iaea human health series publications - SEDIM

8.5.4. Detector ghosting8.5.4.1. ScopeIn this measurement, ghosting (sensitivity variation) or lag (residual signal) from a previous exposure isinduced in a manner similar to clinical operation and the results are quantified. For simplicity, both phenomena arereferred to in this publication as ‘ghosting’. Both qualitative and quantitative (preferred) evaluations may beundertaken, depending on the capability of the acquisition workstation. Here, a quantitative method for assessingghosting and an alternative qualitative technique are described.— Objective: To evaluate the severity of any artefact due to recent previous exposure to the detector in allsystems except photon counting systems.— Minimum frequency: Annually and after replacement of the detector.8.5.4.2. Instrumentation(1) PMMA slab, 45 mm thick.8.5.4.3. MethodologyImportant: Read through these steps and understand them before starting this test. It is important that theelapsed time between acquisition of the initial image (step 4) and the ghost measurement image (step 5) be theshortest time allowed by the mammography unit, as this will replicate the clinical situation.(1) Create a ‘patient’ study with an appropriate name.(2) Place the PMMA slab on the right half of the breast support table with the edge placed at the middle of thetable, running from the chest wall to the anterior edge such that approximately one half of the breast imagingarea is covered (Fig. 32(a)).(3) Lower the paddle so that it is in contact with the breast support plate. Note where the edge of the phantomcrosses the detector.(4) Acquire an image under manual exposure, using typical clinical exposure factors for the average breast. Thisis the ‘ghost creation’ image.(5) For CR, immediately process the cassette and retrieve the same cassette (screen) from the processing station.(6) Reposition the slab on the breast support table so that the middle of the slab is centred on the location of theedge of the uniform attenuator in the ghost creation image (Fig. 32(b)).(7) As soon after the first image is acquired that the unit allows another exposure, or as soon as the same detectorplate is available, acquire a second image using the same manual technique. This is the ‘ghost measurement’image.(8) View the ghost measurement image (the unprocessed version, if possible). Use the ROI tool (area ~4 cm 2 ) totake three measurements in two ROIs in the ghost measurement image at the locations described below:(a) The MVP in the background of the phantom on the side where no attenuator was present in the first image.In Fig. 32(b), the centre of the ROI should be ~20 mm to the left of the line marked D.(b) The MVP in the background of the phantom on the side where the uniform attenuator was present in thefirst image. Here, the centre of the ROI should be ~20 mm to the right of the line marked D.(c) The standard deviation (SD) in the background of the phantom on the side where the uniform attenuatorwas present in the first image (i.e. the same ROI as for step 8(b)).(9) Record the results on the evaluation of ghosting chart (Chart 7 in Annex II).100