System and Method for Measuring the Instantaneous Period of a Quasi-Periodic Signal

• US 20100191118A1
• Filed: 01/29/2009
• Published: 07/29/2010
• Est. Priority Date: 01/29/2009
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Kabakov, Serguei
• Granted Patent: US 8,491,481 B2
• Time in Patent Office: Days
• US Class Current: 600/453
• CPC Class Codes: A61B 5/02411 of foetuses
• CPC Class Codes: A61B 5/344 Foetal cardiography
• CPC Class Codes: A61B 5/35 by template matching
• CPC Class Codes: A61B 5/4362 Assessing foetal parameters
• CPC Class Codes: A61B 8/02 Measuring pulse or heart rate
• CPC Class Codes: A61B 8/0866 involving foetal diagnosis; pre-natal or peri-natal diagnosis of the baby
• CPC Class Codes: A61B 8/5238 for combining image data of patient, e.g. merging several images from different acquisition modes into one image for image registration algorithms G06T7/30
• CPC Class Codes: G01R 23/15 Indicating that frequency of pulses is either above or below a predetermined value or within or outside a predetermined range of values, by making use of non-linear or digital elements (indicating that pulse width is above or below a certain limit)
• CPC Class Codes: G01S 15/52 Discriminating between fixed and moving objects or between objects moving at different speeds
• CPC Class Codes: G01S 15/88 Sonar systems specially adapted for specific applications seismic or acoustic prospecting or detecting G01V1/00

Abstract

A system for measuring the instantaneous period of a quasi-periodic signal includes a transducer that receives a quasi-periodic signal. A correlation computer cross correlates a template with the received quasi-periodic signal to produce a correlation signal. A peak detector identifies peaks of the correlation signal that exceed a threshold. A template creator averages the portions of the quasi-periodic signal that correspond to the identified peaks in the correlation signal to create a new template. A method of measuring the instantaneous period of a quasi-periodic signal includes determining an initial template, performing a cross-correlation between the initial template and a quasi-periodic signal and selecting portions of the signal that correspond to the peaks of the correlation signal. The selected portions are then averaged and another cross correlation is performed between the quasi-periodic signal and a template including the averaged selected portions. The instantaneous period is then measured from the correlation signal.

Claims

1. A method of measuring the instantaneous period of a quasi-periodic signal, the method comprising:

obtaining a quasi-periodic signal;

determining an initial template of a determined size and shift;

performing a cross-correlation between the initial template and the quasi-periodic signal to obtain an initial correlation signal;

establishing a first correlation threshold;

selecting portions of the quasi-periodic signal that correspond to peaks of the initial correlation signal greater than the first correlation threshold;

averaging the selected portions to create a second template;

performing a cross-correlation between the second template and the quasi-periodic signal to obtain a second correlation signal; and

measuring the instantaneous period.

2. The method of claim 1, wherein the step of determining an initial template of a determined size and shift, further comprises:

performing a cross-correlation between the quasi-periodic signal and a template segment of the quasi-periodic signal, the template segment being of a pre-determined size and a pre-determined shift, to obtain a selection coefficient;

incrementing the pre-determined size and the pre-determined shift of the template segment to create an incremented template segment;

repeating the cross-correlation between the quasi-periodic signal and the incremented template segment through a plurality of increments to obtain selection coefficients for each of a plurality of incremented template segments;

selecting the incremented template with the maximum selection coefficient.

3. The method of claim 2 wherein the quasi-periodic signal comprises a first segment and a second segment and the step of determining an initial template is performed using the first segment of the quasi-periodic signal.

4. The method of claim 3 wherein the steps of performing a cross-correlation between the initial template and the quasi-periodic signal and performing a cross-correlation between the second template and the quasi-periodic signal are performed using the second segment of the quasi-periodic signal, and measuring the instantaneous period of the second segment.

5. The method of claim 2, further comprising:

counting the number of peaks in the initial correlation signal greater than the first correlation threshold;

selecting the portions of the quasi-periodic signal that correspond to the peaks of the second correlation signal greater than the second correlation threshold;

averaging the selected portions of the quasi-periodic signal to create a third template; and

performing a cross-correlation between the third template and the quasi-periodic signal to obtain a third correlation signal.

6. The method of claim 2 further comprising:

counting the number of peaks in the initial correlation signal greater than the first correlation threshold;

establishing a second correlation threshold; and

counting the number of peaks in the second correlation signal greater than the second correlation threshold;

wherein if the number of peaks counted in the initial correlation signal is greater than or equal to the number of peaks counted in the second correlation signal, measuring the instantaneous period between the peaks in the second correlation signal, and if the number of peaks counted in the initial correlation signal is less than the number of peaks counted in the second correlation signal;

selecting the portions of the quasi-periodic signal that correspond to the peaks of the second correlation signal greater than the second correlation threshold;

averaging the selected portions of the quasi-periodic signal to create a third template;

performing a cross-correlation between the third template and the quasi-periodic signal to obtain a third correlation signal.

