This article discusses the use of high concentration CT contrast in cardiac imaging applications. Functional Imaging, CT Coronary Angiogrpahy Imaging and future applications are discussed.
William Stanford, MD
Professor of Radiology, Division of Chest and Cardiovascular
Radiology, University of Iowa, College of Medicine, Iowa City,
The use of contrast media in cardiac CT is becoming increasingly
important. Potential applications include CT coronary angiography;
definition of cardiac and great vessel anatomy; and assessment of
ventricular function, intracardiac thrombi and tumors, myocardial
perfusion, and pericardial disease, particularly if diastolic
filling needs to be determined.
Contrast is useful in accurately defining the borders of vessels
and, particularly, the cardiac chambers; identifying intraluminal
filling defects; and facilitating placement of regions-of-interest,
both for myocardial perfusion and in the imaging of coronary artery
When using contrast in cardiac imaging, several considerations
be-come important. First, it is necessary to use nonionic contrast
to prevent perturbations of heart rate. Second, many patients have
low cardiac outputs, with ejection fractions in the high-20% to
low-30% range. Such low outputs have a significant impact on venous
and arterial circulation times; thus, it becomes important to track
contrast arrival times accurately and individually for each patient
in order to optimize vessel and cardiac chamber visualization.
Finally, the physiological impact of the arrival of contrast media
must be kept in mind, as it may result in a sudden Valsalva
reaction, which could impede the arrival of the contrast bolus.
Two types of scanners are used in cardiac CT imaging:
electron-beam and helical. The introduction of electron-beam CT in
the mid-1980s made it possible, for the first time, to image the
heart quickly enough to eliminate motion artifacts. Now we are able
to do 50- to 100-msec acquisitions at 4.5 to 9.5 line-pairs/cm, and
can use both flow and cine sequences. With cine sequences, we can
acquire 17 images/sec and actually observe cardiac contraction.
With continuous-volume scanning, we can acquire a total of 140
3-mm-thick slices in 17 seconds. All of this is done using less
contrast, as will be discussed later, and at a reduced radiation
exposure (1.08 cGy with a 100-msec acquisition time and 3-mm
slices, as compared with 2 to 4 cGy with conventional CT).
Helical technology also plays an important role in cardiac
imaging. With helical CT it is possible to perform 500-msec scans
with effective partial reconstruction scan times of 125, 250, and
320 msec. These improvements, along with continued advances in
multirow detectors and electrocardiographic triggering, have made
helical CT a major player in the evaluation of the heart and great
We use two contrast injection sequences. One involves tracking
the flow of a bolus of contrast (Figure 1). Flow studies are useful
in examining abnormalities in coronary artery anatomy, including
arteriovenous malformations (Figure 2) and shunts. The other type
of injection sequence involves the continuous administration of
contrast throughout the scanning period. Such cine studies are
useful for examining cardiac function (Figure 3) and myocardial
contraction, especially in patients with possible myocardial
Because of the critical need for imaging small structures in the
heart, we routinely use one of several methods to determine
circulation time in order to predict contrast arrival more
accurately. In cardiac imaging, our most common approach is to
determine circulation time by measuring the arm-to-tongue
circulation time with magnesium sulfate. A diluted solution of
magnesium sulfate is injected into the antecubital vein and the
patient tells us when he feels heat in the base of his tongue.
Another way to measure circulation time is by using cardio-green
dye and an earlobe densitometer.
A third frequently used method is to give a 10-mL test bolus of
contrast and observe the time of its arrival in the heart.
Computer-automated techniques such as SureStart (Toshiba Medical
Systems, Tustin, CA) offer an alternative. With this technique, we
place a cursor over the structure of interest and, once the
contrast density attenuation reaches a prespecified point, the
scanner turns on and begins imaging. A final option is to use a
fixed delay between contrast injection and image acquisition, and
this often works quite well.
When examining cardiac function with electron-beam CT, we use
contrast with a concentration of 76% iodine (iopamidol 370),
infusing 75 to 100 mL at 2.3 to 2.4 mL/sec (Table 1). We determine
the circulation time using magnesium sulfate and then delay imaging
for an additional 10 seconds, to ensure that the contrast has
arrived and will continue to opacify the heart during our 3-second
We also use contrast with a concentration of 76% iodine for flow
studies. In this case, we infuse 40 mL of contrast very rapidly, at
10 mL/sec. In order to image the structure of interest during the
arrival and washout of the bolus, we generally subtract 6 seconds
from the circulation time before beginning to image.
