![]() ![]() Due to the porous nature of the forearm compartments, often a single fasciotomy will suffice, releasing tension in the remaining compartments. These two compartments are separated by the radius, ulna, and the intraosseous membrane which spans these bones (Fig. Anatomically, the forearm compartments are often incompletely separated, and for the purpose of this discussion, can be thought of as either dorsal or volar (Table 1). In the four compartment classification, there are dorsal, mobile wad of Henry, volar superficial, and volar deep compartments. The description of the forearm is more variable with two, three, and four compartments cited in the literature. The deltoid muscle is not included in the arm compartment schema. The three heads of the triceps muscle (long, lateral, and medial) occupy the posterior compartment in toto. The long and short heads of the biceps make up the superficial muscles of the anterior compartment, while the coracobrachialis and brachialis make up the deeper muscles of the compartment. ![]() The muscle compartments of the arm are separated into anterior and posterior compartments by the medial and lateral intermuscular septa, which are fascia thickenings that extend from the humerus to the deep fascia (Fig. The upper extremity anatomy can be divided into the arm and the forearm (Table 1). The compartments are space-limited areas, confined by the deep and intermuscular fascia. Accurate localization of the abnormalities can help clinicians understand the extent of disease and whether the process is affecting adjacent structures such as bone, joint space, bursa, tendons, and neurovascular bundles. Potentially, cases of suspected but clinically equivocal ACS may benefit from this type of MRI imaging in the future.Ī thorough understanding of compartmental anatomy is necessary to adequately describe MR imaging findings of myositis and its complications. Incorporating an artificial intelligence automation tool could assist with data processing in order to optimize the turn-around time for these cases. Such an endeavor would require specialized software, a streamlined patient selection protocol, and close coordination between the radiologist and technologist however, with the proper equipment, the idea is imaginable. Ultimately, one could conceive of an abbreviated ACS protocol that uses isotopic 3D volume acquisition combined with P31-MRS and MRE to yield anatomic and biochemical information. It remains to be seen whether the muscle compartment tension associated with myositis and ACS coincides with measurable tissue stiffness afforded by MRE. The inherent tissue stiffness affects the speed of the wave propagation, with stiffer tissue inducing faster wave speed. MRE “measures tissue stiffness by encoding displacements due to the propagation of externally induced acoustic waves into the MR phase signal”. Another emerging technology is MR elastography (MRE). reported a scan time of 4 min using a 3T system. MRS uses the relative chemical shift of inorganic phosphate (Pi) and phosphocreatine (PCr) to calculate pH using the Henderson-Hasselbalch equation. Efforts are in progress to measure muscle compartment pH by using an indwelling catheter or needle however, P31 magnetic resonance spectroscopy (MRS) has proven to accurately measure intracellular pH in a noninvasive manner. Conversely, lactate concentration has been shown to increase, leading to decreased tissue pH. Pyruvate and phosphocreatine levels have been shown to decrease in the setting of muscle glycolysis (anaerobic respiration). The inconsistent performance of intracompartmental pressure measurements has spawned research into the biochemical markers of impending irreversible muscle ischemia. However, with the ever-increasing accessibility, more efficient image acquisition, and the clinical use of techniques formerly reserved for research or less acute entities, emergent MRI could be useful in the diagnosis of myositis and even cases complicated by acute compartment syndrome (ACS). In general, MRI has been criticized for long scan times, difficulty of access, and lack of specificity. Occasionally, paraspinal muscles and shoulders are imaged when clinically indicated. The use of composed MR post-processing technique helps to seamlessly link upper and lower components of the arms or legs. The brevity of the protocol reflects the inherent sensitivity of MR in the detection of abnormal muscle signal over specificity. Currently, MR myositis protocols are characteristically brief, using a fluid-sensitive sequence such as short TI inversion recovery (STIR) or proton density with fat saturation in the axial and coronal planes as well as an axial T1 sequence.
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