Information Technology

Computer System

Basics

byte = 8 bits (0 or 1); kilo = 210 = 1024

integrated circuits = complex electronic circuitry compressed into small plastic packages

silicon chips = silicon/semiconductors with high V = 1, low V = 0

CPU (central processing unit) = microprocessor

  • CU = control unit = interpretation and executions of instructions
  • ALU = arithmetic/logic unit = data manipulation
  • clock rate = measure of speed in computations per sec
  • number of bits (32/64) determines how much data can be processed in parallel

RAM (random access memory) = short-term volatile memory

ROM (read only memory) = non-volatile permanent storage for basic aspects of computer operation

magnetic storage (tape, floppy disk, hard disk) stores by controlling local magnetism at different points

optical disks (CD [compact disk], DVD [digital versatile disk], blu-ray disk)

computer bus = communication pathway for components of computer

software

  • operating system
  • application packages
  • programming packages: editor (writing text), library of subroutines (small programs for common functions), linker (link written program to subroutine library) and compiler (translate written program to digital format)

Digital Image Processor

for image manipulation (eg enhancement) and image analyses (eg extraction of indices)

imaging system produces primary image information, fed to image acquisition circuitry for ADC and manipulation of data with an input look-up table (eg for pre-processing)

image display circuitry uses DAC, manipulation of displayed images to enhance appearance using output look-up table, for image magnification, displaying multiple images, annotation of images

ALU for image data (eg image subtraction in DSA)

array processor performs manipulations at higher speeds (eg FFT, image reconstruction)

high speed bus also connects to image memory, storage and computer interface

Digital Imaging

analogue = time varying signal with pixels influencing each other

digital = pixel/voxel signals are separate in a matrix format

sampling = digitisation of spatial information by dividing into sampling points = picture elements = N x N array of pixels; N is a power of 2

quantisation = digitisation of brightness = pixel values, the range of which is gray scale = G shades of gray; G = 2m where m = number of bits required (256/8, 1024/10, 4096/12) where 0 = black and 1023 = white

number of bits required = b = N x N x m

lossless compression maintains quality whereas lossy reduces quality for better compression ratio

Image Processing

contrast enhancement = windowing = gray-level transformation of pixel values for display purposes using output look-up tables (LUT); level = threshold where lower pixel values displayed as black, window = threshold for white, window width = distance between white and black

kernels are matrices which are centred on each pixel and figures multiplied to the existing pixel values

sharpening – kernel (negative peripheral values) enhancing sharp discontinuities between pixels

edge enhancement – kernel to highlight edges

median filter = form of smoothing – replaces centre of sample with median

Gaussian filtering – replaces central pixel with weighted sum (in Gaussian distribution) of sample

erosion filter – replaces pixels with brightest of surrounding sample

dilation filter – replaces pixel with darkest of surrounding sample

shadow filter – places shadow of opposite density adjacent to structures

threshold filter – changes values to white or black depending on a threshold

Fourier Transform

FT converts image into a combination of sine waves with differing amplitudes (brightness), phases and frequencies (spatial frequency, generating positive and negatives values), ie frequency domain/space; can be plotted with amplitude vs spatial frequency (1D) or fy vs fx with different brightness (2D)

small objects with sharp edges require increasingly high spatial frequencies; a perfectly sharp edge has all spatial frequencies in equal proportion

frequency representation enables data manipulation (eg removal of noise at certain frequencies)

calculated numerically and quickly using the fast Fourier transform (FFT) rather than the slower analytical discrete Fourier transform (DFT)

inverse Fourier transform (iFT) transforms back to image domain/space

Picture Archival and Communication Systems (PACS)

system enabling access to data of imaging modalities and transfer at high speeds (gigabit LAN = local area network using TCP/IP) to remote viewing consoles or archival storage; connects to reporting/clinical workstations, web server, teleradiology server and laser imager

teleradiology sends images to another location to enable viewing elsewhere

images from modalities (incl film digitisers) are sent to modality servers with RAID (redundant array of independent disks) and online archive simultaneously

offline archive is the final long-term storage with magnetic tapes (slow but durable) with copies sent off-site for disaster recover

database servers keep track of image data, move and copy studies between servers, communicate with RIS/HIS and provide modality work-list services (passing orders to modalities)

requires standardisation of image data interchange (DICOM), interface with RIS (radiology information system) and thus HIS (hospital information system) to avoid duplications and erroneous patient data (using HL7 = hospital language 7 standard), high quality image displays, user friendly, efficient distribution of images with short transfer times

DICOM (digital imaging and communications in medicine) defines network transmission protocol, storage information (name, age, NHI etc) and monitor calibration and specification requirements

Diagnostic Monitors

colour LCDs have 3 subpixels/elements (red, green, blue)

medical grade LCDs have greater luminescence (Cd)

greyscale LCDs use these to modulate the greyscale to give more bits to the image; also have higher luminance due to lack of colour filter; require special separate video card

digital LCDs and images systems miss out the DAC step which would otherwise cause noise and less likely to pick up interference

human eye can resolve 1 minute of arc (1/60°) at 600mm ≈ pixel size of 0.175mm

JND (just noticeable difference) = minimum luminance change for average observer to notice a difference with a finite JNDs per luminance range (hence higher luminance = more JNDs); 10bit grey systems provide enough grey levels to access all the JNDs of the human eye

recommend minimum luminance of 170 Cd/m2 and min depth of 8 bits, 2MPX; ideally 500Cd/m2 and 10 bits, 3MPX

response curve = monitor’s unique luminance output to the input range of grey levels

monitors must be calibrated to the DICOM grey scale definition function (GSDF) due to human eye perception not linear, calibrated so perceived brightness corresponds to original greyscale data

Workstations

separate hard drives for operating system and data storage so applications do not slow the system

large memory to enable fast interactive viewing of images

Compression

can be compressed by 20% without any loss of data (lossless compared to lossy which reduces amount of information; takes time to uncompress)

wavelet compression (used by InteleViewer) breaks image into series of blocks (transformations) that overlay to give increasing resolution

Internet

modem (modulator/demodulator)

hub = connect anything to anything with many connections causing packet collisions

switches = connect network ports with traffic directed to recipients

routers = programmable and decide which way to send communications, connecting different networks to buffer and transfer data between

WWW (world wide web) = global assemblage of computer networks

TCP/IP (transmission control protocol/internet protocol) = standard internet protocol

SLIP (serial line internet protocol) and PPP (point-to-point protocol) for communication over phone line

FTP (file transfer protocol) for exchange of files

HTTP (hypertext transfer protocol) = for transfer of documents containing different media

DHCP (dynamic host configuration protocol) = automatic allocation of an IP address

POP (post office protocol), SMTP (simple mail transfer protocol) and NNTP (network news transfer protocol) for electronic mail and newsgroups

MIME (multipurpose internet mail extensions) = for incorporating audio and image files via e-mail

HTML (hypertext mark-up language) for writing HTTP documents

GIF (graphical interchange format), JPEG (joint photographical experts group) and PNG (portable network graphics) = image formats

MPEG (moving picture experts group ) – image sequences format

URL (uniform resource location) = address in format: transfer protocol://www.server.type of environment.country code/document name.document format

applet = small compiled application downloaded with HTML to run on client computer (eg Java)

CGI (common gateway interface) = scripts = software for server-side processing