Transcript Data Acquisition
CT
Seeram Chapter 5: Data Acquisition in CT
Data Collection Basics
X-ray source & detector must be in & stay in alignment Beam moves (scans) around patient many transmission measurements Patient X-Ray beams
Data Collection Basics
Pre-patient beam collimated to pass only through slice of interest shaped by special bow tie filter for uniformity Filter Patient
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 View
# of simultaneously collected rays
Scan Requires Many Data Samples # Data Samples = [# data samples per view] X [# views] # Data Samples = [# detectors] X [# data samples per detector]
Acquisition Geometries
Pencil Beam Fan Beam Spiral Multislice
Pencil Beam Geometry
Tube-detector assembly translates left to right Entire assembly rotates 1 o 1st Generation CT Tube 1 o Detector
Tube
Fan Beam Geometry
3nd Generation Detectors 2nd Generation 4th Generation
Comparing Long vs. Short Geometry Long Geometry • • • • • • • Smaller fan angle Longer source-detector distance Lower beam intensity Lower patient dose More image noise Less image blur Requires larger gantry Scan FOV Scan FOV
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 Slip Rings Interconnect Wiring Tube Detector
What’s a Slip Ring?
Slip Rings Electrical connections made by stationary brushes pressing against rotating circular conductor Similar to electric motor / generator design
X-Ray Generator Configurations with Slip Ring Technology Problem: Supply high voltage to a continually rotating x-ray tube?
Options #1 Stationary Generator & Transformer #2 Stationary Generator Transformer & x-ray tube rotate in gantry #3 Transformer, generator & tube rotate in gantry
Option #1: Stationary High Voltage Transformer Incoming AC Power X-Ray Generator Primary Voltage Secondary Voltage High Voltage Transformer X-Ray Tube
Option #1: Stationary High Voltage Transformer Line Voltage Generator Primary Voltage HV Transformer high voltage must pass through slip rings Secondary Voltage Tube Slip Rings Detector
Option #2: Rotating High Voltage Transformer Incoming AC Power X-Ray Generator Primary Voltage Secondary Voltage High Voltage Transformer X-Ray Tube
Option #2: Rotating High Voltage Transformer Line Voltage Generator Primary Voltage HV Transformer low voltage must pass through slip rings Slip Rings Tube Detector
Rotating Generator
Incoming AC Power X-Ray Generator Primary Voltage Secondary Voltage High Voltage Transformer X-Ray Tube
Rotating Generator
low line voltage must pass through slip rings Line Voltage Generator Slip Rings HV Transformer Tube
Spiral CT Advantages
Faster scan times minimal interscan delays no need to stop / reverse direction of rotation Slip rings solve problem of cabling to rotating equipment Continuous acquisition protocols possible
X-Ray System Components
X-Ray Generator X-Ray Tube Beam Filter Collimators
X-Ray Generator
3 phase originally used Most vendors now use high frequency generators relatively small small enough to rotate with x-ray tube can fit inside gantry
X-Ray Tube
X-Ray Tube
Must provide sufficient intensity of transmitted radiation to detectors Radiation incident on detector depends upon beam intensity from tube patient attenuation beam’s energy spectrum patient thickness atomic # density
Maximizing X-Ray Tube Heat Capacity rotating anode high rotational speed small target angle large anode diameter focal spot size appropriate to geometry distances detector size
Special Considerations for Slip Ring Scanners continuous scanning means Heat added to tube faster No cooling between slices Need more heat capacity faster cooling
Why not use a 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 Coping with x-ray tube energy spectrum heavy beam filtering (see next slide) reconstruction algorithm corrects for beam hardening
CT Beam Filtration Hardens beam preferentially removes low-energy radiation Removes greater fraction of low energy photons than high energy photons reduces patient exposure Attempts to produce uniform intensity & beam hardening across beam cross section Filter Patient
CT Beam Collimation
Pre-collimators between tube & patient Tube Post-collimators • between patient & detector Detector
Pre-Collimation
Constrains size of beam Reduces production of scatter May have several stages or sets of jaws Tube Pre-collimator Detector
Post-Collimation
Reduces scatter radiation reaching detector Helped define slice (beam) thickness for some scanners Tube Post-collimator Detector
CT Detector Technology: Desirable Characteristics High efficiency Quick response time High dynamic range Stability
