JP4686009B2 - X-ray computed tomography system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray computed tomography apparatus of a type that continuously rotates an annular rotary frame on which an X-ray tube, an X-ray detector, and the like are mounted in order to shorten a scan time.
[0002]
[Prior art]
As is well known, the above-described continuous high-speed rotation can be realized by employing a slip ring and a direct drive motor that perform power supply to the X-ray tube and signal transmission / reception with a metal ring and a conductive brush. In such continuous high-speed rotation, it is required to detect the rotational position of the X-ray tube with very high accuracy. For this purpose, a resolver has been introduced. Similar to the direct drive motor, the resolver is composed of a stator and a rotor, and a two-phase coil having a phase difference of 90 ° is arranged in the stator, and position detection is performed based on the phase of the voltage of the coil rotating with the rotor. In principle, when an excitation voltage is applied to each of the two-phase coils of the stator with a sine wave and a cosine wave, the induced voltage of the rotor coil varies depending on the rotational position θ, and this rotational position θ It is given by the voltage phase difference. Therefore, the ratio of the interval (actually the clock counter value) from the excitation voltage zero cross point to the induced voltage zero cross point with respect to 360 degrees is similar to the ratio of the counter position (clock frequency of the counter) / excitation frequency to the rotational position θ. Thus, by solving this, the rotational position θ can be obtained as an absolute value. This resolver has a characteristic that its accuracy can be increased relatively easily by increasing the number of poles.
[0003]
As described above, the resolver has an advantage that the rotational position can be obtained as an absolute value and high accuracy can be easily obtained.
[0004]
However, in order to employ such a resolver in an X-ray computed tomography apparatus, it is necessary to create a resolver having a large aperture of about 1000 mm in accordance with the imaging region into which the subject is inserted. It becomes expensive. Further, since the rotor coil signal is taken out via the slip ring, the frame must be disassembled at the time of failure, which requires much labor and time.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a rotational position detection that is small, inexpensive, and highly maintainable in an X-ray computed tomography apparatus.
[0006]
[Means for Solving the Problems]
The present invention relates to a fixed frame, a substantially circular rotating frame supported rotatably with respect to the fixed frame, an X-ray tube attached to the rotating frame, and an object emitted from the X-ray tube. An X-ray detector that detects transmitted X-rays, and a computer that reconstructs tomographic image data of the subject based on the output of the X-ray detector and the rotational position of the X-ray tube due to the rotation of the rotary frame In the X-ray computed tomography apparatus having a portion, a magnetoresistive sensor that detects the rotational position of the X-ray tube and the rotational drive control of the rotating frame based on the output waveform of the magnetoresistive sensor A first pulse generation circuit configured by an encoder that generates a first pulse train; and a cycle measurement circuit that measures a cycle of an output of the magnetoresistive sensor. According to the measured cycle, the tomographic image data Re A second pulse generating circuit for generating a second pulse train used for forming, characterized in that it comprises a.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the X-ray computed tomography apparatus according to the present invention will be described below with reference to the drawings. As is well known, in an X-ray computed tomography apparatus, an X-ray tube and an X-ray detector are rotated and rotated (ROTATE / ROTATE) generations in which the X-ray tube and the X-ray detector are rotated as one body. There are fixed / rotation (STATIONARY / ROTATE) generations in which only the X-ray tube rotates around the subject, and the present invention can be applied to any of these generations. Here, the rotation / rotation generation that currently occupies the mainstream will be described as an example. In addition, in order to reconstruct one tomographic image, projection data for about 360 degrees around the subject and 180 degrees + view angle (X-ray diffusion angle) is projected even by the half scan method. Data is needed. The present invention can be applied to any method. Here, a description will be given assuming that one piece of tomographic image data is reconstructed from the general projection data set of about 360 degrees.
