Computed tomography apparatus and method for operating same

[0002] 1. Field of the Invention

[0003] The present invention is directed to a computed tomography system equipped with a data-acquisition system, as well as a method for use with such a computed tomography system.

[0004] 2. Description of the Prior Art

[0005] A computed tomography system of the type noted above has an X-ray source that directs a pyramidal X-ray beam through an object that is to be examined, e.g. a patient, onto a radiation detector. The X-ray source and, in some types of computed tomography systems, the radiation detector as well, are mounted on a gantry that can be rotated about the patient. The patient can be lying on a table that may be displaced or moved relative to the gantry along a system axis. This, inter alia, makes it possible to scan a portion of the patient's body in a spiral manner, so that a volume of the patient's body scanned. The values measured during the process are used to reconstruct sectional views of planar slices of the patient.

[0006] The radiation detector of the computed tomography system can have a linear detector array composed of a row of several detector elements aligned adjacent to one another or, as described, for example, in U.S. Pat. No. 5,291,402, several parallel linear detector arrays. The advantage of employing a computed tomography system equipped with a multi-line radiation detector is, for example, the ability to more rapidly scan a volume of the patient's body that is of interest, which, among other things, reduces the recording time. The disadvantage of employing a computed tomography system equipped with a multi-line radiation detector, however, is that it has a larger number of detector elements, which means that more detector elements must be read out during each scanning step than in the case of a computed tomography system equipped with a single-line radiation detector. This, in turn, leads to higher data rates for signals coming from a data-acquisition system that reads out the detector elements.

[0007] A radiological therapy device that has a photodiode array preceded by an X-ray image intensifier is known from U.S. Pat. No. 5,117,445. The photodiode array preceded by an X-ray image intensifier converts X-ray radiation into electrical signals. The electrical signals may be falsified, however, by geometric non-linearities of the X-ray image intensifier, which is why at least some of the output signals at the photodiode array must be interpolated in a suitable manner.

[0008] An object of the present invention to provide a computed tomography system of the type initially described with a data-acquisition system configured so that a prerequisite for lower data rates is satisfied. Another object of the invention is to specify a method for use with such a computed tomography system that will allow the data rates to be reduced.

[0009] The first object is achieved in accordance with invention in a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one linear detector array composed of several detector elements aligned up adjacent to one another, the data-acquisition system reading out the detector elements and forming and further processing difference signals from signals read out from pairs of adjacent detector elements. Since it is highly likely that any differences in the amplitudes of signals read out from pairs of adjacent detector elements will be slight, the rates of change of their difference signals will also be slight. Employing difference signals will thus allow data rates to be reduced compared to the conventional case where signals read out from single detector elements are employed. In a variant of the invention the data-acquisition system has at least one analog/digital converter that digitizes the signals read out from the detector elements. This advantageously makes it possible to form the difference signals at the digital level.

[0010] In an embodiment of the invention the data-acquisition system has at least one analog/digital converter that digitizes the difference signals read out from pairs of adjacent detector elements.

[0011] If, in an embodiment of the invention, the digitized difference signals have a data length of one byte, the data rates for a computed tomography system according to the invention will be low.

[0012] In a further embodiment of the invention the radiation detector is formed of several detector modules each composed of several linear detector arrays, which in turn each have several detector elements aligned adjacent to one another, and each of the detector modules is assigned an analog/digital converter.

[0013] The other object of the invention is achieved in a first embodiment of a method for acquiring signals from a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one detector module with at least one linear detector array with several detector elements aligned adjacent to one another, having the following method steps:

[0014] a.) reading out each detector element of a detector module at each scanning step and

[0015] b.) forming signals S*_j,m,kfrom signals read out from detector elements of the j^thdetector module, where

[0016] S*_j,m,1=S_j,m,1for k=1

[0017] and

[0018] S*_j,m,k=S_j,m,k−S_j,m,k−1for 1<k≦K,

[0019] where S_j,m,kis the signal read out from the k^thdetector element of the m^thlinear detector array of the j^thdetector module and K is the number of detector elements in the m^thlinear detector array.

