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DOI: 10.1148/radiol.2462062092
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(Radiology 2008;246:526-535.)
© RSNA, 2008

Musculoskeletal Imaging

Internal Knee Derangement Assessed with 3-minute Three-dimensional Isovoxel True FISP MR Sequence: Preliminary Study1

Sylvain R. Duc, MD, Christian W. A. Pfirrmann, MD, Peter P. Koch, MD, Marco Zanetti, MD, and Juerg Hodler, MD

1 From the Departments of Radiology (S.R.D., C.W.A.P., M.Z., J.H.) and Orthopedic Surgery (P.P.K.), University Hospital Balgrist, Zurich, Switzerland. Received December 8, 2006; revision requested February 20, 2007; revision received April 10; accepted May 22; final version accepted August 8. Address correspondence to S.R.D., Department of Radiology, University Hospital Geneva, Rue Micheli-du-Crest 24, CH-1211 Geneva, Switzerland (e-mail: csduc{at}dplanet.ch).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 
Purpose: To prospectively evaluate the accuracy of magnetic resonance (MR) imaging of the knee performed by using a three-dimensional (3D) isovoxel sequence involving an acquisition time of approximately 3 minutes, with surgery as the reference standard.

Materials and Methods: The study was institutional review board approved. Written informed consent was obtained from all patients. Thirty knees of 29 patients (14 women, 15 men; mean age, 41 years) were prospectively examined by using a 3D isovoxel true fast imaging with steady-state precession (FISP) sequence with water excitation and secondary multiplanar reformations. All patients underwent arthroscopy within 12 days after true FISP MR imaging. Two blinded readers evaluated the MR images. Accuracy for detection of cartilage defects and anterior cruciate ligament (ACL) and meniscal tears, interobserver agreement, and intermethod agreement were calculated.

Results: Overall sensitivity, specificity, and accuracy of isovoxel true FISP imaging for the diagnosis of cartilage defects were 45%, 83%, and 76%, respectively, for reader 1 and 63%, 82%, and 83%, respectively, for reader 2. Averaged (for readers 1 and 2) sensitivity, specificity, and accuracy of isovoxel true FISP imaging were, respectively, 80%, 95%, and 90% for diagnosis of ACL tear; 100%, 82%, and 90% for diagnosis of medial meniscal tear; and 83%, 83%, and 83% for diagnosis of lateral meniscal tear. The standard MR sequences used at the authors' institution had overall sensitivities, specificities, and accuracies of 39%, 83%, and 71%, respectively, for reader 1 and 37%, 85%, and 76%, respectively, for reader 2. Averaged sensitivity, specificity, and accuracy of the standard MR sequences were, respectively, 70%, 100%, and 90% for diagnosis of ACL tear; 96%, 77%, and 85% for diagnosis of medial meniscal tear; and 83%, 77%, and 78% for diagnosis of lateral meniscal tear.

Conclusion: The diagnostic performance of knee MR imaging performed by using a 3D water excitation isovoxel true FISP sequence and an imaging time of approximately 3 minutes is comparable to the diagnostic performance of the MR sequences used as standards at the authors' institution.

© RSNA, 2008


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 
MR imaging is widely used to assess internal derangements of the extremities (1). There is an interest in decreasing examination times to improve patient comfort and reduce the problems related to claustrophobia and motion artifacts, which occur more commonly with long examinations.

Imaging sequences can be shortened to a certain extent by reducing the repetition time, number of signals acquired, and/or matrix size; or the number of sequences can be reduced, although there are limits to this approach due to current American College of Radiology guidelines (2). The application of newly available technical advances holds promise for shortening imaging times in musculoskeletal imaging. Parallel imaging techniques (3,4) such as integrated parallel imaging, sensitivity encoding, and array spatial sensitivity encoding have facilitated reduced acquisition times. However, these techniques have been used more commonly in cardiovascular imaging (5–7) than in musculoskeletal imaging (8–10).

Another possible approach to shortening imaging times is the use of a volumetric isovoxel sequence (11) with secondary reformations. All required imaging planes can be imaged within a single acquisition. Several three-dimensional (3D) sequences with which image appearances are optimized for the musculoskeletal system have been introduced and include fast low-angle shot (12), double-echo steady state, multiecho data image combination, true fast imaging with steady-state precession (FISP), and other variants of the steady-state free precession technique such as iterative decomposition of water and fat with echo asymmetry and least squares estimations, or IDEAL, balanced steady-state free precession (13) and phase-sensitive steady-state free precession (14).

