The effect of transmit B₁ inhomogeneity on hyperpolarized [1‐¹³C]‐pyruvate metabolic MR imaging biomarkers

Abstract

A specialized Helmholtz-style 13 C volume transmit "clamshell" coil is currently being utilized for 13 C excitation in pre-clinical and clinical hyperpolarized 13 C MRI studies aimed at probing the metabolic activity of tumors in various target anatomy. Due to the widespread use of this 13 C clamshell coil design, it is important that the effects of the 13 C clamshell coil B 1 + profile on HP signal evolution and quantification are well understood. The goal of this study was to | 4901 THE EFFECT OF TRANSMIT B

Model‐constrained reconstruction accelerated with Fourier‐based undersampling for hyperpolarized [1‐13C] pyruvate imaging

“…In addition to noise and PK model accuracy, MoReFUn‐reconstructed images may be impaired by systematic errors, such as B 1 miscalibration, 44 inhomogeneities, 28 and excitation schemes 29 . The related artifacts and distortion will have a similar negatively effect on MoReFUn as on full acquisition.…”

Current methods for hyperpolarized [1‐13C]pyruvate MRI human studies

“…Many human studies have used Helmholtz pair resonators for transmit, including the “clamshell coil,” which has a notably inhomogeneous B 1 + profile, but has been used because of relatively easy integration into the scanner bore. B 1 + variation results in variations in the flip angles that control the use of the HP magnetization and creates errors in common HP metrics 9,76 . The exception are head coils, where birdcage designs with highly homogeneous B 1 + can be placed around the head while easily fitting inside the bore.…”

“…For scan prescriptions, the FOV is designed to provide full prostate coverage and typically to match the orientation of the anatomic imaging used for registration. Flip angles used in current studies are constant through time, as quantification with a variable‐through‐time flip scheme is highly sensitive to bolus timing 8 and errors in the RF transmit (B 1 + ) field 76 …”

“…For HP 13 C imaging studies in humans, RF coils for both 1 H and 13 C nuclei are needed, with 1 H MRI providing an anatomical reference for registration and optional additional multiparametric MRI readouts. At the Larmor frequency of 13 C nuclei, the relative contributions from coil noise compared to sample noise increase compared to 1 H, 75,76 although sample noise still is likely the dominant contributor for human-sized coils at 32.1 MHz-the resonance frequency of 13 C nuclei at 3 T.…”

“…B 1 + variation results in variations in the flip angles that control the use of the HP magnetization and creates errors in common HP metrics. 9,76 The exception are head coils, where birdcage designs with highly homogeneous B 1 + can be placed around the head while easily fitting inside the bore.…”