New-generation Computed Tomography Scanners for Coronary Artery Disease Imaging – Implications for Future Service Provision

The European Society of Cardiology (ESC) published guidelines for the diagnosis and management of chronic coronary syndromes in 2019, including new recommendations for coronary computed tomography angiography (CCTA). (1) We shall discuss the new role of CCTA in the diagnostic work-up of coronary artery disease (CAD) and potential difficulties with its implementation in the health care system. We shall also describe and present difficult-to-scan patients in detail, such as those with extensive coronary calcifications, increased or irregular heart rate, or those who are obese.

Although CAD has heterogeneous morphology, a bivariant clinical perception of cardiovascular continuum according to which acute coronary syndromes are differentiated from chronic coronary syndromes is still dominant. (1) The implementation of CCTA in clinical practice as an accurate diagnostic procedure for ruling out CAD has been quite dynamic. The American Heart Association (AHA) implemented CCTA in its guidelines in 2012 and recommended that it should be considered (Class IIa) for patients with low-intermediate pre-test likelihood for CAD who are unable to exercise, as well as for those with inconclusive stress test, normal stress test but ongoing symptoms, and those unable to undergo stress echocardiography or nuclear medicine myocardial perfusion imaging. (2) The ESC guidelines issued in 2013 stated that CCTA should be considered (Class IIa) as a first-line test in patients with low to intermediate pre-test probability and suspected CAD. (3) Moreover, ESC recommended it in 2015 as a Class IIa examination even for patients with acute chest pain but without ECG changes or elevated enzymes. (4) Finally, in 2017, the National Institute for Health and Care Excellence (NICE) recommended CCTA as a first-line examination for symptomatic patients with suspected CAD regardless of pre-test probability. (5) The new ESC guidelines promoted CCTA as a Class I examination, which means that CCTA or non-invasive functional imaging is recommended or indicated for myocardial ischemia as the initial test for diagnosing CAD in symptomatic patients in whom obstructive CAD cannot be excluded by clinical assessment alone. (1)

Numerous prerequisites must be fulfilled to clinically implement this highly demanding CT examination. Patients who are difficult to scan, such as those with extensive coronary calcifications (>400 Agatston Units), increased (>65 bpm) or irregular heart rate, or those who are obese (body mass index >30) and unable to hold their breath, have been already identified by NICE in earlier scanner generations. For these patients, NICE recommends the new generation of scanners or even particular scanner models, as opposed to the ESC guidelines which do not recommend CCTA. (6) NICE also defines the required technical capabilities of scanner models for accurate CCTA acquisition; these are more demanding than the Society of Cardiovascular Computed Tomography (SCCT) guidelines, in which the minimum requirement is a 64-slice scanner with fast gantry rotation time (≤350 ms). (7) In spite of these differences between ESC and NICE recommendations, an experienced clinical team consisting of a radiographer, radiologist, and cardiologist can obtain good image quality from new-generation CT scanners, even from patients who are difficult to scan. Since an ideal cardiac-dedicated CT scanner is not yet available, manufacturers use different technological solutions for specific patient groups.

For patients with high coronary calcium burden or stents, high spatial resolution in three dimensions (isovoxel) is of the utmost importance to accurately differentiate small coronary stenosis grades. Although 64-slice or higher scanners have improved z-direction spatial resolution resulting from improved detector z-dimension in order to achieve high isovoxel resolution, the desired isovoxel spatial resolution of ≤0.1mm is currently not available. Spatial resolution is crucial to reducing two imaging artefacts in calcified patients – blooming artefacts and partial volume effect. Therefore, when investing in new scanner, it is important to know the z-dimension of each detector row and the overall z-dimension coverage, as well as whether or not it includes a z-flying focal spot, a unique hardware solution that enables the acquisition of overlapping slices (Figure 1,Figure 2). (8)

FIGURE 1 Male patient, 67 years old, with hypertension and obesity (body mass index 37.4 kg/m2; chest/waist circumference 129/122 cm). After intravenous application of four doses of metoprolol, heart frequency remained variable with average rate 71 bpm (A). Longer calcifications were found in proximal segments of the right coronary artery (RCA) and the left anterior descending artery (LAD), but despite the latter the image quality of three major epicardial vessels allowed accurate analysis (B, C, D). Despite minor motion artefacts in some segments, even terminal branches of major vessels are clearly visible (E, F, G, H). Computed tomography fractional flow reserve (CT-FFR) revealed hemodynamically significant stenosis of mid RCA (FFR=0.78) and LAD (FFR=0.80) (I).

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FIGURE 2 Male patient, 65 years old, with a heart rate of 60 bpm after intravenous application of metoprolol, with obesity (body mass index 34.2 kg/m2; chest/waist circumference 104/101 cm). Major epicardial vessels have segmentally longer calcifications but without major “blooming” artefacts (the left main artery and the right coronary artery) (A, B, C). Although images of side and even terminal branches of major epicardial vessels are slightly grainy, they still allow accurate analysis (D, E, F, G, H). Coronary computed tomography angiography revealed non-obstructive coronary artery disease.

