Maximizing contrast to noise with inductively coupled implanted coils.

Magnetic resonance (MR) microscopy with inductively coupled implanted coils has been used previously to follow loss and return of intra-medullary contrast as a result of nephrotoxic acute tubular necrosis with 117 microns resolution over a 2000 microns thick slice. The purpose of the current study was to further investigate the capabilities of in vivo MR microscopy by combining the implanted coil imaging technique with spin echo pulse sequence optimization done through signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) modeling. These models included consideration of the effects of T2* and sampling time on signal-to-noise and contrast-to-noise ratios. They were initially tested with GdCl3 and agar gel phantoms constructed to the relaxation time and spin density specifications of the intra-medullary junction which bridges the outer and inner stripe of the outer medulla. In vivo microscopy was performed using single turn radiofrequency (RF) coils that were surgically implanted around the left kidney of two rats and inductively coupled to an external "birdcage" body coil. The models revealed maximum CNR per unit imaging time at a TR of 800 msec. A TE of 16 msec proved to be the best compromise between loss of transverse magnetization and decreased bandwidth. These CNR predictions were supported by the gel phantom and in vivo data. Maximizing the CNR in the current study enabled us to improve the resolution of in vivo MR microscopy to 78 microns over a 1000 microns slice with an SNR of 40 and a CNR of eight in a total imaging time of 54 minutes.