The document discusses the advantages of multidetector computed tomography (MDCT) in evaluating urologic conditions, specifically urinary calculi, as compared to traditional intravenous urography (IVU). It highlights the effectiveness of maximum-intensity projection (MIP) images for visualizing stones alongside ureteric stents, while also addressing limitations in size measurement. The study concludes that CT urography is the preferred imaging technique for comprehensive evaluation of the urinary tract.

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2. IINNTTRROODDUUCCTTIIOONN
With the introduction of multidetector technology,
CT urography, to date, has emerged as the initial heir
apparent to intravenous urography; many years of
experience have now clearly demonstrated that CT is
the test of choice for many urologic problems,
including urolithiasis, renal masses, urinary tract
infection, trauma, and obstructive uropathy.

3. CT urography provides a detailed anatomic depiction of
each of the major portions of the urinary tract—the
kidneys, intrarenal collecting systems, ureters, and
bladder—and thus allows patients with hematuria to be
evaluated comprehensively ©RSNA, 2009.

4. Because it provides fast results, unenhanced MDCT
has largely supplanted intravenous urography, and,
in the United States, MSCT generally prevails as
the standard examination for detecting ureteral
calculi (Wehrschuetz et. al., 2008).

5. OOBBJJEECCTTIIVVEE
The objective of the current study is highlighting the
rule of MIP images among the various image
processing techniques in the special situation when
ureteric stents are implemented or when the urinary
tract was unintentionally opacified by IV contrast.

6. PPAATTIIEENNTTSS AANNDD MMEETTHHOODDSS
Study Population
A total of 23 consecutive patients (18 males and 5
females, median age 41.2 years, range 2-65) with
suspected urinary calculi were included in this
retropective pilot study from April through September
2009.
Twelve patients were referred for detection of urinary
calculi after inconclusive intravenous uroengraphy
(IVU) and 11 patients were referred after placement of
ureteric stents. In the former group IV contrast was
administered 1-24 hours prior to CT examination.

7. Imaging protocol
Unenhanced MDCT images were acquired with a 64
detector row CT scanner VCT GE, Milwaukee, WI (18
patients) or a 16 detector row CT scanner Brightspeed
S GE, Milwaukee, WI (5 patients).
Five-millimeter contiguous unenhanced axial CT
images were obtained in a cephalocaudal direction
from the diaphragm to the symphysis pubis. No oral or
IV contrast was administered.

8. Scanning protocol
For the 64-slice scanner, scans were obtained with tube
rotation time 0.8sec, pitch 0.984:1, table feed/gantry
rotation 39.75mm, a tube voltage of 140 kVp and
effective tube current-time product of 548mAs.
For the 16-slice scanner, scans were obtained with tube
rotation time 0.8sec, pitch 1.375:1, table feed/gantry
rotation 27.5mm, a tube voltage of 120 kVp and
effective tube current-time product of 248mAs.
The images were then reconstructed at 0.625 mm thick
slices.

9. Image processing
All datasets were sent to a commercially available
workstation (ADW4.4 or ADW4.3). In all patients
5mm MPR images were obtained in addition to the
following:
1) Thick slab MIP.
2)Volume rendering
3)Curved reformats.

10. Image evaluation
Two independent radiologists and a urologist analyzed
the images. The observers were asked to separately
determine whether pyelocalyceal and ureteral calculi
were present. They were instructed to document the
location and size of each calculus using standard
measurement devices.

11. RREESSUULLTTSS
Urinary calculi were detected in 10 patients referred
after inconclusive IVU and in 8 patients with ureteric
stents. In 2 patients with inconclusive IVU ureteric
stricture was the alternative diagnosis.

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13. Distinction between the urinary calculi and/or contrast
and ureteric stents was not possible in the standard
window for viewing the abdomen. In the VR images
calculi can be demonstrated against ureteric stent by
their morphology despite of having the same density
but they could not be demonstrated within the
opacified collecting system. All the stones were clearly
depicted against the contrasted lumina (by having
higher density) or the ureteric stents (by having lower
density) in thick slab MIP at bone window setting.

14. Bone windowing also facilitates visualization of stones
against the pelvic bones or the spine.
The major weakness of MIP images was the inability
to measure the size of the stones. VR or MPR were
then reviewed for that context.

15. RREEPPRREESSEENNTTAATTIIVVEE CCAASSEESS

23. DDIISSCCUUSSSSIIOONN
The use of maximum-intensity projection (MIP) and
images in CT urography was initially described by
McNicholas et al (1998). To our knowledge it was
applied in the to date puplished literatures describing
various techniques of CT urography. Since the
majority of these studies were dedicated to detection of
urothelial abnormalities they reported lower sensitivity
of MIP images than thin section MPR images (Caoili
et al. 2005), (Chow and Sommer 2001) in detection of
similar lesions.

24. In our study, however, higher sensitivity of MIP
images was evident. This is due to higher density of
the stones compared with the low density uroepithelial
lesions that may be masked by the contrast
concentrated within the collecting system. Moreover
the low contrast density in the collecting system
caused by poor renal contrast concentration helped
detection of the calculi.

25. The use of bone window for distinguishing stones from
stent was reported by Tarnikut et al. (2004). In a series
of a 3 patients they used bone window to distinguish
stones from nephrostomy tubes in axial CT images
after measuring the in vitro density values for stents.

26. The higher sensitivity of MIP images compared with
that of VR images is related to the rendering technique
as stated by Fishman et al (2006). VR images are
formed by binary technique; voxels in volume are
considered as either containing the specified densi
Inaccurate size measurement of the recorded
abnormality at MIP images is explained by lack of
depth (or Z-axis perception) in the 2D image that
represent the MIP slab.

27. CONCLUSION AANNDD RREECCOOMMMMEENNDDAATTIIOONNSS
The CTU has become the radiologists most robust
imaging tool for the evaluation of the kidneys, upper
urinary tracts, and UB.
Complete imaging of the kidneys and urinary tracts can
be performed with MDCT. Studies are tailored to the
clinical question and may be performed as noncontrast,
combined non-contrast and post-contrast or post-contrast
imaging studies only.

28. Finally radiologists need to educate emergency room
and referring physicians about the limitations of
unenhanced CT scans, as it does not detect infarcts,
pyelonephritis, small renal cell carcinomas, or small
ureteral tumors.

29. TTHHAANNKK YYOOUU