How does PET differ from SPECT?

PET and SPECT are both tomographic nuclear medicine imaging techniques, but they differ fundamentally in how photons are detected and localised.

SPECT detects single gamma photons emitted directly from the radionuclide and uses mechanical collimation to determine their direction. PET detects pairs of 511 keV photons produced by positron–electron annihilation and uses electronic coincidence detection to determine the line along which the event occurred.

PET uses coincidence detection of annihilation photon pairs without mechanical collimation, whereas SPECT detects single photons using lead collimators, resulting in major differences in sensitivity and resolution.

Because PET does not rely on mechanical collimators, it has substantially higher sensitivity and generally better spatial resolution and quantitative accuracy than SPECT.

Understanding the physics

The difference begins at the level of radioactive decay.

In SPECT, radionuclides such as Tc-99m emit a single gamma photon. The gamma camera must use a lead collimator to determine photon direction. Most emitted photons are absorbed by the collimator, which limits sensitivity.

In PET, radionuclides such as F-18 emit a positron. After travelling a short distance in tissue, the positron annihilates with an electron, producing two 511 keV photons emitted approximately 180° apart. The PET detector ring registers these photons simultaneously. If two detectors detect photons within a very short timing window, the system assumes they originated from the same annihilation event.

This method, known as electronic collimation, determines that the event occurred somewhere along the line connecting the two detectors (the line of response). Because no physical collimator blocks photons, PET systems detect a much higher fraction of emitted radiation.

These differences lead to several practical consequences:

PET generally has higher sensitivity than SPECT.
PET has improved quantitative accuracy.
PET spatial resolution is limited by positron range and photon non-collinearity rather than collimator geometry.
SPECT systems are typically less expensive and more widely available.

Attenuation correction is important in both modalities, but PET systems routinely incorporate CT-based attenuation correction as part of standard acquisition.

Where this matters clinically

PET is widely used in oncology due to its high sensitivity and quantitative capability. SPECT remains important in myocardial perfusion imaging, bone scintigraphy, and many other applications. Understanding their physical differences clarifies why PET images often appear smoother and more quantitative than SPECT images.