Background

Myocardial Perfusion Imaging (MPI) is a popular non-invasive diagnostic tool for individuals with coronary artery disease (CAD). It helps to estimate myocardial blood flow, detect ischemia and evaluate myocardial viability. The technique’s capacity to incorporate functional and anatomical insights into heart health has rendered it crucial in clinical cardiology.

In 1973, H. William Strauss introduced the exercise stress-test myocardial scan, and Elliot Lebowitz used thallium-201 myocardial perfusion imaging for the first time in the clinic. These breakthroughs marked the beginning of nuclear imaging in cardiology, allowing for the measurement of myocardial perfusion in a variety of physiological conditions. Significant advancements in the discipline include the creation of single-photon emission computed tomography (SPECT) imaging, pharmacologic stress testing and electrocardiogram (ECG)-gated imaging. ECG-gated SPECT improved diagnostic capabilities by allowing for the simultaneous measurement of global left ventricular ejection fraction (LVEF), regional wall motion and wall thickness under stress or rest.

MPI SPECT is a diagnostic imaging technique that uses a gamma-emitting radioactive tracer to assess the viability and perfusion status of the myocardium. Technological advancements have improved diagnostic accuracy and image quality. Advanced reconstruction algorithms, such as iterative reconstruction, enhance the quality of images. CZT detectors a semiconductor that converts gamma radiation into electrical signals, offer higher detection sensitivity and spatial resolution while reducing scanning time and radiation exposure. Hybrid SPECT/CT imaging integrates SPECT with computed tomography (SPECT/CT) enabling attenuation correction and improving diagnostic precision. These systems also reduce scanning time and enhance the ability to co-localize anatomical and functional abnormalities.

Analysis:

Evaluate raw pictures or rebuilt highest intensity projected image in cine mode as well as the linogram and sinogram image to identify probable reasons for image distortion and extracardiac tracer activities distributions.

Interpret pictures based on size, location, degree of severity, and reversible change of perfusion abnormalities, cardiac chamber size, and absence or presence of enhanced pulmonary uptake (particularly 201Tl).

Assess the findings of the quantitative perfusion testing.

Assess functional data derived from gated photos.

Consider stress ECG, clinical data and the hemodynamic data since these may impact the study’s ultimate interpretation.

Indications

In patient who have stable and acute chest pain, coronary artery disease, it is used. It is more effective than ECG in diagnosing myocardial ischemia. Specific indication like determining the hemodynamic importance of aberrant coronary arteries, coronary aneurisms and muscle bridging in the presence of the Kawasaki disease. It is possible to identify posterior and inferior anomalies, tiny patches of infarction, blocked blood arteries and an accumulation of viable and infarcted myocardium. Stress pictures in SPECT can help to evaluate the level of viable myocardium or inducible ischemia susceptible to revascularization after a heart attack.

Contraindications

Absolute contraindication to dynamic stress:

Acute Myocardial Infarction (in 2 to 48 hours): Risk of exacerbating ischemia or arrhythmias.

Unstable Angina (not stabilized with treatment): It may lead to ischemia or acute coronary events.

Severe Aortic Stenosis: It can be increased risk of syncope or sudden cardiac death due to limited cardiac output.

Uncontrolled Arrhythmias: It can lead to hemodynamic instability.

Symptomatic Heart Failure: Stress may worsen heart failure symptoms.

Acute Aortic Dissection: Exercise increases wall stress and risking rupture.

Acute Pulmonary Embolism or Deep Vein Thrombosis: Risk of exacerbating pulmonary or systemic complications.

Relative contraindications to dynamic stress:

Severe Hypertension (SBP > 200 mmHg or DBP >110 mmHg): Risk of adverse cardiovascular events.

Physical Limitations: Orthopedic, neurological or musculoskeletal conditions preventing adequate exercise.

High-Degree AV Block or Bradycardia: It reduced ability to increase cardiac output.

Tachyarrhythmias: It increased risk of worsening arrhythmias.

Electrolyte Abnormalities (hypokalemia or hypomagnesemia): It may predispose to arrhythmias during stress.

Absolute contraindication to vasodilator stress:

Severe Bronchospasm or Asthma: Vasodilators can provoke significant bronchoconstriction.

Severe Hypotension (< 90 mmHg): It may worsen hemodynamic instability.

High-Degree AV Block or Sinus Node Dysfunction (without a pacemaker): It can exacerbate bradycardia or heart block.

