Effective Use of Imaging

Although valuable as a diagnostic tool, imaging should only be requested if the result will be used to guide management and is most effective when used to confirm or exclude a specific clinical hypothesis or to narrow a clinical differential. In this setting, the test has a specific purpose, and the result has direct clinical meaning. The importance of a thorough clinical assessment therefore cannot be overemphasized, because it is only on the basis of specific questions that arise from the history and physical examination that the most appropriate imaging test(s) can be chosen. A valid clinical role for imaging can include the detection or exclusion of sinister or systemic disease when clinical “red flags” are present (Table 3-1).

It is important that the request form should, apart from describing the general symptomatic context (e.g., forefoot pain), further state the actual question(s) to be answered (e.g., “stress fracture second metatarsal?”). This allows the supervising radiologist to both optimize the imaging protocol and provide a clinically relevant report.

Incidental imaging findings are common and can include normal variants, age-related degenerative changes, old posttraumatic changes and old surgical changes. Such findings can be problematic, particularly when imaging is used in place of a proper physical examination.

Choice of Test

Any imaging workup should generally commence with plain radiographs. This low-cost approach carries an acceptable low radiation dose and provides basic diagnostic information that cannot be readily obtained by any other means (e.g., bone alignment in the weight-bearing foot). When combined with a careful clinical assessment, plain films alone are often sufficient for both diagnosis and management. When negative, they are still useful in ruling out common differential considerations and demonstrating features that sophisticated tests alone, such as magnetic resonance imaging (MRI) and ultrasound, may otherwise entirely miss or misinterpret (e.g., soft tissue calcifications, foreign bodies, bone spurs and loose bodies, accessory centers of ossification, deformities and malalignments, old injuries, anatomic predispositions, bone tumor characterization). Particularly in the current era of MRI, the fundamental and enduring importance of plain radiographs should never be forgotten.

When further imaging is required, the choice of exactly which test to order can sometimes be challenging because a wide range of imaging tests and variations in imaging technology are available. In general, the specialist foot and ankle surgeon who is expected to resolve a difficult clinical problem should select the locally available test that is most reliable for the diagnosis under consideration. This may vary in any given locale for reasons such as equipment availability and radiologist expertise. Other factors that may influence the choice of imaging test can include (1) patient concerns, such as cost, convenience, discomfort; (2) safety risks, including patient age, radiation dose, and contrast sensitivity; and (3) costs to third parties, such as sporting clubs, insurers, or the taxpayer.

Radiation Safety

Plain radiographs, computed tomography (CT) scans and isotope bone scans all expose patients to ionizing radiation that carries a risk of both cancer induction and genetic damage. This risk is well established at high doses (e.g., Hiroshima victims) but remains uncertain at low doses, such as those used in medical imaging. Because all populations are exposed to the natural background radiation of cosmic rays (Table 3-2) and also have an incidence of cancers arising from causes other than radiation, it is statistically difficult to evaluate the cancer risk associated with medical radiations at doses of 100 mSv or less.²⁵ A cautious “linear no-threshold model” has therefore been adopted that assumes that even the smallest radiation dose carries a level of risk and that this risk increases linearly with cumulative lifetime exposure. It is also well recognized that, depending on age, children can be up to 10 times more sensitive to ionizing radiation than adults, and an actual effect equivalent to one excess case of leukemia and one excess case of brain tumor per 10,000 head CT scans has recently been shown for the first time in children younger than 10 years who underwent multiple CT head scans with a cumulative dose of 50 to 60 mGy.⁵⁷

