David A. Dowe, MD

Coronary CT angiography came of age in the last decade. This occurred because CT technology moved into the 16- and 64- detector era. These technical advances, together with refinements in technique, made the procedure possible on a routine basis for the vast majority of patients at sites that had the multislice technology and properly trained technologists and physicians.

Enthusiasm for a noninvasive method of examining the coronary arteries was so great that it spawned a cottage industry: coronary CT angiography (CCTA) education, with courses popping up out of nowhere. We were one of the first. In 2006-2007, more than 1000 radiologists were trained in our private office alone. Thousands of others were taught at remote, destination-style courses we hosted. This produced gross revenue in the millions of dollars. Multiply this by the numerous sites at major academic institutions, for both cardiologists and radiologists, and you get a feel for how quickly CCTA came to be viewed as a major breakthrough in medical imaging.

The medical literature has been clear that CCTA stacks up very favorably against stress tests as an initial exam and against catheter coronary angiography for examining the coronary arteries for obstructive coronary artery disease. The cost savings of using a protocol that incorporates early use of CCTA in emergency and nonemergency settings for low- to intermediate-risk coronary artery disease patients has been well documented. The Rule Out Myocardial Infarction/Ischemia Using Computer Assisted Tomography (ROMICAT-II) and CT for Systematic Triage of Acute Chest Pain Patients to Treatment (CT-STAT) trial results will soon document the advantages of CCTA in the emergency department. The Coronary CT Angiography Evaluation for Clinical Outcomes (CONFIRM) registry, encompassing more than 10,000 patients, will reveal that CCTA has prognostic value in patients without known coronary artery disease.

Paradoxically, despite all these achievements, CCTA has had a very difficult time gaining traction in routine clinical care protocols and in the reimbursement world. Utilization of CCTA is actually down from 2005 levels. The number of physicians seeking training in the procedure has diminished greatly, and it is harder than ever to obtain reimbursement for it. A confluence of unfortunate events, some medical and some economic, are the cause.

The furor over radiation exposure from medical imaging tests began in the U.S. at the 2007 RSNA meeting. A study documenting radiation exposure from CT scans used in the trauma setting

 made its way into the national spotlight and there is no going back. This led to increased awareness of radiation dose, technical improvements to reduce dose, and increased awareness of self-referral and inappropriate utilization of CT. This, in turn, has led to a decline in CT utilization of anywhere from 10% to 20% in the outpatient imaging centers with which I have close contact.

Unfortunately, CCTA has suffered more from the radiation dose scare than other CT exams for reasons that are less than true. When CCTA burst on the scene, it was done using retrospective gating. Effective doses using retrospectively gated CCTA were in the range of 10 to 30 mSv, depending mostly on the patient’s body mass index. This dose level exceeded that commonly quoted for catheter coronary angiography, which ranged from 3 to 10 mSv for noncongenital work.

In addition to this, radiation dose became a political football at local, national, and international levels and was used to scare people away from CCTA by those who wished to maintain the treatment status quo for coronary artery disease: nuclear stress tests leading to catheter coronary angiography. What was conveniently hidden by this crowd was the radiation dose of SPECT nuclear stress tests. Using a sestamibi-sestamibi protocol,

 radiation doses range from 12 to 25 mSv. When thallium is incorporated into the protocol, as either a single or dual isotope, radiation doses can easily exceed 22 to 30 mSv per test. Prospectively gated CCTA with adaptive iterative reconstruction is now performed with a dose range of 0.4 to 10 mSv, which is well below the SPECT stress test range.

So where are the radiation dose police regarding the use of SPECT stress exams, utilization of which (according to Medicare data) has seen exponential growth since the mid-1990s? Why wasn’t CCTA trumpeted as a low-dose, low-cost first-line alternative to SPECT stress tests? Some will argue that the answer is the need for functional imaging and myocardial perfusion. But my personal experience is that if a patient has a CCTA showing no stenosis greater than 50%, that patient will eventually get an echocardiogram to document wall motion and valve function and, if it’s negative, will have no cardiac events at six months follow-up. It is my strong opinion that the radiation dose of CCTA was used to delay its clinical adoption and to preserve the income stream of those who provide SPECT stress tests.

The explosion of medical imaging utilization as a leading cause of increasing healthcare costs has been well documented. The causes of this are many and include the increased ability of medical imaging to contribute to the diagnosis of patients’ problems, the speed at which it can achieve this, the increased availability of high-end imaging equipment, medicolegal concerns about missed diagnoses, and self-referral.

These factors have put medical imaging smack in the center of the insurance industry’s radar screen. Initial “soft” attempts to decrease utilization were tried in the form of patient and referring physician education, but the approach was quickly abandoned. Insurance companies began contracting with radiology business management companies (RBMs) in an effort to stem the tide of high-end medical imaging. RBMs, in turn, created “hard stops” and roadblocks in the imaging referral process by a variety of means. In their favor, RBMs heavily research their positions for what can be approved or disapproved in medical imaging. They defend their positions aggressively and have been able to reduce inappropriate utilization. However, the buck never stops there.

I have presented at meetings of RBM medical directors and spoken with executives of RBMs numerous times about how the use of CCTA, with its 100% negative predictive value, could virtually eliminate the elective negative diagnostic coronary catheterization. I have documented that this is the number one reason for referrals to me from cardiologists. I have produced literature on the safety and cost-effectiveness of this approach. I have explained how this would break up any abuse of self-referral resulting from the echo cardiogram to stress test to cath referral pattern so common with both equivocal and negative stress tests.

