Abstract:
To acquire tomographs, two detectors ( 1, 2 ) are secured together in a support structure ( 3 ) in such a manner that their detection fields are inclined relative to each other. The angle of inclination between them is preferably 90°. This pair of detectors is then placed in such a manner that the plane ( 9 ) bisecting the two detection fields includes the axis of rotation ( 4 ) around which the pair of detectors is to rotate to perform tomography on a subject. For fat subjects, the pair is moved away ( 13 ) from the axis, and for thin subjects it is moved towards it. It is shown that to avoid detector displacement interfering with tomography computation, it suffices merely to change computation parameters in the reconstruction algorithm.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to tomograph acquisition apparatus usable, in particular, in the medical field. It relates essentially to acquiring tomographs with scintigraphy detectors that are easy to use. 
     The principles of scintigraphy detectors is known in nuclear medicine. They are as follows. A radioactive marker, generally technetium, is injected into a patient. As a function of its nature, the marker is distributed from its point of injection into various portions of the patient&#39;s body. In the patient&#39;s body, the marker is to be found in the organ under investigation and it reveals the function thereof. The marker produces gamma photons. After passing through a collimator, the gamma photons are detected by a scintillator crystal, whence the term “scintigraphy detector”. The crystal transforms the gamma photons into light photons. The light photons are in turn detected by photomultiplier tubes placed looking at the scintillator. The currents that flow through the photomultiplier tubes are in the form of pulses and are a function of the magnitude of the scintillation produced. These currents are applied to a resistance matrix. The resistance matrix outputs “locating” pulses that specify the position at which scintillation took place facing the photomultiplier tubes. 
     A counter unit connected to the output from the resistance matrix serves to sum the number of such pulses occurring at each location on the scintillator. It is then possible to create an image representative of the activity of the marker in the body by attributing brightness to each image point as a function of the number of strikes that have been counted for each of said locations. Such a method is known in the state of the art under the name “Anger” method. With very fast scintigraphy cameras, e.g. capable of counting up to 200,000 strikes per second, an image constituting a projection of a portion of the human body can be generated in about 30 seconds. 
     The detector is mounted in a rotary assembly called a gamma camera which also serves to aim the detector. If the detector is aimed in different directions relative to the body, multiple images can be acquired under the same conditions. By acquiring a sufficient number of images for different aiming directions of the detector, the set of image signals can be subjected to processing suitable for obtaining tomographs of the body by algorithmic reconstruction. Given that the accuracy of such tomographic images increases with the number of projection images, it can be seen that such a method leads to periods of examination that are relatively long. 
     Proposals have already been made to remedy this problem by constructing gamma cameras provided with two, three, or even more detectors. Under such circumstances, the duration of an examination is reduced, substantially pro rata the number of detectors. 
     However, another problem arises. To obtain projection images, and consequently tomographs, that have very good resolution, it is necessary for the detectors to be placed as close as possible to the body. Unfortunately, patients to be examined are not all the same size, some are fat, others are thin. In addition, depending on the examination being performed, it may be necessary for the detectors to be at various different distances from the body. For example, an examination around the belly requires the detectors to be at a different distance from the patient than an examination around the head, since the diameter of the head is smaller. A known way of solving this problem is to mount the detectors on telescopic arms and to move the detectors initially as close as possible to the patient. During an examination, the detector travels around a circle whose diameter depends on said distance from the patient. A device of this kind is for example depicted in U.S. Pat. No. 4,368,389. 
     The drawback of such a telescopic mechanism is that it is twice as complex when there are two detectors instead of only one, and so on. The telescopic mechanism is itself mounted on a rotary assembly enabling the detectors to be pointed in different image-taking directions. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to remedy these drawbacks firstly by taking account of the need to place more than one detector on the rotary assembly in order to accelerate acquisition, and secondly to simplify the handling of the various detectors. To solve these problems, the invention begins by securing the two detectors to each other and also by giving them a certain angle of inclination relative to each other. 
