其图像与正常对照比较判断有无伪影。结果作多因素分析。结果 轻度位移伪影的特征为：x轴位移表现为下壁突出的结节状热区;y轴位移表现为间隔和侧壁呈对称分布的热区;z轴位移表现为前壁的局部热区;这些表现仅见于短轴像上。重度伪影表现为“三角形”分布的壁内热区以及典型的“双三角形”改变。位移距离相同,方向相反,伪影的“冷”“热”分布的壁内热区以及典型的“双三角形”改变。位移距离相同,方向相反,伪影的“冷”“热”区位置相反。伪影与位移帧数和轴向有关,与起始点无关。结论 不同轴向位移伪影各有特征。移动帧数和y轴位移对伪影产生的影响最大。     

Rapid line scan NMR angiography

This paper describes a new technique for NMR angiography based on rapid line scan projection imaging and presaturation of stationary spins. The resulting line scan angiograms are free from both misregistration artifacts common to subtraction methods and motion artifacts encountered in Fourier imaging. Moreover, line scan angiograms may be recorded within seconds and offer arbitrary fields of view as well as gradient zooming without problems due to aliasing. Three-dimensional information is obtained by rotation of the read gradient axis generating multiple view angles. Experimental line scan angiograms of the forearms of healthy volunteers have been recorded using a Bruker 2.35-T 40-cm magnet.     

Assessment of myocardial function with interactive non-breath-hold real-time MR imaging: comparison with echocardiography and breath-hold Cine MR imaging

PURPOSE: To compare a spiral gradient-echo sequence with a radial steady-state free precession sequence and to compare these two interactive real-time cardiac magnetic resonance (MR) imaging examinations with harmonic two-dimensional echocardiography (ECHO) for the evaluation of regional myocardial function. MATERIALS AND METHODS: Electrocardiographically triggered breath-hold steady-state free precession (BH-SSFP) MR imaging was the reference standard. Thirty-five nonselected patients scheduled for routine ECHO were included. Data from corresponding two-, three-, and four-chamber long-axis views and a midventricular short-axis view were acquired with each modality. Image quality and depiction of segmental wall motion were scored semiquantitatively by using the 16-segment model of the American Society of Echocardiography. Repeated-measures analysis of variance was performed to assess differences in image quality and wall motion depiction scores among the four imaging methods. Agreement was assessed by using Cohen kappa statistics. RESULTS: Compared with the image quality achieved with BH-SSFP MR imaging, the image quality achieved with radial MR imaging was similar (nonsignificant difference), but that achieved with spiral MR imaging and ECHO was significantly inferior (P <.0001). There were no significant differences in the image quality of the long- and short-axis views between the radial and BH-SSFP sequences, while the image quality of the long-axis spiral (P <.05) and the short- and long-axis ECHO (P <.0001) views was lower than that of the BH-SSFP views. Compared with the mean wall motion score for BH-SSFP MR imaging, the mean wall motion score for radial MR imaging was not significantly different, but those for ECHO (P <.05) and spiral MR imaging (P =.0003) were significantly lower. Cohen kappa coefficients for agreement with the BH-SSFP sequence regarding wall motion scoring were 0.47 for ECHO, 0.67 for the spiral sequence, and 0.89 for the radial sequence. CONCLUSION: The radial sequence enables similar accurate assessment of regional wall motion compared with the BH-SSFP sequence and yields image quality that is superior to that yielded by the spiral sequence and ECHO.     

A comparison of MRI and echocardiography in hypertrophic cardiomyopathy.

This study compares MRI and echocardiography as imaging modalities in hypertrophic cardiomyopathy, with particular reference to measurement of left ventricular wall thickness and mass. 10 subjects underwent echocardiography and MRI. Contiguous 10 mm short axis 35 degrees flip angle cine gradient recalled echo MR images were acquired from the apex to the base of the left ventricle at 1.5 tesla. Standard M-mode and cross-sectional echocardiographic views of the left ventricle were obtained. Excellent agreement between measurements occurred with MRI and M-mode echocardiographic assessment of the thickness of the anterior interventricular septum (95% limits of agreement -1.5 to +1.5 mm). Other comparisons of MRI vs M-mode echocardiographic measurements had the following limits of agreement: posterior free wall -3.3 to +2.9 mm; end-diastolic dimension -5 to +8 mm, left ventricular mass -291 to +55.5 g. Comparing MRI with cross-sectional echocardiographic measurements, the limits of agreement were: anterior interventricular septum -2.4 to +1.7 mm, posterior interventricular septum -2.4 to +2.9 mm, posterior free wall -3.4 to +2.5 mm, anterior free wall -2.4 to +1.7 mm, end-diastolic dimension -4.1 to +8 mm. MRI estimates of LVM in systole vs diastole showed good agreement with 95% limits of agreement of -20 to +17 g, with excellent interobserver variability in diastole (-9 to +5 g) and in systole (-7 to +12 g). In conclusion, MRI is superior to echocardiography for the quantification of ventricular mass in the abnormal left ventricle because it does not make invalid geometrical assumptions. Comparisons of wall thickness show greater discrepancy with increasing distance from the echocardiographic transducer. This study suggests that sequential echocardiography could rationalize the need for MRI in left ventricular hypertrophy. A change in anterior septal thickness of > or = 3 mm on echocardiography merits a further MRI study.     

