METHOD AND APPARATUS FOR RESPIRATION-CORRELATED COMPUTED TOMOGRAPHY IMAGING

Abstract

In a method and apparatus for respiration-correlated computed tomography imaging, a patient-specific breathing curve is recorded and is evaluated online, and a CT scan, providing a number of raw images of a region of interest of a patient, is controlled synchronously with the patient-specific breathing curve according to the results of the online evaluation.

Claims

1. A method for respiration-correlated computed tomography (CT) imaging, comprising: while a patient is situated in a CT scanner, obtaining a breathing curve, representing respiration of the patient, providing said breathing curve to a processor, and evaluating said breathing curve online in real time in said processor; while said breathing curve is evaluated online, operating the CT scanner to implement a CT scan to obtain a plurality of sets of raw CT data controlled synchronously with the breathing curve according to results of the online evaluation, and thereby obtaining respiration-correlated sets of raw CT data; and making the sets of respiration-correlated raw CT data available in electronic form, as at least one data file, from said processor.

2. A method as claimed in claim 1 comprising, in said processor, determining a start time and an end time for said scan from said breathing curve, and controlling said CT scan from said processor to begin at said start time and to terminate at said end time.

3. A method as claimed in claim 2 comprising determining said start time and said end time so that at least one breathing cycle of the patient occurs therebetween.

4. A method as claimed in claim 3 comprising, from said processor, operating said CT scanner to execute a first CT scan of the patient in parallel time with a first breathing cycle of the patient, with the patient at a fixed position in a longitudinal direction of the patient and, upon completion of said first CT scan, operating said CT scanner from said processor to execute a second CT scan in parallel time with another breathing cycle of the patient, with the patient at a position longitudinally downstream of said position of the patient during said first CT scan.

5. A method as claimed in claim 2 comprising determining said start time as a time of occurrence of an extreme value of said breathing curve corresponding to a first inhalation maximum, and determining said end time as a time of occurrence of a subsequent extreme value of said breathing curve, corresponding to a second inhalation maximum.

6. A method as claimed in claim 5 comprising determining at least one of said start time or said end time as corresponding to the time of occurrence of the respective extreme value only if an absolute value of an amplitude of the respective extreme value exceeds a predetermined set value.

7. A method as claimed in claim 1 comprising additionally evaluating said breathing curve of the patient offline after completion of said CT scan.

8. A method as claimed in claim 7 comprising, in said offline evaluation, identifying irregularities in said breathing curve that occurred during said CT scan, and correlating said irregularities with a position within the patient along a longitudinal direction of the patient.

9. A method as claimed in claim 8 comprising, for any position along said longitudinal direction of the patient at which one of said irregularities is identified, re-scanning the patient at that position.

10. A method as claimed in claim 1 comprising determining a representative breathing curve for the patient as a predefined breathing curve or a learned breathing curve that is learned based on a plurality of breathing curves of the patient, and evaluating the breathing curve online during said CT scan dependent on said representative breathing curve.

11. A method as claimed in claim 10 comprising determining a start time and an end time for said CT scan online from the breathing curve obtained from the patient relative to the representative breathing curve.

12. A method as claimed in claim 11 comprising determining the start time and the end time as respective times of occurrence of extreme values in said representative breathing curve.

13. A method as claimed in claim 12 comprising, in said processor: determining tuples of amplitude and derivatives with respect to time from values of the representative breathing curve; determining coordinates of a center from said tuples and converting the tuples, with respect to said center, into polar coordinates, and assigning a respective angle to each extreme value of the representative breathing curve in said polar coordinates, said extreme values corresponding to respective inhalation maximums; concerting values of the breathing curve obtained from the patient during said CT scan into polar coordinates with respect to said center determined from said representative breathing curve; and determining a curve polar angle in said polar coordinates for each value of the breathing curve of the patient obtained during said CT scan, and determining a time of occurrence of said extreme values online by comparing the currently determined angle of the breathing curve of the patient obtained during the CT scan with the angle assigned to an extreme value of said representative breathing curve in said polar coordinates.

14. A computed tomography (CT) apparatus comprising: a CT scanner adapted to receive a patient therein; a breathing detector configured to generate a breathing curve that represents respiration of the patient in the CT scanner; and a control computer configured to operate the CT scanner to implement a CT scan of the patient, said control computer being supplied with said breathing curve online during said CT scan and being configured to evaluate said breathing curve in real time and to control said CT scan in real time dependent on the evaluation of said breathing curve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows an example of a patient-specific breathing curve.

[0033] FIG. 2 shows a representative breathing curve determined from the real breathing curve.