7. The method of claim 2, wherein the quasi-periodic signal comprises a first segment, a second segment, and a third segment and the step of determining an initial template is performed using the first segment.

8. The method of claim 7 further comprising:

measuring the instantaneous period in the second segment by determining the initial template using the first segment, cross correlating the initial template with the second segment and using the cross-correlation between the initial template and the second segment to obtain a correlation signal of the second segment; and

measuring the instantaneous period in the third segment by determining the second initial template using the using the second segment by cross correlating the second initial template with the third segment, and using the cross-correlation between the second initial template and the third segment to obtain a correlation signal of the third segment.

9. The method of claim 7, further comprising:

performing a cross-correlation between the initial template and the first segment to obtain an initial correlation signal;

establishing a first correlation threshold;

selecting portions of the first segment that correspond to peaks of the initial correlation signal greater than the first correlation threshold;

averaging the selected portions to create a second template;

measuring the instantaneous period of the second segment using the second template;

performing a cross-correlation between the initial template and the second segment to obtain a second correlation signal;

establishing a second correlation threshold;

selecting portions of the second segment that correspond to peaks of the second correlation signal greater than the second correlation threshold;

averaging the selected portions to create a third template; and

measuring the instantaneous period of the third segment using the third template.

10. The method of claim 7 further comprising:

cross-correlating the initial template with the first segment to create an initial correlation signal of the first segment;

determining a first final template by;

counting the number of peaks in a previous correlation signal, wherein the previous correlation signal is initially the initial correlation signal of the first segment; and

counting a number of peaks in a current correlation signal, wherein the current correlation signal is initially the second correlation signal of the first segment;

wherein if the number of peaks counted in the previous correlation signal is greater than or equal to the number of peaks counted in the current correlation signal;

establishing the current template as the first final template; and

measuring the instantaneous period between the peaks in the second correlation signal;

wherein if the number of peaks counted in the previous correlation signal is less than the number of peaks counted in the current correlation signal;

selecting the portions of the first segment that correspond to the peaks of the current correlation signal;

averaging the selected portions of the first segment to create a current template; and

cross-correlating the current template with the first segment to determine a new current correlation signal of the first segment;

repeating the above steps until the number of peaks counted in the current correlation signal is greater than or equal to the number of peaks counted in the previous correlation signal;

cross-correlating the first final template with the second segment to create an initial correlation signal of the second segment; and

measuring the instantaneous period in the second segment using the initial correlation signal of the second segment.

11. The method of claim 10 further comprising:

determining a second final template by;

counting the number of peaks in a previous correlation signal, wherein the previous correlation signal is initially the initial correlation signal of the second segment; and

counting a number of peaks in a current correlation signal, wherein the current correlation signal is initially the second correlation signal of the second segment;

wherein if the number of peaks counted in the previous correlation signal is greater than or equal to the number of peaks counted in the current correlation signal;

establishing the current template as the second final template; and

measuring the instantaneous period between the peaks in the current correlation signal;

wherein if the number of peaks counted in the previous correlation signal is less than the number of peaks counted in the current correlation signal;

selecting the portions of the second segment that correspond to the peaks of the current correlation signal;

averaging the selected portions of the first segment to create a current template; and

cross-correlating the current template with the second segment to determine a new current correlation signal of the second segment;

repeating the above steps until the number of peaks counted in the current correlation signal is greater than or equal to the number of peaks counted in the previous correlation signal;

cross-correlating the second final template with the third segment to create an initial correlation signal of the third segment; and

measuring the instantaneous period in the third segment using the initial correlation signal of the third segment.

12. A method of measuring a fetal heart rate from an ultrasound signal, the method comprising:

obtaining an ultrasound signal comprising a first segment and a second segment;

processing the first segment of the ultrasound signal by;

selecting a portion of the first segment as a first template;

performing an autocorrelation between a first template and the first segment to obtain an initial correlation signal with a first peak and a second peak; and

measuring the fetal heart rate of the first segment between the first peak and the second peak;

reporting the measured instantaneous period of the first segment; and

processing the second segment by;

establishing a first correlation threshold;

performing a cross-correlation between the first template and the second segment to obtain a first correlation signal; and

measuring the fetal heart rate across the second segment.

13. The method of claim 12, wherein the processing of the second segment further comprises:

selecting portions of the second segment that correspond to peaks of the first correlation signal greater than a first correlation threshold;

averaging the selected portions to create a second template; and

performing a cross-correlation between the second template and the second segment to obtain a second correlation signal.

