Advanced morphological 3D magnetic resonance observation of cartilage repair tissue (MOCART) scoring using a new isotropic 3D proton-density, turbo spin echo sequence with variable flip angle distribution (PD-SPACE) compared to an isotropic 3D steady-state free precession sequence (True-FISP) and standard 2D sequences

Authors


Abstract

Purpose

To evaluate a new isotropic 3D proton-density, turbo-spin-echo sequence with variable flip-angle distribution (PD-SPACE) sequence compared to an isotropic 3D true-fast-imaging with steady-state-precession (True-FISP) sequence and 2D standard MR sequences with regard to the new 3D magnetic resonance observation of cartilage repair tissue (MOCART) score.

Materials and Methods

Sixty consecutive MR scans on 37 patients (age: 32.8 ± 7.9 years) after matrix-associated autologous chondrocyte transplantation (MACT) of the knee were prospectively included. The 3D MOCART score was assessed using the standard 2D sequences and the multiplanar-reconstruction (MPR) of both isotropic sequences. Statistical, Bonferroni-corrected correlation as well as subjective quality analysis were performed.

Results

The correlation of the different sequences was significant for the variables defect fill, cartilage interface, bone interface, surface, subchondral lamina, chondral osteophytes, and effusion (Pearson coefficients 0.514–0.865). Especially between the standard sequences and the 3D True-FISP sequence, the variables structure, signal intensity, subchondral bone, and bone marrow edema revealed lower, not significant, correlation values (0.242–0.383). Subjective quality was good for all sequences (P ≥ 0.05). Artifacts were most often visible on the 3D True-FISP sequence (P < 0.05).

Conclusion

Different isotropic sequences can be used for the 3D evaluation of cartilage repair with the benefits of isotropic 3D MRI, MPR, and a significantly reduced scan time, where the 3D PD-SPACE sequence reveals the best results. J. Magn. Reson. Imaging 2011;33:180–188. © 2010 Wiley-Liss, Inc.

MAGNETIC RESONANCE IMAGING (MRI) of the knee has been reported as the method of choice to depict cartilage injuries or the postoperative constitution of cartilage repair tissue (1–7). A widespread array of surgical cartilage repair techniques require objective, noninvasive, and reliable follow-up examination, for which the MR observation of cartilage repair tissue (MOCART) score is claimed to be a reliable, reproducible, and accurate tool for assessing cartilage repair tissue (8, 9). In several recent studies the MOCART score was used and helped to depict the morphological constitution of the repair tissue and the adjacent structures in the follow-up after different cartilage repair procedures (3, 6–8, 10–14). This basic MOCART score comprises standard MR sequences and is performed, depending on the location of the cartilage repair site, in the sagittal, axial, or coronal 2D planes using high spatial in-plane resolution together with a slice thickness of 2–4 mm. These sequences offer excellent image quality with high spatial resolution, but there are still some limitations in the exact depiction of the repair tissue, its borders, and the adjacent cartilage due to thick slices, the curvature of cartilage layers, and the limited oblique reconstruction. To use the capabilities of new isovoxel 3D sequences and their 3D multiplanar-reconstruction (MPR) with no loss of spatial resolution, the 3D MOCART scoring system was recently introduced, and was based on an isotropic 3D true fast imaging with steady-state precession (True-FISP) sequence (15). The benefits of this new 3D MOCART score and the 3D evaluation of repair tissue could be clearly demonstrated; however, an obvious limitation of the 3D True-FISP sequence in the postoperative evaluation of cartilage repair tissue was the high number of susceptibility artifacts in the repair tissue and the subchondral bone with this gradient echo-based technique. A recently developed isotropic 3D proton-density, turbo spin-echo sequence, called PD-SPACE (sampling perfection with application-optimized contrasts using different flip angle evolutions), might, because of its spin-echo signal behavior, overcome these limitations with potentially fewer artifacts and better suitability for postoperative imaging of a joint (16).

