Nuclear Stress Test: What is it?
A nuclear stress test is a minimally invasive diagnostic imaging procedure designed to evaluate the perfusion of blood through the coronary arteries to the heart muscle. Images of the heart are acquired when the heart is in a baseline resting state and again after the heart has been exercised, or stressed. Images are taken using specialized cameras (gamma or SPECT cameras) and radioactive tracers. Patients can be “stressed” by physical exercise, pharmacologically, or a combination of both. Other names for a nuclear stress test include cardiac stress test, myocardial perfusion imaging, cardiac SPECT scan, and thallium stress test.
Nuclear Stress Test: Who needs one?
Nuclear stress tests are ordered by cardiologists and other types of physicians for patients that may be at risk for Coronary Artery Disease (CAD), a condition in which the arteries that supply blood to the heart muscle can become blocked, which could lead to a myocardial infarction, more commonly known as a heart attack. Risk factors such as diabetes, family history of CAD, hyperlipidemia (high cholesterol), hypertension (high blood pressure), and more can lead to a higher chance of CAD. Risk factors along with symptoms, such as angina (chest pain), dyspnea (shortness of breath), decreased exercise tolerance, among others can indicate that a patient may currently have CAD.
Nuclear stress tests can be used to diagnose the potential presence of coronary artery disease. Nuclear stress tests are usually performed in an outpatient setting, whether in a diagnostic imaging clinic, a doctor’s office, or a hospital radiology/nuclear medicine department.
Nuclear Stress Test: How does it work?
The heart pumps blood to every part of the body, and it too needs blood to work, just like any other muscle. When the heart muscle is exercised, it requires more blood to work harder. Sometimes, due to blockage within one or more coronary arteries, blood flow can be partially impeded or fully blocked from reaching the heart muscle tissue. Inadequate blood supply to the heart is called ischemia. Ischemia can cause chest pain, shortness of breath, and decreased exercise tolerance in patients.
Images of the blood supply to the heart can be acquired by using an injection of a radioactive tracer and viewing the tracer with a specialized radioisotope detecting camera. This is accomplished by establishing an intravenous catheter, usually called an IV, in a vein in the patient’s arm. A radioactive tracer, also referred to as a radiopharmaceutical or radioisotope, is a small amount of radioactive material bound to a molecule that can enter the heart muscle cells (cardiac myocytes). The radioactive tracer itself does not cause the patient to feel any differently, and the radiation exposure received from the tracer is low, similar to many other diagnostic imaging procedures. The most common radioisotope used is Technetium-99m (Tc-99m). For nuclear stress tests, Tc-99m is most commonly bound to molecules called sestamibi or tetrofosmin. Both of these molecules have an affinity to attach to the cells of the myocardium (heart muscle).
Nuclear stress tests can be performed in many different ways, and each method produces equivalent results. With the Rest/Stress procedure, images of the patient’s heart are acquired first at rest and then at stress. The Stress/Rest protocol is the reverse, imaging first under stress conditions, then second under rest conditions. It is becoming more common for some patients, especially new patients being seen for the first time, to be assessed with the Stress Only protocol; the patient is stressed, images are acquired, then reviewed by a cardiologist or radiologist. If an abnormality is seen, the patient is then imaged under rest conditions for comparison. If the Stress Only image is normal and the reading physician is confident that the patient is at low risk, no resting image is needed. However, the standard of most clinics is to acquire both rest and stress images, so having to be imaged twice is no cause for concern.
The outline for the Rest/Stress protocol is as follows:
- The patient arrives at the clinic fully prepared for the test as per the clinic’s instructions.
- A nuclear medicine technologist or other medical professional inserts an IV into the patient’s arm.
- A nuclear medicine technologist injects the radioactive tracer through the IV.
- The patient waits a certain amount of time (usually 30 to 60 minutes) to allow the tracer to distribute into the heart muscle.
- The patient is placed on the camera and images are acquired, taking anywhere from 2 minutes to 20 minutes depending on the type of camera.
- The patient is connected to an electrocardiogram (ECG) so the heart rhythm can be monitored by a physician or other medical professional during the stress test.
- If the patient is able to physically exercise, a treadmill or stationary bicycle is used to elevate the patient’s heart rate. If not, the patient will receive a pharmacologic stress medicine to chemically induce the exercised (stressed) state.
- At peak stress, a second radioactive tracer is injected to acquire the second set of images.
- The patient is imaged again, usually within 15 to 45 minutes after the stress test.
- A nuclear medicine technologist and/or physician preliminarily reviews the images for quality and technical problems.
- The patient is then usually sent home.
- A radiologist or cardiologist reads the images and creates a report detailing any abnormalities.
- If the images are abnormal and indicate ischemia, the patient may be referred to further testing or scheduled for an arteriogram.
Many different types of gamma cameras can perform nuclear stress test imaging. The cameras come in all shapes and sizes and have different benefits in terms of image quality, image acquisition time, and patient comfort. The most common gamma (SPECT) cameras will be similar to the following:
A nuclear stress test A three-dimensional image of the heart is acquired during a nuclear stress test. This allows the heart to be “sliced” in three different perspectives:
- Short Axis (top to bottom)
- Vertical Long Axis (side to side)
- Horizontal Long Axis (front to back)
The nuclear medicine technologist processes the images using specialized software. This software creates the slices of the heart, cleans up noise and artifacts, and applies a color filter for easier viewing. A normal nuclear stress test image is shown below.
A nuclear stress test can be thought of as a comparative test, i.e. two sets of images that are compared side by side for differences. In many cases, CAD and ischemia present themselves upon exertion of the patient, causing symptoms like chest pain and shortness of breath. Symptoms from ischemia are caused by the heart not receiving the blood it requires to function.
Normal versus Abnormal:
To demonstrate what a normal nuclear stress test looks like compared to an abnormal, let’s jump right into some images.
The picture above contains both Stress and Rest images. The Stress slices start at the very top and alternate with the Rest slices, which are displayed on every other row. It is clear that there is virtually no difference in the blood flow, and thus, the tracer distribution between the stress and rest images.
This image is in the same format as the normal example above, but the differences are clear. There is a large portion of the heart that did not receive the radiotracer due to blockage in one or more coronary arteries. Therefore, this portion appears “dark” on the image.
Nuclear Stress Test: The Results
After completion of the test, a cardiologist or radiologist will review the images. The specialized software quantifies and labels areas of concern, and the physician will generate a report that discusses:
- The number of defects
- It is possible to have multiple arteries with blockages at one time, which creates multiple image defects in the heart.
- The size of the defects
- Some arteries affect larger areas than others.
- The severity of the defects
- The degree to which an artery is blocked affects the appearance of the defect.
- Defects caused by a partial blockage can appear much differently than defects caused by a total blockage.
- Reversibility of the defects
- If a defect is purely caused by a temporary lack of blood flow, it is said to be reversible. This type of defect can usually be corrected by restoring blood flow through the artery by inserting a stent or performing angioplasty.
- If a defect is caused by a permanent lack of blood flow (also known as an infarct), it will appear the same on both rest and stress images and is said to not be “reversible.”
Nuclear stress tests provide a minimally-invasive view of the blood flow of the heart. They can be used as both a first-look diagnostic test in patients who are at risk of CAD and as a routine test in patients with chronic CAD to monitor the progression of their disease. This article was written by Jay Roberson CNMT.