Computed Tomography Scanner Technology

The original CT scanners were introduced for clinical medical imaging in 1972. These first-generation CT scanners consisted of a gantry in which x-rays produced at one end traveled through the patient and then to a detector 180 degrees away, which would measure the amount of attenuation of the x-ray beam. The x-ray source and the detector would translate around the patient, and the information from this slice would be converted to a gray-scale, cross-sectional image with the use of a computerized filtered back projection. In this image, each pixel represented a measurement of the mean x-ray attenuation. Structures that attenuate x-rays more than water (i.e., cortical bone and muscle) will have positive CT numbers or Hounsfield units, whereas structures that attenuate x-rays less than water (i.e., air and fat) have negative CT Hounsfield units. High-attenuation structures would appear white whereas at the opposite end of the scale would appear black. The imaging of the patient would take up to 30 minutes as the patient moved through the gantry in a stepwise manner, 1 cm at a time, until the region of interest was imaged. The next-generation CT scanners increased the number of detectors in one row so that the x-ray beam fanned out from its source and hit multiple detectors at one time; this reduced imaging time. Additional improvements in 1989 included the spiral or helical scanning of the patient so that the patient would move continuously through the gantry; this again reduced imaging time. With this technique, a series of images could be obtained during the holding of a single breath.

The next generation of CT scanners then added rows of detectors; this technology is referred to as multislice CT, multichannel CT, or multidetector CT (MDCT). The initial MDCT scanners introduced in 1992 could image only two slices at the same time, but this number has since increased; currently available commercial scanners can image 64, 128, or even 256 rows or slices. There are several benefits of such MDCT scanners. One significant benefit is the markedly reduced time that it takes to acquire images; an entire extremity can be imaged with this type of CT with less than 1 minute of imaging time. Another benefit is that high x-ray tube current, which is measured as milliampere-seconds, can be achieved to allow imaging through metal hardware; however, this advantage is offset by the increased radiation dose required. Another significant benefit is that slice thicknesses of less than 1 mm are now attainable, which allows for high-resolution imaging and, more important, for high-resolution reformatted images in any plane.

Two- and Three-Dimensional Reformatted Computed Tomography Images

Before the invention of MDCT scanners, the ability to reformat the original axial data set into other imaging planes was markedly limited. The resulting images were often distorted with a venetian-blind effect with steplike contours, and thus they were of limited diagnostic quality. With the advent of MDCT scanners, however, this has dramatically improved. The reason behind this is the concept of isotropic imaging. If a volume of tissue (i.e., a voxel) is imaged at a very small quantity such that the length, width, and height of the volume are equal, then a reformatted image retains high resolution as compared with the axial images (Figure 5-1, A and B). It is possible to obtain these images with the use of MDCT scanners with 16 or more detector rows that allow for a slice thickness of less than 1 mm. A standard protocol for the imaging of any extremity with CT is to reconstruct the original data at a slice thickness of less than 1 mm with 50% overlap of each slice and to then produce two-dimensional reformatted images 1- to 2-mm thick in the axial, sagittal, and coronal planes. MDCT scanners now have several options or tools that allow for three-dimensional reformatted imaging and surface rendering. At an independent workstation, these data can be manipulated to remove overlying soft tissues, osseous structures, or hardware and to produce a rotating volumetric data set (Figure 5-1, C and D).

Figure 5–1 Normal CT of the hip. A, Axial CT image. B, Coronal multiplanar reformatted CT image. C and D, Surface rendering three-dimensional CT images show a normal left hip.

Radiation Exposure

Although imaging quality and resolution improve with the use of MDCT, one must understand that this is at the expense of radiation dose. The effective radiation dose takes into account the tissue being irradiated and the type of radiation, and it is measured in units of rem or sievert (Sv), with 100 rem being equal to 1 Sv. The effective patient dose for CT of the pelvis is approximately 3 mSv to 4 mSv. To put this into perspective, a single posteroanterior chest radiograph is approximately 0.02 mSv, and the average annual background radiation dose in the United States is 3.6 mSv. A statistically significant increase in cancer is seen among individuals who have been exposed to 50 mSv or more; this was determined on the basis of a study of atomic bomb survivors in Japan. The radiation dose from CT is not insignificant, and this becomes even more problematic in the patient who is young or potentially wants to bear children. As with any imaging study, the medical necessity of the test should be weighed against the risks of the examination.

