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Urinary Tract Imaging
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The
advantages of multidetector CT urography over conventional plain film
excretory urography (also known as IV pyelography and IV urography) and
ultrasound for the evaluation of the urinary tract are numerous.
Dedicated CT protocols have been developed for these new high speed
machines for different clinical indications including "stone protocol"
for the evaluation of urinary tract calculi, CT urography for the
evaluation of patients with hematuria and "renal mass protocol" for the
characterization of known renal masses. Multidetector CT scanning is
fast, taking around 15 seconds for image acquisition from the kidneys
to the bladder during a single breath-hold. The images have good
spatial resolution, little mis-registration due to respiratory
movement, and the acquisition of multiple thin slices allows excellent
two- and three-dimensional reconstructions of the abdominal anatomy,
making it possible to detect pathologies outside the urinary tract as
well those within. Iodinated contrast agents are not usually required
for the detection of renal stones, thus avoiding the risk of adverse
reactions to these agents, but are routinely used in CT urography and
the renal mass protocol.
In comparison to CT, plain
film excretory urography offers excellent delineation of calyceal and
papillary anatomy, the ureters and bladder, but it is inferior to multi
detector CT for imaging of the kidney parenchyma. Both CT and plain
film excretory |
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Three
dimensional coronal reconstruction of CT urography image, showing
contrast-enhanced renal collecting system, ureters, and bladder. Note
duplicated system on left side.
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urography
are associated with a substantial radiation dose. Ultrasound is good
for imaging the kidney parenchyma and for detecting hydronephrosis,
does not require the administration of iodinated contrast, and avoids
radiation exposure. However, ultrasound is not good for detecting
urinary tract calculi, and does not adequately image the renal
collecting system or ureters. For these reasons, multidetector CT
imaging has become the gold standard for the diagnosis of urinary tract
calculi, the investigation of hematuria, and the characterization of
renal masses and has largely replaced both plain film excretory
urography and ultrasound examinations for these purposes.
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Urinary Tract Imaging Protocols
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Stone Protocol
For detection of renal, ureteral, or bladder stones
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Non-contrast CT imaging from kidney to bladder.
(May be necessary infrequently to use iodinated contrast agent to
distinguish between ureteral stones and phleboliths)
Follow-up imaging with non-contrast plain film radiography
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CT Urography (Hematuria Protocol)
For evaluation for common causes of persistent hematuria, i.e. stones, urothelial tumors, renal tumors
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Non-contrast followed by contrast CT imaging from kidney to bladder
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Renal Mass Protocol
For characterization of renal masses detected by other imaging studies, e.g. ultrasound, MRI
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Non-contrast followed by contrast CT imaging of kidneys only
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Axial image from "stone protocol" CT showing left ureteral stone.
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Urolithiasis and Nephrolithiasis
Almost
all ureteral and renal stones, including those containing uric acid,
can be detected by non-contrast CT imaging. The accuracy of the
technique in diagnosing urolithiasis in patients presenting with acute
flank pain has been determined to be as high as 97%, with a sensitivity
of 95% and specificity of 98%. This compares to sensitivities in the
range of 45-58% for non-contrast plain film radiography and 64-87% for
plain film excretory urography.
However, it is
occasionally difficult to distinguish between non-obstructing distal
ureteral calculi and pelvic phleboliths on non-contrast CT images. In
these cases, it may be necessary to use intravenous contrast agent, so
that the relationship of the calculus to the opacified ureter can be
determined. Another situation in which intravenous contrast can be
helpful is in the detection of stones in HIV positive patients on
protease inhibitors such as Indinovir. These calculi are typically
non-radio opaque and may go undetected on stone protocol CT scans. The
use of 3-D reconstruction techniques of contrast-enhanced pyelographic
phase images can be helpful in all of these situations.
The
disadvantage of "stone protocol" CT is that the radiation dose is high
(about 500 mrem) compared to that needed for plain film excretory
urography (about 150-350 mrem) and non-contrast plain film radiography
(about 13 mrem). This exposure is a significant concern, especially as
urinary stones frequently affect young people. For this reason, it is
better to avoid CT for follow up studies wherever possible and to use
non-contrast plain film radiography instead. The initial CT scan and
reconstruction images can be used to aid subsequent detection of stones
on follow-up plain film radiographic images since the detection rate of
stones increases from 45% identified on non-contrast radiographic films
alone to 78% on films viewed together with 3-D reconstructions of the
initial diagnostic CT images.
