Synchronous systems with trigger delays.

Abstract

The synchronization system comprises a sensor device for detecting a trigger-based event. The analysis module and the computing unit are configured to access a priori information regarding a time delay between detection of the trigger-based event by the sensor device and a start of an acquisition time interval for acquiring imaging data. The start of the acquisition time interval is calculated from the detected trigger-based event and the priori information of the time delay. Although the time delay between detection of the trigger-based event by the sensor device and the acquisition time interval may vary between individual subjects, for each individual subject the time delay is well reproducible and can therefore be calibrated for each subject.

Description

[Technical field]

[0001] The present invention relates to a synchronization system with a trigger delay, and in particular to a synchronization system for triggering the acquisition of imaging data. [Background technology]

[0002] Such synchronization is known from the literature N. Spicher, M. Kukuk, S. Maderwald, and ME Ladd. "Initial evaluation of prospective cardiac triggering using photo plethysmography signals recorded with a video camera compared to pulse oximetry and electrocardiography at 7T MRI", in Biomedical engineering online 2016;15(1):126. Summary of the Invention [Problem to be solved by the invention]

[0003] The known synchronization system is in fact an observational setup for synchronization between MR imaging data (k-space data) acquisition and the subject's cardiac activity in order to reduce imaging artifacts due to (cardiac) motion. Non-contact triggering is performed by estimating the cardiac cycle phase from a remote photoplethysmography (rPPG) signal obtained from skin color changes using a video camera. That is to say, the above mentioned paper describes an MR image acquisition based on video triggering itself.

[0004] It is an object of the present invention to provide synchronization that more accurately determines trigger-based events that form the basis for synchronization of imaging data acquisition during subject motion. [Means for solving the problem]

[0005] This object is achieved by a sensor device of the present invention which detects a trigger-based event, the sensor device having an analysis module and a calculation unit configured to access prior information regarding a time delay between the detection of a trigger-based event by the sensor device and the start of an acquisition time interval for acquiring imaging data, and to calculate the start of the acquisition time interval from the detected trigger-based event and the prior information of the time delay, the analysis module and the calculation unit being configured to access the prior information contained in image information of a subject, and to derive, based on the accessed prior information, a time delay between the detection of a trigger-based event by the sensor device and the start of an acquisition time interval for acquiring imaging data.

[0006] The insight of the present invention is that a synchronization system based on a sensor device performing a relatively indirect measurement of a trigger-based event will produce a time delay between the actual occurrence of the trigger-based event and the instantaneous time at which the sensor device detects the trigger-based event. Another insight of the present invention is that this time delay can be compensated for. The synchronization system of this aspect of the present invention is The sensor device detects a trigger-based event representative of a next acquisition time interval during which imaging data, such as k-space data or CT attenuation profiles, can be acquired by a tomographic (MRI, CT, NM (PET, SPECT) diagnostic imaging system in the presence of low levels of perturbations or even in the absence of perturbations, e.g. due to motion of the imaged subject (patient being examined). The sensor device may be a camera system that records skin color changes reflecting heartbeats. The sensor device may be a simple conventional (IR) fingertip sensor. According to this aspect, a priori information is accessed about the time delay between the detection by the sensor device of the trigger-based event and the occurrence of the acquisition time interval. The a priori information includes information or includes image information representative of anatomical distances and sizes. In particular, these distances and sizes determine the path that blood needs to travel from the patient's heart to the location where the skin color changes are recorded. From the prior information, anatomical size and distance can be calculated by image analysis (e.g., automatic pattern recognition or identification of anatomical landmarks by a human user). Then, using a predetermined value or an estimate of the patient's blood flow velocity, a time delay between the detection by the sensor device of the trigger-based event and the actual occurrence of the trigger-based event can be calculated. Alternatively, a trained neural network can be provided in the analysis module, whereby the trained neural network in the analysis module can also return the time delay from the input image information. This forms a machine learning analysis module. This calculation of the time delay from the image information can be performed by a suitably trained machine learning model or a trained neural network. Explicit calculation of anatomical distance from the image information is an implementation that uses relatively simple feature recognition and geometric calculations.

