Radionuclide Imaging and SPECT

Radionuclide Imaging

• Employs a gamma camera to image after injecting radioactive material.
• Evaluates cardiac valvular, congenital, and other cardiac disorders, cardiomyopathy, and coronary artery disease (CAD).
• Delivers similar radiation doses as CT scans.
• May trigger radiation alarms in patients for days post-testing due to retained radioactive material.

Single-Photon Emission Computed Tomography (SPECT)

• SPECT is more common in the United States than planar techniques, which produce a 2-dimensional image and are rarely used
• Uses a rotating camera and tomographic reconstruction to produce 3-dimensional images.
• Multihead SPECT systems complete imaging in ≤ 10 minutes.
• SPECT can identify inferior/posterior abnormalities, small infarctions, and vessels causing infarction.
• The mass of infarcted and viable myocardium can be quantified, aiding prognosis.

Myocardial Perfusion Imaging

• Intravenous radionuclides are absorbed by cardiac tissues in proportion to perfusion.
• Areas of reduced uptake indicate relative or absolute ischemia.
• Attenuation from overlying soft tissue may cause false positives and is common in women due to breast tissue.
• Diaphragm and abdominal contents can cause spurious inferior wall defects in both sexes, but more common in men.
• Attenuation is more likely with technetium-99m (99mTc) than with radioactive thallium 201 (Tl-201).
• Used with stress testing to evaluate chest pain, determine the significance of coronary artery stenosis and collateral vessels, assess reperfusion intervention success, and estimate post-myocardial infarction prognosis.

Protocols and Imaging Agents

• Protocols vary based on the imaging agent used.
• Agents include radioactive thallium-201 (Tl-201), technetium-99m (Tc-99m) markers, iodine-123 (I-123)-labeled fatty acids, and I-123 metaiodobenzylguanidine (MIBG).
• Tl-201, a potassium analog, was the first stress-testing tracer, injected at peak stress, imaged with SPECT, and reimaged after a reduced dose at rest, but usage has declined.
• Technetium-99m (Tc-99m) markers include sestamibi (commonly used), tetrofosmin, and teboroxime because the imaging of Tl-201 for the gamma camera is not good

Technetium-99m (Tc-99) Myocardial Perfusion Markers

• Tc-99m sestamibi uptake is slower than thallium but allows flexible timing and immediate imaging for acute symptoms; however, uptake depends on blood flow and low flow areas may be mistaken for scar; studies occur over single or multiple days, and with ECG-gated imaging, ventricular characteristics can be estimated.
• Tc-99m tetrofosmin characteristics are similar to sestamibi.
• Tc-99m teboroxime has high first-pass extraction and rapid washout, making treadmill exercise difficult; studies suggest stress-redistribution testing can be done quickly with pharmacologic stress, and coronary artery disease can be found without stress via myocardial washout analysis.
• Protocols include 2-day stress-rest, 1-day rest-stress, and 1-day stress-rest.
• Dual isotopes (Tl-201 and Tc-99m) are sometimes employed, with sensitivity around 90% and specificity about 71% for detecting coronary artery disease.
• Imaging at rest may be skipped in 2-day protocols if the initial stress test is normal.
• High Tc-99m doses facilitate first-transit function studies with ventriculography for perfusion imaging.
• Other radionuclides include iodine-123 (I-123)–labeled fatty acids, gallium citrate-67 (Ga-67), and I-123 metaiodobenzylguanidine used for research.

Infarct Avid Imaging

• This technique uses radiolabeled markers which accumulate in damaged myocardium.
• Markers include Tc-99m pyrophosphate and antimyosin.
• Images are positive 12-24 hours after acute myocardial infarction and remain so for about a week.
• Seldom utilized now due to the availability of other diagnostic tests for myocardial infarction and because it cannot give any information about the size of an infarct.
• Tc-99m pyrophosphate now assesses cardiac involvement in transthyretin amyloidosis.
• A high uptake ratio indicates transthyretin cardiac amyloidosis, potentially eliminating the need for myocardial biopsy.

Radionuclide Ventriculography

• Evaluates ventricular function, measuring resting/exercise ejection fraction in various heart conditions.
• Replaced by echocardiography due to cost and radiation; however, some doctors prefer this method for the seriel assessment of ventricular function.
• Tc-99m–labeled red blood cells are injected intravenously, and LV/RV function can be evaluated by:
  • First-transit studies occur from beat-to-beat
  • Gated (ECG-synchronized) blood pool imaging (MUGA) occurs over several minutes

First-Transit Studies vs. MUGA

• Either study can be done during rest or after exercise.
• First-transit studies are faster, but MUGA provides better images.
• First-transit studies image 8-10 cardiac cycles as the marker mixes with blood, ideal for assessing RV function/intracardiac shunts.
• MUGA synchronizes imaging with the ECG R wave, taking multiple images of short cardiac cycle sections over 5-10 minutes, synthesizing a continuous cinematic loop.
• MUGA quantifies ventricular function, including regional wall motion, ejection fraction (EF), volume ratios, and relative volume overload and EF is the most common.
• MUGA during rest has virtually no risk and is used to evaluate RV/LV function, monitor patients on cardiotoxic drugs, and assess CABG, angioplasty, and thrombolysis but arrhythmias are a contraindication.
• MUGA is useful for detecting left ventricular aneurysms.
  • Sensitivity and specificity are > 90% for typical anterior/anteroapical aneurysms, but conventional imaging shows inferoposterior LV aneurysms less clearly.
  • Gated SPECT imaging takes longer than planar views but shows ventricles better.

Right Ventriculography

• MUGA assesses right ventricular function in lung disorders or when inferior left ventricular infarcts involve the RV.
• RVEF is normally lower than LVEF.
• Low RVEF is common in pulmonary hypertension, RV infarction, or cardiomyopathy affecting the RV.
• Idiopathic cardiomyopathy usually causes biventricular dysfunction.

Valve Assessment

• MUGA with rest-stress protocols assesses valvular disorders causing left ventricular volume overload.
• Aortic regurgitation: reduced resting EF or no increase with exercise indicates deteriorating function and possible valvular repair.
• MUGA calculates the regurgitant fraction of any valve.
• LV stroke volume exceeds RV stroke volume in left-sided valvular regurgitation.

Shunt Assessment

• MUGA and computer programs quantify congenital shunt size via stroke volume ratio or the ratio of abnormal early pulmonary recirculation to total pulmonary radioactivity during the marker's first transit.