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Coronary artery disease has an important impact on the morbidity and mortality statistics and health economics worldwide. Diagnosis of coronary artery disease is important in risk stratification and guides further management. Invasive coronary angiography is the traditional method of imaging the coronary arteries and remains the gold standard. It detects luminal stenosis but provides little information about the vessel wall or plaques. Besides, not all anatomical lesions are functionally significant. This has lent itself to a wide variety of imaging techniques to identify and assess a flow-limiting stenosis. The approach to diagnosis of coronary artery disease is broadly based on anatomical and functional imaging. Coronary CT and MRI of coronary arteries provide an anatomical assessment of coronary stenosis. Coronary calcium score and coronary CT assess subclinical atherosclerosis by assessing the atherosclerotic plaque burden. The haemodynamic significance of a coronary artery stenosis can be assessed by stress radioisotope studies, stress echocardiography and stress MRI. The more recent literature also focuses on plaque assessment and identification of plaques that are likely to give rise to an acute coronary syndrome. There is an explosion of literature on the merits and limitations of the different imaging modalities. This review article will provide an overview of all the imaging modalities in the diagnosis of coronary artery disease.
Chest pain is a significant healthcare burden, accounting for 1.5% of visits to the general practitioner , 6% of visits to the emergency department and 27% of emergency hospital admissions . Chest pain may be cardiac related or may be due to non-cardiac causes such as musculoskeletal or gastrointestinal pain. It is important to identify the subgroup of patients with ischaemic heart disease presenting with chest pain. This is because coronary artery disease (CAD) is the biggest killer in the UK, accounting for the death of one in five men and one in six women . It incurs an annual cost of 9 billion pounds, 36% of which is attributed to direct healthcare costs, 43% to production losses and 21% to informal care of people with CAD . The problem is exacerbated by the growing prevalence of Type 2 diabetes, obesity and an ageing population.
Clinical assessment is important in the preliminary assessment of chest pain and in determining whether it is due to CAD . Further assessment with imaging is important in the:
The diagnosis of CAD is not restricted to the diagnosis of luminal stenosis. It also includes a study of plaques, including plaque volume and plaque characteristics. The functional significance of a stenosis refers to whether the lesion is significant enough to cause ischaemia. Viability refers to live myocardium. Assessment of viability is important in predicting functional recovery following revascularisation. Left ventricular function (LVF) is an important prognostic consideration in the assessment of ischaemic heart disease.
The various imaging modalities available for investigation of chest pain due to suspected CAD can be broadly divided into the categories below.
The article provides an overview of the various imaging techniques, including:
The article does not discuss the imaging techniques and the interpretation of findings in detail. It also does not discuss the role of various imaging modalities in the context of an acute coronary syndrome. These are described elsewhere in this issue.
Development of angina is the end result of a sequence of events resulting from an imbalance between myocardial oxygen consumption and myocardial oxygen delivery called the ischaemic cascade. It refers to the temporal sequence of pathophysiological events that occurs within seconds of occlusion or chronic stenosis of a coronary artery (Figure 2) .
The ischaemic changes become irreversible after 30 min, when myocardial necrosis sets in. It starts in the subendocardium and moves towards the epicardium as a wave front phenomenon. This is reflected in the pattern of delayed enhancement seen on MRI. Enhancement due to an infarct is initially typically subendocardial in location before progressing towards the epicardium.
The ischaemic cascade forms the basis of functional imaging. This can be assessed by demonstrating perfusion or wall motion abnormalities on stress MRI, SE, SPECT and PET imaging brought on by “stressing the heart” with adenosine, dipyridamole, exercise or dobutamine.
Stress is achieved either in the form of:
Adenosine and dipyridamole are predominantly vasodilatory agents. Exercise and dobutamine are predominantly ionotropic stress agents.
Perfusion abnormalities occur earlier than wall motion abnormalities in the ischaemic cascade. This understanding of the ischaemic cascade explains the better sensitivity of techniques that identify perfusion abnormalities and better specificity of techniques that identify wall motion abnormalities in the diagnosis of ischaemia .
