Self-gated cardiac cine MRI.

The need for ECG gating presents many difficulties in cardiac magnetic resonance imaging (CMRI). Real-time imaging techniques eliminate the need for ECG gating in cine CMRI, but they cannot offer the spatial and temporal resolution provided by segmented acquisition techniques. Previous MR signal-based techniques have demonstrated an ability to provide cardiac gating information; however, these techniques result in decreased imaging efficiency. The purpose of this work was to develop a new "self-gated" (SG) acquisition technique that eliminates these efficiency deficits by extracting the motion synchronization signal directly from the same MR signals used for image reconstruction. Three separate strategies are proposed for deriving the SG signal from data acquired using radial k-space sampling: echo peak magnitude, kymogram, and 2D correlation. The SG techniques were performed on seven normal volunteers. A comparison of the results showed that they provided cine image series with no significant differences in image quality compared to that obtained with conventional ECG gating techniques. SG techniques represent an important practical advance in clinical MRI because they enable the acquisition of high temporal and spatial resolution cardiac cine images without the need for ECG gating and with no loss in imaging efficiency.     

Electrocardiograph-independent, "wireless" cardiovascular cine MR imaging.

A new electrocardiograph (ECG)-independent, "wireless" gating technique for cine magnetic resonance (MR) imaging was evaluated in 23 cases of cardiovascular disease; in each case, standard ECG-dependent image loops were available for comparison. The ECG-independent strategy references cine MR imaging data retrospectively to inherent periodic changes in MR signal related to the cardiac cycle. With a "double-section" method, both timing data reflecting such changes and imaging data can be acquired simultaneously. "Artificial R waves" are extracted from the timing data acquired with a projection approach. The ECG-independent image loops were diagnostic in 91% of cases. Their overall image quality was at least equal to that of available ECG-dependent versions in only 39% of cases, but this proportion increased to 53% if cases with suboptimal imaging orientations for monitoring of the motion-dependent signal changes were excluded. Orientation appeared to be the primary technical limitation associated with this ECG-independent technique; however, poor ventricular function also significantly impaired performance. Improvement in the performance of the ECG-independent strategy is anticipated with technical advances.     

Cardiac and respiratory self-gated cine MRI in the mouse: comparison between radial and rectilinear techniques at 7T.

ECG-gated cardiac MRI in the mouse is hindered by many technical difficulties in ECG signal recording inside high magnetic field scanners. The present study proposes a robust rectilinear method of acquiring cardiac and respiratory self-gated cine images in mouse hearts. In this approach, a motion-synchronization MR signal is collected in the center of k-space simultaneously with imaging data in each readout of a nontriggered rectilinear acquisition. This signal is then used for both cardiac and respiratory retrospective gating before cine image reconstruction. The value of this approach for overcoming ECG-gating failure was demonstrated by performing cardiac imaging in eight mice with myocardial infarction. Comparison with an auto-gated radial k-space sampling technique, previously reported for cardiac applications in the mouse, found the rectilinear strategy more robust, thanks to a more reliable self-gating signal, while the radial strategy was less sensitive to motion and flow artifacts.     

Doppler US gating of cardiac MR imaging

RATIONALE AND OBJECTIVES: Electrocardiographic (ECG) gating of cardiac magnetic resonance (MR) imaging has been problematic for many reasons. The purpose of this study was to demonstrate the feasibility of using Doppler ultrasound (US) gating, either directly off the moving cardiac wall or the systolic upstroke of the arterial signal from the great vessels in neck, in alternative gating modes. MATERIALS AND METHODS: A 2.5-MHz, range-gated Doppler US device was used with A-mode guidance for gating directly off left ventricular wall motion. A 4- or 8.1-MHz, continuous-wave (CW) Doppler US device was used for gating off the systolic upstroke from the great vessels in the neck. The subject undergoing imaging held the transducer against his chest for range-gated Doppler US and against his neck for 8.1-MHz CW Doppler US. The 4-MHz transducer was strapped to the subject's neck. Modified Doppler signals were fed back into the gating circuitry of the MR imager to achieve cardiac synchrony. RESULTS: Cardiac gating was achieved by using both the range-gated technique directly off the cardiac wall and the CW method off blood flow from the great vessels. Problems occurred with radiofrequency shielding during the range-gated method; however, these problems were almost completely removed by use of the CW Doppler probes. CONCLUSION: Doppler US gating of MR images is possible and potentially could overcome many shortcomings of ECG gating. Subsequent embodiments of the technique will require improved radiofrequency shielding in the range-gated technique.     

Fiber-optic stethoscope: a cardiac monitoring and gating system for magnetic resonance microscopy.

