DOI: 10.1148/radiol.2382050165
(Radiology 2006;238:706-711.)
© RSNA, 2006
Evaluation of the Acetabular Labrum at 3.0-T MR Imaging Compared with 1.5-T MR Arthrography: Preliminary Experience1
Thorsten P. Sundberg, MD,
Glen A. Toomayan, MD and
Nancy M. Major, MD
1 From the Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710. Received January 31, 2005; revision requested March 29; revision received April 25; accepted May 19; final version accepted June 1.
Address correspondence to G.A.T. (e-mail: glen.toomayan{at}duke.edu).
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ABSTRACT
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Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. The purpose of this study was to prospectively compare imaging of the acetabular labrum with 3.0-T magnetic resonance (MR) imaging and 1.5-T MR arthrography. Eight patients (four male, four female; mean age, 38 years) with hip pain suspicious for labral disease were examined at both MR arthrography and MR imaging. Presence of labral lesions, paralabral cysts, articular cartilage lesions, subchondral cysts, osteophytes, and synovial herniation pits was recorded. There was arthroscopic correlation of findings in five patients. MR imaging depicted four surgically confirmed labral tears that were identified at MR arthrography, as well as one that was not visualized at MR arthrography. MR imaging helped identify all other pathologic conditions that were diagnosed at MR arthrography and helped identify one additional surgically confirmed focal articular cartilage lesion. These results provide encouraging support for evaluation with 3.0-T MR imaging over 1.5-T MR arthrography.
© RSNA, 2006
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INTRODUCTION
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The acetabular labrum is a rim of fibrocartilaginous tissue with fibrovascular bundles that is directly attached to the osseous rim of the acetabulum. Functionally, the labrum increases the depth of the ball-and-socket hip joint. Labral lesions can be caused by a variety of mechanisms. Acetabular dysplasia, congenital abnormalities, prior trauma, and abnormal bony anatomy involving the neck of the femur can predispose the labrum to tears or degeneration (1–4). Degenerative tears in elderly patients are not infrequent. Paralabral cysts have also been associated with a torn labrum (5). Disorders of the acetabular labrum have been well documented as a source of chronic hip pain, and patients may present with acute or chronic hip pain characterized by a clicking or snapping sensation with movement (6). Atypical clinical presentations for labral lesions are not uncommon.
While the association between hip pain and labral lesions has been well established (7–9), there continue to be numerous studies seeking to determine the most appropriate method of imaging the joint (10–13). Conventional radiography is not useful in the diagnosis of labral injury, and invasive magnetic resonance (MR) arthrography, which is performed with use of an intraarticular injection of contrast material, has become the most popular modality for imaging labral lesions (14). The injection of contrast material helps in the visualization of labral lesions by distending the hip capsule to allow distinction between the labrum and soft-tissue capsule and by filling clefts associated with labral tears or detachment. The use of 1.5-T MR arthrography remains moderately effective in the diagnosis of a damaged labrum, but 1.5-T MR arthrography has continued to outperform conventional 1.5-T MR imaging and is the current clinical standard (1,14–17). We hypothesized that the increased signal-to-noise ratio provided by 3.0-T MR imaging would allow noninvasive hip imaging to equal or surpass MR arthrography with respect to diagnosis of labral lesions. Thus, the purpose of our study was to prospectively compare imaging of the acetabular labrum by using 3.0-T MR imaging and 1.5-T MR arthrography.
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MATERIALS AND METHODS
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Institutional review board approval was obtained for our Health Insurance Portability and Accountability Act–compliant study, and all subjects (volunteers and patients) provided informed consent.
