Sampada Bhave^{1}, S. Sivaram Kaushik^{2}, Robert D Peters^{2}, and Kevin M Koch^{1}

The application of diffusion weighted imaging (DWI) is rapidly increasing in musculoskeletal system. DWI is useful in imaging diverse range of musculoskeletal pathologies like soft-tissue tumors, bone lesions, vertebral fractures pre and post treatment follow up. In this work, our goal is to quantitatively compare the accuracy of ADC estimation of EPI, and PROPELLER based techniques. We also optimize the imaging parameters for PROPELLER and MSI-PROPELLER techniques and provide a correction method to improve the accuracy of ADC estimation.

[1] Bhojwani, N., Szpakowski, P., Partovi, S., Maurer, M. H., Grosse, U., von Tengg-Kobligk, H., ... & Robbin, M. R. (2015). Diffusion-weighted imaging in musculoskeletal radiology—clinical applications and future directions. Quantitative imaging in medicine and surgery, 5(5), 740.

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3] Kealey, S. M., Aho, T., Delong, D., Barboriak, D. P., Provenzale, J. M., & Eastwood, J. D. (2005). Assessment of apparent diffusion coefficient in normal and degenerated intervertebral lumbar disks: initial experience. Radiology, 235(2), 569-574.

[4] Yakushiji, T., Oka, K., Sato, H., Yorimitsu, S., Fujimoto, T., Yamashita, Y., & Mizuta, H. (2009). Characterization of chondroblastic osteosarcoma: Gadolinium‐enhanced versus diffusion‐weighted MR imaging. Journal of Magnetic Resonance Imaging: An Official Journal of the International Society for Magnetic Resonance in Medicine, 29(4), 895-900.

[5] Koch, K. M., Bhave, S., Gaddipati, A., Hargreaves, B. A., Gui, D., Peters, R., ... & Kaushik, S. S. (2018). Multispectral diffusion‐weighted imaging near metal implants. Magnetic resonance in medicine, 79(2), 987-993.

Figure
1: NIST diffusion phantom: The NIST diffusion phantom containing vials filled
with varying polymer concentrations is shown here. The data from 6 vials (Vial
1-6) with polymer concentrations varying from 0% to 50% shown in (a) was used
for all the ADC calculations. (b) shows the T_{2}-weighted image for
the center slice of the phantom. The known mean ADC values for these vials are given in the table.

Figure
2: Bar chart for quantitative comparison of 6 techniques: The quantitative
comparison for EPI, MUSE, Multiband EPI, PROPELLER, MSI PROPELLER and FOCUS-EPI
in the 6 vials with varying polymer concentrations is shown here. 3 groups of 6
bars each (one bar for each technique) represent the mean ADC values for b-value
of 350s/mm^{2}, 600s/mm^{2} and 1000s/mm^{2}
respectively. The black line denotes the known ADC value for that vial. EPI and
FOCUS-EPI have the optimal and consistent performance across b-values and
vials. The PROPELLER based techniques show elevated mean ADCs values.

Figure
3: Effect of bandwidth (BW) and echo train length (ETL) for PROPELLER technique:
The effect of BW and ETL on the ADC values is shown for each of 6 vials. Each
subplot corresponds to one vial and shows change in the mean ADC values for
different bandwidths. The ETL of 32 (shown in red) yields ADCs values close to
the known values. Vials 5-6 have low signal intensity and hence yield better
performance at BW = 62.5 kHz, whereas other vials have similar or better
performance at BW of 15.6 kHz.

Figure
4: Correction strategy for PROPELLER based methods: The comparison of the known
ADC values and those obtained from PROPELLER and MSI PROPELLER techniques
before and after correction is shown here. We see that a 2nd degree
polynomial fitting corrects the elevated ADCs in both the PROPELLER
methods.