Image Display

Correct display of pixel values.

Overview

In general, the pixel values that are stored in a DICOM file cannot be directly displayed to the screen. Instead, the pixel values go through a display pipeline that changes the values to something suitable for viewing. Unless the pixels are stored in the file as RGB or as 8-bit grayscale, the display pipeline will generally do one of two things: it will map the values through a palette to generate RGB pixels, or it will rescale the values (sometimes in multiple steps) for monochrome display.

The vtkDICOMReader will, by default, produce the Modality Values of the pixels, which are the values associated with the modality that produced the images (e.g. Hounsfield values for CT). If desired, the AutoRescaleOff() method can be called on the reader to force it to produce Stored Values instead, which are the pixel values stored in the file. Note that Modality Values are equal to Stored Values for many modalities, including MRI and Ultrasound.

Photometric Interpretation

The Image Pixel Module of the DICOM standard defines the Photometric Interpretation attribute as a hint for how the stored pixel values should be interpreted by a DICOM image display pipeline. Typical values are:

• MONOCHROME1 - negative image, where higher values are darker (e.g. radiographic film )
• MONOCHROME2 - positive image, where higher values are brighter (e.g. CT, MR)
• PALETTE COLOR - indexed color with palette
• RGB - full-color image with separate RGB components

In addition to the above, the YBR family of photometric interpretations indicate that the image was stored with one luma channel and two color channels, which is commonly done when a color image is compressed. The vtkDICOMReader automatically converts YBR images to RGB, so display pipelines should interpret YBR the same as RGB.

• YBR_FULL - used by ultrasound, usually with RLE compression
• YBR_FULL_422 - used by JPEG lossy compressed images
• YBR_PARTIAL_420 - used by MPEG lossy compressed video
• YBR_ICT - used by JPEG2000 lossy compressed images
• YBR_RCT - used by JPEG2000 lossless compressed images

Here is a short example of how to interpret the output of vtkDICOMReader for display:

reader->Update();
vtkDICOMMetaData *meta = reader->GetMetaData();
vtkDICOMValue photometric = meta->Get(DC::PhotometricInterpretation)
if (photometric.Matches("MONOCHROME1"))
{
  // display with a lookup table that goes from white to black
}
else if (photometric.Matches("MONOCHROME2"))
{
  // display with a lookup table that goes from black to white,
  // or display with a suitable pseudocolor lookup table
}
else if (photometric.Matches("PALETTE*"))
{
  // display with palette lookup table (see vtkDICOMLookupTable),
  // or convert to RGB with vtkDICOMApplyPalette
}
else if (photometric.Matches("RGB*") || photometric.Matches("YBR*"))
{
  // display RGB data directly
}

Grayscale image display

DICOM defines a grayscale display pipeline that converts the Stored Values in the file to Presentation Values that are suitable for display. The details of how this is done are described in the Lookup Tables and Presentation States chapter of the DICOM standard. A brief summary of the steps in the grayscale display pipeline is as follows:

1. Stored Values are rescaled to generate Modality Values
2. Modality Values are windowed to generate Windowed Values (or Values of Interest)
3. Windowed Values are mapped to Presentation Values

The Modality Values are real values in units that are appropriate to the modality, for example, Hounsfield units for CT or counts for PET. The RescaleIntercept and RescaleSlope define a linear mapping from Stored Values to Modality Values:

ModalityValue = RescaleSlope * StoredValue + RescaleIntercept

For modalities such as ultrasound and MRI that do not have any units, the RescaleSlope and RescaleIntercept are absent and the Modality Values are equal to the Stored Values.

The Windowed Values (or more generically, Values of Interest or VOIs) represent the range of values to be displayed. Decreasing the WindowWidth increases the perceived contrast of the image, and decreasing the WindowCenter increases the perceived brightness of the image. Note that, as shown in the pseudocode below, the DICOM standard uses a definition of "window" and "level" that might be different from that to which you are accustomed. The DICOM WindowWidth must be greater than or equal to 1.

n = number of output gray levels (usually 256 or 1024)
w = WindowWidth - 1.0
c = WindowCenter - 0.5

if (ModalityValue <= c - 0.5*w), then WindowedValue = 0
else if (ModalityValue > c + 0.5*w), then WindowedValue = n - 1
else WindowedValue = ((ModalityValue - c)/w + 0.5) * (n - 1)

For ultrasound (and for 8-bit images in general) the WindowWidth and WindowCenter may be absent from the file. If absent, they can be assumed to be 256 and 128 respectively, which provides an 8-bit identity mapping. More information can be found in the VOI LUT Module of the DICOM standard. Note that a DICOM display application should allow the user to adjust the window according to his or her needs.

The Presentation Values are the values that are ultimately displayed to the user. If the PhotometricInterpretation is MONOCHROME2, then the Windowed Values should be displayed directly. If the the PhotometricInterpetation is MONOCHROME1, then the grayscale lookup table should be inverted such that a value of 0 is displayed as white and the maximum value is displayed as black.

Color image display

There are several categories of color display for DICOM, the most important of which are:

• RGB (or YBR) values stored directly in the file
• PALETTE COLOR where a palette LUT is applied to the stored values
• A supplemental palette LUT applied to some or all stored values
• A pseudo-color palette LUT applied to the windowed values

For convenience, the vtkDICOMApplyPalette filter can convert a PALETTE COLOR image to RGB, or can convert an image with a supplemental palette to RGB, but will pass through all other images. For more information about supplemental palettes, see the Supplemental Palette Lookup Table Module. As an alternative to vtkDICOMApplyPalette, the palette lookup table can be retrieved from a DICOM data set with the vtkDICOMLookupTable class.

The final category of color display, pseudo-color, is displayed as per the grayscale pipeline described above, except that a pseudocolor LUT is used in place of the black-to-white (MONOCHROME2) or white-to-black (MONOCHROME1) LUT.