[0008]
First, the basic configuration of the X-ray computed tomography apparatus will be described. An X-ray computed tomography apparatus is a computer system that is given a role of reconstructing tomographic image data by processing a scan main body of the large-scale structure that collects projection data and processing of the scan main body and projection data. It is divided into and. The computer system re-processes the tomographic image data based on the projection data corrected by the pre-processing unit and the pre-processing unit that perform pre-processing such as correcting non-uniform sensitivity between channels on the detected projection data. A reconstruction processor to be configured, a display for displaying the tomographic image data, an input device for inputting user instructions, a scan controller for controlling the scanning operation of the gantry, and the like are provided.
[0009]
FIG. 1 shows the internal structure of the gantry part by cutting out a part of the gantry part. FIG. 2 shows a schematic side view of the gantry diagram, and FIG. 3 shows a schematic front view of the gantry diagram. Further, FIG. 4 shows a cross-sectional view taken along the line AA of FIG. Reference numeral 10 denotes a substantially ring-shaped rotary frame, which is rotatably supported by a fixed frame 11 disposed substantially perpendicular to the stand 12. A permanent magnet and an annular magnetic rotor yoke 17 are attached to the rotating frame 10 at equal intervals along the circumference, and the rotating frame 10 is directly connected to the rotating frame 10 by a coil 18 attached to the fixed frame 11. The direct drive motor system is used.
[0010]
In the rotary frame 10, for example, an X-ray tube 13 of a type that emits X-rays in a cone beam shape and a multi-slice X-ray detector 14, for example, a subject P placed on a bed 16 are placed. It is mounted in a positional relationship facing each other. In addition, the rotary frame 10 includes an IV converter that converts a current signal from the X-ray detector 14 into a voltage, an integrator that periodically integrates the voltage signal, and an amplifier that amplifies the output signal of the integrator. A box 15 for storing a data acquisition system (generally called a DAS (data acquisition system)) including an analog / digital converter for converting the output signal of the preamplifier into a digital signal, and an X-ray tube 13 A slip ring function for power supply to and signal transmission / reception is provided in a balanced manner.
[0011]
In the present embodiment, the magnetoresistive method is adopted in order to provide a rotational position detection that is small, inexpensive, and highly maintainable. This method will be described below. As shown in FIG. 5, this magnetoresistive system has a magnetic detection gear 21 that is directly attached to the rotary frame 10 and a direction in which the detection surface faces a predetermined distance G from the tooth tip of the detection gear 21. This is realized by the magnetoresistive sensor 22 attached to the fixed frame 11. The detection gear 21 has a large diameter tip circle diameter, and teeth 23 of the same shape are regularly formed on the outer periphery thereof at a constant interval pitch. Note that the detection gear 21 may be provided on the bed 16 side as long as the detection gear 21 rotates with the rotary frame 10. Alternatively, the detection gear 21 may be attached to the fixed frame 11 and the magnetoresistive sensor 22 may be attached to the rotating frame 10.
[0012]
The tooth shape of the detection gear 21 is preferably involute, and the type may be any of a helical tooth, a tooth, a flat tooth, and a bevel as shown in FIG. In particular, helical teeth have an effect of reducing high-frequency magnetic flux and can be said to be preferable from the viewpoint of improving sensor accuracy. A plurality of slits 26 are formed on the side surface of the detection gear 21 along the circumferential direction as shown in FIG. Alternatively, a plurality of grooves 27 are radially formed on the side surface of the detection gear 21 in a direction parallel to the radial direction or slightly inclined with respect to the radial direction as shown in FIG. When the detection gear 21 rotates with the rotary frame 10, the air (fluid) inside the gantry flows through the slits 26 and grooves 27 formed on the side surfaces of the detection gear 21. Thereby, the cooling effect of the direct drive motor or the X-ray tube 13 can be expected. Furthermore, in order to improve this cooling effect, as shown in FIG. 7D or FIG. 7E which is a DD cross-sectional view of FIG. 7C and FIG. You may make it incline the end surface of this with respect to a rotating shaft.
