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Health Program > Products and Procedures > Diagnostic X-Ray > Resource Manual for Compliance Test Parameters of Diagnostic X-Ray Systems > Addendum to Resource Manual
Addendum to Resource Manual
ADDENDUM
TO RESOURCE MANUAL
OPERATIONAL CHARACTERISTICS OF THE MDH 1015 X-RAY MONITOR
- Introduction
We felt this addendum to our resource manual was necessary, because the
material in the resource manual and testing manual does not provide adequate
information on the functioning of the MDH 1015, particularly the PULSE
DURATION mode and the use of the thumbwheel switch for establishing the
measurement threshold. The MDH 1015 is a unique instrument in that it can
accurately measure exposure time based on the accuracy specifications
supplied by the manufacturer and the measurement basis (bases) used by them
to arrive at their accuracy specifications. These measurements could be made
with other instruments, but the MDH 1015 combines functions that would
necessitate several instruments.
- History and Design Considerations
When the Center for Devices and Radiological Health (formerly the Bureau
of Radiological Health) initiated a diagnostic x-ray compliance testing
procedure, there was a need for a more specialized instrument to conduct
this testing. The instrument used prior to the development of the MDH 1015
X-ray Monitor was an ion chamber instrument with the following features:
- an ion chamber attached to a cable.
- an analog (needle) meter with one numerical range (ranges could be
changed by means of a selector switch).
- no exposure timing option.
This situation presented the following problems:
- the cable attached to the ion chamber created spurious currents or
"noise" whenever the cable was moved or the instrument
repositioned.
- if the radiation level from the x-ray source was unknown, it was
difficult to choose the appropriate range on the instrument.
- a separate instrument was needed to measure exposure time.
The MDH 1015 was designed by the Division of Electronic Products (DEP),
to fulfill the Center's need for an instrument that would solve the problems
mentioned above. The MDH 1015 eliminated the cable noise problem by placing
a current-to-frequency converter (explained in paragraph B, below) at the
ion chamber instead of at the instrument housing. It eliminated the range
problem by employing an autoranging feature (explained in paragraph C,
below), and included PULSE DURATION and PULSE EXPOSURE modes to measure
exposure time and the radiation produced in that time interval (explained in
section II.).
The instrument housing, cable, converter box, and ion chamber assembly
are all designed for portability and rugged field use. The battery power
supply enables the MDH 1015 to be used at any location without the need of
external power.
- Current-to-frequency converter
The MDH 1015 utilizes a current-to-frequency converter to measure
exposure. When the ionization chamber is exposed to radiation, the
ionization of air molecules in the chamber establishes a current in the
instrument. The current-to-frequency converter measures the current and
generates electrical pulses based on the amount of current. The higher the
current from the ion chamber, the higher the frequency of the pulses emitted
from the current-to-frequency converter. The pulses are measured in the MDH
1015 by a pulse counter which measures the number of pulses in a given time
interval (PULSE EXPOSURE and EXPOSURE modes). For the EXPOSURE-RATE mode,
the instrument samples the number of pulses every 1.2 seconds and computes
the rate per unit time.
The advantage of this type of design is that the amplitude or strength of
these pulses is much higher than any cable noise. This prevents the pulse
counter from reading anything but pulses generated by radiation to the ion
chamber.
- Autoranging
Most ion chamber instruments used for x-ray exposure measurements have an
analog meter with one or more ranges on the meter and a range selector
switch. If one range is present on the scale, the operator must multiply or
divide the reading on the scale to obtain a correct value for the
measurement. If the meter goes beyond the maximum value on a particular
range, the operator must switch to the higher range to obtain a measurement.
When the operator does not have sufficient exposure at one of the higher
ranges, he must switch the unit to one of the lower ranges. In some cases,
the operator might lose the measurement of an exposure that he can't
duplicate.
The MDH 1015 does not have a selector switch for ranges due to the
autoranging feature. This enables the unit to go from one range of exposure
to another, automatically, with a floating decimal point similar to a
portable calculator. In the EXPOSURE-RATE mode, the unit will automatically
switch from mR/minute to R/minute when the ion chamber receives an exposure
rate of that magnitude and the "R" LED indicator lights up instead
of the "mR" indicator.