Despite all this I have found no clear acceptance of CCTA. Why? Because there are often underlying competing economic motives. RBMs may be reimbursed by how well they decrease utilization. RBMs have also taken on risk-sharing contracts with insurance companies whereby they are reimbursed a flat fee for medical imaging expenses and are allowed to keep whatever they can by further squeezing down the costs of imaging.

Recently, I have come across an additional disincentive, this one put on the backs of patients in need of CT, MRI, or PET/CT scans. Copays have been shifting from employers to employees. Traditionally, the copay is based on the encounter and is the same whether it is for an x-ray or a CT. It has never depended on the type of exam the patient is receiving. A local self-funded PPO, however, has announced plans to increase its copay based on the type of imaging exam performed. The patient copay may be $10 for a mammogram but could be $300-out of pocket-for a CT or MRI. This undoubtedly discourages patients from having unnecessary exams, but may also cause them to shun any exam to avoid a severe financial burden. I regard this as unethical and I hope it will be met with rigorous opposition.

Long-term thinking takes a back seat to financial performance in the current economic climate. My efforts to convince RBM and insurance executives of the great value CCTA has to offer their patients and their bottom line are greeted with courteous yawns and thank-yous, and the game remains unchanged.

Opinions on the appropriate utilization of CCTA coming out of the cardiology community could hardly be classified as unified. Cardiologists, appropriately, are seen as the most qualified physicians to speak on the topic of coronary artery disease and how it is imaged.

Their views about CCTA are strong and very divergent. This is best illustrated by an interaction that occurred after the appearance of a paper in the 

. Titled “Pay now, benefits may follow-the case of cardiac computed tomographic angiography”

 by lead author Dr. Rita Redberg, this paper attacked the view that CCTA has immediate benefit to patients. In defense of Dr. Redberg, she is an outspoken opponent of inappropriate utilization of many tests, not just CCTA.

Nonetheless, many cardiologists and radiologists disagreed with her opinions. Dr. Tony DeFrance, a highly respected cardiologist, responded, “I am a cardiologist, but there is a heavy vested interest against [CCTA] succeeding because of people’s revenue streams. It is no secret that cardiologists make the majority of their money from nuclear scans-or a large percentage of it-and invasive catheter angiography. So some people don’t want the status quo to change.”

 It is this internal conflict, which mixes both medical and economic considerations, that is seized upon by the insurance/RBM industry as a reason for not approving any utilization of CCTA. They have a “call us when you can agree” attitude.

There is a good side of RBMs though. In New Jersey, a nationally prominent RBM has made it extremely difficult for nonradiologists to use CT, MRI, and PET/CT scanners in their offices through the in-office ancillary service loophole in the Stark II law. The negative ramifications of self-referral have been known for decades and recently documented in a series of articles in the 

Journal of the American College of Radiology

The insurance industry is beginning to act on self-referral. This will eliminate many unnecessary exams and their costs from the healthcare system; they simply will not be done at all. Although these exams will not shift to radiology, there will be a benefit for radiology because the specialty’s most uncontrollable source of competition, self-referral, will be diminished.

What will be the affect on CCTA? As long as it competes with a stress test as a first-line test, probably not much, but the use of stress tests has significantly decreased. Many RBMs are taking the decision of which test to use out of the hands of cardiologists. Over the long term, I suspect that the declining reimbursement for stress tests will make it unattractive to do them and therefore open the door for CCTA. And if RBMs prohibit nonradiologists from owning CT scanners, CCTA utilization could take off.

Radiologists were very quick to recognize how important CCTA could become to patients and to their own practices. They were eager to become trained in the procedure; thousands now meet the American College of Radiology and Level II American College of Cardiology criteria for performing CCTA.

Radiologists have been leaders in the development of cardiac imaging for decades. Echocardiography, SPECT nuclear stress tests, and catheter angiography have all had their beginnings in radiology only to be lost to cardiology in subsequent turf wars. CCTA was seen by radiologists as an opportunity to get back into cardiac imaging and they jumped on this opportunity with fervor.

Add in the Medicare/insurance industry/RBM alignment against selfreferral and you have a major advantage for radiology in grabbing CCTA market share. There will always be turf wars within hospitals, which cardiologists traditionally win. But there may be an advantage held by radiologists in the ability to provide 24/7/365 coverage. Radiologists are responsible, either by themselves or through the use of a teleradiology service, for this level of coverage. Adding another exam to the mix is logical and easier for them than it is for cardiologists. But there is a caveat: teleradiology has softened the underbelly of radiology. Nobody wants more after-hours work. Groups fear the effect increasing after-hours work will have on their ability to recruit new radiologists. There is a sense of entitlement that exists among some, though not all, radiology recruits that they will never work the graveyard shift. Certainly many established radiologists do not want to expand after-hours work and deal with the negative effects on their lifestyles, even if that attitude undermines a major advantage they hold in getting into CCTA.

But they say that “fat cats don’t hunt.” The cure for that condition is decreasing reimbursement, which, unfortunately is occurring now and will continue for the foreseeable future.

