Robust Arterial Spin Labeling T2 Measurements

Abstract

Perfusion and bolus arrival time are often quantified physiological parameters. By additionally quantifying permeability it is hoped to better understand tissue physiology or monitor small permeability changes in cerebrovascular diseases, which are difficult to detect by traditional gadolinium contrast agents. A problem of gadolinium contrast agents is the size of the macro molecules. They cannot pass an intact or slightly damaged blood brain barrier, whereas hydrogen molecules are small enough to pass it. In arterial spin labeling (ASL) the hydrogen spins of the water molecules in inflowing blood are selectively inverted and can be used as a non-invasive endogenous contrast agent. In standard ASL modules, generally only T1 effects are considered. This is sufficient for perfusion and arrival time but not for capillary wall permeability quantification, due to the small difference of blood and tissue T1 times. In contrast to this, T2 times of blood and tissue are considerably different, which makes arterial spin labeling T2 measurements a potential method for permeability estimations if current modules are extended and optimized to include T2. Yet, accurate T2 calculations are often very time consuming, limiting the acquisition for an individual patient and the general implementation in clinical routine. In the present work, acquisition techniques and fitting routines of a multi-TI multi-TE 3D-GRASE (gradient and spin echo) sequence are presented allowing a fast and reliable T2 calculation at every inflow time, which can be used for permeability estimations. An optimized acquisition scheme and T2 calculation algorithm is found with regard to accuracy, measurement time, and signal-to-noise ratio (SNR). The concept of adaptive averaging is presented, which allows the acquisition of longer inflow times more often than shorter ones, improving SNR at long inflow times by 1.5 and more, but by simultaneously keeping the total scan time constant. The impact on T2 calculations from stimulated echoes in combination with different kinds of crusher gradients at several refocusing flip angles is evaluated in phantoms and volunteers. The fitted T2 results are compared to simulations and gold standard single spin echo T2 values. Finally, T2 values at multiple inflow times with different turbo factors (TF) are obtained. The impact of the turbo factor on the T2 calculation is simulated and verified in a volunteer study, resulting in a suggestion for an optimised imaging scheme for TF>1. For TF1 the multi-TE ASL 3D-GRASE sequence is already capable of T2 measurements at multiple inflow times, very close to T2 values obtained with the gold standard single spin echo sequence. The T2 values could be directly incorporated in a two compartment model for perfusion quantification, to further study tissue functions or disease related to permeability changes.