Topic 8 - External Dosimetry

External Radiation Protection

Types of X-ray Machines

• Three Principal Uses
  • Diagnostic
  • Therapeutic
  • Non-medical radiographic devices
• X-ray tubes
  • Housed in heavy lead casing
  • Aperture for primary (useful) beam
• Metal filters for beam (Al, Cu)
• Collimators
• Tube Housing
  • Conforms to specifications to limit “leakage”

Acceptable Leakage Limits

• Diagnositic X-ray tubes
  • Leakage @ 1 m < 0.1 R h-1
  • when tube operated continuously at maximum current and potential
• Therapeutic machines
  • Peak potential < 500 kVp
    • Leakage @ 1 m < 1 R h-1
  • Peak potential > 500 kVp
    • Leakage @ 1 m < 1 R h-1 or 0.1% of useful beam exposure rate at 1 m from the target (whichever is greater)
  • when tube operated continuously at maximum current and potential
• Non-medical radiographic
  • Housing conforms to at least requirements for therapeutic devices

Schematic View of X-ray Room

Design of X-ray facilities

• Shielding
  • Source shielding (housing)
  • Structural shielding
    • (e.g., lead-lined wall)
    • Fixed in place at any direction beam can be pointed
  • Secondary protective barrier
    • Designed to reduce exposure from both leakage and scattered radiation fields
    • Concrete walls may suffice
    • Additional shielding may be required
  • Designed to limit dose rates outside room
    • 1 mSv wk ^-1 in controlled areas
    • 0.1 mSv wk^-1 in uncontrolled areas

Uncontrolled vs Controlled Areas

• Controlled area
  • Access and occupancy are regulated in conjunction with operation of the X-ray machine
• Uncontrolled area
  • Operator of the X-ray facility has no jurisdiction
• Design rates based on annual limits:
  • 50 mSv (occupational)
  • 5 mSv (non-occupational)
• In Std units
  • 0.1 R wk^-1 (occupational)
  • 0.01 R wk^-1 (non-occupational)

Design of Primary Protective Barrier

• “Cookbook” approach
  • Attenuation of primary X-ray beams through different thicknesses of various shielding materials measured
  • Data plotted to yield attenuation curves
  • These are used to design primary protective barriers
• Primary beam intensity transmitted through shield:
  • Strong function of peak operating voltage
  • Very little effect of filtration applied to beam
  • At fixed kVp, exposure is proportional to beam current (mA-min) - this is the time integral of the beam current

Selecting Shielding - K values

• Attenuation curves for shielding material and peak voltages (kVp) are available
• Ordinate, K, gives exposure of attenuated beam in R mA^-1 min^-1 at reference distance of 1 m
• Abscissa gives shield thickness

Graph

Attenuation in lead of X rays produced with (peak) potential differences from 250 kVp to 400 kVp. (National Bureau of Standards Handbook 76, 1961, Washinton DC)

Graph 2

Attenuation in lead of X rays produced with (peak) potential differences from 500 kVp to 3000 kVp. (National Bureau of Standards Handbook 76, 1961, Washington, DC)

Graph 3

Attenuation in concrete of X rays produced with (peak) potential differences from 50 kVp to 400 kVp. (National Bureau of Standards Handbook 76, 1961, Washington, DC)

Graph 4

Attenuation in concrete of X rays produced with (peak) potential differences from 500 kVp to 3000 kVp. (National Bureau of Standards Handbook 76, 1961, Washington, DC)

Shielding

• Example
  • 1 m behind 2 mm lead for 150 kVp machine is 10^-3 R mA^-1 min^-1 (read from graph)
  • If machine is operated with a beam current of 200 mA for 90 s, the exposure will be:
    • 200 X1.5 min x 10^-3 R mA^-1 min^-1 = 0.3 R
  • Same result if beam operated at 300 mA for 60 s
• Exposure at other distances obtained by inverse square law (why?)
• 2 mm lead can be located anywhere between X-ray tube and the point of interest

Determining K Values

• Where
  • P is the maximum permissible exposure rate
    • P is 0.1 R wk^-1 for controlled areas
    • P is 0.01 R wk^-1 for uncontrolled areas
  • W is the workload, or weekly amount of use of the X ray machine, expressed in mA min
    wk ^-1
  • U is the use factor - or fraction of the workload during which the useful beam is pointed in a direction under consideration
• Where
  • T is the occupancy factor - takes into account the fraction of the time that an area outside the barrier is likely to be occupied
    • Note - average weekly exposure rates may be greater than P in areas not occupied full time.  The allowed average exposure rate in an area is P/T R wk^-1
• Where
  • d is the distance, in meters from the target on the tube to the location under consideration.
• With P in R wk^-1, d in m, and W in mA min wk^-1, K gives exposure of transmitted radiation in R mA^-1min^-1 at 1 m

Occupancy Factors (T)

Example

Diagnostic machine operated

125kVp

220 mA for an average of 90 s wk^-1

Calculate primary barrier thickness if lead or concrete were to be used to protect an uncontrolled hallway 15 ft from the tube target

Useful beam is directed horizontally toward the barrier 1/3 of the time and vertically into the ground the rest of the time

Schematic Used in Example

Densities of Commercial Building Materials

Half-Value Layers for X Rays (Broad Beams) in Lead and Concrete

Example, continued

With the previous example, an existing 3 in. sand plaster wall separates the X-ray room and the hallway. 

What thickness of lead must be added to the wall to provide the primary protective barrier?

Design of a Secondary Protective Barrier

Secondary barrier is designed to protect areas not in line of the useful beam

Protects from scattered and leakage radiation

“Quality” of these two components can be very different

Shielding requirements are computed separately, then summed

Calculation methods vary, this is one alternative to Cember

Use factor (U) = 1

Shielding from Leakage Radiation

Leakage limits previously given

Express as Y (R h^-1 @ 1 m)

Given Y, secondary barrier can be computed as # half-value layers needed to restrict exposure to allowed levels.

t is tube operation time, min wk^-1

B is the required reduction for the X-ray intensity

I is the average beam current in mA

Calculating number of half-value layers needed for shielding

The number of half-value layers that reduces the radiation to the factor B of its unshielded value is given by

B=2^-N

N = -lnB/ln2 = -lnB/0.693

Example

Same setup as previous example, but a therapy machine is installed.

Machine has a continuous tube current of 26mA at 300 kVp. 

Average workload in the facility is 24,000 mA min wk^-1.

How many HVLs of shielding would be needed to protect the laboratory from leakage?

Scattered Radiation

Based on tube operating potential

If tube < 500 kVp, then scattered Xrays assumed to be same as primary beam

If tube potential > 500 kVp, then scattered photons are treated like primary ones at 500 kVp.

Value of K for scattered radiation:

Example

Calculate the number of HVLs needed to protect the laboratory from scattered radiation from the therapy machine

What thickness of lead must be added to an existing 2.5 in plaster wall between the X-ray room and the laboratory to provide an adequate level of shielding?

Values for Scattered Radiation