These risks should be kept in perspective. The radiation dose associated with a basic three-view foot radiograph is at least 10 times lower than the exposure associated with a single trans-Atlantic air flight (Table 3-2), and it is generally accepted that medical imaging tests are justified if there is a reasonable likelihood they will provide a health benefit or inform patient management.⁶ Nevertheless, medical exposures should always be kept “As Low As Reasonably Achievable” (ALARA principle), and alternative tests, such as ultrasound and MRI, which avoid ionizing radiations altogether are preferable to CT or isotope studies whenever they offer equivalent or superior diagnostic efficacy, particularly in children and especially for follow-up imaging. In clinical practice, radiologists normally exercise caution and specify imaging protocols that minimize the ionizing radiation dose. The surgeon should also feel comfortable that the doses involved in foot and ankle imaging are mostly extremely low (see Table 3-2). Last, the American Association of Physicists in Medicine has recently issued a reassuring policy statement that asserts that “the risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent.”¹

1. Ankle impingement: Lateral dorsiflexion or plantar-flexion views (Fig. 3-12); “lazy” lateral view (Fig. 3-13)

2. Talocalcaneal coalition: Harris-Beath view (Fig. 3-14), oblique views (Fig. 3-15), Brodén views (Fig. 3-16)

3. Trauma: AP view of proximal fibula if Maisonneuve fracture suspected (Fig. 3-17); reverse oblique view of ankle if medial malleolus fracture suspected (Fig. 3-18)

4. Ankle instability: Stress views of ankle joint (Figs. 3-19 and 3-20); stress view of distal tibiofibular syndesmosis (Fig. 3-21)

5. Heel: Lateral and axial views of calcaneus (Fig. 3-22)

6. Navicular: Navicular view (Fig. 3-23)

7. Talus: Talar neck view (Fig. 3-24).

8. Tarsometatarsal joints or base of second metatarsal: Plantar-dorsal projection of midfoot (Fig. 3-25)

9. Toes: Lateral and lateral-oblique phalangeal views (Fig. 3-26)

10. Sesamoids: Axial, medial-oblique, lateromedial, and stress views of sesamoids (Figs. 3-27 to 3-30).

Basic Science

Ultrasound (US) uses the SONAR (SOund NAvigation and Ranging) principle to build up cross-sectional images of soft tissue anatomy. Sound waves above the pitch of human hearing (typically in the 5- to 18-MHz range for musculoskeletal imaging) are generated by a handheld transducer applied to the skin. These propagate through the tissues and are reflected by various acoustic tissue interfaces as returning echoes of differing intensity and time-of-flight that are then assembled line by line to form a “gray-scale” image. The whole process occurs rapidly enough to allow real-time imaging.

Use of Ultrasound

The advantages of US include low cost, patient comfort, an absence of hazardous ionizing radiation, the clinically interactive nature of the test (Fig. 3-38), the ability to assess tissue dynamics such as tendon glide and stability (Fig. 3-39), the ability of Doppler techniques to detect soft tissue hyperemia and neovascularity (Fig. 3-40), and the ability to guide diagnostic and/or therapeutic injections accurately (Fig. 3-41). The clinical aspect of directly probing the visualized anatomy with an US transducer allows the operator to accurately correlate any site(s) of pain and/or tenderness with the imaging appearance. This has been described as “sonopalpation” and can be very helpful in confirming a symptomatic structure or clinically relevant finding. Similarly, the demonstration of locally increased tissue vascularity using Doppler techniques serves to increase the likelihood of a gray-scale finding being relevant to symptoms.⁶²

The limitations of US include an inability to see through bone/gas/metal, image quality degraded by obesity and marked subcutaneous edema, reduced resolution and sensitivity at tissue depths greater than 3 cm, the local availability of a reliable and well-trained operator, the “keyhole” nature of images that may be difficult for the surgeon to orient and understand, and image artifacts that can limit visualization of some tendon segments or be a diagnostic pitfall for the inexperienced (Fig. 3-42).

US is well suited to the assessment of many superficial tendons and ligaments (Figs. 3-43 and 3-44), foreign bodies, ganglion cysts (Fig. 3-45), and other superficial masses (Fig. 3-46; see Fig. 3-38). In addition, because metal does not distort the sonographic image, US is a good alternative to MRI for the assessment of soft tissue structures adjacent to orthopedic hardware (Fig. 3-47).

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