Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

Fig. 9.31 Plantar fibromatosis is best seen on coronal and sagittal MR images of the foot. (A) The lesion typically has low to intermediate signal intensity (arrowhead) on T1-weighted images and is directly contiguous to the plantar fascia. (B) On T2-weighted images, there is

low to intermediate signal and soft-tissue edema and/or signal alternation in the deep musculature. On this fat-suppressed T2-weighted image, the signal within the mass (arrowhead) seems relatively high because of fat suppression.

tendon sheath predominantly a ects the peroneal and flexor tendon sheaths. The MRI appearance is similar to that of intraarticular PVNS, but the lesion is typically less di use and more defined.53

The more common benign bone tumors involving the foot and ankle are the following:

•Simple bone cyst

•Aneurysmal bone cyst

•Intraosseous lipoma

•Giant cell tumor

•Enchondroma

•Osteoid osteoma

Malignant bone tumors of the foot and ankle are rare, but metastasis, Ewing sarcoma, chondrosarcoma, and osteosarcoma are occasionally seen in this region.

Fig. 9.32 A sagittal fat-suppressed T2-weighted image of the ankle in a patient with PVNS shows heterogeneous signal intensity within a region of synovial proliferation in the anterior (left arrowhead) and posterior (right arrowhead) tibiotalar joint. There is also a large posterosuperior extension of the synovial space.

Plantar Fasciitis

Plantar fasciitis is a degenerative condition that occurs at the origin of the medial calcaneal tuberosity.54 Less commonly, it can also occur secondary to an enthesopathy from systemic disease (such as RA, diabetes, or lupus) or after trauma, but typically it does not involve an inflammatory infiltrate, so “fasciitis” is a misnomer. The diagnosis is usually made clinically. Patients typically complain of pain at the plantar medial aspect of the heel that is severe first thing in the morning, after prolonged sitting, and toward the end of the

Fig. 9.33 A sagittal fat-suppressed T2-weighted image of a patient with plantar fasciitis, showing thickening of the fascia at its origin, intrasubstance changes, and edema of the calcaneus (arrowhead).

day after activity. The patient has point tenderness at the medial edge of the calcaneal tuberosity. Although not necessary at early presentation, MRI may be useful in di erentiating plantar fasciitis from other conditions such as calcaneal stress fractures. Sagittal images are most helpful in assessing the plantar fascia. MRI shows thickening of the fascia at its origin, intrasubstance changes, and sometimes edema of the calcaneus (Fig. 9.33).

metatarsal ligament.58 The neuroma is isointense compared with muscle on T1-weighted images and hypointense to fat on non–fat-suppressed T2-weighted images.58

■ Postoperative Findings

Although radiography remains the predominant postoperative imaging modality for the ankle and foot, the excellent soft-tissue contrast of MRI is particularly useful for assessment after ligament and tendon surgery.

Tendon Repair

The treatment of tendon injuries consists primarily of immobilization and surgical reconnection of the apposed tendon ends. During the first month after tendon repair, granulation tissue bridges the anastomosis (week 1), followed by increased vascularization of the paratenon zone (week 2),

A

Morton Neuroma

Morton neuroma refers to entrapment or impingement of an interdigital nerve between two adjacent metatarsals near the MTP joint.55 The nerve emerges from beneath the intermetatarsal ligament to bifurcate and course distally to the two adjacent toes at that web space. It most commonly a ects the third interspace; the second interspace is the second most often a ected area.56 The patient typically complains of stabbing, burning, or radiating pain and often numbness in the corresponding toes.57 Histologically, the nerve shows thickening, perineural fibrosis, and axonal demyelination.57 MRI can be a useful diagnostic adjunct to clinical examination because conventional radiographs do not image the soft tissues e ectively. Axial images are best for evaluating the interspaces (Fig. 9.34). On cross-sectional imaging, the nerve appears enlarged and spindle-shaped plantar to the inter-

B

Fig. 9.34 Morton neuroma. (A) A coronal T1-weighted image of the left foot shows a small nodular mass (arrowhead) plantar to the intermetatarsal ligament connecting the second and third metatarsals.

