[Oradlist] AC vs DC

Fri Oct 16 06:34:52 PDT 2009

On a occiliscope, when looking at image wave, a dc machine goes up to and stays at 70kv or whatever the machines kv is.  On an AC machine, the image wave goes from negative 35 kv to positive 35 kv creating a ripple.  At exposure times under .1 sec on  a digital sensor, on an AC machine, current is either on the way up or on the way down, therefore creating an inconsistent image, sometime to much, sometimes too low,,,,underexposed or overexposed,,,,sometimes called blooming,,,the sensor is over saturated depending where on the wave that the exposure was taken.

Steve Desch

-----Original Message-----
From: Lennart Flygare <Lennart.Flygare at nll.se>
To: 'Oral Radiology Discussion Group' <oradlist at lists.ucla.edu>
Sent: Fri, Oct 16, 2009 1:31 am
Subject: Re: [Oradlist] AC vs DC

n exposure times under 0.1s the exposure rate in any AC machine may vary up 
ill 40% pending the frequency of the net-current (usually 50-60Hz) whereas in a 
C-unit the same variation in exposure rate doesn't exceed 10%. An AC-machine is 
lso more sensitive to changes in the net-voltage whereas a DC-generator better 
ompensate for such fluctuations.
In theory a DC-machine is thus much more suitable for the low exposure times 
mployed in digital dental radiology. In practice it may be that the difference 
s insignificant as Allan points out especially if you live in an area where the 
et-voltage is stable.
The X-ray spectrum produces by a DC-machine is different from an AC machine. 
hen the kV is set on the machine it is defined as the top of the spectrum. As 
he X-ray spectrum in a DC-machine has a shift toward higher energies than in an 
C-machine the kV-setting should be approximately 5kV lower to achieve similar 
ontrast and dose as in an AC-machine.
If I where to buy new equipment I think there is only one alternative i.e. DC.
f I already had well working AC equipment I wouldn't exchange it unless forced 
o.
Best wishes

------------------------------------
ennart Flygare, Odont Dr
enior Consultant
entomaxillofacial Radiology
ept of Radiology
underby Hospital
E-971 80 Luleå
WEDEN

el+46-920-282931
ob: +46-70-6743858
ax:+46-920-282942
----Ursprungligt meddelande-----
rån: oradlist-bounces at lists.ucla.edu [mailto:oradlist-bounces at lists.ucla.edu] 
ör Allan G Farman
kickat: den 16 oktober 2009 00:22
ill: Oral Radiology Discussion Group
mne: Re: [Oradlist] AC vs DC
In my tests with digital detectors I have found no disadvantages for AC
ver DC. Both X-ray generator types generally provide excellent provided
hat the system is not "worn out."  An inconsistent machine is
nconsistent whether AC or DC. AGF
Allan G. Farman, BDS, PhD, MBA, DSc, Diplomate ABOMR
Prof. Radiology & Imaging Science
niv. Louisville School of Dentistry: SUHD
01 South Preston Street,
ouisville, Kentucky 40292, USA 
Tel: +1(502) 852.1241
ax: +1(502)852.1626

>> Andrés Briner <andresbriner at gmail.com> 10/15/2009 05:04 PM >>>
ear Axel,
1.-  DC equipments produces more homogeneous X-ray photons in terms of
ts
nergy, since the potential difference is constant and high.
2.-  AC equipments deliver pulses of x-rays with periods of silence in
etween them, so the total exposure time is at least the double, giving
ore
ossibility to have blurring due to sensor motion.
3.-  AC pulses are not compatible with the short exposure times needed
n
igital x-rays, so it is very easy to have under or over exposures.
4.-  Most of the AC equipment are old and they have a bigger focal spot
ue
ts use, and the higher temperatures they produce in the anode.
Kind Regards
Dr. Andres Briner
max Radiological Centers
antiago
hile
On Thu, Oct 15, 2009 at 4:41 PM, Ruprecht, Axel
axel-ruprecht at uiowa.edu>wrote:
>  Hello

 I have had a question put to me by a former student about whether a
C unit
 used with digital imaging produces better images than an AC unit. Do
ny of
 you have any hands on experience in this regard, that you would be
illing
 to offer to me?

 Rgds

 Axel

 [image: Signature no AR]

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