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

These masses typically cause pain on palpation or motion of the involved joint or musculature, and they may or may not feel warm to the touch. Locations vary and can include almost any joint. More common locations include the prepatellar bursa and the trochanteric bursa; in such locations, advanced imaging usually does not aid in the diagnosis.

MRI can be useful in making the diagnosis of bursitis in locations other than the prepatellar and greater trochanteric regions, such as at the following52–54:

•Ankle

•Pes anserine insertion

•Olecranon

It is important to remember that the diagnoses of nonseptic, septic, and inflammatory bursitis share many of the same MRI findings, although septic bursitis di ers in that postgadolinium images show enhancement of the infectious process.52,53 Conventional radiographs add little to the diagnosis other than the exclusion of other conditions. T1-weighted images show a cystic structure that is isointense compared with muscle, and T2-weighted images show increased signal intensity within the lesion secondary to the fluid collection. The bursa margins enhance, as does edema, which commonly arises in surrounding tissues (Fig. 15.13). As stated above, it is important to note that inflammatory processes

Fig. 15.12 PVNS of the right elbow. (A) A coronal T1-weighted image shows a lobular hypointense soft-tissue mass (arrow) with areas of signal dropout corresponding to hemosiderin deposition. (B) A sagittal STIR image shows several loci of the PVNS process (arrow) with high signal intensity, along with areas of signal dropout cor-

such as RA, gout, or infection show enhancement on postgadolinium images.52,53,55

Aneurysm

The formation of aneurysms in the extremities is rare, and the primary etiology is trauma. Other etiologies include the following:

•Atherosclerosis

•Mycosis

•Congenital disease

•Ehlers-Danlos syndrome

These lesions occur most commonly in the fifth or sixth decade of life and a ect males more frequently than females. Extremity aneurysms are often false (pseudoaneurysms), and aneurysms below the knee occur infrequently.56 In the lower extremity, an aneurysm occurs most frequently in the popliteal artery.57 The location of the aneurysm dictates the symptomatology, and some reports have described the occlusion of tibial veins by the posterior tibial artery and the dysfunction of the peroneal nerve by an aneurysm

responding to hemosiderin deposition. (From Papp DF, Khanna AJ, McCarthy EF, Carrino JA, Farber AJ, Frassica FJ. Magnetic resonance imaging of soft tissue tumors: determinate and indeterminate lesions. J Bone Joint Surg Am 2007;89(suppl 3):103-115. Reprinted by permission.)

of the anterior tibial artery.58,59 In addition to compromising the function of surrounding neurovascular structures, aneurysms can present as painful or nonpainful masses; frequently, they are pulsatile. Ischemic consequences, such as claudication or even thrombosis, can occur. When these ischemic conditions are found, the evaluation should include investigation of the contralateral leg and imaging of the abdominal aorta. Up to 50% of patients with a popliteal artery aneurysm have concomitant abdominal aortic aneurysm.60

MRI of an aneurysm shows the diameter of the a ected vessel to be at least 50% larger than that of the adjacent normal region. Mural thrombus may or may not exist in the lesion and does not enhance to the same degree with the administration of gadolinium as the vessel itself. MRI may be better suited than ultrasound for discerning the extent of thrombus, if present60 (Fig. 15.14).

Indeterminate Lesions

When a diagnosis cannot be established based on the imaging features, the lesion is categorized as indeterminate. As

Fig. 15.13 Olecranon bursitis. (A) A sagittal T1-weighted image shows a hypointense lesion (arrows) posterior to the olecranon process, with a well-defined ovular shape. (B) A sagittal T2-weighted

Fig. 15.14 An axial T1-weighted image of the right knee showing a pseudoaneurysm of the popliteal artery (arrows) with vessel dilatation of >50% of previous sections and signal dropout consistent with hemosiderin deposition. This particular lesion was associated with an osteochondroma (O) of the tibia.

image shows the lesion (arrows) to be well-circumscribed and hyperintense, compatible with the fluid present in the olecranon bursa. Motion artifact is seen on the T2-weighted image.

