Comparison of self-gated cine MRI retrospective cardiac synchronization algorithms

PURPOSE: To determine whether improved self-gating (SG) algorithms can provide superior synchronization accuracy for retrospectively gated cine MRI. MATERIALS AND METHODS: First difference, template matching, and polynomial fitting algorithms were implemented to improve the synchronization of MRI data using cardiac SG signals. Cine datasets were acquired during short-axis, two-, three-, and four-chamber cardiac MRI scans. The root-mean-square (RMS) error of SG synchronization positions compared to detected R-wave positions were calculated along with the mean square error (MSE) and peak signal-to-noise ratio (PSNR) comparing SG to electrocardiogram (ECG)-gated images. Overall image quality was also compared by two expert reviewers. RESULTS: RMS errors were highest for the first difference method for all orientations. Improvements for both template matching and cubic polynomial fitting methods were significant for two-, three-, and four-chamber scans. MSE values were lower and PSNR were significantly higher for the cubic method compared to the first difference method for all orientations. Reviewers scored the images to be of comparable quality. CONCLUSION: Template matching and polynomial fitting improved the accuracy of cardiac cycle synchronization for two-, three-, and four-chamber scans; improvements in SG synchronization accuracy were reflected in improvements in analytical image quality. Implementation of robust postprocessing algorithms may bring SG approaches closer to clinical utilization.     

Retrospective gating for mouse cardiac MRI.

Cardiac MR imaging in small animals presents some difficulties due to shorter cardiac cycles and smaller dimensions than in human beings, but prospectively gated techniques have been successfully applied. As with human imaging, there may be certain applications in animal imaging for which retrospective gating is preferable to prospective gating. For example, cardiac imaging in multiple mice simultaneously is one such application. In this work we investigate the use of retrospective gating for cardiac imaging in a mouse. Using a three-dimensional imaging protocol, we show that image quality with retrospective gating is comparable to prospectively gated imaging. We conclude that retrospective gating is applicable for small animal cardiac MRI and show how it can be applied to the problem of cardiac MRI in multiple mice.     

Wireless self-gated multiple-mouse cardiac cine MRI.

Despite the excellent image-contrast capability of MRI and the ability to synchronize MRI with the murine cardiac cycle, this technique is underused for assessing mouse models of cardiovascular disease because of its perceived cost and complexity. This perception stems, in part, from complications associated with the placement and adjustment of electrocardiographic leads that may interact with gradient pulses and the relatively long acquisition times required with traditional gating schemes. To improve the efficiency and reduce the cost and complexity of using cardiac MRI in mice, we combined wireless self-gating techniques (with which we derived cardiac synchronization signals from acquired data) with an imaging technique that acquires multislice cardiac cine images from four mice simultaneously. As a result, the wireless self-gated acquisitions minimized animal preparation time and improved image quality. The simultaneous acquisition of cardiac cine data from multiple animals greatly increased throughput and reduced costs associated with instrument access.     

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.     

Metric optimized gating for fetal cardiac MRI

Phase‐contrast magnetic resonance imaging can be used to complement echocardiography for the evaluation of the fetal heart. Cardiac imaging typically requires gating with peripheral hardware; however, a gating signal is not readily available in utero. No successful application of existing technologies to human fetal phase‐contrast magnetic resonance imaging has been reported to date in the literature. The purpose of this work is to develop a technique for phase‐contrast magnetic resonance imaging of the fetal heart that does not require measurement of a gating signal. Metric optimized gating involves acquiring data without gating and retrospectively determining the proper reconstruction by optimizing an image metric. The effects of incorrect gating on phase contrast images were investigated, and the time‐entropy of the series of images was found to provide a good measure of the level of corruption. The technique was validated with a pulsatile flow phantom, experiments with adult volunteers, and in vivo application in the fetal population. Images and flow curves from these measurements are presented. Additionally, numerical simulations were used to investigate the degree to which heart rate variability affects the reconstruction process. Metric optimized gating enables imaging with conventional phase‐contrast magnetic resonance imaging sequences in the absence of a gating signal, permitting flow measurements in the great vessels in utero. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Prospective self‐gating for simultaneous compensation of cardiac and respiratory motion

Segmented cardiac acquisitions generally require the use of an electrocardiogram (ECG) in combination with a breathhold or a respiratory navigator placed on the diaphragm. These techniques necessitate patient cooperation and increase the complexity of cardiac imaging. The ECG signal may be distorted inside the magnet by interferences from radiofrequency and gradient action. Breathhold acquisition limits the total scan time, while navigators on the diaphragm might not fully reflect respiratory‐induced motion of the heart. To overcome some of these problems, several self‐gating (SG) or “wireless” techniques have recently been presented. All of these approaches, however, are based on either cardiac triggering or respiratory gating, or the data are processed retrospectively, reducing the efficiency of data acquisition. In this work a prospective SG approach for free‐breathing imaging is presented that requires neither ECG gating nor respiratory navigation. The motion data used for cardiac triggering and respiratory gating are extracted from the repeatedly acquired k‐space center. Based on computer simulations and in vivo data of the heart, it is shown that cardiac as well as respiratory motion can be accurately extracted in real time. Using the method proposed, the scan efficiency could be significantly increased while preserving image quality relative to retrospective SG approaches. Magn Reson Med 60:683–690, 2008. © 2008 Wiley‐Liss, Inc.     

Reconstruction from free‐breathing cardiac MRI data using reproducing kernel Hilbert spaces

This paper describes a rigorous framework for reconstructing MR images of the heart, acquired continuously over the cardiac and respiratory cycle. The framework generalizes existing techniques, commonly referred to as retrospective gating, and is based on the properties of reproducing kernel Hilbert spaces. The reconstruction problem is formulated as a moment problem in a multidimensional reproducing kernel Hilbert spaces (a two‐dimensional space for cardiac and respiratory resolved imaging). Several reproducing kernel Hilbert spaces were tested and compared, including those corresponding to commonly used interpolation techniques (sinc‐based and splines kernels) and a more specific kernel allowed by the framework (based on a first‐order Sobolev RKHS). The Sobolev reproducing kernel Hilbert spaces was shown to allow improved reconstructions in both simulated and real data from healthy volunteers, acquired in free breathing. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

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.     

Whole-heart cine MRI using real-time respiratory self-gating.

Two-dimensional (2D) breath-hold cine MRI is used to assess cardiac anatomy and function. However, this technique requires cooperation from the patient, and in some cases the scan planning is complicated. Isotropic nonangulated three-dimensional (3D) cardiac MR can overcome some of these problems because it requires minimal planning and can be reformatted in any plane. However, current methods, even those that use undersampling techniques, involve breath-holding for periods that are too long for many patients. Free-breathing respiratory gating sequences represent a possible solution for realizing 3D cine imaging. A real-time respiratory self-gating technique for whole-heart cine MRI is presented. The technique enables assessment of cardiac anatomy and function with minimum planning or patient cooperation. Nonangulated isotropic 3D data were acquired from five healthy volunteers and then reformatted into 2D clinical views. The respiratory self-gating technique is shown to improve image quality in free-breathing scanning. In addition, ventricular volumetric data obtained using the 3D approach were comparable to those acquired with the conventional multislice 2D approach.