4 Positron Emission Tomography

Positron emission tomography (PET) is a diagnostic method whereby images are formed through the detection of radiation from the emission of positrons. A positron is the antimatter counterpart of electrons. They are emitted by some radioactive isotopes like fluorine-18, which will be covered later in this section. This application of nuclear medicine is best known for its diagnosis of cancer through the identification of tumors in the nervous system.  PET can also be used for imaging overall brain activity when investigating cognitive defects (Open Neuroscience Initiative, 2021). This is done using radiolabeled glucose molecules, and the biochemical activity of the areas of interest in the brain.

How the image is acquired

The scanner consists of a detector that surrounds a patient., The patient is injected with a radionuclide tracer. A tracer is the radiolabeled glucose molecule because the body uses glucose as a source of energy. The PET scanner receives the signal of the gamma rays given off by the tracer, as it decays. The scanner is able to can measure the amount of metabolic activity and the computer takes the said signals and rearranges them into an image  (Cognitive Psychology and Cognitive Neuroscience/Behavioural and Neuroscience Methods, 2021). When looking at brain activity the most common tracer used is fluorodeoxyglucose-F18 also known as FDG,  this tracer is a radioactive analog of glucose, making it an excellent tracer because glucose is the main source of cellular energy (Open Neuroscience Initiative, 2021). Rapidly reproducing cells such as cancer cells have a higher energy demand causing more metabolic activity in the area which then leads to more tracer which causes the area in the scan to show up brighter than the surrounding area.

History of the method

In 1950, Gordon Brownell and William Street developed the first human positron imaging device using sodium iodide to detect brain tumours (Portnow, L. H., Vaillancourt, D. E., & Okun, M. S. 2013). The first PET scanner was an improvement from Brownell and Street’s with an increase of detectors. This new scanner allowed for heightened sensitivity, and was unveiled at the 1972 meeting on Tomographic Imaging in Nuclear Medicine As they moved forward they added more detectors to increase the sensitivity and the first PET scanner was unveiled at the 1972 meeting on Tomographic Imaging in Nuclear Medicine (Portnow, L. H., Vaillancourt, D. E., & Okun, M. S. 2013) . Now, PET imaging can help diagnosis,  manage the following: Parkinsonian dementia, Alzheimer’s, ‘s disease dementia, and epilepsy along with many others. Alzheimer’s and other memory-related conditions are often not diagnosed until autopsy. PET allows doctors to detect and act on early warning signs by observing the decline in glucose metabolism associated with decreased cognitive function. The affected regions and regions that would be watched in scans are the temporal and parietal lobes. , This which is made possible by the advancements in technology and computer-based algorithms that are used to enhance image resolution.

PET in action

Nakamoto et al. (2022), reviewed articles observing the use of the combined modalities of PET and MRI (magnetic resonance Imaging). They found that when imaging epilepsy patients the ability to identify the epileptogenic zone was improved. This was done using the morphometric analysis program (MAP) and quantitative FDG PET (QPET).  explores how PET/MRI has improved the assessment of the extent of rectal cancers, outperforming  MRI in evaluating tumour size. They also look into how the use of PET/MRI is helping the research of new drugs in relation to Alzheimer’s disease.   Jack Jr. et. Al.,   I(2018) discusses the use of tau tracers in a longitudinal study. Tau tracers are tracers that bind specifically to tau neurofibrillary tangles. These tangles are important as they connect synaptic loss and Alzheimer’s disease (Ossenkoppele & Hansson, 2021). In their study on the accumulation of Tau PET shows the ability to use tau tracers and perform longitudinal studies to see the accumulation of pathological tau (2018). Jack Jr. et. Al., (2018)  used this information to develop the theory that pathological tau continues to accumulate in the area it was previously found in. Tau is a protein that helps to stabilize the internal skeleton of neurons in the brain and is a protein that can build up in the brain and cause Alzheimer’s disease (Bloom G. S., 2014). With the use of tau tracers, they were able to track early and late Alzheimer’s disease over the longitudinal study.

Limitations

One limitation of the PET scan is that the patient is exposed to radioactive compounds and gamma wave radiation. While precautionary methods are put in place to minimize risk to patients, the possibility of mutation is still present. Boundaries of tissue are often difficult to determine therefore the scan is often performed simultaneously with a CT scan. This is due to the imaging relying on the emission of gamma rays from the positron which causes there to be a blur in the boundaries. A cluster of cells smaller than a cubic centimetre will be impossible to detect with the current status of spatial and temporal resolution (Open Neuroscience Initiative, 2021).