     Thus, any one detector has a substantially plane detection surface constituting its detection field. At present, such detection fields are rectangular in shape. They have a length and they have a width. In the invention, two detectors are placed against each other so that the normals to the centres of their detection fields intersect, and so that these detection fields are adjacent to each other along one side of each of them, which sides are called “lengths”. In the following explanation, the adjacent sides are called lengths, but that does not mean that the detection field is necessarily longer in that direction than it is along a direction at rightangles. 
     It is preferable for the normals to intersect at an angle of 90°. However, this is not essential and the angle could be acute or obtuse. The two detectors are thus secured to each other in this configuration in such a manner that the bisector plane including the point of intersection of the normals and the adjacent lengths of the two detectors also contains the axis of rotation of the tomography. 
     With small patients, the assembly is displaced radially towards the axis of rotation. The corner formed by the two detectors can thus be moved closer to a small patient on the axis of the machine. Alternatively the corner can be moved further away when examining a fat patient, likewise on the axis of the machine. This simplifies the displacement mechanics. 
     However, by acting in this way, the information acquired during projection is not properly situated since the axis of rotation of the machine does not necessarily pass through the point of intersection of the normals to the detection fields. To be able to use the same reconstruction algorithms, it is therefore necessary to transform the image signal processing parameters as a function of the distance between the pair of detectors and the axis of rotation. It is shown that exactly the same algorithms can be used to reconstitute tomography images, while also obtaining a substantial saving on the mechanical equipment which is much simpler. 
     The invention thus provides an apparatus for acquiring tomographs of a subject, the apparatus comprising a pair of plane scintillation detectors carried by a support rotating about an axis of rotation, and connected to an image processor, the apparatus being characterised in that: 
     the detection fields of these two detectors are inclined at an angle relative to each other; 
     the bisector plane bisecting the angle formed between these detection fields includes the axis of rotation; 
     and in that the apparatus includes: 
     means for displacing the pair of detectors together relative to the subject in a direction which is radial to the axis of rotation; and 
     modification means for modifying an effective detection field of these detectors as a function of said displacement. 
     The invention will be better understood on reading the following description and examining the accompanying drawings. The drawings are given merely by way of indication and do not limit the invention in any way. In the drawings: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of acquisition apparatus of the invention; 
     FIG. 2 is a section through the mechanism for rotating the detectors and moving them relative to the axis of rotation; 
     FIG. 3 is a perspective diagram of a mechanical detail enabling the detectors and their counterweights to be moved simultaneously; 
     FIG. 4 is a block diagram of the set of means implemented by the invention; and 
     FIGS. 5 a  to  5   c  are diagrams of the diameters that can be fully reconstructed for three different sizes of patient. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows tomography acquisition apparatus of the invention. It includes a pair of plane detectors  1  and  2  carried together by a common support  3 . The support  3  rotates about an axis of rotation  4 . The axis of rotation lies slightly above the top surface  5  of the bed  6  that carries a patient being examined by the machine. By acting in this way, the axis  4  is caused to lie substantially in the middle of the patient. Both of the detectors  1  and  2  are connected to an image processing apparatus that is explained below. The detection fields  7  and  8  of the detectors  1  and  2  respectively are at an angle relative to each other. Their adjacent edges, named length, are spaced apart from each other, in one example with a 2 cm space, but preferably this space is less that 8 or 10 cm. 
     In the example shown they are inclined at an angle of substantially 90° to each other and their respective centre-of-field normals  9  and  10  intersect at a right angle. A bisector plane  11  contains the bisector of the angle formed by the detection fields  7  and  8  and also contains the axis of rotation  4 . The set of two detectors  1  and  2  is rotatable about the axis of rotation  4  by rotating a ring  32  carried by a circular cover  12 . According to an essential characteristic of the invention, the set  3  of detectors  1  and  2  is displaceable radially as shown by arrow  13  contained in the plane  11  and intersecting the axis  4 . 