(B) A coronal fat-suppressed T2-weighted image of the same patient shows relative hyperintensity of the lesion (arrowhead).

Fig. 9.35 An axial T1-weighted image of the right ankle showing an intact anastomosis (arrowhead) between the FHL and the Achilles tendon after tendon transfer surgery for Achilles repair.

collagen fibril formation (week 3), and, ultimately, the resolution of edema (week 4).59 In cases where tendon rupture is left untreated for extended periods of time, the muscle becomes retracted and grafting may be required.60 In such cases, available grafting strategies include free tendon grafts, tendon lengthening, and tendon transfers.

In the early postoperative period, the repaired tendon appears to be thickened.59 In the late postoperative period, the principal findings can include generalized thickening and moderate heterogeneity of signal.61 In the presence of a tendon transfer, the postoperative MR appearance includes nonanatomic connections between the relevant tendons (Fig. 9.35). Recurrent and new tears can be diagnosed as new areas of focal high signal on proton-density and T2-weighted images. T1-weighted images can also be reviewed for anatomic detail (Fig. 9.36). Comparison with previous studies is often useful for distinguishing postoperative intrasubstance changes and degeneration from re-tears. Postoperative infections such as abscess formation and osteomyelitis are best imaged using sequences before and after contrast enhancement.

Lateral Ankle Ligament Reconstruction

Surgical reconstruction of the lateral ligament complex is indicated for patients with chronic lateral instability as a sequela of ankle sprain.62 In such cases, knowledge of the relevant surgical procedure is essential because the post-

operative appearance is characterized by nonanatomic anastomoses and osseous tunnels.63 In the modified Evans procedure, the peroneus brevis tendon is transected, passed through a fibular tunnel, and anastomosed at the transaction site.63 In the Lee procedure, the peroneus brevis is transected proximally, and the distal portion is looped through a fibular tunnel and anastomosed to its distal portion. The proximal peroneus brevis is then anastomosed to the peroneus longus.63 In the Watson-Jones procedure, the peroneus brevis tendon is transected proximally, looped through two fibular tunnels and a tarsal tunnel, and sutured to itself distally.63 As in the Lee procedure, the proximal portion is anastomosed to the peroneus longus. In the Christman-Sook procedure, the peroneus brevis is split, and the free half is passed beneath the talar periosteum, fibula tunnel, and calcaneal periosteum where it is anastomosed to the tethered portion of the peroneus brevis and adjacent peroneus longus in the region of the distal fibula (Fig. 9.37).63

Other Foot and Ankle Surgical Procedures

Other common surgical procedures of the foot and ankle include plantar fasciotomy, tarsal tunnel surgery, arthrod-

Fig. 9.36 A sagittal T1-weighted image showing several foci of susceptibility artifact within the anterior tibialis tendon, suggestive of previous repair. In addition, there is discontinuity within the anterior tibialis tendon (arrowhead), representing recurrent tear with some retraction.

Fig. 9.37 Postoperative imaging in lateral ankle ligament reconstruction in a patient after a Chrisman-Snook procedure. (A) An axial T1-weighted image of the right ankle showing the split peroneus brevis tendon traversing two osseous tunnels through the fibula (ar-

esis, and hallux valgus repair.59 After fasciotomy, the plantar fascia is thickened with indistinct margins, with a lack of the fascial edema that characterizes plantar fasciitis.64 After tarsal tunnel surgery, MR shows scarring of the subcutaneous fat and fascia medial to the tarsal tunnel visible as abnormal areas of low T1 signal.54 Postoperative imaging after arthrodesis typically involves using radiography

rowheads). (B) A sagittal T1-weighted image showing the split peroneus brevis tendon traversing through additional tunnels traversing the talus (black arrowhead) and calcaneus (white arrowhead).

