Coronary Imaging Considered

Recent years have seen the capability of computed tomography angiography (CTA) and magnetic resonance imaging (MRI) improve significantly, leading to much greater use of these techniques in cardiac studies. Of the two, CTA has shown the fastest rise in popularity. In 2006, the CT field became well-known as cardiologists adopted the method of CTA, to the point where many observers feel it will soon replace conventional angiography in diagnostic applications. Nevertheless, the debate about the merits of MRI and CTA continues.

"Cardiac MRI is a hazardless technology and gives a large amount of data on damage, though it is less clear on coronary arteries than 64-slice CT," says Blase Carabello, an imaging expert from the American College of Cardiology. "More sophisticated cardiac MRI and multi-slice scanning technology give you immediate images and an enormous amount of data, but it is likely that hospitals may not buy both technologies."

It seems that the balance is tilting towards cardiac CTA on the basis that emerging systems are set to provide ever more detail on the condition of coronary arteries. Many feel this is worth the hazards associated with the scanning technique – namely the use of dye and the dose of radiation administered to the patient.

CT is already used in many areas of medicine as a key source of images that determine diagnosis and treatment, from neurology to nephrology. Improvements in the technology suggest that evolving scanning techniques and image quality suit it to use in cardiology. However, studies have yet to conclude whether superior image quality ensures better diagnoses or therapeutic outcomes. Such studies are ongoing, but the sheer pace of technology development limits their relevance.

"The trials are worth doing, but they will be out of date when they are published,’ believes Geoffrey D Rubin, chair of the American College of Radiology (ACR) Committee on Cardiovascular Imaging. "The first multi-site trial of CT angiography was recently published using 16-row CT. By the time of its publication the technology had moved on to 64-row."

Carabello agrees: "There is no question that technology continues to evolve faster than our ability to know when and where to utilise it. No one technology is better than another. It depends on who uses the technology and how."

ACCELERATING EVOLUTION

CTA passed a landmark in 1998 with the introduction of the first multi-detector row scanners, which used four rows of scanners on each rotation. The technology moved incrementally but rapidly from four-row to 64-row scanning, as systems vendors focused largely on adding more banks of detectors to improve image quality and scanning speed.

Similarly, all vendors have subsequently focused their efforts on increasing the rotation speed of the scanner, though now developers seem to be going off on different tangents in pursuit of different methods to achieve the same goals.

Some strive to produce thinner sections to increase the level of image detail, while others continue to add more detectors, with 256-row systems the next milestone. One vendor has even tested a combination of two scanners in one machine to improve temporal resolution by cutting the scan time in half.

It seems certain that these different approaches will yield a broader range of CT technologies in the next few years, which some feel will benefit the market as a whole and improve its applications in coronary imaging.

"We will see a differentiation of CT technology from different vendors, which will lead to more choice," says Rubin. "Some are looking to add capability as before and taking the technology to 256 rows could, for instance, enable us to image the whole heart in one rotation. As you can see, the heart is influencing vendors’ approaches."

In fact, applications in cardiology are now the defining factor in the development of CT imaging. "CT has been used successfully for 15 years for the rest of the vascular system outside the field of coronary artery imaging," he adds. ‘But in 2004, it became clear that all of the technology improvements in CT are now directed specifically towards cardiac scanning. Given that CT is a key element in the diagnosis of many diseases and abnormalities, this focus on the heart is striking."

Improving temporal resolution, for instance, specifically benefits cardiac imaging, as it addresses the problem of the constant motion of the coronary arteries. Freezing this motion is vital for the production of more useful images, so faster scanning is a must for coronary CT angiography.

WHO SHOULD CONTROL CT?

While CT imaging is used in the diagnosis of many diseases, it is only in its application to coronary imaging that a debate has arisen regarding who should control the use of the technology.

Cardiologists are accustomed to having an important role in the imaging processes on which they rely. They will, for instance, be involved in the process of catheter angiography, so they naturally expect to have a similar role in cardiac CTA. This, however, throws into question the role of radiologists, whose skills in the use of the technology and the interpretation of data suggest that they, too, have a role to play in imaging the heart and its surrounding structures.

"The issue of control is surfacing, and it is complex," notes Rubin. "Cardiologists have evolved to become more involved in imaging than other specialists. They have been imaging coronary arteries exclusively for 20 years. They still want to do it and they want to use the latest tools."

The appetite of cardiologists has been further enhanced by a quantum leap in the capabilities of cardiac imaging technologies. Rubin describes catheter angiography as searching in a dark room with a flashlight: you only see where you look In comparison, coronary CT is like finding the switch to turning on all the lights and seeing everything at once.

One problem is that cardiologists require training in the interpretation of cross-sectional images and the much greater volume of data that CT imaging provides. Radiologists, however, already have these skills.

"Radiologists have been trained from day one to interpret cross-sectional images – particularly CT images – so there is a process that is ingrained in them," argues Rubin. "They must examine the full data set, including the surrounding organs, to detect early stage or unusual manifestation of a disease. Cardiologists will focus exclusively on the heart."

"Imaging is not just about image capture, but about interpretation. Radiologists are focused on that – it is part of their job description," he adds.

Furthermore, US legislation places responsibility for the scanning process squarely on the shoulders of radiologists, so compliance issues lend weight to their claim to the control of coronary CTA. In terms of patient care and the quality of the final outcome, however, the feeling on both sides is that whoever is responsible for the imaging process should be the best trained person.

CLOSER COOPERATION REQUIRED

Overall, there is a growing feeling that radiologists and cardiologists should work together more closely. "The quality of the image depends on the quality of the artist," says ACC’s Carabello. "It is not just about the technology. It requires people with background knowledge of how to make it work. There are good and bad cardiologists, just as there are good and bad radiologists."

"The question of who should have control is an old one," he continues. "It arose with echocardiography, which is now used by many practitioners, including anaesthesiologists. At the end of the day, it comes down to who is best trained to use the technology, whether they are a cardiologist or a radiologist."

As CT technology differentiates, treatment centres will no doubt adopt different strategies for cardiac imaging. In some it will be the preserve of cardiologists, in others radiologists, and in others still a collaborative approach between the two disciplines. The latter is increasingly seen as desirable.

"It’s about capability, not about specialism," says Carabello."The best solution would be a collaboration between radiologists and cardiologists," concurs Rubin.

Whether this can be successfully achieved remains to be seen, as the debate over CT technology continues.