Frequency plot not working properly when using IFFT

I have impoted an ultrasound signal to matlab and I was to look at this signal in the frequency spectrum, but I am not sure if I am missing some information, but my plot doesn't work. What I have tried is as follows. My imported file contains 8600 x 1 arrays for my Data and Time vectors. Any input would be appreciated :)

FName = 'Tit_10MHz_110F.flxhst';

MyData = read_history(FName);

Time = MyData.TimeRecB.Time;
Data= MyData.DataRecB(1).Data;

fft_points = 2 ^ nextpow2(size(Time, 1));
time_step=(Time(2)-Time(1));
frequency_spectrum = fft(Time, fft_points);
frequency_spectrum = frequency_spectrum(1:end / 2, :);
frequency_step = 1 / (fft_points * time_step);
frequency = ([1 : fft_points / 2] - 1) * frequency_step;
figure(5)
plot(frequency, abs(frequency_spectrum));

Answer 1

[VBBV]

time_step=(Time(2)-Time(1));

Check with timestep. It may not be constant difference between first two values in Time vector

[David Harra]

Hey VBBV, I think this seems to be the case. It is not exactly constant. I am not to sure how to fix this issues. Any suggestions?