COMPUTED TOMOGRAPHYCOMPUTED TOMOGRAPHY INSTRUMENTATION AND INSTRUMENTATION AND

OPERATIONOPERATION

Dr. Halima HawesaLecture 3-RAD 466

OUTLINEOUTLINE CT SYSTEM COMPONENTS – DEFINITION OF CT SYSTEM COMPONENTS – DEFINITION OF

A SCANNER A SCANNER SCANNER COORDINATE SYSTEM – XYZ, SCANNER COORDINATE SYSTEM – XYZ,

ISOCENTERISOCENTER IMAGING SYSTEMIMAGING SYSTEM COMPUTER SYSTEM COMPUTER SYSTEM DISPLAY, RECORDING, AND STORAGE DISPLAY, RECORDING, AND STORAGE

SYSTEMSSYSTEMS

CT MAIN SYSTEMS

IMAGING SYSTEMIMAGING SYSTEM COMPUTER SYSTEMCOMPUTER SYSTEM DISPLAY, RECORDING, STORAGE DISPLAY, RECORDING, STORAGE

SYSTEMSYSTEM DATA ACQUISITION SYSTEMDATA ACQUISITION SYSTEM

CT SYSTEMCT SYSTEMGANTRY

DETECTORS

S/H

ADC

ARRAY PROCESSOR

HOST COMPUTER

STORAGECONSOLE

SCAN CONTROLLER

DAC

GANTRY CONTROL

HIGH VOLTAGE GENERATOR

X-RAY TUBE

SCANNERSCANNER

SCANNER

GANTRYGANTRY PATIENT COUCHPATIENT COUCH

GANTRY HOUSES:GANTRY HOUSES:

X-RAY TUBEX-RAY TUBE GENERATOR (LOW VOLTAGE DESIGN)GENERATOR (LOW VOLTAGE DESIGN) COLLIMATORSCOLLIMATORS DETECTORSDETECTORS

GANTRY CHARACTERISTICS

APERTUREAPERTURE TILTING RANGETILTING RANGE

MOST OF THE SCANNERS MOST OF THE SCANNERS HAVE 70CM APERTUREHAVE 70CM APERTURE

70 CM

COORDINATE SYSTEMCOORDINATE SYSTEM

X

COORDINATE SYSTEMCOORDINATE SYSTEM

Y

COORDINATE SYSTEMCOORDINATE SYSTEM

Z

ISOCENTERISOCENTER

TILTING RANGE OF MOST TILTING RANGE OF MOST SCANNERS- +30 TO -30 SCANNERS- +30 TO -30

DEGREESDEGREES

PATIENT COUCH :PATIENT COUCH :

450 LBS (204 KG) 450 LBS (204 KG) DISTRIBUTED WEIGHT LIMITDISTRIBUTED WEIGHT LIMIT

SCANNABLE RANGE:SCANNABLE RANGE:

COVERAGE FROM HEAD TO COVERAGE FROM HEAD TO THIGH (162CM) THIGH (162CM)

MAX. SCANNABLE RANGEMAX. SCANNABLE RANGE

IMAGING SYSTEMIMAGING SYSTEM

PRODUCTION OF X-RAYSPRODUCTION OF X-RAYS SHAPING OF X-RAY BEAM ENERGYSHAPING OF X-RAY BEAM ENERGY FILTERING X-RAY BEAMFILTERING X-RAY BEAM

IMAGING SYSTEM COMPONENTS

X-RAY TUBEX-RAY TUBE GENERATOR –GENERATOR –HIGH VOLTAGEHIGH VOLTAGE COLLIMATORSCOLLIMATORS FILTER FILTER DETECTORSDETECTORS DETECTOR ELECTRONICSDETECTOR ELECTRONICS

