The clinical uses of computed tomographic angiography (CTA) and CT venography (CTV) in the urinary tract fall into three broad categories. The ﬁrst is in the examination of renal arterial disease, including renal artery stenosis and stents, and renal aneurysms. The second is in the assessment of renal vascular anatomy, for surgical planning prior to nephron-sparing surgery, evaluation of living renal transplant donors, or examination of crossing vessels in ureteropelvic junction obstruction. The third is in the examination of renal vein thrombosis, including tumor thrombus.
is Head of the Section of Abdominal Imaging, Division of
Radiology, Cleveland Clinic Foundation, Cleveland, OH.
The clinical uses of computed tomographic angiography (CTA) and
CT venography (CTV) in the urinary tract fall into three broad
categories. The first is in the examination of renal arterial
disease, including renal artery stenosis and stents, and renal
aneurysms. The second is in the assessment of renal vascular
anatomy, for surgical planning prior to nephron-sparing surgery,
evaluation of living renal transplant donors, or examination of
crossing vessels in ureteropelvic junction obstruction. The third
is in the examination of renal vein thrombosis, including tumor
In any CT examination of the kidney, certain considerations must
be kept in mind. In many examinations, it is necessary to image
both the renal arterial and renal venous systems. The examination
necessarily includes the renal parenchyma; therefore, it is
important to follow basic renal imaging principles, including the
use of a consistent slice thickness and collimation for all scan
phases and the incorporation of techniques that enable
identification of calcifications and characterization of renal
lesions. It is also important to determine whether the examination
will include the collecting system. Furthermore, the kidneys may
differ in size, position, and function. Renal vascular anomalies
are common and highly variable.
The kidneys actively concentrate contrast. Poor kidney function
is often associated with a reduction in blood supply, which may
limit the effectiveness of CTA. Therefore, adequate renal function
is often necessary for disease detection. Finally, contrast
material must be used with care in patients with renal
in-sufficiency to reduce the risk of contrast-associated
Various definitions have been used to describe
contrast-associated nephropathy. It has been reported in the
literature as an increase above the baseline creatinine level of
20% to 50%, or an absolute increase in serum creatinine of 0.5 to
There are many risk factors for contrast-associated nephropathy,
including pre-existing renal insufficiency (serum creatinine
>1.5 mg/dL), diabetes mellitus, age >70 years, and
The combination of diabetes and pre-existing renal insufficiency
poses the highest risk for contrast-asso-ciated nephropathy, as
much as 5 times the general risk.
In most cases, contrast nephropathy is self-limiting and its
clinical course is predictable. Typically, the serum creatinine
level begins to rise within 24 hours of contrast exposure, peaks
within 96 hours (4 days), and then returns to baseline, with little
Therefore, concern over the possibility of contrast-associated
nephropathy should not preclude a contrast-enhanced CT examination
in a patient who needs it.
At the Cleveland Clinic, our practice when performing CTA and
CTV of the kidneys in patients with a normal serum creatinine level
(<1.5 mg/dL) is to use 150 mL of low-osmolar, nonionic contrast
media (Table 1). Given an iodine concentration of 300 mg/mL, this
results in a total iodine load of 45 g.
In patients with a mildly elevated serum creatinine level (1.5
to 1.9 mg/dL) and no other risk factors, we use our standard
low-osmolar nonionic contrast agent and standard contrast load, but
hydrate with oral fiuids after the procedure to ensure that
dehydration does not increase the risk of contrast-associated
nephropathy. If the patient has additional risk factors, including
diabetes, older age, or previous renal insufficiency, we begin
hydration prior to the procedure and consider using an isosmolar
contrast agent, such as iodixanol.
In patients with moderate renal insufficiency (serum creatinine
2.0 to 2.4 mg/dL), it is important to consider using magnetic
resonance imaging (MRI) or ultrasound (US) as an alternative
imaging method. If contrast-enhanced CT is necessary, the patient
should undergo intravenous (IV) hydration before the procedure, and
oral or IV hydration afterward. The patient should also receive an
isosmolar contrast agent to minimize the risks of
In patients with a serum creatinine level >2.5 mg/dL,
contrast-enhanced CT is generally not recommended, as poor renal
function will likely preclude adequate assessment of the kidneys
and renal vasculature. MRI and US are acceptable alternatives in
most patients. When CT is necessary, it is essential to hydrate the
patient intravenously both before and after the procedure and use
an isosmolar contrast agent.
