- Many radiologists approach advanced imaging of the elbow with some degree of trepidation. This likely is more due to a lack of familiarity with the relevant anatomy and pathology encountered in the elbow region than to any related complexity of diagnosis. The elbow is, in fact, a fairly straightforward articulation to master for the practicing radiologist. It is the rarity of elbow MR imaging referrals (relative to the more commonly encountered cases involving the knee, shoulder, and ankle) that often limits practical experience and, therefore, comfort level for many radiologists. This section explores the commonly, and some of the not so commonly, encountered pathologic conditions for which advanced imaging lends advantage.
Terminology and Conventions
- In proper anatomic planes, the radial aspect of the elbow is labeled lateral and the ulnar aspect medial. The terms coronal and sagittal used in these chapters refer to the anatomic planes as defined by the inherent anatomy of the elbow and, thus, frequently will be oblique planes (with reference to the machine axis) for any given patient. As in other chapters, degenerative changes within a tendon are referred to as tendinopathy and not as tendinitis or tendinosis in an effort to stay true to the appropriate etymologic meanings of these terms.
- The elbow is, at its simplest, a hinge joint at the ulnohumeral articulation. There is also a rotational component at the radiocapitellar and proximal radioulnar joints to allow pronation and supination of the forearm. Capsular thickenings of the joint medially and laterally form the ulnar collateral ligament complex and the lateral collateral ligament complex, respectively. Tendon insertions of the muscles of elbow flexion (biceps brachii and brachialis) and extension (triceps brachii) occur on the bones immediately adjacent to the joint. Origins of the major flexor and extensor tendons of the wrist and fingers arise from the medial and lateral humeral epicondyles, respectively. Three major nerves pass through the elbow region and can be injured here. The ulnar nerve is particularly vulnerable because of its superficial location in the cubital tunnel, but the median and radial nerves can be affected by a variety of congenital and acquired conditions as they pass through their respective courses in the elbow.
- Several imaging pitfalls in the evaluation of the elbow, particularly for MR imaging, have been described.
- Injury to the elbow may be due to a discrete traumatic episode, which more often results in osseous injury or ligament tear, or to chronic repetitive injury, which may result in a variety of pathology involving bones, ligaments, or tendons. Injury patterns also vary depending on the age of the patient. When relevant, issues specific to the pediatric patient have been identified. Fractures of the distal humerus, proximal ulna, and proximal radius are usually the result of a discrete episode of trauma and may be associated with substantial soft tissue injury around the elbow joint as well as other traumatic lesions elsewhere in the body. Grading systems for these injuries tend to focus on criteria that help determine the method of treatment and are described in the appropriate chapters.
- Ligament tears can be due to either acute trauma or repetitive injury and may manifest as chronic pain or with sudden onset of symptoms during activity. Ulnar collateral ligament tear is the most common and significant elbow ligament injury and often affects overhead-throwing athletes. One type of repetitive injury in particular has been paid special attention: athletes who perform overhead-throwing motions are prone to develop a particular pattern of injury due to valgus stress placed on the elbow during these activities, resulting in impaction injuries in the lateral aspect of the elbow and distraction injuries in the medial aspect. This pattern is important to recognize: when caught early in its course, appropriate treatment can make the difference between return to play and a career-ending injury.
- Radiographic evaluation of the elbow should include, at a minimum, AP and lateral views. In the setting of trauma, an oblique view should be added, usually a dedicated radial head view to search for subtle radial head fractures. This involves 90° of elbow flexion and a 45° tube angulation toward the radial head to best profile the radial head and neck.
- CT of the elbow usually is performed for surgical planning in the setting of complex fractures or to assess reduction of fracture-dislocations. As such, most elbow CTs are performed with the elbow in a cast or splint with the elbow flexed. Ideally, the patient will place the elbow overhead for scanning, reducing artifact that occurs when scanned at the patient's side or with the elbow lying on the abdomen. Unfortunately, many trauma patients cannot achieve the overhead position, and images are degraded by inclusion of the adjacent abdomen, spine, and ribs within the scan beam. Moreover, the cast material degrades image quality to some degree. With the elbow flexed, but often not at precisely 90°, the technologists must be diligent to provide reformatted images that are precisely orthogonal to the planes of the distal humerus (axial, coronal, and sagittal) as well as for the proximal forearm.
- US evaluation of the elbow has been shown to be effective for tendon pathology. Injury to the biceps, triceps, flexor, and extensor tendons can be evaluated accurately using US imaging. US has also been shown to be useful in the assessment of the ulnar collateral ligament of the elbow. US provides the additional benefit of offering dynamic, real-time assessment of structures with the elbow in various positions of function in order for nondisplaced ligament tears, lax ligaments, and subluxating structures to be assessed. Limited evaluation of some articular cartilage surfaces can be achieved, but a high false-negative rate indicates that MR imaging remains the strategy of choice in the evaluation of articular cartilage and marrow.