[0007] For example, the trigger-based event may be an R-peak in an electrocardiogram (ECG) of the patient's heart, and the acquisition time interval may be a resting cardiac phase following the R-peak, in which there is no or little cardiac motion, and the acquired k-space data or attenuation profile is little or not affected by the motion. The insight of this aspect of the invention is that although the time delay between the detection by the sensor device of the trigger-based event and the acquisition time interval may vary between individual subjects, for each individual subject, the time delay is well reproducible and can therefore be calibrated for each subject. The calibrated time delay can synchronize the acquisition of imaging data with the subject's motion, whereby the imaging data can be acquired. Thus, in this aspect of the invention, the acquisition of imaging data can be based on the trigger-based event immediately preceding the acquisition time interval. In a practical example, the trigger-based event may be an R-peak in an electrocardiogram, and the acquisition interval may be in a resting phase immediately following the detected R-peak. That is, the resting interval immediately following the detected R-peak can be used for the acquisition of imaging data. From the calibrated time delay and the average time interval between trigger-based events, image acquisition can be adapted to the time available between the detection of the current trigger-based event and the expected next trigger-based event.

[0008] From US 2010 / 0308823 it is known per se that photoplethysmography (PPG) signals appear with a time delay from ventricular contraction, which delay can be estimated from blood flow velocity changes from separate measurements of blood flow velocity per pixel without using an ECG signal.

[0009] In another example of the inventive synchronization system, the calculation of the start of the acquisition time interval also takes into account the latency of the sensor device. This is particularly effective when the sensor device is realized as a camera-based sensor device. This latency represents the technical delay caused by the technology of the sensor device. In the case of a camera, in fact there may be a delay of 1-2 frames until the software of the sensor device can determine that blood is flowing to the skin, for example to the patient's face. However, the insight of the invention is still that a large part of the sensor device delay is caused by the pulse transit time from the heart to the face. Further details of the compensation of the latency of the sensor device are described in European Patent Application No. 20185605.1.

[0010] In another embodiment of the synchronization system, a priori information regarding the time delay between the actual trigger-based event and the timing of the acquisition interval is obtained from a separate (for imaging data acquisition) calibration in which the timing of the trigger-based event and the acquisition interval are directly measured, i.e., in a test separate from the synchronous acquisition of imaging data, the following is performed: (a) direct measurement of the actual instantaneous time of the trigger-based event and the acquisition time interval, and (b) simultaneous detection of the trigger-based event by the sensor device.

[0011] This calibration may not require a comparison of direct measurements with detection of trigger-based events by the actual individual sensor devices used to synchronize acquisition of imaging data by the imaging system. Typically, it is sufficient for the calibration to include a combination of direct measurements and determinations by sensor devices that are sufficiently similar (same type and class) to the individual sensor devices used in image acquisition at later points in the patient's care cycle.