Intravenous hydration may be contraindicated in patients with
severe heart failure or other conditions that require fiuid
restriction. In general, however, outpatients who require hydration
receive a 3-hour infusion of 500 mL of half-normal saline before
the examination and are instructed to push oral fiuids
It is possible to control hydration more closely in hospitalized
patients. At our institution, inpatients receive an infusion of
half-normal saline at 100 mL/hr for 12 hours before, and 12 hours
after, the examination,
an approach that has been developed in association with our team of
Contrast injection protocols
Factors to consider in developing contrast injection protocols
include the rate of injection, contrast viscosity, scan duration,
injection duration, contrast volume, and cost. When CTA is
performed alone, it is preferable for the injection duration and
scan duration to be equivalent to maintain high levels of con
trast enhancement, regardless of the scan delay. If, however,
CTA is performed in combination with diagnostic CT, it will be
necessary to inject a certain minimum iodine load to enable imaging
of the liver and kidneys. For studies that combine diagnostic CT
with CTA, we use 150 mL of 300 mg I/mL contrast material. After a
20-mL test bolus, we inject the remaining 130 mL at 4 mL/sec for
approximately 30 seconds (Table 2).
For renal CTA alone, we use a higher-concentration agent (370 mg
I/mL) and reduce the intravenous injection rate to 3 to 3.5 mL/sec
to accommodate the higher viscosity of the contrast material. Total
contrast volume is approximately 100 mL, and injection duration is
approximately 33 seconds.
With most studies, we determine optimal scan timing through
injection of a test bolus of contrast. Scanning the upper abdominal
aorta, we use a single slice and no table movement. Starting 10
seconds after contrast injection, we scan every second for 30
seconds. The time to peak enhancement determines the scan delay;
however, 5 seconds should be added to the scan delay if the study
will include examination of the renal veins.
Use of a 20-mL saline fiush can improve enhancement when
injecting low volumes of high-concentration contrast media. We do
not routinely use a saline fiush when CTA studies are performed
with a 300 mg I/mL contrast agent. We do, however, routinely
hand-inject a 100-mL saline fiush during CT urography, as we find
that it results in better contrast opacification of the collecting
The three-phase helical CT scan we routinely use to image the
kidneys consists of an unenhanced phase, a vascular or
corticomedullary phase (which is useful for performing both CTA and
CTV), and a nephrographic or parenchymal phase (which can also be
used for performing CTV).
The unenhanced CT scan localizes the kidneys in anticipation of
the contrast-enhanced examination. Calcifications can also be seen
during the unenhanced phase. These may be renal calculi or vascular
calcifications, such as at the renal artery ostia or in an
aneurysm. Calcifications may also be present in the wall or septae
of a complex cyst. The unenhanced scan is also required for renal
lesion characterization, as it provides the baseline attenuation
for assessing en-hancement after contrast.
The vascular-phase scan generally extends from above the celiac
axis through the common iliac arteries.
Recently, we have expanded the scan range to include the diaphragm,
as renal veins and arteries are widely variable in both their
origin and course (Figure 1). The renal arteries can arise from
anywhere along the abdominal aorta or even the common iliac
During the corticomedullary or vascular phase, scanning is timed
to coincide with renal arterial and venous enhancement. Generally,
the scan delay is approximately 20 to 35 seconds after injection,
but the exact timing is best determined with a timing bolus. In the
corticomedullary or vascular phase, the renal cortex enhances, but
there is little contrast concentration by the renal medulla.
Contrast will be observed in the collecting system only if a
preload or timing bolus is used.
The nephrographic or parenchymal phase is most sensitive for the
detection and characterization of renal lesions.
