Nuclear Cardiology and Other Cardiac Scans

There is a growing body of evidence to support myocardial perfusion imaging is a clinically effective and cost effective method of managing patients with actual or suspected cardiac disease. However, its full potential has yet to be explored in the UK.¹

• Myocardial perfusion scintigraphy with SPECT (single photon emission computed tomography ) uses a radio-pharmaceutical that is taken up into heart muscle in proportion to localised blood flow and stays in myocardial cells whilst scan is performed. Most use either thallium-201, technetium-99m in proprietary compounds.
• Procedure:²
  • Stress test, e.g. treadmill, bicycle, IV adenosine or IV dobutamine
  • Thallium or technetium then injected
  • Image of myocardial perfusion then taken immediately and again over four hours after stress
• The technique is used for:
  • Prognostic assessment
  • When conventional stress testing fails
  • Assessing patients with atypical chest pain when the resting ECG is normal or not diagnostic³
  • Screening patients at high risk of acute myocardial infarction
  • Assessing significance of stenoses in coronary vessels seen on angiography
  • Investigating myocardial function after a myocardial infarction
  • Investigating restenosis after angioplasty or bypass surgery
  • Risk stratification prior to non-cardiac surgery
• Results:
  • 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, e.g. diabetes mellitus
• Infarct-avid imaging:
  • Can be used in the diagnosis of myocardial infarction.⁴
  • Commonly uses technetium-99m-stannous pyrophosphate that concentrate in damaged myocardial cells for 'hot-spot' scanning.
  • Scans are best performed 24-96 hours after myocardial infarction. Alternatively, can use cold-spot scanning where non-viable heart muscle does not take up the radio-pharmaceutical.
  • Unfortunately, because of the delay the method is of no value in identifying patients suitable for thrombolytic therapy.

• 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 patients own red blood cells are labelled with technetium-99m-pertechnate. Regional wall motion studies can also be performed with this method.
• Indications:
  • Can assess both left ventricular and right ventricular function after myocardial infarction
  • Left ventricular ejection fraction measurement
  • Monitor anthracycline cardiotoxicity
• Procedure:
  • Radio-labelled Technetium red cells are injected followed by imaging
  • Combination with stress test allows assessment of cardiac reserve
• Uses:
  • 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

• Cardiac MRI is non-invasive, has high spatial resolution and avoids use of potentially nephrotoxic contrast agents or radiation.⁵
• 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.
• May also be used to image the coronary vessels and disease of the aorta, including aortic dissection.
• Is contraindicated 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.
• The role of cardiac MRI is very likely to continue to increase for assessing cardiac structure and function as well as detecting coronary artery disease and the assessment of those patients who may need angioplasty.⁵

• 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 sec for a single slice so needs ECG gating and IV contrast with very fast electron-beam scanner to image the heart.