Femoroacetabular Impingement and Computed Tomography Arthrography

Impingement between the proximal femur and the acetabulum may be classified as cam type, pincer type, or mixed type. With the cam type of femoroacetabular impingement (FAI), a nonspheric femoral head with an abnormal contour of the femoral head–neck junction directly impinges on the acetabulum and labrum with flexion, adduction, and internal rotation of the hip. Proposed causes for this type of impingement include prior slipped capital femoral epiphysis, prior trauma, and growth disturbance with distortion of the physis. With the pincer type of FAI, there is abnormal contact between the acetabulum and the proximal femur from acetabular causes (e.g., retroversion, protrusio, acetabular rim prominence) or femoral causes (e.g., coxa magna, coxa profunda).

Many of the imaging features of FAI include bony abnormalities. Although magnetic resonance imaging (MRI) has also been used to demonstrate these findings, CT is well suited for characterizing bone abnormalities (Figure 5-2, A). With the cam type of FAI, the abnormal contour at the femoral head–neck junction is measured as the alpha angle, which indicates where the bone contour of the femoral head extends beyond the confines of the femoral head. An angle of more than 55 degrees measured on a sagittal–oblique image parallel to the femoral neck is considered abnormal and correlates with the cam type of FAI (Figure 5-2, B). Other bony changes associated with the cam type of FAI are well demonstrated with CT, including fibrocystic changes at the anterosuperior femoral neck (Figure 5-2, C). Such fibrocystic changes are more common among patients with FAI, and they may be directly caused by impingement. CT has an advantage over radiography for showing such cortical changes. Other radiographic signs of the cam type of FAI, such as the abnormal contour of the femoral head–neck junction (pistol grip deformity), are also well delineated on CT, because patient positioning may not optimally profile the bone contour deformity.

Figure 5–2 Femoroacetabular impingement, cam type. A, Axial CT image shows abnormal contour (arrow) at the femoral head–neck junction. B, Abnormal alpha angle, which measures more than 55 degrees, and incidental proximal femur intramedullary nail. C, Coronal multiplanar reformatted CT image from a different patient shows cortical defects with sclerotic margins (arrow).

With regard to CT of the pincer type of FAI, bony abnormalities such as acetabular protrusion (Figure 5-3) and acetabular retroversion may be demonstrated. When assessing for acetabular retroversion on radiography, the crossover sign (i.e., the anterior acetabular wall projects lateral to the posterior acetabular wall) may be affected by patient positioning. CT avoids this pitfall by directly measuring the acetabular version, which is described as 23 degrees in females (range, 10 to 37 degrees) and 17 degrees in males (range, 4 to 30 degrees). A retroverted acetabulum is associated with the pincer type of FAI and with hip osteoarthrosis. CT is also effective for measuring anterior and posterior acetabular sector angles in the setting of hip dysplasia.

Figure 5–3 Femoroacetabular impingement, pincer type. A, Axial and B, coronal multiplanar reformatted CT images show (arrow) left acetabular protrusio. Note the right acetabular fracture.

One of the benefits of MRI and, more important, of MR arthrography for the assessment of FAI is that cartilage abnormalities may also be diagnosed in addition to the previously described bony abnormalities. Although the evaluation of the cartilaginous structures is limited with routine CT, the use of intra-articular iodinated contrast in conjunction with CT (or CT arthrography) can effectively diagnose labral and hyaline cartilage abnormalities, which are seen with FAI. With the use of isotropic imaging that can produce a submillimeter slice thickness, CT arthrography can diagnose a labral tear with 97% sensitivity, 87% specificity, and 92% accuracy. Similarly, CT arthrography can diagnose articular cartilage disorders with 88% sensitivity, 82% specificity, and 85% accuracy. In the setting of hip dysplasia, labral and hyaline cartilage abnormalities commonly coexist. The use of radial reformatted CT is also possible with submillimeter slice thicknesses and isotropic imaging.

Hip Trauma

CT is often used to evaluate the hip and acetabulum after hip dislocation and other pelvic trauma (Figures 5-3 and 5-4). When an acetabular fracture is identified by radiography, CT can further characterize the fracture pattern with the use of multiplanar reformatted images and three-dimensional surface rendering (see Figure 5-3). Associated abnormalities such as intra-articular bodies and pelvic hematoma are also well demonstrated with CT. After hip dislocation, CT demonstrates the position of the femoral head and the coexisting femoral head fracture (see Figure 5-4, A). A sign of prior hip dislocation on CT is the presence of a bubble of gas, which is most commonly seen at the anterior aspect of the hip joint (see Figure 5-4, B).

