Note: Descriptions are shown in the official language in which they were submitted.
PHN. 3803.
The invention relates to a device for comput r
tomography, comprising an X-ray source for generatin~ a
flat, fan-shaped beam of X-rays, and an X-ray detector
comprising a series of detection elements which are
arranged on an arc of a circle, said X-ray source and
X-ray detector being rotatable about a common axis of
rotation which is directed transversely of the plane of
the fan-shaped beam.
; A device of this kind is particularly suitable
for X-ray diagnosis. During such an examination, a part
of the body of a patient is irradiated by the flat, fan-
shaped beam from different directions. Locally trans-
mitted radiation is measured and, using the measuring
data thus obtained, a computer calculates the density
distribution of the part of the body of the patient in
the irradiated plane, the result being displayed, for
example, on a television monitor.
A device of the described kind is known from
Netherlands Patent Application No. 7,503,520 by EMI
; 20 Limited which was filed on March 24, 1975 and laid open
to public inspection on September 25, 1975. This spec-
ification describes a device in which use is made of a
flat, fan-shaped beam of X-rays which completely encloses
the part of the body to be examined in at least one
direction. In order to obtain an adequate number of
measuring data, the X-
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` P~ 8803
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ray source and the X-ray detector rotate together about
the patient which is arranged in the vicinity of a common
axis of rotation, The device comprises means to compensate
for the effect exerted on the measuring signals by dif-
ferences in the sensitivity of the various detection
elements.
The X-ray ~ource and the X-ray detector of the
kno~n device rotate together around the axis of rotation
at a uniform speed during examination. The output signals
of the detection elements are integrated over a short
period of time in order to obtain reliable measurements,
the X-ray source and the X-ray detector rotating through
a small angle of, for example 1 during said period of
time. Subsequently, the logarithms of the intensity
measurements are determined. During examination, each
detection element thus supplies a set of logarithms of
the intensities of X radiation transmitted by the patient
in different directions. During the examination, these
data are stored in a first electronic memory and, after
completion of exannination, they are sorted according to
sets of logarithms, originating from different detection
elements, of the intensities of X-radiation transrnitted
in parallel directions. The sorted sets are stored in a
second electronic memory. Subsequently, the density of
each picture point in the irradia-ted plane of the part
of the body examined is calculated by means of a recon-
struction techni~ue, which means that the data of the
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PHN 8803
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sorted sets which cover the relevant picture point are
summed.
In the known device, the occurrence of disturb~
ing ring-like interference patterns in the display of the
calculated density distribution, referred to as ring
artefacts, is suppressed by compensation of the dif-
ferences in sensitivity of the detection elements. To
~ this end, a fast movement of the X-ray source is sepa-
; rately superposed on the uniform, common rotary movement
of X-ray source and X-ray detector, so that each measure-
ment by each detection element is repeated by a neigh-
bouring detection element during examin~tion. On the basis
of the measurements thus obtained, the detection elements
are compared with each other and differences in sensiti-
vity are compensated Ior; for this purpose, an addi-
tional electrollic processing network is included.
The present invention has for its object to
provide a device for computer to-nography in which on ths
; one hand differences in the sensitivity of detection
elements do not become manifest in the occurrence of
ring artefacts, whereas on the other hand each detec-
tion element provides, d-uring the examination, a set
; of measurements of the intensity of the X-radiation
transmitted in parallel directions, so that the sort- ¦
ing of the measurements af`ter termination of the exami~
nation can be dispensed wi-th. To this end, the de-~ice
for computer tomography in accordance with the inven-
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PI~N 8803
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tion is characterized in that the X~ray source has a com-
paratively small emissive surface area, the detection
elements having a comparatively small radiation entrance
surface area, the distan.ce betl~een the radiation entrance
surf`ace of the detection elements and the axis of rota-
tion being substantially equal to the distance bet~een the
emissive surface of the X-ray source and the axis of ro-
tation, the X-ray source and the X-ray detector being
. rotatable about the axis of rotation with a substantial-
.: 10 ly equal angular velocity, but in an opposite sense.