[0020] As explained above, it is highly likely that signals read out from pairs of adjacent detector elements will have slight differences in amplitude. Since the rates of change of the difference signals will thus also be slight, employing the difference signals will allow data rates to be reduced compared to the conventional case where signals read out from single detector elements are employed.

[0021] According to the invention, the second object also is achieved in a second embodiment of a method for acquiring signals from a computed tomography system equipped with a data-acquisition system and a radiation detector that has at least one detector module with at least one detector column with several detector elements aligned adjacent to one another, having the following method steps:

[0022] a.) reading out each detector element of a detector module at each scanning step and

[0023] b.) forming signals S*_j,m,kfrom signals read out from detector elements of the j^thdetector module, where

[0024] S*_j,1,k=S_j,1,kfor m=1

[0025] and

[0026] S_j,m,k=S_j,m,k−S_j,m−1,kfor 1<m≦M,

[0027] where S_j,m,kis the signal read out from the mth detector element of the k^thdetector column of the j^thdetector module and M is the number of detector elements in the k^thdetector column.

[0028] The reduced rates of change of the difference signals may thus be exploited in an advantageous manner for reducing the data rates.

[0029] In a variant of the invention, the signals S_j,m,kare further processed in digital form in the method. It is thus advantageously possible to form the difference signals at the digital level.

[0030] In an embodiment of the invention, the signals S*_j,m,kare further processed in digital form in the method.

[0031] If, in a further embodiment of the method, the digital difference signals have a data length of one byte, data rates may be kept low in an advantageous manner.

[0032] FIG. 1 shows a computed tomography system according to the invention.

[0033] FIG. 2 is a block diagram illustrating the acquisition of difference signals at the digital level in accordance with the invention.

[0034] FIGS. 3 and 4 are block diagrams illustrating the acquisition of difference signals at the analog level in accordance with the invention.

[0035] FIG. 5 shows an exemplary embodiment of a circuit for acquiring analog difference signals in accordance with the invention.

[0036] FIG. 6 shows plots for explaining the operation of the circuit illustrated in FIG. 5.

[0037] FIG. 7 shows another exemplary embodiment of a circuit for acquiring analog difference signals in accordance with the invention.

[0038] FIG. 8 shows plots for explaining the operation of the circuit illustrated in FIG. 7.

[0039] FIG. 1 schematically depicts a computed tomography system according to the invention equipped with an X-ray source 1, emitting a pyramidal X-ray beam 2, whose marginal rays are represented by the dot-dashed lines in FIG. 1, that penetrates an object 3 to be examined, for example a patient, and that is incident on a radiation detector 4. In the exemplary embodiment, the radiation detector 4 has four linear detector arrays, 5athrough 5d, adjacent to one another, with several detector elements, 6athrough 6x, aligned adjacent to one another.

[0040] The X-ray source 1 and the radiation detector 4 are, in the exemplary embodiment, mounted opposite to one another on an annular gantry 7. The gantry 7 is mounted on a mounting fixture that is not shown in FIG. 1 such that it may be rotated (cf. arrow a) around a system axis 8 that passes through the center of the annular gantry 7.

[0041] In the exemplary embodiment, the patient 3 is lying on a table 9 that is transparent to X-rays and that is mounted such that it can be displaced (cf. arrow b) along the system axis 8 using a supporting mechanism that is not shown in FIG. 1.

[0042] The X-ray source 1 and the radiation detector 4 thus form a metrological system that can be rotated around the system axis 8 and displaced along the system axis 8 relative to the patient 3 so that the patient 3 can be X-rayed at various projection angles relative to said system axis 8 and at various positions along the system axis 8. The resultant output signals of the individual detector elements, 6athrough 6x, are read out by a data-acquisition system 10 mounted on said gantry 7. The data-acquisition system 10 forms difference signals from adjacent detector elements 6athrough 6x. The signals are sent to a signal processor 12, that computes an image of the patient 3 that, in turn, can be displayed on a monitor 13, by means of an electrical cable 11 that incorporates a slip-ring system, or a wireless transmission link in a manner that has not been illustrated. In the exemplary embodiment, the monitor 13 is connected to the signal processor 12 by an electrical cable 14.

