From: "John Ryan" <[EMAIL PROTECTED]> Ian, A recent happening not directly related but close. I was shown a film, horizontal beam lateral of thoraco-lumbar spine, which had linear artifacts. Having seen the pattern before I was able to say it was an image of the back of the cassette. I accused people of having left the cassette in an inappropriate place and subjecting it to scatter from
other exposures. The staff rejected my accusation and I have to report they conducted experiments to conclusively demonstrate the scatter was from the wall behind the cassette - small amount of egg on face! This was a reminder of the significance of "backscatter". Like you I'm not aware of measurement of such scatter with chest exposures. I will be surprised if it hasn't been done and written up - probably many years ago and therefore may not be available through on line searches. As an extension of the unknowns. We will be introducing CR in the forseeable future. My understanding is the CR plates are more sensitive to scatter. Do CR cassettes have lead backing. What other defensive actions can be taken to reduce the effect of scatter when using CR for mobile chest x-rays? John Ryan >From: Ian and Louise Smalley <[EMAIL PROTECTED]> >Reply-To: AIRNEWS <[EMAIL PROTECTED]> >To: undisclosed-recipients: ; >Subject: RE: [AIRNEWS] Pb protection for mobile chests >Date: Fri, 12 Sep 2003 10:19:12 +1000 > > From: Ian and Louise Smalley <[EMAIL PROTECTED]> > >Here is my take on this Ric, > >Assuming the LBD accurately portrays the area of the incident beam, the >best control the dose from primary beam is through careful collimation. >Now for scatter: Assuming careful alignment of the beam and the >patient to eliminate the presence of unwanted artefact (drip stands, >structural columns etc), the first opportunity for the generation of >scatter is obviously the patient and scatter continues to be generated >at just about every interface through to and including the wall. Major >scatter is generated by the bed frame of which a fair chunk deflects >back toward the patient. Placing Pb protection on the waist (on the >tube side) of the patient is an effective way of preventing scatter >from exiting the patient, similarly placing Pb protection on the >cassette side will prevent scatter exiting the patient, but this >approach will prevent much of the bed-generated scatter actually >entering the patient. > >With respect to scatter generated within the patient, there is not a >lot we can do to protect the internal organs (including ovaries), but >as the testes are external (mine are kept on the fridge) there are >options for the very keen radiographer. > >An old boss of mine (sorry about the "old" Glen Burt) was an advocate >of attempting every mobile chest x-ray as a PA examination with the >patient sitting with legs off the side of the bed. This gave good >geometry and eliminates sources of backscatter. It also sent the >residual beam off to the patient in the next bed, but distances usually >made this acceptable. > >Ultimately, dose reduction on mobiles is not too different from fixed >equipment. Maintain your equipment, collimate carefully and get >positioning and exposure right the first time. Too easy! > >The above views on scatter are to be best of my knowledge totally >untested by scientific study. > >Which brings me to my "Why do they do that?" question. Why do >radiographers place Pb protection on the lap of a patient having a knee >examination when the sources of scatter (the knee, the cassette and >table/bucky structures) are ALL underneath the Pb protection? Surely >rather than teach this nonsense to students we should calculate dose to >the gonads (is it actually a significant dose?), or encourage >protection to be placed under the patient and/or curved between the >thighs as indicated by each view. > > > > >Ian Smalley +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+= This message is being broadcast by AIRNEWS, the Australian Institute of Radiography list Server Send messages to [EMAIL PROTECTED] For assistance send mail to [EMAIL PROTECTED] Commands should be sent to [EMAIL PROTECTED] Opinions expressed on this list are not necessarily those of the moderator, his assistants or those of the A.I.R.
