Measurement of Absolute Cerebral Blood Volume using Velocity-Selective Pulse Trains
Feng Xu1,2 and Qin Qin1,2

1Radiology, Johns Hopkins University, Baltimore, MD, United States, 2F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States

Synopsis

Current MRI techniques for quantification of absolute cerebral blood volume (CBV) are all contrast-based. To reduce associated risks and cost, we proposed a non-contrast-enhanced (NCE) MRI method using a velocity-selective (VS) spin labeling approach for CBV measurement at 3T. Gray matter CBV values across 5 subjects are 2.4±0.2 mL/100g for blood flowing between the encoded cutoff velocities of 0.5cm/s and 3.1cm/s, with a GM/WM ratio of 1.9±0.3.

INTRODUCTION

Cerebral blood volume (CBV) is an important hemodynamic parameter for monitoring many brain disorders, such as stoke and brain tumor. Gadolinium-based dynamic susceptibility contrast (DSC-MRI)1-3 have been widely used in the clinic for imaging total CBV. More recently, Gadolinium-based vascular-space-occupancy (VASO) MRI4 and Ferumoxytol-based steady-state approach5 have been introduced for quantitative CBV mapping. The only non-contrast-enhanced (NCE) MRI method is based on spatially-selective arterial spin labeling (ASL), which estimates the volume of the arterial blood compartment6. Velocity-selective (VS) ASL has been applied for mapping cerebral blood flow (CBF) 7-10 and oxygen extraction fraction11-12. In this work we aim to develop a VS spin labeling method for absolute CBV measurement.

Methods

A basic VS labeling module7,8 consists of ±90° hard pulses enclosing a pair of adiabatic refocusing pulses with surrounding velocity encoding gradients (Fig. 1). When assuming laminar flow, the VS module crushes the signal of blood flowing above the cutoff velocity (VcVS). In contrast, spins moving below the Vc, including the static tissue, only experience the T2 weighting during the TVS and diffusion weighting by the motion-sensitized gradients. For this work, double refocused hyperbolic tangent refocusing pulses were chosen (5 ms, tanh/tan, maximum amplitude of 575 Hz and a frequency sweep of 8 KHz); four alternating triangle gradient lobes with a ramp time of 1.2ms and maximum amplitude of GVS=27.8mT/m yield the VcVS=0.5cm/s, which is close to the velocity of capillary blood (0.2~0.9cm/s)13-14. A 3.5ms gap between each gradient and RF pulse is kept to minimize the effect of eddy currents. For this VS modual, TVS=35ms, bVS=2.7sec/mm2. The corresponding control module has gradients turned off. A tailored hard-pulse train15 for global saturation was applied following acquisition with delay of Trecover=3.6s before the label/control modules (Fig. 1). A bipolar gradient (BP) (GBP=33mT/m, TBP=2.5ms×2, VcBP=3.1cm/s) is inserted before the EPI acquisition to suppress the signal from large vessels.

Experiments were performed on a 3T Philips scanner using a 32-channel head coil for reception. To evaluate effects of eddy currents, a silicone oil phantom were tested with the proposed control/label modules placed 10 ms before image acquisition10. Using a TR=4.0s, the total measurement time after 24 repetitions was about 3.3min. Proton density-weighted image of signal intensity (SIPD) was also acquired with TR=10s. The parameters used for the phantom images were identical with the human studies. A total of 7 healthy volunteers (age: 25~51yrs, 3 females) were enrolled with informed consent during the development. Acquisition parameters: the transverse FOV was 213x186mm2 with 10 slices acquired at a slice thickness of 4.4mm with no gaps; the acquisition resolution was 3.3mm in plane and the reconstructed voxel size was 1.9mm. A subgroup of subjects (three) were first scanned comparing configurations with different gradient strengths for VS module and BP to examine CBV for blood moving at velocities (VcVS <V< VcBP). A double inversion recovery (DIR) image was also acquired to visualize gray matter only. The proposed sequence was evaluated for five subjects. For quantification, CBV=100× (SIcontrol-SIlabel)/[SIPD×exp(-TVS/T2,b)×exp(-TBP/T*2,b)×(1-exp(-Trecover/T1,b))], where T1,b=1850ms and T2,b is assumed to have 30% contribution of arterial blood (T2,a=150ms) and 70% of venous blood (T2,v=70ms).

Results and Discussion

Fig. 2 displays the effects of gradient imperfections for three orthogonal directions across 10 slices of the phantom. The normalized subtraction errors (mean±STD) along A-P, L-R and S-I directions are 0.07± 0.17%, 0.13 ± 0.18%, and 0.24±0.17%, respectively. Representative data from one subject are shown in Fig. 3 with (a) SIPD; (b) DIR; (c) VcVS =0.5cm/s (GVS=27.8mT/m), VcBP=∞ (GBP=0mT/m); (d) VcVS =0.5cm/s (GVS=27.8mT/m), VcBP=3.1cm/s (GBP=33mT/m); (e) VcVS =1.5cm/s (GVS=9.3mT/m), VcBP=∞ (GBP=0mT/m); (f) VcVS =1.5cm/s (GVS=9.3mT/m), VcBP=3.1cm/s (GBP=33mT/m). When lowering VcVS from 1.5cm/s to 0.5cm/s, more signal remain in the obtained CBV maps (Fig. 3c,d) than results of (Fig. 3e,f). Large arteries are much less visible in Fig. 3d,f than Fig. 3c,e when increasing GBP. The averaged GM and WM CBV values across 5 subjects are 2.4±0.2 and 1.3±0.2 mL/100g, with a GM/WM ratio of 1.9±0.3, for blood flowing between the encoded cutoff velocities of 0.5cm/s and 3.1cm/s (Table 1).

CONCLUSION

A new method for measuring absolute CBV values using velocity-selective spin labeling approach has been developed at 3T. The technical feasibility was shown by the reasonable results among healthy subjects. Further optimization of the reported technique is under way and will be tested in various clinical applications.

Acknowledgements

Funding Source: NIH K25 HL121192 (QQ) and P41 EB015909.

References

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Figures

The pulse sequence diagram of the proposed method for CBV measurement using velocity-selective (VS) pulse train for labeling. The bipolar gradient (BP) before EPI acquisition is to reduce signal from large vessels.

Signal difference errors on a phantom for the employed VS control/label modules caused by eddy current along different gradient orientations. All acquired 10 slices are shown with the averaged error percentage (mean±STD) displayed at the top of each row.

Representative data from one subject are shown with (a) SIPD; (b) GM-only image from DIR; VcVS =0.5cm/s: (c) VcBP=∞; (d) VcBP=3.1cm/s; VcVS =1.5cm/s, (e) VcBP=∞; (f) VcBP=3.1cm/s.

Table 1: Averaged CBV and SNR values (mean±STD, n=5) in GM and WM ROIs and their GM/WM ratios.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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