Patients with high or/and irregular heart rate represent another challenge that requires good temporal resolution in order to reduce coronary motion artefacts. Temporal resolution rests on gantry rotation time, and manufacturers have therefore developed several solutions to improve it. Only half (180^o) of the tube rotation can be utilized for fast data acquisition in order to prevent motion artefacts with single-source scanners; with dual-source scanners, only one fourth (90^o) can be utilized. Currently, only one manufacturer provides such dual-source technology. It allows a temporal resolution of 66 ms, while the majority of currently available single-source CT scanners enable temporal resolutions between 100 and 250 ms, thus making them more suitable for patients with a stable heart rate ≤65 bpm. (8) For patients with irregular heart rate, full coverage of the cardiac anatomy with long z-detector array (>12 cm) enables CCTA acquisition in a single heartbeat. Currently, several manufacturers provide such a solution. A long detector array is also an important technical advantage for scanning patients with coronary grafts. Some vendors use table movement in phases to obtain artefact-free images, while dual-source CT scanners use fast and continuous table movement to enable high-pitch scanning in a single heartbeat. Only one vendor utilizes stationary table image acquisition for single-heartbeat CCTA, which permits less cardiac motion and breath-hold artefacts along with a lower radiation dose (≤1 mSv). Together with these technological solutions, which enable a shorter breath hold, the key prerequisites for good image quality in arrhythmic patients (≥65 bpm) are the application of beta-blockers and the selection of the best heart cycle phase for image reconstruction (Figure 3).

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FIGURE 3 Female patient, 68 years old, with atrial fibrillation, variable and high average heart frequency of 86 bpm, and voluminous breasts (chest circumference 104 cm) (A). Despite the latter, the image quality of great epicardial vessels allows accurate analysis (B, C, D). Side branches of three major epicardial vessels are clearly presented (E, F, G). Detailed lumen analysis reveals significant ostial stenosis of the right coronary artery (D).

Scanning obese patients and those with a large chest (e.g. voluminous breasts) requires a higher radiation dose and results in grainy images due to high noise production; however, scanning with a high X-ray tube current (>500 mA) and reduced tube voltage (<120 kV) provides sufficient image quality along with a lower radiation dose. A powerful generator is an important technological prerequisite for high X-ray tube current, a consideration that is not always taken into account when purchasing a new CT scanner. Additionally, dual-source scanners provide superior CCTA image quality compared with single-source scanners. (9-11) CT scanners for difficult-to-scan patients should feature dual-source technology with two powerful current generators in order to provide good temporal resolution; they should also have a long z-detector array in order to ensure high spatial resolution and volume coverage. Good image quality requires appropriate patient preparation and the adjustment of scan protocols to individual patient characteristics. (Figures 1, 2 and 3{ label needed for fig[@id='f2'] }{ label needed for fig[@id='f3'] })

For the past decade, nearly half of the CT scanners in Croatia have been outdated and unsuitable for CCTA. (12) Recently, numerous public hospitals have purchased scanners that fulfil SCCT technical guidelines, but these are neither cardiac-dedicated nor recommended for difficult-to-scan patients. The availability of technical resources could influence the diagnostic work-up of patients with CAD, since unsatisfactory image quality can facilitate downstream indications for invasive coronary angiography (ICA) and increase potential overuse. A previous study has confirmed patient referral for ICA instead of CCTA, which may contribute to Croatia’s ICA utilization rate being the second highest in Europe. (13,14) CCTA should be perceived as a gatekeeper for ICA, especially in low-intermediate risk patients; new ESC guidelines confirm the previously emphasized need to implement this clinical practice. (15-17) The high burden of CAD in Croatia can be explained through the high overall prevalence of risk factors: overweight or obesity (57.4%), smoking (33%), and hypertension in 50% of men and 44% of women. (18-21) Atrial fibrillation is most likely similar to the European prevalence of 1-3%. (22,23) Considering the burden of risk factors and the 2019 ESC guidelines, referrals to CCTA are expected to rise at the national level, just as in other countries with a similar cardiovascular burden, although clinical practice may vary. Despite the proven cost-effectiveness of new-generation CT scanners for difficult-to-scan patients in Great Britain, ESC does not yet recommend CCTA for these patients. (24) Public hospitals worldwide are struggling to provide sufficient CCTA services to make up for the shortage in educated radiographers and radiologists, as well as for the lack of new-generation cardiac-dedicated CT scanners. The president of the Cardiac Section of the Croatian Society of Radiology (Assist Prof Dr Maja Hrabak Paar) has emphasized existing constraints in human resources, the need to educate radiology residents in cardiac imaging, and the administrative recognition of Level III education as an official subspecialisation. (25)