Recent Use of Xanthine Derivatives (theophylline or caffeine): These substances antagonize vasodilator effects.

Relative contraindication to vasodilator stress:

Unstable Angina or Recent Myocardial Infarction (within 2 to 4 days): It increased risk of adverse cardiac events.

Severe Aortic Stenosis or Obstructive Cardiomyopathy: Vasodilation may worsen outflow obstruction.

Severe Autonomic Dysfunction: It may lead to unpredictable blood pressure responses.

Significant Bradycardia (<40 bpm): Risk of worsening bradycardia.

Pregnancy: Avoid radiation exposure when possible as vasodilator use has not been extensively studied in pregnancy.

Absolute contraindication to dobutamine stress:

Acute Myocardial Infarction or Unstable Angina: Risk of inducing ischemia, arrhythmias, or worsening infarction.

Severe Aortic Stenosis: Increased risk of syncope or sudden cardiac death.

Uncontrolled Hypertension (>180/110 mmHg): Increases the risk of adverse cardiovascular events.

Severe Arrhythmias (e.g., ventricular tachycardia): Dobutamine’s inotropic and chronotropic effects can exacerbate arrhythmias.

HOCM with Obstruction: Dobutamine can exacerbate outflow obstruction.

Aortic Dissection: It increased heart rate and contractility elevate wall stress.

Relative contraindication to dobutamine stress:

Recent Stroke or Transient Ischemic Attack: Dobutamine-induced hypertension or arrhythmias could lead to neurologic deficits.

Tachyarrhythmias or Pre-Existing Atrial Fibrillation: It may worsen due to dobutamine’s chronotropic effects.

Severe Anxiety or panic disorders: Patients may experience heightened symptoms during the test.

Electrolyte Imbalances (hypokalemia): Predispose to arrhythmias under stress.

Outcomes

Equipment

This kind of diagnostic test uses a radioisotope. Technetium-99m-2-methoxy-isobutyl-isonitrile (99mTc MIBI), thallium-201 thallous chloride (thallium) and 99mTc 1,2-bis [bis(2-ethoxyethyl) phosphino] ethane (tetrofosmin) are the most widely utilized tracers. Commercially accessible are thallium-201 and the 2 technetium-labeled radiopharmaceuticals (tetrofosmin and MIBI).  The preferred treatment for people who are unable to exercise enough or who have paced rhythm or LBBB is a pharmacologic stress with vasodilators. Dipyridamole and adenosine are the two most often utilized vasodilators. When dynamic exercise is not practical or there are known risks to vasodilator stress, dobutamine infusion is frequently employed.

Medication:

Radiopharmaceuticals agents:

• Thallium-201
• Technetium-99 Labeled Radiotracers

Vasodilator agents:

• Adenosine (Adenoscan)
• Dipyridamole

Inotropic Agents

• Dobutamine

Patient preparation

Dynamic exercise:

The patient must stop taking the medications which can interfere with physiological exercise. Calcium channel and beta antagonists must be stopped for 5 half life. Patient must avoid beverages, caffeine containing food and medications before the 12 hours. Patient is instructed to dress properly. Fasting is not necessary.

Pharmacologic stress:

For at least 12 hours before to the test and ideally for 24 hours, patients under the effect of the vasodilator dipyridamole or adenosine (vasodilator stress) must refrain from caffeine-containing meals, drinks and medications. Stop taking theophylline and aminophylline for 24 hours before to the test. Dipyridamole users should stop taking the medication at least 24 hours before to vasodilator stress. Unless otherwise specified, beta antagonists must be stopped for 5 half-life or minimum 24 hours before to the test in patients under dobutamine stress.

No anesthesia is necessary.

Patient position

Patient must be supine with arms are above the head.

Since the patient’s comfort is crucial to minimizing motion, knee support can be helpful. Although prone imaging might result in anterior artifacts and is not advised in isolation, it has been used in certain locations to lower the prevalence of inferior attenuation artifact.

Technique

Dynamic stress test technique

Step 1: Pre-Test Preparation

• Confirm patient’s contraindications and obtain informed consent. Ensure test conducted by trained healthcare professionals including cardiovascular stress specialists.
• Ensure access to advanced life support personnel.
• Secure and flush intravenous line for patency confirmation.
• Record heart rate, blood pressure and 12-lead ECG for baseline abnormalities detection.

