Charles McGrath

All Publications

• Referenceless 4D flow MRI using radial balanced SSFP at 0.6 T MAGNETIC RESONANCE IN MEDICINE Mcgrath, C., Dirix, P., Vousten, V., Smink, J., Ercan, E., Boernert, P., Kozerke, S. 2025; 94 (2): 625-639

  Abstract

  To implement four-dimensional-flow MRI using phase-contrast balanced steady-state free precession (bSSFP) at 0.6 T using a free-running three-dimensional (3D) radial trajectory and referenceless background phase correction.A free-running, wobbling Archimedean spiral approach including bipolar velocity-encoding gradients (3D PC-bSSFP) was implemented on a 0.6T prototype scanner. Bipolar rewinder gradients were added to ensure first-moment nulling per repetition time. Velocity encoding was performed using a three-point encoding scheme (i.e., omitting a reference measurement). Advanced computer simulations were carried out to validate the approach. Image reconstruction was performed using a locally low-rank approach. Results for anatomical visualization and flow quantification were reconstructed separately with different regularization factors. Background phase correction was achieved using phase estimation on time-averaged reconstructions. In vivo data were acquired in 6 healthy subjects during free breathing. Additional two-dimensional (2D) phase-contrast spoiled gradient-echo (2D PC-GRE) breath-hold data were obtained for reference to compare flow values in the ascending aorta, descending aorta, and pulmonary trunk.Velocity data acquired with 3D PC-bSSFP compared well with 2D PC-GRE (root mean square error = 3.96 cm/s), with minor underestimation of velocities (-0.52 cm/s). Cardiac phase-dependent signal-to-noise ratios normalized for differences in scan time and resolution between 3D PC-bSSFP and 2D PC-GRE demonstrate relatively steady values for 3D PC-bSSFP when compared to 2D PC-bSSFP with some reduction during phases of high flow.Free-running, referenceless, four-dimensional-flow MRI using radial 3D PC-bSSFP is feasible on a lower-field 0.6T system, producing adequate flow quantification while yielding simultaneously reasonable cine images for concurrent flow and functional assessment of the heart and great vessels.

  View details for DOI 10.1002/mrm.30503

  View details for Web of Science ID 001447521300001

  View details for PubMedID 40106793

• CMRsim-A python package for cardiovascular MR simulations incorporating complex motion and flow. Magnetic resonance in medicine Weine, J., McGrath, C., Dirix, P., Buoso, S., Kozerke, S. 2024; 91 (6): 2621-2637

  Abstract

  To present an open-source MR simulation framework that facilitates the incorporation of complex motion and flow for studying cardiovascular MR (CMR) acquisition and reconstruction.CMRsim is a Python package that allows simulation of CMR images using dynamic digital phantoms with complex motion as input. Two simulation paradigms are available, namely, numerical and analytical solutions to the Bloch equations, using a common motion representation. Competitive simulation speeds are achieved using TensorFlow for GPU acceleration. To demonstrate the capability of the package, one introductory and two advanced CMR simulation experiments are presented. The latter showcase phase-contrast imaging of turbulent flow downstream of a stenotic section and cardiac diffusion tensor imaging on a contracting left ventricle. Additionally, extensive documentation and example resources are provided.The Bloch simulation with turbulent flow using approximately 1.5 million particles and a sequence duration of 710 ms for each of the seven different velocity encodings took a total of 29 min on a NVIDIA Titan RTX GPU. The results show characteristic phase contrast and magnitude modulation present in real data. The analytical simulation of cardiac diffusion tensor imaging with bulk-motion phase sensitivity took approximately 10 s per diffusion-weighted image, including preparation and loading steps. The results exhibit the expected alteration of diffusion metrics due to strain.CMRsim is the first simulation framework that allows one to feasibly incorporate complex motion, including turbulent flow, to systematically study advanced CMR acquisition and reconstruction approaches. The open-source package features modularity and transparency, facilitating maintainability and extensibility in support of reproducible research.

  View details for DOI 10.1002/mrm.30010

  View details for PubMedID 38234037

• Self-gated cine phase-contrast balanced SSFP flow quantification at 0.55 T. Magnetic resonance in medicine McGrath, C., Bieri, O., Kozerke, S., Bauman, G. 2024; 91 (1): 174-189

