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December 05, 2008

PET Scan: Particle Physics And Electronics in Medical Imaging

Numerous imaging modalities are there to view anatomical structures in our body. They include X rays, Ultrasound imaging, MRI and many other procedures where we can see normal or diseased tissues in our body. They tell us ‘the location’ (position) in the body the image corresponds to. If we wanted to see what were happening in these locations, we would then need to perform functional imaging techniques like PET scan or fMRI.

In Positron Emission Tomography or PET, a radioactive isotope that decays by positron emission is introduced into the body. Positron emitting radioisotopes are prepared by bombarding stable atomic nuclei by protons. Protons are speeded up in a particle accelerator called cyclotron which then impinge upon the stable nuclei, and knocks out one neutron from its nucleus. The proton now occupies the position where the ousted neutron once stayed. But this atomic configuration is unstable, so the proton now decays. It decays by emitting positron, a particle resembling an electron in all aspects except that the charge is positive and not negative. In other words, a positron is an antimatter: an anti-electron.

positron annihilation and formation of two collinear gamma ray photonsMany different radioisotopes are there, such as Fluorine18, Oxygen15, and Carbon11. 18F is the most commonly used isotope. It replaces hydroxyl (OH) group in molecules of interest. We can use 18F Fluorodeoxyglucose (18FDG) an analog of glucose for probing the activity of brain. Our brain uses glucose for its metabolism, so when it encounters 18FDG, it stores them. The FDG in the brain begins emitting positrons. These particles travel only a short distance before they meet nearby electrons and annihilate. Two gamma ray photons, each having 511 keV of energy are produced--- Photons because they are electromagnetic waves, and gamma ray because the frequencies correspond to the gamma ray spectrum of electromagnetic waves.

So by detecting these photons, we can find out where they came from, since we know that these photons are emitted back to back, 180 degrees apart. (They aren’t exactly collinear as their initial velocity is not zero, and some computational error always creeps in). For the detection part, we need a detection array which will convert these photons into electrons. This is done by scintillators. Bismuth germanate, Luterium oxyorthosilicate (LSO) are some of them. Photons which are incident on them produce electrons by photoelectric effect. These electrons are then guided through a vacuum tube, which has many positive electrodes (dynodes) held at successively higher voltages. These dynodes of this photomultiplier tube accelerate these electrons, which in turn knock-off more electrons from the dynode plates. Thus we get more electrons than what we started with. The signal has now been amplified and we now have a measurable current.

With advancement in detection technology, silicon avalanche photodiodes (silicon APD) has now shown promise to replace the vacuum technology (photomultiplier tube). As the name suggests, APDs work in a similar way an avalanche gains its momentum as it descends from the mountain-avalanche effect. Detecting photons aren’t sufficient. We need to detect only co-linear (coincident) photons. Each collinear photon pair (i.e. 180 degrees apart) will constitute an event. All other photons (noise) must be rejected. About 10^7 to 10^8 or more ‘events’ must be registered in order to have a good signal to noise ratio. Image faithfulness varies proportionally with the square root of the number of events.

By acquiring a large number of events, the computer software is able to determine exactly where these radioactive tracers are located. This, in our case, means the locations where the neurons are accumulating (accumulation is a function of utilization of glucose) 18FDG. Thus we get a functional map. In order to know ‘what’ these structure were, we need to combine anatomical imaging like MRI or CT with PET. This combined PET-CT or PET-MRI let us know what structures are doing how much.

PET scan is very useful in neuroscience researches, clinical diagnoses like cancer detection, receptor analyses and even watching gene expression in molecular biology.

References: A good site with animation: PET animation
Physicsworld

Last Mod: 10 Mar, 2014

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