Physics of CT Scanning
Lecture 2
Digital Imaging
Analog vs. Digital Information
Analog
• continuous information
• Can have any of an infinite
number of values
Digital
• discrete information
• Can have a finite number of
values limited by
» # of digits on display
» # of bits used to represent value
Analog vs. Digital Images
Digital Analog
• discrete spatial • continuous spatial
information information
Digitizing a Picture
Commercial scanner
Renders a photograph into numbers
311, 255, 309, 78,
43, 99, 124,…
Analog vs. Digital Images
Analog
• continuous gray
shade information
Digital
• Discrete gray
shade information
Digital Image Formation
Screen Wire Mesh Clinical Image
Digital Image Formation:
Sampling
Place mesh over
image
Assign each square
194
(pixel) a value based
on density
Pixel values form 73
the digital image
22
Digital Image Formation:
Sampling
Each pixel assigned
a value
Value averages
194
entire pixel
• Any spatial variation
within a pixel is lost 73
The larger the pixel,
the more variation
22
Digital Image Formation
The finer the mesh (sampling), the more accurate the
digital performance
What is this?
12 X 9 Matrix
Same object, smaller squares
24 X 18 Matrix
Same object, smaller squares
48 X 36 Matrix
Same object, smaller squares
96 X 72 Matrix
Same object, smaller squares
192 X 144 Matrix
Digital Image Bit Depth
bitdepth (1, 2, 3,….) controls # of possible values a
pixel can have
increasing bit depth results in
• more possible values for a pixel
• better contrast resolution
Bits Values # Values
1 0, 1 21=2
2 00, 01, 10, 11 22=4
3 000, 001, 010, 011, 100, 101, 110, 1112 3 = 8
. . .
. . .
. . .
8 00000000, 00000001, ... 11111111 2 8 = 256
Digital Image Formation
Quantization (A to D Conversion)
Process of assigning
a number to a gray 88 ? 89
shade The middle pixel attenuates
Only discrete #’s between the other two. What
# will the A to D converter
assigned
assign it?
• can lose information
because of discrete #
assignment
Analog to Digital Converter
88 ? 89
Since there are no #’s between
88 & 89 (88.5 not allowed), the
A to D converter will assign
pixel either a 88 or a 89.
The fact that the center pixel is
darker than the left one and
lighter than the right one is
forever lost.
Contrast Resolution
difference in x-ray attenuation required
for 2 pixels to be assigned different
digital values
88
89
Gray Scale
themore candidate values for a
pixel
• the more shades of gray image can
be stored in digital image
• The less difference between x-ray
attenuation required to guarantee
different pixel values
» See next slide
Display Limitations
not possible to display all shades of gray
simultaneously
window & level controls determine how pixel
values are mapped to gray shades
numbers (pixel values) do not change; window &
level only change gray shade mapping
17 = Change
17 =
window /
level
65 = 65 =
Presentation of Brightness Levels
pixel values assigned brightness levels
• pre-processing
manipulating brightness levels does not affect
image data
• post-processing
» window
» level
125 25 311 111 182 222 176
199 192 85 69 133 149 112
77 103 118 139 154 125 120
145 301 256 223 287 256 225
178 322 325 299 353 333 300
# of Possible Values & Contrast
Resolution
The more possible values for a pixel, the more
gray shades & the better the contrast.
4 grade shades 256 grade shades
Digital Image Sources
CT
MRI
CR
DigitalSubtraction Angiography
Ultrasound
Nuclear Medicine
Why Digital?
Required for use with Enhancement
computers • Edge enhancement
Perfect image copies • Smoothing (noise
Compression reduction)
Manipulation Analysis
• Rotation • Image statistics
• White/black reversal • Pattern recognition
• Window/level
• Zoom
