Case Study

Use of MDCT Coronary Angiography

Imaging of coronary arteries with selective conventional angiography based on severity of luminal stenosis remains the basis for planning and guiding catheter-based and surgical myocardial revascularisation treatment in the assessment of advanced stages of CAD

Dr Kapisoor Singh Consultant Cardiac & Interventional Radiologist KG Hospital, Coimbatore

Since its introduction in the 1960s, catheter-based cine Coronary Angiography (CAG) has largely defined our understanding of normal and pathologic coronary anatomy. The precise angiographic depiction of luminal narrowing remains the basis for catheter-based or surgical revascularisation of the myocardium in the distribution of significantly diseased coronary arteries. However, it is now appreciated that the accumulation of atherosclerotic plaque in the coronary arterial wall begins much earlier than the development of luminal stenosis. In fact, most acute coronary syndromes are initiated by sudden disruption of atherosclerotic plaques that are not causing significant stenosis. Therefore, the assessment of these early stages of Coronary Artery Disease (CAD) has emerged as an important goal in preventing both CAD progression and complications of atherosclerotic CAD (for example, myocardial infarction). Because early atherosclerotic plaque accumulation is typically associated with compensatory vessel expansion (positive arterial remodeling), the description of these important early changes of CAD on the basis of alterations in coronary luminal dimensions (evaluated by CAG-luminograms) alone, is not sufficient.

Approximately 50 per cent of all acute coronary syndromes occur in previously asymptomatic subjects, hence there obviously is a need to identify these subjects before coronary atherosclerosis clinically manifests and irreversible damage occurs by progression to myocardial infarction or cardiac death.

While invasive techniques will remain vital to the diagnosis and treatment of significantly stenotic coronary lesions, the comprehensive and serial assessment of asymptomatic or minimally symptomatic stages of CAD for preventive purposes will eventually need to rely on non-invasive imaging techniques. Cardiovascular imaging with tomographic modalities, including Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), has great potential for providing this information.

Intravascular Ultrasonography (IVUS), Optical Coherence Tomography (OCT), virtual histology using backscatter radiofrequency, thermography (infrared imaging) are some of the techniques requiring selective coronary catheterisation that have allowed excellent visualisation of the coronary arterial wall and direct assessment of atherosclerotic plaques. However, these are invasive and scarcely available.

Experience with 128 slice MDCT Angiography of Coronary Arteries

CT Angiographic Coronary Anatomy in Symptomatic Patients: Advanced CAD is characterised by multiple stenotic lesions of variable severity, causing chronically reduced coronary blood flow and potentially gradual or sudden vessel occlusion. Consequently, advanced disease is clinically manifested by a wide variety of disease states, including stable angina pectoris, chronic heart failure or acute coronary syndromes. In these clinical situations with a high pre-test likelihood of significant stenotic atherosclerotic disease, the definitive identification and accurate quantification of luminal stenoses in the entire epicardial coronary tree are necessary. Selective conventional angiography provides this information with high spatial (0.1-0.2mm) and temporal (4-7m sec) resolution and allows simultaneous catheter-based therapeutic interventions (angioplasty, stent placement). Therefore, selective angiography will remain vital for guiding interventional or surgical (bypass grafting) treatment of significantly stenotic coronary lesions.

However, in other clinical situations, the comprehensive assessment of obstructive and non-obstructive CAD will eventually increasingly rely on non-invasive imaging techniques. CCTA can demonstrate the presence of atherosclerotic plaque over a decade earlier than other tests including invasive cardiac catheterisation. Coronary angiography with use of multi-detector row CT is typically performed in a spiral scan mode, which permits three-dimensional scanning of the entire heart in a single breath hold. Data is retrospectively referenced to the ECG signal to reconstruct images during the diastolic phase or other phases of the cardiac cycle.

Adjacent overlapping transverse images are reconstructed with a minimum section thickness of as little as 0.6 mm and a maximum in-plane spatial resolution of approximately 0.3 x 0.3mm. Temporal resolution is 40-160mm depending on image reconstruction algorithm/ protocol and whether a single source or a dual source scanner is being used. Patient radiation dose with current high-end multi-detector row CT scanners is approaching that at conventional coronary angiography (3.5-6 milli Severt).

