CORRECTION OF DETECTING DEPTH AND CALCULATION OF SPEED OF MOVING OBJECTS BASED ON TIME OF FLIGHT OF ULTRASOUND PULSES

Abstract

During transmission, speed of ultrasound pulses gradually reduces due to their energy loss from acoustic impedance. So, calculating a detecting depth with fixed speed of the ultrasound pulses will distort ultrasound images. Correction of the detecting depth will rectify a depth registration and improve the imaging quality. The thickness and density of piezoelectric elements (PZT) decide a quantity of the ultrasound pulses, which affect their detecting depth. The density and sound speed in PZT elements decide the frequency of the ultrasound pulses, which can be used to increase the detecting depth for a high frequency ultrasound. Moving objects can change speed of reflected ultrasound pulses, which change their TOF and TOF shift. Therefore the TOF shift can be used to calculate the velocity of the moving objects in a continuous and a pulsed wave and a color ultrasound, and correct aliasing of the pulsed wave and the color ultrasound.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No. 13/341,928, filed on Dec. 31, 2011, which claims priority to Provisional Application No. 61/508,333 filed on Jul. 15, 2011, and U.S. patent application Ser. No. 14/305,074 filed on Jun. 16, 2014, the entire contents of all of which are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to the field of ultrasound technology and, more particularly, relates to a method for correction of detected depth and calculation of speed of moving objects based on time of flight (TOF) of domain analysis for a continuous wave, a pulsed wave and a color ultrasound.

BACKGROUND

Transmission of ultrasound pulses is actually energy traveling of acoustic pulses in transmitting medium. Currently, it is supposed that speed of the ultrasound pulses is fixed in the same medium during the transmission. But, in the invention, the speed of the ultrasound pulses is considered as gradually reduced during the transmission due to acoustic impedance of the transmitting medium, which depletes energy of the ultrasound pulses. If there is acoustic impedance during the transmission of ultrasound pulses, the acoustic impedance will resist the movement of the ultrasound pulses. The acoustic impedance is directly related to the speed of the ultrasound pulses in the transmitting medium. Higher speed of the ultrasound pulses will meet higher resistance and consume more its energy during the transmission. So, the question is if the speed of the ultrasound pulses can still keep the same as currently supposed when its energy is gradually reduced until exhausted? If the speed of the ultrasound pulses is gradually reduced during transmission, the detected depth will be wrong based on calculating the detecting depth with fixed ultrasound speed.

Comparing lower frequency of the ultrasound pulses, higher frequency of the ultrasound pulses has larger attenuation coefficient and thus is more readily absorbed in the transmitting medium, which limits the detecting depth of the ultrasound pulses. So, if there is way to increase the detecting depth for high frequency ultrasound?

Ultrasound pulses can be reflected by motionless or moving objects, and it is currently considered that forward moving objects can compress the frequency of the ultrasound pulses and reversely moving objects decompress the frequency of the ultrasound pulses. So, Doppler has been widely used to measure the velocity of the moving objects based on frequency shift, such as medical ultrasound machine and Doppler radar. In Doppler of the pulsed wave ultrasound, aliasing is explained with insufficient Doppler sampling rate of the frequency domain analysis. But, the theory of the frequency domain can not completely solve the aliasing problem in Doppler of the pulsed wave ultrasound and the color ultrasound.

Thus, there is a need to overcome above problems to provide methods for more accurately calculating the detecting depth of ultrasound pulses, increasing the detecting depth of high frequency ultrasound, correctly calculating the speed of moving objects and correcting the aliasing for the pulsed wave and the color ultrasound.

BRIEF SUMMARY OF THE DISCLOSURE

One aspect of the invention, correcting the transmitting distance of ultrasound pulses can rectify the registration of the detecting depth, which improves the quality of ultrasound images. Current ultrasound theory is based on the fixed speed of ultrasound pulses in the same transmitting medium. But, the invention is based the speed reduction of ultrasound pulses during the transmission in the medium due to the loss of their energy caused by acoustic impedance. Calculating of the detecting depth of ultrasound pulses based on the fixed speed of the ultrasound pulses will cause miscalculation of the detecting depth due to the speed reduction of ultrasound pulses. As the result, the calculated detecting depth is larger than its actually traveling depth, which distorts the ultrasound images. Because the speed of ultrasound pulses is inversely correlated to acoustic impedance and attenuation coefficient, they can be used to calculate the ultrasound speed reducing coefficient and correct the registration of detecting depth of ultrasound pulses, which improve the quality of images.

In another aspect of the invention, changing thickness and density of piezoelectric elements (PZT) and sound speed in the PZT elements can regulate quantity and density of the ultrasound pulses, which affect the detecting depth and frequency of the ultrasound pulses. The detecting depth of the ultrasound pulses is not directly related to their frequencies, but related to the quantity of the ultrasound pulses. The quantity of the ultrasound pulses is decided by the thickness and the density of PZT elements. The more thickness and density of the PZT elements will generate the greater quantity of the ultrasound pulses, which increase their detecting depth. At the same time, selecting the PZT with greater density and higher speed of ultrasound pulses in the PZT elements will generate the ultrasound pulses with greater quantity and higher frequency, which increase the detecting depth for high frequency ultrasound.