The functional significance of a lesion can also be determined invasively by measuring the fractional flow reserve (FFR) at the time of a coronary angiogram. FFR is expressed as a ratio of the coronary pressure distal to a stenosis to simultaneous aortic pressure measurement during maximal hyperaemia. An FFR less than 0.75 is considered to be functionally significant .
The debate regarding percutaneous intervention (PCI) vs optimum medical therapy (OMT) continues. The COURAGE (Clinical Outcomes Utilizing Revascularization and AGgressive drug Evaluation) trial showed that in stable CAD with more than 70% stenosis, the adverse event rate (defined as all-cause mortality, non-fatal myocardial infarction, stroke and hospitalisation for acute coronary syndrome) was similar in patients randomised to an initial strategy of OMT alone and those treated with PCI and OMT .
Recent publications such as the nuclear substudy of the COURAGE trial , the 1 and 2 year follow-up of the FAME study (Fractional flow reserve versus Angiography for Multivessel Evaluation) [16,17] and the 2 and 5 year follow-up of the DEFER study [18,19] highlight the importance of looking for objective evidence of ischaemia prior to PCI. These studies highlight several important clinically relevant points.
These conclusions have important clinical ramifications. The decision to revascularise has traditionally been based on visual anatomical assessment of stenosis on invasive coronary angiogram. However, there is a lack of correlation between anatomical severity and functional significance. The classic study of Gould and Lipscomb  showed that a stenosis of less than 50% is unlikely to be functionally significant. Based on FFR measurement, stenoses of severity between 50% and 90% and, in particular, between 50% and 70% show a wide variability in their functional significance (Table 2) .
Factors that may make a 50–70% lesion functionally significant include:
On the other hand, the presence of collateral vessels may ensure enough blood flow to a region supplied by a severely stenotic artery so as to not show objective evidence of ischaemia .
An individualised approach to multivessel CAD based on intervention targeted to culprit functionally significant lesions is likely to improve clinical outcome.
The different imaging modalities provide complementary information about different aspects of CAD. An integrated approach refines risk stratification by providing incremental information over any single technique. In a study involving 541 patients who underwent both SPECT imaging and coronary CT, both modalities provided independent prognostic information. The annualised hard event rate (defined as an all-cause mortality or non-fatal myocardial infarction) was 1.8% for none/mild CAD and 4.8% for significant CAD on CT. The annualised hard event rate was 1.1% for a normal SPECT and 3.8% for an abnormal SPECT study. The two techniques, when combined, provided additional prognostic information (Table 3) .
Addition of plaque analysis further enhanced the risk prediction to 10.8%. The plaque composition also influenced prognostication. The risk increased with two or more segments with non-calcified plaques, three or more segments with mixed plaques and four or more segments with calcified plaques .
Coronary calcium and coronary CT diagnose the presence of plaques. In an asymptomatic patient or in a patient with no objective ischaemia on functional imaging, their presence is an indicator of subclinical atherosclerosis. Those with subclinical atherosclerosis may benefit from lifestyle modifications and aggressive medical therapy as part of secondary prevention strategies .
NICE is an independent UK-based organisation. It provides evidence-based national guidelines on various matters relating to health. The NICE guideline, issued in March 2010, recommends a structured, evidence-based cost-effective model incorporating anatomical and functional assessment in a patient with stable chest pain of recent onset .
The salient features of the NICE guideline are:
It is anticipated that there will be a substantial increase in the number of patients undergoing non-invasive imaging and invasive angiography . This necessitates a considerable improvement in the infrastructure of imaging facilities in terms of the equipment, manpower and technical expertise. Local guidelines, although broadly based on the NICE guideline, need to take into account the availability of local resources and expertise in implementing the NICE recommendations.
