METHODS AND APPARATUS FOR ULTRASOUND IMAGING

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 a 2 , 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 b 1 f PRF , wherein if the highest maximum frequency high f max is greater than the current positive maximum frequency limit b 1 f 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 b 1 f PRF , comparing the highest maximum frequency high f max with a low level threshold b 2 b 1 f PRF , wherein if the highest maximum frequency high f max is less than the low level threshold b 2 b 1 f 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−b 1 )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−b 1 )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−b 1 )f PRF , comparing the absolute value of the lowest minimum frequency low f min with the absolute value of a low level threshold −b 2 (1−b 1 )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 −b 2 (1−b 1 )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 a 2 , 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 b 2 b 1 f PRF is calculated; and

comparing the highest maximum frequency high f max with the positive maximum frequency limit b 1 f PRF and the positive low level threshold b 2 b 1 f PRF wherein if the highest maximum frequency high f max is less than the positive low level threshold b 2 b 1 f PRF , the PRF is decreased until the positive maximum frequency limit b 1 f PRF equals the highest maximum frequency high f max , or aliasing starts to occur at the negative maximum frequency limit −(1−b 1 )f PRF whichever comes first, and wherein if the highest maximum frequency high f max is greater than the positive maximum frequency limit b 1 f 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 −b 2 (1−b 1 )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−b 1 )f PRF and the absolute value of the low level threshold −b 2 (1−b 1 )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 −b 2 (1−b 1 )f PRF , the PRF is decreased until the absolute value of the negative maximum frequency limit −(1−b 1 )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−b 1 )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 b 1 f PRF and the negative maximum frequency limit −(1−b 1 )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 b 2 b 1 f PRF is calculated; and

means for comparing the highest maximum frequency high f max with the positive maximum frequency limit b 1 f PRF and the positive low level threshold b 2 b 1 f PRF wherein if the highest maximum frequency high f max is less than the positive low level threshold b 2 b 1 f PRF , the PRF is decreased until the positive maximum frequency limit b 1 f PRF equals the highest maximum frequency high f max or aliasing starts to occur at the negative maximum frequency limit −(1−b 1 )f PRF whichever comes first, and wherein if the highest maximum frequency high f max is greater than the positive maximum frequency limit b 1 f 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 −b 2 (1−b 1 )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−b 1 )f PRF and the absolute value of the low level threshold −b 2 (1−b 1 )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 −b 2 (1−b 1 )f PRF , the PRF is decreased until the absolute value of the negative maximum frequency limit −(1−b 1 )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−b 1 )f PRF , the PRF is increased to equal the lowest minimum frequency low f min .