During operation, the output signals of th.e detection
elements are integrated over a shor-t period O r time in
order to achieve adecluate measuring accuracy, the X ray
source and the X-ray detector being rotated through. a
small angle of, for example, 1 during said period o~
time. Because the X-ray source and the X-ray detec-tor
move in opposite directions, the part of the bocly to be
examined is scanned by the X-ray source and a detection
elemellt in mutually substantially parallel, consecutive
20 . paths. This means that each calculation of the density
in each picture point of the irradiated plane includes
- a measurement by each detection element. ~s a result,
interference patterns in the display of the calculated
density distribution, caused by differences in the sen~
sitivity of the detection elements, cannot occur. The
said ring artefacts are thus avoided.
~. preferred enlbodiment of the device for com-
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PHN 8803
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puter tomography in accordance with the inventio-n is
characterized in that the X-ray detector consists of a
closed circle array of detection elements. As a result,
sets of measurements of the intensity of the X-radiation
transmitted in parallel directions can be determined for
all directions in the plane of examination~
One embodiment in accordance with the in~rention
will be described in detail hereinafter, by way of exam-
ple, with reference to the accompanying diagrammatic
drawing.
Fig. 1 is a diagrammatic longitudinal sectional
; view of a device for computer tomography in accordance
with the invention;
; Fig. 2 is a diagrammatic cross-sect:Lonal view
of the device shown in Fig. 1, taken along the line
II - II;
i~ Fig. 3 shows a circuit diagram o~ a device for
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comp~lter tomography in accordance with the invention;
Fig. 4 diagrammatically S~lOWS the position of
the X-ray source and a detection element with respect
to each other at successive instants;
Fig. 5 diagrammatically shows an Y-ray detec-
tor which is particularly suitable for use as a detector
element, and
Fig. 6 illustra-tes Ihe scanning possibilities
offerad ~y the device shos~ll in the Figs. 1 and 2.
Fi gs, 1 and 2 are a di.agramrnE~ t L c l ongi ta di nal
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PHN 8803
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sectional view and a diagrammatic cross-sectional view,
respectively, of the same device for computer tomography
in which a patient 1, resting on patient table 2, is ir-
radiated by a flat, fan-shaped beam of X-rays 3. The beaM
' 5 of X-rays 3 has an angle of aperture, referred to herein-
- after as fan angle ~ , of, for example, 30 in the plane
of the drawing of` Fig. 2, and is comparatively flat in
" the direction perpendicularly thereto, its thickness be-
' ing approximately 10 mm. The fan angle is so large that
the beam 3 encloses the entire patient 1 in the fan direc-
tion. The X-ray beam is generated by an X~ray tube 4,
comprising a rotary anode (not shown)~, Because the emis-
sive surface of th,e rotary anode (the actual X-ray source)
is comparatively small, its length and its width being
approximately 2 mm, the X-ray source may be considered
to be point-shaped ~rom a radiation-technical point of
view. The radiation transmitted by the patient is measur-
ed by an X-ray detector 5 which comprises a series of,
'~ for example, 400 detection elements 6 which are arrang-
ed on a circle. The detection elements 6, to be describ-
ed hereina~ter and to be considered to be point-shaped
-- f`rom a radi,ation-technical point of view consiclering the
comparatively smal.l dimensiorls, comprise, for example,
a scintil,]ation crystal and a light detector. The X-ray
tube 4 is mounted on a ring 7 which is journalled on
wheels 10 which are secured to the housing 9 alld which
is rotatable, by means o~ a drive motor 12, abcut an axis
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14 whieh extends transversely of the plane of the fan-
shaped beam 3, The aetual X-ray source - the emissive
surface of the rotary anode - rotates in a circular path
15. The X-ray deteetor 5 is connected to a ring 8 which
is journalled on wheels 11 connected to the housing 9
and which is rotatable about the axis 14 by means of a
drive motor 13. The ring 7 and the ring 8 rotate in op-
posite directions during an examination. The detection
elements 6 rotate in a oircular path~ the radius of
which is substantially equal to the radius of the cir-
eular path 15 in which the actual X-ray source xotates;
the radii are, for example, 90 cm and 85 cm, respective-
ly. The processing of the measuring data will be describ-
ed in detail herinaftel with reference to F:ig. 3.