X-RAY TUBE AND X-RAY X-RAY TUBE AND X-RAY PRODUCTIONPRODUCTION

CATHODE --------CATHODE --------MADE OF TUNGSTENMADE OF TUNGSTEN

IN CT – STILL SMALL AND LARGE

THERMIONIC EMISSIONTHERMIONIC EMISSION

CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS TO LIBERATE ELECTRONS FOR TRANSIT TO ANODE

FOCAL SPOT- CT UTILIZES FOCAL SPOT- CT UTILIZES DIFFERENT FOCAL SPOTS DIFFERENT FOCAL SPOTS

THE FILAMENT SIZE – LENGTH – THE FILAMENT SIZE – LENGTH – FOCAL SPOTFOCAL SPOT

SMALLER FOCAL SPOT - Low mA

SMALLER FOCAL SPOT – sharper image

ANODE +++++ MADE OF ANODE +++++ MADE OF TUNGSTEN AND TUNGSTEN AND MOLYBDENUMMOLYBDENUM

TUNGSTENTARGET

TARGET MADE OF TUNGSTEN AND

RHENIUM

mA – tube currentmA – tube current

The number of electrons flowing from The number of electrons flowing from cathode to anodecathode to anode

kVpkVp

Potential difference between cathode Potential difference between cathode and anode (Volts) kilo means 1,000 and anode (Volts) kilo means 1,000 x.x.

S –time of exposureS –time of exposure

mAs tube current for mAs tube current for certain length of timecertain length of time

X-RAY PRODUCTION RESULTS X-RAY PRODUCTION RESULTS IN A LOT OF HEAT AND VERY IN A LOT OF HEAT AND VERY

LITTLE LITTLE X-RAYS BEING GENERATEDX-RAYS BEING GENERATED

HEAT UNITS CALCULATION

HU= kVp X mA x time

MOST CT TUBES HEAT CAPACITY 3-5 MILLION HU

REDUCTION OF HEAT UNITS – REDUCTION OF HEAT UNITS – TECHNIQUE COMPENSATIONTECHNIQUE COMPENSATION

kVp kVp

mAmA

TimeTime

INCREASED NOISE

TOO LOW OF kVp:TOO LOW OF kVp:

NOISE !!!!NOISE !!!!

Why changingWhy changing mA or time mA or time

Avoiding motion – mA timeAvoiding motion – mA time Pediatric technique modificationPediatric technique modification Reducing noise - mAsReducing noise - mAs

NOISE MOTION

Tube voltage (kVp) CHANGETube voltage (kVp) CHANGE

kVp

INTENSITY -

ENERGY –

15% INCREASE OF KVP = 2 * mAs

kVp IN CTkVp IN CT

80-14080-140 TOO LOW – NOISE TOO LOW – NOISE

(NOT ENOUGH PENETRATION OF THE PATIENT )(NOT ENOUGH PENETRATION OF THE PATIENT )

PHOTON STARVATION - NOISE!!!!!PHOTON STARVATION - NOISE!!!!!

HIGH VOLTAGE GENERATOR HIGH VOLTAGE GENERATOR –(HVG)–(HVG)

GENERATES HIGH VOLTAGE GENERATES HIGH VOLTAGE POTENTIAL BETWEEN CATHODE AND POTENTIAL BETWEEN CATHODE AND ANODE OF AN X-RAY TUBEANODE OF AN X-RAY TUBE

CT GENERATOR

5-50 kHz5-50 kHz 30-60 kW30-60 kW

KVP SELECTION:

80, 100, 120, 130,140

mA selection:

30, 50, 65, 100, 125, 150, 175, 200, 400

Filter

Patient

DEFINES SLICE THICKNESS

REDUCES SCATTER RECHING THE PATIENT

Detector

CT Imaging Steps

COLLIMATION IN CTCOLLIMATION IN CTBASIC DATA AQUSITION SCHEME IN CTBASIC DATA AQUSITION SCHEME IN CT

ADC

PRE-PATIENT COLLIMATION

POST-PATIENT COLLIMATION

FILTRATION

X-RAY EMISSIONX-RAY EMISSIONTUBE CURRENT CHANGETUBE CURRENT CHANGE

CURRENTINTENSITY

ENERGY – NO CHANGE

2 * mA = 2 * number of photons4 * mA = 4 * number of photons

FILTRATION CHANGEFILTRATION CHANGE

FILTRATION

INTENSITY

ENERGY

FILTRATION MATERIALFILTRATION MATERIAL

ALUMINIUM ( SPECIAL FILTER IN CT)ALUMINIUM ( SPECIAL FILTER IN CT)