Problems for analysis of regional wall motion using ECG gated myocardial SPECT imaging]

Several issues arose during analysis of cardiac function using ECG-gated myocardial SPECT imaging. First, the analysis program did not depending on a direct radioactivity counts as in equilibrium radionuclide angiography, but on the extraction of the myocardial margin. Consequently, spatial resolution was an important consideration in the analysis. The auto-analyzing program could be used in cases with poor quality images. Poor time resolution was also a weak point of the gated SPECT program. In the present study, we compared observation of regional wall motion obtained by Quantitative gated SPECT program (QGS) and echocardiography in patients with subacute phase myocardial infarction. The QGS indices of regional wall motion were correlated with the echocardiography indices, although mild asynergy could not be detected by any of the QGS indices. Further basic research on the analysis of regional wall motion using gated SPECT is needed.     

Assessment of cardiac function and left ventricular regional wall motion by 99mTc multigated cardiac blood-pool emission computed tomography]

Forty-three patients underwent the analysis of left and right ventricular (LV and RV) volumes, and LV regional wall motion by multigated cardiac blood pool single photon emission computed tomography (SPECT) with 99mTc. To calculate the cardiac volume correctly, the optimal cutoff level in relation to background level was first obtained by a phantom study. Left ventricular end-diastolic, end-systolic volume (EDV and ESV) and ejection fraction (EF) calculated thus with SPECT were correlated well with the data obtained with left ventriculography (LVG) and magnetic resonance imaging (MRI), especially using horizontal long axial image. RV stroke volume (SV) without shunt or valvular diseases was also correlated well with that of LV when it was calculated using horizontal long axial image. However, SV ratio (LVSV/RVSV) was not necessarily ideal numerical 1. In addition LV wall motion was evaluated by multicontour systolic display and phase analysis in SPECT and gated planar images. The results obtained with SPECT were better correlated with those of LVG than gated planar images. It is concluded that multigated cardiac blood pool SPECT is a clinically useful method for an evaluation of cardiac function and left ventricular regional wall motion.     

Real-time acquisition,display, and interactive graphic control of NMR cardiac profiles and images

A highly interactive MRI scanner interface has been developed that allows, for the first time, real-time graphic control of one-dimensional (1D) and two-dimensional (2D) cardiac MRI exams. The system comprises a Mercury array processor (AP) in a Sun SPARCserver with two connections to the MRI scanner, a data link that passes the NMR data directly to the AP as they are collected, and a control link that passes commands from the Sun to the scanner to redirect the imaging pulse sequence in real time. In the 1D techniques, a cylinder or “pencil” of magnetization is repeatedly excited using gradient-echo or spin-echo line-scan sequences, with the magnetization read out each time along the length of the cylinder, and a scrolling display generated on the Sun monitor. Rubber-band lines drawn on the scout image redirect the pencil or imaging slice to different locations, with the changes immediately visible in the display. M-mode imaging, 1D flow imaging, and 2D fast cardiac imaging have been demonstrated on normal volunteers using this system. This platform represents an operator-“friendly” way of directing real-time imaging of the heart.     

Prospective self-gating for swallowing motion: a feasibility study in carotid artery wall MRI using three-dimensional variable-flip-angle turbo spin-echo

Three-dimensional black-blood MRI is a promising noninvasive imaging technique for the assessment of atherosclerotic carotid artery disease. However, this technique is inherently susceptible to motion. In particular, swallowing can result in considerable wall motion at the carotid bifurcations, which may induce drastic image degradation or substantial overestimation of wall thickness. Self-gating techniques have previously been shown to be capable of resolving and compensating for cardiac or respiratory motion during MRI. This work presents a self-gating-based prospective motion gating scheme that is combined with a three-dimensional variable-flip-angle turbo spin-echo sequence (SPACE) for detecting swallowing motion. Self-gating signal readouts along the superior-inferior direction during each repetition time period are used to derive the projection profiles of the imaging volume. Based on cross-correlation analysis between the projection profiles and the corresponding reference profiles, swallowing motion can be detected and the motion-contaminated data will subsequently be discarded and reacquired in the next repetition time. The self-gated SPACE sequence was validated on eight healthy volunteers and two patients and, when compared with the conventional SPACE sequence, proved to be more resistant to swallowing motion and significantly improved image quality as well as the sharpness of carotid artery wall boundaries.     

Two-dimensional echocardiography in mitral annulus calcification

Mitral annular calcification, established by fluoroscopy, was studied by M-mode and two-dimensional echocardiography in 18 patients. M-mode echocardiography revealed the typical dense, linear echo of mitral annular calcification, but not its extent. Two-dimenstional echocardiography demonstrated a dense, elipsoidal echo at the junction of the left atrium and ventricle in the long axis view, and an area of calcification below the mitral valve in the short axis view. These patterns were not seen in 20 control patients without mitral annular calcification by fluoroscopy. Two-dimensional echocardiography establishes the site and extent of mitral annular calcification, minimizes the potential for diagnostic error associated with M-mode echocardiography, and is superior to M-mode echocardiography for the recognition of mitral annular calcification.     