[0043] The computed tomography system depicted in FIG. 1 can be employed for performing either sequential scans or spiral scans.

[0044] In the case of sequential scans, scanning of the patient 3 takes place in slices. The X-ray source 1 and the radiation detector 4 are rotated around the patient 3 relative to the system axis 8, and the measurement system formed by the X-ray source 1 and the radiation detector 4 records numerous projections in order to scan two-dimensional slices of the patient 3. Sectional views representing the scanned slices are reconstructed from the resultant measurement data. The patient 3 is in each case moved along the system axis 8 between scans of consecutive slices. This procedure is repeated until all slices of interest have been scanned.

[0045] During spiral scans, the measurement system formed by the X-ray source 1 and the radiation detector 4 is rotated around said system axis 8, and the table 9 is continuously moved in the direction of the arrow b, i.e., the measurement system is continuously moved along a spiral path c relative to the patient 3 until such time as the entire portion of the patient 3 that is of interest has been scanned, thereby generating a dataset representing a volume. The signal processor 12 computes planar data from which, as in the case of sequential scanning, sectional views may be reconstructed from the volume data record by employing an interpolation method.

[0046] FIG. 2 depicts a block diagram of the data-acquisition system 10 shown in FIG. 1, which, inter alia, forms difference signals from signals read out from said detector elements 6athrough 6xof the radiation detector 4 at each scanning step.

[0047] In the exemplary embodiment, each detector element 6a through 6xis followed by an integrator 20athrough 20xin the form of a capacitor. FIG. 2 shows only integrators 20aand 20x. As a variation of those shown in FIG. 2, the integrators 20athrough 20xmay also include amplifier stages or be incorporated into the detector elements 6athrough 6xto the extent that the detector elements 6athrough 6xhave an auto-integrating capability.

[0048] Charges created in the detector elements 6athrough 6xby absorption of X-rays are integrated over a specified time interval by the integrators 20athrough 20xfor each scanning step and, in the exemplary embodiment, are sequentially read out row by row by a multiplexer 21, and are amplified, by an amplifier 22. Thus signals read out first from detector element 6a, then from detector elements 6bto 6f, then from detector elements 6gto 6letc. will be sequentially available at the output of said electronic element 22. Alternatively, the detector elements 6athrough 6xmay be read out column by column.

[0049] The signals from the detector elements 6athrough 6xthat have been read out and amplified by the electronic element 22 are subsequently sequentially digitized by an analog/digital converter 23 and, in the exemplary embodiment, are sent to an arithmetic logic unit (ALU) 24. In the arithmetic logic unit 24, in the case where the detector elements 6athrough 6xhave been read out row by row, difference signals are formed from signals from pairs of adjacent read-out detector elements 6athrough 6xof the linear detector array 5athrough 5d, in accordance with the following method.

[0050] The signals S*_m,1are formed as

[0051] S*_m,1=S_m,1

[0052] where

[0053] 1≦m≦4.

[0054] In the exemplary embodiment signal S_1,1being the digitized signal read out from the first detector element 6a, S_2,1is the digitized signal read out from the second detector element 6g, S_3,1is the digitized signal read out from the third detector element 6m, and S_4,1is the digitized signal read out from the fourth detector element 6s, of the first detector column 25aof the radiation detector 4. The radiation detector 4 has four linear detector arrays 5athrough 5d. The aforementioned signals are present at the output of the ALU 24 in digitized form. S*_m,1are the signals that are available at the output of the ALU 24, and sent to the signal processor 12 shown in FIG. 1.

[0055] The difference signals S*_m,k, with, in the present exemplary embodiment, 1<k≦, are formed as

[0056] S*_m,k=S_m,k−S_m,k−1.

