Positron Emission Tomography (PET) is a medical imaging technique that uses radioactive tracers to produce images of organs and tissues inside the body. It is a powerful diagnostic tool that helps physicians detect and diagnose diseases such as cancer, heart disease, and neurological disorders. Scintillation crystals play a crucial role in the PET imaging process.
What are Scintillation Crystals?
Scintillation crystals are materials that emit light when they are exposed to radiation. In PET imaging, scintillation crystals are used to detect the gamma rays emitted by the radioactive tracers used in the procedure. When a gamma ray interacts with a scintillation crystal, it produces a burst of light that is detected by a photodetector. The photodetector then converts the light into an electrical signal, which is used to create the PET image.
Types of Scintillation Crystals Used in PET
There are several types of scintillation crystals that are commonly used in PET imaging. The most commonly used scintillation crystals are made of bismuth germanate (BGO) and lutetium oxyorthosilicate (LSO).
Bismuth Germanate (BGO) Crystals
BGO crystals have been used in PET imaging for over 30 years. They are known for their high light output and excellent energy resolution, which makes them well-suited for use in PET detectors. BGO crystals are also relatively inexpensive and have a long lifespan.
Lutetium Oxyorthosilicate (LSO) Crystals
LSO crystals are a more recent development in the field of PET imaging. They offer several advantages over BGO crystals, including higher light output, better energy resolution, and faster decay time. These properties make LSO crystals well-suited for use in high-resolution PET detectors.
Other Types of Scintillation Crystals
Other types of scintillation crystals that are sometimes used in PET imaging include:
Advantages of Scintillation Crystals in PET Imaging
Scintillation crystals offer several advantages over other types of radiation detectors, including:
A common myth is that to detect radiation effectively, a detector must be sensitive to both alpha and beta radiation. However, professional equipment costing tens of thousands of dollars, used by special services or customs officers, is often not sensitive to alpha and beta radiation. So why do professionals ignore alpha and beta radiation? There are several reasons for this:
This is especially evident when compared to Geiger Counters, which detect radiation through the interaction of light gas in the Geiger tube. Due to the low density of the gas, much of the radiation passes through without interaction and is not registered. In contrast, scintillation instruments use high-density crystals for detection. As gamma rays pass through the dense crystal, they interact and are registered much more efficiently. The device records 5-10 pulses every second even in natural background conditions. With such a volume of data, the device can easily detect any changes in the radiation background.
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Detectors register particles too infrequently, so Geiger counter devices cannot quickly show small changes in the radiation background. For example, the most popular Geiger counter, the SBM-20, takes 40 seconds to detect a statistical difference between 0.08 µSv/h and 0.20 µSv/h. A scintillation sensor like the Radiacode can register a similar change in less than 1 second. For special services or customs, such a long measurement time is unacceptable.
Scintillation detectors have a special characteristic: the lower the energy of gamma radiation, the better they detect it. On an X-ray of bones, we see how weak gamma radiation does not pass through dense bones. The scintillation crystal, which is the basis of the scintillator, is also very dense. The lower the energy of gamma radiation, the more the detector interacts with radiation, and the higher the detection efficiency. Simply put, the dense crystal does not allow radiation to pass through, and registers all of it.
All beta and alpha radiation sources emit weak gamma (X-ray) radiation. Alpha decay almost always leads to the instability of the atomic nucleus, which results in weak gamma radiation. This radiation often has very low energy, which scintillation detectors register best. Beta decay is characterized by bremsstrahlung gamma radiation, which appears when a beta particle hits the atoms of a substance. The efficiency of this process is not very high, but due to the incredible sensitivity of scintillators to weak gamma radiation, it is sufficient for detection. Also, beta and alpha emitters often have their X-ray and gamma lines and decay products.
High sensitivity to low-energy gamma radiation in scintillation instruments can lead to incorrect dose rate readings, so compensation for this effect should be applied in such devices. Radiacode has this compensation. This is why two independent values and graphs are displayed. The first is the registered pulses per second by the detector. The second is the energy-compensated dose rate, where the device makes corrections. By displaying two independent values, the device benefits in both sensitivity and accuracy.
This results in a paradoxical situation: scintillation instruments often have a higher response to beta and alpha radiation sources than devices capable of registering beta and alpha radiation. Additionally, it’s important to note that alpha and beta radiation are easily shielded. Placing the source in a plastic box is enough for alpha and beta radiation to disappear. However, weak gamma radiation easily passes through the box, and the scintillation detector registers it.