TO MAKE THE BEAM HARDER AND MORE MONOENERGETIC

CT DETECTORSCT DETECTORS

DETECTOR TYPES: DETECTOR TYPES: SCINTILLATIONSCINTILLATION

PM TUBE

S. CRYSTALS. CRYSTAL

PHOTODIODE

SCINTILLATION CRYSTALS SCINTILLATION CRYSTALS USED WITH PM TUBES:USED WITH PM TUBES:

SODIUM IODIDE SODIUM IODIDE CALCIUM FLUORIDECALCIUM FLUORIDE

BISMUTH GERMANATEBISMUTH GERMANATE

S. CRYSTAL USED WITH S. CRYSTAL USED WITH PHOTODIODEPHOTODIODE

CALCIUM TUNGSTATECALCIUM TUNGSTATE RARE EARTH OXIDES - CERAMICRARE EARTH OXIDES - CERAMIC

DETECTOR TYPE: GAS IONIZATIONDETECTOR TYPE: GAS IONIZATION

XENON GAS

30 ATM

EFFICIENCY OF DETECTORS- EFFICIENCY OF DETECTORS- QDEQDE

SCINTILLATION – 95% - 100%- SCINTILLATION – 95% - 100%- COMMONLY USED IN III & IV COMMONLY USED IN III & IV GENERATION SCANNERSGENERATION SCANNERS

GAS – 50% - 60%GAS – 50% - 60%

COMPUTER SYSTEMCOMPUTER SYSTEM

RECONSTRUCTION AND RECONSTRUCTION AND POSTPROCESSINGPOSTPROCESSING

CONTROL OF ALL SCANNER CONTROL OF ALL SCANNER COMPONENTSCOMPONENTS

CONTROL OF DATA ACQUSITION, CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.PROCESSING, DISPLAY.

DATA FLOW DIRECTION DATA FLOW DIRECTION

COMPUTER SYSTEM IN CTCOMPUTER SYSTEM IN CT

MINICOMPUTERSMINICOMPUTERS

COMPUTER SYSTEM COMPUTER SYSTEM COMPOSED OF:COMPOSED OF:

HARDWAREHARDWARE SOFTWARESOFTWARE

COMPUTER PROCESSING IN COMPUTER PROCESSING IN CTCT

SEQUENTIAL PROCESSINGSEQUENTIAL PROCESSING MULTITASKINGMULTITASKING MULTIPROCESSINGMULTIPROCESSING

SOFTWARE –PROGRAM (S) SOFTWARE –PROGRAM (S) HELPING CT USER TO HELPING CT USER TO

COMMUNICATE WITH THE COMMUNICATE WITH THE CT SYSTEMCT SYSTEM

CT OPERATING SYSTEM-CT OPERATING SYSTEM-PROGRAMS THAT CONTROL PROGRAMS THAT CONTROL

THE HARDWARE THE HARDWARE COMPONENTS AND THE COMPONENTS AND THE OVERALL OPERATION OF OVERALL OPERATION OF

THE CT COMPUTERTHE CT COMPUTER

CT OPERATING SYSTEMCT OPERATING SYSTEM

UNIXUNIX WINDOWSWINDOWS

HOST COMPUTER

CONTROL OF ALL COMPONENTSCONTROL OF ALL COMPONENTS CONTROL OF DATA ACQUSITION, CONTROL OF DATA ACQUSITION,

PROCESSING, DISPLAY.PROCESSING, DISPLAY. DATA FLOW DIRECTION DATA FLOW DIRECTION

ARRAY PROCESSORARRAY PROCESSOR

TAKES DETECTOR MEASUREMENTS TAKES DETECTOR MEASUREMENTS FORM HUNDREDS OF PROJECTIONS. FORM HUNDREDS OF PROJECTIONS. RESPONSIBLE FOR RETROSPECTIVE RESPONSIBLE FOR RETROSPECTIVE RECONSTRUCTION AND RECONSTRUCTION AND POSTPROCESSING OF DATA. POSTPROCESSING OF DATA.

THE MORE PROCESSORS IN THE COMPUTER THE SHORTER THE RECONSTRUCTION TIME

DATA ACQUISITION SYSTEM DATA ACQUISITION SYSTEM (DAS)(DAS)

SET OF ELECTRONICS BETWEEN SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER. DETECTORS AND HOST COMPUTER.

