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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Osteoarthritis Cartilage. Author manuscript; available in PMC Jan 1, 2012.
Published in final edited form as:
PMCID: PMC3098496
NIHMSID: NIHMS256148

UTE-T2* mapping of human articular cartilage in vivo: a repeatability assessment

Ashley Williams, MS, Yongxian Qian, PhD, and Constance R. Chu, MD

Abstract

Introduction

Ultrashort echo-time enhanced T2* (UTE-T2*) mapping of articular cartilage is a novel quantitative MRI technique with the potential to visualize deep cartilage characteristics better than standard T2 mapping. The feasibility and intersession repeatability of UTE-T2* mapping of cartilage in vivo has not previously been evaluated.

Methods

Eleven asymptomatic subjects underwent repeat UTE-T2* imaging on a whole-body 3T MRI scanner on three consecutive days. Full-thickness, superficial and deep regions of interest (ROIs) were evaluated in the central weight-bearing medial femoral condyle (cMFC) and tibial plateau (cMTP). Intersession precision error across subjects was evaluated by the root-mean-square average coefficients of variation (RMSA-CV) and by the median of intra-subject standard deviations (SD) of UTE-T2* values in each ROIs.

Results

UTE-T2* values in vivo were found to be repeatable with relative (RMSA-CV) intersession precision errors of 8%, 6%, 16% for full-thickness, superficial and deep cMFC ROIs, corresponding to absolute errors (SD) of 1.2, 1.5, 1.5ms, respectively. In cMTP tissue, UTE-T2* relative repeatability was 8%, 8%, 13%, corresponding to absolute repeatability of 1.0, 1.5, 2.1ms (full-thickness, superficial, deep). UTE-T2* values were higher in superficial cartilage compared to deep in both cMFC (p<<0.001) and cMTP (p=0.0004) regions.

Conclusion

In vivo 3-D UTE-T2* mapping at 3T is feasible and can be implemented using a standard clinical MRI scanner and knee coil. Intersession precision error of UTE-T2* values in full-thickness ROIs in the weight-bearing regions of asymptomatic subjects is under 1.2ms or 8% (RMSA-CV). Significant zonal and regional variations of UTE-T2* were seen.

Keywords: Ultrashort echo time, T2* Mapping, Cartilage

INTRODUCTION / SUMMARY

Arthritis is the most common cause of disability in the United States and rates of osteoarthritis (OA) are expected to increase as the population ages 1. Therefore, there is a crucial and growing need for the non-invasive diagnosis and staging of articular cartilage degeneration. Radiography and conventional magnetic resonance imaging (MRI) tend to be unreliable at detecting early articular disease 2, 3 particularly in the deep and calcified zones of articular cartilage, where highly organized collagen fibrils restrict proton mobility and cause rapid T2 relaxation 4-7.

Ultrashort echo-time enhanced T2* (UTE-T2*) mapping of articular cartilage is a novel quantitative MRI technique with the potential to visualize deep cartilage characteristics better than standard T2 mapping 8. Previous spectroscopic approaches to measuring cartilage UTE-T2* have determined that UTE-T2* values in deep tissue are short, perhaps as short as 1ms in calcified cartilage, and increase toward the articular surface 9, 10. Recently, UTE-T2* mapping of human articular cartilage explants has been shown to be more robust in deep cartilage than the standard T2 mapping, and UTE-T2* values have been shown to increase with increasing collagen matrix degeneration 8.

Knowledge of the in vivo repeatability, of UTE-T2* mapping is necessary to establish the clinical utility of this technique for detection and staging of articular cartilage degeneration. In this work, we show that the intersession precision error of UTE-T2* values in full-thickness ROIs in the weight-bearing regions of asymptomatic subjects is under 1.2ms or 8% (RMSA-CV). Moreover, we also show that 3-D UTE-T2* mapping at 3T is feasible in the clinical setting and can be implemented using a standard clinical MRI scanner and knee coil.

METHODS / RESULTS / DISCUSSION

Eleven asymptomatic human subjects with no known or suspected knee injury or disease (27.7 ± 4.5 yrs, BMI 25.1 ± 4.5, 5 female, 11 left knees) were enrolled in this study. All subjects gave informed consent and all studies were approved by the institutional review board of the University of Pittsburgh.

MRI Methods

Subjects (n=11) underwent MR imaging for UTE-T2* mapping of the left knee, one time daily for 3 consecutive days. To reduce variability due to diurnal variations, subjects were scanned at the same time each day ± 1 hour 11. One subject missed the Day1 UTE-T2* scan, therefore, data from only 2 time-points are available for that subject.

