DIGITAL IMAGE FORMATION

• Acquiring of digital image: digital image requires conversion from analog to digital signal/electrical signal
• To be suitable for computer processing, an image function must be digitized both spatially and in amplitude
• Image sampling: digitization of the spatial coordinates and is related to pixel size, reflective of matrix size and affects spatial resolution
• Image gray level quantization: digitization of the amplitude or brightness
• image gray level quantization is determined by computer bit depth
• Image gray level quantization is reflected in the image contrast resolution
• The process of digital image production includes scanning of the analog image line-by-line to obtain a continuous analog signal representing the variations in image brightness
• SPATIAL SAMPLING: process of dividing the analog signal into individual pixels which is performed in equal intervals
• contrast quantization: process of converting the amplitude into a digitized numerical pixel value
• analog-to-digital (ADC) converter: turns the quantized level into binary code
• Sampling: involves digitizing coordinate values to reduce the scanned image into a series of amplitude values

  • involves the x-axis
• Quantization: digitizing the amplitude values

  • involves y-axis
• Quantization gives color to the values taken:

  • 0 = black
  • in between = gray
  • 256 = white
• Matrix: a two-dimensional pixel
• Pixel: represents a volume of tissue
• Voxel: represents a pixel with depth
• A digital image stored in a computer is rectangular in format and made up of small squares called pixels
• A typical digital chest x-ray might contain 2000 columns of pixels and 2500 rows of pixels for a total of 5 million pixels
• Header: where information about the patient, type of examination, and place of examination in an x-ray image is stored
• One problem with digital medical images is that they take up a relatively large amount of storage space and need to be transferred from the examination room to the radiologist and then need to be archived
• In PACS, digital medical images are stored on a medium that allows for quick access until the examination results are reviewed by a radiologist or other physician. Then the examination results are typically sent to a cheaper type of storage device that takes longer to access for archiving
• Images are transferred via a network of usually fiber optic lines (internet connections) that run throughout the hospital or facility
• Having all of these digital images available on a network has made reading medical imaging examination results extremely convenient
• there is no need for a hard copy of an x-ray film or other study
• physicians merely need a fast network connection to obtain digital copies of the study they would like to read. This has led to the practice of teleradiology
• teleradiology: specifically identifies the physician/radiologist not present in the hospital for reading