Method for synchronising an emitter and a detector in a computed tomography scanner

In a computed tomography scanner provided with an emitter for emitting a beam of radiation through an object to be analyzed and a detector for acquiring radiographies of the object, the detector generates a synchronization signal corresponding to its state of activation for acquiring the radiographies, and the emitter is controlled via the synchronization signal in such a way that the beam of radiation will be emitted when the detector is already activated.

The present invention relates to a method for synchronising an emitter and a detector in a computed tomography scanner.

In particular, the present invention finds advantageous, but not exclusive, application in computed tomography scanners used in the sector of dentistry, to which the ensuing description will make explicit reference, without this implying any loss of generality.

BACKGROUND OF THE INVENTION

In the sector of dentistry computed tomography scanners are used, in the present state of the art, of the type comprising an x-ray source-detector assembly designed to rotate about an area of analysis in which the head of a patient is positioned for acquiring volumetric tomographic data of one or both of the dental arches of the patient. The source-detector assembly comprises a rotating support, typically constituted by an arm that is motor-driven so as to rotate about a horizontal axis traversing said area of analysis, an x-ray emitter, mounted on a first end of the arm for emitting an x-ray beam through the area of analysis, and an x-ray detector, mounted on the opposite end of the arm and facing the emitter for receiving the beam after it has traversed the area of analysis.

The tomography scanner1further comprises a control unit, connected to the source-detector assembly for controlling emission and reception of the beam in a way synchronous with rotation of the arm, and a processing unit connected to the detector for receiving, storing, and processing the volumetric tomographic data so as to reconstruct images of the object.

In particular, the control unit generates a first signal for operating the detector for a given exposure time, and a second signal for operating the emitter so that it emits an x-ray beam only after the detector has been operated to prevent useless doses of x-rays from being administered to the patient. The two signals for operation, respectively, of the detector and of the emitter are generated starting from a single pre-defined synchronisation signal, generated by the control unit, for example as a function of the angular position of the arm.

The fact of having a number of units, i.e., the emitter and the detector, controlled according to a single synchronisation signal generated by a control unit, gives rise to latency times due to the propagation of the synchronisation signal along the connections between the control unit and the various controlled units, said latency times not always being predictable and being of the order of magnitude of the times involved, i.e., of the order of magnitude of a few microseconds or of some tens of microseconds. Said latency times generate synchronisation errors between the controlled units.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a method for synchronizing an emitter and a detector in a computed tomography scanner and to provide a detector for a computed tomography scanner and a computed tomography scanner implementing said method that will enable the drawback described above to be overcome and, at the same time, will be easy and inexpensive to implement.

According to the present invention a method for synchronising an emitter and a detector in a computed tomography scanner, a detector for a computed tomography scanner and a computed tomography scanner are provided as claimed in the attached claims.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1, designated as a whole by 1 is a computed tomography scanner of the type used in dentistry, comprising a frame2, a couch3for supporting a patient (not illustrated) lying down, and an x-ray source-detector assembly4, designed to rotate about an area of analysis5located in a region corresponding to a headrest6of the couch3for acquiring volumetric tomographic data of a part of the patient's head, for example a maxillofacial complex such as the dental arches, the mandibular bone, or else the maxillary bone of the patient, which hereinafter will be referred to as “object” for reasons of simplicity.

The source-detector assembly4comprises: a rotating support, constituted by an arm7mounted on the frame2and motor-driven so as to rotate about a substantially horizontal axis8of rotation traversing the region of analysis5; a position sensor7afor detecting the angular position of the arm7with respect to the axis8; an x-ray emitter9, mounted on a first end of the arm7and facing in the direction of the axis8for emitting a conical beam10of x-rays towards the area of analysis5; and an x-ray detector11, mounted on the opposite end of the arm7and facing in the direction of the axis8for receiving the beam10after it has traversed the area of analysis5and thus acquiring one or more radiographies of the object per unit angle of rotation.

The tomography scanner1further comprises a control unit12, connected to the source-detector assembly4for controlling emission and reception of the beam10in a way synchronous with rotation of the arm7, and a processing unit13, connected to the detector11for receiving, storing, and processing the volumetric tomographic data so as to reconstruct images of the object and to the control unit12for activating the source-detector assembly4on the basis of commands imparted by an operator or of instructions with which the processing unit13itself is programmed. The processing unit13is constituted, for example, by a personal computer provided with a monitor14for displaying the reconstructed images, a keyboard15, and a mouse16for acquiring data supplied and/or commands imparted by the operator.

With reference toFIG. 2, the detector11has a first output11a, designed to supply a synchronisation signal SYNC for controlling activation of the emitter9through a format conversion performed by the control unit12. In other words, the emitter9is operated on the basis of the synchronisation signal SYNC. In particular, the control unit12implements a conversion block17for carrying out a conversion of the format of the synchronisation signal SYNC, said conversion being substantially transparent from the standpoint of the information content and producing a corresponding signal, designated by SYNC′ inFIG. 2, which can be used directly by the emitter9. From the functional standpoint, then, the emitter9is controlled directly by the detector11via the synchronisation signal SYNC.