[0012] For example, in the case of cardiac MR imaging, the R-peak of the electrocardiogram is used as the trigger-based event. The direct measurement of the R-peak can be based on electronic measurement of the heart's electrical activity by a set of electrodes placed on the patient's chest. The acquisition interval is timed to the resting (mid-to-end) diastole between successive R-peaks in the electrocardiogram. The electrocardiogram can be measured directly electronically by picking up the electronic heart signal by electrodes attached to the patient's chest. Along with direct detection of the trigger-based event, the trigger-based event is also detected by a sensor device, which may be camera-based. From a comparison of the detection of the trigger-based event by one sensor device (configuration) and the direct measurement by the other, the time delay between the actual occurrence of the trigger-based event and its detection by the sensor device (configuration) can be calibrated. The insight of the present invention is that while this time delay may vary between individual patients, the time delay is well reproducible for a single individual patient. In practice, the direct measurement of the trigger-based event in the form of the R-peak of the electrocardiogram is performed early in the patient's care cycle. This direct measurement, when combined with the detection of a trigger-based event by a sensor device (apparatus), provides a calibrated time delay that can also be used in subsequent imaging procedures. To that end, the ECG recording system can be equipped with a trigger-based event (PPG) sensor device, which can be a camera-based sensor device or a conventional fingertip sensor device. Such an integrated ECG recording system with a PPG sensor device can be installed at a local point-of-care, such as a general practitioner's office. The camera-based PPG sensor device detects changes in the skin color of the examined patient, which are indicative of pulsatile blood flow. Typically, the skin color is detected from the patient's face, for example, the forehead or temples. Alternatively or additionally, a PPG sensor device in adhesive contact can be placed on the patient's skin. Furthermore, a conventional fingertip PPG clamped to the patient's fingertip can also be used.Such a combined ECG recording and PPG sensor configuration can estimate a patient-specific delay between the actual trigger-based event, i.e., the actual R-peak, and the PPG trigger marker detected by the PPG sensor device. This estimation provides a calibration of the patient-specific delay of the PPG sensor placement relative to the actual R-peak, which is recorded almost instantly by an electrode-based ECG recording system on the patient's chest. This calibrated delay value can then be used in a PPG-based triggered magnetic resonance imaging protocol. The calibrated delay can also be used to accurately time-stamp the magnetic resonance image relative to the R-peak that forms the trigger-based event. The calibrated delay between the PPG sensor device's recording of the trigger-based event (R-peak) and the actual occurrence of the trigger-based event by the PPG sensor device can be stored in the patient's digital health record and made available during subsequent magnetic resonance imaging procedures.

[0013] Furthermore, the calibrated delays found from a comparison of direct (ECG) and (PPG) sensor-based measurements can be compared to delays calculated from image information on the subject that represents, among other things, anatomical distances and sizes. This comparison provides a quantitative measure of the accuracy of the delay times derived from the image information.

[0014] The delay between the detection by the sensor device of the trigger-based event (R-peak) and the trigger-based event itself is caused by the pulse transit time (PTT) from the heart to the PPG site, which delays the cardiac trigger. However, even here, face-based camera-PPG has an advantage, since the respective PTT is significantly shorter than for fingertip-PPG. The PTT is known to vary with various physiological parameters, such as heart rate and blood pressure. It is very common for a stress ECG (e.g. during physical stress by cycling) to be acquired in a general practitioner or cardiologist prior to referral to a magnetic resonance imaging examination. It has been proposed to equip this stress ECG device with a PPG camera, or to equip it with a contact PPG sensor device at the temple, which is less susceptible to motion artifacts in the PPG signal. During the examination, the heart rate varies significantly. It is proposed to map the above delay as a function of heart rate. This mapping can be used during the MR examination, where the current heart rate is known from the PPG signal.

[0015] One aspect of the present invention relates to a computer program comprising instructions for accessing prior information regarding a time delay between detection of a trigger-based event by a sensor device and a start of an acquisition time interval for acquiring imaging data, and calculating the start of an acquisition time interval from the detected trigger-based event and the prior information of the time delay.

[0016] Another aspect of the invention relates to a computer program comprising instructions for performing direct measurement of an actual instantaneous time of a trigger-based event and an acquisition time interval, simultaneously detecting the trigger-based event by a sensor device, and determining a time delay between the actual instantaneous time of the trigger-based event and the detection of the trigger-based event by the sensor device.

[0017] Thus, according to these computer program aspects of the invention, the invention may be realized at least in part in providing the technical effects of the invention when installed on a processor or computer of a synchronization system.