Although many of these sequences involve relatively long acquisition times, the true FISP sequence used in combination with water excitation enables 3D isovoxel data sets to be obtained within an acquisition time of approximately 3 minutes with good image quality and the contrast properties required for musculoskeletal imaging, including hyperintense cartilage and joint fluid and hypointense bone marrow. Thus, the purpose of our study was to prospectively evaluate the accuracy of MR imaging of the knee performed by using a 3D isovoxel sequence involving an acquisition time of approximately 3 minutes, with surgery as the reference standard.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 
Patients
Between February 2005 and January 2006, 30 knees of 29 patients were included in our prospective study. The study protocol was approved by the institutional review board of the Kanton of Zurich. Written informed consent was obtained from all patients (14 women, 15 men; mean age, 41 years; age range, 18–78 years). Inclusion criteria were age 18 years or older and knee surgery planned on the basis of criteria established at our institution. Exclusion criteria were prior knee surgery, surgical intervention that prevented sufficient review of the internal structures of the knee, lack of availability of the MR imaging unit, and contraindications to MR imaging. The informed consent form clearly indicated that an MR examination would be performed specifically for the purpose of this study and that the resultant images would be evaluated only after surgery. Thus, MR imaging information was not available to the orthopedic surgeons during surgery. However, all patients underwent the appropriate imaging work-up before surgery, as required according to the institution's policy.

During the study inclusion period, surgical interventions around 606 knees were performed. Thirty-nine of these knees were those of patients younger than 18 years. Prior surgery had been performed in 250 of the remaining 567 knees. For 39 of the remaining 317 knees, a slot was available for MR examination. In nine of these 39 knees, a prosthesis was surgically implanted for advanced osteoarthritis. In the patients with these prostheses, the intraarticular knee structures were not consistently evaluated intraoperatively; thus, these patients were excluded from our study. For the remaining 30 knees, intraoperative assessment of the intraarticular knee structures was sufficient for comparison of the surgical and MR examination results. Therefore, 30 knees of 29 patients were evaluated in our study (Fig 1).


Figure 1
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  		Figure 1: Flowchart of patient selection process.

 
MR Imaging
The MR examinations were performed 0–12 days (mean, 2 days) before surgery. A 1.5-T MR imaging unit (Siemens Avanto; Siemens Medical Solutions, Erlangen, Germany) with 18 independent radiofrequency channels and an eight-channel receive-only extremity coil was used. For the 3-minute MR imaging protocol (Table 1), a 3D water excitation isovoxel true FISP sequence was performed in the coronal plane. With use of the isovoxel true FISP sequence, multiplanar reformations (MPRs) with a 2-mm partition thickness (30 partitions per slab) were acquired in the transverse, coronal, and sagittal planes at a separate workstation (Leonardo; Siemens Medical Solutions). Postprocessing was performed by one staff radiologist (S.R.D.). The total postprocessing time, including the time to download the patient data to the workstation and the time for image reconstruction in the transverse, coronal, and sagittal planes, was 4 minutes.


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  		Table 1. MR Sequence Parameters

 
The sequences used as standard MR examinations at our institution were coronal 2D fast STIR, coronal 2D T1-weighted spin echo, sagittal 2D intermediate-weighted fast spin echo with frequency-selective fat saturation, standard sagittal 3D water excitation true FISP, and transverse 2D multiecho data image combination.

Evaluation of MR Images
Two independent readers with 19 (J.H.) and 10 (C.W.A.P.) years experience in musculoskeletal radiology evaluated the MR images while blinded to the clinical and surgical results. For the first 15 patients, evaluation started with review of the isovoxel true FISP images, including the reformatted images. After 3 weeks, the images obtained by using the standard sequences in these same 15 patients were evaluated. The knees of the remaining 15 patients were evaluated in reverse order after another interval of 3 weeks. All evaluations were performed on picture archiving and communication system workstations (Cedara I-Read 5.2 P11; Cerner Image Devices, Idstein, Germany).

The following structures were evaluated: the articular cartilage (Fig 2), the anterior cruciate ligament (ACL) (Fig 3), the medial (Fig 4) and lateral menisci, and the subchondral bone marrow. The articular cartilage was divided into six surfaces: medial and lateral femoral condyles, medial and lateral tibial plateaus, trochlea, and patella. MR findings were graded by using a modified Noyes score, which is used by the surgeons at our institution. Five grades were used: 0 for normal cartilage, 1 for signal intensity abnormality but normal surface; 2 for superficial fraying, erosion, or ulceration involving not more than 50% of the cartilage thickness; 3 for defect affecting a depth of greater than 50% but less than 100% of the cartilage thickness; and 4 for defect extending to the subchondral bone. If more than one cartilage abnormality was found in a single region, the highest grade assigned was used for the purposes of our study.


