Seems like I have missed out something critical too. There is also this Patient Position (0018, 5100) tag that we have to factor in as well. The axes of the patient based coordinate system varies at different patient positions. The following images should illustrate this well enough:
Fig. 1 - Head First-Supine (HFS)
Fig. 2 - Feet First-Supine (FFS)
Fig. 3 - Head First-Prone (HFP)
Fig. 4 - Feet First-Prone (FFP)
Fig. 5 - Imaging Equipment
The figures above can be found from here as well, and there are also some comprehensive explanations:
http://xmedcon.sourceforge.net/Docs/OrientationXMedConPatientBased
When facing the front of the imaging equipment (refer to Fig. 5),
- Head First is defined as the patient’s head being positioned toward the front of the imaging equipment.
- Feet First is defined as the patient’s feet being positioned toward the front of the imaging equipment.
- Prone is defined as the patient’s face being positioned in a downward (gravity) direction.
- Supine is defined as the patient’s face being in an upward direction.
- Decubitus Right is defined as the patient’s right side being in a downward direction.
- Decubitus Left is defined as the patient’s left side being in a downward direction.
Here are the following terms that can be found in the Patient Position (0018, 5100) tag:
- HFP (Head First-Prone)
- HFS (Head First-Supine)
- HFDR (Head First-Decubitus Right)
- HFDL (Head First-Decubitus Left)
- FFP (Feet First-Prone)
- FFS (Feet First-Supine)
- FFDR (Feet First-Decubitus Right)
- FFDL (Feet First-Decubitus Left)
After a short discussion with Zac yesterday, I decided to look a little more into the patient-based coordinate system, in particular, to find out where the origin is. Since the position and orientation of the dicom images are provided to us with respect to the patient-based coordinate system, we would have to know where the origin is in order to map it into our models in our application. Looked around for quite some time but I cannot seem to find any sources that talks about the origin of the patient-based coordinate system.
Zac and I then had a short discussion again, and somehow we realized that we do not have a need to know the origin of the patient-based coordinate system at all. We were only loading the specific object models, not the whole human body. Hence, we would only need to know where the images should face, and at which orientation using the following 3 tags in the dicom images:
- Patient Position
- Image Position
- Image Orientation
The newly renamed images now look like this:
IM-0001-0001_x-74.798828125y-268.798828125z-82.5r100c010pFFS.png
In the case above,
x = -74.798828125
y = -268.798828125
z = -82.5
r (row vector) = (1, 0, 0)
c (column vector) = (0, 1, 0)
p (Patient Position) = FFS (Feet First-Supine)
Finally, it is time for me to move on back to work on the pan gesture to translate feature.
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