PCr/ATP ratio mapping of the human head by simultaneously imaging of multiple spectral peaks with interleaved excitations and flexible twisted projection imaging readout trajectories at 9.4 T

Abstract

Quantitative (31)P magnetic resonance imaging of the whole human brain is often time-consuming even at low spatial resolution due to the low concentrations, long T(1) relaxation times, and low detection sensitivity of phosphorus metabolites. We report herein the results of combining the increased detection sensitivity of an ultra-high field 9.4 T scanner designed for human imaging with a new pulse sequence termed simultaneously imaging of multiple spectral peaks with interleaved excitations and flexible twisted projection imaging readout trajectories to rapidly sample multiple resonances in the (31)P spectrum. The phosphocreatine and γ-adenosine triphosphate images, obtained simultaneously from the entire human head, are demonstrated at 1.5 cm isotropic nominal resolution in a total acquisition time of 33 min. The phosphocreatine/γ-adenosine triphosphate ratio calculated for brain parenchyma (1-2) and the superficial temporalis muscle (3-5) are in agreement with literature values.

Figure 1

(a) Data acquisition scheme for the SIMPLE-TPI sequence. In each TR, signal from γ-ATP is collected twice while that from PCr is acquired once. The same readout gradient waveforms are played out for all three excitations in each TR. Typical gradient waveforms are shown for the dual-echo readout (an FID and an echo). (b) Data acquisition scheme for the SIMPLE-RARE sequence. In each TR, one complete slice-encoding step is collected for PCr with excitation. One complete slice-encoding step is collected for ATP with two excitations, each sampling half of the plane in k-space.

Figure 2

Phantom images obtained with (a,b) the SIMPLE-TPI sequence in 16.5 minutes and (c, d) the SIMPLE-RARE sequence in 17 minutes show clean separation between the solution (8mM inorganic phosphate) in the sphere (16cm diameter) (a, c) and solution in the three tubes filled with different concentrations of aqueous inorganic phosphate solutions (3, 6, 9 mM) (b, d).

Figure 3

(a) A ³¹P spectrum (real part) of the whole human head collected with a 0.2ms hard RF pulse. (b) A ³¹P spectrum (real part) of the whole human head collected with a 7.2ms minimal phase spectrally selective RF pulse with a pass band of 380Hz centered at the PCr resonance frequency. (c) A ³¹P spectrum (real part) of the whole human head collected with the same spectral selective RF pulse with its pass band centered on the γ-ATP resonance frequency. All spectra were collected at 9.4T in approximately 2 minutes with a TR of 8s. The spectra in (b) and (c) were acquired with the same RF power and are displayed with the same scale. The PCr/ATP ratio for the spectrum of the head includes brain and muscle and falls between the ratios found separately in each tissue resolved by imaging.

Figure 4

PCr images and γ-ATP images collected simultaneously with the SIMPLE-TPI sequence (TR_seq/TR_PCr/ TR_/γ-ATP=8s/8s/4s, scan time: 33 minutes) were used to generate PCr/γ-ATP ratio maps. (a) PCr images reconstructed from the FID. (b) PCr images reconstructed from the gradient echo with a TE of ~11.0ms. (c) ATP images reconstructed from the 1^st echo. The nominal spatial resolution of the PCr and ATP images is 1.5cm isotropic. (d) Reference sodium images reconstructed with the same nominal resolution. (e) PCr/γ-ATP ratio maps. The ratio is 1-2 in brain tissue and 3-5 in muscle. (f) Superimposed partially transparent PCr/γ-ATP ratio maps (right) over sodium images, showing high PCr signals over muscles of the head.

Figure 5

Superimposed partially transparent PCr (left, purple), γ-ATP (middle, purple) and the PCr/γ-ATP ratio images (right, red) superimposed over sodium images (gray scale) collected with the SIMPLE-TPI sequence in 33 minutes. The nominal spatial resolution is 1.5cm isotropic and the TR_seq is 8s (thus TR_PCr/TR_/γ-ATP=8s/4s). The reference sodium images were collected with the flexTPI sequence in 10 minutes with 3.3mm isotropic resolution. The TR used is 160 ms. A mask generated from the sodium images and the ATP images was applied to the ratio maps to remove the background noise as well as voxels with low ATP signal. The vertical red color bar shows the color scale of the ratio maps.