Another aspect of the invention is detecting the speed of moving objects based on TOF shift of time domain analysis for a continuous wave, a pulsed wave and a color ultrasound. It is based on the speed changes of reflected ultrasound pulses by the moving objects, which change its TOF and TOF shift of the ultrasound pulses. As the ultrasound pulses are emitted from activated PZT, the ultrasound pulses contain their quantity. No matter in the continuous wave or the pulsed wave or the color ultrasound, when checking the speed of blood flow, the ultrasound system always detects the reflected ultrasound pulses from fixed locations where ultrasound beam cross with blood vessels to calculate the TOF shift. So, the speed of the moving objects can be calculated based on the TOF shift. The moving objects may change the rebounding forces to the ultrasound pulses, which is decided by the speed and angle of the moving objects with the ultrasound beam. Faster forward speed and/or greater angle of the moving objects with the ultrasound beam will produce greater rebounding force, which generates greater reflected speed of the ultrasound pulses and results in shorter TOF and greater TOF shift. On the contrary, faster reversed speed and/or smaller angle of the moving objects with the ultrasound beam will reduce the rebounding force, which produces slower reflected speed of the ultrasound pulses and results in longer TOF and greater TOF shift. For the continuous, the pulsed wave and the color ultrasound, the speed of the moving objects can be calculated based on the TOF shift.

The theory of above TOF can be used to completely correct the aliasing for the pulsed wave and the color ultrasound no matter how fast the speed of the moving objects will be. The calculated TOF is based on the general speed of ultrasound pulses in the transmitting medium and distance between transducer and the gate. The detected TOF is the time that the ultrasound system interprets from emitted pulses and reflected pulses. Before the aliasing limits, the detected TOF is the actual TOF, and after aliasing the detected TOF is the aliasing TOF. The detected TOF will be affected by the moving objects. If the speed of the moving objects is too fast, which makes the actual TOF excesses its aliasing limits, the ultrasound system will misinterpret the reflected ultrasound pulses and generate the aliasing TOF. For the forward moving objects, the aliasing limit for the actual TOF is less than the value of half of calculated TOF. If the actual TOF is smaller than the aliasing limit, the ultrasound system will misinterpret the reflected ultrasound pulse and add a value of calculated TOF into the actual TOF, which generates the aliasing TOF. Then the aliasing TOF is greater than the calculated TOF. So, the aliasing TOF shift is below the baseline, which represents the moving objects toward opposite direction. For reversely moving objects, their TOF aliasing limit is that the actual TOF is greater than the value of one and half calculated TOF. If the actual TOF is greater than the aliasing limit, the ultrasound system will misinterpret the detected TOF and subtract a value of calculated TOF from the actual TOF. Then the aliasing TOF is smaller than the calculated TOF. So, the aliasing TOF shift is above the baseline, which represents the moving objects as forward direction. So, in the invention, a computer program is designed to identify the aliasing TOF shift, and the corrected TOF shift can be used to correct aliasing when the detected TOF excesses its aliasing limit in the pulse wave or the color ultrasound no matter how fast the speed of the moving objects will be. Identifying and correcting the aliasing TOF shift can also be used to differentiate the colors of aliasing from the colors of the disturbed flow, which benefits clinical judgment and diagnosis.

Based on the speed reduction of the ultrasound pulses during the transmission, the quantity of the ultrasound pulses, and the TOF changes of ultrasound pulses by the moving objects, TOF shift can more accurately present the relationship between the TOF of the ultrasound pulses and the speed of the moving objects, and it also better explains the effect of the velocity and angle of the moving objects on the ultrasound pulses. So, the calculated speed of the moving objects from the TOF shift should be more accurate than the results from the Doppler shift. Other aspects or embodiments of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure.

FIG. 1 is a schematic illustration of rebound force of forward flow to ultrasound pulse;

FIG. 2 is a schematic illustration of rebound force of reversed flow to ultrasound pulse;

FIG. 3a is a schematic illustration of spectrum for TOF shift of continuous wave ultrasound;

FIG. 3b is a schematic illustration of TOF shift and the profile of TOF shift for forward moving objects of pulsed wave ultrasound;

FIG. 3c is a schematic illustration of TOF shift and profile of TOF shift for reversely moving objects of pulsed wave ultrasound;

FIG. 4 is a schematic illustration of aliasing TOF and aliasing TOF shift;

FIG. 5a is a schematic illustration of profile of aliasing TOF shift for forward flow of pulsed wave ultrasound;

FIG. 5b is a schematic illustration of profile of corrected TOF shift for forward flow of pulsed wave ultrasound;

FIG. 6a is a schematic illustration of profile of aliasing TOF shift for reversed flow of pulsed wave ultrasound;

FIG. 6b is a schematic illustration of profile of corrected TOF shift for reversed flow of pulsed wave ultrasound;

FIG. 7 is a schematic illustration of computer program to calculate TOF shift of continuous wave ultrasound;

FIG. 8 is a schematic illustration of computer program to identify and correct aliasing TOF shift, and calculate the speed of moving objects for pulsed wave and color ultrasound;

FIG. 9 is a schematic illustration of the color of aliasing in color ultrasound;

FIG. 10 is a schematic illustration of the colors of disturbed flow in color ultrasound; and

FIG. 11 is a schematic illustration of computer program to differentiate the color of disturbed flow from the color of aliasing and correct color of aliasing based on TOF shift.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of the disclosure, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

Speed of Ultrasound Pulses Gradually Reduces During Transmission

Transmitting in a medium, a sound pulse contains three factors: quantity, density and energy. The quantity of the sound pulse is related to thickness of materials that create the sound. The thicker material gives greater quantity of the sound, which is like different sounds from different chords of a violin. The density of the sound is related to density of materials that create the sound. The higher density of the materials is, the greater density of the sound will be, such as the sounds launched from wood or metal. The energy of the sound is related its speed. A sound pulse with greater energy will travel faster. The speed of the sound will gradually reduce due to the impedance of transmitting medium, which gradually depletes the energy of the sound.