Figure 5–4 Femoral head fracture and dislocation. A, Axial CT image shows a posterior dislocation of the right hip with a comminuted femoral head fracture (arrow). B, Axial CT image with soft-tissue windows shows an air bubble (arrow) anterior to the hip joint.

It is important to understand the advantages and disadvantages of CT for the evaluation of fracture. CT is most accurate for demonstrating fractures of cortical bone (Figure 5-5, A). In an osteopenic patient in whom the cortex is thin, accuracy will decrease, especially when the fracture is not displaced. This becomes even more problematic for the diagnosis of an intramedullary fracture. In an osteopenic patient in whom the trabeculae are thin or resorbed, a fracture may not be apparent on CT. MRI has been shown to be more accurate than CT for the evaluation of proximal femur fractures in patients more than 50 years old where CT led to a misdiagnosis in 66% of patients. In addition, CT may not show the entire intramedullary extent of a presumed isolated greater trochanteric fracture. As a general rule, CT is most effective for diagnosing fractures of cortical bone in younger patients, and it is relatively limited with regard to intramedullary fractures among the elderly (e.g., insufficiency-type stress fractures). By contrast, a chronic-fatigue–type stress fracture is well demonstrated with CT given the associated sclerosis (Figure 5-5, B).

Figure 5–5 Stress fracture, fatigue type. A, Axial CT image shows an acute cortical fracture as linear low attenuation (arrow). B, Coronal multiplanar reformatted CT image from a different patient shows a chronic cortical fracture (arrow) as a linear low-attenuation fracture line and a thickened high-attenuation cortex.

Hip Arthroplasty

Although radiography is the imaging method of choice for the routine evaluation of the hip after arthroplasty, CT does have a role in specific scenarios, such as the evaluation of infection, osteolysis, and component position. The technical advance that permits for the CT evaluation of metal with reduced artifact is MDCT, which allows increased x-ray tube current to image through metal. The individual components of an arthroplasty as well as the adjacent soft tissues and bone can be visualized with CT (Figure 5-6). This is helpful for displaying fracture (Figure 5-7) and for the diagnosis of soft-tissue infection adjacent to a prosthesis (Figure 5-8). In the presence of component wear and particle disease, CT can directly show the polyethylene component wear as well as the adjacent osteolysis (Figure 5-9). Although radiography adequately screens for osteolysis, CT more accurately measures the volume of osteolysis. CT can also be used to measure component version after hip arthroplasty.

Figure 5–6 Total hip arthroplasty, normal. A, Axial and B, coronal multiplanar reformatted CT images show a cementless total hip arthroplasty. Note the ceramic acetabular and femoral head (arrow) surfaces, which are lower in attenuation as compared with metal.

Figure 5–7 Component failure, total hip arthroplasty revision. A, Coronal multiplanar reformatted CT image, and B, three-dimensional rendering with partially transparent bone and red-colored metal shows femoral component failure (arrow).

Figure 5–8 Infection, total hip arthroplasty. Axial CT images after intravenous contrast in A, bone, and B, soft-tissue windows, show cortical destruction and adjacent soft-tissue fluid collection (arrow).

Figure 5–9 Particle disease, total hip arthroplasty. Axial CT image in soft-tissue windows shows focal areas of bone lysis (arrows) of the acetabulum replaced with soft-tissue attenuation.

Miscellaneous Hip Abnormalities

Other hip disorders that involve the bone or that produce calcification or ossification can be evaluated with CT. Primary synovial osteochondromatosis is a benign neoplastic condition in which hyaline cartilage nodules form in the subsynovial tissue of a single large joint. If these nodules ossify, they are readily demonstrated on CT as multiple uniform ossific bodies in the joint (Figure 5-10). Secondary osteoarthrosis and associated erosions may also be present. The hip is the second most common joint affected by this condition, after the knee.

Figure 5–10 Synovial osteochondromatosis. Axial CT image shows several ossified cartilaginous bodies (arrow).