Essentially nothing, if you look at the exam itself and how it is relevant to patient care. It is the politics and economics of healthcare that have gotten in the way of a truly remarkable test that could save the lives of many (

). I don’t see that situation improving anytime soon, but stay tuned…

Articles

Coronary CT angiography came of age in the last decade. This occurred because CT technology moved into the 16- and 64- detector era.

Coronary CTA really works, but why isn't its use soaring?

, I wrote a brief article describing the benefits of using prospective gating when performing coronary CT angiography.1 Since then, a number of software and technique improvements have occurred that make possible the use of prospectively gated CCTA in the everyday clinical setting. In our practice, prospective gating is used in 98% of our CCTA patients as the initial and only scan technique. It has become rare for us to default initially to retrospective gating or reinject a patient to do a retrospectively gated CCTA because of a failed prospectively gated acquisition.

Prior to prospectively gated CCTA, the most effective radiation dose reduction technique was ECG dose modulation, used when performing retrospectively gated CCTA. It reduces dose by decreasing the mA of the x-ray beam in the systolic and near-systolic portions of the cardiac cycle, when the data are unlikely to be used to postprocess the coronary artery images. This results in a reduction in radiation exposure of approximately 30%. What prevents this technique from reducing radiation dose further is that the x-ray beam remains on throughout the cardiac cycle, even during the reduced exposure phases, when no images will be reconstructed. This results in unnecessary radiation being delivered to the patient and is inherent in all spirally acquired CCTA exams.

Prospective gating avoids this extraneous radiation dose by completely turning off the x-ray beam during most of the cardiac cycle. The portion of the cardiac cycle that is to be radiated is selected before the scan, hence the term prospective. The radiated window (length of time the tube is turned on) may be left wide, making it possible to reconstruct vessels in a range of cardiac phases, though this comes at the expense of an increased radiation dose. Before scanning a patient, the user must select the phase of the cardiac cycle in which the reconstruction will be centered. I routinely use the 75% phase, which, in patients with heart rates < 65 bpm, ensures scanning takes place during diastole.

In the prospectively gated CCTA protocol, there is a default time added to the window for tube on-time (dynamic padding), based on the patient's heart rate. However, if the patient's heart rate is < 65 bpm, one can safely conclude that the postprocessing will be successful; i.e., diagnostic images of all 15 American Heart Association coronary artery segments will be obtained with the single 75% phase available for reconstruction. For this reason, I always override the default padding and manually enter a 10-msec padding surrounding the 75% phase. For example, in a patient with a heart rate of 60 bpm, the duration of the cardiac cycle is 1000 msec. By radiating only 10 msec before and 10 msec after the 75% phase, only 2% of the cardiac cycle beyond the window-centered at the 75% phase-is radiated.

Using prospectively gated CCTA with the default padding reduces radiation by 52% to 74% compared with retrospectively gated CCTA, with no loss of image quality. By manually decreasing the padding to 10 msec, the radiation exposure can be reduced by up to 90%. This dramatic reduction in radiation exposure more than compensates for the "inconvenience" of administering beta blockers to bring the heart rate down to 65 bpm or less. Prospectively gated CCTA may, in fact, be used in patients with heart rates between 65 bpm and 70 bpm but is somewhat less consistent in obtaining all 15 AHA coronary artery segments at postprocessing. Functional evaluation of the heart is not possible, since images are not collected at all phases of the cardiac cycle.

Before the topics of spatial and temporal resolution as they pertain to CCTA can be considered, images with a signal-to-noise ratio adequate to visualize the coronary arteries must be obtainable. The demands for an increased SNR are most apparent in obese and morbidly obese patients. In these patients, the use of 650 to 800 mA may be necessary to obtain diagnostic images. In scanners whose maximum tube current is less than this, it may be necessary to increase voltage to 140 kVp from the usual 120 kVp. Each incremental increase of 20 kVp results in a 38% increase in radiation exposure. Conversely, a reduction of 20 kVp decreases radiation exposure by 38%.

It is important to titrate your technical factors to the minimum needed for patients of different body mass indices. Body mass index is the only variable used to select the kVp and mA for both prospectively and retrospectively gated CCTA. From the development of prospectively gated CCTA has come the realization that image quality can be preserved in many patients by using 100 kVp instead of the 120 kVp I routinely used with retrospectively gated CCTA, resulting in ultralow radiation dose CCTA. In many patients, the dose from this scan is at or less than that needed for coronary artery calcium scoring.

In my opinion, retrospectively gated CCTA was unfairly labeled as having excessive radiation exposures. In fact, the radiation dose resulting from retrospectively gated CCTA is in the range of technetium-99m sestamibi SPECT nuclear stress tests and less than the radiation exposure resulting from thallium-201 SPECT or SPECT dual-isotope nuclear stress tests.2,3 Prospectively gated CCTA has made this point moot. Taken to an extreme, 100-kVp, 10-msec padded prospectively gated CCTA in a patient with a BMI < 24 results in a 90% dose reduction when compared with Tc-99m sestamibi SPECT (see table).

