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1
Department of Radiology, University of California, 505 Parnassus Ave., San
Francisco, CA 94143-0628.
2
San Francisco Magnetic Resonance Center, 3333 California St., Ste. 105, San
Francisco, CA 94118.
3
National Orthopedic Imaging Associates, 1260 S. Eliseo Dr., Greenbrae, CA
94904.
Received May 7, 2001;
accepted after revision June 12, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Seattle, May 2001.
Abstract
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SUBJECTS AND METHODS. Twenty consecutive patients who underwent both MR imaging and arthroscopy of the knee joint for suspected internal derangement were examined. A modified three-point Dixon MR sequence with a single radiofrequency echo single-scan method for water and fat separation with correction of the static field inhomogeneities was performed on a 0.35-T open magnet to obtain fat suppression. The MR images were prospectively evaluated for the presence and grade of articular cartilage defects.
RESULTS. Uniform fat suppression was obtained in all patients using the modified three-point Dixon technique. Fifty-nine cartilage abnormalities were identified in 19 patients on the basis of arthroscopy. Forty-seven of 59 arthroscopically proven abnormalities were prospectively detected on MR imaging. Compared with arthroscopy, the overall sensitivity of the modified three-point Dixon technique in detecting cartilage lesions was 80% and the specificity was 73%. Sixty-five percent of the cartilage abnormalities were graded identically on MR imaging and arthroscopy.
CONCLUSION. The modified three-point Dixon sequence is a useful technique for achieving fat suppression in the knee joint on a 0.35-T open magnet. It is a sensitive and specific technique for the assessment of cartilage abnormalities in the knee.
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A method for fat suppression using a low-field magnet is the phase-contrast method, described by Dixon [9], that is based on the chemical shift phenomenon. This technique has been applied successfully in MR imaging of adrenal masses [10] and bone marrow [11]; however, this method requires a homogeneous static magnetic field. An alternative imaging method that can be applied to a low-field magnet is the modified three-point Dixon sequence with a single radiofrequency echo single-scan method for water and fat separation with correction of the static field inhomogeneities [12]. The purpose of our study was to assess the diagnostic performance of the newly developed modified three-point Dixon sequence for evaluating articular cartilage defects in the knee joint on a 0.35-T open magnet, correlated with arthroscopy.
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The images were obtained with an open 0.35-T superconductive open magnet (Opart; Toshiba America Medical Systems, South San Francisco, CA) with the use of a solenoid-type knee coil. To provide fat suppression, we used a short tau inversion recovery (STIR) sequence and a modified Dixon sequence that uses a single radiofrequency echo that is "sandwiched" between two gradient echoes [12]. Imaging parameters were as follows: coronal two-dimensional STIR (TR, 2000 msec; inversion time, 95 msec; flip angle, 90°, flop angle, 180°), water—fat sagittal field-echo two-dimensional three-point Dixon (TR/TE, 2600/20), and water—fat axial spin-echo two-dimensional three-point Dixon (1200/36). The imaging flip angle in the Dixon sequence was 30°; section thickness, 5 mm; number of signals acquired, 1-2; matrix, 160-256x256; field of view, 16x16 cm. No phase wrap was used to avoid aliasing. Imaging time for the three-point Dixon sequence in the axial plane was 5 min 11 sec and for the sagittal plane 7 min 20 sec.
Sagittal proton density (1000/20) and coronal T1-weighted spin-echo (600/20) images were also included in the routine protocol but not assessed in this study.
The MR images were prospectively evaluated and graded by two musculoskeletal radiologists at the time of each patient's initial evaluation. Differences of opinion were resolved by consensus. Six articular surfaces were assessed: the patella, the trochlear groove, medial and lateral femoral condyles, and medial and lateral tibial plateaus. For MR imaging interpretation, an orthopedic grading system was adopted that corresponded to the arthroscopic grading system used by the referring orthopedist: grade 0, normal; grade 1, signal intensity abnormality only; grade 2, surface irregularity; grade 3, partial thickness loss, not down to the bone; grade 4, full thickness loss, down to the bone.
