Fwd: X-Ray Survey Equipment/X-ray spectra

[Ccja@aol.com]

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Forwarded message:
From:	micus@DENMARK-C.IT.EARTHLINK.NET (toader micu)
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	micus@earthlink.net
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-08-19 02:55:00 EDT

Dear All,

Bob is quite right, Radcal is currently working with  NIST to develop
a crystal diffraction x-ray spectrometer for mammography. The
instrument has a demonstrated capability to measure kVp with an
accuracy of better than 50 eV in the critical mammo range. Best of
all it shows the whole spectrum and it is self calibrating since it
uses known features such as k-lines as a reference.

Bob is also right that there is no clear, mathematical definition of
the kVp that everyone agrees on.  For what it's worth, there is an
IEC committee (of which we are a part of) trying to define exactly
what kVp is.

It is also correct to say that non-invasive meters are faster and
easier to use than invasive dividers. If one knows the all the
relevant spectral information (filtration, tube target material,
waveform, etc.) one can get reproducible kVp values for a waveform
that doesn't have much ripple. Unfortunately, different meters have
do have very different dependencies on the relevant spectral factors
and may give different results. However, I beg to differ with Bob on
several points:

1. Though no international definition of kVp yet exists, there is a
single device currently used by virtually all non-invasive kVp meter
manufacturers, x-ray machine manufacturers and calibration
facilities world-wide as their calibration standard. This device is
the DYNALYZER (or the HV-1 divider) and is viewed by many in the
industry as a working standard.

2. Determining the true voltage applied to the tube is very
important. Many service engineers still use high-quality invasive
dividers when installing a new x-ray machine because they believe
exactly this. The ability to make sure the anode and cathode are well
balanced, the mA waveform is proper and that there are no
high-frequency spikes or irregularities up to 100 kHz,  are
capabilities that non-invasive meters typically cannot provide.

Put another way, if kVp wasn't important, maybe the output of these
non-invasive meters should be a number not called kVp (kilo volts
peak, the emphasis is on volts). Maybe the x-ray machines dials
should have a relative number called the KRN (K. Relative Number),
not kVp. Maybe...

My personal feeling is that for many applications, the accuracy of
the information given by a non-invasive meter is just as important as
its precision.

While it is definitely true that it is possible to minimize meter
to  meter variations by using only one kind of instrument, this
approach  doesn't address the industry's need for non-invasive
instruments that provide accurate results under non-laboratory
conditions.

Please feel free to comment directly to me. If this issue is of
interest, I will try to summarize all different inputs and post them
on this listserv.




Ted
Ted Micu
Sales Manager
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