CT Detector Efficiency
Ability to absorb & convert x-ray photons to electrical signals
Efficiency Components
Capture efficiency fraction of beam incident on active detector Absorption efficiency fraction of photons incident on the detector which are absorbed Conversion efficiency fraction of absorbed energy which produce signal
Overall Detector Efficiency
Overall detector efficiency = capture efficiency X absorption efficiency X conversion efficiency
Capture Efficiency
Fraction of beam incident on active detector
Absorption Efficiency
Fraction of photons incident on the detector which are absorbed Depends upon detector’s atomic # density size thickness Depends on beam spectrum capture efficiency X absorption efficiency X conversion efficiency
Conversion Efficiency
Ability to convert x-ray energy to light GE “Gemstone Detector” made of garnet
Conversion Efficiency
Ability to convert x-ray energy to light Siemens UltraFastCeramic (UFC) CT Detector • Proprietary • Fast afterglow decay UFC Material UFC Plate
Response Time
Minimum time after detection of 1st event until detector can detect 2nd event If time between events < response time, 2 nd detected event may not be Shorter response time better
Stability
Consistency of detector signal over time Short term Long term The less stable, the more frequently calibration required
Dynamic Range
Ratio of largest to smallest signal which can be faithfully detected Ability to faithfully detect large range of intensities Typical dynamic range: 1,000,000:1 much better than film
Detector Types: Gas Ionization
X-rays converted directly to electrical signal Filled with Air
X-Rays +
Ionization Chamber
-
- + 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
X-Rays
thickness
Older Style Scintillation Detectors
X-rays fall on crystal material Crystal glows Light flash directed toward photomultiplier (PM) tube Light directed through light pipe or conduit PM tube converts light to electrical signal signal proportional to light intensity PM Electrical Signal
Detector Types: Scintillation
X-ray energy converted to light Light converted to electrical signal
X-Rays
Light Scintillation Crystal Photomultiplier Tube Electrical Signal
Photomultiplier Tubes
Light incident on Photocathode of PM tube Photocathode releases electrons + -
X-Rays
Scintillation Crystal Light Photocathode Dynodes PM Tube
Photomultiplier Tubes
Electrons attracted to series of dynodes each dynode slightly more positive than last one + + + +
X-Rays
Scintillation Crystal Light Photocathode + Dynodes PM Tube
Solid State Detectors
Crystal converts incident x-rays to light Photodiode semiconductor current proportional to light
X-Rays
Light Photodiode Semiconductor Electrical Signal
Photodiode
Made of two types of materials p-type n-type Lens focuses light from crystal onto junction of p & n type materials
X-Rays
Light Lens p n Junction
Photodiode
Light controls resistance of junction Semiconductor current proportional to light falling on junction
X-Rays
Light Lens p n Junction
Solid State Detectors
Output electrical signal amplified Fast response time Large dynamic range Almost 100% conversion & photon capture efficiency Scintillation materials cadmium tungstate high-purity ceramic material
Detector Electronics
From Detector Pre-Amplifier Increases signal strength for later processing Logarithmic Amplifier Analog to Digital Converter To Computer Compresses dynamic range; Converts transmission intensity into attenuation data
Logarithms
Log 10 x = ? means 10 ?
= x?
logarithms are exponents log 10 x is exponent to which 10 is raised to get x log 10 100 =2 because 10 2 =100
Logarithms
Input 100,000 10,000 1,000 100 10 1 Logarithm 5 4 3 2 1 0 Using logarithms the difference between 10,000 and 100,000 is the same as the difference between 10 and 100
Compression
1,000 3 = log 1000 2 =log 100 1 = log 10 0 = log 10 Hard to distinguish between 1 & 10 here 1 10 100 1000 Input 100,000 10,000 1,000 100 10 1 Logarithm 5 4 3 2 1 0 1 10 100 1000 Difference between 1 & 10 the same as between 100 & 1000 Logarithms stretch low end of scale; compress high end
Logarithmic Amplifier
accepts widely varying input takes logarithm of input amplifies logarithm logarithm output dynamic range now appropriate for A/D conversion Input Logarithm 100,000 10,000 1,000 100 10 1 5 4 3 2 1 0
Improving Quality & Detection
Geometry Smaller detectors Smaller focal spot Thinner slices Larger focus-detector distance Smaller patient-detector distance less patient variation over slice thickness distance