Computer Terms Used in Radiography
To understand current literature about computers in radiography,
some basic understanding of computer terms is required. The following is a short discussion of some of the common terms
and how they apply to radiography.
Algorithm
A series of mathematical instructions to solve a problem, different from a formula. An algorithm
is a detailed set of instructions executed by the computer. Different algorithms varying in speed or complexity may
apply the same image processing formula to the image, achieving the same result.
Applications Software
Software that performs a specific job or application, such as word processing, image
processing, or network communications.
Aspect Ratio
The ratio of the width of an image to the image height, standard television has a 4:3 aspect
ratio.
Bandwidth
Used as a measure of capacity of a network, expressed in bits per second.
Bit
Binary digit, a single digit in the binary number system with a value of either 0 or 1.
Bridge
A connection between two similar networks that allows communication between them while it keeps
messages on the local network, if these messages do not have remote destinations. A busy network can be divided into
two segments with a bridge between them to reduce traffic on each segment and improve throughput.
Bus
An internal data "highway" in a computer, depending on the design, the bus may be 16 bits, 32 bits or
more, wide.
Byte
A grouping or string of bits that are operated on as a unit. Byte usually refers to a string of bits,
which can represent decimal values from 0 to 225 where 0 is represented by the binary string 00000000 and 255 is
represented by 11111111. A byte can also represent a single text character.
Compression Ratio
Comparison of the file size before and after compression, a file size reduced from 100
bytes to 50 bytes is said to have undergone 2:1 compression
Data Base
A file that contains data, frequently in table form.
Data-Base Management System (DBMS)
Software that enable a user to build, manage, and query a data base. A
relational DBMS is based on tables that can be linked together.
Data Quantities - Memory Requirements Image Storage
512 X 512 = 262 Kbytes
1,024 X 1,024 = 1 Mbyte
2,048 X 2,048 = 4 Mbytes Greater than eight bits of gray scale information, requires two bytes:
e.g.,
2,048 X 2,048 X 12 = 8 Mbytes
Text Storage
Average Letter
= 250 words / page
= 1500 letters
= 1.5
Kbytes
Digital Image Resolution
A digital image's size is often described by its dimensions in pixels. To specify
the spatial resolution of a digital image in object space, each pixel matrix dimension is divided by the corresponding
dimension of the original object. Take the example of a 35 x 35 cm image digitized to 2,048 X 2,048 pixels:
| image width (pixels) = | 2,048 pixels = | 5.85 (pixels) |
 | | |
| image width |
350mm | mm |
To express this pixel resolution in more familiar terms of line pairs per millimeter (often incorrectly called lines
per millimeter), the pixel resolution must be halved:
| 5.85 pixels |  | 1 line pair | | 2.93 line pairs |
| X | | X |
|
| mm | | 2 pixels | | mm |
Be aware that pixels are not necessarily always square, and resolution may not be the same in each dimension.
Display
Workstations and display stations enable soft copy viewing of radiographic images. A display
station generally consists of a computer, monitor, and (sometimes) special purpose hardware. In addition, users
manipulate images by means of a keyboard, mouse, or trackball. To alter the appearance of images, users might typically
change the window and level, magnify the image, flip or rotate the image, or perform filtering operations. Filtering
generally applies an image processing algorithm to an image and thereby changes its appearance. Common filters can
smooth a noisy image, sharpen the edges, or display the image in reverse video. Some display terms are defined herein.
DR11-W
A parallel data communication standard used to connect to DEC Computer buses at high speed, 15 meter
limit.
EPROM - Erasable PROM
Can be programmed, erasable with UV light
Ethernet
A network system generally implemented as a broadcast network in which every message is broadcast
to all other systems but only the destination station receives it. A system with a message to transmit listens to the
network and then, if the network is inactive, sends the message. If another system sends a message at the same time, a
collision occurs and both systems terminate their attempts to transmit, wait a short random time, and then attempt
transmission again.
Ethernet Network
| Media | Twisted pair
Coax
Fiber
|
| Topology | Bus
Branched bus (tree) |
| Signaling rate | 10 MBITS / second
Effective throughput
1-4
MBITS / second |
Fiber Distributed Data Interface (FDDI)
A fiber optic based ring network with higher throughput than
Ethernet and token ring networks.
| Media | Fiber |
| Topology | Ring |
| Topology | Ring |
| Signaling rate | 100 MBITS / second |
| Effective throughput | 80 - 100 MBITS / second |
Field
A scan of the image. In non-interlaced video, a field is equivalent to a frame. In interlaced video,
frames are composed of two fields, odd and even scan lines.
Fixed Discs
Hard disc, Winchester drives, parallel transfer discs, RAID (Redundant Array of Inexpensive
Discs) Capacity up to 1.2 GB, access time - 20ms, $/Mbyte - $2.70-5.30.
