Confirming the existence of five peaks in 129Xe rat head spectra.

A series of experiments were performed to investigate why two peaks (D and E) of the five dissolved phase peaks in hyperpolarized (129)Xe rat head spectra appeared inconsistently in previous work. Specifically, spectra were acquired under conditions of various shim states, anaesthetics, and arterial ligation. The shimming experiments showed that slice-shimming can be used to improve resolution of the dissolved phase peaks, but even so, subtle changes in the shim state that may dramatically alter the shape of peak E remain poorly understood. Also, the inability to shim gas spaces and tissue simultaneously may explain why inconsistent chemical shift values have been reported in the literature. A possible solution for this problem is suggested. The results of pre- and postligation spectra from the same animal indicated that two peaks (A and E) originate from brain. Changing the anaesthetic was found to have no effect on the number of dissolved peaks in xenon spectra.

A method for measuring the decay time of hyperpolarized 129Xe magnetization in rat brain without estimation of RF flip angles.

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.

129Xe spectra from the heads of rats with and without ligation of the external carotid and pterygopalatine arteries.

After rats inhaled hyperpolarized (129)Xe gas, in vivo spectra from their heads revealed a dominant peak around 195 ppm, another easily resolvable peak near 189 ppm, a broad peak around 210 ppm, and two minor peaks around 198 ppm and 192 ppm. However, the source of each peak remains controversial. To further study the origin of each peak, we compared spectra obtained from the heads of normal rats with spectra taken from the heads of rats that had undergone ligation of the external carotid (ECA) and pterygopalatine (PPA) arteries, the major feeding vessels of nonbrain tissue in the rat head. The amplitude of the peak at around 189 ppm was greatly reduced in the ECA/PPA-ligated rats, while the peak around 195 ppm persisted. We conclude that the signal that originates from the rat brain after inhalation of (129)Xe gas is overwhelmingly dominated by the single resonance at 195 ppm.

Magnetic resonance spectra of hyperpolarized (129)xe in human blood and living rat chest.

We constructed a gas polarization system to test the feasibility of using hyperpolarized (129)Xe gas as an NMR (nuclear magnetic resonance) probe to explore brain function. Both in vitro and in vivo experiments were performed with a 4.7 T NMR spectrometer. Xenon spectra from human blood confirmed the existence of two peaks corresponding to red blood cells and plasma. In rat studies, three peaks at around 200 ppm were observed. Our results are consistent with previously reported data.