This publication was supported by an educational grant from Amersham Health, Princeton, NJ. The opinions expressed in this publication are those of the authors and not necessarily those of Amersham Health.

Dr. Bae reports relationships with Tyco Healthcare and Mallinckrodt through patent agreements and as a consultant. Dr. Fishman reports relationships with Siemens Medical Solutions and Amersham Health as a consultant. Dr. Foley reports a relationship with GE Medical Systems through an investigator agreement. Dr. Naidich reports a relationship with Siemens Medical Solutions through its Advisory Board and as a consultant. Dr. Saini reports a relationship with GE Medical Systems through research support. Dr. Becker, Dr. Sahani, Dr. Siegel, Dr. Tahktani, and Dr. Zinreich report that no such relationships exist.

Dr. Naidich is a Professor of Radiology at the New York University School of Medicine, New York, NY.

We are living in a time of extraordinary technological prog-ress in computed tomography (CT). Some 11 years after its introduction, multislice CT continues to improve, expanding both its range and accuracy in thoracic imaging. Today, multislice CT can easily visualize the aorta in acute aortic syndromes, during postoperative evaluation, and in congenital disease; diagnose pulmonary embolism; assess regional lung perfusion; and characterize lung nodules, among other applications. In the future, flat-panel detectors will undoubtedly revolutionize our approach to CT once again, as will wide-spread use of computer-assisted diagnosis.

Contrast enhancement plays a central role in thoracic CT and there are many theoretical considerations governing its use. When discussing contrast delivery and image processing, it is important to keep the day-to-day realities of a busy imaging department in mind, however. In addition to the need to employ straightforward contrast-enhancement strategies, it is important to remember that many community radiologists do not have experience using advanced workstations or have picture archiving and communications systems. Whether in large medical centers, such as NYU Medical Center, where we perform >100 CT scans a day, or in smaller institutions, protocols for contrast delivery must be developed with an eye to practicality.

Technique

Advances in CT technology have enabled us to perform faster scans while using thinner collimation to acquire data across larger and larger volumes of interest. We have progressed from using a single-detector scanner with 5-mm collimation to examine the main pulmonary arteries in 1991, to using a 4-slice scanner with 1-mm collimation to examine many of its branches in 2000. With the introduction of 8-, 10- and 16-slice CT scanners, it is now possible to use submillimeter collimation, which enables true isotropic imaging. One indication of this progress is a reduction in the number of indeterminate CT pulmonary arteriograms, to 5% to 10%, with multidetector CT scanners. ^1,2

We, and others, are interested in pushing CT to its limits, and today that means using 16-detector technology. Table 1 provides generic guidelines for performing CT angiography (CTA) using 4- or 16-slice scanners. Although these guidelines must be modified for specific clinical situations, they cover nearly all potential indications for CTA in the thorax.

We routinely use 1- to 1.25-mm collimators for data acquisition, and 0.75-mm collimators only rarely. Images are then reconstructed at 1.5- to 5-mm intervals, as clinically indicated. We typically use a contrast volume of 100 to 150 mL, with con-centrations of 240 to 300 mgI/mL. We rarely use contrast material with a concentration as high as 370 mgI/mL. In our practice, the use of monomeric nonionic contrast material is the norm.

We inject contrast at a maximum rate of 3 to 5 mL/sec. Although it is feasible to consider the use of faster injection rates, as has been done in Europe, presently there are no routine indications for this practice. In early studies of CT examination for pulmonary embolism, investigators in France reported in-jection rates of 6 to 7 mL/sec using contrast material with a concentration of 120 mgI/mL. ³ While this approach proved adequate for identifying central clot, it proved less efficacious for identifying peripheral segmental and subsegmental pulmonary arteries.

To time image acquisition, we use either real-time bolus tracking or a 20-mL test bolus of contrast material. In the latter case, we acquire images at 5, 10, 15, and 20 seconds, as a guide to optimizing contrast enhancement. In our experience, the main purpose for a test bolus is to determine whether initial flow rates provide optimal contrast enhancement of the central pulmonary arteries. In cases of suboptimal contrast enhancement, we increase the flow rate to a maximum of 5 mL/sec. Although a test bolus is effective in ensuring a good-quality study, in our experience, bolus tracking has proved more reliable for gauging when to acquire images.

