as a filtered agent without active excretion or uptake from the renal tubules. Several methods have been developed for esti- mating the GFR from dynamic nuclear medicine data, but all are hampered by the poor counting sta- tistics of such dynamic studies and the problem of accounting for the extrarenal component of the sig- nal. Recently, several groups have applied the methods developed for nuclear medicine to dy- namic MR imaging data acquired in conjunction with an injection of the contrast agent gadoli- nium-diethylenetriamine pentaacetic acid. In ap- plying these techniques to MR imaging data, several issues must be addressed. First, although nu- clear medicine measures the activity, and hence the concentration, of the contrast agent directly, in MR imaging the contrast agents change signal by alter- ing the relaxation times of the tissue, producing a linear relationship with the concentration over only a limited range of concentrations. Second, the exact relationship between the signal and con- centration depends on the flip angle used, and be- cause the flip angle varies across the slice in 2D studies, time-consuming corrections are required for 2D data, making these unsuitable for routine clinical applications. Third, to obtain an adequate signal-to-noise ratio, it generally is necessary to use surface array coils for the reception of the signal, which in turn can lead to local variations in signal intensity that complicate the analysis of the data. One approach that the present authors have advo- cated [10] addresses these problems by using a slow injection of contrast over 10 seconds to limit the arterial concentration, by using a 3D technique and discarding the outer slices to ensure a uniform flip angle, and by using the precontrast signal to correct for spatial variations in the signal intensity. Calculation of the individual RBF and GFR from gadolinium-enhanced MRNU can be coupled with measurement of the individual kidney volumes (cortex plus medulla). This technique makes it possible to determine RBF and GFR in proportion to a unit measure of kidney volume that can be ex- pressed, for example, as RBF or GFR per milliliter of kidney. This value may provide an additional func- tional parameter for monitoring renal dysfunction and response to interventions, which previously was not possible in the clinical setting (Fig. 3). Potential applications range across the full spec- trum of renal diseases. Imaging techniques Gadolinium-enhanced renal perfusion-distribution imaging Both 2D and 3D GRE techniques have been pro- posed to capture the critical period when the infused gadolinium arrives in the renal artery. The principle that has been adopted is that the blood flow to the kidney can be determined in the first few seconds as the gadolinium contrast agent perfuses the renal parenchyma; the GFR then can be measured by measuring the total amount of gadolinium agent within the entire kid- ney parenchyma as a function of time with the data collected up to the point of urinary excretion. The strength of 2D techniques is that a turbo-flash sequence can be implemented providing a fast ac- quisition method that is relatively insensitive to motion, as has been used to evaluate cardiac perfu- sion. A limitation of this approach is that volumet- ric determination of total kidney signal and volume is less accurate. Using 3D GRE provides volumetric data for more accurate evaluation of to- tal kidney signal and volume. A challenge has been to acquire 3D GRE with a sufficiently short acqui- sition time to provide the necessary temporal reso- lution demanded from the kinetic modeling. Volumetric GRE also is more motion sensitive. The present authors have approached this problem by using 3D GRE with a high degree of accelera- tion to achieve the necessary short acquisition time and to reduce motion sensitivity. Use of sur- face coils with parallel processing inherently cor- rects for coil element sensitivity profile and helps overcome the problem of positional changes in signal intensities within the field of view. The authors have adopted a technique to achieve a long infusion period combined with a minimal gadolinium concentration. The objectives are to produce a more uniform arterial gadolinium con- centration over the period of data collection and to maintain the gadolinium concentration at the lowest detectable level, to minimize susceptibility effects. They administer the gadolinium agent using a dual-syringe power injector at a dose of 0.1 mmol/kg diluted into a total volume of 60 mL with normal saline and injected at a rate of 0.6 mL/s. Renal perfusion imaging is performed during the first pass using a coronal 3D GRE tech- nique with fat saturation and centric-radial k-space acquisition using a 430-mm 2 field of view, 96 ma- trix (60% scan percentage, reconstructed to 256), recovery time/echo time/flip angle of 3.7/1.7ms/ 30 , 30 slices at a 2.8-mm slice thickness, 120 k-lines/segment, and a sensitivity encoding factor of 3. These parameters result in an acquisition time of 0.9 seconds per dynamic scan. The resul- tant images have an acceptable signal-to-noise ratio and provide adequate spatial resolution. A benefit of this highly accelerated acquisition time is that the imaging may be performed during normal breathing with negligible motion-related image deterioration. Magnetic Resonance Nephrourography 15 Nuclear Imaging in the Genitourinary Tract: Recent Advances and Future Dir ections Wei He, MD,AlanJ.Fischman,MD, PhD * For almost 3 decades, noninvasive radionuclide procedures for the evaluation of renal disease have been important components of nuclear medi- cine practice [1–3]. With the introduction of new imaging agents and procedures, these techniques can provide valuable data on perfusion and func- tion of individual kidneys. In general, these proce- dures are easy to perform and carry a low radiation burden, and sedation is not required. Moreover, radionuclide imaging of the genitourinary tract has become an invaluable asset to clinicians in the evaluation of renal parenchyma and urologic ab- normalities [4]. Nuclear medicine procedures in addition to other modalities, such as CT, MR imaging, and ul- trasound (US), constantly are evolving and finding greater and greater applications in nephrology and urology. The specific areas in which radionuclide techniques play a key role include measurement of renal function, assessment of obstruction, RADIOLOGIC CLINICS OF NORTH AMERICA Radiol Clin N Am 46 (2008) 25–43 Division of Nuclear Medicine, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit Street, Boston, MA 02114, USA * Corresponding author. E-mail address: fischman@pet.mgh.harvard.edu (A.J. Fischman). - Camera-based radionuclide assessment of glomerular filtration rate using 99m Tc- labeled diethylenetriamine pentaacetic acid Indications Pitfalls and limitations Future prospects - Determination of glomerular filtration rate by CT and MR imaging - Diuretic renography Indications for diuretic renography Pitfalls and limitations Future prospects - Other imaging modalities Clinical applications - Angiotension-coverting enzyme inhibition renography Indications Pitfalls and limitations - Other imaging modalities Future prospects Clinical applications Indications Pitfalls and limitations Future prospects - Other imaging modalities Clinical applications Indications Pitfalls and limitations Future prospects - Other modalities Clinical application: pyelonephritis and renal cortical scarring - Renal transplant evaluation - Summary - References 25 0033-8389/08/$ – see front matter ª 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.rcl.2008.01.006 radiologic.theclinics.com . range across the full spec- trum of renal diseases. Imaging techniques Gadolinium-enhanced renal perfusion-distribution imaging Both 2D and 3D GRE techniques have been pro- posed to capture the. a coronal 3D GRE tech- nique with fat saturation and centric-radial k-space acquisition using a 430-mm 2 field of view, 96 ma- trix (60% scan percentage, reconstructed to 25 6), recovery time/echo. prospects - Other modalities Clinical application: pyelonephritis and renal cortical scarring - Renal transplant evaluation - Summary - References 25 003 3-8 389/08/$ – see front matter ª 20 08 Elsevier