Knee Joint: Comprehensive Assessment with 3D Isotropic Resolution Fast Spin-Echo MR Imaging—Diagnostic Performance Compared with That of Conventional MR Imaging at 3.0 T

Abstract

Purpose

To determine whether a three-dimensional isotropic resolution fast spin-echo sequence (FSE-Cube) has similar diagnostic performance as a routine magnetic resonance (MR) imaging protocol for evaluating the cartilage, ligaments, menisci, and osseous structures of the knee joint in symptomatic patients at 3.0 T.

Materials and Methods

This prospective, HIPAA-compliant, institutional review board–approved study was performed with a waiver of informed consent. FSE-Cube was added to the routine 3.0-T MR imaging protocol performed in 100 symptomatic patients (54 male patients with a median age of 32 years and 46 female patients with a median age of 33 years) who subsequently underwent arthroscopic knee surgery. All MR imaging studies were independently reviewed twice by two musculoskeletal radiologists. During the first review, the routine MR imaging protocol was used to detect cartilage lesions, ligament tears, meniscal tears, and bone marrow edema lesions. During the second review, FSE-Cube with multiplanar reformations was used to detect these joint abnormalities. With arthroscopic results as the reference standard, the sensitivity and specificity of FSE-Cube and the routine MR imaging protocol in the detection of cartilage lesions, anterior cruciate ligament tears, and meniscal tears were calculated. Permutation tests were used to compare sensitivity and specificity values.

Results

FSE-Cube had significantly higher sensitivity (P = .039) but significantly lower specificity (P = .003) than the routine MR imaging protocol for detecting cartilage lesions. There were no significant differences (P = .183–.999) in sensitivity and specificity between FSE-Cube and the routine MR imaging protocol in the detection of anterior cruciate ligament tears, medial meniscal tears, or lateral meniscal tears. FSE-Cube depicted 96.2% of medial collateral ligament tears, 100% of lateral collateral ligament tears, and 85.3% of bone marrow edema lesions identified on images obtained with the routine MR imaging protocol.

Conclusion

FSE-Cube has similar diagnostic performance as a routine MR imaging protocol for detecting cartilage lesions, cruciate ligament tears, collateral ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint at 3.0 T.

© RSNA, 2009

Introduction

Three-dimensional (3D) sequences with isotropic resolution have the potential to improve the quality and efficiency of musculoskeletal magnetic resonance (MR) imaging. Current musculoskeletal MR imaging protocols can be time consuming and often consist of two-dimensional (2D) fast spin-echo (FSE) sequences repeated in multiple planes. Although these 2D sequences have high in-plane spatial resolution, they have relatively thick sections and gaps between sections that can lead to partial-volume artifacts. Three-dimensional isotropic resolution sequences can reduce partial-volume artifacts through the acquisition of thin continuous sections through joints. Furthermore, the isotropic source data can be used to create multiplanar reformations (MPRs), thereby eliminating the need to repeat sequences with identical tissue contrast in multiple planes. The use of 3D isotropic resolution sequences in clinical practice could markedly decrease MR imaging examination times, which would improve patient comfort, reduce motion artifacts, and increase the clinical efficiency of the MR imaging unit.

Until recently, the use of 3D sequences with isotropic resolution in musculoskeletal MR imaging has been limited by their long acquisition and postprocessing times (1,2). However, with the development of more efficient imaging techniques and the availability of high-performance MR imaging workstations, evaluating the knee joint by using 3D isotropic resolution sequences has become clinically feasible (3–5). Most currently used 3D sequences with isotropic resolution are balanced steady-state free precession sequences (1–5). Three-dimensional FSE sequences with intermediate-weighted contrast have also been developed, but their use in clinical practice is currently limited by their anisotropic resolution and relatively long acquisition times (6).

FSE-Cube is a new 3D FSE sequence that can be used to evaluate the knee joint. FSE-Cube can produce multiplanar 3D intermediate-weighted images with 0.7-mm isotropic resolution at 3.0 T after a single 5-minute acquisition (7,8). The use of FSE-Cube for multiplanar evaluation of the knee joint in asymptomatic volunteers has been previously reported (9). However, clinical studies with arthroscopic correlation are needed to assess the strengths and weaknesses of FSE-Cube and to determine whether the 3D sequence can replace currently used 2D sequences for evaluating the knee joint in clinical practice. Thus, this study was performed to determine whether FSE-Cube has similar diagnostic performance as a routine MR imaging protocol for evaluating the cartilage, ligaments, menisci, and osseous structures of the knee joint in symptomatic patients at 3.0 T.

