Cardiac nuclear imaging - single photon emission computed tomography (SPECT) and cardiac positron emission tomography (PET)

• cardiac SPECT and PET are both non-invasive nuclear imaging techniques
  • rely on intravenous administration of radiotracers that are extracted by normal myocardium, such as thallium and sestamibi for SPECT and Fluorodeoxyglucose (FDG) for PET
  • images are derived from radioactive emission data acquired using either a gamma camera (SPECT) or a PET scanner and processed to provide an image of the radiotracer distribution in the myocardium
  • both techniques generate low resolution, high sensitivity 3D images of radiotracer distribution within the myocardium
  • primary role is in the quantative assessment of myocardial perfusion and myocardial viability
  • have no role in anatomical imaging
    
    
• SPECT and PET data can be acquired during pharmacological stress (adenosine, dipyridamole) to determine whether there are inducible areas of ischaemia within the myocardium
  • assessment of myocardial ischaemia by SPECT involves comparing tracer distribution during stress with that at rest. A mismatch indicates reversible ischaemia, whereas a 'fixed' defect indicates myocardial infarction. Consequently a SPECT myocardial perfusion study requires two imaging sessions separated by sufficient time to allow for decay of the previous dose. Cardiac SPECT is much more widely available than PET and uses tracers with long halflives
  • cardiac PET is less available and uses short half life tracers whose short half-life requires that they have to be generated near to the scanner and used within a short time of production
    • advantage of PET is its ability to use many different tracers to interrogate several aspects of myocardial pathophysiology
• total scanning time is usually around 30 minutes.The data are displayed in the three standard cardiac planes and may include both static and dynamic images. Images can be read both qualitatively and quantitatively
• combined PET/CT scanners provide the opportunity to acquire complimentary anatomical data such as CT coronary angiography and functional data such as perfusion in a single imaging session

Uses of cardiac nuclear imaging:

• assessment of myocardial perfusion (SPECT)
• assessment of left ventricular ejection fraction
• assessment myocardial viability (PET) - FDG-PET can identify viable myocardium that is functioning poorly as a result of reduced perfusion (hibernating myocardium) that would benefit from revascularization

Notes:

• cardiac PET imaging with FDG is the gold standard for assessment of myocardial viability
• cardiac SPECT is relatively cheap and widely available
• as with cardiac CT, nuclear imaging involves ionizing radiation. For example, the radiation dose for a SPECT perfusion study is approximately 15mSv for a thallium scan. Consequently, nuclear imaging is not appropriate for serial follow up studies

Reference:

• 1) British Heart Foundation Factfile (January 2010). Cardiac imaging with MRI, CT and Nuclear techniques