Is it possible to retrieve beam real world position from RT PLAN

Question

I am currently working on some radiotherapy plan generation and I am trying to retrieve the beam source position from a DICOM RTPLAN file and point it on a related CT-Scan 3D image.

With the RTPLAN, I am able to access the isocenter position of each beam but this is in patient coordinates and I am note quite sure how to find the coordinates in the basis that is used by the 3D CT-Scan image.

I have access to the attributes ImagePosition and ImageOrientation of the DICOM of the CT. Moreover, the CT DICOM-like file (it is in practice a json regrouping some DICOM information) and the RTPLAN share the same FrameOfReference (Does it mean that they share the Patient coordinate system ?).

What does ImagePosition truely indicates ? As well as I can understand I think this is the position of the point (0, 0, 0) of the CT-3DImage in the Patient Coordinates. I am also a bit confused about the ImageOrientation attribute.

Answer 1

As you can read in this answer here, the ImagePosition attribute gives you the x, y, and z coordinates of the upper left hand corner of the image, in mm, i.e. the coordinates of the center of the upper left pixel of the image.

For your convenience I copy-paste below a table from the DICOM Documentation Part 3 (page 561).

Regarding the ImageOrientation attribute, as described in the documentation, gives you the direction cosines of the first row and the first column with respect​ to the patient. To understand better this attribute take a look at the very useful website, DICOM is Easy, by Roni Zaharia. In one of his images (below), you can clearly see that when the attribute is not equal to 1\0\0\0\1\0, then the coordinate system of the image is not align with the coordinate system of the patient. To align them, you have to use the direction cosines provided by the attribute and apply a rotation (take a look at the transformation matrix at page 562 of the aforementioned DICOM documentation).