  \textit{In vivo} spin spin relaxation time ($T_2$) heterogeneity of
hyperpolarized \textsuperscript{13}C urea in the rat kidney was investigated.
Selective quenching of the vascular hyperpolarized \textsuperscript{13}C signal
with a macromolecular relaxation agent revealed that a long-$T_2$ component of
the \textsuperscript{13}C urea signal originated from the renal extravascular
space, thus allowing the vascular and renal filtrate contrast agent pools of
the \textsuperscript{13}C urea to be distinguished via multi-exponential
analysis. The $T_2$ response to induced diuresis and antidiuresis was performed
with two imaging agents: hyperpolarized \textsuperscript{13}C urea and a
control agent hyperpolarized
bis-1,1-(hydroxymethyl)-1-\textsuperscript{13}C-cyclopropane-$^2\textrm{H}_8$.
Large $T_2$ increases in the inner-medullar and papilla were observed with the
former agent and not the latter during antidiuresis suggesting that $T_2$
relaxometry may be used to monitor the inner-medullary urea transporter (UT)-A1
and UT-A3 mediated urea concentrating process. Two high resolution imaging
techniques - multiple echo time averaging and ultra-long echo time sub-2 mm$^3$
resolution 3D imaging - were developed to exploit the particularly long
relaxation times observed.
