Transcript No Slide Title

The fifty Cent Version of Digital Imaging
Bits
Bytes
Pixels
Matrix
Dynamic range
Machine language
Processors (8,10,12 bits etc.)
Base 10 numbering
Binary numbering system
Hounsfield units (CT numbers)
The Basics of Digital Imaging
OFF
In this analogy the light bulb is a
pixel on a monitor
The switch is a bit (binary digit)
in the computer circuitry
The Basics of Digital Imaging
ON
Flip the switch (binary digit or bit) and light up
the bulb (pixel)
Row and columns of pixels
form the matrix
Dynamic range of 2
(21)
How is contrast added to
an image?
Magic switches. The more
of them In the ON position,
the brighter the light
ON
OFF
ON
ON
Dynamic range of 256 (28)
1
OFF
1
ON
3
3
4
0
2
2
OFF
ON
OFF
OFF
OFF
OFF
ON
ON
OFF
OFF
5
6
ON
OFF
ON
ON
ON
ON
ON
OFF
ON
More bits, in various configurations,
create a greater dynamic range
(analogous to the radiographic scale
of contrast)
ON
ON
ON
OFF
OFF
OFF
OFF
OFF
An 8 bit processor can display a dynamic range of 256 (28)
8 bits = 1 byte
ON
ON
ON
ON
ON
ON
ON
ON
In computer circuitry 8 bits are grouped together
8 bits = 1 byte
10011011
11101001
00010111
01011001
OFF
0
This is a machine language computer code
ON
ON
1
1
OFF
0
OFF
0
ON
1
ON
1
ON
1
The language computers speak is binary (two numbers)
That’s because bits are either ON (1) or OFF (0)
Human beans count in decimals (base 10)
because
They have 10 fingers and 10 toes
4
10 10 3 10 2 10 1 10 0
Ten is raised to every power
of itself to create columns
of higher orders
Binary numbering
4
3
2
1
0
2 2 2 2 2
By definition: Any number raised to the power of 0 is 1
Any number raised to the power of 1 is the number itself
* Count from right to left
* Each column is twice the previous
* Pick any column and it will be one more than the sum of all the previous columns
Binary numbering system (344): In the
base 2 numbering system all values are
represented by 0 or 1. Zero = OFF when
representing an open switch. One = ON.
Columns are
derived from:
Columns:
Binary numbering
2
4
3
2
1
0
2 2 2 2
256 128 64 32 16 8 4 2 1
Examples:
0
0
0
0
1
1 1
0
0
1
1 1 1 0 = 14
1 0 1 0 = 42
1 1 1 1 = 127
An 8 bit processor displays a dynamic range of 256
8
2
= one byte = 256 shades of gray
256 128 64 32 16 8 4 2 1
1
1
1
1
1
1
1
1 = 255
0
0
0
0
0
0
0
0=0
When all the bits are ON a pixel is illuminated as brightly as it could
get. There are 255 possibilities of a bit or bits being ON.
When all the bits are OFF the pixel is black.
255 + 1 = 256: Ergo, 8 bits stores a dynamic range of 256 from
dark to bright.
An 8 bit processor displays a dynamic range of 256
512
What will a 9 bit processor display?
10 bit? 1024
11 bits 2048
12 bit? 4096
13 bits 8192
14
16384
15 bits 22768
16 bit? 44536
What effect does increasing the dynamic range have?
It increases the memory requirements dramatically!
10011011
11101001
00010111
01011001
OFF
0
ON
ON
1
1
OFF
0
32 16
What does this byte of computer
memory do in this midsagittal
MRI of the head?
It defines how brightly one pixel, in
the matrix of pixels, will be illuminated.
In this case the decimal equivalent
of the binary 00110111 is 55.
OFF
0
ON
ON
ON
1
1
0
4
2
1
= 55
This print out of the data in
each pixel of the image
matrix shows that the
decimal number 55 is the
shade of gray for the pixel
in X axis 250, Y axis 217.
Image data like this may be
printed out by service
technicians for QA,
calibration, and repair.
The numbers in this area
are fairly homogeneous
and could represent the
cerebrospinal fluid in the
The computer’s binary
body of the lateral ventricle
code is represented in
since 39-55 is relatively
decimal for the convenience close to zero, which is the
of us
value of water on the
Hounsfield scale.
CT Numbers (Hounsfield Units)
The Hounsfield scale, or units, commonly called CT numbers, is the
scale on which the brightness of pixels is based in digital systems.
Bone = + 1000
Based on
Water = 0
Air = - 1000
CT numbers (HU),
express attenuation
values relative to water