The purpose of this study was to evaluate a new isotropic 3D PD-SPACE sequence compared to an isotropic 3D True-FISP sequence and to a set of 2D standard MR sequences with regard to the new 3D MOCART score. The results of the 3D MOCART scoring using these three sequences were correlated and subjective image quality and sensitivity to artifacts was also graded.

MATERIALS AND METHODS

Patient Selection

The medical university ethics commission approved the study protocol and written informed consent was obtained from all patients prior to enrolment in the study.

Sixty MR scans were prospectively included in this study between October 2008 and October 2009. MRI was performed during the clinical routine at standard follow-up intervals of 1, 3, 6, 12, 24, 48, 60, and 72 months after matrix-associated autologous chondrocyte transplantation (MACT) of the knee joint, with a mean follow-up of 24.3 ± 24.1 months. The frequency of the follow-up intervals in this cross-sectional evaluation was: 1 month (n = 8); 3 months (n = 8); 6 months (n = 10); 12 months (n = 7); 24 months (n = 10); 48 months (n = 2); 60 months (n = 13); and 72 months (n = 2). The 60 MRI scans were performed in 37 patients with a mean age of 32.8 ± 7.9 years. The 60 MR measurements comprised the right knee joint in 33 and the left knee joint in 27 measurements, with 47 knees of male and 13 knees of female patients. Cartilage repair surgery was performed on the medial femoral condyle (n = 28), the lateral femoral condyle (n = 12), the patella (n = 10), and the trochlea (n = 10). In the 60 consecutive MRIs, considering the standard follow-up examination, 23 patients were included once, nine patients were included twice, three patients were included three times, and two patients were included four times. MACT was performed as a two-step surgical procedure. In a first arthroscopic step a biopsy was obtained from a nonweight-bearing area of the knee. After cell extraction and cultivation, the chondrocytes were transferred onto a biomaterial. In a second step, a mini-arthrotomy was performed to debride the cartilage defect to the subchondral bone. The cell matrix transplants were cut to a suitable size and implanted. The edges were fixed with fibrin glue (17).

Image Acquisition

The clinical routine MRI was performed on a 3T MR system (Magnetom Tim Trio, Siemens Healthcare, Erlangen, Germany) using a dedicated eight-channel knee coil (InVivo, Gainesville, FL). All patients were positioned consistently with the joint space in the middle of the coil and the knee extended in the coil. The MR protocol was identical for all 60 MRI examinations and consisted of a set of localizers in all three planes, as follows. 1) A standard MR protocol with a sagittal or axial (sagittal for both femoral condyles; axial for the patella and the trochlea) high-resolution, nonfat-saturated proton-density turbo spin-echo (PD-TSE) sequence, a sagittal (or axial) PD, respectively, T2-weighted dual fast spin-echo (dual-FSE) sequence, and a coronal fat-saturated proton-density turbo spin-echo (FS-PD-TSE) sequence. Furthermore 2) a coronal isotropic 3D True-FISP sequence, as well as 3) a sagittal isotropic 3D PD-SPACE sequence was acquired. The FS-PD-TSE sequence, as well as both isotropic sequences, was performed in all patients in the same direction. The 3D True-FISP sequence and the 3D PD-SPACE sequence were subsequently reconstructed in every direction using MPR. Sequence parameters are provided in Table 1. Two exemplary patients after MACT of the femoral condyle are provided in Figures 1–4. Whereas Fig. 1 and 3 represent the standard 2D seqeunces, Fig. 2 and 4 represent both 3D isotropic approaches.

