Pseudo-continuous transfer insensitive labeling technique

Abstract

Transfer insensitive labeling technique (TILT) was previously applied to acquire multislice cerebral blood flow maps as a pulsed arterial spin labeling (PASL) method. The magnetization transfer effect with TILT is well controlled by using concatenated radiofrequency pulses. However, use of TILT has been limited by several challenges, including slice profile errors, sensitivity to arterial transit time and intrinsic low signal-to-noise ratio (SNR). In this work, we propose to address these challenges by making the original TILT method into a novel pseudo-continuous arterial spin labeling approach, named pseudo-continuous transfer insensitive labeling technique (pTILT). pTILT improves perfusion acquisitions by (i) realizing pseudo-continuous tagging with nonadiabatic pulses, (ii) being sensitive to slow flows in addition to fast flows, and (iii) providing flexible labeling geometries. Perfusion maps during both resting state and functional tasks are successfully demonstrated in healthy volunteers with pTILT. A comparison with typical SNR values from other perfusion techniques shows that although pTILT provides less SNR than inversion-based pseudo-continuous ASL techniques, the modified sequence provides similar SNR to inversion-based PASL techniques.

Copyright © 2011 Wiley-Liss, Inc.

Figures

Figure 1

Schematic of the labeling geometry for (a) TILT, (b) global pTILT, (c) localized pTILT for visual area and (d) localized pTILT for motor area. Orange box: imaging slices and dashed yellow box: labeling slab. pTILT pulse sequence for control (e) and label (f) sessions. Tps: RF pair spacing, τ: RF spacing, Ts: total labeling duration, w: post-labeling delay.

Figure 2

Simulated label (dashed line), control (dotted line) and combined (solid line) efficiency as a function of blood velocity with the pTILT sequence. (a) Efficiency comparison between different RF pair spacings (Tps=20 ms, red, and Tps=30 ms, black) with the same labeling slice thickness 10 mm. (b) Efficiency comparison between different labeling slice thickness (10 mm, red, and 20 mm, black) with the same RF pair spacing (Tps=20 ms). T1 and T2 relaxations of blood are not considered in this simulation. The blue dashed line in (a) denotes the labeling efficiency of flow-driven adiabatic inversion pCASL methods for comparison (12).

Figure 3

Flow-weighted images (ΔM/Mo) acquired by using TILT labeling applied (a) below and (b) above the imaging slices, and using pTILT labeling applied (c) below and (d) above the imaging slices. Note that no flow signal is expected when labeling above the imaging slices.

Figure 4

(a) An example of multi-slice, whole-brain perfusion-weighted images with pTILT. (b) Coronal perfusion-weighted images of visual cognitive area with localized pTILT. (c) Axial perfusion-weighted images of motor cognitive area with localized pTILT. (d) Z-score activation maps by co-acquired BOLD (1st column) and pTILT (2nd column) during a visual stimulus task. (e) Z-score activation maps for the motor task, same arrangement as in (d).