Piezoelectric elements (PZT) in a transducer of ultrasound machine emit ultrasound pulses with their quantity, density and energy. The energy of the ultrasound pulses is transmitted in a medium. Nowadays, the speed of the ultrasound pulses is considered as fixed in the same medium. But, actually the speed of the ultrasound pulses is not fixed at the same speed as supposed during the transmission, and it will gradually reduce due to acoustic impedance of the transmitting medium. As bullets shooting from a machine gun, their speed is gradually reduced due to loss of their energy caused by resistance of air. But, frequency of the bullets at any sites of trajectory may be kept the same. The transmission of the ultrasound pulses has the similar mechanism. During the transmission, the speed of the ultrasound pulses gradually reduces due to the loss of their energy caused by the acoustic impedance, which will finally exhausts the energy of the ultrasound pulses. A rate of its speed changes is decided by attenuation coefficient and acoustic impedance. But, Ultrasound pulses keep the same frequency during the transmission, including reflected frequency.

The acoustic impedance is decided by density of the transmitting medium and the speed of the ultrasound pulses in the transmitting medium.

Acoustic impedance (rayls)=density (kg/m3)×speed (m/s)

Total attenuation (dB)=attenuation coefficient×distance

So, the higher the speed of the ultrasound pulse is, the greater the acoustic impedance will be, which consume more its energy during the transmission. As the energy transmission of acoustic pulses in the medium, the speed of the ultrasound pulses should be directly related to its energy. Therefore its speed will gradually reduce due to the loss of its energy during its transmission until the exhaustion of its energy.

Rectifying the Registration of Detecting Depth of Ultrasound Pulses Improves the Quality of Images

One aspect of the invention is rectifying errors of the registration of detecting depth of the ultrasound pulses due to the speed reduction of the ultrasound pulses during the transmission. Under effect of the acoustic impedance and the attenuation coefficient, the speed of the ultrasound pulses will gradually reduce as it transmits in the transmitting medium. Nowadays, calculating the detecting depth is based on fixed general speed of the ultrasound pulses in the transmitting medium, which will cause miscalculation of the detecting depth due to speed changes of the ultrasound pulses, and distort the ultrasound images. The longer the ultrasound pulses travel, the slower the speed of the ultrasound pulses will be. So, actually detecting depth of the ultrasound pulses is smaller than the calculated detecting depth. As the speed reduction of ultrasound pulses is directly correlated to the acoustic impedance and the attenuation coefficient. A speed reducing coefficient of the ultrasound pulses can be used to correct the errors of the calculated detecting depth. Distance shift of the ultrasound pulses is a value of multiplication result of the speed reducing coefficient and traveling time of the ultrasound pulses. V is the general speed of the ultrasound pulses in the transmitting medium. For instance, the general ultrasound speed in soft tissues is 1540 meter/second. t is the traveling time from emitting to receiving the ultrasound pulses. Calculated detecting depth is the depth based on the general speed and the traveling time of the ultrasound pulses between emitting to receiving. The corrected detecting depth is a difference between the calculated detecting depth and depth shift.

Speed reducing coefficient=acoustic impedance×attenuation coefficient

Depth shift=speed reducing coefficient×t/2

Calculated detecting depth=V×t/2

Corrected detecting depth=calculated detecting depth−depth shift

the Quantity of the Ultrasound Pulses Affect their Detecting Depth

In another aspect of the invention, increasing the quantity of the ultrasound pulses by increasing the thickness and the density of PZT elements increases the detecting depth of the ultrasound pulses.

In current ultrasound theory, the attenuation coefficient is directly proportional to the frequency of the ultrasound pulses. The lower the frequency of the ultrasound pulses is, the smaller the attenuation coefficient will be. As the frequency of the ultrasound pulses is inversely proportional to the thickness of the PZT elements, the more thickness of PZT has lower frequency of the ultrasound pulses.

Attenuation Coefficient (dB/cm)=frequency (MHz)/2

Frequency=sound speed in PZT/2×PZT thickness

Actually, the thickness and the density of the PZT elements are directly related to the quantity of the ultrasound pulses. The more thickness and the density of the PZT elements, the more PZT elements will be activated, which generate greater quantity of the ultrasound pulses. Therefore, the attenuation coefficient is actually decided by the quantity of the ultrasound pulses, which is related the thickness and the density of the PZT elements. As a heavier ball has ability of further traveling distance, the ultrasound pulses with greater quantity will have greater penetrating depth because it has lower attenuation coefficient, and have smaller reducing rate of its speed comparing to the ultrasound pulses with smaller quantity. So, changing the thickness and the density of the PZT elements can regulate the quantity of the pulses and its detecting depth.

Quantity of ultrasound pulse=PZT thickness×PZT density

Attenuation Coefficient (dB/cm)=sound speed in PZT/4×quantity of ultrasound pulse

Increasing Detecting Depth for High Frequency Ultrasound by Increasing its the Density of PZT Elements and Sound Speed in PZT elements

In another aspect of the invention, increasing the density of the PZT elements and sound speed in the PZT elements will increase the detecting depth for high frequency ultrasound. The greater density of the PZT elements, the more PZT elements are activated, which generate the ultrasound pulses with greater density. At the same time, greater density also generates higher sound speed in the PZT elements. Currently, in order to increase the frequency of the ultrasound pulses, the thickness of the PZT elements is reduced, which decreases the quantity of the ultrasound pulses and their detecting depth. But, in the invention, by creating a transducer with greater density of the PZT elements and higher sound speed in the PZT elements, but not decreasing the thickness of the PZT elements, it will increase the frequency as well as the quantity of the ultrasound pulses. As the result, it increases the detecting depth for high frequency ultrasound.

