Link between Image Position & Image Orientation Patient tags

[supraja]

Hi,

I work with DICOM CT data and I am confused about the various image
orientations.

1. How are the Patient Position (0018,5100) & Image Orientation
Patient (0020,0037) tags related:

Would it be right to say the following -

HFS : ( -1,0,0,0,1,0)

HFP : (-1,0,0,0,-1,0)

FFS: (1,0,0,0,1,0)

FFP: (1,0,0,0,-1,0)

I am a little confused since I have seen some datasets with Image
orientation (1,0,0,0,1,0) having Patient position value as FFS while
some others having a value HFS.

2. Does an FFS orientation mean that the patient is lying with his
feet towards the gantry?

3. I have read that Patient Position follows the equipment based co-
ordinate system while Image Orientation follows Patient-based co-
ordinate system. How are they different?

I work with 3D volumes so if I need to handle any processing of these
datasets in various orientations, would it suffice to take care of the
Image orientation tag and ignore the Patient position tag?

Please help.

Thanks,
Supraja

[luigipb]

1. Patient Position (0018,5100) and Image Orientation (Patient)
(0020,0037) are not related together: the Image Orientation Patient is
related to the patient body, regardless how she's placed in the
machine, while Patient Position specifies the position of the patient
relative to the imaging equipment (let me say relative to the gantry).

The machine manufacturer has to know the Patient Position (that is,
the way the patient has been put into the machine) to be able to
calculate the Image Orientation (Patient) from the image orientation
cosines in the frame of reference of the machine.

After that, you do not have to take care anymore of the Patient
Position, it is just an annotation for understanding the way the exam
was done, not to find the orientation of the image you are looking at.

2. Yes.

3. See above.

Regards.

Luigi Pampana-Biancheri
mailto:luigi....@REMOVETHIS.esaote.com