GB2152680A - Multichannel current measuring apparatus - Google Patents

Abstract

To achieve high speed current measurement in an X-ray computerized tomograph, off-sets are measured (when signal currents I1-In are not supplied) by closing switches Sr, S1-Sn to charge capacitors C1-Cn with any off-set currents. Switches S1-Sn are then opened in sequence to discharge capacitors C1-Cn through amplifier U and A/D converter 3 into memory 7. Sr and S1-Sn are then again closed simultaneously to repeat the cycle. The data stored in memory 7 is averaged for each channel in arithmetic unit 9 and returned to memory 7. When signal currents I1-In are supplied, a similar cycle is used but the signals received in sequence from the various channels pass to memory 5. Outputs from memories 5, 7 pass to arithemetic unit 11 which supplies a true signal from which any off-set portion has been removed. <IMAGE>

Description

SPECIFICATION Multipoint current measauring apparatus This invention relates to a multipoint current measuring apparatus for applying a multichannel signal current sent from an X-ray detector of an X-ray computerized tomograph to each of a plurality of capacitors once and for reading the voltage with which the capacitor has been charged to determine the signal current in the channel associated with the capacitor.

The invention further relates to a capability of compensating for an offset produced in the current measuring system and to measuring the currents at high speeds.

A prior X-ray computerized tomograph is so constructed that an object to be examined is roentgenized (i.e. subjected to X-ray scanning) and that the transmitted X-rays are detected by an X-ray detector comprising a number of ionization chambers to obtain projection data relating to the object based on the signal given by the detector. In this case, the tomograph is provided with a multipoint current measuring apparatus used to measure the strength of the output current of the ionization chamber in each of the channels.

The output currents from the multichannel ionization chambers in this prior art arrangement are thus converted into the voltage signals by way of capacitors as aforesaid for measurement. In this case, a leakage current may flow into each capacitor used for current/voltage conversion. In addition, because the amplifier for amplifying the charged voltage of the capacitor is affected by any off set that cannot be ignored, it has become necessary to correct these error factors, if highly accurate measurement is required.

It is an object of the present invention to provide a current measuring apparatus capable of correcting the aforesaid error factors to obtain highly accurate measurement and to maintain high-speed measurement while effecting the foregoing error correction.

According to the present invention, a multipoint current measuring apparatus has a plurality of channels including respective capacitors chargeable by currents flowing through the channels from an X-ray detector, the currents in the respective channels being measurable by successively reading voltages across the capacitors, respectively.The multipoint current measuring apparatus comprises off-set measuring means for discharging the capacitors in a period in which signal currents to be measured are not supplied, applying other components than the signal currents to the respective capacitors for fixed intervals of time determined respectively for the capacitors after the latter have been discharged, and then measuring the voltages across the capacitors to determine off-sets in the respective channels, off-set storage means for storing the off-sets in the respective channels, and measuring and arithmetic means for successively reading the voltages across the capacitors charged by the signal currents in the channels from the X-ray detector after the capacitors have been discharged, upon elapse of the fixed intervals of time after the capacitors have been discharged, reading the off-sets in the channels from the off-set storage means, and subtracting the off-sets from the measured signal currents.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a prior art circuit and a preferred embodiment of the present invention are shown by way of illustrative example.

Figure 1 is a circuit diagram of a conventional multipoint current measuring apparatus: Figure 2 is a circuit diagram, partly in block form, of a multipoint current measuring apparatus according to the present invention: and Figure 3 is a timing chart illustrative of operation of the multipoint current measuring apparatus of the present invention.

Figure 1 of the accompanying drawings illustrates a conventional current measuring apparatus in which signal currents 11-ln to be measured flow from an X-ray detector. The signal currents l1-In are pulsed currents which are converted by respective capacitors C1-Cn into voltage signals. The voltages charged across the capacitors C1-Cn are selected one at a time by sampling switches S1-Sn, and the selected voltage is amplified by an amplifier u and then issues out.

The present invention will hereinafter be described with reference to Figures 2 and 3.