[0057] The signals S_m,kare the digitized signals read out from the detector elements 6athrough 6xof the m^thof the four linear detector arrays 5athrough 5d(in the exemplary embodiment, 5ais the first of the linear detector arrays, 5bis the second of the linear detector arrays, 5cis the third of the linear detector arrays, and 5dis the fourth of the linear detector arrays) and the k^thcolumn of the six detector columns 25athrough 25f(in the exemplary embodiment, for example, the first of the detector columns is 25a, the second of the detector columns is 25b, etc.) which are available at the input of the ALU 24 in digitized form. The difference signals S*_m,k(1≦k<6) are available at the output of the ALU 24 and are sent to the signal processor 12 shown in FIG. 1.

[0058] Since the amplitudes of signals read out from pairs of adjacent detector elements 6athrough 6xare highly unlikely to differ by very much, it is highly likely that the rates of change of the difference signals S*_m,k(1<k≦6) will be less than those of signals S_m,kread out from single detector elements 6athrough 6x. Consequently, the difference signals S*_m,k(1<k≦6) can be supplied to the signal processor 12 using shorter data lengths than in the case of signals S_m,kread out from single detector elements 6athrough 6x.

[0059] In the exemplary embodiment, the difference signals S*_m,k(1<k≦6) are encoded by the ALU 24 using a data length of only one byte, one bit being used to represent the algebraic sign, four bits being used to represent the mantissa, and three bits being used to represent the exponent. The signals S*_m,1, i.e. those signals corresponding to the signals read out from detector elements 6a, 6g, 6m, and 6sof said first detector column 25a, are not difference signals and, in the exemplary embodiment, have greater data lengths than the difference signals S*_m,k(1<k≦6) while they are being supplied to the signal processor 12. Since at least the difference signals S*_m,k(1 <k≦6) are supplied from the data-acquisition system 10 to the signal processor 12 with shorter data lengths than the signals corresponding to a detector element 6athrough 6xread out, the data rate for transmission from the data-acquisition system 10 to the signal processor 12 is reduced.

[0060] Alternatively, instead of the difference signals S*_m,k(1<k≦6) of a linear detector array 5athrough 5d, difference signals derived from signals read out from pairs of adjacent detector elements 6athrough 6xof the detector columns 25athrough 25fcan be formed. In this alternative difference signals S*_m,k(1<m≦4) are formed from signals from the detector elements 6athrough 6xof the six detector columns 25athrough 25f, in accordance with the following method:

[0061] The signals S*_1,kare formed as

[0062] S*_1,k=S_1,k

[0063] where

[0064] 1<k≦6,

[0065] the signals S_1,kbeing the digitized signals read out from the detector elements, 6athrough 6f, of the first linear detector array 5a.

[0066] The difference signals S*_m,k, where, in the case of the present exemplary embodiment, 1<m≦4, are formed as

[0067] S*_m,k=S_m,k−S_m−1,k.

[0068] The statements made above regarding the data lengths of said signals S*_1,kand the difference signals S*_m,k(1<m≧4) apply analogously.

[0069] The difference signals also can be formed at the analog level. FIG. 3 schematically depicts an exemplary embodiment of a data-acquisition system 30 that, inter alia, forms difference signals at the analog level from the detector elements, 6athrough 6x, of said radiation detector 4 of the computed tomography system shown in FIG. 1 read out by the data-acquisition system 30.

[0070] In a manner similar to that shown in FIG. 2 and described above, the detector elements 6athrough 6xare followed by integrators 31athrough 31x. Only the integrators 31aand 31xare depicted in FIG. 3.

[0071] Analogously to the case described above, the integrators 31athrough 31xintegrate the charges created in said detector elements 6athrough 6xon absorption of X-radiation over a specified time interval for each scanning step.

