Normal Ankle and Foot

Primary initial evaluation of a majority of foot and ankle pathologic processes, including trauma, infection, tumor, arthritis, congenital or biomechanical dysfunction, hardware placement, and foreign bodies

Detailed evaluation of bone integrity, contour, and joint alignment and efficient initial evaluation of soft tissue and fat interfaces

Readily available, fast, efficient, and inexpensive

Readily distinguishes calcification, ossification, bone proliferation, spurring, periosteal and cortical pathologic processes, and otherwise inconspicuous cortical avulsion fractures

Exposure to ionizing radiation (minimal)

Limited sensitivity and specificity of soft tissue pathology

High-definition film/screen combination for foot and toes

Medium-speed film/double-screen combination for ankle

Details fracture pattern, alignment, apposition, and healing response

Details transarticular osseous pathology, including arthritis, coalitions, and acute or chronic traumatic arthrosis

Primary osseous lesions in patients with contraindication for MRI

Distinguishes osteoid versus chondroid tumor matrix

CT provides excellent osseous detail, and multiplanar and surface reconstructed images provide 3D anatomic and osseous transarticular detail.

A 32- to 64-slice volume acquisition, 3D multiplanar imaging provides reduced beam-hardening artifact seen with conventional CT and allows improved evaluation of hardware placement and alignment.

Reconstructed multiplanar imaging details complex fractures and is used for surgical planning.

Exposure to ionizing radiation

Expensive

Limited sensitivity and specificity of soft tissue pathologic processes; sensitivity has shown to be improved by intravenous use of a contrast agent.

Osteonecrosis, avascular necrosis, soft tissue and osseous tumors, occult fractures, osteochondral pathology, sinus tarsi syndrome, arthropathies, neuropathy, diabetic foot, myopathies, entrapment syndromes, osteomyelitis, synovitis, ligament and tendon pathologic processes, sesamoid disorders, plantar plate pathology, Morton neuroma, turf toe, plantar fascial disorders, non-radiopaque retained foreign bodies

Provides excellent spatial resolution and tissue contrast of soft tissue pathology and delineates intratrabecular, marrow, cortical, and periosteal bone pathologic processes

Allows localization of pathologic processes of osseous, articular, chondral, neurovascular, ligament, tendon, capsular, retinacular, and supporting structures

High sensitivity and specificity for evaluating intra­articular pathology, including nonossified debris and bodies

No ionizing radiation

Expensive

Claustrophobic in closed magnets (high-field open magnet provides high-resolution imaging as an alternative for claustrophobic patients)

Long examination times, leading to patient motion

Contraindicated in patients with many types of implanted mechanical devices, such as pacemakers. Manufacturer guidelines should be referred to before the examination.

Contraindicated for patients with metal in orbits, intracranial aneurysm clips, and surgery within a 6- to 8-week period

Risk of MRI to fetus is unknown in the first trimester of pregnancy and considered a relative contraindication.

There are several variations in imaging protocols: proton density–weighted and proton density–weighted, fat-saturated sequences are obtained in three planes; alternatively, T1- and T2-weighted fast spin-echo, fat-saturated sequences in three planes are also commonly used.

T1-weighted sequences are preferred for demonstrating trabecular detail and osseous pathology and should be used for suspected bone lesions and marrow pathology, such as osteomyelitis.

Short echo-time sequences produce intermediate signal artifact within normal tendons when tendons are at an orientation of greater than 55 degrees to the main magnetic field. This creates false-positive findings. For this reason, higher echo-time proton density, T2-weighted sequences are preferred over T1-weighted sequences for routine imaging of the foot and ankle and, in particular, when a pathologic process of a tendon pathology is clinically questioned. This occurs particularly in the sagittal plane at the retromalleolar course of the peroneal tendons, posterior tibial tendon, and flexor digitorum longus and flexor hallucis longus tendons.

Short tau inversion recovery (STIR) sequences alternatively may be used in place of fat-saturated sequences and provide greater sensitivity for fluid signal, but proton density–weighted and T2-weighted, fat-saturated sequences provide higher spatial resolution, allowing localization of fluid signal denoting a pathologic process.

STIR sequences may be more sensitive for evaluating subtle diffuse pathologic processes of muscle.

To decrease metal hardware susceptibility artifact: Maximize the bandwidth, decrease the echo time, increase the interecho train length, and use STIR sequences in place of fat-saturated sequences. Phase and frequency encoding direction should be parallel to the shaft of the hardware or parallel to the area of interest.

Multiple sequences have been used for chondral pathologic processes. Research continues with specialized sequences, but, to date, proton density–weighted, fast spin-echo, fat-saturated, and STIR fast spin-echo sequences are preferred and widely used.

Postcontrast, fat-suppressed T1-weighted images are useful in imaging inflammatory processes and vascular lesions.

Indirect MR arthrography when images are obtained after 30 minutes has been found to be useful in increasing contrast and spatial resolution of ligament and tendon tears and in visualization of joint debris and chondral defects and pathologic processes.

Direct MR arthrography allows visualization of distention of the joint space and intravasation of contrast medium through the suspected ligament, diagnosis of capsular or retinacular tears, as well as evaluation of stability of osteochondral lesions and detection of intraarticular bodies.

Parameters using a 1.5-T magnet:

• Phased-array four- or eight-channel send-receive coil (standard knee coil with foot chute for ankle and midfoot; wrist coil for forefoot and toes); surface coil for isolated toe pathologic process can be used, but newer phased-array coils provide higher resolution.

• Slice thickness in three planes is generally 3 to 5 mm with field of view of 12 to 16 cm for ankle and midfoot; 2 to 4 mm with field of view of 12 to 14 cm for forefoot; and matrix 256 × 192 to 256.

If signal-to-noise ratio allows, smaller slice thickness can be used for fine anatomic detail of plantar plates, intermetatarsal space, and hallux sesamoid complex.

Tendon tears and pathologic processes, ligament tears, muscle lesions or tears, muscle perfusion, bursal inflammation or fluid collection, soft tissue foreign bodies, nerve inflammation or focal lesions, dynamic imaging of tendon dysfunction, joint effusions, ganglion cysts, and plantar fascial fibromas or tears

Used as a tool to complement MRI and CT

Quick, efficient, dynamic, and noninvasive

Relatively inexpensive

Highly dependent on skill of the technologist and experience of the interpreting radiologist

Dependent on high-resolution and high-frequency transducer and advanced hardware and software ultrasonographic capabilities

Needs 5- to 10-MHz transducer for deep soft tissues and broader field of view

Needs 10- to 15-MHz transducer for superficial ligaments, tendons, and soft tissue structures

Needs 17-MHz transducer or higher for detailed resolution of small field of view and superficial structures

Tissue harmonic imaging and compound imaging provide finer resolution of deep structures and improve real-time spatial resolution.

Power Doppler imaging and use of contrast agents improve the detection of blood flow and muscle and soft tissue perfusion.