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

RA is a systemic autoimmune inflammatory disorder that primarily a ects the small joints of the hands and feet but may involve the hip joints late in the disease process. MRI is useful for evaluating erosions, e usion, and synovial pannus formation.68 The synovial proliferation is typically a masslike prominence of tissue within the joint that is hyperintense on fat-suppressed sequences.46 The pannus may be heterogeneous or homogeneous in appearance, and pannus hemorrhages can form with areas of low signal hemosiderin.68 As with osteoarthritis, the MR findings of inflammatory arthritis correlate with the radiographic appearance, including any of the following68:

•Symmetric joint space narrowing

•Periarticular erosions

•Juxtaarticular osteoporosis

•Protrusion acetabulae

Other findings may include the following46:

•Hyperintense joint e usion

•Joint capsule distention

•Hyperintense marrow edema

Fig. 7.14 An oblique axial FSE image of the left hip showing an increased αangle in a nonspherical femoral head with a cam lesion. (© 2009 Hospital for Special Surgery, New York, NY.)

the role of MRI has not been well defined, and conventional radiography is the standard for diagnostic evaluation.69

Ankylosing spondylitis is another form of inflammatory arthritis. It usually involves the spine and larger joints, so the hip is frequently a ected and may be so early in the disease process.46 The hip may be involved in 17 to 35% of patients, but

■ Synovial Disorders

A systematic examination of a hip MRI must include careful scrutiny of the synovium because synovial proliferative

A

Fig. 7.15 Synovial osteochondromatosis. (A) A coronal FSE image of the left hip showing synovial osteochondromatosis with cartilaginous bodies in the inferior capsule (arrows). (B) A coronal fat-suppressed

B

FSE image of the pelvis also showing large detached cartilaginous bodies (arrow). (© 2009 Hospital for Special Surgery, New York, NY.)

disorders are not uncommon. In the hip, negative intraarticular pressure and the tight capsule may result in more prominent erosions than those associated with synovial proliferative disorders in other joints such as the knee, which has several patulous synovial recesses.46 Asymmetric fluid is seen on body coil fat-suppressed images, and fine synovial debris is present on the surface coil images in the setting of synovial osteochondromatosis, which is a monoarticular synovium-based cartilage metaplasia that commonly involves the hip joint (Fig. 7.15).46,70 The development of intraarticular loose bodies may converge to create large pressure erosions of the femoral neck and head or result in destruction of the hyaline cartilage. Typical MRI characteris-

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■Specialized Pulse Sequences and Protocols

For MRI of the knee, the patient is positioned with the leg in comfortable external rotation. Sagittal, coronal, and axial imaging is obtained with fat-suppressed acquisition in at least one plane. To optimize spatial resolution, thinner segments are preferred, but the choice of segment thickness must be balanced with signal-to-noise and scan time constraints. In general, MRI systems with higher field strengths can achieve thinner segments with a better signal-to-noise ratio in less time than can units with lower field strength. For most SE protocols designed for knee imaging, the e ective segment thickness is 3 to 4 mm; that for gradient-echo imaging is 1 to 3 mm.

A typical low magnetic field strength protocol for knee imaging may include T1-weighted and T2-weighted sagittal acquisitions, a coronal STIR sequence, and an axial FSE T2weighted sequence. A sagittal 3D gradient-echo sequence represents an additional option, particularly with low field strength systems; this high-resolution technique o ers a relatively thin segment with a superior signal-to-noise ratio, useful for meniscal assessment. A high field strength approach to knee imaging may include the following sequences:

•Sagittal FSE proton-density fat-suppressed sequence to evaluate the following:

οMenisci

οArticular cartilage

οCruciate ligaments

οExtensor mechanism

οArticular fluid

•Coronal FSE fat-suppressed T2-weighted sequence to assess the following:

οOsseous injury

οArticular and meniscal cartilage

οCollateral supporting structures

οFluid distribution

•Sagittal T1-weighted sequence to evaluate the following:

οEvaluate marrow and subcutaneous fat

οDetect heterotopic ossification

οProvide further evaluation of meniscal integrity

•Axial FSE T2-weighted sequence primarily to assess the patellofemoral joint

■Traumatic Pathology

Acute Hemarthrosis

Acute knee trauma commonly presents with pain and hemarthrosis. ACL tears are seen in more than 70% of acute hemarthroses.1 Usually, ACL tears are associated with other injuries such as meniscal tears, osteochondral fractures, patellar dislocations, and ligament and tendon tears. MRI is commonly used to discern the etiology of an acute hemarthrosis, especially when the knee is too tender or the patient is too anxious for a thorough physical examination. MRI is especially helpful when conventional radiographs are negative. Typically, acute hemarthrosis appears as fluid within the joint with high signal intensity on T2-weighted images and intermediate signal intensity on T1-weighted images. Similar findings can be seen with inflammatory processes, resulting in a nontraumatic knee e usion, and the two processes may be di erentiated by the clinician, based on the history and physical examination. The presence of blood in joint fluid causes a relatively brighter appearance on T1 imaging than that observed with nonhemorrhagic fluid because of the e ect of methemoglobin. Hemosiderin deposition within joint fluid, associated with chronic hemorrhage or PVNS, appears dark on all pulse sequences.