Aortic disease

Multidetector CT can be used to evaluate virtually any clinical condition involving the aorta. While acute aortic syndromes, such as aortic aneurysm and dissection, are among the most common, CT is also useful in postoperative examinations and in the evaluation of congenital disease. More recently, attention has focused on the use of CTA for the evaluation of stent placement in patients with aortic dissection. ⁴

Unlike imaging of the abdominal aorta, in which identification of branch vessels is of critical importance, in the evaluation of the thoracic aorta we routinely use thicker sections (5 mm). In select cases, in which identification of bronchial, intercostal, or anomalous vessels is needed, we may obtain 1- to 3-mm sections. In all cases, additional routine or curved multiplanar sections are reconstructed to simulate the appearance of the aorta as seen angio-graphically. Depending on available technology, we also use both maximum intensity projection images (MIPs) or volumetric reconstructions (Figure 1).

Although volumetric renderings, in particular, elegantly demonstrate aortic anatomy, they are clearly dependent on optimization of contrast administration. Equally important is the interface between the radiologist and advanced imaging technology. While it is possible to optimize contrast enhancement throughout the entire abdomen and chest, it is unlikely that CTA will gain widespread acceptance by general radiologists until easier, more automated methods of advanced imaging techniques are introduced. Only then will multislice CT reach its full potential.

CT angioscopy is another promising application, not only for evaluating the aorta but, potentially, for evaluating both the pulmonary and coronary arteries as well (Figure 2).

Pulmonary arterial disease

Pulmonary embolism

The ability to perform increasingly sophisticated CTA has revolutionized the diagnosis of diseases of the pulmonary arteries and veins. Evaluation of acute pulmonary embolism, for example, represents the most important opportunity today for clinical expansion of CT in the chest. A decade ago, CT had virtually no role in the diagnosis of pulmonary embolism; now it is one of the most commonly requested scans in our institution.

With the aid of contrast enhancement, we are able to image through the entire thorax, routinely visualizing small pulmonary emboli even in fourth-, fifth-, and sixth-generation pulmonary arteries. The volume-rendered images in Figure 3 depict a small clot in the right pulmonary artery to the right upper lobe. Such superb spatial resolution, as well as the ability to acquire data through the entire thorax using a single contrast injection, has now become common. Figure 2 demonstrates the potential for pulmonary angioscopy to be used as an alternative method to evaluate patients with suspected pulmonary embolism, specifically those who might require embolectomy.

To image small pulmonary arteries, it is necessary to acquire the thinnest possible sections. When data are acquired using a single-detector CT and 3-mm-thick slices, subsegmental artery visualization is possible in just 37% to 39% of patients. ² By comparison, when multidetector CT is used to acquire data in 1.25-mm thick slices, visualization is possible in 71% to 76% of patients. Now, with the capability of acquiring data in 0.75-mm thick sections throughout the thorax, our ability to make more precise diagnoses is even better.

Scan delay

A key issue in pulmonary arterial CT is the optimal delay between contrast administration and the initiation of scanning. A study by Hartmann et al ⁵ compared the use of a fixed 20-second scan delay with the use of contrast transit time in a total of 107 patients. With a single-detector CT scanner, investigators found no difference between the two groups in image quality. They concluded that a 20-second fixed delay was acceptable for the evaluation of pulmonary embolism.

In our experience, however, use of a fixed scan delay results in a level of contrast enhancement insufficient to make subtle diagnoses in segmental and subsegmental pulmonary arteries in approximately 5% of cases, even with a 4-detector scanner. Therefore, we now prefer to use real-time bolus tracking.