[0034] FIG. 3 shows real breathing curves and the representative breathing curve in an amplitude/speed graph.

[0035] FIG. 4 shows an apparatus for respiration-correlated computed tomography imaging, comprising a CT scanner and a sensor for capturing breathing curves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 1 shows a typical breathing curve 1 of a patient. The breathing curve 1 is measured/recorded as a breathing surrogate using an external sensor, e.g. a spirometer or a belt with chest expansion transducers. To control a CT scan providing a plurality of raw images of a region of interest of the patient, the acquired patient-specific breathing curve is evaluated online. The scan is then controlled according to the results of the online evaluation. A corresponding apparatus for this purpose and the corresponding apparatus components are shown in FIG. 4.

[0037] Controlling the scan via the online evaluation of the synchronously acquired breathing curve ensures in particular that, at a position z.sub.k of the region of interest of the patient, sufficient consistent raw data for a complete breathing cycle is recorded in order to be able to reconstruct 3D images of the region of interest without artifacts caused by breathing irregularities.

[0038] The breathing curve 1 shown in FIG. 1 in this case comprises five breathing cycles 3. A second breathing cycle 5 thereof exhibits irregularities in its amplitude. This may be due to the measurement data or to a changed respiration of the patient. To perform a scan, the current breathing curve 1 is evaluated online after points 7 of maximum inhalation. In this case these are points of maximum amplitude, i.e. extrema 8 along the breathing curve 1. The breathing cycle 5 also shows an extreme 9. However, this is at a significantly reduced amplitude compared to the other extremes 8.

[0039] After the start of the examination process, a first scan 10 shall be performed at a first fixed position z.sub.k in the longitudinal direction of the patient. For this purpose the occurrence of the first extreme 8 is determined from the online evaluation of the breathing curve 1 as the start time 11. On reaching or detecting the start time 11, the first scan 10 begins. While the first scan is being carried out, with a plurality of raw images being captured from different directions, the breathing curve 1 continues to be synchronously evaluated, i.e. in parallel time. The next extreme 9 of the second breathing cycle 5 is not detected as a relevant inhalation maximum, as its amplitude value is small compared to a predefined setpoint value. It is only in a further breathing cycle 3 that the online evaluation identifies another extreme 8 in the breathing curve 1, as the amplitude is sufficiently high. Only this extreme 8 of the further breathing cycle 3 is evaluated by the online evaluation as an end time 13, the detection or attainment of which terminates the first scan 10.

[0040] It can be seen that the first scan 10 extends over a period of two breathing cycles 3. The second, irregular breathing cycle 5 is identified as irregular. Consequently, the scan 10 is continued until sufficient raw data have been acquired over each phase of a regular breathing cycle 2. Raw data are used that are not from an inhalation phase of the irregular, second breathing cycle 5, but are from an inhalation phase of the next regular breathing cycle 3, in order to reconstruct a 3D image at the position z.sub.k.

[0041] On completion of the first scan 10, a second scan 15 is carried out with the patient's positioning changed (table feed control), at a position z.sub.k+1. In contrast to the first scan 10, the simultaneously further evaluated breathing curve 1 here shows breathing irregularities. On detection or attainment of a point 7 of maximum inhalation, i.e. the extreme 8 of a breathing cycle 3, the second scan 15 is started. On detection or attainment of a point 7 of maximum inhalation, i.e. the extreme 8 of the subsequent breathing cycle 3, the second scan 15 is terminated. The amplitude values at the two extrema 8 exceed a predefined setpoint value in each case. The second scan 15 lasts for a duration of one breathing cycle.

[0042] The method is then continued, moving the patient along until a scan has been performed at all the desired positions z.sub.k. It will be immediately apparent that, using the method specified, the duration of a respective scan is directly linked to the breathing rate. A longer or shorter breathing cycle results in a corresponding adjustment to the scan duration.

[0043] In an advantageous variant, after a scan has been performed or when scanning of all the positions is complete, offline evaluation of the acquired breathing curves 1 is carried out. If the breathing curve if found to have irregularities or to be of very poor quality, the corresponding position z.sub.k is re-scanned, in particular prioritized according to the severity of the irregularity. An irregularity resulting in re-scanning is, in particular, missing amplitude information in the breathing curve, which information is necessary for phase-selected reconstruction of the 3D image.

