Elsevier

Magnetic Resonance Imaging

Volume 55, January 2019, Pages 86-92
Magnetic Resonance Imaging

Original contribution
The comparison of the performance of 3 T and 7 T T2 mapping for untreated low-grade cartilage lesions

https://doi.org/10.1016/j.mri.2018.09.021Get rights and content

Abstract

Objective

To investigate T2 mapping as a possible marker for low-grade human articular cartilage lesions during a one-year follow-up, possible changes during the follow-up and compare the reliability and sensitivity of these measurements on high-field (3 T) and ultra-high-field (7 T) MRI scanners.

Design

Twenty-one patients with femoral, tibial and patellar cartilage defect in the knee joint participated in the study. The MRI protocol consisted of morphological, as well as three-dimensional triple-echo steady-state (3D-TESS) T2 mapping sequences with similar parameters at 3T and 7T. Patients were scanned at five time-points up to 12 months. T2 values were evaluated in the lesion and healthy-appearing regions for superficial and deep cartilage zone. The repeated ANOVA was used to determine differences in T2 values at various time points.

Results

A significant decrease in T2 values was observed between baseline and six months in the superficial layer of the lesion in patients at 3 T (decrease from 41.89 ± 9.3 ms to 31.21 ± 7.2 ms, which is a difference of −5.67 ± 2.2 ms (p = 0.031)), and at 12 months in the superficial layer of the lesion in patients at 3 T (decrease from 41.89 ± 9.3 ms to 35.28 ± 4.9 ms, which is a difference of −6.60 ± 4.4 ms (p = 0.044). No significant differences were recorded at 7 T.

Conclusion

The change in T2 values acquired with 3 T 3D-TESS appears to be reflecting subtle changes of cartilage composition in the course of low-grade lesion development. 7 T T2 mapping does not reflect these changes probably due to completely decayed short T2 component.

Introduction

Cartilage degeneration is a major source of pain and disability in Western societies. Biochemical changes of the extracellular matrix often precede the morphological changes [1]. Thus, early diagnosis of articular cartilage degradation is crucial for successful treatment. However, the detection of low-grade cartilage lesions can be rather challenging. MRI is the modality of choice because of its non-invasive nature, good reliability, and diagnostic power [[2], [3], [4]].

Mature cartilage tissue is characterized by small amount of cells which account only for small volume of hydrated cartilage. Cartilage hydration and the amount various extracellular elements are dependent on age and species. The basic matrix consists of collagen fibers typically oriented in three cartilage layers and glycosaminoglycan molecules responsible for dynamic mechanical properties [5].

There is a strong demand for a noninvasive tool that would enable early diagnosis but also treatment monitoring and benchmark of success for new therapeutic interventions. Such a tool would allow physicians to monitor the patients treated with any treatment options and therapeutic advice to slow down the onset or progression of osteoarthritis (OA) disease. MRI has been previously successfully used for quantification of the cartilage thickness and for volumetry [6], water content [7], as well as proteoglycan and collagen content assessment [[8], [9], [10]]. MRI can also detect focal cartilage disorder that precedes subsequent structural changes of the tissue. The changes in articular cartilage during OA or in acute lesions result in an increased hydrodynamic fluid pressure and increased stress throughout the matrix. This leads to the proteoglycan-collagen matrix degeneration and cartilage tissue loss [11].

Quantitative MRI has shown a great potential to non-invasively detect cartilage tissue alterations, especially with T2 mapping, which is sensitive to collagen matrix anisotropy/organization and water content [9]. There are several methods currently used for T2 mapping in articular cartilage. As a gold standard for T2 mapping, a multi-echo spin echo sequence is usually employed [12,13]. It provides robust, reproducible, and fast T2 mapping; however, its use at ultra-high-fields is compromised by specific-absorption rate (SAR) limitations due to multiple 180° pulses needed for repeated refocusing. The recently introduced technique of 3D Triple-Echo Steady-State (TESS) T2 mapping has been shown a faster and more reproducible alternative to multi-echo spin-echo T2 mapping [14,15]. The reproducibility of T2 mapping with 3D-TESS observed in this study was comparable to the previously published works [16,17]. At ultra-high-field MRI, the advantage of 3D-TESS over multi-echo spin echo is even more prominent since TESS imaging is not prone to specific absorption rate (SAR) issues due to the low optimal excitation flip angles and the derived T2 maps are intrinsically insensitive to transmit field (B1) inhomogeneities. It was shown previously, cartilage tissue has multiple water compartments resulting in multi-component T2 values [18]. Reiter et al. demonstrated in bovine cartilage three components of T2, specifically T2,1 = 2.3 ms, T2,2 = 25.2 ms, and T2,3 = 96.3 ms, with fractions w1 = 6.2%, w2 = 14.5%, and w3 = 79.3%, respectively, using multi-echo spin-echo sequence at 9.4 T [19]. With conventional multi-echo spin-echo the minimal echo time is rarely below 10 ms, however by using TESS T2 mapping with the echo time as short as 5 ms, there are still some residuals from the shortest component which might contribute to the global T2. As this short component is said to be related to bound water molecules, it may bring an interesting information about the cartilage tissue composition. T2* might also provide an interesting information on cartilage composition [20,21], especially because it allows for ultra-short echo times [22,23], however, the previous studies showed this information is not substantially different for T2 [24].

