DICOM: relating Slice Location to Image Orientation and Patient Position

Question

If the Image Orientation (Patient) tag (0020,0037) reads [1,0,0,0,1,0] and the Patient Position tag (0018, 5100) reads ‘HFS’, how do I interpret Slice Location tag (0020,1041), assuming that it exists? I know that it represents the `Relative position of the image plane in millimeters', I'm just having trouble relating the end points of the range to the Z axis in the DICOM Reference Coordinates System (RCS).

Example: I have an sequence of Slice Location numbers in the range: [-1873.382, -771.782]

Since the numbers are increasing and in the DICOM RCS, the Z axis increases in the Inferior to Superior direction, can I conclude that '-1873.382' is the position of the most Inferior slice?

Also, just to note that the z coordinate of my Image Position (Patient) (0020,0032) attribute for each slice, contains the same information as my Slice Location tag.

Answer 1

I still advise against using the Slice Location attribute for sorting. In MR imaging, slices can have arbitrary orientation and even in CT the gantry can be tilted, so you cannot rely that all slices are parallel to the xy-plane. So you actually do not know to which axis the Slice Location refers.

What I do is to subtract ImagePositionPatient from two slices which gives me the direction in which the slices in the stack are moving. Ordering can be done by the amount of the difference vectors.

Answer 2

Image Position (Patient) (0020, 0032) is the x, y, and z coordinates of the upper left hand corner of the image and Image Orientation (0020, 0037) says the direction of the first row and the first column with respect patient (farther defined by patient orientation). X-axis increasing direction is towards the left hand side of the patient, y-axis increasing is towards the posterior side and z-axis increasing is toward the head of the patient.

In your case, if the Z axis is changing and increase is towards the head, I would use the Z-axis values for sorting the stack. It is more reliable than Slice Location. Yes the smallest value (e.g. value -1873.382) is the most Inferior slice.