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

[Ccja@aol.com]

---------------------
Forwarded message:
From:	BKOBISTEK@prodigy.net (Bob Kobistek)
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	BKOBISTEK@prodigy.net
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-08-24 15:26:29 EDT

toader micu wrote:
>
> 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
>  __        __
> |__|      |
> | \ adcal |__orp.
> ISO-9001 Cert.
> 426 West Duarte Road
> Monrovia, CA 91016, USA
> tmicu@radcal.com (work)
> micus@earthlink.net (personal)
> Phone:818-357-7921
> Fax:818-357-8863

The X-ray spectrum contains all the information there is to know about
the quality of the X-ray beam.  Why then is the instrument of choice
(Dynalyzer) one that only measures one point on the spectrum? (and,
perhaps, one of the least meaningful points)

In my opinion, absolute peak kilovolt measurement is meaningless, except
to the service engineer who must know it in order to adjust the
machine.  If absolute kVp were meaningful, all technique charts would be
the same.  Instead, each machine in the facility has its own technique
chart, based on the behavior of the individual machine.  Obviously a
high frequency machine set at 80 kVp will give a higher contrast than a
single-phase machine at the same setting.  Suppose I calibrated an X-ray
machine low by 5 kV, and then I added 5 kVp to all the techniques on the
technique chart.  Who'd know the difference?  I'm not saying we should
toss out kVp measurement.  We obviously need to know it for adjusting
filtration, calculating shielding, dose, and of course, to be sure the
electrical adjustments of the machine are correct.  I'm also not saying
that I disregard kVp accuracy.  If the measured kVp differs from the
dial setting by more than 5%, I advise my customer to have the machine
serviced.  kVp may not be the optimum measure of beam quality, but at
the moment, it's all I have to work with.

What I was saying in my original message is that AT THIS TIME IN
HISTORY, there is no uniform standard.  The Dynalyzer may supposedly be
a standard, but it is not an option for a consulting physicist like
myself.  Among noninvasive devices, there are as many standards as there
are manufacturers.  I chose my instrument because of its ubiquity.  With
so many in the hands of service engineers and equipment manufacturers,
the chances are good that everyone involved is on the same page.

I will now offer my answer to the question posed in the first
paragraph.  We measure kVp because we CAN measure kVp.  As Radcal and
NIST certainly have demonstrated by their work on the spectrometer, kVp
is not the best indicator of beam quality, but it is and always has been
easy to measure.  This is not the only example of widespread adoption of
a less than optimum quantity.  The Roentgen is another example.  It
evolved along with the device used to measure it, but I've never yet met
an air-equivalent patient.

Robert J. Kobistek, MS
National Physics Consultants, Ltd.
Cleveland, Ohio