Frequency=sound speed in PZT/2×PZT thickness

Quantity of ultrasound pulses=PZT thickness×PZT density

Moving Objects Change TOF and TOF Shift of the Ultrasound Pulses

As containing the quantity and the energy, the ultrasound pulses can be reflected by motionless or moving objects. No matter in the continuous wave or the pulsed wave or the color ultrasound, when checking speed of blood flow, the ultrasound system always detects the reflected ultrasound pulses from fixed locations where the ultrasound beam cross with blood vessels to calculate TOF shift of the reflected ultrasound pulses. So, it is unlike changes of sounds from coming or leaving motorcycle, because distance of the motorcycle is changing. But more like playing table tennis, a racket hits a ball at fix location and changes speeds of the reflected ball, which changes TOF of the ball. Comparing to motionless objects, moving objects will change the rebounding force to the ultrasound pulses. As in the FIG. 1, forward moving objects will generate the forward rebound force shift against the ultrasound pulses. The forward rebounding force shift is decided by speed and angle θ of the moving objects with the ultrasound beam. The faster speed of the moving objects and greater angle θ will generate greater forward rebounding force shift, which increases the speed of the reflected ultrasound pulses. So, its TOF is decreased and smaller than the TOF from motionless objects (baseline). As the result, the TOF shift is increased and above the baseline. On the contrary, as in the FIG. 2, reversely moving objects will generate reversed rebounding force shift with the same direction of emitted ultrasound pulses, which reduces the rebounding force. The faster speed of the moving objects and smaller angle θ will create greater reversed rebounding force shift, which decreases the reflected speed of the ultrasound pulses. So, its TOF is increased and greater than the baseline. As the result, the TOF shift is below baseline.

As the quantity of the ultrasound pulses is directly related the thickness and density of the PZT elements, changing the quantity of the ultrasound pulses also affects their TOF and TOF shift. For the forward moving objects, increasing the quantity of the ultrasound pulses will have smaller rate of increased speed of the reflected pulses. It elongates their actual TOF and reduces their TOF shift. Decreasing the quantity of the ultrasound pulses have greater rate of increased speed of the reflected ultrasound pulses, which will shorten their TOF and increase their TOF shift. For reversely moving objects, increasing the quantity of the ultrasound pulses will have smaller rate of decreased speed of the reflected ultrasound pulses. It shortens its actual TOF and decreases their TOF shift. Decreasing the quantity of the ultrasound pulses have greater rate of decreased speed of the reflected ultrasound pulses, which elongates their TOF and increase their TOF shift.

The density of the PZT elements also affects the TOF shift. The greater density of the PZT elements generates higher density of the ultrasound pulses with higher speed of the ultrasound pulses in the PZT elements, which will shorten its TOF and increase its TOF shift.

$\mathrm{frequency}=\mathrm{sound}\mathrm{speed}\mathrm{in}\mathrm{PZT}/2\times \mathrm{PZT}\mathrm{thickness}$ $\mathrm{Rebounding}\mathrm{force}\mathrm{shift}=\mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \cos \theta$ $\mathrm{Reflected}\mathrm{speed}\mathrm{shift}=\mathrm{rebounding}\mathrm{force}\mathrm{shift}/\mathrm{pulse}\mathrm{quantity}$ $\mathrm{TOF}\mathrm{shift}=\frac{\begin{array}{c}2\times \mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \\ \mathrm{transducer}\mathrm{frequency}\times \cos \theta \end{array}}{\mathrm{pulse}\mathrm{propagation}\mathrm{speed}}$ $\mathrm{TOF}\mathrm{shift}=\frac{\begin{array}{c}\mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \cos \theta \times \\ \mathrm{sound}\mathrm{speed}\mathrm{in}\mathrm{PZT}\end{array}}{\mathrm{PZT}\mathrm{thickness}\times \mathrm{pulse}\mathrm{propagation}\mathrm{speed}}$

So, one aspect of the invention is calculating the speed of the moving objects based on the TOF shift for the continuous wave or the pulsed wave or the color ultrasound. As mentioned above, the speed of the ultrasound pulses will gradually reduce, and the moving objects will generate the rebounding force shift, which changes the speed of the reflected ultrasound pulses, their TOF and TOF shift. Therefore, the TOF shift can more accurately present relationship between the speed of the moving objects and characters of the ultrasound pulses.

Calculating Speed of Moving Objects Based on TOF Shift of Continuous Wave Ultrasound

Currently, it is considered that speed of the ultrasound pulses is fixed in the same medium during the transmission. The moving objects will change the frequency of the reflected ultrasound pulses. The forward moving objects will compress the reflected frequency, which is higher than the emitted frequency. Its Doppler shift is above the baseline. The reversely moving objects will decompress the reflected frequency, which is lower than the emitted frequency. Its Doppler shift is below the baseline. So, calculating Doppler shift of the continuous wave (CW) ultrasound is based on difference between the reflected frequency and the emitted frequency.

$\mathrm{Doppler}\mathrm{shift}=\mathrm{reflected}\mathrm{frequency}-\mathrm{emitted}\mathrm{frequency}$ $\mathrm{Doppler}\mathrm{shift}=\frac{\begin{array}{c}2\times \mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \\ \mathrm{transducer}\mathrm{frequency}\times \cos \theta \end{array}}{\mathrm{pulse}\mathrm{propagation}\mathrm{speed}}$

The invention discloses that TOF shift of the CW ultrasound is used to calculate the speed of the moving objects. The TOF shift of the CW ultrasound is difference between a time of emitting period and a time of reflected period. There are two PZT parts in CW ultrasound transducer. As in the FIG. 7, the emitting PZT part emits the continuous wave ultrasound pulses with certain emitted period between previous and following emitted ultrasound pulses (105), which is decided by the ultrasound system. The receiving PZT part receives the reflected ultrasound pulses and detects the time of the reflected period between previously and following reflected ultrasound pulses (106). The time of the reflected period is decided by the speed of the moving objects and the angle of the moving objects with the beam of ultrasound pulses. The emitted period is set as baseline, and the TOF shift equals zero at the baseline. Then, the ultrasound system obtains the TOF shift from difference between the emitted period and the reflected period, and calculates speed of the moving objects according to the equation of the TOF shift (108).

