6720 Lecture 7_digital imaging - Lecture VII Radiographic Imaging Digital Methods VII-1 Overview Digitization and processing 1 2 3 4 5 Digitization and

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