Osteoid osteoma is a benign bone lesion of uncertain origin that involves a vascularized nidus being present within the bone, typically the cortex. When this occurs in an extra-articular location, the nidus is associated with significant sclerosis and periostitis (Figure 5-11). When it is intra-articular, there is associated effusion and synovitis. CT shows the nidus as a round area of low attenuation with surrounding sclerosis that may calcify. CT can be used to effectively guide the percutaneous thermoablation of osteoid osteomas.

Figure 5–11 Osteoid osteoma. Axial CT image shows a low-attenuation nidus (arrow) with partial calcification. Note the surrounding high-attenuation sclerosis.

CT may also be used to characterize other bone abnormalities. When a sclerotic focus is present within the bone, CT can show the uniform sclerotic density and spiculated margins that are typical of a bone island or enostosis (Figure 5-12). The calcified matrix of a chondroid tumor such as chondroblastoma or chondrosarcoma (Figure 5-13) or the ossified matrix of an osteosarcoma can be demonstrated with CT, which assists with the characterization of a primary bone tumor. CT is the typical imaging method used for the percutaneous imaging-guided biopsy of a bone tumor that involves the pelvis or the proximal femur.

Figure 5–12 Bone island (enostosis). Axial CT image shows a high-attenuation sclerotic focus with speculated margins (arrow).

Figure 5–13 Chondrosarcoma. Axial CT image shows low-attenuation bone destruction with a calcified matrix (arrow).

Ultrasound Technology

Ultrasound is a unique imaging method in that it does not involve ionizing radiation but rather makes use of sound waves to produce images. A sound wave is transmitted from the ultrasound transducer, which is placed on the skin surface with acoustic coupling gel. The sound wave propagates through the soft tissues, interacts at soft-tissue interfaces, and then returns to the transducer, where the sound waves are converted into an image. At interfaces where there is a significant difference in impedance, nearly all of the sound waves are reflected back to the transducer, which produces a bright echo that is displayed on the image. The echoes on the ultrasound image are described as hyperechoic (bright echo), isoechoic (equal echo), hypoechoic (low echo), or anechoic (no echo) as compared with the surrounding tissues. Cortical bone is hyperechoic with shadowing, tendon is hyperechoic and fibrillar, muscle is hypoechoic with interspersed hyperechoic fibroadipose septae, and simple fluid is anechoic. An important technical option on most ultrasound machines is color or power Doppler imaging, which shows blood flow as color on the ultrasound image.

Joint Abnormalities

Ultrasound can be used to identify a hip effusion, which characteristically distends the anterior recess over the femoral neck (Figure 5-14). Joint effusion is diagnosed when there is 2 mm or more of fluid between the anterior capsule and posterior capsular reflection, when the hip capsule over the femoral neck measures 7 mm or more, or when there is a difference in distention of more than 1 mm as compared with the normal contralateral side. The collapsed joint recess with its capsular reflection may measure up to 7 mm and appear to be hyperechoic, but this can appear hypoechoic if it is not imaged perpendicular to the ultrasound beam or if the patient is large. A simple effusion is anechoic, whereas complex fluid will appear to be hypoechoic with possible internal echoes. Synovitis is characterized by hypoechoic to variable echogenicity distention of the joint recess with possible increased flow on color or power Doppler imaging (Figure 5-15). Ultrasound cannot differentiate septic from aseptic effusion; therefore, aspiration should be considered when there is concern about infection. The accuracy of ultrasound for the diagnosis of hip effusion depends on the quantity of fluid and the size of the patient; ultrasound is very accurate in children and thin adults, whereas the diagnosis may be difficult in adults with a large body habitus. When screening for joint effusion, a negative ultrasound should be followed with a fluoroscopy-guided aspiration in a large patient when there is high clinical concern regarding infection. Although the anterior labrum can be visualized with ultrasound as a hyperechoic triangle-shaped structure, the diagnosis of labral tear is limited as a result of the thickness of the soft tissues over the labrum and the inability to visualize the entire labrum.

Figure 5–14 Hip joint effusion. A, Sagittal–oblique ultrasound image parallel to the femoral neck shows anechoic joint fluid (arrow) in between the anterior capsule and the posterior capsular reflection (braces). B, Normal contralateral hip joint with opposed capsule layers (braces). In both images, the left side of the image is superior, and the right side is inferior. FH, femoral head; FN, femoral neck.

