Experimental approaches to image localized human³¹P NMR spectroscopy

Abstract

Experimental procedures for obtaining localized 31P NMR spectra of humans by means of the ISIS sequence are discussed in detail. The technique is optimized for use with volume coils and with surface coils in order to measure localized 31P NMR spectra of different tissues and organs. Selective frequency-modulated (FM) inversion and excitation pulses are applied for optimal inversion or excitation despite B1 inhomogeneity. Pulse imperfection may lead to spurious signal contributions from outside the selected volume; this contamination is reduced by using long pulse intervals, by properly ordering the ISIS acquisitions, and by using FM excitation pulses. Simultaneous measurement of multiple volumes was implemented by including an additional selective inversion pulse, and an extension of the ISIS addition/subtraction scheme. Localized T1 measurements with surface coils are implemented by using a B1-insensitive inversion pulse in the inversion recovery sequence. The quantitative reproducibility of localized 31P NMR spectra was verified. Absolute metabolite concentration can be determined after a suitable calibration of the 31P NMR spectrum. Localized shimming is required to obtain localized 31P NMR spectra of excellent spectral resolution. This is done by monitoring the 1H NMR signal from water by a single-shot localization technique. The techniques discussed can be applied to obtain spectra of brain, liver, heart, and other organs. 31P NMR spectra of intracranial tumors demonstrate its applicability in the examination of patients.

Selective maximization of 31P MR spectroscopic signals of in vivo human brain metabolites at 3T

“…[5] we can deduce that the flip angle ␣, which maximizes the signal, increases as T 1 increases, such that as T 1 3 inf, ␣ 3 180°, and decreases as TR increases, such that as TR 3 inf, ␣ 3 90°. One should note that though similar in appearance to the classic Ernst angle expression (9), Eq.…”

“…The optimal flip angle required to maximize the signal of each brain metabolite depends on the TR of the sequence and the T 1 of the respective metabolite, as given by Eq. [5]. As such, the LFAs needed to maximize all of the human brain metabolites suggest long T 1 relaxation times (i.e., T 1 relaxation times equal to or greater than the 2-second-TR).…”

“…Estimates of T 1 relaxation times were calculated from Eq. [5] based on the flip angles that maximized PCr, PE, PC, P i , GPE, and GPC, which were 140°, 150°, 120°, 120°, 130°, and 150°, respectively, and 120°for ␣-, ␤-, ␥-ATP. The approximate T 1 values for these metabolites are on the order of 7.5, 13.9, 2.9, 2.9, 4.5, and 13.9 seconds for PCr, PE, PC, P i , GPE, GPC, respectively, and 2.9 seconds for ␣-, ␤-, ␥-ATP.…”

“…However, the Cramer-Rao lower bounds, which actually reach a minimum at the optimal flip angle necessary to maximize each metabolite peak, suggest that the ability to estimate the spectral peak area and gain was not adversely affected by flip angle error resulting from increasing RF power. Instead, this observation of data spread as flip angle increased, as observed with the PCr peak, could be better explained by the Ϯ20% variability in volunteer T 1 relaxation times observed in multiple in vivo studies (5)(6)(7). Such T 1 variability would cause the differential response observed; at the 2s-TR used, larger gains would occur for those volunteers with longer PCr T 1 relaxation times, with maximal gain at larger optimal flip angles.…”

“…The gain of a specific peak is maximized with a unique flip angle, but it is possible to determine an optimal flip angle ␣ opt that maximizes multiple peaks over a relevant range from Eq. [5]. As such, a linear optimization function with a T 1 weighting factor for each of the peaks of interest could be represented as:…”