Seeram Chapter 5: Data Acquisition in CT

Data Collection Basics Patient X-ray source & detector must be in & stay in alignment Beam moves (scans) around patient many transmission measurements 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 Patient Filter

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 Pencil Beam 1st Generation CT Tube-detector assembly translates left to right Entire assembly rotates 1o 1st Generation CT 1o Tube Detector Pencil Beam

Fan Beam Geometry Fan Beams 3nd Generation 2nd Generation Tube Detectors 3nd Generation Fan Beams 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 Short Long

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

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 Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

Option #1: Stationary High Voltage Transformer Generator Secondary Voltage Line Voltage Primary Voltage Tube Detector Slip Rings HV Transformer high voltage must pass through slip rings

Option #2: Rotating High Voltage Transformer Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

Option #2: Rotating High Voltage Transformer Generator Primary Voltage Line Voltage Tube Detector Slip Rings HV Transformer low voltage must pass through slip rings

High Voltage Transformer Rotating Generator Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

Rotating Generator low line voltage must pass through slip rings Line Tube Slip Rings Generator HV Transformer

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 Patient Filter

CT Beam Collimation Post-collimators between patient & detector 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 Detector Pre-collimator

Post-Collimation Reduces scatter radiation reaching detector Helped define slice (beam) thickness for some scanners Tube Detector Post-collimator

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 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 Plate UFC Material

Response Time Minimum time after detection of 1st event until detector can detect 2nd event If time between events < response time, 2nd event may not be detected 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 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

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 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

Photomultiplier Tubes Electrons attracted to series of dynodes each dynode slightly more positive than last one + + + - + + X-Rays Light Scintillation Crystal Photocathode PM Tube Dynodes

Solid State Detectors Crystal converts incident x-rays to light Photodiode semiconductor current proportional to light Photodiode Semiconductor Electrical Signal X-Rays Light

Photodiode Made of two types of materials p-type n-type Lens focuses light from crystal onto junction of p & n type materials p n Lens Junction X-Rays Light

Photodiode Light controls resistance of junction Semiconductor current proportional to light falling on junction p n Lens X-Rays Light 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 Increases signal strength for later processing Pre-Amplifier Compresses dynamic range; Converts transmission intensity into attenuation data Logarithmic Amplifier Analog to Digital Converter To Computer

Logarithms Log10x = ? means 10? = x? logarithms are exponents log10x is exponent to which 10 is raised to get x log10100 =2 because 102=100

Logarithms 100,000 10,000 1,000 100 10 1 5 4 3 2 Input Logarithm Input Logarithm Using logarithms the difference between 10,000 and 100,000 is the same as the difference between 10 and 100

Compression 100,000 10,000 1,000 100 10 1 5 4 3 2 Input Logarithm 1000 Hard to distinguish between 1 & 10 here 100,000 10,000 1,000 100 10 1 5 4 3 2 Input Logarithm 3 = log 1000 2 =log 100 Difference between 1 & 10 the same as between 100 & 1000 1 = log 10 0 = log 10 Logarithms stretch low end of scale; compress high end 1 10 100 1000

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 5 10,000 4 1,000 3 100 2 10 1 1

Improving Quality & Detection Geometry Smaller detectors Smaller focal spot Larger focus-detector distance Smaller patient-detector distance Thinner slices less patient variation over slice thickness distance