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A comparison of multi-echo spin-echo and triple-echo steady-state T2 mapping for in vivo evaluation of articular cartilage

Objectives To assess the clinical relevance of T2 relaxation times, measured by 3D triple-echo steady-state (3D-TESS), in knee articular cartilage compared to conventional multi-echo spin-echo T2-mapping. Methods Thirteen volunteers and ten patients with focal cartilage lesions were included in this prospective study. All subjects underwent 3-Tesla MRI consisting of a multi-echo multi-slice spin-echo sequence (CPMG) as a reference method for T2 mapping, and 3D TESS with the same geometry settings, but variable acquisition times: standard (TESSs 4:35min) and quick (TESSq 2:05min). T2 values were compared in six different regions in the femoral and tibial cartilage using a Wilcoxon signed ranks test and the Pearson correlation coefficient (r). The local ethics committee approved this study, and all participants gave written informed consent. Results The mean quantitative T2 values measured by CPMG (mean: 46±9ms) in volunteers were significantly higher compared to those measured with TESS (mean: 31±5ms) in all regions. Both methods performed similarly in patients, but CPMG provided a slightly higher difference between lesions and native cartilage (CPMG: 90ms→61ms [31%],p=0.0125;TESS 32ms→24ms [24%],p=0.0839). Conclusions 3D-TESS provides results similar to those of a conventional multi-echo spin-echo sequence with many benefits, such as shortening of total acquisition time and insensitivity to B1 and B0 changes. Key points • 3D-TESS T2mapping provides clinically comparable results to CPMG in shorter scan-time. • Clinical and investigational studies may benefit from high temporal resolution of 3D-TESS. • 3D-TESS T2values are able to differentiate between healthy and damaged cartilage.


Introduction
Transverse relaxation time (T 2 ) mapping in articular cartilage has been previously reported as a reliable marker for the evaluation of collagen matrix and water content. The changes in T 2 values have been demonstrated to correlate with alterations in water content, as well as collagen structure and organization, and are linked to changes in hyaline cartilage and its depletion [1][2][3]. To date, T 2 mapping of cartilage has been successfully used in many applications related to biomechanical and biochemical analysis of cartilage tissue [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Although the loss of glycosaminoglycans is an early sign of osteoarthritis (OA) and collagen depletion occurs in later stages, the potential of T 2 as a marker for the grading of cartilage degeneration in OA has also been demonstrated [18]. T 2 maps are typically calculated from a series of T 2weighted images acquired with different echo times. However, the accuracy and reliability of the resulting T 2 maps is highly dependent on the method used to record the initial T 2 -weighted images.
A multi-spin-echo pulse sequence [e.g., Carr-Purcell-Meiboom-Gill (CPMG) [19]] is a widely used method for in vivo T 2 mapping. This sequence acquires signals from multiple time points along the T 2 decay for each k-space line during a single repetition time. However, this is accompanied by the consequent signal contamination by stimulated echoes [20]. Acquiring the complete T 2 decay curve is also time-consuming and frequently requires segmented imaging strategies.
Another possibility for acquiring T 2 -weighted images is a single-spin-echo strategy. It is well known that maps generated with a series of single-echo spin-echo (SE) images suffer from steady state effects (i.e., different T 1 weightings) in the images. To prevent this, long transverse relaxation (TR) times are necessary, which may limit clinical applications. In addition, the total scan time is substantially longer compared to CPMG because the sequence must be repeated for each echo time separately [21].
Recently, a t riple-echo steady-sta te (TESS) relaxometry method was introduced for fast and accurate T 2 and T 1 mapping [22,23]. This sequence has several advantages over conventional T 2 mapping: it is very fast, the data are acquired in a single scan, and the quantification of T 2 is markedly insensitive to B 1 . To date, the clinical performance of 3D-TESS for T 2 mapping in cartilage has not yet been evaluated.
Therefore, the purpose of this study was to assess the clinical relevance of T 2 relaxation times, measured by threedimensional (3D) triple-echo steady-state (3D-TESS), in knee articular cartilage compared to conventional multi-echo spinecho T 2 -mapping.

Patients
The right knees of 10 patients (35.1±10.5 years) with a focal cartilage lesion and 13 age-matched volunteers (31.4±9.4 years) were measured in this prospective study. The local ethics committee approved this study, and all participants gave written, informed consent. Inclusion criteria for volunteers were: no signs of any cartilage lesion in the knee joint on standard morphological magnetic resonance (MR) sequences; and no meniscal and ligament damage. In patients, only subjects with focal lesions in the femoral cartilage were included. Three patients underwent surgical procedures on the femoral cartilage (matrix-associated chondrocyte transplantation [MACT], Pridie drilling, and mosaicplasty), and the remaining patients were diagnosed with chondropathy grade 2 to 4 according to the International Cartilage Repair Society (ICRS) classification.