Prospectively gated CCTA should become the first-line test in any patient suspected of having coronary artery disease, especially given its superior sensitivity to a stenosis of > 50% (SPECT studies usually are not positive until there is at least a 70% stenosis).4,5 Also in CCTA's favor are its ability to detect eccentric plaque invisible to stress tests and many coronary catheterizations (the same eccentric plaques whose rupture accounts for up to 85% of all myocardial infarctions) and its relatively low cost-and now the dramatic dose reduction that is possible with the modality. The only downside I see is the risk that comes with injecting iodinated contrast and the lack of guidelines for determining which patients should be on statins when plaque is detected in patients with normal cholesterol profiles.6,7

With the introduction of 64-slice CT scanners using a 4-cm detector composed of 0.625-mm elements, expanded coverage in the z-axis became possible. Simultaneous arterial-phase opacification of the thoracic aorta, coronary, and pulmonary arteries also became possible. Called the triple rule-out because of its diagnostic capability to rule out aortic pathology, coronary artery disease, and pulmonary emboli in one scan, this exam has drawn major interest from physicians, especially those in an emergency department setting.

In the ER, establishing the etiology of a patient's symptoms is often difficult, resulting in erroneous discharge of 4% of patients with an acute myocardial infarction.8 These patients have a threefold increase in mortality over the next 72 hours compared with their cohorts who are correctly admitted to the hospital. The cost to the patient with retrospectively gated CCTA triple rule-out exams was radiation exposure in the 20 to 30 mSv range.

With the dramatic reduction in dose possible with prospectively gated CCTA, it is now a reality to perform triple rule-out exams with more acceptable levels of radiation exposure. This may usher in the era of CTA exams with even greater z-axis coverage. Quadruple rule-out exams (adding carotid stenosis), which extend coverage from the skull base through the thorax, may be useful in patients with syncope, transient ischemic attacks, and cerebrovascular accidents resulting from carotid stenoses (Figures 1 and 2).

Quad rule-out exams require 20 cc more contrast than triple rule-out exams and a 20-second-long breath-hold, but they result in a radiation dose of 5.8 to 10 mSv depending on the patient's BMI. In the future, it is conceivable that the circle of Willis could be added to the exam, which may diagnose an embolic cardiovascular accident but also the possible origin of the embolus. In essence, this would be a "half-body CCTA." At the moment it exists only in theory, but it may be brought to a clinical reality with further improvements in z-axis coverage.

Prospective gating is now achievable in the overwhelming majority of patients needing CCTA. It requires a heart rate of < 65 bpm, which is easily and safely obtained with the administration of beta blockers. Given the massive reduction in radiation exposure, I feel that prospectively gated CCTA will soon become the standard of care and will likely force the hand of all CT vendors to develop this capability. I question whether the additional expense of dual-source CT scanners can be justified in an era when the use of beta blockers can bring heart rates low enough that it's possible to reduce radiation exposures to levels never before seen in cardiac CT and CCTA. Given that the radiation dose is now far less than that used with SPECT studies, CCTA should quickly become the initial exam used in patients suspected of having CAD.

Dr. Dowe is medical director of coronary CTA at Atlantic Medical Imaging in Galloway, NJ.

Dowe D. Prospectively gated computed tomographic angiography dramatically reduces radiation dose. Diagnostic Imaging 2006:28(10):S1-S5.

Perisinakis K, Theocharopoulos N, Karkavitsas N, Damilakis J. Patient effective dose and associated risk from transmission scans using 153Gd line sources in cardiac SPECT studies. Health Phys 2002;83(1):66-74.

Einstein A, Moser K, Thompson R, et al. Radiation dose to patients from cardiac diagnostic imaging. Circulation 2007;116(11):1290-1305.

Raff G, Gallagher M, O'Neill WW, Goldstein JA. Diagnostic accuracy of noninvasive coronary angiography using 64-slice spiral computed tomography. J Am Coll Cardiol 2005;46(3):552-557.

Leschka S, Alkadhi H, Plass A, et al. Accuracy of MSCT coronary angiography with 64-slice technology: first experience. Eur Heart J 2005;26(15):1482-1487.

Dowe D. The case in favor of screening for coronary artery disease with coronary CT angiography. J Am Coll Radiol 2007;4(5):289-294.

Black W, Czum J. Screening coronary CT angiography: no time soon. J Am Coll Radiol 2007;4(5):295-299.

Mehta RH, Eagle K. Missed diagnoses of acute coronary syndromes in the emergency room-continuing challenges. NEJM 2000;342(16):1207-1210.

In the October 2006 issue of Diagnostic Imaging, I wrote a brief article describing the benefits of using prospective gating when performing coronary CT angiography.

Low-dose 'quadruple rule-out' portends era of half-body CTA

 By reducing radiation exposure while maintaining image quality, technique could make coronary CTA viable as screening tool 

Initial clinical studies of coronary angiography with 64-slice CT show that it is even more accurate than 16-slice CCTA. Sensitivity, specificity, and positive and negative predictive values are all excellent when compared with coronary catheterization in detecting 50% or greater stenoses.1,2 With the help of a prospectively gated technique, 64-slice systems may soon also dramatically reduce radiation dose incurred during CCTA.

Like its predecessors, 64-slice CCTA involves a radiation dose of 7 to 20 mSv, even after the routine use of ECG-driven tube current modulation. The dose may exceed this range if the exam performed requires sizable coverage in the z-axis such as bypass graft or triple rule-out cases. Although the radiation exposure associated with CCTA is comparable to or even less than nuclear studies,3 the ALARA principle suggests that CT scanner manufacturers and clinicians continuously seek ways to reduce radiation exposure with CCTA.

