Computed TomographyRAD309

Data Acquisition

Data Acquisition

Data acquisition represents the first step in process of image production

X-ray tube & detectors collect information systematically

Collect large number of x ray transmissions around the patient

Data Collection Basics

X-ray source & detector must be in & stay in alignment

Beam moves (scans) around patientmany transmission

measurements taken

Patient

X-Ray beams

Data Collection Basics

Pre-patient beamcollimated to pass only through slice of interestshaped by special filter for uniformity

Data Collection Basics (cont) Beam attenuated by patient Transmitted photons detected by scanner Detected photon intensity converted to

electrical signal (analog) Electrical signal converted to digital value

A to D converter Digital value sent to reconstruction

computer

CT “Ray”

That part of beam falling onto a single detector

Ray

Each CT Ray

attenuated by patient projected onto one detector

detector produces electrical signalproduces single data sample

CT Projection -or- View

# of simultaneously collected rays

Acquisition Geometries

Pencil Beam

Fan Beam

Spiral

DA Geometries

1. Parallel beam, translate rotate motion2. Fan beam, translate rotate motion3. Fan beam, complete rotation tube/detector4. Fan beam, complete rotation of tube around

stationary ring of detectors5. Special: high speed CT, stationary/stationary,

multiple targets tube6. Spiral, rotate/translate 7. Multiple detector rows

Spiral Geometry

X-ray tube rotates continuously around patient

Patient continuously transported through gantry

No physical wiring between gantry & x-ray tube

Requires “Slip Ring” technology

Tube

DetectorSlipRings

InterconnectWiring

X Ray System

Initially used low energy gamma rays Problem: low radiation intensity rate, large

source size, low source strength, high cost

Use of X ray tubes Benefit: high radiation intensity, high

contrast ct scanning Problem: heterogeneous beam , does not

obay Lamber-Beer Exponential Law

Radioactive Source instead of an X-Ray Tube?

High intensity requiredX-ray tubes produce higher intensities than

sources Single energy spectrum desired

Produced by radioactive sourceX-ray tubes produce spectrum of energies

Patient

CT Beam Filtration

Shapes beam to appear monochromatic and satisfy

reconstruction process 1. Hardens beam

Removes greater fraction of low-energy photons than high energy photons

reduces patient exposure

2. Shapes energy distribution to produce uniform intensity & beam cross section

Filter

Patient Protection

Pre-collimatorsbetween tube & patient Tube

Detector

Post-collimators• between patient &

detector

Pre-Collimation

Constrains size of beam Reduces amount of scatter produced Designed to minimize beam divergence Often consists of several stages or sets of

jawsTube

Detector

Pre-collimator

Post-Collimation

Helps define slice (beam) thickness Reduces scatter radiation reaching detector Improves image quality

Tube

Detector

Post-collimator

Detectors

Capture radiation from patient Converts to electrical signal Then they are converted to binary

coded information

CT Detector Characteristics

Efficiency Response time Dynamic range Reproducibility and Stability

1. Efficiency

Ability to capture, absorb & convert x-ray photons to electrical signals

Efficiency Componentsa. Capture efficiencya. Capture efficiency

Efficiency of detector to obtain transmitted photons from patient

Size of detector area, distance between 2 detectors

b. Absorption efficiencyb. Absorption efficiencyno. of photons absorbedZ , density, size, thickness of detector

c. Conversion efficiencyc. Conversion efficiencyfraction of absorbed energy which produce signal

Overall Detector Efficiency

capture efficiencyX

absorption efficiencyX

conversion efficiency

Absorption Efficiency

Depends upon detector’satomic #densitysizethickness

Depends on beam spectrum

2. Response Time

“Speed with which detector can detect an x ray event and recover to detect the next one”

Minimum time after detection of 1st event when detector can detect 2nd event

If time between events shorter than response time, second event may not be detected

Shorter response time better

3. Dynamic Range

Ability to faithfully detect large range of intensities

“Ratio of largest signal to be measured to the precision of the smallest signal to be discriminated”

Typical dynamic range: 1,000,000:1much better than film

4. Stability

“Steadiness” of detector system Consistency of detector signal over

time The less stable, the more frequently

calibration required

Detector Types

2 principles: Convert x-ray into light ---electrical

signal Scintillation detector

Convert x-ray directly into electrical signal

Gas ionization detector

Scintillation Detectors

Crystal couple to photomultiplier tube X ray falls on crystal ---light flashes

(glow) Light directed to PM Light hits Photocathode in PM and

releases electrons

Scintillation

X-ray energy converted to light Light converted to electrical signal

X-Rays

Photomultiplier Tube

Light ElectricalSignal

ScintillationCrystal

Photomultiplier Tubes Light incident on PhotocathodePhotocathode of PM tube Photocathode releases electrons

X-Rays Light

ScintillationCrystal PM

TubePhotocathode

-+

Dynodes

Gas Ionization Detector

Series of individual chambers separated by tungsten plates

X ray falls on each chamber– (+/- ions) + ions move to – plate, - ions to + plate The migration produces electrical signal

Gas Ionization

X-rays converted directly to electrical signal

X-Rays

IonizationChamber

ElectricalSignal

-+

+ -

Filled with Air

CT Ionization Detectors

Many detectors (chambers) used adjacent walls shared between

chambers Techniques to increase efficiency

Increase chamber thickness• x-rays encounter longer path length

Pressurize air (xenon)• more gas molecules encountered per unit

path length

X-Rays thickness

Detector Array

Slice by Slice – one arc of detector array Volume – one arc of detector array,

acquires volume of tissue then separated by computed to slice by slice

DAS

Detector electronics Location: between detector and computer Role of translator

Measure transmitted radiation beamEncodes measurement to binary dataTransmits binary data to computer

Components of DAS

Amplifier Log Amplifier Analog to Digital Converter (digital data) Digital Transmission to computer

Log Amplification

Transmission measurement data must be changed into attenuation and thickness data

Attenuation = log of transmission x thickness

Detector Electronics

Amplifier

Analog to DigitalConverter

Logarithmic Amplifier

FromDetector

ToComputer

Compresses dynamic range;

Converts transmission intensity into

attenuation data

Increases signal strength for

later processing

DA and Sampling

Radiation falling on detector Each samples the beam intensity on it Not enough samples = artifacts

appear To increase number of

measurement/samples available for reconstruction and improve image quality

Improving Quality & Detection

Geometry Smaller detectors Closer packed detectors Smaller patient-detector

distance Thinner slices

less patient variation over slice thickness distance