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.

Angiography is, by its very nature, a high radiation dose procedural area with some cases using multiple thousands of millirem per patient. In ideal laboratory environments, photon-counting detectors have been shown to reduce dose by up to 60% to produce the same images as traditional TFT detectors. Adapting photon-counting detectors into traditionally high-dose imaging modalities could benefit not only the patient, but the intraprocedural staff as well.

Time, distance, and shielding have long been the cornerstones of radiation safety. And while these measures work well in most imaging modalities, interventional applications make all three of these challenging to implement, particularly in tandem. Companies are actively attempting to solve the issue of high-dose exposure to the physician (who in the traditional procedural setup is positioned closest to the patient and primary beam) by increasing shielding and distance options through heavily shielded suspended suits and remote shielded cockpits. Procedural requirements and setups leave few options to improve radiation protection for non-physician staff members. In these procedures, the scrub (i.e., the staff member directly assisting the physician at bedside) and the circulator (i.e., the primary staffer administering necessary medications to the patient during the procedure) are typically wearing wraparound lead aprons.

Steep angles are routinely necessary to properly visualize anatomical structures. The use of steep angles in an interventional case increases dose to the patient, thus increasing scatter radiation, and increases the chances a staff member will be required to stand in the primary beam. Aprons and shields are not designed to protect the wearer from the intensity of primary irradiation.

Adapting a photon-counting detector for use in angiography suites could open the possibility of better patient imaging at an overall reduced dose. Improved visualization of anatomy during interventions can drive better patient outcomes as devices can be placed accurately and efficiently, decreasing the procedure times and resulting in faster turnover of procedural spaces. The improvement in visualization of structures may open the possibility for new minimally invasive procedures previously impossible because of limitations in anatomic structure visualization. By replacing traditional surgical cases with minimally invasive procedures, health systems can decrease hospital stay lengths while increasing patient satisfaction. This emerging technology has the potential to revolutionize medical imaging in general — and if the technology is implemented in interventional areas, the benefits are magnified for all parties: patients, staff, and hospital leaders. It will be interesting to see how this technology performs in CT applications and whether the technology is truly adaptable to other imaging modalities.

About the author: Camille Allred is a clinical advisor in cardiac and interventional imaging for symplr.