1. Module C Computed Tomography Physics, Instrumentation, and Imaging

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3. Data Acquisition Three step process: 1. Data Acquisition 2. Image Reconstruction 3. Image Display

4. CT components Gantry Gantry Gantry Apertures range from Gantry Apertures range from 50 – 85 cm Current table limit Current table limit 450 lbs. Diagnostic Ct tables are curved Diagnostic Ct tables are curved Radiation Therapy tables are flat Radiation Therapy tables are flat

5. CT components Tilt 30 degrees cephalad or caudal Tilt 30 degrees cephalad or caudal Tilt capabilities absent using Positron Emission Tomography (PET) Tilt capabilities absent using Positron Emission Tomography (PET) Table and scanning controls Table and scanning controls Gantry Gantry table table

6. CT X-Ray Tube More than one filament More than one filament tungsten tungsten Focusing cup Focusing cup Anode Anode Stationary Stationary rotating rotating

7. Anode Rotating most common today Rotating most common today Shorter exposure times Shorter exposure times Small focal spots for greater detail Small focal spots for greater detail High heat load capacity High heat load capacity Produces heterogeneous beam from large rotating anode Produces heterogeneous beam from large rotating anode Large anode disc and small focal spot facilitate spatial resolution requirements Large anode disc and small focal spot facilitate spatial resolution requirements

8. Anode Made of rhenium Made of rhenium Tungsten Tungsten Molybdenum alloy (RTM) Molybdenum alloy (RTM) Additional types of discs- Additional types of discs- All metal disk All metal disk Brazed graphite disk Brazed graphite disk Chemical vapor disk Chemical vapor disk

9. Anode Target angle is generally 12 degrees Target angle is generally 12 degrees Rotational speed is either Rotational speed is either 3,600 rpm 3,600 rpm 10,000 rpm 10,000 rpm Heat storage capacity Heat storage capacity 2-5 million heat units (HU) 2-5 million heat units (HU)

10. Anode Heat dissipation rates are in the area of 400,000 HU/m. Heat dissipation rates are in the area of 400,000 HU/m. Achieved using Achieved using Oil cooled Oil cooled Liquid cooled Liquid cooled Fan cooled Fan cooled

12. X-ray Beam Polychromatic Polychromatic CT has to have Monochromatic CT has to have Monochromatic Bowtie Filter Bowtie Filter Remove the long wavelength x-rays from the beam Remove the long wavelength x-rays from the beam Shape the beam Shape the beam

13. Collimation Two sets of collimators Two sets of collimators Patient radiation dose Patient radiation dose Quality of the CT images Quality of the CT images Together called PRE-detector Together called PRE-detector Separately called PRE and POST patient Separately called PRE and POST patient

14. CT Detectors Positioned within the gantry Positioned within the gantry Generation determines the type Generation determines the type Rotating Rotating or or Stationary Stationary

15. Detector Characteristics 1. Efficiency 2. Response time 3. Dynamic Range 4. High Reproducibility 5. Stability

16. Types of Detectors Gas Ionization Gas Ionization Solid-State Solid-State

17. Xenon Detectors Inert gas Inert gas Fast rate of decay Fast rate of decay Used in third generation scanners Used in third generation scanners Fourth generation scanners have non- rotating detectors. Fourth generation detectors rock back and forth. Fourth generation scanners have non- rotating detectors. Fourth generation detectors rock back and forth.

18. Continue……. Detectors Solid State Detectors Solid State Detectors Also called Scintillation detectors Also called Scintillation detectors Made of: Made of: Bismuth germinate Bismuth germinate Cesium iodide Cesium iodide Cadmium tungstate Cadmium tungstate

19. Sampling Governed by Nyquist Theorem Governed by Nyquist Theorem Increasing sampling methods Increasing sampling methods Adjustments in slice thickness Adjustments in slice thickness Use of closely packed detectors Use of closely packed detectors Quarter-shifted detectors Quarter-shifted detectors

20. Array Processor Primary function – reconstruct the projected raw data Primary function – reconstruct the projected raw data Solves all mathematically complex problems Solves all mathematically complex problems Used in retrospective reconstruction and post processing Used in retrospective reconstruction and post processing Number and type used affects the reconstruction time of the scanner Number and type used affects the reconstruction time of the scanner

21. DAS Data Acquisition system Data Acquisition system A set of electronics that are located between the detector array and the CT host computer A set of electronics that are located between the detector array and the CT host computer DAS functions: DAS functions: Measures the transmitted beam Measures the transmitted beam Logarithmic conversions Logarithmic conversions Changes the measurements into digital form Changes the measurements into digital form Sends the digital signals to the array processor for processing Sends the digital signals to the array processor for processing