The number of shades of gray depends clearly from the number of bits used to store the sample that, in this case, coincide with the pixel depth. A scale of shades of gray from black to white is used to display the images. Monochrome images have one sample per pixel and no color information stored in the image. To specify if color information is or is not stored in the image pixel values, we introduce the concept of samples per pixel (also known as number of channels). The photometric interpretation specifies how the pixel data should be interpreted for the correct image display as a monochrome or color image. Therefore, complex data typically have a pixel depth twice that used to represent a single real number.įrom the previous overview emerges that the pixel depth is a concept related to the memory space necessary to represent in binary the amount of information we want to store in a pixel. Complex data have a real and an imaginary component which are referred as pairs of real numbers.
Although unusual, pixels can store complex numbers. The standard addresses the problem of the precision with which a finite number of combination obtainable with a sequence of n-bit (2 n−1) can represent a continuous range of real numbers. The Institute of Electrical and Electronics Engineers created a standard (IEEE-754) in which defines two basic formats for the encoding in binary of floating-point numbers: the single precision 32-bit and the double precision 64-bit.
With a pixel depth of 2 bytes per pixels, it is possible to codify and store integer numbers between 0 and 65,535 (2 16−1) alternatively, it is possible to represent integer numbers between −32,768 and +32,767 using 15 bits to represent the numbers and 1 bit to represent the sign. This means that if a 256 by 256 pixels image has a pixel depth of 12 or 16 bits, the computer will always store two bytes per pixel and then the pixel data will require 256 × 256 × 2 = 131,072 bytes of memory in both cases.
Bytes are group of 8 bits and represent the smallest quantity that can be stored in the memory of a computer. Every image is stored in a file and kept in the memory of a computer as group of bytes. Pixel depth is the number of bits used to encode the information of each pixel. Finally, the limits and strengths of the reviewed formats are discussed, with some considerations about the future directions in the field of medical image file formats. Then, a description of the four major file formats used in medical imaging, Analyze, Neuroimaging Informatics Technology Initiative ( Nifti), Minc, and Digital Imaging and Communications in Medicine ( Dicom), is provided. These concepts include: pixel depth, photometric interpretation, metadata, and pixel data. First, some basic concepts common to all image file formats are reviewed.
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The file format describes how the image data are organized inside the image file and how the pixel data should be interpreted by a software for the correct loading and visualization.
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A medical image data set consists typically of one or more images representing the projection of an anatomical volume onto an image plane (projection or planar imaging), a series of images representing thin slices through a volume (tomographic or multislice two-dimensional imaging), a set of data from a volume (volume or three-dimensional imaging), or multiple acquisition of the same tomographic or volume image over time to produce a dynamic series of acquisitions (four-dimensional imaging). Image file formats provide a standardized way to store the information describing an image in a computer file.
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