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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
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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
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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
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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
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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
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red
green
blue
VII-1
Digitization and processing
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PHYS 6720 - Lecture VII
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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
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VII-1
Digitization and processing
Digitization approaches
(1) Direct conversion:
x-ray photons
Æ
electric charges
Æ
A/D converter

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- Spring '10
- HU
- Image processing, Pixel, Image sensor, Charge-coupled device, Digital radiography
-
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