It also provides more consistent opacification of the renal veins
and the inferior vena cava (IVC). Therefore, this is often the best
phase to use for imaging of the branch vessels, such as the
adrenal, lumbar, and gonadal veins.
In the past, we used a 4-detector-row scanner for renal CTA.
Today, however, we scan renal patients on a 16-row multidetector CT
scanner whenever possible. In addition to providing greater image
detail, the 16-row scanner enables much more rapid reconstruction
and avoids tube-cooling delays.
With the 16-row scanner, we use a 0.75-mm collimation for all 16
rows, which results in a table movement of 12 mm per rotation. At
0.5 seconds per revolution, and a coverage of 24 mm/sec, we can
complete the vascular phase of the scan relatively quickly.
Standard settings include 120 kV and an effective mA of 200.
After image acquisition, we create 2 sets of images. One, a
diagnostic set intended for review on the picture archiving and
communications systems (PACS) and for filming, uses 3-mm-thick
slices and a 3-mm reconstruction interval. Creating this set of
images takes no additional time on our scanner (Siemens Sensation
16, Siemens Medical Solutions, Malvern, PA). The second set of
images, intended for both two- (2D) and three-dimensional (3D) CTA,
uses 1-mm-thick slices and a 0.8-mm reconstruction interval.
With the 4-detector-row scanner, we use 2.5-mm collimation for
all 4 rows and a table speed of 10 mm per rotation. At 0.5 seconds
per rotation, the resulting 20 mm/sec coverage is only slightly
less than that achieved with our 16-row scanner. It is also
possible to use a 1-mm collimation and a table movement of 4 mm per
rotation, which results in a coverage of 8 mm/sec at a 0.5-second
rotation time. This approach results in one-third the coverage of a
16-row scanner, however, and prohibits scanning from the diaphragm
through the iliac arteries.
With the 4-detector-row scanner, we create 1 set of diagnostic
images for PACS review, filming, and 2D and 3D CTA, using
3-mm-thick slices and a 1.5-mm reconstruction interval.
Renal arterial disease
The imaging of renal arterial disease encompasses renal artery
stenosis, renal artery aneurysms, and renal vascular diseases, such
as fibromuscular dysplasia. Renal artery stenosis remains a
difficult imaging challenge. A thorough assessment requires both
anatomic and functional information, which may not be possible with
a single test.
A key goal of noninvasive imaging of renal artery stenosis is to
avoid renal arteriography. Although arteriography is considered the
"gold standard" for the assessment of renal artery stenosis, the
risks of an invasive procedure may not be justified, given the low
prevalence of renal artery stenosis as a cause of hypertension (3%
At the same time, noninvasive assessment must achieve a high
sensitivity to be an effective screening study.
According to the American College of Radiology criteria for the
radiologic investigation of renal artery stenosis, this condition
is defined as a reduction in vessel diameter of >50%.
Without functional information, however, the diagnosis is actually
achieved in reverse: If treatment reduces the blood pressure, then
the diagnosis of renal artery stenosis is considered, in
retrospect, to have been correct.
There are several options for the diagnosis of renal artery
stenosis, including US, MR angiography, CTA, digital subtraction
angiography, and measurement of selective renal vein renins.
Ultrasound is noninvasive, inexpensive, and does not require
iodinated contrast media or radiation. Its sensitivity, however, is
reported in the scientific literature to range from 0% to 90% based
on peak systolic velocities and the parvus-tardus waveform.
Gadolinium-enhanced MRA is highly sensitive in the proximal renal
vessels (77% to 100%)
and is useful in patients with reduced renal function.
CT angiography requires the use of radiation and iodinated
contrast media. It is sensitive in the proximal vessel as well,
with a sensitivity for stenosis of 88% to 96%.
Its overall use is limited, however, because many patients with
suspected renal artery stenosis have renal insufficiency and are at
risk for contrast-associated nephrotoxicity.
It is important to note that the evaluation of renal artery
stenosis is complicated by the existence of multiple renal arteries
in 25% to 33% of patients, or 15% to 25% of kidneys. Such
anatomical features as a small accessory artery, an early branch
artery, or 2 closely situated branch arteries present major imaging
challenges, particularly for US.
