METHODS AND APPARATUS FOR ULTRASOUND IMAGING

• US 20080242994A1
• Filed: 10/29/2007
• Published: 10/02/2008
• Est. Priority Date: 03/29/2007
• Status: Abandoned Application

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Aloka Company Limited (Hitachi, Ltd.)
• Inventors: Tamura, Tadashi
• Application Number:
  US11/926,251
• Publication Number:
  US 20080242994A1
• Time in Patent Office: Days
• US Class Current: 600/453
• CPC Class Codes: A61B 8/06 Measuring blood flow measuring volume flow in general G01F, e.g. G01F1/66, G01F1/72; measuring speed of fluids in general G01P5/00
• CPC Class Codes: A61B 8/488 involving Doppler signals
• CPC Class Codes: G01S 15/584 with measures taken for suppressing velocity ambiguities, i.e. anti-aliasing
• CPC Class Codes: G01S 15/8979 Combined Doppler and pulse-echo imaging systems

Abstract

The maximum frequency in a Doppler spectrum is obtained and used as an aliasing detector. When aliasing occurs, frequencies greater than a frequency limit change from one frequency region to another. When aliasing is detected, a zero frequency baseline is shifted to prevent future aliasing.

Claims

1. A method for detecting and correcting aliasing in a Doppler frequency spectrum comprising:

receiving a Doppler frequency spectrum signal over time;

calculating maximum frequencies f_max and minimum frequencies f_min from the Doppler frequency spectra

tracking the maximum f_max and minimum f_min frequencies over time;

detecting whether aliasing is occurring from the maximum frequencies f_max if frequencies in a positive frequency region change (wrap) to a negative frequency region, or detecting whether aliasing is occurring from the minimum frequencies f_min if negative frequencies in the negative frequency region change (wrap) to the positive region; and

if aliasing is detected, shifting a zero frequency baseline separating the negative and positive frequency regions of the Doppler spectrum in either a positive or negative direction according to a maximum frequency deviation f_a.

2. The method according to claim 1 wherein the Doppler spectrum signal is from the group consisting of an amplitude spectrum a, a power spectrum a², or a power raised to a power a^b, where b is a real number.

3. The method according to claim 1 wherein the maximum frequencies f_max are percentile values of the Doppler frequency spectra.

4. The method according to claim 1 wherein the minimum frequencies f_min are percentile values of the Doppler frequency spectra.

5. The method according to claim 1 further comprising determining the maximum frequency deviation f_a from the magnitude of frequencies that have wrapped from one region (positive or negative) to the other (negative or positive).

6. The method according to claim 1 further comprising:

adding a frequency safety margin f_s to the maximum frequency deviation f_a; and

shifting the frequency spectrum baseline by the maximum frequency deviation plus safety margin ±

(f_a+f_s).

7. A method of determining a pulse repetition frequency for an ultrasound system comprising:

receiving a Doppler frequency spectrum signal over time;

calculating maximum frequencies f_max from the Doppler frequency spectra;

calculating minimum frequencies f_min from the Doppler frequency spectra;

tracking the maximum f_max and minimum f_min frequencies over time;

capturing a highest value high f_max of the maximum f_max frequencies and a lowest value low f_min of the minimum f_min frequencies tracked;

comparing the highest value high f_max and the lowest value low f_min to determine whether the maximum f_max frequencies and minimum f_min frequencies are bipolar, or negative or positive unipolar;

if bipolar;

determining a frequency span based on a difference between the highest maximum frequency high f_max and lowest minimum frequency low f_min;

comparing the frequency span to a current PRF setting value;

if the frequency span is greater than the current PRF setting value, increase the PRF setting value;

if the frequency span is less than a predetermined fraction of the current PRF setting value, decrease the PRF setting value; and

if the frequency span is less than the current PRF setting value but greater than the predetermined fraction of the current PRF, use the current PRF setting value;

if positive unipolar;

comparing the highest maximum frequency high f_max with a current positive maximum frequency limit b₁f_PRF, wherein if the highest maximum frequency high f_max is greater than the current positive maximum frequency limit b₁f_PRF the current PRF setting value is increased to a setting corresponding to the highest maximum frequency high f_max;

if the highest maximum frequency high f_max is less than a current positive maximum frequency limit b₁f_PRF, comparing the highest maximum frequency high f_max with a low level threshold b₂b₁f_PRF, wherein if the highest maximum frequency high f_max is less than the low level threshold b₂b₁f_PRF, the PRF is decreased until equal to the highest maximum frequency high f_max; and

if negative unipolar;

comparing the absolute value of the lowest minimum frequency low f_min with the absolute value of a current negative maximum frequency limit −