Figure 2A
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  		Figure 2a: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 2B
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  		Figure 2b: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 2C
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  		Figure 2c: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 2D
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  		Figure 2d: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 2E
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  		Figure 2e: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 2F
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  		Figure 2f: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a femoral cartilage defect (arrowheads) with subchondral bone marrow alterations (arrows). The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3A
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  		Figure 3a: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3B
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  		Figure 3b: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3C
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  		Figure 3c: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3D
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  		Figure 3d: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3E
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  		Figure 3e: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 3F
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  		Figure 3f: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show an acute ACL tear (white arrowheads). The distal stump of the cruciate ligament (black arrowheads) is dislocated anteriorly. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4A
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  		Figure 4a: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4B
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  		Figure 4b: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4C
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  		Figure 4c: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4D
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  		Figure 4d: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4E
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  		Figure 4e: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 

Figure 4F
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  		Figure 4f: (a, b, d, e) Standard-protocol and (c, f) 3-minute-protocol MR images show a horizontal meniscal tear (arrowheads) involving the posterior horn and reaching the undersurface of the meniscus. The following images are shown: (a) 2D intermediate-weighted fat saturation sagittal (3020/15); (b) 3D true FISP sagittal (9.25/3.17); (c) 3D isovoxel true FISP (9.44/3.45), MPR sagittal; (d) 2D T1-weighted spin-echo coronal (450/11); (e) 2D STIR coronal (5460/34); and (f) 3D isovoxel true FISP (9.44/3.45), MPR coronal.

 
The ACL was judged to be intact (no signal intensity alteration, continuity maintained), degenerated (signal intensity altered but continuity maintained), or torn (loss of continuity). The menisci were judged to be intact or torn. The subchondral bone marrow was considered to be normal when no focal signal intensity alteration was present and abnormal when any focal signal intensity alteration was noted. Each reader entered these data onto an Excel 2003 (Microsoft, Redmond, Wash) spreadsheet.

Surgery as Reference Standard
Surgery was performed by four in-house orthopedic surgeons (P.P.K. with 12 years experience and three others with 10, 7, and 7 years experience). All patients underwent the diagnostic work-up normally performed to make therapeutic decisions at our hospital, including the appropriate imaging examination. For the purpose of this study, and after informed consent was obtained, additional imaging was performed only for evaluations within the context of this study—that is, not for patient treatment. The orthopedic surgeons intraoperatively assessed the cartilage defects in the six compartments of the knee and completed an evaluation form specifically designed for this study. The worst cartilage defect in each compartment was noted. For the other structures evaluated, the descriptions found in the surgical reports were used as reference standards.

Statistical Analyses
Agreement between the two readers (interobserver agreement) in evaluating the isovoxel true FISP and standard MR findings and agreement between the isovoxel true FISP and standard MR findings (intermethod agreement) of cartilage, ACL, medial and lateral meniscal, and subchondral bone marrow abnormalities were calculated by using {kappa} statistics (SPSS software, version 11, 2001; SPSS, Chicago, Ill) (15) (Table 2). P < .05 was considered to indicate significance.


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  		Table 2. Interobserver and Intermethod Agreement

 
For cartilage defects, data were dichotomized into defect (grades 2–4) and no defect (grades 0–1) categories. Because for cartilage there were more outcome measures than knees, the sensitivity, specificity, and accuracy were calculated separately for each knee. Thereafter, mean (for the two readers) sensitivities, specificities, and accuracies for the isovoxel true FISP and standard MR sequences were calculated. Sensitivity, specificity, accuracy, and corresponding 95% confidence intervals for the detection of ACL and meniscal tears were calculated by using the Wilson scoring method.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 
Reference Standard
Fifty-five cartilage defects, 10 ACL tears, 13 medial meniscal tears, and six lateral meniscal tears were detected intraoperatively. In one knee, the cartilage of the patellar surface was not sufficiently visible, so this surface was not considered in the statistical analysis.

