Computed Tomography RAD309 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 Patient X-ray source & detector must be in & stay in alignment Beam moves (scans) around patient many transmission measurements taken X-Ray beams

Data Collection Basics Pre-patient beam collimated to pass only through slice of interest shaped 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 signal produces single data sample

CT Projection -or- View # of simultaneously collected rays

Acquisition Geometries Pencil Beam Fan Beam Spiral

DA Geometries Parallel beam, translate rotate motion Fan beam, translate rotate motion Fan beam, complete rotation tube/detector Fan beam, complete rotation of tube around stationary ring of detectors Special: high speed CT, stationary/stationary, multiple targets tube Spiral, rotate/translate Multiple detector rows

Spiral Geometry X-ray tube rotates continuously around patient Detector Slip Rings Interconnect Wiring X-ray tube rotates continuously around patient Patient continuously transported through gantry No physical wiring between gantry & x-ray tube Requires “Slip Ring” technology

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 required X-ray tubes produce higher intensities than sources Single energy spectrum desired Produced by radioactive source X-ray tubes produce spectrum of energies

CT Beam Filtration Shapes beam to appear monochromatic and satisfy reconstruction process Hardens beam Removes greater fraction of low-energy photons than high energy photons reduces patient exposure Shapes energy distribution to produce uniform intensity & beam cross section Patient Filter

Patient Protection Post-collimators Pre-collimators between tube & patient Tube Post-collimators between patient & detector 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 jaws Tube 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 Components a. Capture efficiency Efficiency of detector to obtain transmitted photons from patient Size of detector area, distance between 2 detectors b. Absorption efficiency no. of photons absorbed Z , density, size, thickness of detector c. Conversion efficiency fraction of absorbed energy which produce signal

Overall Detector Efficiency capture efficiency X absorption efficiency X conversion efficiency

Absorption Efficiency Depends upon detector’s atomic # density size thickness 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: 1,000,000:1 much 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 Photomultiplier Tube X-Rays Light Electrical Signal Scintillation Crystal

Photomultiplier Tubes Light incident on Photocathode of PM tube Photocathode releases electrons + - X-Rays Light Scintillation Crystal Photocathode PM Tube 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 + - Ionization Chamber X-rays converted directly to electrical signal Filled with Air Ionization Chamber X-Rays Electrical Signal + - - +

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 thickness X-Rays

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 beam Encodes measurement to binary data Transmits 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 From Detector Increases signal strength for later processing Amplifier Compresses dynamic range; Converts transmission intensity into attenuation data Logarithmic Amplifier Analog to Digital Converter To Computer

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