Low-grade cartilage lesions are challenging to diagnose since morphological alterations are often subclinical, but are accompanied by changes in water content and disruption of collagen fibers [25]. As a result, T2 mapping could be a very helpful tool for the initial diagnosis of cartilage lesions as well as for monitoring their subsequent development over time. Several longitudinal studies have used quantitative MRI to describe the cartilage repair process. Krusche-Mandl et al. used quantitative MRI parameters to follow-up patients with autologous chondrocyte transplantation where T2 was the only quantitative parameter that correlated with the modified Lysholm score [26]. A strong correlation between qualitative MRI and clinical score was also found by Salzmann et al.; the correlation with quantitative MR (T2 mapping), on the other hand, was only moderate [27]. However, to establish sensitivity of T2 mapping to cartilage tissue alteration, the natural course of cartilage degradation should be established. To date, no study compared the clinical value of quantitative MRI at different field-strengths in patients with untreated, low-grade cartilage lesions.

Therefore, in this study we investigated 3D-TESS T2 mapping as a possible marker of changes in cartilage status over one year after baseline examination in patients with low-grade cartilage lesions and with risk factors for further cartilage degeneration, such as a meniscal or anterior cruciate ligament tear. The sensitivity and reproducibility of T2 mapping were evaluated at two field-strengths, 3 and 7 Tesla.

Section snippets

Subjects

The ethics committee of the Medical University of Vienna approved the study protocol (No. 1978/2014) and all patients gave written, informed consent. Twenty-one patients (mean age ± standard deviation, 46.3 ± 11.1 years; 12 males/9 females) were enrolled. All of them had femoral (N = 18), patellar (N = 2), and tibial (N = 1) cartilage defect(s) in the knee joint (ICRS Grade I or II [28,29]). The grading of the lesion was performed according to the ICRS classification system by one radiologist

Morphological appearance

Morphologically, no change in lesion grade in the time course was found for any patients. The size of the lesions ranged from 23.4 mm3 to 368.1 mm3 (on average, 162.5 ± 107 mm3) and did not change significantly during the observation period.

Reproducibility and inter-observer variability

The mean coefficient of variation of T2 mapping between the baseline and the eight-day follow-up measurement ranged, in various cartilage regions, from 4.5% to 10.1% at 3 T (on average, 7.8 ± 2.0%) and from 8.9% to 12.8% at 7 T (on average, 11.0 ± 1.5%). The

Discussion

The results of this study showed that the cartilage structure can be detected in patients with low-grade lesions and followed-up over time. The results also suggest that the T2 values at 3 T might be more beneficial for detecting collagen organization than those obtained at 7 T. It is generally accepted that T2 values slightly decrease as the field-strength increases [32]. Indeed, T2 values in this study appeared lower when measured at 7 T compared to 3 T.

Conventional MR sequences typically

Conclusion

3D-TESS T2 mapping is highly sensitive for the detection of the status of low-grade lesions in human articular cartilage and for observing the development of the lesions over time. 3 T T2 mapping appears to be more sensitive to subtle alteration in cartilage tissue due to residuals of short T2 component (usually attributed to bound water molecules) which is mostly decayed at 7 T. In the future, T2 mapping might be a valuable marker for monitoring cartilage development after regenerative

Acknowledgements

The authors would like to thank Harry Haber for his help with statistics. This work was supported by the NOVARTIS PHARMA AG PJMR0062112, Austrian Science Fund (FWF) KLI541-B30 and Slovak Grant Agency APVV-15-0029.

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