MR Imaging of Avascular Necrosis of the Femoral Head: Value of Small- Field-of-View Sagittal Surface-Coil Images

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1 1073 William P. Shuman1 Arthur A. Castagno1 2 Richard L. Baron1 Michael L. Richardson1 Received December 21, 1987; accepted after revision January 21, I Department of RadiOlOgy. SB-05, 1kiversity of Washington Hospital, Seattle, WA Address reprint requests to W. P. Shuman. 2 Present address: Radiology Associates, 3822 Colby Ave., Everett, WA AJR 150: , May X/88/ C American Roentgen Ray Society MR Imaging of Avascular Necrosis of the Femoral Head: Value of Small- Field-of-View Sagittal Surface-Coil Images We compared coronal, large-field-of-view, body-coil MR images with sagittal, smallfield-of-view, surface-coil images of 30 hips for their sensitivities in establishing the diagnosis of avascular necrosis; spatially localizing the avascular necrosis; and detecting joint-space narrowing, femoral head collapse, articular cartilage fracture, and joint fluid. We also compared the two separate plane/coil combinations for detection of the double-line sign (high signal inside a band of low signal, believed to be characteristic for avascular necrosis) and intertrochanteric conversion of hematopoletic marrow to fatty marrow. Coronal, large-field-of-view, body-coil images provided an adequate screening examination for the presence of avascular necrosis (sensitivity of 94%) and were preferred In all cases for mediolateral localization of focal abnormality. They were also better for assessing joint fluid and detecting fatty conversion of marrow. Sagittal, small-field-of-view, surface-coil images were preferred for anteropostenor localization in all cases and for superoinfenor localization of focal abnormality in 15 of 18 cases. They detected additional cases of joint-space narrowing, articular cartilage fracture, and the double-line sign missed by coronal, body-coil images. Sagittal, small-fleld-of-view, surface-coil images are a valuable adjunct to MR evaluation of femoral avascular necrosis because they provide additional information that may be useful for planning surgical therapy. Several studies have shown that MR is a sensitive technique for detecting the presence of avascular necrosis (AVN) in the femoral heads [1-4]. Most of these studies have used relatively low-resolution, large-field-of-view, coronal images produced with the imager body RE receiver coils. Beyond detecting the presence of AVN (sensitivity), MR may be able to provide additional information about the condition that is useful in planning surgical treatment. Such additional information might include improved specificity (to ensure that the imaged abnormality is indeed AVN), precise localization of foci of AVN within the femoral head, and some characterization of the degenerative changes associated with AVN. We prospectively evaluated the ability of small-field-of-view, sagittal, surface-coil images to produce information useful for surgical planning beyond that available on coronal, body-coil images. Subjects and Methods We compared coronal, body-coil and sagittal, surface-coil MR images from 30 hips in 15 patients with suspected AVN of the femoral heads. The patients were years old; six were men and nine were women. Fourteen patients had hip pain (1 1 unilateral, three bilateral). Thirteen patients had risk factors predisposing to AVN (prolonged steroid therapy in 11 patients, alcohol abuse in two patients). Plain-film radiographs were available in 25 hips in 13 patients. Seven patients had Tc-methylene diphosphonate bone scans that included the femoral heads. MR studies were performed on a 1.5-T clinical imager (General Electric, Milwaukee, WI) with 2.0- or 2.5-version software. In all hips, the imager body coil was used for RF reception for coronal images, and a butterfly-configuration, loop-gap surface coil (Medical Advances,

2 1074 SHUMAN ET AL. AJR:150, May 1988 Milwaukee, WI) was used for reception for sagittal images. The surface coil had two connected round resonators, each 1 1 cm in diameter, that could be positioned so that one resonator was on thc pelvic skin anterior to each femoral head (Fig. 1). This loop-gap surface coil was tuned and matched for each individual patient to optimize reception; tuning and matching required about 3 mm and was done in a preparation area adjacent to the magnet room just before the MR study. In all hips, coronal, body-coil images were Ti - weighted, /20, 25 (TR/TE) (requiring 8 mm to obtain), or spin-density-weighted, 2000/20; in 22 hips, they were T2-weighted, /60, 80 (requiring 1 7 mm to obtain). Coronal, body-coil slices were 5 mm thick (22 