Standardization and Quantitative Imaging With Photon-Counting Detector CT

Computed tomography (CT) images display anatomic structures across 3 dimensions and are highly quantitative; they are the reference standard for 3-dimensional geometric measurements and are used for 3-dimensional printing of anatomic models and custom implants, as well as for radiation therapy treatment planning. The pixel intensity in CT images represents the linear x-ray attenuation coefficient of the imaged materials after linearly scaling the coefficients into a quantity known as CT numbers that is conveyed in Hounsfield units. When measured with the same scanner model, acquisition, and reconstruction parameters, the mean CT number of a material is highly reproducible, and quantitative applications of CT scanning that rely on the measured CT number, such as for assessing bone mineral density or coronary artery calcification, are well established. However, the strong dependence of CT numbers on x-ray beam spectra limits quantitative applications and standardization from achieving robust widespread success. This article reviews several quantitative applications of CT and the challenges they face, and describes the benefits brought by photon-counting detector (PCD) CT technology. The discussed benefits of PCD-CT include that it is inherently multienergy, expands material decomposition capabilities, and improves spatial resolution and geometric quantification. Further, the utility of virtual monoenergetic images to standardize CT numbers is discussed, as virtual monoenergetic images can be the default image type in PCD-CT due to the full-time spectral nature of the technology.

Automated summary

Computed tomography (CT) images are highly quantitative and display anatomic structures in 3D. They are used for various applications, such as 3D printing of anatomical models and radiation therapy treatment planning. CT numbers, derived from pixel intensity, represent the x-ray attenuation coefficient and are widely used for quantitative measurements. However, the dependence of CT numbers on x-ray beam spectra limits their robust application. This article reviews the challenges faced by CT and introduces the benefits of photon-counting detector (PCD) CT technology, such as multienergy capability and improved spatial resolution.