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Fwd: KVP MEASUREMENT/DEFINITION
---------------------
Forwarded message:
From: joelgray@M3.SPRYNET.COM (Joel E. Gray)
Sender: medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to: joelgray@sprynet.com
To: MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-08-26 15:57:19 EDT
John,
There is something that many folks overlook-- it is what comes out of
the patient that effects image quality and what comes of the tube that
effects patient dose! This is a bit simplistic, so let me explain.
Small differences in x-ray spectra due to the waveform are usually lost
by the time the beam reaches the intensifying screen. For example, some
older generators used to have a small, low energy pulse on the front end
of the waveform, i.e., premagnetization energy as it was called.
However, this never got through the patient to alter image quality.
Since it never got through the patient it only contributed to excessive
patient dose.
Small differences in ripple, e.g., high frequency vs three phase vs
constant potential, are lost as they go through the patient, i.e., the
output spectra are swamped with scatter, etc. Consequently, try as hard
as you like, it will be virtually impossible to demonstrate CLINICALLY
SIGNIFICANT diffferences in image quality. Furthermore, if you add
additional filtration, e.g., set a minimum HVL of at least 3.2 mm of Al
at 80 kVp (as opposed to 2.3 mm required by federal regulations) you
will see no difference in image quality but you will reduce patient
doses significantly (30 to 40%).
Theorists will tell you that as the HVL goes up the R to rad conversion
factors also increase, which is true. However, the entrance skin
exposure is reduced more rapidly thant the increasing R to rad
conversion factor so the dose to the patient decreases.
I have never seen an effect from screens with age, i.e., loss of
absorption, etc. If this does occur, what is the mechanism? Screen
absorption depends on the components of the phosphor and these are not
changing with age. Perhaps some of the older screens experienced loss
of speed due to changes in the dyes used in some screens; discoloration
of the binder materials; and due to abrasion of the screen surface (and
spilled coffee and coke on the screen!). IIs do change their
characteristics dramatically with time (I usually think of about a five
year useful life for most intensifieres).
Just some food for thought on this topic!!
Joel
Joel E. Gray, Ph.D., Consultant
2804 Second Street Southwest, Suite 334
Rochester, Minnesota 55902
U.S.A.
Phone 507-286-8910
Cell Phone 507-269-4247
Fax 507-286-8910
e-mail joelgray@sprynet.com
On Aug 26, 11:29am, John Jezioranski wrote:
> Subject: Re: KVP MEASUREMENT/DEFINITION
> 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).
>
> ----------
> 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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> |> ._ O | "Politeness is the poison of all good
> __{_{ _ / //\. | collaboration in science." - Francis
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> i{_{{_{{_{ \>> | | "Things should be made as simple as
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> /|{_{{_{{_{ \\ | but not simpler." - Albert Einstein
> (_|_|__|__|_, | "It's necessary to be slightly
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> ^^^^^^^^^^^^^^&^^^ | "There are two kinds of men who never
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>
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>-- End of excerpt from John Jezioranski