A 4DCT phantom
was created with MATLAB®, the phantom consists of 50 slices and 10
phases (512x512 slices with 1x1x2 mm3 voxel size).
The contour of
a synthetic 4D organ was designed within the phantom according to the following
2D polar equation
Rθ=a+b(sin(θ) - cisin(θ)cos(θ) + dicos(θ)) Eq.1
Where [6≤a≤8, -1≤b≤1.5, -3≤ci≤1,
1≤di≤2.5, i=slice number]
The contour was deformed through individual slices
and phases by applying different values to the a, b, c and d
coefficients of Eq.1, the values were chosen to achieve a smooth deformation
between consecutive phases and slices like human organs. The intensity values
within the contour were assigned to five discrete gradual values (1000 - 3000
HU) following the same polar equation and coefficients (see Fig.1).
![](https://www.estro.org:443/ESTRO/media/Abstracts/267/233195e9-da38-4f14-bb63-6bf6ce4197da.jpeg)
Since coefficients (a) and (b)
are constants across phases, the radius Rθ of Eq.1 at any fixed
angle (θ) depends linearly on the coefficients (a,b,ci,di). Thus, the ground-truth Mid-P
contour in a slice is generated by using the mean coefficients of the 10
phases (a, b, mean(ci), mean(di))
in Eq.1. By repeating this to all slices we can create a time-weighted average
contour of the organ.
The intensity
values within the Mid-P contour were assigned similarly to the phantom. The
created phantom was used to test a Mid-P automated research prototype
(developed by Mirada Medical Ltd). The constructed Mid-P CT by the prototype
was compared to the ground-truth Mid-P CT using root mean-squared error (RMSE)
and the organ’s contour of the constructed Mid-P was compared to the
ground-truth contour using Dice similarity coefficient (DSC) and Hausdorff
distance.