Most published CCTA studies, especially those involving patients with low heart rates, report that a limited reconstruction phase range can be used for diagnosis. This observation is the design principle behind ECG-driven tube current modulation as well. Prospectively gated CCTA, which dramatically reduces radiation exposure with no compromise in image quality or diagnostic accuracy, is soon to be a clinical reality. Our preliminary experience with the technique demonstrates up to a 70% reduction in radiation exposure for patients with heart rates below 65 bpm.

Multislice CCTA has always been performed using retrospective gating, which occurs when the CT scanner collects anatomical data continuously and an ECG monitor separately collects temporal data such as cardiac rhythm. It is only after the scan is complete that the reconstruction engine generates images for a selected set of cardiac phases, usually defined as the R-R interval, from the anatomical scan data using the temporal data. The reconstruction phases are selected so that they will produce images with the best temporal and spatial resolution possible. Because this is performed retrospectively, the x-ray beam is on continuously throughout the scan, resulting in deliberate oversampling of the tissue. This in turn results in an increase in radiation exposure.

Prospective gating has been used for years to produce coronary artery calcium scores. During the exam, we are not attempting to obtain images of flowing intravascular contrast, and therefore there are no timing constraints. We simply acquire as many sequential step-and-shoot scans as needed to cover the heart. The lesions in question, coronary artery calcifications, are stationary in the wall of the coronary arteries.

Recent advances in CT technology have made prospectively gated CCTA possible:

- 4-cm detector coverage with a 64 x 0.625-mm slice profile to enable covering the whole heart with thin slices in three or four scans;

- a sufficiently powerful x-ray tube with faster gantry speeds to maintain image noise at acceptable levels; and

- fast table and x-ray tube control to adapt the scan acquisition to real-time ECG information and enable completion of the exam while maintaining peak arterial contrast enhancement.

In addition, advanced conebeam reconstruction algorithms generate consistently high quality images in the axial scanning mode.

With prospective gating, the temporal information gathered by the ECG monitor is used in real-time to turn the x-ray beam on and off during the user-selected phases in the cardiac cycle and to quickly move the patient to the next scanning position. Radiation exposure is markedly diminished as there is no oversampling of the tissue (Figure 1).

Phantom studies suggest that up to a 70% reduction in radiation exposure is possible without any compromise in image quality. One drawback of the technique is that images are acquired during a limited, preselected phase range. If patients have a high or irregular heart rhythm, the best time to visualize their coronary arteries may be during a different phase in the R-R interval. These patients need to be identified before the exam and instead have a retrospectively gated CCTA.

Our study involved 31 male patients (women were not included to avoid x-ray exposure to breasts), aged 38 to 60. They were scanned using prospective gating followed by retrospective gating. Patients provided their informed consent of the additional x-ray exposure, which was approved by an outside institutional review board. Each patient received 100 mg of metoprolol orally one hour before the study with a goal of achieving a heart rate of < 65 bpm. Patients' heart rates during scans ranged from 38 bpm to 67 bpm, with an average of 54 bpm. Heart rate variations during scanning ranged from 0 bpm to 9 bpm with an average of 3 bpm.

Technical parameters for the techniques are listed in Table 1. The major differences exist in the phases of the cardiac cycle during which patients were radiated, and slight differences arose in the mAs used during the scans. Contrast administration among patients differed only slightly, with an additional 10 cc used during prospectively gated CCTA (Table 2).

All patients were scanned using a GE LightSpeed VCT 64-channel scanner; contrast was injected using a Medrad Stellant injector. Images of the prospectively gated CCTA exams were acquired using GE's SnapShot Cine option and were compared with retrospectively gated CCTA images to verify diagnostic accuracy.

About half of the patients required scanning at three locations and over five heartbeats to cover the entire heart; the other half required four locations and seven heartbeats. Total scan times varied from 4.5 seconds to 10 seconds with an average of 6.7 seconds. Diagnostic images of all 15 American Heart Association coronary artery segments were obtained using both gating techniques in all patients.

The retrospectively gated CCTA images were processed and interpreted immediately. Prospectively gated CCTA images were reconstructed and given a segment-by-segment rating.

A comparison of image quality and diagnostic confidence was performed by the same radiologist to ensure consistency.

A few initial cases showed a slight increase in image noise relative to the helical counterpart, prompting the addition of 50 mAs to the subsequent prospectively gated studies to equalize noise measurements. The expert radiologist reviewing the cases rated the image quality as excellent, with pathology considered identical using the two separate gating techniques. No pathology was missed when using the same or the higher mA setting (Figures 2 through 5). Most important, x-ray exposure was reduced by 50% to 70% using prospective gating compared with retrospective gating with ECG-driven tube current modulation.

Additional improvements are anticipated in the short term once the technique is perfected, possibly using an overlay of different filters resulting in improved x-ray beam homogeneity.

Prospectively gated CCTA dramatically decreases radiation exposure. In patients with heart rates < 65 bpm, it can routinely replace retrospectively gated CCTA. This level of heart rate control is easily achieved using one dose of oral beta blockers.

With the reduced radiation exposure made possible with prospective gating, combined with short-term optimization of the technique, we may be approaching dose levels that could make CCTA a viable screening tool.

Dr. Dowe is chief operating officer and medical director of Atlantic Medical Imaging in Galloway, NJ. He is a consultant to GE Healthcare and MedRad and has also received grants and research support from both of these companies as well as Bracco.