A study by Kirchner et al ⁶ involved 179 patients undergoing lung CT using a 4-slice scanner and real-time bolus tracking. The study design had certain limitations. For example, although investigators used contrast material with a concentration of 300 mgI/mL, they injected at a rate of only 2 mL/sec. In addition, most patients were undergoing evaluation for thoracic neoplasia rather than pulmonary embolism.

Nonetheless, the study yielded several interesting observations. Investigators triggered imaging when contrast enhancement reached 100 HU. Although, on average, scanning was initiated after a delay of 21 seconds, the range was quite wide, 12 to 48 seconds, reflecting substantial variation in patient physiology. Investigators used an average of 10 monitoring images, a number that would not be expected to substantially increase radiation dose. They found no correlation between bolus geometry and patient age, body surface area, or even weight in this study group. In addition, contrast volume averaged only 48 mL (range 38 to 71 mL). It is unlikely that such a low volume would be feasible for CT of the aorta or pulmonary arteries, however.

It is worth noting that in CT of the chest, single-phase imaging is the norm. The complicated multiphasic contrast injections that characterize CT of the liver, for example, are generally unnecessary.

New directions

Perfusion imaging, although still in its infancy, shows considerable promise as a correlative method for assessing pulmonary embolism. Wildberger et al ⁷ reported preliminary data on the use of multidetector CT to produce perfusion-weighted color maps of lung paren-chyma. Using a 4-slice CT scanner, 120 mL of contrast material, and a 30-second scan delay, investigators acquired data in 1.25-mm thick slices and reconstructed it in increments of 0.8 mm. From source images, they performed an automated 3D segmentation of the lungs, followed by vessel cutting and adaptive filtering. The filtered volume data were used to create a color-coded display of lung density, which was fused with the original CT images.

In the 4 patients who did not have evidence of pulmonary embolism on previous multislice CT, perfusion imaging showed a homogeneous distribution of color-coded densities in the lung (Figure 4A). In the 2 patients with established pulmonary embolism,
perfusion-weighted color maps showed low-density values distal to the occluded pulmonary arteries (Figure 4B). Investigators concluded that lung densitometry with color-coded display of densities in the lung parenchyma may provide additional information in the evaluation of pulmonary embolism.

Whether lung perfusion imaging will become clinically practical is uncertain. Clearly, it is technologically feasible to create such images, and to examine not just the embolus, but lung physiology as well.

The potential to combine pulmonary angiography and pulmonary venography represents another area of active research. ^8-12 The published literature offers no consensus on the clinical indications for such studies or the best way to perform them. Nonetheless, they demonstrate that we are capable of finding clot in the peripheral veins and suggest that CT venography may be an important adjunct in the imaging of pulmonary embolism. Determining the optimal method for contrast enhancement, including scan delay, will require further investigation.

Computer-assisted diagnosis of pulmonary embolism also has great potential. In Figure 5, segmented views of the pulmonary arteries to the right lower lobe show the presence of multiple filling defects that were identified using computer-assisted diagnosis. This tool will likely become important, particularly for detecting small subsegmental pulmonary emboli.

The best ways to administer contrast and project data are also the subject of intense research. One approach would be to use bicolor coding of multiplanar reconstructions of the pulmonary arteries to display differences in contrast density suggestive of embolism (Figure 5).

Miscellaneous applications

In addition to routine applications, CTA can be of value in evaluating virtually any intrathoracic disease in which abnormal vasculature is present. In addition to assessing thoracic aortic aneurysms and dissections, CTA has also proved valuable for assessing aortitis, identifying congenital anomalies, and evaluating posttraumatic thoracic aortic injuries. ^13,14

Similarly, although CTA has most often been used to assess acute pulmonary embolism, less frequent indications include chronic pulmonary embolism, pulmonary arteritis, pulmonary vascular neoplasms, and traumatic pulmonary vascular injuries, among others. ¹⁵

Additional indications include vascular anomalies, such as those that occur in patients with pulmonary sequestration or arteriovenous malformations (Figure 6), and abnormalities of the systemic and pulmonary veins, including the superior vena caval syndrome. ¹⁶

Nodule enhancement

Although nodule enhancement does not come under the heading of CT angiography, it is an appropriate topic for any discussion of the practical applications of contrast-enhanced thoracic CT. Nodule enhancement has been well described in the literature. ¹⁷ The data suggest that the use of contrast enhancement in the evaluation of pulmonary nodules is extremely helpful for differentiating benign from malignant disease.

