DE102012205290A1 - Method for artifact reduction in computed tomography recordings, involves calibrating counting rate directly before scanning of object, during which x-ray detector is radiated by variation of tube current of used X-ray tube - Google Patents

Abstract

The method involves calibrating the counting rate directly before the scanning of the object, during which an x-ray detector is radiated by variation of the tube current of a used X-ray tube. A computed tomography scan of the object is prepared, and the object is placed on a movable table. The object is calibrated by determining multiple counting rates for a detector element depending on different actually applied dose performances on the detector element. The object is placed in the scanning field. Independent claims are included for the following: (1) a computed tomography system with a controlling- and computing unit; and (2) a C-arm system with a program storage.

Description

The invention relates to a method for artifact reduction in computer tomographic images, which are reconstructed from detector data of a scan of an object, in particular of a patient, with a quantum-counting X-ray detector. Moreover, the invention also relates to a CT system with at least one emitter-detector system for scanning an object, in particular a patient, connected to at least one control and arithmetic unit, which has a program memory for computer programs.

One way to further reduce radiation exposure in clinical computed tomography is to use quantum counting detectors rather than the energy integrating detectors used today. The individual registration of individual X-ray quanta leads to an increase of contrasts and slightly reduced image noise - which allows a noticeable reduction of the patient dose. In addition, the energy selectivity of quantum-counting detectors enables the application of novel material identification methods, such as multi-spectral imaging or K-edge imaging, in everyday clinical practice.

Quantum counting detectors typically consist of a semiconductor sensor that serves as a direct converter for X-ray quanta. Due to their very good detection efficiency for the X-ray spectra used in CT, CdTe and CdZnTe are now considered as sensor materials of choice. In practice, the operation of CdTe or CdZnTe detectors proves to be difficult even at lower flows, ie above 106 MeV / (s · mm ² ). As confirmed by direct measurements, the intense irradiation leads to a millisecond change in the electric field in the detector material. The consequence of the altered electric field is a change in the signal characteristic, which ultimately leads to uncorrectable miscounts, commonly known as counting rate drift. This phenomenon persists even after the intensive irradiation has ended and sounds comparatively slow.

On the one hand, clinical CT imaging has a high dynamic range of approximately five orders of magnitude in terms of the count rate signal, and on the other hand, systematic measurement errors must be well below 1%. For this reason, even small miscounts lead to significant ring artifacts in the CT slice images.

It is therefore an object of the invention to find a method and a CT system in which the artefacts occurring during the reconstruction of computed tomographic images by the counting rate drift of the quantum counting detectors are reduced or completely avoided.

This object is solved by the features of the independent claims. Advantageous developments of the invention are the subject of the subordinate claims.

The inventor has recognized that it is possible to eliminate the negative effect of the counting rate drift of the detector during a measurement on the patient in that the detector is conditioned by an irradiation taking place immediately before the actual measurement, ie is transferred into a type of saturation range which are caused by the subsequent irradiation during the actual measurement no significant changes more. Accordingly, after the conditioning and before the actual measurement, calibration data can be determined, which can then be used to determine the correct dose during the scanning of the patient and show no drift.

Thus, since otherwise the electric field in the detector material does not correspond to the desired constant state, a so-called pre-scan is carried out immediately before a planned CT scan. This planned CT scan can be used to calibrate, adjust the system, or examine a patient.

As a pre-scan is the calibration using a dose rate bandwidth corresponding to the expected dose rate bandwidth in the sample to understand, with optional conditioning of the detector, ie an irradiation of the detector without in the beam path object, possibly with additional filtration of the X-ray beam may precede , For this purpose, the examination table of the CT scanner, possibly with a patient lying on it, is driven out of the beam path, the pre-scan is performed, and then returned to the original position. The gantry rotation speed may be identical to that scheduled for the subsequent scan. The filtration should be installed so that there is no excess of stray radiation to which the patient may be exposed.

If this is complied with, the pre-scan will add a relatively small and negligible amount of extra dose to the patient. With a correspondingly short duration of the pre-scan and depending on the characteristics of the table speed, this additional scan represents only a minor extension of the examination protocol.

This pre-scan thus generates a conditioning of the detector material by targeted Introduction of X-radiation and subsequent analysis of the detector signal to extract parameters used to counter-rate correct the data of at least one subsequent scan.