- When making the choice between US and MR for the elbow, often the question comes down to whether the goal is to assess the presence or absence of a specific injury or abnormality. If so, then US may be most expeditious. If the clinical picture is less certain, MR offers a more comprehensive assessment of all tissues within the elbow.
- MR imaging of the elbow can be challenging to perform well. Two positioning strategies are available. The preferred method puts greater demands on the patient (in terms of positioning and comfort) and the MR imaging technologist (in terms of correctly positioning the patient and performing the study) but results in more clinically satisfying images. This usually involves placing the extended elbow in a volume coil (such as a clamshell-phased-array knee coil), which serves as a rigid frame in which to steady the extremity but may cause uncomfortable compression of tissues. The use of such a coil often requires that the patient be scanned with their arm raised over the head since the coil must be placed in or near the magnetic isocenter. Most patients prefer to accomplish this while lying prone as opposed to supine. Generous padding with cushions and pillows may mitigate some of the positional discomfort, but the technologist should warn the patient that the extremity may become painful or even numb during the imaging process as a result of the position in which it is kept. A tailored approach to imaging, in which the most clinically desired sequences for the patient's symptoms are acquired 1st, is advised.
- The 2nd strategy involves letting the patient lie prone, arm at the side (with elbow extended), and using a flexible surface coil to image the elbow. Though more comfortable for the patient and generally easier for the technologist, this method often renders less desirable images due to limited signal-to-noise ratio (SNR), poor fat suppression due to inhomogeneity of the magnetic field at its periphery, surface coil artifact (brighter tissue signal near the coil with rapid drop-off toward the deep portion of the extremity), and artifact from motion of the adjacent torso (respiration and other movements). In fact, the arm-at-the-side position may not be possible to accomplish in some larger patients due to restrictions of the magnet bore caliber.
- A 3rd option is to acquire images of the elbow in an open-sided magnet for the elbow to remain in the center of the magnetic field, but the arm can be held only slightly abducted to the side. Field strength of such magnets is usually limited, and coil selection is more limited.
- Whichever method is employed for elbow imaging, the forearm will likely be held in some degree of pronation. Though true anatomic position implies supination of the forearm, in clinical practice, it can be unreasonable to expect patients to hold their arm in a supinated position for 20-30 minutes. A moderate degree of pronation is acceptable and is to be expected. This generally causes no diagnostic challenge, but one will notice that the radial tuberosity is rotated medially with associated curving of the distal biceps tendon.
- Once appropriate positioning has been achieved and scout images acquired, it is important that true anatomic planes of imaging be produced. For coronal images, slices should parallel a line drawn on a true axial image through the widest point of the medial and lateral humeral epicondyles. Sagittal images are prescribed 90° to the coronal images. Educating the MR technologists working in one's practice regarding these positioning and scan prescription guidelines will prove invaluable and should curtail the incidence of suboptimal scans and patient callbacks.
- The use of MR arthrography has been advocated for the detection of certain conditions, chiefly undersurface partial tears of the ulnar collateral ligament and cartilage defects. With modern MR imaging equipment and appropriate sequence selection, MR arthrography can add much to the diagnosis and characterization of these lesions. Some authors have advocated the use of indirect arthrography, in which gadolinium contrast is injected intravenously, and delayed imaging of the elbow is performed after exercising the extremity to increase synovial blood flow and, thus, diffusion of intraarticular contrast into the joint fluid (as an alternative to direct puncture arthrography). Similarly, the addition of the FABS (elbow flexed, shoulder abducted with supination of forearm) position for evaluation of the distal biceps tendon has been described. An example is provided in the corresponding chapter. Intravenous contrast administration is usually reserved for the assessment of inflammatory or neoplastic pathology (synovitis, infection, or soft tissue masses).
- As with other joints discussed in this book, personal preferences regarding sequence selection vary widely among individuals who perform and interpret MR imaging of the elbow. Rather than take a purely prescriptive approach, it is best to state general guidelines for this purpose. At least 1 sequence in each anatomic plane (axial, coronal, and sagittal) should be acquired. If an isotropic volume acquisition is used and diagnostic reformatted images can be created in multiple planes, then of course the 3-plane rule may not apply. It also is advisable to acquire at least 1 T1-weighted sequence without fat suppression as a way to properly characterize tissue signal and marrow. A fat-suppressed proton density-(intermediate-) weighted or fat-suppressed T2-weighted (or STIR) MR sequence in some or all planes allows one a way of studying both anatomy and tissue characteristics. Proton density-weighted sequences without chemical fat suppression provide excellent anatomic detail with high SNR but are not particularly useful for tissue characterization. In general, one tends to develop a protocol that provides an acceptable combination of speed, variety, and reliability and become comfortable with it.
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