Abstract
MRI of hyperpolarized media, such as 129Xe and 3He, shows great potential for clinical applications. The optimal use of the available spin polarization requires accurate flip angle calibrations and T1 measurements. Traditional flip angle calibration methods are time-consuming and suffer from polarization losses during T1 relaxation. In this paper, we propose a method to simultaneously calibrate flip angles and measure T1 in vivo during a breath-hold time of less than 4 seconds. We demonstrate the accuracy, robustness and repeatability of this method and contrast it with traditional methods. By measuring the T1 of hyperpolarized gas, the oxygen pressure in vivo can be calibrated during the same breath hold. The results of the calibration have been applied in variable flip angle (VFA) scheme to obtain a stable steady-state transverse magnetization. Coupled with this method, the ultra-short TE (UTE) and constant VFA (CVFA) schemes are expected to give rise to new applications of hyperpolarized media.
Highlights
MRI of hyperpolarized media, such as 129Xe and 3He, shows great potential for clinical applications
The Spatial Modulation of Magnetization (SPAMM) method is not sensitive to flip angles smaller than 45° and has limited signal-to-noise ratio (SNR) due to the need to acquire two images separated by a so-called “SPAMM preparation” period done within a single breath-hold
The results demonstrate that the single-breath method described above yields repeatable values of both the angle calibration and T1 measurement
Summary
MRI of hyperpolarized media, such as 129Xe and 3He, shows great potential for clinical applications. The results of the calibration have been applied in variable flip angle (VFA) scheme to obtain a stable steady-state transverse magnetization Coupled with this method, the ultra-short TE (UTE) and constant VFA (CVFA) schemes are expected to give rise to new applications of hyperpolarized media. By measuring the spin-lattice relaxation time (T1) of the gas, the partial oxygen pressure can be determined[6,7,8], which is an important parameter related to the efficiency of gas exchange in the lung These pre-clinical studies illustrate the potential of hyperpolarized gas MRI for clinical applications. The Bloch–Siegert shift method calculates flip angles via the accumulated phase shift during an off-resonance RF pulse, showing good robustness in different situations It is limited by the long Fermi pulse between on-resonance excitation and signal acquisition. Most previous studies neglected or avoided the effects of T1 relaxation while calibrating flip angles[18,19,21,22]
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