DICOM Slice Ordering

The origin of the patient coordinate system is arbitrary, and selected by the imaging modality. I assume that the modality is free to choose a different origin for each series of images, but the origin is fixed for a given series.

If you wish to annotate images when displayed with notations such as (L) or (R) to indicate which side of the 2D image is considered the patient's left or right side, you need the Patient Position (0018, 5100) Attribute from the General Series module.

This attribute contains the position of the patient relative to the imaging equipment space.Valid values are:

1.Head First-Prone 
2.Head First-Supine
3.Head First-Decubitus Right
4.Head First-Decubitus Left
5.Feet First-Decubitus Left
6.Feet First-Decubitus Right
7.Feet First-Prone
8.Feet First-Supine. 

Definitions:

Head First means the patient was laying on the imaging couch with the head facing the imaging device first.

Feet first means the patient was laying on the imaging couch with the feet facing the imaging device first.

Prone means the patient is laying on his/her stomach. (Patient's face being positioned in a downwards (gravity) direction.)

Supine means the patient is laying on his/her back. (Patient's face being in an upwards direction.)

Decubitus Right means the patient is laying with his/her right side in a downwards direction.

Decubitus Left means the patient is laying with his/her left side in a downwards direction.

NOTE -- Patient Position (0018, 5100) is a type 1 required attribute for both the CT and MR modalities. This attribute is VERY IMPORTANT for accurately interpreting the patient's orientation.

You do not need to consider the patient position tag to label dicom images orientations when presented on screen. You only need the image orientation patient. Since that's the orientation of the image in the (patient) reference coordinate system. So, for example you will get different values for IOP from a CT scanner if the patient is prone or supine.

Patient Position (0018, 5100) will tell you if the patient was scanned head-first supine, feet-first prone, head-first prone, etc. Instance Number (0020, 0013), also commonly known as slice number, contains no information about spatial location and isn't even guaranteed to be unique. Slice Location (0020, 1041) is useful, if it exists, but you can't count on it always existing because it's a Type 3 (optional) attribute. To have a robust solution, you need to use Image Position Patient (0020, 0032) together with Image Orientation Patient (0020, 0037) and Patient Position (0018, 5100) to properly order the slices in space. Image Position Patient gives you a vector from the origin to the center of the first transmitted pixel of the image. Image Orientation Patient gives you vectors for the orientation of the rows and columns of the image in space. Patient Position tells you how the patient was placed on the table relative to the coordinate system.