Background suppression in multi slice arterial spin labeling

Introduction In brain perfusion measurements with Arterial Spin Labeling (ASL), blood is used as an endogenous tracer by inversion of the longitudinal magnetization of spins in blood. Because typical perfusion signal is low (1-2%) as compared to background signal, alternate images are acquired with and without labeling rendering perfusion images after subtraction. However, subtraction does not render complete cancellation of the background signal due to physiological noise that fluctuates over time. To counter this problem, a background suppression (BS) scheme can be added to the scan sequence, commonly consisting of a saturation pulse that precedes labeling and one or two inversion pulses during the waiting period between labeling and imaging. In BS, timing needs to be configured such that signals of all tissues have equal sign during acquisition of a slice (or volume), to prevent artifacts due to sign changes over time or due to intra voxel cancellation when tissues have signals of opposite sign. When timing is optimized, BS may result in improvement of perfusion SNR of up to 100 % [1]. Whereas for single shot 3D GRASE sequences BS can be made optimal for the complete acquisition volume, in multi slice 2D imaging the BS is only ideal for the first slice, while background signal will regrow for subsequent slices. However, multi slice 2D imaging has the advantage of higher in-plane resolutions and the possibilities of large coverage, due to the limitations of total read-out duration in single shot 3D acquisitions. In the present study we investigated the effectiveness of an extended BS scheme with inter-slice inversion pulses during image acquisition. To evaluate detrimental effect on the label, efficiency of BS inversion pulses was investigated first and secondly the relation between the amount of background suppression and SNR was established. This allowed for computer simulations of 2D acquisition with different BS inter-slice inversion schemes that subsequently were tested in vivo.