[0072] In the exemplary embodiment, the outputs of the integrators 31athrough 31xare selected row by row using a column multiplexer 32 connected to electronic elements 33athrough 33f, i.e. the electronic elements 33athrough 33ffirst read out detector elements 6athrough 6fof the first linear detector array 5ain parallel, then read out detector elements 6gthrough 6lof the second linear detector array 5bin parallel, then read out detector elements 6mthrough 6rof the third linear detector array 5cin parallel, and finally read out detector elements 6sthrough 6xof the fourth linear detector array 5din parallel, and amplify those respective signals.

[0073] The output signals from the electronic elements 33athrough 33fare sent to a difference former 34 that, inter alia, forms difference signals from signals from pairs of adjacent read-out detector elements 6athrough 6xof a detector column 25athrough 25f, similarly to the method described above:

[0074] Said signals S*_1,kare formed as

[0075] S*_1,k=S_1,k

[0076] where

[0077] 1≦k≦6.

[0078] In the exemplary embodiment the signals S_1,kbeing read out from the detector elements 6athrough 6fof the first linear detector array 5a.

[0079] The difference signals S*_m,k, with, in the exemplary embodiment, 1<m≦4, are formed as

[0080] S*_m,k=S_m,k−S_m−1,k.

[0081] The signals S*_1,k(1≦k≦6) and the difference signals S*_2,k(1≦k<6), S*_3,k(1<k≦6), and S_*4,k(1≦k≦6), are, in turn, converted by a multiplexer 35 into a serial data stream that is digitized by an analog/digital converter 36, and are thus available at the outputs of the difference former 34, for each scanning step.

[0082] Since the difference signals S*_m,k(1<m≦4), generally have rates of change less than those of the signals S_m,kread out from said detector elements 6athrough 6x, they can be supplied to the signal processor 12 shown in FIG. 1 using shorter data lengths. Similarly to the data-acquisition device 10 shown in FIG. 2, their data length may be only one byte, where one bit is used for the algebraic sign, four bits are used for the mantissa, and three bits are used for the exponent.

[0083] If the detector elements 6athrough 6xare alternatively read out column by column, a row multiplexer with appropriate outputs is provided instead of the column multiplexer 32 shown in FIG. 3, in which case the difference former forms difference signals from signals read out from pairs of adjacent detector elements 6athrough 6xof a linear detector array 5athrough 5d.

[0084] FIG. 4 schematically depicts an alternative data-acquisition system 40 for reading out and forming difference signals at the analog level for the computed tomography system shown in FIG. 1. Unless otherwise stated, the components of the data-acquisition system 40 shown in FIG. 4 that are largely identical in form and function to those of the data-acquisition system 30 that is shown in FIG. 3, and has been described above, have the same reference symbols.

[0085] In the data-acquisition system 40 shown in FIG. 4, the electronic elements 33athrough 33fthat read out the detector elements 6athrough 6xare followed by a multiplexer 41 that converts signals from the detector elements 6athrough 6x, which are read out row by row, into a sequential data stream, i.e. the signals read out from detector elements 6a, 6b, 6c, through 6xare available, one after the other, at the output of the multiplexer 41, for each scanning step. The signals are sent to a difference former 42 that, as in the method described above, forms difference signals S*_m,k(1<k≦6) from signals read out from pairs of adjacent detector elements 6athrough 6xof the linear detector arrays 5athrough 5f. The signal S*_1,1, the difference signals S*_1,k(1<k≦6), the signal S*_2,1, the difference signals S*_2,k(1<k≦6), the signal S*_3,1, the difference signals S*_3,k(1<k≦6), the signal S*_4,1and the difference signals S_*4,k(1<k≦6), that are digitized by an analog/digital converter 43, are thus available, one after the other, at the output of said difference former 42, for each scanning step. The difference signals S*_m,k(1<k≦6), can be supplied in a manner similar to that in the exemplary embodiments described above using data lengths that are shorter than those for signals read out from a single detector element 6athrough 6x.

[0086] If the detector elements 6athrough 6xare read out column by column, the column multiplexer 32 shown in FIG. 4 is a row multiplexer with appropriate outputs, and the difference former 42 forms difference signals from signals read out from pairs of adjacent detector elements 6athrough 6xof a detector column 25athrough 25f.

