Computed tomography instrumentation and operation
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COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION. OUTLINE. CT SYSTEM COMPONENTS – DEFINITION OF A SCANNER SCANNER COORDINATE SYSTEM – XYZ, ISOCENTER IMAGING SYSTEM COMPUTER SYSTEM DISPLAY, RECORDING, AND STORAGE SYSTEMS. CT MAIN SYSTEMS. IMAGING SYSTEM COMPUTER SYSTEM

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COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION

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COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION


OUTLINE

  • CT SYSTEM COMPONENTS – DEFINITION OF A SCANNER

  • SCANNER COORDINATE SYSTEM – XYZ, ISOCENTER

  • IMAGING SYSTEM

  • COMPUTER SYSTEM

  • DISPLAY, RECORDING, AND STORAGE SYSTEMS


CT MAIN SYSTEMS

  • IMAGING SYSTEM

  • COMPUTER SYSTEM

  • DISPLAY, RECORDING, STORAGE SYSTEM

  • DATA ACQUISITION SYSTEM


CT SYSTEM

GANTRY

X-RAY TUBE

GANTRY CONTROL

DETECTORS

HIGH VOLTAGE

GENERATOR

DAC

S/H

ADC

SCAN CONTROLLER

ARRAY PROCESSOR

HOST

COMPUTER

CONSOLE

STORAGE


SCANNER


SCANNER

  • GANTRY

  • PATIENT COUCH


GANTRY HOUSES:

  • X-RAY TUBE

  • GENERATOR (LOW VOLTAGE DESIGN)

  • COLLIMATORS

  • DETECTORS


GANTRY CHARACTERISTICS

  • APERTURE

  • TILTING RANGE


MOST OF THE SCANNERS HAVE 70CM APERTURE


70 CM


COORDINATE SYSTEM

X


COORDINATE SYSTEM

Y


COORDINATE SYSTEM

Z


ISOCENTER


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


PATIENT COUCH :450 LBS (204 KG) DISTRIBUTED WEIGHT LIMIT


SCANNABLE RANGE:COVERAGE FROM HEAD TO THIGH (162CM)


MAX. SCANNABLE RANGE


IMAGING SYSTEM

  • PRODUCTION OF X-RAYS

  • SHAPING OF X-RAY BEAM ENERGY

  • FILTERING X-RAY BEAM


IMAGING SYSTEM COMPONENTS

  • X-RAY TUBE

  • GENERATOR –HIGH VOLTAGE

  • COLLIMATORS

  • FILTER

  • DETECTORS

  • DETECTOR ELECTRONICS


X-RAY TUBE AND X-RAY PRODUCTION


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

IN CT – STILL SMALL AND LARGE


THERMIONIC EMISSION

CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS

TO LIBERATE ELECTRONS FOR TRANSIT TO ANODE


FOCAL SPOT- CT UTILIZES DIFFERENT FOCAL SPOTS

  • THE FILAMENT SIZE – LENGTH – FOCAL SPOT

SMALLER FOCAL SPOT - Low mA

SMALLER FOCAL SPOT – sharper image


ANODE +++++ MADE OF TUNGSTEN AND MOLYBDENUM

TUNGSTEN

TARGET

TARGET MADE OF TUNGSTEN

AND

RHENIUM


mA – tube current

  • The number of electrons flowing from cathode to anode


kVp

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


S –time of exposuremAs tube current for certain length of time


X-RAY PRODUCTION RESULTS IN A LOT OF HEAT AND VERY LITTLE X-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 – TECHNIQUE COMPENSATION

  • kVp

  • mA

  • Time

INCREASED NOISE


TOO LOW OF kVp:

  • NOISE !!!!


X-RAY EMISSION


TUBE CURRENT CHANGE

INTENSITY

ENERGY – NO CHANGE

CURRENT

2 * mA = 2 * number of photons

4 * mA = 4 * number of photons


Why changing mA or time

  • Avoiding motion – mA time

  • Pediatric technique modification

  • Reducing noise - mAs

MOTION

NOISE


Tube voltage (kVp) CHANGE

INTENSITY -

ENERGY–

kVp

15% INCREASE OF KVP = 2 * mAs


kVp IN CT

  • 80-140

  • TOO LOW – NOISE

    (NOT ENOUGH PENETRATION OF THE PATIENT )

    PHOTON STARVATION - NOISE!!!!!


