REMOTE FOCUS X-RAY TUBE

TUBE DE RADIOLOGIE A MISE EN FOYER PAR TELECOMMANDE

English Abstract

PHD 77-028
8-3-1978

ABSTRACT:

Remote-focus X-ray tubes, in particular dental
X-ray tubes, have the problem that the focus is shifted as
a result of which the emission distribution changes. It
has now been found that a cause of said shifting is the
earth magnetic field and, sometimes, magnetic fields pro-
duced by electromagnetic elements of the apparatus. In
order to eliminate these magnetic fields the anode tube is
surrounded by a soft-magnetic layer which is dimensioned
so that it serves at the same time as prefilter for the
useful radiation emitted by the target.

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Claims

PHD 77-028


THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A remote-focus X ray tube, electrons emitted
by the cathode following a comparatively long path to a
conical target, characterized in a tube of a soft-magnetic
material surrounding the electron path serves as prefilter
for useful radiation emitted by the target.
2. A remote-focus x-ray tube as claimed in claim
1 characterized in that an anode tube in which interior the
target is positioned has an outer diameter which is smaller
than the inner diameter of the tube of soft magnetic mater-
ial surrounding the anode tube.

Description

PHD 77--028

The invention relates to a remote-focus X~ray
tube, electrons emitted by the cathode following a compara-
tively long path to a preferably conical target. Such x-ray
tubes are employed in particular for intracavitary diagnos-

tic and therapeutic purposes. The anode and the X-ray emit-
ting target respectively are then generally located at an
end of a comparatively long tube, which can be inserted into
the body cavity.
Such X-ray tubes are ~or example employed in
dentistry for recording the overall teeth condition, the
anode tube containing the target being inserted into the
patient's mouth. The target which is disposed at the end of
said anode tube has the shape of a cone, on whose ape~ the
electron beam is centred by means of suitable focussing
coils. These dental X-ray tubes are known (German Offen-
legungsschrift 15 64 738 which issued to Siemens AG with an
Of~enlegungstag date of January 2, 1970).
If in such X-ray tubes the point of the -target
is disposed only a fraction off the centre of the (round
100 mm long) electron path, a non uniform radiation distri-
bu~ion results, which may for example lead to the radiation
intensity at the location of the right half of the jaw being
substantially greater than at the location of the left half
of the jaw, which may give rise to an incorrect exposure of
at least one half of a film which records the teekh condit-
ion, which film is placed on the skin o~ the patient's face.
In order -to prevent the focus from being sh.ifted
out of a position which is syrnmetrical relative to the cone




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8~ PHD 77~028

apex, use has been made of flexible retaining members bet-
ween the housing of the X-radiator and the X-ray tube which
can take up a shock to which the anode kube is subjected,
without said tube being bent, but even in such X-ray tubes
the focus may still be shifted.
It is therefore the object of the present inven-
tion to construct a remote-focus X-ray tube of the type men-
tioned in the preamble in such a way that said shifting of
the focus can be prevented or at least mitigated.
In accordance with the invention this object is
achieved by a layer of a soft-magnetic material which sur
rounds the electron path, which at the same time serves as
prefilter for the useful radiation emitted by the target.
The invention is based on the recognition that
external magnetic fields, in particular the earth magnetic
field, may give rise to shifting of the focus or a substant-
ial departure from the desired symmetrical intensity distri-

bution. The influence of these magnetic fields on the elec- ,
tron path is substantially eliminated by the soft-magnetic
layer which surrounds the electron path. Owing to the pre-
liminary filtration of the useful radiation by said layer,
which is achieved by a suitable dimensioning of the thick-
ness of said layer, for which allowance is to be made for
the filtration of the effective radiation by further compon-

ents, as for example the anode tube of the X-ray tube, an
additional prefilter may be dispensed w.ith. This i8 a
special advantage ~or dental X-ray tubes, because the part

of the X-ray tuhe to be inserted into the patient's mouth
should have a minimal diameter.




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PHD 77~028



In accordance with a further embodiment of the
inven~ion which may be used in remote focus X-ray tubes with
an anode tube, the soft-magnetic layer has the shape of a
tube, whose inner diameter is greater than the outer dia-

meter of the tube portion constitu-ting the anode tube. If
desired, this tube may be slid over the anode tube. It may
be arranged in an applicator, which during an X-ray exposure
is slid over the actual anode tube and which serves to
shield the areas not to be exposed against X-radiation.
The invention will be described in more detail
with reference to the drawing.
The drawing shows a dental X-ray tube, which i5
located in the interior of a protective housing 3 and of
which only the target 1 and the anode tube 2, through which
the electron beam passes, are shown, because further this
X-ray tube may fully correspond to the known X-ray tubes in
accordance with German Offenlegungsschrift 15 64 738.
The useful radiation beam emitted by the conical
target substantially has an aperture designated ~ and is
rotation-symmetrical relative to the longitudinal axis of
the anode tube 2. In the area through which the useful rad-
iation passes it may consist in known manner of beryllium
tGerman Offenlegungsschrift 15 64 738), but it may also be a
thin ceramic tube.
In accordance with the invention a tube 4 is
slid over the anode tube 2, which tube ~ extends from the
end face of the tube shield in the direction of the target

at least so far that the useful radiation always passes
through this tube. The tube consists of an extremely



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so~t-magnet:ic rnateri.lL o:f hi.gh perrlleabi:L.ity and wlthout
remanence. A part:icllla:rly sui-tab:le tnc~ter:ia:l ls :t`ound -to l~e
the shielding materia:L which is available in sheet f-orm
~rom the Magne-tic Shield Division, Per~ection Mica Co.; USA,
under the trade name ~CO-N~TIC". However, it is alternat:ively
possible -to employ a mu-metal tube. T:he thickness o:f this
-tube should be between 0.05 mm and 0.1 mm, so t:ha-t in additi~
to magnetic shield:ing a satis~actory prelimincLry f`iL-tra-tion
is obtained.
Although the inven-tion has been described in
connection w:i-th a den-tal X-ray -tube, it may also be employed
in other remote-focus X-ray tubes with a long electron path,
pre~erably in such tubes in which the target is situated
at the end of a hollow anode -tube; the target may also have
a shape other than that of a cone~