Stress fractures (fatigue or insufficiency fracture) are caused by the mismatch between bone strength and chronic stress applied to the bone. The vast majority of these fractures occur in the lower extremity. Early-stage diagnosis is crucial to optimize patient care. Appropriate imaging is relevant in confirming diagnosis after clinical suspicion of stress fractures. Radiographs have low sensitivity, so a relevant number of fractures go undetected. MRI has a high sensitivity, but its availability is limited, and its respective examination time is prolonged. This study investigates the diagnostic accuracy of PCCT in lower extremity stress fractures as a dose-saving technology, guaranteeing an examination according to the ALARA-principle (as low as reasonably achievable).

Stress fracture is caused by the mismatch between bone strength and chronical stress applied to the bone, which is insufficient to cause an acute fracture, but a stress fracture does not heal itself. One can subclassify it into fatigue fracture (overuse of a healthy bone) and insufficiency fracture (normal use of a weakened bone). Fatigue fractures usually happen in healthy athletes or military recruits, whereas insufficiency fracture appear in patients with underlying metabolic or nutritional disorder (e.g. osteoporosis). On radiographs and Computed Tomography (CT), stress fractures are defined as round or linear intracortical lucency or an intertrabecular sclerotic line, which rarely intersects the cortex. Radiograph is a cost-effective and highly available modality in detecting fractures, showing however a moderate sensitivity in detecting stress fractures: 15-35% on the initial and 30-70% on the follow-up imaging. CT, another modality highly available in most hospital settings, shows a similar moderate sensitivity of 32-38% with however a corresponding high specificity of 88-98% on initial imaging. Similar specificity values can be observed for magnetic resonance imaging (MRI) and nuclear scintigraphy. Although their availability is limited and their respective examination time is prolonged, they outperform the x-ray based technologies in term of sensitivity (68-98% MRI and 50-97% nuclear scintigraphy, respectively).

The introduction of dual-energy technology advanced CT from a pure anatomical evaluation tool to a combined anatomical and functional modality. Every material has a specific absorption number, which can be assessed by applying two different energies (high and low x-ray tube voltages). This method of multispectral imaging has been established and clinically implemented in detecting gout and characterizing renal stones. Further studies have shown that DECT can depict bone marrow edema, a marker of early stress fracture and a common finding in MRI. However, this has yet not been implemented in clinical practice.

The photon-counting-computed-tomography (PCCT) has been introduced recently, enabling an energy dependent separation of photons over the whole x-ray energy spectrum. This results in reduced background noise, improved image resolution and multispectral imaging without the necessity of an additional acquisition at a different energy level. An initial study has shown already shown the superiority of PCCT by better detecting and characterizing small renal stones, when compared to conventional dual-energy computed tomography (DECT).

In this project the investigators aim to include clinically referred patients with suspected stress fracture of the lower extremity who will have an MRI to confirm the diagnosis of a suspected stress fracture. The subjects will be scanned on the new PCCT system with dose saving technology, guaranteeing an examination according to the ALARA-principle (as low as reasonably achievable). The investigators will not inject iodine contrast media and they will expect a median dose of 2-4 mSv (millisieverts). Since this will not exceed the threshold of 5 mSv, this project will be classified as category A.

Inclusion Criteria:

• ≥ 16 years of age. Minor study subjects can have an additional signature by the parent or legal guardian
• Clinically suspected stress or insufficiency fracture of the lower extremity
• Written consent of study participation
• Patients who will have an MRI to confirm the diagnosis of a suspected stress fracture

Exclusion Criteria:

• < 16 years of age
• Pregnancy
• Metal implants
• Postoperative situation
• Infection or tumorous disease affecting the lower extremity

In cooperation with the Foot Clinic at the Balgrist University Hospital Zurich, potential study candidates with suspected stress fracture who will undergo an MRI to confirm the diagnosis are prospectively screened and proposed to join the study.