HIGH VOLTAGE GENERATOR –(HVG)

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


CT GENERATOR

  • 5-50 kHz

  • 30-60 kW

KVP SELECTION:

80, 100, 120, 130,140

mA selection:

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


COLLIMATION IN CT

PRE-PATIENT COLLIMATION

POST-PATIENT COLLIMATION

ADC


BASIC DATA AQUSITION SCHEME IN CT

FILTRATION

ADC


FILTRATION CHANGE

INTENSITY

ENERGY –

FILTRATION


FILTRATION MATERIAL

  • ALUMINIUM ( SPECIAL FILTER IN CT)

BOWTIE

TO MAKE THE BEAM HARDER AND

MORE MONOENERGETIC


Filter

DEFINES SLICE

THICKNESS

Patient

REDUCES SCATTER

RECHING THE PATIENT

Detector


CT DETECTORS


DETECTOR TYPES: SCINTILLATION

S. CRYSTAL

S. CRYSTAL

PHOTODIODE

PM TUBE


SCINTILLATION CRYSTALS USED WITH PM TUBES:

  • SODIUM IODIDE –AFTERGLOW + LOW DYNAMIC RANGE ( USED IN THE PAST)

  • CALCIUM FLUORIDE

  • BISMUTH GERMANATE


S. CRYSTAL USED WITH PHOTODIODE

  • CALCIUM TUNGSTATE

  • RARE EARTH OXIDES - CERAMIC


DETECTOR TYPE: GAS IONIZATION

XENON GAS

30 ATM


EFFICIENCY OF DETECTORS- QDE

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

  • GAS – 50% - 60%


COMPUTER SYSTEM

  • RECONSTRUCTION AND POSTPROCESSING

  • CONTROL OF ALL SCANNER COMPONENTS

  • CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.

  • DATA FLOW DIRECTION


COMPUTER SYSTEM IN CT

  • MINICOMPUTERS


COMPUTER SYSTEM COMPOSED OF:

  • HARDWARE

  • SOFTWARE


COMPUTER PROCESSING IN CT

  • SEQUENTIAL PROCESSING

  • MULTITASKING

  • MULTIPROCESSING


SOFTWARE –PROGRAM (S) HELPING CT USER TO COMMUNICATE WITH THE CT SYSTEM


CT OPERATING SYSTEM-PROGRAMS THAT CONTROL THE HARDWARE COMPONENTS AND THE OVERALL OPERATION OF THE CT COMPUTER


CT OPERATING SYSTEM

  • UNIX

  • WINDOWS


HOST COMPUTER

  • CONTROL OF ALL COMPONENTS

  • CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.

  • DATA FLOW DIRECTION


ARRAY PROCESSOR

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

THE MORE PROCESSORS IN THE COMPUTER

THE SHORTER THE RECONSTRUCTION TIME


DATA ACQUISITION SYSTEM (DAS)

  • SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER.

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


AMPLIFIER

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


ADC

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


SAMPLE/HOLD UNIT (S/H)

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


DAS

GANTRY

X-RAY TUBE

GANTRY CONTROL

DETECTORS

HIGH VOLTAGE

GENERATOR

DAC

S/H

ADC

SCAN CONTROLLER

ARRAY PROCESSOR

HOST

COMPUTER

CONSOLE

STORAGE


IMAGE DISPLAY, RECORDING, STORAGE

  • DISPLAYS IMAGE ( OUTPUT FROM COMPUTER)

  • PROVIDES HARD COPY OF THE IMAGE

  • FACILITATES THE STORAGE AND RETRIEVAL OF DIGITAL DATA

  • COMMUNICATES IMAGES IN THE NETWORK


IMAGE DISPLAY


IMAGE RECORDING SYSTEMS(LASER PRINTERS)

  • SOLID STATE LASER PRINTERS

  • GAS LASER PRINTERS


HARD COPY


IMAGE STORAGE MEDIA

  • MAGNETIC TAPES

  • MAGNETO-OPTICAL DISK (MOD)

  • CD


COMMUNICATION

  • PACS


OVERREAD NETWORK

While most teleradiology systems purchased over the last decade were intended for on-call purposes, the past two years have seen a rapid increase in the use of teleradiology to link hospitals and affiliated satellite facilities, other primary hospitals, and imaging centers. A number of the enabling technologies needed for effective overread networks, such as more affordable high-speed telecommunications networks and improved data compression techniques, have matured in recent years.


NightHawk Radiology Services has developed an innovative approach to the delivery of radiology services by operating centralized, state-of-the-art reading centers in Sydney, Australia and Zurich, Switzerland. Staffing U.S.-trained, board-certified radiologists specializing in emergency radiology, these locations are ideally situated for U.S. care because when it’s the middle of the night in Boston, it’s daytime “Down Under.” When it’s early morning in Los Angeles, it’s daytime in the Alps. From the centralized reading centers, NightHawk radiologists interpret exams and report the results to attending physicians in real-time, usually less than 20 minutes.


CT ROOM LAYOUT


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