Variants and Pitfalls in Musculoskeletal Imaging

The practice of musculoskeletal radiology has become increasingly complex, with growing utilization of advanced imaging modalities. High-resolution ultrasonography (US) and magnetic resonance (MR) imaging are both powerful diagnostic imaging tools that are currently used in daily practice for the evaluation of a whole spectrum of musculoskeletal disorders. These applications include trauma to various structures such as bones, joints, tendons, ligaments, muscles, and nerves, as well as various diseases affecting these structures such as inflammatory arthritides, infections, tumors, and tumor-like conditions.

Although the latest US and MR imaging machines are able to show a larger number of musculoskeletal structures in more detail and with greater spatial resolution than ever before, a downside is the increased detection of clinically unsuspected anatomical variants and a generation of a variety of artifacts peculiar to these imaging modalities. Failure to recognize these anatomical variants and imaging artifacts may lead to diagnostic error and misinterpretation, and possible medicolegal problems. Inadequate imaging technique, lack of training/inexperience, and failure to correlate with other imaging findings, particularly radiographs, are other potentially correctable pitfalls that may affect radiologists.

This issue of Seminars in Musculoskeletal Radiology aims to highlight variants and pitfalls commonly encountered in musculoskeletal radiology. Contributors from various centers in diverse localities worldwide share their experiences in detecting and dealing with artifacts produced during musculoskeletal US and MR imaging. Variants and pitfalls in MR imaging of various regions such as the spine, shoulder, knee, and foot and ankle, and imaging pitfalls affecting the diagnosis of musculoskeletal tumors and infections are also discussed.

For the diagnosis of some musculoskeletal injuries and diseases, high-resolution US is comparable with MR imaging. US is advantageous in many circumstances due to its superior spatial resolution and ability for dynamic assessment. However, musculoskeletal US is highly operator dependent, and the examiner has to be knowledgeable about the complex anatomy of the musculoskeletal system, as well as US imaging technique. Additionally, familiarity with several common imaging artifacts in musculoskeletal US, which may be mistaken for pathology, as well as artifacts that frequently accompany pathologic conditions, is a key factor. Taljanovic and colleagues from Arizona discuss common artifacts seen in musculoskeletal US, and they highlight techniques to avoid or minimize these artifacts during clinical US examination.

MR imaging has become an important diagnostic tool in evaluation of a vast number of pathologies, particularly in evaluation of the spine, joints, and soft tissue structures of the musculoskeletal system. However, MR imaging is susceptible to various artifacts that may affect the image quality or even simulate pathologic lesions. Some of these artifacts have become more prominent with higher field strength magnets and with increasing use of MR imaging in postoperative patients, especially those with implants. Artifacts may arise from patient motion or periodic motion and from various protocol errors producing saturation, wraparound, truncation, shading, partial volume averaging, and radiofrequency interference artifacts. Susceptibility artifact occurs at interfaces with different magnetic susceptibilities and assumes special importance with the increasing use of metallic implants. Magic angle phenomenon is a special type of artifact that occurs in musculoskeletal MR imaging. Singh et al emphasize the importance of recognizing these artifacts and suggest ways to correct or minimize them.

In many centers in Singapore, MR imaging is the imaging tool of choice for the assessment of lesions affecting the spine, as well as injuries affecting the shoulder, knee, and foot and ankle. Although it enables excellent visualization of soft tissue anatomy and produces good soft tissue contrast and tissue characterization, numerous potential pitfalls may affect interpretation of MR imaging of these structures that may lead to diagnostic errors. These errors may be due to various factors such as poor technique, imaging artifacts, failure to recognize normal structures or variants, and lack of clinical or radiographic correlation. Most of these factors are potentially preventable, and their recognition is crucial in providing an accurate diagnosis. Articles authored by Shikhare et al, Al-Riyami et al, Tan et al, and Othman et al aim to illustrate the various types of potential pitfalls that may be encountered during interpretation of MR imaging of the spine, shoulder, knee, and foot and ankle, respectively, and ways to identify and counter them.

Although MR imaging has been used extensively in the evaluation of musculoskeletal tumors, nontumoral or tumor-like lesions may have similar imaging findings. The imaging features of certain normal, reactive, benign neoplastic, inflammatory, traumatic, or degenerative processes in the musculoskeletal system may mimic malignant tumors. Misinterpretation of the imaging findings can lead to inappropriate clinical management of the patient. Arkun and Argin from Turkey review and describe the MR imaging characteristics of nontumoral bone lesions that are located in the marrow cavity, cortical bone, or in both, and soft tissue lesions, which may be misinterpreted as sarcoma.

US has become the first-line investigation for investigation of soft tissue masses in some centers. With increasing experience, most soft tissues masses, particularly superficial ones, can be diagnosed, based on a conundrum of clinical and US findings. Putting a specific label onto a soft tissue mass, such as a lipoma, nerve sheath tumor, or giant cell tumor of tendon sheath, minimizes the need for percutaneous biopsy, greatly enhances clinical efficiency with regard to discussions on management and outcome, as well as immediately reduces patient anxiety with regard to the presence of malignancy. Hung and Griffith from Hong Kong address the general approach to US of soft issue masses, highlighting in particular the common pitfalls encountered in the diagnosis of soft tissue tumors.

Early diagnosis of musculoskeletal infection helps lead to timely and appropriate treatment and the prevention of potentially harmful complications. However, diagnosis can be challenging because the clinical patterns are variable, depending on the virulence of pathogen and host immune response. Imaging studies helps detect infection, define extent, and localize target areas for intervention. Many pitfalls in the imaging of musculoskeletal infections are related to the nonspecific imaging features of the disease that can mimic other entities. Imaging may fail to detect infection in the early stage or under- or overestimate the severity and extent of the disease. Pattamapaspong and colleagues from Thailand highlight some pearls and common pitfalls in the interpretation of imaging of musculoskeletal infections. To avoid these pitfalls, radiologists have to be familiar with the patterns of infections and oversee selection of appropriate imaging modalities.

In summary, recognition of various artifacts, variants, and other potential pitfalls encountered in musculoskeletal imaging should help the practising radiologist achieve a more accurate diagnosis in daily clinical practice.