Optimization of image acquisition technique and contrast administration protocol is crucial for performing high-quality hepatic CT angiography.
This publication was supported by an educational grant from
Amersham Health, Princeton, NJ. The opinions expressed in this
publication are those of the authors and not necessarily those of
Dr. Bae reports relationships with Tyco Healthcare and
Mallinckrodt through patent agreements and as a consultant. Dr.
Fishman reports relationships with Siemens Medical Solutions and
Amersham Health as a consultant. Dr. Foley reports a relationship
with GE Medical Systems through an investigator agreement. Dr.
Naidich reports a relationship with Siemens Medical Solutions
through its Advisory Board and as a consultant. Dr. Saini reports a
relationship with GE Medical Systems through research support. Dr.
Becker, Dr. Sahani, Dr. Siegel, Dr. Tahktani, and Dr. Zinreich
report that no such relationships exist.
Dr. Saini i
s the Director of Computed Tomography at Massachusetts General
Hospital and is a Professor of Radiology at Harvard Medical
School, Boston, MA.
is an Instructor of Radiology at Harvard Medical School and is a
Staff Radiologist at Massachusetts General Hospital.
CT angiography (CTA) of the liver is invariably performed as a
preoperative study in patients with metastatic or primary malignant
liver disease for whom hepatic resection or placement of a
chemotherapy infusion pump are treatment options.
In this setting, preoperative knowledge of any variations from
normal hepatic arterial anatomy is critical for optimal surgical
Although the primary goal of hepatic CTA is to visualize the main
branches of the common hepatic artery, depiction of the hepatic
venous anatomy is also important for segmental localization of
liver tumors. The traditional goals of liver imaging, hepatic
lesion detection, and characterization are less important in these
patients since these tasks will have already been accomplished
during an initial diagnostic CT.
A more technically challenging but less common application of
CTA of the liver is the evaluation of individuals who may donate
part of their liver for living-related hepatic transplantation.
In such cases, it is important to visualize all three of the
hepatic vascular systems: the arterial system, the portal venous
system, and the hepatic venous system.
Of these, the most challenging is the hepatic arterial system,
since it is necessary to demonstrate not only the main trunk
hepatic artery and its branches, but also the arterial supply to
various hepatic segments, particularly segment 4.
Technical issues associated with CTA of the liver can be divided
into image acquisition protocol and contrast administration
protocol. Key factors in image acquisition protocol include scan
range, table speed (which is determined by gantry rotation speed,
pitch, number of data-channels, and effective detector
configuration), and the reconstructed slice thickness. Although it
is seldom discussed, selection of kVp is important in determining
Along with mA, kVp also determines radiation dose, which should be
minimized in living-related donor candidates. Key factors in
contrast administration protocol are iodine dose, which is
determined by the concentration and administered volume of contrast
medium, and the rate of contrast media injection. Finally, scan
delay or timing is the crucial link between image acquisition
techniques and contrast administration protocol, and this
coordination must be done precisely due to the short scan times
associated with multidetector CTA.
Image acquisition protocol
For high-quality CTA of the liver, breath-hold whole-liver
acquisitions are necessary to avoid anatomic gaps. Hence,
thin-slice scanning at sufficiently rapid table speed is critical.
For example, a z-axis coverage of 200 mm in a 20-second breath-hold
requires a table speed of at least 10 mm/sec. Obviously, faster
table speeds will allow acquisitions in shorter breath-holds. Table
1 lists the typical image acquisition parameters for 16- and
4-slice CT scanners. The advantages of the 16-slice CT acquisition
over the 4-slice acquisition are a shorter breath-hold and better
quality three-dimensional (3D) CTA reformations due to the
availability of submillimeter slices. Although data are
reconstructed at the thinnest possible slices with 50% overlap for
3D reformations, for axial viewing, slightly thicker slices (1.25
to 1.5 mm) are adequate.