discussed above, dialogue with a multidisciplinary team can help with diagnosis, although occasionally the MRI findings and physical examination do not provide enough information for diagnosis.7 In this situation, the lesion must be biopsied for identification. However, excision of the lesion without previous biopsy, or excisional biopsy, should be avoided for indeterminate lesions. Although certain lesion characteristics (size >5 cm, firm mass, mass deep to the fascia, and lesion adherent to surrounding tissues) are indicative of a malignant process, such examination findings are not completely reliable in terms of reaching a diagnosis.6 Similarly, MRI findings, such as low signal intensity on T1-weighted imaging and high signal intensity on T2, are nonspecific. The physician must investigate these types of lesions further.

Liposarcoma

Liposarcoma arises from primitive mesenchymal cells and di erentiates into adipose tissue. Depending on the subtype, varying amounts of fat are found. Liposarcomas with little adipose formation are often categorized as indeterminate lesions, and their appearance is similar to that of most aggressive soft-tissue sarcomas. These types include the

round-cell, dedi erentiated, and pleomorphic liposarcomas. For these lesions, MRI shows low signal on T1-weighted images and high signal on T2-weighted images. Varying degrees of necrosis or hemorrhage may exist in the lesion, as may heterogeneity.8 One cannot diagnose these lesions with noninvasive methods; they must be biopsied.

Myxoid liposarcoma, the most common type of liposarcoma, also cannot be diagnosed definitively without a tissue sample, but its appearance on MRI di erentiates it from more aggressive lesions. Myxoid liposarcoma tumors consist of a myxoid matrix of soft-tissue elements, including dedif-

ferentiated lipoblasts and a plexiform formation of vessels.61 The MRI appearance of this lesion varies, but it is typical to see scattered high-intensity centers on T1-weighted images within a low signal intensity lesion. The adipose tissue in these lesions has been described as “lacy” or amorphous in nature.62 On T2-weighted images, these lesions often have high signal intensity, given the myxomatous nature of the lesion. In fact, di erentiating these lesions from cystic masses can be di cult. With gadolinium, intense enhancement commonly occurs, which can help di erentiate this lesion from others (Fig. 15.15).8,62

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Osteoid Osteoma

This lesion most commonly presents in young patients and is seen more commonly in males than in females.80 Classically, this benign lesion presents with progressively worsening pain that can disappear completely with the administration of nonsteroidal antiinflammatory medications. The lesion itself is termed the nidus, and reactive intense bone formation occurs around the lesion. Although the lesion can be diagnosed with conventional radiographs or CT, MRI allows for definitive diagnosis in many cases. However, one should note that the literature describes several cases of incorrect diagnosis of more aggressive lesions based on MRI findings without other imaging modalities.28 Ideally, one should see the nidus, which should not be >15 mm in diameter. On T1-weighted images, the nidus has low to intermediate signal intensity. T2-weighted images often show high signal intensity. Gadolinium causes enhancement of the lesion that can be better identified when combined with fat suppression techniques.81 When an osteoid osteoma is suspected, T1-weighted imaging with gadolinium can provide superior results, showing osteoid osteoma 82% of the time on the arterial phase and with conspicuity equal to that of thin-segment CT.28 Associated bone marrow edema, synovitis of associated joints, and associated soft-tissue involvement with increased vascularity and inflammatory cell infiltration have also been associated with osteoid osteoma. These findings may confuse the diagnosis and lead to impressions of infection or malignancy. Ultimately, T1weighted imaging with gadolinium enhancement provides the best sensitivity for MRI-based evaluation, but correlation with conventional radiography or CT provides definitive diagnosis.

The diagnosis of osteoid osteoma highlights the fact that MRI is not always the ideal imaging modality for all lesions.

