Determination of spin compartment in arterial spin labeling MRI

Abstract

A major difference between arterial-spin-labeling MRI and gold-standard radiotracer blood flow methods is that the compartment localization of the labeled spins in the arterial-spin-labeling image is often ambiguous, which may affect the quantification of cerebral blood flow. In this study, we aim to probe whether the spins are located in the vascular system or tissue by using T2 of the arterial-spin-labeling signal as a marker. We combined two recently developed techniques, pseudo-continuous arterial spin labeling and T2-Relaxation-Under-Spin-Tagging, to determine the T2 of the labeled spins at multiple postlabeling delay times. Our data suggest that the labeled spins first showed the T2 of arterial blood followed by gradually approaching and stabilizing at the tissue T2. The T2 values did not decrease further toward the venous T2. By fitting the experimental data to a two-compartment model, we estimated gray matter cerebral blood flow, arterial transit time, and tissue transit time to be 74.0 ± 10.7 mL/100g/min (mean ± SD, N = 10), 938 ± 156 msec, and 1901 ± 181 msec, respectively. The arterial blood volume was calculated to be 1.18 ± 0.21 mL/100 g. A postlabeling delay time of 2 s is sufficient to allow the spins to completely enter the tissue space for gray matter but not for white matter.

Fig. 1

Schematic diagram of the TRUST-PCASL sequence. The sequence consists of interleaved acquisitions of label and control scans. For each scan, after a train of RF pulses which provide the pseudo-continuous flow driven inversion, a series of non-slice-selective T2-preparation pulses are inserted before the slice-selective excitation pulse to modulate the T2-weighting, the duration of which is denoted by eTE. The subtraction of the control and labeled images yields the ASL signal.

Fig. 2

Illustration of the two-compartment perfusion model for PCASL. The labeled spins are shown by the gray areas. The box indicates the imaged voxel which consists of vessels and tissue. T_2,a and T_2,t are the transverse relaxation times. δ_a and δ are the time it takes for the labeled spin to reach the arterial and tissue compartments, respectively. τ and w are imaging parameters indicating the labeling duration and post-labeling delay, respectively.

Fig. 3

CBF-weighted images (control-label) from a representative subject. The images were acquired with post labeling delays from 200 to 2000 ms and effective echo times from 0 to 160 ms. The scale bar indicates ΔM/M₀ (%).

Fig. 4

An example of the mono-exponential fitting of the ROI data. The symbols and curves indicate the experimental data and the fitting, respectively. Error bars indicate standard deviations of the signals over the ROI. The plots are for the four delay times of 200 ms, 850 ms, 1525 ms and 2000 ms from the subject shown in Fig. 3. The fitting for the vessel ROI is not good at longer delay times (1525 ms and 2000 ms) due to lower SNR as shown in Table 1.

Fig. 5

Amplitude of ASL signal in ROIs containing vessels (red), gray matter (blue) and white matter (green). The error bars indicate the standard deviations across the subjects. Note that the error bars for gray and white matters are very small.

Fig. 6

T2 of ASL signal (solid line) and control signal (dashed line) for gray matter (a), white matter (b), and vessel ROIs (c). The error bars indicate the standard deviations across the subjects. Note that the white matter T2 at delay time of 200ms had a large error bar because the ASL signal intensity was low.

Fig. 7

Fitting of experimental data to the perfusion models in a representative subject. Both two-compartment (solid curves) and one-compartment (dashed curves) models were tested. For the two-compartment model, the procedure was conducted in two steps in which spin T2 (left) and ASL signal amplitude (right) were separately fitted (see Methods for details). The T2 curve was primarily affected by the tissue transit time whereas the amplitude curve was predominantly determined by CBF and the arterial transit time. The one-compartment model fitting only used the amplitude data in accordance with previous reports.