[0013]
When the detection gear 21 rotates with the rotation of the rotary frame 10, the magnetic flux density fluctuates in the vicinity of the magnetoresistive sensor 22 attached to the fixed frame 11 due to the irregularities on the outer periphery thereof, and this fluctuation causes the magnetoresistive type to change. The output Vout of the sensor 22 changes. In order to magnetically shield the magnetoresistive sensor 22 against the permanent magnet and the coil 18 attached to the rotor yoke 17 of the direct drive motor, a magnetic shield plate is provided between the coil 18 and the magnetoresistive sensor 22. 27 is arranged. The magnetic shield plate 27 may be attached to the rotary frame 10, but is preferably attached to the fixed frame 11 together with the magnetoresistive sensor 22.
[0014]
As shown in FIGS. 8 and 9, the magnetoresistive sensor 22 has a small (or large) resistance value when the magnetic flux density is sparse, and conversely, a large (or small) resistance value when the magnetic flux density is dense. A pair of magnetoresistive elements MR1 and MR2 having properties are provided. The pair of magnetoresistive elements MR1 and MR2 are arranged along the moving direction of the detection gear 21, that is, the direction orthogonal to the rotation axis, and a permanent magnet 24 for applying a magnetic bias is disposed behind the pair of magnetoresistive elements MR1 and MR2. Yes. The pair of magnetoresistive elements MR1 and MR2 are connected in series between the DC power supply E and the ground GND, and the sensor output is drawn from the middle point M thereof.
[0015]
Here, as shown in FIG. 10, when the magnetic teeth (convex portions) 23 face the magnetoresistive elements MR1 and MR2, the magnetic flux density increases and the resistance value increases. Conversely, when the magnetic tooth groove (concave portion) 28 faces the magnetoresistive elements MR1 and MR2, the magnetic flux density is conversely lowered and the resistance value is reduced. When the teeth 23 of the detection gear 21 pass one after the other immediately before the magnetoresistive elements MR1 and MR2, the potential Vout of the midpoint M of the magnetoresistive elements MR1 and MR2 varies with the variation of the resistance value. Specifically, when the resistance of the magnetoresistive element MR1 is R1, the resistance of the other magnetoresistive element MR2 is R2, and the direct current is E, the potential Vout at the midpoint M is
Vout = E × R2 / (R1 + R2)
Given in. When the rotary frame 10 rotates and the teeth 23 come close to the magnetoresistive element MR1 as shown in FIG. 10A, the resistance value R1 of the magnetoresistive element MR1 becomes smaller and R1 <R2. Further, when the tooth 23 comes directly below the middle point M as shown in FIG. 10B, the resistance values R1 and R2 of both become equal, and as shown in FIG. 10C, the tooth 23 becomes the magnetoresistive element MR1. , The resistance value R2 of the magnetoresistive element MR2 becomes smaller and R2 <R1. Therefore, the midpoint potential Vout changes in a substantially sine wave as the tooth 23 approaches, passes, and repeats. By detecting the passage of the tooth 23 from the time change of the potential Vout, the change of the rotational position can be recognized.
[0016]
The detection accuracy of the rotational position basically depends on the machining accuracy of the teeth 23 of the detection gear 21, but the other important factor is the distortion of the output waveform of the sensor 22. Further, the distortion of the output waveform of the latter sensor 22 is determined mainly depending on the shape of the tip of the tooth 23. Here, the tooth tip shape that reduces the distortion of the output waveform of the sensor 22 will be considered. FIG. 11 shows dimensional parameters of the teeth 23. The tooth tip shape is defined by tooth width r / single pitch λ. Although details will be described later, in the present embodiment, the zero-cross point of the output waveform of the sensor 22 is detected and a pulse is output based on that point, and the distortion of the output waveform of the sensor 22 is small. If this is the case, the magnitude of the error between the theoretical pulse width and the measured pulse width (hereinafter referred to as pulse accuracy) is inversely proportional.
