The latest advances in neuroradiology were presented at the 41st Annual Meeting of the American Society of Neuroradiology (ASNR).
is a Professor of Radiology, the Gerald D. Dodd, Jr.
Distinguished Chair in Diagnostic Imaging, and Director of
Academic Development; and
are Assistant Professors of Radiology at the University of Texas
M.D. Anderson Cancer Center, Houston, TX.
[Note: Part I of this article was published in the May 2003
Silverman PM, Szklaruk J, Tamm E. Contrast usage for liver
imaging in the era of multislice (MSCT), multidetector (MDCT) CT:
In the early 1990s, dual or split detector systems became
available allowing the acquisition of 2 slices during a single
gantry rotation. In 1998, the first multislice systems were
introduced with 4 data channels providing a quantum leap in CT
These scanners have been referred to by various names, including
multidetector, multidetector row, and, most appropriately,
multislice CT (MSCT). In the past couple of years, 8-slice,
16-slice, and even 32-slice detector systems have been developed,
and manufacturers are now testing the incorporation of flat-panel
detectors in scanners in an attempt to extend this technology to
near-instantaneous data acquisition with volumetric data
Scanners already allow the acquisition of multislice data sets
with sub-second scan times.
These advances allow the acquisition of thinner slices, shorter
scanning times, and greater volumetric coverage in the z-axis
(Figure 1). Depending on the specific clinical application,
relative trade-offs between speed and collimation are tailored to
create optimal protocols for multislice CT.
The development of MSCT has also allowed for near-isotropic and, in
the past year, isotropic voxels that can provide the ability to
image in multiple planes without loss of resolution.
Three-dimensional (3D) imaging is not only practical but results in
extremely high-quality images devoid of previous stair-step
artifacts. Reconstructions from very thinly collimated images
provide exquisite depiction of anatomy when coupled with optimized
timing of scanning with contrast enhancement usually using
bolus-tracking approaches. Three-dimensional images readily
performed in the arterial and venous phases are valuable for
staging tumors, as well as for the evaluation for vascular
anomalies and vascular disease as part of a detailed preoperative
assessment. Such imaging
is also of great value in planning
The liver, because of its unique dual blood supply (20% from the
hepatic artery and 80% from the portal venous system), remains just
as much or more of a challenge for optimizing protocols in the
current era of MSCT. These scanners offer a quantum leap in speed
and flexibility when compared with even standard single-slice
helical scanners. The previously termed portal venous phase (PVP)
for single-slice CT (SSCT) has now been more appropriately named
the hepatic venous phase (HVP) on MSCT, as this phase captures the
opacification of these veins and maximal liver enhancement (Figures
2 through 8).
The main impact of MSCT scanners has been to provide the ability to
examine an organ, such as the liver, in multiple phases of contrast
dynamics with the hope of allowing increased detection of lesions
as well as improved lesion characterization.
Optimizing protocols for multiphasic imaging includes adding
phase(s) to the HVP (ie, dual-phase imaging) and/or inclusion of a
very early arterial phase for 3D imaging of the vascular system,
(ie, triple-phase imaging). In contrast to helical SSCT, MSCT is
able to define three distinct phases of contrast enhancement,
rather than just two. With SSCT, the two phases are the hepatic
arterial dominant phase (HADP) and the PVP; with MSCT these phases
have been termed the hepatic arterial phase (HAP), late arterial
phase (LAP) or portal venous inflow phase (PVIP), and a hepatic
venous phase (HVP).
The first two phases were incorporated in the HADP described with
SSCT (Figures 9 through 12). The ability to scan rapidly with MSCT
allows one to separate these and scan in two phases what could only
be done in one phase previously. Hypervascular lesions, either
primary or metastatic, are usually best seen in the LAP; however,
some lesions are seen only in either the HAP or LAP phases (Figures
13 through 15). Hypervascular lesions have always presented a
challenge to the radiologist.
Failing to image hypervascular lesions during the HAP results in an
insensitive examination similar to failing to image hypovascular
lesions in the PVP. The HAP is best identified 10 to 20 seconds
after the administration of contrast and is characterized by
enhancement of the hepatic artery. The LAP is best identified 25 to
30 seconds after injection and shows enhancement of the hepatic
artery and some enhancement of the portal venous structures. The
is marked by opacification of the hepatic veins at the dome of the
liver and enhancement of the portal veins. The speed results in one
of the most important challenges in developing optimized protocols
for this new, robust technology. Although multiphasic studies could
be performed with helical scanners, high-quality, whole-organ
imaging with multiple phases awaited the introduction of MSCT.
Higher concentration contrast in multislice CT
Detection of liver lesions is dependent on scanning during the
phase that optimally distinguishes normal from abnormal tissue as
discussed. Optimized imaging requires using adequate amounts of
contrast, ie, grams of iodine.
The grams of iodine have a direct impact on the difference in
hepatic attenuation relative to lesion detection that defines the
relative conspicuity of lesions (normal hepatic attenuation liver
lesion attenuation = lesion conspicuity).
Most recently, with the rapid proliferation of MSCT technology,
the concept of using higher concentrations of contrast material has
begun to be explored.
The impetus for this has been that the standard contrast
concentrations of 300 to 320 mg I/mL have required volumes on the
order of 150 mL to deliver adequate grams of iodine to image the
liver effectively. This is in contrast to examining other areas of
the body, such as the chest, where the dose and volume of iodinated
contrast can be significantly reduced (ie, 150 to 100 mL [helical
CT] to 60 to 75 mL [MSCT]). Imaging of liver lesions requires more
precise protocols. Studies of the liver with less than optimal
contrast enhancement result in compromised lesion detectability.
Fortunately, to date, prices of contrast material are not directly
tied to grams of iodine within the product, but are most closely
linked with the volume of contrast. Thus, if we can use lower
volumes and higher concentrations of contrast, it has the
additional benefit of becoming highly cost effective. With SSCT and
helical scanning, protocols for body CT required volumes of
contrast in the range of 150 mL with 300 mg I/mL and 320 mg I/mL to
be able to have optimal enhancement of the liver and also provide
adequate enhancement of abdominal and pelvic structures. With MSCT,
this can be accomplished without requiring such large volumes since
scans can be completed so rapidly. Thus, it becomes the challenge
for radiologists to adopt new protocols to take advantage of this
continually evolving technology.
Higher concentrations of contrast, 350, 370, and even 400 mg
I/mL, have been developed and are being used clinically. If a
target range of 37 to 48 grams of iodine is considered to image the
liver, then this can be achieved by a number of different
permutations of volume and concentration of contrast (Table 1).
Higher concentrations of contrast also allow contrast delivery of
the same grams of iodine per second to the target organ at lower
rates. For example, the administration of 150 mL of 300 mg I/mL at
5 mL/sec delivers an iodine dose of 1.5 g/sec whereas the
administration of 100 mL of 370 mg I/mL at only 4 mL/sec delivers
essentially the same iodine dose of 1.48 g/sec. The ability to
decrease the total volume of contrast will result in overall
substantial cost savings in a busy clinical CT service.
The introduction of MSCT has created a new challenge for
radiologists. These very fast scanners provide a great deal of
flexibility for body imaging, especially in the liver. It also
provides for very high-quality 3D vascular imaging, which can aid
in surgical and therapeutic planning. It is only by understanding
the flexibility of this new technology and new developments made by
contrast companies in providing a variety of concentrations of
contrast material that we can take full advantage and harness its
potential for the benefit of our patients.