Radiation Physics - Safety and Dose

Question 1

What is the primary physical issue of CT imaging and is exhibited by a volume of tissue?

Answer 1

measurement of attenuation

Question 2

What is recognized as the 2D map of the measured attenuation values?

Answer 2

reconstructed CT image

Question 3

What is described as the reduction in intensity of a radiation beam as it passes through a substance?

Answer 3

attenuation

Question 4

What process causes radiation quanta to be absorbed as it transmits through an object?

Answer 4

photoelectric effect

Question 5

occurs when energy of an incoming x-ray photon is completely absorbed through the ionization of an inner-shell electron of the target atom

Answer 5

photoelectric absorption

Question 6

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?

Answer 6

Compton scattering

Question 7

What occurs when an incoming x-ray photon has a sudden loss of energy and change in direction?

Answer 7

ionization

Question 8

What two factors does attenuation depend on?

Answer 8

beam quality/beam energy and atomic density of tissue

Question 9

What is transmitted radiation incident upon?

Answer 9

array of CT detectors

Question 10

What distance ratio says that as distance from the x-ray tube to detectors decreases, dose increases?

Answer 10

source-detector distance

Question 11

What law is tube to detector distance used to compare resultant radiation dose?

Answer 11

inverse square law

Question 12

What distance ratio describes the distance from the x-ray source to the center of the gantry opening, where the patient should be positioned?

Answer 12

focus-to-isocenter distance

Question 13

What distance ratio describes the distance between the x-ray source and the detector array?

Answer 13

focus-to-detector distance

Question 14

How will a structural increase in either focus-to-isocenter or detector distance affect patient dose?

Answer 14

decrease patient dose

Question 15

What is the range and type of filtration material used within the CT tube?

Answer 15

6-9mm aluminum or the equivalent

Question 16

What type of x-ray photons does filtration act to remove from the radiation beam?

Answer 16

low quality photons

Question 17

Why is it important to remove low quality radiation from the beam?

Answer 17

to reduce the extra radiation exposure that low quality radiation contributes to by increasing absorption into a patient’s tissues

Question 18

What other types of filtration are added to the x-ray tube in order to compensate for the noncylindrical nature of most body shapes?

Answer 18

beam-shaping or bowtie filters

Question 19

How does extra filtration along the periphery of the x-ray beam affect radiation dose and why?

Answer 19

dose decreases due to unnecessary radiation being absorbed by the filters

Question 20

How should the patient be positioned in the gantry when a bowtie filter is in place?

Answer 20

isocentered

Question 21

What can happen if the patient is not isocentered when a bowtie filter is in place?

Answer 21

poor image quality and variations in dose

Question 22

What other factor in regards to detectors plays an important role in patient radiation dose?

Answer 22

detector efficiency

Question 23

What describes the ability of the detector element to capture transmitted x-ray quanta and produce the appropriate response?

Answer 23

inherent absorption efficiency

Question 24

What can an inefficiency of detector absorption result in?

Answer 24

loss of data and subsequent compensatory increase in patient dose

Question 25

What describes the spatial arrangement of detector elements, including the amount of interspace material required between adjacent elements?

Answer 25

geometric efficiency

Question 26

What material in-between each detector element absorbs incoming radiation but gives no response?

Answer 26

interspace material

Question 27

What is the result of radiation being absorbed in the interspace material and what must be done to fix the issue?

Answer 27

loss in data/signal and increase in radiation dose and technical factors

Question 28

What type of CT scanner requires an increase amount of interspace material between detectors because of the arrangement of multiple detector rows?

Answer 28

MDCT

Question 29

What type of beam shape is used in MDCT to cover the wide detector array?

Answer 29

cone-shape

Question 30

What level of intensity must all of the detectors be exposed to by radiation?

Answer 30

equal intensity

Question 31

What is prevented when the beam is expanded to expose more of the detectors?

Answer 31

penumbra

Question 32

What is the process of penumbra referred to as?

Answer 32

overbeaming

Question 33

Is all of the CT image data acquired from the exposed detectors?

Answer 33

no

Question 34

What was designed to control the position of the x-ray beam onto the detectors, reducing overbeaming and subsequent radiation exposure?

Answer 34

focal spot tracking systems

Question 35

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?

Answer 35

overranging

Question 36

How much of a spiral is used in overranging on average?

Answer 36

up to a half rotation

Question 37

What algorithms are employed in the reconstruction process to reduce the displayed noise within the CT image?

Answer 37

noise reduction algorithms or adaptive filters

Question 38

What does a reduction in noise allow for with technical factors?

Answer 38

decrease in milliamperage used for scan

Question 39

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?

Answer 39

MDCT