BOLD and Sodium MRI of the Human Kidney: Preliminary Experience with Patients

Na MRI of the human kidneys, as was recently developed in our lab, enabled the determination of the corticomedullary sodium gradient, and has been proven sensitive to changes in water homeostatsis in the body (4). Both imaging techniques are able to map these physiological parameters in the various regions of the kidneys at 2-3 mm in-plane spatial resolution, and therefore may lead to new insights regarding the pathogenic processes underlining various renal diseases. This ongoing initial study is aimed at monitoring the modulations in renal oxygenation levels and in the corticomedullary sodium gradient along the progression of various types of renal failure, investigating the possible correlation between these two parameters, and determining whether the pathophysiological patterns observed are diffuse or focal in presentation. Methods Subjects: MR studies were performed on 5 patients and 1 healthy volunteer so far. Three of the patients had transplanted kidneys, two of them with chronic renal failure (CRF) and 1 with normal renal function, and the other 2 patients had CRF of their intact kidneys. Data acquisition: MRI scans were performed on a 3.0 T scanner (Signa LX, General Electric, Waukesha, WI). The subjects lay supine and entered the magnet with their feet first. The imaging protocol included two parts: Part I – Anatomical imaging and BOLD, using a torso array 8-channel coil: 1) Fast 3D SPGR (LAVA) with TR/TE=4/1.8 ms, flip angle=10°, FOV=40 cm, slice thickness=5 mm, matrix=256x192, 0.8 phase FOV sampled. 2) FGRE in/out of phase, 2D, TR=250 ms, flip angle=70°, FOV=40 cm, slice thickness=5 mm, slice spacing=0, matrix=320x160, 0.9 phase FOV sampled. 3) BOLD imaging using a 3D multiple GRE sequence, with an echo train of 8 echoes, TE’s ranging from 2 to 23 ms, TR=25.3, flip angle=10°, BW=83.3 KHz, FOV=40x40x50 cm, slice thickness=5 mm (10 slices prescribed, 6 are eventually reconstructed), matrix=256x160, 0.7x0.7 phase FOV sampled, acq. time=23 sec. The sequence was supplied by GE, in collaboration with Pottumarthy V. Prasad, PhD, Northwestern University, Chicago, Illinois. Part II – Reference proton images (body coil) followed by 23 Na imaging of the kidney using a sodium surface coil: 1) FGRE in/out of phase, 2D, TR=250 ms, flip angle=70°, FOV=40 cm, slice thickness=15 mm, slice spacing=0, matrix=320x160, 0.8 phase FOV sampled. 2) 23 Na MRI using a 3D-GRE sequence modified for multinuclear imaging, with FOV=38×38×24 cm, matrix size=128×128×16, TE/TR =1.8/30 ms, 24 averages, acquisition time=24 min. The short TE was achieved by applying a 66% partial Fourier echo, along with a hard, non-slice selective, 300 sec RF excitation pulse. All scans were prescribed to acquire coronal images of intact kidneys or sagittal images of transplanted kidneys. In each part the scans were prescribed with matched slice locations. All scans except for the 23 Na MRI were acquired during breathholds. Data processing: The anatomical images were evaluated for kidneys size, morphological findings and corticomedullary differentiation. The BOLD data was processed off-line using the freeware ImageJ (5) to fit each pixel intensity curve to an exponential signal decay. The resulting T2* maps were converted to pixel-by-pixel R2* maps (R2*=1/T2*). The quality of fit (r 2