how to calculate power of fraction without getting complex double ?

Hi I am trying to make a map (Q). Everything goes well except for the section that delatR2_st = deltaR2_star .^(2/3); where this command creates a double complex matrix, and I dont want it to be complex. And this creates an error for the last command where I try to create the map Qmap(xsize_se,ysize_se) = deltaR2./deltaR2_st ; The errors I get are : Undefined function or variable 'deltaR2_st'.

Error in MVD (line 73) Qmap(xsize_se,ysize_se) = deltaR2./deltaR2_st ;

The code is attached bellow. Can you please help me out what to do.

SE = analyze75read('10_ep2d_ge_se_128_DCE_slice8.hdr'); GE = analyze75read('10_ep2d_ge_128_DCE_slice8.hdr'); TE = 120; % echo time 120 ms [xsize_se, ysize_se, tsize_se] = size(SE); [xsize_ge, ysize_ge, tsize_ge] = size(GE);

Qmap = zeros(xsize_se, ysize_se); IM_Slice = zeros(xsize_se, ysize_se);

for x = 1:xsize_se for y = 1:ysize_se

        IM_signal_SE = squeeze(SE(x,y,:));
        IM_signal_SE_baseline = squeeze(SE(x,y,4:15));
        Spre = mean(IM_signal_SE_baseline); 
        Spre = double(Spre);
        Spost = IM_signal_SE ; % post contrast signal SE
        Spost = double (Spost);
        deltaR2 = 1/TE .* log(Spre./Spost);

    end
end 
for n = 1:xsize_ge 
    for m = 1:ysize_ge

        IM_signal_GE = squeeze(GE(n,m,:));
        IM_signal_GE_baseline = squeeze(GE(n,m,4:16));
        Spre_star = mean(IM_signal_GE_baseline); 
        Spost_star = IM_signal_GE ;  % post contrast signal GE
        Spre_star = double (Spre_star);
        Spost_star = double (Spost_star);
        deltaR2_star = 1/TE .* log(Spre_star./Spost_star);

    end
end
delatR2_st = deltaR2_star .^(2/3); 
Qmap(xsize_se,ysize_se) = deltaR2./deltaR2_st ;

[dpb]

Well, I'm not sure what you're after, but as soon as you take log(anything) where anything<1 then log() will be negative and the 2/3rds root of a negative value is complex.

You need some other kind of scaling to keep deltaR2_star>0; what that should be I've no klew...

Answer 1

[James Tursa]

Not sure what the overall calculation is doing, but if you will allow negative inputs, then maybe this formulation is what you can use:

delatR2_st = nthroot(deltaR2_star,3).^2;

[Hoda]

Great, this worked, no complex values. But Im still getting an error when i try to run the last command Qmap(xsize_se,ysize_se) = deltaR2./deltaR2_st ; (the commands are according to the theory of making a specific map, called the Qmap, deltaR2 is the intensity of one image and deltaR2star is the intensity of another image. Their ration gives us the Qmap) Thanks alot.

the error I get is the following: Undefined function or variable 'deltaR2_st'.

Error in MVD (line 73) Qmap(xsize_se,ysize_se) = deltaR2./deltaR2_st ;