Foot/Ankle at 7T MRI

Background:

Dedicated imaging of the foot (either forefoot or hindfoot/ankle) is one of the most frequently performed MR examinations in daily routine in a musculoskeletal specialized radiologic department as ours.

Specifically, high resolution MRI is required for accurate diagnosis given the broad spectrum of pathology and complexity of foot and ankle anatomy (23).

A variety of pathologies can be assessed, ranging from degeneration (i.e. osteoarthrosis) over traumatic injuries of the bones and soft tissues (i.e. plantar plate of the metatarsophalangeal joints) to tears of the various ligaments and tendons of the foot, most of them can assessed exclusively with MR imaging.

Another reason for referring patients for MR imaging of the forefoot is Morton's interdigital neuroma, which refers to a neuropathy of the interdigital nerve and is a common cause of forefoot pain (14). Repetitive trauma and compression to the nerve are believed to result in vascular changes, endoneurial edema, and excessive bursal thickening leading to perineural fibrosis (15). That is why Morton neuroma is not considered as true neuroma but as degenerative perineural fibrosis (16,17). The diagnosis is made through case history, clinical examination and imaging (18). Imaging is usually needed to confirm the diagnosis and to exclude other aetiology (19). MRI and ultrasound have been proven to be the most sensitive and specific exams for diagnosing Morton neuroma (20). However Morton neuroma is frequently found at imaging in asymptomatic volunteers, up to 33% (21). A correlation between clinical examination and imaging is therefore mandatory. But the clinical examinations can be difficult, especially when degenerative changes are present or in post-operative cases.

Degeneration or tears of small structures like the plantar plate or the hindfoot tendons are sometimes difficult to assess even with the best technique in MR scanners that are clinical available (3 Tesla).

In this regard it seems very useful to establish new imaging tools to get a better depiction of the complex foot anatomy as well as to diagnose the above mentioned different pathologies of the foot (i.e. Morton neuroma, tendon tears, plantar plate tears etc.). Since the apparition of 7T MRI a decade ago, a number of scientific studies have been undertaken in the musculoskeletal field and recently the first clinical 7 Tesla MRI system has become available (22). To the best of our knowledge, only few studies have been evaluating foot at 7T MRI (23,24) and no one with focus on the forefoot.

Due to its higher signal to noise ratio (SNR) and contrast to noise ratio (CNR) than 1,5T or 3T MRI (25). Therefore spatial resolution in morphological imaging of small structures, such as those found in the foot and ankle can theoretically be significantly improved in 7T MRI compared to lower field strengths (1.5T or 3T).

Study design: Prospective observational, qualitative and quantitative national single center study.

Outcome(s): Primary outcomes: The sensitivity/specificity for detection of various pathologies in the foot/ankle, for exampleMorton neuroma, plantar plate tears and tendon injuries at 7T MRI

Secondaryoutcomes:

Detailed visualization of the microanatomy of the complex soft tissue structures of the foot and ankle (especially the plantar plate) and to establish normal findings in asymptomatic individuals.

Measurements and procedures: Clinical MRI sequences of the forefoot and/or hindfoot (without contrast medium) will be acquired on a 7 Tesla MRI for assessing the anatomy (i.e. plantar plate) and various pathologies (i.e. Morton neuroma, tendon tears). Patients who have undergone a 1,5 or 3T MRI for clinical indications (or for previous study) will be enrolled through consecutive ongoing recruitment by the project leader and referring physicians in daily clinical practice at Balgrist University Hospital Zurich. They will undergo a MR examination of the forefoot at 7 Tesla. The examination will take approximately up to 30 min. The depiction of the respective pathologies (i.e. Morton neuroma) and the anatomy of the respective structures (i.e plantar plate anatomy) will be assessed by two board-certified musculoskeletal radiologists. The diagnostic accuracy for each examination will be evaluated and compared. In case of pure anatomic descriptions the frequency of findings will be assessed. Interreader reliability will be assessed in all cases.