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-d
Trang 1as 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, producalter-ing
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 acquisiaccelera-tion 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-mm2field 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
Trang 2Nuclear Imaging in the
Genitourinary Tract: Recent
Advances and Future Directions
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,
R A D I O L O G I C
C L I N I C S
O F N O R T H A M E R I C A
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 using99m
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