(1−

b₁)f_PRF, wherein if the absolute value of the lowest minimum frequency low f_min is greater than the absolute value of the current negative maximum frequency limit −

(1−

b₁)f_PRF, the current PRF setting value is increased to a setting corresponding to the absolute value of the lowest minimum frequency low f_min;

if the absolute value of the lowest minimum frequency low f_min is less than the absolute value of the current negative maximum frequency limit −

(1−

b₁)f_PRF, comparing the absolute value of the lowest minimum frequency low f_min with the absolute value of a low level threshold −

b₂(1−

b₁)f_PRF, wherein if the absolute value of the lowest minimum frequency low f_min is less than the absolute value of the low level threshold −

b₂(1−

b₁)f_PRF, the PRF is decreased to equal the absolute value of the lowest minimum frequency low f_min.

8. The method according to claim 7 wherein the Doppler spectrum signal is from the group consisting of an amplitude spectrum a, a power spectrum a², or a power raised to a power a^b, where b is a real number.

9. The method according to claim 7 wherein the maximum frequencies f_max are percentile frequencies of the Doppler frequency spectra.

10. The method according to claim 7 wherein the minimum frequencies f_min are percentile frequencies of the Doppler frequency spectra.

11. The method according to claim 7 further comprising:

detecting whether aliasing is occurring from the maximum frequencies f_max if frequencies in a positive frequency region wrap to a negative frequency region;

detecting whether aliasing is occurring from the minimum frequencies f_min if negative frequencies in the negative frequency region wrap to the positive region; and

if aliasing is detected, shifting a zero frequency baseline separating the negative and positive frequency regions of the Doppler spectrum in either a positive or negative direction according to a maximum frequency deviation f_a.

12. The method according to claim 7 further comprising:

detecting whether aliasing is occurring from the maximum frequencies f_max if frequencies in a positive frequency region wrap to a negative frequency region;

detecting whether aliasing is occurring from the minimum frequencies f_min if frequencies in the negative frequency region wrap to the positive region; and

if aliasing is detected, adding aliased frequency deviations to the aliased maximum frequencies and to the aliased minimum frequencies.

13. The method according to claim 11 wherein the frequency deviation further comprises:

determining the maximum deviation f_a from the magnitudes of frequencies that have wrapped from one region (positive or negative) to the other (negative or positive) to calculate the maximum frequencies f_max and the minimum frequencies f_min.

14. The method according to claim 13 further comprising:

adding frequency safety margins to the maximum frequency deviation f_a; and

shifting the frequency spectrum baseline by the maximum frequency deviation f_a plus safety margins.

15. The method according to claim 7 further comprising:

adding frequency safety margins to the frequency span, highest maximum frequency high f_max, or the absolute value of the lowest minimum frequency low f_min; and

comparing with the current PRF.

16. A method of determining a pulse repetition frequency for an ultrasound system comprising:

setting an initial pulse repetition frequency;

receiving a Doppler frequency spectrum signal over time;

calculating maximum frequencies f_max from the Doppler frequency spectra;

calculating minimum frequencies f_min from the Doppler frequency spectra;

tracking the maximum f_max and minimum f_min frequencies over time;

capturing a highest value high f_max of the maximum frequencies f_max and a lowest value low f_min of the minimum frequencies f_min tracked;

comparing the absolute value of the highest maximum value high f_max with the absolute value of the lowest minimum frequency low f_min to determine whether the positive or negative frequency region takes precedence;

if the highest maximum value high f_max is greater, the positive frequency region takes precedence and a positive low level threshold b₂b₁f_PRF is calculated; and

comparing the highest maximum frequency high f_max with the positive maximum frequency limit b₁f_PRF and the positive low level threshold b₂b₁f_PRF wherein if the highest maximum frequency high f_max is less than the positive low level threshold b₂b₁f_PRF, the PRF is decreased until the positive maximum frequency limit b₁f_PRF equals the highest maximum frequency high f_max, or aliasing starts to occur at the negative maximum frequency limit −

(1−

b₁)f_PRF whichever comes first, and wherein if the highest maximum frequency high f_max is greater than the positive maximum frequency limit b₁f_PRF, the PRF is increased to equal the highest maximum frequency high f_max;

if the absolute value of the lowest minimum frequency low f_min is greater, the negative frequency region takes precedence and a low level threshold −

b₂(1−

b₁)f_PRF is calculated;

comparing the absolute value of the lowest minimum frequency low f_min with the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF and the absolute value of the low level threshold −

b₂(1−

b₁)f_PRF wherein if the absolute value of the lowest minimum frequency low f_min is less than the absolute value of the low level threshold −

b₂(1−

b₁)f_PRF, the PRF is decreased until the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF is the absolute value the lowest minimum frequency low f_min or aliasing starts to occur at the positive frequency limit whichever comes first, and wherein if the absolute value of the lowest minimum frequency low f_min is greater than the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF, the PRF is increased to equal the lowest minimum frequency low f_min.

