Imaging of the Hip

Conventional Radiography

Radiography is the first line of imaging for the investigation of all hip pathologies, because it can demonstrate gross osseous alignment, morphology, and integrity. The advantages of plain radiography include ease of access, rapid acquisition, lack of any post processing, and flexibility in patient positioning. However, the shape, contour, and spatial relationships of bones may not be demonstrated by radiography to the same degree as by cross-sectional modalities such as computed tomography (CT) and magnetic resonance imaging ( MRI ). Radiography is also limited by its inability to demonstrate internal joint structures, including the cartilage, labrum, capsule, and surrounding periarticular soft tissues.

Many radiographic projections can be used in the investigation of the hip; these views are variably selected depending on the nature of the pathology and on the anatomic regions of concern. Standard imaging protocols have been established to ensure consistent positioning and optimal visualization of regions of interest.

Computed Tomography

CT is a modality that allows accurate depiction of hip joint anatomy and many pathologies. It can provide very high spatial resolution images, revealing detailed information on osseous morphology, alignment, and integrity. CT acquires images in the axial plane, although images in the coronal, sagittal, and other oblique planes can be secondarily constructed by software reformation. Three-dimensional volume surface shaded images, models, and rotational series can be generated using advanced computer software algorithms. These postprocessing algorithms and filters can be applied to selectively optimize evaluation of the bones or soft tissues. Various structures, including bone, soft tissue, and even hardware, can be subtracted or emphasized with postprocessing techniques. The acquisition time of a CT scan is very short; thus, motion artifacts are not significant.

The indications for CT evaluation of the hip include acute trauma (fracture, dislocation, intraarticular bodies, preoperative planning), treatment follow-up (healing assessment, joint congruity, hardware fixation devices), characterization of mineralized components of bone and soft tissue tumors and neoplasms, and characterization of congenital and developmental abnormalities.

In certain situations and pathologies, CT can be used to diagnose and characterize hematomas, fluid collections, and gross changes of inflammation and infection. However, it should be noted that, compared with MRI, CT has a relatively lower contrast resolution, which can limit evaluation of nonosseous and nonmineralized articular and periarticular structures and soft tissues of the hip. A potential concern with CT is the radiation dose that patients receive, especially to the gonads. Therefore, CT parameters should be optimized to minimize the radiation dose to the patient, with consideration of the degree of detail required given the clinical situation.

CT may be combined with intraarticular injection of iodinated contrast agents (CT arthrography) to assess labral and chondral integrity and to detect intraarticular bodies, although this technique is undertaken only when MRI is contraindicated.

Magnetic Resonance Imaging

MRI offers exquisite contrast resolution, allowing optimal characterization of nonosseous and nonmineralized articular and periarticular structures, including the labrum, cartilage, joint space, capsule, marrow space, and regional soft tissues. It can assess compact and cancellous bone as well, although sometimes not as accurately as CT.

MRI has multiplanar image acquisition capability. Images in standard axial, sagittal, coronal, or any oblique planes can be directly acquired or secondarily constructed with the use of postprocessing reformation software. Recent technological advances in MRI field strength and sequence protocols allow the preparation of three-dimensional sequences with fairly high resolution. It is important to note that MRI does not involve ionizing radiation.

Indications for MRI of the hip are broad because of its ability to evaluate both osseous and regional soft tissue structures. Indications include trauma (detection of radiograph and CT occult fractures and stress or insufficiency fractures), avascular necrosis (AVN), bone marrow edema syndromes (transient osteoporosis, migratory osteoporosis), oncology (detection and characterization of lesions), arthritis (detailed cartilage assessment), internal joint derangement (labral evaluation), osseous deformity (FAI, hip dysplasia ), infection (joint, bone, surrounding soft tissues), myotendinous injury, and soft tissue inflammation (bursitis).

The combination of MRI with intraarticular injection of gadolinium-based contrast agents—direct magnetic resonance arthrography (MRA)—is often used to facilitate evaluation of small structures in the hip joint, including the labrum, hyaline cartilage, and intraarticular bodies. The distention effect of the arthrogram may cause separation of capsular, labral, and osteochondral structures, resulting in increased spatial resolution. Injected contrast solution outlines both normal anatomic structures and abnormal pathologies, further improving contrast resolution and increasing the conspicuity of intraarticular pathology ( Fig. 29.4A–C ).

