(E1) System Operation and Instrumentation

components of the CT system

• operating console

• computer

• gantry (donut)

the operating console is the

key point of interaction between technologist and imaging system (how the tech and system communicate)

examples of graphical monitors:

• technologist console

• work stations for reconstruction

• physicians work station

operating console:

1. annotation of patient data (on image)

2. identificiation for each image

3. adjust various imaging parameters

4. allows reviewing of resulting image (original) before transfer to hard copy or a PACS system for the physician view

‘Host computer’ is the

primary link between technologist and imaging system

• hard drives, archiving, and networking

the host computer is where _____ equations are solved

mathematical

what is reconstruction time

the time between the end of imaging and the appearance of an image

• array processor

the retrieving capabilities of a “host” computer can be ____

networked

archiving portion of “host” computer

• optical disk/tape

• magnetic disk (floppy disk)/tape

• film/CD/ digital videotap

• PACS

other capabilites of the host computer:

• quality assurance checks

• calibration checks

• record system issues to the service engineer

what are the two types of information in the “host” computer

1. analog

2. digital

what is analog

• continuous wave pattern used by machinery

  • gantry, table

what is digital

• discrete number used by computers

what are the key components of the gantry?

• Slip rings and high voltage generator

• Cooling system

• CT x-ray tube

• detectors

• collimators

• table (slides through gantry)

what are slip rings

brush like apparatus that allows for continuous scanning (1989)

what is a generator

high voltage transmitted to the x-ray tube (kilowatts)

what is the cooling system?

blowers, filters, devices that perform oil-to-air heat enchange

• could produce 30 scans per exam, 10-20 exams per day= 10,000 exposures per month

what is the CT x-ray tube

• mounted on a rotating assembly

• travels around the patient

what is multislice spiral CT

• high termal demand

  • tube energized 60 seconds continuously

what happens in the cathode

filament (thoriated tungsten wire) is heated and electrons are boiled off

small focal spot size

• strikes a smaller region of the target; narrow electron beam

• good for thin slices and high spatial resolution imaging

  • extremities, high resolution chest, cardiac imaging

what is spatial resolution

ability of a system to define small objects distinctly

large focal spot size

• used for larger scans- strikes larger portion of the anode

• chest, abdome, thorax

the temperature of the filament determines what

how many electrons flow through the tube

• controlled by the current in the wire(mA)

x-ray quantity is _____ to the number of electrons comin gfrom the cathode

proportional

energy level of the photons is controlled by ____

kVp

• quality of the beam

• determines energy of photons ability to penetrate

• tube voltage

the anode:

• high heat temperature are generated

• made of tungsten alloy

• rotates to minimize heat build up on any one spot

what is inter-scan delay

occurs briefly to allow cooling of the tube

what is the detector array

the entire collection o fdetectors inculded in the CT system

• situated in an arch or ring

what s a detector

a single element or single type of detector used in the CT system

what are xenon ionizaztion chanbers- gas detector

• contain xenon pressurizzed to 30 times normal atmospheric pressure

• airtight chamber that is 1 millimeter wide

where can a xenon ionizaiton chamber NOT be used

in helical scanning

where can a xenon ionization chamber BE USED

only in 3rd generation scanners- not common anymore

how does a xenon ionization chamber- gas detector work?

• signal is produced when a photon collides with a xenon atom

• impulse is amplified and sent to DAS

• digitized by the ADC and sent to the array processor to make the image

what are the two detector types

• solid state crystal

• scintillation detector

Solid state crystal or scintillation detectors can be used in

all helical and MSCT scanners

the detector material of solid state crystal or scintillation detectors is

solid crystalline

• cadmium tungstate

characteristics of cadmium tungstate

• more dense than xenon gas

• more sensitive to incoming x-rays at various angles

How do the detector types work?

• x-ray photon is detected by the solid state material

• reaction between the photon and crystal creates a flash of light

• light is converted to a usable electrical signal then is amplified and sent to the DAS and digitized

advntages of solid state crystal or scintillation detectors

• smaller and cheaper

• less positioning limitations

• more sensitive

  • 90% of x-rays to detector being absorbed

important characteristics of detectors

• capture efficiency

• absorption efficiency

• conversion efficiency

what is capture efficiency

how well the detectors recieve photons from the patient

what is absorption efficiency

how well the detectors convert/absorb incoming x-ray photons

what is conversion efficiency

determined by how well the detector converts the abrosbed photon info to a digital signal for the computer

what are the 4 conditions needed for detection

1. photons need to enter the detector

2. photon needs to collide with an atom

3. conversion (light or ion) is produced

4. amplification of the signal occurs

function of collimation

• restrict the x-ray beam to a specific area reducing scatter radiation

  • limits photons to area of interest

  • imporves contrast resolution; decreases patient dose

• controls slice thickness

what are the two types of collimation

1. pre-patient

2. post-patient (pre-detector collimation)

what is pre-patient collimation

• found between the tube and the patient

• used to choose slice thickness by widening or narrowing the beam

• reduces the patient dose

what is post-patient collimation

• found between the patient and the detector

• stops scatter radiation from entering the detector

• works with pre-patient to better define slice thickness and reduce scatter

characteristics of the patient table

• referred to as a “couch”

• flat or curved

• marde of carbon graphite fiber(decreases beam attenuation)

• able to support entire weight while fed through the gantry

table movement: serial scan/conventional

• table stops and image is taken

• table moves fo rthe next slice aquisition

• table stops and image is taken

table movement: helical and scout images

continuous table movement

patient is landmarked in___ and ___ coordinated

X- width (L to R); sagittal

Y- height (ant. to post.); coronal

what does Z (axis) determine

the thickness of the slice

scout image/ localizer scan: AP or PA

tube in 0 degree position

scout image/ localizer scan: lateral

tube in 90 degree position

what does a power injector do

automatically delivers IV contrast to patient during CT scan

programmable features of the power injector

• amount of contrast

• rate of injection (cc/sec)

• pressure

  • can be decreased for smaller gauge needles

  • can be increased for central lines

a single power injector does what

pushes contrast only

and double power injector does what

• combination of contrast and saline

• can be deliveered separately or simultaneously

  • used to decrease the amount of contrast needed and also used to push it through quicker

safety issues of a power injector

• only used with compatible IV lines

  • incompatible lines can break off and travel through vessels

• increased chance of extravasation

what is extravasation in CT

accidental infusion of contrast into surrounding tissues