Imaging Semiology: Ultrasound and MRI in the Assessment of Muscle Injury

Fig. 6.1

Extensive grade 1 muscle injury on MRI. Coronal T2-weighted fat-suppressed image depicts typical diffuse feathery appearance (of semimembranosus muscle in this example). This effect is due to fluid deposition within the muscle, which leads to a clear definition of the secondary muscle bundles. No circumscribed hematoma is seen in grade 1 lesions

Fig. 6.2

A 27-year-old footballer with a grade 1 injury on ultrasound, presenting with left calf pain. (a) On ultrasound examination, a small hypoechogenic area measuring 1 × 0.4 cm was noted (arrows), corresponding with a grade 1 strain of the medial head of the gastrocnemius. (b) No evidence of hematoma or other abnormality was observed, but only circumscribed, non-fluid-equivalent hypoechogenicity (arrows). MRI was not performed

In the presence of partial tears of fibers without retraction (grade 2 injuries), there is a mild loss of muscle function. On MRI, in addition to interstitial edema and hemorrhage, hematoma at the musculotendinous junction and perifascial fluid collection appear as fluid-equivalent hyperintensity on fluid-sensitive sequences. On ultrasound, these pathologic features are depicted as hypoechogenic. Disruption of muscle fibers will be depicted as notable echo inhomogeneity (Fig. 6.3). Treatment of partial tears is also conservative.

Fig. 6.3

A 26-year-old football player with a Grade 2 muscle injury of the rectus femoris muscle. (a) Axial T2-weighted MRI shows diffuse hyperintensity surrounding the central tendon (arrows). In addition, there are small patches of fluid-equivalent signal intensity (arrowhead) defining this lesion as a grade 2 muscle strain. (b) Longitudinal ultrasound image shows liquid-equivalent hematoma (hypoechogenic area depicted by arrows). In addition, there are more diffuse edematous changes seen surrounding the hematoma (arrowheads)

Complete musculotendinous rupture (grade 3 injury) is commonly accompanied by a hematoma. The diagnosis is usually made on clinical grounds, i.e. complete loss of muscle function, with palpable gap and muscle fiber retraction. Surgical repair is an option, depending on the location of the rupture [22], and both MRI and ultrasound may be useful for preoperative assessment of the extent of retraction [11]. Extensive acute edema and hemorrhage may limit accurate evaluation of the injured muscle. If the tears are left untreated, the ends may become rounded and tether to adjacent muscles or fascia [23].

6.2.2 Muscle Contusion

Muscle contusions result from direct trauma [18]. The injury consists of a well-defined sequence of events involving microscopic rupture and damage to muscle cells, macroscopic defects in muscle bellies, infiltrative bleeding, and inflammation. As a complication, myositis ossificans traumatica may develop [24]. Unlike strains, these traumas usually occur deep in the muscle belly and tend to be less symptomatic than strains. Severity depends on the site of impact, the activation status of the muscles involved, the age of the patient, and the presence of fatigue [25].

On ultrasound, muscle contusion is characterized by discontinuity of normal muscle architecture, with ill-defined hyperechogenicity that may cross fascial boundaries [21]. MRI varies according to severity of injury, but typically there is a feathery appearance of diffuse muscle edema on short tau inversion recovery and fat-suppressed T2-weighted images [9] (Fig. 6.4). Increased muscle girth can be observed but there are no other architectural changes, such as fiber discontinuity or laxity. In case of severe trauma with muscle fiber disruptions, deep intramuscular hematoma is seen [11]. Signal intensity within the hematoma is influenced by the concentration of protein, methemoglobin, magnetic susceptibility at high field strength, and tissue clearance [26]. Acute hematomas (<48 h) are typically isointense on T1-weighted images, and subacute hematomas (<30 days) appear hyperintense relative to muscle on both T1-weighted and fluid-sensitive sequences secondary to methemoglobin accumulation [19]. As the hematoma evolves, a wide range of MR signal intensity can be seen within the collection, depending on the age of degradation products (Fig. 6.5). Chronic hematoma characteristically shows a hypointense rim on all pulse sequences due to hemosiderin. As blood degradation products are reabsorbed over 6–8 weeks, the size of the hematoma will decrease [27].

Fig. 6.4

A 22-year-old rugby player presenting with stiffness of the right thigh after receiving a direct blow to the right anterior thigh during a tackle. Sagittal STIR MRI show diffuse intramuscular hyperintensity, consistent with contusion injury of the rectus femoris, vastus lateralis and vastus intermedius muscles. A large epifascial hematoma is noted superficially. This sagittal image demonstrates the longitudinal extent of the contusion injury and the hematoma

Fig. 6.5

Complete tear of the semimembranosus muscle with minor retraction and hemorrhage. (a) Coronal proton density-weighted fat-suppressed MRI shows linear hematoma and complete disruption of the muscle belly (arrows). (b) Axial proton density-weighted fat-suppressed MRI shows extent of hematoma involving the whole cross-sectional area of the muscle and sedimentation within the hematoma characterized by susceptibility artifacts (arrowheads)

6.2.3 Avulsion Injury

Acute avulsion injury results from extreme, unbalanced and often eccentric muscular contractions, and patients with such injuries present with severe pain and loss of function [28]. Adolescents are particularly vulnerable to avulsion injuries because of the inherent weakness of the apophyses. The many apophyses in the pelvis and hip are common sites of avulsion injuries. The single most common site of apophyseal avulsion is at the ischial tuberosity [29]. Cheerleaders, sprinters, gymnasts, track athletes, American football players, and baseball players are commonly affected [29]. Treatment for avulsion injury is generally conservative and the prognosis is good, but non-union may occur.

