The authors review the rapid progress in the development and implementation of computed tomographic urography (CTU) for detailed evaluation of the urinary tract in patients presenting with painless hematuria. With improved spatial resolution afforded with multidetector CT, CTU has the potential to replace the need for excretory urography in many cases. Optimization of CTU techniques continues in order to resolve several remaining limitations of this approach.
is a PGY1 Resident in the Department of General Surgery, and
is an Assistant Professor in the Department of Radiology,
Stanford University School of Medicine, Stanford, CA.
will be beginning a residency in diagnostic radiology at the
University of California, San Diego.
Painless hematuria is a relatively common condition that is an
important warning sign of urologic malignancy. The prevalence of
microscopic hematuria has been reported to occur in 2.5% to 21.1%
of the population.
Urologic malignancy appears to occur in 2% to 3% of all patients
with microscopic hematuria
and has been reported to occur in up to 8% to 9% of those >50
years of age.
Although the causes of hematuria include many benign etiologies,
malignancies, such as renal cell and transitional cell carcinoma,
must be ruled out by radiologic and urologic examination. It is
widely accepted that the development of gross hematuria
necessitates a thorough diagnostic evaluation. For microscopic
hematuria, it is suggested that detection of >2 to 3 red blood
cells (RBCs) per high-power field (HPF) warrants a full diagnostic
Thorough evaluation for a finding of 1 to 3 RBCs per HPF is also
encouraged in patients with formal risk factors for urologic
malignancy, including: age >45 years, smoking history, exposure
to arylamines found in chemicals or dyes, history of pelvic
irradiation, and history of phena-cetin or cyclophosphamide use.
The full diagnostic work-up for hematuria includes evaluation of
the renal parenchyma and urothelium. Traditionally, this work-up
has been performed with cystoscopy to evaluate the bladder and
excretory urography (EU) to evaluate the upper urinary tract and
kidneys. In the most recent American Urological Association (AUA)
Best Practice Policy Recommendations, it was suggested that either
EU or computed tomography (CT) is appropriate for the standard
screening of adult patients with painless hematuria.
Although EU can define the pelvicalyceal structures and ureters in
exquisite detail, it is insensitive in detecting small renal
parenchymal masses, even when conventional tomography is employed.
The sensitivity of EU for detecting renal masses >3 cm in size
was reported to be 85%, with the sensitivity dropping off to 52%
for masses 2 to 3 cm in size and 21% for masses <2 cm in size.
Although ultrasound is very effective at detecting renal cystic
lesions, this modality also has poor sensitivity for detecting
solid renal lesions <3 cm.
For these reasons, even prior to the publication of the AUA
recommendations, many institutions had already added
cross-sectional imaging, including magnetic resonance imaging (MRI)
or contrast-enhanced CT, to the standard work-up of hematuria to
increase the sensitivity for detecting renal parenchymal
It is now widely accepted that nonenhanced CT is superior to EU
for detecting urinary tract calculi in patients presenting with
acute fiank pain.
Additionally, contrast-enhanced helical CT is the most common
cross-sectional imaging modality used to effectively evaluate the
renal parenchyma. However, to date, CT has not replaced EU for
evaluation of the upper urinary tract because conventional CT
protocols do not depict the renal collecting structures and ureters
to good advantage. Its sensitivity for detecting small urothelial
lesions has been reported to range from 50% to 90%,
with several limiting factors, including poor longitudinal
resolution, poor distention of the collecting system, and
obscuration of the urothelium by dense intraluminal contrast. Given
these limitations, many medical centers have been reluctant to
completely replace EU with CT and instead prefer to use both
examinations to complement their sensitivities for detecting
urothelial lesions, renal parenchymal pathology, and urinary
In order to simplify the approach, shorten the work-up duration,
and reduce costs, there has been a drive to develop a single
examination to effectively evaluate the kidneys and upper urinary
tract to-gether. Various strategies have been used to achieve this
end. Hybrid techniques have included following EU with immediate CT
or following contrast-enhanced CT with immediate EU
without using additional contrast medium. However, these techniques
are limited by the logistical difficulties of transferring patients
rapidly between the CT scanner and conventional radiography rooms.