~; l5 Fig. 3 diagrammatically shows a device of the
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described kind, with a patient 20, an X-ray source 21
and an X-ray detector 22 wbich comprises a series of cle-
tection elements ~3 which are arranged on a circle The
X-ray source 21 and the detection elements 23 rotate in
opposite directions during an examination, in the same
circular path 24 in a first approximation. ~ll detec-
tion e]ements are connected, even thougll this is shown
for only three elements, to an integration circuit 2~
in which the measuring signals of the detection elements
23 are integrated over a short period of tine of, for
example, 10 ms in order to ach:ieve adecluate measurillg
accuracy. Because the X-ray source 21 and the X-ray de-
P~IN 8803
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tector 22 move in opposite direetions, the part of the
body 20 to be examined is scar~led in mutually suhstan-
tially parallel, consecutive paths as shown in Fig. 4~
: in which the posi.tion Or the X-ray source 21 and one de-
tection element 23 is indi.cated with respect to each
other a1 consecutive instants ta, tb, t , ... th. The
X-ray source 21 and the X-ray detector 23 in first ap-
proximation follow the same circular path 24 during the
examination, so that the distance between the X~ray source
and a detection element ch.anges during the examin.ation,
with the result that the intensities measured by the de-
tection elements change during the examination, indepen-
dent of the local. absorption of the patient. These changes
are eorrected in the circuits 26. ~fter correc-tion., the
logari.thms of` the measuri.ng signals of the detection ele-
ments are determi.ned in logarithrnic ampl.ifie:rs 27 and the
signals thus treated are stored in a memory 28. Thus, dur-
ing examination each detecti.on el.ement 23 produces a set
of logarithms of the intensities of X-radiation trans--
mitted i.n substantia].ly parallel directions th.rougll the
pati.ent. ~fte:r completion. Or -the examination, th.e com-
puter 29 calculates, by way of a reconstruct:ion techn:ique,
the c1ensity distributio:n of -the irradiatecl part of the
body 20; th.is :is disp1.a~ed~ for exarnple, o n a te]e~-isi.on
monitor 30.
Fi.g~ 5 diagramlllatical]y S110WS a:n X-~ay deiec-
; tor 31 whi.ch is particular]y sui-tabLe for use as a de-
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. 30.1.7~
tection element in the described device for computer
tomography. The detector 31 comprises a cylindrical scin-
tillator 32, having a diameter of, for example, 5 mm and
a length of, for example, 20 mm, and a light detector 33
which is axially coupled thereto. The two bea.ms of` ~-rays
34 and 3~, diagrammatically shown and inciden-t on the de
tector from dif`ferent directions, are detected in the
same manner due to the cylinder-s-ymmetry thereof. Thi.s
property renders the detector particularly su:itab.~e for
said application, because the direction of the X-rad:i.a-
tion incident on a d.etecti.on element continuously changes
during the examinatic)n. This change i.s independent of the
.. local absorption of thc patient and, therefore, i.t should
not influence the measuri.n.g signal
1~ Subsequently, the scannlng possibiliti.es of
the described devi.ce will be elaborated wi.th reference
to Fig. 6. Therein, the circle ~l1 denotes the path to be
followed by a poi.nt-shaped X~ray source 42 and an X-ray
detector 43 comprising a series of detection elements
4-4~ The position of the X-ray source /12 is shown at three
instants, i.e~ a.t the beginning of the examinati.on t = 0,
at the end OL the examilla1ion t = T, and at an :instant
i therebetween. The posi-ti.on of the detector 43 is shown
; at -t = 0. During the exami.Ilt.ltion~ the source moves a-t an
2~ uniform angular ~eloci.ty ~, so the tOt~l]. anglllar dis-
p].acement of the source and the detector is ~'.T. The
fan angle GC of the X-ray beam is determined by a di.a-
~1~740~7 3 0 . 1 . 7 8
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grammati.cally shown collimator 45 which moves wi.th the
source so that the centre of the beam of X-rays is always
directed OlltO t'he centre M of the circle ~l1. During the
exarrlination, the detection elements produce sets o~
measurements o~ the intensity of X-radiation transmitted
by the patient in mutually parallel directions, the direc-
tiOIl bei.ng s~ uated be-tween the diagrammatically shown.