Table 1. 
Sequence protocolsi) Standard MR Seqeuncesii) 3D-True-FISPiii) 3D-PD-SPACE
PD-TSEDual FSEFS-PD-TSE
  1. Parallel Aquisition technique (PAT) was applied with the given acceleration factor using a generalized auto calibrating partially parallel acquisition (GRAPPA)

Repitition time (TR)2130ms5090ms4250ms7.8ms1200ms
Echo time (TE)36ms12ms; 85ms27ms3.8ms30ms
Flip angle (FA)180°180°180°30°180°
Field of view (FoV)120 × 120mm160 × 160mm150 × 150mm192 × 192mm160 × 160mm
Pixel matrix448 × 448448 × 448384 × 384384 × 384324 × 324
Slice thickness2mm3mm3mm0.5mm0.6mm
Number of slides323036240192
Voxel size0.27 × 0.27 × 2mm0.36 × 0.36 × 3mm0.39 × 0.39 × 3mm0.5 × 0.5 × 0.5mm0.6 × 0.6 × 0.6mm
Fat supressionNoneNoneFat supressionWater excitationFat saturation
PATOffOff222
Bandwidth180Hz/Px180Hz/Px150Hz/Px352Hz/Px540Hz/Px
Aquisition time5:27min.6:03min.2:47min.6:21min.7:15min.

Data Analysis

The new 3D MOCART score, published by Welsch et al (15), with the variables 1) defect fill, 2) cartilage interface, 3) bone interface, 4) surface, 5) structure, 6) signal intensity, 7) subchondral lamina, 8) chondral osteophyte, 9) bone marrow edema, 10) subchondral bone, and 11) effusion, was assessed using i) the standard clinical sequences (with the limitation that the evaluation of the transplant could not be performed in 3D using MPR); ii) the MPR of the isotropic-3D True-FISP dataset; and iii) the MPR of the isotropic 3D PD-SPACE dataset. This evaluation is detailed in Table 2. The evaluation was performed on a Leonardo Workstation (Siemens) by a senior musculoskeletal radiologist (25 years of experience) and an orthopedic surgeon with a special interest in MRI (10 years of experience), in consensus. During their analysis both were blinded to patient name and postoperative follow-up interval and were advised only about the localization of the cartilage transplants. The evaluation of the 60 knee MRIs was performed in random order with regard to the respective sequences i), ii), or iii).

Table 2. Three-dimensional (3D) magnetic resonance observation of cartilage repair tissue (MOCART) score assessed using a i) set of standard MR sequences, the ii) MPR of the 3D-True-FISP sequence and iii) the MPR of the 3D-PD-SPACE sequence
Variablesi) Standard 2Dii) 3D-True-FISPiii) 3D-PD-SPACE
  1. Variables 1–11 for 3D MOCART score; subcategories “localization” optional.