There is a wide variation in literature on the subject owing to different methodologies involved, different prevalence of disease in the populations studied and different definitions of significant CAD. Most studies define significant stenosis as 50% or more based on the original study by Gould . However, 70% or more stenosis is considered severe stenosis in day-to-day practice  and this cut-off has been used in some studies to define significant stenosis. Invasive coronary angiogram is still considered to be a gold standard. This, in itself, is a flaw as an anatomical yardstick is used to validate the diagnostic accuracy of functional assessment. The rest of the article looks at the role of different imaging modalities, with particular reference to their accuracy in the diagnosis of CAD, prognostication and assessment of viability.
SPECT imaging has been available since the 1970s and has given us a large body of evidence confirming its diagnostic and prognostic value. The commonly used radio-isotopes are thallium-201 and technetium-based agents such as 99Tcm sestamibi and 99Tcm tetrofosmin. Ischaemia is suspected when there is reduced tracer uptake on the stress acquisition which is reversible on the rest acquisition (Figure 3). A fixed defect, i.e. a defect present on both stress and rest acquisitions, is suggestive of an infarct provided attenuation artefacts are ruled out (Figure 4).
The sensitivity and specificity for the diagnosis of significant coronary stenosis (defined as ≥50% stenosis) were 86% and 74%, respectively .
A false negative study may be a feature of three-vessel and left main stem disease because SPECT assesses relative perfusion. Normal perfusion is seen in up to 13–15% of patients with left main stem disease on account of balanced ischaemia in multivessel disease [30,31].
False positive tests due to attenuation artefacts lower the specificity. For example, an elevated diaphragm results in an apparent fixed defect in the inferior wall in men, and breast artefact gives rise to an apparent defect in the anterior wall in women. Implementing gated studies , attenuation correction algorithm  and prone imaging  help improve the specificity by reducing the number of equivocal scans in such cases.
Referral bias, introduced by the fact that only patients with a positive test will undergo an invasive angiogram, also falsely lowers the specificity.
A negative study confers an annualised risk of less than 1% of adverse cardiac events .
The warranty period refers to the frequency of follow-up testing following a negative study. The warranty period is approximately 5 years in a clinically stable patient with no new symptoms or signs. The individual risk and the warranty period are dependent on the age and sex of the patient, stress-induced ECG changes and associated comorbidities such as diabetes and renal dysfunction. The annual risk varies from 1.4% to 1.8%. The risk is highest for an 80-year-old female diabetic patient, in whom the warranty period is only 1–2 years .
Viability assessment on SPECT is based on demonstration of the integrity of the cell membrane following an injection of a perfusion tracer. Pooled meta-analysis of thallium and tetrofosmin studies suggests good sensitivity of 83–88% and a modest specificity of 49–69% for prediction of regional functional recovery after revascularisation . This suggests that it has a good negative predictive value. The poor positive predictive value is due to the poor spatial resolution of the technique. Subendocardial infarcts are beyond the spatial resolution of SPECT and are likely to be missed leading to overestimation of viability .
Analysis of LVF on gated studies correlated well with MRI in a meta-analysis of nine studies. However, the margin of error was higher in women with smaller left ventricular volumes, with dilated cardiomyopathy and in the presence of global subendocardial perfusion defects .
The European and American guidelines recommend the stress electrocardiography test as a first-line of investigation in patients with an intermediate pre-test probability owing to its wide availability [39,40]. A perfusion scintigraphy scan is recommended for the diagnosis of CAD in the following conditions:
In patients with known CAD, it assesses the haemodynamic significance of a coronary lesion and is involved in risk stratification and prognosis.
PET consists of perfusion imaging with a perfusion tracer (rubidium-82, nitrogen-13 ammonia or oxygen-15 water) and functional metabolic imaging with 18F-fluorodeoxyglucose (FDG).
Mismatch between flow and metabolism, i.e. reduced flow with normal or increased FDG uptake, suggests reversible ischaemia. Matched reduction in blood flow and metabolism suggests an infarct.