As 101 in FIG. 3, the emitted period is the time between the previously and following emitted pulses, which forms the baseline. The reflected period is the time between previously and following reflected ultrasound pulses. If the ultrasound pulses are reflected from the moving objects that are vertical to the ultrasound beam (flow N), the emitted period equals to its reflected period, and the TOF shift is zero. But, if the ultrasound pulses are reflected from the forward moving objects (flow M), the speed of the reflected ultrasound pulses will be accelerated due to the increasing rebounding force, which shorten the TOF M′. So, the reflected period will be less than the time of the emitted period, which generates TOF shift M′ and is above the baseline. On the contrary, for the reversely moving object (flow O), the TOF O′ will be elongated due to the reduced rebounding force and the speed of the reflected ultrasound pulses. So, the reflected period will be greater than the time of the emitted period, which generates the TOF shift O′ and is below the baseline.

$\mathrm{emitted}\mathrm{period}=\mathrm{the}\mathrm{time}\mathrm{between}\mathrm{previous}\mathrm{and}\mathrm{following}$ $\mathrm{emitted}\mathrm{pulses}$ $\mathrm{reflected}\mathrm{period}=\mathrm{the}\mathrm{time}\mathrm{between}\mathrm{previously}\mathrm{and}\mathrm{following}$ $\mathrm{reflected}\mathrm{pulses}$ $\mathrm{TOF}\mathrm{shift}=\mathrm{emitted}\mathrm{period}-\mathrm{reflected}\mathrm{period}$ $\mathrm{TOF}\mathrm{shift}=\frac{\begin{array}{c}2\times \mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \\ \mathrm{transducer}\mathrm{frequency}\times \cos \theta \end{array}}{\mathrm{pulse}\mathrm{propagation}\mathrm{speed}}$

As 100 in FIG. 3, for the CW ultrasound, a transducer receives all of the reflected ultrasound pulses from an area under the transducer. If there are several moving objects with different velocities toward the transducer, they will rebound the ultrasound pulses with different reflected speeds and TOFs, which generate different TOF shifts related to these moving objects. Then the ultrasound system will compare a list of these reflected pulses and respectively present these TOF shifts on TOF shift spectrum. For CW ultrasound, because there are usually multiple moving objects under the transducer with different velocities, such as multiple blood vessels, its TOF shift spectrum often presents as spectral broadening as 102 in FIG. 3a. So, a computer program can be used to calculate the speed of the moving objects based on the values of the TOF shift as in FIG. 7.

Calculating Speed of Moving Objects by TOF Shift of Pulsed Wave and Color Ultrasound

There is only one PZT in a transducer of the pulsed wave ultrasound, which sends and receives ultrasound pulses. So, the transducer has to receive previously reflected pulses before sending next emitted pulses. In order to detect speed of the moving objects, a gate is set with a certain distance. So, based on the general speed of the ultrasound pulses and the distance between the transducer and the gate, a calculated TOF can be obtained as A in FIG. 3b, which is set as the baseline, and the TOF shift at the baseline equals to zero. The detected TOF is that the ultrasound system interprets TOF from the emitted and reflected ultrasound pulses, which can be affected by the moving objects. The actual TOF is the time the ultrasound pulses actually travel between the transducer and the gate. The aliasing TOF is the misinterpreted TOF by ultrasound system after the actual TOF excesses its aliasing limit. Before aliasing, the detected TOF is the actual TOF, and after aliasing, the detected TOF is the aliasing TOF. The TOF shift is difference between the calculated TOF and the detected TOF. For a forward moving object, it accelerates the speed of the reflected pulses, which shorten its actual TOF as B in FIG. 3b. So, the actual TOF is smaller than the calculated TOF, and the TOF shift is above the baseline. As increasing the speed of the moving objects, the value of the detected TOF decreases and the value of the TOF shift increases, tip of the profile of the TOF shift is away from the baseline (80 in FIG. 3b). On the contrary, reversely moving objects elongate their actual TOF, which is greater than the baseline, and the TOF shift is below the baseline. As the speed of the moving objects increases, the value of the detected TOF and the value of the TOF shift both increase, and the tip of the profile of the TOF shift is away from the baseline (82 in FIG. 3c). Then the speed of the moving objects can be calculated according the value of the TOF shift.

$\mathrm{TOF}\mathrm{shift}=\mathrm{calculated}\mathrm{TOF}-\mathrm{detected}\mathrm{TOF}$ $\mathrm{TOF}\mathrm{shift}=\frac{\begin{array}{c}2\times \mathrm{speed}\mathrm{of}\mathrm{moving}\mathrm{objects}\times \\ \mathrm{transducer}\mathrm{frequency}\times \cos \theta \end{array}}{\mathrm{pulse}\mathrm{propagation}\mathrm{speed}}$

Identifying and Correcting Aliasing for Pulsed Wave Ultrasound

For the pulsed wave ultrasound, there is aliasing, which is caused by the ultrasound system misinterpreting the detected TOF from the reflected pulses. If the speed of the moving objects is too fast, and makes its actual TOF excesses its aliasing limit, the ultrasound system will misinterpret it and the detected TOF becomes an aliasing TOF. Then the aliasing TOF shift is located on opposite side of the baseline, which presents the moving objects as toward opposite direction. The aliasing TOF shift also disrupts continuation of the profile of the TOF shift.

