Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"~lethocl o~ detcrnining ~-oltage and current valuec: associ~ed
~ith a given s~itch-on duration of an X-ray source, and circui-t
arrangemcnt for performing said method".
The invelltion relates to a method of determinin~
voltage and current values associated -ith a given switch-on
d~lration of an X-ray source having a given operating range
limitation nomogram and to be applied to said X-ray source in
which a desired culrent time product is determined with the
aid of an X-ray exposure meter for a desired radiation qu~nti-
ty.
Such a method is usually based on experience
and is performed by those skilled in the art of operating
X-ray equipment.
The invention has for its object to simplify
this method so that also those less experienced and skilled
in the art of operating X-ray equipment can perform this
method with a very favourable result.
To this end a method of the kind described in
the preamble according to the inv ntion is characterized in
that the current time product is determined at a given voltage
applied to the radiation source while with the aid of a
voltage-cur~ent function applying to the desired radiation
quantity, which function is pussed through the current inten~
sity associated with the desired current time product, and
with tho aid of the operating range limitation nomogram
with the same current scale as the voltage-currellt function
the point of intersection of the voltage-current function
2~ and the operating range limitation nomG~ram with the lowest
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voltage ~ralue is determined and that said lowest voltage
value is taken as a target value for the determination of
the current with the aid of the load nomogram.
Furthermore the invention rclates to a circuit
arrangement for performing such a method and is character-
ized in that~he X-ray e~posure meter includes a circuit for
determining the logarithm of the desired current value
associated with the applied voltage and a chosen period and
for supplying a voltage proportional to this logarithm to
10 its output which is coupled to a first input of an adder
and subtractor circuit a second input of which i5 coupled
to an nutput o~ a clamping circuit an input of which is
coupled to an output of a voltage-current function genera-
tor which supplies an output voltage proportional to the
15 logarithm o~ a current value associated with an input voltage,
while a third input of the adder and subtractor circuit is
` coupled to an output of an operating range limitation nomogram
function generator supplying ~n output voltage proportional
to the logarithm of a current value associated with an input
` 20 voltage~ W~ILE AN INPUT OF said ~unction generators is coupled
to an output of the adder and subtractor circuit, said output
¦ being furthermore coupled to a control signal input of a
~` voltage control circuit of the radiation source.
The invention will be described in greater
t 25 detail with reference to the drawing.
~ Fig. 1 shows a method according to the invention
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!~ by way of a voltage-current characteristic,
J , Fig. 2 illustrates by way of a block-schematic
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diagraDI a circuit arrangement for performing a method
according ~o the invention.
Fig. 3 illustrat0s by way of a block-schematic
diagralll an X-ray exposure mcter for a circuit arrangement
according to Fig. 2 and
Fig. 4 sho~Ys by wa~ of a block-schematic dia-
graltl a clamping circuit and an adder and subtractor circuit
~or a circuit arran~ement accordin~ to Fig. 2.
Fig. 1 shows a voltage-current range for voltage
and currents of an X-ray source with a logarithmic current
scale denoted b~ log I on th0 vertical axis and a voltage
scale which, if desired, may also be logarith~ic, but is de-
noted by V on the horizontal axis.
Furthermore the reference numeral 1 denotes
an operating range limitation nomogram of an X-ray source
; and 2 and 3 denote some curves corresponding to some radiat-
ion quantities suitable for photo or film for observation
purpose obtained by irradiation of an object.
The operating range limitation nomogram 1 of
the X-ray source under pick-up conditions gives the relation-
,; ship for a given switch-on duration of the radiation source
of the maximum suitable current value for a chosen voltage
for energizing this source.
~ ~ The method is as follows. At a given voltage
s 25 Vte9t applied to an X-ray source an exposure measurement
at tho area of an observation device to be expos0d is per-
~ormed with a giv0n switch-on duration of the radiation
source. With this measurement a desired current value I
W1
a,5,sociated with thi8 voltage Vt t is found which will
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yiel~ a certain density for photo or fil~. In case of
films and photos the switch on duration ~or an exposure
mea~urement is equal to that for pic~-up while ~ith the
use of a film exchanger a certaill ratio bet~Yeen these
switcll-on durations can be taken. Through the point in
rlg~ 1 corresponding to the desired current value I and
tho given voltage value Vtest the curve 2 is provided
which connects points with voltage and current values which
would causq tlle same density. l~en the point of intersection
with the lowest voltage value of one of the two points of
intersec~tion of this curve 2 with the nomogram 1 is de-
termined by V i and a current value Il, this is the most
suitable adjustment for the radiation source because in
case of a maximum con-,trol of the source according to a nomogram
~alue `,the ~ radiation is obtained at the lowest possible
voltage which yields the optinum contrast for photo or film.''