In a meta-analysis of 19 studies, PET had a sensitivity of 92% and a specificity of 85% for diagnosing significant CAD (defined as ≥50%) .
A study of 1441 patients who underwent rubidium PET study confirmed that the all-cause mortality increased with increasing severity of perfusion defect and with decreasing left ventricular ejection fraction (LVEF) over a follow-up period of 2.7 years . The annualised cardiac event rate was 0.4% in patients with normal studies. It increased to 2.3% with mild perfusion abnormalities and 7% with moderate to severe perfusion defects .
As with SPECT, PET has good sensitivity and moderate specificity for predicting functional recovery post-revascularisation .
In a study looking at age-, gender- and body mass-matched patients who had PET (n = 112) or SPECT (n = 112), PET was superior to SPECT in many aspects . PET had higher diagnostic accuracy than SPECT (87% vs 71%) for stenosis of more than 50%. It correctly diagnosed multivessel disease in more patients. There was less gut interference and fewer attenuation artefacts, resulting in superior quality images. It was quicker than SPECT because of the short half-life of the perfusion tracers. Rubidium-82 has a half life of 72 s, nitrogen-13 ammonia has 10 min and oxygen-15 water has 2 min.
Unlike SPECT, PET has the advantage of being able to measure myocardial blood flow in absolute units, which is important in the assessment of the distal coronary microcirculation .
Despite its superiority over SPECT imaging, the widespread use of PET is hampered by the requirement for expensive PET cameras and for cyclotron or rubidium generators.
SE is a low-cost, widely available procedure which is based on assessment of regional wall motion abnormality induced by exercise or increasing doses of dobutamine. Stress-induced new or worsening regional or global wall motion abnormality is a reliable predictor of ischaemia.
The sensitivity and specificity of SE in the diagnosis of CAD varies with the technique used. The sensitivity is 80%, 85% and 78% and specificity is 86%, 76% and 91% for dobutamine, exercise and dipyridamole, respectively [47,48]. The sensitivity and specificity are reduced in patients with severe left ventricular dysfunction.
A negative SE confers a 0.5–0.8% risk of cardiac death or non-fatal myocardial infarction [49,50]. An abnormal SE is associated with an increased risk of adverse cardiac events. The risk is increased with resting left ventricular dysfunction, extensive ischaemia and extensive wall motion abnormality [48,51].
Viability imaging is based on demonstration of contractile reserve, i.e. the ability of dysfunctional myocardium to contract with low doses of an ionotropic agent. SE had a sensitivity of 84% and a specificity of 81% for prediction of functional recovery. False negatives may be due to the presence of fibrosis or the disruption of the contractile apparatus in viable tissue. Contractile response of the myocardium adjacent to an infarct can give rise to a false positive result .
LVF is mainly assessed by M mode and two-dimensional echocardiography . It is widely available, inexpensive and does not involve the use of radiation. However, it is unreliable in progressive ventricular dilatation as it relies on geometric assumptions regarding the left ventricular cavity .
Pooled meta-analyses show that SPECT is more sensitive and SE is more specific, both in the diagnosis of significant CAD and in the assessment of viability .
Indications for SE include:
It is most useful in patients in whom stress ECG is contraindicated, not feasible, equivocal or submaximal .
Stress is achieved by the same mechanisms as for other techniques, i.e. exercise or pharmacological. The myocardium is imaged during the first pass of a bolus of gadolinium during stress. The normally perfused myocardium enhances with contrast.
Reversible ischaemia is visually assessed as a reversible low-signal defect in the absence of delayed enhancement (Figure 5). An infarct shows up as an enhancing area as opposed to normal myocardium, which is black or “nulled” on the delayed enhancement sequences with gadolinium (Figure 6).
A recent meta-analysis confirmed a high sensitivity of 89% and a moderate specificity of 80% for the diagnosis of significant CAD in a population with a high prevalence of CAD of 57% . The value of stress CMR in low-prevalence populations is not clear. False positive tests can be attributed to the presence of artefacts due to susceptibility (called dark rim artefacts), poor gating and motion artefacts .
