Applied Radiology

William Stanford, MD
Professor of Radiology, Division of Chest and Cardiovascular Radiology, University of Iowa, College of Medicine, Iowa City, IA

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 plaque.

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. ¹

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). ²

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 vessels. ³

Contrast Administration

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 infarction.

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 acquisition.

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.

Functional Imaging

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. ⁴ 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 ⁵ 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 tortuosity.

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. ⁶ 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 CT ⁷ (Figure 7).

Additional Applications

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 9).

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 constrictive process.

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). ⁸ 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 plaque.

Conclusion

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. *