Overall Study Design
The preliminary study was focused on the comparison of a variety of imaging sequences to evaluate the resolution capabilities of 3.0-T MR imaging in hips of cadaveric specimens and a human volunteer with no suspicion of hip disease. After establishing which sequences best displayed normal labral anatomy with the 3.0-T unit, we proceeded to a prospective clinical study by enrolling patients from orthopedic clinics with hip pain suspicious for labral disease to undergo both 3.0-T MR imaging and 1.5-T MR arthrography. Findings at hip arthroscopy (for patients who proceeded to surgery) were used as the reference standard with which 3.0-T MR imaging and 1.5-T MR arthrography findings were compared. Cadaveric hip specimens were obtained through the anatomic gift program at our institution and were evaluated according to institutional guidelines. Informed consent for the use of specimens in research was obtained from next of kin in accordance with institutional policy.
Preliminary Study and Findings
In the preliminary study, right hips from two female cadavers (age and clinical information unknown) with overlying soft tissue intact and both hips in a 24-year-old male volunteer with no history of hip or groin pain and normal results at physical examination were examined with a 3.0-T MR imaging unit (Siemens, Erlangen, Germany) by using a multichannel phased-array body coil (USA Instruments, Aurora, Ohio) and software (Numaris 4 syngo MR 2003T; Siemens) to optimize a protocol for evaluation of the acetabular labrum.
The hips of the cadavers and volunteer were imaged in the oblique transverse and oblique coronal planes by using multiple sequence parameters (Table 1). Images were assessed by one musculoskeletal radiologist (N.M.M.) with 8 years of experience interpreting musculoskeletal MR images for their ability to delineate the interface between labrum and articular cartilage, provide contrast between labrum and adjacent capsular soft tissue, provide contrast between joint fluid and adjacent labrum and articular cartilage, and help identify signal intensity changes within the substance of the articular cartilage.
Images obtained with both of the intermediate-weighted oblique transverse sequences (sequences 1 and 2) provided contrast between the labrum and adjacent joint fluid, but images from sequence 2 demonstrated better contrast between labrum and articular cartilage and between labrum and soft tissue. Images obtained with both of the 3D double-echo steady state oblique transverse sequences (sequences 3 and 4) demonstrated well-defined signal intensity changes within the substance of the articular cartilage and provided excellent contrast between joint fluid and adjacent labrum and articular cartilage. Sequence 3 clearly delineated the labroacetabular interface, while sequence 4 did not allow for this distinction to be made. The three 3D double-echo steady state oblique coronal sequences (sequences 5, 6, and 7) provided good contrast between joint fluid and labrum, between labrum and soft tissue, and between labrum and articular cartilage. Sequences 5 and 7 both yielded poorly defined areas of signal intensity alteration within the substance of the articular cartilage. Images from sequence 6 showed better definition of the borders of these areas of signal intensity alteration, which allowed for a more precise localization of abnormal signal intensity within the articular cartilage. On the basis of these findings, the optimized protocol used in the clinical study included intermediate-weighted oblique transverse images (sequence 2), 3D double-echo steady state oblique transverse images (sequence 3), and 3D double-echo steady state oblique coronal images (sequence 6).
Clinical Study
The clinical study was performed prospectively. Patients who were enrolled presented with anterior groin or thigh pain that was considered by the orthopedic surgeon to be suspicious for labral disease, and these patients were referred for diagnostic 1.5-T MR arthrography. Eight patients (four male, four female; mean age, 38 years; five right hips, three left hips) were recruited by means of physician referral from the orthopedic clinics at our institution over a 6-month interval. Patients underwent 3.0-T MR imaging with the same Siemens apparatus used in the preliminary study employing the optimized 3.0-T protocol. Patients were placed supine with legs in slight internal rotation, which provided for about 10° of internal rotation of the hip. Immediately after completing the 3.0-T protocol, each patient underwent anterior hip injection at the level of the femoral neck, which was performed by using fluoroscopic guidance, with 10–15 mL of a 1:200 dilution of gadolinium in sterile saline without epinephrine. Patients then immediately underwent 1.5-T MR arthrography (Signa LX; GE, Milwaukee, Wis) with a surface coil or torso phased-array coil by using a protocol previously optimized for evaluation of labral disease and validated by means of arthroscopic correlation (14). T1-weighted spin-echo (500–800/8–15) images and T2-weighted fast spin-echo (4000–6000/50–80) images (section thickness, 4.0 mm; field of view, 18–24 cm; matrix, 256 x 192) were obtained in oblique coronal and oblique transverse planes, all with frequency-selective fat suppression; two signals were acquired. Total imaging time for the 3.0-T MR imaging protocol was 25 minutes compared with 40 minutes for the 1.5-T MR arthrography protocol.
