On Jul 16, 1:37=A0am, Mauro <mauro.ita...@[EMAIL PROTECTED]
> wrote:
> Hi all,
> I would like to understand better the idea of frequency (in the
> spatial domain) of an image in relation with an imaged object. Is the
> frequency measured in pixels/cm or cm/pixels?
> Mainly becuase I would like to see whether the Nyquist frequency is
> satisfied over an imaged object in a 3d volume (CT).
>
> thanks,
> Mauro
Mauro,
There can be several ways of estimating the frequency of a signal.
Take a simple case of a 1 dimensional signal, a finite ****tion of
which has been digitized (sampled along equal intervals using some
spread function). Also, lets assume you know that width of the finite
****tion in some spatial units, lets say cm.
In this case, then, the frequency of the signal as estimated in the
digitized spatial domain will be: (number of pixels in digitized
signal / length in cm of the signal ****tion).
Meaning, higher the number of pixels in the 1D image, the finer you
sampled over the original signal =3D> higher frequency.
To ensure your sampling method is using at least a Nyquist frequency
or higher, you will first need to know the frequency of the original
signal. Then you will nede to make sure you sample it enough times
(enough pixels) that will capture even the finest detail in the signal
that would occur in the smalles relative spatial region.
For a 3D CT case, I assume you are talking about resampling an already
digitized image data. Is that correct? If so, you will need to find
out the highest frequency of a feature that you dont want to lose in
the image after resampling. If the image is not isotropic (has
different frequency along three dimensions), you will need to repeat
this for all three directions and get the highest frequency of all for
simplicity. Then you will need to make sure the frequency of the
resampling kernel is higher than the Nyquist rate, given the highest
frequency.
Hope this helps.
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