5 Computed Tomography Flashcards
What is the tube voltages range in CT?
Tube voltages range from 80 to 140 kV.
What is the tube currents range in CT?
Tube currents can range up to 1,000 mA.
What is the time for 1 rotation in CT?
Time for a 360-degree rotation of the x-ray tube currently ranges between 0.3 and 2 seconds.
What is the power range in CT?
Power loading on CT x-ray tubes can be as high as ∼100 kW.
What is the position of the anode-cathode in the CT? Why?
Perpendicular to the imaging plane to reduce the heel effect.
What are the types of filters used in CT?
Copper or aluminum filters are used to filter the x-ray beam.
What is the HVL of Al in CT?
- The typical filtration on a CT x-ray tube is ∼6 mm Al.
* The heavy filtration used with CT scanners typically produces a beam with an aluminum (HVL) of up to 10 mm Al.
What is Bow tie filter?
*Bow tie filters attenuate little in the center, but attenuation increases with increasing distance from the central ray.
*Bow tie filters are made of a low Z material such as Teflon to minimize beam hardening differences.
–Bow tie filters also reduce scatter and patient dose.
Bow tie filter used in what?
- A bow tie filter is used to minimize the dynamic range of exposures at the detector.
- Bow tie filters also reduce scatter and patient dose.
What is the beam width on 64 slice CT?
It is ∼40 mm.
What is the beam width on 320 slice CT?
It is ∼160 mm.
Define antiscatter collimation?
The form of thin lamellae (e.g., 100-μm tantalum sheets). Antiscatter collimation is located between the detector elements oriented along the long patient axis and aligned with the x-ray focus.
How much the geometric efficiencies for detectors that are 1 mm wide?
Geometric efficiencies are ∼90%.
What are the characteristics of the CT detector?
- good temporal response and rapid signal decay.
- low afterglow characteristics.
- quantum efficiency of >90%.
What is quantum efficiency?
It is the percentage of incident x-ray photons that are absorbed.
What are the type of detectors in CT?
The most common material used in solid-state detectors is cadmium tungstate (CdWO4), Cesium iodide, calcium fluoride, and bismuth germanate.
Define isotropic resolution?
Isotropic resolution permits nonaxial reconstructions without stretching pixels.
Define sinogram?
A graphic plot of projections as a function of x-ray tube angle.
How to generate an image from the acquired data (Image reconstruction)?
Determining the linear attenuation coefficients of the individual pixels in the image matrix.
Define filtered back-projection?
Calculate the histogram model of a feature and then use it to find this feature in an image.
Define CT images?
Maps of the relative linear attenuation values of tissues.
Define CT numbers ( Hounsfield units)?
The relative attenuation coefficient (μ).
What is the formula of Hounsfield units?
HUx = 1,000 × (μx − μwater)/μwater
What is helical CT?
The patient is moved along the horizontal axis as the x-ray tube rotates around the patient.