The conditioning should be done at high flux of X-ray quanta, with the X-ray spectrum used plays a minor role. The analysis of the detector signal, ie the calibration, should then take place at a moderate flux of X-ray quanta and with a moderately hardened spectrum such that the quantum flux and the incident at the detector X-ray spectrum by adjusting tube voltage, tube current and / or beam filtration values of quantum flux and the X-ray spectrum at the detector in the subsequent sampling of a patient in good approximation. Thus, a calibration taking into account these variables can take place or the detector data obtained during the sampling can be corrected taking into account the incident dose rate and the incident X-ray spectrum. The comparison of the measurement data with the expectation defined by the physically applied radiation field then provides access to the parameters for the count rate correction. A simple but efficient example is the channel-specific determination of the ratio of measured data and expectation. The data of the subsequent scan can then be easily corrected with the reciprocal values of these channel-specific ratios. The efficiency of this rather simple procedure has already been confirmed experimentally in the laboratory.

A possible improvement of this method can be done by an additional post-scan, in which, however, no more conditioning takes place, but only the analysis of the detector response to the incoming radiation takes place. For counting rate correction of the detector data of the at least one preceding scan, the data from the pre-scan and from the post-scan can be interpolated. In this case, a simple, preferably detector-element-specific, averaging can be used or the temporal decay behavior, optionally with individual consideration of the detector element-specific irradiation history, can be taken into account in the interpolation of the correction values based on the specific time of the measurements performed on the patient.

The proposed method allows the efficient reduction of ring artifacts in CT images caused by the phenomenon of pronounced counting rate drift. In this way, the use of materials with pronounced but relatively slow decaying counting rate drift in clinical CT scanners is possible.

According to these findings, the inventor proposes a method for artifact reduction in computed tomographic images, which are reconstructed from detector data of a scan of an object, in particular a patient, with a quantum counting X-ray detector, wherein according to the invention immediately before the scanning of the object a count rate calibration takes place, in which the X-ray detector is irradiated by varying the tube current of a used X-ray tube with a dose rate bandwidth, which corresponds to the, expected in the later scanning of the object, dose rate bandwidth at the detector.

• – Vorbereitung einer CT-Untersuchung des Objektes,
• – platzieren des Objektes auf einem verfahrbaren Tisch,
• – Kalibrierung durch ermitteln einer Vielzahl von Zählraten für mindestens ein Detektorelement in Abhängigkeit unterschiedlicher Dosisleistungen an dem mindestens einen Detektorelement,
• – unmittelbar darauf folgende Einbringung des Objektes in das Abtastfeld,
• – abtasten des Objektes mit dem mindestens einen Strahler-Detektor-System,
• – Korrektur der bei der Abtastung des Objektes gewonnenen Detektordaten und,
• – Rekonstruktion mindestens einer computertomographischen Aufnahme des Objektes.

Advantageously, in this method, the following method steps in the CT examination of the object with at least one emitter-detector system, each having a beam path and a scanning field are performed:

• Preparation of a CT examination of the object,
• Placing the object on a movable table,
• Calibration by determining a plurality of count rates for at least one detector element as a function of different dose rates at the at least one detector element,
• Immediately following introduction of the object into the scanning field,
• Scanning the object with the at least one emitter-detector system,
• Correction of the detector data obtained during the scanning of the object and
• - Reconstruction of at least one computed tomographic image of the object.

It is particularly favorable if the X-ray detector is additionally conditioned by irradiation immediately before the first calibration and scanning of the object. As a result, the detector is placed in a relatively stable state, in which much less drift occurs than in a measurement without previous irradiation. For the purpose of such conditioning, in the absence of the object in the beam path, irradiation of the at least one X-ray detector with a multiplicity of detector elements can be carried out.

It is furthermore particularly advantageous if a further determination of a multiplicity of count rates for the at least one detector element as a function of different dose rates at the at least one detector element takes place immediately after the scanning of the object, and the detector data measured during the scanning of the object be determined by interpolated intermediate values of the measurements before and after the sampling.

Furthermore, for conditioning and / or for determining the dose-rate-dependent ratios of count rates and dose rate applied to the detector, additional filtering for hardening the X-ray spectrum can be performed, compared to the X-ray spectrum used for scanning. In this case, the filtering should essentially correspond to the passage of the radiation through the examined object, in order to obtain the most comparable possible irradiation situations.

