Fwd: Anomalous HVL Measure Update

[Ccja@aol.com]

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Forwarded message:
Subj:    Re: Anomalous HVL Measure Update
Date:    97-02-24 20:17:23 EST
From:    Ccja
To:      MEDPHYS@lists.wayne.edu

Dear Medphys Folks
Thank you for responding last week to the half value layer question between
3phase and High Frequency. I would like to provide more information on how
this question came to be.  It actually started with acceptance testing on a
newly installed piece of equipment where the measured value seemed a little
high at 80kvp, measuring 3.77mm.  The kVp was measured and found to be
actually around 78.
Three separate instruments were used and were in agreement with our standard
MDH model 1515 measuring between 3.6 and 3.8.  Because the filtration in the
collimator is fixed at 2.0 mm at 100 KVP we concluded that the added
filtration must be coming from the xray tube itself.  We removed the
collimator from the measurement and found indeed that the tube was measuring
around 1.7mm at 80kvp.  Most xray tubes have a sticker on the side that
states the inherent HVL to be around .6 mm minimum, but we were measuring
almost triple that amount at 80KVP.  This led to the question of an upper
limit tolerance for acceptance testing.
When we contacted the tube manufacturer their first response indicated that a
typical number should be .7 to 1.0  and he made the comment that a tube
probably shouldn't leave the plant over 1.0.  Our local service provider for
this equipment measured several similar units in the area and found all of
them to be in agreement.  We replaced the original tube in this room with a
new one and came up with the same numbers.  We decided to back off on asking
for a third xray tube because we thought that the numbers would be the same.
 The person who originally had indicated typical values of .7 to 1.0 inherent
filtration seemed not to be to sure about his numbers the next day.
So I quess the real question is that since collimators are made without
interchangeable filtration in todays world, should there be a tolerance
window that should be specified in the bid specs for rejectability of
inherent tube filtration?  The 3phase to High Frequency question came up when
we were measuring tube inherent filtration around 1.8 at 80kvp with HF
compared to the Tube Manufacturers measurement of .7 to 1.0 with a 3phase
generator.  I would be interested in any response you might have.  Thanks
again.

Lonny Horton
ccja@aol.com