Three-dimensional high-resolution T₁ mapping and perfusion imaging in the normal and ischemic rat brain with...

To develop methods for three-dimensional relaxometry and quantitative perfusion imaging using hyperpolarized [2-¹³C]tertiary-butyl alcohol (TBA) in the rat brain under normal and ischemic conditions at 9.4 T. TBA is a freely diffusible tracer that readily traverses the blood-brain barrier, resulting in high tissue signal and long residence times.

A hybrid method consisting of rapid two-dimensional imaging of the arterial input followed by slower three-dimensional variable tip-angle balanced steady state free-precession imaging of the brain is implemented. Image data are analyzed to extract the signal amplitude and T_1,2 decay rates. Knowledge of the tracer's kinetics in tissue is used to determine blood flow.

Effective T₁ relaxation rates in the rat brain range from about 15 to 20s. T₂ ranges from about 60 to 250 ms, with the shortest relaxation times found in the brainstem. In ischemic regions, the effective T₁ relaxation time is prolonged due to slower washout, whereas T₂ is largely unchanged. The technique yields flow rates in cortical gray matter ranging from 140 mL/100 g/min in normal brains to less than 30 mL/100 g/min in ischemic cases.

Hyperpolarized TBA provides sufficient sensitivity and tissue residence time to enable three-dimensional mapping of relaxation and blood flow at 1.2-1.5-mm³ isotropic resolution in the rat brain. The technique has adequate signal-to-noise ratio in tissue with restricted flow. Raw images of the tracer can be acquired at 0.48 mm³ isotropic resolution and signal-to-noise ratio of about 13 in cortical gray matter.