[Insight-users] Dicom Series Read Series Write: change in intensity values

[Steve M. Robbins]

On Wed, Nov 28, 2007 at 08:55:15AM +0100, Mathieu Malaterre wrote:
> On Nov 28, 2007 4:40 AM, Steve M. Robbins <steve at sumost.ca> wrote:
> > On Mon, Nov 26, 2007 at 11:22:25PM +0100, Mathieu Malaterre wrote:
> >
> > >  To go into further details, PET for instance has a dynamic
> > > slope/intersept.
> >
> > What do you mean by "dynamic slope/intercept"?  That the rescale
> > slope and intercept are specified per-image, even when the image
> > series forms a volume?  That's not limited to PT; CT has the
> > same behaviour, doesn't it?
> 
> Hum...
> I have never seen this happen with CT images (due to the nature of
> Houndsfield Units this is not typical to go beyond 16 bits well at
> least for human composition, in which case a single rescale
> slope/intercept should be enough).

True.  I asked the question not because I have seen it in practice,
but to make sure I understood what you and Bill were talking about.
Somewhere in that conversation it was mentioned that PET scales each
*pixel* individually, which made me wonder whether I understood DICOM
correctly.  (I think the comment about scaling by pixel was an error
and scaling by slice was intended.)


> Do you have by any chance a sample dataset, or do you recall the
> private vendor doing this ?

I don't recall any CT series in which rescale slope and intercept
change.

However, let me tell you about a series I did come across in the wild.
It was MR and it had different *Bits Stored* across the series: a few
images had 12 bits, the next few had 13, some more had 14.  I have no
idea why they needed to do this, but it did expose a bug in our code.
I can't recall for certain, but I don't think this was a volumetric
data set.

The moral of the story is only that a DICOM reader ought to be
prepared for almost anything and verify assumptions (such as "all
rescales and slopes are equal") on the actual input.

Regards,
-Steve

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