Figure 5–15 Synovitis. Sagittal–oblique ultrasound image shows hypoechoic synovitis (arrows). Note the reflective surface and artifact deep to the femoral head and neck of the total hip arthroplasty. The left side of the image is superior, and the right side is inferior. FH, Femoral head; FN, femoral neck.

Ultrasound for the diagnosis of hip joint effusion after hip arthroplasty is more limited as compared with a native adult hip, likely as a result of postsurgical changes in the soft tissue and possible patient body habitus size. Although ultrasound may show a large effusion, a small effusion may be overlooked. A negative ultrasound should be followed with a fluoroscopic-guided hip joint aspiration if there is a high concern about infection. Ultrasound does have a significant role in the evaluation of infection after arthroplasty for evaluating the overlying soft tissues for abscess before fluoroscopic aspiration, thereby avoiding the passage of a needle through an occult abscess with resulting contamination of the joint. Ultrasound may also show bursae or other fluid collections that will not be visible during fluoroscopy (Figure 5-16). The region deep to a skin incision should be evaluated for postoperative fluid collection or abscess. The identification of soft-tissue fluid that extends from the hip joint suggests infection.

Figure 5–16 Infection, total hip arthroplasty. A, Axial ultrasound image shows hypoechoic and heterogeneous fluid collection (arrows) adjacent to the femur. Note the bright artifact deep to the fluid collection. B, Corresponding CT image after intravenous contrast administration. F, Femur.

Bursal Abnormalities

The iliopsoas bursa is located anterior to the hip and communicates with the hip joint in up to 15% of individuals; this number is increased in the presence of hip pathology. Distention of the iliopsoas bursa produces a characteristic shape that is concave lateral as it wraps around the iliopsoas tendon (Figure 5-17). Communication between the iliopsoas bursa and the hip joint can be demonstrated with imaging. Similar to the hip joint, the distention of the bursa may appear as anechoic simple fluid, mixed echogenicity complex fluid, and hypoechoic to variable echogenicity synovitis with possible flow on color or power Doppler imaging. A chronically distended iliopsoas bursa may enlarge significantly and extend cephalad into the abdomen. It is important in this situation to not mistake a large iliopsoas bursa for a psoas abscess.

Figure 5–17 Iliopsoas bursa. A, Axial color Doppler ultrasound image of right hip shows hypoechoic distention of the iliopsoas bursa (arrow). Note the communication with the hip joint (arrowhead) medial to the iliopsoas tendon and the color flow in the adjacent femoral artery and vein. The iliopsoas bursa (arrow) is also shown on the following: B, corresponding axial CT image as low attenuation; C, T2-weighted axial MRI as high signal; and D, T1-weighted fat-saturation axial MRI after intravenous gadolinium administration as ring enhancement. Note the communication between the iliopsoas bursa and the hip joint (arrowhead). IP, Iliopsoas tendon.

There are three bursa that are located near the greater trochanter of the femur: the subgluteus minimus bursa (anterolaterally between the gluteus minimus and greater trochanter), the subgluteus medius bursa (laterally between the gluteus medius and the greater trochanter), and the trochanteric bursa (posterolaterally between the gluteus maximus and the greater trochanter with some extension over the gluteus medius). It is important to evaluate the circumference of the greater trochanter and to evaluate for the distention of these bursae between their respective tendons and the greater trochanter. As with the iliopsoas bursa, distention can range from anechoic to mixed echogenicity, depending on whether there is simple fluid, complex fluid, or synovitis distention (Figure 5-18).

Figure 5–18 Trochanteric bursitis. Axial ultrasound image shows anechoic distention of the trochanteric bursa (arrow) adjacent to the greater trochanter. GT, Greater trochanter.

Muscle and Tendon Abnormalities

Muscle or tendon tear is characterized as the disruption of tendon fibers with anechoic or hypoechoic fluid or hemorrhage. The presence of tendon retraction suggests a full-thickness tear. By contrast, the hypoechoic thickening of a tendon without tendon fiber disruption is characteristic of tendinosis. If a tendon has a tendon sheath, the distention of the tendon sheath indicates tenosynovitis; similar to joint recess distention, this can range from anechoic to mixed echogenicity distention. Ultrasound is useful for the diagnosis of iliopsoas impingement from an adjacent hip arthroplasty, because prosthesis artifact occurs deep and away from the overlying soft tissues (Figure 5-19).

