Download Functional Renal Imaging Vivian S. Lee The kidneys

ISMRM, May 2008
Functional Renal Imaging
Vivian S. Lee
The kidneys maintain homeostasis by filtering and excreting metabolic waste products,
regulating acid-base balance, and moderating blood pressure and fluid volume. Since decreasing
renal function accompanies renal disease, monitoring renal function permits assessment of
disease progression and prognosis and is used to guide patient management and therapy. Many
non-invasive tests of renal function are commonly used, but all have their drawbacks. Serum
creatinine levels and creatinine clearance are insensitive measures of global function and cannot
supply information about individual renal function. Renal scintigraphy can assess renal function
but provides limited anatomic information and exposes the patient to radiation. Computed
tomography (CT) and intravenous urography (IVU) can provide functional and anatomic
information, but both employ nephrotoxic contrast agents and also expose the patient to
radiation. MRI is the only single imaging modality with the potential to deliver a comprehensive
anatomic and functional examination of the kidneys with minimal risk to the patient.
The MRI evaluation of renal function typically centers on visualizing the passage of
contrast material through the kidney. The contrast agents used in MRI are usually gadolinium
chelates such as Gd-DTPA, also known as gadopentetate dimeglumine, although there are also
more novel approaches such as arterial spin labeling. The paramagnetic properties of gadolinium
cause a decrease in the T1 and T2 relaxation times of nearby tissues and fluids. The physiologic
behavior of gadolinium is governed by the properties of the agent to which it is chelated. DTPA
is a substance that, like inulin, is freely filtered by the glomerulus and is neither resorbed nor
secreted by the renal tubules, rendering it a convenient marker of glomerular filtration. When
bound to gadolinium, its path through the kidneys can be traced with T1-weighted MRI. We
refer to such imaging examinations as magnetic resonance renography (MRR).
The high spatial resolution that MRI provides allows visualization of gadolinium contrast
material within distinct intrarenal regions such as the cortex, the medulla, and the collecting
system [1-4]. Enhancement of the cortex primarily reflects perfusion and glomerular filtration,
whereas enhancement of the medulla and collecting system, although dependent on filtration,
primarily reflects the condition of the renal tubules. Given that different diseases affect different
portions of the vascular-nephron system, MRR has the potential, unique among all non-invasive
tests, to distinguish glomerular from tubulointerstitial pathology.
In this talk, we review renal anatomy and physiology, concentrating on two important
renal functional parameters, renal blood flow and glomerular filtration rate. Finally, we explore
specific applications of functional renal MRI in the arenas of renovascular disease,
hydronephrosis, and renal transplantation.
References
1. Prasad PV. Functional magnetic resonance imaging of the kidney. Methods in Molecular
Medicine. 124:197-224, 2006.
2. Grattan-Smith JD. Jones RA. MR urography in children. Pediatric Radiology. 36(11):111932, 2006
3. Renken NS. Krestin GP. Magnetic resonance imaging of the kidney. Seminars in Ultrasound,
CT & MR. 26(3):153-61, 2005.
4. Lee VS. Rusinek H. Bokacheva L. et al. Renal function measurements from MR renography
and a simplified multicompartmental model. Am J Physiol - Renal Physiology. 292(5):F1548-59,
2007.