1. Defect fill (degree of defect repair and filling of the defect in relation to the adjacent cartilage)
 ○ 0%0 (0)0 (0)0 (0)
 ○ 0–25%0 (0)0 (0)0 (0)
 ○ 25–50%0 (0)0 (0)0 (0)
 ○ 50–75%9 (15)4 (6.7)5 (8.3)
 ○ 75–100%11 (18.3)13 (21,7)12 (20)
 ○ 100%28 (46.7)29 (48.3)28 (46.7)
 ○ 100–125%7 (11.7)11 (18.3)11 (18.3)
 ○ 125–150%5 (8.3)3 (5)3 (5)
 ○ 150–200%0 (0)0 (0)0 (0)
 ○ >200%0 (0)0 (0)0 (0)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ < 50%
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
2. Cartilage Interface (integration with adjacent cartilage to border zone in two planes)
 Sagittal (Femur, Patella, Trochlea, Tibia)
 ○ Complete41 (68.3)40 (66.7)30 (50)
 ○ Demarcating border visible (split-like)12 (20)13 (21.7)23 (38.3)
 ○ Defect visible <50%4 (6.7)5 (8.3)6 (10)
 ○ Defect visible >50%3 (5)2 (3.3)1 (1.7)
 Coronal (Femur, Tibia); Axial (Patella, Trochlea)
 ○ Complete 38 (63.3)34 (56.7)
 ○ Demarcating border visible (split-like) 18 (30)21 (35.0)
 ○ Defect visible <50% 4 (6.7)5 (8.3)
 ○ Defect visible >50% 0 (0)0 (0)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ <50%
 ○ Weight-bearing   ○ Non weight-bearing
3. Bone interface (integration of the transplant to the subchondral bone; integration of a possible periosteal flap)
 ○ Complete53 (88.3)54 (90)52 (86.7)
 ○ Partial delamination7 (11.7)6 (10)8 (13.3)
 ○ Complete delamination0 (0)0 (0)0 (0)
 ○ Delamination0 (0)0 (0)0 (0)
Localization
 ○ Weight-bearing   ○ Non-weight-bearing
4. Surface (constitution of the surface of the repair tissue)
 ○ Surface intact31 (51.7)24 (40)26 (43.3)
 ○ Surface damaged <50% of depth19 (31.7)24 (40)22 (36.7)
 ○ Surface damaged >50% of depth6 (10)9 (15)7 (11.7)
 ○ Adhesions4 (6.7)3 (5)5 (8.3)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ < 50%
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
5. Structure (constitution of the repair tissue)
 ○ Homogeneous22 (45)15 (25)12 (16.7)
 ○ Inhomogeneous or cleft formation38 (55)45 (75)48 (83.3)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ < 50%
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
6. Signal intensity (Intensity of MR signal in of the repair tissue in comparison to the adjacent cartilage: normal = identical to adjacent cartilage; nearly normal = slight areas of signal alterations; abnormal = large areas of signal alteration)
 ○ Normal29 (48.3)24 (40)27 (45)
 ○ Nearly normal25 (41.7)32 (53.3)32 (53.3)
 ○ Abnormal6 (10)3 (6.7)1 (1.7)
Localization
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
7. Subchondral lamina (Constitution of the subchondral lamina)
 ○ Intact33 (55)36 (60)29 (48.3)
 ○ Not intact27 (45)24 (40)31 (51.7)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ < 50%
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
8. Chondral Osteophytes (Osteophytes within the cartilage repair area)
 ○ Absent37 (61.7)31 (51.7)31 (51.7)
 ○ Osteophytes < 50% of repair tissue16 (26.7)14 (23.3)17 (28.3)
 ○ Osteophytes > 50% of repair tissue7 (11.7)15 (25.0)12 (20)
Localization
 Size: ——mm (plane: ——) × —— mm (plane: ——)
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
9. Bone marrow edema (Maximum size and localization in relation to the cartilage repair tissue and other alterations assessed in the 3D MOCART score).
 ○ Absent14 (23.3)33 (55)15 (25)
 ○ Small (< 1cm)14 (23.3)15 (25)13 (21.7)
 ○ Medium (< 2cm)20 (33.3)8 (13.3)20 (32.3)
 ○ Large (< 4cm)9 (15)4 (6.7)10 (16.7)
 ○ Diffuse3 (5)0 (0)2 (3.3)
Localization
 Size: ——mm (plane: ——) × —— mm (plane: ——)
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
 ○ Relation to other alterations within this score of variable No. ——
10. Subchondral bone (Constitution of the subchondral bone)
 ○ Intact31 (33.3)34 (56.7)39 (65)
 ○ Granulation tissue18 (30)25 (33.3)18 (30)
 ○ Cyst4 (6.7)4 (6.7)3 (5)
Localization
 ○ Whole area of cartilage repair   ○ > 50%   ○ < 50%
 ○ Central   ○ Peripheral   ○ Weight-bearing   ○ Non weight-bearing
11. Effusion (Approx. size of joint effusion visualized in all planes)
 ○ Absent17 (28.3)10 (16.7)11 (18.3)
 ○ Small26 (43.3)31 (51.7)29 (48.3)
 ○ Medium14 (23.3)16 (26.7)17 (28.3)
 ○ Large3 (5)3 (5)3 (5)