Materials and Methods

Study Group

One author (R.F.B.) is an employee of GE Healthcare (Waukesha, Wisconsin), and another author (G.E.G.) receives research support from GE Healthcare. These authors did not have control of inclusion of any data or information. The study was performed in compliance with Health Insurance Portability and Accountability Act regulations, with approval from the institutional review board of the University of Wisconsin, and with a waiver of informed consent.

Between December 1, 2007, and October 1, 2008, 272 symptomatic patients (143 male patients [age range, 16–72 years; median age, 35 years] and 129 female patients [age range, 16–81 years; median age, 36 years]) undergoing routine MR imaging of the knee at our institution were enrolled in a prospective clinical study investigating the ability of FSE-Cube to provide comprehensive knee joint assessment. All 272 patients enrolled in the study were imaged with our routine knee MR imaging protocol, which consists of multiplanar 2D FSE sequences, and the FSE-Cube sequence. One hundred of these 272 patients subsequently underwent arthroscopic knee surgery. The study group consisted of these 100 consecutive patients (54 male patients [age range, 16–66 years; median age, 32 years] and 46 female patients [age range, 16–79 years; median age, 33 years]) who were evaluated with MR imaging and arthroscopy. No patient was excluded from the study on the basis of any factor, including age, weight, severity of knee injury, history of prior knee surgery, or quality of the MR imaging examination.

MR Imaging

All 100 patients in the study group were imaged with the same 3.0-T MR imaging unit (Sigma Excite HDx; GE Healthcare) by using an eight-channel phased-array extremity coil (Precision Eight TX/TR High Resolution Knee Array; Invivo, Orlando, Fla). All MR imaging examinations consisted of an axial frequency selective fat-suppressed T2-weighted FSE sequence, a coronal intermediate-weighted FSE sequence, a coronal frequency selective fat-suppressed intermediate-weighted FSE sequence, a sagittal intermediate-weighted FSE sequence, a sagittal frequency selective fat-suppressed T2-weighted FSE sequence, and a sagittal FSE-Cube sequence. The imaging parameters of all sequences are summarized in Table 1. The FSE-Cube sequence was performed with a 2D autocalibrating parallel imaging reconstruction technique (ARC; GE Healthcare) with an acceleration factor of three to reduce imaging time. The FSE-Cube isotropic source data were used to create sagittal, coronal, and axial reformatted images of the knee joint with 1.5-mm section thickness. The postprocessing was performed by a technologist on the imaging workstation immediately after the MR imaging examination.

Table 1.

Parameters for MR Imaging Sequences

Arthroscopic Knee Surgery

Arthroscopic knee surgery was performed in all 100 patients in the study group within 3 months (time range, 3–81 days; mean, 37.3 days ± 18.0 [standard deviation]) of their MR imaging examination. All arthroscopic knee surgeries were performed by one of three experienced orthopedic surgeons at the University of Wisconsin who specialized in sports medicine and who had between 10 and 25 years of clinical experience. The decision to perform arthroscopic surgery was based on clinical findings and the official interpretations of the MR imaging studies. The official interpretations of the MR imaging studies were made by one of seven fellowship-trained musculoskeletal radiologists at our institution (including R.K., K.W.D., and A.A.D.S.) by using the routine MR imaging protocol, as mandated by our internal review board.

All articular surfaces of the knee joint were graded at arthroscopy by using the Noyes classification system (grade 0 = normal, grade 1 = cartilage softening, grade 2A = superficial partial-thickness cartilage lesion < 50% of the total thickness of the articular surface, grade 2B = deep partial-thickness cartilage lesion > 50% of the total thickness of the articular surface, and grade 3 = full-thickness cartilage lesion) (10). The presence of anterior cruciate ligament tears, posterior cruciate ligament tears, and meniscal tears was also documented at arthroscopy. The anterior cruciate ligament tears and posterior cruciate ligament tears were classified as partial thickness or full thickness. No attempt was made to classify the meniscal tears. The orthopedic surgeons were aware of the official interpretations of the MR imaging studies in all patients at the time of arthroscopy.