To overcome this limitation, the use of an auxiliary CT tabletop,
as described by McCollough et al,
allows the acquisition of plain radiographs with the patient on the
CT table and, thus, permits the acquisition of a CT and EU study in
a single sitting. This strategy is limited by the requirements for
significant hardware modifications. Ideally, an examination
performed with a single imaging modality would greatly simplify the
The advent of multidetector-row computed tomography (MDCT) has
dramatically improved the speed with which isotropic CT data can be
acquired. By combining the improved spatial resolution of MDCT with
strategies to distend the urinary tract and scanning during
appropriate phases of enhancement, it is possible to perform a
study capable of showing the renal parenchyma, uro-thelium, and
urinary tract calculi in a single examination, which is called CT
A CTU protocol should be designed to optimize visualization of
renal stones, the renal parenchyma, and the urothelium. Therefore,
it is recommended that patients be imaged at three phases of renal
enhancement: the precontrast, nephrographic, and excretory phases.
Urinary tract calculi are optimally detected using nonenhanced
images with very high sensitivity and specificity.
Postcontrast images are less sensitive in diagnosing
nephrolithiasis, as contrast material may obscure calculi. Thus,
the first phase of a CTU study involves a nonenhanced, helical
acquisition through the abdomen and pelvis, much like a renal colic
The detection of renal parenchymal abnormalities relies most
heavily on images acquired during the nephrographic phase.
Corticomedullary phase images are of limited value in this clinical
context, as several studies have suggested that, compared with
images at the nephrographic phase alone, they offer little added
sensitivity for detecting renal parenchymal lesions.
Nonenhanced images are also important in this context because they
help depict enhancement of potential renal lesions, distinguishing
neoplasms from simple cysts.
The urothelium is best depicted during the delayed, excretory
phase, with opacification of the renal collecting structures,
pelvis, and uteters (Figure 1). In order to best evaluate these
urothelium-lined structures, spatial resolution must be maximized.
The in-plane resolution of CT of approximately 0.6 mm combined with
a longitudinal resolution (slice-thickness) of 1.0 to 1.25 mm
should be sufficient to depict urothelial lesions of interest.
Clearly, a multidetector-row scanner is necessary to obtain such a
dataset within a single breath-hold.
While MDCT technology has resolved the problems with
longitudinal spatial resolution that were previously encountered
with CT, limitations from incomplete opacification of the ureters
and dense intraluminal contrast still need to be addressed.
Incomplete opacification of the ureters can result from inadequate
distension or from peristalsis. With these phenomena, urothelial
lesions can escape detection if they lie within unopacified
segments. Oral or intravenous hydration immediately prior to the
examination promotes a natural diuresis, which, in turn, helps
distend the urinary tract more uniformly.
More adequate distension can be achieved when hydration is
complemented with other techniques. One of these supplemental
strategies is the use of abdominal compression, a technique that
has been employed in EU for decades. Infiation of the balloon
promotes distension of the renal collecting structures and
abdominal ureters, and release of compression allows pooled
contrast and fiuid to fiood the distal ureters. In several studies,
CTU with balloon compression appeared to yield equal or better
opacification of the urinary collecting system compared with EU.
With this technique, however, the collecting structures/abdominal
ureters and the pelvic ureters must be imaged in separate
acquisitions yielding two separate volumetric datasets.
Additionally, one must be cognizant of contraindications to
abdominal compression that include abdominal aortic aneurysms with
or without stent grafts, recent abdominal surgery, abdominal pain,
urinary tract diversions, severe ascites, horseshoe kidney, and
inferior vena cava filters. An alternate strategy used to achieve
uniform opacification is pharmacologic diuresis (Figure 2).
Nolte-Ernsting et al
described the use of 10 mg of furosemide intravenously prior to
scanning in 16 patients and achieved complete or near-complete
opacification of the ureters and pelvicaliceal systems in 94% and
100% of cases, respectively.
Difficulty in evaluating the urothelium can also arise with high
intraluminal contrast density. High concentrations of contrast can
result in linear streak artifacts and loss of fine calyceal detail
that can prevent the detection of small urothelial lesions.
Adequate prehydration not only helps minimize potential
but also helps maximize image quality by diluting excreted contrast
material. Pharmacologic diuresis may also help dilute contrast
material sufficiently to better depict calyceal anatomy and
eliminate streak artifact. In the study performed by Nolte-Ernsting
patients receiving furosemide achieved 4- to 5-fold lower
attenuation in the collecting structures and more even distribution
of excreted contrast compared with patients receiving only 250 mL
of normal saline. The authors also noted that the high density of
contrast in the proximal collecting system of the patients who
received saline alone resulted in blurring and blunting of the fine
calyceal structures, which was not seen in patients who received
Finally, because unopacified segments may also result from
peristalsis, a digital scout image of the abdomen and pelvis
acquired at each phase of the examination can be helpful. The
additional scout images provide opportunities for visualizing any
nondistended segments resulting from peristalsis and can provide
complementary information to the CT acquisition
without a substantial increase in radiation dose (Figure 1).