A negative adenosine stress perfusion CMR conferred a low cardiac event rate of 1%, both in the low to intermediate risk population and in patients with known CAD [57–59]. An abnormal adenosine stress CMR was associated with a 12-fold increased risk of a cardiac event, and an abnormal dobutamine stress perfusion was associated with a 5-fold risk of a cardiac event over a follow up period of 2.3 years .
Viability imaging on CMR relies on the demonstration of scar tissue 10–20 min after the administration of gadolinium on the delayed enhancement inversion gradient echo sequence. In a study involving 50 consecutive patients who were imaged before and after revascularisation, Kim et al  showed that the extent of the infarct on delayed enhancement sequences predicted functional recovery after revascularisation (Table 5). The extent of the infarct was expressed as a percentage of the myocardium that enhanced with contrast.
Thus, absence of enhancement and enhancement of more than 75% of myocardium were the best predictors of functional recovery 79±36 days after revascularisation.
This is considered to be the gold standard for global and regional left ventricular functional analysis. It is superior to echocardiography for the following reasons:
A comparison of stress CMR and SPECT in 234 patients in 18 centres worldwide was the basis of the MR-IMPACT (magnetic resonance imaging for myocardial perfusion assessment in coronary artery disease) trial. It showed that stress CMR using 0.1 mmol kg–1 gadolinium performed better than SPECT (area under the curve of 0.86 vs 0.67) and that stress MR performed better than SPECT in the diagnosis of multivessel disease .
Delayed enhancement CMR has a better negative predictive value than SE, particularly in segments with severe dysfunction . Dobutamine echocardiography has a low sensitivity of 26% in severe left ventricular dysfunction , the very segments whose viability assessment needs to be accurate. This is because contractility depends on the delivery of adequate amount of oxygen to an intact contractile apparatus. In severely dysfunctional segments, ionotropic reserve is hampered owing to an exhausted coronary flow reserve to a possibly disrupted contractile apparatus.
This is a marker of subclinical atherosclerosis and reflects the total atherosclerotic burden including calcified and non-calcified plaques. The most widely used method for quantifying coronary calcium is the Agatston score . It is based on identifying calcium on the basis of its density (>130 Hounsfield units) (Figure 7). It assigns a CT factor to the coronary calcium based on the Hounsfield unit in all the coronary arteries and compiles a total score based on age and gender .
Absent coronary calcium was associated with an event rate of less than 1.01% over 50 months. A coronary calcium score of zero was associated with a low incidence of significant coronary artery stenosis on invasive angiogram, a low likelihood of acute coronary syndrome and a low incidence of an abnormal myocardial perfusion scintigraphy scan .
Although most of the prognostic data are derived from studies involving asymptomatic patients, the prognostic role also extends to the symptomatic population. In a meta-analysis of 7 studies involving 3924 patients, 1.8% of patients with no coronary calcium had a cardiovascular event compared with 8.99% of symptomatic patients with coronary calcium followed over 42 months .
Coronary calcium has an important role to play in patients presenting with stable chest pain, with a pre-test probability of 10–29% as suggested by the NICE guideline (Table 5) . The presence of coronary calcium is sensitive but not specific for the diagnosis of significant coronary artery stenosis . A clear relationship exists between increasing coronary calcium scores and the severity of coronary stenosis and the number of stenotic vessels . Absent coronary calcium is indicative of the absence of significant stenosis. However, this has to be interpreted with caution. Both the Multi-Ethnic Study of Atherosclerosis  and a recent meta-analysis  showed that absent coronary calcium may be associated with significant coronary artery stenosis in 2–4% of patients. These patients were younger, and hence absent coronary calcium should be interpreted with caution in patients under 50 years.
