Atrial fibrillation cardiac radioablation target visibility on magnetic resonance imaging

Abstract

Magnetic resonance imaging (MRI) guided cardiac radioablation (CR) for atrial fibrillation (AF) is a promising treatment concept. However, the visibility of AF CR targets on MRI acquisitions requires further exploration and MRI sequence and parameter optimization has not yet been performed for this application. This pilot study explores the feasibility of MRI-guided tracking of AF CR targets by evaluating AF CR target visualization on human participants using a selection of 3D and 2D MRI sequences.MRI datasets were acquired in healthy and AF participants using a range of MRI sequences and parameters. MRI acquisition categories included 3D free-breathing acquisitions (3D_acq), 2D breath-hold ECG-gated acquisitions (2D_ECG-gated), stacks of 2D breath-hold ECG-gated acquisitions which were retrospectively interpolated to 3D datasets (3D_interp), and 2D breath-hold ungated acquisitions (2D_real-time). The ease of target delineation and the presence of artifacts were qualitatively analyzed. Image quality was quantitatively analyzed using signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and non-uniformity. Confident 3D target delineation was achievable on all 3D_interp datasets but was not possible on any of the 3D_acq datasets. Fewer artifacts and significantly better SNR, CNR and non-uniformity metrics were observed with 3D_interp compared to 3D_acq. 2D_real-time datasets had slightly lower SNR and CNR than 2D_ECG-gated and 3D_{interp n} datasets. AF CR target visualization on MRI was qualitatively and quantitatively evaluated. The study findings indicate that AF CR target visualization is achievable despite the imaging challenges associated with these targets, warranting further investigation into MRI-guided AF CR treatments.

Keywords: Atrial fibrillation; Cardiac; Cardiac radioablation; MRI; MRI-guided; Motion management.

Fig. 1

Illustration of overall study method. CNR contrast-to-noise ratio, ROIs region of interest, SNR signal-to-noise ratio

Fig. 2

Exemplar illustration of the diameter size and location of ROIs used to quantitatively calculate image quality metrics. Background (pink), lung (red), left atria (yellow), right pulmonary vein region near the target (green), left pulmonary vein region near the target (blue) ROIs were used

Fig. 3

Exemplar images to illustrate the differing image quality and presence of artifacts between MRI acquisition categories and datasets. The best and worst scoring 3D acquisition MRI datasets (3D_acq) in terms of qualitative score and quantitative analysis are compared to 2D acquisitions. 2D balanced steady-state free-precision sequence (TrueFISP) and gradient echo fast low angle shot (FLASH) dataset comparisons are shown to illustrate the increased presence of artifacts with TrueFISP. Near real-time 2D acquisition without ECG-gating (2D_real-time) datasets had lower signal-to-noise (SNR) and contrast-to-noise (CNR) compared to cardiac-gated datasets (2D TrueFISP & 2D FLASH) and this image quality reduction can be visually observed in these images. Images are for healthy participants rather than atrial fibrillation (AF) participants unless otherwise indicated. SNR and CNR were slightly lower in datasets acquired from AF participants compared to healthy participants. Artifacts detected included motion artifacts, flow artifacts, and signal voids. Arrows indicate the approximate target region

Fig. 4

Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) between the evaluated ROI(s) and lung ROI, and non-uniformity image quality metrics for 3D_acquisition datasets. Results are given for the average of the left and right PV ROIs (a) and for the left atria (b)

Fig. 5

Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) between the evaluated ROI(s) and lung ROI, and non-uniformity image quality metrics for 2D_ECG-gated datasets. Results are given for the average of the left and right PV ROIs (a) and for the left atria (b). The bars and error bars represent the average and standard deviation of the image quality metric throughout the cardiac cycle respectively

Fig. 6

Variation of average contrast-to-noise ratio (CNR) between the two PV ROIs and surrounding lung ROI a throughout the cardiac cycle for balanced steady-state free-precision sequence (TrueFISP) in blue and gradient echo fast low angle shot sequence (FLASH) in orange for participant Vol03 and b with time for an exemplar 2D_real-time dataset. The larger variation in CNR in TrueFISP compared to FLASH observed in a and the periodic variation observed in b loosely corresponding to heart-rate were typical of what was observed in other participants