This article discusses the use of higher concentration CT contrast agents in Multislice CT applications.
Paul M. Silverman, MD
Professor of Radiology; Gerald D. Dodd, Jr. Distinguished Chair;
Chief, Section of Body Imaging; Department of Radiology; M.D.
Anderson Cancer Center; Houston
Advances in multislice CT technology have resulted in a 4- to
6-fold increase in imaging speed and have enabled both the
acquisition of thinner slices and an increase in z-axis coverage.
Together, these improvements are prompting reconsideration of
conventional approaches to the use of contrast media. Now, along
with setting appropriate parameters for pitch, slice thickness, and
the number of passes to make during various phases of contrast
enhancement, radiologists tailor the delivery of iodinated contrast
by adjusting volume, concentration, rate, and the timing between
injection and scanning.
The organ whose imaging is most affected by advances in
multislice CT is the liver. Identification of pathology within the
liver is related directly to differences in normal hepatic
parenchyma and abnormal tissue. In the vast majority of cases,
metastatic disease presents as lower-density, hypovascular lesions
against a bright background, made so by substantially greater
The goal in hepatic imaging is to increase the difference in
enhancement between the two tissues, so that the conspicuity of
lesions is maximized. The best way to do this is by giving adequate
amounts of contrast material, as gauged by the total grams of
iodine. The rate of contrast administration determines the timing
of peak contrast enhancement.
The enhancement curve of the liver is characterized by a fairly
steep uprise. It then reaches a plateau, followed by a slower
downward curve. It is at the peak that the maximal difference in
the enhancement of the liver and hypovascular liver lesions becomes
apparent. It is, therefore, imperative that scanning be performed
during the maximal phase of contrast enhancement.
Single-slice helical scanners require a significant amount of
time to scan the liver, and therefore must include some of the
upward phase of the contrast enhancement curve, as well as the
plateau and the downward phase (Figure 1). Scanning during the
early upward phase of the enhancement curve is inefficient, as
images are obtained when the concentrations of contrast are still
low, even though the patient has received a large contrast
Scanning during the downward phase of the enhancement curve is
worse. As contrast reaches equilibrium, the aortic enhancement
curve and the liver enhancement curve begin to run parallel, and
hypovascular liver lesions begin to fill in from the periphery.
During this equilibrium phase, liver lesions become much less
conspicuous, they appear smaller, and, finally, they disappear.
This results in a less accurate representation of the extent of
disease and could, for example, give the false impression that
lesions were improving with chemotherapy or radiation therapy when,
in fact, their change in size was simply a hemodynamic artifact
caused by encroachment of contrast into the lesion.
A major advantage of multislice CT over standard helical CT is
that it takes significantly less time to scan a target organ.
Consequently, less of the upward and downward portions of the
enhancement curve need be included. Instead, most of the scanning
closely surrounds the area of peak enhancement of the liver (Figure
Ideally, imaging would capture just the peak of liver
enhancement, with none of the up- or downslope of the enhancement
curve. To do that, image acquisition in the entire volume of the
liver would need to be completed within only seconds. Although we
have not yet achieved that goal, the development of 4-, 8-, and
even 16-slice CT scanners has brought us much closer.
As 8- and 16-slice CT scanners become available and, in the
future, volumetric CT is introduced, it will become even more
critical to time scanning precisely to correspond to the peak
contrast enhancement of the liver. We can do this by using either a
test bolus of contrast material or, even better, computer automated
scanning technology (CAST). SmartPrep (GE Medical Systems,
Milwaukee, WI) is one example of this technology.
Using a test bolus has several disadvantages. It requires giving
additional contrast material, at additional expense. In addition,
it is done separately from routine scanning and, therefore, adds to
the length of the examination.
By comparison, CAST enables contrast to be given at an
acceptable rate (2.5 to 6 mL/sec) and a series of
low-radiation-dose scans taken at short intervals until a specific
enhancement threshold over baseline is achieved in the liver. The
computerized software is now sophisticated enough to automatically
track with cursor placement in aortic and liver enhancement
Our research has shown that an enhancement threshold of 50 HU
results in excellent lesion detection in the liver.
