CEA-Leti has developed a new X-ray photon-counting detector module to enhance CT scanning (Photo courtesy of CEA-Leti)

Siemens and CEA-Leti partner on photon counting CT detector module

September 08, 2020
by John R. Fischer, Senior Reporter
France’s technology research institute CEA-Leti has developed a novel X-ray photon-counting detector module (PCDM) that has shown promise for improving CT scanning.

Integrated in a CT scanner prototype from Siemens Healthineers for clinical trials, the PCDM was found to increase spatial resolution, reduce X-ray exposure to patients, and decrease image noise and artifacts for better image quality, and to distinguish multiple contrast agents from one another.

"The CT scanner required a very high performing detector in terms of spatial resolution, quantum efficiency, count rate, power consumption, linearity and reliability," CEA-Leti researcher Loick Verger, business development manager in technologies for imaging systems, told HCB News. "The detector performance is the result of material properties, an optimize detector geometry and a specific electronic readout and PCB, by considering the system architecture of the CT. The challenge was enormous, no available detector existed at this time. We started with Siemens from scratch."

Siemens Healthineers approached CEA-Leti about designing, integrating, manufacturing, and testing a new generation of PCDM that would be mature enough to be integrated into an X-ray CT scanner prototype, because of the advantages they offer in imaging.

X-ray PCDMs based on cadmium telluride (CdTe) allow simultaneous acquisition of high-spatial-resolution and multi-energy images. With the use of a small-pixel size detector, the higher spatial resolution translates to greater image quality and clearer images for identifying very fine structures, such as small airways in the lungs, trabeculae in bones, and thin wires in coronary stents, than current scanner technology.

The multi-energy feature enables images to be colored, compared to the grey-level images of conventional detectors, and for the atomic number of chemical elements present in the body to be precisely determined.

CTs rely on computer-processed combinations of many X-ray measurements taken at different angles to create cross-sectional images of scanned objects, and currently use energy-integrating detectors to produce images, a process that is based on indirect conversion technology. PCDMs, in contrast, directly convert X-ray photons into electronic signals with a higher conversion yield.

And while the EIDs do not discriminate between low- and high-energy photons while registering the total energy deposited in a pixel during a fixed period of time, PCDMs count each photon, which improves the contrast-to-noise ratio of the image. It also allows the energy classification of the detected photons to be used to produce a color image that can show a precise determination of the atomic number of any chemical element, and distinguishes multiple contrast agents present in the body.

Researchers at Mayo Clinic in the U.S. have applied the Siemens Healthineers’ PCDM to phantoms, cadavers, animals and humans, and produced images of more than 300 patients that, they say, consistently show the theoretical benefits the technology has to offer clinically.

“Publications by our research team have shown improved spatial resolution, decreased radiation or iodine contrast dose requirements, and decreased levels of image noise and artifacts," said Cynthia McCollough, professor of Medical Physics and Biomedical Engineering at the Mayo Clinic, in a statement. "Additionally, the ability to simultaneously acquire multiple 150-micron-resolution datasets, each representing a different energy spectrum, is anticipated to lead to new clinical applications."

CEA-Leti is focusing its efforts on improving the efficiency of the PCDMs by taking part in the development of a CdTe-based energy resolved PCDM for the detection of individual X-ray photons with two energy threshold counters.

"Despite these very encouraging results, the spectral quality of the PCDMs’ response is far from optimum and there are still technical challenges to overcome, such as charge sharing, charge induction and pile-up," said Verger. "Solving these issues will be the next step for a new generation of PCDM-based CT."

To do this, the company has developed a corrected photon counting detector (CPCD) that features a "real time cleaned spectrum" inside the pixel before digitizing the signal. It is based on the hybridization of a CdTe layer with a new CMOS Read-Out Integrated Circuit in which each pixel integrates on-chip charge sharing correction, charge induction and pile-up rejection together with an interface memory in order to relax the off-chip post-processing constraints.

"The next challenge will be to design a new chip featuring 25 Mcps/pixel Output Count Rate and a low power consumption (few mW/pixel) with a pixel pitch of 0.5mm including all correction," said Verger. "The resulting CPCD should be able to measure the required flux of 2.108 count per second/mm² for an optimal response of the medical PCDM-based CT."