In case of a large deviation between Vt t
and V min1 it may occur that the-desity at the values
found for V i and I1 will exhibit a slight deviation
relative to the desired density. A subsequent exposure
measurement is then performed at the found voltage value
Vmin yielding a desired current value Iw which may
slightly deviate from the current value I1 found. The curve
~ 3 for a density associated with these new voltage and cur-
A 25 Vmin1, Iw is provided throu~h the point with
the last determined values and the point of intersection Vmin
with the nomogram yields an associated value I2 for the
current which will the~ produce a more accurate approximation
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of thc correct value. Although generally one or two e~posure
measure~lents are sufficient this procedure may bc repeated,
if dosired t ~ultil tl~e values fouIld ~or I and I sub-
~n~1 n
stantially coincide wllich will generally be the case after
three e~posure measurements.
By using a logari*hmic currcnt scale the curve
~ may be obtained by shifting the curve 2 in the vertical
direction so that, for example, ~ith the aid of a jig moved
along a ruler this curve can be drawn in a simple manner
through an arbitrary point in the graph so that the voltage
and current values to be adjùsted can be determined very
quickly and simply even by persons that are not very skilled
in the art or have little experience in the use of X-ray
equipment~
In ~ig. 2 an X-ray source 5 is connected to a
voltage control circuit 7 and a current control circuit 9
each-having control signal inputs 11 and 13, respectively.
The control signal input 11 is connected to an output 15
of a control loop 17 and the control signal input 13 is
connected to an output 19 of a change-over switch 23 which can
be operated by means of an operating device 21 and has a first
input 25 connected to an output 27 of the control loop 17
and a second input 29 applying a signal obtained from an
output 31 of an operating range limitation function generator
32 to the control signal input 13. Consequently, when per-
for~ning exposure measurements, in which the change-over switch
23 assumes the posit-ion, shown the current through the
- ra~iation source 5 is maintained at a given value I with
test
thc aid of thc curr~nt control oircult 9. ~liS circuit 9 is
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active in the f;lament circuit of the radiation source 5 cannot
sufficiently quickly be controlled to a deviating value so as
to make a quick succession of experimental exposures at
different current values possible. The current control circuit
9 may be formed as described in our Canadian Patent Application
211,234 which wàs filed in October 1974.
The voltage originating from the output 3 of the
operating range limitation function generator 32 is also
applied to an input 33 of an X-ray exposure meter 35 a further
input 37 of which is connected to an output 39 of a radiation-
sensitive element 41. The exposure meter 35 has an input 43
which is connected to an output 45 of a pulse signal source 47.
A further output 46 of the pulse signal source 47 is connected
to an input 49 of the voltage control circuit 7. The pulse
signal source 47 thus ensures that a given switch-on duration
of the radiation source 5, which duration is adjustable with the
aid of the operating device 21, can be obtained while also the
exposure meter 35 is activated at the correct instant. The
exposure meter 35 may receive information from the operating
device, for example, about a film sensitivity to be used.
After performing an exposure measurement at a
voltage Vtest the exposure meter 35 applies a signal to an
output 51 which signal is proportional to the logarithm of
the current value Iw desired for a given density and
associated with the voltage Vtest or Vmjn. T g
hereinafter be referred to as log Iw ~
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'Nle signal log I~, is applied toa first
input 53 of an adder and subtractor circuit 55. A second
input 57 of this adder and subtractor circuit 55 is con-
nected to an output 59 of a clamping ci.rcuit 61 which
: 5 recci~-e.s a pulse sig~al at an input 63 at the instant Or
tlle exposure measuretnent so that its output voltage is
clamped at zero.