The 1.5-T MR arthrography studies were interpreted by one of five trained musculoskeletal radiologists, with 4, 8 (N.M.M.), 20, 22, and 25 years of experience, who were aware that each examination was ordered to evaluate for a labral tear but were unaware that patients were included in this study. The 3.0-T MR images were interpreted by one trained musculoskeletal radiologist (N.M.M.) who was aware that each patient was included in this study but was blinded to patient identities and results of the 1.5-T MR arthrography studies.
For all studies, tears were noted when contrast material or joint fluid was seen to disrupt the acetabular-labral interface or when linear or diffuse high signal intensity representing contrast material or joint fluid was identified within the substance of the labrum. Abnormal labral structure identified by means of an amorphous, round area of high signal intensity within the substance of the labrum was considered degenerative and not diagnostic of a tear (16). Images were also assessed for findings other than labral tears. A synovial herniation pit was identified by means of a small rounded focus with the signal intensity of fluid, located in the cortex of the anterior femoral neck. A paralabral cyst was identified by means of a well-defined paraarticular focus with the signal intensity of fluid. Subchondral cysts were identified as rounded foci with fluid signal intensity deep to the bone-cartilage interface. Thinning or loss of articular cartilage was noted as high or low signal intensity within the substance of the cartilage, with loss of smooth contour or normal thickness.
Arthroscopy
Five of the eight patients who underwent imaging underwent subsequent hip arthroscopy, which allowed surgical comparison for presence or absence of labral disease with 1.5-T MR arthrography and 3.0-T MR imaging depiction of labral tears. The remaining three patients did not undergo arthroscopy owing to resolution of symptoms following MR imaging.
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RESULTS
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1.5-T MR Arthrography
Of the eight patients who underwent imaging, six were diagnosed with labral tears at 1.5-T MR arthrography (Table 2). In three patients, detachment of the anterosuperior labrum was demonstrated at MR arthrography, and in the other three patients, high signal intensity that represented intraarticular contrast material disrupting the surface of the anterosuperior labrum was demonstrated. Tears or detachments were identified in the anterosuperior labrum in all five patients who underwent subsequent arthroscopy. In four of these patients, labral tears were diagnosed on 1.5-T MR arthrograms, while in one patient the tear was misdiagnosed as a normal labrum on 1.5-T MR arthrograms. Findings of articular cartilage thinning or loss, presence of osteophytes, subchondral cysts, paralabral cysts, and synovial herniation pits diagnosed at 1.5-T MR arthrography are shown in Table 3.
3.0-T MR Imaging
At 3.0-T MR imaging, seven of eight patients were shown to have labral tears (Table 2). In three patients anterosuperior labral detachments were visualized, while in four patients abnormal high signal intensity that represented joint fluid disrupting the surface of the anterosuperior labrum was demonstrated. In all five patients with a diagnosis of labral tears at arthroscopy, labral tears were also diagnosed at 3.0-T MR imaging. Findings of articular cartilage thinning or loss, and presence of osteophytes, subchondral cysts, paralabral cysts, and synovial herniation pits diagnosed at 3.0-T MR imaging are shown in Table 3.
Comparison of 1.5-T MR Arthrography and 3.0-T MR Imaging
The interpretations at 3.0-T MR imaging and 1.5-T MR arthrography were in agreement regarding the labrum in seven of the eight patients (Fig 1). In the one patient in whom the interpretations from the two examinations disagreed (patient 6), the labrum demonstrated a marked increase in intrasubstance signal intensity indicative of a tear at 3.0-T MR imaging, while a normal labrum was shown at 1.5-T MR arthrography (Fig 2). At arthroscopy, the labrum was found to be torn.