Agreement
Interobserver agreement was similar for the evaluation of all structures except the ACL, for which agreement was moderate with use of isovoxel true FISP imaging and almost perfect with use of the standard MR sequences. For reader 1, intermethod agreement was worst for evaluation of ACL tears ({kappa} = 0.44, moderate agreement) and best for evaluation of medial meniscal tears ({kappa} = 0.87, almost perfect agreement) (Table 2). For reader 2, intermethod agreement was worst for evaluation of overall cartilage defects ({kappa} = 0.43, moderate agreement) and best for evaluation of medial meniscal tears ({kappa} = 0.93, almost perfect agreement) (Table 2). The correlation coefficient for the difference between the intraoperative and MR grades, averaged for readers 1 and 2, was 0.69 for both the isovoxel true FISP sequence and the sequences used as standards at our institution.

Diagnostic Performance
For reader 1, mean sensitivity, specificity, and accuracy values for the detection of cartilage defects, calculated for each evaluated knee, were 45%, 83%, and 76%, respectively, for the isovoxel true FISP sequence and 39%, 83%, and 71%, respectively, for the standard sequences (Table 3). For reader 2, mean sensitivity, specificity, and accuracy values were 63%, 82%, and 83%, respectively, for the isovoxel true FISP sequence and 37%, 85%, and 76%, respectively, for the standard sequences.


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  		Table 3. Diagnostic Performance for Detection of Cartilage Defects

 
For the diagnosis of ACL tears (Table 4), the two MR protocols had similar value. The isovoxel true FISP sequence was slightly more sensitive (80%) and slightly less specific (95%) compared with the standard protocol (70% sensitivity, 100% specificity). For the diagnosis of medial and lateral meniscal tears, the averaged (for the two readers) performance values for the isovoxel true FISP sequence were better than those for the standard sequences. Readers 1 and 2 detected bone marrow signal intensity abnormalities in 20 and 27 knees, respectively, with isovoxel true FISP imaging and in 27 and 24 knees, respectively, with the standard MR examinations.


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  		Table 4. Performance of 3-minute True FISP and Standard MR Protocols in Diagnosis of ACL and Meniscal Tears

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 
For MR imaging of the knee, instead of shortening the acquisition time per sequence one can reduce the number of sequences used if an isovoxel 3D sequence is performed with secondary reformation of the other sequences. Primary volumetric acquisitions with secondary MPRs are widely used in computed tomography. They are also used in MR examinations, predominantly for cardiovascular (16) and abdominal imaging (17). Until recently, one of the drawbacks of using volumetric MR acquisition with secondary MPR was the long acquisition times in high-quality 3D sequences (18,19). In addition, the time to perform reformations has to be included in any evaluation.

Gay and colleagues (20) used sagittal 3D FISP MR data sets with MPRs to evaluate internal derangements of the knee. They found this technique to be comparable to standard interpretations for the evaluation of ACL and meniscal injuries but noted an increase in both the total procedural time (processing time plus reading time) and the isolated radiologist reading time. Wieslander et al (21) compared standard coronal and sagittal MR sequences with a 1.2-mm-section sagittal MR sequence performed with subsequent MPR and observed no substantial difference in diagnostic effectiveness between the two MR techniques. However, the 10-minute reduction in data acquisition time in the MPR-based sequence was offset by an approximate 20-minute increase in postprocessing time. At that time, the authors concluded that use of MPR yielded no additional diagnostic value or time savings. However, the time to perform standard MPRs is no longer a problem. Owing to the use of high-performance workstations and optimized work flow that enable the technician to perform semiautomated reformations with a few mouse clicks, postprocessing is no longer an important factor.

Isotropic MR imaging of the joints has not been evaluated frequently. Harms and colleagues (22) used a 9-minute 3D data acquisition with nearly isotropic voxels to reformat images in all planes without a substantial loss in image quality. Similar to us, they found the 3D images to be superior or equal to the 2D images in depicting meniscal tears, ligament tears, bone marrow disease, and osteochondral defects. However, the 3D sequence used by Harms et al was not truly isotropic (voxel size, 1.50 x 0.55 x 0.55 mm), the examination time was too long to facilitate a major reduction in total imaging time (approximately 9 minutes), and on the basis of the corresponding illustrations that we reviewed, the quality of the 2D images was equivalent to or better than that of the 3D images. These factors may explain why there was no breakthrough in the use of this MR acquisition method.

In our study, the diagnostic performance of the isotropic acquisition in the detection of cartilage defects was comparable to that of the standard MR protocol used at our institution. It is interesting that indirect signs of cartilage defects, such as subchondral bone marrow alterations, were well seen not only with use of our institution's standard protocol, which includes a STIR sequence—considered quite sensitive for the detection of bone marrow abnormalities—but also with use of isovoxel true FISP imaging, a gradient-echo examination.