When the contrast reaches this threshold, the technologist simply
performs the diagnostic scan. The time it takes to switch from
enhancement detection scanning to the actual diagnostic study is
acceptable for parenchymal organs, such as the liver, though it may
be slightly longer than is ideal in some applications, such as CT
The development of CAST was prompted by the observation that
contrast circulation times vary from person to person. The average
time from injection to portal venous phase scanning of the liver is
65 to 70 seconds. While this time frame would likely be appropriate
for a very fit person, a patient with cardiovascular disease would
benefit from CAST, which automatically adapts to the patient's
decreased cardiac output and delays the liver scan until
enhancement is optimal.
Computer automation may also reduce the volume of contrast used.
Research in this area has demonstrated that the degree of liver
enhancement achieved with a 125-mL bolus of contrast using CAST is
equivalent to that achieved with 150 mL of contrast using a
conventional time delay between injection and imaging.
Another technique used to achieve better contrast enhancement is
the "saline-push." By chasing the contrast with saline, a greater
degree of contrast enhancement can be achieved. This approach
requires additional time, a more experienced technologist, and may
compromise contrast sterility if not well managed. In the
appropriate clinical environment, however, it can be a useful
method to improve contrast enhancement.
For most CT examinations, it is typical to use approximately 150
mL of contrast with a concentration of 300 to 320 mgI/mL. As
multislice CT technology advances, the ability to deliver higher
concentrations of contrast in smaller volumes will become
important. With faster scanners, including those capable of
volumetric scans of an entire organ in one acquisition, it is
absolutely critical to deliver contrast rapidly. Volume, on the
other hand, becomes less important. With increasingly fast
scanners, it is no longer necessary to use volume to prolong the
examination, as has been done in the past. Higher concentrations
(for example, 370 to 400 mgI/mL) and lower volumes likely represent
the future of contrast administration as scanners become faster and
Some caution is warranted, however. Rates of contrast
administration have increased significantly in the past decade. Two
surveys conducted by the Society of Computed Body
Tomography/Magnetic Resonance, in 1995 and 1998,
indicated contrast delivery rates for routine body examinations
have increased from a range of 1 to 1.5 mL/sec to 2.5 to 3 mL/sec,
and in the case of evaluation for hypervascular lesions, to 5 to 6
mL/sec. To remain comfortable in delivering contrast at such high
rates, we must be sure that there is an extremely low incidence of
In our practice, we are committed to universal use of
low-osmolality contrast media, and have been able to achieve
exceptional efficiency with an extremely low incidence of adverse
reactions. Careful attention to safety will be even more critical
as we venture into intravenous use of higher-concentration
iodinated contrast material in CT.
Another potential problem may be the use of relatively high
concentrations of iodinated contrast when scanning such areas as
the neck and upper thorax, where streak artifacts emanating from
undiluted contrast material can be detrimental. Despite this,
higher concentrations of contrast, assuming they can remain safely
within the low-osmolar range, will likely become widely used.
The use of prefilled syringes is now becoming more popular, as
scan times are reduced and workplace efficiency critically
examined. At the University of Texas M.D. Anderson Cancer Center,
we perform nearly 300 CT procedures each day. If the cost of using
prefilled syringes is comparable to that of manually drawing up
contrast for each procedure, the added advantages of sterility and
time savings for nurses and technologists are likely to be deciding
factors that propel the use of prefilled syringes.
Another cost-saving approach might be to buy contrast in bulk,
perhaps in 1000-mL containers, rather than the 500-mL containers
now more generally in use. We could then spike these containers
once and, using a series of one-way valves to protect against
bacterial and viral contamination, fill syringes for multiple
patients. This approach is actually being used now in many
hospitals, as the contents of today's 200- to 500-mL contrast
bottles are used for multiple examinations in multiple
Current and future multislice CT scanners will generate interest
in using higher concentrations of contrast, lower volumes, and
techniques to clearly define contrast circulation and peak
enhancement, such as computer automated scanning. Prefilled
syringes will probably be more widely used as pressure to increase
efficiency continues. Purchasing contrast in bulk containers may be
another effective means of reducing costs. *