Review of MR Imaging Studies

All MR imaging studies were independently reviewed twice in separate sittings by two fellowship-trained musculoskeletal radiologists (R.K. and K.W.D., with 10 and 14 years of clinical experience, respectively). The radiologists were unaware of the arthroscopic findings in each patient when they reviewed the MR imaging studies. To prevent recall bias, the radiologists reviewed the MR imaging studies in separate sittings at least 4 months apart.

During the first review of the MR imaging studies, the radiologists used all the sequences in the routine MR imaging protocol together to detect cartilage lesions, anterior cruciate ligament tears, posterior cruciate ligament tears, meniscal tears, medial collateral ligament tears, lateral collateral ligament tears, and bone marrow edema lesions within the knee joint. During the second review, the radiologists used the FSE-Cube sequence with MPRs to detect these joint abnormalities. During both reviews, the radiologists graded all articular surfaces of the knee joint by using a modified Noyes classification system (grade 0 = normal cartilage, grade 1 = increased T2 signal intensity of morphologically normal cartilage, grade 2A = superficial partial-thickness cartilage lesion < 50% of the total thickness of the articular surface, grade 2B = deep partial-thickness cartilage lesion > 50% of the total thickness of the articular surface, and grade 3 = full-thickness cartilage lesion) (11–13). No attempt was made to classify the anterior cruciate ligament tears, posterior cruciate ligament tears, or meniscal tears.

A third review of the MR imaging studies was performed by both radiologists to obtain a consensus interpretation of the images acquired with the routine MR imaging protocol in terms of the presence or absence of medial collateral ligament tears, lateral collateral ligaments tears, and bone marrow edema lesions. This consensus interpretation was used as the reference standard to determine the diagnostic performance of FSE-Cube for detecting these joint abnormalities. The medical records of all patients with a consensus interpretation of medial collateral ligament tears or lateral collateral ligament tears with the routine MR imaging protocol were reviewed. All 15 patients with MR imaging findings of a medial collateral ligament tear had medial joint line pain and tenderness at palpation of the medial collateral ligament. Two patients also had valgus instability at examination with anesthesia. All five patients with MR imaging findings of a lateral collateral ligament tear had lateral joint line pain and tenderness at palpation of the lateral collateral ligament.

Statistical Analysis

All statistical analyses were performed by using the R programming environment (R: A Language and Environment for Statistical Computing, version 2.3.1; R Foundation of Statistical Imaging, Vienna, Austria, 2006 [http://www.r-project.org]). For all statistical tests, differences were considered to be significant if the P value was less than .05.

Statistical analysis was used to compare the demographic data of the 100 patients in the study group who underwent arthroscopic knee surgery with those of the 187 patients enrolled in the prospective clinical study who did not undergo arthroscopic knee surgery and who were thus excluded from the study group. The Wilcoxon test was used to compare the median ages of the male and female patients. χ² Tests were used to compare the proportions of male and female patients and the proportions of patients with right and those with left knee pain.

Statistical analysis was used to compare the demographic characteristics of the 54 male patients and 46 female patients in the study group. The Wilcoxon test was used to compare the median age. We used t tests to compare the mean time interval between MR imaging and arthroscopic knee surgery. χ² Tests were used to compare the proportions of patients with right and those with left knee pain, the proportions of patients undergoing arthroscopic surgery performed by each orthopedic surgeon, and the proportions of patients with each surgical indication (ie, anterior cruciate ligament reconstruction, medial meniscus repair or resection, lateral meniscus repair or resection, and osteochondral autograft transplantation).

At statistical analysis to assess the diagnostic performance of FSE-Cube and the routine MR imaging protocol for detecting cartilage lesions, ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint, the data from the independent reviews of both readers were combined. This was done to increase statistical power for a comparison between FSE-Cube and the routine MR imaging protocol.

