Functional magnetic resonance imaging with continuous arterial spin labeling: A novel pulse sequence and quantitative model.

Arterial spin labeling (ASL) offers a number of advantages over the blood oxygenation level dependent technique for functional brain imaging. However, widespread adoption of ASL for functional imaging has been hampered by inferior SNR and temporal resolution. A novel pulse sequence, Turbo-CASL, allows acquisition of perfusion images with improved temporal resolution and increased sensitivity to neuronal-activity induced perfusion responses. It is demonstrated through both experiment and computer simulation that the technique is very sensitive to arterial transit time. This sensitivity is exploited to enhance detection of cerebral blood flow (CBF) changes in the presence of neuronal activation-induced reductions in transit time. By acquiring data at a range of timing parameters, the technique can also be used as a means of transit time measurement. One consequence of transit time sensitivity is increased difficulty in making quantitative measures of CBF under conditions of changing transit time. For this reason we propose a dynamic model which uses the transport equation to govern the arrival of labeled blood at the imaging plane. By incorporating time-varying velocity and CBF functions into the model, the resulting ASL signal was calculated under conditions of dynamic changes in these parameters. If the resting state transit time is known and a linear relationship between changes in CBF and mean arterial velocity is assumed, then CBF can be estimated from the measured ASL data. The assumption of a linear relationship for change in CBF versus mean arterial velocity was investigated by having subjects undergo four different tasks expected to invoke separate CBF response levels. By measuring both transit time and CBF under each of these conditions, it was determined that in humans there is a linear relationship between percentage changes in mean velocity and CBF at blood flow levels relevant to functional imaging experiments.Ph.D.Applied SciencesBiomedical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126755/2/3276222.pd