Re: Fwd: Anomalous HVL Measure Update

[tdc@ehssun.lbl.gov]

>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.

Actually - you seem to be confusing inherent aluminum equivalent
filtration and half-value layer.  They are NOT the same!!  It is NOT even
a linear relationship.

Using your numbers and some plots I made of measurements made by Dale
Trout and published in Radiology MANY MANY years ago, and some numbers I
got relating HVL/Al equivalent from 4 pulse (single phase) to 3 phase 6
pulse or 12 pulse (using the latter in this case _AND_ assuming you did a
proper narrow beam HVL with very tight collimation, the detector well up
off the table AND the filters 1/2 way between the anode and detector _AND_
assuming your filters were pure Al:

I calculate:

3.77 mmAL hvl at 80 kVp 12 pulse 3 phase implies 4.8 mm Al equivalent.

1.7 mmAL hvl at 80 kVp 12 pulse 3 phase implies 1.3 mm Al equivalent.

Personally I think an inherent of .8 for a tube sounds a bit low and 1.3
sounds more like what I am familiar with.  Anyhow - if several new tubes
all read the same - they must be OK.  After all - the only thing to harden
the beam would be the glass envelope!

Bottom line is that HVL can be used to make relative judgements of
inherent filtration BUT it is not terribly accurate for measuring same.
The lower the filtration - the less accurate the method.

You CAN verify the Al equivalent of the collimator or anything external to
the tube - but the only means I can think of to actually measure the
equivalent of the glass envelope of the tube would be destructive!

Ted de Castro
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
Bldg 90 Rm 0026B
Berkeley, CA 94720
(510) 486-5256
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