With arthroscopy as the reference standard, the sensitivity, specificity, and accuracy of FSE-Cube and the routine MR imaging protocol for detecting each grade and all grades of cartilage lesions within the knee joint were calculated for both readers combined and for all articular surfaces combined. For calculating sensitivity, specificity, and accuracy, the cartilage grades assigned at MR imaging were classified as either “disease negative” (ie, MR imaging grade 0) or “disease positive” (ie, MR imaging grades 1, 2A, 2B, and 3). The proportions of cartilage lesions graded identically and the proportions of cartilage lesions graded within one grade at arthroscopy and at MR imaging were calculated for FSE-Cube and the routine MR imaging protocol for both readers combined. Standard errors of the mean were calculated by “bootstrapping” patients to account for dependence within patients among the six articular surfaces and among the two readers. On the basis of the standard errors of the mean, permutation tests were used to compare differences between FSE-Cube and the routine MR imaging protocol.

With arthroscopy as the reference standard, the sensitivity, specificity, and accuracy of FSE-Cube and the routine MR imaging protocol for detecting anterior cruciate ligament tears, posterior cruciate ligament tears, medial meniscal tears, and lateral meniscal tears within the knee joint were calculated for both readers combined. On the basis of standard errors of the mean calculated by “bootstrapping” patients, permutation tests were used to compare differences between FSE-Cube and the routine MR imaging protocol. Using the consensus interpretations of the routine MR imaging data as the reference standard, the sensitivity, specificity, and accuracy of FSE-Cube for detecting medial collateral ligament tears, lateral collateral ligament tears, and bone marrow edema lesions were calculated for both readers combined.

κ Statistics were used to measure interobserver agreement between readers for determining the presence or absence of cartilage lesions, anterior cruciate ligament tears, posterior cruciate ligament tears, medial meniscal tears, lateral meniscal tears, medial collateral ligament tears, lateral collateral ligament tears, and bone marrow edema lesions. Interobserver agreement was assessed according to the recommendations of Landis and Koch (14), in which a κ value of 0.00–0.20 indicates slight agreement; a κ value of 0.21–0.40, fair agreement; a κ value of 0.41–0.60, moderate agreement; a κ value of 0.61–0.80, substantial agreement; a κ value of 0.81 to less than 1.00, almost perfect agreement; and a κ value of 1.00, perfect agreement. On the basis of standard errors of the mean calculated by “bootstrapping” patients, permutation tests were used to compare differences between FSE-Cube and the routine MR imaging protocol.

Results

There were no significant differences between the 100 patients in the study group who underwent arthroscopic knee surgery and the 187 patients enrolled in the prospective clinical study who did not undergo arthroscopic knee surgery with regard to the median age of male patients (P = .09), the median age of female patients (P = .23), the proportions of male and female patients (P = .82), and the proportions of patients with right and those with left knee pain (P = .96).

There were no significant differences between the 54 male patients and the 46 female patients in the study group with regard to median age (P = .89), the mean time interval between MR imaging and arthroscopic knee surgery (P = .47), the proportions of patients with right and those with left knee pain (P = .89), the proportions of patients undergoing arthroscopic surgery performed by each orthopedic surgeon (P = .96), and the proportions of patients with each surgical indication (P = .91–.99).

As shown in Table 2, the sensitivity, specificity, and accuracy, respectively, for detecting 189 cartilage lesions within the knee joint were 72.8%, 88.2%, and 83.3% for FSE-Cube and 68.2%, 92.8%, and 85.1% for the routine MR imaging protocol (Figs 1 and 2). FSE-Cube had significantly higher sensitivity (P = .039), significantly lower specificity (P = .003), and similar accuracy (P = .062) compared with the routine MR imaging protocol for detecting cartilage lesions. There was no significant difference between FSE-Cube and the routine MR imaging protocol in the proportions of cartilage lesions graded identically (33.9% [128 of 378] for FSE-Cube and 28.8% [109 of 378] for the routine MR imaging protocol, P = .228) or within one grade of the arthroscopic grade (72.0% [272 of 378] for FSE-Cube and 68.0% [257 of 378] for the routine MR imaging protocol, P = .203).

Table 2.

