Single-Photon Emission Computed Tomography, or SPECT, is a specialized nuclear medicine imaging procedure. Unlike standard X-rays or CT scans, which primarily visualize anatomical structures, SPECT provides three-dimensional images that map out organ function. This technique reveals how well blood is flowing to tissues and the metabolic activity occurring within them. By measuring these physiological processes, SPECT helps physicians diagnose conditions affecting the heart, brain, and bones long before structural changes become apparent.
How SPECT Imaging Works
The process begins with the introduction of a radiotracer, a radioactive pharmaceutical typically injected into a vein. This tracer is designed to travel through the bloodstream and accumulate in the targeted organ or tissue. As the radiotracer decays within the body, it releases gamma rays, which are single photons of energy.
A specialized device called a gamma camera, or SPECT scanner, detects these emitted photons. This camera system consists of detectors that rotate in a circular or elliptical path around the patient. As the camera rotates, it captures a series of two-dimensional images from hundreds of different angles around the area of interest.
The raw data collected by the gamma camera is sent to a computer system for processing. The computer applies complex mathematical reconstruction algorithms, similar to those used in CT scanning, to combine the multiple two-dimensional projections. This reconstructs a detailed, three-dimensional tomographic image that displays a map of the tracer concentration, reflecting functional activity like blood flow or cellular metabolism.
Key Clinical Uses of SPECT Scans
SPECT scans are frequently used in cardiology to assess blood flow to the heart muscle, a procedure known as myocardial perfusion imaging. A radiotracer is injected both while the patient is at rest and again during stress, allowing physicians to identify areas of the heart that are not receiving adequate blood supply. This comparison is invaluable for diagnosing coronary artery disease and evaluating damage following a heart attack.
In neurological practice, SPECT provides insight into brain function by mapping cerebral blood flow. Specialized tracers, such as those used in a DaTscan, can specifically target and visualize the dopamine transporters in the brain. This capability is useful for distinguishing Parkinson’s disease from other movement disorders or for localizing seizure foci in patients with epilepsy.
The technology also plays an important role in identifying issues within the skeletal system, often referred to as a bone scan. By detecting areas of increased bone turnover, SPECT can pinpoint stress fractures that may not be visible on standard X-rays. This functional imaging is effective at detecting the early spread of cancer (metastases) or identifying sites of infection or inflammation in the bone.
Preparing for and Undergoing a SPECT Scan
Preparation for a SPECT scan varies depending on the specific organ being imaged. Patients are generally instructed to wear comfortable clothing and remove all metallic objects. Some procedures may require fasting for several hours before the appointment to ensure the radiotracer is absorbed correctly by the target tissue. Reviewing the patient’s medication list is also common practice, as some drugs might interfere with the tracer’s distribution.
The procedure begins with the injection of the radiotracer into a vein. Following the injection, there is a waiting period, which can range from minutes to several hours, allowing the tracer time to circulate and accumulate in the target organ. Once the tracer has localized, the patient is positioned on a narrow table that slides into the SPECT machine.
During the scan, the gamma camera slowly rotates around the patient, taking pictures for approximately 20 to 45 minutes. It is important for the patient to remain still during this time to prevent image blurring. The radiation exposure from the radiotracer is low, and the substance naturally leaves the body over a short period.
SPECT vs. PET
Both SPECT and Positron Emission Tomography (PET) are functional nuclear medicine scans, but they differ fundamentally in the type of radioactive tracer they employ. SPECT tracers emit a single gamma ray photon as they decay, which the rotating gamma camera directly detects. Conversely, PET tracers emit a positron that collides with an electron, resulting in the simultaneous emission of two photons traveling in opposite directions.
This difference in detection mechanism means that PET imaging offers a higher spatial resolution, typically around 5 to 7 millimeters, compared to SPECT’s resolution of about 10 to 20 millimeters. PET’s superior resolution makes it effective for oncology applications, where detecting small lesions is crucial. However, SPECT scanners are more widely available across hospitals and imaging centers, and the procedures are generally less expensive than PET scans.
While both modalities provide functional information, SPECT is often preferred for specific applications like routine cardiac perfusion studies due to the longer half-life of its tracers. PET excels at measuring precise metabolic processes, often using the glucose analog fluorodeoxyglucose (FDG) to map cellular energy consumption. The choice between the two is determined by the specific clinical question and the availability of the appropriate radiotracer.