Introduction

DWI and ADC maps are key components of multiparametric MRI (mp-MRI) in the prostate^{1, 2}. Recently, it has been shown that T1 may help to differentiate prostate cancer (PCa) from normal prostate tissue (NPT)^3-5. We hypothesized that this T1 difference between PCa and NPT can be exploited to improve diffusion contrast, and reported the usefulness of short TR DWI⁶, based on the specific feature that T1 of PCa is lower than that of NPT (T1 in lesions is typically longer than in normal tissue of other organs). It has been suggested that short TR (of 1850ms) DWI in prostate has the potential to replace conventional (long TR) DWI⁶. DWI with shorter TR of less than 1850ms enables to improve contrast further, however it suffers from reduced SNR, which results in decreased accuracy of ADC. Therefore, it might be difficult to replace conventional DWI with DWI using shorter TR of less than 1850ms. We explored the possibility of generating an additional DWI with shorter TR (e.g. 1000ms) based on DWIs with two different TR (e.g. conventional [long] and additional short TR such as 6000 and 2000ms). We refer to this as synthetic DWI (synDWI). ADC can be calculated from DWI with conventional TR (6000ms). The goal of this study is to investigate clinical feasibility of synDWI for PCa.

Methods

The study was approved by the IRB, and written informed consent was obtained from all subjects. Three patients underwent mp-MRI on an Ingenia Elition 3.0T with an anterior coil and posterior built-in coils (Philips Healthcare, Best, The Netherlands). DWI was scanned with TR of 6000ms (dynamic1) and 2000ms (dynamic2) in one single scan with the following parameters: TE 70ms, FOV 300mm, acquisition voxel size 3.1 x 3.1 x 3.0 mm³, the number of packages 1, the number of slices 20, b-values 0-1000 s/mm², NSA 4(b=0) 10(b=1000). synDWI can be calculated on a pixel-by-pixel basis from DWIs with two different TRs, following the procedure below. The signal intensity (SI) of spin echo type DWI is given by
SI = cte PD×(1-e^-TR/T1)×e^-TE/T2×e^-bD (1)
The T1 value can be determined dividing equations 2 and 3, keeping TE and b constant:
SI in DWI with long TR = cte PD×(1-e^{-long TR/T1})×e^-TE/T2×e^-bD (2)
SI in DWI with short TR = cte PD×(1-e^{-short TR/T1})×e^-TE/T2×e^-bD (3)
Next, the [cte PD × T2 term × diffusion term] factor can be calculated using the SI, rewriting e.g. Eq. 2 as
cte PD×e^-TE/T2×e^-bD = SI in DWI with long TR / (1-e^{-long TR/T1}) (4)
Finally, SI in DWI with any TR can be calculated according to Eq.1. This procedure was implemented in Python (Python 3.8). synDWI at b = 1000 s/mm² (synDWI₁₀₀₀) with TR of 500, 1000, 2000, and 4000ms were generated, and contrast ratio (CR) in synDWI₁₀₀₀ with TR of 500, 1000, 2000, and 4000ms were compared in each patient. Moreover, CR and visual scoring were compared between synDWI₁₀₀₀ with TR of 1000ms and DWI₁₀₀₀ with TR of 6000ms.
CR = (PCa_ave-NPT_ave) / (PCa_ave+NPT_ave)
where PCa_ave and NPT_ave is the average signal in cancerous and normal tissue respectively.
Visual evaluation for comparison of contrast between PCa and NPT was conducted by a radiologist with twenty-four years of experience in MRI. It was graded using a 3-pont scale, where 1 = weak; 2= moderate; 3 = strong contrast.

Results and Discussion

CR in synDWI₁₀₀₀ had a tendency to show higher value in shorter TR (Figure 1). Figure 2 and 3 show representative clinical cases with PCa. synDWI₁₀₀₀ with shorter TR provided better contrast compared to DWI₁₀₀₀ with long TR. CR in synDWI₁₀₀₀ with TR of 1000ms was higher than that of DWI₁₀₀₀ with TR of 6000ms in all three patients (Figure 4a). Visual scoring in synDWI₁₀₀₀ with TR of 1000ms was equal to or greater than that of DWI₁₀₀₀ with TR of 6000ms (Figure 4b). These results indicated that shorter TR had improved clinical value of DWI, probably thanks to shorter T1 value in PCa compared to NPT. Further clinical investigation is ongoing.

Conclusion

Our preliminary results show that synDWI₁₀₀₀ with shorter TR of 1000ms had a tendency to have better diffusion contrast compared to DWI₁₀₀₀ with long TR of 6000ms. Further clinical investigations are needed to assess if synDWI with shorter TR can provide better diagnostic performance than DWI with long TR.

Acknowledgements

No acknowledgement found.