Quantifying myocardial perfusion during MR-guided interventions without exogenous contrast agents: intra-arterial spin labeling.

Purpose: To test intra-arterial spin labeling (iASL) using active guiding catheters for myocardial perfusion measurements during magnetic resonance (MR)-guided interventions in a pig study.

Methods: In this work, a single-loop radiofrequency (RF) coil at the tip of a 6F active coronary catheter was used as a transmit coil for local spin labeling. The transmit magnetic RF field (B₁) of the coil and the labeling efficiency were determined, and iASL was tested in two pigs after the catheter was engaged in the aortic root, the ostium of the left coronary artery (LCA) under MR-guidance. The iASL effect was assessed by the signal difference between spin-labeling On and control (spin-labeling OFF) images, and in a cross-correlation between ON/Off states of spin-labeling a binary labeling paradigm. In addition, quantitative myocardial perfusion was calculated from the iASL experiments.

Results: The maximum B₁ in the vicinity of the catheter coil was 2.1 µT. A strong local labeling effect with a labeling efficiency of 0.45 was achieved with iASL both in vitro and in vivo. In both pigs, the proximal myocardial segments supplied by the LCA showed significant labelling effect up to distances of 60 mm from the aortic root with a relative signal difference of (3.14 ± 2.89)% in the first and (3.50 ± 1.25)% in the second animal. The mean correlation coefficients were R = 0.63 ± 0.22 and 0.42 ± 0.16, respectively. The corresponding computed myocardial perfusion values in this region of the myocardium were similar to those obtained with contrast perfusion methods ((1.2 ± 1.1) mL/min/g and (0.8 ± 0.6) mL/min/g).

Conclusion: The proposed iASL method demonstrates the feasibility of selective myocardial perfusion measurements during MR-guided coronary interventions, which with further technical improvements may provide an alternative to exogenous contrast-based perfusion. Due to the invasive nature of the iASL method, it can potentially be used in concert with MRI-guided coronary angioplasty.