Volumetric real-time imaging for interventional CMR at 0.55T (RF_TH_239)

Background: Interventional cardiovascular MRI (iCMR) uses rapid high-contrast imaging to guide invasive procedures [1]. New 0.55T MRI systems offer reduced heating of metallic devices and provide superior image quality for real-time dynamic imaging due to reduced off-resonance effects [2]. Typically, rapid multi-planar 2D imaging (3-10 slices per second) is used to provide valuable real-time feedback to interventionists during procedures. These scans require a skilled MRI technician rapidly switching between pre-planned scan planes. Here, we propose the acquisition and reconstruction of the entire 3D volume in real-time to improve workflow and coverage for iCMR procedures.

Methods: Images were acquired in a sagittal plane with an isotropic 3.5 mm resolution using stack-of-spirals sampling, bSSFP contrast, and rate-2 kz acceleration (FOV 392 x 392 x 126mm, 1144ms per volume). Experiments were conducted using a 0.55 T system (investigational MAGNETOM Free.Max, Siemens Healthineers, Erlangen, Germany) equipped with high-performance gradients (33 mT/m amplitude, 160 T/m/s slew rate ). Nine healthy volunteers were imaged with approval from our Institutional Review Board after providing written informed consent.

Low-latency online reconstruction used a GPU-based gridding operation with the Gadgetron reconstruction framework [3]. A 2-minute pre-scan was used for calculation of coil sensitivity maps and density compensation factors [4, 5] to minimize online reconstruction computation. The central nine slices were displayed to the operator in a montage view for procedural guidance.

Results: Figure 1 illustrates the reconstruction pipeline and latency measurements . Image reconstruction required 217 ± 18ms per volume and therefore introduced minimal latency in image display to the interventionist. Figure 2 contains an example online reconstruction montage of 9 out of 36 sagittal images, which capture the subject’s respiratory motion, as well as resliced cardiac views. These images provide sufficient resolution and contrast for procedural guidance and have minor residual aliasing artifacts. Real-time volumetric imaging was feasible in all nine volunteers.

Conclusion: We demonstrate real-time 3D isotropic image acquisition and reconstruction for iCMR, achieving a frame rate of ~1.1 volume/s and minimal reconstruction latency of 217 ms. The frame rate is competitive with current best practices of 3 intersecting slices per second [2]. The ability to re-slice isotropic images is particularly useful for interventional procedures, where following a balloon-tip catheter is crucial. Future work will include testing with interventional devices, higher spatial resolution, and improved reconstructions [6].