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Reducing T2-shinethrough effects in prostate diffusion-weighted imaging with Stimulated Echo imaging
Yuxin Zhang1,2, Shane A Wells2, Benjamin L Triche2, Frederick Kelcz2, and Diego Hernando1,2

### Synopsis

The bright appearance of long-T2 tissues in DWI, termed “T2-shinethrough”, reduces the contrast between healthy tissue and cancer and is prominent in spin-echo based DWI acquisitions. In prostate DWI, the need to avoid T2-shinethrough has led to the acquisition of very high b-values in clinical practice, which may result in low SNR and other image artifacts. In this work, we have assessed the ability of stimulated-echo DWI to provide high contrast between PCa and healthy peripheral zone, without the need for high b-values. Preliminary results in 19 patients show reduced T2-shinethrough effects in stimulated-echo DWI compared with spin-echo DWI.

### Introduction

Diffusion MRI is widely used for the assessment of prostate cancer (PCa). Diffusion MRI is typically acquired with spin echo (SE) based diffusion-weighted (DW) sequences. However, T2-shinethrough remains a problem in SE-DWI due to the long echo times needed for diffusion encoding1. In these sequences, long T2 tissues (eg: the healthy peripheral zone) appear bright on DW images due to T2-shinethrough. This has led to the acquisition of very high b-value SE-DWI (b=1500-2000s/mm2) in research and clinical practice1-4. However, very high b-value acquisitions result in low SNR and may introduce other image artifacts3-4. In contrast, STimulated Echo (STE) DWI sequences5 enables the acquisition of diffusion MRI with short TEs (ie: reducing T2-shinethrough). In this project, we assessed the ability of STE-DWI to provide high contrast between PCa and surrounding tissues, without the need for very high b-values.

### Methods

Simulation: Both diffusion-weighted SE and STE signals were simulated in Matlab with T2=80ms for PCa and T2=130ms for healthy peripheral zone (PZ). The ADC values for PCa and healthy PZ were 1.00mm2/s and 1.50mm2/s, respectively.6-7 Other relevant parameters were Gmax = 50mT/m, SRmax = 100T/m/s, T_90=5.3ms, T_180=6ms and readout-to-echo time=11ms. Different maximum b-values were simulated with TE minimized for each b-value. Different mixing times (TM) were also used for STE sequences. Ignoring T1 relaxation effects (T1 times are generally larger than one second in the prostate8), SE and STE signals are calculated by $$S_{SE}=Ae^{-\frac{TE}{T2}}e^{-bADC}$$ and $$S_{STE}=\frac{1}{2}Ae^{-\frac{TE}{T2}}e^{-bADC}$$.

Signal-to-Noise Ratio (SNR) of the healthy PZ and Contrast-to-Noise Ratio (CNR) between PCa and healthy PZ were calculated for each simulated DWI signal.

### Results

Fig.1(a-b) illustrates the change of TE over b-values and the signal decay curves for both SE- and STE-DWI in the simulation. The double-head arrows with the same length in Fig.1(b) point to signal difference between PCa and healthy PZ tissues, indicating the same contrast can be expected with STE b=800s/mm2 as with SE b=1500s/mm2. Fig.1(c-d) presents the comparison of SNR and CNR between SE and STE sequences, where STE can reach similar CNR at around b=800s/mm2 as SE b=1500s/mm2, but STE provides higher SNR. Fig.1(e) shows that the CNR of STE-DWI is increasing over TM in tissues with long T1.

An example patient case is shown in Fig.2 with prostate cancer in the PZ. Preliminary results from the reader study in Table 2 did not show significant difference between different sequences except for SNR. Figure 3 shows the ROI measurements of SNR, CNR and ADC. The SNR of STE b=800s/mm2 is significantly higher than SE b=1500s/mm2 while the CNR is significantly higher than both SE b=800s/mm2 and 1500s/mm2 (although the averaged difference in CNR between STE b=800s/mm2 and SE 1500s/mm2 is small). As expected, the ADC of PCa is significantly lower than healthy PZ in all three sequences.

### Discussion

In this study, we have characterized the performance of STE-DWI for assessment of PCa, compared with standard SE methods. From simulations and patient scans, STE-DWI is able to provide high contrast between healthy PZ and cancer without the need for a very high b-value. The disagreement between the reader assessment of image quality and the quantitative evaluation of SNR and CNR is unclear in this preliminary study. Future work will include optimization of mixing time and b-values in STE-DWI, as well as a larger biopsy correlation study to evaluate the performance of STE-DWI compared to SE-DWI.