hips) or 1 0 mm thick (eight hips) with 1- or 2-mm interval between slices; all coronal images were Obtained with a 40- or 48-cm field of view, two repetitions, and a 256 x 256 matrix. Sagittal, surface-coil images were Ti-weighted or spin-density-weighted in all hips and T2-weighted in 20 hips. Sagittal, surfacecoil slices were all 5 mm thick with a 1 -mm interval between slices (Fig. 2); they were obtained with a 1 6- or 20-cm field of view, two repetitions, and a 256 x 256 matrix. On most of the sagittal, surfacecoil images, the direction of phase-encoding was superior to inferior on the image so that flow artifacts generated by the femoral artery did not extend across the femoral head. Two separate sagittal series were required, one through each femoral head; because the surface coil had dual resonators, these two series could be accomplished without moving either the patient or the coil. MR studies were reviewed independently by two of the authors who were unaware of all patient information such as clinical history, surgical results, plain-film findings, and results of radionuclide studies. Differences in MR findings arising from independent review were resolved by agreement when both authors subsequently reviewed the studies together. Both coronal, body-coil and sagittal, surfacecoil images were reviewed for the presence or absence of AVN as defined by focal nonanatomic low-signal areas in each hip [1, 4-7]. When present, AVN was further characterized as focal or diffusely mottled in appearance for each reception-coil/plane combination in each hip. Localization of AVN, when present, was qualitatively assessed by choosing (on the basis of the definition of the border between normal and abnormal tissue) the preferred reception coil/ plane combination for mediolateral localization, anteroposterior localization, and craniocaudad localization in each hip. Detection of various degenerative changes-joint-space narrowing, articular cartilage fracture, femoral head collapse, and significant (grades 2 and 3) joint effusion [8]-was noted for each reception-coil/plane combination; r --,,. qualitative preference for one coil/plane combination over the other for detection of these changes was also recorded for each hip. Detection of the double-line sign (high signal intensity inside a lowintensity rim on T2-weighted images [4, 9, 10]) and detection of the conversion of intertrochantenc hematopoietic (dark) marrow to fatty (bright) marrow [1 1] were noted for each reception-coil/plane combination; qualitative preference for one coil/plane combination over the other for detection of each of these signs was also recorded for each hip. MR findings were compared with surgical and clinical diagnoses. Surgery was performed in 12 hips; tissue sent for pathologic evaluation confirmed AVN in all 1 2. The remaining 1 8 hips were diagnosed clinically by using other imaging results, signs and symptoms, and follow-up of 6-i 8 months. In six of these 18 hips, a combination of positive symptoms, presence of a significant risk factor, characteristic radionuclide and/or plain-film findings (scierosis/lucency and trabecular coarsening), and characteristic MR findings, was considered positive for AVN. In all six of these hips, surgeons elected to observe rather than operate on the AVN because of the patient s general condition. Of the 18 hips with AVN, the condition was bilateral in three patients and unilateral in 12. The remaining 12 of the 18 hips evaluated clinically were diagnosed as negative for AVN because each had normal imaging studies and because each was asymptomatic during the follow-up period. Patients whose hips were in this latter group (negative for AVN) and who had not been seen for at least 6 months after the diagnosis (n = 5) were recontacted by phone to confirm continued absence of symptoms. Results Detection of AVN Large-field-of-view, Ti -weighted, coronal, body-coil images detected 1 7 of the 1 8 cases of AVN. Small-field-of-view, Ti - weighted, sagittal, surface-coil images detected all 1 8 cases of AVN. In the one additional case detected by sagittal, surface-coil images, a small anterior dark focus was missed on the coronal, body-coil images; this focus was biopsied with the guidance of the sagittal images and was histologically documented to be AVN. On coronal, body-coil MR images, I tfi#{228}:;.photo9ra,h of patient t (head to left) showing butterfly-contlguration loop-gap surface coil positioned on patient so one round resonator Is anterior to each femoral head. Small block of foam rubber (arrow) stabilizes coil.. I Fig. 2.-Localizer coronal MR Image indicates relative position of sagitr. tal surface-coil images.

