[vtkusers] Hounsfield Units : strange result (Volume rendering)

[Kevin Osborn]

By definition the Hounsfield value of air is -1000. Water has a value of 
0 HU and all other CT densities (or strictly speaking electron 
densities) represent a linear scale around these two values. In practice 
the densities don't fit into a narrow Hounsfield range due to variations 
in calibration of the xray detectors, noise within CT systems and so 
called partial volume artefact where some image voxels contain both air 
and soft tissue and therefore have a density intermediate between air 
(-1000 HU) and soft tissue (approx 40 HU). To set an appropriate range 
to render lung, you will always therefore render the air/skin interface 
(and also stomach and bowel gas if the scan includes the upper abdomen). 
As Frederic Perez said, a region growing algorithm is your best bet.

Regards, Kevin


wassim_belhadj at topnet.tn wrote:
> Hi,
>
> Thank you for your reply. 
>
> According to Hounsfield Units, the pixel values that form the air is
> between -1000 <--------> -995.
>
> I want to display ONLY the lungs (not skin ...) so I set my function
> transfer like this:
>
> opacityFun->AddPoint(-3024,      0.0);
> opacityFun->AddPoint(-878.674,   0.0);
> opacityFun->AddPoint(-864.466,   1.0);
> opacityFun->AddPoint(-594.524,   1.0);
> opacityFun->AddPoint(-395.62,    1.0);
> opacityFun->AddPoint(3071,       0.0);
>
> Hounsfield Units for lungs are   -950 <--------> -550 
>
> I Attached a NEW screenshot that illustrates the problem.
>
> I vary the values of the transfer function but no result.I did not manage
> to remove the part that surrounds the lungs.
>
> Best regards,
> BELHADJ wassim
>
>
>
>
>
> On Tue, 17 Feb 2009 18:06:18 +1100, Kevin Osborn <planck at netspeed.com.au>
> wrote:
>   
>> Don't forget air surrounds the patient. What your image shows is the 
>> air/skin interface, not the skin itself. As the lung is mostly air, your 
>> Hounsfield range between -600 and -400 not only gives lung but also 
>> includes air around the patient.
>>
>>
>> wassim_belhadj at topnet.tn wrote:
>>