[Insight-users] Question about DICOM and CT scanner

[Mark Rabotnikov]

Hi Remi,

CT values are measured in HU and can be negative (water is 0, so air is 
negative). When stored in DICOM format, these values are encoded to 
another pixel format (usually unsigned integers), called "stored values", 
by applying a linear transformation. Each CT DICOM image specifies the 
transformation from the stored values to HU by providing Rescale Slope 
(tag 0028,1053) and Rescale Intercept (tag 0028,1052):

HU = m*SV+b

Here m is the slope and b is the intercept (SV stands hare for a stored 
value).

Now when you have the values in HU, you need to display them. For this 
purpose window center and window width tags are used.
Window Center (0028,1050) and Window Width (0028,1051) specify a linear 
ramp that converts from HU values to the grey levels to be displayed. 
Usually the user will adjust these values interactively.

Generally, there are few more tags specifying different pixel formats in 
DICOM (signed/unsigned integer, number of relevant bits per pixel, where 
the relevant bits are located, etc.), but in the common case the situation 
is simple.

Hope this helped,

Mark Rabotnikov
EBW team
CT Engineering
Philips Medical Systems Technologies LTD
Phone: +972-4-8310646









Remi Vieux <revieux at yahoo.fr> 
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insight-users-bounces+mark.rabotnikov=philips.com at itk.org
13/03/2006 06:07

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Subject
[Insight-users] Question about DICOM and CT scanner
Classification







Hi everyone,

My question is not about ITK but I thought somebody here could help me
with that: it's about CT scanner and DICOM image file format.

So, basically, what I learned during my lectures on medical imaging is
that a CT scan acquire its data, in Hounsfield Units, which may vary
depending on the actual scanner you are using. Then, depending on what
contrast you want to emphasize, you can choose a combination of window
width/window level, and save this view into a DICOM file, hence mapping
the H.U. into grey levels from 0 to 4095.

Now, I'm doing a project for my university dealing with CT images in
DICOM format. First thing I realised in the exams I have in my hand is
that the data I'm retrieving are not grey levels from 0 to 4095 but from
-1000 to 3095. Ok, not much harm done there, it is still 4096 grey
levels. But then, answering to a previous question of mine in this
mailing list, I've been told that I could not assume that it is always
the case (or so I understood).

So my question is: what exactly is saved in a DICOM file during a CT
scan exam? Is it the 4096 grey levels representing a "windowed" view of
the exam as I was teached? Is it the H.U. as I thought it could be for a
moment? Is it both? None? Whatever..?

That was quite a long speech for a short question, but, pardon me, I'm
kind of lost here... If anybody could help me out, answering it or
simply pointing me some reference in the litterature, I would deeply
appreciate it!

Best Regards,

Remi.
-- 
Remi Vieux <revieux at yahoo.fr>


PS: By the way, if somebody knows a good algorithm to perform a
segmentation of the teeth in a CT exam, it would also be appreciated,
but that's just because I'm stuck :-p


 

 
 
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