Frame
The entire video image, composed of at least one field.
Frame Buffer
A board, installed in the computer, capable of holding images for fast transfer to the image
buffer. This board enhances work station hardware to enable a user to quickly page among several screens of images and
is very useful for multiple image studies.
Gateway
A connection or computer between two different networks that allows communication between them.
Graphic User Interface (GUI)
An interface that sets up windows on the display with buttons, pull down
menus, slider bars, and icons to run programs. Typical GUIs include Xwindows (developed at Massachusetts Institute of
Technology, Cambridge), Windows (Microsoft, Redmond, Washington), and the Macintosh System software (Apple Computer,
Cupertino, California).
Histogram
A graph of the number of pixels (y axis) by their pixel value (x axis) used to assess images of
different ranges of pixel values and with histogram equalization display more of the image, reducing windowing, level
setting.
Horizontal
The number vertical line pairs that can be visualized in a displayed image, a characteristic of
the monitor. Horizontal resolution is usually best at the center of the CRT and quoted as such by the manufacturer.
Image Buffer
A board, installed in the computer, that produces images on the display monitor.
Characteristics of this board determine how fast images can be changed on the screen (e.g., how fast it refreshes the
screen whenever a user changes the window or level).
Interlaced
A type of video in which the odd and even fields are rapidly displayed (interlaced) to make a
complete frame.
LAN
Local Area Network
Level
The pixel values about which a window operates, used in combination with window width to select a
portion of the image data display.
Line Rate
The number of horizontal lines in a video frame, in many common video applications, there are 525
lines/frame.
Local and Remote Storage
Local storage is typically fast, fixed magnetic disk storage at the workstation.
This local storage allows the operator to rapidly retrieve and switch between copies of images of interest from the
archive without having to access the remote central archive through the network during the review session. Remote
archive typically employs larger, slower optical storage. Images from the archive must be sent over the network to the
workstation. In sophisticated systems, images which are expected to be required at the workstation can be fetched in
advance for the workstation. This provides the operator with relatively fast access to images while maintaining the
economy and control of a central archive.
Lossless Compression
Data is not lost or changed in compression, decompression produces an exact replica of
the original file. Lossless compression at 2.5:1 is readily achieved.
Lossy Compression
Some "superfluous" data is lost in compression, and exact replica of the original file is
impossible to achieve. Lossy compression of 20:1 is being investigated.
Luminance
Brightness of the display screen. This quality is important whenever one assesses the usefulness
of a display system for radiography.
LUT
Look Up Table — A set of output values corresponding to each possible input value, often used to
modify contrast or correct overall density.
Magnetic Disks
These disks are either "floppy" disks, which typically hold between 700 Kbytes and 2 Mbytes,
or "hard" disks, which can hold a maximum of approximately 1.5 Gbytes. Magnetic disks are subject to wear and
eventually will fail; such failure can destroy their contents. This type of storage generally provides fastest access.
Magnetic Tape
This kind of tape provides sequential, usually long term storage. Because access to a data
item requires a successful search for it, retrieval of a data item from magnetic tape tends to be slower than retrieval
from disk. Tape is used as archival media by many manufacturers of radiography equipment, although the trend is to use
magneto-optical disks on new equipment. Tape is usually the least expensive storage option.
Magneto-Optical Discs
MO (rewritable optical) Discs, Capacity 650MB, access time 38 - 50 ms, $/Mbyte - $.50
Magneto-Optical Disks
Erasable optical disks that combine the reliability of optical technology with the
ability to erase or rewrite data.
MAN
Metropolitan Area Network
Million Instructions Per Second (MIPS)
The average number of machine language instructions that a computer
performs in a second. More MIPS means faster computing.
Modem
Modern telecommunication channels are designed to carry analog (voice) information, unlike the
telegraph systems which carried digital (Morse code) information. To change the digital data to an analog form, a
device called a modem (modulator/demodulator) is used to change the digital data to analog form suitable
for transmission over telecommunication channels.
The speed of modems is characterized in baud and bits per second.
For the simplest modems, a baud is equal to one bit per second. For most modems, a baud and protocols used in imaging,
the bit per second transfer rate is higher than the baud rate. (The baud rate is actually the rate of change of states
of the line.)
Network Components
| Gateway | connects two or more networks together, can adapt protocols, controls
traffic between networks |
| Bridges | connect two or more network legs together, manages
traffic |
| Repeater | extends the distance limitation for a network
Transceiver connects devices to the network |
Non-Interlaced (Progressive Scan)
A type of video in which the entire frame content is contained in a
single field. The frame is presented by sequential display of image lines.
