Field camera versus phantom-based measurement of the gradient system transfer function (GSTF) with dwell time compensation.

PURPOSE: The gradient system transfer function (GSTF) can be used to describe the dynamic gradient system and applied for trajectory correction in non-Cartesian MRI. This study compares the field camera and the phantom-based methods to measure the GSTF and implements a compensation for the difference in measurement dwell time. METHODS: The self-term GSTFs of a MR system were determined with two approaches: 1) using a dynamic field camera and 2) using a spherical phantom-based measurement with standard MR hardware. The phantom-based GSTF was convolved with a box function to compensate for the dwell time dependence of the measurement. The field camera and phantom-based GSTFs were used for trajectory prediction during retrospective image reconstruction of 3D wave-CAIPI phantom images. RESULTS: Differences in the GSTF magnitude response were observed between the two measurement methods. For the wave-CAIPI sequence, this led to deviations in the GSTF predicted trajectories of 4% compared to measured trajectories, and residual distortions in the reconstructed phantom images generated with the phantom-based GSTF. Following dwell-time compensation, deviations in the GSTF magnitudes, GSTF-predicted trajectories, and resulting image artifacts were eliminated (< 0.5% deviation in trajectories). CONCLUSION: With dwell time compensation, both the field camera and the phantom-based GSTF self-terms show negligible deviations and lead to strong artifact reduction when they are used for trajectory correction in image reconstruction.

A cardiovascular magnetic resonance (CMR) safe metal braided catheter design for interventional CMR at 1.5 T: freedom from radiofrequency induced heating and preserved mechanical performance.

BACKGROUND: Catheter designs incorporating metallic braiding have high torque control and kink resistance compared with unbraided alternatives. However, metallic segments longer than a quarter wavelength (~ 12 cm for 1.5 T scanner) are prone to radiofrequency (RF) induced heating during cardiovascular magnetic resonance (CMR) catheterization. We designed a braid-reinforced catheter with interrupted metallic segments to mitigate RF-induced heating yet retain expected mechanical properties for CMR catheterization. METHODS: We constructed metal wire braided 6 Fr catheter shaft subassemblies using electrically insulated stainless-steel wires and off-the-shelf biocompatible polymers. The braiding was segmented, in-situ, using lasers to create non-resonant wire lengths. We compared the heating and mechanical performance of segmented- with un-segmented- metal braided catheter shaft subassemblies. RESULTS: The braiding segmentation procedure did not significantly alter the structural integrity of catheter subassemblies, torque response, push-ability, or kink resistance compared with non-segmented controls. Segmentation shortened the electrical length of individually insulated metallic braids, and therefore inhibited resonance during CMR RF excitation. RF-induced heating was reduced below 2 °C under expected use conditions in vitro. CONCLUSION: We describe a simple modification to the manufacture of metallic braided catheters that will allow CMR catheterization without RF-induced heating under contemporary scanning conditions at 1.5 T. The proposed segmentation pattern largely preserves braid structure and mechanical integrity while interrupting electrical resonance. This inexpensive design may be applicable to both diagnostic and interventional catheters and will help to enable a range of interventional procedures using real time CMR.

Hepatic arterial spin labelling MRI: an initial evaluation in mice.

The development of strategies to combat hepatic disease and augment tissue regeneration has created a need for methods to assess regional liver function. Liver perfusion imaging has the potential to fulfil this need, across a range of hepatic diseases, alongside the assessment of therapeutic response. In this study, the feasibility of hepatic arterial spin labelling (HASL) was assessed for the first time in mice at 9.4 T, its variability and repeatability were evaluated, and it was applied to a model of colorectal liver metastasis. Data were acquired using flow-sensitive alternating inversion recovery-arterial spin labelling (FAIR-ASL) with a Look-Locker readout, and analysed using retrospective respiratory gating and a T1 -based quantification. This study shows that preclinical HASL is feasible and exhibits good repeatability and reproducibility. Mean estimated liver perfusion was 2.2 ± 0.8 mL/g/min (mean ± standard error, n = 10), which agrees well with previous measurements using invasive approaches. Estimates of the variation gave a within-session coefficient of variation (CVWS) of 7%, a between-session coefficient of variation (CVBS) of 9% and a between-animal coefficient of variation (CVA) of 15%. The within-session Bland-Altman repeatability coefficient (RCWS) was 18% and the between-session repeatability coefficient (RCBS) was 29%. Finally, the HASL method was applied to a mouse model of liver metastasis, in which significantly lower mean perfusion (1.1 ± 0.5 mL/g/min, n = 6) was measured within the tumours, as seen by fluorescence histology. These data indicate that precise and accurate liver perfusion estimates can be achieved using ASL techniques, and provide a platform for future studies investigating hepatic perfusion in mouse models of disease.