Cesium Iodide: The Future of Solid State Imaging

What is Solid State?

Surprisingly, the majority of gamma cameras still used in nuclear medicine today utilize Anger-based technology; vacuum tubes and hygroscopic sodium iodide (NaI) crystals, technologies that are now more than 50 years old.

In response to the demands for better sensitivity and high energy resolution, a new generation of solid state pixelated cameras has been developed. Solid state imaging is classified by the absence of vacuum tubes, and replaces antiquated photomultiplier tubes (PMTs) with segmented crystals and semiconductors. The major advantage of these cameras is that they can determine locations with complete certainty. They accomplish this by pixelating the camera’s detector head into tiles instead of a continuous sheet of tubes.

There is a widespread misconception that only cadmium zinc telluride (CZT) is used in solid state nuclear imaging. In fact, there are actually two types of solid state nuclear imaging technology: direct conversion, which uses cadmium zinc telluride (CZT); and indirect conversion, which uses cesium iodide (CsI) with a photodiode.

Issues with CZT

There are claims that CZT is ten times more sensitive than NaI/PMT. However, in actual practice, a CZT camera’s photopeak detection efficiency is only 64% of either NaI/PMT or CsI/Photodiode. This poor intrinsic peak efficiency is due to a common problem in direct conversion solid state detectors called hole tailing. The resulting loss of sensitivity means that the energy window must be set wider than necessary for the specified energy resolution to recover the lost photopeak counts. Additionally, there is no improvement in scatter rejection with CZT, resulting in higher costs and reduced precision.

Next Generation of Solid State

Although Digirad’s first generation of solid state detectors used CZT, the material was abandoned in the late 1990s to pursue indirect conversion solid state detector heads using CsI with a photodiode. CsI is a scintillator which has 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.

Digirad’s solid state detectors, optimized with CsI and Digirad’s patented photodiode, offer the following benefits:

  • They produce high quality, solid state imaging.
  • They are much lighter and compact, providing the ability to create new gantry designs for rapid imaging.
  • They can acquire both SPECT and CT, requiring only one gantry.
  • CsI/photodiode is more affordable than CZT because it is less expensive to produce the material.

Using CsI coupled with Digirad’s patented photodiode, instead of CZT is beneficial to both the patient and your office because it allows you to provide superior imaging quality at a lower cost. Digirad’s CsI/photodiode technology is the newest generation of solid state nuclear imaging.


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