beam restriction rad

Penumbra

Geometric unsharpness around the periphery of an image also known as edge unsharpness

Becomes greater with large focal spot and small SID

Why do shutters reduce penumbra?

Because of their greater distance from the focal spot

Aperture diaphragms

a flat sheet of metal, usually lead that has an opening in the center it is attached to the x ray tube head. Simplest of the beam restrictors. Designed in fixed applications with fixed image receptors and fixed SID.

Allows field to cover area just smaller than image receptor can be supplemented with cones and cylinders

Used in dedicated chest or head units

Dental units

Trauma units

Aperture diaphragm disadvantages

Greater amount of penumbra and off focus radiation due to proximity to tube port

Cones/cylinders

Essentially circular aperture diaphragms with metal extensions

one has an extension that flares or diverges with upper donated smaller than bottom flared end the cylinder does not flare

Used for greater detail visibility in a small area reduction in scatter makes images seem sharper

Application of cones and cylinders

Head work- sinuses or skulls

Spine work- spot films

Cross- table hip

Dental

Cylinders

Reduce the amount of irradiated material therefore reduce patient dose reduce scatter and increase contrast

Beam restricting devices

Ideally restrict field size to anatomy of interest

Reducing amount of tissue irradiated

Reducing scatter

Increasing contrast

More visibility of detail

Ancillary devices

Generally designed with a special need in mind.

Examples are lead blockers and lead masks.

Tailored to restrict the beam to a specific shape for a specific exam

Lead blocker

Ancillary device that can be placed on the radiographic table during examination of the lower spine in the lateral position to absorb scatter produced by the soft tissue of the patients back

Lead masks

Ancillary device that was cut to correspond to the field size desired then secured to the end of the collimator. Ones in the shape of a keyhole were used in cerebral angiography to reduce scatter.

Compton scatter

Dependent on the thickness of the irradiated material (more matter is more electrons and more scatter) also depends on kvp since scatter increased relative to photoelectric effect with increased kvp

Grids

Can absorb scatter radiation that has already been produced. Improves image contrast at the expense of patient dose since we need to increase technique to compensate since these remove part of the primary beam.

Collimation

Decreases the amount of radiation that interacts with the patient and reduces patient dose. Also reduces scatter since we reduce the amount of matter the photons can interact with

Pros of collimation

Reduce patient dose

Reduce scatter radiation

Increase contrast

Reduce unnecessary exposure by the primary beam

Collimation rules

Should be limited to 1 inch beyond the anatomy of interest on all sides tabletop

Should never exceed field size

Should not exceed size of image receptor

Positive bean limitation device

PBL devices most collimators are equipped with this. Automatically collimates the beam to the size of the IR

Collimator function

Permits infinite number of field sizes

Provided light source for radiographer

Reduces penumbra

Entrance shutters

Help reduce off focus radiation. Or x rays created in other parts of the tube not at the target

Second set of shutters

This is the set of collimator shutters that the technologist controls. The length and width is controlled independently by technologist

Light field

Uses a light reflected off a mirror to project area of the x ray beam. Proper adjustment of mirror is necessary to display location of exposure field

X ray beam coincidence testing

Should be part of quality control needs to be accurate within +- 2% of the SID

Penny test at 40 SID diameter of penny is .80 which is 2% of 40

Light x Ray field can’t fall outside penny

Filtration

Use of collimator results in this of the primary beam since the beam passes through the mirror

The mirror is about 1mm Al/Eq

Feature of collimator housing

Central ray must be marked

Some units project location of photo timer sensors into light field

Alignment light helps center beam with image receptor