The CT evaluation of renal artery stent patency calls for the
use of either coronal multiplanar reformations (MPRs) or curved
MPRs to depict the entire renal artery. CT angiography can depict
the interior of the stent as well.
CT angiography can easily detect renal artery aneurysms. In
addition, by depicting the neck of the aneurysm and branch vessels,
it can help target surgical treatment or embolization. It is
important that the examination include 3D renderings, however. A
bright area of contrast on the axial images may be mistaken for the
renal pelvis, for example, whereas on 3D imaging the same finding
is more easily indentified as an aneurysm in the renal hilum
We generally reconstruct CTA data as thin-slice maximum
intensity projections (thin MIPs) or MPRs, as it eases
visualization of curved structures that run in and out of a scan
plane. It is possible to scroll back and forth through a series of
thin-slab MIPs, easily identifying the renal vasculature. This
approach also eliminates the need for editing when doing MPRs.
The Cleveland Clinic Foundation is a major referral center for
nephron-sparing surgery. The evaluation of candidates for this
procedure is the most common reason for performing renal CTA and
CTV in our institution.
Nephron-sparing surgery is indicated in patients with renal
neoplasms who would otherwise require dialysis after conventional
surgery, or radical nephrectomy. Such patients may have a solitary
functional kidney, bilateral renal tumors, or underlying renal
disease that would necessitate dialysis following radical
The purpose of CTA and CTV in such cases is to localize all of
the arteries, veins, and major branch vessels. It is important to
remember that the renal arteries are end-arteries; therefore,
preserving renal function necessitates preserving the renal
There are 4 segmental branches of the renal artery, each named
for the renal parenchymal segment it supplies. These branches are
the apical, basilar, anterior, and posterior segmental branches.
With renal CTA, it is usually possible to identify at least 3-and
often all 4-of these segmental renal branch vessels.
Renal arterial anatomy is highly variable. Accessory right renal
arteries can course anterior to the IVC, rather than in the normal
posterior position. A patient may have multiple renal arteries on
one or both sides (Figure 3). Even in the case of a single renal
artery, the point of origin can vary widely from patient to
patient, so it is important that the scan extend through the common
It is also important to look for early branch vessels. Some
branches, such as the apical polar branch shown in Figure 4, go
directly into the renal cortex, rather than entering at the renal
hilum. Others may originate near the renal artery ostia, an
anatomical variation that the surgeon must be aware of when
Renal vein anatomy
The right renal vein may be a single vessel or multiple vessels,
and may enter the IVC from anterior, lateral, or posterior
positions. The right gonadal vein typically enters the IVC
directly, but in some cases joins an inferior accessory right renal
The left renal vein usually courses anterior to the aorta, with
the adrenal and gonadal veins emptying directly into it. There are,
however, circumaortic and retroaortic variants of the left renal
vein (Figure 5), as well as variants of the adrenal and gonadal
Living renal donors
Multiple studies have shown that CTA and MRA compare favorably
with angiography for the evaluation of potential living renal
CTA yields better venous assessment and, when combined with 3D
reconstructions, better depicts anatomic relationships.
In addition, CTA is a single, minimally invasive test that can
replace a combination of tests, such as US or conventional CT
combined with angiography and postangiography intravenous
urography. Thus, the use of CTA reduces costs and inconvenience to
When evaluating a potential renal transplant donor, the primary
goal is to ensure the health of the donor, thus each donor first
completes a full medical screening. Once they are determined to be
a suitable donor candidate, each donor then undergoes an assessment
of the renal vascular anatomy to determine the preferred side for
donation in order to maximize the likelihood of graft success.
In potential donors with a single artery and conventional venous
anatomy, the left side is preferred because of the longer left
renal vein. The disadvantage of using the left kidney is the
presence of the adrenal, gonadal, and lumbar renal vein branches.
If one of the donor's kidneys has a cyst or a small calculus, that
kidney will be the one selected for transplantation.
