Background

Single-shot echo-planar imaging (EPI) is widely used in diffusion-weighted imaging (DWI); however, switching the diffusion gradient on and off to induce the currents (eddy currents) of the magnetic field that is changing with time, resulting in a non-uniform magnetic field [1] that causes artifacts, which affect the apparent diffusion coefficient (ADC) [1]. Hence, DWI with turbo spin echo (TSE), which is superior to magnetic field inhomogeneity, is used as an alternative when artifacts appear with EPI [2]. There were no significant differences between EPI and TSE in terms of the calculated ADC [3]. On the other hand, ADC has been reported to reduce with a decrease in signal-to-noise ratio (SNR) [4, 5]. In other words, when quantitatively assessing the brain regions with ADCs, it is necessary to recognize the influence of the position. When set up from the lower cerebellum to the parietal region, as in a typical MRI scan of the brain, ADC values may vary depending on the slice position; however, the extent of this variation has not been clarified. If the ADC changes due to slicing, it may interfere with quantitative assessment and site-by-site comparison.

Purpose

The position dependence of the ADC in magnetic resonance imaging (MRI) by EPI- and TSE-DWI was assessed using phantoms based on the relationship between SNR and B₁ maps.

Methods

Five pure water-filled containers were placed parallel to the direction of the static magnetic field from the center of the magnetic field to the foot direction and a cross-section image was scanned at the center of each container using a 1.5-T MRI scanner. Five slice positions were set for each container (from the center of the static magnetic field and receiving coil "position A" to the direction of the foot "position E", the slice interval is appoximately 33.5 mm.). Diffusion times for both EPI- and FSE-DWI were matched. The slice thickness was adjusted to match the SNR at the center of the magnetic field for both sequences. A B₁ map was analyzed. Pearson correlation coefficients between ADC and SNR and between ADC and flip angle (FA) were calculated.

Results

ADC decreased with distance from the center of the magnetic field for both EPI-DWI and TSE-DWI. The Pearson correlation coefficient between ADC and SNR was 0.77 in EPI-DWI and 0.54 in TSE-DWI, there was no significant difference between ADC and SNR in EPI-DWI (P = 0.07) and TSE-DWI (P = 0.21), whereas that between ADC and FA was 0.99 in EPI-DWI and 0.96 in TSE-DWI, there were significant differences (P <0.01). The FA decreased by 0.047 degrees per mm from the center of the magnetic field to foot direction.

Discussion

The measured SNR in the present study was affected by the uniformity of the static magnetic field and the sensitivity of the receiving coil. The ADC is expected to be involved in the linearity of the static magnetic field or the gradient magnetic field. From these results, we considered that SNRs may be involved in ADC changes but not necessarily in a strong way.

Conclusion

ADC depends on the slice position and decreases with an increase in distance from the magnetic field center. Caution should be taken when comparing and quantitatively evaluating the ADC at sites shifted in the long-axis direction. The effect of B₁ inhomogeneity was more significant than SNR.

Acknowledgements

No acknowledgement found.

References

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