M6 - Beam Restrictions

Penumbra

_______ is the result of x-ray photons created in all areas of the focal spot rather than at just a single point.

1. Aperture diaphragm

2. Cones or cylinders

3. Variable aperture collimatir

There are three types of beam restricting devices:

Scatter radiation

_______ is produced during Compton interaction.

1. Restricting the x-ray beam; and

2. By using a grid.

To lessen Compton interaction, this can be accomplished by:

1. Kilovoltage (kVpj

2. Irradiated material

The principal factors that affect the amount of scatter produced are:

field size and patient thickness

The volume of irradiated material is controlled by _______ and ______.

increases

As the volume of irradiated tissue increases, the amount of scatter _______.

Aperture diaphragm

______ is a flat sheet of metal, usually lead, with a hole cut in the center and attach to an x- ray tube port.

Penumbra

geometric unsharpness around the periphery of the image

Penumbra

Principal Disadvantage of Aperture Diaphragm:

Off-focus radiation

__________ can result in images, similar to shadows of the patient, beyond the exposed field of radiation.

Cones or cylinders

______ or _________ are essentially circular aperture diaphragms with metal extensions.

cones

______ has extension which flares or diverges with the upper diameter smaller than the bottom flared end.

Cylinders

_________ do not flare and has the same diameter at the bottom of the extension as it has at the top.

cylinders

May be equipped with an extension sleeve which can expand or collapse to vary the degree of beam restriction.

Collimator

_________ is most commonly employed beam restrictor in radiography.

Collimator

It consists of sets of lead shutters at right angles to one another which move in opposing pairs. Each set moves symmetrically from the center of the field.

Bottom shutters

_______ reduce penumbra along the periphery of the beam of their greater distance from the focal spot.

Upper shutters

_________ help in reducing the amount of off-focus (stem) radiation reaching the image receptor by absorbing this radiation before it exists the tube.

Positive beam limitation (PBL)

________ devices are automatic collimators.

Ancillary Devices

________ are generally designed with a special need in mind.

lead blocker

A _______ is simply a sheet of lead-impregnated rubber that can be cut to any size or shape.

Lead mask

________ is usually cut to correspond t the particular field size desired and is then secured to the end of the collimator.