This article discusses the advances in Computed Tomography (CT) and CT angiography (CTA) due the introduction of multidetector scanners and improvements in scan rotation speeds. Also discussed timing of contrast and image acquisition and future applications.
Dennis Foley, MD
Director, Section of Digital Imaging, Medical College of
Wisconsin, Milwaukee, WI
The introduction of helical technology in the late 1980s
revolutionized computed tomography and made noninvasive CT
angiography (CTA) a reality. Multidetector scanners and
improvements in scan rotation speeds have had nearly as profound an
influence, making it possible to image the vasculature more quickly
and with thinner slices, thus improving both temporal and spatial
resolution.
1
Vascular coverage depends on several factors, including the
number of detectors, detector collimation, beam width, pitch, and
scan rotation speed. In general, CTA is optimized by coupling the
thinnest possible image slices with the table speed needed to cover
the targeted cephalocaudad dimension during the first circulation
of a bolus of contrast media.
At the Medical College of Wisconsin, we use a 4-slice General
Electric LightSpeed CT scanner (GE Medical Systems, Milwaukee, WI).
A beam width of 5 mm and a detector collimation of 1.25 mm produce
the thinnest possible image slices. Using a pitch of 6 and a scan
rotation speed of 0.5 seconds, it is possible to achieve a coverage
speed of 15 mm/sec. Image acquisition can be modified to account
for varying requirements in the amount of coverage needed and the
speed with which images must be acquired. By doubling detector
collimation to 2.5 mm and beam width to 10 mm, for example, it is
possible to cover 30 mm/sec.
Timing of Contrast and Image Acquisition
It is important that image acquisition be timed to correspond
with the first circulation of contrast, based on a test mini-bolus;
that the duration of the acquisition interval match that of the
injection interval; and that the intra- arterial iodine
concentration produce an arterial attenuation of 250 to 300
Hounsfield units (HU) throughout the full sequence of the
acquisition.
1
Bolus-tracking software may one day be fast enough to precisely
match acquisition interval to injection interval but, in my
opinion, that is not the case today.
The acquisition interval depends not only on contrast
circulation time but also on scanner performance. Figure 1 shows
how an improvement in scan rotation speed from 0.8 seconds to 0.5
seconds reduces the image acquisition interval from 30 seconds to
20 seconds in an aortoiliac CT arteriogram encompassing
approximately 30 cm of cephalocaudad coverage. The injection
interval is similarly reduced.
Breathhold capacity is another factor to consider in determining
the image acquisition interval; however, most patients can easily
accommodate the 20- to 30-second breathhold typically required for
CTA.
More Channels
There are many potential advantages of scanning with more than 4
channels. Among these are increased flexibility, the opportunity to
balance the competing demands of speed and resolution, and the
ability to reduce the contrast load.
In aortoiliac CTA, just as reducing scan rotation speed from 0.8
seconds to 0.5 seconds on a 4-slice scanner produces a 50 mL
savings in contrast, imaging on an 8-slice scanner while
maintaining detector collimation constant enables a further
reduction in both injection and acquisition intervals, and an
additional 50-mL reduction in contrast load (Figure 1C).
In a scan of a potential kidney donor, Figure 2 demonstrates
clear definition of the renal arteries from the aortic ostium to
the renal hilum. The image was acquired with an 8-channel scanner,
following injection of 50 mL of contrast media. The contrast
material in the renal collecting system reflects the preliminary
mini-bolus.
Thoracoabdominal aortic CTA (Figure 3) provides an example of
how an increased number of detector channels enables improvements
in longitudinal resolution. With a cephalocaudad target coverage of
55 to 60 cm, a 4-detector scanner with a scan rotation speed of 0.5
seconds uses a 2.5-mm detector collimation and a beam width of 10
mm. Contrast is delivered at 5 mL/sec for 20 seconds, for a total
of 100 mL. When an 8-detector scanner is used, the injection and
acquisition intervals remain the same, as do the beam width and
scan rotation speed. Detector collimation is reduced from 2.5 mm to
1.25 mm, however, improving longitudinal resolution. The resulting
3-dimensional image is shown in Figure 4.
The next advance in helical CT technology will be the
introduction of scanners with 16 channels. These advanced scanners
will enable the acquisition of slices as thin as 0.625 mm. One
result will be a clear improvement in spatial resolution, as shown
in Figure 5.
Future Applications
As a result of technological ad-vances in CT scanners, CT
angiography is capable not only of evaluating the thoracoabdominal
aorta and proximal abdominal visceral branch vessels, but also of
improving the definition of vessels within such organs as the
kidney, liver, and pancreas. Additional applications include
angioportography; abdominal visceral imaging to detect visceral
pathology, such as pancreatic lesions and hyper- and hypovascular
liver lesions; and renal venography.
Under such circumstances, iodine load becomes an important
consideration, as does imaging of various circulatory phases. In
hepatic and pancreatic imaging, a 42-gram iodine load is necessary
to acquire high-quality images not only in the venous phase, but
subsequently in the parenchymal phase as well.
3,4
Similarly, in renal imaging, the desire to reduce the contrast load
must be tempered by the need for venous imaging and for
demonstrating pathology in the abdominal viscera. Figure 6
demonstrates angioportography in a patient with cholangiocarcinoma
of the right hepatic lobe involving the right portal vein. A
cut-off of the right portal vein is clearly delineated on venous
phase imaging.
Even so, studies that produce first-pass images with excellent
delineation of the hepatic arteries may only partly visualize
small-vessel neovasculature in focal liver lesions. It is probably
not yet possible to demonstrate abnormal vascularity within focal
liver lesions as well as could be done with selective hepatic
arteriography.
When CT is used to evaluate pulmonary embolism, the contrast
load is determined by the need for indirect CT venography. The
contrast injection interval extends well beyond
the acquisition interval for the pulmonary arterial phase, so that
studies can be obtained at 2.5 to 3 minutes after injection of the
peripheral veins (Figure 7).
Conclusion
As radiologists begin to perform CT angiography on scanners with
8 or more detector channels, imaging will become faster and
resolution better. The contrast load, at least for arterial
imaging, can be reduced. Under certain instances, the volume of
contrast material used in CTA will be
as little as 40 to 50 mL, approximating the amount of gadolinium
used for magnetic resonance angiography. Aortoiliac CT angiography,
as discussed earlier, is one example.
Practical considerations related to CT angiography must be taken
into account as well, among them, workflow, workstation design, and
improvements in software for three-dimensional display. *