Radiology Shielding and Radiation Intensity

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.