Quantitative CT imaging: Where are we, and what is missing?

Quantitative computed tomography (CT) enables the measurement of biophysical processes characterized by morphology, composition, flow and/or motion to aid in clinical diagnosis and intervention. Since its initial application for determining bone mineral density for skeletal fragility assessment, quantitative CT has continued to evolve alongside CT's technological advancements. A key challenge facing quantitative CT is the lack of standardization pertinent to image acquisition, reconstruction and image analysis. With the introduction of spectral CT involving dual-energy approaches and photon-counting detectors (PCD), we are now able to obtain detailed information regarding mass densities of endogenous and exogeneous materials. Further, energy-resolved CT yields spectral image types (e.g. virtual monoenergetic image) that are, in principle, independent of tube potential and patient size. This paves the way for workflow standardization and to improve the consistency and reproducibility of CT-derived measurements. In this article, we review clinical applications of quantitative CT, discuss key challenges associated with quantitative CT and its adoption into routine practice, and outline the unique benefits ushered by new CT technologies such as PCD-CT towards improving quantitative imaging.