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Measuring Dental X-Ray with Keithly Triad
I received a number of questions regarding the comment I made about
measuring dental x-ray kVp and time with the Keithly Triad. I thought that
instead of just replying to the individual I'd post it so others interested
might have the same information.
I perform lots of surveys and have used many different multimeters. When I
was conducting surveys on dental x-ray units (interoral) I noticed that
there were often problems with both time and kVp measurements. Being a
science geek, and wanting to play with toys, I thought that I'd figure out
what the problems were.
Procedures: I got a hold of a Seimens Heliodent 70, Model D3152. This
happens to be a portable model of the Heliodent but is essentially the same
as the fixed units. I performed a calibration of the unit according to the
manufacturer's procedures. The system was calibrated and operated very
well.
I then took the Keithly Triad and connected a duel channel digital storage
O'Scope to the kV detector. On the second channel I connected a Radcal MDH
1515 with the 6 cc ion chamber. What I wanted to do was to compare the
output waveform with the kV waveform. Having done a lot of work with the
MDH, I knew that it did a very good job of measuring time on dental systems.
I then exposed both detectors (Triad and MDH) simultaneously to the X-Ray
system. The O'Scope recorded both the kV waveform and the output waveform
from the exposure. I then plotted both waveforms on a graph and compared
the two.
Results: What I found was a very unusual kV waveform and a typical output
waveform. The output waveform correctly measured the time while the Keithly
Triad misreported the time by between 150-200 milliseconds. I then
carefully reviewed the schematics for the Heliodent. Siemens designed their
x-ray systems in an unusual way. Most general purpose x-ray systems have a
method of applying kV to the tube quickly. This allows for quick production
of x-rays. On these systems, the mA is applies to the filament, then when
the exposure is initiated, the kV is quickly applied making the x-ray
exposure. However the electronics to make this happen are expensive and
bulky. So apparently Siemens worked a way around this problem.
What Siemens did is instead of applying the mA and then the kV. They apply
the kV then the mA! This is what caused the problems with the Triad. They
way the Heliodent works is there is an inductive heater in the focusing cup
on the filament. This inductive heater is used to heat the filament to the
proper temperature prior to applying the mA. While the filament is heating,
the tube is developing the kV. Then once the filament is up to a
predetermined temperature, the mA is applied. At the moment the mA is
applied, the timer is activated. So what ever time is used to heat the
filament is not considered exposure time. This 'preheat' time is on the
order of 150-200 milliseconds.
Here's the problem. As long as there are electrons and the kV is applied,
there are x-rays produced. It is true that the number of x-rays produced
are very low - but the Keithley Triad is VERY sensitive. The Triad measures
time from the kV waveform. The MDH measures time from the output waveform.
Further the MDH has a thumb wheel that adjusts when (by % of the max height
on the output waveform) the time starts and stops measuring. What the
O'Scope showed was that the Triad was measuring the time of the FULL
exposure - including preheat. The MDH through the use of the thumb wheel
settings did not.
The other problem is this. Since the Triad measures the 'full' exposure
including preheat, the kV is also affected. What is seen on the kV waveform
is a classic 'ski slope' curve during the preheat section of the waveform.
After the mA is applied, the waveform stabilized and the waveform then
becomes a classic single phase full wave rectified waveform. When the Triad
produces the kV value for output it first bins the measured values, then it
averages the bins. This is the reported kV. Since the lead edge of the
waveform is a 'ski slope' the average kV is often reported lower than
expected. Further, if a long exposure time is used, the preheat part of the
waveform becomes less significant to the average so the kV appears to
improve. This can lead to the incorrect assumption that the kV is not only
off, but also not reproducible.
Conclusions: The bottom line is that if you use the Triad on dental x-ray
systems for timing, you will measure the preheat time in addition to the set
time. In addition, the preheat time is variable depending on the
temperature of the filament. You will also often find the reported kV is
too low and some times you may conclude it is not reproducible.
Work Around: To get around this problem, you can always save the waveform.
If you do this, you can look at the flat portion of the waveform and
determine the approximate kV and see if it is steady. Further, you can
subtract the time from the 'ski slope' portion of the waveform from the
measured time and you'll find the time is probably accurate if the system is
truly calibrated properly.
Caution: A work of caution is provided. If you disagree with the Medical
Maintenance or the Service Rep where you contend the system is out of
calibration and they insist that their equipment says its OK, there's a good
chance you're both right. When they measure time, it is invasively and the
location on the board where they connect their pulse counter is after the
switch activated by the thermostat. So their times measured do not include
the preheat whereas yours do.
=========================================================
+ 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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