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6720 Lecture 7_digital imaging

6720 Lecture 7_digital imaging - Lecture VII Radiographic...

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1 PHYS 6720 - Lecture VII 1 Lecture VII Radiographic Imaging – Digital Methods 1. Digitization and processing 2. DICOM and PACS 3. Computed radiography (CR) 4. Screen-CCD systems 5. Digital Radiography PHYS 6720 - Lecture VII 2 VII-1 Digitization and processing Overview Analog images are very difficult for processing, storage, communication and reconstruction for 3D imaging Digital files can be separated into two categories: (i) standard character files using the ASCII encoding method (American Standard Code for Information Interchange) (ii) binary files using numerical or binary numbers ASCII uses 127 7-bit binary numbers to define control characters (32) and English letters/decimal numbers/symbols (94) Started in 1990s, an Unicode is defined for about 100,000 characters in different languages in which the first 127 characters are those in ASCII PHYS 6720 - Lecture VII 3 VII-1 Digitization and processing Overview Image file are binary files containing only number (pixels) Medical image file need storage of both characters and numbers Medical images are 2D image based with image intensity interpreted as the projection of 3D distribution of physical parameters (attenuation coefficient in x-ray image) onto the 2D image sensor plane 3D image reconstruction is achieved by stacking 2D imaging along the “vertical” axis Many other advantages: image database and integrated medical record database, telemedicine through internet, “infinite” shelf life time, … PHYS 6720 - Lecture VII 4 VII-1 Digitization and processing Basic terms • Pixels : image=matrix of pixels spatial resolution = number of pixels (row × column) Each pixel in a gray-scale image is represented by a number I indicating the signal intensity Each pixel intensity number I is a n-bit binary number between 0 (zero intensity) and 2 n -1 (maximum intensity), with n called as pixel depth or pixel resolution Pixel density : dpi = pixels (dots) per inch pitch = pixel-pixel distance PHYS 6720 - Lecture VII 5 VII-1 Digitization and processing Basic terms Most monitors or display devices are designed for 8-bit pixel resolution since human eyes are not good at resolving more than 256 (=2 8 ) gray-scale shades For digital image processing, however, high pixel resolution is desired for reducing processing errors Color images: each pixel is represented by a set of three numbers corresponding to three primary colors: (R,G,B) PHYS 6720 - Lecture VII 6 red green blue VII-1 Digitization and processing
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2 PHYS 6720 - Lecture VII 7 VII-1 Digitization and processing Basic terms Example: conversion between dpi, pixel pitch and lp/mm standard 35mm color film: 6000dpi ~ 6000/25.4/2 (lp/mm) ~ 120 (lp/mm) consumer zoom lens or film projector: 1500dpi ~ 30 (lp/mm) a CCD sensor of 23.7x15.7mm 2 and 4288x2848 pixels (12 megapixels): 180pixels/mm ~ 5.5mm pixel pitch ~ 90 (lp/mm) Canon 9950F Scanner: 4800 x 9600 dpi 4800x9600 dpi ~ 94x190 (lp/mm) PHYS 6720 - Lecture VII 8 VII-1 Digitization and processing Digitization approaches (1) Direct conversion: x-ray photons Æ electric charges Æ A/D converter
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