Further evaluation was performed to classify image quality for the 3D MOCART scoring using i) the standard MR sequences (PD-TSE, dual FSE and FS-PD-TSE), ii) the 3D True-FISP sequence, as well as iii) the 3D PD-SPACE sequence to guarantee a sufficient analysis of the cartilage repair tissue and of the adjacent cartilage. Image quality was also graded by the two observers, in consensus, after they performed the MOCART scoring. A four-level scale was used in which a score of 4 indicated excellent image quality; a score of 3, good image quality; a score of 2, acceptable image quality; and a score of 1, poor image quality (18), which indicated that the transplant could not be evaluated sufficiently. In addition, since the diagnostic performance at high-field MRI also suffers from artifacts (19), a scaling of possible artifacts and their impact on the evaluation of the cartilage repair tissue and the surrounding cartilage was performed; again, separately for the three sequences i), ii), and iii). Visible artifacts, including motion, susceptibility, metal/postsurgical, banding, etc., were subjectively graded as absent (4), mild (3), moderate (2), and severe (1) (20).

Statistical Analysis

Statistical analysis was performed to compare the different sequences using SPSS v. 16.0 (Chicago, IL) for Mac (Apple, Cupertino, CA). First, the 3D MOCART scoring was correlated for the 11 single variables. To address the problem of multiple correlations, a Bonferroni correction was additionally performed. Second, the subjective quality and the visible artifacts were compared. For the whole evaluation, the i) standard sequences were compared/correlated to the ii) 3D True-FISP sequence, and to the iii) 3D PD-SPACE sequence. In addition, the ii) 3D True-FISP sequence and the iii) 3D PD-SPACE sequence were compared (respectively correlated). Correlation was achieved using a bivariate correlation with the Pearson correlation coefficient. Comparison for quality and artifacts was prepared using Student's t-test. For all evaluations a P-value < 0.05 was considered statistically significant.

RESULTS

3D MOCART Score

The results for the 3D MOCART evaluation, as performed by i) the standard MR sequences, ii) the 3D True-FISP sequence, and iii) the 3D PD-SPACE sequence, are listed in Table 2. With respect to the single variables, comparable results for all three sequences could be achieved for 1) defect fill, 2) cartilage interface, 3) bone interface, 4) surface, 7) subchondral lamina, 8) chondral osteophytes, and 11) effusion. However, when comparing the different sequences i) to iii), the cartilage repair tissue and its surrounding structures were assessed with slightly inferior results by the 3D True-FISP sequence, compared to the 3D PD-SPACE sequence, with respect to the standard MR sequences. Compared to the above-listed variables, the depiction of the variables 5) structure, 6) signal intensity, 9) bone marrow edema, and 10) subchondral bone showed larger differences between the three sequences i) to iii) (Table 2).

Correlations

The correlation between the 3D MOCART scoring as performed by i) the standard MR sequences, ii) the 3D True-FISP sequence, and iii) the 3D PD-SPACE sequence was significant for the variables 1) defect fill, 2) cartilage interface, 3) bone interface, 4) surface, 7) subchondral lamina, 8) chondral osteophytes, and 11) effusion, with and without Bonferroni correction, with Pearson coefficients ranging from 0.514–0.865. The variables 5) structure, 6) signal intensity, 9) bone marrow edema, and 10) subchondral bone demonstrated lower correlations, especially between the i) standard MR sequences and ii) the 3D True-FISP sequence, with Pearson coefficients ranging from 0.242–0.383. The correlations between the i) standard sequences and iii) the 3D PD-SPACE sequence and between ii) the 3D True-FISP sequence and the iii) 3D PD-SPACE sequence revealed higher (Pearson: 0.307–0.633) correlations. Whereas the correlation between the ii) the 3D True-FISP sequence and the iii) 3D PD-SPACE sequence was again significant with and without Bonferroni correction, the correlations between the i) standard MR sequences and ii) the 3D True-FISP sequence, for these four variables, were not significant based on the Bonferroni correction. Results are provided in detail in Table 3.

