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Cards In This Set
| Front | Back |
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How a CT is formed (6 steps)
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Thinly collimated X-ray beam
Multiple Projects/views Detectors to measure attenuation/transmission Data Acquisition system Image reconstruction algorithm (FBP) Film/Monitors for display |
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X-ray Tube used
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Same as radiography - boil e- off cathode, aim at anode, produce bremss and charac. x-rays
Tube is continuously on, so need higher heat capacity and cooling system |
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How is field flattened and filtered?
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Bow tie filter
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Possible detectors (2)
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Solid State/Photodetectors
Xn gas (not common) |
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Solid State/ Photodetectors
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Uses doped ceramic to scintillate. Light is collected by Si-based photodetector and produces a voltage which is converted by analog to digital converter (ADC)
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Path of an X-ray into detector to signal (5 steps)
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Xray is scintillated and collected by photodiode
Goes to Data acquisition system (DAS) Integrator A/D converter Digital value goes to reconstruction |
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Attenuation (mu) is a function of _____ and _____
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Z and photon energy
rho* mu(m) rho* Ng * mu(e) rho(e) * mu(e) mu(e) is function of A and photon energy |
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How to find mu's?
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Large number of projections
Take projection values and filter, then backproject |
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Filtered Backprojection
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Convolve calculated attenuation from backprojection with a filter function
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Shape of reconstruction filter affects ____ and _____
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Spatial resolution and noise
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CT number (equation)
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(mu(tissue)-mu(water) / mu(water)) * 1000 in HU
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Water HU
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Mu=mu(water) -> 0 HU
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HU of Air
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Mu = 0 -> -1000 HU
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HU of bone
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Mu = 2*mu(water) -> +1000 HU
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Window
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Width of input gray level range to display
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