     The assembly  3  is therefore preferably displaceable radially relative to the cover  12 . In which case the bed  6  includes lift means  14  for placing its top surface  5  at an appropriate height, together with a translation motor for placing the portion of the patient to be investigated level with the pair of detectors  1  and  2 . These translation movements take place along arrows  15  in a direction referred to as the length direction of the detection fields  7  and  8  of the detectors  1  and  2  respectively. This is the preferred solution. However, the invention produces exactly the same results if the bed  6  is provided with means for displacing its top surface horizontally in a direction  16  perpendicular to the translation direction  15  and to the lift direction  14 . It can be shown that displacements along arrows  16  in combination with displacements along arrow  14  are equivalent to displacing the set  3  of detectors radially along arrow  13 . The preferred solution is described below, but it should not be forgotten that the other solution is also possible. FIG. 1 also shows a swivel-mounted display console  17  having a handle  18  enabling an operator to observe, inter alia, various machine settings. 
     FIG. 2 shows the means for displacing the set of two detectors  1  and  2  relative to a patient to be examined in a direction  13  which extends radially relative to the axis of rotation  4 . This figure shows the set  3  of detectors  1  and  2 . It also shows the outline of the cover  12 . The cover  12  is fixed to a pedestal  19 . Also supported by the pedestal  19  is a cylinder  20  of thickness e and coaxial with the cover. This cylinder is held behind the figure by the back of the cover  12  and it projects towards the observer perpendicularly to the plane of the FIG.  2 . Near its front end, this cylinder has a circular groove  21 , or optionally a projecting rib. This rib or groove  21  guides a certain number of running wheels such as  23  to  31  whose axles are held by the plane ring  32 . The plane of the plane ring is perpendicular to the axis of the cylinder  20 - 21 . The plane of the ring  32  lies in the plane of FIG.  2 . The wheels retain the ring  32 . 
     A toothed groove  33  at the periphery of the ring  32  receives a drive chain  34  driven by a motor  35 . The chain  34  runs round the ring  32 , passes over two driving sprockets  36  and  37 , and engages a control sprocket  38 . The driving sprockets are driven by the motor  35 . The two driving sprockets  36  and  37  are mounted on a plate  39  which may be moved away from or towards a stand  40  which supports the sprocket  38 . Thus by acting on actuators such as  41  and  42  it is possible to ensure that the chain  34  is properly tensioned in the groove  33  on the ring  32 . When the motor  35  is caused to rotate, the ring  32  is thus driven around the axis  4 . 
     The ring  32  carries another motor  43  whose driving gear wheel  44  rotates a gear wheel  45  whose axis is fixed substantially perpendicularly relative to the plane of the ring  32  (and is thus parallel to the axis  4 ). The motor  43  is the motor that is used for moving the pair  3  of detectors  1  and  2  away from the axis  4 . To this end, the pair  3  of detectors is fixed to a T-shaped bar  46 . The two flanges of the T-shaped bar  46  are fixed to the edges of the pair  3 , e.g. by bolts  47  to  50 . The web  51  of the T-shape of the bar  46  lying in a plane perpendicular to the plane of the figure is provided on either side with respective racks  52  and  53 . These racks mesh with respective gear wheels  54  and  55  themselves driven by the gear wheel  45  via a return pulley  56  and a belt  57 . The belt  57  also passes between two studs  58  and  59  mounted eccentrically on a circular plate  60  for tensioning the belt  57 . When the circular plate  60  is rotated, the two studs  58  and  59  deform the path followed by the belt  56  and change its tension. 
     When the motor  43  rotates, the T-shaped bar  46  moves up or down in the direction of arrow  13  relative to the axis  4 . The pair  3  of detectors  1  and  2  is thus easily moved closer to a patient. 
     One of the problems to be solved with a mechanism of this kind is obtaining a corresponding displacement of a counter weight system  61  and  62 . Since the set of two detectors is relatively heavy, about  140  kg, it is important to balance it so that the tension exerted on the chain  34  is not excessive. 