For forward moving objects, their aliasing limit of the actual TOF is less than the value of half calculated TOF. if the actual TOF is smaller than its aliasing limit, the ultrasound system will misinterpret the reflected pulses, and the aliasing TOF is a value of a actual TOF adding a calculated TOF, which is larger than the calculated TOF (104 in FIG. 4). So, the aliasing TOF shift becomes below baseline, which misrepresents the moving objects moving toward opposite direction. As the result, before the actual TOF excesses its aliasing limit, the value of the TOF shift is above the baseline (from E to F in FIG. 5). But, after the actual TOF excesses its aliasing limit, the value of the aliasing TOF shift is below the baseline (G and H in FIG. 5a); As the speed of the moving objects increases, both the value of the aliasing TOF and the value of the aliasing TOF shift decrease; and the tip of the profile of the aliasing TOF shift is toward the baseline (81 in FIG. 5a), which discontinues the profile of the TOF shift.

Aliasing TOF=actual TOF+calculated TOF

Aliasing TOF shift=calculated TOF−aliasing TOF

Aliasing TOF shift=−actual TOF

So, in the invention, a computer program is designed to identify and correct the aliasing TOF shift. For the forward moving objects, the actual TOF is smaller than calculated TOF, and its TOF shift is above the baseline. As the speed of moving objects is increased, its actual TOF keeps decrease and smaller than the calculated TOF, and the TOF shift keeps increase and above baseline. But, after the actual TOF excesses its aliasing limit, the aliasing TOF becomes greater than the calculated TOF, and the aliasing TOF shift becomes below the baseline. The computer program will trace and compare the value of the following TOF and TOF shift with the value of the previous TOF and TOF shift. If the value of the TOF and the TOF shift approaches the value of half calculated TOF, and the value of following TOF shift is below the baseline, which discontinues the profile of the TOF shift. It is a aliasing TOF shift. After identifying the aliasing TOF shift, the ultrasound system will register the corrected TOF shift by subtracting the value of the aliasing TOF shift from one calculated TOF (116 in FIG. 8).

TOF shift=calculated TOF−actual TOF

Aliasing TOF shift=−actual TOF

corrected TOF shift=calculated TOF−|aliasing TOF shift|

After rectifying the registering errors of TOF shift, the value of the corrected TOF shift will keep increase as increase of the speed of the moving objects, and the tip of the profile of the TOF shift is away from the baseline (84 in FIG. 5b), which reestablish the continuation of the profile of the TOF shift (FIG. 5b), and the value of the correct TOF shift can be used to calculated the speed of the moving objects.

For the reversely moving objects, the rebounding force is reduced, which decreases the reflected speed of the ultrasound pulses and increases their TOF, which is greater than the value of the calculated TOF. So, the value of the TOF shift is below the baseline. For the reversely moving objects, the aliasing limit of the actual TOF is larger than the value of one and half calculated TOF. If the value of the actual TOF excesses its aliasing limit, the ultrasound system will misinterpret the reflected ultrasound pulses and the aliasing TOF is the value of the actual TOF subtracting a calculated TOF, which is smaller than the calculated TOF. So, the aliasing TOF shift will be above the baseline; as the speed of the reversely moving objects keeps increase, the aliasing TOF is increased but the aliasing TOF shift is decreased, which makes the tip of the profile of TOF shift is toward baseline (83 in FIG. 6a). As a result, the continuity of the profile of TOF shift is disrupted (FIG. 6a). In the invention, the computer program is used to identify the aliasing. As the value of actual TOF is close to the value of one and half calculated TOF and TOF shift approaches the value of half calculated TOF, if following TOF shift is above the baseline, the aliasing TOF shift is identified.

Aliasing TOF=actual TOF−calculated TOF

Aliasing TOF shift=calculated TOF−aliasing TOF

Aliasing TOF shift=2×calculated TOF−actual TOF

After identifying the aliasing TOF shift, the computer program will rectify the aliasing TOF shift by subtract the value of a calculated TOF from the value of the aliasing TOF shift, which is based on following equations:

TOF shift=calculated TOF−actual TOF

aliasing TOF shift=2×calculated TOF−actual TOF

correct TOF shift=aliasing TOF shift−calculated TOF

After correcting the aliasing TOF shift, the corrected TOF shift will increase as the speed of the moving objects keeps increase, which makes the tip of the profile of the corrected TOF shift away from the baseline. The corrected TOF shift will reestablish the continuation of the profile of the TOF shift (FIG. 6b), and it can be used to calculate the speed of the moving objects.

Differentiating Color of Aliasing from Color of Disturbed Flows for Color Ultrasound

For the color ultrasound, colors are used to represent direction of the moving objects. But, there are similar color patterns between color of aliasing and color of disturbed flows. For aliasing pattern, there can be the color that mistakenly presents as the moving objects toward opposite side after the actual TOF excesses its aliasing limit. For the disturbed flows, there can be color that truly presents the moving objects toward opposite side. So, this will make the difficulties for clinical judgment and diagnosis for pathological situations. In the invention, differentiating the color of aliasing from the color of disturbed flows is based on the characters of the TOF shift of different colors.

For the color of aliasing in FIG. 9, when a forward flow (S) passes a narrow part of vessel, the speed of a blood flow will be accelerated within the narrow part. If its actual TOF excesses its aliasing limit, the aliasing TOF shift marks the flow with a color of aliasing (T) at the narrow part, which represents the blood flow as toward opposite direction. Color of U represents the flow between the color of no-aliasing S and the color of aliasing T, and the value of TOF shift for color U is close to the value of half calculated TOF because the actual TOF for the color U is closing to its aliasing limit. From the color T to the color U, or from the color S to the color U, their TOF shift is gradually increased until close to the value of half calculated TOF. For the color of aliasing, the profile of aliasing TOF shift will be more close to the value of half calculated TOF with its tip of the profile of the aliasing TOF shift toward baseline. But for the color of no-aliasing (color S), the profile of no-aliasing TOF shift will be more close to the baseline with the tip of the profile of the no-aliasing TOF shift away from the baseline. Correcting the aliasing TOF shift is based on the direction of no-aliasing flow as forward or reversely moving direction. Then the color of aliasing can be corrected based on the corrected TOF shift. The designed computer program in FIG. 11 will trace and identify the characters of the profile of the TOF shift for theses colors, and correct the color of aliasing by rectifying their aliasing TOF shift.