The clamping circuit 61 is controlled at an
input 65 b~ a signal originating from an output 67 of a
current voltage ~unction generator 69 receiv*ng at an input
71 a voltage from the output 15 of the control loop 17 and
thus being a measure of the voltage applied to the radiation
source 5. A voltage proportional to the logarithm of a
current value for a given density associated with a given
voltage at ~he radiation source 5 is produced at its output
67. This signal is hereinafter referred to as log I .
This signal appears at the output 59 of the
~ clamping circuit 61 and is shifted due to the clamping cir-
cuit 61 in such a manner that the generated function passes
through the point which a~ter the nth exposure measurement
is associated with a desired current value Iw found for a
certain voltage value. This signal is then proportional to
~; log Iz _ log Iw .
The adde~ and subtractor circuit 55 has a third
; 25 input 73 which is connected to an output 73 of the operating
range limitation nomogram function generator 32 an input
. 79 of which i.s connected to the output 15 of the control
;~ loop 17 and thus also receives a voltage which is a msasure
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of the voltage applied by the ~tolta~e con~rol circuit 7 to
the radiation source 5. The si~nal at the output 75 is
then proportional to the logarit~ of the nomogram current
value associated with that voltage. Th;s signal is herein-
after refQrred to as log I
An output 81 of the adder and subtractor cir-
cuit 55 is connected through a switch 83 which can be operated
by means of the operating devico to a storage circuit 85
represented in this case by a capacitor. The output 15 of
; 10 the control loop 17 is connected to this capacitor 85. Im-
mediately after an exposure measurement the switch 83 is
temporarily closed and the control loop 17 is active. This
control loop will attempt to render the total input vo]tage
of the adder and subtractor circuit 55 zero.
When the first input 53 and the second input
57 is an adder input and the third input 73 is a subtractor
- input, the total input signal of the adder and subtractor
circuit will become.
g Wn + (Log Iz - Log I~" ) - Log I
This will be zero for log Iz = log I ,i.e.
for a point of intersection in the load nomogram and the
curve for the desired density. By the choice sf the phase ~f
the feedbackin the control loop this point of intersec-tion
is chosen at the lowest voltage value. The control loop then
automatically applies to its output 15 the control voltage
for obtaining V i applied to the voltage control circui~ 7.
The above described circuit arrangement per-
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forms on a ~'irst co~and of t}l~ operating dev:ice 21 a first
exposu~e measurement ~ith the aid of a voltage Vt t which
is deterlr.ined by a voltage applied bcfore a series of
measurements to the capacitor S5 and is obtained frorn an
output ~7 of the operating device 21 whicll voltage produces ihe
voltage Vt st at the radiation source 5. After the first
exposure measurement the pOillt corresponding to the current
value I1 is found by the radiation source 5 with the asso-
ciated voltage value V in~ After the second exposure measure-
ment we find I and V . and so forth. After termination
2 mln2
of a series of exposure measurements the control loop 17 i9
interrupted ~ith the aid of the switch 83 and the last
value found for V . is stored in the capacitor 85 and
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passes through the operating range limitation function
generator 3~ the control voltage for the current value I to
be adjusted to the change-over switch 23 which is then set to
' the position not sho-rn and controls the current control cir-
cuit 9 at the desired current intensity.
The function generators 69 and 32 may be
realized in kno~n manner with the aid Or resistance, ampli-
fication and diode net~orks.
2 Fig. 3 uses the same reference numerals for
corresponding components as Fig. 2.
The input 37 of the exposure meter 35 receives
a current upon an exposure measurement of the radiation-
-sonsitive element 41~ which current is a measure of the
I radiation quantity passed by an object to be examined. This
-~ current i~ applied through a switch 89, which is closed
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during the exposure measurcment, to an input 91 of a dif-
ference amplifier 93 the other input of which is connected
to ground and an output 95 through a capacitor 97 is fed
back to the input 91. This amplifier constitutes, with the
capacitor 97, an integrator circuit passing on the integrated
input current to nn input 101 of a logarithm-forming circuit
103. The capacitor 97 is --hunted by a switch 99 which dis-
charges the capacitor 97 every time a~ter an e~pocure
measurement.
