Accuracy and precision of electrical permittivity mapping at 3T: the impact of three B1+ mapping techniques

Purpose: To investigate the sequence-specific impact of B-1(+) amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region. Methods: B-1(+) maps obtained with actual flip angle imaging (AFI), Bloch Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these B-1(+) maps with Helmholtz -based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2 compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer's pelvis. Results: Accuracy was evaluated with noiseless simulations on the phantom. The maximum B-1(+) bias relative to the true B-1(+) distribution was 1% for AF1 and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity' of 7% to 20% for AFT and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in B-1(+) of 0.2% of the B-1(+) average value. As regards B-1(+) precision, in vivo and phantom -measurements were comparable. Conclusions: Our simulation framework quantitatively predicts the different impact of B-1(+) mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence-specitic bias and noise perturbation in the B-1(+) map. These findings arc supported by the experimental results.