Lesion enhancement in radio-frequency 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) sequences relative to conventional T1-weighted spin-echo techniques.
METHODS: 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 dimeglumine solute from 0 to 10 mM. Predictions of signal strength were verified in vitro by imaging of a doped water phantom. The effects of standard (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: The measured amplitude of undoped water and the rate of increase of doped 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 those of SPGR (35/5/145 degrees). Greater increases in SPGR doped water signal and its slope were produced by increasing TR than by varying echo-time or flip angle. The subjective lesion conspicuity and measured lesion contrast at 0.3 mmol/kg were greater with spin-echo (600/11/1/90 degrees) than with SPGR (35/5/145 degrees) in all three patients; the measured lesion enhancement was similar for both techniques in two patients and decreased for SPGR in the third patient.
CONCLUSIONS: The phantom studies suggest that the short repetition time of 35 msec, typically used in clinical SPGR imaging, is largely responsible for a reduced signal amplitude and a diminished rate of increase of signal with increasing gadopentetate concentration, relative to spin-echo. Phantom and clinical studies suggest that the dose of paramagnetic agent required to achieve SPGR lesion conspicuity with short repetition time comparable with spin-echo would have to be higher than the dose in current clinical use.