Arthroscopy was performed within 6 weeks of the MR examination. At the time of surgery, the orthopedic surgeons had available to them the MR imaging report with the location of the lesion but without the exact grading. At arthroscopy, the cartilage of each surface was graded on a similar five-point scale: grade 0, normal; grade 1, softening; grade 2, shallow ulceration or blisterlike swelling; grade 3, deep ulceration or fibrillation, not extending to bone; grade 4, ulceration with exposure of subchondral bone.
After arthroscopy, the MR images were compared with the arthroscopy report and the exact location and extent of the lesion was identified. The arthroscopic results were used as the standard of reference for determining sensitivity and specificity of the three-point Dixon technique for cartilage abnormalities.
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Using axial and sagittal three-point Dixon sequences together and considering only detection of an arthroscopically observed defect, we recorded 22 false-positive and 12 false-negative results at MR imaging. The 12 false-negatives involved three lateral tibial plateau lesions, one medial tibial plateau lesion, two medial femoral condyle lesions, one lateral femoral condyle lesion, one trochlear groove lesion, three medial patellar facet lesions, and one lateral patellar facet lesion. The 22 false-positives involved the medial tibial plateau in two patients, the lateral tibial plateau in three, the medial femoral condyle in two, the lateral femoral condyle in four, the lateral patellar facet in five, the medial patellar facet in four, and the trochlear groove in two patients. Each imaging plane was evaluated separately for cartilage defects. Sensitivity and specificity for the axial three-point Dixon sequence was 66% and 70%, respectively. For the sagittal three-point Dixon sequence, sensitivity was 71% and specificity, 68%. Compared with arthroscopy, the overall sensitivity of a combination of axial and sagittal planes of three-point Dixon sequence in detecting cartilage abnormalities was 80% and the specificity was 73%. Coronal STIR showed a low sensitivity (39%) but high specificity (94%) for the evaluation of cartilage abnormalities.
A total of 140 articular surfaces in 20 patients were graded at MR imaging and arthroscopy (Table 1). Eighty-one of these (58%) were considered normal (grade 0), and 59 surfaces (42%) were considered abnormal on the basis of arthroscopic findings.
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Of the 140 articular surfaces, 91 (65%) were graded identically on MR imaging and arthroscopy. Of the 59 arthroscopically proven lesions, 32 (54%) were graded identically at arthroscopy and MR imaging (Figs. 2,3,4). The three-point Dixon sequence showed a low sensitivity in grading early stages of chondromalacia (Figs. 5 and 6). Grades 1 and 2 lesions were sometimes difficult to differentiate from the high signal intensity of joint fluid or inflamed synovium (Fig. 5). Sensitivity for correctly identifying grade 1 lesions was 50%; sensitivity for grade 2 lesions was 27%. Three-point Dixon MR imaging was sensitive in correctly identifying advanced chondromalacia. The sensitivity for grades 3 and 4 lesions was 74% and 57%, respectively. The sensitivity of images obtained in the axial and sagittal planes of the three-point Dixon sequence combined was 59% in grading early stages of chondromalacia (grades 1 and 2 combined) and 83% in grading advanced stages of chondromalacia (grades 3 and 4 combined).
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Two patients had an isolated cartilage abnormality without other knee abnormalities. Findings in the remaining patients showed meniscal tears (n=15), abnormalities of the anterior and posterior cruciate ligament (n=6), and abnormalities of the medical collateral ligament (n=2) associated with the cartilage defect.
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Fat-suppressed MR imaging has been shown to be helpful in evaluating cartilage abnormalities in the knee joint [4, 7, 14, 15]. Our study used a modified three-point Dixon technique to provide fat suppression on a 0.35-T open configuration magnet. The basic principle of the original Dixon method is the acquisition of a pair of images with the water and fat signals in-phase in one image and out-of-phase in the other. The Dixon method is error-free only when the chemical shift between the two components is the only source of phase shifts [9]. In routine MR imaging of the knee joint, however, several other sources of phase shifts occur, caused by inhomogeneous penetration of the patient by radiofrequency pulses or equipment-related delays in the digitalization of the signals. Also, magnetic field inhomogeneities cause phase shifts. Shimming can reduce these irregularities to a certain extent but it is very time-consuming [16, 17].
