Whole-brain cerebral blood flow mapping using 3D echo planar imaging and pulsed arterial tagging

Abstract

Purpose: To quantitate cerebral blood flow (CBF) in the entire brain using the 3D echo planar imaging (EPI) PULSAR (pulsed star labeling) technique.

Materials and methods: The PULSAR technique was modified to 1) incorporate a nonselective inversion pulse to suppress background signal; 2) to use 3D EPI acquisition; and 3) to modulate flip angle in such a manner as to minimize the blurring resulting from T1 modulation along the slice encoding direction. Computation of CBF was performed using the general kinetic model (GKM). In a series of healthy volunteers (n = 12), we first investigated the effects of introducing an inversion pulse on the measured value of CBF and on the temporal stability of the perfusion signal. Next we investigated the effect of flip angle modulation on the spatial blurring of the perfusion signal. Finally, we evaluated the repeatability of the CBF measurements, including the influence of the measurement of arterial blood magnetization (a calibration factor for the GKM).

Results: The sequence provides sufficient perfusion signal to achieve whole brain coverage in ≈ 5 minutes. Introduction of the inversion pulse for background suppression did not significantly affect computed CBF values, but did reduce the fluctuation in the perfusion signal. Flip angle modulation reduced blurring, resulting in higher estimates of gray matter (GM) CBF and lower estimates of white matter (WM) CBF. The repeatability study showed that measurement of arterial blood signal did not result in significantly higher error in the perfusion measurement.

Conclusion: Improvements in acquisition and sequence preparation presented here allow for better quantification and localization of perfusion signal, allowing for accurate whole-brain CBF measurements in 5 minutes.

Figure 1

Schematic representation of the 3D-IR-PULSAR sequence. The saturation pulse provides bolus definition while the inversion pulse improves background suppression and leads to reduced perfusion signal fluctuation. DAC denotes the data acquisition window which uses 3D TFEPI acquisition.

Figure 2

Simulations depicting source of blurring and correction scheme (A) Variable flip angles used with 3D TFEPI acquisition with maximum flip angle of 30° (top-l) (B) transverse magnetization with constant flip angle scheme and with the modulated flip angle scheme (top-r) (C) PSF for the constant and variable flip angle schemes (bottom-l) and (D) the effect of the PSF on the slice profile (bottom-r). Center slice of thickness 4 mm is considered. Constant flip angle scheme results in increased dispersion of the magnetization signal into neighboring slices while variable flip angle scheme provides an almost ideal slice profile.

Figure 3

CBF maps obtained with (A) 3D-PULSAR and (B) IR-3D-PULSAR. Every fourth slice from a 24 slice acquisition is shown. Scale on the right is in ml/100g/min.

Figure 4

Plot shows gray matter perfusion values for five volunteers with and without background suppression pulse. Average GM perfusion across volunteers is 58.2 ml/100g/min for 3D PULSAR and 58.9 ml/100g/min for IR-PULSAR case. Good correspondence between the two (3D PULSAR and 3D-IR-PULSAR) values for each volunteer is seen.

Figure 5

Correction for blurring shows improved localization of the perfusion signal. Six transverse slices and reformatted sagittal and coronal slices obtained with (A, C) constant flip angle and (B, D) modulated flip angle are shown. Scale on right is in ml/100g/min and window/level is the same for all images.

Figure 6

Standard deviation maps obtained with repeatability study. Top row (A) shows the segmented GM CBF map for three slices while (B) shows the σ map across four repeated scans with 3D-PULSAR and (C) shows the σ map with IR-3D-PULSAR. Notice the poorer reproducibility of perfusion signal in areas close to CSF with 3D-PULSAR due to increased background noise.