Collimator Maintenance and Testing Procedures PDF

Collimators The collimator, sometimes referred to as the light beam diaphragm (LBD), provides the radiograoher with an easy to use, accurate, method of controlling the size of the X- ray field and placing it over the area of interest, thus reducing the radiation dose to the patient and improv...

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