Imaging studies are a component of the clinical evaluation of a patient with a condition affecting the foot or ankle. After completion of a careful history and physical examination, imaging studies usually are obtained as the next step in determining a diagnosis and treatment plan. Each of the available imaging modalities has appropriate indications and applications.

Plain radiography is the foundation of most diagnostic foot and ankle imaging. Bone anatomy, integrity, and alignment as well as joint congruency can be assessed through a routine set of radiographs, which usually can be obtained during the patient’s initial office visit. Weight-bearing radiographs must be obtained whenever possible. Although a patient with an acute traumatic injury may only tolerate non-weight-bearing radiography to assess structural anatomy and rule out the presence of fracture, these radiographs do not show the foot or ankle in a physiologic position and therefore may not facilitate detection of malalignment or some pathologic conditions.

The popularity of digital radiology has eclipsed that of printed radiographic studies during the past few years. Radiographs are captured digitally in two ways.¹ Computed radiography is affordable, offers excellent image quality, and uses existing radiography systems.

True digital radiography requires more expensive technology than traditional radiography or computerized modification but offers greater efficiency and the capacity for higher image quality and a lower radiation dosage. Images are stored in a picture archiving and communication system rather than on hard copy radiograph. The most commonly used format for medical images is Digital Imaging and Communication in Medicine (DICOM), which allows safe, high-quality off-site viewing and analysis of radiographs. Digital radiography also allows computer-assisted measurements of distances or angles to be generated for use in diagnosing pathologic foot and ankle conditions.

Plain radiographs can be used to observe secondary changes caused by tendon dysfunction but cannot be used to evaluate the tendons themselves. Tenography involves the injection of contrast material into the tendon sheath under fluoroscopy, often followed by a steroid injection. Tenosynovitis, stenosing tenosynovitis, and a tendon tear or rupture can be seen. Tenography can be particularly useful in a posttraumatic setting in which the value of other diagnostic modalities is limited by the presence of anatomic deformity or associated hardware (eg, in peroneal impingement after calcaneal fracture).⁶ In the absence of a tendon tear, intrasheath steroid injection with tenography relieves symptoms in some patients with tenosynovitis around the foot and ankle.⁷ This cost-effective but invasive technique is associated with a small risk of tendon rupture. Tenography has been less frequently used for diagnostic purposes since the widespread adoption of MRI.

Ultrasonographic evaluation of tendons around the foot and ankle offers several advantages over other modalities. Ultrasonography is cost-effective, safe, noninvasive, and not based on radiation. Structures are evaluated in real time. Dynamic evaluation of tendon function allows conditions such as peroneal subluxation or dislocation to be directly visualized. The transducer can be manipulated to avoid interference from any metallic implant in the ankle. This ability is useful in evaluating tendon injury after hardware placement in the foot or ankle, which would create interference during MRI. Ultrasonography also avoids the possibility of the so-called magic angle phenomenon on MRI (artifact seen in tendons oriented 54.7° to the magnetic field), which can inaccurately suggest the presence of intratendinous pathology. Ultrasonography is slightly less sensitive than MRI for use in diagnosing tibialis posterior tendon pathology, but a recent study found that discrepancies did not result in altered clinical management.⁸ Ultrasonography has high sensitivity and specificity for detecting peroneal tendon tears and 100% positive predictive value for detecting peroneal subluxation, as correlated with intraoperative findings.⁹

For evaluating tears in the lateral ankle ligament, ultrasonography has sensitivity and specificity comparable to that of MRI. Morton neuroma, recurrent interdigital neuroma, and other soft-tissue masses or cysts can be detected on ultrasonography after an equivocal clinical evaluation.

Most modern ultrasonography machines are able to evaluate regional blood flow. The movement of blood cells within the vessels causes a change in the pitch of reflected sound waves called the Doppler effect. A computer converts the Doppler sounds into colors that are overlaid on the musculoskeletal ultrasonographic image. The presence of local hyperemia secondary to inflammation can confirm that the ultrasonographic findings are consistent with the pathology or symptoms.

The primary disadvantage of ultrasonography is that it is a technician-dependent modality. Many medical centers lack a radiologist trained and experienced in interpreting the results of musculoskeletal ultrasonographic studies. Ultrasonography offers only poor visualization of bone. Because direct skin contact and the application of gel are required, ultrasonography cannot be used through a cast or splint.