Figure 5–19 Iliopsoas impingement after arthroplasty. Sagittal–oblique ultrasound image shows abnormal hypoechoic soft tissue (arrow) between the metal femoral head and the overlying iliopsoas. A, Acetabulum; FH, femoral head; FN, femoral neck; IP, iliopsoas.

Ultrasound is well suited for evaluating for snapping tendon abnormalities around the hip given its dynamic capabilities, which allow for examination during hip or leg motion. Snapping hip syndrome represents a number of pathologies that cause painful snapping of the hip during motion. These conditions can be divided into internal (hip joint) and external (iliopsoas, iliotibial tract, or gluteus maximus) causes. With regard to the external causes, these relate to the abnormal snapping of a tendon or muscle during motion. Iliopsoas tendon snapping occurs when the leg is straightened from a frog-leg position, which causes the abrupt movement of the iliopsoas tendon in the region of the iliopectineal eminence of the ilium (Figure 5-20). Iliotibial tract or gluteus maximus snapping may occur over the greater trochanter with hip flexion and extension, and there will be abrupt snapping of the representative structure. In each situation, a palpable snap may be felt through the transducer, which corresponds with the abrupt motion of the structure during ultrasound and when the patient is experiencing symptoms. Ultrasound-guided injection may be used for treatment.

Figure 5–20 Snapping iliopsoas tendon. Axial–oblique ultrasound images of left hip show abrupt movement of the iliopsoas tendon (arrow) when comparing A with B. The left side of each image is medial, and the right side is lateral.

Other Hip Abnormalities

Ultrasound has been used effectively for the evaluation for hip dysplasia, and it is accepted as a screening tool for this diagnosis. The ultrasound examination includes static images with measurements and various dynamic stress maneuvers similar to Barlow or Ortolani tests to demonstrate subluxation and dislocation. A newborn with abnormal clinical findings and an unstable hip has an ultrasound examination at 2 weeks of age, whereas one with abnormal clinical findings and a stable click will have ultrasound at 4 to 6 weeks of age. A newborn with normal clinical findings and the presence of risk factors will also have an ultrasound at 4 to 6 weeks of age. The benefit of ultrasound is its ability to visualize the unossified epiphysis and to incorporate stress maneuvers. As the epiphysis ossifies, ultrasound becomes more limited for the evaluation for dysplasia as radiographs become more important.

Soft-tissue abscess and other fluid collections can be demonstrated with ultrasound in a way that is similar to what was described previously with regard to hip joint effusion and bursal fluid. The patient typically can indicate the area of concern, or signs and symptoms may help to localize the area to be imaged. The limitation of ultrasound is difficulty with screening large areas of soft tissue, especially when the pathology is located deep, where resolution will be lower. In addition, the involvement of adjacent bone may be overlooked. In the setting of a negative ultrasound examination, MRI is usually considered if there is high clinical concern for occult infection or other pathology.

Joint Injection

Percutaneous injection of the hip joint may be guided with fluoroscopy, CT, or ultrasound. The use of fluoroscopy is most common, and it can be completed in minutes with few complications. With the patient supine and oblique toward the opposite hip, the femoral artery is identified, and the skin directly over the femoral neck (lateral to the artery) is marked and prepared in typical sterile fashion. A 20-gauge spinal needle with a stylet is inserted into the femoral neck from an anterior or anterolateral approach. Needle-tip placement laterally near the femoral head–neck junction is usually successful, although any location along the femoral neck to the level of the intertrochanteric line is intra-articular (Figure 5-21). Because the needle tip may be located within the capsular reflection immediately adjacent to the bone, thus inhibiting low-resistance injection, minimal rotation of the needle or backing the needle out 1 mm during a test injection will assist with the finding of the joint recess. Before injecting the diagnostic or therapeutic agents, a test injection with iodinated contrast is used to confirm correct needle placement; the needle can then be repositioned as necessary with a repeat test injection of iodinated contrast. It is important to combine the diagnostic or therapeutic agents with iodinated contrast to visualize the structures that are being injected. The extension of diagnostic or therapeutic agents beyond the intended target (e.g., the filling of the iliopsoas bursa) must be reported. A diagnostic or therapeutic injection may be combined with dilute gadolinium for subsequent MR arthrography. The technique of hip injection with the use of CT is the same as described previously with the use fluoroscopy, in which a test injection of iodinated contrast is used before the injection of contrast with the diagnostic and therapeutic agents. CT is not typically used for hip injections, except for cases in which extensive heterotopic ossification may make fluoroscopic-guided injection difficult.