Like or corresponding parts in Figure 2 are denoted by like or corresponding reference characters in Figure 1, and will not be described in detail. A plurality of illustrated sampling switches may be replaced with a multiplexer. A reset switch Sr is capable of discharging capacitors C,Cn. An amplifier U has a high-input impedance for receiving voltages across the capacitors C1-Cn and serves to amplify these voltages and transmit them to a next stage.The apparatus also has an A/D converter 3 for converting analog signals from the amplifier U into digital signals, memories 5, 7 for storing the digital signals fed from the A/D converter 3, an arithmetic unit 9 for picking up a plurality of items of stored data from the memory 7 and computing the average of the data items, and an arithmetic unit 11 for picking up data items from the memories 5, 7 and for adding and subtracting them. i,in added to the capacitors C,Cn in parallel represent leakage currents.

Operation of the apparatus of Figure 2 thus constructed will be described with reference to Figure 3.

During the period before no X-ray pulses are emitted and hence before the signal currents I1-ln are not inputted, the off-set portion in each channel (noise signal containing no signal current to be measured) is first measured in the following order: In Figure 3, among the waveforms indicating the operation of the switches, the switch is assumed in ON and OFF states during the time the waveform is at "high" and "low" levels, respectively.

(1) As shown in Figure 3, when the reset switch Sr is turned on at P1, each of the sampling switches S1-Sn is simultaneously turned ON. Accordingly, the charges that the capacitors C1-Cn have been charged with are discharged and the voltage of each capacitor becomes zero.

(2) Subsequently, the switches Sr and S1-Sn are all turned OFF and the current based on the off-set is allowed to flow into each of the capacitors C1-Cn and integrated therein.

(3) The sampling switch S1 is first turned ON at time T1 after the capacitors C1Cn have been discharged. The voltage of the capacitors C, is read out and stored in the memory 7 via the amplifier U and the A/D converter 3.

(4) Next the sampling switch S2 is turned ON the time T2 after the capacitors C1-Cn have been discharged. The voltage of the capacitor C2 is read out and stored in the memory 7 via the same course as that in (3).

(5) Then the voltages of the capacitors C3-Cn are all stored in the memory 7 through the same operation as above.

(6) Each of the sampling switches S1-Sn is turned ON at the same time that the reset switch Sr is again turned On at P2, and the voltages of the capacitors C1 -Cn becomes zero.

(7) The operations (1)-(5) are repeated so that the data on the off-set in each channel are stored in the memory 7 one after the other.

(8) The repetition of the above operations causes the off-set data in each of the channels to be stored in the memory 7, the data being collected during the predetermined period of time (T1, T2, ... Tn) on a channel basis after the capacitors C1-Cn have been discharged.

(9) The data stored in the memory 7 are taken out by the respective channels to generate the average value of the data items from the arithmetic unit 9. For instance, only the data fetched from the capacitor C, are used to produce the average value and the average value of the off-set portion in the channel 1 is again stored in the memory 7.

(10) The average value of the off-set portion in each channel is computed through the same operation and stored in the memory 7.

In the present invention, the average value of the off-set portion in each channel during the period before the signal currents l1-ln have not been inputted is computed and stored in the memory.

On the other hand, the signal currents I1-In are measured as follows: (11) As shown in Figure 3, each of the sampling switches S,--Sn is turned ON simultaneously when the reset switch Sr is turned ON at PA.

and the capacitors C - Cn are discharged.

(12) Subsequently, as shown in Figure 3, X-ray pulses are emitted and each of the current signals l1-ln flows into one of the capacitors C1n in the respective channels.

and is converted into a voltage.

(13) The switch S, is turned ON the time T, after the capacitors CXCn have been discharged and the voltage data of the capacitor C, is stored in the memory 5 via the amplifier U and the A/D converter 3.

(14) Next the sampling switch S, is turned ON the time T2 later and the voltage of the capacitor C, is read out. The value obtained is stored in the memory 5 through the same route as that in (13).

(15) By the same operations as those in (13) and (1 4), the voltages of the capacitors C3n are all stored in the memory 5 hereinafter.