A well-recognised limitation in the assessment of coronary stenosis with CT angiography is related to 'beam hardening artefact' produced by dense objects in coronary walls/ their vicinity - like densely calcified plaques, metallic stents, surgical clips and implants. The resulting 'blooming' effect of coronary arterial calcification causes difficulties in assessing adjacent plaque structures, potentially resulting in a false-positive detection of stenosis or overestimation of the degree of narrowing. Similarly, surgical clips and coronary stent struts produce streaky artefacts which preclude confident detection and grading of adjacent coronary artery lumen or in-stent restenosis. However, differentiation between stent patency and occlusion, as well as evaluation of stenosis at the leading or trailing ends of a stent is often directly or indirectly (aided by assessment of flow distal to the stent) possible.

Curved MPR 128-slice MDCTA image showing soft and mixed plaques causing diffuse disease in right coronary artery

3D volume rendered 128-slice MDCTA image in a post bypass graft (CABG) patient

In spite of the surgical clips, CT angiography has advantage in assessment of bypass grafts because of unknown/ variable positions of the graft ostia pose difficulty in their selective catheterisation.

An important advantage of Multi Detector row CT Angiography (MDCT-CA) over conventional angiography is that additional information about cardiac and non-cardiac anatomy is simultaneously provided. This information is of great clinical importance because, for example, the demonstration of an anomalous artery tracking between the aorta and the pulmonary artery may define an indication of surgical correction. Additional non-atherosclerotic coronary abnormalities detectable at multi-detector row CT angiography include the intra-myocardial course of a coronary artery due to a myocardial bridge. In acute onset severe chest pain CTA is the one-stop shop for "triple rule-out" of aortic dissection, pulmonary embolism and acute coronary syndrome.

MDCT-CA seems to be effective as a pre-operative screening test prior to non-coronary cardiac surgery. In this era of cost containment and optimal care of patients, MDCT-CA is able to provide coronary vessel and ventricular function evaluation and may become the method of choice for the assessment of a cardiovascular risk profile prior to major surgery.

It is important to understand that the lower spatial resolution of CT angiography is more problematic in the assessment of highly stenotic lesions, a subtotal coronary occlusion with a lumen size below the resolution of CT cannot be differentiated from a total occlusion. Also, very small coronary arterial branches and sub-branches that are visualised on CAG may not be visualised on CTA because of lower spatial and temporal resolution.

Also, about five to ten per cent of CTA studies are suboptimal because of an unexpected rise in heart rate or an unexpectedly wide beat-to-beat variability at the time of acquisition in spite of an optimal heart rate just before the start of contrast injection during CTA. This results in motion artefacts rendering many arterial segments non-evaluable.

On the other hand, CT angiography has the advantage of vessel wall and plaque depiction in addition to its ability to enable assessment of luminal dimensions. The assessment of size and composition (calcification and fat content) of coronary arterial lesions and the associated changes in vessel architecture, for instance, arterial remodeling may have important clinical implications.

Plaque Burden Assessment and Plaque Characterisation in Asymptomatic or Minimally Symptomatic Patients: The number of significant stenoses in the coronary arterial tree as assessed with selective conventional angiography has prognostic value. However, serial angiographic observations of existing severe stenoses do not allow the assessment of the effect of risk-factor modification. Generally, changes in atherosclerotic plaque size are not well reflected in luminal dimensions, because plaque progression and regression are associated with arterial expansion (positive remodeling) and shrinkage (negative remodeling). This limitation of angiographic lesion assessment has become evident from prevention trials with lipid-lowering medications that showed a statistically significant reduction in clinical events but only minimal change in the severity of existing angiographic stenoses.

In addition, findings of several studies have shown that most acute coronary syndromes are initiated by sudden changes of mildly stenotic lesions, commonly found in positively remodelled arterial lesions, rather than from progression of lesions already causing significant luminal narrowing. Therefore, it has been postulated that the identification of mildly stenotic but vulnerable atherosclerotic lesions and the overall plaque burden could provide better markers of coronary risk than do measures of luminal stenosis. This hypothesis is currently being examined in intravascular US studies. Multi-detector row CT has a potential to address several important parameters about early atherosclerotic changes in the coronary arteries.

Calcium Scoring

It is a very low radiation-dose CT scan (<1 mSv) that does not involve injecting of intravenous contrast medium. It has been shown that the negative predictive value of a zero calcium score in asymptomatic individual is 97-99 per cent and in patients with atypical angina symptoms, it is 90-95 per cent. The calcium score has been shown to predict cardiac events independently of standard risk factors and enhance Framingham risk stratification categories. The score provides a more accurate rationale for determining the necessity for LDL-lowering therapy and risk factor management. Calcium scoring helps identify the calcified portion of the overall atherosclerotic plaque burden. However, non-calcified atherosclerotic plaque (which in fact could have micro-calcifications below the threshold of detection with calcium scoring techniques) may be more unstable and prone to rupture, causing acute coronary syndromes.

