Imaging 2 M1L1

• important to understand the acquisition and processing steps of the DR and CR systems to: Prevent errors, Understand results of image and exposure indicators and To minimize patient dose
• major components of DR system include: data acquisition, computer data processing, image display and image storage
• analog data is continuously changing, may be described as infinite
• digital data is describes as finite or discrete
• example of analog data: x-ray with infinite number of shades of grey
• example of digital data: digital clock where only hours and minutes are displayed
• computers in imaging can only process data that is in a digital form. An ADC is used to convert.
• Digitization is aka as quantization.
• Digitization: The analog data on the plate is divided into a matrix of pixels and each one is assigned a number that represents the amount of light that was emitted from that part of the plate. This is required because the computer can only analyze numerical data.
• digitization: Digital images are recorded as multiple numeric values that are divided into an array of pixels that can be processed in many different ways. The numbers are converted back into shades of grey for viewing.
• Data transferred internally in the computer in binary form is considered digital
• digital data contains less error
• Analog data can become distorted the more it is transmitted causing possible errors
• Once analog information is in digital form the chance of error is less
• Digital information does not “accumulate” errors. Safeguards against errors are in place
• analog representation of data: Commonly more radiation =black on image, less radiation =white on image and a wide range in between
• 3 steps for ADC: scanning, sampling and quantization
• scanning (step 1 for ADC): image is divided into an array of pixels
• sampling (step 2 for ADC): measuring the brightness of each pixel
• quantization (step 3 for ADC): the brightness levels are assigned numbers called a gray level. Total number of gray levels = gray scale
• When performing ADC not all analog points can be considered. Only certain points are selected = sampling. Each analog sample is then converted into digital format = digitization or quantization
• Sampling rate – # of times per second analog signal is sampled and digitized. Most DI systems require a high sample rate
• disadvantages of ADC: conversion of analog to digital causes a loss of information. Loss is due to sampling frequency. The digital signal is only an approximate of analog signal.
• Digital to analog conversion (DAC): required to display image on monitor (converts numerical info into visible light).
• Information lost from ADC is not restored during DAC