Rapid T2 Relaxometry of the Meniscus, Tendons, and Ligaments with an Ultrashort Echo-Time Double-Echo Steady State Sequence
Akshay S Chaudhari1,2, Bragi Sveinsson1,3, Marcus T Alley1, Emily J McWalter1, Garry E Gold1,2, and Brian A Hargreaves1,2,3

1Radiology, Stanford University, Stanford, CA, United States, 2Bioengineering, Stanford University, Stanford, CA, United States, 3Electrical Engineering, Stanford University, Stanford, CA, United States

Synopsis

Quantitative MR imaging has the potential to provide information regarding the biochemical state of tissues and to track biochemical changes before the onset of morphological changes. However, with current MRI techniques, it is challenging to obtain signal from tissues with short-T2 relaxation times in the knee, let alone to perform quantitative imaging on them. Ultrashort echo-time double-echo in steady state (UTEDESS) is a pulse sequence that offers echo times of 50μs, and high resolution imaging with high SNR efficiencies. In this study, UTEDESS was also used to measure T2 relaxation times of the menisci, tendons, and ligaments in human knees.

Introduction

MR imaging of connective tissues in the knee such as the meniscus, ligaments, and tendons is challenging due to their short-T2 relaxation times. These tissues have been shown to be integral in osteoarthritis progression [1], which makes it compelling to track their early biochemical changes before the onset of morphological change. Double-echo in steady state (DESS) is a 3D pulse sequence that has been used for morphological imaging and T2 relaxation time measurements in cartilage [2,3], but the imaging of tissues with short T2 times is a challenge. Ultrashort-echo-time DESS (UTEDESS), seen in Fig. 1, is a 3D radial variation of DESS offering ultrashort echo times, fat-water separation, simultaneous acquisition of multiple contrast mechanisms, high isotropic resolution, and excellent SNR efficiency [4]. This study aims to utilize UTEDESS to measure T2 relaxation times for the short-T2 fibrocartilage tissues in the knee, using an improved T2 fit model.

Methods

The two echoes (S+ and S-) of DESS have been used to estimate the T2 relaxation times of tissues, assuming relatively high flip angles [3]. The simple exponential approach in ref. [3] can be improved by accounting for T1 and the lower flip angles necessary for imaging short-T2 tissues with short repetition times.

UTEDESS was used to scan 7 healthy volunteers (FOV=16cm, isotropic resolution=0.63mm, TE/TR=0.4ms/4.8ms, flip angle=10°, bandwidth=±250kHz, scan time=8:10 minutes, 2x radial undersampling). A Cartesian DESS sequence (FOV=16cm, resolution=0.63x0.63x3mm, TE/TR=5.1/17.7ms, flip angle=35°, bandwidth=±31.25kHz, scan time=3:01 minutes) was also used to calculate T2 in meniscus as a best reference for the UTEDESS T2 maps. Statistical differences between the two were calculated by a paired Wilcoxon signed rank test. SNR estimates were calculated using noise from the same background ROI for both sequences.

T2 maps were generated for manually segmented regions of the patellar tendon, posterior cruciate ligament (PCL), along with the body, posterior and anterior horns of the medial and lateral meniscus in single slices. For the improved T2 fit, T1 for the short-T2 tissues was approximated at 1s [5] while flip angles of 10° and 35° were used for UTEDESS and DESS respectively. UTEDESS scans were repeated three times on five volunteers to assess repeatability and a root-mean-square repeatability coefficient of variation (rCV) per tissue was calculated.

Results

The T2 values for the patellar tendon were 9.5±1.5ms (average rCV=2.5%) and for the PCL were 14.6±1.1ms (average rCV=2.9%). SNR comparisons amongst UTEDESS and DESS are seen in Fig. 2. Comparisons of the meniscal T2 values measured with DESS and UTEDESS are shown in Fig. 3. The repeatability of T2 measurements with UTEDESS is seen in the rCV plots in Fig. 4, where the average rCV is 5.1% in the six sub-sections of the meniscus over three repeated scans. Sample T2 maps in the lateral menisci and patellar tendon are shown in Fig. 5.

Discussion

A minimum TE significantly shorter than the expected T2 range is necessary to generate stable and regionally sensitive T2 measurements in connective tissue [6]. Ideally, a spin-echo sequence would have been used for validation, but for comparable resolution, it would have a minimum TE=8.5ms. Cartesian DESS has been demonstrated to offer robust T2 measurements [7], and offers TE=5.1ms, so it was used as a comparison since it provides higher SNR than spin-echo in meniscus, which is requisite for accurate T2 exponential regressions [8]. The background ROI to estimate SNR gave noise estimates closely matching the noise measured by subtracting two UTEDESS scans, and is therefore a reasonable comparison metric here.

