Single Photon Emission Computed Tomography

Updated : December 18, 2024

Single Photon Emission Computed Tomography (SPECT) is a highly adaptable imaging technology which integrates nuclear medicine ideas with modern tomographic imaging methods. It uses gamma-ray-emitting radiopharmaceuticals which are injected, inhaled or ingested as per its research objectives. These radiopharmaceuticals target specific tissues or organs and produce radiation which can be detected by a gamma camera spinning around the patient to provide cross-sectional pictures.

SPECT provides a 3-D picture of the movement of a radioactive tracer (also known as a probe) that is injected into the circulation and then absorbed by certain organs. This is carried out with specialist nuclear medicine camera. Thus, SPECT helps the clinicians to evaluate the perfusion and functioning of specific tissue. The capacity to measure tissue functioning and physiology distinguishes SPECT imaging from other anatomic imaging methods like computed tomography, magnetic resonance imaging (MRI), and roentgenography.

Myocardial perfusion testing using SPECT has a sensitivity of 82% and a specificity of 76% in identifying coronary artery disease. Furthermore, patients with normal myocardial SPECT findings had a yearly risk of unfavorable cardiac events of less than 1%. In brain imaging for Alzheimer’s dementia diagnosis, SPECT has a sensitivity of 92%, specificity of 100%, positive predictive value of 92%, and negative predictive value of 57%.

To evaluate the patient who has suspected dementia
To diagnose encephalitis
To localize the epileptic foci before the operation
To monitor and assess the vascular span because of the subarachnoid hemorrhage
To map the brain perfusion during the surgery
To detect and evaluate the cerebrovascular disease
To predict the prognosis of patient who have cerebrovascular disease

The American Society of Nuclear Cardiology has some indication of SPECT on cardiac disease like:

To evaluate the patient for coronary artery disease
To assess the treatment response and for the future treatment in diseases like coronary artery disease, heart failure, and cardiomyopathy
To diagnose coronary artery disease in patient who is not able to perform the standard exercise stress test
To evaluate the patient who have confirmed or suspected coronary artery disease before the surgery

Pregnancy

Allergy to radiotracer

Gamma camera with the rotating head

A collimator

A radio labeled probe for a particular tissue (Technetium-99m, iodine-123)

Patient preparation

Obtain detailed medical history including current medications, allergies, and previous imaging studies.
Assess for contraindications like claustrophobia and conditions that may impair patient’s stillness.
Review recent nuclear imaging procedures to avoid interference from residual radiopharmaceuticals.
Ensure patient understands the purpose of the radiotracer and any specific instructions related to its administration.
Encourage adequate hydration before and after the procedure.
Fasting for 4 to 6 hours may be required for certain studies.
Avoid fatty or high-protein meals if fasting is not required. Avoid caffeine and alcohol for 12 to 24 hours before the procedure.
Temporarily discontinue beta-blockers, nitrates, or calcium channel blockers for cardiac studies.
Avoid sedatives, stimulants, or other medications that may affect cerebral perfusion.
Wear loose, comfortable clothing without metallic elements. Remove all metallic objects before the procedure.

Patient position

Patients are usually supine on the imaging table, but in some cases, prone or seated positioning may be necessary due to target region or patient limitations.

The patient receives a radio-labeled probe chemical. For cerebral tests, patients are situated in a dimly lit room and instructed not to read or speak for at least 10 minutes.
Sedation is given following the tracer injection.
The wait time for tracer to circulate varies, ranging from 90 minutes for brain examinations to 15 minutes for cardiac stress testing.
The patient is placed in the detection device and administered cardiac stimulants if cardiac stress testing is necessary.
The detector spins around the subject, capturing planar images every 3 to 6 degrees.
SPECT has limits in detecting tiny metabolic abnormalities without comparable anatomical imaging.
A combination SPECT/computed tomography procedure is being developed to assess both functional and anatomical problems concurrently.
Careful patient selection for SPECT imaging, based on a clearly defined indication, minimizes unnecessary radiation exposure and reduces the likelihood of errors.
Technetium-99m-based SPECT protocols (e.g., sestamibi and tetrofosmin) result in lower radiation exposure compared to thallium-201 protocols (e.g., stress/redistribution or stress/reinjection). For evaluating chest pain and diagnosing ischemia, technetium-99m protocols are both safer and preferred.
To optimize the required radioactivity dose, radiotracer administration should be weight-based.
Advanced imaging technology, such as cadmium zinc telluride detectors, offers higher sensitivity to ionizing radiation along with improved energy and spatial resolution. Employing these detectors, along with Anger cameras, enhances radiation efficiency.
SPECT stress-only protocols using technetium-99m radiotracers can lower radiation exposure by approximately 25% compared to traditional rest/stress studies.
Stress-first imaging is recommended for patients with a low to moderate pretest probability of coronary artery disease, particularly younger patients, as it reduces overall exposure.
Two-day rest/stress technetium-99m protocols (14 mSv) can be optimized to stress-only (7 mSv) or single-day rest/stress protocols (10 mSv) to further minimize radiation.
Acquisition practices can also be adjusted to reduce the radiotracer dose. For cooperative patients, extending acquisition times allows for dose reduction.
Keeping the imaging camera as close to the patient as possible during acquisition helps lower the required dose.
Modern reconstruction algorithms enable image quality to be preserved even with reduced radiotracer doses. For example, wide-beam reconstruction algorithms allow for superior imaging with up to a 50% reduction in radiation exposure.
Additionally, when CT is used for attenuation correction, acquisition protocols should be optimized to use the lowest possible dose, such as using a rod-source approach.