FOV Modification for Image Reconstruction

[Mencu]

Hi,
First of all, thank you for KomaMRI, it's an amazing tool!
I was trying to do some custom simulations with Koma on the EPI_SE sequence with different custom phantoms. After the simulations, I also wanted to look at some reconstructions. As my phantoms are fairly small (20x20 spins), the resulting image reconstruction is fairly zoomed out. I figured it's only a problem of FOV, but I do not understand where to change the FOV for this reconstruction. Where should I change it? In the raw signal params (reconSize)? Does that suffice? Could you please point me to the object attribute I need to change? I am new to Julia.

I have not changed any acquisition points in the resulting signal, the kspace is also the same. Thank you for the help!

Best,
Andrei

[gsahonero]

Hi,

I think you should just crop the image. In a way, changing the FOV requires changing the sequence.

Best,
Guillermo

[cncastillo]

Hi sorry for the delay, I was at ISMRM in Singapore. Can you give a little more detail to reproduce the problem?

In theory, as @gsahonero says, the FOV is encoded in the sequence. One way of scaling the FOV of the sequence is to multiply by a diagonal matrix:

julia> using KomaMRI

julia> seq = PulseDesigner.EPI_example()
Sequence[ τ = 62.846 ms | blocks: 204 | ADC: 101 | GR: 205 | RF: 1 | DEF: 5 ]

julia> seq.GR[:, 3]
3-element Vector{Grad}:
 Grad(51.0578 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms) # <------------------ BEFORE SCALING
 Grad(0.0 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms)
 Grad(0.0 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms)

julia> S = [2.0 0.0 0.0; 0.0 2.0 0.0; 0.0 0.0 1.0] 
3×3 Matrix{Float64}:
 2.0  0.0  0.0
 0.0  2.0  0.0
 0.0  0.0  1.0

julia> seq2 = S * seq # <---------------------------------------------- APPLY LINEAR OPERATOR
Sequence[ τ = 62.846 ms | blocks: 204 | ADC: 101 | GR: 205 | RF: 1 | DEF: 5 ]

julia> seq2.GR[:, 3]
3-element Vector{Grad}:
 Grad(102.1157 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms) # <------------------ AFTER SCALING
 Grad(0.0 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms)
 Grad(0.0 mT, 0.2 ms, ↑0.1021 ms, ↓0.1021 ms)