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Figure 1a: Coronal MR images of the right hip in a 36-year-old man with right groin pain after external rotation hip injury. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (750/14) shows a torn superior labrum with linear high signal intensity (arrow) within the labrum. (b) Nonenhanced 3.0-T double-echo steady state image (15.5/4.5) shows a torn anterior labrum (arrow). A torn right labrum was found at arthroscopy.
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Figure 1b: Coronal MR images of the right hip in a 36-year-old man with right groin pain after external rotation hip injury. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (750/14) shows a torn superior labrum with linear high signal intensity (arrow) within the labrum. (b) Nonenhanced 3.0-T double-echo steady state image (15.5/4.5) shows a torn anterior labrum (arrow). A torn right labrum was found at arthroscopy.
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Figure 2a: Coronal MR images of the right hip in a 36-year-old woman with right hip pain. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (600/8) shows an intact anterosuperior labrum with no high signal intensity seen within the substance of the labrum (arrow). No labral tear or cartilage abnormalities were identified. (b) Nonenhanced 3.0-T double-echo steady state image (15.0/4.5) shows an abnormal-shaped labrum with abnormal intrasubstance signal intensity consistent with an anterosuperior labral tear (arrow) and an articular cartilage defect (arrowhead). A torn right labrum and marked loss of articular cartilage were found at arthroscopy.
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Figure 2b: Coronal MR images of the right hip in a 36-year-old woman with right hip pain. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (600/8) shows an intact anterosuperior labrum with no high signal intensity seen within the substance of the labrum (arrow). No labral tear or cartilage abnormalities were identified. (b) Nonenhanced 3.0-T double-echo steady state image (15.0/4.5) shows an abnormal-shaped labrum with abnormal intrasubstance signal intensity consistent with an anterosuperior labral tear (arrow) and an articular cartilage defect (arrowhead). A torn right labrum and marked loss of articular cartilage were found at arthroscopy.
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The findings at 1.5-T MR arthrography and 3.0-T MR imaging were in agreement in the diagnosis of other hip abnormalities in six of the eight patients (Table 3). All findings of subchondral cysts, paralabral cysts, and synovial herniation pits were identified on images from both examinations. In one patient (patient 6), a focal loss of articular cartilage was identified on 3.0-T MR images, but the findings were normal on 1.5-T MR arthrograms (Fig 2); at arthroscopy, this patient was found to have a focal loss of cartilage in the acetabulum. One patient (patient 3) was found to have thin, irregular cartilage in the superior aspect of the femoral head on 3.0-T MR images, but this was not identified on the 1.5-T MR arthrograms; no surgical correlation was available for this patient. In one patient (patient 5) who had evidence of femoral and acetabular cartilage loss at both 1.5-T MR arthrography and 3.0-T MR imaging, an intraarticular osteophyte that was demonstrated on 3.0-T MR images was not appreciated on 1.5-T MR arthrograms (Fig 3); the intraarticular osteophyte was confirmed at arthroscopy.
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Figure 3a: Coronal MR images of the right hip in a 44-year-old woman with right hip pain. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (800/14) shows a defect in the articular cartilage of both the femoral head and acetabulum noted by filling with contrast material (large arrow). A paralabral cyst (arrowhead) and related superior labral tear (small arrow) are also seen. (b) Nonenhanced 3.0-T double-echo steady state image (15.0/4.5) shows loss of articular cartilage (arrows) with intraarticular osteophyte formation on both the femoral head and acetabulum. A paralabral cyst (arrowhead) and tear are once again appreciated.
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Figure 3b: Coronal MR images of the right hip in a 44-year-old woman with right hip pain. (a) Intraarticular gadolinium–enhanced 1.5-T T1-weighted spin-echo image (800/14) shows a defect in the articular cartilage of both the femoral head and acetabulum noted by filling with contrast material (large arrow). A paralabral cyst (arrowhead) and related superior labral tear (small arrow) are also seen. (b) Nonenhanced 3.0-T double-echo steady state image (15.0/4.5) shows loss of articular cartilage (arrows) with intraarticular osteophyte formation on both the femoral head and acetabulum. A paralabral cyst (arrowhead) and tear are once again appreciated.