Pregnant patients
should be evaluated initially with ultrasound imaging, to avoid
exposure to any unnecessary radiation, and MR urography is an
alternative imaging technique for evaluating the renal system in
pregnant women, children, and patients with contraindications to
contrast agents.
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Hematuria
The
main causes of hematuria are urinary tract calculi, renal tumors,
urothelial tumors, and infection. CT urography is the best single
diagnostic examination for diagnosing all of these pathologies, with
the exception of infection, which is effectively diagnosed in most
cases by microbiological analysis of the urine.
CT
urography requires the use of contrast agent to opacify the collecting
system, the ureters, and the bladder. In addition to optimal
opacification, distension appears to be an important requirement for
thorough evaluation of the renal collecting system and ureter. For this
reason, intravenous saline is given at the same time as the contrast
material to aid in the detection of subtle filling defects and the
discrimination between urothelial neoplasms and other filling defects.
Image reconstruction techniques are used to create images of the entire
length of the urinary system from the kidneys to the bladder. Multi
planar 3-D reconstruction can provide the anatomic detail required to
correlate the finding with retrograde ureterography or to perform an
endoscopic evaluation. CT has been shown to detect parenchymal masses
in the kidney with a sensitivity of 94%, compared to 67% for plain film
excretory urography and 79% for ultrasound.
Another potential advantage of CT is that reconstructed images can show
tumors in a filled bladder opacified with contrast agent ("virtual
cytoscopy"). However, conventional cytoscopy remains the gold standard
for the detection of tumors of the bladder, as it will detect early
color-changing mucosal lesions that do not deform the contour of the
bladder wall. In addition, cytoscopy has the added capability of biopsy
of suspicious lesions.
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Axial CT urography image showing a filling defect in the right renal pelvis consistent with a large urothelial tumor.
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Renal masses
Many renal lesions are incidentally detected on a variety of imaging
tests, but cannot usually be characterized at the time of detection.
Currently, at this institution "renal-mass protocol" CT is the gold
standard for the characterization of renal masses. This protocol
acquires thin section images of the kidneys before and after
intravenous contrast administration to evaluate the important
characteristic of solid lesions, the unequivocal demonstration of
lesion enhancement post contrast. Lesions that demonstrate unequivocal
enhancement require histologic diagnosis either by image-guided biopsy
or by surgical resection.
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Scheduling
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Further Information
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CT
imaging of the urinary system has essentially replaced conventional
plain film excretory urography at MGH. CT scanning for stones,
hematuria, or for evaluating renal masses are performed at Mass General
West Imaging in Waltham, Mass General Imaging in Chelsea, or the main
MGH campus. The appropriate CT protocol will be selected by the
radiologist based on the clinical history of the patient. CT imaging
studies can be ordered by calling 4-XRAY (617-724-9729). Results are
made available to physicians online within 24-48 hours.
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For further questions on CT urography, contact
Dr. Michael Maher, MGH Department of Radiology,
617-726-8396
This article provided useful information about the appropriate use of imaging studies:
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References
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Kawashima A, Glockner JF, King BF (2003) CT Urography and MR Urography. Radiol Clin North Am. 41:945-61.
Kim, JK, Ahn, JH, Park, T, Ahn, HJ, Kim, CS, Cho, KS. (2002) Virtual Cytoscopy of the Contrast Material-Filled Bladder in Patients with Gross Hematuria. Am J. Roentgenol 179: 763-768.
Maher, MM, Kalra, MK, Mueller, PR, and Saini, S. (2003) Multidetector CT Urography in Imaging the Urinary Tract. in press.
McCollough CH, Bruesewitz MR, Vrtiska TJ, King BF, LeRoy AJ, Quam JP, Hattery RR. (2001) Image
Quality and Dose Comparison among Screen-Film, Computed, and CT Scanned
Projection Radiography: Applications to CT Urography. Radiology. 221: 395-403.
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Portis AJ and Sundaram CP (2001) Diagnosis and Initial Management of Kidney Stones. American Family Physician 63: 1329-1338.
Van Beers, BE, Dechambre, S, Hulcelle, P and Materne, R, and Jamart, J (2001)
Value of Multislice Helical CT Scans and Maximum-Intensity Projection
Images to Improve the Detection of Ureteral Stones at Abdominal
Radiography. Am J Roentgenol 177: 1117-1121.
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