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Variable Refocusing Flip Angle Single-Shot Fast Spin Echo Imaging of Liver Lesions: Improved Lesion Contrast and Speed

Robert M Hicks¹, Andreas M Loening², Shreyas S Vasanawala², Michael A Ohliger¹, and Thomas A Hope¹

¹Radiology, University of California, San Francisco, San Francisco, CA, United States, ²Radiology, Stanford University School of Medicine, Stanford, CA, United States

Synopsis

Variable refocusing flip angle single-shot fast spin echo (vrfSSFSE) decreases T2-decay related blurring, allows increased K-space coverage while maintaining clinically relevant echo times, and reduces specific absorption rate. We evaluated clinical image quality and acquisition time of vrfSSFSE for imaging liver lesions in comparison with conventional SSFSE in patients undergoing 68Ga-DOTA-TOC PET on a 3T time-of-flight PET/MRI. Quantitative analysis of 53 lesions in 27 patients demonstrated higher mean CNR with vrfSSFSE (9.9 vs. 6.7, p<0.001) and 1.7-fold increase in speed compared to SSFSE. vrfSSFSE improves liver lesion contrast with shorter scan times acquired using a single 20 second breath hold.

PURPOSE

Fast spin echo imaging of the liver is limited by long acquisition times and motion artifact.¹ Compared to conventional single shot fast spin echo (SSFSE, also known as HASTE), variable refocusing flip angle SSFSE (vrfSSFSE) decreases T2-decay related blurring, allows for increased K-space coverage while maintaining clinically relevant effective echo times due to T2-decay prolongation, and reduces specific absorption rate (SAR) due to the lower flip angles employed.² We evaluated clinical image quality and acquisition time of vrfSSFSE for imaging of liver lesions in comparison with a conventional SSFSE sequence.

METHODS

Variable refocusing flip angle SSFSE (vrfSSFSE) sequence was acquired in 46 patients with known pancreatic neuroendocrine tumors undergoing 68Ga-DOTA-TOC PET on a simultaneous time-of-flight 3.0T PET/MRI (Signa, GE Healthcare). The following imaging parameters were used for SSFSE (224 phase encodes, TE/TR of 102/1139, NEX of 54, slice thickness of 6 mm) and vrfSSFSE (224 phase encodes, TE/TR of 100/679, NEX of 71, slice thickness of 6 mm). Liver lesions ≥1.5cm with radionucleotide uptake were analyzed. Patients without metastases or metastases <1.5cm were excluded. Contrast-to-noise ratios (CNRs) were obtained using region of interest (ROI) measurements and calculated as the absolute difference in signal intensity of the lesion compared to adjacent hepatic parenchyma, divided by the standard deviation of the adjacent hepatic signal intensity. Difference in CNR was assessed using the Student’s two-tailed paired t-test with p<0.05 considered statistically significant.

RESULTS

68Ga-DOTA-TOC PET/MRI identified 53 eligible lesions in 27 patients. vrfSSFSE demonstrated higher mean CNR compared to SSFSE (9.9 vs. 6.7, p<0.001). Mean repetition time (TR) was 679±97 milliseconds for the vrfSSFSE sequence compared to 1139±106 milliseconds for SSFSE, corresponding to a 1.7-fold increase in speed and 20 vs. 34 seconds to acquire a 30-slice sequence.

DISCUSSION

Compared to conventional SSFSE, vrfSSFSE reduced acquisition times and improved lesion contrast on simultaneous time-of-flight 3.0T PET/MRI in patients with liver metastases. Due to the decreased SAR demands of the sequence, TR could be significantly decreased using the vrfSSFSE sequence, allowing coverage of the entire liver in a single twenty-second breath hold. This is particularly important in the setting of PET/MRI, where scan time is limited by the necessity of whole body image acquisition as well as bed specific imaging. Additionally, removing breath holds from the protocol decreases demands on the patient.

CONCLUSION

Time-of-flight 3.0T PET/MRI utilizing vrfSSFSE improves liver lesion contrast and achieves shorter scan times compared to conventional SSFSE.

Acknowledgements

No acknowledgement found.

References

1. Yang RK, Roth CG, Ward RJ, deJesus JO, Mitchell DG. Optimizing abdominal MR imaging: approaches to common problems. Radiographics 2010;30:185-99.

2. Loening AM, Saranathan M, Ruangwattanapaisarn N, Litwiller DV, Shimakawa A, Vasanawala SS. Increased speed and image quality in single-shot fast spin echo imaging via variable refocusing flip angles. J Magn Reson Imaging 2015;42:1747-58.

3. Mannelli L, Wilson GJ, Dubinsky TJ, et al. Assessment of the liver strain among cirrhotic and normal livers using tagged MRI. J Magn Reson Imaging 2012;36:1490-5.

Figures

Figure 1: vrfSSFSE image (A) of a 2 cm metastasis (solid arrow) in the right lobe of the liver and an image of the common bile duct (B, dashed arrow), and corresponding SSFSE images (C,D). Note the improved lesion to liver contrast with vrfSSFSE and increased intensity of the common bile duct when compared with SSFSE images.

Figure 2: vrfSSFSE (A) and SSFSE (B) images of a metastasis in the left lobe of the liver (arrow). Note the decreased overall noise and increased conspicuity of hepatic vessels with vrfSSFSE. There is minimal cardiac motion-related signal loss, an artifact previously described with reduced refocusing flip angles and a known challenge when imaging the left lobe of the liver.^2,3

Figure 3: vrfSSFSE (A) and SSFSE (B) images of a large, heterogeneous metastasis in the hepatic dome (solid arrows). Note the increased conspicuity of the fine septations within the tumor, as well as the plicae circularis of the small bowel (dashed arrows) with vrfSSFSE. The common bile duct (arrowheads) is seen in the same plane.

Figure 4: Comparison of vrfSSFSE and SSFSE contrast-to-noise ratios. Average CNR was significantly higher for vrfSSFSE (p<0.001).

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)

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