Imaging Renal Urea Handling in Rats at Millimeter Resolution Using Hyperpolarized Magnetic Resonance Relaxometry

spin spin relaxation time () heterogeneity of hyperpolarized [C,N]urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized C signal with a macromolecular relaxation agent revealed that a long- component of the [C,N]urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the [C,N]urea to be distinguished via multi-exponential analysis. The response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized [C,N]urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-C-cyclopropane-H. Large increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis. Therefore, [C,N]urea relaxometry is sensitive to two steps of the renal urea handling process: glomerular filtration and the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Simple motion correction and subspace denoising algorithms are presented to aid in the multi exponential data analysis. Furthermore, a -edited, ultra long echo time sequence was developed for sub-2 mm resolution 3D encoding of urea by exploiting relaxation differences in the vascular and filtrate pools.