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Questions and Answers
The half-value layer (HVL) is the thickness of absorber that reduces the radiation intensity to one-tenth its original value.
The half-value layer (HVL) is the thickness of absorber that reduces the radiation intensity to one-tenth its original value.
False (B)
The linear absorption coefficient (µ) is measured in units of m-1.
The linear absorption coefficient (µ) is measured in units of m-1.
False (B)
A 0.5 mm Pb protective apron can reduce exposure to approximately 90%.
A 0.5 mm Pb protective apron can reduce exposure to approximately 90%.
False (B)
The shielding thickness can be calculated using the formula I=Ioe-µx.
The shielding thickness can be calculated using the formula I=Ioe-µx.
The half-value thickness (HVT) is calculated using the formula HVT=1/µ.
The half-value thickness (HVT) is calculated using the formula HVT=1/µ.
Lead is the only material used for shielding in diagnostic radiology.
Lead is the only material used for shielding in diagnostic radiology.
The effective dose is the equivalent dose to a specific organ or tissue.
The effective dose is the equivalent dose to a specific organ or tissue.
The National Committee on Radiation Protection (NCRP) provides guidelines for radiation protection.
The National Committee on Radiation Protection (NCRP) provides guidelines for radiation protection.
The lead shield thickness required to reduce the x-ray beam intensity by 30% is approximately 0.62cm.
The lead shield thickness required to reduce the x-ray beam intensity by 30% is approximately 0.62cm.
The radiation intensity is directly proportional to the square of the distance from the source.
The radiation intensity is directly proportional to the square of the distance from the source.
The shielding thickness must be at least 25.7cm to reduce the radiation intensity to 2mR/hr.
The shielding thickness must be at least 25.7cm to reduce the radiation intensity to 2mR/hr.
The weighted average dose is calculated by summing the doses to each organ and tissue.
The weighted average dose is calculated by summing the doses to each organ and tissue.
Proper shielding of the source can reduce the radiation dose.
Proper shielding of the source can reduce the radiation dose.
The longer the time of exposure, the lower the radiation dose.
The longer the time of exposure, the lower the radiation dose.
If the time during which one is exposed to radiation is doubled, the exposure will be quadrupled.
If the time during which one is exposed to radiation is doubled, the exposure will be quadrupled.
A radiation worker exposed to 2.3 mGy/hr for 36 minutes will have a total occupational exposure of 82.8 mGy.
A radiation worker exposed to 2.3 mGy/hr for 36 minutes will have a total occupational exposure of 82.8 mGy.
The parent of a patient may remain next to the patient during fluoroscopy for an unlimited amount of time, regardless of the radiation exposure level.
The parent of a patient may remain next to the patient during fluoroscopy for an unlimited amount of time, regardless of the radiation exposure level.
A fluoroscope emits 42 mGy/min at the tabletop for every milliampere of operation, and the patient exposure is directly proportional to the milliampere and time.
A fluoroscope emits 42 mGy/min at the tabletop for every milliampere of operation, and the patient exposure is directly proportional to the milliampere and time.
As the distance between the source of radiation and the person increases, radiation exposure increases rapidly.
As the distance between the source of radiation and the person increases, radiation exposure increases rapidly.
The inverse square law is used to calculate the dose received at a point with the radiation from the tube as a variable.
The inverse square law is used to calculate the dose received at a point with the radiation from the tube as a variable.
Positioning shielding between the radiation source and exposed persons has no effect on the level of radiation exposure.
Positioning shielding between the radiation source and exposed persons has no effect on the level of radiation exposure.
Radiation shielding is typically used to increase the radiation exposure level.
Radiation shielding is typically used to increase the radiation exposure level.
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Study Notes
Shielding in Diagnostic Radiology
- Shielding in diagnostic radiology typically consists of lead, although conventional building materials are also used.
- The amount of radiation intensity reduction by a protective barrier can be estimated if the half-value layer (HVL) or tenth-value layer (TVL) of the barrier material is known.
- One TVL is the thickness of absorber that reduces the radiation intensity to one-tenth its original value.
Protective Apparel
- Protective aprons usually contain 0.5 mm Pb.
- Such aprons reduce exposure to approximately 10% due to scattered x-rays incident on the apron at an oblique angle.
Calculating Shielding Thickness
- Shielding thickness is calculated from the formula: I = Ioe^(-µx)
- Half-value thickness (HVT) is the thickness needed to reduce the initial intensity to its half value, calculated as HVT = 0.693/µ
- Example: Calculate the radiation intensity at 1m from a 185TBq 60Co source, then calculate the shielding thickness of lead necessary to shield the source to a radiation level less than 2mR/hr at 1m.
Effective Dose
- Effective dose is the equivalent whole-body dose, which is the weighted average of the radiation dose to various organs and tissues.
- The National Committee on Radiation Protection (NCRP) has identified various tissues and organs and their relative radiosensitivity.
Golden Rules for Radiation Protection
- Time: The longer the time of exposure, the higher the radiation dose.
- Distance: The intensity of radiation is inversely proportional to the square of the distance from the source.
- Activity: Choose the lowest activity of source that is sufficient for the needed job.
- Shielding: Proper shielding of the source and exposed persons greatly reduces the level of radiation exposure.
Radiation Exposure Calculations
- If the time of exposure is doubled, the exposure will be doubled.
- Example: Calculate the total occupational exposure if a radiation worker is exposed to 2.3 mGy/hr from a radiation source for 36 minutes.
- Example: Calculate the maximum time a parent can remain next to a patient during fluoroscopy without exceeding the allowable daily exposure.
Maximizing Distance
- As the distance between the source of radiation and the person increases, radiation exposure decreases rapidly.
- This decrease in exposure is calculated using the inverse square law.
- Example: Calculate the radiation exposure at a distance of 350 cm from an x-ray tube with an output intensity of 26 mGy/mAs at 100-cm source-to-image receptor distance.
Using Shielding
- Positioning shielding between the radiation source and exposed persons greatly reduces the level of radiation exposure.
- Example: Calculate the approximate occupational exposure of a radiologic technologist at different positions during a fluoroscopic examination.
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