[Show abstract][Hide abstract] 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.
Magnetic Resonance in Medicine 02/2011; 65(2):399-408. DOI:10.1002/mrm.22638 · 3.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A novel method is presented for the three-dimensional mapping of the B(1) -field of a transmit radio-frequency MR coil. The method is based on the acquisition of phase images, where the effective flip angle is encoded in the phase of the nonselective hard pulse excitation. The method involves the application of a rectangular composite pulse as excitation in a three-dimensional gradient recall echo to produce measurable phase angle variation. However, such a pulse may significantly increase the radio-frequency power deposition in excess of the standard acceptable SAR limits, imposing extremely long TRs (>100 msec), which would result in acquisition times significantly greater than a single breath-hold. In this study, the phases of the radio-frequency excitation are modified, resulting in a different pulse sequence scheme. It is shown that the new method increases sensitivity with respect to radio-frequency inhomogeneities by up to 10 times, and reduces the total duration of the pulse so that three-dimensional B(1) mapping is possible with (3) He in lungs within a single breath-hold. Computer simulations demonstrate the increase in sensitivity. Phantom results with (1) H MRI are used for validation. In vivo results are presented with hyperpolarized (3) He in human lungs at 1.5T.
Magnetic Resonance in Medicine 04/2011; 65(4):1166-72. DOI:10.1002/mrm.22683 · 3.57 Impact Factor
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