1. Leschka S, Alkadhi H, Plass A, et al. Accuracy of MSCT coronary angiography with 64-slice technology: first experience. Eur Heart J 2005;26(15):1482-1487.

2. Mollett NR, Cademartiri F, van Miegham CA, et al. High resolution spiral computed tomography coronary angiography in patients referred for diagnostic conventional coronary angiography. Circulation 2005;112(15):2318-2323.

3.Perisnakis K, Theocharopoulos N, Karkavitasis N, Damilakin T. Patient effective radiation dose and associated risk from transmission scans using Gd-153 line sources in cardiac SPECT studies. Health Phys 2002;83(1):66-74.

By reducing radiation exposure while maintaining image quality, technique could make coronary CTA viable as screening tool

Prospectively gated CTA dramatically reduces dose

Coronary CT angiography (CCTA) was initially performed on four-slice multidetector CT scanners. These examinations were confined to evaluation of the proximal coronary arteries because the studies were limited in spatial and temporal resolution and were plagued by long acquisition times that required intolerably long breath-holds. Sixteen-slice scanners produced revolutionary improvements in CCTA technique: For the first time, complete coronary circulation exams with breath-holds in the 15 to 20-second range could be routinely obtained.

Breath-holds of this length are easily tolerable in patients who do not have chronic obstructive pulmonary disease (COPD). Submillimeter slice thickness significantly improved spatial resolution, while gantry speeds of less than a half-second improved temporal resolution. The most important improvement, however, is the increased signal-to-noise ratios achieved with high-milliamperage generators. These generators provide enhanced small vessel visualization, better spatial resolution, and a significant reduction in blooming of calcified plaques.

Sixteen-slice CCTA achieved very favorable results in detecting a > 50% stenosis compared with catheter angiography, far exceeding any sensitivity achieved with nuclear stress tests.

 More important, 16-slice CCTA demonstrated negative predictive values > 97%, effectively ruling out coronary artery disease. Sixty-four slice scanners are building on the improvements seen with 16-slice CT scanners. They promise to make CCTA the first diagnostic exam in the workup of patients with chest pain and to replace catheter angiography as the diagnostic tool for evaluation of the coronary arteries. This development is similar to the replacement of catheter angiography by noninvasive imaging in every vascular bed in the body with the exception of the heart.

I use a 64-slice LightSpeed volume CT (VCT) scanner (GE Healthcare) to perform all CCTA examinations. A number of technical modifications have dramatically improved image quality. The most immediate of these is the reduction in acquisition time that results from the ability to obtain 64 x 0.625-mm MSCT scans. This detector provides true 4-cm coverage. Acquisition times and, hence, breath-hold times for CCTA are now five to six seconds. Even patients with severe COPD can tolerate this breath-hold, but all patients benefit from the reduction in phase misregistration because we use data from no more than six heartbeats. The subsequent reduction in reconstruction time has greatly improved workflow. Gantry speeds continue to decline; the gantry speed of the VCT is 0.35 second. This reduces temporal resolution to 175 msec with single-sector analysis, 87.5 msec with two-sector analysis, and 44 msec with four-sector analysis. Higher heart rates are no longer an obstacle to scanning and reconstruction using single-sector analysis (Figure 1).

Due to the dramatic reduction in acquisition time for CCTA, exams can be accomplished at a maximum mA of 800 on serial patients without confronting tube cooling issues. Image quality has thus improved for CCTA's most difficult patients, the morbidly obese.

Blooming of calcified plaques has been the major limitation in evaluating the coronary arteries, accounting for the persistent positive predictive value in the 80% range compared with the negative predictive value of > 97%. Technical advances in spatial and temporal resolution with CCTA have reduced the blooming artifact (Figure 2).

Patient selection remains the same for 64-slice CCTA as for 16-slice examinations. The top four indications are chest pain with CAD risk factors, chest pain without CAD risk factors, patients being considered for a diagnostic coronary catheterization, and asymptomatic high-risk patients.

Patient preparation is also the same. I still use oral beta blockers for every CCTA exam. With the 64-slice scanner, my goal heart rate is < 70 bpm as opposed to < 60 bpm with a 16-slice MSCT scanner. Higher heart rates can easily be accommodated without the use of beta blockers, but I prefer to use single-sector analysis, which renders superior image quality. Beta blockers have advantages in addition to decreasing the heart rate: They limit variability, diminish anxiety, and assist in decreasing radiation dose when combined with routine use of ECG-gated dose modulation.

I favor oral rather than intravenous beta blockers. Oral beta-blockers are well tolerated, the patient can take them at home prior to the exam, and the patient does not need the extra time in the CT scanner and extra attention from a nurse that use of intravenous beta blockers requires.

Given the true 4-cm coverage of the detector, 0.625-mm slice thickness is possible in all patients, even those with bypass grafts (Figure 3). With 16-slice CCTA, the volume of tissue coverage in a bypass graft case required a breath-hold of > 25 seconds. For this reason, 16-slice CCTA scanned bypass grafts at 1.25 mm, which reduced the acquisition time by 50% but sacrificed spatial resolution in the z-axis.

