RE: Query re measuring II input doserates

["North, David" <DNorth@Lifespan.org>]

Re: Image intensifier input dose rates
A good reference for this subject can be found in a chapter by Keith Strauss
in "Physical and Technical Aspects of Angiography and Interventional
Radiology", which is a course syllabus from the 81st Annual Meeting of the
RSNA in 1995. In my experience, one needs to use copper attenuators, not
aluminum, at a minimum thickness of 2.5 mm of Cu. Sometimes more Cu, say 3.0
mm or so, is needed to bring the kV of the generator up via the automatic
brightness control if there is added Cu in the clinical beam. I build up the
attenuator Cu for a minimum 75 kV or so in such a system. For a unit with
manual kV control I don't hesitate to go up to 120 kV for a beam that has
only about 2.5 mm Al-equivalent total filtration. The idea is that this is
an efficiency measurement, and one wants to bring the energy of the x-rays
impinging on the intensifier up above the K-edge of the input phosphor
(about 31 keV for CsI) so that one is measuring only radiation that the
imaging system can use.
David North
Rhode Island Hospital
Providence, RI

> ----------
> From: 	Chris Alston
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Wednesday, November 3, 1999 08:22
> To: 	Multiple recipients of list
> Subject: 	Query re measuring II input doserates
> 
> >I would like some advice on acceptable kVps for measuring II input
> >doserates. We use 20 mm of Al as the beam attenuator, and
> >measure the doserate at the II face in ABC mode.
> >
> >In "Specification, Acceptance Testing and Quality Control of
> >Diagnostic X-Ray Imaging Equipment" (Seibert, Narnes & Gould,
> >eds), 1994, AAPM, it is noted that for a 3.8 cm (1.5 in) thick Al
> >attenuator, the measured doserate is fairly independant of kVp
> >ranging from 50 to 110 (p 487). There are further data indicating
> >that for 22 mm total Al attenuation, the conversion efficiency varies
> >by about 15% between 60 and 110 kVp (p 443).
> >
> >However, I have also heard it said that for II input doserate
> >measurements, the kVp should ideally be kept to around 70 to 80
> >kVp, otherwise the doserate measured may be high.
> >
> >I would appreciate some thoughts on this.
> >
> >Thanks
> >
> >John Burrage
> >---
> >Department of Medical Physics
> >Royal Perth Hospital, Perth, Western Australia
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