MR examinations
All subjects underwent MR examinations of the knee joint on a 3 Tesla Tim TRIO MR scanner (Siemens HealthCare, Erlangen, Germany). An eight-channel knee coil (Siemens HealthCare, Erlangen, Germany) was used for all MR scans. The examination consisted of two methods: a multi-echo multi-slice spin echo sequence (CPMG) as a reference method for T 2 mapping; and a 3D-TESS with the exact same geometry settings as in the CPMG sequence but with two modifications, a standard (TESSs, acquisition time 4:35 min) and a quick (TESSq, acquisition time 2:05 min) sequence. For TESSq, the total acquisition time was reduced by using a phase-and-slice partial Fourier acquisition. The abbreviation TESS refers to TESSs throughout the entire manuscript. With TESS imaging, the three steady-state free precession signal amplitudes (F 1 , F 0 , and F -1 ) were acquired within separate radiofrequency cycles (each signal within one at the same echo time) to enable a shorter repetition time and mitigate possible susceptibility effects [22]. CPMG T 2 maps were calculated using the Syngo console tool Mapit (Siemens, Erlangen, Germany), and, for 3D-TESS, the scripting framework IceLuva was used [24]. To ensure the possibility of evaluating all T 2 maps at once by copying regions-ofinterest (ROIs), the frequency calibration was doublechecked before each scanning to avoid chemical shift artefacts. The summary of all sequence parameters is in the Table 1.

Statistical analysis
The difference between absolute T 2 values calculated with different mapping techniques (CPMG versus 3D-TESS and 3D-TESSq versus 3D-TESSs) was expressed as a P-value calculated by a Wilcoxon signed ranks test. Pearson correlation coefficients were calculated, and standard deviations (SDs) of the ROIs were compared. Both absolute values and percentage differences between CPMG and 3D-TESS were evaluated. In patients, the T 2 values from focal chondral lesions were compared to the adjacent native cartilage and the ability to differentiate between healthy and depleted cartilage was assessed. All statistical calculations were performed in SPSS version 19.0 (SPSS Inc., Chicago, IL) and a P-value less than 0.05 was considered statistically significant.
The SDs for TESS-T 2 were slightly lower than for CPMG-T 2 ; however, this difference was not statistically significant (28±6 % and 30±12 %, P = 0.579). When comparing cartilage layers, the SD did not demonstrate any significant differences between the superficial (TESS 26±8 %, CPMG 32±10 %) and the deep zone (TESS 29±7 %, CPMG 29±8 %).  Both T 2 mapping techniques were able to detect the changes between chondral lesions and native cartilage. For CPMG, the mean T 2 in cartilaginous lesions was 90 ±50 and in native cartilage, it was 62±25 ms. For 3D-TESS, the mean T 2 in cartilaginous lesions was 32±6, and in native cartilage, it was 24±8 ms. Example T 2 maps of native and damaged cartilage are depicted in Fig. 5.
The comparison between TESSq and TESSs showed almost no difference between T 2 values. On average, T 2 (3D-TESSq) was 31 ±2 ms and T 2 (3D-TESSs) was 31±3 ms. The mean Pearson correlation coefficient calculated from the whole cartilage in volunteers and patients was 0.816, ranging from 0.750 to 0.911 in different cartilage regions. The example of correlation in the femoral and tibial cartilage is depicted in Fig. 6.

