Cardiovascular

Introduction

Nuclear medicine studies of the cardiovascular system have become so important to cardiologist that it is recognized as a subspeciality: Nuclear Cardiology. Nuclear cardiology has proven to be very versatile, not only is it capable of detecting cardiac abnormalities but also the functional consequences of disease. Nuclear cardiology encompasses the evaluation of coronary artery perfusion by myocardial perfusion imaging, the assessment of ventricular pump equilibrium and the diagnosis and evaluation of acute myocardial infarction. This section will focus primarily on myocardial perfsuion imaging.

Myocardial Perfusion Imaging (MPI)

MPI is an important tool for managing patients with cornary artery disease (CAD) and those at risk of developing CAD. The main purpose of MPI is the determine the adequacy of blood flow to the myocardium under resting conditions and during stress. Thallium-201 chloride, tc99m sestamibi and tc99m tetrofosmin are the radiopharmaceuticals employed for MPI.

Perfusion abnormalities in the myocardium are caused by stenoses (A stenosis is the narrowing of a blood vessel). The perfusion abnormalites caused by stenoses show up as photon deficient defects on the screen but even some severe stenoses may not produce detectable defects because under resting conditions the flow rate is not great enough the produce a significant difference in flow below the stenosis (Stenoses of up to 90% may not produce detectable defects). To improve detection efficiency the flow rate must be increased to cause a greater difference in flow. This is achieved by some form of physical exercise or by pharmalogical intervention. Under stress, stenoses of 50% or more are detectable . By comparing rest and stress images, areas of reduced coronary reserve cause by stenoses and stress induced ischemia can be identified.

Imaging

As mentioned above there are three choices of radiopharmaceuticals for MPI. A quick look at how each is distributed and behaves in the human body will help in understanding how images are interpreted.

Potassium is a key element in cardiovascular function, so the obvious direction is to use a potassium analog for imaging. This was initially attmepted but none of the analogs used were suitable. The next direction then is to use an element which is close to being a potassium analog. Thallium fits this description, it is not a true potassium analog but it behaves in its organ and tissue distribution much like potassium. It is cleared rapidly from the blood after intravenous injection with peak uptake in the myocardium 10 to 20 minutes after injection. The initial uptake period is followed by a process called redistribution. Redistribution is defined as the simultaneous washout of thallium from myocardial tissue and re-extraction from the systemic blood pool. Several hours (~4 hours) after the initial administration an equilibrium in the distribution of extra- and intracellular thallium is achieved. Stress imaging is performed first to take advantage of peak uptake. Rest images are taken 4 hours later during the redistribution phase. Defects on stress images which "disappear" on redistribution indicate some form of reversible stress induced ischemia. Defects on both sets indicate scarred or stunned myocardial tissue.

Sestamibi and terofosmin do not belong to the same compound group but because they are both technetium based and for the sake of simplicity this section will apply to both. Because of its low energy x-rays and its half-life (73 hours) thallium is not ideally suited for scintigraphy. For this reason technetium based radiopharmaceuticals were sought because of their superior imaging characteristics and short half-life. Unlike thallium, neither sestamibi nor tetrofosmin undergoes redistribution and as a consequence imaging protocols and interpretation are different. A clearance half-time exceeding 5 hours permits of time window of several hours after administration to perform imaging but because of the absence of redistribution a second dose of higher activity is required for the second set of imaging. A higher activity ensures that the majority of photons detected are from the second dose.
The wall motion of the heart can have a degrading effect on images, by affecting contrast and creating artifacts. To counteract this an ECG input is used by the computer to synch image acquisition with the heart beat. This is called a gated acquisition. Gated acquisition allows the creation of animations of wall motion and calculation of ejection fractions. Gated acquistion can only be achieved with technetium.

Clinical Indications and Applications

CAD

For any patient with or at risk of developing CAD there are many questions about the extent or severity of the disease that the cardiologist needs to answer that could potentially impact the method of treatment.