But, for the color of disturbed flow in FIG. 10, the color of X represents a forward flow that enters in an enlarged part of a blood vessel. The flow will become disturbed at the enlarged part of the vessel, and the color of Y represents a reversed blood flow. The color of Z represents the flow at the edge between flow of X and flow of Y. The TOF shift for color of Z will be close to zero because its actual TOF is close to its baseline. The TOF shift from one color to the edge of another color is gradually reduced until close to the zero, because the speed of the flow is gradually decreased to the edge area. The tip of the profile of the TOF shift for both colors is away from the baseline. The colors of flows are assigned based on their TOF shift.

So, tracing and differentiating the aliasing TOF shift for the color of aliasing from the TOF shift for the color of disturbed flows, and correct the colors of aliasing, will benefit the clinical judgment and diagnosis for truly pathological conditions.

Increasing the Quantity of Ultrasound Pulses Helps Improving Aliasing

Increasing the quantity of the ultrasound pulses by increasing the thickness and the density of PZT will reduce the rate of speed changes for the reflected ultrasound pulses, which improves the aliasing. For the forward moving objects, increasing the quantity of the ultrasound pulses will have smaller rate of increased speed of the reflected pulses, which elongates their actual TOF and delays their value reaching their aliasing limit. For the reversely moving objects, increasing the quantity of the ultrasound pulses will have smaller rate of decreased speed of the reflected pulses, which reduces their actual TOF and delays their value reaching their aliasing limit.

Decreasing Detecting Depth Helps Improving Aliasing

The speed of the ultrasound pulses is gradually reduced during the transmission. But, the calculated TOF is based on the fixed general speed of ultrasound pulses in the transmitting medium, which will cause the systematic errors in the calculation. Decreasing the detecting depth will decrease the systematic errors. So, selecting the moving objects that are closer to the transducer will reduce the systematic errors and improve the aliasing.

Other applications, advantages, alternations, modifications, or equivalents to the disclosed embodiments are obvious to those skilled in the art and are intended to be encompassed within the scope of the present disclosure.

Claims

1. A method for rectifying errors of registration of a detecting depth due to speed reduction of ultrasound pulses during a transmission, the method comprising:

obtaining a calculated detecting depth from a general speed of the ultrasound pulses and a traveling time between emitting and receiving ultrasound pulses, wherein the calculated detecting depth is a half value of a multiplication result of the general speed of the ultrasound pulses in a transmitting medium and the traveling time between the emitting and the receiving ultrasound pulses;

calculating a speed reducing coefficient of the ultrasound pulses, wherein the speed reducing coefficient of the ultrasound pulses equals to a multiplication result of acoustic impedance and attenuation coefficient;

obtaining a depth shift of the ultrasound pulses, wherein the depth shift of the ultrasound pulses equals to a half value of a multiplication result of the speed reducing coefficient and the traveling time; and

determining a corrected detecting depth, wherein the corrected detecting depth is a difference between the calculated detecting depth and the depth shift.

2. The method of claim 1, further comprising: changing one or more of thickness and density of piezoelectric elements (PZT) to regulate quantity and density of the ultrasound pulses, wherein the quantity of the ultrasound pulses is directly proportional to the thickness and the density of the PZT elements, which comprising:

Quantity of ultrasound pulses=PZT thickness×PZT density.

3. The method of claim 2, further comprising: changing the quantity of the ultrasound pulses to regulate their detecting depth wherein increasing the quantity of the ultrasound pulses increases the detecting depth.

4. The method of claim 2, further comprising: changing the density of the PZT elements to regulate the density of the PZT and a speed of sound in the PZT elements wherein increasing the density of the PZT increases the density of the ultrasound pulses and the speed of sound in the PZT elements.

5. The method of claim 4, further comprising: creating a transducer with greater density of the PZT elements and a faster speed of sound in the PZT elements to generate the ultrasound pulses with larger quantity and higher frequency wherein the transducer with high frequency has a deeper detecting depth.

6. A method of using time of flight (TOF) shift of the ultrasound pulses to calculate a speed of moving objects in a continuous wave, a pulsed wave and a color ultrasound, the method comprising: TOF   shift = 2 × speed   of   moving   objects × transducer   frequency × cos   θ pulse   propagation   speed

setting a baseline of TOF wherein the baseline is a traveling time of the ultrasound pulses emitted from a transducer and reflected from motionless objects at the same depth as from the moving objects, and a TOF shift equals to zero at the baseline, the baseline is decided by a ultrasound system;

obtaining a detected TOF wherein the detected TOF is the time that ultrasound system interprets from the emitting and receiving ultrasound pulses, the detected TOF is decided by the speed of the moving objects and an angle of the moving objects with a ultrasound beam, and the quantity of the ultrasound pulses;

calculating a TOF shift wherein the TOF shift is a difference between the baseline and the detected TOF; and

calculating the speed of the moving objects based on an equation wherein θ is the angle of a ultrasound beam made with the moving objects, a pulse propagation speed is determined by a transmitting medium for the ultrasound pulses, and the TOF shift is the TOF shift for the continuous wave, the pulsed wave and the color ultrasound, the equation is:

7. The method of claim 6, further comprising: changing the angle of the ultrasound pulses with the moving objects to regulate a rebounding force wherein the changes of the rebounding force alter the TOF and the TOF shift of the ultrasound pulses.