Discussion
The results of our study indicate that T 2 relaxation times in the cartilage of the knee joint, calculated from a novel 3D-TESS sequence, are highly consistent with those calculated by a classic CPMG sequence. Although the absolute T 2 values were significantly lower for 3D-TESS, this decrease was systematic, and the T2 values were highly correlated between 3D-TESS and CPMG in most of the selected regions. In Ref. [23], the authors demonstrated that TESS-based T 2 quantification provides accurate T 2 values in excellent agreement with single-echo spin echo reference measurements, while CPMG resulted in a substantial overestimation of T 2 due to stimulated echo contributions. The observed significant difference between TESS-T 2 and CPMG-T 2 may, thus, be mainly attributed to stimulated echo contamination present in the CPMG scans. In addition, the increase of T 2 values in the region affected by the magic angle effect (posterior femoral condyle in our case) was observable with both techniques. According to  Mosher et al., the greatest variation in cartilage T 2 with regard to the magic angle effect is observed in the superficial 20 % of cartilage [25]. This is in concordance with the presented results, although we divided the cartilage into two layers rather than three because of limited in vivo resolution.
The great advantage of 3D-TESS for cartilage T 2 mapping is the decrease of total scan time. At our institute, the routine (CPMG) T 2 mapping at 3T requires four and a half minutes. In our study, for f comparison purposes, we primarily used a total-acquisition-timematched version of 3D-TESS. However, we were able to decrease the acquisition time of 3D-TESS down to 2:05 min by using a phase-and-slice partial Fourier sampling, keeping the resulting cartilage T 2 values without a statistically significant change, compared to the totalacquisition-time-matched version of 3D-TESS. This feature would be extremely beneficial in studies where a higher temporal resolution is desired. A typical example would be the physiological analysis of cartilage changes during in vivo loading.
For instance, Nishii et al. used CPMG T 2 mapping in their in vivo loading study with a 13-minute long CPMG sequence version [26]. Mamisch et al. evaluated the unloading effect on articular cartilage prior to and during the MR scan with a 4:09-minute long CPMGbased sequence [27]. In another study, the changes in cartilage water content in cartilage under compression were detected using T 2 mapping, with an acquisition time of 7:30 minutes [28]. The limitation with all such experiments is that the decrease of T 2 values during the MR experiment under loading cannot be excluded with certainty. Therefore, there is a need to shorten the acquisition time of T 2 experiments to a minimum. In this case, 3D-TESS seems to be the ideal T 2 mapping technique for such investigational applications. However, fast T 2 mapping with 3D-TESS could also be beneficial for routine clinical applications, providing increased patient comfort and reducing costs.
We also hypothesized that the SD of T 2 from ROIs selected on cartilage would be lower with 3D-TESS due to its more robust T 2 calculation. The results, however, showed the SDs were the same, no matter which technique was   [31]. Since all of these studies were performed in vitro, it can be assumed that motion artefacts did not contribute to those high SDs. High SDs of T2 in cartilage tissue could be attributed to relatively large tissue anisotropy and heterogeneity due to collagen fibre organization [32].
In has been previously demonstrated that focal cartilage lesions, as well as the cartilage repair tissue, are characterized by T 2 elevation [14,[33][34][35]. This focal T 2 increase was attributed either to collagen disorganization or to cartilage swelling, accompanied by water volume increase. Similarly, in our study, we observed an increase in T 2 differences between native cartilage and lesions. For CPMG, this difference was slightly larger (32 %) compared to 3D-TESS (24 %). Since both differences were statistically significant, the clinical relevance of both techniques is comparable.
Our study has some limitations. Our patient group was relatively small and the diagnoses were inconsistent. It would be necessary to validate these findings in a larger patient group with the same type of cartilage lesions or repair tissues. Moreover, the lesions selected in our study were quite severe, resulting in relatively large T 2 value changes. It would be interesting, however, to investigate the sensitivity of 3D-TESS to subtle T 2 changes, where 3D-TESS would be expected to perform better as it provides more accurate T 2 values. This is primarily because 3D-TESS is less prone to errors, while CPMG substantially overestimates T 2 due to stimulated echo contributions.
3D-TESS is insensitive to B 0 and B 1 heterogeneities and may, thus, provide more accurate T2 values with lower SDs. Moreover, the three-dimensional nature of 3D-TESS allows reformatting of T 2 maps in arbitrary planes, expanding the usefulness of this sequence to other tissues (e.g., ligaments, menisci) or more challenging joints for MR imaging (MRI; e.g., hip, shoulder, temporo-mandibular). The 3D-TESS Fig. 5 A: T 2 map acquired by CPMG, pseudo-colour-coded and overlaid on a morphological image of a 41-year-old patient with Pridie drilling on the medial femoral condyle (TE=11.9 ms); the ratio of native and lesion cartilage T 2 s is 0.81. B: T 2 map acquired by 3D-TESS, pseudo-colourcoded and overlaid on the second acquired MR image (F 0 ); the ratio of native and lesion cartilage T 2 s is 0.83. Focal increase in T 2 values is observable with both techniques ('lesion'). Bone oedema is also present (large arrow). The dynamic range of the CMPG T 2 map is higher compared to the 3D-TESS T 2 map, therefore, the lesion appears to be more diffuse, although the ratios of native and lesion cartilage T 2 s are comparable between CPMG and 3D-TESS

Conclusion
The results of this study demonstrate the clinical utility of a 3D-TESS sequence for T 2 mapping of human knee cartilage. 3D-TESS provides results similar to those of a conventional multi-echo spin-echo sequence with many benefits, such as shortening of total acquisition time and insensitivity to B 1 and B 0 changes. T 2 mapping with 3D-TESS was able to differentiate between healthy and damaged cartilage tissue, indicating its great potential for clinical studies.
Acknowledgments The scientific guarantor of this publication is Prof. Siegfried Trattnig, MD. The authors of this manuscript declare no relationships with any companies whose products or services may be related to the subject matter of the article. This study has received funding by the Austrian Science Fund (FWF) P 25246 B24 and Slovak Grant Agency APVV-0431-12. One of the authors has significant statistical expertise. Institutional review board approval was obtained. Written informed consent was obtained from all subjects (patients) in this study. Methodology: prospective, cross sectional study, performed at one institution.
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