Magnetic resonance imaging of the forefoot : A concise , practical overview

The forefoot may be affected by a wide variety of pathologies that require imaging. Magnetic resonance imaging is the modality of choice for soft-tissue pathologies and occult bone lesions. Correct imaging protocols and an awareness of anatomy are essential for accurate diagnosis.


Introduction
A sound knowledge of forefoot anatomy, a basic understanding of applicable biomechanics, and a structured differential diagnosis, are essential when performing imaging of the forefoot.

Imaging planes
Imaging planes are prescribed from the scout views. 3Initially, three-axis short scout views are obtained (acquisition time 40 seconds), followed by slightly high resolution scout views in the short and long axis (acquisition time 50 seconds), after which the first sagittal diagnostic sequence is prescribed (Figure 2).

Anatomical planes for forefoot imaging
The sagittal plane is usually prescribed in the long axis of the second metatarsal, but may be adjusted according to the ROI.The short axis is prescribed in the axis of the second or third metatarsal perpendicular to the sagittal and long axis planes.The long axis is prescribed in the axis of the second or third metatarsal on sagittal views. 2 It is important to avoid confusion when identifying the imaging planes.We prefer to use short and long axis terminology, rather than coronal and axial descriptions, to avoid confusion because positioning (prone versus supine) can influence the relative plane in relation to the long axis of the body. 3

Sequences
A combination of non-fat-saturated (proton density (PD)/ T1) and fluid-sensitive fat-saturated (PD FS/T2 FS) image sequences are obtained, with all sequences preferably in three imaging planes.Typical echo times for fluid-sensitive sequences are 30 ms -60 ms but are adjusted to vendor variations and magnetic field strength.The field of view is 120 mm-220 mm and slice thickness is 2.5 mm -3 mm, with a small slice gap (10% -20%).

Additional sequences
A STIR sequence is obtained if adequate fat saturation is not obtained, or when dealing with metal or other artefacts.Gradient echo sequences are added when searching for foreign bodies, blood products or calcifications.T1 FS with or without intravenous gadolinium is reserved for special cases such as mass lesions and infections.Routine use of contrast when evaluating Morton's neuroma is debatable.

MR short axis anatomy
Compartmental anatomy is crucial when evaluating forefoot pathology and is best imaged in the short axis plane.The deep transverse metatarsal ligament divides the forefoot into the dorsal and plantar compartments.The dorsal compartment contains the metatarsals and intermetatarsal spaces including the intermetatarsal bursae.The plantar compartment contains the important neurovascular bundles, specifically important when imaging for a possible Morton's neuroma (Figure 3).The first metatarsophalangeal and sesamoid complex anatomy is also visualised in the short axis plane (Figure 4).

Biomechanics
Human bipedalism has evolved over a period of approximately 4 million years.During this process, the first tarso-metatarsal joint has not evolved into an adequately rigid joint, with the medial longitudinal arch also not sufficiently rigid to absorb continuous high-stress loads.When the medial longitudinal arch starts to sag, usually owing to old age or underlying foot pathology such as rheumatoid arthritis or other joint-centred pathology, secondary load transference occurs to the second digit and lateral arch, with also a relatively rigid second tarso-metatarsal joint.
Stress-load transference may lead to tibialis posterior dysfunction, and stress changes in the second metatarsal with secondary pressure changes in the second metatarsal head and peripheral soft-tissue pressure point or plantar plate.A short first metatarsal index (Greek/Morton's foot) also contributes to an increased risk and earlier occurrence of this cascade of events. 4,5
A sound knowledge of normal anatomy, biomechanics, differential pathology and the most common causes of forefoot pain is essential when imaging the forefoot.High-resolution anatomical imaging in three planes should be obtained with sequences specifically tailored for the forefoot.Fat-saturated fluid-sensitive as well as nonfat-saturated PD or T1 sequences should be included.In specific instances, STIR, gradient echo or post-contrast image sequences may be added.If there is uncertainty about the specific region of pathology, a large FOV STIR study may be obtained to identify the region of pathology.

FIGURE 1 :
FIGURE 1: Sagittal T2 FS MR image demonstrating a vitamin E capsule (arrow) used to indicate the region of discomfort.

FIGURE 2 :
FIGURE 2: Scout views used to plan imaging planes.(a) sagittal plane, (b) long axis plane and (c) short axis plane.

FIGURE 3 :
FIGURE 3: Schematic diagram of forefoot anatomy in the short axis view.

FIGURE 4 :
FIGURE 4: Schematic diagram of the anatomy of the first metatarsophalangeal sesamoid complex in the short axis view.

5 FIGURE 5 :b
FIGURE 5: Schematic diagram illustrating the location of (a) Morton's neuroma occurring in the webspace between the 3rd and 4th toes; and (b) the region of peripheral nerve where excision should occur to prevent the formation of a stump neuroma.

Teaching points 1 .
High-resolution true plane anatomical imaging is important for accurately diagnosing forefoot pathology and abnormalities.

FIGURE 9 :
FIGURE 9: Short axis T2 FS MR image of the foot demonstrating oedema in the medial sesamoid (arrow) owing to sesamoid stress changes.

FIGURE 10 :
FIGURE 10: Long axis T2 FS MR image of the foot demonstrating synovitis, erosions and bone oedema in a patient with rheumatoid arthritis.

FIGURE 11 :
FIGURE 11: Short axis T1 FS MR image of the foot demonstrating a giant cell tumour in the tendon sheath of the 3rd toe.

FIGURE 8 :
FIGURE 8: Long axis T2 FS MR image of the foot demonstrating oedema in the 2nd metatarsal head owing to Freiberg's disease.

FIGURE 7 :
FIGURE 7: Long axis STIR MR image of the foot demonstrating oedema in the 4th metatarsal owing to a stress fracture.

FIGURE 6 :
FIGURE 6: Short axis T2 FS MR image of the foot demonstrating an intermetatarsal bursitis.