Table 3. Pearson correlation coefficients with given p-value (p) and Bonferroni corrected p-value (pB) for the 3D - MOCART score as performed by a i) set of standard MR sequences, the ii) MPR of the 3D-True-FISP sequence and iii) the MPR of the 3D-PD-SPACE sequence
Correlationsi) Standard 2D vs. ii) 3D-True-FISPi) Standard 2D vs. iii) 3D-PD-SPACEii) 3D-True-FISP vs. iii) 3D-PD-SPACE
Variables:
1. Defect fill0.790 (p < 0.001) (pB = 0.033)0.649 (p < 0.001) (pB = 0.033)0.764 (p < 0.001) (pB = 0.033)
2. Cartilage interface0.793 (p < 0.001) (pB = 0.033)0.747 (p < 0.001) (pB = 0.033)0.848 (p < 0.001) (pB = 0.033)
3. Bone interface0.514 (p < 0.001) (pB = 0.033)0.717 (p < 0.001) (pB = 0.033)0.720 (p < 0.001) (pB = 0.033)
4. Surface0.613 (p < 0.001) (pB = 0.033)0.568 (p < 0.001) (pB = 0.033)0.662 (p < 0.001) (pB = 0.033)
5. Structure0.280 (p = 0.031) (pB = 1.000)0.414 (p < 0.001) (pB = 0.033)0.566 (p < 0.001) (pB = 0.033)
6. Signal intensity0.383 (p < 0.002) (pB = 0.066)0.347 (p = 0.006) (pB = 0.198)0.478 (p < 0.001) (pB = 0.033)
7. Subchondral lamina0.672 (p < 0.001) (pB = 0.033)0.511 (p < 0.001) (pB = 0.033)0.658 (p < 0.001) (pB = 0.033)
8. Chondral Osteophytes0.807 (p < 0.001) (pB = 0.033)0.811 (p < 0.001) (pB = 0.033)0.865 (p < 0.001) (pB = 0.033)
9. Bone marrow edema0.242 (p = 0.061) (pB = 1.000)0.633 (p < 0.001) (pB = 0.033)0.410 (p < 0.001) (pB = 0.033)
10. Subchondral bone0.378 (p = 0.003) (pB = 0.099)0.307 (p = 0.016) (pB = 0.528)0.535 (p < 0.001) (pB = 0.033)
11. Effusion0.519 (p < 0.001) (pB = 0.033)0.601 (p < 0.001) (pB = 0.033)0.815 (p < 0.001) (pB = 0.033)

Image Quality and Artifacts

In the assessment of the subjective quality and possible artifacts, i) the standard MR sequences revealed the highest scoring (quality: 4.0 ± 0.0; artifacts: 3.93 ± 0.3), with no significant difference compared to iii) the 3D PD-SPACE sequence (quality: 3.87 ± 0.3; artifacts: 3.77 ± 0.5) (P ≥ 0.05). For ii) the 3D True-FISP sequence, the image quality showed comparable values (quality: 3.78 ± 0.5; P ≥ 0.05), whereas artifacts were significantly more often visible (artifacts: 3.30 ± 0.7; P < 0.05) when compared to i) standard sequences and the iii) 3D PD-SPACE sequence.

DISCUSSION

The present study indicates that with the use of i) standard MR sequences, ii) a 3D True-FISP sequence, or iii) a 3D PD-SPACE sequence the recently introduced 3D MOCART score (15) can be obtained with comparable results. Thus, the new 3D MOCART score can be calculated with the MPR of only one isotropic 3D sequence, indicating advantages in the depiction of knee cartilage and especially the cartilage repair tissue (6, 21–23). The evaluation can be performed in more detail with 3D isotropic MRI, possible changes can be detected, and additional pathologies in other structures in the joint can be diagnosed (7). The isotropic data set is used to move/scroll through the knee joint in three planes, localize the repair tissue, and use the right angulations to depict the structure of the repair tissue and the adjacent cartilage. The variables of the 3D MOCART score, where the more detailed information gained by the isotropic sequences and the associated MPR might be beneficial, are the cartilage interface, the surface of the repair tissue, the subchondral osteophytes, and the effusion. These parameters revealed, in the present study, more “normal” cases for the i) standard 2D evaluation compared to the ii) / iii) isotropic 3D evaluation. This higher number of irregularities in the repair tissue on isotropic sequences might be due to the easier assessment of the repair tissue site by the MPR of the isotropic dataset, with visualization in every plane. Another reason might be the much lower slice thickness of the isotropic sequences of ≈0.5 mm to 2–3 mm of the standard 2D sequences. Although it has to be mentioned that for both the 3D isotropic and the 2D standard sequences, available sequences, optimized for their use in cartilage repair patients, were used. Hence, no further optimization was obtained to, eg, decrease the slice thickness of the standard MR sequences to reach a more fair comparison.

