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What is the difference between Rescale slope & intercept AND scale slope & intercept?

I'm quite new to DICOM and I'm having difficulties in understanding the difference between rescaling and scaling factors.

To transform the stored values, in a MRI DICOM image, into a quantitatively meaningful image, it seems that I should use the Rescale Slope (0028, 1053) and the Rescale Intercept (0028, 1052).

However, for the same transformation, some MRI manufacturers (e.g. Philips) provide an additional Private Tag - the Scale Slope. In that case, the transformation requires the use of, not only the Rescale Slope and Rescale Intercept, but also the Scale Slope.

I can't understand what exactly is this Scale Slope. Can anyone please provide any help?

Solution

  • The Rescale Intercept (0028,1052) and the Rescale Slope (0028,1053) are standard DICOM tags.

    As you already said in question, the Scale Slope (probably (2005,100E)) is a Private Tag - specific to the Equipment Manufacturer. So, only manufacturer may say something reliable about this.

    Private tags do not have standard names; generally, manufacturers mention the details of private tags they created in their DICOM Conformance Statement or other similar document. They may also name the tag in that document. Please refer to this answer for more details.

    Now, to your question -- what is the difference?
    Considering what I said above, it is hard to answer this question. Meaning and usage of standard tags can be found in standards. It is not the case with private tags. You have to go through vendor specific documents to understand it in details (if they mention it in details).

    Even so, a quick googling give me this and this. One of the post in thread discusses about usage of Scale Slope in a mathematical formula.

    If you open a PAR/REC header you will see the Phiips description of these values
    //# === PIXEL VALUES =============================================================
    //# PV = pixel value in REC file, FP = floating point value, DV = displayed value on console
    //# RS = rescale slope, RI = rescale intercept, SS = scale slope
    //# DV = PV * RS + RI FP = DV / (RS * SS)

    and

    Inputs:
    R = raw stored value of voxel in DICOM without scaling
    WS = RealWorldValue slope (0040,9225) "PhilipsRWVSlope"
    WI = RealWorldValue intercept (0040,9224) "PhilipsRWVIntercept"
    RS = rescale slope (0028,1053) "PhilipsRescaleSlope"
    RI = rescale intercept (0028,1052) "PhilipsRescaleIntercept"
    SS = scale slope (2005,100E) "PhilipsScaleSlope"
    Outputs:
    W = real world value
    P = precise value
    D = displayed value
    Formulas:
    W = R * WS + WI
    D = R * RS + RI
    P = D / (RS * SS)