The detector11moreover has a second output11b, connected to the processing unit13for supply of the radiographies acquired, designated by DATA, to the processing unit13itself.

The synchronisation signal SYNC is constituted by a waveform that informs the emitter9of the periods of time in which the detector11is ready for acquiring radiographies. In other words, the detector (11) generates the synchronisation signal (SYNC), which corresponds to its state of activation for acquiring the radiographies, and the emitter (9) is controlled via the synchronisation signal (SYNC) in such a way that the beam of radiation (10) will be emitted when the detector (11) is already activated.

With reference toFIG. 3, the synchronisation signal SYNC comprises a succession of activation pulses30, during which the detector11is activated, i.e. it is ready for acquiring radiographies, in alternation with idle or de-activation periods31, during which the detector11does not acquire any radiographies in so far as it transfers the radiographies acquired (DATA) to the processing unit13. In other words, the detector11is operated for acquiring radiographies on the basis of synchronisation signal SYNC generated by the detector11itself. The activation pulses30each have a duration equal to a given exposure interval TEX characteristic of the detector11. The duration of the exposure interval TEX is optionally configurable via the processing unit13. The detector11is configured for continuous supply of the synchronisation signal SYNC as soon as it is electrically supplied.

The information content of the signal SYNC′ is substantially the same as that of the synchronisation signal SYNC in the sense that the signal SYNC′ comprises a succession of pulses (not illustrated) designed to activate the emitter9, each pulse being synchronised with the rising edge of a corresponding activation pulse30generated by the detector11.

The emitter9is configured for emitting the pulsed beam10; in particular, the emitter9emits an x-ray pulse32as soon as it receives, via the signal SYNC′, the rising edge of an activation pulse30, i.e., after it has learned that the detector11is ready for acquiring a radiography. Consequently, the detector11functions as master, and the emitter9functions as slave. The x-ray pulse32has a duration of emission TEM that is established as a function of the dosage of x-rays required for acquiring the individual radiography.FIG. 3illustrates the case where the duration of emission TEM is shorter than or equal to the exposure interval TEX so that the emitter9emits an x-ray pulse32at each activation pulse30comprised in a sub-succession of activation pulses30defined by the control unit12on the basis of instructions supplied by the processing unit13.

FIG. 4illustrates, instead, the case where the duration of emission TEM is greater than the exposure time TEX. In these conditions, the emitter9is configured for emitting a beam10that is continuous in time, i.e., the emitter9remains turned on by a first usable activation pulse30, defined by the control unit12, until a time-out TOUT, defined as a function of the exposure interval TEX and of a total number of radiographies to be acquired established for analysis of the object. In this way, useless transients of turning-on and turning-off of the emitter9are prevented during the de-activation periods31.

With reference once again toFIG. 2, the control unit12implements: a command block18, designed to supply a command signal SR for issuing a command for rotation of the arm7, and a number of radiographies NR per unit angle of rotation, said signal SR and number of radiographies NR being obtained by the command block18according to instructions received from the processing unit13; and a verification block19, which receives the synchronisation signal SYNC, the number of radiographies NR, and a position signal SP supplied by the position sensor7a, and supplies at output an error signal ERR, which informs whether the current number of radiographies acquired is correct. In particular, the verification block19is configured for counting the number of radiographies acquired, starting from the synchronisation signal SYNC and compares said number with a value obtained as a function of the number of radiographies NR and of the position signal SP.

The control unit12moreover implements an inhibition block20, designed to inhibit, on the basis of the error signal ERR, propagation of the synchronisation signal SYNC to the emitter9in the case where the current number of radiographies acquired is not correct. In this way, emission of useless doses of x-rays is blocked when the source-detector assembly4does not function correctly.

Operation of the source-detector assembly4of the tomography scanner1described above does not require further explanations in so far as it results clearly from the foregoing description.

From the above description, it is moreover clear that the method for synchronisation between the emitter9and the detector11of a tomography scanner1according to the present invention is applicable to any type of tomography scanner and not only to a tomography scanner for use in dentistry, and for acquiring radiographies of any part of the human body or of any object of biological or non-biological matter. In fact, the method is completely independent of the type of mechanical structure of the tomography scanner1and of the technology of emission and detection of radiation used in the emitter9and in the detector11, respectively.

The main advantage of the method for synchronisation between emitter and receiver of a tomography scanner described above is that of not requiring generation of a synchronisation signal by an external unit, said generation being a source of frequent synchronisation errors that lead to errors of acquisition and hence administration of useless doses of x-rays to the patient. In addition, the detector11according to the present invention can be simply integrated in any tomography scanner1with minimum modifications of the electronics of the control unit12.