REF010 - Myocardial Perfusion Imaging

The purpose of this document is to assist clinicians in deciding on the appropriateness of a Nuclear Medicine myocardial perfusion investigation and to give guidance on the mechanism of referral.

• Consultant cardiologists and renal physicians for those patients as part of renal transplant assessment.
• Vascular surgeons for MPI prior to AAA or major bypass surgery.
• Other secondary care clinicans may refer if the test has been recommended by one of the above.

Referring clinicians are reminded of the requirement to avoid unnecessary exposure to ionising radiation. The investigation may already have been requested at this hospital or elsewhere and clinicians are asked to be particularly vigilant to ensure that their request is not a double request.

Requests will only be accepted if the diagnostic question fulfills one of the referral criteria detailed in section 'Referral Criteria'. Appropriate clinical information must be supplied to allow the investigation to be justified. It is not acceptable to simply request the investigation without appropriate clinical information. Requests will be returned to the referrer if:

• The diagnostic question is unclear.
• The diagnostic question cannot be answered by the investigation.
• Insufficient clinical information is supplied to justify the investigation.

Requests must be submitted electronically via WebV. Test Required, Clinical Details, Diagnostic Question and Medication should be fully and appropriately completed.

It is very important that sufficient information is provided to allow the appropriate stressing regime to be chosen in advance of the investigation. Relevant information could include inability to exercise, previous sub-optimal exercise or if the patient is asthmatic, suffering from chronic obstructive airways disease or suffering from an unstable coronary syndrome.

Sufficient clinical information must be provided to justify the investigation proceeding.

The clinical indications are detailed below.

Referrals for patients with congenital abnormalities of the cardiovascular system or patients who are 40 years old or under must be authorised by an ARSAC Licence Holder only, not delegated to Cardiac Stressing Practitioners. Referrers are encouraged to discuss such cases directly with the ARSAC holder before submitting a referral.

In cases of known or suspected angina provoked by exertion, an exercise ECG can provide valuable information. Exercise ECG does, however, have poor sensitivity and specificity for coronary artery disease especially and only provides limited information regarding the localisation and extent of disease. A normal study does not exclude disease. The value of the test arises from estimates of the VO2max value (METS) derived from the time on the treadmill using predefined protocols. Coronary anatomy alone, as defined by the coronary angiogram, is not able to define blood flow at the myocardial level because of it’s limited ability to assess stenoses of intermediate severity or the degree of protection afforded by collateral circulation. Myocardial perfusion imaging (MPI) can assess the overall effects of coronary artery disease irrespective of actual anatomy.

Myocardial blood flow distal to coronary artery stenoses is normal at rest unless the occlusion is greater than 90% of luminal diameter or a myocardial infarction is present. Under conditions of maximal stress, however, the normal increase in myocardial blood flow becomes impaired distal to the stenosis of approximately 75% or greater. Myocardial perfusion imaging is performed by injecting a radiopharmaceutical intravenously after increased coronary blood flow has been induced by maximal stress. These stress images are compared to a set of resting images obtained on a different day.

Adequate stress and pulse rate response are necessary to increase the myocardial blood flow to normal myocardium sufficiently to bring out the contrast between normal and abnormal myocardium where the blood flow cannot increase because of the stenosed supplying vessel. It is important that maximal levels of stress are reached when treadmill exercise is utilised. Submaximal stress will result in a false negative result in some patients with coronary artery disease. If patients are unable to exercise to their full potential due to leg claudication, physical inability or unwillingness to make the physical effort necessary, then pharmacological stress is an appropriate alternative. These agents remove the need for patient cooperation or motivation. A variety of pharmacological stress agents are available, two of the most widely used being adenosine and dobutamine.

Adenosine is a vasodilator. It works by stimulating the A1/A2 receptors and produces apparent defects in radiopharmaceutical uptake by a differential effect on the territories of normal and stenosed coronary arteries. It is administered intravenously and can result in unwanted effects such as angina, arrhythmia and bronchospasm in asthmatics, in whom it is contraindicated. Adenosine can be combined with light exercise such as leg raising to increase blood flow still further.

Dobutamine is a beta receptor agonist which is widely used as a stress agent in echocardiography and magnetic resonance imaging. Whereas the vasodilator adenosine can produce defects of radiopharmaceutical uptake without actual ischaemia, dobutamine regularly results in wall motion abnormality. It is safe in asthmatics, but it does have some risk of arrythmias and is contraindicated in unstable coronary syndromes. It is administered as an intravenous infusion with progressively increasing dosage over several minutes. Dobutamine can also be combined with light exercise such as leg raising to increase blood flow still further. If the response to dobutamine stress is poor, atropine can be administered intravenously to increase heart rate. Atropine can also be used to combat the rather common vasovagal reaction

Myocardial perfusion imaging has been performed for many years using the radiopharmaceutical thallium-201 thallous chloride. Although thallium-201 is still sometimes used at NLAG (mainly for investigation of suspected myocardial hibernation) most myocardial pefusion imaging at NLAG is performed using technetium-99m labelled tetrofosmin (Myoview).

Uptake of myocardial perfusion tracers in the myocardium is regionally distributed in proportion to blood flow at the time of injection which means that if the tracer is administered during peak stress, images taken later represent myocardial perfusion during peak stress. With tetrofosmin, a further injection is given at rest on a different day to image myocardial perfusion at test. Thallium uptake redistributes slowly and images taken at 3-4 hours after injection at stress are similar to those that would be achieved with a resting injection.

The presence of myocardial ischaemia is characterised by an area, or areas, of reduced perfusion on the stress images which return to normal perfusion on the resting images. Infarcted tissue is characterised by an area, or areas, of reduced perfusion on the stress images which does not return to normal on the resting images.

In cases of suspected myocardial infarction, a resting only study may be performed when it is desirable not to stress the patient. Whilst this technique cannot differentiate between ischaemic and infarcted areas, a normal distribution does exclude a myocardial infarction.

If myocardial hibernation is suspected a stress tetrofosmin study may be combined with a rest/redistribution thallium study. See separate referral guideline REF020 - Myocardial Perfusion Scan for Hibernating Myocardium (Thallium Rest/Redistribution)