Indirect MRA involves intravenous injection of gadolinium contrast, followed by a variable delay and/or physical activity regimen. Gadolinium contrast distributes within the joint space, diffusely enhancing the synovial fluid. This strategy provides greater contrast resolution between the joint fluid and the labrum, cartilage, and capsule. The benefit of indirect MRA is that it is less invasive for the patient and does not require fluoroscopically guided joint injection. However, the distention effect that direct MRA provides is not realized. Enhancement of background extraarticular soft tissues and vascular structures, both normal and pathologic, will also be noted. This property may make articular pathology less conspicuous.

MRI pulse sequence selection depends on the technique used; multiple, variable combinations of sequence classes can provide equally diagnostic studies. Spin echo, fast spin echo, and gradient recalled echo sequences can be used in the MRI investigation of hip pathology. Three-dimensional volume and isotropic voxel acquisitions with multiplanar reformations have become feasible in terms of time and imaging quality and can be applied to MRI and MRA of the hip. Multiplanar imaging with at least one sequence or reformation in each plane should be standard.

For nonarthrographic MRI, the protocol should include T1-weighted (T1W) imaging without fat suppression to demonstrate anatomy, joint alignment, marrow abnormality, or fracture. A fluid-sensitive sequence (T-2 weighted [T2W] or proton density [PD] weighted) with fat suppression, or short tau inversion recovery (STIR), should always be part of any protocol to detect abnormal edema within marrow and soft tissues, as well as periarticular fluid collections or cysts. In addition, fluid-sensitive sequences increase the contrast between the joint fluid and the adjacent labrum, cartilage, bone, and capsule. With nonarthrographic MRI studies, higher resolution and an increased number of sampling averages are used; this condition may offset the lower contrast and spatial resolution of MRI compared with MRA.

With direct MRA, the most commonly used sequence is T1W, with or without fat suppression, in addition to fluid-sensitive sequences. With indirect MRA, the same sequence selection as for direct MRA is adequate, although with a few caveats. Fat suppression is strongly advised for maximal contrast resolution because the concentration of gadolinium within the joint may not be as high as with direct injection. In addition, multiphasic imaging may be considered to understand the vascular physiology of the joint.

The drawbacks of MRI include the relatively longer acquisition times, susceptibility to motion artifacts, magnetic susceptibility artifacts owing to regional metal prostheses, and/or postsurgical artifacts, along with several other absolute and relative contraindications (claustrophobia, non–MRI-compatible prostheses, some makes of electronic implanted devices and cardiac pacer equipment, orbital metal bodies).

Ultrasound Examination

Ultrasound (US) is not as widely used as other modalities in radiologic investigation of the adult hip. It is very important in the pediatric hip for the evaluation of developmental dysplasia. US is limited to the assessment of soft tissues, fluid structures, and nonmineralized structures. It can detect the presence of an effusion, synovitis, and capsular thickening. In addition, tendon abnormalities such as tendinosis, tenosynovitis, or tears can be diagnosed. US may also assess regional soft tissues for muscle tears, fluid collections, and bursitis. US is unique in that it allows for the dynamic, real-time visualization of periarticular structures with patient feedback. No parameters for US of the hip have been defined, although a high-frequency linear transducer should be used for high-resolution imaging of periarticular soft tissues. It is important to note that no ionizing radiation is associated with US. However, US may be limited by patient habitus such as obesity, it can be operator dependent, and unlike other modalities it does not provide any gross information on regional osseous structures.

Nuclear Medicine

Nuclear medicine studies may be useful in providing insight into the physiologic state of the hip joint. A radioactive compound is injected into the bloodstream, followed by a variable imaging regimen. Several protocols and various radiotracers can be used in isolation or in combination to generate images that track the flow and accumulation of tracers. Although traditionally two-dimensional images were acquired, three-dimensional datasets can be obtained with the use of multiple rotational detectors, yielding greater spatial information. Indications for nuclear medicine studies include trauma (detection of occult acute and stress fractures), AVN, infection, and oncologic workup.