3 AJR:150, May 1988 MR OF FEMORAL AVASCULAR NECROSIS i of the 1 7 hips positive for AVN showed a focal, low-signal pattern; four were more diffusely mottled (Fig. 3A). On sagittal, surface-coil MR images, i 6 of the 1 8 hips positive for AVN had a pattern of focal or linear low signal; two were mottled. In two hips that appeared diffusely mottled on body-coil, coronal images, a more discrete focal or linear dark abnormality was seen on the small-field-of-view, surface-coil images (Fig. 3B). Localization of AVN Fig. 3.-Bilateral avascular necrosis. A, Coronal body-cell large-field-of-view MR image, spin-density-weighted. Right femoral head has mottled pattern, whereas left femoral head has focal area of necrosis. Mediolateral localization of focal left abnormality Is well defined. B, Sagfttal, spin-density-weighted, surface-coil, small-field-of-view MR Image of rlghtfemoral head. AbnOrmalIty that was mottled on coronal image appears as serpiginous black lines. A = anterior P = posterior. C, S.gIttaI, spin-density-weighted, surface-coil, small-field-of-view MR Image of left femoral head. Linear dark abnormality surrounds brighter region anteriorly (arrow). Femoral and acetabular cartilage (arrowheads). A = anterior, P = posterior. D, Sagfttal, T2-weighted, surface-coil, small-field-of-view image of left femoral head. Focal bright area (arrow) Is spatially well localized in anteroposterlor and superoinferlor directions. A = anterior P = posterior. Fig. 4.-Focal left avascular necroala. A, Coronal, body-coil, large-field-ofview, TI-weighted MR Image. Focal dark region In left femoral head has reasonably well defined medlolateral localization. B, Saglttal, TI-weighted, surfacecoil, small-field-of-view MR image of left femoral head. Focal dark region Is well demarcated In anteroposterlorand superoinferlor planes. Surgeon s eye view. A = anterlor P = postedor PS = psoas muscle. The coronal plane was qualitatively preferred over the sagittal for mediolateral localization of the AVN process in i 7 of the 1 8 cases (Figs. 3A and 4A). Despite the lower resolution in the large-field-of-view, coronal images, mediolateral localization was accomplished more easily by direct observation of coronal images than by inference from stacked, sequential, small-field-of-view sagittal images. However, the sagittal, small-field-of-view, surface-coil images were qualitatively preferred over the coronal images for anteroposterior localization of the AVN process in all i 8 cases, and they were preferred for superoinferior localization in i 5 of the 1 8 cases (Figs. 3C and 4B). The ability to locate AVN by direct observation (rather than by inference from stacked sequential images) accounted for this preference in the anteroposterior plane; the superior resolution of the small-field-of-view, surface-coil images accounted for the preference in the superoinferior plane. In the three cases in which the coronal plane was preferred for superoinferior localization, the abnormality had extended somewhat into the proximal femoral neck, and the extension was better appreciated on the coronal image. D

4 1076 SHUMAN ET AL. AJR:150, May 1988 Detection of Associated Findings Of the 18 hips with AVN, both plain films and MR detected joint-space narrowing in three hips and femoral head collapse in two hips. These findings were all detected by the sagittal, surface-coil images (Fig. 5B); two cases ofjoint-space narrowing were missed on the coronal body-coil images. Eemoral head articular cartilage fracture was detected by MR in three hips; in all three cases, it was seen only on small-field-of-view, sagittal surface-coil images (Figs. 5B and 6). The sagittal surface-coil images were qualitatively preferred in all hips for assessment of these degenerative findings, primarily because of the higher spatial resolution provided by the small-field-ofview, surface-coil technique. Grade 2 joint fluid (enough to surround the femoral neck) [8] was detected in five hips by the coronal, body-coil images (Fig. 5A) and in four hips by the sagittal, surface-coil images. Grade 3 joint fluid (distension of the capsule recesses) was detected in two hips by the coronal, body-coil images; this grade of joint fluid was difficult to detect on sagittal images. The coronal plane was qualitatively preferred for assessing joint fluid, primarily because of the ability to depict the joint capsule around both the femoral head and neck on a single slice. The double-line sign (low-signal band with high-signal inner border) was evaluated only on T2-weighted images [4, 9, 1 0]. Of the 1 8 hips with AVN, T2-weighted images were available from both the coronal body coil and sagittal surface coil in 1 6. The double-line sign was detected in 14 of the 16 hips with the sagittal, surface-coil images (Fig. 5C) but in only 1 0 hips with the coronal, body-coil images. In all 1 6 hips, the surface-coil images were qualitatively preferred for evaluation V1,lFI ( 1 Ik,..,, 1(.rH. 4 Fig. 5.