A fundamental problem associated with using the conventional electrocardiograph (ECG) to monitor a subject's cardiac activity during magnetic resonance imaging (MRI) is the distortion of the ECG due to electromagnetic interference. This problem is particularly pronounced in MR microscopy (MRI of small animals at microscopic resolutions (< 0.03 mm(3))) because the strong, rapidly-switching magnetic field gradients induce artifacts in the animal's ECG that often mimic electrophysiologic activity, impairing the use of the ECG for cardiac monitoring and gating purposes. The fiber-optic stethoscope system offers a novel approach to measuring cardiac activity that, unlike the ECG, is immune to electromagnetic effects. The fiber-optic stethoscope is perorally inserted into the esophagus of small animals to optically detect pulsatile compression of the esophageal wall. The optical system is shown to provide a robust cardiac monitoring and gating signal in rats and mice during routine cardiac MR microscopy.     

Variability of myocardial signal on magnetic resonance images

The diagnosis of myocardial disease by magnetic resonance (MR) imaging depends on accurate measurement of myocardial signal intensity. The authors performed 15 experiments in four rabbits at 1.9 T with spin-echo MR imaging to study the variability of myocardial signal intensity throughout the cardiac cycle and to measure myocardial T2 values. Variability in signal from the myocardium throughout the cardiac cycle was observed in all experiments. During systole, a significant increase in myocardial signal was noted, when data acquisition was performed with electrocardiogram (ECG)-gating and controlled ventilation (P = .02). An inverse relationship between myocardial signal and phase noise was found, indicating the motion-related nature of the variation of myocardial signal. A similar inverse relationship was observed in images obtained from a normal human volunteer. Ex vivo myocardial T2 values of rabbit myocardial tissue were significantly higher than the in vivo values (P = .003), reflecting residual motion despite cardiac gating and controlled ventilation.     

Assessment of motion gating strategies for mouse magnetic resonance at high magnetic fields

PURPOSE: To assess the performance of motion gating strategies for mouse cardiac magnetic resonance (MR) at high magnetic fields by quantifying the levels of motion artifact observed in images and spectra in vivo. MATERIALS AND METHODS: MR imaging (MRI) of the heart, diaphragm, and liver; MR angiography of the aortic arch; and slice-selective 1H-spectroscopy of the heart were performed on anesthetized C57Bl/6 mice at 11.75 T. Gating signals were derived using a custom-built physiological motion gating device, and the gating strategies considered were no gating, cardiac gating, conventional gating (i.e., blanking during respiration), automatic gating, and user-defined gating. Both automatic and user-defined modes used cardiac and respiratory gating with steady-state maintenance during respiration. Gating performance was assessed by quantifying the levels of motion artifact observed in images and the degree of amplitude and phase stability in spectra. RESULTS: User-defined gating with steady-state maintenance during respiration gave the best performance for mouse cardiac imaging, angiography, and spectroscopy, with a threefold increase in signal intensity and a sixfold reduction in noise intensity compared to cardiac gating only. CONCLUSION: Physiological gating with steady-state maintenance during respiration is essential for mouse cardiac MR at high magnetic fields.     

High-resolution contrast-enhanced MRI of atherosclerosis with digital cardiac and respiratory gating in mice.

Atherosclerosis initially develops predominantly at the aortic root and carotid origin, where effective visualization in mice requires efficient cardiac and respiratory gating. The present study sought to first compare the high-resolution MRI gating performance of two digital gating strategies using: 1) separate cardiac and respiratory signals (double-sensor); and 2) a single-sensor cardiorespiratory signal (ECG demodulation), and second, to apply an optimized processing technique to dynamic contrast-enhanced (CE) carotid origin vessel-wall imaging in mice. High-resolution MR mouse heart and aortic arch images were acquired by ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (MathWorks, USA). Double-sensor gating used a respiratory sensor while single-sensor gating used breathing-modulated ECG to generate a demodulated respiratory signal. Pre- and postcontrast T(1)-weighted images were acquired to evaluate vessel-wall enhancement with a gadolinium blood-pool agent (P792; Guerbet, France) at the carotid origin in vivo in ApoE(-/-) and C57BL/6 mice, using the optimized cardiorespiratory gating processing technique. Both strategies provided images with improved spatial resolution, less artifacts, and 100% correct transistor-to-transistor logic (TTL) signals. Image quality allowed vessel-wall enhancement measurement in all the ApoE(-/-) mice, with maximal (32%) enhancement 27 min postinjection. The study demonstrated the efficiency of both cardiorespiratory gating strategies for dynamic contrast-enhanced vessel-wall imaging.     