The MDH 1015 employs a LCD (liquid crystal display) readout for the
numerical exposure value and LED (light-emitting diodes) for range
indication. The LCD on the unit displays three digits with a decimal point.
These three digits are the most significant digits for each measurement. The
MDH 1015 can, for example, go from reading 998 mR/minute to 1.02 R/minute
with the autoranging feature. On an instrument without autoranging, the 1.02
would most likely be off the scale.
- Modes of Operation
The MDH 1015 offers four modes of operation. These are the EXPOSURE-RATE,
EXPOSURE, PULSE EXPOSURE, and PULSE DURATION modes.
- EXPOSURE-RATE mode
In the EXPOSURE-RATE mode, the MDH 1015 measures exposure per unit time.
The instrument measures the exposure over a 1.2 second interval and
automatically calculates the exposure-rate per minute or per hour. The MDH
1015 can be used with several different ion chambers, the two most common of
which are the 6 cc chamber and the 180 cc chamber. The smaller 6 cc chamber
is used for measuring moderately high radiation intensities while the 180 cc
chamber is used for very low levels. The instrument has a feature that
senses which chamber is connected to the converter box. If the 6 cc chamber
is connected, then the instrument measures exposure rate per minute. When
the 180 cc chamber is connected, it measures the rate per hour.
- EXPOSURE mode
The EXPOSURE mode provides for the integrated measurement of exposure.
The MDH 1015 measures the total exposure to the ion chamber for the entire
time that the instrument is set in the MEASURE position. A design feature is
provided that updates and displays the accumulated exposure every 1.2
seconds during the integrated measurement. This mode is very sensitive and
can resolve exposures as low as 0.02 mR. This is important for measuring
such things as primary protective barrier transmission of fluoroscopic
systems, where very low exposures can be found. Also, the EXPOSURE mode will
continue to measure and accumulate the exposure until it is reset. This
permits the summing of individual exposures in a series, which is useful for
determining the total exposure to film or direct-print paper.
- PULSE EXPOSURE mode
The PULSE EXPOSURE mode (and the PULSE DURATION mode discussed later) are
atypical of operational modes usually found on radiation monitors. These
modes were specifically designed into the MDH 1015 to evaluate x-ray machine
performance in accordance with the Federal standards. In order to understand
the operation of the PULSE EXPOSURE mode, consider the x-ray output emerging
from a simple single-phase x-ray machine. The voltage waveform supplied to
the x-ray system is sinusoidal in nature (Figure 1A), but because only the
positive pulses are useful in producing X rays, the circuitry is designed to
either clip the negative pulses (half-wave rectified system, Figure IB) or
to flip them over to positive pulses (full-wave rectified system, Figure
IC).
-
Figure 1A
Figure 1B
Figure 1C
Thus, the x-ray output is a series of pulses each rising from zero
intensity up to a maximum (peak) and falling again to zero.
The MDH 1015 is designed to measure the x-ray exposure of such output in
the PULSE EXPOSURE mode. When the exposure is initiated and the voltage
waveform begins its positive rise, X-rays begin to emerge from the tube. At
some point, the MDH 1015 will start to measure the x-ray exposure and will
continue measuring until the exposure ends. This beginning measurement
point, which was selected and designed into the MDH 1015 by the instrument
developers, is where the x-ray intensity reaches 10 mR/sec on the rise, and
the ending point is where the intensity drops to 10 mR/sec on the fall
(Figure 2).
