What Is Single Photon Emission Computed Tomography?

Single photon emission computed tomography (SPECT) is a nuclear medicine imaging technique that produces three-dimensional maps of radiotracer distribution within the body by detecting gamma photons emitted from radiopharmaceuticals administered to the patient. Unlike X-ray CT, in which the radiation source is external, SPECT places the radiation source inside the body in the form of a tracer that accumulates preferentially in specific tissues based on its biochemical properties. The technique provides functional and physiological information, such as blood perfusion, receptor binding, and metabolic activity, that anatomical imaging modalities alone cannot supply. SPECT draws on nuclear medicine, radiation physics, signal processing, and tomographic reconstruction mathematics, and has been a standard clinical imaging tool since the 1980s.

The modality is closely related to positron emission tomography (PET) but uses radionuclides that emit single gamma photons rather than positron-annihilation photon pairs, allowing a broader selection of radioisotopes and less expensive gamma camera detector systems.

Image Acquisition and Radiotracers

SPECT acquisition uses a gamma camera equipped with one or more scintillation detectors that rotate around the patient, collecting two-dimensional projection images at angular intervals over a 180-degree or 360-degree arc. The spatial resolution of the projections depends on collimator geometry, detector crystal thickness, and the energy of the emitted photons. The most widely used radiotracer is technetium-99m, which emits 140-keV gamma photons, has a six-hour physical half-life suited to clinical logistics, and can be labeled to a wide variety of pharmaceuticals that target the heart, brain, kidneys, bone, and thyroid.

As described in the NCBI Bookshelf chapter on SPECT mathematics and physics, the fundamental imaging equation relates the measured projections to the three-dimensional activity distribution through the Radon transform, the same mathematical framework underlying X-ray CT reconstruction.

Image Reconstruction

Reconstruction of the three-dimensional activity map from the set of planar projections is the central computational task in SPECT. Filtered back-projection (FBP), a direct analytical method applying a ramp filter in frequency space before back-projecting the data into image space, was the dominant clinical algorithm for decades. Iterative methods, particularly the ordered-subset expectation maximization (OSEM) algorithm, have largely replaced FBP in modern systems because they incorporate physical models of photon attenuation, detector response, and scatter more naturally, producing images with improved contrast and reduced noise artifacts at equivalent acquisition times.

A clinical review of SPECT imaging from NCBI StatPearls outlines how modern SPECT/CT hybrid scanners combine functional SPECT data with anatomical CT for precise attenuation correction and anatomical localization of tracer uptake, substantially improving diagnostic accuracy for oncological and cardiac studies.

Clinical Imaging

SPECT's ability to image perfusion and receptor density has established it in several clinical diagnostic categories. Myocardial perfusion imaging uses Tc-99m sestamibi or tetrofosmin to map regional blood flow in the heart at rest and under pharmacological or exercise stress, identifying coronary artery disease with high sensitivity. Brain SPECT in neurotherapeutics research published via PMC documents applications in epilepsy localization, dementia differential diagnosis, and psychiatric conditions, where regional cerebral blood flow patterns correlate with underlying neuropathology.

Bone scintigraphy with Tc-99m methylene diphosphonate remains one of the most sensitive whole-body surveys for osseous metastases, and sentinel lymph node mapping with radiolabeled colloids guides surgical planning in breast cancer and melanoma.

Applications

Single photon emission computed tomography has applications across a range of medical and research disciplines, including:

• Cardiology, for myocardial perfusion imaging and viability assessment in coronary artery disease
• Oncology and cancer staging, including detection of bone metastases and sentinel node mapping
• Neurology, for regional cerebral perfusion imaging in epilepsy, stroke, and neurodegenerative disease
• Endocrinology, for thyroid function assessment and parathyroid adenoma localization
• Pharmaceutical research, for in vivo receptor occupancy studies during drug development