In addition to the method described above, the inventor also proposes an improvement of a CT system or a C-arm system, this with at least one emitter-detector system for scanning an object, in particular a patient, connected to at least one control and Arithmetic unit, which has a program memory for computer programs, is equipped and wherein at least one program is stored in the program memory of the control and processing unit, which executes the method according to one of the preceding method claims.

In the following the invention will be described in more detail with the aid of the figures, wherein only the features necessary for understanding the invention are shown. The following reference numerals are used: 1: CT system / C-arm system; 2: first X-ray tube; 3: first detector; 4: second X-ray tube; 5: second detector; 6: gantry housing; 7: swivel arm; 8: examination couch; 9: system axis; 10: control and computing unit; 11: contrast agent applicator; 12: ECG lead; A: decay interval; D: dose; K1: conditioning phase; K2: calibration phase; K3: recalibration; M: measuring interval; Prg ₁ prg _n : computer programs; S1.1: preparation of a CT examination of an object; S1.2: placing the object on a movable table; S1.3: irradiation of the X-ray detector for conditioning; S1.4: calibration; S1.5: introduction of the object into the scanning field; S1.6: scanning the object; S1.7: correction of the detector data; S1.8: reconstruction and output of image data; S2.1: preparation of a CT examination of an object; S2.2: placing the object on a movable table; S2.3: pre-calibration; S2.4: introduction of the object into the scanning field / scanning of the object / removal of the object from the scanning field; S2.5: post-calibration; S2.6: correction of the detector data with data from pre- and post-calibration; S2.7: reconstruction and output of the image data; V: time interval; Z: count rate; Z / D (t): time course from count rate to dose.

They show in detail:

1 a schematic representation of the time course of the ratio of registered quanta to received dose in the context of a CT scan with quantum counting detector according to the prior art,

2 a schematic representation of the time course of the ratio of registered quanta to received dose in the context of a CT scan with quantum counting detector with pre-scan,

3 a schematic representation of the time course of the ratio of registered quantum to

received dose as part of a CT scan with quantum counting detector with pre-scan and post-scan,

4 a flow chart of a method according to the invention of a CT examination of a patient with quantum-counting detector with pre-scan with conditioning and calibration,

5 a flow chart of a method according to the invention of a CT examination of a patient with quantum-counting detector with pre-scan without conditioning and post-scan for post-calibration,

6 a CT system configured to carry out the method according to the invention and

7 a C-arm system configured to carry out the method according to the invention.

In the 1 schematically shows the time course Z / D (t) of the ratio of the number Z of registered quanta in a quantum-counting detector element of a detector to the received dose D as part of a CT investigation with quantum-counting detector according to the current state of the art. In this case, the time interval V describes the preparation phase of the examination, in which no radiation dose is delivered in the CT system, but the patient to be examined is prepared, for example by storage on the patient table and placement of an access for a contrast agent bolus. This is followed by the measurement interval in which the patient is scanned by X-ray radiation and measurement of the absorption of the radiation as it passes through the body with the quantum-counting detector. Here, the curve Z / D (t) begins, which shows-schematically - how the ratio of the quanta counted per dose-individually due to the radiation impinging on the detector material of the detector and the electric field built up in the detector material considered detector element - changed. in the Measuring interval decreases the count rate Z per received dose steadily and non-linearly. Following the measurement interval M, ie after completion of the irradiation, the detector material slowly recovers in a slightly rising curve shown in the decay interval A. The ratio Z / D is therefore during the measurement of a strong temporal and the individually received dose-dependent change subjected, whereby the correct determination of the received dose very difficult or errors arise that leads to the above-mentioned image artifacts in the utilization of the detector data by image reconstruction.

According to the invention, these image artifacts can now be avoided if - as in the 2 shown schematically - before the actual measurement interval, a pre-scan, consisting of a conditioning phase K1 and a calibration phase K2, takes place. Like in the 2 After the preparation interval V, first of all, in the interval K1, a conditioning of the detector is carried out by application of an irradiation to the detector, so that the latter changes into a substantially saturated state, which is stable with respect to the Z / D ratio. Subsequently, in this state, a calibration or determination of correction values is carried out immediately with respect to an original calibration. Whereby this analysis should be carried out with dose rate values on the detector and X-ray spectra representative of the subsequent CT examination of the patient. Immediately after this first calibration, the patient is scanned in the measurement interval M, with only small changes in the response of the detector resulting due to the now occurred saturation. After the completion of the measurement interval M, as in the prior art, the decay interval A follows.

