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Pixel values and real values

In medical imaging, pixel values are frequently stored as bytes or shorts, but there is a generally a real value associated with each pixel as well. This real value is obtained by a scale factor and offset associated with each image or image volume. The MINC standard indicates how pixel values should be interpreted.

Image data in the MIimage variable can be stored as bytes, shorts, ints (32-bit), floats or doubles. NetCDF conventions use the attributes MIvalid_range or MIvalid_max and MIvalid_min to indicate the range of values that can be found in the variable. For short values, for example, we might have a valid range of 0 to 32000. To convert these integers to real values, we could use a scale and offset. However, these values would have to change if the data where converted to bytes in the range 23 to 228. If we specify an image maximum and minimum to which MIvalid_max and MIvalid_min should be mapped by an appropriate scale and offset, then we can convert type and valid range without having to change the real maximum and minimum. To allow the maximum dynamic range in an image, we use the variables MIimagemax and MIimagemin to store the real maximum and minimum -- these can vary over any of the non-image dimensions of MIimage.


next up previous contents
Next: General convenience functions Up: The MINC standard Previous: MINC coordinate system   Contents
Robert VINCENT 2004-05-28