Radiation Physics - Safety and Dose Flashcards
What is the primary physical issue of CT imaging and is exhibited by a volume of tissue?
measurement of attenuation
What is recognized as the 2D map of the measured attenuation values?
reconstructed CT image
What is described as the reduction in intensity of a radiation beam as it passes through a substance?
attenuation
What process causes radiation quanta to be absorbed as it transmits through an object?
photoelectric effect
occurs when energy of an incoming x-ray photon is completely absorbed through the ionization of an inner-shell electron of the target atom
photoelectric absorption
What is the process when incident x-ray photons interact with a target atom’s outer-shell electron, a transfer of energy may occur which ejects the outer-shell electron resulting in loss of energy and change in direction?
Compton scattering
What occurs when an incoming x-ray photon has a sudden loss of energy and change in direction?
ionization
What two factors does attenuation depend on?
beam quality/beam energy and atomic density of tissue
What is transmitted radiation incident upon?
array of CT detectors
What distance ratio says that as distance from the x-ray tube to detectors decreases, dose increases?
source-detector distance
What law is tube to detector distance used to compare resultant radiation dose?
inverse square law
What distance ratio describes the distance from the x-ray source to the center of the gantry opening, where the patient should be positioned?
focus-to-isocenter distance
What distance ratio describes the distance between the x-ray source and the detector array?
focus-to-detector distance
How will a structural increase in either focus-to-isocenter or detector distance affect patient dose?
decrease patient dose
What is the range and type of filtration material used within the CT tube?
6-9mm aluminum or the equivalent
What type of x-ray photons does filtration act to remove from the radiation beam?
low quality photons
Why is it important to remove low quality radiation from the beam?
to reduce the extra radiation exposure that low quality radiation contributes to by increasing absorption into a patient’s tissues
What other types of filtration are added to the x-ray tube in order to compensate for the noncylindrical nature of most body shapes?
beam-shaping or bowtie filters
How does extra filtration along the periphery of the x-ray beam affect radiation dose and why?
dose decreases due to unnecessary radiation being absorbed by the filters
How should the patient be positioned in the gantry when a bowtie filter is in place?
isocentered
What can happen if the patient is not isocentered when a bowtie filter is in place?
poor image quality and variations in dose
What other factor in regards to detectors plays an important role in patient radiation dose?
detector efficiency
What describes the ability of the detector element to capture transmitted x-ray quanta and produce the appropriate response?
inherent absorption efficiency
What can an inefficiency of detector absorption result in?
loss of data and subsequent compensatory increase in patient dose
What describes the spatial arrangement of detector elements, including the amount of interspace material required between adjacent elements?
geometric efficiency
What material in-between each detector element absorbs incoming radiation but gives no response?
interspace material
What is the result of radiation being absorbed in the interspace material and what must be done to fix the issue?
loss in data/signal and increase in radiation dose and technical factors
What type of CT scanner requires an increase amount of interspace material between detectors because of the arrangement of multiple detector rows?
MDCT
What type of beam shape is used in MDCT to cover the wide detector array?
cone-shape
What level of intensity must all of the detectors be exposed to by radiation?
equal intensity
What is prevented when the beam is expanded to expose more of the detectors?
penumbra
What is the process of penumbra referred to as?
overbeaming
Is all of the CT image data acquired from the exposed detectors?
no
What was designed to control the position of the x-ray beam onto the detectors, reducing overbeaming and subsequent radiation exposure?
focal spot tracking systems
What occurs when radiation dose is applied before and after the acquisition volume to ensure sufficient data collection for interpolation algorithms inherent in spiral CT?
overranging
How much of a spiral is used in overranging on average?
up to a half rotation
What algorithms are employed in the reconstruction process to reduce the displayed noise within the CT image?
noise reduction algorithms or adaptive filters
What does a reduction in noise allow for with technical factors?
decrease in milliamperage used for scan
decrease in focus-to-detector distance, use of a cone beam instead of a fan beam tightly collimated along the z-axis, increase in the number of phases of acquisition due to decreased scan times and thinner section widths for improvement of 3D and MPR, are all indications that which type of CT system tends to increase patient dose?
MDCT