Step 2: Exercise Protocol

• Use treadmill or bicycle ergometer based on patient’s physical condition and clinical setting.
• Continuously observe heart rate, blood pressure and ECG to detect ischemic changes and arrhythmias.
• Monitor patient symptoms like chest pain, dyspnea or dizziness.
• Target heart rate to 85% of patient’s age-predicted maximum heart rate.
• Terminate test if significant symptoms or ECG changes occur.

Step 3: Tracer Injection

• Inject at peak exercise or before target heart rate.
• Encourage further exercise for 1 to 2 minutes post-injection.

Step 4: Recovery Phase

• Gradual decrease in exercise intensity to prevent abrupt hemodynamic changes.
• Monitoring heart rate, blood pressure and ECG during recovery. Continue patient observation until vital signs stabilize and symptoms resolve.

Step 5: Imaging

• Perform SPECT imaging twice: after radiotracer injection at rest and stress test.
• Both phases essential for detecting reversible ischemia.

Pharmacological Stress Testing Technique

Step 1: Pre-Test Preparation

• Ensure dietary and medication restrictions are followed especially avoiding caffeine for 12 to 24 hours.
• Place the patient supine and establish IV access.

Step 2: Baseline Assessment

• Record baseline heart rate, blood pressure, and ECG.

Step 3: Pharmacologic Agent Administration

• Administer a vasodilator (adenosine, dipyridamole) or an inotropic agent (dobutamine) according to standard dosing protocols:

Adenosine: 140 µg/kg/min IV infusion over 4 to 6 minutes.

Dipyridamole: 0.142 mg/kg IV infusion over 4 minutes.

Dobutamine: Begin at 5 µg/kg/min and increase every 3 minutes to 20, 30 and 40 µg/kg/min, if necessary.

Step 4: Monitor for Side Effects

• Observe for symptoms (flushing, chest discomfort or dyspnea) and manage adverse effects as needed (aminophylline for vasodilators or beta-blockers for dobutamine).

Step 5: Tracer Injection

• Administer the radiotracer thallium-201 approximately 2 minutes after starting the pharmacologic agent or at peak effect.

Step 6: Recovery Phase

• Monitor ECG, blood pressure and symptoms until baseline conditions are restored.

Image acquisition:

Step 1: Patient Preparation

• Position the patient supine with arms raised above the head to minimize attenuation artifacts.
• Ensure the patient remains still throughout the scan to avoid motion artifacts.

Step 2: Radiotracer Administration

• Administer the radiotracer (technetium-99m agents or thallium-201).
• Allow time for myocardial uptake of the tracer.

Step 3: Gamma Camera Setup

• Use a gamma camera to acquire multiple 2D projections at defined intervals (typically every 3 to 6°).
• For optimal tomographic reconstruction, perform a full 360° rotation around the patient.
• Multiheaded cameras can reduce acquisition time by simultaneously capturing projections from multiple angles.

Step 4: Projection Acquisition

• Acquire each projection over 15 to 20 seconds, with a total scan time of 15 to 20 minutes for single-head systems.
• Dual-headed systems require only 180° rotation and triple-headed systems require 120° reducing imaging time to under 10 minutes.

Step 5: Reconstruction of 3D Images

• Use tomographic reconstruction algorithms (iterative reconstruction) to convert 2D projections into a 3D dataset.
• Adjust slices along desired axes for analysis of myocardial perfusion.

Step 6: Artifact Correction

• Use attenuation correction maps from integrated CT images to reduce artifacts from gamma ray attenuation in deep tissues.
• Compensate for patient movement using motion correction algorithms if needed.

Step 7: Stress and Rest Imaging Protocol

• If both rest and stress imaging are required, perform them on separate days due to radionuclide redistribution.
• Stress-first imaging may eliminate the need for rest imaging if stress results are normal.

Complications:

Chest pain or angina

Arrhythmias like premature ventricular contractions or atrial fibrillation

Rarely, myocardial infarction

Fatigue, dizziness, or syncope

Musculoskeletal pain or injury from physical exertion

References

https://www.ncbi.nlm.nih.gov/books/NBK539772/

https://pmc.ncbi.nlm.nih.gov/articles/PMC3221136/

References

https://www.ncbi.nlm.nih.gov/books/NBK539772/

https://pmc.ncbi.nlm.nih.gov/articles/PMC3221136/