Medical imaging: What is the difference between PET and SPECT?

PET (positron emission tomography) and SPECT (single photon emission computed tomography) are two of the main imaging methods used in nuclear medicine for visualizing the way a particular tissue or organ is working.

These techniques use radioactive substances called radioisotopes (in very small amounts) attached to biological molecules which are injected into the patient and the signal produced from the isotope once it is on the body is tracked (visualized) when a patient is scanned. The information can be analyzed in a visual and in a quantitative manner to determine where the tracer goes and how much of it accumulates in different areas of the body before clearing away from the circulation. The imaging of molecules with this approach is also referred to as molecular imaging.

The molecular imaging information is used to inform on physiology, metabolic activity or even the presence of a protein in particular organs, which can help determine if they are working in a normal healthy way. PET and SPECT use different scanners and the underlying physical principles behind them are also different.

Here are some of the main differences between the two techniques and how they are used in the clinic today:

Imaging isotopes: SPECT uses gamma ray-emitting radioisotopes (e.g. technetium-99m, iodine-123, iodine-131, indium-111) while PET uses positron emitting radioisotopes (e.g. fluorine-18, gallium-68, copper-64, zerconium-89).

Medical applications: SPECT has been mainly used for heart disease monitoring while PET is generally used for cancer detection, monitoring and evaluating patient response to treatment.

Imaging timeframe: Because of the differences in isotope half-life, PET is associated with a shorter scanning time than SPECT allowing imaging multiple areas of the body and rapid monitoring of biological processes over short periods of time (for example uptake of metabolites into tissues). SPECT on the other hand allows the study of slower biological processes, for example uptake of therapeutic antibodies into tissues.

Injected activity: The short half-life of some PET radiotracers allows injection of higher activity leading to increased detection sensitivity on a scan while in SPECT, particles tend to release less energy than PET so radiation dose exposure (dosimetry) tends to be more acceptable.

Energy levels and multi-isotope imaging: While in PET all radioisotopes have the same energy level so it is difficult to carry out simultaneous imaging of different tracers (though methods are being developed), SPECT isotopes have different energy levels so could permit simultaneous imaging of different processes. This could potentially reduce image acquisition times and allow co-registration (correlation) of data in space and time. Contamination between different energy levels is however a concern. So for multi-tracer imaging, PET imaging using different isotopes with short half-lives can work quite well, for example using a 11C- labelled tracer (11C has a half-life of 20 minutes and thus signal decays very quickly) followed by administration of a second 18F-labelled tracer when the 11C-signal has completely disappeared.

PET affords higher image quality, contrast and spatial resolution compared to SPECT and has higher sensitivity (100-1000 times higher than SPECT) in addition to being a more quantitative imaging method than SPECT. However, SPECT scanners are more widely available that PET scanners and the technology is more affordable that PET. So the use of either PET or SPECT will largely depend on the clinical question and available imaging infrastructure.

The following are two examples of how PET and SPECT are used in nuclear medicine:

¹⁸F-FDG-PET: This uses a modified glucose molecule with a radioactive fluorine (¹⁸F) attachment (¹⁸F-fluorodeoxyglucose, FDG). The scan allows doctors to determine how much glucose is used in different parts of the body, with high glucose accumulation often being an indicator of cancer or inflammation. The brain is an organ that uses high amounts of glucose and will also give high signal on an FDG-PET scan.

 ^99mTc-tetrofosmin (Myoview^TM) SPECT: This uses small amounts of a drug that is attached to a radioactive technetium tracer (^99mTc) which is rapidly taken up by the heart and patients are scanned under conditions of stress (exercise) and rest.  The information obtained is used to report on overall heart health, determine blood perfusion and identify diseased areas (for example infarct) within the heart.