Figure 2
Referring again to Figure 1A, it can be seen that the voltage pulses are
only 8.33 milliseconds in duration whereas most routine radiographic x-ray
exposure times are in excess of 50 milliseconds, thus a typical x-ray
exposure will consist of a number of radiation pulses, each one rising and
falling through the 10 mR/sec "trigger" point. How then does the
MDH 1015 "know" to count all the pulses in a full x-ray exposure
rather than count only the first pulse and then terminate when the intensity
drops to 10 mR/sec the first time? This is achieved by a built-in memory
buffer and delay circuit. When the MDH 1015 begins measuring (x-ray
intensity reaches 10 mR/sec on the rise), it accumulates the exposure until
the intensity drops to 10 mR/sec on the fall at which time it stores the
accumulated exposure into a memory buffer. The MDH 1015 now waits for 2
seconds to see if any more radiation (second pulse) comes in. If so, it will
accumulate the exposure of this pulse and add it to the exposure already in
the memory buffer. It will continue this process until no more radiation
pulses are detected within 2 seconds of the previous pulse. The MDH 1015
will then display the sum of the accumulated exposures in each of the
pulses. The 2 second delay was selected as a reasonable delay time to allow
for measuring the exposure of half-wave rectified systems which produce a
series of pulses separated by a non-radiation producing gap. The assumption
being that any radiation coming into the MDH 1015 within 2 seconds of the
previous pulse is considered to still be part of the first exposure, and any
radiation detected after 2 seconds is actually a second exposure.
In the PULSE EXPOSURE mode, the MDH 1015 "resets" automatically
between exposure measurements. For example, during an exposure, if the
exposure rate drops below 10 mR/sec for longer than 2 seconds and then comes
back up above 10 mR/sec again, the MDH 1015 interprets this as two separate
exposures. It then resets itself after the first exposure and displays the
value of the second exposure. This permits the measurement of several,
distinct radiation exposures without the total radiation being added
together, as it does in the EXPOSURE mode. Since in this mode the instrument
does not sum an exposure series, it is possible to take several separate
exposures without having to reset the instrument between exposures. This is
important, since it is not always convenient to reset the instrument
manually.
- PULSE DURATION mode
The PULSE DURATION mode provides a means to determine the length of time
that the x-ray tube is producing radiation. This mode is a desired feature
of the MDH 1015, because before its development, it was necessary to use a
separate instrument for measuring exposure time.
The provisions of the Federal performance standards (21 CFR Subchapter J)
require manufacturers of x-ray systems to establish and specify the
measurement basis for exposure time. This measurement base is generally
expressed in terms of percent of the voltage waveform. of the high voltage
output through the x-ray tube. Simply stated, the exposure time is the time
radiation is produced beginning at a certain percentage of the voltage
waveform on the rise until that same percentage is reached on the
fall of the last pulse in the exposure interval (Figure 3).
Figure 3
For three-phase systems, there is, in effect, only one long pulse, so the
time interval begins and ends at a certain percent of the same pulse (Figure
4).
Figure 4
The Federal standards do not restrict the manufacturer's selection of the
measurement base, hence the specified "triggering" percent point
may be different for different models of x-ray systems. The selection of the
triggering percent point is influenced by several factors such as an
asymmetrical first pulse in which the rise of the first pulse is somewhat
jagged in shape (Figure 5) and the manufacturer wants to exclude that part
of the pulse before starting the timer measurement.
Figure 5
Because the measurement base can vary, it was-necessary to design the MDH
1015 with the capability for measuring the exposure time in accordance with
the manufacturer's specifications. This is accomplished by use of the
PULSE-FRACTION-THRESHOLD thumbwheel. It is important to note that this
thumbwheel works only in conjunction with the PULSE DURATION mode and has no
connection to or effect on any of the other three modes. The thumbwheel
provides selectable dial settings from 0.1 to 0.9, corresponding to an
adjustable range of 10% to 90% of the radiation pulse. Although the
manufacturer specifies the time measurement base as it relates to the
voltage waveform, the MDH 1015 is only capable of detecting radiation, hence
it "triggers" at a certain percent of the radiation pulse (as
determined by the thumbwheel setting) rather than at a percentage of the
voltage pulse which it is unable to detect. Unfortunately, the shape of the
radiation pulse(s) is not the same as the voltage pulse(s). This is
illustrated in Figure 6.
Figure 6
From the figure it can be seen that the radiation pulse is not congruent
with the voltage pulse but "lags" behind on the rise and drops
more rapidly on the fall. This phenomenon occurs because at lower voltages
(just as the voltage pulse begins to rise or has reached nearly zero on the
fall) less energetic X rays are produced which are readily absorbed in the
tube housing glass, beam-limiting device filters, and other components in
their path such that the radiation output is low until peak voltages are
reached.
