The impacts of time of echo and time of repetition on slow diffusion coefficient (SDC) measure and the potential age- and gender-related variations in liver and spleen SDC values: studies of healthy volunteers.

BACKGROUND

The time of echo (TE) and time of repetition (TR) impact the apparent diffusion coefficient (ADC) value. This study investigates the impact of TE and TR on slow diffusion coefficient (SDC) values of the liver and spleen. An age-related decline of liver ADC has been described for both men and women. However, a lower perfusion level, a higher fat level and a higher iron level can all lead to a decrease in liver ADC value in older populations. This study also investigates whether there is an age-related decrease in the liver slow diffusion as measured by SDC.

METHODS

For dataset 1, liver and spleen diffusion weighted imaging (DWI) data was acquired at 3.0 T with six young healthy volunteers totaling 23 scans, and b-values included 400 and 600 s/mm2. When TE changed with steps of TE =46, 52, 60, 70, 80 ms, TR was fixed at 2,000 ms. When TR changed with steps of TR =200, 300, 500, 1,000, 2,000 ms, TR was fixed at 46 ms. For dataset 2, liver and spleen DWI data acquisition was acquired at 1.5 T with healthy volunteers (TR: 1,600 ms, TE: 63 ms), and b-values included 400, 600 s/mm2. Dataset 2 had 68 men (mean age: 50.2 years, range: 25-70 years) and 43 women (mean age: 45.6 years, range: 20-71 years).

RESULTS

Dataset 1 showed, following the increase in TE from 46 to 80 ms, SDC decreased by 63.9% (median) for the liver and by 31.0% (median) for the spleen, and SDCspleen/SDCliver ratio increased by 150.3% (median). Following the increase in TR from 200 to 2,000 ms, SDC increased by 478% (median) for the liver, and there was a biphasic change in spleen SDC value with SDC value being highest when TR was 1,000 ms, SDCspleen/SDCliver ratio remained unchanged across TR values. Dataset 2 showed, there was no age-related change in SDC value for men's liver, for men's spleen, and for the livers of women aged <54 years (n=27). There was an age-related decrease in SDC value for women's spleen, and for the livers of women aged ≥54 years (n=16). The liver median SDC values were 'women aged <54 years' > 'women >54 years' > 'men'. The spleen median SDC values were also 'women aged <54 years' > 'women >54 years' > 'men'. For all comparisons, spleen SDC values were higher than liver SDC values.

CONCLUSIONS

The TE and TR have very notable impacts on SDC values. There may be no true tissue diffusion change following aging for men and women. The difference in liver SDC between pre-menopausal women value and post-menopausal women value suggest that iron-related T2* shortening lead to a lower SDC measure. SDC may be insensitive to tissue perfusion and liver fat signal.