[0044] FIG. 2 shows a representative breathing curve 20 (continuous bold line) averaged or learned from a number of actually measured previous breathing curves 18 of a patient over the duration of a breathing cycle. At a position t.sub.max, the representative breathing curve 20 has a point 7 of maximum inhalation, i.e. its extreme 8, as is typically the case for regular breathing curves 3 as shown in FIG. 1. The representative breathing curve 20 is specifically obtained in advance prior to performing an examination, e.g. based on a number of recorded breathing curves 18 of the patient. It is also possible to predefine the representative breathing curve 20 in a patient-specific manner on the basis of empirical values, for which purpose appropriate databases are accessed.

[0045] The representative breathing curve 20 is used for online evaluation of a breathing curve 1 according to FIG. 1 in order to control a scan for CT imaging during an examination or treatment of the patient.

[0046] To that end FIG. 3 shows the representative breathing curve 20 for a duration of one breathing cycle and a family of currently recorded breathing curves 1 in an amplitude/rate graph. The respective amplitude values R are here plotted against the rate values V determined as a time derivative. Because of the periodicity of the signal, the plot of the value pairs or tuples (R.sub.repr, V.sub.repr) of the representative breathing curve 20 produces, for a breathing cycle, the rotation of a characteristic circle that is angle-dependent in its radius. Corresponding online evaluation of a currently recorded breathing curve 1 produces a curved path rotating along this circle, or with few deviations.

[0047] From the tuples (R.sub.repr, V.sub.repr) of the representative breathing curve 20, a geometric (mid-point) or center C predefined via amplitude and rate values is determined, in respect of which the tuples of the breathing curves 1, 20 are converted into polar coordinates, i.e. distance P and phase angle φ. The center C is marked with its coordinates (C.sub.R, C.sub.V) in FIG. 3. In particular, there is produced for the representative breathing curve 20 a characteristic angle φ.sub.max that is assigned to the extreme 8 or more precisely the instant of maximum inhalation 8. In the graph according to FIG. 3, the corresponding tuple is marked with (P.sub.max, φ.sub.max). For online evaluation of a current breathing curve 1 in which the phase angle φ(t) is determined continuously or in time-discrete increments, the resulting unambiguous criterion for establishing the occurrence of the corresponding extreme is φ(t.sub.i)<φ.sub.max and φ(t.sub.i+1)>φ.sub.max. In this case the extreme has been attained or exceeded, and the corresponding time value can be used as the start time or end time for commencing/terminating a scan as described above.

[0048] Depending on the selection of the center C, in the case of an irregular breathing cycle with low amplitude, no phase angles are determined in the region of φ.sub.max by the online evaluation described if the corresponding breathing curve according to FIG. 3, for example, lies below the center C. In this case, the scan is continued until a regular abort criterion has been established on the basis of a regular breathing cycle.

[0049] Actually measured breathing curves 1 of the patient are used at regular intervals to adjust the representative breathing curve 20. Shifts, particularly baseline drift, in the respiration signals are therefore reacted to. Also the representative breathing curve 20 is continuously matched to the actual breathing of the patient. Depending on the particular representative breathing curve 20, the calculation of the center C is adjusted accordingly.

[0050] FIG. 4 shows an apparatus 81 for respiration-correlated computed tomography imaging. The apparatus 81 includes a CT scanner 83 having a rotatable gantry 85 comprising a fan beam X-ray source 87 and a circular segmented, flat panel detector 89. To perform a scan of a region of interest 91 of a patient 93, said scan providing a plurality of raw images, the apparatus has a sensor 97 for recording a patient-specific breathing curve 1, and a control computer 99 designed to carry out the method as claimed in one of the preceding claims.

[0051] As preparation for e.g. radiotherapy, the computed tomography scanner 83 is used to perform a CT scan of a region of interest 91 of the patient 93. Consecutive positions are successively scanned. To carry out the examination, i.e. imaging, the table 95 on which the patient is positioned is fed to a first fixed position z.sub.k. The gantry 85 rotates about the patient at this position z.sub.k until consistent raw data for at least one complete breathing cycle has been obtained. The duration of a scan at a position of the patient 93 is determined by means of online evaluation of a synchronously recorded breathing curve of the patient 93. The breathing curve itself is acquired as a breathing surrogate during the CT scan by means of a sensor 97. A belt with chest expansion transducers, for example, is used as a sensor 97. Alternatively, a spirometer is used. The table 95 on which the patient is positioned is then moved along the longitudinal direction of the patient 101 to a next fixed position z.sub.k+1 and a new scan is performed.

[0052] Each scan is controlled at the respective position z.sub.k depending on the results of the online-evaluated patient-specific breathing curve. An appropriately implemented control unit 99 is used for this purpose. The control computer 99 is designed to carry out the method for controlling a scan on the basis of online evaluation of the current breathing curve as described above.

[0053] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.