Example of disease-specific CBF quantification
Sickle cell disease (SCD) has provided a naturally occurring condition in which to study the reproducibility and variability of ASL measurement and quantification. The fact that in pCASL the labelling efficiency depends on flow-driven adiabatic inversion makes the measurement inherently flow-dependent [Wong 2014]. As such, variability is then also dependent on the variability of arterial flow velocity, which can be dramatically increased in patients with SCD. Labelling efficiency can be simulated to find velocity-specific efficiencies [Maccotta NMR Biomed 1997; Wu 2007 MRM], and the CBF quantification subsequently corrected by phase-contrast MRI measurements of velocity, resulting in velocity-derived labelling efficiency [Vaclavu et al ISMRM 2018]. It should be noted that whether the labelling efficiency is increased or decreased in patients with SCD compared to controls depends on the sequence parameters of the pCASL implementation with respect to the RF pulse and gradients. A more direct method is to measure the labelling efficiency during the pCASL experiment [van Osch 2017]. Due to the higher arterial flow velocities in SCD one can also expect significant reductions in the bolus arrival time [Kawadler 2018; Juttukonda 2017] and correcting for this may further prevent overestimation of CBF in this population. Another source of quantification error arises from differences in B1 inhomogeneities [Bush et al MRI 2018], B0 inhomogeneities and differences in T1 of blood, the latter of which is known to vary with haematocrit [De Vis 2014; Varela 2011]. In patients with SCD, CBF quantification can benefit from using an additionally acquired T1 blood measurement [Vaclavu et al2016].
Lena Vaclavu (email@example.com)
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