[0018] According to the invention, the delay between the detection of the trigger-based event by the sensor device and the trigger-based event itself (trigger delay) is derived from image information of the subject. The image information represents anatomical information of the subject, such as height, arm length, shoulder width, distance from the aortic valve to the patient's forehead or fingers. This anatomical information is relevant for determining the trigger delay. The accessed image information may be (color or grayscale, infrared) images acquired by a camera system. The camera system can be mounted outside the examination zone of the tomographic diagnostic imaging system and controlled to acquire images during preparation of the subject to be examined in the tomographic diagnostic imaging system. The camera system can also be mounted in or next to the examination zone. Such an in-bore camera system can acquire image information during the tomographic imaging procedure. Image information can also be accessed from preparatory magnetic resonance images, such as low-resolution survey images. This embodiment of the synchronization achieves an accurate determination of the trigger delay taking into account differences between individual patients. A precise trigger delay can be used to synchronize the acquisition of imaging data by a tomographic diagnostic imaging system with repetitive motions, such as cardiac motion of a subject. A precise trigger delay can also be used for precise timing of magnetization preparation (e.g., inversion recovery, magnetization transformation techniques, and black blood angiography) and excitation in magnetic resonance imaging. A precise delay can further be used to optimize scan efficiency by optimally utilizing the time between the sensor device detection of a trigger-based event and the subsequent trigger-based event for magnetization preparation and imaging data acquisition.

[0019] In another example of the synchronization system of the present invention, the trigger delay is estimated based on anatomical size and distance. These parameters seem to mainly determine the subject-to-subject variation of the trigger delay. For example, the trigger delay may be due to the transit time of blood from the patient's heart to the anatomical location (forehead, fingertip). In another implementation, the estimation of the trigger delay can be performed by machine learning (e.g., convolutional networks). This can be achieved in two variants: a) direct estimation of the delay by a network with images as input parameters, or b) estimation of anatomical features as arm length by a network based on the input images and regression of the delay based on these anatomical features.

[0020] In another implementation of the synchronization system of the present invention, at least two trigger-based events, for example based on PPG measurements at different locations, are also used to estimate the pulse transit velocity PTV. This approach is based on the insight that the PTV also depends on physiological parameters such as arterial stiffness and blood pressure. Based on the PTV and the known anatomical distance from the heart to the face, a pulse delay of detection of a trigger-based event, for example by PPG measurements at the patient's face, is calculated.

[0021] A further aspect of the invention relates to a computer program comprising instructions for controlling the sensor device to detect trigger-based events at different locations on the body of the examined subject and to derive a pulse transit rate from the time difference between the detected trigger-based events and the derived anatomical distance between these different locations. According to this aspect of the invention, the derivation of the pulse transit rate can be realized in software. A further aspect of the invention relates to a computer program comprising instructions for accessing image information of the examined subject and deriving, based on the accessed image information, a time delay between the detection of the trigger-based sensor device and the start of an acquisition time interval for acquiring imaging data. According to this aspect of the invention, the derivation of the time delay from the image information can be realized in software. These and other aspects of the invention are further elaborated with reference to the embodiments defined in the dependent claims.

[0022] These and other aspects of the invention will be explained with reference to the embodiments described hereinafter and with reference to the accompanying drawings. [Brief description of the drawings]