True 4-cm coverage also has affected CCTA of the morbidly obese patient. Using 16-slice CCTA, I would sacrifice spatial resolution by acquiring data at 1.25 mm in order to shorten the acquisition time, which in turn allowed me to use higher mA. The higher mA greatly improved my signal-to-noise ratio at the expense of increased radiation exposure. On the 64-slice scanner, 0.625-mm slice thickness in obese patients is routinely possible at 800 mA, due to the five to six-second acquisition time made possible by this detector's 4-cm coverage.

The shorter acquisition time enables me to use a lower contrast volume. I currently use a triphasic injection technique (40 cc contrast at 5 cc/sec, 20 cc contrast plus 30 cc saline admixture at 5 cc/sec, 20 cc saline at 5 cc/sec), for a total administration of 60 cc of contrast. When combined with the manual timing bolus run, the patient receives a maximum of 80 cc of contrast. I do not taper the injection rate. Once circulation time has been calculated, my goal is to get the highest peak injection in the coronary arteries by maintaining a high injection rate.

A triple rule-out examination is a single acquisition that results in simultaneous arterial phase opacification of the pulmonary arteries, coronary arteries, and aorta. The radiologist can rule out pulmonary embolism, CAD, and aortic dissection or aneurysm with a single 12-second scan. Triple rule-out exams require the use of 110 cc of contrast. Postprocessing includes routine reconstruction of the coronary arteries (Figure 4), wide field-of-view coronal reconstructions of the lungs (Figure 5) and aorta, and "candy cane" oblique sagittal reconstructions of the thoracic aorta.

Radiation dose for this exam is increased secondary to the necessary coverage. In my opinion, this exam will revolutionize the imaging of patients with acute chest pain. This represents an enormous volume of work originating in the emergency room, and radiology groups will struggle to manage this work. The only factors that may control use of this examination are the risks imposed by the increased radiation dose and increased volume of contrast.

Noncalcified, or soft, plaques are more vulnerable to rupture than calcified, or hard, plaques. Coronary artery calcium scoring is limited to the detection and quantification of calcified plaque burden. The presence of coronary atherosclerosis can be verified based on the presence of calcified plaques. Atherosclerotic plaque burden, measured with the calcium score, in turn implies the likelihood of a flow-limiting stenosis. The calcium score is also an independent predictor of mortality and is therefore considered additive to the Framingham score.

 It is limited, however, in younger patients whose disease has not existed long enough to calcify, and it does not detect the soft, more vulnerable plaques that may be present in any age group regardless of calcium score.

Patients of any age with any calcium score value may have stenoses from soft plaques or any degree of overall soft plaque burden (Figure 6). CCTA has the advantage of detecting hard and soft plaques and assessing the degree of stenosis for the entire coronary arterial circulation with minimal increased risk above and beyond the calcium score. Compared with intravascular ultrasound, 16-slice CCTA has shown an acceptable sensitivity for detecting soft plaques,

 which greatly increased when the study was limited to detection of plaque in the proximal vessels.

 The importance of this finding is that the risk of sudden death and large, disabling myocardial infarcts decreases dramatically when a plaque ruptures distal to the first septal perforator branch of the left anterior descending artery.

Plaque characterization by Hounsfield unit (HU) measurement has been criticized as inaccurate, based on signal-to-noise ratio limitations. Although this criticism is partly valid, the plaque composition itself contributes to this variation. Plaques are heterogeneous in their composition, reflecting various rates of healing. Visualization of atheromatous tissue within a heterogeneous plaque is possible thanks to the improved temporal and spatial resolution of modern scanners (Figure 7). Applying a color scale to HU measurements may increase the accuracy of soft plaque detection and characterization and improve the ability to assess the degree of stenosis by the application of color to contrast HU ranges (Figure 8). This technique requires further validation by intravascular ultrasound. In the future, it will be possible to quantify plaque burden and subdivide that value into the percentages of atheromatous, fibrous, and calcified tissue present within the total plaque burden.

Detection of in-stent and stent margin restenosis first became possible with 16-slice scanners compared with cardiac catheterization

 and should improve still further with 64-slice scanners. However, stents that are old and overgrown by endothelium, distal stents, and small stents must be treated with caution. In these cases, it may be difficult to directly visualize the stent lumen. An early observation with 64-slice scanners is that they may greatly enhance our ability to evaluate stents compared with 16-slice CCTA (Figure 9).

CCTA imaging of bypass grafts has always been accurate and fast. The saphenous vein grafts are large and extracardiac and therefore are less prone to motion artifact and have no branches. Arterial conduits, although smaller in caliber, share these imaging advantages. The major advancement with 64-slice CCTA is that we now image bypass grafts with the superior spatial resolution of 0.625-mm slice thickness made possible by the 4-cm-long detector. Visualizing bypass graft stenoses is equally easy, but the enhanced spatial resolution results in more accurate evaluation of stenoses (Figure 10). With more complete assessment of stents and bypass grafts, we are able to triage patients with new symptoms or new stress test findings into two categories: patients with widely patent stents and grafts showing progression of CAD within their native vessels, and those with stenoses in their interventions.

Radiation dose remains a serious issue with CCTA. Increased dose is inevitable because of the very low pitch values (0.20 to 0.26:1) and retrospective gating. ECG dose modulation decreases dose by reducing the mA of the x-ray beam during portions of the cardiac cycle when we are unlikely to reconstruct the coronary arteries. I use peak mA during the 40% to 80% window of the cardiac cycle only. This limits the radiation dose outside of this time window but still affords me the flexibility I need for multiple phase reconstruction, should the patient's heart rate change significantly during the exam. Many other dose-reducing technical advances are available on the 64-slice scanners, including cardiac filter modes and bowtie filters, but discussion of these is beyond the scope of this article.

