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

■ Background

The management of soft-tissue masses presents an interesting quandary for physicians that has important implications. Although it has been suggested that 1 of 100 soft-tissue lesions seen by a physician is malignant, the precise overall number is unknown.1 The incidence of soft-tissue sarcomas has been estimated to be approximately 8,100 cases per year in the United States.2 In most cases, soft-tissue masses are benign. Indeed, many of these soft-tissue lesions have no potential for metastasis or local invasion and can simply be observed. The danger in simple observation occurs when the physician observes a lesion without having a firm diagnosis; this situation can lead to errors in management with resultant poor outcomes, including local invasion of neurovascular structures and metastatic disease. Similarly, excision of a lesion without a definitive diagnosis can also result in catastrophic outcomes. With an incorrect diagnosis, excision of a malignant lesion can lead to the contamination of una ected tissues, recurrence, and, in some cases, eventual amputation of an a ected limb.

For many years, the use of tissue biopsy was the only means of obtaining a definitive diagnosis. Today, the increased use of MRI has substantially improved the diagnosis and management of soft-tissue tumors. MRI provides excellent soft-tissue resolution and allows the physician to di erentiate various soft-tissue types based on imaging characteristics via the use of various pulse sequences,3,4 a feature not a orded by other imaging modalities such as conventional radiographs and CT. The excellent spatial resolution provided by MRI also provides sharp delineation of soft-tissue boundaries and highlights the boundaries between the soft-tissue tumor and the adjacent normal tissues. This information can help guide the determination of the diagnosis of a soft-tissue lesion, which can obviate tissue biopsy. Another advantage of MRI relates to its use in preoperative planning. With the guidance of multiplanar imaging provided by MRI, neurovascular structures can be avoided during the approach to the lesion, eliminating or decreasing the potential for contamination during planned biopsies or providing a means to ensure adequate margins when resection is planned. With all of its inherent capabilities, MRI has become a powerful tool in the diagnosis and management of soft-tissue tumors.

In addition to its value in diagnosing soft-tissue tumors, the diagnosis of bone tumors has been enhanced by the use of MRI in conjunction with conventional radiographs and CT. Although conventional radiographs often provide enough information to make a diagnosis of a given bone tumor, MRI can provide additional details that will help guide the management of the lesion. MRI also allows for the visualization of pathology that cannot be seen on conventional radiographs, such as fluid–fluid levels or the degree of invasion of the adjacent soft-tissue structures. A lesion’s tissue composition also can be identified more easily with MRI than with conventional radiographs. This information can a ect the surgeon’s management decisions, such as the choice of neoadjuvant or postoperative chemotherapy.

The clinician must properly diagnose the soft-tissue lesion before planning any type of treatment, including simple observation. Understanding this point and the consequences of misdiagnosis, the surgeon must take a methodical, systematic approach to the diagnosis of these lesions. Therefore, a diagnostic and treatment algorithm directed toward soft-tissue and bone lesions depends on information from the history, physical examination, conventional radiographs, CT scans, and MRI studies. This chapter provides such an algorithm based on the concept of determinate versus indeterminate soft-tissue tumors, reviews common soft-tissue and bone lesions, and describes them and their characteristic MRI findings.

■ Soft-Tissue Tumors

As with any medical condition, the evaluation of a soft-tissue tumor begins with the history and physical examination. Although they are essential parts of the diagnostic process, these two items often do not provide a definitive diagnosis for soft-tissue lesions. Most soft-tissue tumors are slowgrowing lesions. A history of trauma, although suggestive of a hematoma or heterotopic ossification, does not ensure that the lesion is a benign, posttraumatic process. The trauma may merely alert the patient to a soft-tissue tumor that had previously been present in the area. In addition, it is important to note that a history of pain is not a reliable indicator of the benign or malignant nature of a lesion and that only

Fig. 15.3 Atypical lipoma in the left thigh. (A) An axial T1-weighted image shows a large, high signal intensity lesion (arrows) with multiple septations, suggestive of an atypical lipoma. (B) An axial STIR image shows suppression of most of the signal within the lesion

(arrows) but also shows multiple fibrous septations with a complex, heterogeneous composition. Because of its large size and heterogeneous appearance, this lipoma was biopsied.

Fig. 15.4 Hemangioma. Axial T1-weighted (A) and axial fatsuppressed T2-weighted (B) images of the right calf show a mass (arrow) in the medial head of the gastrocnemius muscle. The mass is minimally high signal on the T1-weighted image and heterogeneous and serpentine high signal on the STIR image. (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.)

A

Fig. 15.5 Ganglion cyst. Coronal STIR (A) and coronal contrastenhanced, fat-suppressed T1-weighted (B) images show the typical appearance of a ganglion cyst (arrow) along the dorsum of the midfoot. The lesion is round, well-circumscribed, and hyperintense compared with muscle on the STIR image with a thin rim of en-

B

hancement, which is compatible with the fluid-filled, cystic nature of a ganglion cyst. (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.)

