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Improving the Conspicuity of Meniscal Tears in Knee MRI at 3T and 7T
Venkata Veerendranadh Chebrolu1, Peter Kollasch1, Benjamin Howe2, Matthew Frick2, Suzanne L Carlson2, Daniel B Spence2, and Kimberly K Amrami2

1Siemens Healthineers, Rochester, MN, United States, 2Department of Radiology, Mayo Clinic, Rochester, MN, United States

Synopsis

In this work we propose the application of a small percentage of fat-saturation (fat-sat) using Spectral Heterogeneity Adaptive RF Pulses (SHARP) to improve the conspicuity of meniscal tears at 3T and 7T. Knee Turbo-spin-echo (TSE) MRI without fat-sat was compared with TSE MRI with a small percentage of fat-sat applied using SHARP to demonstrate the improvement in conspicuity of meniscal tears with SHARP at 3T and 7T.

Introduction

Identifying meniscal tears with confidence is an important element of routine musculoskeletal (MSK) MRI. Sagittal knee MR images without fat-saturation (fat-sat) are often used for the diagnosis of meniscal tears (1,2). Studies analyzing the accuracy of MRI for meniscal tear diagnosis reported varying degrees of sensitivity and specificity (1,3,4). One systematic review reported a moderate 78% (95 % CI 66–87%) sensitivity of MRI for lateral meniscal tear diagnosis (3). Identifying meniscal tears is a critical aspect of knee imaging and high confidence is required for meaningful results. There are few studies assessing the accuracy of 3T for meniscal tears (5) and none for 7T.

Turbo-spin-echo (TSE) sequence is the most often used sequence in clinical MSK MR imaging. Improving the conspicuity of meniscal tears with TSE could improve the diagnostic accuracy of MRI for detecting meniscal tears.

Recently, a novel method, Spectral Heterogeneity Adaptive RF Pulses (SHARP), with functionality to control the degree of “darkness” of fat was proposed and was shown to achieve uniform fat-sat with TSE MRI at 3T (6). The darkness of fat was controlled by SHARP using the flip-angle of the saturation RF pulse (spectrally selective to fat). A fat-sat RF flip-angle of 180° provided the darkest appearance for fat with SHARP and a reduction in RF flip-angle from 180° reduced the darkness of fat, with 0° flip-angle providing non-fat-saturated images.

In this work, we propose to test the ability SHARP to control the degree of darkness of fat to optimize the dynamic range of image intensities in TSE MRI and improve the conspicuity of meniscal tears at 3T and 7T.

Methods

5 subjects with suspected meniscal tears were imaged both at 3T (Magnetom Skyra, Siemens Healthineers, Erlangen, Germany) and 7T (Manetom Terra, Siemens Healthineers, Erlangen, Germany) under the guidelines of an Institutional Review Board. A single-channel transmit, 15-channel receive phased-array knee coil (QED, Quality Electrodynamics, Mayfield Village, OH, USA) was used for imaging at 3T and single-channel transmit, 28-channel phased-array receive knee coil (QED, Quality Electrodynamics, Mayfield Village, OH, USA) was used for imaging at 7T.

A TSE sequence with typical acquisition parameters for proton density (PD)-weighting was used for imaging and SHARP was used to control the darkness of fat-sat.

Three different sequences were compared for conspicuity of meniscal tears: i) PD-weighted sagittal MRI without fat-sat; ii) PD-weighted sagittal MRI with a small percentage of fat-sat applied using SHARP with 30° fat-sat RF flip-angle; iii) ) PD-weighted sagittal MRI with fat-sat applied using SHARP with 50° fat-sat RF flip-angle (50° flip-angle provides higher percentage of fat suppression compared to 30° flip-angle).

The conspicuity of meniscal tears was evaluated by three MSK fellowship trained radiologists.

Results and Discussion

Figures 1 and 2 compare the meniscal tear conspicuity of PD-weighted sagittal MRI without fat-sat with SHARP PD-weighted sagittal MRI with 30° and 50° fat-sat RF flip-angles at 7T. Figure 3 shows the same comparison between PD-weighted sagittal MRI without fat-sat and SHARP at 3T.

