Vendor-agnostic, open-source 4D flow

Motivation: The need for a vendor-independent 4D flow MRI sequence to aid in multi-center based investigation of rare diseases. Goal(s): To develop and validate both an open-source 4D flow MRI sequence and a gradient-probing sequence to map gradient directions to physical coordinates. Approach: A photoplethysmogram-triggered 4D flow sequence and a gradient probing sequence (to map gradient direction to physical coordinates) were implemented in Pypulseq. Both were tested on a 3T Siemens Prisma, with in vivo validation on two healthy subjects. Results: The 4D flow sequence's z-axis velocity measurements in the popliteal artery matched a vendor-provided sequence, validating its potential for muscle stimulation studies. Impact: This work provides open-source, vendor-agnostic implementations of 4D flow and gradient probing sequences. The latter enables mapping of gradient directionality, crucial to accurately interpret results. Together, they represent a building block towards a reproducible dynamic imaging protocol for skeletal muscles.