Flip‐angle measurement by magnetization inversion: Calibration of magnetization nutation angle in hyperpolarized ³He magnetic resonance imaging lung experiments

Abstract

The aim of this work was to establish a new, fast, and robust method of flip-angle calibration for magnetic resonance imaging of hyperpolarized 3 He. The method called flip-angle measurement with magnetization inversion is based on acquiring images from periodically inverted longitudinal magnetization created using the spatial modulation of magnetization technique. By measuring the width of the area where the magnetization was inverted by the spatial modulation of magnetization preparation in phase images, the flip angle can be generated using a simple equation. To validate and establish the limits of the proposed method, flip-angle measurement with magnetization inversion acquisitions were simulated and applied to proton and hyperpolarized 3 He phantoms. Then, the calibration procedure was applied during hyperpolarized 3 He magnetic resonance imaging in a healthy volunteer. The advantage of the flip-angle measurement with magnetization inversion method compared with the conventional method based on the assessment of radiofrequency-decay is that it is free of errors induced by relaxation due to oxygen, by imperfect excitation slice profile and by any diffusion of 3 He into and out of the slice. Another advantage is that it does not require image processing with external software and therefore can be performed using the implemented tools on the magnetic resonance workstation. Magn Reson Med 65:399-408,

Transmit gain calibration for nonproton MR using the Bloch–Siegert shift

“…TG calibration becomes somewhat more involved when imaging nonproton nuclei, such as 13 C, 3 He or 129 Xe 1–7. The signal from natural‐abundance, thermally polarised spins is typically not detectable.…”

Fast Determination of Flip Angle and T1 in Hyperpolarized Gas MRI During a Single Breath-Hold

“…Traditional flip angle calibration methods in 1 H NMR experiments, such as the “Spin Echo-Stimulated Echo (SE-STE)” method 13 implemented on commercial NMR systems, are not suitable for hyperpolarized MRI, due to the rapid and non-optimal consumption of polarization by numerous big-angle RF pulses. Special calibration procedures adapted for hyperpolarized MRI have been proposed based on phase methods 14 15 16 17 such as the Bloch–Siegert shift 14 15 and Spatial Modulation of Magnetization (SPAMM) 16 . 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.…”

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

“…A series of RF excitation pulses or spoiled gradient echo images with constant power can be used to measure the B 1 calibration or generate B 1 maps from the signal variation with number of excitations [114,115]. This is similar to the variable power method, except that it requires approximate initial estimate of the B 1 calibration and T 1 relaxation time, but uses only small flip angles.…”

“…The RF pulses have flip angle above 45°, so that magnetization is inverted (more than 90° effective flip angle) after both pulses, in a band where the gradient field is weakest. The flip angle of the RF pulses can then be determined from the width of the inverted band of magnetization, and is unaffected by oxygen-related relaxation, imperfect slice profiles, or diffusion [114].…”