According to the invention, due to the correction data obtained in a timely manner, ie immediately before the scanning of the patient, and due to the preceding conditioning of the detector material, a very good and constant precision can be achieved in the determination of the dose from the quantum counts determined.

Another improvement of the new process will be in the 3 shown. Here is in addition to the expiration of the 2 after the measuring interval M, a post-scan is carried out in the interval K3 before the decay interval begins. For this purpose, immediately after the scan, the patient and the patient table are removed from the beam path of the CT system and the analysis, as has already been carried out in the interval K2, repeated again. However, since the dose rates or count rates resulting from the previous measurement in the interval M are known in the interval M, the dose rate range to be analyzed can be better maintained.

Since correction values are present before and after the patient's actual scan, they can be used for interpolation and the actual measured values can be evaluated even more precisely.

For a better understanding of the invention is in the 4 a flow chart of the method according to the invention of a CT examination of a patient with quantum-counting detector using a pre-scan with conditioning shown. Accordingly, the preparation of a CT examination of an object, usually a patient, is first carried out in step S1.1. In step S1.2, the object is placed on a movable table, so that the sampling can be started without a time delay. However, the object is not yet spent in the beam path of the CT system. It then follows in step S1.3, the irradiation of the at least one X-ray detector with its plurality of detector elements for the purpose of conditioning in the absence of the object in the beam path. This is followed by the calibration in step S1.4, wherein the ratio Z / D is determined for the detector elements as a function of different dose rates. Immediately thereafter, the object is introduced into the beam path at step S1.5, followed by the scanning of the object at step S1.6. In step S1.7, the measurement data of the detector are evaluated on the basis of the correction data determined during the calibration or analysis and the reconstruction of at least one computed tomographic image of the object is carried out with the thus determined exact detector data in step S1.8 and this can be at least one image being represented.

An alternative variant of a flowchart of the invention is shown in FIG 5 - shown here using a pre-scan without conditioning and additional post-scan. Accordingly, the preparation of a CT examination of an object, usually a patient, is first carried out in step S2.1. In step S2.2, the object is placed on a movable table, so that the sampling can be started without a time delay. However, the object is not yet spent in the beam path of the CT system. This is followed by a pre-calibration in step S2.3, wherein the ratio Z / D is determined for the detector elements as a function of different applied dose rates or quantum fluxes. Immediately thereafter, the object is introduced with the step S2.4 in the beam path, scanned and brought out of the beam path again. Then, in step S2.5 immediately after the scanning and the removal of the object, a post-calibration of the detector, wherein again the bandwidth of the quantum fluxes occurring in the previous scan is applied to the detector. In step S2.6, the measurement data of the detector are evaluated on the basis of the correction data determined in the pre-calibration and the post-calibration in order to know the correction factors applied between the calibrations as precisely as possible in order to arrive at optimally corrected detector data with these corrections , In step S2.7, the reconstruction of at least one computed tomographic image of the object takes place using this corrected detector data and this at least one image can be displayed.

In addition, it should be noted that according to another variant of the invention also includes a method in which in addition to the 5 described conditioning - according to the process step S1.3 from 4 - Before the step S2.3 the pre-calibration is performed.

The 6 shows an example of a CT system 1 with which the method according to the invention can be carried out. The CT system 1 shows a first tube / detector system with an x-ray tube 2 and an opposite quantum counting detector 3 on. Optionally, this CT system 1 via a second x-ray tube 4 with a second opposite quantum counting detector 5 feature. Both tube / detector systems are located on a gantry in a gantry housing 6 is arranged and during the scan around a system axis 9 rotates. Patient P is on a sliding examination couch 8th that are either continuous or sequential along the system axis 9 through the gantry housing 6 is pushed scanning field, wherein the attenuation of the X-rays emitted by the X-ray tubes is measured by the detectors.

During the measurement, the patient P can use a contrast agent applicator 11 a contrast agent bolus be injected so that blood vessels can be better recognized or a perfusion measurement can be performed. In cardio recordings may additionally, with the help of an ECG lead 12 , cardiac activity is measured and an ECG gated scan is performed.