ECG-synchronized cardiac MR imaging: method and evaluation

An electrocardiographic (ECG) sensing and gating device compatible with a 0.35-tesla (T) magnetic resonance (MR) imager has been developed and used to produce 802 MR images of the heart in 30 patients. The instrument consists of an isolated acquisition module, an electrically floating preamplifier, and a monitor gating module. Two spin-echo images were acquired for each of five, 0.7-cm thick, transaxial sections from the base to the apex of the heart during each ECG-synchronized imaging run. Image quality was assessed in a blind study by two investigators, on a scale from 0 to 3, as diagnostic [2-3] or nondiagnostic [0-1]. There was agreement in 91.4% of their assessments of diagnostic images (68.1% of the images studied). Resolution of heart anatomy on the MR images was adversely affected by prolonged spin-echo time delay, imaging in late diastole, image acquisition at the cardiac apex, irregular triggering, and artifacts. The synchronization of gradient pulses to the ECG at 0.35 T appears safe for patients, permits diagnostic resolution of images, allows image acquisition at distinct points during the cardiac cycle, and enables monitoring of patients during imaging.     

Real-time gating system for mouse cardiovascular MR imaging.

Mouse cardiac MR gating using ECG is affected by the hostile MR environment. It requires appropriate signal processing and correct QRS detection, but gating software methods are currently limited. In this study we sought to demonstrate the feasibility of digital real-time automatically updated gating methods, based on optimizing a signal-processing technique for different mouse strains. High-resolution MR images of mouse hearts and aortic arches were acquired using a chain consisting of ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (The MathWorks, Inc., Natick, MA, USA). The signal-processing algorithms used were respectively low-pass filtering, nonlinear passband, and wavelet decomposition. Both updated and nonupdated gating signal generation methods were tested. Noise reduction was assessed by comparison of the ECG signal-to-noise ratio (SNR) before and after each processing step. Gating performance was assessed by measuring QRS detection accuracy before and after online trigger-level adjustments. Low-pass filtering with trigger-level adjustment gave the best performance for mouse cardiovascular imaging using gradient-echo (GE), spin-echo (SE), and fast SE (FSE) sequences with minimum induced delay and maximum gating efficiency (99% sensitivity and R-peak detection). This simple digital gating interface will allow various gating strategies to be optimized for cardiovascular MR explorations in mice.     

An electrocardiograph-respiration gating device for MR studies.

A versatile gating device for magnetic resonance (MR) spectroscopy and imaging is presented. The device uses electrocardiograph (ECG) and respiration signals as input, applies appropriate signal conditioning, and generates control signals for ECG, respiration, or combined gating studies. In the combined ECG and respiration mode, in conjunction with a proper MR pulse program, one can acquire MR data gated by the ECG signal within a selected window of the respiration cycle, while maintaining a steady level of magnetization saturation during the remainder of the respiration cycle, by gating the radio-frequency excitation with the ECG while inhibiting data acquisition.     

Valvular regurgitation: dynamic MR imaging

Cine magnetic resonance (MR) imaging is a new technique that combines short repetition times, limited flip angles, gradient refocused echoes, and cardiac gating. This technique has a temporal resolution of up to 32 time frames per cardiac cycle and accentuates signal from flowing blood. Cine MR images of 56 valves in 27 patients were evaluated and compared with either Doppler echocardiograms or cardiac catheterization images. An area of decreased signal that correlated spatially and temporally with regurgitant blood flow was seen in all instances in which valvular incompetence was demonstrated on either Doppler echocardiograms or cardiac catheterization images (20 valves). This abnormality was seen in nine of 36 cases without valvular incompetence. Cine MR imaging may be sensitive to turbulence and thus sensitive to valvular regurgitation.     

Cardiac and respiratory double self-gated cine MRI in the mouse at 7 T.

ECG-gated cardiac MRI in the mouse is hindered by many technical difficulties in ECG signal recording inside static and variable high magnetic scanner fields. The present study proposes an alternative robust method of acquiring auto-gated cardiac and respiratory cine images in mouse heart. In our approach, a motion synchronization signal is extracted from the echo peak MR signal of a non-triggered radial acquisition. This signal is then used for both cardiac and respiratory retrospective gating before cine image reconstruction. Highly asymmetric echoes were acquired to achieve the radial k-space sampling in order to avoid radial acquisition related artifacts and to increase auto-gating robustness. In vivo experiments demonstrated the feasibility and robustness of self-gated cine-MRI in the mouse heart at 7T. The signal-to-noise and contrast-to-noise ratios of the self-gated and ECG-gated images were comparable, all parameters being equal. Magn Reson Med, 2006. (c) 2006 Wiley-Liss, Inc.     

