Lawrence N. Tanenbaum, MD
Section Chief, Neuroradiology, MRI & CT, New Jersey
Neuroscience Institute, JFK Medical CenterEdison Imaging
Associates; Assistant Professor, Neuroscience, Seton Hall
University; Edison, NJ
Since the introduction of the first slip-ring scanners,
neurological CT has been characterized by substantially shorter
scan times, a trend that has accelerated with rapid advancements in
multidetector systems. Today, we can complete a CT angiogram that
extends from the thoracic aorta to the toes in less than one
minute, and a CT of the neck in mere seconds.
Although the ability to scan more quickly has been the dominant
advantage of recent improvements in CT technology, an increasing
number of detector channels also presents the option to balance
speed with improvements in image quality. Either approach can have
a substantial impact on the selection of contrast agents, the
volume used, and the administration method. Iodine concentration is
of particular importance. In many applications of neurological CT,
higher-concentration, lower-volume contrast media produces the best
With single-channel CT scanners, we conduct most neurological
examinations at a pitch of 1.5. (Pitch is defined as the distance
the table travels per tube revolution, divided by the detector
Under some circumstances, we select a higher pitch when using a
single-detector scanner, for example when we need very rapid
coverage in order to chase a small bolus of contrast, to image a
long area of interest, or to scan an uncooperative patient. In
general, however, a pitch of 1.5 remains our internal reference for
image quality and slice profile.
With 4-channel systems, our ap-proach for most applications is
to use a pitch of 3, which enables us to double scanning speed
while improving the slice profile. With a 15-mm table
incrementation per tube revolution and a collimation of 4 * 1.25
mm, it is possible to cover 150 mm in 10 revolutions in as little
as 5 seconds. Under certain circumstances, such as CT angiography
(CTA) from the aortic arch to the Circle of Willis and studies of
the soft-tissue neck, we take advantage of the increased scan speed
that a pitch of 6 provides. With a table speed of 30 mm per tube
revolution, it is possible to cover 300 mm in 10 revolutions in the
same 5 seconds, with an acceptable increase in slice profile.
An 8-channel system offers even more flexibility. Because of its
greater speed, we can use a moderate pitch of 7 to 10.8 for most
applications, including imaging the soft-tissue neck.
Alternatively, we can push scan speed to the maximum the system can
achieve in order to do CT angiography or studies that necessitate
long breathholds, such as 1.25-mm collimation scans of the abdomen
and pelvis. Alternatively, we reduce pitch to 5 for head CTA. Here
speed is less critical and, instead, the primary objective is the
acquisition of superior images of low contrast resolution and least
possible image noise.
In addition to multiple detectors, CT scanners now also offer
extreme-ly fast scan rotation speeds, ranging from 0.5 to 1 second.
A scan rotation speed of 0.5 seconds, which we typically use for
vascular opacification or CTA, yields twice the coverage for a
given breathhold. This gives us an opportunity to be more effective
in our utilization of contrast.
In neurological CT, one of our primary goals in using contrast
is to achieve optimal depiction of the breakdown of the blood-brain
barrier (Table 1). Detection of neurological disease is in part
governed by the fact that, while normal brain tissue will not
enhance, lesions that do not maintain the blood-brain barrier will.
In other areas, such as the soft-tissue neck, we are looking for
mucosal enhancement. Under other circumstances, such as CTA and
neck CT, we want to maintain maximum vascular opacification. We
also want to maximize image contrast resolution, particularly in
One way to take advantage of today's extraordinary imaging
speeds while still delivering the appropriate amount of iodine for
a specific imaging application is to use higher injection rates. We
use injection rates of 2 mL/sec for our routine applications, 3.5
mL/sec for CT angiography, and 4 mL/sec for perfusion CT.
In a busy outpatient setting, injection rates are limited by a
number of practical concerns. These include extravasation risk, as
well as the time it takes to obtain intravenous access suitable for
high injection rates and its impact on throughput. We scan 4
patients every hour on our 8-channel system, and establishing a
large-bore intravenous line can cause unacceptable delays.
Patient tolerance to contrast injection is also key in enabling
the use of high injection rates. The local effects of contrast,
such as a feeling of warmth or a burning sensation, must be
considered, as must the potential for nausea and vomiting. It is
important to remember that an interrupted exam is a lost exam. We
feel most comfortable using nonionic contrast media for
applications that require high injection rates, a choice that has
been made progressively easier by the reduced cost differential
between ionic and nonionic contrast agents.
As in most practices, we work with a variety of helical CT
scanners, including 1-, 4-, and 8-channel systems. We hope to add a
16-channel system in the near future. To accommodate such a wide
range of technology, our approach has been to match the
concentration and volume of contrast to both the specific
application and each scanner's capability.