or CT to assess the extent of osseous fusion, although MRI can be useful for assessing soft-tissue complications such as infection, secondary degenerative changes, and tendinous and ligamentous stability. Lastly, MR is a useful adjunct in postoperative assessment of hallux valgus repair to evaluate for complications such as infection, osteonecrosis, and nonunion.59

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■ Specialized Pulse Sequences and

Protocols

Although imaging protocols of the cervical spine for specific indications can vary among institutions, standard MRI of the cervical spine for degenerative pathologies usually includes the following pulse sequences:

•Sagittal T1-weighted SE

•Sagittal T2-weighted FSE

•Axial gradient-echo

•Axial T2-weighted FSE

A detailed discussion of all the imaging sequences used in the cervical spine is beyond the scope of this chapter; however, salient features of commonly used sequences are discussed below.

T1-weighted images are useful in identifying fracture lines. Because they are sensitive to the presence of gadolinium contrast, they are also used for contrast-enhanced imaging, which is helpful in assessing neoplasms, infections, and the postoperative spine. Typically, fat-suppressed postgadolinium T1-weighted images are used to make lesions more conspicuous. T2-weighted images are sensitive to water (and thus edema) and are useful in identifying areas of potential pathology. However, care must be taken with regard to interpreting bone marrow edema because it may be seen with a variety of conditions, including infection, inflammation, trauma, and degeneration. Although edema may focus attention toward an abnormality, many of these conditions can coexist, so additional analysis is required before finalizing a conclusion. FSE is now routinely used to acquire T2-weighted images at speeds up to 64 times faster than conventional SE T2-weighted images. Sometimes the di erentiation of fat, water, and lesions can be di cult, especially on T2-weighted FSE images, and therefore fat suppression is used to make these areas more conspicuous. This sequence can be obtained by applying a fat-suppression pulse to produce fat-suppressed T2-weighted images or by obtaining a STIR sequence. Visualizing edema is helpful in identifying ligamentous injuries, and such visualization is best achieved with STIR or fat-suppressed T2-weighted images. T2-weighted images are also most sensitive for evaluating the cord parenchyma for lesions and edema, which are seen as abnormally bright signal, although the sagittal orientation

is subject to linear bright artifact within the cord (Gibbs phenomenon). For this reason, axial T2-weighted images serve as a useful tool for detecting cord abnormalities and confirming lesions suspected on sagittal T2-weighted images.

Gradient-echo images are very susceptible to magnetic artifacts; this important characteristic makes them useful for detecting small areas of hemorrhage, such as with cervical spine trauma and vascular malformations. However, these images can also overestimate the degree of canal and foraminal stenosis secondary to artifact from the adjacent bone. Because of the rapidity with which gradient-echo images are acquired, studies can be obtained with higher resolution than that required for other pulse sequences and even as a 3D volume set, which allows for isotropic voxels and reformations in multiple planes. This volume set can then allow one to characterize the cervical foramina in the appropriate oblique plane.

For evaluation of vascular structures in the neck, MR angiography can be obtained without contrast, using 2D or 3D time-of-flight or phase-contrast imaging. These sequences create contrast between flowing and stationary structures. Phase-contrast imaging may also provide flow-velocity information. As a result of the technique, time-of-flight imaging shows fat or subacute thrombus as bright signal and may be useful in detecting small, subtle thrombi. The 3D techniques require more time and are slightly less sensitive to slow flow states. Gadolinium-enhanced MR angiography may also be obtained and is extremely accurate.

■ Traumatic Conditions

Although the cervical spine is injured in only 2% to 3% of blunt trauma accidents,1 the potential for instability and critical neurologic injury makes prompt identification and management of cervical spine injuries important. Patients with suspected cervical spine injury should be evaluated initially with conventional radiographs (AP, lateral, and open-mouth odontoid views). CT imaging o ers greater osseous detail than does conventional radiography and may reveal fractures or details that are not detected with radiography. CT is especially helpful in assessing fractures of the occipital condyles and cervicothoracic junction, where osseous overlap on conventional radiographs makes fracture detection dif-