Fast multiphase MR imaging of aqueductal CSF flow: 1. Study of healthy subjects

Gradient-echo MR sequences are more sensitive to flow phenomena than spin-echo sequences are. We investigated aqueductal CSF flow by fast multiphase imaging. Fast multiphase imaging offers the opportunity to perform a dynamic study of fluid motion that is synchronous with the cardiac cycle. A section perpendicular to the cerebral aqueduct was imaged in 18 healthy volunteers. Serial, gated (every 50 msec from the ECG R wave), flow-compensated modulus images with 70 degrees-flip-angle excitation pulses were obtained with a single acquisition. The behavior vs time of CSF signal in the aqueduct was compared with that in the lateral ventricles. The former showed a peak at 0.47 +/- 0.1 fractions of a heart cycle after the R wave. No periodicity with the heart rate was observed for the ventricular CSF signal intensity. The mean CSF signal intensity in the aqueduct was found to range from about twice to three times that in the lateral ventricles over a cardiac cycle. Fast multiphase imaging is a sensitive and practical sequence for the MR investigation of aqueductal CSF flow. Its potential in patients with hydrocephalus is studied in a companion article.     

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.     

Magnetic resonance microscopy of the rat thorax and abdomen

With magnetic resonance (MR) microscopy, high-resolution volumetric imaging (3DFT) of small animals is possible. Although these techniques are suitable for imaging the head and other small stationary objects, breathing and cardiac motion degrade the quality of body images. Scan synchronous ventilation and cardiac gating methods have been developed that permit acquisition of high-resolution images from anywhere in the body of small animals (150 to 400 g). Anesthetized rats were ventilated in synchrony with three-dimensional Fourier spin warp (3DFT) sequence (TR = 400 to 1000 ms, TE = 20 ms). Eight or 16 slices (1.2 or 2.5 mm thick) were acquired simultaneously. Effective pixel size was 200 X 200 mu. Imaging was performed in a 1.5 T, 1-m bore research system using a 28-cm diameter high field gradient coil and a 6-cm diameter radio frequency coil. For thoracic imaging, acquisitions were gated to the QRS of the ECG. Scan synchronous ventilation eliminated breathing motion artifacts and permitted visualization of peripheral vascular structures in the lung and liver. In images that were also cardiac gated, cardiac chambers and major thoracic vessels, including the coronary arteries, were well demonstrated. Thus, thoroughly characterized rodent models can now be studied with MR not only to explore noninvasively the intricacies of mammalian pathomorphology, but also to test the capabilities of MR and aid in interpreting MR data.     

周围门控技术在胸部血管磁共振成像的应用

目的：探讨周围门控技术在胸部血管MR成像中的应用。材料和方法：选取正常健康志愿者5例，患者5例，应用Philips GyroscanNTl．5T磁共振成像仪，采用快速梯度回波序列加心电(ECG)门控和周围(PPU)门控对胸部血管进行成像。结果：采用快速梯度回波序列，加ECG门控或PPU门控扫描分别获得胸部血管图像，发现PPU门控亦能很好地抑制心脏和血流搏动造成的伪影。结论：采用快速梯度回波技术结合PPU门控技术可以获得与ECG门控技术相类似的成像效果。     

Automated rectilinear self-gated cardiac cine imaging.

ECG-based gating in cardiac MR imaging requires additional patient preparation time, is susceptible to RF and magnetic interference, and is ineffective in a significant percentage of patients. "Wireless" or "self-gating" techniques have been described using either interleaved central k-space lines or projection reconstruction to obtain MR signals synchronous with the cardiac cycle. However, the interleaved, central line method results in a doubling of the acquisition time, while radial streak artifacts are encountered with the projection reconstruction method. In this work, a new self-gating technique is presented to overcome these limitations. A retrospectively gated TrueFISP cine sequence was modified to acquire a short second echo after the readout and phase gradients are rewound. The information obtained from this second echo was used to derive a gating signal. This technique was compared to ECG-based gating in 10 healthy volunteers and shown to have no significant difference in image quality. The results indicate that this method could serve as an alternative gating strategy without the need for external physiological signal detection.     

Cardiac MR imaging with external respirator: synchronizing cardiac and respiratory motion--feasibility study

The feasibility of electrocardiography (ECG)-synchronized respiration with an external cuirass-type respirator in cardiac magnetic resonance (MR) imaging was evaluated. Cardiac MR imaging was performed in 10 nonsedated healthy volunteers with an ECG-triggered external respirator that was modified for use in the MR environment. Coronary MR angiograms and multiphase gradient-echo cine images were acquired with one respiratory cycle performed per cardiac cycle. The technique was feasible and in this group of volunteers resulted in equivalent image quality but shorter acquisition times than those of conventional free-breathing and breath-holding techniques.