|An image of the left anterior descending artery from a dedicated coronary CT angiogram performed on a multidetector CT scanner. Image courtesy of Robert C. Gilkeson, MD, University Hospitals of Cleveland, Case Western University.
Recent years have seen remarkable advances in imaging technology. This has been particularly true in the field of computed tomography, where the introduction of multidetector CT, continued improvement in 3-D imaging capabilities, and greater sophistication in interventional CT techniques have strengthened CT’s central role in diagnostic radiology.
In any discussion of advances in CT technology, the introduction of multidetector CT (MDCT) must be identified as the greatest development of the past decade. Just as the introduction of spiral CT revolutionized the clinical capabilities of CT imaging, MDCT technology has again exponentially expanded CT’s diagnostic capabilities.
Advances in MDCT have been made possible by the advent of multidetector systems that allow the acquisition of multiple slices in the time traditionally used to acquire a single volume of data. Different vendors have chosen different detector designs for the purpose: the asymmetric detector system favored by some vendors allows greater flexibility in slice and pitch choices with greater dose efficiency. More recent advances in x-ray tube design now allow tube rotation times of 500 msec. These advances have significantly improved imaging times and image quality. If, for example, we compare a standard chest CT done at 1-cm slice thickness and a pitch of 1, the acquisition time is generally between 25 and 35 seconds. If a similar multislice CT protocol is utilized using the same slice thickness and a tube rotation of 500 msec, the acquisition time is 4-5 seconds. Because of this markedly improved speed, it is possible to choose to decrease slice thickness and allow for greater lesion conspicuity. Conversely, it is also possible to choose to image a larger imaging volume and increase our anatomic coverage.
The improved imaging flexibility afforded with multislice CT therefore enables the radiologist to tailor the examination according to the indication. For example, in the CT evaluation of pulmonary nodules, the protocol at University Hospitals of Cleveland is to perform 2.5-mm slices through the chest at a pitch of 1. This allows complete coverage in 15-20 seconds, with the enhanced lesion detection enabled by 2.5-mm slice images. In the evaluation of pulmonary emboli, 1-mm slice thickness can be obtained even in critically ill patients in less time than the conventional single slice CT scanner can obtain 3-mm slices. These improved imaging parameters have been proven to be more sensitive to smaller vascular structures, particularly segmental and subsegmental emboli.
One of the most exciting new applications of multidetector CT is in the evaluation of cardiovascular disease. New software advances now allow ECG gating in conjunction with multislice acquisitions. ECG gating can be performed prospectively or retrospectively. Prospective ECG gating allows assessment of coronary artery calcification, an application previously reserved for electron beam computed tomography (EBCT). A number of EBCT studies have shown that a scan showing no calcification has a nearly 100% negative predictive value for a future coronary event in the next 2-5 years. Conversely, a high coronary artery calcification score correlates with angiographically significant disease. Several studies have been published recently showing excellent correlation between EBCT and MDCT in the evaluation of coronary calcification. In fact, early data suggest that because of its superior image quality, MDCT may have greater sensitivity to the presence of small amounts of calcification than EBCT. While there is still controversy regarding the clinical significance of coronary artery calcification, the recent increase in EBCT scanners offering coronary artery screening is testimony to the public’s perception of its importance. The recent data, however, suggest that MDCT may be able to provide data similar to that of EBCT with more imaging flexibility.
While the assessment of coronary artery calcification is clearly a benefit with the new multidetector scanners, research in CT coronary angiography has also demonstrated the ability to image significant portions of contrast-enhanced coronary arteries. The use of retrospective ECG gating has enabled radiologists to examine the coronary arteries during the diastolic phase of the cardiac cycle, minimizing the motion artifacts that previously limited spiral CT’s ability to visualize these small vessels. While a number of imaging issues need to be answered, this technology is being used at a number of clinical centers to assess coronary artery disease. Future advances in MDCT technology suggest that in the race for assessment of coronary artery disease, CT may have an advantage over coronary MRA. In addition, the use of cardiac gating has allowed us to evaluate ventricular function and certain types of cardiac arrhythmias in ways previously reserved for cardiac MR. Regardless of the outcome, many radiologists working in the field are excited about the prospect of a comprehensive cardiac evaluation performed in the radiology department.
Clearly, however, the assessment of smaller structures with MDCT is not limited to the coronary arteries. In the field of neuroradiology, CT myelography with MDCT has offered superior imaging of nerve/disc path pathology, CT angiography has allowed visualization of intracranial aneurysms and the circle of Willis, and the ability to perform 0.5-mm slices has markedly improved evaluation of diseases of the inner ear.
While this superior assessment of small structures has been a clear advantage of MDCT, the advantage of greater anatomic coverage has been equally important. A comprehensive CT evaluation of the trauma patient can be performed in 30-40 seconds, with accurate assessment of the aorta, intra-abdominal organs, and musculoskeletal structures. CT angiograms of the aorta, iliac vessels, and lower extremity runoffs can be performed in a similar time, often obviating the need for conventional angiography.
The significant advances of MDCT outlined above have opened the door to a number of issues, particularly the concept of CT screening. A number of independent sites and entrepreneurs provide the “whole body CT scan,” offering to screen the patient for occult lung cancers, coronary artery disease, and colon cancer. While early data support the role of CT screening in lung cancer, the lack of any controlled trial regarding this approach makes it difficult to assess its cost-effectiveness or clinical utility. However, 6-month waiting lists for these CT screening examinations are testimony to the public’s interest in this technology, and may drive its future use.
It is clearly an exciting time for radiology. However, as often happens, our technological advances have outpaced our ability to clinically evaluate its role in future patient care. One thing is certain: these advances will only accelerate in the near future, and it will be a continuing challenge to assess and effectively use these advances in the radiology department of the future.
Robert C. Gilkeson, MD, is the section head of chest radiology and assistant professor of radiology at University Hospitals of Cleveland and Case Western Reserve University. He is actively involved in a number of research protocols evaluating the utility of multidetector CT.