Real‐time gating system for mouse cardiovascular MR imaging

Abstract

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), spinecho (SE), and fast SE (FSE) sequences with minimum induced delay and maximum gating efficiency (99% sensitivity and Rpeak detection). This simple digital gating interface will allow various gating strategies to be optimized for cardiovascular MR explorations in mice. Magn Reson Med 57:29 -39, 2007.

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

“…A solid‐state ultra‐low pressure sensor (SURSENSE; Honeywell, IL, USA), connected to an air‐filled latex belt surrounding the abdomen of the mouse, was used to generate a respiratory signal. Using the real‐time toolbox of Simulink (The MathWorks Inc., Natick, MA, USA), in‐house written applications were developed to acquire and monitor the physiological signals and parameters (ECG R‐waves and respiration‐expiratory phases) and to control image acquisition (17).…”

“…The ECG signal was processed using a digital filter composed of low‐pass infinite impulse response (IIR) filtering, followed by signal amplification. The filter order of 3 was selected for better MR interference elimination with minimal delay, and the cutoff frequency was set to 12 Hz to cope with the QRS component bandwidth (17). For both denoised ECG and respiratory digital signals, threshold detection with conversion of the physiological signals into transistor‐to‐transistor logic (TTL) window acquisition signals was applied.…”

“…QRS complexes were detected by applying a Schmitt trigger to the denoised signal. For efficient, correct R‐wave detection, the threshold was updated by automatic adjustment; when automatic adjustment was not sufficient, a manual adjustment mode was selected (17). The expiratory phases were selected as time segments with sensor voltage below a constant threshold (1.95 V in our particular case).…”

“…The present study developed a new approach, referred to as “single‐sensor gating,” based on a breathing‐modulated ECG signal, generating a demodulated respiratory signal and consequently a cardiorespiratory gating window. The method derived from a digital real‐time cardiac gating system recently published (17).…”

Real-time optical gating for three-dimensional beating heart imaging

“…Alternative approaches that do not suffer from this issue include optical postacquisition processing using a simple metric based on the amplitude at the k-space center, 10 and gating using a fiber-optic stethoscope. 11 However, despite the continued growth in computing power, such interventional approaches remain challenging, and high-resolution "prospective" gating performed in real time has only recently been demonstrated, 12,13 with latencies that are far too great to give the high-precision results required for optical microscopy imaging, particularly in small animal embryos. Prospective gating in MRI remains an open problem.…”