As for renal vein anomalies, it is not crucial to preserve small
branch vessels in renal donation, as collateral venous fiow will be
sufficient. The short right renal vein remains a problem, however,
prompting surgeons to avoid selecting the right kidney for
The approach to multiple renal arteries (Figure 6) will depend
on their proximity to the renal ostia. An early arterial branch
(within 1 to 2 cm of the renal ostia) will usually be treated as an
accessory artery. An accessory artery may be anastomosed to the
main renal artery or graft. Small segmental accessory arteries are
likely to be sacrificed, however.
Renal CTA and CTV are important tools in the evaluation of
ureteropelvic junction (UPJ) obstruction and are used in the
selection of the most appropriate treatment method. Endourologic
repair has a lower success rate in patients with crossing vessels
at the UPJ.
Intravenous urography, angiography, or endoluminal US can be used
to evaluate UPJ obstruction, but CTA is the preferred examination,
as it is minimally invasive and provides direct visualization of
both the UPJ and vessels. The size and location of vessels are
important in the evaluation of UPJ vasculature. Of interest are
branch or accessory arteries or veins with a diameter of >2 mm,
and any vessel located within 1 to 2 cm of the UPJ.
CT venography for the evaluation of renal vein thrombosis is
often requested in conjunction with a conventional CT of the
abdomen and pelvis. Aside from renal cell carcinoma, which can
cause renal vein thrombus, the most common causes for renal vein
thrombosis are nephrotic syndrome, malignancy, and hypoalbuminemia
For renal CTA and CTV, we usually use MIP reconstructions of
coronal oblique slices angled parallel to the aorta or IVC. The
slice thickness is 3 mm, and the reconstruction interval is 2.5 mm.
Thin-slab MIPs avoid the need for editing and enable a
technologist, using a predefined protocol, to create a series of
images to scroll through.
Volume rendering is the most sophisticated reconstruction
technique and the hardest to learn. It is also the most fiexible
rendering technique and can provide quite specific anatomic detail.
It is possible to vary the windows and levels to look at either the
arterial system or the venous system.
Occasionally, we use MPRs or curved MPRs of CTA and CTV data. As
with thin-slab MIPs, this technique produces a series of images for
review. It is more difficult to draw curved planes accurately,
Generally, technologists do most of the MIPs and MPRs at our
institution, whereas physicians do the volume renderings, as well
as some MPRs and MIPs. We prefer to save CT examinations with a
large number of images to a CD-ROM, which we give to referring
physicians. This enables us to give referring physicians the entire
set of images, including a full set of MIPs.
When preparing laser film instead of CDs for referring
physicians, we suggest printing only selected images. Digital image
files and movies are alternatives to film. These, however, require
electronic transfer, which is more technology-intensive.
One of the first steps in optimizing CTA is to get the
technologist involved. The use of preset protocols that specify
collimation, slice thickness, and reconstruction interval enables
the technologist to acquire the same high-quality data set every
time. It is also possible to use preset reconstruction protocols,
but it is essential to educate technologists as to what the CTA
should look like in the end.
It is also important to work with the application specialist to
learn how to get the optimal performance from the CT scanner and
workstations. Frequent practice at the workstation is also
Finally, talk with referring physicians. Finding out what their
specific needs and interests are-and fulfilling those needs-is
essential to increasing CTA referrals.
ELLIOT K. FISHMAN, MD:
That was terrific, Brian. Are there any questions from the
LEO P. LAWLER, MD, FRCR:
Thank you, Brian; that was very comprehensive. But I have two
questions. You probably have the same experience as we do, in that
CTA is often performed with something else. They want to see the
renal parenchyma as well as the vasculature, etc. I had a question
about partial nephrectomy patients with follow-up. Do you think the
examinations require noncontrast, arterial, and venous enhancement
images of the renal parenchyma to look for recurrence, as well as
just the simple single-phase image?
BRIAN R. HERTS, MD:
It depends upon how soon after the surgery you are talking about.
My experience with our surgeon has been that recurrences,
especially with the small tumors, do not really occur until 2 or 4
years after surgery. So, if the follow-up is being done in the
first 6 months, you can just do a standard abdomen/pelvis CT.
