ESTRO 2023 - Abstract Book

S1390

Digital Posters

ESTRO 2023

Diffusion sequences were acquired using a uniform spherical (SP) filled with aqueous nickel chloride hexahydrate solution and another specifically realised for diffusion measurements (DWI phantom) with four inserts with certified ADC values. Images were acquired on both phantoms by varying number of averages, b-values and slice thickness while keeping the others unchanged. The relative ADC maps (4 maps for each acquisition considering 3 gradient direction and the average of the latter) were then calculated using a single-exponential model. On these maps, mean values of ADC, SNR and uniformity were evaluated to find the best combination of parameters considering the acquisition time. In addition, a repeatability test was performed, acquiring two identical sequences under the same temperature and humidity conditions 24 hours apart. Results Results obtained for the SP are shown in Fig1. SNR (panels 'a' and 'd') seems to grow proportionally to slice thickness and averages, although in the latter case the growth seems to stop after 10 averages. The same can be deduced for the uniformity trend (panels "b" and "e"). ADC value this seems to be strongly underestimated with respect to the reference value (1600-2000 10^-6 mm^2/s - Italian survey on 1.5-3.0 T scanners) until the 6 mm value is reached (panel "c"). The ADC value appears to be unaffected by the number of averages used in the sequence (panel “f”). Acquisition times as a function of the number of averages vary between 26 s (1 av.) and 390 s (15 av.) while remain unchanged for slice thicknesses. Results for the DWI phantom are summarised in Fig2. The ADC values for the 4 inserts do not seem to depend on either the number of averages (panel “a”) or the number of b-values (panel “b”). For the repeatability the result is that the difference in ADC values between the two measurements is compatible with the background within the experimental errors. The average of the ADC values obtained for each of the 4 inserts results to be (mean (standard dev.) 10^-6 mm^2/s):

insert 1: 1007(90), insert 2: 719(75), insert 3: 302(100), insert 4: 178(65)

to compare with 1900, 1450, 1000 and 500 10^-6 mm^2/s respectively.