Fwd: KVP MEASUREMENT/DEFINITION

[Ccja@aol.com]

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Forwarded message:
From:	john_jezioranski@CLINPHYS.PMH.TORONTO.ON.CA (John Jezioranski)
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	MEDPHYS@LISTS.WAYNE.EDU (Medical Physics Mailing List)
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-08-26 20:16:26 EDT

I'd love a universal set of technique factors too.  Has anyone thought of a
way
around any of the following problems?

While it's true that the x-ray spectrum integrated over the exposure time
together
with tube current contains all the information needed to describe what come
out of
the x-ray tube, technique factors also depend on the age of an intensifying
screen
or II phosphor as well as its properties when new, etc.

Additionally, the spectrum depends on more than the type of generator,
target and filtration.  The ripple on one of our high frequency generators
depends
substantially on tube current as well (slightly different HVL).  Different HF
generators
rise (and overshoot or undershoot) to the HV at different rates at beam on.
This depends on another kind of quality -
the quality of your service personnel.  It also means that the spectrum
integrated
over the exposure is different for a short exposure than a long one
(splitting hairs now).

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From:   Robert G. Zamenhof[SMTP:zamenhof@MIT.EDU]
Sent:   Tuesday August 26, 1997 9:39 AM
To:     Multiple recipients of list MEDPHYS
Subject:        KVP MEASUREMENT/DEFINITION

I am finding this discussion very interesting, because it touches on
something I have always felt as well: that measuring the kVp is probably
the best SIMPLE measure of a complicated parameter that strongly influences
both patient dose and image quality (and is, therefore, of critical
importance in QA).  However, as Bob Kobistek points out it is far from the
whole story.  Beam filtration and waveform also affect these quantities
(incidentally, Bob, I think that a single-phase 80 kVp waveform produces
BETTER contrast than a high-frequency 80 kVp waveform...), so why don't we
recalibrate the "kVp" knob in "HVL" or "PENETRATION" units instead?
True,the HVL would need to be measured using water-equivalent absorbers
instead of aluminum; but so what? More applications for solid water!
Perhaps if that were done, and a correction to the technique charts were
made for the differences in absolute output of different tubes, then indeed
all x-ray machines might be able to use exactly the same technique factors
for given patients.

Yours,

Robert G. Zamenhof, Ph.D.
Director, Section of Radiological Physics
Department of Radiology
Beth Israel Deaconess Medical Center, Harvard Medical School
1 Deaconess Road, Boston, MA 02215, U.S.A.
Tel: (617) 667-0175  Fax: (617) 975-5233  E-Mail: zamenhof@mit.edu or
rzamenho@bidmc.harvard.edu
*** "Non iligitimati carborundum est" ***

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