I also have a second question, also about a group of patients in
whom clinicians want both the CTA and the follow-through CT
urogram, such as in living donors. Despite the fact that CT does
not have the line-pair resolution of an intravenous urogram, do you
think that performing 3D processing after the arterial-venous phase
may obviate the need for direct visualizations of the urothelium in
a patient with hematuria?
Our CT urogram protocol is much more delayed than our CTA and CTV.
Our urogram protocol takes scans out to 8 and 10 minutes with or
without compression, and we use a saline chaser, so we get a much
better look at the urothelium. We would add that to this protocol
if that were the indication.
GEOFFREY D. RUBIN, MD:
Brian, your discussion of the treatment of patients with various
degrees of azotemia is very useful. Have you considered the use of
renal protective agents, particularly acetylcysteine and
fenold-opam, in your protocol? There has been a lot of controversy
in the literature about the relative benefits.
We came up with a hospital-wide policy, in conjunction with the
nephrologists and the vascular surgeons. We do consider the use of
acetylcysteine, although it is not recommended or mandatory. We had
theophylline on our list 5 years ago, and we have now removed it as
the controversies arose. So, we put that as a guideline, but we do
not mandate it now.
Since formalizing your protocol, have you followed patients to note
the frequency of contrast-induced nephrotoxicity, particularly in
the groups of the higher azotemics that you delineated?
We have not formally followed them. We talk to urologists a fair
amount because so many of these patients had elevated creatinines
and are going on to partial nephrectomy. But that is a redundant
reason: that is why they have elevated creatinines and that is why
they need partial nephrectomies. It is obviously complicated by
surgery, but the patients are actually doing quite well and
generally do not have a problem. Most of the experience with
contrast-induced nephropathy when you are talking to nephrologists
comes out of the cardiac catheterization laboratory; very little
comes from the radiology department.
Do you think we might be overly conservative in this regard?
I think we are.
W. DENNIS FOLEY, MD:
Brian, I have a quick question for you about preparing patients
with diabetes who are taking metformin. Is your protocol to take
them off metformin for 48 hours after the procedure, as most people
do? Then do you remeasure their creatinine?
If they have a normal creatinine, we do exactly that. We have them
stop taking metformin; they can take it up to the day of the
procedure. Then, we stop it for 48 hours; we do not routinely
U. JOSEPH SCHOEPF, MD:
You mentioned the use of saline chasing in your protocols. Can you
quickly explain what exactly its role is in the imaging of the
renal vasculature in your institution? What systems do you use it
Most of the time, we are really trying to image the renal
parenchyma at the same time. So, in those cases, we give a full
load of contrast and the saline chaser is not as important. If we
are really just looking at the renal vasculature and we have a
smaller volume of contrast, then we will use a saline chaser, just
20 mL, because we are using a much smaller volume of contrast. That
has been shown to maintain the contrast level in the blood vessels
by clearing the line and getting a little bolus behind it.
What is the role of the saline injection? You mentioned an
in-jection for a CT urography.
It actually provides a little bit of a diuresis, and we are still
collecting our data. We initially started with 50 mL and thought it
was simple, that they would inject from a 50-mL syringe. But we
were not getting the opacification that we wanted. When we
increased the saline injection volume to 100 mL, we were actually
getting much better contrast opacification in the collecting
system, much more consistently.
You mentioned that you use 300 mg I/mL concentration contrast
routinely, except when you are doing dedicated vessel studies. Can
you ex-plain your rationale for that?
Part of the issue for us is that we are also looking at the liver
and other organs as well. Our volume of contrast injection has been
150 mL. If we go down to the smaller doses, we tend to get a little
less iodine in if we go to a higher concentration, unless we
increase the volume, which we did not really want to do. That,
obviously, would drive up costs. The other issue for us is that
when you use the higher concentration agents, the contrast tends to
get a little brighter in the kidneys, and I think you get a little
more image artifact off the renal cortex.