Open System Interconnection / International Standards Organization (OSI/ISO) Reference Model
A model of
computer communication architecture. This layered model defines the protocols that should be developed at each level of
communication.
Operating System
Software that controls the input, output, and memory U.S.A.ge of the computer. Application
software requests the use of computer resources from the operating system. Common operating systems are UNIX, DOS (disk
operating system) and the Macintosh System.
Optical Discs
WORM (Write-Once Read-Many) Discs, Capacity up to 6GB, $/Mbyte - $15 - 20
Optical Disks
Disks recorded with optical technology. These disks are considered less volatile and less
likely to lose information.
Parallel
A type of port or connection in which multiple bits(8, 16, or more) are transmitted simultaneously
over multiple wires.
Port
An electrical "jack" to make an external data connection to a computer or device.
PROM — Programmable ROM
Can be programmed once.
RAM — Random Access Memory
Can be changed in course of program, used for data storage, information is
lost when power is turned off, Capacity 1-32 MB, Access Time 80-120ns, $/Mbyte - $ 25-40.
Random Access Memory (RAM)
The memory in the computer that stores programs and data while they are active.
When the computer is turned off or the computer program is terminated, the memory no longer holds the information. For
example, suppose that a users is typing a letter with a word processor and the letter has not been saved to disk. If
the electricity fails, the letters is lost. If the letter was saved on the disk before the loss of electricity, the
only characters lost are those typed since the save was performed. Data in RAM are accessed faster than data on storage
devices such as disks or tapes.
Removable Discs
Bernouli, Syquest, (e.g., RAS Systems), Floppy Discs, Capacity up to 88 MB, access time -
20ms, $/Mbyte - 1.10.
ROM — Read Only Memory
Not erasable, permanent, used for instruction storage.
RS-232
A specific, standardized type of serial communication, slow (110 - 19,600 baud).
SCSI
A parallel data communication standard: Small Computer Systems Interface.
Serial
A type of port or connection in which data is transmitted serially, bit by bit, over one wire.
SMPTE
The Society of Motion Picture and Television Engineers, sponsors of recommended practice RP133, also
known as SMPTE test pattern for setting up and maintaining quality control of diagnostic displays.
Storage
Images, programs, and other data are stored in computer memory and on disks and tape. The capacity
of each type of storage is usually expressed in kilobytes (Kbytes), units of 1,024 bytes (sometimes considered,
loosely, to be 1,000 bytes, because the letter K, an abbreviation of the Greek word kilo, means thoU.S.A.nd, and the
number of bytes in memory is always a multiple of 1,024, the power of 2 closest to 1,000); megabytes (Mbytes), units of
1,048,576 bytes; or gigabytes (Gbyte), units of 1,073,741,824 bytes (1,024 x 1,024 x 1,024 bytes).
Tape
9 Track tape
Tape cartridges
Reel tapes, capacity up to 1TB
Token ring
A network system that is implemented as a ring. A bit string token is transmitted around the
ring from one node (such as a workstation) to another. When the token is received by a system, that system has
permission to transmit its message; once the message has been transmitted, the node transmits the token for another
node to use.
Transmission
Once in digital form, images are generally communicated to a display or workstation or to
archive storage over a digital network. A network is generally implemented on fiber optic or copper cable (which is
connected to a computer by network interface hardware) and is controlled through software. For one computer to talk
with another, both must use the same protocol, which is a combination of hardware, electrical and software agreement.
Transmission Control Protocol / Internet Protocol (TCP/IP)
The best known network protocol. Many software
and hardware vendors incorporate the TCP/IP into their products and thereby make network communications possible.
Ultranet Media Fiber
Topology Meshed Star
Signaling rate 1000 MBITS/second (hub)
250
MBITS/second (Star's effective throughput depends upon implementation)
Unsharp Mask
A digital copy of the original image which has been intentionally blurred (high frequency
information is lost). The difference between the original and blurry image is obtained by mathematically subtracting
the two images, pixel-for-pixel. This difference image is added to the original image to produce one with increased
sharpness. Unsharp masking may be linear (the blurring factor is constant) or non-linear (the blurring factor is a
function of density). FCR offers nonlinear unsharp mask enhancement.
Vertical
The number of active horizontal lines displayed in a displayed image. A nominal "525 line" monitor
usually displays fewer video lines which are inactive (lost during vertical retrace).
WAN
Wide Area Network
Window
A selection of a range of pixel values from an image to be displayed or transmitted; display devices
cannot display all of the data in most images simultaneously.
Word
A group or string of bits that are operated on as a unit. A word is usually a string of 16 bits, which
can represent decimal values from 0 to 65,535.
Write-Once Read-Many (WORM) disks.
Nonerasable disk. These are typically optical disks.
