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fracture fragments adjacent to the lateral acetabulum (arrow). (B) Computed tomography (CT) scan reveals impaction fractures of the femoral head and posterior wall of the acetabulum (arrowheads) as well as fracture fragments from the posterior wall (arrows).

CT examination of the hip at our institution is performed using a multislice helical scanner (General Electric, Milwaukee, WI). Imaging is performed in the axial plane and extends from just cephalad to the anterior inferior iliac spine to just caudal to the lesser trochanter. A bone algorithm is used with 2.5-mm collimation at 1.25-mm intervals at a table speed of 0.75 seconds. Two-dimensional (2D) coronal and sagittal reformations are often generated from the data set to aid in spatial orientation. Three dimensional (3D) images are less often generated but may be helpful for surgical planning.

Magnetic Resonance Imaging

MRI has become the secondary imaging examination of choice in the evaluation of unexplained hip pain. MRI

FIGURE 4.2. Intraarticular fracture fragments after reduction of a posterior hip dislocation. CT scan reveals multiple intraarticular fragments (arrows) and large defect (large arrow) in the posterior wall of the acetabulum.

has the unique ability to demonstrate soft tissue and marrow-based abnormalities that cannot be seen on plain radiographs or CT. The spectrum of pathology of the hip demonstrated with MRI has expanded well beyond detecting osteonecrosis, for which hip MRI gained its initial success. MRI is effective in demonstrating in-traarticular and extraarticular pathology. Extraarticu-lar disorders that are well demonstrated with MRI include muscle injuries,23,24 iliopsoas and trochanteric bursitis,25,26 sacroiliitis, and pelvic neoplasms. Intra-articular hip disorders depicted on MRI include joint effusions,27 osteonecrosis,28,29 stress fractures,30,31 occult fractures,32-35 osteoarthritis, and inflammatory arthropathies.36 Unfortunately, conventional MRI has had poor success with demonstrating articular surface cartilage37 and acetabular labral5 abnormalities. Future development and improvements in MRI technology may lead to successful noninvasive evaluation of these structures.

Protocols for MRI of the hip vary among institutions and with the type of scanner used. Likewise, the information gained from MRI of the hip greatly depends on the field strength of the scanner, selection of sequences, and the experience and knowledge of the radiologist interpreting the examination. Currently, low-field-strength scanners do not provide the image quality of high-field-strength scanners. Examinations that do not include high-resolution small field of view images of the affected hip do not allow accurate evaluation of articular surface and labral abnormalities. Although there is no correct set of sequences, some important principles exist when developing protocols. MRI of the hip should include at least one coronal T1-weighted sequence and preferably at least one T2-weighted sequence that includes the pelvis and both hips. This protocol is based on the need to evaluate for occult pelvic pathology (insufficiency fractures, sacroiliitis, and tumors) that may manifest clinically as hip pain and to evaluate the contralateral femoral head, because osteonecrosis is often bilateral. A T2-weighted sequence is also helpful in comparing the amount of joint fluid present in the hips. At least one small field of view high-resolution sequence of the affected hip should be obtained to allow better visualization of chondral, subchondral, and labral abnormalities.17

At our institution, MRI of the hip is performed using a General Electric (Milwaukee) 1.5-T superconducting magnet. A torso coil (General Electric, Milwaukee) is used for the images of both hips and an extremity coil (General Electric, Milwaukee) is used for high-resolution images of the affected hip. Coronal T1-weighted images (TR = 550, TE = minimum, 4.0 mm, 1.0-mm gap, 256X192, 2 NEX, FOV = 34 cm) of both hips depict anatomy and marrow-based abnormalities (osteonecrosis, occult fractures, and so forth). Fat-suppressed T2-weighted fast spin echo (FSE) images of both hips in the coronal (TR = 3300, TE = 102, 4.0 mm, 1.0-mm gap, ET = 8, 256X192, 3NEX, FOV = 34 cm) and axial (TR = 5000, TE = 102, 4.0 mm, 1.0-mm gap, ET = 14, 256X224, 3 NEX, FOV = 34 cm) planes are helpful in demonstrating intraartic-ular and extraarticular fluid collections, osteonecro-sis, marrow edema, stress fractures, sacroiliitis, and paraarticular muscle injuries. High-resolution proton density fat-suppressed FSE images (TR = 3500, TE = 34, 3.0 mm, 1.0-mm gap, ET = 8, 256X192, 3 NEX, FOV = 16 cm) are helpful in assessing the acetabular labrum, articular cartilage, and articular surface of the femoral head.

Magnetic Resonance Arthrography

Several reports have documented the success of MR arthrography of the hip in detecting labral pathol-ogy.4,5,,8,9,12,14 In addition to labral pathology, this examination has the potential to demonstrate loose bodies and abnormalities of the articular cartilage10 and ligamentum teres.17 MR arthrography allows better visualization of normal intraarticular anatomy (Figure 4.3) and pathology than conventional MRI by distending the capsule from the underlying bone and surrounding normal structures. MR arthrography of the hip is thus helpful when conventional MRI is non-contributory and there is clinical suspicion for labral injury or other intraarticular abnormality.