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DISCUSSION
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Three-dimensional double-echo steady state MR imaging is basically a modified 3D fast imaging with steady-state precession, or FISP, protocol. The initial FISP sequence provides high fidelity anatomic resolution of bone and cartilage; double-echo steady state examination adds an additional late echo (reversed FISP), which provides increased T2 weighting to the resulting image for musculoskeletal applications (18). The steady state examination uses small flip angles combined with extremely short repetition time and echo time, which improves signal-to-noise ratios in a given acquisition time. Improved signal-to-noise ratio, combined with the increased signal intensity provided by higher magnet field strength, allows for section thicknesses on the order of 1.0 mm. With such finely sectioned images, software applications have been developed to allow the double-echo steady state images obtained in one plane to be reformatted to display in any other plane desired; the data acquired in two dimensions becomes applicable to any plane (3D). While we have not thus far used 3D-reformatted images in our study, the ability to reformat would allow a single session of data acquisition to be redefined according to physicians' interests.
In our study, a 3.0-T MR imaging protocol was used to successfully identify all labral tears noted at 1.5-T MR arthrography and to identify one additional surgically proved labral tear that was not diagnosed at 1.5-T MR arthrography. Findings such as subchondral cysts, synovial herniation pits, and paralabral cysts were evident whether the patient underwent MR arthrography or MR imaging. Perhaps the most encouraging finding with the 3.0-T imaging was the ability to demonstrate abnormalities in the articular cartilage of the femoral head and acetabulum. In a hip without articular cartilage abnormality, an even, continuous, homogeneous strip of intermediate signal intensity can be demonstrated overlying the femoral head and separated from the articular cartilage of the acetabulum by a narrow band of high signal intensity representing joint fluid. Joints with pathologic changes of the articular cartilage appear to have thin, irregular areas of heterogeneous high and low signal intensity, which suggest cartilage abnormalities. In our study, one of the patients with normal results at MR arthrography was found to have a normal labrum, but thin, irregular cartilage in the superior aspect of the femoral head was demonstrated at 3.0-T MR imaging; this finding could help account for this patient's hip pain. In another patient, 3.0-T MR images accurately demonstrated a surgically proved loss of acetabular articular cartilage, but findings were interpreted as normal on 1.5-T MR arthrograms.
Our study had limitations. The number of patients in our series was small, and arthroscopic comparison was not available in all patients. The 3.0-T MR images were read by a single radiologist, while clinical MR arthrograms were read by several blinded radiologists. Future studies regarding 3.0-T MR imaging interpretation will be needed to establish interobserver variation. A lesion severity scale may have provided a more accurate comparison of 1.5-T MR arthrography and 3.0-T MR imaging. Future 3.0-T MR imaging testing may result in improved imaging parameters that will afford improved diagnostic results.
The information learned from our preliminary study is encouraging. Patients who now have labral disease evaluated with intraarticular injections of contrast material and 1.5-T MR imaging can potentially be imaged at 3.0 T without injection. Larger studies will be needed, however, to determine the diagnostic performance of 3.0-T MR imaging compared with 1.5-T MR arthrography.
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FOOTNOTES
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Abbreviations: 3D = three-dimensional
Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, T.P.S., G.A.T., N.M.M.; study concepts/study design or data acquisition or data analysis/interpretation, T.P.S., G.A.T., N.M.M.; manuscript drafting or manuscript revision for important intellectual content, T.P.S., G.A.T., N.M.M.; approval of final version of submitted manuscript, T.P.S., G.A.T., N.M.M.; literature research, T.P.S., G.A.T., N.M.M.; clinical and experimental studies, T.P.S., G.A.T., N.M.M.; statistical analysis, T.P.S., G.A.T., N.M.M.; and manuscript editing, T.P.S., G.A.T., N.M.M.
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ABSTRACT
INTRODUCTION