[0087] FIG. 5 schematically depicts an exemplary embodiment of the analog difference former 42, that forms differences derived from pairs of consecutive signals. In the exemplary embodiment, said difference former 42 has a buffer, 50, which sequentially receives input signals from the read-out detector elements 6athrough 6xshown in FIGS. 1 through 4. The output of the buffer 50 may be switched to a coupling capacitor C₁using a switch S₁. The coupling capacitor C₁is, in turn, connected to an inverting input of an amplifier stage, for example an operational amplifier 51. In addition, the inverting input of the operational amplifier 51 is connected to the output of the operational amplifier 51 using a sampling capacitor C_Sthat can be discharged using a switch S_S. The coupling capacitor C₁can be discharged using the switch S₁and a switch S_K. In addition, the non-inverting input of the operational amplifier 51 is grounded.

[0088] FIG. 6 depicts, schematically and byway of example, sample plots explaining the operation of the difference former 42 shown in FIGS. 4 and 5, which forms difference signals from signals from pairs of adjacent, read-out detector elements 6athrough 6xof the linear detector arrays 5athrough 5d.

[0089] At time t₀, there is no input signal (U_E=0) at the input of the buffer 50, and the switch S₁is open. At time t₁, the sampling capacitor C_Sis discharged by the closing of switch S_S, and the coupling capacitor C₁is discharged by means of the closed switch S_K.

[0090] At time t₂, the switches S_Kand S_Sare opened, the switch S₁is closed, and an input signal U_E,1is applied to the input of the buffer 50. The signal U_A,1=−U_E,1is thus present at the output of the operational amplifier 51. The input signal U_E,1is, in the exemplary embodiment, the signal S_1,1read out from detector element 6aof the radiation detector 4, i.e. the output signal U_A,1=−S_1,1.

[0091] At time t₃, the switch S_Sis closed, discharging the sampling capacitor C_S, and there is no signal present at the output of the operational amplifier 51. The coupling capacitor C₁stores the input signal U_E,1, i.e. S_1,1.

[0092] At time t₄, the switch S_Sis reopened and an input signal U_E,2is applied to the input of the buffer 50. The input signal U_E,2is, in the exemplary embodiment, the signal S_1,2read out from detector element 6b. Due to the capacitive coupling of the buffer 50 and the operational amplifier 51 using the coupling capacitor C₁the signal present at the output of the operational amplifier 51 is then—(U_E,2−U_E,1), i.e. the output signal U_A,2) =−(S_1,22−S_1,1)=−S*_1,2.

[0093] At time t₅, the switch S_Sis closed again, the sampling capacitor C_Sis discharged, and there is, once again, no signal present at the output of the operational amplifier 51. The coupling capacitor C₁then stores the input signal U_E,2, i.e. S_1,2.

[0094] At time t₆, the switch S_Sis reopened and an input signal U_E,3is applied to the input of the buffer 50, which input signal U_E,3is, in the exemplary embodiment, the signal S_1,3read out from detector element 6c. The signal present at the output of the operational amplifier 51 is thus U_A,3=−(U_E,3−U_E,2)=−S*_1,3. The difference former 42 shown in FIG. 5 thus is able to form difference signals derived from consecutive signals present at the input of the buffer 50.

[0095] The difference former 42 shown in FIG. 5 also is suitable for forming the difference signals of signals from pairs of adjacent read-out detector elements 6athrough 6xof a detector column 25athrough 25f.

[0096] The difference former 34 shown in FIG. 3 has several inputs and outputs. The difference former 34 may, for example, be designed such that it is formed by a number of difference formers shown in FIG. 5, this number corresponding to the number of inputs and outputs on the difference former 34.

[0097] FIG. 7 schematically depicts another embodiment of a difference former 70 that is capable of storing several (in the present exemplary embodiment, six) signal values. Unless otherwise stated, components of the difference former 70 shown in FIG. 7 that are largely identical in form and function to those of the difference former 42 that is shown in FIG. 5 and has been described above have the same reference symbols.