The morphologic characteristics of unstable or vulnerable plaques are being understood. Differences between stable and unstable coronary plaques have been examined with invasive catheter-based methods like coronary intravascular US (hypoechoic plaque has been associated with the clinical presentation of unstable angina), optical coherence tomography (capable of differentiating lipid and fibrous components as well as calcification at higher resolution than IVUS), intravascular thermography and with non-invasive methods like nuclear imaging (PET-CT, SPECT-CT) using multiple radiopharmaceutical approaches.

On CTA, the vulnerable plaques appear as soft plaques with excessive (thick) lipid content (hypodense, <30 Hounsfield unit) and thin fibrous cap (fibrous tissue is more dense, 50-80 HU) with or without calcification (>300HU).

According to a recent Korean study published in Journal of the American College of Cardiology July, 2008; screening coronary angiography (64-slice MDCT) of 1,000 asymptomatic middle aged (50+9 yrs) Korean subjects was done to detect occult CAD as part of a general health evaluation. Atherosclerotic plaques were identified in 22 per cent individuals, four per cent had only non-calcified plaques, five per cent subjects had significant (>50per cent ) diameter stenosis and 2per cent had severe (>75per cent ) stenosis. Midterm follow-up (17 + 2 months) revealed 15 cardiac events only in those with CAD on CTA: 1 unstable angina requiring hospital stay and 14 revascularization procedures. 87per cent events occurred within 90 days of index CTA.

Thus, the prevalence of occult CAD in apparently healthy individuals was not negligible and CTA has a potential to provide a better insight about the occult CAD in this population.

The problem in applying CTA as a screening tool for asymptomatic population is the radiation exposure at CT Angiography. The Food and Drug Administration announced that there is a small chance (one in 2,000) of developing a fatal cancer due to a 10-mSv CT study. However, with newer MDCT-scanners allowing coronary CTA in 3.5-8 mSv radiation dose (depending on the protocol chosen), persons with low and intermediated risk for heart attack (risk factors include diabetes, obesity, smoker (past or current), high blood pressure, family history of heart disease, high cholesterol and age above 45 years for males and age above 50 years for females can be considered for screening CTA examination. With the chances of having a heart attack in this subgroup much greater than developing breast and other cancer following a coronary CTA, screening can save many lives.

With rapidly evolving MDCT technology, the radiation dose for CTA would likely be soon minimised to less than 3 mSv (less than the level of annual background natural radiation exposure).

Overview

Imaging of coronary arteries with selective conventional angiography based on severity of luminal stenosis remains the basis for planning and guiding catheter-based and surgical myocardial revascularisation treatment in the assessment of advanced stages of CAD. In contrast, prevention of coronary events requires identification of early stages of atherosclerosis and the associated abnormalities in coronary architecture before the development of luminal stenosis. Here comes the role of coronary plaque imaging methods like IVUS, OCT, MDCT, PET/CT, SPECT/CT. Catheter-based diagnostic techniques, like IVUS and OCT allow high-resolution visualisation of a wide range of coronary lesions/ plaques, independent of their luminal dimensions but they are invasive, complicated and scarcely available.

Non-invasive simple and rapid MDCT coronary imaging technique is rapidly gaining ground for the routine examination of asymptomatic low-risk or minimally symptomatic patients with moderate risk despite all the politics involved. MDCT-CA can be applied to both the assessment of significant luminal stenosis and the identification of non-stenotic atherosclerotic plaques. It could become complementary to conventional angiography in the assessment of selected patients with stenotic atherosclerotic or non-atherosclerotic coronary disease providing an overall assessment of cardiac anatomy beyond coronary imaging, including cardiac function assessment, paralleling capabilities of cardiac MR imaging.

It has already a well-established role in non-coronary causes of chest pain. The non-invasive characterisation and quantification of atherosclerotic plaque burden has important implications for risk stratification and prevention of CAD progression and/ or its complications. In centres where both angiographic and tomographic imaging modalities are available, the integration of tomographic image information will facilitate comprehensive non-invasive imaging of patients with early and those with late stages of CAD.

The clinical impact of non-invasive tomographic imaging technologies, such as multi-detector row CT, on imaging of coronary arteriosclerosis will need further elucidation, but the prospects are exciting.

kapisoor@gmail.com