Imaging Modalities

Because of the common nature of these injuries, many muscle strains are treated clinically. However the clinical scenario may be unclear and grading of injury may be difficult. Imaging may help delineate the presence and extent of muscle injury. The main modalities used for evaluation almost exclusively include MRI and ultrasound.

Radiographs are useful for evaluating bony avulsion injuries in adolescents particularly of the pelvis that can be missed with MR and ultrasound. Subtle areas of soft tissue swelling and unexpected bone-related problems (tumor, stress fracture, and so forth) might be detected with plain films. While cross-sectional imaging findings of muscle strain were originally described with computed tomography (CT), currently CT has little role for evaluating acute muscle injury because of its relative lack of tissue contrast as compared with MR [5]. It is useful to evaluate osseous structures associated with avulsion injuries and complications like myositis ossificans.

At some institutions ultrasound may be the preferred primary modality for evaluation of muscle injury because of its portability, ease of use, and decreased cost. While ultrasound does have excellent spatial resolution, the contrast resolution is not as good as MR particularly in the subacute or chronic phases when injury-related edema begins to resolve. Also, because sound waves dissipate and do not reflect over long distances, evaluation of deep structures in athletes with bulky musculature may be difficult. Evaluation of more superficial structures such as the patellar tendon is easier with ultrasound. Another relative disadvantage is the significant reliance on operator skill and expertise that can only be achieved with dedication and practice.

At our institution we prefer evaluation of muscle injuries with MR because of its superior soft tissue contrast, excellent spatial resolution, and reproducibil-ity. Our typical protocol uses a combination of T1- and T2-weighted sequences to emphasize anatomy and pathologic edema. Fatty structures appear bright on T1-weighted images (and some T2-weighted images, ie, fast spin echo) and muscle has intermediate signal intensity allowing for excellent anatomic detail of fat planes. In general, fluid-sensitive or T2-weighted images, allow easy visualization of mobile water protons, which means that pathologic processes involving edema, like muscle strains, are easily detected. Contrast resolution is increased when fat signal is nullified on fast spin echo T2-weighted images with specific chemical fat-saturation pulse (ie, fat saturation). Alternatively, fluid sensitivity may be achieved when a more diffuse nullifying signal is employed that limits non-water signal (ie, inversion recovery [IR] or STIR sequences). Either sequence is considered fluid sensitive and essential for the evaluation of muscle strain injury.

Anatomic coverage includes long and short axis imaging of the region or muscle of interest. We generally use a body coil to include both thighs and lower legs depending on the area of concern to allow for comparative analysis of anatomy in the symptomatic and asymptomatic extremity. Others prefer dedicated unilateral imaging of the injured extremity. Studies have shown that hamstring injuries can occur at multiple sites and involve multiple muscles and therefore thorough evaluation along the course of the muscle group is needed not just the area of pain [9,13,14].

For the screening protocol of the thigh or lower leg we include coronal T1, coronal IR, axial T1, and axial T2 fat-saturated images. Depending on the clinical scenario we may add additional sagittal T1 or fluid-sensitive sequences perhaps in the case of an ischial tuberosity avulsion. The important concept is to include short and long axis imaging of the structures of interests with T1 and fluid-sensitive sequences for each.

Intravenous gadolinium contrast is used very sparingly for routine cases of clinically suspected muscle injury. Some have suggested that low-grade injuries that appear normal on fluid-sensitive sequences may be seen with postintra-venous contrast imaging although this report was only a case series of four athletes with high clinical suspicions of injury [15]. Others have found intravenous contrast imaging useful for evaluating symptomatic proximal adductor insertional injuries. These contrast-enhanced images revealed enhancing teno-periosteal granulation tissue associated with symptoms and partial healing [16]. Contrast should be used when cases of infection, tumor, or myositis are within the differential (Fig. 1).