[0098] As in the case of the difference former 42 shown in FIG. 5, the difference former 70 shown in FIG. 7 has a buffer 50 with an input and receiving signals from the sequentially read detector elements 6athrough 6xshown in FIGS. 1 through 4. The output of the buffer 50 may be switched to coupling capacitors C₁through C₆using switches S₁through S₆. The coupling capacitors C₁through C₆are, in the exemplary embodiment, in turn connected to an inverting input of an amplifier stage, for example an operational amplifier 51. In addition, the inverting output of the operational amplifier 51 is connected to the output of the operational amplifier 51 using a sampling capacitor C_S. The sampling capacitor C_Smay be discharged using a switch S_S. In addition, the non-inverting input of the operational amplifier 51 is grounded. For clarity, the switches for discharging the coupling capacitors C₁through C₆are not shown in FIG. 7.

[0099] In the exemplary embodiment, said difference former 70 shown in FIG. 7 may be used instead of the difference former 42 shown in FIG. 4. At the input to the difference former 70, there are signals S_m,kthat have been read out in sequence, row by row, from the detector elements 6athrough 6x. Since the difference former 70 has six coupling capacitors C₁through C₆, six signals can be stored. In particular, for each scanning step, those signals S_1,k, read out from the detector elements 6athrough 6fof the first linear detector array 5a, may be stored first, followed by those signals S_2,kread out from the detector elements 6gthrough 61 of the second linear detector array 5b. The switches S₁through S₆are cyclically closed and reopened so that signals from the detector elements 6athrough 6xof the linear detector array 5athrough 5dcan be read out one after the other and respectively stored in one of the coupling capacitors C₁through C₆.

[0100] FIG. 8 depicts, schematically and by way of example, sample plots explaining the operation of the difference former 70 shown in FIG. 7, to which signals from said read-out detector elements 6athrough 6xare applied sequentially and row by row.

[0101] At time t₀, there is no input signal (U_E=0) at the input of the buffer 50, and the switches S₁through S₆are open. At time t₁, the sampling capacitor C_Sis discharged by the closing of switch S_Sand the coupling capacitors C₁through C₆are discharged by means of switches that are not shown.

[0102] At time t₂, the switch S_Sis opened, the switch S₁is closed, and an input signal U_E,1is applied to the input of the buffer 50. The signal U_A,1=−U_E,1thus is present at the output of the operational amplifier 51. The input signal U_E,1is, in the exemplary embodiment, the signal S_1,1read out from detector element 6aof the radiation detector 4, i.e. the output signal U_A,1=−S_1,1.

[0103] At time t₃, the switch S_Sclosed, the switch S₁is opened, the sampling capacitor C_Sis discharged, and there is no signal at the output of the operational amplifier 51. The coupling capacitor C₁therefore stores the input signal U_E,1, i.e. the signal S_1,1.

[0104] At time t₄, the switch S_Sis reopened, the switch S₂is closed, and an input signal U_E,2is applied to the input of the buffer 50. The input signal U_E,2is, in the exemplary embodiment, the signal S_1,2read out from detector element 6b. The signal at the output of the operational amplifier 51 thus is the signal U_A,2=−S*_1,2.

[0105] At time t₅, the switch S_Sis closed again, the switch S₂is opened, the sampling capacitor C_Sis discharged, and there is, once again, no signal at the output of the operational amplifier 51. The coupling capacitor C₂then stores the input signal U_E,2, i.e. signal S_1,2.

[0106] At time t₆, the switch S_Sis reopened, the switch S₃is closed, and an input signal U_E,3is applied to the input of the buffer 50. The input signal U_E,3is, in the exemplary embodiment, the signal S_1,3read out from detector element 6c. The signal at the output of the operational amplifier 51 thus is the signal U_A,3=−S_1,3.