Thin-slice scanning enables exquisite visualization of the main
trunk branches of the common hepatic artery (Figure 1). Within the
liver, it is also possible to routinely visualize the segmental
branches, which is necessary in living-related donor work-up
(Figure 2). Anatomic variants, such as replaced or accessory
hepatic arteries, are also easily depicted (Figure 2). On
portal-phase images, the hepatic venous (Figure 3) and portal
venous (Figure 4) structures are readily seen. Since these
structures are comparatively larger than hepatic arteries, this
phase can be reconstructed at 1.25 mm thickness. Typical scan delay
for this phase is 70 seconds.
Contrast administration protocol
Figure 5 presents simulated contrast enhancement curves for the
aorta and liver based on administration of 150 mL of 300 mgI/mL
concentration contrast media injected at 4 mL/sec and assuming
normal cardiac output.
Since arterial-phase scans should be obtained prior to significant
portal venous enhancement, Figure 5 also shows that the optimal
period for hepatic CTA imaging is approximately 20 to 40 seconds
after contrast administration. Although aortic CTA studies can be
performed at much lower total iodine dose, liver CTA requires a
total iodine dose of 45 g in order to permit imaging of the hepatic
venous and portal venous systems. Hence, with a 300 mgI/mL
concentration, 150 mL is injected at 5 mL/sec, while with 370
mgI/mL iodine concentration, it is possible to inject the 45-g
iodine dose with a volume of 120 mL and inject it at a rate of 4
mL/sec. With both these contrast administration protocols, the rate
of iodine infusion is identical.
For accurate timing of image acquisition, standard delays are
commonly used because this simplifies workflow. With this approach,
a scan delay of 20 seconds is appropriate for the hepatic arterial
system, followed by imaging of the venous systems at 70 seconds.
However, use of bolus tracking techniques is preferred, because it
permits customization of scan delay for individuals with reduced
For hepatic CTA, bolus-tracking scans that monitor aortic
attenuation are acquired every 2 seconds beginning at 10 seconds
after start of injection, and imaging is triggered at aortic
enhancement of 150 HU with the portal/hepatic venous phase imaging
performed at 70 seconds. Figure 6 illustrates the value of using
bolus tracking to precisely time image acquisition due to
variations in cardiac status. Note, in patients with poor cardiac
output, the aortic enhancement is delayed and bolus tracking allows
initiation of scanning with an optimized longer delay.
The targeted arterial density during CTA is 250 to 350 HU. It is
possible, however, to achieve higher levels of enhancement or lower
iodine dose by using a kVp of 80, at which X-ray photon energy is
closer to the k-edge of iodine, which is 33 keV. According to Huda
iodine density is 50% lower at 140 kVp, compared with 80 kVp.
Lowering kVp, however, requires increasing the mAs so that image
noise is constant. Unfortunately, while an 80-kVp acquisition would
be optimal for attenuation of X-rays by iodine, the mA requirements
for overcoming image noise are so great that they cannot be met by
current generation CT scanners. Hence, we are currently undertaking
CTA scanning with kVp of 100 to 120 with a slight increase in mAs
compared with the 140 kVp technique. This approach also permits
reduction of radiation dose, although the images are slightly
noisier. Initial experience suggests that the higher image noise
can be tolerated in CTA studies, due to the high soft-tissue
contrast between enhancing vessels and background liver. This is
especially true for submillimeter slices, since a reduction in
slice thickness also increases image noise, which provides another
need for increasing tube current. Therefore, axial viewing is
undertaken at a slice thickness of 1.25 to 1.5 mm with thinner
slices used for 3D reformations. For CT scanners with automatic
exposure control, a noise index of 20 HU appears to be suitable for
hepatic CTA. However, considerable research needs to done in this
arena for optimizing image quality and radiation dose.
In order to perform high-quality hepatic CTA, it is essential
that optimal image acquisition techniques and contrast
administration protocols are utilized. In addition, X-ray
parameters, including peak tube-potential (kVp) and tube-current
(mA), should be chosen carefully, since they have an important
effect on image noise, vascular enhancement, and radiation dose.
Today, all preoperative vascular evaluation of the liver is
undertaken with multidetector CT, which makes catheter angiog-raphy
Hepatic CTA demonstrating normal hepatic arterial anatomy.
Hepatic CTA demonstrating variation in hepatic arterial anatomy.
