RE: Errors in Dental X-Ray Eqt Test using Keithley's TRIADsystem

[Tonry Louie L MAJ <Louie.Tonry@se.amedd.army.mil>]

Mr. Bala;

There are a number of ways to determine what's going on with the kVp.  I
agree the best way is to use the waveform.  When you look at the waveform,
you will see a representation of the kVp and how it changes over time.  If
the kV is unstable over time or you are getting an unusually high peak on
the lead edge, this can result in errors in the reported kV.

Some suggestions that might help.  Often with dental systems, it's hard to
get a complete coverage of the detectors active area.  We have often seen
very high kVs when we expose one detector and not the other.  If you
understand how the detector works this makes sense.  The kV detector has two
detectors; one is a reference detector (with a reference filter) and the
other is the measurement filter (with an appropriate filter for the energy
of interest).  The voltages from these two detectors are compared and the
calibration table is referenced to determine the actual kV.  If the
reference detector isn't fully exposed, the measured voltage will be too low
and the ratio will be too high leading to a higher reported kV.

Try exposing the detector at different distances and see if that helps.

A second suggestion is to use a higher technique.  Some times if the
exposures are too low, the signal from the detectors is inadequate to give a
good reading.  This sort of counters what I recommended above but try moving
the tube head closer to the detector.

Here's another technique you might want to use.  Get an old x-ray cassette
and tear out the intensifing screen.  The screen can then be used to
visualize the x-ray field size during setup of your equipment.  This way you
will know if you've indeed got adequate coverage of the detectors prior to
measurement.  I always have one in my case for just this purpose.

The biggest problem when measuring dental kV and time non-invasively is the
way the manufacturer's make their x-ray units.  If you compare the output
waveform and the kV waveform you will see what's actually happening.  There
is a preheat time built into most dental x-ray systems.  During this time,
kV is applied to the x-ray tube but the mA isn't.  However, there are still
electons available on the filament which will produce x-rays.  The intensity
is very low BUT the multi-meters are sensitive enough that it will 'see'
them.  Since during this time, the kV is stabilizing (one of the reasons
this is done), often you will see varing kV.  Once the mA is applied, the kV
is usually stable and usually will be pretty accurate.

I've attached a waveform picture for you to look at.  This is a 320 mS
exposure from a Heliodent 70 x-ray system.  The top graph is the kV and the
bottom is the output.  As you can see, the kV waveform starts well before
any apprecable output is recorded.  This is the preheat period.  Once the mA
is applied to the filament, the output jumps up and the kV flattens out.

I've used the PMX many times.  It's pretty good but has some disadvantages.
It's a little hard to learn to use and the software is DOS based which I
don't like.  They tell me that a windows based software is due to be
released this summer.  I hope so since I like the PMXs flexability.

Hope this is helpful.  If you have other questions, let me know.

Louie Tonry


     =========================================================
     + LOUIE TONRY, CHP
     + MAJOR, MS
     + Chief, Radiation Protection Division
     +
     + Eisenhower Army Medical Center
     + ATTN: MCHF-LOG-HP (Radiation Protection)
     + Box 264
     + Ft. Gordon, Georgia 30905-5650
     +
     + Voice:     DSN: 773-4692/6392      Civilian: (706) 787-4692/6392
     + Facsimile: DSN: 773-3427           Civilian: (706) 787-3427
     + Internet: Louie.Tonry@SE.AMEDD.ARMY.MIL
     =========================================================




-----Original Message-----
From: Bala, Vaidy LAB [mailto:vaidy.bala.lab@govmail.gov.sk.ca]
Sent: Monday, August 09, 1999 5:13 PM
To: 'Louie.Tonry@se.amedd.army.mil'
Subject: Errors in Dental X-Ray Eqt Test using Keithley's TRIADsystem


Dear Sir: I did  read your comments about the use of Triad for testing the
dental X-ray units. We read always a higher kVp and the vendors have
disagreed on this point. This is particularly true for single phase Siemens
X-ray units. Do you suggest any correction factors for this application?  

Your comments would be appreciated.  Keithley's head office told me unless I
do a wave form analysis, they would not be able to help. We are planning to
use the mini RTI Sweden equipment for non-invasive kVp measurement.

Any publication you may have on this subject, copy is a desired to be sent
to Sask Labour, rad. safety Unit, 1870 Albert Street, REGINA,Saskatchewan
S4P 3V7 CANADA

Thanking you in advance


Vaidy Bala
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