A limited number of studies of patients presenting to the emergency department with acute chest pain show that in patients with negative cardiac enzymes and no electrocardiographic changes, absent coronary calcium excludes acute coronary syndrome with a high sensitivity of 99% and a negative predictive value of 99% . These patients can be discharged from the unit promptly and safely with a low risk of a cardiac event .
Coronary calcium is deposited in a non-atherosclerotic process in the tunica media as a consequence of altered calcium metabolism . It is high and progressive in renal patients, particularly in patients on dialysis . It is associated with a significantly increased cardiovascular risk . The role of coronary calcium is not clear because of limited studies on the subject. More prospective studies are required to clearly ascertain the relationship between increased cardiovascular risk and coronary calcium.
Several studies demonstrate that:
Absent coronary calcium is associated with a low incidence of ischaemia on SPECT/PET. Only 6% of patients with absent coronary calcium had ischaemia compared with 20% of patients with a high coronary calcium score [81,82]. Increasing coronary calcium is associated with an increased risk of an adverse cardiac event, both in patients with normal perfusion scans and in those with perfusion abnormalities on PET imaging .
Technological advancements in hardware and software of newer generation CT scanners have improved spatial and temporal resolution and z-axis coverage while reducing the radiation dose. Faster gantry rotation times, wide detector designs, dual-source technology and newer reconstruction methods have resulted in improvements in image acquisition, reconstruction, work flow and analysis.
Diagnosis of CAD involves detection of stenosis and plaque analysis (Figure 8).
Multicentre prospective studies [28,84,85] and a recent meta-analysis  confirm an excellent sensitivity of 99% for 64 slice coronary CT. Coronary CT rules out coronary stenosis with a high degree of confidence in low-, intermediate- and high-risk populations [28,84,85].
However, its specificity varies between 64% and 89% [84,86-88]. The relatively modest specificity, particularly in high-risk patients, is due to a number of factors, including heavily calcified vessels , smaller vessels  and the presence of stents . Artefacts introduced by poor image quality due to high heart rates, arrhythmias and motion may be falsely interpreted as stenoses . Thus, there is a tendency to overestimate stenoses, particularly in high-risk patients.
Assessment of the atherosclerotic burden by CT is based on plaque volume assessment. There is a good correlation between plaque volume estimation on intravascular ultrasound and on CT . Plaque volume estimation has the potential for monitoring response to lipid-lowering therapy . However, it is affected by several variables including the image quality and interobserver variability .
Plaques may also be implicated in the development of acute coronary syndrome. Low attenuation plaques (i.e. non-calcified plaques), plaques with spotty calcification (mixed plaques) and those associated with constrictive remodelling are more likely to result in an acute coronary syndrome .
There are few data regarding long-term prognostic information, particularly with the newer generation of CT scanners. In a recent meta-analysis, the annualised cardiac event rate was 0.17% for a normal coronary CT over a median follow-up of 20 months. The adverse annual event rate was 8.8% for an abnormal coronary CT and the risk rose with increasing severity of coronary stenosis .
Most of the data regarding the diagnostic accuracy of coronary CT, as described above, relate to stable patients with suspected CAD. The high negative predictive value in intermediate-risk patients safely reduces unnecessary referrals for invasive angiograms . Its modest positive predictive value limits its diagnostic accuracy, particularly in high-risk patients . Thus, coronary CT is of most value in the intermediate-risk population.
Patients presenting to the emergency department with acute chest pain can be stratified on the basis of coronary CT provided they have no ECG changes and have normal cardiac enzymes [97,98]. Small studies show that up to 50% of such patients do not have significant coronary stenosis  and can be safely discharged with a low 1 year event rate . Coronary CT also has a role to play in a “triple rule-out” test, which aims to rule out important non-cardiac causes of chest pain such as an acute aortic syndrome and pulmonary embolism. There are issues such as increased radiation dose and difficulty in ensuring adequate contrast opacification in three vascular territories . More data, particularly from large randomised controlled trials, are needed to evaluate the clinical utility of the triple rule out test in day-to-day practice.
