Aquilion PRIME CT scanner for imaging coronary artery disease in adults in whom imaging is difficult

Introduction

Coronary artery disease (CAD) is a condition in which an atherosclerotic plaque, made up of fat, cholesterol, calcium and other substances found in the blood, builds up inside the coronary arteries (NHS Choices 2014). Over time, the plaque hardens and narrows the coronary arteries, restricting the flow of oxygen‑rich blood to the heart (ischaemia). Sometimes, an area of the plaque can rupture, causing a blood clot to form on the surface of the plaque. If the clot becomes large enough, it can largely or completely block blood flow through a coronary artery. The most common symptoms of CAD are shortness of breath and chest pain or discomfort. Chest pain caused by CAD is called angina, which is broadly divided into 2 categories:

If left untreated, CAD can lead to myocardial infarction and death. In the UK, an estimated 2.3 million people are living with CAD and an estimated 74,000 deaths were caused by CAD in 2012 (Townsend et al. 2014).

When a person presents with acute chest pain the NICE clinical guideline on the assessment and diagnosis of chest pain of recent onset, before the 2016 update, recommended that if acute coronary syndrome is not suspected, but CAD (stable angina) cannot be ruled out after clinical assessment, the person's estimated likelihood of having CAD should be determined. This is calculated on the basis of symptoms, age, sex and risk factors. The guideline recommended that computed tomography coronary angiography (CTCA) should be offered if the estimated likelihood of CAD is 10–29% and that people in this category should be offered calcium scoring (a measure of atherosclerotic plaque burden), followed by CTCA if the calcium score is between 1 and 400. CTCA, therefore, would rule out a diagnosis of CAD, rather than confirm it (British Heart Foundation 2015). A calcium score above 400 indicates that imaging using earlier generation CT scanners would be difficult, and the guideline recommended invasive coronary angiography (ICA), the gold standard for evaluating coronary anatomy in these cases, if this was considered clinically appropriate.

NICE diagnostics guidance on new generation cardiac CT scanners also recommends CTCA as an option for first‑line evaluation of disease progression to establish the need for revascularisation, in people with known CAD in whom imaging with earlier generation CT scanners is difficult. Furthermore, according to the NICE guideline on stable angina, CTCA should also be offered to people with stable angina whose symptoms are not satisfactorily controlled with medical treatment.

CTCA involves combining multiple X‑rays into cross‑sectional images (slices) of the heart to assess potential coronary artery narrowing or occlusion, as well as coronary plaque morphology and density. A standard intravenous cannula is inserted into a suitable vein in the arm. A contrast medium is injected and the CT image is acquired as the person moves through the gantry (the doughnut‑shaped opening containing the X‑ray source and detectors; see figure 1). The blood vessels containing the contrast medium are visible on the CT scan and any narrowed or blocked vessels can be seen (NHS Choices 2013).

Figure 1 Schematic of CT scanner main components (a) in x–y plane and (b) in z‑axis direction.

In some patient populations, it is difficult to get good‑quality CTCA images unless the CT scanners have particular technical specifications. The following common conditions can make CTCA difficult:

• A high heart rate that cannot be lowered with drugs. The movement associated with a high heart rate creates blurring in the scan (motion artefact).

• An irregular heart beat (arrhythmia), which causes artefacts (distortions unrelated to the subject being scanned) in the resulting image. More than 2 million people a year experience arrhythmias (NHS Choices 2015).

• Excessive coronary calcium. Calcified plaques appear proportionally larger on the CT scan than they actually are, and can obscure the lumen of the blood vessel (blooming artefact). According to NICE diagnostics guidance on new generation cardiac CT scanners a calcium score above 400 suggests imaging using earlier generation CT scanners would be difficult.

• Obesity. CT images of patients with obesity may be poor quality because of increased image noise, making it difficult to distinguish different structures (due to poorer contrast resolution). According to the Health Survey for England, 67% of men and 57% of women were overweight (with a BMI between 25 kg/m² and 30 kg/m²) or obese (with a BMI over 30 kg/m²; Health & Social Care Information Centre 2012). A higher BMI increases the risk of CAD, so a significant proportion of people who could benefit from cardiac CT imaging may be overweight or obese.

