What's the meaning of slice location (0020, 1041)?

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I am confuzed abot the slice location (0020, 1041). The explanation (Slice Location Attribute – DICOM Standard Browser) is:

Slice Location (0020,1041) is defined as the relative position of the image plane expressed in mm. This information is relative to an unspecified implementation specific reference point.

What’s the meaning about an unspecified implamentation specific reference point? Where is this point?

What’s the relationship between slice location (0020, 1041) and image position (patient) (0020, 0032))? If the image position is [x, y, z], is slice location the z value? In a dicom viewer, the slice location and image position is:

(0020,0032) DS -102.96828174586\-170.6410164833\76.9224891098919 Image Position (Patient)
(0020,1041) DS 0                                                  Slice Location

We can see that the slice location is not the z value of image position.

Any suggestion is appreciated~~~

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Hello @zhang-qiang-github,

Good question. The standard doesn’t seem to provide more information to get a clear sense of what this value is used for. I haven’t encountered it before and the value of this field is not used when positioning the 2D slice in 3D, as shown in equation C.7.6.2.1-1.

Possibly @dclunie can provide additional insight.

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Short version - do not use it for anything.

Long version:

As noted above, the standard defines it as being the relative position of the image plane expressed in mm, relative to an unspecified implementation specific reference point. No more, and no less:

http://dicom.nema.org/medical/dicom/current/output/chtml/part03/sect_C.7.6.2.html#sect_C.7.6.2.1.2

In practice, this is a vendor and implementation specific value that generally corresponds to the position shown on the modality console or in the vendor’s viewer, which the user may find helpful when referencing a slice, or manually estimating the cross-plane extent of something.

If present at all, it may or may not be the distance along the Z plane (consider tilted CT slices), or along the normal to the plane orientation (for MR, untitled CT transverse slices, etc.). Very frequently it is not the same value (esp., different origin) and there are cases when the increment is not the same either.

The origin may or may not be the origin of the Frame of Reference, or isocenter, the operator landmarked position, or something else manufacturer specific, or something completely unexpected.

The (sign of the) direction may not match the Patient coordinates system either (e.g., it may be gantry/table rather than patient relative).

It is not intended to be a substitute for precise 3D computations using Image Position (Patient) (0020,0032), and should never be used in any calculations or reconstructions. It is purely “informative” (intended to allow reproduction of original vendor’s displayed information only).

E.g., It’s value should correspond to GE private data element Image Location (“GEMS_IMAG_01”,0027,xx41).

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Thank you for your explanation.