  Abstract

  To implement cine phase-contrast balanced SSFP (PC-bSSFP) for low-field 0.55T cardiac MRI by exploiting the intrinsic flow sensitivity of the bSSFP slice-select gradient and the in-plane phase-cancelation properties of radial trajectories, enabling self-gated and referenceless PC-bSSFP flow quantification at 0.55 T.A free-running, tiny golden-angle radial PC-bSSFP approach was implemented on 0.55T and 1.5T systems. Cardiac and respiratory self-gating was incorporated to enable electrocardiogram-free scanning during breath-hold and free-breathing. By exploiting the intrinsic in-plane phase-cancelation properties of radial acquisitions and background phase fitting, referenceless single-point PC-bSSFP was realized. In vivo data were acquired in the ascending aorta of healthy subjects at 0.55 T and 1.5 T during breath-hold and free-breathing. Flow data, SNR, and velocity-to-noise ratio were compared relative to data obtained with phase-contrast spoiled gradient-echo variants.Velocities acquired with PC-bSSFP compared well with data from phase-contrast spoiled gradient-echo (RMSEv  = 5.8 cm/s). PC-bSSFP at 0.55 T resulted in high-quality cine magnitude images and phase maps with sufficient SNR and velocity-to-noise ratio. Breath-hold and free-breathing PC-bSSFP performed very similarly, with comparable flow quantification (RMSEv  = 5.7 cm/s). Referenceless single-point PC-bSSFP results agreed well with two-point PC-bSSFP (-1.8 ± 5.2 cm/s) while reducing scan times 2-fold.PC-bSSFP is feasible on low-field 0.55T systems, producing high-quality cine images while permitting simultaneous aortic flow measurements during breath-hold and free-breathing and without the need for electrocardiogram gating.

  View details for DOI 10.1002/mrm.29837

  View details for PubMedID 37668108

• Ramping down a clinical 3 T scanner: a journey into MRI and MRS at 0.75 T. Magma (New York, N.Y.) Guenthner, C., Peereboom, S. M., Dillinger, H., McGrath, C., Albannay, M. M., Vishnevskiy, V., Fuetterer, M., Luechinger, R., Jenneskens, T., Sturzenegger, U., Overweg, J., Koken, P., Börnert, P., Kozerke, S. 2023; 36 (3): 355-373

  Abstract

  Lower-field MR is reemerging as a viable, potentially cost-effective alternative to high-field MR, thanks to advances in hardware, sequence design, and reconstruction over the past decades. Evaluation of lower field strengths, however, is limited by the availability of lower-field systems on the market and their considerable procurement costs. In this work, we demonstrate a low-cost, temporary alternative to purchasing a dedicated lower-field MR system.By ramping down an existing clinical 3 T MRI system to 0.75 T, proton signals can be acquired using repurposed 13C transmit/receive hardware and the multi-nuclei spectrometer interface. We describe the ramp-down procedure and necessary software and hardware changes to the system.Apart from presenting system characterization results, we show in vivo examples of cardiac cine imaging, abdominal two- and three-point Dixon-type water/fat separation, water/fat-separated MR Fingerprinting, and point-resolved spectroscopy. In addition, the ramp-down approach allows unique comparisons of, e.g., gradient fidelity of the same MR system operated at different field strengths using the same receive chain, gradient coils, and amplifiers.Ramping down an existing MR system may be seen as a viable alternative for lower-field MR research in groups that already own multi-nuclei hardware and can also serve as a testing platform for custom-made multi-nuclei transmit/receive coils.

  View details for DOI 10.1007/s10334-023-01089-9

  View details for PubMedID 37171689

  View details for PubMedCentralID PMC10386956

• Fundamentals of turbulent flow spectrum imaging. Magnetic resonance in medicine Dillinger, H., McGrath, C., Guenthner, C., Kozerke, S. 2022; 87 (3): 1231-1249

  Abstract

  To introduce a mathematical framework and in-silico validation of turbulent flow spectrum imaging (TFSI) of stenotic flow using phase-contrast MRI, evaluate systematic errors in quantitative turbulence parameter estimation, and propose a novel method for probing the Lagrangian velocity spectra of turbulent flows.The spectral response of velocity-encoding gradients is derived theoretically and linked to turbulence parameter estimation including the velocity autocorrelation function spectrum. Using a phase-contrast MRI simulation framework, the encoding properties of bipolar gradient waveforms with identical first gradient moments but different duration are investigated on turbulent flow data of defined characteristics as derived from computational fluid dynamics. Based on theoretical insights, an approach using velocity-compensated gradient waveforms is proposed to specifically probe desired ranges of the velocity autocorrelation function spectrum with increased accuracy.Practical velocity-encoding gradients exhibit limited encoding power of typical turbulent flow spectra, resulting in up to 50% systematic underestimation of intravoxel SD values. Depending on the turbulence level in fluids, the error due to a single encoding gradient spectral response can vary by 20%. When using tailored velocity-compensated gradients, improved quantification of the Lagrangian velocity spectrum on a voxel-by-voxel basis is achieved and used for quantitative correction of intravoxel SD values estimated with velocity-encoding gradients.To address systematic underestimation of turbulence parameters using bipolar velocity-encoding gradients in phase-contrast MRI of stenotic flows with short correlation times, tailored velocity-compensated gradients are proposed to improve quantitative mapping of turbulent blood flow characteristics.

  View details for DOI 10.1002/mrm.29001

  View details for PubMedID 34786764

  View details for PubMedCentralID PMC9299145