• Cardiac stents. Stents can cause image artefacts (blooming; Mahnken 2015) and therefore may not be well defined in CT images. Over 71,500 percutaneous coronary intervention procedures, in which stents are commonly inserted to widen coronary arteries, were carried out in the UK in 2014 (Health & Social Care Information Centre 2015).

• Previous coronary artery bypass grafts. These surgical procedures are carried out in people with severe CAD who will have narrowed and calcified coronary arteries, which are difficult to image. Over 16,000 coronary artery bypass grafts were conducted in the UK in 2012 (Townsend et al. 2014).

The NICE clinical guideline on the assessment and diagnosis of chest pain of recent onset recommends using 64‑slice (or above) CT scanners, which are the most commonly used scanners in the NHS (Clinical Imaging Board 2015). NICE diagnostics guidance on new generation cardiac CT scanners recommends 4 specific scanners (Aquilion ONE, Brilliance iCT, Discovery CT750 HD and Somatom Definition Flash), all of which have technical enhancements that can improve CTCA image acquisition in people with suspected or known coronary artery disease in whom imaging is difficult with earlier generation CT scanners.

The following technical performance parameters are helpful when performing CTCA where imaging is difficult (also see figure 2):

• X‑ray output: a measure of the intensity of the X‑ray beam produced by a scanner. This must be at a level which is sufficient to provide images that distinguish between different tissues. It can be expressed as CT dose index weighted (CTDI_w).

  • Higher CTDI_w values will result in lower image noise and improved contrast resolution.

  • Maintaining image quality with increased X‑ray intensity can be difficult when scanning people with obesity.

• Volume coverage: the length of anatomy in the z‑axis (that is, head to toe) that can be scanned in 1 axial rotation of the scanner.

  • With a large volume coverage, the cardiac volume can be acquired in a smaller number of heartbeats (ideally a single heartbeat), reducing image artefacts. Fast volume coverage can improve imaging for people with arrhythmia or grafts.

• Temporal resolution: the period of time over which images are reconstructed.

  • The intrinsic temporal resolution is defined as the time taken to acquire 180° of data, the minimum usually necessary for image reconstruction. The effective temporal resolution is the time taken when various methods for improving the intrinsic temporal resolution (for example, motion correction algorithms, multi‑segment reconstruction) are applied.

  • A high temporal resolution (lower values represent better resolution) is important for acquiring CTCA images that are free from blurring in people with a high heart rate.

• Spatial resolution: the ability of the imaging system to depict small anatomical features in an image. It is defined as the minimum distance (mm) between 2 small, high‑contrast objects that allows both structures to be seen in the image.

  • Spatial resolution in the x‑y plane (across the body) and on the z‑axis (down the body) often differ. Several algorithms used to reconstruct the images have been developed by different CT scanner manufacturers. These provide different effective spatial resolution in the x‑y plane for general CTCA scans and improved high‑contrast resolution for CTCA in people with stents or high calcium scores. High spatial resolution (lower values represent better resolution) can reduce the amount of blooming from stents or high calcium levels.

There is no single scanner on the market that optimally addresses all clinical challenges for all patient subgroups. NICE diagnostics guidance on new generation cardiac CT scanners recommends a range of CT scanners as a first‑line option for people in whom cardiac imaging is difficult, because they have advanced features to produce better images at lower radiation doses, and avoid blooming or motion (blurring) artefacts. The Aquilion PRIME was not covered by the NICE guidance but also has some of these features. Figure 2 highlights the clinical challenge for each patient group in whom imaging is difficult and matches this to the key technical feature of scanners designed to improve the image quality for that group. The highest specification commercially available for each technical feature is used to 'create' a hypothetical scanner.

Figure 2 Clinical challenges for each subgroup of patients in whom imaging is difficult, matched to key technical features of CT scanners intended to improve the image quality for each subpopulation. The optimal specification for each technical feature is used to 'create' a hypothetical scanner

Abbreviations: ALARP, as low as reasonably practicable; CTDIw, CT dose index weighted (the average absorbed dose across the field of view in a standard phantom); kV, kilovolt; mA, milliamp; mGy, milligray; mm, millimetre; ms, millisecond.