FAST INVERSION-RECOVERY MR - THE EFFECT OF HYBRID RARE READOUT ON THENULL POINTS OF FAT AND CEREBROSPINAL-FLUID

PURPOSE: To evaluate the effect of the hybrid RARE (rapid acquisition with relaxation enhancement) readout, commonly coupled to inversion-re covery pulse sequences, on the null inversiton time (TI) of fluid and fat using both phantoms and human volunteers. METHODS: Two phantoms, s imulating fat (phantom A) and cerebrospinal fluid (phantom B), respect ively, were imaged using a fast inversion-recovery sequence that coupl ed an inversion-recovery preparation pulse to a hybrid RARE readout. A t repetition times (TRs) ranging from 700 to 20 000, the TI necessary to null the signal from each phantom (null TI) was determined for an e cho train length of 4, 6, 8, 10, 12, 14, 16, 18, and 20, respectively. Plots of null TI versus echo train length at different TRs were gener ated for both phantoms. Fast inversion-recovery MR imaging of the cerv ical spine and brain was performed in healthy volunteers. At a fixed T R and TI, the adequacy of signal suppression from bone marrow and cere brospinal fluid was assessed as a function of echo train length. RESUL TS: There was a gradual decrease of null TI for both phantoms with ech o train length. This decrease persisted at longer TRs for phantom B (T 1 = 3175 +/- 70 milliseconds) than for phantom A (T1 = 218 +/- 5 milli seconds). In the human volunteers, there was a gradual loss of suppres sion of signal from bone marrow and cerebrospinal fluid, with changes in the hybrid RARE readout. CONCLUSION: To optimize specific tissue su ppression, radiologists implementing fast inversion-recovery MR imagin g should be aware of the effects of the hybrid RARE readout on null TI .