Sensitivity, Specificity, and Accuracy of FSE-Cube and Routine MR Imaging Protocol in Detection of Cartilage Lesions within Knee Joint for Both Readers Combined and for All Articular Surfaces Combined

Note.—Data in parentheses were used to calculate the percentages (numerators and denominators represent the combined data from the independent reviews of the two readers); data in brackets are P values for comparison of the two imaging techniques. P < .05 indicates a significant difference.

Figure 1a:

Sagittal MR images in 43-year-old man with surgically confirmed grade 2B cartilage lesion on the femoral trochlea that was detected by neither reader with the routine MR imaging protocol and by both readers with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show normal-appearing articular cartilage on the femoral trochlea (arrow). (c) Corresponding FSE-Cube image shows a deep partial-thickness cartilage lesion on the femoral trochlea (arrow).

Figure 1b:

Sagittal MR images in 43-year-old man with surgically confirmed grade 2B cartilage lesion on the femoral trochlea that was detected by neither reader with the routine MR imaging protocol and by both readers with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show normal-appearing articular cartilage on the femoral trochlea (arrow). (c) Corresponding FSE-Cube image shows a deep partial-thickness cartilage lesion on the femoral trochlea (arrow).

Figure 1c:

Sagittal MR images in 43-year-old man with surgically confirmed grade 2B cartilage lesion on the femoral trochlea that was detected by neither reader with the routine MR imaging protocol and by both readers with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show normal-appearing articular cartilage on the femoral trochlea (arrow). (c) Corresponding FSE-Cube image shows a deep partial-thickness cartilage lesion on the femoral trochlea (arrow).

Figure 2a:

Sagittal MR images in 41-year-old woman with surgically confirmed normal articular cartilage on the lateral femoral condyle that was detected by both readers with the routine MR imaging protocol and by neither reader with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show a smooth articular surface of the lateral femoral condyle (arrow). (c) Corresponding FSE-Cube image shows an irregular articular surface of the lateral femoral condyle (arrow); this was thought by both readers to represent a superficial partial-thickness cartilage lesion.

Figure 2b:

Sagittal MR images in 41-year-old woman with surgically confirmed normal articular cartilage on the lateral femoral condyle that was detected by both readers with the routine MR imaging protocol and by neither reader with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show a smooth articular surface of the lateral femoral condyle (arrow). (c) Corresponding FSE-Cube image shows an irregular articular surface of the lateral femoral condyle (arrow); this was thought by both readers to represent a superficial partial-thickness cartilage lesion.

Figure 2c:

Sagittal MR images in 41-year-old woman with surgically confirmed normal articular cartilage on the lateral femoral condyle that was detected by both readers with the routine MR imaging protocol and by neither reader with FSE-Cube. (a) Intermediate-weighted FSE image and (b) fat-suppressed T2-weighted FSE image of knee joint show a smooth articular surface of the lateral femoral condyle (arrow). (c) Corresponding FSE-Cube image shows an irregular articular surface of the lateral femoral condyle (arrow); this was thought by both readers to represent a superficial partial-thickness cartilage lesion.

As shown in Table 3, there were no statistically significant differences (P = .183–.999) in sensitivity, specificity, and accuracy between FSE-Cube and the routine MR imaging protocol for detecting 33 full-thickness and three partial-thickness anterior cruciate ligament tears, 52 medial meniscal tears, and 35 lateral meniscal tears (Figs 3–5). FSE-Cube also had similar specificity (P = .999) and accuracy (P = .999) compared with the routine MR imaging protocol for detecting posterior cruciate ligament tears. When the consensus interpretation of the routine MR imaging data was used as the reference standard, the sensitivities of FSE-Cube for detecting 13 medial collateral ligament tears, five lateral collateral ligament tears, and 149 bone marrow edema lesions were 96.2%, 100%, and 85.3%, respectively (Table 3, Fig 6). FSE-Cube had high specificity and accuracy in the detection of these joint abnormalities.

Table 3.

Sensitivity, Specificity, and Accuracy of FSE-Cube and Routine MR Imaging Protocol in Detection of Knee Joint Abnormalities for Both Readers Combined

Note.—Data in parentheses were used to calculate the percentages (numerators and denominators represent the combined data from the independent reviews of the two readers); data in brackets are P values for comparison of the two imaging techniques. P < .05 indicates a significant difference. NA = not applicable.