MR arthrography of the hip involves intraarticular injection of either a dilute solution of gadolinium (1-2 mmol) or saline under fluoroscopic guidance followed by multiplanar MRI of the hip. Concomitant injection of anesthetic15,17 as a diluent adds the advantage of providing clinical information helpful in distinguishing intraarticular from extraarticular pathology. At our institution, if the patient has not undergone conventional MRI before MR arthrography, precontrast

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FIGURE 4.3. Normal anatomy seen on magnetic resonance (MR) arthrography. Coronal T1-weighted fat-suppressed image of the left hip demonstrates the lateral labrum, perilabral sulcus, transverse ligament, ligamentum teres, and zona orbicularis.

imaging of the hip is performed similar to the protocol outlined previously. This step can yield useful information regarding the presence or absence of a joint effusion and may demonstrate obvious intraarticular or extraarticular pathology that obviates the need for MR arthrography. Written informed consent is obtained on all patients undergoing MR arthrography of the hip. The technique for gaining access to the hip is identical to that described later in this chapter using a 22-gauge spinal needle. Approximately 1 mL iodi-nated contrast (Conray 60; Mallinkcrodt, St. Louis, MO) is injected to verify an intraarticular position of the needle. After verification of an intraarticular position of the needle, a mixture of 0.05 mL gadolinium (Omniscan, Nycomed, Princeton, NJ) and 3 mL iodi-nated contrast is administered into the joint, followed by the injection of 5 to 6 mL 0.5% bupivacaine HCl. This combination results in an intraarticular dilution of gadolinium of approximately 1:200. MRI is performed within 45 minutes of the injection. Postinjection imaging using the extremity coil includes fat-suppressed T1-weighted images (TR = 750, TE = minimum, 4.0 mm, 0 gap, 256X256, 2 NEX, FOV = 16 cm) in the axial, coronal, sagittal, and oblique ax-ial10 (image plane oriented parallel to the femoral neck) planes. Although radial reconstructions can be obtained and have been reported to improve detection of labral pathology,38 we have not employed this technique on a routine basis.

In cases in which MRI is contraindicated, CT arthrography can be used. Single-contrast technique should be adequate to evaluate the labrum and to identify loose bodies. At our institution, a combination of 5 mL 0.5% bupivicaine HCL and 5 mL Conray 60 (Mallinkcrodt, St Louis, MO) is used for single-contrast hip CT arthrography. CT is performed simi lar to the method outlined previously with coronal and sagittal reformations.

flammation-avid agents in imaging hip disease is beyond the scope of this text.

Nuclear Scintigraphy

With the advent of MRI, nuclear scintigraphy has had a diminished role in the evaluation of hip pain. The bone scan is the most common scintigraphic examination used in the evaluation of skeletal disorders. Ra-dionuclide bone scanning is sensitive to bone turnover and may reveal information regarding local blood flow and can help distinguish monostotic from polyostotic disease.39 This examination employs the use of ra-diopharmaceutically labeled bone-avid agents and gamma camera technology to produce images of the whole body or specific region of interest. In singlephase examinations, a radiopharmaceutically labeled diphosphonate agent (typically technetium-99 methylene diphosphonate, MDP) is injected intravenously, and planar images are obtained 2 to 4 hours after injection. Although seldom used to evaluate the hip, the triple-phase bone scan is a variant of this method typically used to distinguish cellulitis from osteomyelitis. The triple-phase examination includes immediate sequential images of a specified region to evaluate blood flow, immediate static imaging to assess blood pool activity, and delayed images to evaluate bone turnover. Osteomyelitis is distinguished from cellulitis by demonstrating focal increased osseous activity on the delayed images. Other variations in radionuclide bone scanning that may improve lesion detection include the use of pinhole collimation and single photon emission computed tomography (SPECT).39 SPECT yields multiplanar tomographic images that also may also aid in lesion localization. The use of tumor- and in

Hip Arthrography, Injection, and Aspiration

Historically, conventional arthrography of the hip in adults was used in cases of suspected hip infection, intraarticular loose bodies, and synovial proliferative disorders and to evaluate potential loosening or infection of a hip prosthesis.40 Today, a limited form of conventional arthrography (Figure 4.4) is still used in conjunction with hip injections and aspirations and CT and MR arthrography. At our institution, an anterior approach is used (Figure 4.5) to gain intraarticular access using sterile technique, buffered 1% lido-caine HCl as a local anesthetic, and small-gauge spinal needles (3.5 inch). Diagnostic aspirations are typically performed in cases of suspected septic arthritis or infected hip prosthesis. In the case of a native hip, an entry site mark is placed on the skin lateral to the vascular bundle directly anterior to the lateral portion of the femoral neck. Using sterile technique and local anesthetic, a 20-gauge needle is advanced to the lateral aspect of the junction of the femoral head and neck. Aspiration with a 10- to 12-mL syringe is then performed. A small amount (1-2 mL) of iodinated contrast is injected to verify an intraarticular position of the needle. When an aspiration of a total hip arthro-plasty is performed, an entry site is chosen just lateral to the base of the neck of the femoral component so that the needle can be visualized. If a dry tap occurs, 10 to 20 mL sterile water (nonbacteriostatic) is injected into the joint and reaspirated.

Intraarticular injection of a long-acting anesthetic and corticosteroid can be used to distinguish intraar-

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