Abstract
PurposeThe scattering matrix (S‐matrix) of a parallel transmit (pTx) coil is sensitive to physiological motion but requires additional monitoring RF pulses to be measured. In this work, we present and evaluate pTx RF pulse designs that simultaneously excite for imaging and measure the S‐matrix to generate real‐time motion signals without prolonging the image sequence.Theory and MethodsThree pTx waveforms for measuring the S‐matrix were identified and superimposed onto the imaging excitation RF pulses: (1) time division multiplexing, (2) frequency division multiplexing, and (3) code division multiplexing. These 3 methods were evaluated in healthy volunteers for scattering sensitivity and image artefacts. The S‐matrix and real‐time motion signals were calculated on the image calculation environment of the MR scanner. Prospective cardiac triggers were identified in early systole as a high rate of change of the cardiac motion signal. Monitoring accuracy was compared against electrocardiogram or the imaged diaphragm position.ResultsAll 3 monitoring approaches measure the S‐matrix during image excitation with quality correlated to input power. No image artefacts were observed for frequency multiplexing, and low energy artefacts were observed in the other methods. The accuracy of the achieved prospective cardiac gating was 15 ± 16 ms for breath hold and 24 ± 17 ms during free breathing. The diaphragm position prediction accuracy was 1.3 ± 0.9 mm. In all volunteers, good quality cine images were acquired for breath hold scans and dual gated CINEs were demonstrated.ConclusionThe S‐matrix can be measured during image excitation to generate real‐time cardiac and respiratory motion signals for prospective gating. No artefacts are introduced when frequency division multiplexing is used.
Highlights
For many cardiovascular MR applications, physiological motion and a reliable monitoring of such remains the major challenge for image quality
Other external devices that are independent to the electrical activity of the heart are based on finger plethysmography, Doppler ultrasound,[7] or on heart tones.[8]
We assumed that cardiac and respiratory changes are linear and independent, and we model the time‐ resolved S‐matrix S(t) with 3 additive terms: S0, which is static S‐matrix and represents the coil properties and the non‐movable tissue, ΔSresp (t), which represents respiratory induced changes, and ΔScardiac (t), which reflects the change resulting from cardiac motion
Summary
Engineering and Physical Sciences Research Council (EPSRC) and Medical Research Council (MRC), Grant/Award Number: EP/L016052/1; Clarendon Fund and Keble College de Breyne Scholarship; Siemens Healthcare GmbH; BHF Centre of Research Excellence, Grant/Award Number: RE/13/1/30181. We present and evaluate pTx RF pulse designs that simultaneously excite for imaging and measure the S‐matrix to generate real‐time motion signals without prolonging the image sequence. Theory and Methods: Three pTx waveforms for measuring the S‐matrix were identified and superimposed onto the imaging excitation RF pulses: (1) time division multiplexing, (2) frequency division multiplexing, and (3) code division multiplexing. These 3 methods were evaluated in healthy volunteers for scattering sensitivity and image artefacts. Conclusion: The S‐matrix can be measured during image excitation to generate real‐ time cardiac and respiratory motion signals for prospective gating. KEYWORDS cardiac self‐gating, cardiovascular magnetic resonance, dual gating, parallel transmit, respiratory motion, ultra‐high field MRI
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