In this case, there is a replaced right hepatic artery. Note the
clear visualization of the intrahepatic branches, such as the
segment 4 branch that arises from the left hepatic artery.
Hepatic CTA of the portal venous system showing trifurcation of
the main portal vein.
Hepatic CTA of the hepatic venous system showing a variation in
hepatic venous anatomy. In this case, there is an accessory right
hepatic vein. Recognition of this variation is especially
important in living-related liver donors.
Simulated enhancement curves of the aorta and liver after
injection of 150 mL of 300 mgI/mL concentration at a rate of 4
mL/sec with normal cardiac output. Note that the optimal period
for hepatic arterial imaging is approximately 20 to 40 seconds
after contrast administration, before significant portal venous
enhancement occurs. (Image courtesy of Kyongtae T. Bae, MD,
Mallinckrodt Institute of Radiology, Washington University School
of Medicine, St. Louis, MO.)
Bolus tracking allows precise timing of image acquisition in
patients with (A) normal or (B) reduced cardiac output. Note the
arterial enhancement would not have been adequate in the latter
case if a fixed delay of 20 seconds had been used since the
aortic attenuation of even 150 HU is not achieved until 30
ELLIOT K. FISHMAN, MD:
Thank you. That's an interesting comment about the lower kVp.
Depending upon how long your acquisition is, and how high you
adjust the mA, is tube heating going to be any limitation?
SANJAY SAINI, MD:
We tried 80 kVp on a patient for an aortic CTA study. The mAs were
fixed at 190, which the tube could support possibly because the
scan did not extend into the legs. Although the image was a bit
noisy, it was more than adequate for visualization of the aorta.
Another concern with a lower kVp is a higher absorbed X-ray dose.
Hence the lower mAs may be appropriate for this reason as well.
In terms of some of the 3D rendering, as you were showing the
hepatic arteries, can you comment on the impact that the
concentration or the contrast would have depending on which
rendering techniques you use? For example, would the use of volume
rendering versus MIP make a significant difference?
Until recently, we were scanning with 300 mgI/mL concentration and
have since used higher concentrations as well. There is no visible
difference in the images with the higher concentration. It seems
that the advantage of the higher concentration contrast media is
that you can use a lower injection volume and a lower injection
Contrast volume makes some impact on volume rendering, and would
probably make a much bigger impact on the MIP images, but I don't
know how much impact.
KYONGTAE T. BAE, MD, PhD:
In the pediatric population, we would use 80 kVp, correct?
MARILYN SIEGEL, MD:
But I think the low kVp can be problematic in large patients for
scanning the pelvis, where you have to penetrate the large bony
Correct. However, I understand that CT scanners are being
introduced this year that will allow mAs up to 1000.
DAVID P. NAIDICH, MD:
I have a question about technique. I was curious about looking at
really small vessels, especially in the liver and in small
pulmonary arteries. Are you using a specific reconstruction
Standard soft-tissue algorithm.
You're using a standard algorithm. You haven't played around with
W. DENNIS FOLEY, MD:
The vascular-phase image was an odd mixture of the hepatic artery
and the portal vein in some cases. In other cases, it's the hepatic
artery without the portal vein. Now that you are going to dispel
this tracking technique, are you separating out hepatic artery
visualization from the portal vein? Or do you like to see both
I don't necessarily think we like to see both together. In all
studies, we get a portal-venous-phase scan as well. I haven't
looked critically enough to know that we are better off today than
we were yesterday, so I don't know the answer to that.
One thing we've done is to deliberately separate out the two. So
you can have what might be called an early
arterial phase, in which you simply define arteries. Then we scan
in the late arterial phase, which actually turns out to be the best
timing to detect hypervascular lesions and generate a vascular map
of the portal vein.
But we would do that only if the patient is a candidate for
surgery, transplantation, or chemoembolization.
Is this early arterial phase at peak aorta plus 10 seconds?
Well, actually, no, the other arterial phase begins at peak aortic.
That's determined from a mini-bolus. But for the late arterial
phase, it's aortic peak plus 15 seconds.
The early arterial phase imaging would be at 20 seconds?
It varies, somewhere be-tween 12 seconds with very fast circulation
and up to 30 seconds in patients with very slow circulation.