N.2.1 Grayscale Transformations

N.2.1.1 Modality LUT

The Modality LUT operation applies only to grayscale values.

The Modality LUT transformation transforms the manufacturer dependent pixel values into pixel values which are meaningful for the modality and which are manufacturer independent (e.g., Hounsfield number for CT modalities, Optical Density for film digitizers). These may represent physical units or be dimensionless. The Modality LUT in the Presentation State is modality dependent and is analogous to the same module in an Image.

Notes: 1. In some cases, such as the CT Image Storage SOP Class, the same conceptual step as the Modality LUT is specified in another form, for example as Rescale Slope and Rescale Intercept Attributes in the CT Image Module, though the Modality LUT Module is not part of the CT Image IOD.

2. Image pixel values with a value of Pixel Padding Value (0028,0120) in the referenced image, or within the range specified by Pixel Padding Value (0028,0120) and Pixel Padding Range Limit (0028,0121) (if present in the referenced image) shall be accounted for prior to entry to the Modality LUT stage. See the definition of Pixel Padding Value in PS 3.3. Neither Pixel Padding Value (0028,0120) nor Pixel Padding Range Limit (0028,0121) are encoded in the Presentation State Instance.

In the case of a linear transformation, the Modality LUT is described by the Rescale Slope (0028,1053) and Rescale Intercept (0028,1052). In the case of a non-linear transformation, the Modality LUT is described by the Modality LUT Sequence. The rules for application of the Modality LUT are defined in PS 3.3 Modality LUT Module.

If the Modality LUT or equivalent Attributes are part of both the Image and the Presentation State, then the Presentation State Modality LUT shall be used instead of the Image Modality LUT or equivalent Attributes in the Image. If the Modality LUT is not present in the Presentation State it shall be assumed to be an identity transformation. Any Modality LUT or equivalent Attributes in the Image shall not be used.

N.2.1.2 Mask

The Mask operation applies only to grayscale values.

The mask transformation may be applied in the case of multi-frame images for which other frames at a fixed frame position or time interval relative to the current frame may be subtracted from the current frame. Multiple mask frames may be averaged, and sub-pixel shifted before subtraction.

This transformation uses the Mask Module as used in the X-Ray Angiography Image Storage SOP Class, though it may be applied to any Image Storage SOP Instance that contains a multi-frame image.

In the case of X-Ray images, the subtraction is specified to take place in a space logarithmic to X-Ray intensity. If the stored pixel values are not already in such a space, an implementation defined transformation to such a space must be performed prior to subtraction. If a Modality LUT Module is present as well as a Mask Module, then the Modality LUT shall specify a transformation into such a logarithmic space, otherwise it shall not be present (even though a Modality LUT may be present in the referenced image(s) which shall be ignored).

Notes: 1. In the case of an XA or XRF image, if the Pixel Intensity Relationship (0028,1040) in the image is LOG, then even though a Modality LUT would be present in the image (to map pixel values back to linear to X-Ray intensity), no Modality LUT would be present in the presentation state (i.e. the Modality LUT would be an identity transformation) since log values are required for subtraction. See PS 3.3 C.8.7.1.1.2.

2. In the case of an XA or XRF image, if the Pixel Intensity Relationship (0028,1040) is LIN, then no Modality LUT would be present in the image, but a Modality LUT would need to be present in the presentation state since log values are required for subtraction.

3. In the case of an XA or XRF image, if the Pixel Intensity Relationship (0028,1040) in the image is DISP, then even though a Modality LUT may or may not be present in the image (to map pixel values back to linear to X-Ray intensity), a different Modality LUT would be present in the presentation state if the creator of the presentation state could create a transformation from DISP pixel values to a logarithmic space for subtraction, or the Modality LUT in the presentation state would be an identity transformation if the DISP pixel values were known to already be log values required for subtraction.

The result will be a signed value with a bit length one longer than the source frames.

When there is no difference between corresponding pixel values, the subtracted image pixel will have a value of 0.

If a pixel in the current frame has a greater value than in the mask frame, then the resulting frame shall have a positive value. If it has a lesser value, then the resulting frame shall have a negative value.

N.2.1.3 VOI LUT

The VOI LUT operation applies only to grayscale values.

The value of interest (VOI) LUT transformation transforms the modality pixel values into pixel values that are meaningful for the user or the application.

Note: Photometric Interpretation (0028,0004) is ignored, since its effect is defined by the application of the grayscale transformations.

The Softcopy VOI LUT Module in the Presentation State is analogous to the VOI LUT Module in an Image.

In the case of a linear transformation, the VOI LUT is described by the Window Center (0028,1050) and Window Width (0028,1051). In the case of a non-linear transformation, the VOI LUT is described by the VOI LUT Sequence. A VOI LUT Function (0028,1056) may be present to define a potentially non-linear interpretation (e.g., SIGMOID) of the values of Window Center (0028,1050) and Window Width (0028,1051). The rules for application of the VOI LUT are defined in PS 3.3 Softcopy VOI LUT Module.

The VOI LUT may have sections with negative slope.

Note: In the Basic Print Service Class a VOI LUT may not have negative slope.

If a VOI LUT is part of both the Image and the Presentation State then the Presentation State VOI LUT shall be used instead of the Image VOI LUT. If a VOI LUT (that applies to the Image) is not present in the Presentation State , it shall be assumed to be an identity transformation. Any VOI LUT or equivalent values in the Image shall not be used.

N.2.1.4 Presentation LUT

The Presentation LUT operation applies only to grayscale values.

The Presentation LUT transformation transforms the pixel values into P-Values, a device independent perceptually linear space as defined in PS 3.14 Grayscale Display Function Standard. It may be an identity function if the output of the VOI LUT transformation is in P-Values.

Note: If the Presentation LUT and VOI LUT step are identity transformations, and the Mask Module is absent, then the output of the Modality LUT must be, by definition, P-Values.

No output space other than P-Values is defined for the Grayscale Softcopy Presentation State Storage SOP Classes.

In the case of a linear transformation, the Presentation LUT is described by the Presentation LUT Shape (2050,0020). In the case of a non-linear transformation, the Presentation LUT is described by the Presentation LUT Sequence. The rules for application of the Presentation LUT are defined in PS 3.3 Softcopy Presentation LUT Module.

Notes: 1. Since the grayscale transformation pipeline fully defines all transformations applied to the stored pixel values in the referenced image object, the value of Photometric Interpretation (0028,0004) in the referenced image object is ignored and overridden. This implies that either the creator of the presentation state chose a pipeline that reflects the Photometric Interpretation (0028,0004) , or chose to ignore or override the Photometric Interpretation, and invert the image relative to what is specified by Photometric Interpretation. If the Modality LUT and VOI LUT do not have a negative slope, one can achieve the effect of inversion of the polarity of an image by choosing Presentation LUT Shape of IDENTITY or INVERSE that displays the minimum pixel value as white rather than black in the case of a Photometric Interpretation of MONOCHROME2, or black rather than white in the case of a Photometric Interpretation of MONOCHROME1. If Presentation LUT Data is sent, then one can invert the value of the entries in the LUT table to achieve inversion of polarity.

2. The minimum P-Value (zero) always commands that the lowest intensity be displayed.

3. No separate Polarity transformation is defined.

A Softcopy Presentation LUT Module is always present in a Presentation State. If a Presentation LUT is present in the Image then the Presentation State Presentation LUT shall be used instead of the Image Presentation LUT.