Chondroblastoma

Chondroblastomas, which occur most commonly in the first through third decades of life and which occur with a slight predominance in males, originate in the epiphysis and extend to the metaphysis in two thirds of patients.29 The femur, tibia, and humerus are most commonly a ected. As with other osseous lesions, pain and swelling are the most common clinical features, although an e usion occurs in up to 30% of cases.29 MRI shows a lesion with low signal intensity on T1-weighted images and a heterogeneous appearance on T2-weighted images (Fig. 15.17). The rim of the tumor is of low signal intensity, and associated bone marrow edema is common.29,83 A fluid–fluid level within the tumor is seen in approximately one of five cases, although it may occur in up to 50% of chondroblastomas in the foot.29 Although chondroblastomas are often benign, these tumors have the capacity to metastasize. In addition, if not excised properly, they recur locally in approximately 2% of patients.84

Periosteal Chondroma

Although a rare lesion, periosteal chondroma has a characteristic appearance on conventional radiographs and MRI that can make diagnosis relatively simple without biopsy. The lesion most commonly appears in patients during the third and fourth decades of life on the metaphysis of long bones or at the insertion sites of tendons or ligaments. All lesions abut the cortex of the a ected bone, with lobules of cartilage and a minimal amount of pressure erosion of the involved cortex. The periosteum appears intact on T2-weighted images. On T1-weighted images, the lesion is hypointense to isointense compared with muscle and shows a sharp demarcation. As with other cartilaginous lesions, it appears bright

Fig. 15.17 Chondroblastoma. (A) A sagittal T1-weighted image shows a low signal intensity lesion (arrows) at the posterior aspect of the medial femoral condyle. (B) A sagittal T2-weighted image

on T2-weighted or STIR images. If enhancement occurs with gadolinium, it commonly occurs at the periphery, where the vascular supply for the lesion is located.76

Enchondroma

This common benign bone lesion appears in the metaphysis of long bones, although it also often arises in the bones of the hands and feet. On conventional radiographs, the lesion appears lucent, often with characteristic rings and stipples common to cartilage lesions that have calcified; however, this lesion can appear without these findings if the cartilaginous components of the lesion have not calcified. MRI displays the cartilage better, with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images and with a pattern similar to that of articular cartilage (Fig. 15.18). These lesions enhance with gadolinium. When diagnosed with certainty, there is no reason to biopsy them if they are asymptomatic. However, a painful lesion should be monitored with follow-up examinations because it may be a slow-growing, low-grade chondrosarcoma.85,86

shows a primarily high signal intensity lesion (arrow) with heterogeneity and a slight lobular pattern. Conventional radiographic findings would also aid in the diagnosis of this lesion.

Osteochondroma

Osteochondromas, the most common benign bone tumor, are thought to arise from a region of the growth plate that grows diagonally or perpendicularly to the surface of the bone. One study indicated that the prevalence of osteochondroma in the general population is as high as 2%.87 Although the lesion is commonly asymptomatic, patients may complain of pain with range of motion (mechanical irritation), or they may have decreased range of motion. Neurologic or vascular sequelae rarely occur.76,87 Conventional radiographs are often diagnostic. The lesion extends from the involved bone and is contiguous with the medullary cavity. MRI (especially T1-weighted images) has the advantage of showing the tumor’s relationship with the a ected bone’s medullary canal. T2-weighted images show high signal intensity in the cartilage cap, secondary to the high water content (Fig. 15.19). Moreover, MRI can be used to assess the malignant transformation of an osteochondroma to a chondrosarcoma. If the cartilage cap exceeds 2 cm in adults and 3 cm in children, malignant transformation is considered to be more likely.84

Fig. 15.18 A coronal T1-weighted image of the right knee shows a lesion in the distal femur (arrows) with a hypointense signal compatible with the calcification seen in enchondromas. The conventional radiographic findings also confirmed the diagnosis of enchondroma.

toms. The formation of an aneurysmal bone cyst occurs secondary to preexisting lesions in up to one third of cases and a ects females more than males.91 The primary lesions that may predispose to the development of an aneurysmal bone cyst include the following:

•Chondroblastoma

•Nonossifying fibroma

•Giant cell tumor

•Fibrous dysplasia

Other lesions can also predispose to the development of an aneurysmal bone cyst.29,91

