Sources of Radiation
Primary Beam: This refers to the x-ray beam prior to any interaction with the patient, grid, table or image intensifier.
Exit Beam: The beam that interacts with the detector is termed the exit beam and will have been significantly attenuated. However, the beam will have been heavily filtered and, consequently, will be harder and more penetrating than the primary beam.
Leakage Radiation: This is leakage from the x-ray tube housing. However, this is limited to a maximum of 1 mGy/hr at 1 metre from the focus and, in practice, is usually much less. It doesn’t contribute significantly to staff dose.
Scattered Radiation: This is a direct result of the Compton effect in the patient and contributes the most to staff radiation dose. The amount (fluence) of scatter depends on:
- Field size
- Volume of patient
- Quality of primary beam.
An increase in scatter can be caused by:
- Increased kV – number of photons proportional to square of applied potential
- Increased mA – number of photons directly proportional to tube current
- Increased energy
- Increased exposure time – no. photons directly proportional to length of time of exposure
- Larger volume exposed – more tissue for photons in x-ray beam to interact with. The tighter the collimation the less the scatter
- Position relative to patient – on the exit side of the patient the scatter is less as it has been attenuated by the patient as compared to scatter on the tube side (i.e. angular dependence, the greater the angle from the exit beam the greater the scatter)
Minimising Staff Dose
- Pulsed operation in fluoroscopy
- Last image hold in fluoroscopy enables decisions to be made without further exposure
- Virtual collimation
During acquisition phase, only essential personnel remain in the room and they are shielded. Radiation dose falls with distance as demonstrated by the inverse square law.
Most rooms are designed so that any member of the public passing outside will not receive a dose from x-ray procedures being performed in that room of more than 0.3 mSv. Typical shielding for a busy x-ray room is 150 mm thick concrete walls or 2.0 mm lead ply strapped to an existing wall.
The operator’s console/control room should provide areas behind which doses and dose rates are sufficiently low such that members of staff do not need to wear additional protective equipment. These include ceiling mounted, table mounted, intensifier mounted and mobile protective screens that are usually at lead equivalences of 0.5mm Pb.
Radiation risk assessment
Local rules dictate what PPE is appropriate. These provide no protection against primary radiation. Lead gloves are often not recommended due to the risk of them getting in the primary beam, which would lead to the system increasing the exposure factors and giving a higher dose to both the patient and the staff.
|Lead aprons||0.25 mm for 100 kV|
0.35 mm for 150 kV
|Thyroid shields||0.5 mm|
|Lead glasses||0.25 x 1.0 mm|
|Lead gloves||0.25 mm|
Modern gloves have 0.5 or 1.0 mm
Fluoroscopy and Interventional
1. Overcouch configuration: x-ray tube above the patient and beam directed downwards. This exposes the operator to more radiation as the scatter occurs upwards towards the upper body of the operator. These are used in C-arm systems but care must be taken to orient tube away from the operator during oblique views.
2. Undercouch configuration: the intensifier is positioned above the patient as close as possible and, therefore, provides some shielding to the operator. The dose to staff is 10x less than in overcouch.
Pregnant / Breast Feeding Staff
When pregnancy is declared, a risk assessment should be carried out by the employer to make sure dose limits are adhered to. For declared pregnant staff the dose limit is 1 mSv to the fetus over the duration of the pregnancy which, generally, translates to a 2 mSv dose limit to the abdomen of the pregnant employee.
No extra precautions are necessary if breast feeding except if working in the nuclear medicine department.