8. The method of claim 6, further comprising: changing the quantity of the ultrasound pulses to regulate the speed of the reflected ultrasound pulses wherein the changes of the speed of the reflected ultrasound pulses alter the TOF and the TOF shift of the ultrasound pulses.

9. The method of claim 6, further comprising: changing the density of the PZT elements and the sound speed in the PZT elements to regulate the TOF shift of the ultrasound pulses.

10. The method of claim 6, further comprising a method of calculating the speed of the moving objects for the continuous wave ultrasound comprising:

setting a time of a emitted period as the baseline wherein the time of the emitted period is the time between previously and following emitted pulses, the time of the emitted period is decided by ultrasound system;

obtaining a time of a reflected period as the detected TOF wherein the time of the reflected period is the time between previously and following reflected pulses;

calculating a TOF shift wherein the TOF shift is a difference between the time of the emitted period and the time of the reflected period; and

using the TOF shift to calculate the speed of the moving objects based on the equation.

11. The method of claim 6 further comprising a method of calculating the speed of the moving objects for the pulsed wave and the color ultrasound comprising:

setting a time of a calculated TOF as the baseline wherein the calculated TOF is the time that ultrasound system calculates according to a distance between a transducer and a gate and the general speed of the ultrasound pulses in the transmitting medium;

obtaining the detected TOF wherein the detected TOF is the time that ultrasound system interprets from ultrasound pulses traveling between the transducer and the gate, before a aliasing the detected TOF is an actual TOF, and the actual TOF is a truly traveling time of the ultrasound pulses;

calculating a TOF shift wherein the TOF shift is a difference between the value of the calculated TOF and the value of the detected TOF; and

using the TOF shift to calculate the speed of the moving objects based on the equation.

12. The method of claim 11, further comprising a method to correctly calculate the speed of the moving objects after a aliasing for the pulsed wave and color ultrasound comprising: corrected   TOF   shift = 2 × speed   of   moving   objects × transducer   frequency × cos   θ pulse   propagation   speed

identifying an aliasing of the ultrasound pulses, wherein as the actual TOF excesses its aliasing limit, the detected TOF is misinterpreted by ultrasound system to generate a aliasing TOF, the aliasing TOF shift is on opposite site of the baseline and disrupts continuity of the profile of the TOF shift, and the tip of the profile of the aliasing TOF shift is toward the baseline;

obtaining a corrected TOF shift by rectifying the aliasing TOF shift to correct registration of the reflected ultrasound pulses after the actual TOF exceeds the aliasing limit; and

using the corrected TOF shift to calculate the speed of the moving objects based on an equation:

13. The method of claim 12, wherein for forward moving objects, the speed of the moving objects is correctly calculated after the aliasing by:

identifying the aliasing for the forward moving objects wherein a aliasing limit for the actual TOF is less than the value of half calculated TOF; after the actual TOF excesses the aliasing limit, the ultrasound system misinterprets the detected TOF by adding a value of one calculated TOF to a value of the actual TOF to form an aliasing TOF; the value of the aliasing TOF is greater than the value of the baseline and its TOF shift is located on opposite site of the baseline, and the tip of the profile of the aliasing TOF shift is toward the baseline; and the aliasing TOF disrupts the continuation of the profile of the TOF shift;

obtaining the corrected TOF shift by subtracting a value of the aliasing TOF shift from a value of the calculated TOF to reestablish the continuation of the profile of the TOF shift; and

using the corrected TOF shift to calculate the speed of the forward moving objects based on the equation.

14. The method of claim 12, wherein for reversely moving objects, the speed of the moving objects is correctly calculated after the aliasing by:

identifying the aliasing for the reversely moving objects wherein a aliasing limit for the actual TOF is greater than the value of one and half calculated TOF; after the actual TOF excesses the aliasing limit, the ultrasound system misinterprets the detected TOF by subtracting a value of one calculated TOF from a value of the actual TOF to form the aliasing TOF; the value of the aliasing TOF is smaller than the value of the baseline and the aliasing TOF shift is located on opposite site of the baseline, the tip of the profile of the aliasing TOF shift is toward the baseline; and the aliasing TOF shift disrupts the continuation of the profile of the TOF shift;

obtaining the corrected TOF shift by subtracting a value of the calculated TOF shift from a value of the aliasing TOF shift to reestablish the continuation of the profile of the TOF shift; and

using the corrected TOF shift to calculate the speed of the reversely moving objects based on the equation.

15. The method of claim 12, further comprising a method to differentiating a color of aliasing from a color of disturbed flow and rectifying the color of aliasing for color ultrasound comprising:

identifying the aliasing TOF shift for the color of aliasing wherein from a flow within one color to a flow on edge of another color, the value of the TOF shift is gradually increased until close to the value of half calculated TOF; the profile of the aliasing TOF shift is more close to the value of half calculated TOF and the tip of the profile of the aliasing TOF shift is toward the baseline; the profile of no-aliasing TOF shift is more closer to the baseline, and the tip of the no-aliasing TOF shift is away from the baseline, the no-aliasing flow is a flow before its TOF excesses the aliasing limit;

identifying the TOF shift for the color of disturbed flow wherein from a flow within one color to a flow on edge of another color, the value of the TOF shift is gradually decreased until close to zero, the tip of the profile of the TOF shift for the colors is away from the baseline; and

rectifying the color of aliasing wherein the aliasing TOF shift is corrected according to the direction of a no-aliasing flow, and the color of aliasing is corrected based on the value of the corrected TOF shift.

16. The method of claim 2, further comprising: changing the quantity of the ultrasound pulses to regulate a rate of speed changes for the reflected pulses wherein increasing the quantity of the ultrasound pulses improves the aliasing.

17. The method of claim 6, further comprising: selecting the moving objects that are closer to the transducer to reduce the systematic errors and to improve the aliasing.