Chapter 24: Digital Imaging processing
Analog (343): Information display that is continually changing. Every value, to
infinity, exists. For examples: a sweep second hand on a clock, a mercury
thermometer, and the gray scale of an x-ray film. (See digital, see appendix I)
Digital (343): Information (or a display of information) that is discrete. Values
are absolute, with nothing in between. For examples: the second by second
increments of a digital clock, a digital thermometer, and the gray scale of a digital
image. (See analog, appendix I)
Data (343): The smallest unit of information. If a brick building were used for
analogy, a single brick would be data. (See information)
Information: Processed data. If a brick building were used for analogy, a single
brick would be data, and the building itself would be information. (See data)
Bit (Binary digit) (344): In the binary numbering system, two symbols, 0 and 1,
are used to represent any value from null to infinity. Like base 10 numbering,
binary (base 2), is derived from the base number raised to every exponent of itself:
20, 21, 22, 23 etc. Bit depth refers to the dynamic range (gray scale) a
microprocessor can display. A 4 bit processor displays 32 shades of gray, an 8 bit
processor 256. (See appendix I)
Byte (344): A group of 8 bits, used in computer programming for organization of
data. An 8 bit processor (28) stores 256 bits of data. When the data is an image,
than image can have up to 256 shades of gray. (See appendix I)
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.
(Appendix I)
Program (343): Operating instructions in the form of operating systems or
application programs. (See software)
Software (343): Software is the program (machine language, BASIC, Visual
BASIC, Fortran, C, C+, C++, Magic) that directs a computer’s function by
configuring the switches in the semiconductor material of a computer chip. (See
program, hardware)
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Machine language: (344) is a programming language that sets the configuration
of switches in a computer chip, which determines how data is processed. Though
cryptic to humans (10010111), machine language is the most efficient
programming code, for it does not require interpretationfor the computer uses it.
(See bits, bytes, appendix I)
Hardware (344): The computer chips or integrated circuits (IC), the mother
board they reside on, the DVD, hard drive, ports, plugs, box, and anything else
coffee can be spilled on.
Central processing unit (CPU) (344): The major components of a digital
computer: the arithmetic logic unit (ALU), the control unit (CU), and primary
memory (Random access memory or RAM), comprise the CPU. Input and output
devices, as well as secondary memory are peripherals (CD, DVD, floppy drives,
flash drives) that communicate with the CPU. (See appendix I)
Random access memory (RAM) (345): Semiconductor switches in microchips
have addresses: locations which may be accessed directly (randomly): a quick,
electronic process. When a program (such as word processing) is loaded from a
secondary storage device (such as a hard drive) it is copied into a RAM chip. As
changes are made they are made in RAM. The program must be saved back to the
hard drive to be retained. RAM is also called volatile memory, which means that
when power is turned off data in RAM is lost. (See appendix I)
Read only memory (ROM) (345): Memory that can only be read (used) and not
written to (changed).
Pixel (Picture element) (346): Commonly thought of as the dots on a monitor or
TV screen, but pixels also refer to the elements of certain digital detectors. The
term conveys the concept of reducing information into a matrix of discreet
elements (See matrix)
Matrix (346): An array of pixels arranged in rows and columns. The pixels of a
monitor or TV screen form a matrix. A large matrix (more pixels) = better spatial
resolution(See pixels)
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Voxel (Volume element) (346): Although each pixel on a monitor displays a two
dimensional representation of data, the data did not come from a finite plane. For
example, a sectional image of a CT scan is sampled by a fan beam which has a
thickness from 1mm to a cm. The volume of the tissue within the sampled tissue
contributes to the brightness of the pixels. This is the voxel (See appendix I)
Bandwidth (Bandpass) (350): The range of frequencies in a signal.
Conventional commercial TV has a 525 x 525 matrix (1-2 lp/mm), which is very
poor spatial resolution. Increasing the matrix size (more pixels) improves spatial
resolution but also requires the pixels to be scanned faster, which requires the
electron beam to modulate (change from pixel to pixel) more rapidly. The
frequency of modulation is measured in Hz, and is referred to as the bandwidth.
Commercial TV has a bandwidth of 4 MHz. A 1000 line (actually 1050 pixels
vertically) high definition monitor requires 20 MHz, and resolves 5-7 lp/mm.
Filtering (Convolution) (350-35): With image data stored as binary numbers,
mathematical algorithms (repeated applications of a formula, applied to raw data)
are used to enhance the appearance of the image. Digital filters do not add to the
data. They accentuate features by suppressing frequencies that may obscure detail,
like blowing dust from a written page enhances readability without altering the
print. This is done by suppressing spatial frequencies (which enhances others).
An example of anatomy with low spatial frequencies is the liver, for it is a
homogeneous shade of gray. High spatial frequencies would occur in trabecular
bone, which demonstrates heterogeneous shades of gray. (See appendix I)
Band-pass filtering: Demonstration (display) of a selected range of
frequencies.
High-pass filtering: Suppression of all but high frequency signals.
Also called sharpening or edge enhancement.
Low-pass filtering: Suppression of all but low frequency signals.
Smoothing filter: A low-pass that averages adjacent pixels
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Masking (350): A term used to denote filtering, as in the masking of suppressed
frequencies.
Signal to noise ratio (SNR) (347): Along with a signal (such as the video signal)
there are stray electrical currents that degrade image information. The higher the
ratio the better the signal. For example, a vidicon camera has a SNR of 200:1 due
the heat produced by an electron tube. A digital signal must be no less than
1000:1 to be of acceptable quality.
Windowing (Window level and width) (349): Digital images may have a
dynamic range of thousands of shades of gray (depending on the bit depth of the
processor) but the human eye can only distinguish about 32. To utilize this much
information a feature called windowing allows selective display of the dynamic
range. (See appendix I)
Window width: If 256 shades of gray were acquired during imaging,
but only half of those were chosen to be displayed on the monitor, the
window width would be 128. Widow width can be described as
controlling density (or brightness).
Window level (or center): The displayed dynamic range could be
at the high or low end of the scale, or anywhere in between. The
window level (or center) is the number the display is centered on. For
example, if the window width were 105, and the window center was 70,
the display of grays would extend from 18 to 122. (52 shades of gray
below 70, and 52 above, which = 105 shades of gray. Window level
can be described as controlling contrast.