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Convert Point Spread Function translate to MTF

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Zak Ali
Zak Ali on 22 Mar 2023
Answered: Drishti on 7 Oct 2024 at 9:41
Hi
I have a guassian point spread function and i would like to find the equivalent MTF in frequency domain.
I have the following code with assistance from : https://www.mathworks.com/matlabcentral/answers/274848-point-spread-function-translate-to-mtf
x=[1,2,3,4,5,6,7,8,9,10,11,12,13]
y=[0,1,3,21,46,156,222,140,111,25,14,3,2]
LSF=[x,y];
OTF = fftshift(fft(LSF)); % OTF
MTF = abs(OTF); % absolute value of OTF
MTF = MTF./max(MTF); % normalize
% correct sampling frequency for conversion on frequency bins to frequency
Size = length(MTF);
spacing = 1.12e-6;% pixel size
fsx = abs(1/spacing); % turn into sampling frequency
a = linspace(-Size/2,Size/2,Size); % form scale for conversion based on frequency bins
conversionx = fsx./Size; % conversion factor for frequency bin units to frequency (unit^-1)
Psi = a.*conversionx; % frequency (unit^-1)
figure
plot(Psi,MTF)
xlim([0,Psi(end)])
xlabel('cycles/unit')
ylabel('MTF')
could someone please help me convert the X axis from cycles/unit to lp/mm , i am not sure how this conversion is peformed

Answers (1)

Drishti
Drishti on 7 Oct 2024 at 9:41
Hi Zak,
To convert the x-axis from cycles/unit to lp/mm (line pairs/millimetre), you need to first convert the spacing from metres to millimetres.
By multiplying the spatial frequency (Psi) by ‘spacing_mm’, you will obtain the x-axis values in lp/mm.
Refer to the code snippet provided below for better understanding:
% convert spacing from meters to millimeters
spacing_mm = spacing * 1e3;
% convert to lp/mm
Psi_lpmm = Psi * spacing_mm;
I hope this resolves the query.

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