Technical parameters and reconstruction techniques
Image acquisitions for CTU should be performed on a
multidetector-row scanner in order to maximize spatial resolution.
With 4-, 8-, and 16row scanners, each phase of the study can be
performed with a single breath-hold of <25 seconds. All CT
images are obtained helically with a high pitch value. For
nonenhanced CT acquisitions, axial sections of 3.75-mm slice
thickness and 2.5-mm increment are generated for primary
interpretation. Enhanced CT acquisitions are reconstructed with
overlapping 1.0 or 1.25 mm nominal section thickness in order to
produce two-dimensional reconstruction images of highest quality.
Drawbacks for acquiring such thin sections include decreased
signal-to-noise ratios and large numbers of axial source
Coronally reformatted images through the kidneys and collecting
structures can be produced rapidly on the source scanner and do not
require a separate workstation, yet they present the relevant
anatomy in a more intuitive display plane. If the institution has
access to a dedicated workstation, oblique coronal sliding
thin-slab images can also be postprocessed with maximum intensity
projections (MIP) and minimum intensity projections (MINIP).
Coronal sliding thin-slab reconstructions, initially des-cribed for
use in the thorax by Napel et al,
display the calices and infundibula individually in an intuitive
anatomic plane. Lesions within these structures become easier to
detect as superimposition of overlying structures is eliminated.
This technique also provides an additional plane to help
characterize anatomic relationships (Figure 3). However, with MIP
and average projection (AP) images, smaller filling defects may be
missed if they are contained within the same voxel as intraluminal
contrast due to volume averaging and MIP effects. For this reason,
thinner sections (¡Ü2.5 mm) should be used to minimize voxel size
or additional MINIP images should be produced. Both of these
techniques have the benefit of obviating the need for manual
editing of osseous structures.
Anteroposterior thick-slab (35 to 50 mm) MIP and AP images of
both kidneys and proximal ureters in an oblique coronal plane can
also be helpful by providing images similar to those created by
traditional EU, to which many clinicians are accustomed.
Additionally, a MIP image of the distal ureters and urinary bladder
can be generated to give clinicians another global look at the
lower urinary tract on a single image. Although suboptimal for
detecting small lesions and subtle pathology, both of these
reconstructions together can enhance communication with referring
physicians by depicting the relevant urinary tract anatomy on only
two or three images.
Strengths and weaknesses of various protocol
Although obtaining multiple postcontrast scans during
nephrographic and excretory phases can be advantageous by providing
redundancy, which allows for the potential to capture an
unopacified segment of ureter or collecting system on another
imaging pass, this strategy yields more images for interpretation
and results in higher radiation doses. For a three-phase
multidetector-row CT protocol, McTavish et al
reported an estimated skin dose of 74.1 mGy and an estimated total
effective dose of 22.6 mGy.
Additionally, average effective radiation doses for
multi-detector-row CTU are reported to range from 5 to 10 mSv to 25
to 35 mSv, depending on protocol design.
In order to reduce radiation exposure and the number of images
generated, intravenous contrast material can be administered in a
split bolus to allow for only a single contrast-enhanced scan. By
injecting intravenous contrast material in two se-quential boluses
separated by an appropriate time delay, imaging during synchronous
nephrographic and excretory phases is possible.
If other technical factors are held constant, this strategy reduces
the effective radiation dose associated with the CTU examination
simply by reducing the number of CT acquisitions obtained. The
split-bolus protocol amounts to a roughly equivalent radiation dose
as a standard pre- and post-contrast CT of the abdomen and pelvis,
which many patients will undergo in addition to EU in their
diagnostic evaluation. Therefore, CTU could potentially reduce
overall radiation dose by eliminating the need for two separate
Although it is widely accepted that prehydration provides
nephroprotective effects and is likely to provide imaging benefits
in this context, it is currently not clear whether abdominal
compression or pharmological diuresis provides better results, as
both techniques have been separately shown to improve urinary tract
distension. Abdominal compression can be advantageous because many
radiologists and technologists are familiar with this technique
from their experience with EU and it does not require the
administration of an additional medication. Indeed, to date, most
studies comparing EU with CTU were performed with this technique.