[0017]
In order to obtain a tooth tip shape suitable for reducing distortion of the output waveform of the sensor 22, (tooth width r / tooth gap m) is made smaller than a predetermined value, and (tooth width r / single pitch λ). ) Within the predetermined range. Further, the gap between the tooth tip of the detection gear 21 and the detection surface of the magnetoresistive sensor 22 is also set within a predetermined range in order to obtain the necessary pulse accuracy even with this tooth profile.
[0018]
By designing the tooth tip under such conditions and keeping the gap within a predetermined range, the pulse accuracy could be improved until the specification was satisfied.
[0019]
Next, a pulse generation circuit that outputs a pulse train based on the output waveform of the magnetoresistive sensor 22 will be described. FIG. 12 shows a configuration of a pulse generation circuit that outputs a pulse train in accordance with fluctuations in the output waveform of the magnetoresistive sensor 22. Here, the number of teeth 23 of the detection gear 21 will be described as 432. The output of the magnetoresistive sensor 22 is distributed to two systems of an encoder (first pulse generation circuit) 31 and a pulse generation circuit (second pulse generation circuit) 32. The former encoder 31 generates pulses at the same frequency based on the peak value of the output waveform of the magnetoresistive sensor 22, and accordingly, 432 pulses are generated per rotation. The encoder 31 has a characteristic that the output from the magnetoresistive sensor 22 to the pulse output is almost in real time, but the pulse accuracy (rotational position measurement accuracy) has a dependency on the waveform distortion. It is effective to use it for feedback control to a direct drive motor.
[0020]
On the other hand, unlike the encoder 31, the latter pulse generation circuit 32 has no dependency on the waveform distortion, that is, the pulse accuracy is high, but there is a slight difference between the output of the magnetoresistive sensor 22 and the pulse output. It has the characteristic that time delay occurs. This characteristic is effective when used for image reconstruction.
[0021]
13 shows the configuration of the pulse generation circuit 32, and FIG. 14 shows a waveform diagram for explaining the operation of the pulse generation circuit 32. As shown in FIG. First, the output signal (Vout) from the magnetoresistive sensor 22 periodically varies every time the individual tooth 23 passes through the magnetoresistive sensor 22. That is, the output signal (Vout) periodically amplitudes at a frequency that is the reciprocal of one rotation time / the number of teeth. The waveform of the output signal basically has a substantially sine wave shape as described above. The output signal is first taken into the zero cross point detection comparator 41. The zero cross point detection comparator 41 detects the zero cross point ZC of the output signal.
[0022]
In the time measurement timer circuit 42, the time from the previous zero-cross point ZC detected by the zero-cross point detection comparator 41 to the current zero-cross point ZC, that is, each cycle of the output signal from the magnetoresistive sensor 22, is converted into a clock circuit. For example, a clock of 12 MHz from 46 is measured as a minimum unit. Actually, it is measured as the number of clock pulses CY included in the cycle. The measured period information of the output signal from the magnetoresistive sensor 22 is divided by the measurement time division circuit 43 by a preset divisor (4 · n) (CY / (4 · n)). Note that n is a multiplication factor of the frequency of the output pulse of the pulse generation counter circuit 45 with respect to the frequency of the output signal from the magnetoresistive sensor 22, and in this example, n = 25. This value is arbitrary and can be freely set and changed by the operator. This division result (CY / (4 · n)) is sent to the pulse generation counter circuit 45 via the delay circuit 44. The delay time of the delay circuit 44 is provided to fix the time delay from the signal output from the magnetoresistive sensor 22 to the pulse output from the pulse generation counter circuit 45 to a predetermined time, in this example, two cycles. (Preferably at least 2 cycles before).
[0023]
The pulse generation counter circuit 45 is a two-phase pulse of phase A and phase B having a frequency n times the frequency of the output signal from the magnetoresistive sensor 22 (the phase of the phase B pulse is shifted by 90 ° relative to the phase A pulse). Output). Specifically, as shown in FIG. 15, in the pulse generation counter circuit 45, the current count value of the clock circuit 46 and the previous count value of the clock circuit 46 are compared with the division result (CY / (4 · n)). When the difference (Cn-Cn-1) coincides, in this case, the B phase pulse falls. In the next period, the A phase pulse is started. In this way, the two-phase pulses are alternately lowered / rise according to (CY / (4 · n)). Such an operation is repeated (4 · n) times while maintaining the increment of the count value of the clock circuit 46, and the periodic operation is completed. At this time, the count value of the clock circuit 46 is reset, and the same operation is performed in the next cycle based on the measurement cycle.
