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A method for measuring the decay time of hyperpolarized 129Xe magnetization in rat brain without estimation of RF
Atsushi Wakai1, Kazuhiro Nakamura, Jeff Kershaw
1Akita Research Institute of Brain and Blood Vessels, 6-10 Senshu-Kubota Machi, Akita 010-0874, Japan. wakai@akita-noken.go.jp
Purpose:
The decay time of hyperpolarized 129Xe in brain tissue depends on the cerebral blood flow (CBF) as well as the longitudinal relaxation time in the tissue (T(1,tissue)). Therefore, the decay time is an important parameter for investigating the potential of Xe for cerebral studies. Previous attempts to measure the decay time have been performed after correction of the MR signal for the costheta decay induced by multiple radiofrequency (RF) excitation pulses. However, since this method requires accurate knowledge of the RF pulse flip angle, the use of a surface coil is restricted because of its nonuniform RF power, distribution. We present a two-pulse protocol for estimating the decay time without the need for flip-angle estimation and demonstrate it in the rat brain.
Method:
After rat inhalation of hyperpolarized Xe, two MR spectra of the rat head were obtained at various delay times (4-16 s) and the logarithmic ratio of the two amplitudes was calculated. The decay time was obtained from the slope of the logarithmic ratio against the delay time. The MR measurements were performed with a 4.7T imaging spectrometer with a surface coil located over the head of the anesthetized rat. The gas (25 cc) was smoothly introduced to the lung for 40 s before each measurement began.
Result:
From 18 experiments on 11 rats, the decay time was estimated to be 17.7+/-1.9 s.
Discussion:
Assuming a normal rat CBF value, T(1,tissue) can be estimated from the decay time to be 26+/-4 s.
