Crystals are an essential component of nuclear medicine imaging systems, and manufacturers use several different types of crystals to capture and power the cameras. With the number of options in the market, it’s easy to become overwhelmed by all the choices. This post reviews the primary crystals used in nuclear medicine imaging systems.
Cesium iodide (CsI) crystals work via indirect conversion and are the most affordable and efficient crystals available for solid-state gamma cameras. In addition to nuclear medicine, CsI crystals are used in many different modalities. These crystals are often incorporated in cameras with a small physical footprint, such as the Digirad X-ACT+.
CsI is a scintillator with good stopping power for low-energy gamma rays, emits more optical photons than sodium iodide, and has lower manufacturing costs. CsI also allows for the same gamma-ray reaction as CZT, but the conversion is done indirectly using a photodiode.
In addition to the benefits of a solid-state detector, CsI crystals also enable the use of Fluorescence Attenuation Correction. With SPECT/FAC, instead of performing a SPECT/CT, the CT portion of the camera has been replaced by fluorescence. This change results in a high-quality image while dramatically reducing radiation exposure to the patient compared to traditional CT imaging. It’s the best of both worlds.
CsI crystals not only detect gamma radiation they also function as the fluoro detector. This dual-purpose, solid-state crystal technology is unique among nuclear medicine technology and has dramatically advanced nuclear imaging.
Cadmium zinc telluride (CZT) crystals are used in direct conversion, solid-state gamma cameras. While they are a good material to be used in nuclear medicine cameras, they also significantly impact the price point of these systems.
During the production phase of these cameras, the crystal material has proven to be challenging to produce, and there is a considerable amount of wasted material and failing crystal elements.
Unfortunately for nuclear medicine professionals, the expense of producing a large batch of crystal elements with only a small portion being acceptable for use is passed onto the customer. While these systems can produce excellent images, a cost comparison to CsI crystal-based systems makes CZT cameras challenging to justify.
Thallium-activated sodium iodide crystals are typically associated with older PMT technology. While this has traditionally been the go-to crystal for general nuclear medicine camera systems, the industry is trending away from its use.
Due to the thickness required of the crystals to be efficient, strict environmental requirements, and detector technology with which it is paired, these systems have taken a back seat behind the available solid-state systems.
Despite their dominance in the past, advancements in solid-state technology have these crystals into obsolescence as there are no definitive benefits that the NaI(Tl) provides over the solid-state gamma camera crystals.
Is one better than the other?
At the very least, both CsI and CZT produce high-quality imaging, which is a significant improvement over older Anger technology. Your decision will likely depend on the individual machine, its features, and what studies you plan on performing with the camera.
Ultimately the choice comes down to a mix of price, features, availability, and finding a system you are comfortable working with. Most SPECT cameras are used for 8+ years, so the decision is one you and your patients will experience for years to come. If you are interested in learning more about the X-ACT+ from Digirad or have questions about the information covered in this post, feel free to contact the Digirad team.