Recent advances in technology have led to improvements in the quality, portability, and cost of ultrasonography machines, and office-based ultrasonography is now feasible. To complement the clinical examination and standard radiographic evaluation, a diagnostic ultrasonographic examination can be done during the patient’s initial visit to the foot and ankle surgeon. Ultrasonography can be particularly useful in making a subtle diagnosis and planning treatment; for example, a determination that the patient has a Morton neuroma in the second web space rather than early plantar plate dysfunction precludes the use of a cortisone injection, which can cause metatarsophalangeal instability.¹⁰ In-office ultrasonography also can be used to identify tendinosis, a tendon injury around the foot and ankle, a mass or tumor, or a nonradiopaque foreign body such as a wood splinter. Ultrasonography can be used to improve the accuracy of diagnostic injections in the foot or ankle, guide injections for a condition such as Morton neuroma or plantar fasciitis, and guide the aspiration of cysts. The use of ultrasonography was found to improve the accuracy and outcome of therapeutic injections and thereby reduce the need for additional procedures.¹¹

CT provides high-resolution thin-slice studies of the foot and ankle. The development and implementation of multidetector-row slip-ring technology has resulted in improved imaging of osseous structures. Source images acquired in the axial, coronal, or sagittal plane allow three-dimensional views of the bony anatomy for diagnosis and therapeutic planning. Relatively new computer software creates multiplane reconstructions without increasing the patient’s exposure to ionizing radiation; for example, coronal images can be constructed from axial images, and three-dimensional reconstruction models can be created from the initial source images (Figure 1).
Around the foot and ankle, studies with 3-mm or thinner slices are routinely recommended. The foot position in the scanner is important because the source image planes are obtained parallel to the osseous structures of the foot in each plane using a tomographic scout localizer image. The presence of cast or splinting material around the region does not interfere with CT but may create difficulty in positioning the foot. The advent of multidetector-row systems has permitted rapid image acquisition and high-quality three-dimensional reconstruction to be achieved with decreased radiation dosages.

In the foot and ankle, CT is most commonly used to assess bony abnormalities and is particularly useful in providing fine osseous detail not obtainable with plain radiography. Fracture, infection, osteochondral injury (requiring accurate assessment of the bony anatomy and lesion size), arthritis, osteonecrosis, and osseous tumor or congenital abnormality can be diagnosed using CT. This is particularly useful in suspected subtle fracture, joint diastasis, or articular incongruity not detectable on plain radiography, as in suspected Lisfranc injuries, syndesmotic injuries, and tibial plafond fractures. In addition, CT can be used to assess the extent of joint comminution in calcaneal or plafond fractures and to assist in preoperative planning (Figure 2).

CT has high accuracy and sensitivity for the detection of fracture nonunion and is more reliable than serial radiographic studies for assessing the extent of union after surgical arthrodesis¹² (Figure 3). The use of micro-CT and nano-CT technology is likely to further enhance the accuracy of this modality. The interpretation of postoperative CT can be negatively influenced by metallic artifact or scatter from internal hardware. Relatively new software uses metal deletion techniques to limit this effect and improve the accuracy of study interpretation.

Current CT scanners use multidetector configurations in which a narrow, fan-shaped x-ray beam and detector rotates around the patient to quickly acquire multiple image sections. This technology cannot be used with the patient in a weight-bearing stance, as is needed to evaluate extremity alignment and deformity during physiologic loading. Axial loads can be applied to joints to simulate weight bearing when the patient is supine, but the resulting studies have questionable accuracy. The recently developed cone-beam CT technology uses a pyramid-shaped x-ray beam and a large-area detector to obtain volumetric data from multiple projections through a single rotation around the patient, who is in a standing, weight-bearing position.¹³ The technique for image reconstruction is similar to that used for multidetector CT. The three-dimensional data obtained from cone-beam CT allow a much improved quantitative analysis of many pathologies. Other advantages of cone-beam CT over multidetector CT include more rapid acquisition of images, superior image quality, and lower radiation dosages (approximately 9 mGy compared with 27-40 mGy). The compact, portable design of the cone-beam CT machine has positive implications for workflow and storage. Cone-beam CT technology has the potential
to be extremely valuable in musculoskeletal extremity imaging from both clinical and economic viewpoints.

Weight-bearing CT (WBCT) has recently become available for clinical use. This technology allows for three-dimensional imaging of the foot and ankle under the axial load of the patient’s body weight. Various potential advantages of WBCT over non-weight-bearing CT have been proposed including the improved ability to evaluate deformity throughout the foot and ankle, determine the extent of joint space narrowing of arthritic joints, and assess for bony impingement that may only be radiographic apparent with weight bearing.¹⁴ WBCT has been used as a research tool to define normal functional anatomic and physiologic relationships including the orientation of the subtalar joint, fibular rotation, and translation at the distal tibiofibular joint with tibiotalar motion and talus rotation within the ankle mortise with weight bearing.^15,16