Conventional radiographs show an expansile, lytic lesion that expands the cortex into the surrounding soft tissues. MRI shows a rim of low signal intensity, with multiple lobules and septations (Fig. 15.20). Overall, the lesion is heterogeneous, with each loculated collection having different signal characteristics. These lesions commonly show fluid–fluid levels. Telangiectatic osteosarcoma (see below), which presents in a similar fashion, shows nodularity in its septations along the rim, which correspond to nests of tumor cells. Similarly, an associated soft-tissue mass occurs in telangiectatic osteosarcoma in 89% of cases.10,29,92

Giant Cell Tumor

Giant cell tumors of bone most commonly a ect young adults and are found in a metaphyseal-epiphyseal location. The most common sites include the femur, tibia, and distal radius. Although benign (approximately 2% metastasize88), the lesion can cause destruction of a ected bones. Given its predilection for the periarticular region, the subchondral bone may be specifically a ected. Conventional radiographs show a poorly marginated lytic lesion with no sclerotic rim. The multiplanar images provided by MRI can be useful for detailing soft-tissue involvement. Approximately 60% of all giant cell tumors appear dark on all pulse sequences because of hemosiderin deposition.89 The remaining 40% appear dark on T1-weighted and bright on T2-weighted images.89 T1weighted images show the destructive nature of the lesion best and can detail any soft-tissue extension, which is usually contained by a reactive osseous rim.89,90

Aneurysmal Bone Cyst

Aneurysmal bone cysts occur most commonly in young adults, and pain and swelling are the most common symp-

Fig. 15.19 A coronal T2-weighted image of the right knee shows an osteochondroma (arrows) of the distal femur. Note the continuity with the normal cortex. The cartilage cap of the lesion shows high signal intensity.

Fig. 15.20 A coronal T2-weighted image of the distal femur shows an aneurysmal bone cyst with multiple fluid–fluid levels (arrows). The lesion is expansile, destroying the cortex.

Nonossifying Fibroma

This cortically based lesion occurs in the metaphysis of long bones (most commonly, the femur and tibia) and slowly migrates into the diaphysis. Conventional radiographs show a sclerotic rim, with scalloped margins.93 As with other fi- brous lesions, the MRI findings can be variable and depend on the amount of fibrous tissue relative to other tissue content within the lesion, including hemorrhage, collagen, and osseous trabeculae. All nonossifying fibromas present as low signal intensity lesions on T1-weighted images (Fig. 15.21). Most nonossifying fibromas are hypointense compared with muscle on T2-weighted images, with clear evidence of septations.88 However, not all lesions present in this manner, and it is important to compare the MR images with conventional radiographs. Unless these lesions cause pathologic fracture, they typically need no therapy.

Fibrous Dysplasia

Fibrous dysplasia is a developmental disorder in which there is a failure to form normal lamellar bone. It is typically

Fig. 15.21 A coronal T1-weighted image shows a metaphyseal lesion at the posterior aspect of the proximal right tibia, which is isointense compared with muscle. The conventional radiographic findings help confirm the diagnosis of nonossifying fibroma.

asymptomatic.29,93 One in five patients present with polyostotic disease, and in such cases fibrous dysplasia has an association with McCune-Albright syndrome and other endocrine abnormalities.91 Conventional radiographs are often diagnostic, with lesions having a “ground glass” appearance and a ected bones often showing gross deformity and narrowing of the cortex.93 As with other sclerotic lesions, the lesion appears dark on T1-weighted images.85 T2-weighted imaging does not always present a consistent picture: in one series, 79% of all lesions were hypointense and 21% were hyperintense compared with muscle (Fig. 15.22).94 Gadolinium enhancement occurs in approximately 70% of cases.94 Given this mixed picture on MRI, correlation with conventional radiographs is recommended.

Malignant Tumors

Osteosarcoma

Osteosarcoma, the most common primary malignant mesenchymal bone tumor, develops most frequently in young adults or adolescents. There are varying patterns of growth,