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RE: dental
Mark;
I'll give you my two cents worth.
On some dental systems, a preheat time is used. This preheat simplifies the
electronics required to produce x-rays. Let me explain why. A good x-ray
system will produce a consistent potential once activated (ie. stable kVp),
and will produce x-rays at the required kVp quickly after activation of the
exposure.
On GP x-ray systems this is accomplished with a large capacitor bank in the
tank which can deliver the high voltage necessary immediately on request.
Since dental systems do not have (or want to have) this large tank (and the
associated costs), they design their systems differently.
When an exposure is initiated, the kV is immediately applied to the tube.
This kV does not have to be particularly stable since no mA is applied at
this time. This lack of mA ensures the number of x-rays produced are
minimized (ie. the exposure is low). This process also allows the kV to
"develop"; to establish a stable electric potential across the tube prior to
application of the mA current.
During this "preheat" time, the kV stablizes, and the filiment in the tube
is heated. On the Philips Heliodent, the filiment is heated through an
inductive heater. This heater is located in the focusing cup which
surrounds the filiment. Once the filiment reached a preset temperature, the
mA is applied to the filiment and the exposure timer is started (NOT
BEFORE).
What this means is two things. First, the filiment has to reach a
predetermined temperature. If multiple shots are taken, the time required
to reach this temp will vary due to residual heat in the focusing
cup/filiment. So if you measure time on a system and start measuring time
on a cold system (same time setting) as more and more shots are taken in a
row, you will see a decrease in measured time as the tube gets hotter. This
is because the time require to get to the predetermined temp gets less.
One other comment on time with dental systems is that if you research the
electronics, you will find that the test points used by maintenance to set
the time are AFTER the preheat circuits. What this means is the preheat
time has no effect on what maintenance measures for the shot duration since
the timing does not start (as far as they can measure) until the preheat is
finished. So you may measure something completely different than they do
and you may both be "right".
I would suggest to you the same thing I recommended to Mr. Bala. Take a
look at the waveform. You will see graphically some interesting
information. The kV waveform (on the Philips system) shows an initial rapid
rise in kV followed by a fairly rapid falloff. This is then followed by a
rapid rise to the set kV and a stable waveform. What this is showing is the
preheat. If you look at the waveform and disregard the time prior to the
first valley, you will be seeing the actual exposure time.
If you can get a storage O-Scope and connect it to an MDH 1015 or 1515, you
can see the actual output waveform. This will show you the radiation being
put out by the system and you will see during the first portion of the
exposure (the preheat) the output is very low. Once the preheat is
completed, the output jumps up (this is when the mA is applied to the
filiment). Once you see this the whole problem you're experiencing will
make sense.
Bottom line, setting a delay of 220 mS will not give you reproducable times
since the preheat time varies.
----
As far as the international tooth and the film type and linearity. Dental
systems have a fixed mA. The tooth indication and the film type indications
only adjust time. The film type is a multiplier to the time. If you select
film type 2 you may be multipling a 1.1 factor to all times. The tooth
indicator simply applies a different time (ie. .3 sec or .7 sec). Therefore
if you select a .3 sec tooth and a film type 2 you will be selecting a total
exposure time of 0.33 seconds. Again though, the preheat will make it
difficult to get good numbers.
My feelings on linearity is as follows. Linearity is used to determine if
the mA stations on systems which have variable mA stations are set properly.
It is also used to provide an indirect check on the accuracy of the mA.
This is done by a comparison to standard output tables. If you find the
output is twice the recommended output it is likely the mA is inproperly
set. On dental systems, linearity is not really appropriate since there is
only one mA station. If you check "linearity" on a dental system what
you're really looking at is the timer - are you getting twice the exposure
for twice the time. This isn't really linearity.
You asked when I was concerned with time. My feelings are that I get
concerned when exposures are not reproducible. Technicians are not really
concerned with absolute time. They will select a time station that gives
them a good film. What will cause them problems is once a technique is
determined if the time is not reproducible, them will have repeats which is
bad.
I hope this helps.
Lou
Louie L. 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: Facsimile:
DSN: 773-4692/6392 DSN: 773-3427
Commercial: (706) 787-4692/6392 Commercial: (706) 787-3427
Internet: Louie.Tonry@SE.AMEDD.ARMY.MIL
-----Original Message-----
From: Mark_Dirksen@moh.govt.nz [mailto:Mark_Dirksen@moh.govt.nz]
Sent: Monday, November 29, 1999 5:40 PM
To: Louie.Tonry@SE.AMEDD.ARMY.MIL
Subject: dental
I am hoping that if you have a minute you could answer my questions
regarding
dental x-ray monitoring.
I use a PMX III to measure kVp output and timer accuracy. My questions has
to
with any Trophy IRIX/CCX.
It seems that I get a fairly reliable kVp output reading. But when it comes
to
the timer accuracy it gets confusing. Seems like if I were take into
account
the 220 mS preheat time - my time recorded on the PMX should be the selected
Trophy time plus 220 mS. As shown by typical it is @160mS but then I have
the
PMX disregarding the first 50 mS.
Where I get confused is when do I have a concern with a timer? I seems like
I
would be concern with the atypical responses since they seem to fall short
on
time? Do you have any thoughts?
Typical
kVp Short time Long time
65 340 480 1.12 1.26
67 300 462 1.08 1.24
69 460 623 820 983
68 300 463 1.1 1.27
Atypical
66 360 384 1.06 1.1
? 340 400 670 745
Also, I have a question about the Trophy 708 with the international tooth
symbol
selection and the 'film type' selector. Have you ever monitored one of
these
units and if so did you have a problem with linearity?
thanks
Mark
This is how I got your name.
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
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