# [vtkusers] How to understand the slab thickness, slab resolution, spacing

David Gobbi david.gobbi at gmail.com
Wed Aug 2 11:26:59 EDT 2017

```Hi Zhang,

The links in David Cole's email get to the heart of the issue, so please
read them.  I think that we are using different definitions of "spacing"
and "thickness".  The reason I think this, is because of your statement "obtain
the image whose thickness is 0.4 from the image whose thickness is 0.8",
which makes no sense to me.  Let us start with my definitions of "spacing"
and "thickness":

The "spacing" is sample spacing, e.g. the distance from the center of one
slice to the center of the next slice.

The "thickness" refers to the collimated X-ray beam width in CT, or to the
selection slice thickness in 2D MRI.  Usually "thickness" is only
approximate, because the slice profile is not uniform.  Essentially, the
"slice profile" is the point-spread function in the slice direction, and
"slice thickness" is a measure of the width of the slice profile.

Do you agree with these definitions?

So if a CT is acquired with a slice thickness of 0.8 mm, it is impossible
to convert it to 0.4 mm.  that would break the laws of physics, so to
speak.  It is possible to increase the slice thickness by averaging slices
together or by applying other filtering operations, but decreasing the
thickness is not possible.

The operations that vtkImageReslice can do are: 1) increasing or decreasing
the spacing (by resampling with interpolation), and 2) increasing the
thickness (by averaging, or by maximum intensity projection).

- David

On Wed, Aug 2, 2017 at 8:48 AM, David Cole via vtkusers <vtkusers at vtk.org>
wrote:

> vtkImageData is a regularly spaced collection of *point* samples,
> there is no "thickness"...
>
> Please refer to the classic "A Pixel Is Not A Little Square" :
> http://alvyray.com/Memos/CG/Microsoft/6_pixel.pdf
>
> Also, a shorter summary here:
> http://graphics.cs.wisc.edu/WP/cs559-fall2014/2014/08/29/
> what-is-a-pixel-and-what-is-a-point-sample/
>
>
> HTH,
> David C.
>
>
>
> On Wed, Aug 2, 2017 at 10:39 AM, zhang qiang <zhang07101 at 126.com> wrote:
> > Dear David:
> >
> >      Thanks a lot. And the vtkImageData also do not record the thickness
> > information. I wonder why VTK ignore the thickness information? I think
> the
> > thickness information is quite important for DICOM image. My guess is
> that
> > we can not obtain the image whose thickness is 0.4 from the image whose
> > thickness is 0.8. So,  the thickness information is useless in
> vtkImageData?
> > Am I right?
> >
> > zhang qiang
>
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