There is a growing body of evidence to support myocardial perfusion imaging as a clinically effective and cost-effective method of managing patients with actual or suspected cardiac disease. It is becoming increasingly used as the investigation of choice for several clinical indications. The British Nuclear Cardiology Society was founded in 1981 to promote the use of this branch of science in the UK.
Using techniques such as cardiac single photon emission computed tomography (SPECT) and positron emission tomography (PET) has achieved excellence in coronary artery disease diagnosis and risk stratification. However, developments in other cardiac imaging modalities (eg, cardiac CT, cardiac MRI and echocardiography) have raised expectations in terms of diagnostic accuracy and achieving high-quality images with little or no ionising radiation exposure.
One review found that SPECT, cardiac magnetic resonance (CMR) and PET all achieve high sensitivity, although a broad range of specificity was seen. SPECT is widely available and most extensively validated. PET achieved the highest diagnostic performance. CMR may provide an alternative without ionising radiation and a similar diagnostic accuracy as PET.
Myocardial perfusion scintigraphy imaging
- Radionuclide myocardial perfusion scintigraphy (MPS) has become established as the main functional cardiac imaging technique for ischaemic heart disease (IHD).
- MPS with SPECT uses a radio-pharmaceutical that is taken up into heart muscle in proportion to localised blood flow and stays in myocardial cells whilst scanning is performed. Most use either thallium 201 or technetium 99m in proprietary compounds.
- Guidance released by the National Institute for Health and Care Excellence (NICE) in 2003 recommended that MPS with SPECT should be used in the following circumstances:
- As a first-line diagnostic tool for the exclusion of coronary artery disease in patients for whom exercise stress testing poses problems in interpreting or where sensitivity is poor. This may include women, patients with cardiac conduction defects (eg, left bundle branch block) and for people who have difficulty using a treadmill for whatever reason.
- As part of an investigation strategy in patients with a lower risk of coronary artery disease (based on risk factor calculations) in the prediction of likely future cardiac events, as a less invasive alternative to coronary angiography.
- In patients with established coronary artery disease with persistent symptoms after a myocardial infarction or after a reperfusion intervention.
- Infarction causes matched perfusion defects during stress and at rest. Reversible ischaemia shows defects during stress which re-perfuse at rest.
- Severity, extent and number of reversible defects are a good prognostic indicator. A normal study implies a risk of an adverse cardiac event of less than 0.5% per annum.
- Less sensitive in multiple small vessel coronary disease - eg, diabetes mellitus.
- Infarct-avid imaging:
- Can be used in the diagnosis of myocardial infarction.
- Commonly uses technetium99m stannous pyrophosphate concentrating in damaged myocardial cells for 'hot-spot' scanning.
- Scans are best performed 24-96 hours after myocardial infarction. Alternatively, cold-spot scanning can be used where non-viable heart muscle does not take up the radio-pharmaceutical.
- Unfortunately, because of the delay, this method is of no value in identifying patients suitable for thrombolytic therapy.
Cardiac single photon emission computed tomography
- Cardiac SPECT is widely used and non-invasive nuclear imaging for investigating IHD.
- SPECT is appropriate for all aspects of detecting and managing IHD, including diagnosis, risk assessment and stratification, assessment of myocardial viability, and evaluation of left ventricular function.
- Hybrid images combining the functional imaging of SPECT and CT coronary angiography enable improved diagnosis, risk stratification, and treatment planning for patients with suspected coronary artery disease.
Radionuclide ventriculography - multiple gated acquisition scans
- Scintigraphy can be used to estimate ventricular ejection fraction as an indicator of left and right ventricular function.
- In first-pass method, technetium 99m pertechnate is injected as an IV bolus and its passage through the heart recorded every second. The change in radioactivity recorded with time is related to the ejection fraction.
- With the gated cardiac blood pool method, the patient's own red blood cells are labelled with technetium 99m pertechnate. Regional wall motion studies can also be performed with this method.
- Can assess both left ventricular and right ventricular function after myocardial infarction.
- Left ventricular ejection fraction measurement.
- Monitor anthracycline cardiotoxicity.
- Monitor treatment response in cardiac failure and cardiomyopathy.
- Useful for serial measurements during anthracycline therapy.
- Reliable in unechogenic subjects. Otherwise the procedure carries a high radiation dose and echocardiography is preferable for most patients.
- Cardiac dysrhythmias may interfere with results.
- One study found that this method contributed to a significant improvement in risk stratification and secondary prevention strategy in patients with known coronary artery disease.
Magnetic resonance imaging
- Cardiac MRI is non-invasive, has high spatial resolution and avoids use of potentially nephrotoxic contrast agents or radiation.
- It has an important part in diagnosing congenital heart disease as well as disorders of the pericardium, cardiac tumours, atrial or ventricular thrombus, pericardial thickening, myocardial hypertrophy and valvular disease.
- It may also be used to image the coronary vessels and disease of the aorta, including aortic dissection.
- Stress cardiac MRI is a non-invasive option for the diagnosis of coronary artery disease. The advantages of cardiac MRI are that it does not pose the radiation burden associated with SPECT, can also assess left and right ventricular dimensions, viability, and cardiac mass, and may also avoid the need for invasive diagnostic coronary angiography in patients with intermediate risk factors for coronary artery disease.
- It is contra-indicated for many prosthetic valves and for patients with cardiac pacemakers.
- The slowness of scanners available at most centres limits its usefulness but the introduction of new faster techniques will allow breath-hold and real time scans.
- One study found that MRI might be more sensitive than scintigraphy in detecting small areas of myocardial necrosis.
- MRI is considered to be the gold-standard imaging modality for the assessment of aetiology, myocardial anatomy, regional and global function and viability in patients with ischaemic heart failure. In patients with non-ischaemic heart failure it also allows assessment of fibrosis, infiltration and iron overload.
- Various enhancements have recently been developed including cine MRI. Stress perfusion MRI is superior to stress perfusion SPECT in patients with multivessel coronary artery disease. MRI is not as widely used as other modalities in the investigation of cardiac disease because it is technically difficult. However, education and training should eventually rectify this.
- Cardiac CT has several advantages over cardiac MRI. It is much faster and can accommodate patients with implanted devices. It is the imaging modality of choice for the assessment of vascular rings or slings.
- It is mainly used to demonstrate aortic dissection, pericardial thickening and fluid, cardiac tumours and coronary calcification. CT with IV contrast is the most reliable and practical technique for the investigation of aortic dissection.
- Spiral CT is the investigation of choice for pulmonary embolism.
- With latest spiral CT, scanners can acquire whole volume of heart in a single breath-hold but still needs 0.3 seconds for a single slice so needs ECG gating and IV contrast with a very fast electron-beam scanner to image the heart.
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