Figure 5–21 Normal single-contrast hip arthrogram. Anteroposterior fluoroscopic image of the right hip shows high-attenuation iodinated contrast (arrow) distending the hip joint. Note the 20-gauge spinal needle (arrowhead).

Ultrasound may also be used when injecting the hip joint. The benefit of ultrasound is the lack of ionizing radiation and the real-time visualization of the needle entering the hip joint with capsular distention during injection. Ultrasound guidance is most successful for smaller patients (i.e., resolution improves when imaging near the transducer) and when there is a sonographic target (e.g., a distention of the hip joint recess). The technique is similar to that described with fluoroscopy because of where the femoral artery is located and because the 20-gauge spinal needle is inserted into the femoral neck. The imaging and needle plane when ultrasound is used is typically parallel to the femoral neck, with the needle angled cephalad. Ultrasound may be difficult in larger patients, because the needle may be difficult to visualize. In addition, one significant disadvantage of ultrasound as compared with fluoroscopy or CT is that the extension of diagnostic and therapeutic agents beyond the hip joint may not be easily identified. One solution to this limitation is mixing iodinated contrast with the diagnostic and therapeutic agents and then immediately obtaining a radiograph to show the extent of the injection.

Bursa Injection

Both CT and ultrasound can be used to inject the bursae near the hip. Each imaging test is most accurate when there is distention of the bursa before the procedure, because the bursa is then visible and can be used as a target. With the use of sterile technique, a spinal needle is guided to the bursa for the injection, which will demonstrate low resistance to injection as compared with the surrounding soft tissues (Figure 5-22). A collapsed hip bursa is usually not identified during CT or ultrasound, which makes the injection of a collapsed bursa near the hip very difficult and most times unsuccessful. In this scenario, the needle is guided to where the bursa should be located, and test injections of an anesthetic agent are used while the needle is repositioned until the low resistance and distention of a bursa are found. If a bursa is not located, many times the area around the bursa is infiltrated with the agent, and this is noted in the procedural report. Fluoroscopy has also been described for the injection of the iliopsoas bursa, although ultrasound has the advantage of direct visualization of the bursa.

Figure 5–22 Iliopsoas bursal injection. Axial ultrasound image shows a hyperechoic needle (arrowheads) entering into the hypoechoic iliopsoas bursa (arrow) from lateral to medial.

Peritendinous Injection

In the setting of snapping hip syndrome, the soft tissues between the snapping tendon or muscle and the adjacent bone may be injected. Ultrasound is ideal for this procedure, because, unlike CT, it allows for real-time imaging during needle positioning and for the targeting of soft tissues between the snapping structure and the bone, which is difficult with fluoroscopy. During ultrasound, if a bursa is identified, this can be the target of the injection. If a bursa is not visualized and not found during test injections, the tissues between the snapping structure and the bone are targeted. Injection for a snapping iliopsoas tendon under ultrasound guidance is completed in the axial plane, with the spinal needle entering from lateral to medial. The needle tip is placed between the iliopsoas tendon and the adjacent iliopectineal eminence, and the diagnostic and therapeutic agents are injected (Figure 5-23). During injection, fluid will be demonstrated to be pooling between the iliopsoas tendon and the ilium. It is important to inject around a tendon and to not place corticosteroids within the tendon because of the theoretic risk of tendon rupture. It is also important to avoid the injection of other structures (e.g., the hip joint), because this would create confusion with regard to the anatomic origin of symptoms and limit the effectiveness of diagnostic injection. Therapeutic injection around the iliopsoas tendon has been described for the treatment of impingement after total hip arthroplasty. With regard to a snapping iliotibial tract or gluteus maximus over the greater trochanter, ultrasound guidance is similarly used to inject the soft tissues between the snapping structure and the adjacent bone, including a bursa, if present.

Figure 5–23 Iliopsoas peritendinous injection. Axial ultrasound image shows a hyperechoic needle (arrowheads) with the needle tip (arrow) located where the iliopsoas tendon is adjacent to the ilium. IP, Iliopsoas.