Fig. 15.7 Hematoma. Sagittal T1-weighted (A) and sagittal fat-sup- pressed T2-weighted (B) images of the leg show a mass lesion in the anterior compartment (arrow in B). The T1-weighted image shows areas of hyperintensity (arrowheads), reflecting methemoglobin. The high signal intensity areas on the fat-suppressed T2-weighted image are related to soft-tissue edema and hemorrhage. Clinical correlation

diagnoses of pseudoaneurysm and arteriovenous malformation can often be made via a duplex ultrasound examination.

Myositis Ossificans

Heterotopic ossification is the formation of extraskeletal, mature, lamellar bone. This process most often occurs after direct trauma, as a complication of orthopaedic procedures and spinal cord injury, and in the burn patient. In posttraumatic cases, the process is termed myositis ossificans. Myositis ossificans occurs at sites of previous hematoma formations, although the process by which a hematoma involutes or evolves into myositis ossificans or a chronically expanding hematoma is not fully understood. Most patients present in the third decade of life, and the most common locations include the quadriceps and the brachialis muscles.31,32 The process is thought to arise after a direct impact to the a ected muscle; the more severe the injury, the higher is the likelihood of hematoma formation.33

Radiographs show a well-circumscribed, calcified lesion in the pattern of mature, lamellar bone peripherally when the lesion has matured.15 In such cases, a conventional radiograph may be su cient for diagnosis. However, when the diagnosis is in doubt, MRI can be helpful. T1-weighted im-

is especially helpful for the diagnosis of hematoma, and attention should be given to the presence or absence of coagulopathy, history of surgery, or other trauma. (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.)

ages often show a lesion in the belly of the muscle, with a signal intensity the same as or slightly higher than that of the adjacent muscle. At times, this similarity may lead to the recognition of the lesion on T1-weighted images solely by noting the distortion of fascial planes.32 For lesions in patients who present early in their course, T2-weighted images show central high signal intensity with an external ring of low signal intensity. This configuration is pathognomonic and represents the zonal pattern of growth of myositis ossificans in which the external edges of the lesion ossify first. Surrounding edema may or may not be seen (Fig. 15.8).

Myonecrosis (Diabetic and Idiopathic)

The diagnosis of myonecrosis should be considered when a patient presents with a rapidly growing, painful mass that involves at least one extremity. Patients who present with myonecrosis most frequently have involvement of the lower extremities, especially the quadriceps and calf muscles. Diabetes is the most frequently associated cause, but myonecrosis also has been associated with alcohol abuse and other, less common, entities. Diabetic myonecrosis has been reported in a previously healthy woman as a presenting symptom of her diabetes,34 although patients with diabetes more

Fig. 15.8 Myositis ossificans. (A) An axial T1-weighted image of the right leg shows a lesion (arrows) with the same intensity as the surrounding muscle, making the lesion di cult to see. The disruption of fascial planes is the only indication of abnormality. (B) An axial STIR

often have other existing sequelae, such as nephropathy or other forms of end-organ damage.35,36 Laboratory values are typically within normal limits, although elevated creatinine serum kinase levels have been reported.34,36 The ultimate pathophysiology of the process is not fully understood; it is best presented as a mixture of activated coagulation factors, impaired fibrin degradation, and endothelial damage from diabetic microangiopathy.37 These factors ultimately overcome the skeletal muscle’s abundant blood supply, leading to necrosis.

The early recognition of myonecrosis, before tissue biopsy, can be important because patients with diabetes frequently have wound-healing problems. For this reason, nonsurgical management approaches are preferred.38 Conventional radiographs and CT imaging provide little data about the nature of this entity; however, MRI provides valuable information that facilitates diagnosis. T1-weighted imaging shows swelling and disruption of the involved muscles along the fascial planes. The muscle fiber pattern persists after the necrosis occurs, and there is no infiltration of the fascia. Although the resultant loss of striations is seen grossly on MRI, the overall structure remains unchanged. T2-weighted imaging shows a di use increase in signal intensity in the muscle, indicative of edema, and areas of necrosis. The muscles show heterogeneous signal intensity on T2-weighted images, which likely corresponds to fiber regeneration. The findings with myo-

image shows that the lesion (arrows) has a higher signal intensity than the surrounding muscles. Absence of a low signal intensity ring identifies it as an early lesion. Conventional radiographs with a zonal pattern of calcification aid with the diagnosis.

necrosis contrast with those seen with neoplasms, which often disrupt the surrounding anatomy. Contrast-enhanced images may show a mixture of enhancing linear or serpentine fibers within low signal intensity regions. The dark (low signal intensity) areas represent necrotic tissue, and the enhancing areas represent viable or inflamed tissue (Fig. 15.9). The lesions frequently show peripheral enhancement on postgadolinium T1-weighted images.34,36,39

Neurofibroma

Neurofibromatosis type 1, formerly known as von Recklinghausen disease, is an autosomal dominant disorder characterized by neurofibromas and other systemic complications, such as the following:

•Skeletal dysplasias

•Café-au-lait spots

•Lisch nodules

•Vascular malformations

•Learning disabilities

•Optic gliomas

In addition, these patients are at risk for other malignancies, such as nerve-sheath tumors and rhabdomyosarcoma, as well as for dedi erentiation of their neurofibromas into neurofibrosarcomas.40 With an incidence of approximately

Fig. 15.9 Myonecrosis. Axial T1-weighted (A) and T2-weighted (B) images of the right leg show a well-defined area (arrows) of low signal within the muscles of the anterior compartment of the leg in a

1 in 3500, neurofibromatosis type 1 is more common than neurofibromatosis type 2 and is associated with the formation of true neurofibromas; the formation of schwannomas is more characteristic of neurofibromatosis type 2.40

On examination, the lesions can be superficial or deep and are not always painful. Despite their association with nerves, neurologic findings are uncommon. The MRI findings of neurofibromatosis are quite specific and therefore can help with diagnosis. The T1-weighted images show a lesion that is hyperintense relative to skeletal muscle. The appearance is fascicular or nodular. T2-weighted imaging classically shows a target sign, manifesting as an area of low signal centrally situated within a high signal intensity lesion. The myxoid nature of the neurofibroma accounts for the bright periphery, whereas the dark central region represents the compressed nerve fibers41,42 (Fig. 15.10). This specific finding simplifies the diagnosis and allows for ease of continued follow-up of the multiple lesions, which can help monitor for conversion to a neurofibrosarcoma. Superficial lesions do not always present with a target sign, can appear homogeneous, and usually extend to the skin.42

patient with a history of diabetes mellitus. Note that the surrounding fascia and tissue planes are well preserved.

unknown soft-tissue lesions. Muscle tears commonly have a specific inciting incident; patients often remember hearing a snapping sound or feeling that their strength is “giving way,” followed by a period of muscle swelling, tenderness, ecchymosis, and/or edema. However, this scenario does not always occur. In fact, a study at the Walter Reed Army Medical Center showed that, in six of seven patients with rectus femoris muscle tears presenting as soft-tissue masses, there was no specific event; rather, the tear appeared insidiously, and only half of the lesions were painful.43 As old muscle tears scar and retract, they can present in a manner similar to that of soft-tissue tumors.44,45 The physician who understands this presentation can avoid unnecessary biopsy and its associated complications.

Radiographs are not helpful; however, as with other softtissue lesions, MRI facilitates diagnosis. T1-weighted imaging shows clear muscle or tendon deformity. T2-weighted images vary, depending on the time course of the process: acute injuries show increased signal intensity compatible with edema, whereas chronic injuries have low to intermediate intensity (Fig. 15.11).

The presentation of a muscle tear masquerading as a softtissue mass occurs often enough that the orthopaedic surgeon should consider this possibility when addressing

Most patients present with chronic pain and swelling about the involved joint, but other nonspecific symptoms, such as warmth and e usion, also occur.46 PVNS usually

A

Fig. 15.10 Neurofibroma. Sagittal T1-weighted (A) and coronal STIR

(B) images of the ankle show a well-circumscribed mass (arrow) in the plantar aspect of the foot that is hypointense compared with muscle on the T1-weighted image. The STIR image shows the typical “target” sign in which the periphery of the lesion is bright and

involves the large joints (e.g., the knee and hip), with the knee being a ected most frequently. It presents as a nodular process or a di use disease involving the entire joint. This distinction determines the prognosis for surgical removal, with nodular disease being less likely to recur. If the disease is left untreated, progression to secondary arthritis may occur; therefore, surgical removal of the diseased tissue will likely improve the natural history and patient outcome.47,48

Conventional radiographs provide a good starting point for diagnosis, although characteristic findings are not always present. An e usion typically is seen. Over time, well-defined erosions occur with relative maintenance of the joint space. When joint space narrowing occurs, it is usually concentric. Although rare, with an incidence of approximately two per million cases, PVNS is readily identifiable on MRI because of its characteristic imaging findings.45 MRI allows for a more thorough evaluation of the joint and disease process than does conventional radiographic imaging (Fig. 15.12). PVNS appears as a low signal intensity entity on both T1-weighted and T2-weighted images because of its high hemosiderin content. The mass often is nodular, with varying amounts of joint involvement and associated erosion. Gradient-echo

B

the center is dark. (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.)

sequences show the PVNS lesions as “blooming” from the joint capsule.46 More recently, specific sequences, such as a “fast field” echo, show hemosiderin in a manner superior to that of the more conventional SE sequences.49 It is important to remember, however, that if MRI is obtained early in the disease process, hemosiderin deposition may not have occurred, thus leaving the diagnosis uncertain.47,50

Bursitis

Patients presenting with a mass near a joint or tendon sheath should prompt the physician to consider bursitis, especially if the presentation is acute or subacute. It is important to keep in mind, however, that bursitis can also be a chronic process. Chronic bursitis and its associated growth have been reported to mimic a tumor.51 The causes of bursitis include the following51–53:

•Infection

•A single traumatic event

•Repetitive trauma

•Gout

•RA