The control of the CT system is carried out with the aid of a control and processing unit 10 , in which there are computer programs Prg ₁ to Prg _n , which can also carry out the method according to the invention described above. In addition, via this control and processing unit 10 also the output of image data takes place.

Alternatively, this method can also be used in C-arm systems equipped with a quantum-counting detector. In this regard, the shows 7 a C-arm system 1 with which both tomographic and projective images, especially in the context of interventional angiography, can be generated. The C-arm system shown here 1 also has an x-ray tube 2 with an oppositely planar quantum-counting detector 3 , Both systems are using a swivel arm 7 in any position to pivot around the patient P. The patient P is on a patient bed 8th additionally using a contrast agent application system 11 to inject contrast media, if necessary, to visualize blood vessels. The system is controlled by a control and processing unit 10 which has in its memory computer programs Prg ₁ to Prg _n , which can inter alia also carry out the method according to the invention with a pre-scan preceding the scanning and, if appropriate, a subsequent post-scan.

Although the invention has been further illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims (8)

Method for artifact reduction in computer tomographic images, which consists of detector data of a scan of an object, in particular of a patient (P), with a quantum-counting x-ray detector ( 3 . 5 ) are reconstructed, characterized in that immediately before the scanning of the object (P) a count rate calibration takes place, in which the X-ray detector ( 3 . 5 ) by varying the tube current of a used X-ray tube ( 2 . 4 ) is irradiated with a dose rate bandwidth which corresponds to the dose power bandwidth at the X-ray detector to be expected in the later scanning of the object (P) ( 3 . 5 ) corresponds. Method according to the preceding claim 1, characterized in that at least the following method steps in the CT examination of the object (P) with at least one emitter-detector system ( 2 . 3 ; 4 . 5 ) are each carried out with one beam path and one scanning field: Preparation of a CT examination of the object (P), placing the object (P) on a movable table ( 8th ), Calibration by determining a plurality of count rates for at least one detector element as a function of different currently applied Dose outputs at the at least one detector element, immediately following introduction of the object (P) in the scanning field, scanning the object (P) with the at least one emitter-detector system ( 2 . 3 ; 4 . 5 ), Correction of the detector data obtained during the scanning of the object (P), reconstruction of at least one computed tomographic image of the object (P). Method according to one of the preceding claims 1 to 2, characterized in that in addition immediately before the calibration and scanning of the object (P) of the X-ray detector ( 3 . 5 ) is conditioned by irradiation. Method according to the preceding claim 3, characterized in that for the purpose of conditioning in the absence of the object (P) in the beam path irradiation of the at least one X-ray detector ( 3 . 5 ) is performed with a plurality of detector elements. Method according to one of the preceding claims 1 to 4, characterized in that immediately after the scanning of the object (P), a further determination of a plurality of count rates for the at least one detector element in response to different dose rates on the at least one detector element, and in the Sampling of the object (P) measured detector data by interpolated intermediate values of the measurements before and after the scan are determined. Method according to one of the preceding claims 1 to 5, characterized in that for the conditioning and / or for the determination of the dose rate dependent ratios of count rates and at the detector ( 3 . 5 ) applied dose rate one, compared to the X-ray spectrum used for scanning, additional filtering to harden the X-ray spectrum is performed. Method according to the preceding claim 6, characterized in that the additional filtering is carried out such that the filtering substantially corresponds to the passage of the radiation through the object (P) under examination. CT system ( 1 ) with at least one emitter-detector system ( 2 . 3 ; 4 . 5 ) for scanning an object, in particular a patient (P), connected to at least one control and computing unit ( 10 ), which has a program memory for computer programs, characterized in that in the program memory of the control and computing unit ( 10 ) at least one program (Prg ₁ -Prg _n ) is stored, which executes the method according to one of the preceding method claims in operation. C-arm system ( 1 ) with a radiator-detector system ( 2 . 3 ) for scanning an object, in particular a patient (P), connected to at least one control and computing unit ( 10 ), which has a program memory for computer programs, characterized in that in the program memory of the control and computing unit ( 10 ) at least one program (Prg ₁ -Prg _n ) is stored, which executes the method according to one of the preceding method claims in operation.

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