Abstract
PurposeTo establish a cardiac signal from scattering matrix or scattering coefficient measurements made on a 7T 8‐channel parallel transmit (pTx) system, and to evaluate its use for cardiac gating.MethodsMeasurements of the scattering matrix and scattering coefficients were acquired using a monitoring pulse sequence and during a standard cine acquisition, respectively. Postprocessing used an independent component analysis and gating feature identification. The effect of the phase of the excitation radiofrequency (RF) field ( B1+ shim) on the cardiac signal was simulated for multiple B1+ shim configurations, and cine images were reconstructed from both the scattering coefficients and electrocardiogram (ECG).ResultsThe cardiac motion signal was successfully identified in all subjects with a mean signal‐to‐noise ratio of 33.1 and 5.7 using the scattering matrix and scattering coefficient measurements, respectively. The dominant gating feature in the cardiac signal was a peak aligned with end‐systole that occurred on average at 311 and 391 ms after the ECG trigger, with a mean standard deviation of 13.4 and 18.1 ms relative to ECG when using the scattering matrix and scattering coefficients measurements, respectively. The scattering coefficients showed a dependence on B1+ shim with some shim configurations not showing any cardiac signal. Cine images were successfully reconstructed using the scattering coefficients with minimal differences compared to those using ECG.ConclusionWe have shown that the scattering of a pTx RF coil can be used to estimate a cardiac signal, and that scattering matrix and coefficients can be used to cardiac gate MRI acquisitions with the scattering matrix providing a superior cardiac signal. Magn Reson Med 80:633–640, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Highlights
Cardiovascular MRI at a magnetic field strength of 7T is a promising field of research [1,2,3,4] but requires reliable, accurate, and user-independent triggering and gating
Assuming that a standard deviation (SD) of greater than 40 ms limits the suitability for cardiac MRI, the figure shows that 71% of random Bþ1 shim combinations were successful and showed sufficient cardiac information available in ~GAðtÞ
The endsystolic peak was detected with a sensitivity of 98.9%, a positive predictive value of 98.5%, a mean trigger delay of 391 ms after the ECG R-wave, and a mean SD of 18.1 ms relative to ECG
Summary
Cardiovascular MRI at a magnetic field strength of 7T is a promising field of research [1,2,3,4] but requires reliable, accurate, and user-independent triggering and gating. The current gold-standard gating method, based on the electrocardiogram (ECG), is affected by strong magnetic fields through magnetohydrodynamic effects [5]. Whereas ECG gating is possible, lead re-positioning is frequently required [2] and increases subject setup time. Real-time imaging acquisitions, which do not require cardiac gating, can be used [10]. For more standard imaging acquisitions, external devices based on heart tones [11] or pilot tone signal modulation [12,13,14] were developed for cardiac gating. There remains a need for a cardiac gating method that does not require additional hardware or expert setup
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