CT- Physics and Instrumentation
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The general process of CT imaging may be divided into several steps:
- data acquisition- the measurement of the attenuation that occurs along each path through the patient from x-ray tube to detector
- data reconstruction- the computerized processing of the transmission measurements into the CT image
- multidimensional image display- display of the reconstructed gray-scale image in two-dimensional 2D and/or three-dimensional 3D format;a representation of the attenuation that occurred across the scanned volume of tissue
- image archival and communication- display as well as storage, both short-term and long-term (archival), of images on computer workstations
what is a scout image?
- the most basic scan mode utilized by a CT system. It is a digital radiograph obtained by keeping the xray tube and detector array in a fixed position while the patient bed is translated through the gantry.
- used as a localizer for the prescription of the subsequent cross-sectional CT acquisition
what is azimuth?
the angle of the tube and detectors in relationship to the patient position during scout acquisition
what parts does the CT gantry contain?
generator, xray tube, assorted collimators, the data acquisition system (DAS), slip-rings, and the detectors
what do slip-rings do (located inside the gantry) for CT scanners?
- eliminates the need for cables for the xray and detector components which originally prevented continuous rotation within the gantry
- it utilizes a system of contact brushes that supply electricity to power the system and enable the passage of transmission data to the computer system
- allows for the continuous helical acquisition that has become the standard in modern CT imaging
what is an automatic tube current modulation (ATCM)?
- adjusts the mA throughout an acquisition to reduce pt dose to a minimum
- similar to AEC for xray
what is a Dual-energy CT system?
- systems that are capable of applying multiple xray energies during a single CT acquisition
- transmission data are acquired from tissue interaction with both high and low kvp radiation
- differentiation and characterization of tissue composition are made possible by the difference in attenuation between the two xray spectra
- complex voltage-switchin systems may be employed to achieve dual-energy ct scanning. As the single tube rotates around the pt, the applied peak kvp is switched at an extremely high rate for each successive projection.
- alternatively, dual-source CT systems utilize two completely separate xray tubes and detector arrays positioned 90 degrees from each other within the gantry. the two xray tubes acquire data simultaneously at different kvp values
Dual-energy CT has expanded clinical opportunities, what are they?
- improved resolution of soft tissue structures (ligaments, tendons) during musculoskeletal imaging
- the ability to visualize atherosclerotic plaque within contrast-enhanced vasculature during cardiac CT studies
- contrast medium subtraction techniques for the demonstration of pre-contrast images from a single scan acquired after IV contrast
- characterization of the biochemical composition of urinary tract calculi
what are bow-tie filters?
filters that help shap the beam to reduce pt dose. They are thicker at the ends than in the middle which reduces beam intensity toward the outer margins resulting in a lower pt dose
what type of generators do current CT scanners use?
high frequency generators
what beam shape comes out of the tube aperature for pre-patient collimator for SSCT and MSCT?
- SSCT- fan shape
- MSCT- cone shape
what do SSCT and MSCT beam collimation consist of?
- pre-patient collimation- a mechanical device controls the aperture dimension at the xray tube, directly limiting the beam to the desired section width
- post-patient collimation- used to improve image quality by limiting scatter radiation before it reaches the detectors.
as the number of detector rows in an MSCT system increases, what happens to the divergence of the cone beam needed for coverage?
what is pitch?
- describes the relationship between collimation and table movement during scanning.
- pitch = table feed per rotation/section width
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