In the soft-tissue neck, we achieve excellent opacification with
a total iodine dose of about 23 grams, delivered as 75 mL of Isovue
300 (Bracco Diagnostics, Princeton, NJ) (Table 2). In the orbit, we
use the same dose. In the head, we use 37 grams of iodine,
delivered as 100 mL of Isovue 370. In perfusion imaging, the goal
is to maximize the difference in attenuation between baseline and
peak enhancement. It is in this type of application that a
high-density contrast agent can have an enormous impact. We deliver
approximately 15 grams of iodine in 40 mL of Isovue 370.
CT angiography is accomplished using a range of iodine doses. We
do brain CTA with 50 mL of Isovue 370, achieving an excellent exam
with high image quality using only 18 grams of iodine. When we do a
full neurological angiogram, covering from the aortic arch through
the intracranial vasculature, we use approximately 75 mL of Isovue
370 and deliver a total of 28 grams of iodine.
Neck CT presents an unusual set of challenges. The speed offered
by multidetector helical CT is not necessarily helpful in this
instance, as the mucosa and mucosal lesions need time to enhance
before images are acquired. It is also desirable to limit the
overall contrast dose to enhance patient tolerance and safety, and
cost-effectiveness. In addition to imaging the mucosa, we scan the
neck while vessels are still opacified to enable differentiation of
lymph nodes from vascular structures.
We balance this competing set of goals with 23 grams of iodine
administered in 75 mL of Isovue 300. We then tailor scanning
parameters to suit each generation of CT scanner technology. With a
single-channel scanner we use a pitch of 1.5, whereas with a
4-channel scanner we use a pitch of 6, and with an 8-channel
scanner we use a pitch of 11. We image after a scan delay of 20 to
We can take advantage of advances in technology by selecting
slice thickness as well. On a 1-channel scanner, we use 3-mm
collimation, whereas on a 4-channel system we use 2.5-mm
collimation. On an 8-channel system it is possible to accomplish
all of the competing goals of neck CT with 1.25-mm source images.
The tighter the collimation the better and more seamless our
Figure 1 offers a classic example of how neck CT can be
optimized by balancing the capabilities of fast scanners and
higher-concentration contrast media. The patient has squamous cell
carcinoma at the tongue base and tonsillar pillar. A large lymph
node is seen easily, even with poor vascular opacification. Note
that smaller nodes are easily differentiated from small vascular
branches because of the excellent contrast resolution between the
vessels and the lymph nodes.
In imaging the orbit, contrast serves primarily to characterize
lesions, and a moderate dose of a moderate-concentration contrast
agent will suffice. An iodine dose of 23 grams, delivered as 75 mL
of Isovue 300, accomplishes the task very well. Speed is not a key
factor in obtaining high-quality images of the orbit. Accordingly,
we use a 30-second delay between injection and imaging to allow
time for enhancement. We use 1- to 1.25-mm collimation and then
image at a pitch that is appropriate for the particular scanner:
1.5 with a single-channel system, 3 with a 4-channel system, and 7
with an 8-channel system. Because we are very conscious of
radiation dose when we scan the orbit, on our multichannel scanners
we obtain all of our studies with a single scan in the axial plane
and reformat the coronal and, occasionally, the sagittal oblique
Figure 2 is an example of an orbital pseudotumor. The enlarged
medial rectus muscle on the right is contrasted with the normal
appearing medial rectus on the left.
Head CT poses an interesting set of contrast-related challenges.
Contrast dynamics are not critical, nor is speed. However, a
dose-sensitivity relationship has been well-demonstrated in head
We use 100 mL of Isovue 370 to approximate a standard contrast
dose, delivered as an intravenous drip. This approach is helpful on
2 fronts: It creates a delay between administration and imaging
while the contrast is dripping in, which is critical to depiction
of the abnormal blood-brain barrier, and it enables us to avoid the
costs of using disposable supplies for injection.
Figure 3 provides an example of how lesion detection often
hinges on imaging the abnormal blood-brain barrier with contrast.
In this case, noncontrast images suggest an abnormality in the
frontal lobe and, perhaps, the right cerebellar hemisphere. When
the blood-brain barrier is imaged with the appropriate dose of
contrast, a lesion in the right cerebellar hemisphere is enhanced,
as well as one in the frontal lobe, differentiating tumor from
edema. Figure 4 demonstrates a moderate-grade temporal lobe
Whether modern scanners improve lesion visualization has not
been studied. It is reasonable to think they might, given their
vastly superior contrast resolution, ability to acquire thinner
slices, and reduction in beam hardening or partial volume artifacts
in the base of the brain, middle cranial fossa, and posterior
Figure 5 exemplifies how more advanced CT technology can produce
better images. It is easy, for example, to see the tiny nodular
metastasis in the right cerebellar hemisphere, as well as the
enhancing metastasis on the left. With previous generations of CT
scanners, it might not have been possible to detect any abnormality
in the posterior fossa, as a result of artifact.