Table 8.1 Grading of Meniscal Tears

Grade Pathologic and MRI Findings

1Degenerative process; focal, globular intrasubstance increased signal; no extension to articular surface

2Degenerative process; horizontal, linear intrasubstance increased signal; no extension to articular surface

3Meniscal tear; increased signal extends to or communicates with at least one articular surface

4Complex tear/macerated meniscus

Source: From Khanna AJ, Cosgarea AJ, Mont MA, et al. Magnetic resonance imaging of the knee: current techniques and spectrum of disease. J Bone Joint Surg Am 2001;83:128–141. Reprinted by permission.

Degenerative flap tear (A)

Peripheral tear (D)

Peroneal nerve

Biceps tendon

LCL

Horizontal cleavage tear (B)

Iliotibial tract

Patellar retinaculum

Fat

Patellar tendon

A

B

PCL

Medial

Complex tear (C)

Joint capsule

MCL

Radial tear (E)

Transverse ligament

ACL

Bursa

C D

E

Vertical signal oriented parallel to the curvature of the meniscus.

“Double-PCL” sign; displaced fragment often seen parallel to the PCL in the intercondylar notch on sagittal images

Characteristics of each tear type or fragmented/ macerated

Increased signal between the edge of the meniscus and the capsule

low intensity on all sequences.3–10 Most tears involve the medial meniscus, but most acute tears involve the lateral meniscus (Table 8.22,11). Special attention should be given to the menisci in the presence of a chronic ACL tear because associated meniscus tears are common. The medial meniscus is commonly torn in the chronically ACL-deficient knee because the meniscus plays a secondary stabilizing role to anterior tibial translation.

The types of meniscal tear morphologies have been well described (Table 8.22,11). In early publications on MRI evaluation of meniscal pathology, detection focused primarily on meniscal signal intensity patterns and their relationship to the meniscal surface.12,13 Grade I and grade II9 meniscal tears referred to increased intrameniscal signal that appeared punctate or linear, respectively, and were particularly evident on T1-weighted images. The grade III designation indicated

A

Fig. 8.2 Horizontal tear in the posterior horn of the medial meniscus. Sagittal T1-weighted (A) and fat-suppressed proton-density

(B) images showing a horizontal tear (arrow on each) communicat-

B

ing with the undersurface of the meniscus. Note the small meniscal cyst posterior to the meniscus on the fat-suppressed proton-density image.

A

Fig. 8.3 Horizontal medial meniscal tear with large meniscal cyst in the right knee. (A) A sagittal T1-weighted image shows a horizontal tear (arrow) of the posterior horn of the medial meniscus. (B) A coronal fat-suppressed T2-weighted image shows a large meniscal cyst (arrows) extending medial to the medial compartment. (C) An axial T2-weighted image shows a multiloculated meniscal cyst (arrow).

that the signal intensity pattern communicated with the superior or inferior meniscal surface, and therefore most likely corresponded to an arthroscopically identifiable meniscal tear.9 If grade II meniscal pathology was reported in an MRI interpretation as a grade II tear, however, it could create the erroneous impression that meniscal surface pathology existed. Although grade II intrameniscal signal can reflect the presence of a meniscal contusion or an intrameniscal tear, it may also denote myxoid internal meniscal degeneration, artifact (magic angle e ect or truncation), or normal meniscal vascularity in a child or young adult.14,15 This grading system also does not consider morphologic alterations in characterizing meniscal pathology. Because the morphology of a meniscal tear impacts its relative clinical significance and influences surgical planning, a more specific classification

B

C

system, currently a orded wide acceptance, uses MRI findings to categorize tears as follows13:

•Horizontal

•Vertical radial

•Vertical longitudinal with/without flap displacement

•Complex

Horizontal tears divide the meniscus into superior and inferior components, identified on sagittal and coronal imaging series (Fig. 8.2). The tear often communicates with the inferior meniscal surface; sometimes a concomitant meniscal cyst is identified, involving the perimeniscal soft tissue (Fig. 8.3).

Fig. 8.4 Vertical radial meniscal tear in the posterior horn of the medial meniscus in the right knee. (A) A coronal fat-suppressed T2weighted image shows a vertical defect (arrow) involving the posterior horn of the medial meniscus. (B) A sagittal fat-suppressed

Vertical radial tears exhibit a linear signal pattern. They violate the superior and inferior meniscal surfaces and are oriented perpendicular to the meniscal axis of curvature.16 As such, if a vertical radial tear involves the anterior or posterior portion of the meniscus, the tear will show a linear vertical configuration on coronal images (Fig. 8.4A) and an abrupt truncation of the central meniscal contour on sagittal images (Fig. 8.4B). With this type of tear, the change

proton-density image, obtained in the plane of the tear, shows truncation (arrow) and abnormal signal intensity involving the posterior horn. Note the normal anterior horn.

in meniscal morphology, appreciated in the imaging plane oriented parallel to the tear, is equally as important as the surface communication of linear abnormal meniscal signal intensity, noted in the imaging plane perpendicular to the axis of the tear. If the vertical radial tear involves the midbody of the meniscus, a linear meniscal defect is noted on sagittal images (Fig. 8.5A), and blunting or truncation of the central meniscal contour is observed on coronal images (Fig.

Fig. 8.5 Vertical radial meniscal tear in the mid-body lateral meniscus in the left knee. (A) A sagittal fat-suppressed proton-density image shows a vertical tear (arrow) of the central edge of the lateral meniscus, oriented perpendicular to the curvature of the meniscus.

Note the superimposed horizontal tear of the anterior horn. (B) A coronal fat-suppressed T2-weighted image shows the same tear with truncation of the central meniscal margin (arrow).