Computed Tomography Data Acquisition

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

Acquisition Geometries Pencil Beam Fan Beam Spiral Multislice

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?

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

Spiral CT Advantages Faster scan times minimal interscan delays no need to stop / reverse direction of rotation Slip rings allow continuous rotation of tube & detector Continuous acquisition protocols possible

Special Considerations for Slip Ring Scanners continuous scanning means Heat added to tube faster No cooling between slices Need more heat capacity faster cooling huge tubes

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 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 (0-1) fraction of beam incident on active detector Absorption efficiency (0-1) fraction of photons incident on the detector which are absorbed Conversion efficiency (0-1) fraction of absorbed energy which produce signal

Overall Detector Efficiency (Also 0-1) 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

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

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