Isotropic 3D MR sequences are usually gradient echo-based. Among others, available sequences are called SPGR (spoiled gradient echo), FLASH (fast low-angle shot), or VIBE (volume interpolated breathold examination), DESS (double-echo steady-state), and SSFP (steady-state free precession) or the sequence used in this study, the True-FISP sequence (24–28). Isotropic, 3D, fast spin-echo sequences that have recently become available are called fast spin-echo (FSE) extended echo-train acquisition (XETA), or, as applied in this study, PD-SPACE (16, 29). The 3D TrueFISP sequence facilitates high-resolution isotropic MRI and is recommended for the potential improvement in the assessment of articular cartilage (26). The performance of the 3D True-FISP sequence has been studied in detail by Duc et al (28, 30, 31) and was seen to provide accurate information about the constitution of articular cartilage in the knee joint. In the initial description of the 3D MOCART score by Welsch et al (15), the 3D True-FISP sequence was reported to provide results comparable to standard 2D MR sequences; however, only the variables of the older basic 2D MOCART score were correlated and no data were provided on the depiction of bone marrow edema. In the present study this new variable of the 3D MOCART score was the variable with the overall lowest correlation between i) the standard 2D MR sequences and ii) the isotropic 3D True-FISP sequence. Hence, the high number of MRIs, where a possible bone marrow edema was graded as “absent” in the 3D True-FISP evaluation, might represent one weakness of this sequence, particularly when considering the comparable results between the i) standard MR sequences and the iii) 3D PD-SPACE sequence. This can be explained by the well-known low sensitivity of gradient-echo sequences for bone marrow and soft tissue abnormalities. This could even be more emphasized by the correlation based on the Bonferroni correction. When comparing the i) standard MR sequences and the ii) 3D True-FISP sequence, the variables structure, signal intensity, bone marrow edema, and subchondral bone did not correlate significantly with overall low correlation coefficients.

In a recent study by Notohamiprodjo et al (16), the 3D PD-SPACE sequence was studied to evaluate its signal- and contrast-to-noise efficiency and its diagnostic performance within the knee joint compared to a standard 2D PD-TSE sequence. Whereas the signal-to-noise efficiency of the 3D PD-SPACE sequence was four to five times higher than that for the 2D-PD-TSE sequence, all other studied parameters were comparable. This indicates, similar to the present study, that by using the 3D PD-SPACE sequence and its MPR, reliable results can be achieved in a relatively short scan time.

When comparing the 3D True-FISP sequence and the 3D PD-SPACE sequence in the present study, the cartilage repair tissue, represented by variables, such as defect fill, cartilage interface, bone interface, and surface, can be diagnosed with comparable results. The variables concerning the structure of the repair tissue and its signal intensity, as well as the subchondral bone, showed lower correlation coefficients between the two isotropic sequences.

For the 3D MOCART score, the signal intensity is graded in comparison to the adjacent cartilage as “identical,” with “slight areas of signal alterations,” or with “large areas of signal alterations” (15). This enables all isotropic MR sequences to be utilized for 3D MOCART scoring (15) in comparison to the standard 2D MOCART score, where the signal intensity is graded as hypointense or hyperintense (8, 9). Thus, in the present study the signal intensity could be assessed using the 3D True-FISP sequence as well as with the 3D PD-SPACE.