IT CONTAINS: AMPLIFIER, ADC, DAC, IT CONTAINS: AMPLIFIER, ADC, DAC, GENERATOR, S/H. GENERATOR, S/H.

AMPLIFIERAMPLIFIER

SIGNAL FROM DETECTORS GOES TO SIGNAL FROM DETECTORS GOES TO AMPLIFIERS FOR SIGNAL AMPLIFIERS FOR SIGNAL MAGNIFICATION AND THEN IS SENT MAGNIFICATION AND THEN IS SENT TO SAMPLE/HOLD UNITTO SAMPLE/HOLD UNIT

ADCADC

CONVERTS ANALOG SIGNAL OUTPUT CONVERTS ANALOG SIGNAL OUTPUT FROM THE SCANNING EQUIPMENT TO FROM THE SCANNING EQUIPMENT TO A DIGITAL SIGNAL SO IT CAN BE A DIGITAL SIGNAL SO IT CAN BE PROCESSED BY A COMPUTER.PROCESSED BY A COMPUTER.

SAMPLE/HOLD UNIT (S/H)SAMPLE/HOLD UNIT (S/H)

LOCATED BETWEEN LOCATED BETWEEN AMPLIFIERAMPLIFIER AND AND ADC ADC PERFORMS SAMPLING AND ASSIGNS PERFORMS SAMPLING AND ASSIGNS SHADES OF GRAY TO THE PIXELS IN THE SHADES OF GRAY TO THE PIXELS IN THE DIGITAL MATRIX CORRESPONDING TO DIGITAL MATRIX CORRESPONDING TO THE STRUCTURESTHE STRUCTURES

DASDASGANTRY

DETECTORS

S/H

ADC

ARRAY PROCESSOR

HOST COMPUTER

STORAGECONSOLE

SCAN CONTROLLER

DAC

GANTRY CONTROL

HIGH VOLTAGE GENERATOR

X-RAY TUBE

IMAGE DISPLAY, RECORDING, STORAGE

DISPLAYS IMAGE ( OUTPUT FROM DISPLAYS IMAGE ( OUTPUT FROM COMPUTER)COMPUTER)

PROVIDES HARD COPY OF THE IMAGEPROVIDES HARD COPY OF THE IMAGE FACILITATES THE STORAGE AND FACILITATES THE STORAGE AND

RETRIEVAL OF DIGITAL DATARETRIEVAL OF DIGITAL DATA COMMUNICATES IMAGES IN THE COMMUNICATES IMAGES IN THE

NETWORKNETWORK

IMAGE IMAGE DISPLAYDISPLAY

IMAGE RECORDING SYSTEMSIMAGE RECORDING SYSTEMS(LASER PRINTERS)(LASER PRINTERS)

SOLID STATE SOLID STATE LASER PRINTERSLASER PRINTERS

GAS LASER GAS LASER PRINTERSPRINTERS

HARD COPYHARD COPY

IMAGE STORAGE MEDIAIMAGE STORAGE MEDIA

MAGNETIC TAPESMAGNETIC TAPES MAGNETO-OPTICAL DISK (MOD)MAGNETO-OPTICAL DISK (MOD) CDCD

COMMUNICATIONCOMMUNICATION

PACSPACS

OVERREAD NETWORKOVERREAD NETWORK While most teleradiology systems purchased over While most teleradiology systems purchased over

the last decade were intended for on-call the last decade were intended for on-call purposes, the past two years have seen a rapid purposes, the past two years have seen a rapid

increase in the use of teleradiology to link increase in the use of teleradiology to link hospitals and affiliated satellite facilities, other hospitals and affiliated satellite facilities, other

primary hospitals, and imaging centers. A number primary hospitals, and imaging centers. A number of the enabling technologies needed for effective of the enabling technologies needed for effective overread networks, such as more affordable high-overread networks, such as more affordable high-

speed telecommunications networks and speed telecommunications networks and improved data compression techniques, have improved data compression techniques, have

matured in recent yearsmatured in recent years..

CT ROOM LAYOUTCT ROOM LAYOUT

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