Effective radiation dose is calculated by multiplying the dose length product by a k factor that represents different tissues' sensitivities to radiation-induced damage. The result is expressed in units of millisievert. The effective radiation dose of other cardiac and noncardiac imaging studies yields a few surprises when compared with the much maligned dose from a CCTA exam (see table). The dose from a typical cardiac catheterization varies from 3 mSv for a routine diagnostic study with only a few projections to 30 mSv for a congenital heart disease exam. A stress SPECT thallium exam is typically around 25 mSv, due to the long half-life of thallium, and a stress SPECT sestamibi exam dose is about 12 mSv.

Using a 64-slice CCTA scanner with an acquisition time of five seconds produces a radiation dose between 6 and 13 mSv, depending on the mA used during the study and the patient's heart rate. This places 64-slice CCTA well within the historically acceptable range of radiation exposure for other invasive and noninvasive cardiac imaging exams. This fact, combined with the additional fact that CCTA is the only exam that noninvasively images the coronary artery wall, the end organ that causes CAD, speaks very well for its widespread use in the future. But such use must be tempered by the effects of radiation to the individual and to society as a whole. This issue becomes more significant in children and women, given the increased sensitivity of breast tissue to radiation damage. We are not a pediatric center, and I use CCTA in pediatric patients only when a coronary anomaly is a concern (Figures 11 and 12). Furthermore, the effective dose of a triple rule-out exam is approximately 20 mSv, and the uncontrolled, ubiquitous use of this CT application in patients who do not need it should be prohibited.

The revolution in noninvasive imaging of the heart has taken a giant leap with the production of 64-slice, 4-cm-wide, thin-slice detectors. The resultant superior spatial resolution, combined with the improved temporal resolution made possible by faster gantry rotation speeds, makes CCTA feasible for almost every patient. The markedly improved sensitivity, specificity, and positive and negative predictive values seen with 16-slice CCTA compared with cardiac catheterization will improve still further. Sixty-four-slice CCTA will soon qualify as a potential replacement for nuclear stress tests as the first-line imaging test for potential CAD patients, at similar or reduced radiation dosages. CCTA should precede every diagnostic catheterization, based on the following: 91% of diagnostic cardiac catheterizations are elective, and of these 40% to 50% are normal, and CCTA has a negative predictive value > 99% in the most recent papers. Using CCTA in this manner would eliminate unnecessary negative diagnostic caths with their high cost and associated morbidity and mortality.

The triple rule-out examination holds great promise for revolutionizing the imaging of patients with acute chest pain. Hospitals are certain to embrace this technique as a noninvasive exam that improves patient care, decreases ER costs, and reduces the medicolegal risk of missing a potentially lethal diagnosis. The use of this exam must be moderated by its requirement for a higher iodinated contrast volume and its effective radiation dose.

1. Nieman K, Cademartiri F, Lemos P, et al. Reliable noninvasive coronary angiography with fast submillimeter multislice spiral computed tomography. Circulation 2002;106: 2051-2054.

2. Ropers D, Baum U, Pohle K, et al. Detection of coronary artery stenosis with thin slice multidetector row spiral CT with multiplanar reconstruction. Circulation 2003;107:664-666.

3. Kuettner A, et al. Diagnostic accuracy of noninvasive coronary imaging using 16 detector slice spiral computed tomography with 188ms temporal resolution. JACC 2005; 45:123-127.

4. Mollet N, Cademartiri F, Kreslin G, et al. Improved diagnostic accuracy with 16-row multislice CT coronary angiography. JACC 2005;45:128-132.

5. Shaw LJ, Raggi P, Schisterman E, et al. Prognostic value of cardiac risk factors and coronary artery calcium screening for all cause mortality. Radiology 2003:3:826-833.

6. Leber AW, Knez A, Becker A, et al. Accuracy of multidetector spiral CT in identifying and differentiating the composition of coronary atherosclerotic plaques: a comparative study with coronary ultrasound. JACC 2004;43(7):1241-1247.

7. Achenbach S, Moselewski F, Ropers D, et al. Detection of calcified and noncalcified coronary atherosclerotic plaque by contrast enhanced, submillimeter spiral computed tomography: a segment-based comparison with intravascular ultrasound. Circulation 2004;109(1):14-17.

8. Kyodo General Hospital. Coronary CT angiography compared with coronary catheterization for detecting in-stent restenosis. Circulation 2004;110(17):suppl III-563, abstract 2622.

9. Perisnakis K, Theocharopoulos N, Karkavitasis N, Damilakin T. Patient effective radiation dose and associated risk from transmission scans using Gd-153 line sources in cardiac SPECT studies. Health Phys 2002:83(1):66-74.

Dr. Dowe is chief operating officer and medical director of Atlantic Medical Imaging in Galloway, NJ. He has received grants/research support from GE Healthcare and Medrad and is a member of the speakers' bureau for both companies.

EFFECTIVE DOSE OF SELECTED RADIOLOGIC EXAMINATIONS

David A. Dowe, MD

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