Thus, the correspondence between the voltage waveform and the radiation
waveform must be known before an accurate measurement of the time interval
can be made. This correspondence was experimentally determined and verified
in the CDRH laboratories using a single-pulse voltage waveform to correlate
the radiation waveform percentages to the voltage waveform percentages. The
approximate correspondence between the thumbwheel setting of the MDH 1015
and percent voltage waveform peak height is given in Table 1.
% Voltage Waveform Peak Height |
Thumbwheel Setting |
90 |
0.7 |
80 |
0.6 |
75 |
0.5 |
70 |
0.4 |
60 |
0.2 |
Table I
Knowing this correlation, the MDH 1015 thumbwheel can be set
appropriately to measure the time interval as specified by the manufacturer.
In order to perform a time measurement using the PULSE-FRACTION-THRESHOLD
thumbwheel, the MDH 1015 must first "know" what the maximum
intensity (peak) is before it can trigger at a preselected percentage of it.
This is accomplished in a two step process. In the first step, a test
exposure is made in which the MDH 1015 triggers automatically at 10 mR/sec
(the same trigger point as in the PULSE EXPOSURE mode), determines the peak
intensity of this exposure, and stores the peak value in the
PULSE-FRACTION-THRESHOLD circuitry's memory. Now, in the second step, when a
subsequent exposure is made, the MDH 1015 will trigger at the preselected
percentage of the radiation pulse as established by the thumbwheel setting.
For example, consider the radiation pulse illustrated in Figure 7. If the
test exposure peak intensity reaches 250 mR/sec, then during the second
exposure, if the thumbwheel is set to 0.1, the MDH 1015 will trigger at 10%
of the pulse, or 25 mR/sec. The time interval will be measured from this
point on the rise of the pulse until the same intensity is reached on the
fall of the last pulse in the time interval.
Figure 7
The MDH 1015 provides an indication of this entire process on the digital
display. When the mode selector is first put into the PULSE DURATION mode,
the display reads - 00.0. The negative sign indicates that the MDH 1015 is
ready for the test exposure so it can determine the peak pulse height
(intensity). After this initial exposure, the display will show a time
reading preceded by a minus sign (for example -438). The negative sign in
this case indicates that the MDH 1015 has now determined the peak intensity
and stored it in memory. Since a time value is present on the display, the
negative sign also acts as a cautionary indicator to inform the operator
that the MDH 1015 began its time measurement by triggering at the 10 mR/sec
point rather than a preselected percentage via the thumbwheel setting. When
a subsequent exposure is made, the display will show a time reading without
the negative sign, indicating that the MDH 1015 has now triggered at the
preselected percentage determined by the thumbwheel setting. And, as long as
the MDH 1015 is not reset, the time interval of any subsequent exposures)
will be measured based on the thumbwheel setting. The MDH 1015 is designed
so that the mode selector can be switched back and forth between PULSE
DURATION and PULSE EXPOSURE without affecting the PULSE-FRACTION-THRESHOLD
setting. This is to allow for reading the time measurement and its
corresponding exposure without having to start the time measurement process
(discussed above) all over again. However, any other switching of the mode
selector or function selector will reset the PULSE DURATION mode back to
-00.0.
- Special Problems with MDH 1015 Measurements
The manufacturer is allowed to specify the measurement basis for timer
accuracy. This specification is typically based on the voltage pulse rather
than the radiation pulse. Thus, the exposure time is the length of time that
power pulses are gated through to the low voltage (primary) side of the high
voltage transformer. Since the MDH 1015 measures time based on the radiation
pulses generated by the voltage on the high voltage (secondary) side, a
discrepancy between the two measurement bases occurs. This discrepancy can
arise from three sources as follows:
(1) The "phantom" pulse
(2) Timing threshold
(3) Preheat pulses
- The "phantom" pulse
The greatest, but least obvious discrepancy is caused by the
"phantom" pulse which results from tests conducted on single-phase
half-wave rectified machines, because the negative phase of the wave cycles
on the low voltage side produce no radiation pulses on the high voltage
side. This affects single-phase half-wave rectified systems only, since both
full-wave rectified and three-phase systems electronically convert the
negative pulses to positive pulses. To understand how the .. phantom"
pulse affects measurement of timer accuracy, consider a 1/10 second (100 ms)
portion of voltage waveform from the primary side of a single-phase source.