The sensitivity, specificity, and accuracy of the isovoxel true FISP sequence for the diagnosis of ACL tears were consistent with our standard MR sequence results and with the pooled weighted sensitivity and specificity values reported by Oei et al in a meta-analysis of MR imaging of the menisci and cruciate ligaments (23). A potential benefit of isotropic acquisition in the diagnosis of ACL tears is the possibility of performing MPR parallel to the course of the ACL. Although the use of this MPR approach does not improve performance in the diagnosis of tears, it does increase diagnostic confidence, as shown by Duc et al (24).

Oei et al (23) reported a pooled weighted sensitivity for MR imaging in the detection of meniscal tears of 79%–93% and a specificity of 88%–96% in their meta-analysis. Ohishi et al (19) evaluated the diagnostic performance of transverse MPR in 3D MR imaging for the detection of meniscal tears and reported sensitivities, specificities, and accuracies of 97%, 79%, and 87%, respectively, and 94%, 88%, and 91%, respectively, for 2D coronal and sagittal MR imaging.

The results obtained with the 3D isovoxel true FISP sequence and the MR sequences used as standards at our institution are comparable to previously published results. Although sensitivity for the detection of lateral meniscal tears is typically lower than that for the detection of medial meniscal tears (23), for reader 2, the sensitivity for lateral meniscal tear detection was slightly lower than expected. Wieslander et al (21) encountered difficulties in diagnosing lateral meniscal tears with both standard MR imaging and 3D MR–based MPR. Harms et al (22) reported comparable performance between standard MR images and MPRs obtained from 3D MR data sets in the diagnosis of meniscal tears. They stressed the utility of these images for depicting meniscal lesions in multiple imaging planes.

In the future, 3D MR techniques, especially isotropic sequences, might be further improved with increasing availability of 3-T magnets, which are already being used for cartilage evaluation (25,26), and with further development of high-performance multichannel coils. Owing to the increasing use of picture archiving and communication systems, MPRs are no longer limited to dedicated workstations; rather, they have become universally available in radiology departments.

Our study had limitations. The true FISP sequence has the same disadvantages as all other gradient-echo sequences, including susceptibility artifacts in the presence of metal or calcified structures, magic angle effects, and limited accuracy in the detection of degenerative changes in tendons and ligaments. The differentiation among six cartilage regions may not prevent the mismatching between MR and surgical findings in some cases. This is also a potential problem for the ACL and the menisci, where lesions were classified only as torn or not torn in our study. With use of a single 3D sequence with secondary MPRs for evaluation of the knee, artifacts such as motion or susceptibility artifacts will be present in all imaging planes and potentially alter the diagnostic performance of the MR examination. This problem may be more prevalent in general patient populations than it was in our study. The prevalence of posterior cruciate ligament tears was too low (one such tear identified at surgery) for assessment of the performance in diagnosing these abnormalities. The degree of joint effusion and the integrity of the collateral ligaments and other ligamentous structures of the knee, especially at the posterolateral corner of the knee, could not be evaluated because of the lack of a reference standard—a common problem in studies of knee MR imaging. Only the presence (not the extent) of subchondral bone marrow alterations was evaluated in this study. This was in keeping with the routine knee MR evaluations performed at our institution, in which subchondral bone marrow alterations are considered mainly indirect signs of cartilage damage.

In conclusion, for the structures evaluated in our study, the diagnostic performance of knee MR imaging performed by using a 3D water excitation isovoxel true FISP sequence and an imaging time of approximately 3 minutes is comparable to the diagnostic performance of the MR sequences used as standards at our institution.


   ADVANCE IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 


   IMPLICATIONS FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
References
 


   FOOTNOTES
 

Abbreviations: ACL = anterior cruciate ligament • FISP = fast imaging with steady-state precession • MPR = multiplanar reformation • STIR = short inversion time inversion recovery • 3D = three-dimensional • 2D = two-dimensional

Guarantor of integrity of entire study, S.R.D.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, S.R.D.; clinical studies, all authors; statistical analysis, S.R.D., C.W.A.P., M.Z., J.H.; and manuscript editing, all authors

Authors stated no financial relationship to disclose.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
IMPLICATIONS FOR PATIENT CARE...
 References
 

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R. Kijowski, D. G. Blankenbaker, J. L. Klaers, K. Shinki, A. A. De Smet, and W. F. Block
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