[0024]
In this way, the period of the output signal from the magnetoresistive sensor 22 is measured, and a pulse is generated with the period divided by (4 · n). Here, the high and low states of the two-phase pulse are alternately switched. If the working accuracy of the detection gear 21 is high with respect to the desired accuracy, the rotational position can be measured with very high accuracy. This high-accuracy pulse is associated with projection data two cycles before in the reconstruction processor, and is used for reconstruction of tomographic image data.
[0025]
The high-accuracy pulse has a delay time of, for example, two periods with respect to the movement of the gear 21 because the period measurement is essential. Since the inertia (inertia) is very large, it can be said that the difference between the previous cycle and the current cycle is close to zero. Therefore, even if it is used for the feedback control of the motor as a real time one, it can be said that substantially no problem occurs.
[0026]
Since the pulse generation circuit 32 measures the rotational position from the period of the output signal from the magnetoresistive sensor 22 in this way, even if the output waveform from the magnetoresistive sensor 22 is distorted, it is almost affected. The rotational position can be detected with very high accuracy.
[0027]
(Modification)
The magnetoresistive sensor 22 described above can be replaced with an optical sensor 50 as shown in FIG. The optical sensor 50 includes a light source 52 that irradiates light to the detection gear 21 and a photodetector 51 that receives reflected light from the detection gear 21 and outputs an electrical signal having an amplitude corresponding to the received light intensity. When the teeth 23 face the optical sensor 50, most of the reflected light is incident on the photodetector 51. On the other hand, when the tooth gap 28 faces the optical sensor 50, the reflected light is not so much. It does not enter. For this reason, when the detection gear 21 rotates in front of the optical sensor 50 along with the rotating frame 10, the output signal of the optical sensor 50 is similar to the case of the magnetoresistive sensor 22, the approach of the individual teeth 23, Amplifies with the opposite and passing. Therefore, the output signal of the optical sensor 50 can be processed in the same manner by the pulse generation circuit.
[0028]
In this case, in order to increase the amplitude of the output signal of the optical sensor 50 and improve the pulse accuracy, the surface of the tooth 23 of the detection gear 21 is coated with a highly reflective material 53 and detected as shown in FIG. The convex part of the gear 21 is made to have a higher reflectance than the concave part, or the bottom surface and the side surface of the tooth groove 27 are coated with the low reflective material 54 so that the concave part of the detection gear 21 has a lower reflectance than the convex part. It may be made to become. Of course, the part and the reflectance are not limited to this combination, and the reflectance may be different between the convex portion and the concave portion.
[0029]
Furthermore, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. Furthermore, the above embodiment includes various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, some constituent requirements may be deleted from all the constituent requirements shown in the embodiment.
[0030]
【The invention's effect】
According to the present invention, since a sensor for detecting the rotational position of the X-ray tube is provided on the fixed frame, and the magnetoresistive type is adopted for the sensor, the rotation is small, inexpensive and highly maintainable. Position detection can be provided.
[Brief description of the drawings]
FIG. 1 is a cutaway view showing an internal structure of a gantry of an X-ray computed tomography apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic side view of the gantry of FIG. 1;
FIG. 3 is a schematic front view of the gantry of FIG. 1;
4 is a cross-sectional view taken along line AA in FIG.
FIG. 5 is a schematic diagram showing a mounting position of the detection gear of FIG. 1;
6 is a schematic diagram showing a gear shape of the detection gear of FIG. 1;
7 is a schematic side view of the detection gear of FIG. 1;
8 is a schematic diagram of the structure of the magnetoresistive sensor of FIG.