Rapid NMR imaging of dynamic processes using the FLASH technique

FLASH (Fast Low-Angle SHot) imaging is a new method for rapid NMR imaging which has been demonstrated to provide abdominal images without artifacts due to respiratory or peristaltic motions. The sequence typically employs 15 degrees radiofrequency excitation pulses and acquires a free induction decay signal in the form of a gradient echo. Here FLASH images are recorded in the presence of dynamic processes with time constants even smaller than the measuring time of about 2 s for an image with a 128 X 128-pixel resolution. Experiments are carried out on flow phantoms and on rabbits yielding heart images without gating of the cardiac motion.     

Rapid autocorrection using prescan navigator echoes.

Autocorrection is an adaptive motion correction algorithm that does not require an in vivo measurement of the motion record. A novel method for ensuring convergence of this algorithm when motion is severe is presented. A limited number of navigator echoes are acquired before the imaging sequence to obtain a "snapshot" of the object. Phase differences between the navigator and image k-space data are used as an estimate of motion-induced phase shifts in the image, followed by autocorrection. In phantom data a six-fold reduction in computation time compared to autocorrection alone was realized. These results indicate that this navigator/autocorrection combination may be useful for reducing motion artifacts and computation time for MR exams when motion along the image phase encoding axis is severe.     

Effects of respiration on size and function of the athletic heart.

This study compares the effects of quiet breathing on the heart structure and function of pentathlon athletes (n = 11) and a less-conditioned control group (n = 12). Two-dimensional echocardiography was employed to direct an M-mode beam through the heart to minimize measurement errors due to respiratory motion of the heart. Respiration was simultaneously recorded with M-mode echocardiograms from a minimum of five respiratory and an average of 25 cardiac cycles. Athletes had greater (p less than 0.01) end-diastolic transverse dimensions (10%), and interventricular septal and left ventricular posterior wall thicknesses (25%). During inspiration right ventricular internal end-diastolic dimensions increased (8 vs 12%) and left ventricular end-diastolic dimension (LVIDd) decreased (7 vs 8%) in athletes and controls, respectively. Although significant reciprocal changes in LVIDd (p less than 0.01) occurred during respiration, no respiratory change was noted in the transverse diameter of the heart, interventricular septal, posterior wall thickness, LV end-systolic volume or ejection fraction. Inspiration caused a 21% fall (p less than 0.01) in LV stroke volume in both groups. Eighty-four to 92% of the inspiratory decrease in LV stroke volume in the subjects studied could be attributed to ventricular interdependence; no significant interaction was found between level of conditioning and respiration.     

Quantitation of abdominal aortic wall dynamics in man by gradient echo NMR imaging

To determine the magnitude of the pulsatile (P), translational (T), and composite (C) aortic wall motion and to predict its effect on endothelial edge definition in vascular NMR imaging, 10 healthy volunteers, mean age 42.5 +/- 13 years, were studied. In each subject a series of transverse high-resolution (0.6 X 0.6 mm; TR = 40 ms) gradient echo images spanning the cardiac cycle were generated and the aortic wall motion dynamics were analyzed. The group data expressed as a mean +/- SD in millimeters, P = 0.87 +/- 0.34 (mm), T = 2.71 +/- 1.07, and C = 3.39 +/- 1.081, indicate that a significant displacement of the endothelial aortic edge (up to 5.6 pixels in high-resolution imaging) occurs during the cardiac cycle in normotensive healthy volunteers. The displacement of this magnitude suggests that cardiac cycle synchronized aortic NMR imaging should be used to improve endothelial edge definition.     

Functional MRI in ischemic heart disease based on detection of contraction abnormalities.

Even though several non-invasive techniques are available for the assessment of coronary artery disease and the detection of myocardial ischemia, many coronary angiograms yield negative results, thus, warranting higher accuracy for non-invasive tests. The detection of obstructive coronary artery disease is only possible during physical or pharmacological stress. Currently, the assessment of wall motion abnormalities by echocardiography is clinically the most widely used method. However, a significant number of patients yield suboptimal or non-diagnostic images despite improvements with harmonic imaging. Cardiovascular magnetic resonance (CMR) imaging allows a non-invasive visualization of the heart with high spatial and temporal resolution. Gradient echo CMR images permit an exact and reproducible determination of global and regional left ventricular function, wall thickness and wall thickening and identical pharmacological stress protocols, as currently used for dobutamine stress echocardiography, can be implemented for CMR imaging. A review of the literature on dobutamine stress CMR for the detection of stress induced wall motion abnormalities is presented and the safety of CMR stress examinations is discussed. The results show, that especially in those patients with suboptimal echocardiographic image quality dobutamine stress CMR is superior in comparison with dobutamine stress echocardiography and may replace echocardiography in these patients. Further possibilities by the use of myocardial tagging or intravascular contrast agents are outlined.