Figure 8
From figure 8, it can be seen that there are six full cycles (at 60 Hz)
and each cycle is 16.67 milliseconds long. Since single-phase half-wave
rectified x-ray system timers always start counting at a positive going
cycle, phase difference at the instant the exposure switch is pressed can be
ignored. On the secondary side of the high voltage transformer, the
resulting six radiation pulses will be 8.33 ms long each (1/2 cycle) and
will be follow by an 8.33 ms "phantom" pulse which has been
canceled by the rectification (Figure 9), for a total of 1/10 second (100
ms). However, since the MDH 1015 only sees" radiation pulses, the 8.33
ms negative pulses that do not produce radiation will not be detected. The
logic circuit of the MDH 1015 does allow it to count the
"negative" pulses that are bracketed by positive pulses (because
of the built-in 2 second delay discussed in section II.A.), but not the
trailing "negative" pulse. The result is that the time measured
will be 91.67 ms. The MDH 1015 will accumulate time only for the period
during which positive pulses are present. As a result, the time period
during which the last wave cycle is in its negative phase is ignored.
Figure 9
1/10 seconds |
= 100 ms |
|
- 8.33 ms
|
|
91.67 ms |
- Timing the radiation pulse and the threshold setting
The second source of disagreement is due to the circuitry within the MDH
1015 that decides when to start and stop timing an exposure, and affects
single-phase half-wave and full-wave rectified as well as three-phase x-ray
system measurements. This function was designed into the MIDH 1015 to permit
accurate measurements in instances where the manufacturer specifies exposure
time as that time between certain percentage points of the voltage waveform.
(as mentioned earlier in section II.D. PULSE DURATION mode).
Since manufacturers typically specify the exposure time interval based on
the voltage waveform whereas the MDH 1015 measures this interval based on
the radiation waveform, a discrepancy can exist in the measured exposure
time versus the preindicated (or specified) exposure time. This is
especially true for single-phase systems where the manufacturer
traditionally specifies the exposure time interval as beginning and ending
at zero percentage of the voltage waveform (the points where the pulse is
just starting or just ending its positive phase) while the MDH 1015 does not
start measuring the time interval until a certain percentage of the
radiation waveform has been reached as determined by the thumbwheel setting.
This is illustrated in Figure 10. For example, if the MDH 1015 thumbwheel
is set to 0.2 (corresponding to 60% of the voltage waveform) the measured
time interval is T1, but the true time is T2. The difference between Tl and
T2 can be significant, especially for single-phase systems where the
discrepancy can be as much as 4-6 ms. For three-phase systems, the
discrepancy is much less and considered negligible. This is due to the fact
that three-phase systems produce radiation that resembles a constant source
of radiation rather than the pulsed radiation from single-phase systems.
Figure 10
Since the voltage waveform characteristics are quite different from that
of single-phase systems, manufacturers of three-phase systems usually
specify the exposure time interval as beginning and ending at some
percentage, well above zero (typically 75%), of the voltage waveform. In
this case, the MDH 1015 thumbwheel can be set to match the corresponding
specified percentage of the voltage waveform thus making T1 and T2 the same.
- Preheat cycle radiation
Another source of disagreement which can affect the time measurement results
from a non-preheated filament. Radiation is produced by the filament, even
though it is not hot enough to produce the selected tube current. Although most
x-ray systems have a pre-heat cycle, some, such as many dental and small
portable systems, do not. For these systems, the first few voltage pulses are
high in amplitude due to the low current flow (because the filament is not yet
heated enough to "boil" off the maximum number of electrons). This is
illustrated in figure 11.