MR appearance of myotendinous injury has been well described [10,17-20]. Type 1 injuries demonstrate bright signal on fluid-sensitive sequences representing fluid and hemorrhage around the myotendinous unit extending into the adjacent muscle creating a feathery appearance. The myotendinous junction usually appears normal and there is typically less than 5% involvement of muscle fibers (Fig. 2A). Type 2 injuries of the myotendinous junction are more severe and may show a thin or irregular appearance of the myotendinous junction itself along with edema and hemorrhage (increased T2 signal intensity) that often tracks along the fascial plane. However, increased T2 signal intensity changes in strain injury may not necessarily be related to hemorrhage. One recent study evaluated hamstring strain injuries and included gradient sequences, which are highly sensitive for detecting blood products, and found only 1 case of 37 had the typical blooming artifact associated with blood products [14]. Another article has characterized hematoma as a pathognomonic finding of type 2 injury [17] (Fig. 2B). Type 3 injuries reveal complete disruption

Unresponsive Patient
Fig. 1. (A) Enhanced axial T1-weighted image with fat saturation of the calf showing enhancing muscle with areas of nonenhancement compatible with necrosis in this patient found unresponsive. (B) Peripheral enhancement of the calf muscles in a patient with dermatomysositis.
Posterior Calf Muscles

Fig. 2. Coronal fluid sensitive images of posterior thighs demonstrating (A) Type 1 muscle strain injury with mild feathery edema along the intramuscular myotendinous junction of biceps femoris in a professional football wide receiver; (B) Type 2 injury of the proximal myotendinous junction of biceps femoris with intramuscular hematoma formation; and (C) Type 3 injury proximal biceps femoris with retraction of the tendon (arrow) in a professional football cornerback.

Fig. 2. Coronal fluid sensitive images of posterior thighs demonstrating (A) Type 1 muscle strain injury with mild feathery edema along the intramuscular myotendinous junction of biceps femoris in a professional football wide receiver; (B) Type 2 injury of the proximal myotendinous junction of biceps femoris with intramuscular hematoma formation; and (C) Type 3 injury proximal biceps femoris with retraction of the tendon (arrow) in a professional football cornerback.

and discontinuity of muscle typically at the myotendinous junction with complete replacement of organized collagen with fluid signal on fluid sensitive sequences. There is often an associated wavy tendon morphology and retraction. Surrounding edema or hemorrhage is usually extensive (Fig. 2C). MR findings usually correlate with the clinical grading scheme and can help differentiate mild injury from partial tears and referred pain in clinically indeterminate cases [21].

Epimyseal or peripheral injury not associated with myotendinous injury has also been described in the hamstring and quadriceps muscles and manifests as peripheral edema in the muscle extending to and around the epimysium [8,9]. Contusions of muscle are a result from direct trauma (ie, football helmet), and may predispose to hematoma formation. Infiltrative focal edema is a typical finding on fluid-sensitive sequences and may resemble muscle strain. MR appearance of contusion is typically that of increased size with intact muscle fibers and increased fluid signal that is diffuse or geographic with feathery margins [10] (Fig. 3).

Hematoma may result from direct trauma associated with contusion or related to myotendinous injury and subsequent bleeding. MRI and ultrasound helps assess size and location and determine if it is intermuscular or intramuscular in nature. Large hematomas may result in compartment syndrome or significant pain and aspiration may be needed.

The MR appearance of hematomas can be variable depending on age and magnetic field strength and T1- and T2-weighted images can help determine the age and relative oxidative state of hemoglobin [22-24]. Acute hematomas are usually isointense to muscle on Tl-weighted images. T2-weighted images show increased signal intensity possibly with central decreased signal related to deoxyhemoglobin (Fig. 4A). Subacute hematomas (>48 hours) have increased amounts of methemoglobin, which has increased T1 signal. Chronic hematomas may have a peripheral dark rim related to hemosiderin. A seroma may ultimately develop with resorption of blood products (Fig. 4B).

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Responses

  • Mark
    What does a calf muscle tear look like on an mri?
    7 years ago

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