     FIG. 3 shows a detail of the mechanism enabling the set  3  of detectors  1  and  2  to be moved synchronously with the system  61 - 62  of counterweights. FIG. 3 is a diagram showing the ring  32 , the top of the counterweight  61 , and the edge  63  of the pair  3  of detectors. The counterweights  61  and  62  are inside the cover  12 , while the detectors  1  and  2  are outside it. When the set  3  moves down in the direction of arrow  64 , the counterweights move symmetrically in the direction of arrow  65 . A device like that shown in FIG. 3 is to be found at opposite ends of a diameter of the ring  32 . Thus, there is a guide  66  fixed on each side of the pair  3  of detectors and a guide  67  fixed to the corresponding counterweight  61  (or  62 ). These guides are members of a generally channel-section shape. Each possesses a rack at the end of and perpendicular to one of the flanges of its channel-section shape, such as the racks  68  and  69 , respectively. Toothed shafts such as  70  and  71  engage in these racks. In practice, there are three toothed shafts in each mechanism. These toothed shafts pass through holes such as  72  formed in the ring  32 . When the ring is stationary, and when the set  3  of detectors is displaced using the motor  43 , the guide  66  is subjected to vertical motion of the same size. In this case, the rack  68  meshing with the shafts  70  and  71  causes them to rotate. These shafts cannot move down since they are held via respective ball bearings (not shown) inside the holes  72 . 
     By reaction, these shafts move the guide  67  symmetrically by engaging with the rack  69 . The ends of the shafts  70  or  71  are fitted with guide wheels such as  73  and  74  which are held in the guides  66  and  67  respectively, firstly by engaging rods such as  75  and secondly by being held against the rack  69  (or  68  as the case may be). By having three shafts such as  70  and by holding the two counterweights  61  and  62  together at their ends, it is possible to provide the overall assembly with good rigidity. However, as can be seen in FIG. 2, to stabilise motion better, the ends of the counterweights  61  and  62  may be provided with gear wheels meshing with two racks that are likewise caused to engage a T-section bar whose web extends perpendicularly to the ring  32 . 
     The motor  35  is the only motor that rotates the assembly, and similarly the motor  43  is the only motor that can be used to displace the detectors and to balance loads. The solution is therefore simple. In practice, the pair of detectors  1  and  2  is situated on one side of the ring  32  while the set of gear wheels linked to the motor  43  is situated inside the cover  12 . Nevertheless, other similar mechanical solutions could be devised, the essential point being that the detectors are displaced radially. 
     FIGS. 5 a  to  5   c  give an idea of the diameter of the part that can be reconstructed depending on whether the patient is fat or thin, respectively. In each of the figures the point of intersection of the axis  4  is shown, with the axis being in alignment with the middle of the patient&#39;s body. In FIG. 5 a , the patient is fat, the detectors therefore need to be moved away, and the edges  81  and  82  respectively of the fields of the detectors  1  and  2  define the circle of full reconstruction. In one example, this circle is shown as having a diameter of 150 mm. The same items in FIGS. 5 b  and  5   c  enable the diameters of the reconstructed space to increase with decreasing patient size. In the invention, in order to be able to apply the reconstruction algorithms to these diameters to be reconstructed, it is necessary to know both the effective width  83  of the field of view FOV and the position of the normal, e.g.  84  at effective width  83 . In the invention, the position of the normal  84  and the effective width  83  are determined by measuring the displacement of the pair  3  of detectors  1  and  2  relative to a standard position. 
     FIG. 4 shows how the means implemented for achieving the object of the invention are organised. Each detector, e.g. the detector  1 , is provided at its inlet face  7  with a collimator  85  lying over a scintillator  86 . The scintillations produced by the scintillator  86  excite the dinodes of an array  87  of photomultiplier tubes. The currents delivered by these photomultiplier tubes are applied to a resistance matrix  88  which delivers a set of locating pulses to a counter unit  89 . The counter unit is also under the control of an arithmetic and logic unit  90  via a bus  91 . The projection images are stored in an image memory  92  which may contain as many pages as there are different projections acquired. 
