A new method for accurate detection of cartilage lesions in femoroacetabular impingement using quantitative T2 mapping: preliminary validation against arthroscopic findings at 3 T
Noam Ben-Eliezer1,2, Akio Ernesto Yoshimoto2, KAI Tobias Block1,2, Roy Davidovitch3, Thomas Youm3, Robert Meislin3, Michael Recht1,2, Daniel K Sodickson1,2, and Riccardo Lattanzi1,2

1Center for Advanced Imaging Innovation and Research (CAI2R), New York University School of Medicine, New York, NY, United States, 2Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, 3Department of Orthopedic Surgery, New York University Hospital for Joint Diseases, New York, NY, United States

Synopsis

Early diagnosis of cartilage defects is critical for the success of corrective surgical procedures in patients with femoroacetabular impingement (FAI). T2 is a biomarker for early biochemical degeneration of cartilage, but in vivo T2 mapping is challenging while commonly used techniques based on exponential fit of multi spin-echo protocols are inaccurate. We used a Bloch simulation based T2 mapping technique – the EMC algorithm – to retrospectively quantify reliable T2 values in the hip cartilage of FAI patients. We then defined a normalized T2-index using an internal reference and showed that it allows detection of surgically confirmed cartilage lesions with 95% accuracy.

Introduction

Femoroacetabular impingement (FAI) is a hip pathology that leads to osteoarthritis if left untreated1. Surgical procedures aimed at correcting the bony defects responsible for FAI are successful only in patients with limited cartilage damage2. T2 relaxation time is a biomarker for early changes in the cartilage collagen structure and water content3, yet quantitative T2 mapping is highly challenging in vivo owing to the extensive scan times of single Spin-Echo imaging protocols and the high resolution required in order to delineate the thin cartilage layers. Fast Multi Spin-Echo (MSE) protocols offers clinically feasible scan durations but, due to intrinsic scanner imperfections, their signal deviates significantly from the theoretical exponential decay S(t)=S0exp[–t/T2 ] and follows instead a more generalized echo modulation curve (EMC). This results in a bias in exponentially fitted T2 values that is, moreover not constant across different T2 distribution but rather depends on the protocol and parameter-set used. As a result, T2 values for the same subject vary between scanners and vendors4,5, thereby preventing the adoption of quantitative T2 as a biomarker for cartilage damage. A recently introduced method – the EMC algorithm6,7 – can overcomes these limitations and deliver accurate and reliable maps of the true tissue T2 values that are independent of the scanner and protocol-implementation. In this work we hypothesize that quantitative EMC-based T2 values which are normalized using an internal reference, can enable accurate detection of surgically confirmed hip cartilage lesions in FAI patients.

Methods

Data acquisition: retrospective analysis of 21 hips (12 left) was performed in 20 patients (9 males) diagnosed with FAI and either torn or detached labrum. Patients (38 ± 12 y/o) underwent pre-operative MRI at 3T after intravenous injection of Gd-DTPA2- for indirect MR arthrography. MSE data were acquired for either a radial or a sagittal section depicting the anterior-superior region of the hip articular cartilage (Figure 1). See Table 1 for a list of imaging parameters.

Surgical assessment: patients underwent routine hip arthroscopy 53 ± 34 days after the MRI to correct the bone defect associated with FAI. All cartilage surfaces were evaluated, providing a ground truth for assessing the capability of the pre-operative T2 values to detect cartilage lesions.

EMC algorithm6,7: Bloch simulations of the MSE protocol were performed using the exact RF pulse shapes and other experimental parameters. Simulations were repeated for a range of T2 and B1+ values (T2=1…1000ms, B1+ = 50…130 % of nominal value), producing a database of EMCs, each associated with a unique [B1+,T2] value pair. Reconstruction: T2 maps were generated using (1) standard monoexponential fit and (2) the EMC algorithm by pixel-by-pixel matching the time-series of MSE DICOMs to the EMC database via l2-norm minimization of the difference between experimental and simulated EMCs.

Statistical analysis: Patient population was divided into lesion (any chondral defect) and non-lesion subgroups, based on surgical findings. Three regions-of-interest were segmented (Figure 2): FEM – the central portion of the femoral cartilage which is usually healthy in early-stage FAI, ACT – denoting the weight-bearing portion of the acetabular cartilage where lesions are typically found in FAI, and ALL – including both femoral and acetabular cartilage. For each section, a normalized T2 index was defined as T2-index=mean(T2ACT) / mean(T2FEM) in order to remove inter-patient baseline variability.