17. The method according to claim 16 further comprising adding frequency safety margins to the highest maximum frequency high f_max and the absolute value of the lowest minimum frequency low f_min.

18. The method according to claim 16 wherein the positive maximum frequency limit b₁f_PRF and the negative maximum frequency limit −

(1−

b₁)f_PRF are determined by the PRF and a zero frequency baseline position which is fixed.

19. The method according to claim 16 wherein the maximum frequencies f_max are percentile frequencies of the Doppler frequency spectra.

20. The method according to claim 16 wherein the minimum frequencies f_min are percentile frequencies of the Doppler frequency spectra.

21. The method according to claim 16 wherein the observation period may be less than a cardiac cycle or a long period of at least one cardiac period.

22. The method according to claim 16 wherein the maximum f_max and minimum frequencies f_min are calculated from the Doppler spectra with or without a noise reduction.

23. A system for detecting and correcting aliasing in a Doppler frequency spectrum comprising:

means for receiving a Doppler frequency spectrum signal over time;

means for calculating maximum frequencies f_max and minimum frequencies f_min from the Doppler frequency spectra;

means for tracking the maximum f_max and minimum f_min frequencies over time;

means for detecting whether aliasing is occurring from the maximum frequencies f_max if frequencies in a positive frequency region change (wrap) to a negative frequency region;

means for detecting whether aliasing is occurring from the minimum frequencies f_min if negative frequencies in the negative frequency region change (wrap) to the positive region; and

if aliasing is detected, means for shifting a zero frequency baseline separating the negative and positive frequency regions of the Doppler spectrum in either a positive or negative direction according to a maximum frequency deviation f_a.

24. A system for determining a pulse repetition frequency for an ultrasound system comprising:

means for setting an initial pulse repetition frequency;

means for receiving a Doppler frequency spectrum signal over time;

means for calculating maximum frequencies f_max from the Doppler frequency spectra;

means for calculating minimum frequencies f_min from the Doppler frequency spectra;

means for tracking the maximum f_max and minimum f_min frequencies over time;

means for capturing a highest value high f_max of the maximum frequencies f_max and a lowest value low f_min of the minimum frequencies f_min tracked;

means for comparing the absolute value of the highest maximum value high f_max with the absolute value of the lowest minimum frequency low f_min to determine whether the positive or negative frequency region takes precedence;

if the highest maximum value high f_max is greater, the positive frequency region takes precedence and a positive low level threshold b₂b₁f_PRF is calculated; and

means for comparing the highest maximum frequency high f_max with the positive maximum frequency limit b₁f_PRF and the positive low level threshold b₂b₁f_PRF wherein if the highest maximum frequency high f_max is less than the positive low level threshold b₂b₁f_PRF, the PRF is decreased until the positive maximum frequency limit b₁f_PRF equals the highest maximum frequency high f_max or aliasing starts to occur at the negative maximum frequency limit −

(1−

b₁)f_PRF whichever comes first, and wherein if the highest maximum frequency high f_max is greater than the positive maximum frequency limit b₁f_PRF, the PRF is increased to equal the highest maximum frequency high f_max;

if the absolute value of the lowest minimum frequency low f_min is greater, the negative frequency region takes precedence and a low level threshold −

b₂(1−

b₁)f_PRF is calculated;

means for comparing the absolute value of the lowest minimum frequency low f_min with the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF and the absolute value of the low level threshold −

b₂(1−

b₁)f_PRF wherein if the absolute value of the lowest minimum frequency low f_min is less than the absolute value of the low level threshold −

b₂(1−

b₁)f_PRF, the PRF is decreased until the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF is the absolute value the lowest minimum frequency low f_min or aliasing starts to occur at the positive frequency limit whichever comes first, and wherein if the absolute value of the lowest minimum frequency low f_min is greater than the absolute value of the negative maximum frequency limit −

(1−

b₁)f_PRF, the PRF is increased to equal the lowest minimum frequency low f_min.