Collimators

Collimators – Light timer – Timer switch – Cables – Plugs – Light bulb • The collimator, sometimes referred to as the light beam – Light/X-ray beam alignment diaphragm (LBD), provides the radiograoher with an easy Collimator to use, accurate, method of controlling the size of the X- • Accuracy of Scales Test – Not everyone uses the ray field and placing it over the area of interest, thus aperture size scales on the collimator to set the aperture size, it is being reducing the radiation dose to the patient and improving the quality of the image • The collimator receives much use and is vulnerable to more common practive to use visual assessment of the light field size. However if staff do practive this method, it is advisable to knocks, often resulting in inaccuracy of the light beam/X- make sure that the scales are accurate ray beam coincidence, blown light bulbs, electrical and – Inaccuracy of scales may mechanical problems. lead to inaccurate collimation Collimators – Frequency of test • every 6 months – Equipment required • Visual Inspection – Rotation • Collimator to be tested • 100 cm rule – Stability – Knobs for shutter control – Accuracy of shutter setting scales – Shutter chain drive Collimator – Shutters – Housing – Window • Accuracy of Scales Test – Method • Set 100cm focus to tabletop distance UNLESS YOU HAVE HAD • Using the 100cm FFD (SID) ADEQUATE INSTRUCTION. scale on the collimator, set If unsure, call an electrician. vaous aperture sizes with the light on and measure the – Frequency resultant light areas at table • As necessary – Equipment required top level • Suitable light bulb • Compare with setting on the • Cloth or tissue collimator • Screwdriver – Evaluation Collimator • light area measured should be the same as the setting on • Changing a light bulb – Method collimator – Action • If settings are not accurate and the solution is not a simple one, call an X-ray engineer. • File a report • Switch off the power. • Remove relevant collimator housing • Check the spare is the correct type • Before removing the old bulb, Collimator check to see if the replacement bulb must be • Changing a light bulb – A light bulb can fail at anytime without warning. It fitted a specific way around • Replace faulty bulb with a new one is important then, to have the correct spare bulb readily available. – Staff should be capable of • If the bulb is of quartz type, ensure that it is not handled with bare fingers, as body oil replacing a faulty bulb. DO on the bulb will shorten its life NOT ATTEMPT THIS • Replace housing and tighten screws • Light beam/X-ray beam • Test alignment test – Frequency of test • Ensure that the spare bulb is • every 6 months replaced as soon as possible • as necessary – Equipment required • One 24x30cm loaded Collimator • Light beam/X-ray beam alignment test – the purpose of the light in a collimator is to allow more cassette • Alignment test tool. Commercially made test tools are available, but simple alternative is to use eight coins or four paper clips accurate collimation of the X-ray beam. The light must, • Lead marker or ninth coins – Method therefore, coincide with the • Make sure that the table is X-ray beam. The light beam level and the central ray at 90 relies on the accurate degrees to the tabletop positioning of the light bulb • Place a loaded cassette on and angled mirror inside the the tabletop face up collimator. • Set a FFD (SID) at 100cm • Switch on the collimator light – Should either the mirror or • Centre to the middle of the the light bulb be dislodged, cassette then errors in collimation could occur, resulting in Collimator areas of interes being excluded from the field, or • Light beam/X-ray beam too large an area being alignment test – Method irradiated. • Collimate to within the edge of the cassette, leaving a 3cm Collimator border all around, that is outside the light field • Place the coins in pairs, so described above, using that, where the coins touch, different field sizes, coincides with the edge of the remembering to re position light area the markers for the second • All four of the border must be exposure marked in this way – Evaluation • Place the lead marker within • For perfect alignment, the one corner of the light field so light field (where the coins that the film can be related to touch) should coincide with the light/X-ray field and hence the X-ray field. • The irradiated area must not the collimator shutters. • Make an exposure sufficient to blacken the film • Process the film – Alternative method • Use four paper clips, each be greater than the area covered by the light. • At 100cm FFD (SID) the irradiated area must not be more than 10mm smaller than bent to form right angels the area covered by the light. This represents a 1% • Place paper clips at corners tolerance of light field Collimator Collimator • Light beam/X-ray beam • Light beam/X-ray beam alignment test – Action alignment test – Note • If the alignment is • Collimators are known to unacceptable it must be become less accurate as the adjusted field size is increased. To check this make two flash • Call an X-ray engineer – Note exposures on the same film, • Cones and diaphragms can following the method also be checked using a similar method • Make an exposure • Fully close the open shutters Collimator and open fully the closed ones • Shutter efficiency test – Closing the shutters in the collimator fully should prevent any radiation from reaching the film. Useful for testing radiation safety • Make another exposure • Process the film – Evaluation • Study the film. If the shutters are efficient the film will not have been affected by when discharging capacitor discharge mobiles or making tube warm up radiation – Action • If the shutter are allowing exposures radiation to pass, call an X– Frequency of test • every 6 months – Equipment needed • One loaded 24/30cm cassette – Method • Place the cassette on the tabletop face up • Set a FFD (SID) of 100cm • Set an exposure of approximately 80kV and 40mAs Collimator • Shutter efficiency test – Method • Open one set of shutters fully, leaving the other closed ray engineer