[0023] [Figure 1] 1 is a schematic diagram of a synchronization system of the present invention; [Diagram 2] FIG. 1 shows an example of a camera PPG signal from a face and a touch sensor on a finger, illustrating the underlying physiology that causes the time delay. [Diagram 3] FIG. 2 is a schematic diagram illustrating the calibration of time delays for the synchronization system of the present invention. [Figure 4] FIG. 1 is a schematic diagram showing an implementation of the synchronization system of the present invention using image information. [Diagram 5] FIG. 1 shows an example of a linear regression of the delay between the PPG trigger and the R peak (herein referred to as "posterior distance" or "PTT") acquired from the face versus height obtained in a volunteer study. [Figure 6] FIG. 2 is a schematic diagram showing an implementation of the synchronization system of the present invention, in which individual trigger-based events are detected from different positions on the body of the examined patient. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Fig. 1 shows a schematic diagram of a synchronization system 1 of the invention. The synchronization system functions to deduce a subsequent trigger base event from a detected current trigger base event (TBE) 11. A trigger base event is an instance that triggers, for example, an image acquisition by a tomographic imaging system 31. The tomographic imaging system can be a magnetic resonance examination system, a computer tomographic system, a nuclear medicine tomographic imaging system. The acquired imaging information can be a k-space profile, an attenuation profile, or a detected gamma (γ) photon. A magnetic resonance image can be reconstructed from a k-space profile, and a computer tomographic image can be reconstructed from (X-ray) attenuation profiles of different orientations. From the detected γ photons, a nuclear medicine tomographic image can be reconstructed. The imaging information in the form of a k-space profile, an attenuation profile, or a detected photon can be acquired in multiple sets of imaging data at successive intervals, in particular during successive equal or comparable motion states of the imaged subject (patient to be examined). For example, from the imaging data sets, equal or comparable to the corresponding cardiac or respiratory phases can be acquired. The synchronization system comprises a sensor 10 that detects a current trigger-based event 11. The synchronization system 1 comprises an analytical model 20 to which the instantaneous time of the detected trigger-based event is applied by the sensor 10. The analysis module has access to prior information regarding the latency (τ) of the sensor 22 and the time interval (Int) 21 between the actual trigger-based event and the timing of the acquisition interval. This prior information can be obtained from a previous calibration (relative to the imaging data acquisition). The analysis module comprises a computational or machine learning unit that calculates or returns a trigger signal (TS) from the detected instantaneous time of the trigger-based event and the prior information. The trigger signal is provided to a control unit of an image acquisition system 31. The synchronization system can be integrated into the image acquisition system or can be a standalone synchronization system that is coupled to the image acquisition system for triggered image acquisition.

[0025] FIG. 2 shows an example of a camera PPG signal, an ECG signal from a face, and a contact PPG signal from a finger, all measured simultaneously during approximately one cardiac cycle. The "camera PPG valley" and the "contact PPG peak" indicate PPG markers suitable for cardiac triggering. They are delayed relative to the R-peak that is traditionally used for triggering. FIG. 2 shows that a trigger-based event detected by a direct ECG from electrodes placed on the patient's chest is detected with negligible trigger delay. The trigger-based event detected by a subject's forehead trigger or by a contact sensor at the fingertip is delayed by the respective trigger delay relative to the directly detected ECG R-peak.

[0026] Fig. 3 shows a schematic diagram of the calibration of the time delay for the synchronization system of the invention. This calibration relies on the insights presented with reference to Fig. 2. The calibration procedure 60 includes performing a prior direct detection 61 with negligible trigger delay for the trigger-based event, i.e. the direct detection 61 is performed by a measurement setup with negligible technical sensor delay and with negligible pulse transit time related to the subject's physiology. In addition, a (same or corresponding) trigger-based event is detected (62) by a sensor device (same or corresponding equivalent) and a trigger delay is determined from a comparison of the instantaneous time of the detection by the sensor device with the direct detection. This trigger delay represents the time span between the detection by the sensor device of the trigger-based event and the actual occurrence of the trigger-based event in a reproducible manner for the individual subject in question. The calibrated trigger delay can thus be used as prior information in the subsequent synchronization of, for example, diagnostic image acquisition. Further improvements can also be implemented to correct the trigger delay for differences in the subject's heart rate during the calibration and during the synchronization of the diagnostic image acquisition.