The acquisition time was higher for the standard MR sequences at ≈15 minutes, compared to ≈7 minutes for the isotropic sequences. One limiting point, however, is that also for the standard 2D MR sequences the acquisition time could have been reduced and vice versa; the isotropic 3D sequences would have taken substantially longer if the same voxel base areas had been encoded.

Other limitations of the present study are the lack of a gold standard; however, invasive follow-up arthroscopies are no longer performed in a clinical setting, and the subchondral osteophytes, the subchondral bone plate, and bone marrow edemas, in particular, cannot be sufficiently evaluated by arthroscopy. In addition, the cross-sectional character of the study avoids any possible conclusions about the potential predictive value of the 3D MOCART score. This leads to the next limitation, which is the lack of clinical parameters and clinical scoring. Nevertheless, the main aim of the present study was to compare different MR sequences for their ability to be used to obtain the 3D MOCART score. The highly important clinical correlations are the subject of ongoing studies. Another limitation is that image quality and image artifacts cannot be assessed independently from each other. However, for the 3D TrueFISP sequence only the postoperative artifacts really reduced the image quality, where the relatively common banding artifacts, in most of the cases, did not alter the quality of the assessment of the small area of the cartilage repair tissue.

In conclusion, the present study demonstrates that in the clinical routine follow-up after cartilage repair the 3D MOCART score can be assessed with i) standard MR sequences, ii) the 3D True-FISP sequence, or iii) the 3D PD-SPACE sequence. Where existing benefits of both isotropic 3D sequences based on its MPR could be described, the 3D PD-SPACE sequence seems to be slightly superior to the 3D True-FISP sequence, possibly due to a robust performance in the subchondral bone and because of the suppression of susceptibility artifacts produced by the implantation procedure itself and previous surgeries. Based on the findings of the present and other studies, in the routine clinical practice in standard follow-up after cartilage repair procedures, the authors nevertheless still use both the standard 2D sequences as presented in this article and a sagittal isotropic 3D PD-SPACE sequence.

Figure 1.

A 35-year-old male patient 24 months after MACT of the medial femoral condyle. Standard 2D MR sequences with a sagittal PD-TSE sequence (TR = 2130 msec, TE = 36 msec, flip angle = 180°) (a), a sagittal T2-weighted Dual-FSE sequence (5090/12;85/flip angle 180°) (first echo (b); second echo (c)) and a coronal FS-PD-TSE sequence (4250/27/flip angle 180°) (d). The cartilage repair tissue is nicely integrated to its border zones, the defect fill is 100%, there are only slight signal alterations visible, and the subchondral bone seems largely intact.

Figure 2.

A 35-year-old male patient 24 months after MACT of the medial femoral condyle (same as in Fig. 1) assessed with the sagittal and coronal reconstruction based on the MPR of the isotropic 3D True-FISP (7.8/3.8/flip angle = 30°) dataset (a,b) and the isotropic 3D PD-SPACE (1200/30/flip angle 180°) dataset (c,d).

Figure 3.

A 22-year-old female patient 48 months after MACT of the medial femoral condyle. Standard 2D MR sequences with a sagittal PD-TSE sequence (TR = 2130 msec / TE = 36 msec / flip angle = 180°) (a) and a coronal FS-PD-TSE sequence (4250/27/flip angle 180°) (b). The cartilage repair tissue seems to be integrated to its border zones; however, a clear split-like lesion (>50% of the thickness of the cartilage) becomes obvious in the coronal plane. The subchondral bone reveals alterations; a still slightly visible bone plug was used during surgery for a bony defect caused by osteochondritis dissecans.

Figure 4.

The same 22-year-old female patient 48 months after MACT as in Fig. 3, evaluated with the sagittal and coronal reconstruction based on the MPR of the isotropic 3D True-FISP sequence (7.8/3.8/flip angle = 30°) (a,b) and the isotropic 3D PD-SPACE sequence (1200/30/flip angle 180°) (c,d). The changes within the subchondral bone are clearer visualized with the 3D PD-SPACE sequence compared to the 3D True-FISP sequence.

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