In a multicenter trial, Swensen et al ¹⁷ used contrast-enhanced CT to study 356 lung nodules 5 to 40 mm in diameter. Nearly half of the nodules were malignant. Investigators found that in malignant nodules, enhancement averaged 38.1 HU, as compared with 10.0 HU in benign nodules ( P <0.001). Using 15 HU as the cutoff, they found nodule enhancement to have a 98% sensitivity and 58% specificity for malignancy.

Evaluation of nodule enhancement is not widely done, however. In part, this results from uncertainty about how the technique should be applied. It is hoped that eventually it will be common to scan each nodule as a volume and to do whole-volume determinations of enhancement at various time phases. It may also be possible to conduct permeability and perfusion studies of pulmonary nodules--additional applications in which optimization of contrast enhancement will play a crucial role.

Conclusion

With the introduction of multidetector CT scanners capable of acquiring thin sections through the entire thorax in a single breath-hold, CTA has now entered the era of routine clinical practice. As outlined above, virtually any study requiring evaluation of intrathoracic vessels may now be performed as a CT angiogram. This has truly revolutionized our approach to diagnostic imaging, as evidenced by the preferred use of CT in patients with a clinical suspicion of an acute aortic syndrome or, especially, a pulmonary embolism. The result has been near complete replacement of the need for routine angiography in our department. It is anticipated that with more sophisticated methods of data acquisition, we will be able to routinely evaluate lung perfusion as well as more accurately characterize focal lung pathology.

Figure Captions

FIGURE 1. (A and B) Volumetric rendering of a Type B aortic dissection showing both the true and false lumens to advantage.

FIGURE 2. (A) Volumetric and (B) angioscopic renderings of clot in the left interlobar pulmonary artery (arrow). Use of virtual angioscopy may prove of value in selected cases in which embolectomy is planned.

FIGURE 3. Volumetric renderings of the central pulmonary arteries show discrete filling defects in both (A) the truncus anterior (arrow) and (B) the right interlobar pulmonary arteries (arrow).

FIGURE 4. (A) Normal lung perfusion is characterized by a normal color-coded anteroposterior lung perfusion gradient in a patient imaged in the supine position. (B) Perfusion image in a patient with multiple pulmonary emboli. Perfusion imaging enables color mapping of the extent of perfusion defects that would not be evaluable on routine CT images. (Reprinted with permission from Wildberger et al. ⁷ )

FIGURE 5. (A and B) Images through the basilar pulmonary arteries using computer-assisted diagnosis (CAD) to identify multiple pulmonary emboli (PE) (arrows). (Images provided courtesy of Carol Novak, Siemens Corporate Research, Princeton, NJ).

FIGURE 6. Pulmonary arteriovenous malformation. (A) Axial and (B) sagittal volumetrically rendered views show to advantage both the enlarged feeding artery and draining vein.

Discussion

ELLIOT K. FISHMAN, MD: Thanks, David. In terms of pulmonary embolism, whether it's with CTA in the future, or just now, one of the problems people typically have is flow-related phenomenon, determining if it is really a clot or is it just flow? What are your thoughts about choosing contrast? Would an iso-osmolar contrast agent be better in that situation?

DAVID P. NAIDICH, MD: Well, we don't really have any experience with iso-osmolar contrast in that context: I can't really answer that. But I think the question of why it is that some contrast studies of the pulmonary arteries don't look as enhanced as they should relative to that 4% or 5% of cases in which doing more detailed timing and volume acquisition really would have played a role.

Although performing bolus tracking has become less of a problem, we have discovered that using a time-delay test bolus may be as effective.

