24 Questions
How does the radiographic technique used in CT imaging affect the radiation dose to the patient?
The higher the mAs and kVp settings used, the higher the dose to the patient. The relationship between mAs and dose is linear.
What is the effect of patient size and body part thickness on radiation dose?
Large patients or thick body parts require radiographic techniques that increase the radiation dose to avoid an unacceptable level of image noise.
What is the cumulative effect of repeat scans on radiation exposure?
Areas of the patient that are rescanned receive additional radiation, and the effect is cumulative.
What is the goal of health physics in medical imaging?
To provide occupational radiation protection and minimize radiation dose to the public.
What are the three cardinal principles of radiation protection?
Time, Distance, and Shielding.
How does the duration of radiation exposure affect the dose to an individual?
The dose to an individual is directly related to the duration of radiation exposure.
What is the purpose of radiation protection in medical imaging?
To minimize the radiation exposure of patients and personnel.
What is the ALARA principle in radiation protection?
As Low As Reasonably Achievable, which aims to minimize radiation exposures.
What are the two components of the rational use of CT relative to patient care?
Appropriate patient selection and minimization of the radiation dose without compromising diagnostic image quality
What is the unit of x-ray exposure in air?
The roentgen (R)
What is the SI unit of absorbed dose, and how does it relate to the traditional unit 'rad'?
The gray (Gy), with 1 Gy equal to 100 rad
What is the relationship between the activity of a radiation source and its unit of measurement?
1 Ci (Curie) is equal to 3.7x10^10 disintegrations per second, which is equivalent to 3.7x10^10 Bq (Becquerel)
How does the absorbed dose relate to the energy absorbed per unit mass?
The absorbed dose describes the amount of energy absorbed per unit mass, with 1 Gy equal to 1 Joule per kilogram
What is the unit of exposure for x-rays and gamma rays, and how does it relate to ionization of air?
The roentgen (R), which measures the ionization of air due to the radiation coming out of the source
How does the absorbed dose in tissue relate to the radiation distribution and scan field diameter?
The absorbed dose in tissue is affected by the radiation distribution and scan field diameter, which can impact image quality and patient radiation exposure
What is the importance of minimizing radiation dose in CT scanning, and how does it relate to patient safety and health risks?
Minimizing radiation dose in CT scanning is crucial to reduce patient radiation exposure and associated health risks, while maintaining diagnostic image quality
If a radiation worker is exposed to a radiation source of 2.5 mGy/hr, how will the exposure change if the time is doubled?
The exposure will be doubled.
A patient is exposed to a radiation source of 4 mGy/hr during a fluoroscopic examination. If the examination lasts 20 minutes, what is the total occupational exposure?
80 mGy (or 8,000 mR)
If a fluoroscope emits 30 mGy/min at the tabletop, what is the patient exposure in an examination that requires 1.5 minutes of fluoroscopic x-ray exposure time?
45 mGy (or 4,500 mR)
How does the distance between the source of radiation and the person affect radiation exposure?
Radiation exposure decreases rapidly as the distance between the source of radiation and the person increases.
An x-ray tube has an output intensity of 20 mGy/mAs at 100-cm source-to-image receptor distance. What would be the radiation exposure 200 cm from the target?
5 mGy/mAs (or 0.5 mR/mAs)
What is the approximate occupational exposure of a radiologic technologist at a position where the exposure rate is 2 mGy/hr, and farther back where the exposure rate is 0.1 mGy/hr, during a fluoroscopic examination that lasts 3 minutes?
6 mGy (or 600 mR)
How does positioning shielding between the radiation source and exposed persons affect radiation exposure?
It greatly reduces the level of radiation exposure.
A radiologic technologist is exposed to a radiation source of 1.5 mGy/hr during a fluoroscopic examination. If the allowable daily exposure is 0.2 mGy, how long may the technologist remain in that position?
8 minutes
Study Notes
Factors Affecting Dose
- Radiographic technique affects radiation exposure to the patient
- Higher mAs and kVp settings increase radiation dose to the patient
- Relationship between mAs and dose is linear
Patient Size and Body Part Thickness
- Large patients or thick body parts require radiographic techniques that increase radiation dose
- Patient size and body composition affect scatter radiation
Repeat Scans
- Rescanning areas of the patient increases cumulative radiation dose
Health Physics
- Concerned with occupational radiation protection and minimizing radiation dose to the public
- Involves designing equipment, calculating and constructing barriers, and developing administrative protocols to maintain radiation exposures as low as reasonably achievable (ALARA)
Cardinal Principles of Radiation Protection
- Minimize radiation exposure of patients and personnel
- Three principles:
- Time: minimize exposure duration
- Distance: maximize distance between radiation source and person
- Shielding: position shielding between radiation source and exposed persons
Minimize Time
- Dose is directly related to exposure duration
- Doubling exposure time doubles the dose
Maximize Distance
- Radiation exposure decreases rapidly with increasing distance
- Inverse square law calculates decrease in exposure
Use Shielding
- Positioning shielding between radiation source and exposed persons reduces radiation exposure
Radiation Dosimetry in CT
- Involves two components:
- Appropriate patient selection
- Minimization of radiation dose without compromising diagnostic image quality
Basic Dose Concepts
- Ionizing radiation used in CT has maximum energy from 120 to 140 keV and average energy near 70 keV
- Units of x-ray exposure in air: roentgen (R)
- Unit of absorbed dose: radiation absorbed dose (rad) or gray (Gy)
Radiation Protection Units
- Activity: amount of radiation coming out of the source (Curie, Becquerel)
- Exposure: ionization of air due to radiation (Roentgen)
- Absorbed dose: amount of energy absorbed in the medium (Gray)
This quiz covers the factors that affect radiation dose in radiography, including radiographic technique, patient size and body part thickness, and repeat scans.
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