Figure 11
As discussed in section III. B., the manufacturer specifies the time
interval based on the voltage pulse whereas the MDH 1015 measures time based
on the radiation pulse. From figure 11, it can be seen that while the
initial voltage pulses are large in amplitude, the radiation pulses are
small. Thus, depending on where the manufacturer specifies the time interval
to begin will greatly affect the agreement between the MDH 1015 reading and
the indicated time setting. For example, if the manufacturer decides to
include the initial pulses in the time interval, the corresponding radiation
pulses are so small that the MDH 1015 will not be triggered, and the
measured time will be shorter than the indicated time.
On the other hand, if the manufacturer ignores the first five or six
pulses before beginning the time interval, the radiation pulses may have
already reached sufficient amplitude such that the MDH 1015 is triggered,
and now the measured time may be greater than indicated. Because of the wide
variation in both the voltage pulse train and the manufacturer's specified
timing base for non-preheat filament systems, it is not always easy to
establish an absolute value of discrepancy between the indicated time versus
the measured time.
- Specific measurements
- The timer accuracy test
The timer accuracy test is a simple straightforward test to compare the
measured time interval to the indicated time interval of the x-ray system.
Any discrepancy is then compared to the accuracy limits specified by the
manufacturer since the Standard does not specify timer accuracy limits but
requires the manufacturer to state them. The test consists of preselecting a
time, taking a series of "exposure duration" measurements with the
MDH 1015 and the 6 cc chamber, choosing the one value (of the series) that
has the maximum deviation from the preselected time, and computing the
percent deviation. This deviation is then compared to the manufacturer's
stated accuracy specifications.
- Phototimed mode
It is necessary when conducting tests of reproducibility on systems with
automatic exposure controls (phototiming) to make sure that the x-ray tube
produces radiation for longer than 1/10 second (100 ms). Since 21 CFR
1020.31(b)(2) requires that all reproducibility measurements with phototimed
systems be made at exposure times of no less than 1/10 second (100 ms).
If when testing a system for reproducibility in the phototimed mode and
the MDH 1015 reads less than 1/10-second, there are certain measures that
can taken to make the system produce radiation for a longer period of time.
The first thing that should be tried is adjustment of the density control
for the phototimer on the main control panel. This density control usually
is identified on the unit by the following possible settings: -3 -2 -1 0 +1
+2 +3. The density referred to is the level of film density or darkness.
Adjusting this control to a higher value (example: -1 to +2), increases the
exposure to the phototimer by increasing the exposure time.
Note that on some systems an increase in the density adjusts some other
technique factors such as tube potential or current without increasing the
exposure time. If this happens, it might help to reduce the kVp or mA. To
get the same radiation to the phototimer, the system may increase the
exposure time to compensate.
Another method of increasing the exposure time without adjusting the
technique factors is increasing the filtration in the x-ray beam. This will
reduce the amount of radiation to the phototimer detector which is similar
to having a larger patient on the x-ray table. Many systems will compensate
for the reduced radiation by increasing the exposure time.
- Fluoroscopic beam quality
It is important when performing the fluoroscopic beam quality test to
reset the MDH 1015 between each measurement. As the aluminum sheets are
removed from the test stand, the shape of the radiation curve is changed
because of the lower energy radiation being included in the measurement. If
the MDH 1015 is not reset between exposures, the threshold setting adds an
error to the exposure time measurements and will start the exposure reading
at different initial exposure rates, depending on each individual radiation
curve. By resetting the MDH 1015, the measurement will always start at 10 mR/second
and the resulting readings can be normalized using the PULSE DURATION
readings for each exposure.
- Reproducibility
For beam quality measurements the radiation pulse shape varies depending
on the amount of aluminum deliberately placed in the beam, thus it is
necessary to reset the MDH 1015 between exposure measurements for the reason
discussed in the preceding paragraph. However, during reproducibility
testing, no factors affecting the radiation between exposures are changed.
Hence, each exposure is affected only by the amount of radiation being
produced by the x-ray system for a selected time interval. Therefore, it is
important not to reset the MDH 1015 between reproducibility measurements so
that accurate measurements of exposure time can be made according to the
measurement basis (bases) specified by the manufacturer. For consistency,
the MDH 1015 must be allowed to trigger at the threshold setting each time
rather than resetting between exposures.