-Bilateral avascular necrosis and right femoral head collapse. A, Coronal, body.coll, large-field-of-view, T2-welghted MR Image. Apparent marked right femoral head collapse. Grade 2 fluid around femoral neck (arrows). B, Sagfttal, spin-density-weighted, surface-coil small-field-of-view MR image of right femoral head. Some collapse Is apparent but is not as severe as ft seemed from coronal Image. Articular cartilage narrowing and fracture (arrow). A = anterior P = posterior. C, Saglttal, spin-density-weighted, surface-coil, small-field-of-view MR image of left femoral head. Note double-line sign. A = anterior P = posterior. D, Sagittal, spin-density-weighted, surface-coil, small-sold-of-view MR Image of left femoral head. Normal femoral and acetabular cartilage (arrowheads). Dark serpiginous linear abnormality has segment of double-line sign (arrow). A = anterior P = posterior. of the double-line sign because superior spatial resolution made this finding more conspicuous. Intertrochanteric conversion of hematopoietic (dark) marrow to fatty (bright) marrow on Ti -weighted images was detected by MR in 1 4 of the 30 hips (Fig. 7). In 1 0 of these i 4 hips, this finding was associated with the presence of AVN; in i 2 hips (six patients) it was bilateral. Coronal, bodycoil images detected this finding in all 1 4 hips; sagittal, surface-coil images detected this finding in only nine hips. The Fig. 6.-Advanced left avascular necrosis. Sag. thai, T2-weighted, surface-coil, small-field-of-view MR image of left femoral head. Focal femoral head collapse (arrow) and diffuse marked articular cartilage narrowing. A = anteder P = posterior.

5 AJR:150, May 1988 MR OF FEMORAL AVASCULAR NECROSIS 1077 Fig. 7.-Evaluation of lntrtrochanteric marrow (two different patients). A, Coronal, large-field-of-view, body-coil, TI-weighted MR image. Note darker hematopoletic intertrochanteric marrow (arrows). B, Coronal, large-field-of-view, body.coil, TI-weighted MR image. Bright fatty lntertrochanteric marrow (arrows) Is similar in Intensity to subcutaneous fat (F). coronal, body-coil images were qualitatively preferred in all cases for assessment of fatty marrow conversion because the intertrochanteric region could be assessed entirely and bilaterally on a single slice in this plane. Discussion Advanced AVN of the femoral head (Ficat stage 4) with extensive loss of articular cartilage, deep compression fractures, and destruction of the acetabulum presents little diagnostic problem; it is usually treated with total hip replacement [i 2]. Less severe AVN with more focal abnormality or with some flattening of the femoral head (Ficat stages 2 and 3) may be treated with transtrochanteric rotational osteotomy if the hip joint space is not narrowed (indicating preservation of cartilage) [1 3, i 4]. This procedure depends on identifying a structurally normal portion of the femoral head (and cartilage) and surgically rotating the femoral head to make a new weight-bearing surface of that normal portion. Treatment of less advanced AVN (Ficat stages 0 and 1) currently is controversial. Several authors have suggested that such early disease should have immediate pressure measurements within the femoral neck and/or head; elevated pressures should be treated with core decompression [1 2, 1 5-i 7]. It is important that the core go into focal areas of abnormality ( breach the sequestrum ), not only to provide decompression but also to provide an avenue for revascularization [1 8, i 9]. Not all authors agree with the efficacy of this approach [1 9, 20]; however, most agree that both earlier diagnosis and spatially precise coring improve the chances of success. Imaging techniques used in support of this overall surgical approach need to be sensitive and specific for the diagnosis of AVN. In addition, imaging needs to be able to locate an area of abnormality within the femoral head and to detect associated degenerative changes so that the appropriate surgical procedure can be selected. We postulated that small-field-of-view, sagittal images of the femoral head obtained with a surface coil might offer improved information about the localization of the AVN process, the MR appearance of the abnormality, and the severity of associated degenerative changes. Loop-gap-resonator, small-field-of-view, surface-coil technology is capable of producing high-spatial-resolution images in conjunction with highfield-strength MR imagers [21]. Arranging two of these resonators in a butterfly configuration can provide a shape that conforms to the anterior pelvic anatomy while centering one coil over each femoral head. We chose the