There is an increased array of diagnostic tools used in the diagnosis and evaluation of prognosis of coronary artery disease. They can be divided into functional assessments: including exercise testing, nuclear perfusion imaging, stress echocardiography, and more recently cardiac MRI perfusion imaging; and anatomical assessment: coronary angiography and cardiac CT angiography (CTA).
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Up to now, coronary angiography (CA) has been the gold standard in the diagnosis of coronary disease. Anatomical confirmation is increasingly sought for both symptomatically limiting and non-limiting angina. However, despite numerous NHS initiatives, access to CA is suboptimal and there remains a small but finite morbidity (0.1%) and mortality (0.2%) risk. There has also been a paradigm shift in recent times away from a purely anatomical diagnosis of coronary disease; to a prognostic approach focused on patients subsequent cardiac risk and management (1). This is increasingly relevant as at best, only 50% of diagnostic procedures proceed to intervention. As the degree of angiographic coronary narrowing has not been shown to correlate with future cardiac events, more relevant investigations offering prognostic information could include stress echocardiography or nuclear myocardial perfusion imaging (2).
The Evolution of Cardiac Computed Tomography:
Computed tomography (CT) scanning provides a non-invasive technique of imaging cardiac structures. In the process, an X-ray beam is attenuated while passing through the tissues of the heart, and this is measured by detectors and translated into images. Numerous technical advances have resulted in freezing cardiac motion (temporal resolution) to obtain excellent image quality and minimizing artefacts. Certainly fast rotation times (up to 0.3 seconds) of the X-ray source around the gantry have significantly improved temporal resolution. Spatial resolution (the degree of blurring and the ability to discriminate small objects) has been optimized by having small multiple detectors (0.6 mm width), thin slice thicknesses and small X-ray focal spots.
Multi slice spiral CT scanning involves a rotating X-ray tube with a simultaneously moving table (hence spiral imaging) and multiple rows of detectors (16, 32, 64, 128, 256…). With this combination of a greater number of detector rows, simultaneous acquisition of a greater number of axial slices can be made per rotation. Thus a greater coverage of the heart can be made per rotation, resulting in a short breath hold. With the Siemens Somatom 128 multislice CT scanner recently installed at the London Independent Hospital (LIH), the whole heart can be scanned in 7-10 seconds.
The Siemens Somatom 128 multislice CT scanner at LIH:
This was installed exactly one year ago and involves both cardiac and non-cardiac applications. The technology has lived up to its promise of being “adaptive” to a variety of scenarios that challenge this type of imaging. Most importantly, obesity is a major challenge for CT imaging and the scanner benefits from a weight limit of up to 300kg, with a large gantry width of 78 cm and most importantly a powerful generator of 100kW.
This scanner with its arrangement of alternating focal spots where the X-ray beams overlap results in a resolution of 0.24mm. The 128 rows of detectors provide excellent volume coverage. A major concern for CT imaging is radiation dose that patients are exposed to. This Siemens scanner is equipped with several features designed to reduce the radiation to about 4.9mSv (3). This includes: adaptive Dose Shield that removes unnecessary exposure at the beginning and end of acquisition, Dose adaption to the patient, ECG pulsing, dose modulation (that lowers the dose during non-acquisition periods), and adaptive features to arrhythmia.
Cardiac Applications of Cardiac CT:
The major applications of cardiac CT include: calcium scoring, coronary angiography, and visualization of other cardiac structures. Additionally with the large volume of data, left ventricular function can be assessed by as a moving image from reconstructions throughout the cardiac cycle.
Calcium Scoring:
In asymptomatic patients the American Heart Association suggests a role for calcium scoring in patients with intermediate risk for coronary disease (5). Electron Beam CT (EBCT) and more recently Coronary Calcium Scoring (CCS) using the Multi-Slice CT scanner have demonstrated the association of coronary calcification and subsequent cardiovascular risk. The absence of coronary calcium is associated with normal coronary arteries. CT coronary calcification does provide the most accurate method currently available for the early detection of coronary artery disease. The presence of a score > 100 (Agatston Score) is associated with a 10x risk of future cardiovascular events. It provides independent and incremental information over risk factors for predicting extent of angiographic CAD. It is perhaps logical with increasing attention to coronary risk, early detection of atherosclerosis may help select patients for earlier and more aggressive treatment.
Coronary calcification is usually present in all angiographically significant coronary disease (>50% diameter stenosis). Thus it can be applied to evaluate the long term risk of atherosclerosis in asymptomatic patients or those with suspected coronary disease who are deemed to have low or intermediate likelihood of disease. In asymptomatic patients with an Agatston score > 400; would indicate the need for further functional assessment for ischaemia. Specifically in the asymptomatic diabetic population, the fact that a 20% incidence of myocardial infarction occurs within 7 years has led the American Diabetic Association to recommend early screening. Radiation dose is very low at 1 mSv.
Coronary Angiography:
The main coronary angiographic applications are seen in Table 1 (adapted from the AHA/ACC guidelines of 2006 (4)). The main coronary application is in symptomatic patients who have low to intermediate probability of disease (5). Ideally patients should be in sinus rhythm with stable heart rates. More recent software (applicable in our Siemens scanner) can obviate the rhythm problems. With the exception of Dual Source CT angiography, most coronary imaging requires optimizing heart rate to 60 beats per minute or less (see later). Compared to cardiac catheterization which only identifies the coronary lumen, CT angiography looks at the entire vessel. In early stages of coronary disease, the lumen is normal in diameter, as the active pathological process involves thickening of the wall. As seen in Figure in 1 where coronary stents are in place, the extent of disease extends beyond the stent insertion. Increasing attention is being paid to the consistency of the plaque that might yield prognostic value. Certainly, CT angiographic estimation of the volume bulk of coronary lesions does offer prognostic value over angiography.
Cardiac Structures:
In addition to the coronary anatomy, numerous other cardiac structures are well visualized, and described in Table 2.
Practical Issues:
The main practical issue in performing CT angiography is maintaining the heart rate at about 60 beats per minute. This can be achieved by pre-medicating the patient in outpatients with oral beta blockers. At the London Independent Hospital, we do not pre-medicate the patient, but assess the need for rate reduction just prior to the procedure. We use iv metoprolol in in 5mg aliquots (half life 5-7 minutes), and when optimal heart rate is achieved, the patient is scanned. In patients undergoing calcium scoring alone, neither beta blockade nor contrast media is required. From a reporting perspective, we follow the wide held view that dual reporting between cardiologist and radiologist is optimal in reporting on cardiac and non-cardiac structures.
Conclusion:
Cardiac CT has advanced rapidly in the assessment of coronary and non-coronary cardiac evaluation. Combined with calcium scoring it can provide both anatomical and prognostic information to more comprehensively assess the cardiac patient.
References:
- Berman DS 2004 Hurst’s The Heart
- Berman DS et al, Roles of Nuclear Cardiology, Cardiac Computed Tomography, and Cardiac Magnetic Resonance: Assessment of Patients with suspected coronary artery disease. Journal of Nuclear Medicine 2006. 47: 74-82
- www.siemens.com/medical
- Hendel RC, ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 Appropriateness Criteria for CT and Cardiac MR Imaging. Journal of Am College of Cardiol. 48: 1477-1497
- Budoff MJ et al, Assessment of coronary artery disease by cardiac computed tomography. Circulation 2006. 16: 1761-91
Biography:
Dr Shakespeare is an invasive cardiologist working in a DGH setting. He has special interests in cardiac MR imaging, and cardiac CT. His CT training was obtained in the USA.
