From the April 2022 issue of HealthCare Business News magazine

By Camille Allred

Historically, angiography and computed tomography (CT) systems have been outfitted with solid state detectors using indirect conversion technology, whereas direct conversion detectors have been better suited to lower-intensity radiation modalities, such as mammography. With the recent FDA clearance of the first photon-counting CT scanners, it is natural to presume this technology may be adapted into other imaging modalities.

The emergence of digital radiography (DR) has been a slow progression following approximately 10-year cycles as emerging technologies mature and come to market. Computed radiography (CR) emerged in the early 2000s as an early form of digital imaging and was subsequently replaced by digital radiography (DR), which is the primary method of image production used today.
There are three main types of DR: indirect, charge-coupled devices, and direct flat-panel detectors. Angiography primarily uses indirect flat-panel technology with thin film transistors (TFT). As the name suggests, indirect flat panels have multiple steps to image production, requiring X-rays to strike the face of the plate where they are collected by the scintillator. The scintillator fluoresces, converting the X-rays into visible light, which is converted into an electrical signal; an array of thin film transistors then read out the measured change, converting the signal into a digital image.

Proton counting detectors eliminate the conversion of X-rays to light, allowing the X-ray to be directly absorbed by a semi-conductor generating positive and negative charges known as a pulse. These pulses can be sorted into different intensity bins depending on the energy at the time the plate is struck. This allows for increased spatial resolution, reduced radiation exposure, and the potential to use alternative contrast agents.

One of the main limitations of photon counting detectors is the speed of transmitting released charges and the subsequent ability to read pulses fast enough. When detectors are not fast enough, an effect called pileup occurs. This phenomenon occurs when two consecutive pulses almost simultaneously strike the detector, registering only one pulse instead of both. There is also a potential for crosstalk, as these detectors do not contain a septum between pixels. The lack of septa leaves these detectors vulnerable to Compton scatter and X-ray florescence. While these scatter interactions are limited by the substrate material, these interactions still occur. Photons striking near a border between neighboring detector elements may also be counted twice.

April 2022 Magazine

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