We try to use as little contrast as possible to maximize delivery. So we initially use 3 mL/sec, and then acquire data at 5, 10, 15, and 20 seconds. I have found that there is a certain percentage of cases, not surprisingly, in which using that acquisition protocol, you can see that none of those images are really sufficient for good contrast enhancement.

The result of that has been when we use a timed bolus with resulting poor vascular opacification, we can increase the rate of injection from 3, to 4, or even 5 mL/sec, an option we lose with bolus tracking. Ultimately, I don't think there is going to be a solution, because I think part of the reason for suboptimal opacification in select cases is both anatomic and, more importantly, physiologic. Especially considering branch or crossing points of smaller vessels and those with an oblique course, it is just difficult to be certain that one is not missing a small peripheral pulmonary embolus.

FISHMAN: Has anyone had experience using an iso-osmolar contrast agent with pulmonary emboli, compared with other kinds of contrast?

NAIDICH: I can't comment, as we haven't been using it.

FISHMAN: We use iso-osmolar contrast routinely in a lot of pulmonary emboli studies. Dr. Pannu was doing some work that's just being analyzed in some animal models, looking at the opacification of vessels and homogeneity of vessels with different contrast agents. Our experience in trying to document this in an animal model has been that with an iso-osmolar agent, you get a more homogeneous opacification of vessel. We've had fewer problems with determining if a finding is an embolus or is flow-related.

I agree with you that in some patients, especially patients with high blood volumes, such as postpartum patients being evaluated for pulmonary emboli, it's impossible to get a good study. I don't care how you inject the contrast and how you time it; in some cases, it seems that nothing gets very bright, no matter what you do.

NAIDICH: Even more than a postpartum patient, a woman in her third trimester is also very hard to evaluate. In the third trimester, the heart rate is 150% or 200% of what it normally would be, with increased blood vol-ume. But, even in the most unexpected patients in whom you don't know of any history of heart disease nor of any history of even pulmonary physiologic abnormalities, sometimes you never quite get the opacification necessary for good interpretation.

SANJAY SAINI, MD: David, I don't do thoracic imaging, but I understand that the density of contrast material and the superior vena cava can create artifacts.

NAIDICH: That's something that has been mentioned. I suppose, initially, if people aren't used to evaluating PA studies they may be a little confusing. It is an artifact that's easily recognizable as an artifact, and it really has not interfered for some time now. It used to be that people would use this to argue for scanning from below up. But I don't think this is really necessary.

SAINI: Does that mean that using higher density contrast material may be less useful, because of that artifact?

NAIDICH: Again, I'm not sure that streak artifacts specifically from the superior vena cava are ever going to be that much of a problem. Once you know what to look for, it's really not an issue.

W. DENNIS FOLEY, MD: This could be an argument for a saline chase.

SAINI: Potentially, yes.

FOLEY : I want to add one question. Do you have any comments on EKG gating, particularly for the left lower lobe and pulmonary emboli? Particularly with the low mAs that you are now using, could it be a prospective study?

NAIDICH: The problem with prospective gating is you need to know which cases would likely benefit. Although, predictably, there are always going to be problems looking at the lingular pulmonary arteries, because cardiac motion with most state-of-the-art prospective gating prolongs studies and makes them more difficult to perform. Most important, prospective gating may limit volume acquisition in a single breath-hold.

KYONGTAE T. BAE, MD, PhD: What threshold do you use for bolus tracking?

NAIDICH: We've arbitrarily been using 125 HU.

MARILYN SIEGEL, MD: When do you use dual-phase or arterial-venous-phase imaging in the chest? Do you ever use it for some of the cardiac studies there?

NAIDICH: We don't really use it. Dual-phase imaging is rarely necessary in the thorax: the sole exception is
nodule enhancement/perfusion studies. For these cases, a saline chase would theoretically be most important to ensure uniform contrast delivery. To my knowledge, this remains a problem, as I believe that there is now only one manufacturer that markets a dual injector. Is that correct?

BAE: There are now two or three of them.

NAIDICH: It has been suggested that a saline chaser be used without a dual injector; unfortunately, this has proved too complicated to gain general acceptance.