PET Scan, which stands for positron emission tomography, is a non-invasive, nuclear imaging device used for diagnosis and functional imaging of the human body. It uses the phenomenon of electron-positron annihilation as its basic principle. This advanced imaging technique is a powerful tool for locating cancer in the body. That’s the basic idea of what a PET Scan is.
But one of the most important parts of a PET Scanner is the gantry which consists of detectors. This article will discuss the engineering behind PET Scan detectors and how they capture gamma rays with precise accuracy to localize cancer.
Procedure: The In-Vivo Annihilation of Matter
The PET scan uses a tracer injected into the patient’s bloodstream, which is produced through a cyclotron. This tracer is a biomolecule-like glucose modified by replacing one oxygen atom with a radioactive atom like Flourine-18, Iodine-124, Bromine-77, Copper-64, and Gallium-68. The tracer circulates in the body and is absorbed by tissues and organs.
The radioactivity of the tracer emits positrons in the body. These positrons when collide with electrons in our body, annihilate into energy. This energy is in the form of two gamma (γ) rays which are 180° to each other. The γ radiation is detected by the gamma camera which is composed of scintillation crystals and photomultiplier tubes. As the radiation falls on the scintillators the emitted light is detected and amplified by the photomultiplier tubes and converted into an electronic signal which is filtered and processed for image reconstruction. You can refer to our introductory article on PET Scan to learn more about the tracers, machine design, and PET Scan’s uses.
Alt-text: Annihilation of electron-positron, producing two gamma rays in the opposite direction
Inside a PET Scan’s Gantry
The gantry or the gamma camera of a PET Scanner detects the radiation from the patient’s body. The combined effect of the following two components converts the gamma rays into an electronic signal which then goes to the processing unit.
- Scintillation crystals/detectors
- Photomultiplier tubes
Scintillator Detectors:
Scintillators receive the gamma rays from the human body and convert that energy into visible light. This makes them the prime signal catchers of the PET scan, hence the scintillator material used and its configuration are of great importance. Detecting wrong signals, and attracting noise at this stage can corrupt the signal, completely distorting or lowering the resolution of the images.
Four materials are popular in PET Scanners as scintillator detectors:
- Bismuth germanate (BGO)
- Cerium-doped gadolinium oxyorthosilicate (GSO)
- Cerium-doped lutetium oxyorthosilicate (LSO)
- Thallium-doped sodium iodide (NaI(Tl))
The scintillator used in the PET Scan has to meet certain criteria. For example, it should have high density and a high effective atomic number to absorb the maximum radiation by stopping the Compton effect and promoting the photoelectric effect. Moreover, the scintillator should have high luminescence to separate the actual signal from the noise improving scatter rejection and energy resolution.
Another important parameter is the scintillation decay time which should be short, meaning the crystal is fast in responding to radiation and discontinues the response as soon as the radiation stops. This reduces noise by eliminating unwanted signals and hence increases image accuracy.
PhotoMultiplier Tube (PMT)
PET Scanners use PMTs as photosensors. There are two major reasons for that.
- PMTs have a low dark count rate due to their vacuumed environment. Meaning they produce very few false signals in the absence of light which increases accuracy.
- They have a short signal rise time allowing them to detect the light instantly making the process faster and more efficient.
The type of PMT used in PET Scanners is Silicon Photomultiplier or SiPM. They are chip-based hence more compact and lighter weight than bulky photomultiplier tubes. SiPMs are resistant to magnetic field which makes the PET Scanner MRI compatible. They also aid in time of flight systems due to their timing resolution which is under 1ns enabling precise detection of the exact moment light is received. SiMPs give high electron gains with less than 100 volts of reverse voltage making them a safe option.
Configuration of the Detectors:
The detectors are arranged in the gantry in rings or polygonal shapes that completely surround the patient. The design captures the gamma rays from all angles allowing the software to reconstruct 3D images of the body.
Block Configuration
Instead of having small scintillator detectors that are attached to a separate PMT, the engineers came up with an efficient, cost and resource-effective method to increase the detectors without using too many crystals and PMTS simplifying the electronics and circuitry. It’s called block configuration. A cubic piece of crystal of 20 to 30mm in size is finely cut to form an array of 3×3 or 4×4 depending upon the requirement. These cuts don’t separate the crystal but rather create a grid.
A reflective material is filled in the cuts to prevent radiation from interfering with other crystal sections. The blocks are connected to PMTs, which convert the light signal into an electronic signal. The sectioning in the block detector increases the number of detector elements, improving resolution. The fewer PMTs and simplified circuitry reduce the cost.
Data Acquisition in PET Scan
The Principle of Coincidence Imaging
When detecting the gamma rays, the goal is to locate the spot of annihilation in the body, hence only those gamma rays should be recorded that are produced as a result of annihilation. Coincidence imaging is used to eliminate false positives and noise. The two gamma rays produced due to annihilation travel 180° apart at the same speed hence reaching two detectors at the exact same time which is called the coincidence window. The detection of two gamma rays at the same time within the coincidence window shows they’re the result of annihilation. The coincidence window is typically 3 to 4 nanoseconds long. Any signal received outside the coincidence window is discarded.
The short decay time of scintillators helps establish a narrow coincidence window which reduces noise and unwanted signal.
Time of Flight (ToF): Increasing Accuracy
Another important technology used in PET scans which highly improved the contrast-to-noise ratio of images by pinpointing the location of annihilation is Time of Flight (ToF). Through coincidence imaging, we know the annihilation occurs along the line of response (LoR) and the Gamma rays produced by a specific annihilation event reach the detector at the same time. However, there’s a difference in picoseconds that can be measured.
The ToF technology measures the difference between two gamma rays hitting the detector. This information helps locate the position of annihilation along the line of response (LoR) as the event would be closer to the detector that received the radiation first. Time of Flight (ToF) has enabled PET scanners to visualize fine details and lesions that were not captured before. It reveals more detailed and precise anatomy of the human body.
Author Bio: A biomedical engineer by qualification and a freelance content writer and strategist by passion and profession, Shanzay combines her two interests to become a technical writer talking about everything the broad umbrella of biomedical engineering covers. Find out more about her.
References:
- https://www.sciencedirect.com/science/article/pii/S0939388922000903
- https://iopscience.iop.org/article/10.1088/1757-899X/870/1/012043/meta
- https://www.siemens-healthineers.com/en-pk/molecular-imaging/pet-ct/biograph-mct
- https://www.blockimaging.com/blog/what-is-time-of-flight-pet-scanning
- Zanzonico, P. (2004, April). Positron emission tomography: a review of basic principles, scanner design and performance, and current systems. In Seminars in nuclear medicine (Vol. 34, No. 2, pp. 87-111). WB Saunders.
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