We typically use 350 mgI/mL contrast. We do not really change it
based on the specific application. My experience has been similar
to yours at the higher contrast concentration. The point is that it
is actually a disadvantage to have higher concentration than 350
mgI/mL for two reasons: 1) the hyperviscosity, which has the
problems with injecting at higher rates, and 2) artifact off the
You really have to work to manipulate the windows and levels to see
a small lesion in the renal cortex if you are using a
high-concentra-tion contrast agent. You can not use the automatic
soft-tissue windows that you pull up the images with; so you have
to look very carefully for renal lesions in those patients.
Some people have suggested that we could use even higher
concentration contrast. Do you think that would have any value, or
would it just be more of a detriment?
If you are talking specifically about vasculature, the higher the
concentration and the higher the density, the better the rendering
will come out. But we are not just talking about the vasculature or
just talking about the kidneys, so I do find it a detriment.
We do use the higher concentration agents for just the vascular
studies and we will do that when we are doing an endovascular stent
and we will push ourselves to use the 370 mgI/mL. But, we are
trying to keep it simple for our technologists; so if we are doing
a routine study, we use 300 mgI/mL contrast for the kidneys. All of
our kidney studies are actually done as three-phase imaging now; we
do not differentiate between different indications for renal
What recommendation can you offer as to what HU attenutation you
want to achieve, especially for renal vasculature imaging? In
cardiac imaging, there is a level of HU attenuation that you want
to achieve to evaluate the vessel but not obscure any high
attenuation lesion. Is that similar in renal vasculature?
That is an interesting question. I have not looked at the actual
attenuation value. I think we get lucky in the kidneys because the
vein is always a little less dense than the artery because of the
filtering in the kidneys, so it actually helps the differentiation
there and with the volume-rendering techniques. I would estimate
that once you get above 250 HU, it is never an issue.
I would like to raise a point relating to higher concentration
contrast and the degree of attenuation. The issue is the kVp that
we are using, particularly with the thin-section technique. As we
get thin-section technique, we also are combining this with X-ray
tubes that now have the capacity to deliver 700 mAs to compensate
for the very fast scan rotation speeds we are using. Do you have
any advice, Brian, in terms of the kVp, that you think would be
optimal for CTA, in conjunction with the concentration of contrast
you would use?
I actually have not spent any time experimenting, but I know that
our scanners do 100 kVp--it is supposedly very high attenuation and
it really does change what you come up with. The problem I have
found with characterizing renal lesions is that I do not know what
is going to happen. I have not gotten a firm answer from the
scanner manufacturer as to what will happen when you look at a
density value on a renal lesion at 100 kVp. That is really been my
big concern because that is one of our biggest uses. That is
certainly something that needs to be considered, but I do not have
an answer for you.
I could speak to that briefiy because we recently did a little
study of that. We took some tubes of contrast and we changed the
kVp while keeping the computed tomography dose index constant. We
found that, although the contrast solution gets brighter when you
lower the kVp and you get more of a difference between soft tissue
and an enhanced vessel, for example, the noise goes up out of
proportion. So, in fact, the contrast-to-noise ratio drops, even
though the contrast goes up. So, if we are thinking of keeping the
same radiation exposure, going to lower kVp, although increasing
the absolute contrast, hurts us by increasing the noise to a
greater extent. To really take advantage of the greater contrast
difference of low kVp, we have to give the patients higher
That is interesting. Maybe we will come back to it later. There
have been several suggestions that by dropping the kVp to 100 and
raising the mAs appropriately, you would actually give 30% less
radiation dose to achieve better or the same image quality. I think
there are a couple of papers coming out on that. So I think you are
right, that will be an area of a lot of interest, because it is
potentially a way to do better CTA at a lower dose. Maybe your
experience is different, but I have seen some data presented that
suggest substantial decreases in radiation dose.
Have you done any work using gadolinium for renal imaging in
patients with poor renal function?
We have used it on occasion, but not as a routine. We have used it
most of the time for stent studies for endovascular stents--not for
the kidneys--because we can do similar evaluations for MR for
nephron-sparing surgery with CTA.