Results

Figure 3 shows representative T2 maps illustrating the non-uniform bias that is typical to exponential fitting and the ability of EMC to produce more faithful delineation of the cartilage and muscle tissues. This was additionally reflected in lower intra-subject and inter-scanner variability for EMC-based T2 values. Figure 4: the lesion and non-lesion patient subgroups could be separated using both mean T2 value within ACT (p-value=0.053) and the T2-index (p-value<0.001). On a patient-specific level, the EMC-based T2-index detected chondral lesions with 100% specificity, 92.3% sensitivity and 95.2% accuracy. The two subgroups were not separable using exponentially fitted T2 values.

Discussion

Previous work using dGEMRIC had shown the benefit of using the central portion of the femoral cartilage as a normalization reference for quantitative T1-based assessment of the hip cartilage in FAI8. This study demonstrated the ability of the EMC-reconstructed T2-index to detect hip cartilage lesions consistently over different protocol parameters and scanners. The higher specificity of the EMC algorithm can thus be used to guide and improve the accuracy of pre-operative radiologic evaluation9,10 and enable longitudinal, cross-platform, and multi-center clinical studies. Future work includes assessing intra- and inter-observer variability in T2-index analysis, and a prospective study on a larger patient population to investigate detection and staging of cartilage damage.

Acknowledgements

Financial support: NIH Grants: P41 EB017183; R01 EB000447. The Helen and Martin Kimmel Award for Innovative Investigation.

References

[1] Ganz R, Parvizi J, Beck M, Leunig M, Notzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res 2003; 417:112-20.

[2] Beck M, Leunig M, Parvizi J, Boutier V, Wyss D, Ganz R. Anterior femoroacetabular impingement: part II. Midterm results of surgical treatment. Clin Orthop Relat Res 2004; 418: 67-73.

[3] Watanabe A, Boesch C, Siebenrock K, Obata T, Anderson SE. T2 mapping of hip articular cartilage in healthy volunteers at 3T: a study of topographic variation. J Magn Reson Imaging 2007; 26(1): 165-171.

[4] Bauer CM, Jara H, Killiany R; Alzheimer's Disease Neuroimaging Initiative. Whole brain quantitative T2 MRI across multiple scanners with dual echo FSE: applications to AD, MCI, and normal aging. Neuroimage. 2010 Aug 15;52(2):508-14.

[5] Annie Horng, Sabine Weckbach, Mike Notohamiprodjo, Malte Münkel, Jürgen Weber, Maximilian F. Reiser, Christian Glaser. Does the Scanner Make a Difference? Interscanner Variability of Tibial Cartilage T2 Relaxation Time – a Comparison of Three 1.5T & One 3T Scanner of One Manufacturer. Proc Int Soc Magn Reson Med, 2011; 19(3218).

[6] Ben-Eliezer N, Sodickon, DK, and Block, KT. Rapid and accurate T2 mapping from multi-spin-echo data using Bloch-simulation-based reconstruction. Magn Reson Med 2015; 73(2): 809-17.

[7] Ben-Eliezer N1, Sodickson DK, Shepherd T, Wiggins GC, Block KT. Accelerated and motion-robust in vivo T2 mapping from radially undersampled data using bloch-simulation-based iterative reconstruction. Magn Reson Med 2015; doi: 10.1002/mrm.25558. [Epub ahead of print].

[8] Lattanzi R et al. Detection of cartilage damage in femoroacetabular impingement with standardized dGEMRIC at 3T; Osteoarthritis and Cartilage, vol 22(3), 2014, p. 447-456.

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Figures

Figure 1: The two slice orientations used in this study consisting of: (a) radial section through the anterior-superior region of the hip cartilage, and (b) sagittal section through the center of the femoral head. In both cases only the anterior-superior region of the articular cartilage was analyzed.

Figure 2: T2 values were extracted for three regions-of-interest within the cartilage. FEM: was delineated over the central portion of the femoral cartilage; ACT: covered the weight-bearing portion of the acetabular cartilage; ALL: included both the femoral and acetabular cartilage, extending from the chondrolabral junction to approximately the fovea.

Figure 3: Representative T2 maps generated based on (a) EMC fitting and (b) conventional exponential fitting. Clear elevation of T2 values is seen in (b) (see color scale), as well as higher artificial heterogeneity of the cartilage and muscle tissues in comparison to EMC fitted map.

Figure 4: Scatter-plots of the T2-index for patients with (red squares), and without (blue rhombi) surgically confirmed lesions. Distribution of EMC-fitted and exponentially fitted T2-indices are shown in panels (a) and (b) respectively, with corresponding boxplots in (c). Dashed lines mark statistically optimal threshold to differentiate the two groups.

Table 1: MRI scanners (system 1-4) and parameter-values used in the current study (AS: Anterior-Superior). Scan durations were ~6 min with x2 GRAPPA acceleration.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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