All MRI scans were performed on a whole-body 3T MRI scanner (Magnetom Trio Tim, Siemens Medical Solutions, Erlangen, Germany) with an 8-channel knee coil (In vivo Inc., Gainesville, Florida, USA). UTE-T2* mapping images were acquired with a 3D AWSOS sequence (acquisition-weighted stack of spirals) 12. MRI images were acquired at 11 echo times (TE=0.6, 1, 2, 3 ,4, 5, 7, 10, 20, 30, 40ms). The values and spacing of the TEs were optimized by Monte Carlo simulation to have sufficient range to cover all possible T2* values in cartilage, including both long and short components 13. TE images were acquired with a field of view (FOV) of 140mm and matrix size of 256. Other acquisition parameters were: 2mm slice thickness, 60 slices, 24 in-plane spirals, 11.52ms spiral readout time, 5μs data sampling interval, and FA/TR 30°/80ms. Scan time was 1.92 minutes per TE-image or 22 minutes for all 11 TE acquisitions. UTE-T2* acquisitions were acquired with sagittal orientation, centered on the femorotibial joint.

UTE-T2* maps were generated with a mono-exponential T2 curve-fitting routine based on error minimization using MRIMapper software (© Beth Israel Deaconess and MIT 2006). All 11 echo images were used in the mono-exponential fit. A representative mono-exponential UTE-T2* curve fit at a single voxel is shown in Figure 1. Voxels were automatically removed from the analysis for poor regression in cases where the fit-routine could not be solved for a single T2* value at a voxel within a pre-set number of iterations (500). Less than 1% of voxels were removed for this reason.

Figure 1
Representative mono-exponential UTE-T2* curve fit at a single voxel. Black circles represent measured signal intensities in this voxel at each of the 11 ‘echo images.’ The black line represents the calculated mono-exponential UTE-T2* curve. ...

Prior to T2 curve-fitting, TE images from the AWSOS sequence were linearly interpolated to a matrix size of 512 (or a pixel size of 273μm) to permit finer image registration. Interpolated images were registered to reduce spatial offsets between images resulting from patient motion during acquisition. Regions of interest (ROIs) were manually segmented, by one individual with 9 years prior segmenting experience (‘expert’), from a single section from the center of the medial condyle of each knee: full-thickness ROIs were drawn in the central weight-bearing zones of the medial femoral condyle (cMFC) and the medial tibial plateau (cMTP). Zonal variations were examined by further segmenting the full-thickness ROIs into 2 approximately equal sections: a deep zone (extending from the subchondral bone to the center of the tissue to encompass the bottom half of the tissue thickness) and a superficial zone (extending from the center to the articular surface), Figure 2.. Superficial and deep ROIs were approximately 4-6 voxels thick, 20-30 voxels wide. The mean UTE-T2* value (in milliseconds) was recorded for each full-thickness and zonal ROI. Voxels were removed from calculatation of mean UTE-T2* if their value was above or below the range of values deemed physiologically reasonable for cartilage: 0-60ms. Less than 2% of pixels were removed for this reason.

Figure 2
Example in vivo UTE-T2* maps of a representative subject from the 3-day intersession repeatability analysis. Full-thickness, superficial and deep ROIs are outlined in the enlarged map below Day 2. Repeated measures of UTE-T2* values were stable within ...

To gauge intra-operator segmentation variation, UTE-T2* maps were segmented twice by the same individual. In each instance, the expert drew full-thickness, superficial, and deep ROIs in the central weight-bearing zone of the medial femoral condyle and the central weight-bearing zone of the medial tibial plateau (cTP) on the same MRI section. The time interval between initial and repeat segmentations was approximately 3 months.

Statistical Analyses

Intra-observer segmentation reproducibility was assessed by interclass correlation (ICC) of UTE-T2* means for each ROI. Intra-observer segmentation reproducibility was further tested by paired two-tailed t-tests to assess UTE-T2* differences between the repeated segmentations. Intersession UTE-T2* value repeatability was evaluated using 2 methods. First, intra-study-subject average UTE-T2* value (mean ±standard deviation (SD)) was calculated across the 3 study-days for each ROI. Relative inter-subject intersession reproducibility across all 11 study subjects was expressed by the root-mean-square average coefficient of variation (RMSA-CV) for each ROI. RMSA-CV was determined by √((∑CV2)/n) where intra-subject CV was calculated by dividing the SD of a subjects’ UTE-T2* values from Day1, Day2 and Day3 by the mean of the subjects’UTE-T2* values from Day1, Day2 and Day3 for each ROI, and where n was the number of subjects. Second, absolute intersession precision was expressed as median of the intra–subject SDs for each ROI. UTE-T2* differences between superficial and deep cartilage zones were assessed with two-tailed t-Tests for each ROI segmented by the expert. ICC values were computed using Interclass Correlation software (obg.cuhk.edu.hk). All other statistical analyses were performed using Excel (Microsoft).