* ACL = anterior cruciate ligament, BME = bone marrow edema, LCL = lateral collateral ligament, LM = lateral meniscus, MCL = medial collateral ligament, MM = medial meniscus, PCL = posterior cruciate ligament.

Figure 3a:

Sagittal MR images in 19-year-old man with surgically confirmed anterior cruciate ligament tear that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Fat-suppressed T2-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show complete disruption of the fibers of the anterior cruciate ligament (arrow).

Figure 3b:

Sagittal MR images in 19-year-old man with surgically confirmed anterior cruciate ligament tear that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Fat-suppressed T2-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show complete disruption of the fibers of the anterior cruciate ligament (arrow).

Figure 4a:

Sagittal MR images in 25-year-old man with surgically confirmed tear of the posterior horn of the medial meniscus that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Intermediate-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show a tear of the posterior horn of the medial meniscus (arrow) with an associated parameniscal cyst (arrowhead).

Figure 4b:

Sagittal MR images in 25-year-old man with surgically confirmed tear of the posterior horn of the medial meniscus that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Intermediate-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show a tear of the posterior horn of the medial meniscus (arrow) with an associated parameniscal cyst (arrowhead).

Figure 5a:

Sagittal MR images in 23-year-old woman show surgically confirmed tear of the posterior horn of the lateral meniscus, which was detected by both readers with the routine MR imaging protocol and by neither reader with FSE-Cube. (a) Intermediate-weighted FSE image of knee joint shows a small undersurface tear of the posterior horn of the lateral meniscus (arrow). (b) Corresponding FSE-Cube image shows a normal-appearing undersurface of the posterior horn of the lateral meniscus (arrow).

Figure 5b:

Sagittal MR images in 23-year-old woman show surgically confirmed tear of the posterior horn of the lateral meniscus, which was detected by both readers with the routine MR imaging protocol and by neither reader with FSE-Cube. (a) Intermediate-weighted FSE image of knee joint shows a small undersurface tear of the posterior horn of the lateral meniscus (arrow). (b) Corresponding FSE-Cube image shows a normal-appearing undersurface of the posterior horn of the lateral meniscus (arrow).

Figure 6a:

Coronal MR images in 35-year-old man with medial collateral ligament tear that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Fat-suppressed intermediate-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show partial disruption of the proximal fibers of the medial collateral ligament (arrowhead). Also note the subchondral bone marrow edema within the lateral femoral condyle and lateral tibial plateau (arrows).

Figure 6b:

Coronal MR images in 35-year-old man with medial collateral ligament tear that was detected by both readers with the routine MR imaging protocol and FSE-Cube. (a) Fat-suppressed intermediate-weighted FSE image and (b) corresponding FSE-Cube image of knee joint show partial disruption of the proximal fibers of the medial collateral ligament (arrowhead). Also note the subchondral bone marrow edema within the lateral femoral condyle and lateral tibial plateau (arrows).

As shown in Table 4, there was no significant difference (P = .073–.999) in interobserver agreement between FSE-Cube and the routine MR imaging protocol for determining the presence or absence of cartilage lesions, anterior cruciate ligament tears, posterior cruciate ligament tears, medial meniscal tears, lateral meniscal tears, medial collateral ligament tears, lateral collateral ligament tears, and bone marrow edema lesions within the knee joint. Both FSE-Cube and the routine MR imaging protocol had interobserver agreement that ranged between moderate and perfect for determining the presence or absence of these joint abnormalities.

Table 4.

Interobserver Agreement with FSE-Cube and Routine MR Imaging Protocol for Determining Presence or Absence of Knee Joint Abnormalities

Note.—Data are κ values.

* ACL = anterior cruciate ligament, BME = bone marrow edema, LCL = lateral collateral ligament, LM = lateral meniscus, MCL = medial collateral ligament, MM = medial meniscus, PCL = posterior cruciate ligament.

† All grades combined.

