Virtual population‐based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B₁+ uniformity

Abstract

Purpose: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B 1 þ uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. Methods: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. Results: Two-port RF shimming, compared to circular polarization mode, can increase the B 1 þ uniformity on average by þ32%. Worst-case SAR excitations increase the local RF power deposition on average by þ39%. In the first level controlled operating mode, induced peak temperatures reach 42.5 C and 45.6 C in patients with normal and impaired thermoregulation, respectively. Conclusion: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. V C 2015 Wiley Periodicals, Inc.

In vitro and in silico assessment of RF‐induced heating around intracranial aneurysm clips at 7 Tesla

“…However, this solver was not used for the conservative exposure scenario because it results in lower tissue temperature if small vessels in the head are taken into account. Third, Murbach et al showed that constant perfusion, which was applied in the present study, leads to conservative values. In addition, the most conservative tissue perfusion found in the The Foundation for Research on Information Technologies in Society (IT′IS) database was used.…”

Simple modification of arm position improves B1+ and signal homogeneity in the thoracolumbar spine at 3T

“…Murbach et al reported that the arms were one of the hotspot locations of the local specific absorption rate (SAR) and temperature using a two-port body transmission coil in 3T abdominal imaging. 12 Arms are generally positioned so as to rest on the bed, next to the patient's sides, for thoracic or lumber spine imaging, and the transmission of RF may be disturbed by the arms when the RF ports are in oblique directions. When the RF ports are located immediately under the arms on the bed, we considered that the RF waves are mostly absorbed by the arms, which may be one of the causes for the B 1 1 inhomogeneity near the vertebra.…”

Intersubject specific absorption rate variability analysis through construction of 23 realistic body models for prostate imaging at 7T

“…For pTx systems, the RF safety assessment becomes more challenging. The local SAR distributions depend on the degrees of freedom that the parallel transmit system has introduced in addition to the already existing intersubject variation . The combined additional safety margins can result in prolonged scan times or less pulse performance.…”

“…The local SAR distributions depend on the degrees of freedom that the parallel transmit system has introduced in addition to the already existing intersubject variation. 10 The combined additional safety margins can result in prolonged scan times or less pulse performance. This goes in particular for prostate imaging at 7T, 6 where the additional safety margins result in more conservative power thresholds.…”