Signal to Noise

Signal to noise (SNR) was estimated by taking the ratio of mean signal intensity in cartilage tissue to the standard deviation of background noise. Representative signal to noise (SNR) estimates were found to be 56, 55, 60, 51, 51, 46, 41, 31, 22, 17 for TE’s 0.6, 1, 2, 3, 4, 5, 7, 10, 20, 30, 40 ms, respectively. Figure 1 indicates that all 11 echoes had sufficient signal for inclusion in T2* curve-fitting.

Intra-observer Segmentation Reproducibility

ICC values for intra-observer reproducibility for UTE-T2* map segmentation ranged 0.80 −0.98 (Table 1). ICC values in the tibial plateau were higher than those in the femoral condyle (full-thickness cMTP ICC = 0.97 vs. full-thickness cMFC ICC = 0.80). Paired t-tests found no significant differences between the UTE-T2* values calculated from the initial and 3-month repeat segmentation sessions among femoral ROIs (p range = 0.14-0.31, Table 1). Similarly, no differences were seen across segmentation sessions in superficial or deep tibial plateau ROIs (p=0.18, 0.51, respectively).

Table 1
In vivo intersession repeatability of UTE-T2* values in small ROIs

Intersession Repeatability

Example UTE-T2* maps from a representative subject of the repeatability analysis are shown in Figure 2. The in vivo UTE-T2* repeatability for each ROI evaluated is summarized in Table 1. UTE-T2* values in full-thickness ROIs segmented by the expert exhibited relative intersession precision errors of 8% and 8% (RMSA-CV) corresponding to absolute precision errors of 1.2ms and 1.0ms (median SD) for cMFC and cMTP, respectively. UTE-T2* intersession precision was better in superficial articular cartilage layers, (6% and 8%; 1.5ms and 1.5ms), compared to deep layers (16% and 13%; 1.5ms and 2.1ms; for cMFC and cMTP, respectively).

Zonal Variations

The mean UTE-T2* value in full-thickness cMFC cartilage in this population of asymptomatic subjects across the 3 study days was 20.0 ±1.2ms (median of intraindividual means ± median of intraindividual SDs), and the mean UTE-T2* value of full-thickness cMTP cartilage was 20.7±1.0ms, Table 1. A representative UTE-T2* profile demonstrating the depthwise variation of UTE-T2* values is shown in Figure 3. Zonal variations of UTE-T2* values were significant on both the cMFC and cMTP. UTE-T2* values in the superficial half of articular cartilage in the cMFC were 88% higher than in the deep half (27.7±1.5ms vs 10.0±1.5ms, respectively, t-Test p<<0.001). In cMTP cartilage, superficial UTE-T2* values were 40% higher than in deep tissue, (24.7±1.5ms vs 16.3±2.1ms, respectively, t-Test p=0.0004). No regional differences were detected between superficial UTE-T2* values of the cMFC and cMTP (t-Test p= 0.40). Regional differences were detected in deep UTE-T2* values (t-Test p=0.003). Deep cMFC values (10.0±1.5ms), were 39% lower than deep cMTP values (16.3±2.1ms).

Figure 3
A representative UTE-T2* profile depicting the depthwise variation of UTE-T2* values across medial femoral condyle cartilage. UTE-T2* values are lowest in the deepest layer of cartilage, next to the subchondral bone, and increase toward the articular ...

Discussion

Differences in UTE-T2* values between normal and degenerate cartilage have yet to be specifically examined. Therefore, implications of the 6-16% RMSA-CV intersession repeatability described above on the diagnostic ability of UTE-T2* mapping in vivo have yet to be determined. However, compared to reports of intersession repeatabilities from other quantitative MRI measures of cartilage physiology in the literature, including standard T2 (7-11%) 14, 15 and dGEMRIC (5-15%) 16-18, UTE-T2* mapping intersession repeatability is comparable to that of other MRI techniques.

The RMSA-CV provides a conservative estimate of intersession precision error because the root-mean-square function accentuates values distant from the center of the distribution. One limitation of employing RMSA-CV as metric of UTE-T2* variation is that RMSA-CV is very sensitive to small changes in the mean. Consequently, the qualitative impression formed in response to a repeatability error of 16% RMSA-CV in deep tissue may be different from the impression formed from knowledge that UTE-T2* values in deep tissue varied within as few as 1.5ms. Although the median of intraindividual SDs better reflects the central tendency of the distribution, it may provide too narrow an estimate of the error. In this work, both metrics, %RMSA-CV and median %SD, were calculated in an effort to bracket the likely intersession precision error.