(1 6) The measured data in each channel stored in the memory 5 and the off-set portion in each channel stored in the memory 7 are taken into the arithmetic unit 11 in which arithmetic operations are effected on the supplied data. Then, a true signal current from which any off-set portion has been removed is issued from the arithmetic unit 11.

(17) Then, whenever the signal currents l1-In in the channels flow in, the correcting arithmetic operation is repeated using the offset portion stored in the memory The arithmetic unit 11 for taking out the data stored in the memories 5, 7 and effecting subtraction on them has been described as comprising a digital arithmetic unit. However, an analog arithmetic unit may be used in the present invention. In such an alternative, it is necessary to provide a D/A converter in the stage prior to the arithmetic unit 11 for converting a digital signal to an analog signal.

In addition, although the above description has referred to two separately installed memories, only one memory may be employed if addresses are used separately.

Through the foregoing operations measured data with off-set influences compensated for are rendered available in each channel. In reality, however, the off-set value varies during scanning, and random noise is present when measuring the off-set and signals. With a so-cal led third-generation CT (rotate/rotate scanning system), a particular detector channel corresponds to a particular circle on the image, and an annular artefact is produced on the image when there is an off-set error in a certain channel.

For obtaining one sectional image 200 through 900 projection measurments are performed. By effecting the off-set measurment cycle (1)+10) in each projection measuring cycle (1 1)-(1 7), the off-set value can be correctly appropriately, even if not stably, as the off-set and projection measurements are carried out in a short period of time in the range of from 10 through 20 msec. However, this arrangement is disavantageous in that the number of off-set measurement cycles is increased, and the scanning time is prolonged.

Where the off-set value remains substantially constant during the scanning period of 5 through 10 seconds, an off-set measuring cycle may be provided immediately before a scanning operation is started, and an off-set value measured prior to the scanning operation may be used in common for all projection data without newly measuring off-sets during the projection measurement. With such a process, the time required for the off-set measuring cycle can be shortened sufficiently as compared with the entire interval of time required for the projection measuring cycle, and hence the scanning period may not be increased. However, where only one off-set measuring cycle is effected, any noise and measuring error contained in the off-set measurement cycle tend to serve as constants affecting all projection data during the scanning period, resulting in an extremely thin ring artefact on the image.Accordingly, even where an off-set measuring cycle is provided immediately prior to a scanning operationto reduce the overall scanning period, the off-set measurement should be performed several times and the average value of the measured off-sets should be used for reducing the noise and measuring error.

With the present invention, as described above, off-sets in the respective channels are pre-measured and stored, and high-speed corrective arithmetic operation is effected on the basis of the stored off-set each time a signal current is measured. Accordingly, a current measuring apparatus can be achieved which is capable of measuring signal currents in short periods while compensating for off-sets.

Although a certain preferred embodiment has been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims.

Claims (3)

1. A multipoint current measuring apparatus having a plurality of channels including respective capacitors chargeable by currents flowing through said channels from an X-ray detector, said currents in the respective channels being measurable by successively reading voltages across the capacitors, respectively, said multipoint current measuring apparatus comprising: (a) off-set measuring means for discharging said capacitors in a period in which signal currents to be measured are not supplied, applying other components than said signal currents to the respective capacitors for fixed intervals of time determined respectively for said capacitors after the latter have been discharged, and then measuring the voltages across said capacitors to determine off-sets in the respective channels; (b) off-set storage means for storing the offsets in the respective channels; and (c) measuring and arithmetic means for successively reading the voltages across said capacitors charged by the signal currents in said channels from said X-ray detector after said capacitors have been discharged, upon elapse of the fixed intervals of time after said capacitors have been discharged reading the off-sets in said channels from said off-set storage means, and subtracting said off-sets from the measured signal currents.

2. A multipoint current measuring apparatus according to claim 1, wherein said off-set measuring means repeats off-set measurement a plurality of times, and said off-set storage means stores average values of the measured off-sets as said off-sets in the respective channels.

3. A multipoint current measuring apparatus substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.