This study aims to compare subjective image quality and diagnostic accuracy of a photon counting detector computed tomography (PCD-CT) scanner compared with an energy integrating detector (EID)-CT scanner as the reference standard.

Computed tomography (CT) imaging plays a key role in diagnostic radiology, and is particularly valuable in assessing a myriad of musculoskeletal conditions (e.g. trauma, degenerative disorders, post-surgical follow-up, inflammatory diseases).

The vast majority of today's CT scanners are equipped with an energy-integrating detector (EID), which converts energy of incoming X-ray photons in a two-step process into electric signals: First, scintillators generate visible light, which in turn is converted to the electric signal by photodiodes.

Recently, photon-counting detector computed tomography (PCD-CT) scanners became commercially available and have been introduced to improve imaging performance through direct transformation of X-ray photons into electron hole pairs upon absorption in a semiconductor layer, generating an electrical signal proportional to the photon energy.

Current literature has shown that PCD-CT can offer higher spatial resolution, increased contrast-to-noise ratio, higher dose-efficiency, and inherent multi-spectral imaging capacity.

Moreover, the results from cadaver studies suggest that PCD-CT can preserve image quality with reduced radiation dose.

However, data on the diagnostic accuracy of PCD-CT compared to EID-CT is currently limited in the field of musculoskeletal radiology.

In addition to conventional CT acquisition, multi-spectral CT imaging in patients with gout and after trauma, is used in clinical routine to delineate materials with a specific absorption coefficient: in gout, urate crystals can be visualized, while after trauma, bone marrow edema can be detected. For EID-CT, different techniques for multi-spectral CT imaging are available (e.g. fast kV switching, dual source CT) which are widely used in clinical routine. Multi-spectral imaging is also inherently available for recently introduced PCD-CT. However, data on diagnostic accuracy of multi-spectral PCD-CT is currently lacking.

Inclusion Criteria:

• Patient admitted for non-contrast enhanced CT scan of the upper or lower extremity at the radiology department of Balgrist University Hospital
• Signed general consent ("Information zur Weiterverwendung gesundheitsbezogener Daten und Proben zu Forschungszwecken")
• >18 years of age
• Written consent to participate in this study

Exclusion Criteria:

• Clinically significant concomitant disease states (e.g., renal failure, hepatic dysfunction, cardiovascular disease, etc.)
• Women: pregnancy
• Age below 18 years
• Known or suspected non-compliance, drug or alcohol abuse
• Inability to consent and/or follow the procedures of the investigation, e.g. due to language problems, psychological disorders, dementia, etc. of the subject
• Enrolment of the PI, his/her family members, employees and other dependent persons
• Body mass above 200 kg.
• Body mass below 31 kg.