     During image processing, in order to generate one or more tomographs, a program memory  93  delivers instructions that are performed by the arithmetic and logic unit  90  on the image signals contained in the image memory  92 . The program memory normally includes a set of parameters contained in a parameter memory  94 . In particular, this parameter memory is shown as containing the width of the field of view and the position of the centre of rotation. 
     Normally, in state of the art apparatus, the centre of rotation corresponds to the intersection of the normals to the centres of the detection fields  7  and  8  of the detectors  1  and  2 . However, in the invention and because of the displacements, the position of the centre of rotation must be modified by an offset δ whose value is a function of the value d of the displacement of the pair  3  of detectors  1  and  2 . In the example where the detectors  1  and  2  point at substantially 90° to each other, the relationship between δ and d is of the type δ=d.2/2+constant. A similar trigonometrical relationship can be determined if the angle of inclination between the two detectors is other than 90°. However, instead of performing such calculations, a conversion table may be provided in the parameter memory enabling each value  95  of the distance d to be associated with a value  96  for the position of the centre of the effective field of view  99  and with a value  97  representing the width of the effective field of view  99 . These values  96  and  97  are then entered into the program contained in the program memory  93  so that the algorithm performed by the arithmetic and logic unit  90  remains the same. 
     The pair  3  can be moved along the arrow  13  by means of a keyboard  98  or by some other control member such as a mouse or a track ball. Under such circumstances, the motor  43  may also be controlled by the microprocessor contained in the arithmetic and logic unit  90 . In one example, a potentiometer  100  may be engaged with any one of the gears linked to the motor  43 . It is electrically connected firstly to a bias voltage and secondly to ground, with its cursor giving a voltage proportional to the displacement that is actually performed. This voltage can be used firstly to servo-control the position that is to be reached and secondly it can be used to evaluate the distance d. Alternatively, the motor  43  may be a stepper type motor and measurement by means of a potentiometer  100  can be eliminated merely by counting the number of steps applied to the motor  43 . The value of d is used in tables  95 - 97 . 
     By acting in this way, it is shown that the invention is easily implemented since there is no need to change the processing program that such machines already possess, while the mechanical simplification is manifest since there is only one displacement to move both heads simultaneously. 
     In an improvement, the pair  3  of detectors may be caused to describe an elliptical path. In which case, each angular position α of the ring  32  can be associated in advance with a distance d. This can be done in the same way as in the correspondence tables  95 - 96  or  95 - 97 . The distance d is then a function of angle α. 
     In the variant where the detectors are secured immobile on the ring  32  and where the bed only is fitted with means for displacement following the arrow  13 , these displacements must be carried out as a function of the position in rotation of the ring  32 . In this case, the angle α of the ring  32  is measured, for example using means similar to those required for measuring the rotation of the gear wheels linked to motor  43 . The angle α thus measured is then converted, using tables of the same type as tables  95 - 96  or  95 - 97 , into bed displacement instructions for following arrows  14  or  16 . It can easily be seen that the movement following  13  is broken down by trigonometrical functions of the first order (sinus or cosinus) into combined movements following  14  and  16 . For a given distance d, determined for instance by instructions using the keyboard  98 , displacements following  14  will be for example of the d sin α type, while those following  16  will be of the d cos α type. 
     The starting procedure is for example as follows. For α=0 (corresponding to the vertical detector  1  as in FIG. 1, for example), the bed is displaced following arrow  13  in response to instructions given by the keyboard  98  and according to a size of the patient. When the desired position of this bed is obtained, d is validated. The value of d is set during this validating operation, since it corresponds to a preferred nearing of the axis of the patient in the bisector plane of detectors  1  and  2 . The value of d is known since it corresponds to imposed and measurable movements of the bed following  14  and  16 , in relation to a central stopping position. During the following tomographic acquisition, and for each value of α, the motors of the bed are caused to displace the bed according to the functions indicated above. The axis of the patient thus describes a circular movement which is concentric to the ring  12 . 
     In a variant, the cover  12  and the ring  32  are not completely circular but form a C-shape leaving a gap through which a an examination bed can be moved closer laterally.