Discussion

Three-dimensional sequences with isotropic resolution are commonly balanced steady-state free precession sequences (1–5). Results of two recent studies (3,5) have shown that these sequences can be used to provide rapid comprehensive knee joint assessment in symptomatic patients. In a preliminary study performed in 30 patients with arthroscopic correlation, Duc and colleagues (3) found that isotropic resolution water excitation true fast imaging with steady-state precession had similar sensitivity and specificity as a routine MR imaging protocol for detecting cartilage lesions, anterior cruciate ligament tears, and meniscal tears. However, a larger study performed by Kijowski and associates (5) described potential limitations of the T2/T1-weighted tissue contrast of balanced steady-state free precession sequences. In that study, isotropic resolution vastly undersampled isotropic projection steady-state free precession was found to have similar sensitivity and specificity as a routine MR imaging protocol for detecting cartilage lesions, ligament tears, and medial meniscal tears in 95 patients with arthroscopic correlation but to have significantly lower (P < .05) sensitivity for detecting lateral meniscal tears and bone marrow edema lesions.

Three-dimensional FSE sequences with isotropic resolution provide another promising method to replace currently used 2D sequences in clinical practice. Three-dimensional FSE sequences can produce images with intermediate-weighted contrast, which is the most commonly used tissue contrast in musculoskeletal MR imaging (13,15–22). In our study, FSE-Cube with MPRs provided similar clinical information regarding the cartilage, ligaments, menisci, and osseous structures of the knee joint as an entire 25-minute routine MR imaging protocol. To our knowledge, only one previous group (6) has reported the diagnostic performance of a 3D FSE sequence for evaluating the knee joint in symptomatic patients. However, that study was limited by a relatively small patient population and the use of a 3D intermediate-weighted sequence with anisotropic resolution (0.6 × 0.6 × 1.0-mm voxel size).

In our study, FSE-Cube had significantly higher sensitivity but significantly lower specificity than the routine MR imaging protocol for detecting cartilage lesions within the knee joint. To our knowledge, no previous group has reported the diagnostic performance of a 3D FSE sequence for evaluating the knee articular cartilage. The higher sensitivity of FSE-Cube for detecting cartilage lesions was most likely secondary to reduced partial-volume averaging. The FSE-Cube images had thinner section thicknesses than the 2D FSE images, with no gaps between sections; this likely provided better visibility of small cartilage lesions. The lower specificity of FSE-Cube was most likely secondary to decreased in-plane spatial resolution and image blurring due to acquisition of high spatial frequencies late in the echo train. These factors may cause a normal articular surface to appear indistinct and ill defined, simulating the appearance of superficial cartilage degeneration.

FSE-Cube had similar sensitivity, specificity, and accuracy as the routine MR imaging protocol for evaluating the ligaments of the knee joint. Yoon and colleagues (6) also found that a 3D intermediate-weighted FSE sequence with anisotropic resolution had high diagnostic performance for detecting 10 anterior cruciate ligament tears and one posterior cruciate ligament tear confirmed at arthroscopy. FSE-Cube may have advantages over 2D sequences for evaluating the knee ligaments. The thin, continuous sections of FSE-Cube minimize the effect of partial-volume averaging, which can be a source of diagnostic error when evaluating the anterior cruciate ligament (17). In addition, the isotropic resolution of FSE-Cube allows reformatted images to be created in any orientation after a single acquisition. As a result, coronal oblique and sagittal oblique images, which are especially useful for evaluating the ligaments of the posteromedial and posterolateral corners of the knee (23–25), can be obtained.

FSE-Cube had similar sensitivity, specificity, and accuracy as the routine MR imaging protocol for evaluating the menisci of the knee joint. Yoon and colleagues (6) also found that a 3D intermediate-weighted FSE sequence with anisotropic resolution had high diagnostic performance for detecting 26 meniscal tears confirmed at arthroscopy. A potential disadvantage of FSE sequences such as FSE-Cube for evaluating the menisci is image blurring (26,27). Despite the use of flip angle modulation to constrain T2 decay over an extended echo train, FSE-Cube images suffer from blurring at subjective analysis because of the acquisition of high spatial frequencies late in the echo train (28). Blurring on FSE-Cube images can be reduced by decreasing echo train length or increasing bandwidth at the expense of acquisition time and signal-to-noise ratio. The interaction between image blurring, acquisition time, bandwidth, and signal-to-noise ratio is complex, with many combinations possible. Additional studies are needed to determine how the bandwidth, echo train length, and parallel imaging acceleration of FSE-Cube should be optimized to minimize blurring while maintaining adequate signal-to-noise ratio and clinically feasible imaging times.

