Comparison of fluid-attenuated inversion-recovery MR imaging with CT in a simulated model of acute subarachnoid hemorrhage

BACKGROUND AND PURPOSE: because MR imaging is becoming integral to the eval uation and treatment of very early stroke, it is critical to prove that MR imaging is at least as sensitive to acute subarachnoid hemorrhage (SAH) as is CT. The present study was conducted to evaluate the possibility of detec ting a small amount of acute SAH diluted by CSF not revealed by CT but iden tified on fluid-attenuated inversion-recovery (FLAIR) MR images in an in vi tro study. METHODS: Acute SAH was simulated with mixtures of artificial CSF and arteri al blood (hematocrit [Hct], 45%) ranging from 0% to 100% by volume, We scan ned these phantoms with CT and turbo-FLAIR MR imaging (9000/119 [TR/effecti ve TE]; inversion time, 2200 ms; echo train length, 7), and we measured T1 and T2 relaxation times of these phantoms at temperatures within 36 degrees C to 37 degrees C. Plots of CT value from the different blood/water mixtur e ratios versus Hct were generated and correlated with the average CT value from normal cortex. We measured T1 and T2 relaxation times of these phanto ms and normal cortex and generated T2 relaxation curves as a function of ef fective TE for a specific inversion time (2200), and determined the TR (900 0) for the turbo-FLAIR sequence by using a theoretical equation for the tur bo inversion recovery signal intensity. RESULTS: Above a Hct of 27% blood, the mixture was denser on CT scans than was the normal cortex, At a selected time longer than an effective TE of 12 0, above a Hct of 22.4% blood, the mixture was more hyperintense than the n ormal cortex on turbo-FLAIR images, At selected times longer than an effect ive TE of 160, above a Hct of 9% blood, the mixture was more hyperintense t han was the normal cortex. CONCLUSION: FLAIR imaging is more sensitive than CT in the detection of a s mall amount of acute SAH diluted by CSF at selected appropriate TE, as dete rmined in an in vitro study.