The image above is a digital image as are all MRIs, ultrasounds and CTs. X-rays can be created as conventional (screen film radiograph) or digital images.
In the above MRI slice a small square of 8 by 8 pixels is enlarged and shown as its image matrix. An image matrix, like any matrix, is a mathematical object. Note that the numbers in this case do not correspond to any standard denotation of signal intensity or Hounsfield units, rather units that the computer uses to display image pixels. Typically, computers display greyscale images on a screen by producing values between 0 and 255 for each pixel, as this makes one byte of data when encoded in binary.
Original raw data from a digital image will usually corresponds to different matrices from which a display image is derived. The process of CT windowing is an example of deriving different display screen images from an original digital image matrix. Either original image matrices or derived image matrices can be processed in machine learning and texture analysis programs with linear algebra.