X-rays 46 a.d 47. These two rays enclose the examination
angle 0. In the tri.angle determined by the source posi-
tions a-t the instants t = 0 and t = T and the intersec-
tion o~ the rays 46 and 1l7, it carl be simply seen that
~/ T = C~ + ~. Because radi.at:ion is rneasured only in
'; Inutually parallel dlrecti.ons s:i.tuated between the ex-
trelne rays 46 and 47, thc X-ray bea1n is shielded, ex-
cept f`or the ray 46 at the instant t = 0, and except for
the ray 47 at -the instant t. = T, by the aperture 48 whicl
moves wi.th the source. ~s a resu.1.t, the radiat:ioIl dose
whereto the patient is exposed i.s substantia].ly reduc--
ed. The ape:rture performs only a trarlsl.atory rnovement~
~rom left to right in ]?igure 6, so that the aperture
angle ~ is equal to ~, as can be readily seen. Durillg
th.e examination, the detcctor 43 follows -the ci.rcular
path /11 in the indicated di.recti.on at an ang~lar vel.o
c:ity ~ . OuriIlg the time T, the de-tection elelrlent wllich
measures the ray 47 a-t the :i.nstant t = T completes the
angl.e ~,~ T In order to enable t}~e exa.min.ation, a de--
tector is reqllired who.se detectio:rl angle ~ equals
PITN 8803
30.1.78
~J ,T + a, for which ~ = 360 - ~. T - 2(1~0- C~
~ = W .T - 2 ~ (as can be seen i.n Figo 6), so that
- ~ = 2 ( ~.T ~ ~ ) = 2 ~. .
The foregoing illustrates that, in order to
perform an examination througll ~ = 180 at a fan angle
= 60 , the X-ray source and the detector must be rotat-
', ed through an angle ~.T = 240 in opposite directions, a
detector having a detection angle ~ = 360 -th.el~ being re-
quired. A moving aperture 38 then preferably has an aper-
ture angle ~ = 180 . It is to be noted that, i.f the cle-
tection angle ~ = 3~0, the examination an6rle i.s i,ncreas-
ed to ~ = 360 by using each detection elelrlent l,L3 twice,
~) .T then being 420.
' For.~ exami.nation angles ~ of 180 and more, the
X-ray tube 4 and the X-ray detector 5 of Figs. 1 and 2
cannot move i.n the same circular path in practice, but
they can move in circular paths whosc radi.i differ by a
few centimeters. During an examination, each detect:ion
element then supplies - at the same angular velocity of
source and detec-tor - a set o* measurements,oI` the in-
tensity oI` the radiation -transmitted by the patient in
a number of directions w]-lich. are not exactly m-u-tuall.y
paral.l.el. This non-para]leli,ty is so smal:L for a I`an
angle of approxilnately 30 and a length of the radii
of the circulax pat:lls of approximately 100 cm, -tl)at the
effect thereof on the calculation of tlle dells:ities in
the irradi.ated plane is negligibl.y srnall.. Correctioll,
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PI-IN 8803
30.1~78
if desired, is possible by rnaking the X-ray source, mov-
in the smallest circular path, rotate slightly faster
than the X~ray detector.
For a fan angle ~ = 30, on]y an angle of 60
; 5 of the detection elernents 44 of Fig. 6 is being used at
any instant, so that the examinatio~ can be accelerated
by using, ~or example, three or five X-ray sources which
are arranged on the circle 41 accordillg to a regular
triangular or pentagon.
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