Photon-counting CT is an emerging imaging technique in which individual photons and their energies are measured separately, rather than assessing the integrated charges of many photons at once, as with energy-integrating detectors. The potential advantages of this approach include reduced electronic noise, higher contrast-to-noise ratios and improved dose efficiencies. Furthermore, the nature of the photon-counting approach allows for multi-energy imaging with only a single tube, single tube potential and single detector.

A research prototype of one such photon-counting CT detector has been developed by Siemens Healthcare. "The unique features of this scanner include a larger field-of-view to cover the human body and a high tube current, which allows CT scans of patients at clinical dose levels and dose rates," says Shuai Leng, a medical physicist at the Mayo Clinic Department of Radiology in Minnesota. "This is the only photon-counting CT scanner currently available that is able to perform whole-body, in vivo patient imaging," he notes, adding that potential clinical applications of the detector are broad, and could include cardiac, body CT, lung, musculoskeletal, neuro and vascular imaging.

Previous studies with the detector have already demonstrated that it can produce clinical-quality images equivalent to those produced by commercial energy-integrating detector systems, and at the X-ray photon flux encountered during routine CT exams. Furthermore, the scanner boasts an improved contrast-to-noise ratio, increased spatial resolution and better Hounsfield unit stabilities at low doses.

So far, however, tests of the detector's potential have only explored traditional image quality, in a single-energy mode. In the latest study, Leng and colleagues put the photon-counting detector's spectral performance to test. They conducted phantom studies to assess the scanner in terms of its iodine quantification accuracy and CT number accuracy.

The former metric assesses one of the multi-energy mode's main advantages over conventional, single-energy CT: the capacity to differentiate and measure the amount of iodine – the most common CT contrast agent – in a patient. "Accurate quantification of iodine provides doctors with reliable quantitative measurements of X-ray attenuation, electron density and effective atomic number," Leng explains. "This can greatly assist in making a more accurate diagnosis."

To test the iodine quantification accuracy, the researchers placed vials containing iodine of five known concentrations in phantoms designed to simulate the attenuation of differently sized patients. They then scanned the phantoms using the photon-counting CT scanner, as well as second- and third-generation dual-source, dual-energy scanners. Following decomposition, they produced iodine maps and compared the reported concentrations for each vial and phantom size combination with the known values.

Secondly, they generated virtual monogenetic images, to compare the measured CT numbers of each vial with a reference standard. CT numbers represent the values of individual pixels in CT images, reflecting the contrast between different tissues; they are also used in radiotherapy planning to guide calculations of planned dose distributions.

The results showed that the detector could perform accurate iodine quantification – with a root mean square error of 0.5 mgl/cc - and accurate CT number of virtual monogenetic images - with a percentage error of 8.9%.

"This study confirms that the dual-energy accuracy is acceptable, on par with existing systems," comments Scott Hsieh, a researcher from the University of California Los Angeles, who was not involved in this study. The proposed advantages of current photon-counting systems with regards to dual energy are less about accuracy and noise than they are about workflow, he says. "In the older dual-source systems, you had to specify a particular protocol that might otherwise be non-ideal. Or you might take a scan in a non-dual-energy mode, and later wish you had access to dual-energy information," he adds. "With photon counting, you can use any spectrum you want, and the dual-energy information is always going to be there."

With their initial studies complete, the researchers are now performing in vivo patient studies.