High-resolution ultrasound and MRI in the evaluation of the forefoot and midfoot

Figures & Tables

Use a dedicated coil, such as an ‘ankle and foot ‘coil or a flexible surface coil. The larger dedicated foot and ankle coil enables the examination to be extended to the ankle region, if necessary, and also helps minimize movement artifacts.
Small field-of-view coils are helpful for localized pathology e.g. subungual glomus tumor.
Due to the small structures involved and the need for high-resolution imaging, 3T imaging is preferable to 1.5 T imaging, if available.
A combination of coronal and sagittal with or without oblique axial sequences are used.
T1W and T2W FS sequences are most commonly used.
Dixon technique is useful to acquire homogeneous imaging as well as the simultaneous acquisition of water and fat images.
Oblique axial imaging parallel to the metatarsal shafts is especially helpful if mid-foot bone pathology, such as osteomyelitis or stress fracture, is suspected.
Intravenous contrast may be helpful to assess the presence of collections within an inflammatory mass, to assess synovitis and vascular tumor. Most MRI examinations can be adequately performed without intravenous contrast.
T1W FS or subtraction images may be helpful before and after contrast administration.
Dynamic contrast-enhanced MRI imaging or other functional imaging techniques are generally not necessary for MRI assessment of the mid- and forefoot.

	Pericapsular fibrosis	Morton’s neuromas
Location	2^nd > 3^rd MTP	3^rd > 2^nd intermetatarsal space
Shape of fibrotic mass	Crescent-shaped	Roundish or ovoid ± ginkgo leaf shape on side-to side compression
Base of fibrotic mass	Abuts inferolateral (or inferomedial) aspect of affected MTP joint over a broad area	Located centrally in intermetatarsal space ± contacts but does not envelope MTP joint capsule
Continuity with common interdigital nerve	No continuity	Continuity may be visible
Tenderness	Maximum over MTP joint region	Maximum over intermetatarsal area
Mulder maneuver	Negative (no displacement of fibrotic mass)	± Positive (fibrotic mass displaces inferiorly)
Plantar plate integrity	Additional 1⁰ or 2⁰ features of plantar plate degeneration / tear usually present	MTP joint usually normal
MTP joint stability (Hamilton-Thompson test)	± Unstable	Stable

Ensure both you and the patient are in a comfortable position to adequately examine the mid- and hindfoot. Examination of the dorsum or plantar aspect of the foot is normally performed with the foot resting on the examination table, as shown in Fig. 1.
Review any radiographs. Take a history and palpate any lump, if present.
Use a linear high resolution (7–18 MHz) linear transducer. Higher resolution transducers such as a ‘hockey stick’ (18 MHz) or 25 MHz transducer can be used to improve the resolution of smaller structures.
Use of copious acoustic gel to minimalize air gap interference.
Start by examining the corresponding area on the contralateral unaffected foot. This enables one to assess normal anatomy for that part and set up the transducer optimally.
Ensure optimization of transducer depth, focal zone, and time-gain curve settings.
Use compressibility and dynamic assessment for tendon, ligament, or plantar plate assessment
Applied specific maneuvers such as Mulder’s maneuver, when appropriate.
Use minimal transducer pressure when assessing vascularity of superficial structures with color Doppler imaging.
Ensure that the abnormality found on US examination concurs with the clinical symptoms. If the pathology does not fully explain the clinical symptoms or if the lesion has not been fully evaluated, arrange an alternative imaging examination, which will usually be MRI.
Use a full aseptic technique, including a sterile transducer cover, for any interventional procedures.