Many modifications to the original Dixon method have been proposed to overcome field inhomogeneities [16, 18,19,20,21]. We used a technique suggested by Zhang et al. [12], the so-called sandwich method developed for a 0.35-T system. This technique sandwiches the radiofrequency pulse between two gradient echoes. These echoes are used to measure field inhomogeneities that are later used to correct for phase shifts. The algorithms used for these corrections rely on a combination of field modeling using polynomial functions and region growing [12].
At a low field strength, the time needed for the water and fat signals to transition from in-phase to out-of-phase is longer than at a high field strength. Therefore, both in-phase and out-of-phase data can be acquired after a single excitation, in contrast to the other Dixon methods that require 2 [9, 17] or 3 [16, 18] excitations. Our Dixon method uses a single radiofrequency echo, which leads to a considerable reduction in imaging time for this sequence [12].
The modified Dixon technique resulted in good visualization of the articular cartilage in all 50 patients. The overall sensitivity of the sagittal and axial three-point Dixon sequences in detecting cartilage lesions was 80% and 73%, respectively. Three-point Dixon showed a low sensitivity and specificity in detecting early stages of chondromalacia (grade 1). Of the 12 cartilage lesion that were missed on MR imaging, six lesions were given grade 1 on arthroscopy. Of the 21 false positive results, 11 lesions were graded 1 on MR imaging but were found to be normal on arthroscopy. Early chondromalacia (increased signal intensity without cartilage thickness loss) was difficult to differentiate from the high signal intensity of joint fluid or inflamed synovium. The tendency of early cartilage abnormalities to be overestimated with the three-point Dixon sequence should be noted when using this technique in clinical practice.
The three-point Dixon method was more sensitive (83%) in accurately identifying advanced stages (grades 3 and 4) of chondromalacia, than in identifying early stages (grades 1 and 2) of chondromalacia (sensitivity 59%). Accurate identification of grades 3 and 4 lesions is important clinically, because orthopedic surgeons may consider subchondral perforation or abrasion arthroplasty for these more severe grades of patellar cartilage abnormalities, whereas no surgical treatment may be necessary for patients with grades 1 and 2 lesions.
Our study had several limitations. The first was that the orthopedic surgeons had the MR imaging report available to them at the time of surgery, which was a potential source of bias. Second, arthroscopy is an imperfect standard of reference, and its accuracy in the evaluation of cartilaginous disorders has been questioned. Arthroscopy may be successful in detecting surface chondral lesions, but estimating the depth of a lesion is inherently inaccurate because arthroscopy visualizes only surfaces unless the subchondral bone is exposed. Additionally, the joint has blind spots that are difficult to assess properly by arthroscopy. A third limitation of our study was that MR imaging interpretation was performed by consensus of two observers. Observer awareness of cartilage lesions probably was greater in our study than it would be in clinical practice, which may have led to improved sensitivity for detecting lesions. A fourth limitation of our study was the lack of comparison with high-field (1.5-T) MR imaging. However, in a previous study using axial and coronal fast spin-echo T2-weighted MR imaging with frequency-selective fat saturation at 1.5 T, we found a sensitivity of 94% and a specificity of 99% in detecting cartilage abnormalities. Sensitivity and specificity for early stages of chondromalacia (grades 1 and 2) were higher (74% and 85%, respectively) compared with the three-point Dixon sequence [4].
Our study showed that the modified three-point Dixon sequence is applicable in a clinical setting on a routine basis. It can be used to achieve uniform fat suppression in the knee joint on a 0.35-T open magnet. Our observation suggests that high-grade cartilage abnormalities can be evaluated reliably with low-field-strength MR imaging using the three-point Dixon sequence.
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