The appearance of ‘better’ intersession precision error, reported as RMSA-CV, in superficial cartilage compared to deep is due to zonal variations in cartilage UTE-T2* values. Superficial cartilage layers have longer UTE-T2* values compared to deep. Zonal variations in human cartilage were significant with superficial cartilage UTE-T2* values found to be 40-88% greater than deep. Consequently, the degree of intersession variation in UTE-T2* relative to the mean UTE-T2* value for a cartilage layer is smaller in superficial ROIs because the denominator in the RMSA-CV calculation is greater for superficial cartilage than for deep cartilage.

Inclusion of boundary pixels at the cartilage/synovium and cartilage/bone interfaces, respectively, may account for a portion of the absolute precision error (SDs) observed in this analysis. Inclusion of boundary pixels at the synovial interface, which tend to be affected by fluid partial-volume artifact, effectively increases the average UTE-T2* calculated for the ROI because fluid has longer UTE-T2* time than cartilage. Conversely, inclusion of boundary pixels at the bone interface decreases the average UTE-T2* for the ROI due to relatively low UTE-T2* values in calcified cartilage and subchondral bone 19. Care was taken to avoid including boundary pixels in the ROIs, however, given the section thickness of 2mm, some amount of partial-voluming must be assumed in the reported ROI UTE-T2* values.

Although UTE-T2* values varied more for deep tissue of the tibial plateau compared to femoral condyle (absolute precision error 2.1ms vs 1.5ms) the percentage variation (%RMSA-CV) was lower in tibial plateau cartilage (13%) compared to femoral cartilage (16%). This discrepancy may be explained by regional variations of UTE-T2* values. Among asymptomatics, UTE-T2* values of the deep tissue of the tibial plateau was found to be significantly higher than that of the femoral condyle. While this difference may reflect regional physiologic differences of the tissues, it may also be due, in part, to relatively more partial-voluming with subchondral bone in the femoral condyle cartilage voxels compared to the tibial plateau voxels as a result of differences in the concavity of these bones.

The day-to-day variation in UTE-T2* values observed in this study is not likely to be strongly affected by segmentation differences. ICC showed good intra-observer segmentation reliability for all regions evaluated. Segmentation was more consistent in tibial plateau ROIs (higher ICC values) compared to femur, however, a small but significant difference was detected in the average UTE-T2* values of full-thickness tibial plateau ROIs between initial and repeat segmentation sessions by the expert.

Limitations of clinical imaging include imperfect registration between echo images within an imaging session and between tissue regions evaluated over time which may introduce error in UTE-T2* repeatability measurements. In the clinical setting, some patient motion during a 22 minute scan is inevitable. Furthermore, section registration across time points is affected by patient positioning and image slab placement making perfect slice registration across time points unlikely. In this work, potential imperfect image registration was addressed with in-plane realignment of serial echo images within an imaging session and careful longitudinal slice selection by visual inspection of anatomical landmarks. The repeatability estimates reported here, therefore, represent a degree of accuracy that is achievable with commonly available image analysis tools and provide a reasonable expectation for UTE-T2* mapping in the clinical setting.

It should be noted that the UTE-T2* value reported here is not a pure measure of any single component of T2* relaxation in cartilage. Rather, the UTE-T2* metric, calculated from a mono-exponential fit routine, represents a weighted combination of all T2* decay components present in the same voxel. The relative weightings depend on the relative contributions of the different decays to the total signal in each voxel. The data is of sufficient quality to permit multi-component T2* analysis as has recently been shown in ex vivo studies 13.

In summary, UTE-T2* mapping in vivo is feasible and repeatable with intersession precision error of less than 10% for full-thickness ROIs. It remains to be determined by clinical UTE-T2* evaluations of subjects with known knee pathology if this degree of repeatability is sufficient for detection of chondral degeneration.

ACKNOWLEDGEMENTS / SOURCES OF FUNDING

The authors gratefully acknowledge the National Institutes of Health (RO1 AR052784-CR Chu; RO1 CA106840 – FE Boada) and the Pilot Imaging Program of the MR Research Center, Department of Radiology, University of Pittsburgh, for providing funding for this work. The authors also wish to acknowledge the contributions of Fernando E. Boada, PhD for providing logistical support to this project and Andrew Kim for his technical assistance.

Footnotes

COMPETING INTEREST STATEMENT The authors have no conflicts of interest to declare, except that Yongxian Qian is one of the inventors of the AWSOS sequence used in this study. The AWSOS sequence was issued a patent by the United States Patent and Trademark Office in July 2010.

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