- Physical and mechanical considerations
- Barometric adjustment
Some MDH 1015 monitors have a barometric correction knob on the
instrument to compensate for barometric pressure at various altitudes. Most
of the instruments provided by FDA don't have this control. The reason for
not having this compensation on the instrument is that most investigators do
not have access to accurate barometric pressure readings. It would add an
inconsistent source of error to the radiation measurements if the
investigator was unable to determine the barometric pressure for their
location and dialed an incorrect setting into the instrument. Another reason
is that most of the measurements made with the MDH 1015 on x-ray systems are
relative measurements. Exceptions to this are ENTRANCE EXPOSURE RATE on
fluoroscopic systems, STANDBY RADIATION on capacitor discharge systems, and
TRANSMISSION THROUGH THE IMAGE RECEPTOR SUPPORT DEVICE on mammographic
systems. Thus, if one of the measurements in a test is off due to the
barometric pressure during relative measurements, all of the other
measurements in that test will be off by the same percentage. When a final
comparison is made, the results are not affected.
This is not true for ENTRANCE EXPOSURE RATE. Since there are specific
values of exposure rate in the Standard, it is important that this
measurement be accurate. If there is a drop in barometric pressure from 760
mm Hg to 600 due to altitude, then a radiation exposure rate of 2.00
R/minute will drop to 1.58 R/minute. To get an accurate measurement, it
would be necessary to multiply the reading by 760/600 or 1.27.
The result of this problem is that an x-ray system tested at a high
altitude may appear compliant with the ENTRANCE EXPOSURE RATE requirement
when it's really noncompliant. This has been taken into account in the FDA
action levels for entrance exposure rate and the fact that most field tests
are conducted at or close to sea level. At higher altitudes, the ENTRANCE
EXPOSURE RATE may be slightly underestimated.
When using an MDH 1015 with a barometric pressure correction knob and the
pressure in millimeters of mercury (mm Hg) is known, then adjustment of the
correction knob to the appropriate setting can be made. However, if the
barometric pressure is not known, the unit should be set to atmospheric
pressure at sea level (760 mm Hg). Since most MDH 1015 monitors do not have
a correction knob, there is no need for adjustment. DO NOT attempt to
correct any MDH 1015 measurement values for barometric pressure prior to
entering these values on the test record.
- Ion chambers
- removing 6 cc chamber from test stand
You will notice on the ion chamber probe assembly that there is a lip at
the end of the ion chamber (see figure 12, item B) which can catch the edge
of the holes in the test stand when it is being removed from the test stand.
Special care must be exercised to keep from pulling the ion chamber off of
the probe assembly.
- changing ion chambers
It is important when changing ion chambers to observe the design of the
MDH ion chamber probe and converter box assembly. As has been indicated in
the test procedures manual, the probe should never be twisted or turned, due
to the conducting metal pins on the probe connector. Another precaution that
should be observed is the fact that the probe connector is spring loaded.
Never attempt to pull the ion chamber probe from the converter box assembly
without first grasping the probe around the spring-loaded ring opposite the
ion chamber end (see figure 12). By sliding your fingers along the beveled
portion of the converter box assembly connector (see figure 12, item A)
toward the probe, the probe connector ring will compress toward the ion
chamber thus disengaging the probe without damaging it. When connecting the
probe to the converter box assembly, line up the "black dots on the
probe and converter box assembly and push into place until it
"clicks".
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Figure 12
REFERENCES
- Operating and Instruction Manual, 1015F X-Ray Monitor, MDH 1015
Industries, Inc., 426 West Duarte Road, Monrovia, California 91016.
- Memorandum, Measurement of Timer Accuracy for Diagnostic X-Ray Systems,
U.S. Army Environmental Hygiene Agency, Aberdeen Proving Ground, Maryland
21010, (1978)
- Error Analysis on Timer Measurements, G.G. Martin, Office of Compliance,
Center for Devices and Radiological Health, unpublished.
- Personnel Communication, Michael Divine to Thomas Lee, Office of Science
and Technology, Center for Devices and Radiological Health, (1984)
Updated December 1, 2000 |