14. The method of claim 13, wherein the processing of the second segment further comprises:

counting the number of peaks in the first correlation signal that are greater than the first correlation threshold;

establishing a second correlation threshold;

counting the number of peaks in the second correlation signal that are greater than the second correlation threshold;

wherein if the counted number of peaks in the first correlation signal is equal to or greater than the counted number of peaks in the second correlation signal, then the fetal heart rate is measured between the peaks in the first correlation signal, and if the counted number of peaks in the first correlation signal is less than the counted number of peaks in the second correlation signal, then the steps for processing the second segment are repeated to produce a new correlation signal, until the counted number of peaks in the new correlation signal is less than or equal to the counted number of peaks in the previous correlation signal.

15. The method of claim 12, wherein the first template is of a determined size and a determined shift and further comprising the steps of:

calculating a correlation coefficient from the cross-correlation between the first template and the first segment;

incrementing the size and the shift of the first template to maximize the correlation coefficient;

defining the first template as having the size and shift that maximizes the correlation coefficient.

16. The method of claim 15, further comprising:

establishing an initial correlation threshold;

selecting the portions of the first segment associated with peaks in the initial correlation signal exceeding the initial correlation threshold;

averaging the selected portion of the first segment to create a second template; and

cross-correlating the second template with the first segment to produce a first correlation signal for the first segment.

17. The method of claim 16 further comprising:

counting the number of peaks in the initial correlation signal greater than the initial correlation threshold;

counting the number of peaks in the in the first correlation signal greater than a first correlation threshold;

wherein if the number of peaks in the first correlation signal is less than or equal to the number of peaks in the initial correlation signal, then the fetal heart rate across the second segment is measured by correlating the second template with the second segment and if the number of peaks in the first correlation signal is greater than the number of peaks in the initial correlation signal, then the portions of the first segment associated with the peaks of the first correlation signal are averaged to create a third template, the third template being cross-correlated with the first segment to produce a second correlation signal.

18. A system for measuring an instantaneous fetal heart rate using an ultrasound transducer, the system comprising:

an ultrasound transducer attached to the abdomen of a patient, the ultrasound transducer receiving a quasi-periodic ultrasound signal;

a correlation computer that applies a current template to an ultrasound signal segment to calculate a cross-correlation between the current template and the ultrasound signal segment to produce a current correlation signal;

a peak detector that identifies a plurality of peaks of the current correlation signal that exceeds a threshold, the peak detector further producing a count of the peaks of the current correlation signal that exceed the threshold;

a template creator connected to the peak detector and the correlation computer and averages the portions of the ultrasound signal segment that correspond to the detected peaks in the current correlation signal to create a new current template, the old current template being redefined as the previous template, the correlation computer applying the new current template to the ultrasound signal segment to produce a new current correlation signal, the old current correlation signal being redefined as the previous correlation signal;

a fetal heart rate detector that measures the instantaneous period of the current correlation signal to determine a measurement of fetal heart rate; and

a graphical display that presents the measurement of fetal heart rate;

wherein a count of the peaks of the previous correlation signal is compared to the count of the peaks of the current correlation signal, and if the count of the peaks of the previous correlation signal are less than or equal to the count of the peaks of the current correlation signal, then the fetal heart rate is measured using the current correlation signal; and

if the count of the peaks of the current correlation signal are greater than the count of the peaks of the previous correlation signal, the template creator averages the portions of the current ultrasound signal segment that correspond to the detected peaks in the current correlation signal to create a new current template that is applied to the ultrasound signal segment.

19. The system of claim 18 further comprising:

a template selector connected to receive the ultrasound signal and output a selected template to the correlation computer; and

a quality detector connected to receive the correlation signal, the quality detector detecting the quality of the correlation signal output from the correlation computer;

Wherein when the quality of the correlation signal is high, an indication is sent to the template selector to select a long term analysis (LTA) template and if the quality of the correlation signal is low, an indication is sent to the template selector to select a short term analysis (STA) template.

20. The system of claim 19 wherein the template selector further comprises:

A template select that is connected to the quality detector and selects between an STA template and an LTA template;

An STA template generator that provides an STA template;

An LTA template generator that comprises a correlation computer an LTA database and a correlation coefficient database, the correlation computer applying each of a plurality of LTA templates from the LTA database to the ultrasound signal to produce a plurality of correlation coefficients that are stored in the correlation coefficient database, the LTA template generator selecting the LTA template that corresponds to the maximum correlation coefficient;

Wherein when the template select receives a low quality indication from the quality detector, the template select selects the STA template and when the template select receives a high quality indication from the quality detector, the template select selects the LTA template.