An output 105 of the circ~lit 103 provides a
voltage which is proportional to the logarithm of the passed
radiation quantity at a voltage Vtest and a current IteSt
` to the radiation source 5.
,~ This voltage is referred to as log Et t. This
voltage i9 passed on through a switch 106 to a storage
capacitor 107 and also remains available after the exposure
~, measurement. An input 109 of a substractor circuit 111 is
`~ connected ~o this capacitor 107 and receives continuously
~ the voltage Log Etest. The voltage Log It t is applied to
i~ 20 another input 113, which voltage is a measure of the current
; intensity used for the radiation source. An output of
the subtractor circuit 111 applies to the output 51 of the
exposure meter 35 a voltage ~hich is equal to Log It t ~
~ Log Etest. This has been rendered equal to Log Iw in a
i~ 25 simple manner by the choice of a parameter for the
- exposure meter.
In case of an exposure measurement there
~ applies for the voltage Et t obtained at the output 95 of
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the integrator 93, 97 that Et t = k It t where I~ t~ as
alrcady mentionod abo~re, is the current through the radiation
source during an expnsure measur0ment. For a desired density
of t~le film tlle ~oltage at the output 95 would have to be
equal t.o E = k I~ wllere I~r is the current through the
radi.ation source 5 required for the desired density. The
rad.io I ~ is equal to the ratio ~---- from whi.ch it
test test
follows that
I - Etest
w w~test
When for the desired density the uutput voltage of the
integrator is chosen to be 1 V, then Ew = 1 and
w = ~ and log I~r = log Itest ~ log Etest
, which is the voltage at the output 51.
i 15 In Fig. 4 the input 65 of the clamping circuit
61 is connected to an input 115 of an adder circuit 117
- an output 119 of which is connected to the output 59 and
through a switch 121 closed during the exposure measurement
~, to an integration circuit which is constituted by a capacitor
t 20 123 incorporated between an output and an input of a
differential ampli~ier 125, while the output of the laitter
amplifier 125 is connected to a second input 127 of the
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adder circuit 117.
When the switch 121 is closed the voltage at
the input 127 of the adder circuit 117 will be adjusted
in such a manner that the input voltage of the difference
'~ . amplifier 125 becomes substantially æero so that the sum
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of the volta~ e at the input 127 and the voltclge at the input
115 will thus become zero. This actuallymeans a shift to
zero of a voltage valu~ applied to the illpUt 11 ) or a zero
cl~mping oL` the voltage at the output 119 during an exposure
measuremellt.
The aci~l~ and fiubtractor circuit 55 includes
an adder circuit 129 to which the illpUtS S3 and 57 are
connected and an output 131 oE` which is connected through
a resistor 133 to an ~I;nput 135 of a difference amp]ifier
137- ~Il output 139 o~ the difference amplifier 137 is
connected to the output 81 and through a resistor 141 to
the input 135. A further input 1~3 is connected through a
potential divider 145, 1~7 to the input 73.
A voltage may be derived, if desired, from the
output 131 of the adder circuit 129, which voltage is also
a measure of the desired current adjustment of the radiation
source 5. For this voltage, however, the store 85 is no
longer active because the clamping circuit is present in
this circuit. The voltage at the output 139 ma~ be used for
example to detect whether,~luring an exposure measurement a
controlled state of the control loop 17 occurs. When this
is not the case the voltage at the output 131 will exceed
a given value and may be used for a variation of one or
more parameters or for blocking a pick-up pos~ibility of the
device.
`' The described arrangement is very suitable
for angiography.
The voltage scale used in the method and the
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cievice for tlle cl~aracteristic ma~r be arbitrari.ly chosen.
~ logarit~uilic scale value, also for the ~oltage coordinate,
may be ad~antageous due to obtain.ing tlle same pereentual
accurac~-.
It ~-ill be evident that the described method
ca.n llot only be ~ormed ~rith mechanical and analog elec~ical
means. but ~or e~ample also ~rith cligital electronic circuits.
l~en using, ~or e~ample, su.f~`icient stores of a eomputer
program or of a multiplier c:ircui.t for obtaining the eurrent
volta~e funetions, even the use o~ a logarithmie eurrent
seale may be umleeessary.
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