sagittal imaging plane for these surface-coil images to be orthogonal to the coronal, body-coil images and to provide a surgeon s eye view as seen from the lateral aspect of the patient (Fig. 4B). We believed the sagittal images might be helpful in planning either the lateral approach to a core biopsy or a rotational osteotomy (because sagittal images depict the relationship between the weight-beann9 portions of the acetabular dome and the femoral head) [i, 6]. Time limitations restricted surface-coil imaging to one sagittal series through each femoral head while this study was ongoing; however, newer software now available is able to image both hips in the sagittal plane with a single series that skips over the large gap between the femoral heads. In this series, the coronal, body-coil images provided an adequate screen for the presence of AVN with a sensitivity of 94% (1 7 of i 8). In localizing the focal abnormalities of AVN, the sagittal, surface-coil images were qualitatively preferred over the coronal, body-coil images for two of the three axes (anteroposterior and superoinferior). These two axes are important when planning a core biopsy, because focal areas of AVN tend to be anterior and superior in the femoral head [12, 1 6, i 9]. These axes also are important when planning rotational osteotomy because they localize spatial relationships between such an abnormality in the femoral head, the weightbearing region of the femoral head, and the acetabular dome. In our series, the small-field-of-view, surface-coil images detected two cases of joint-space narrowing and three cases of articular cartilage fracture missed by body-coil images. The surface-coil, small-field-of-view images were preferred for evaluating such degenerative findings because of their higher spatial resolution. In particular, we believed assessment of articular cartilage thickness was easier on surface-coil images, as has been reported previously [22] (Figs. 3C and 5D). The double-line sign, consisting of a low-signal rim surround-

6 1078 SHUMAN ET AL. AJR:150, May 1988 ing a zone of high intensity on T2-weighted images (considered by some authors to be pathognomonic of AVN [4, 9, 1 0]) was detected more often with the small-field-of-view, surface-coil images than with the large-field-of-view, body-coil images. Superior resolution was the reason cited for preferring surface-coil images when looking for this sign. In addition, surface-coil images detected two femoral heads with a pattern offocal, discrete, dark patches that had appeared mottled on body-coil images; the mottled pattern is considered to be less specific for AVN [1, 2]. Evaluation of conversion of hematopoietic marrow to fatty marrow may also be related to specificity for the diagnosis of AVN, because such conversion correlates with increased intramedullary pressure and decreased intramedullary blood flow [1 1]. Coronal images detected this finding more often and were qualitatively preferred, because the large field of view imaged the entire intertrochanteric region bilaterally on a single slice in the coronal plane. Similarly, the volume of hip joint fluid (which may correlate with the severity of the AVN process [8]) was most easily assessed on the coronal images because all of the joint capsule region bilaterally could be imaged on a single slice. Our series did not propsectively compare MR with other imaging techniques and did not attempt to measure the therapeutic impact of MR findings. However, our series does suggest that coronal, large-field-of-view, body-coil images may provide an adequate screen for the presence of AVN and adequate information about joint fluid and intertrochanteric marrow. Our series also suggests that sagittal, surface-coil images are preferable for anteroposterior and superoinferior localization of a focal abnormality; they better characterize the status of hip-joint articular cartilage and the double-line sign within the femoral head. Whenever AVN is identified on screening coronal, body-coil images, we believe small-field-of-view, sagittal, surface-coil images of the femoral head can be a valuable and worthwhile adjunct to the MR examination because they provide additional information that may be useful in planning surgical therapy. REFERENCES 1. Bassett LW, Gold RH, Reicher M, Bennett LR, Tooke SM. Magnetic resonance imaging in the early diagnosis of ischemic necrosis of the femoral head. C!in Orthop 1987;214: Thkman D, Axel L, Kressel HY, et al. Magnetic resonance imaging of avascular necrosis of the femoral head. Ske!etal Radio! 1986;15: Mitchell MD, Kundel HL, Steinberg ME, Kressel HY, Alavi A, Axel L. 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Presented at the annual meeting of the society of Magnetic Resonance in Medicine, New York, NY, August 1987