Fat signal is hyper-intense on TSE MRI. Imaging with multi-channel receive-coils at 3T and 7T may result in hypo-intensity near the center of field-of-view. Intra-meniscal signal with contact to inferior and/or superior surface of the meniscus is used as one of the primary criterion for meniscal tear diagnosis (2,7). The effects of hyper-intense fat (in bone marrow near the menisci) and receive-coil induced hypo-intensity of meniscal tears coupled together could reduce the conspicuity of intra-meniscal signal at 3T and 7T. The suppression of fat by a small percentage improves the dynamic range of intensities around the menisci, thus improving the conspicuity of the tears.

Conclusions

SHARP improved the conspicuity of meniscal tears at 3T and 7T on PD-weighted turbo-spin-echo MRI by applying a small percentage of fat-sat.

Acknowledgements

No acknowledgement found.

References

1. De Smet AA, Norris MA, Yandow DR, Quintana FA, Graf BK, Keene JS. MR diagnosis of meniscal tears of the knee: importance of high signal in the meniscus that extends to the surface. AJR. Am. J. Roentgenol. 1993;161:101–7. doi: 10.2214/ajr.161.1.8517286.

2. De Smet AA. How I Diagnose Meniscal Tears on Knee MRI. Am. J. Roentgenol. 2012;199:481–499. doi: 10.2214/AJR.12.8663.

3. Phelan N, Rowland P, Galvin R, O’Byrne JM. A systematic review and meta-analysis of the diagnostic accuracy of MRI for suspected ACL and meniscal tears of the knee. Knee Surgery, Sport. Traumatol. Arthrosc. 2016;24:1525–1539. doi: 10.1007/s00167-015-3861-8.

4. Smith BE, Thacker D, Crewesmith A, Hall M. Special tests for assessing meniscal tears within the knee: a systematic review and meta-analysis. BMJ Evidence-Based Med. 2015;20:88–97. doi: 10.1136/EBMED-2014-110160.

5. Sampson MJ, Jackson MP, Moran CJ, Moran R, Eustace SJ, Shine S. Three Tesla MRI for the diagnosis of meniscal and anterior cruciate ligament pathology: a comparison to arthroscopic findings. Clin. Radiol. 2008;63:1106–1111. doi: 10.1016/j.crad.2008.04.008.

6. Chebrolu V V, Frick M, Kollasch P, Schmitt B, Deshpande V, Grinstead J, Spence D, Felmlee J, Amrami KK. Fat Saturation in Musculoskeletal MRI using Spectral Heterogeneity Adaptive RF Pulses (SHARP). In: Proceedings Joint Annual Meeting ISMRM-ESMRMB. ; 2018. p. 5174.

7. Manaster BJ. Magnetic resonance imaging of the knee. Semin. Ultrasound. CT. MR1990;11:307–26.

Figures

Figure 1: Comparison of meniscal tear conspicuity between (A) PD-weighted sagittal MRI without fat-sat, (B) PD-weighted sagittal MRI with a small percentage of fat-sat applied using SHARP with 30° fat-sat RF flip-angle (FA 30°) and (C) PD-weighted sagittal MRI with fat-sat applied using SHARP with 50° fat-sat RF flip-angle (FA 50°) at 7T. Increased conspicuity of meniscal tears with FA 30° and FA 50° is highlighted using arrows. Same window-width and window-level were used for the three images and were chosen to highlight the conspicuity of meniscal tears.

Figure 2: Comparison of meniscal tear conspicuity between (A) PD-weighted sagittal MRI without fat-sat and PD-weighted sagittal MRI with a small percentage of fat-sat applied using SHARP with 30° (B) and 50° (C) fat-sat RF flip-angle (FA 30° and FA 50°) at 7T. Optimizing the dynamic range of image intensities in TSE MRI with a small percentage of fat-sat improved the conspicuity of meniscal tears as highlighted by the arrows. Same window-width and window-level were used for the three images and were chosen to highlight the conspicuity of meniscal tears.

Figure 3: Comparison of meniscal tear conspicuity between (A) PD-weighted sagittal MRI without fat-sat, (B) PD-weighted sagittal MRI with a small percentage of fat-sat applied using SHARP with 30° fat-sat RF flip-angle (FA 30°) and (C) PD-weighted sagittal MRI with fat-sat applied using SHARP with 50° fat-sat RF flip-angle (FA 50°) at 3T. Same window-width and window-level were used for the three images and were chosen to highlight the conspicuity of meniscal tears.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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