[0027] FIG. 4 shows a schematic diagram of an implementation of the synchronization system of the present invention using image information. Image information 51 can be acquired by a camera or from magnetic resonance or computer tomography (survey) images. A geometry analyzer 53 derives anatomical distances from the image information 51 that are applied to the analysis module. Alternatively, a trained neural network 55 returns the anatomical distances from the input image information 51. The trained neural network can be trained based on a training data set of a set of images and separately measured anatomical distances of the anatomical structures represented by said images. Based on the anatomical distances, the analysis module 20 estimates the trigger delay. This can be done based on a simple calculation using representative values ​​of blood flow velocity or by a look-up table stored in or accessed by the analysis module 20. The trigger delay can be estimated based on a linear regression analysis of height versus calibrated measurements of actual trigger delay or pulse transit time. Empirically, pulse transit time appears to be approximately linear with height. This is shown in Figure 5, which shows an example of a linear regression of the delay between the PPG trigger and the R-peak (here called "backward distance" or "PTT") obtained from the face against height obtained in a volunteer study. The labels 80Hz and 20Hz refer to the camera frame rates used in the camera-based detection of the R-peak. In this experiment, a camera is placed above the patient table. This camera can be used to obtain images from which height, shoulder width, arm length, etc. can be obtained. The fixed installation and geometric calibration of the camera allows absolute sizes to be derived directly from the images. This can be done by known state-of-the-art image processing or by machine learning.

[0028] 6 shows a schematic diagram of an implementation of the synchronization system of the present invention, in which individual trigger-based events are detected from different locations on the body of the examined patient. 1,2,3 ...71. The sensor elements have respective arrival times Ta1,2,3 ...73 detects trigger-based events from each location. From the arrival time 73 and detection location 71, the analysis module can calculate the pulse transit velocity PTV. Based on the PTV and the known anatomical distance from the heart to the face, the pulse delay of the PPG measurement at the patient's face is calculated.

Claims

1. A synchronization system, comprising: a sensor device for detecting a trigger-based event; an analysis module and an arithmetic unit configured to access prior information regarding a time delay between detection of a trigger-based event by the sensor device and a start point of an acquisition time interval for acquiring imaging data, and to calculate the start point of the acquisition time interval from the detected trigger-based event and the prior information of the time delay; The analysis module and the arithmetic unit are configured to access prior information in the form of image information of a subject to be examined, and to derive a time delay between detection of a trigger-based event by the sensor device and a start point of the acquisition time interval for acquiring imaging data based on the accessed image information. The synchronization system.

2. The analysis module and the arithmetic unit are configured to derive an anatomical distance and size of the subject to be examined from the accessed image information, and to derive the time delay from the derived anatomical distance and size. The synchronization system according to claim 1.

3. The analysis module has a machine learning module that returns the time delay from the accessed image information. The synchronization system according to claim 1.

4. The calculation of the start point of the acquisition time interval further takes into account the latency time of the sensor device. The synchronization system according to claim 1.

5. The prior information regarding the time delay has been pre-calibrated for the subject to be examined by imaging in a calibration procedure for measuring the acquisition time interval by direct measurement of the trigger-based event. The synchronization system according to claim 1.

6. A method for calibrating a time delay for the synchronization system according to any one of claims 1 to 5, wherein the acquisition time interval is determined in a calibration test separate from synchronized acquisition of imaging data, (i) the actual instantaneous time of the trigger-based event and a direct measurement of the acquisition time interval, and (ii) simultaneous detection of the trigger-based event by the sensor device, measured by. The method.

7. A computer program for controlling the synchronization system according to any one of claims 1 to 5, Access pre - information regarding the time delay between the detection of a trigger - based event by the sensor device and the start point of the acquisition time interval for acquiring imaging data, Calculate the start point of the acquisition time interval from the detected trigger - based event and the pre - information of the time delay, A computer program.

8. A computer program for controlling the calibration of the time delay for the synchronization system according to any one of claims 1 to 5, Perform a direct measurement of the actual instantaneous time of the trigger - based event and the acquisition time interval, Simultaneously detect a trigger - based event by the sensor device, A computer program for determining the time delay between the actual instantaneous time of the trigger - based event and the detection of the trigger - based event by the sensor device.

9. The sensor device is controlled to detect trigger - based events at different positions of the body of the subject to be examined, The analysis module and the arithmetic unit are configured to derive a pulse propagation velocity from the time difference between the detected trigger - based events and the derived anatomical distance between the different positions, the synchronization system according to any one of claims 1 to 4.

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