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Single-Photon Emission Computed Tomography (SPECT) is a radioactivity-based imaging technique which relies on detection of gamma ray emitters which help visualize the metabolism or blood-flow changes underlying functional neuronal activation (1–4).

Technical Parameters

Similar to PET, SPECT is a radioactivity-based technique. SPECT differs from PET in that it uses gamma (single photon) emitters. Therefore, there is no information about the direction of the incoming photon, as there is with the coincidence detection in PET. To still obtain positional information, SPECT makes use of lead collimators, which guarantee that only photons from a predefined direction are accepted (2,5).

This particular characteristic has as a consequence that radiation event localization with SPECT is more challenging, generally leading to spatial resolution (> 1 cm) and especially sensitivity which is much less than that of PET (6). However, the use of single photon emitters in SPECT (see also below) has an advantage over PET as it widens the time window of observation: depending on the substance used, one would be able to detect signal hours or days after administration. Images are acquired by one or multiple gamma cameras which rotate around the region of interest in the patient and acquires images every couple of degrees. Usually, a full circle will take around 20 seconds, with full acquisition times of less than an hour.

SPECT is far less ideal for quantification of signal, such as blood flow, as compared to PET. In fact, most commercial SPECT-devices do not allow such quantification, making use in a functional setting less ideal (7). Functional maps are often presented using similar color map overlays as seen in fMRI (4).

Biological Substrate

In contrast to the radiotracers used in PET, no biological single photon emitters exist to use in SPECT, meaning that non-physiological tracers will have to be used. Usually, for functional brain imaging purposes, the tracer Technetium (99mTc) exametazime (99mTc) is used (8). After injection, the lipophilic molecule passes the Blood Brain Barrier (BBB), where it rapidly converts to its hydrophilic form and remains trapped in the brain (9,10). The attachment to exametazime facilitates tracing of regional cerebral blood flow (CBF), which serves as a proxy for neuronal activation. Similar to PET, combined SPECT/CT scans are also available to allow for coregistration of images.

Intra-operative applicability

SPECT is known to be less expensive as compared to PET, which can be a great advantage in the clinical setting. However, the spatial resolution of SPECT compared to PET make it the least ideal candidate of the two (Figure 1 and 2). In fact, neither SPECT nor PET are routinely used for pre-operative surgical mapping, mostly given the more superior resolution of non-invasive techniques such as fMRI. Currently, SPECT in the clinical domain mostly seems to have a role in myocardial perfusion studies (11).

Figure 1 - Taken from Henderson et al. (4)
Figure 2 - Taken from Warwick et al. (12)


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