X-Ray Beam Restrictors, MMed Physics, Lesson 7

Lesson 7: X-Ray Beam Restrictors,

An x-ray beam restrictor is a device that is attached to the opening of the x-ray tube housing to regulate the size and shape of an x-ray beam.

1.       Aperture diaphragms

2.       Cones and cylinders

3.       Collimators

Aperture diaphragms

Sheet of lead with a hole in the center. The size and shape of the hole will determine the size and shape of the x-ray beam.

a)      Disadvantage: Large penumbra at the periphery of the x-ray beam.

b)      The width of the penumbra can be reduced by placing the aperture diaphragm as far as possible from the x-ray target.

Cones and Cylinders

Conical and Cylindrical beam restrictors.

a)      Cones are less effective as the flare of the cone is greater than the flare of x-ray beam.

b)      Cylinders are more effective, beam restriction takes place at the far end of the barrel, so there is less penumbra.

Disadvantage of aperture diaphragms, Cones and Cylinders: Limit field sizes

Collimators:

1)      Advantages:

a)      It provides an infinite variety of rectangular x-ray fields

b)      A light beam shows the center and the exact configuration of the x-ray field

2)      Two shutters (s1 and s2) control the beam dimensions. Move as a unit with the second one cleaning the penumbra of the first shutter. Each shutter consists of four adjustable lead plates, moving as independent pairs.

3)      X-ray field is illuminated by a light beam from a light bulb in the collimator, reflected of a 45 degree mirror. The target of the x-ray tube and the light bulb are exactly the same distance from the mirror. The distance from the mirror to  the collimator to the target of the x-ray tube is critical.

4)      Positive Beam Limiting (PBL) devices are Automatic collimators. Required to be accurate within 2% of the source-to-image distance (SID).

5)      Testing X-ray Beam and Light Beam Alignment. Why is this important?

a)      The alignment of the x-ray beam and the light beam must be checked periodically because the mirror tends to get out of adjustment from daily use.

b)      If the collimator is mounted too far from the x-ray target, the x-ray source will be further from the second shutter than the light source, and the x-ray beam will be smaller than the light beam.

Function of Restrictors

1)      Function:

a)      To protect the patient and to decrease scatter radiation

b)      Using smaller fields: gets better films with smaller fields

2)      Patient Protection:

a)      the smaller the x-ray field, the smaller the volume of patient that is irradiated. Decreasing the field size from 20 x 20 cm to 10 x 10 cm, the area decreases from 400 to 100 cm2. (area is a square function, exposure volume is significantly reduced).

b)      Field shape should be determined by the area being examined.

3)      Decrease Scatter Radiation with Collimators: The quantity of radiation reaching the an x-ray film depends on the field size; that is the larger the field size, the more scatter radiation. Figure 7.9.

a)      The exact contribution from scatter  depends on the thickness of the being examined and

b)      On the energy of the x-ray beam

c)       The number of primary photons, per unit area is the same for any size field (straight line).

d)      After the x-ray field reaches a size of 30 x 30 cm the total quantity of scatter radiation is near maximum.

e)      Remember to increase exposure factors as you decrease field size to keep density consistant.