[0107] At time t₇, the switch S_Sis closed again, the switch S₃is opened, the sampling capacitor C_Sis discharged, and there is, once again, no signal at the output of the operational amplifier 51. The coupling capacitor C₃then stores the input signal U_E,3, i.e. signal S_1,3.

[0108] At times t₈through t₁₃, which are not shown in FIG. 8, the input signals U_E,4through U_E,6, which correspond to the signals S_1,4through S_1,6read out from detector elements 6dthrough 6f, are applied one after another to the input of the buffer 50. The signals at the output of the operational amplifier 51 are correspondingly the signals U_A,4=−S_1,4through U_A,6=−S_1,6, and the coupling capacitor C₄stores the signal S_1,4, the coupling capacitor C₅stores the signal S_1,5, and the coupling capacitor C₆stores the signal S_1,6.

[0109] At time t₁₄, the switch S_Sis reopened, the switch S₁is closed again, and an input signal U_E,7is applied to the input of the buffer 50, the input signal U_E,7is, in the exemplary embodiment, the signal S_2,1read out from detector element 6gof the second linear detector array 5b.

[0110] Due to the capacitive coupling of the buffer 50 and the operational amplifier 51 using the coupling capacitor C₁, the signal at the output from the operational amplifier 51 is then—(U_E,7−U_E,1), i.e. the output signal U_A,2=−(S_2,1-S_1,1=−S*_2,1, a difference signal between those signals read out from the pair of read-out adjacent detector elements 6aand 6gof detector column 25a.

[0111] At time t₁₅, the switch S_Sis closed again, the switch S₁is reopened, the sampling capacitor C_Sis discharged, and there is, once again, no signal at the output of the operational amplifier 51. Said coupling capacitor C₃then stores the input signal U_E,7, i.e. the signal S_2,1.

[0112] At time t₁₆, the switch S_Sis reopened, the switch S₂is closed again, and an input signal U_E,8is applied to the input of the buffer 50. The input signal U_E,8, in the exemplary embodiment, is the signal S_2,2read out from detector element 6hof the second linear detector array 5b. The signal at the output from the operational amplifier 51 thus is the difference signal U_A,8=−(U_E,3−U_E,8)=−S*_2,2. The difference former 42 shown in FIG. 5 thus, in the exemplary embodiment, is able to form the difference signals between signals from pairs of adjacent read-out detector elements 6athrough 6xof a detector column 25athrough 25f.

[0113] If an appropriate number of coupling capacitors is employed and the input signals are suitable, the difference former 70 shown in FIG. 7 also is suitable for forming the difference signals between signals from pairs of adjacent read-out detector elements 6athrough 6xof the linear detector array 5athrough 5d.

[0114] The difference former 34 shown in FIG. 3 has several inputs and outputs. The difference former 34, for example, can be designed such that it is formed of a number of difference formers shown in FIG. 7, the number corresponding to the number of inputs and outputs.

[0115] The radiation detector 4 shown in FIGS. 1 through 4 may have more or fewer detector elements than the detector elements 6athrough 6xshown in FIGS. 1 through 4, and/or more or fewer linear detector arrays than the linear detector arrays 5athrough 5d, and/or more or fewer detector columns than the detector columns 25athrough 25f.

[0116] It is also possible to employ a radiation detector with several detector modules having at least one linear detector array, each detector module being assigned an analog/digital converter. The difference signals may then be acquired, in accordance with the above-described methods, column by column and/or row by row for each detector module.

[0117] It is also possible to form higher-order difference signals, these being understood to be, for example, row-by-row difference signals of difference signals from pairs of adjacent read-out detector elements of a detector column.

[0118] The methods described above also may be employed in computed tomography systems equipped with a radiation detector formed by a single-line radiation detector, in which case the difference signals involved will be formed from signals read out from adjacent detector elements of the single-line radiation detector.

[0119] The data length of one byte for the difference signals should also be understood as being merely an example.

[0120] The object to be investigated need not necessarily be a human patient, as suggested in FIG. 1.

[0121] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.