Despite different scan parameters, the mean meniscal UTEDESS and DESS T2 relaxation times agreed very well (Fig. 3a), showing no statistical difference and minimal intra-subject variation (Fig. 3b). The variation of measured T2 times was minimal over the three repeated scans and the rCV values were under 10% for all segmented tissues (Fig. 4). However, the meniscal T2 times were slightly higher than those reported previously [9-11]. Exponential T2 fits can be overestimated because of biases due to noise, which is an inherent challenge with short-T2 tissues. Monte-Carlo simulations with UTEDESS showed that approximately 3ms overestimations are possible due to noise. In addition, magic angle effects prevalent in the PCL [12] may also have increased the T2 times. In future work, we plan to characterize the relationship between noise and T2 fits, as well as to optimize UTEDESS to measure T2 of cartilage.

In conclusion, we have demonstrated rapid T2 relaxation time measurements in connective tissues including meniscus, tendons and ligaments. Combined with its other features, UTEDESS is a promising whole-joint method to measure quantitative tissue parameters in short T2 tissues.

Acknowledgements

Research support provided by the NSF DGE-114747, GE Healthcare, NIH/NIAMS R01 AR063643, NIH/NIAMS K24 AR062068, NIH/NIBIB R01 EB002524.

References

1. Loeser, R.F., et al. "Osteoarthritis: a disease of the joint as an organ." Arthritis & Rheumatism 64.6 (2012): 1697-1707.

2. Staroswiecki E., et al. “Simultaneous Estimation of T2 and ADC in Human Articular Cartilage In Vivo with a Modified 3D DESS Sequence at 3T”. Magnetic Resonance in Medicine. 2012;67(4):1086-1096.

3. Welsch G.H. et al. “Rapid Estimation of Cartilage T2 Based on Double Echo at Steady State (DESS) With 3 Tesla.” Magnetic Resonance in Medicine. 2009;62:544–549.

4. Chaudhari A.S., et al. “Rapid, High-Resolution, and Multi-Contrast Knee MRI of Short-T2 Tissues with Ultrashort TE Double-Echo Steady-State.” Proc. Intl. Soc. Mag. Reson. Med. 23. (2015): 4223.

5. Martirosian P., et al. “Relaxometry of tendons, ligaments and menisci in the knee joint at 3T.” Proc. Intl. Soc. Mag. Reson. Med. 20. (2012): 2374.

6. Williams A.,et al. “UTE-T2* mapping detects sub-clinical meniscus injury after anterior cruciate ligament tear.” Osteoarthritis and Cartilage 20 (2012) 486-494.

7 Matzat S.J., et al. “T2 Relaxation time quantitation differs between pulse sequences in articular cartilage.” Journal of Magnetic Resonance Imaging (2014).

8. Raya, J.G., et al. "T2 measurement in articular cartilage: impact of the fitting method on accuracy and precision at low SNR." Magnetic Resonance in Medicine 63.1 (2010): 181-193.

9. McWalter E.J., et al. “T2 and T2* Relaxometry in the Meniscus using a Novel, Rapid Multi-Echo Steady State Sequence”. Proc. Intl. Soc. Mag. Reson. Med. 21 (2013): 0686.

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Figures

Figure 1: The pulse sequence diagram for UTEDESS shows the radial readouts and lower echo times compared to Cartesian DESS. Meniscal signals from both sequences are also seen (arrows). Note: UTEDESS can utilize Dixon based retrospective fat-water separation while DESS uses a fat suppressed RF excitation.

Figure 2: DESS and UTEDESS have comparable SNR measures for all tissues in the S+ echo. However, UTEDESS has higher SNR for S- echoes and is less susceptible to errors in T2 measurements. DESS has higher S+ meniscus SNR due to a higher flip angle and TR. Note: Different scales.

Figure 3: T2 measurements generated by UTEDESS and DESS show high agreement without significant variation (Wilcoxon signed rank test p values = 0.81, 0.82, 0.38, 0.69, 0.58, and 0.22 respectively in 6 sub-regions across subjects). Overall, UTEDESS reported slightly higher T2 values than DESS, on a per-subject basis.

Figure 4: The intra-subject repeatability dispersion measurements showed little variation in T2 values over multiple scans across the meniscal sub-regions (average repeatability CV = 5%). The largest source of error likely arose from partial volume effects and segmentation of smaller sections of tissues.

Figure 5: Typical T2 measurements in the patellar tendon and lateral menisci of a healthy volunteer. Due to the high SNR efficiency of UTEDESS, spatially smooth T2 fits were observed even in the short-T2 patellar tendon.



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