Diagnosing CAD
MPI has been shown to have both a high sensitivity and specificity for the detection of CAD. MPI is comparable to exercise electrocardiography (ECG) in specificity but has a significantly higher sensitivity. This is in part due to the stress/rest technique. MPI though is not intended to replace ECG as the screening exam for CAD. In cases where ECG may be non-diagnostic or a false positive or negative result is suspected, MPI can be used to improve the diagnosis or help clear up any uncertainties or suspicions.

Risk Stratification
MPI can be used as a prognostic tool in assessing the risk of a future cardiac event in patients already diagnosed with CAD. Statistics show only a 1% occurence of a cardiac event in the first year following a normal scan. Risk stratification post myocardial infarction to determine whether further treatment is necessary or if the patient can be discharged is also well established. This also shows favourable results with only a 6% occurence of cardiac events in the low risk group.

Preoperative Risk Assessment

Resting ECG is used as a preoperative indicator of the likelihood of a perioperative cardiac event. With a normal result, surgery is proceded with as scheduled. An abnormal or non-diagnostic result requires further investigation or cardiac intervention before surgery. MPI has become the next step in further investigation.

Viability

Invasive revascularization techniques can restore perfusion and restore cardiac function but will only be successful if the affected myocardium is viable. Poor perfusion after a cardiac event or as a result of blocked arteries can lead to parts of the myocardium becoming permanently scarred or going into a state of hibernation. Scarred myocardium cannot be repaired with revascularization techniques but hibernating myocardium can be repaired. If revascularization is being considered to treat a patient, determining whether the myocardium is scarred or hibernating can save time, money and possibly the patient's life. For myocardium to be considered viable it must exhibit preserved cell membrane function (i.e redistribution of chemicals over time). The standard four hour rest stress exam is sometimes not enough time to make a definitive diagnosis and requires additional imaging at 12 to 24 hours. Because the redistribution phenomenon only occurs when thallium is used, technetium based pharmaceuticals are not ideal when viability is to be determined.

Indications for MPI are categorized as Diagnositic, Prognostic and Therapeutic. Patients suspected of having CAD because of typical cardiac symptoms or patients presented with acute symptoms where coronary syndrome needs to be ruled out, fall under the category of diagnostic. Patients with known CAD at risk of future cardiac events come under prognostic indications and for patients who have undergone therapy MPI is indicated to determine the adequacy of the treatment.

Diagnostic Indications

ECG treadmill testing (ETT) has been and still is used as the initial test for diagnosis of CAD, however it has been shown that ETT may yeild frequent false postive results, particularly in women, and false negative results in those at significant risk for CAD. Because MPI is superior to ETT, many physicians have adopted MPI as their starting point for diagnosing CAD. The higher cost of MPI may detract some physicians but the option should be presented to the patient. Diagnostic indications for MPI are:

To test patients who are at an increased risk of developing CAD.

The initial test for females with suspected CAD because of the reduced accuracy of ETT.

Diabetic patients, with or without coronary symptoms.

Patients with abnormal baseline ECG, causing ETT to be non-diagnostic.

Patients with abnormal ETT without anginal symptoms.

Patients with an intermediate Duke treadmill score.

To determine the functional significance of a blocked artery seen in an angiogram.

Asymptomatic patients who work in fields where others are at risk if they suffer a cardiac event on the job (driver, pilot, machine operator).

Patients unable to perform ETT adequately because of inability to exercise.

To differentiate between CAD and nonischemic etiology.

To determine whether CAD is the etiology responsbile for abnormal ventricular wall motion seen on another imaging modality.

To differentiate between coronary and non-coronary causes of chest discomfort.

To evaluate the presence of obstructive CAD in cardaic transplant patients.

To assess the risk of a patient, scheduled for non-cardiac surgery, suffering a perioperative cardiac event.

Prognostic Indications

To define the extent or severity of CAD.

To determine the extent of ischemia in patients with abnormal ETT to guide future treatment.

To assess viabilty, LV function and define the presence of post myocardial infarction (MI) ischemia in patients with CAD or recent MI.

To determine the adequacy of medical therapy for acute coronary syndromes As a repeat study to redefine the risk in patients with high probabilty of CAD.

To check for the presence of viable myocardial tissue and the suitability of revascularization.

Therapeutic Indications

To assess the efficacy of medical therapy.

To evaluate a patient after revascularization.