### Conclusion

In conclusion, STE-DWI is able to reduce the T2-shinethrough effect in prostate DWI at moderate b-values.

### Acknowledgements

The authors would like to acknowledge research support from GE Healthcare. The authors acknowledge grant support from NIH-NIDDK Wisconsin Multidisciplinary K12 Urologic Research Career Development Program K12DK100022 (PD: D. Bjorling).

### References

[1] Tamada, T., Sone, T., Jo, Y., Yamamoto, A. and Ito, K., 2014. Diffusion‐weighted MRI and its role in prostate cancer. NMR in Biomedicine, 27(1), pp.25-38.

[2] Tamada, T., Kanomata, N., Sone, T., Jo, Y., Miyaji, Y., Higashi, H., Yamamoto, A. and Ito, K., 2014. High b value (2,000 s/mm2) diffusion-weighted magnetic resonance imaging in prostate cancer at 3 Tesla: comparison with 1,000 s/mm2 for tumor conspicuity and discrimination of aggressiveness. PLoS One, 9(5), p.e96619.

[3] Metens, T., Miranda, D., Absil, J. and Matos, C., 2012. What is the optimal b value in diffusion-weighted MR imaging to depict prostate cancer at 3T?. European radiology, 22(3), pp.703-709.

[4] Kitajima, K., Kaji, Y., Kuroda, K. and Sugimura, K., 2008. High b-value diffusion-weighted imaging in normal and malignant peripheral zone tissue of the prostate: effect of signal-to-noise ratio. Magnetic Resonance in Medical Sciences, 7(2), pp.93-99.

[5] Bammer, R., 2003. Basic principles of diffusion-weighted imaging. European journal of radiology, 45(3), pp.169-184.

[6] Langer DL, van der Kwast TH, Evans AJ, Trachtenberg J, Wilson BC, Haider MA. Prostate cancer detection with multi‐parametric MRI: Logistic regression analysis of quantitative T2, diffusion‐weighted imaging, and dynamic contrast‐enhanced MRI. J Magn Reson Imag 2009; 30(2):327-334.

[7] Gibbs P, Tozer DJ, Liney GP, Turnbull LW. Comparison of quantitative T2 mapping and diffusion‐weighted imaging in the normal and pathologic prostate. Magn Reson Med 2001;46(6):1054-1058.

[8] De Bazelaire CM, Duhamel GD, Rofsky NM, Alsop DC. MR imaging relaxation times of abdominal and pelvic tissues measured in vivo at 3.0 T: preliminary results. Radiology. 2004 Mar;230(3):652-9.

### Figures

Figure 1. Simulation of prostate DWI signals demonstrate the potential of STE-DWI to provide high CNR and SNR compared with the standard SE-DWI. (a) Required TE as a function of b-value for SE- and STE-DWI. (b) Example of signal behavior with T2-shinethrough effect for SE and STE DWI. (c) SNR and (d) CNR as a function of b-value, illustrating the ability of moderate b-value STE-DWI to provide similar CNR and better SNR compared to high b-value SE-DWI. (e) Simulated CNR of STE-DWI compared to SE-DWI over different b-values and TMs, where the red rectangle specifies SE-DWI

Table 1. Imaging parameters

Figure 2. A 67-year-old patient with PIRADS 5 prostate cancer in the left peripheral zone (white arrows) was acquired at the GE 750 3T scanner. Significant T2-shinethrough effect between healthy PZ and PCa can be seen in the SE b=800s/mm2 DWI. Both SE b=1500s/mm2 and STE b=800s/mm2 DWI provide improved contrast, clearly showing the lesion.

Table 2. Reading scores from the reader study and the results of Wilcoxon Rank test between sequence pairs.

Figure 3. ROI measurements of SNR (a), CNR (b) and ADC (c) of the three sequences. The SNR of STE b=800s/mm2 is significantly higher than SE b=1500s/mm2 while the CNR is significantly higher than both SE b=800s/mm2 and 1500s/mm2. All three sequences show lower ADC in the PCa than healthy PZ, but SE b=1500s/mm2 and STE b=800s/mm2 series tend to have lower ADC measurements in the same regions compared to SE b=800s/mm2, possibly due to restricted diffusion effects.

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