In acute avulsion injury, periosteal stripping with hematoma at a tendon attachment site can be depicted by MRI. A wavy appearance and retraction of the torn end of the tendon with fragments of bone/cartilage are characteristic. The redundant tendon edge may be lying in a large fluid collection/hematoma (Fig. 6.6). Ultrasound evaluation is useful, but may be difficult due to the presence of a hematoma of mixed echogenicity which has echogenicity similar to the avulsed tendon [21].

Fig. 6.6

A 21-year-old male javelin thrower presenting with a sudden onset of right-sided groin pain secondary to an avulsion injury. Coronal fat-suppressed T2-weighted MRI demonstrates a wavy appearance and retraction of the torn end of adductor longus tendon (arrow) with surrounding fluid-equivalent hyperintensity representing hematoma (arrowhead). Small hypointense fragments of avulsed cortical bone from the symphyseal attachment are also noted

6.2.4 Chronic and Repetitive Injuries

Imaging features of chronic musculotendinous injury include muscle or tendon retraction or compensatory hypertrophy, muscle atrophy and formation of scar tissue (fibrosis) [30]. In chronic injuries, T1-weighted images may be normal in low grade injuries, but the fluid-sensitive sequences are helpful for detection of symptomatic old tears which are depicted as abnormally hyperintense [27]. There may be associated surrounding edema and hemorrhage due to re-injury at the site [27]. Scar tissue may be observed as early as 6 weeks after initial injury [14]. On MRI, scar formation appears as hypointense on all pulse sequences and, on ultrasound, areas of scar tissue have irregular morphological features and show heterogeneous echogenicity [31]. It is important to identify the scar tissue because recurrent injuries can occur in close proximity due likely to elasticity differences and altered contractility [27].

6.3 Imaging Modalities

6.3.1 Magnetic Resonance Imaging

MRI is commonly performed to locate the lesion and assess its severity. Under normal circumstances, images from only the affected area are acquired using a surface coil, but the appropriate coil should be selected to obtain the desired field of view. Imaging of the contralateral side is performed in exceptional cases only (e.g. bilateral injury). Contrast enhancement is rarely needed except to distinguish solid from cystic lesions or to diagnose muscle infarction [10]. To correlate imaging with clinical findings, a skin marker is placed over the area of symptoms (Fig. 6.7). Extent of injuries and associated architectural distortion is assessed using axial, sagittal and coronal images oriented along the long and short axes of the involved musculotendinous unit. The axial plane is useful to assess muscle contours and to delineate the musculotendinous junction and its exact anatomical relation with focal lesions [10], while coronal and sagittal planes are used to assess the longitudinal extent of injury [11].

Fig. 6.7

Placing a skin marker (arrow) prior to the examination is helpful to define the clinically relevant anatomical region. The marker is commonly a capsule filled with fish oil or vegetable oil. This is especially important in cases of repeated injury in order to differentiate old from incident lesions

Normal skeletal muscles show intermediate to low signal intensity on both T1-weighted (short TR/short TE) and T2-weighted or short tau inversion recovery (STIR) (long TR/long TE) images compared to other soft tissues [19]. Alterations in water content in the affected musculotendinous units are common to all forms of acute traumatic injuries (Figs. 6.2, 6.3, and 6.4) [9–11]. Fluid-sensitive sequences, i.e. fat-suppressed T2-weighted or proton density-weighted turbo spin echo, and STIR sequences are suitable for detecting edematous changes (hyperintensity with a “feathery” appearance) in the musculotendinous unit, and to delineate and locate intramuscular or perifascial fluid collections or hematomas as hyperintensity [10, 32]. Such sequences can depict abnormal hyperintensities at the site of symptomatic old tears [27]. T1-weighted turbo spin-echo sequences are used to visualize atrophy and fatty infiltration and to differentiate between hemorrhage/hematoma (hyperintense) and edema (hypointense) [11], but they are less sensitive for depiction of soft tissue abnormalities [19]. In chronic muscle injuries, T1-weighted images may not show any signal abnormalities in small tears [27].

6.3.2 Ultrasound

Ultrasound is inexpensive and widely available, and is helpful in the initial assessment of injury in the clinic. Unlike MRI, ultrasound allows a dynamic examination, which aids in clarifying the diagnosis. Power Doppler is useful for identifying hyperemia associated with acute injuries [27]. Hematomas may be drained under ultrasound guidance after liquefaction of the hematoma has occurred. The sensitivity of ultrasound to post-traumatic fluid collections in the acute stage has been shown to be equal to the sensitivity of MRI [31]. However, the sensitivity of ultrasound for detecting ongoing muscle healing during recovery is not as high as the sensitivity of MRI [4]. A study involving Australian football players showed that follow-up MRI 6 weeks after hamstring injury detected persistent abnormalities in 36 % of athletes, whereas the 6-week follow-up ultrasound demonstrated residual abnormalities in only 24 % of patients. It is postulated that the lower sensitivity of ultrasound in prediction of convalescence time is due to underestimation of the degree of injury and to areas of subtle edema that cannot be detected. Overall, the disadvantages of ultrasound seem to outweigh the advantages compared to MRI especially for follow-up imaging [14], because it cannot differentiate with certainty between old and new lesions and it is very difficult to reproduce exactly the same imaging position/plane at baseline and follow-up visits.

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