LESION ENHANCEMENT IN RADIOFREQUENCY SPOILED GRADIENT-ECHO IMAGING - THEORY, EXPERIMENTAL EVALUATION, AND CLINICAL IMPLICATIONS

PURPOSE: To investigate the lesser lesion conspicuity after gadolinium contrast infusion with radio-frequency spoiled gradient-echo (SPGR) s equences relative to conventional T1-weighted spin-echo techniques. ME THODS: The influences of repetition time, echo time, and flip angle on spin-echo and SPGR signal were studied with mathematical modeling of the image signal amplitude for concentrations of gadopentetate dimeglu mine solute from O to 10 mM. Predictions of signal strength were verif ied in vitro by imaging of a doped water phantom. The effects of stand ard (0.1 mmol/kg) and high-dose (0.3 mmol/kg) gadoteridol on spin-echo and SPGR images were also investigated in three patients. RESULTS: Th e measured amplitude of undoped water and the rate of increase of dope d water signal with increasing gadopentetate concentration (slope) for spin-echo 600/11/1/90 degrees (repetition time/echo time/excitations/ flip angle) and SPGR (600/11/190 degrees) were similar and exceeded th ose of SPGR (35/5/145 degrees). Greater increases in SPGR doped water signal and its slope were produced by increasing TR than by varying ec ho-time or flip angle. The subjective lesion conspicuity and measured lesion contrast at 0.3 mmol/kg were greater with spin-echo (600/21/1/9 0 degrees) than with SPGR (35/5/145 degrees) in all three patients; th e measured lesion enhancement was similar for both techniques in two p atients and decreased for SPGR in the third patient. CONCLUSIONS: The phantom studies suggest that the short repetition time of 35 msec, typ ically used in clinical SPGR imaging, is largely responsible for a red uced signal amplitude and a diminished rate of increase of signal with increasing gadopentetate concentration, relative to spin-echo. Phanto m and clinical studies suggest that the dose of paramagnetic agent req uired to achieve SPGR lesion conspicuity with short repetition time co mparable with spin-echo would have to be higher than the dose in curre nt clinical use.