Although further work in this area is necessary, there are several
potential advantages for using pharmacologic diuresis. Furosemide
achieves distension with dilution of contrast, as described by
Nolte-Ernsting et al.
Another advantage of pharmacologic diuresis is that the entire
urinary collecting system can be imaged by only one acquisition,
allowing for reconstructions that depict the urinary collecting
system in its entirety (Figure 2). When abdominal compression is
employed, separate reconstructions of the urinary tract must be
produced for the abdomen and pelvis.
An interpretation of the initial digital scout image should be
performed prior to the examination. This brief preliminary read is
important in order to rule out the above-mentioned
contraindications to abdominal compression and to ensure proper
positioning of the balloon device. Once the study is complete, the
source axial images are interpreted. This primary interpretation
should be performed on a picture archiving and communication system
(PACS) workstation in a cine mode, as it is impractical to read
such a large number of images on hard copy. While the reformatted
images may simplify communication with referring physicians, these
images should not replace an initial systematic interpretation of
the axial source images. Rather, these reconstructions should be
used as an adjunct that may clarify questionable findings or
supplement sensitivity by providing a different anatomic plane for
The unenhanced series is generally interpreted first in similar
fashion to a renal colic study. Note is made of urinary tract
calculi (Figure 4), and the native attenuation of any renal lesions
may be obtained. Additionally, transitional cell carcinomas can
often be identified on this series, as they are outlined by
slightly lower-attenuation, unopacified urine. Interpretation of
the postcontrast compression images should follow. As these images
capture the kidneys and abdominal ureters in both the nephrographic
and delayed excretory phases, they are optimal for identifying
renal parenchymal lesions (Figure 5) and urothelial lesions (Figure
6). After a thorough evaluation of the renal parenchyma with
standard abdominal window/level settings, a dedicated evaluation of
the collecting structures should be performed with different
window/level settings appropriate for these structures.
Unfortunately, because of the variable concentration of contrast
material within the collecting structures, window/level settings
must often be individually tailored on a case-by-case basis. Bone
windows represent a good first approximation, but further
adjustment is often necessary.
With abdominal compression protocols, review of the compression
volume set should be followed by a review of the postrelease volume
set to evaluate the pelvic ureters and bladder for urothelial
lesions as well. The same window/level settings used in the axial
images of the abdomen should also be used in the axial images of
Larger transitional cell carcinomas are easy to detect,
particularly when an obstruction is present. Smaller lesions,
however, pose a greater diagnostic challenge. Based on CT findings,
Baron et al
classified these lesions into three morphologic categories: focal
intraluminal mass, urethral wall thickening, and infiltrating mass.
Focal intraluminal mass lesions may only appear as filling defects
within the collecting structures and ureters (Figure 7). They may
be confused with blood clots or calculi, which also appear as
filling defects. However, transitional cell carcinomas have a
comparatively lower attenuation value, which is slightly greater
than the water density of the surrounding urine
on unenhanced scans. Also, transitional cell carcinomas exhibit
modest enhancement with intravenous contrast.
While analyzing the collecting structures, it is important not to
confuse renal papilla for pathologic filling defects. This
misinterpretation is easily avoided, as a papilla should be
centrally located within a calyx.
While CTU is a very effective tool for evaluating the upper
urinary tract, the protocol described in this paper (Table 1) is
currently is not optimized for depicting the bladder. Because of
the delayed phase of imaging the pelvis after contrast, mixing
artifacts within the bladder can result in both false-positive and
false-negative interpretations (Figure 8). While CT has been shown
to detect bladder lesions to good advantage with CT cystography
protocols and reconstruction strategies,
conventional cystoscopy remains the standard screening tool, as it
not only effectively detects lesions, but also provides tissue
diagnosis at the time of the procedure. At the author's
institution, patients with painless hematuria generally undergo
both CTU and cystoscopy as part of their evaluation.
There has been rapid progress in the development and
implementation of CTU for detailed evaluation of the urinary tract
in patients presenting with painless hematuria. With improved
spatial resolution afforded with MDCT, CTU has the potential to
replace the need for EU in many cases. Optimization of these
techniques is ongoing to resolve several remaining limitations. The
ability to use only one radiologic test with CTU could potentially
reduce costs, ionizing radiation exposure, iodinated contrast
administration, and the procedural duration and complexity in the
standard work-up of painless hematuria.