First-pass perfusion imaging of the brain is a fairly new
application of contrast-enhanced CT. The concept is to monitor the
first-pass bolus of iodinated contrast through the cerebral
vasculature. The contrast bolus causes a transient rise in
attenuation that is proportional to the amount of agent in a given
region. Then, integration of data over the course of the first pass
of the contrast agent enables the creation of pixel-by-pixel maps
of brain perfusion.
Perfusion imaging is simple to do in the clinical setting and
typically has its greatest role in acute stroke, although tumor is
another potential application. The procedure involves first
selecting a slice or slices covering 3 vascular territories. A
limited amount of contrast, 40 mL, is injected at 4 mL/sec. We use
software that compensates for the modest 4 mL/sec injection rate;
otherwise it would be necessary to inject at 10 mL/sec to
accomplish, in essence, an instantaneous arrival of the contrast
agent at the brain.
Since the goal of contrast administration is to maximize the
difference between baseline and peak enhancement, the
highest-concentration contrast agent should be the most effective,
at least in theory. We use Isovue 370 and the scan takes 45
The clinical utility of perfusion imaging can be compelling.
Figure 6 depicts the diagnostic course of a patient who was
initially believed to have postictal hemiparesis. Noncontrast CT
revealed no abnormality in the right hemisphere of the brain. On CT
angiography, however, the trunk of the middle cerebral artery could
not be visualized and there were filling defects in the MCA
branches. Perfusion imaging demonstrated a large deficit in
cerebral blood flow and a prolongation of mean transit time,
indicating that the patient was experiencing a significant degree
of ischemia. Cerebral blood volume was well-preserved, however,
suggesting that the brain was still viable. Blood flow values were
all within normal range, with the exception of those associated
with the basal ganglia. The patient was treated aggressively with a
thrombolytic agent, protecting all but the basal ganglia from
CT angiography represents another compelling application of
neurological CT. It can be used to evaluate both the carotid
arteries and the intracranial circulation for atherosclerotic
disease and aneurysms.
Using a single-channel helical scanner, we image from the aortic
arch to the Circle of Willis using a 3-mm collimation and a
moderate pitch of 1.5. On a 4-channel CT scanner, we select a
1.25-mm collimation, the smallest collimation the detector allows.
We use a pitch of 6 in the neck, as the goal is to image the
carotid artery before there is significant overlap of the image
from venous opacification. Once we reach the brain, timing is no
longer a major issue, as the veins that opacify generally don't
interfere with arterial visualization. We therefore reduce the
pitch to 3 to achieve a better slice profile.
With an 8-channel scanner we are able to take advantage of the
boost in speed and can improve image quality by using a moderate
pitch of 10.8 in the neck. In the brain, we again use a fairly low
pitch, 5 in this case, to maintain image quality and optimal slice
We always initiate angiography with enhancement-triggered
scanning techniques (SmartPrep, GE Medical Systems, Milwaukee, WI).
There is a latency between detection of the contrast bolus and
initiation of diagnostic scanning, but the delay is shorter with
modern-generation scanners. We use contrast arrival at the left
heart to trigger our exams, initiating imaging from the aortic arch
toward the brain as soon as we see enhancement.
Our scanning parameters change when we image the Circle of
Willis alone. With a single-channel CT scanner, we use the tightest
collimation, 1.0 mm, and scan at a pitch of 2 in order to get
reasonable coverage. To image the Circle of Willis with a 4-channel
system, we again use the thinnest possible slice, 1.25 mm, but
because speed is not critical, we select a pitch of 3. With an
8-channel scanner, we take a similar approach, using the same
1.25-mm collimation but slowing the pitch to 5.
One of the greatest advantages of CT angiography over
competitive techniques, including conventional and magnetic
resonance angiography, is the ability to view blood vessels in
relation to bony landmarks. This information is highly valuable to
neurosurgeons. Figure 7 shows a nicely delineated aneurysm of the
internal carotid artery, with bones in place.
Another strength of CT is the ability to do image processing on
the scanner. We frequently perform CTA in acute stroke. Under such
circumstances, it is important to have a very rapid and easy
rendering method. We image right at the scanner, using
limited-volume maximum intensity projections both obliquely through
the carotids, as well as in standard orthogonal planes through the
brain. This method produces images, and answers, within minutes of
the patient's arrival.
Today, we are able to perform neurological CT with much faster
scanners. Technological advances give us the opportunity, and
perhaps even a mandate, to use a smaller volume of contrast media
and faster injection speeds. Under most circumstances in
neuroimaging, it is therefore preferable to select contrast media
with the highest iodine concentration, in order to maximize
opacification and enhancement while decreasing costs. Contrast
media with an iodine concentration of 370 mg/mL or greater offers
many advantages in neurological CT. The future availability in the
United States of agents of higher iodine concentration (up to 400
mg/mL) should further extend the utility of this approach. *