FSE-Cube had similar sensitivity, specificity, and accuracy as the routine MR imaging protocol for detecting bone marrow edema lesions within the knee joint. Intermediate-weighted FSE sequences such as FSE-Cube may require fat suppression to provide optimal visualization of bone marrow edema lesions (15). Fat suppression also improves detection of superficial cartilage lesions by increasing the contrast between articular cartilage and synovial fluid and improves visualization of edema and hemorrhage within injured ligaments (18,21,22). FSE-Cube uses a spectral inversion recovery pulse for fat suppression that does not prolong acquisition time or reduce anatomic coverage. This is because the majority of each repetition time is spent in signal recovery rather than in acquiring data from adjacent sections (9).

FSE-Cube and the routine MR imaging protocol had similar interobserver agreement for determining the presence or absence of cartilage lesions, ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint. However, the moderate interobserver agreement for determining the presence or absence of medial meniscus tears in our study was much lower than the values observed in previous studies (3,5,16,19). The relatively low interobserver agreement was due to the wide difference between the two readers in the number of false-positive interpretations at evaluation of the medial meniscus. However, the difference between readers in the number of false-positive interpretations was similar for both FSE-Cube and the routine MR imaging protocol, and this resulted in similar interobserver agreements for determining the presence or absence of medial meniscus tears.

Our study had several limitations. One limitation was our small patient population. Many of the P values in our study comparing FSE-Cube and the routine MR imaging protocol that were not statistically significant, such as those comparing the sensitivity for detecting each grade of cartilage lesion, the accuracy for detecting all cartilage lesions, and the sensitivity and specificity for detecting lateral meniscal tears, were relatively low. For this reason, we believe that additional clinical studies with larger patient populations may be needed to detect subtle differences in the diagnostic performance of FSE-Cube and the routine MR imaging protocol. In addition, multiple statistical tests were performed in the same patient population in our study. As a result, two statistically significant results, such as the higher sensitivity and lower specificity of FSE-Cube for detecting cartilage lesions, are not completely independent of one another and may be partially explained by the same underlying phenomena. Another limitation was the presence of selection bias, as our study group consisted of only a proportion of all patients undergoing routine MR imaging of the knee at our institution. Additional limitations included the inability to randomize the order in which the MR imaging studies were reviewed and the inability to blind readers with regard to the MR imaging sequences they were using for comprehensive knee joint assessment. In addition, the orthopedic surgeons were aware of the official interpretations of the MR imaging studies when making clinical decisions and performing arthroscopic surgery, which was based solely on the findings with the routine MR imaging protocol. Our study also did not assess the ability of FSE-Cube to help evaluate the quadriceps and patellar tendons and the ligaments of the posteromedial and posterolateral corners of the knee. Furthermore, the consensus interpretation of the routine MR imaging data was a less than optimal reference standard for determining the sensitivity, specificity, and accuracy of FSE-Cube in the detection of collateral ligament tears and bone marrow edema lesions.

In conclusion, the results of our preliminary study have shown that FSE-Cube has similar diagnostic performance as a routine MR imaging protocol in the detection of cartilage lesions, cruciate ligament tears, collateral ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint at 3.0 T. For this reason, FSE-Cube can be used to provide rapid comprehensive knee joint assessment in patients with severe pain or claustrophobia who cannot tolerate a 25-minute routine MR imaging examination. However, additional studies are needed to determine whether FSE-Cube can replace currently used 2D sequences for evaluating the knee joint in all patients undergoing routine MR imaging.

Advances in Knowledge.

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  A 5-minute sagittal fast spin-echo (FSE)-Cube sequence with multiplanar reformations (MPRs) has higher sensitivity but lower specificity than a routine MR imaging protocol in the detection of cartilage lesions within the knee joint at 3.0 T.
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  A 5-minute sagittal FSE-Cube sequence with MPRs has similar sensitivity and specificity as a routine MR imaging protocol for detecting ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint at 3.0 T.

Implication for Patient Care.

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  A sagittal FSE-Cube sequence with MPRs has the potential to provide rapid comprehensive assessment of the knee joint in symptomatic patients at 3.0 T.