FIG. 9 is an equivalent circuit diagram of the magnetoresistive sensor of FIG.
FIG. 10 is a diagram illustrating the principle by which the magnetoresistive sensor of FIG. 8 detects irregularities of a detection gear.
11 is a diagram showing a tooth tip shape of the detection gear of FIG. 1;
12 is a configuration diagram of a pulse generation circuit that processes the output of the magnetoresistive sensor of FIG. 1;
13 is a block diagram of the pulse generation circuit of FIG.
14 is an operation explanatory diagram of the pulse generation circuit of FIG. 12;
FIG. 15 is a detailed view of a pulse generation counter output waveform of FIG. 14;
FIG. 16 is a diagram showing a modification of the present embodiment.
FIG. 17 is a diagram showing another modification of the present embodiment.
[Explanation of symbols]
10 ... rotating frame,
11 ... fixed frame,
12 ... stand,
13 ... X-ray tube,
14 ... X-ray detector,
15 ... data collection system,
16 ... Sleeper,
17 ... Rotor yoke,
18 ... Coil,
21: Detection gear.

Claims (12)

A fixed frame, a substantially circular rotary frame rotatably supported by the fixed frame, an X-ray tube attached to the rotary frame, and X-rays emitted from the X-ray tube and transmitted through the subject are detected. X-ray detector, and a computer unit for reconstructing tomographic image data of the subject based on the output of the X-ray detector and the rotational position of the X-ray tube due to the rotation of the rotary frame In computed tomography equipment,
A magnetoresistive sensor for detecting the rotational position of the X-ray tube;
A first pulse generation circuit configured by an encoder that generates a first pulse train used for rotational drive control of the rotary frame based on an output waveform of the magnetoresistive sensor;
A second pulse generation circuit for generating a second pulse train to be used for reconstruction of the tomographic image data according to the measured period; and a period measurement circuit for measuring an output period of the magnetoresistive sensor;
An X-ray computed tomography apparatus comprising:   2. The period measuring circuit includes a comparator that detects a zero-cross point associated with output fluctuation of the magnetoresistive sensor, and a time measurement timer that measures the period from the detected zero-cross point. X-ray computed tomography apparatus. The second pulse generation circuit includes a measurement time division circuit for obtaining a time 1 / n of the measured period, and a pulse generation counter for generating a pulse with the obtained time as a period. The X-ray computed tomography apparatus according to claim 1 or 2. It said second pulse train is X-ray computed tomography apparatus according to any one of claims 1 to 3, characterized in that it has a frequency of n times with respect to the first pulse train. Wherein said first time delay relative to the pulse train of the second pulse train, X according to any one of claims 1 to 4 characterized in that it is a two cycles or more of the length of at least the first pulse train Line computed tomography equipment. A direct drive motor that directly rotates the rotary frame;
X-ray computer tomography apparatus according to any one of claims 1 to 5, characterized in that the magnetic shield plate is provided between the coil and the magneto-resistive sensor of the direct drive motor. A detection gear composed of a magnetic material substantially circular tubular having a circumferential having irregularities at predetermined intervals with attached to one of the previous SL fixed frame or the rotary frame,
The magnetoresistive sensor, the fixed frame or X-ray computed tomography according to any one of claims 1 to 6 together is attached to the other of the rotary frame, characterized the Turkey detect the unevenness apparatus.   The X-ray computed tomography apparatus according to claim 7, wherein a slit is formed on a side surface of the detection gear.   9. The X-ray computed tomography apparatus according to claim 8, wherein an end surface of the slit is formed obliquely with respect to the rotation axis.   The X-ray computed tomography apparatus according to claim 7, wherein grooves are formed radially on a side surface of the detection gear. Wherein the detection gear, X-rays computed tomography system according to any one of claims 7 to claim 10, characterized in that it has a toothed helical. Wherein the detection gear, X-rays computed tomography system according to any one of claims 7 to 10, characterized in that it is involute gear.

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