Both electron-beam and helical CT have the potential to play a
significant role in quantifying ventricular function, though to
date, more work has been done with electron-beam technology. On
short-axis images, a cursor is used to identify regions of interest
around the ventricular cavity, both in diastole and systole (Figure
4). The difference between the cavity size in the two images is the
cardiac output for that slice. To determine myocardial mass, an
additional region-of-interest is identified around the outer
border, and the myocardium between the two borders is interrogated.
By multiplying the resulting figure by the specific gravity of
myocardium, 1.05, it is possible to determine myocardial mass.
By adding up all of the slices throughout the heart, it is possible
to generate very accurate determinations of cardiac function,
ejection fraction, and left and right ventricular outputs.
The ability of cardiac CT to evaluate cardiac function
qualitatively is also important. One example is the observation of
abnormal ventricular contraction in the area of a ventricular
aneurysm. Others include global hypokinesis and impaired valvular
function in a patient with a severely depressed ejection fraction,
and a filling defect in the left ventricle of a patient with a
large mural thrombus, which was formed in an area of relative
hypokinesis that resulted from myocardial infarction. Functional
imaging can also demonstrate movement of an atrial myxoma,
especially if it prolapses through the mitral valve and back into
the atrium during cardiac contraction.
Helical and multidetector CT are beginning to demonstrate their
potential to evaluate ventricular function and will likely join
electron-beam technology in playing a major role in functional
imaging in the future.
CT Coronary Angiography
One application of cardiac CT that is receiving a great deal of
attention is CT coronary angiography. When imaging with an
electron-beam scanner, we program a 100-msec scan time, a 3-mm
slice thickness, and a table travel of 2 mm, so that there is
overlap of slices in the data sets (Figure 5). For helical CT
reconstructions, we use 500-msec scan times and 4 * 2.5-mm slice
thicknesses for prospective gating (Figure 6). For retrospective
gating, 4 * 1-mm slice thicknesses are used with table speeds of
1.5 mm/rotation. All applications require a high iodine
concentration (76% iodine) in order to define the coronary anatomy
and/or identify associated stenoses.
CT angiography has been shown by Achenbach et al
to have excellent sensitivity and specificity for the detection of
high-grade stenoses when compared with conventional invasive
coronary angiography (92% and 94%, respectively). The same 1998
study demonstrated that 25% of arteries were not evaluable,
primarily because of the associated dense calcifications and vessel
In addition, CTA enables us to look at the orifice of the
coronary arteries and "fly through" the vessel, observing the
vessel wall and identifying calcified plaques.
This technique of "virtual angioscopy" has tremendous potential for
imaging coronary pathology, as does intravascular ultrasound, but
CT is much less invasive. Bypass grafts are easily seen with fast
Today's fast CT scanners can evaluate valvular function easily,
including valve movement and leaflet thickening. Examples of
valvular abnormalities that can be demonstrated by CT are mitral
valve prolapse (Figure 8) and flail mitral valve leaflet resulting
from rupture of the chordae tendineae.
With perfusion imaging, CT can be used to identify
regions-of-interest, as is done in neuroimaging, and to look at
both the blood pool and the myocardium. It can also be used to
create parametric-type maps of myocardial blood flow (Figure
To image the pericardium, and especially to evaluate possible
pericardial effusion, it is necessary to first define the
ventricular cavity with contrast. It is then often possible to see
the low-density fluid that surrounds the heart (Figure 10). In
pericardial constriction, CT can not only demonstrate the thickened
pericardium, but also evaluate cardiac function and determine
diastolic filling. In this way, it is possible to substantiate
whether functional compromise is present as a result of the
For identifying patients at risk for a cardiac event, such as
myocardial infarction, CT coronary angiography is becoming
increasingly important for looking not only at calcification in the
coronary arteries but also at soft plaque (Figure 11).
Soft plaque can be detected by both helical and electron-beam
scanners, and a great deal of research involving both technologies
is being devoted to trying to identify and characterize soft
Contrast-enhanced CT is becoming an increasingly important
method for evaluating cardiac anatomy, function, and pathology. Its
success is not limited to a few research centers or to a single
type of technology. Instead, cardiac CT is being performed
successfully at centers all over the world.
For most applications of cardiac CT, iopamidol 370 is the
preferred contrast media (since it is the highest concentration
available); the exceptions are the aorta and great vessels. When
used in combination with today's fast CT scanners, it enables
imaging of the coronary arteries as well as the definition of
cardiac anatomy and ventricular function. Precise timing of
contrast administration and the determination of the best scanning
sequence remain challenges. *