This involves free breathing three-dimensional acquisition of the coronary artery with real-time assessment of the diaphragm using a navigator (Figure 9). It has a sensitivity of 72% and a specificity of 87% for diagnosing coronary artery stenosis of 50% or more . Its predictive value is best for proximal and mid-segments . Its clinical utility is limited by the fact that it is time-consuming and 30–50% of segments cannot be evaluated .
The background risk of radiation is 1–3 mSv. Radiological procedures may be associated with an exposure to radiation over and above the background risk of radiation. The radiation dose is expressed as an effective dose in millisieverts (mSv) . The average risk of dying of malignancy due to exposure to radiation is estimated to be 5–7.9 per 100 individuals in the general population per sievert . The effective radiation dose for cardiac imaging procedures is summarised in Table 6.
Technological innovations in myocardial perfusion and plaque imaging have pushed the boundaries of cardiac imaging.
Myocardial contrast echocardiography assesses myocardial perfusion, viability and myocardial blood flow by using intravenous microbubbles of contrast [108,109]. The microbubbles are coated with lipid, albumin or polymers. They generate an ultrasound signal as they transit through the myocardial capillary circulation. This helps visualise the myocardial capillary bed during a continuous infusion of microbubbles of contrast during vasodilatory or ionotropic stress . The use of intravenous contrast agent combined with three-dimensional echocardiography has made real-time three-dimensional myocardial contrast echocardiography an emerging technique for assessing tissue perfusion. It has the advantage of volumetric data acquisition and delineation of perfusion defects .
A shift from sodium iodide crystals to newer generation crystals such as cadmium zinc telluride and caesium iodide has led to improved detector efficiency. Improved detector configurations have resulted in ultra-fast SPECT cameras. Iterative reconstruction methods have led to quicker acquisition with better contrast resolution and reduced radiation dose. The introduction of the new selective vasodilatory agent, regadenoson, will result in fewer side-effects.
Innovations in the detector crystals, hardware design including detector configuration, reconstruction methods and vasodilator stress agents have led to faster acquisition times with better spatial and contrast resolution.
CT perfusion studies with adenosine on 64 slice, 256 slice and dual-source scanners are associated with sensitivity and specificity comparable to those obtained with SPECT imaging with acceptable radiation doses [112,113].
Initial studies with CT adenosine stress suggest a potential for demonstrating coronary anatomy, ischaemia and infarct in a single procedure. However, more studies are needed before it can be considered for widespread clinical use.
Rupture or fissuring of plaques leads to an acute coronary syndrome. Plaque composition has an important role to play in plaque rupture. Lipid-rich plaques with a thin fibrous cap containing macrophages are more likely to rupture . Identification of plaque composition is the basis of CMR and CT imaging. CMR imaging focuses on identification of the lipid core, the fibrous cap and haemorrhage within the plaque. The use of plaque-avid contrast agents is an important area of research in MRI . PET concentrates on demonstration of metabolic activity in a vulnerable plaque . Invasive methods such as intravascular ultrasound and optical coherence tomography are catheter-based techniques used at the time of coronary angiography to assess plaque composition. Intravascular ultrasound is based on a greyscale image generated by the reflection of ultrasound . Optical coherence tomography is based on reflection of light resulting in high-resolution tomographic images . Both techniques assess plaque composition and identify rupture or fissuring of the fibrous cap.
The different cardiac imaging modalities provide complementary information about various aspects of CAD. Coronary CT and MRI of the coronary arteries provide anatomical information. Functional significance of a coronary stenosis is assessed by stress CMR, radio-isotope studies and SE. Subclinical atherosclerosis is assessed by coronary calcium score and coronary CT. The NICE guideline provides a framework which incorporates anatomical and functional imaging in the setting of stable chest pain of recent onset. An integrated approach combining anatomical and functional imaging is important in guiding treatment options and in risk stratification.