Imaging of urea using chemical exchange-dependent saturation transfer at 1.5T

The purpose of this study was to screen for slow proton chemical exchange between water and kidney metabolites using a standard clinical 1.5-T scanner. Imaging was performed using a fast spin-echo sequence with a magnetization transfer (MT) preparation pulse train. Off-resonance saturation ranging from +/-50 to +/-1000 Hz was used on urea and urine phantoms and normal human subjects imaged through the kidneys. The positive frequency was used as the control for each frequency pair. Results of frequency sweeps show an asymmetric MT effect peaking at approximately 100 Hz ( thick similar1 ppm) for urea, urine, and renal parenchyma. Varying differences (5%-25%) occurred with different human subjects. Few differences were observed from phantom water or subject muscle tissue. Chemical exchange is detectable in the kidney near 1 ppm at 1.5 T, attributable to urea. This technique was used to produce in vivo distribution maps of this metabolite in vivo.