Calibration of RF transmitter voltages for hyperpolarized gas MRI

MRI with hyperpolarized gases, ³He, ¹²⁹Xe, ¹³C, and others, has the potential to become an important diagnostic technique for clinical imaging. Due to the nonreversible loss of magnetization in hyperpolarized gas imaging, the choice of the flip angle is a major factor that influences the signal intensity, and hence, the signal-to-noise ratio. Conventional automated radiofrequency (RF) calibration procedures for ¹H imaging are not suitable for hyperpolarized gas imaging. Herein, we have demonstrated a simple procedure for RF calibration for magnetic resonance imaging (MRI) with hyperpolarized gases that is easily adaptable to clinical settings. We have demonstrated that there exists a linear relationship between the RF transmitter voltages required to obtain the same nutation angle for protons (V^1H) and hyperpolarized gas nuclei (V^3He). For our ¹H and ³He coils we found that V^3He = 1.937 V^1H with correlation coefficient r² = 0.97. This calibration can be done as a one-time procedure during the routine quality assurance (QA) protocol. The proposed procedure was found to be extremely robust in routine scanning and provided an efficient method to achieve a desired flip angle, thus allowing optimum image quality.