CA1295409C - Backscatter data collection technique for ultrasound - Google Patents
Backscatter data collection technique for ultrasoundInfo
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- CA1295409C CA1295409C CA000580098A CA580098A CA1295409C CA 1295409 C CA1295409 C CA 1295409C CA 000580098 A CA000580098 A CA 000580098A CA 580098 A CA580098 A CA 580098A CA 1295409 C CA1295409 C CA 1295409C
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- ultrasound
- fundamental frequency
- intensity
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Abstract
BACKSCATTER DATA COLLECTION TECHNIQUE FOR ULTRASOUND
Abstract of the Disclosure A rapid, non-invasive realtime modality for detecting differential backscatter in ultrasound is achieved using narrowband interrogating frequencies to maximize signal-to-noise ratio and to minimize location errors. Additional improvement in signal-to-noise ratio and separation of tissue populations are achieved by cyclically alternating through a fixed number of predetermined burst lengths and by employing multiple narrowband bursts of differing fundamental frequencies.
Abstract of the Disclosure A rapid, non-invasive realtime modality for detecting differential backscatter in ultrasound is achieved using narrowband interrogating frequencies to maximize signal-to-noise ratio and to minimize location errors. Additional improvement in signal-to-noise ratio and separation of tissue populations are achieved by cyclically alternating through a fixed number of predetermined burst lengths and by employing multiple narrowband bursts of differing fundamental frequencies.
Description
BACKSCATTBR DATA COLL~CTIOH TBCH~IQU~ FOR ULTRASO W D
BacX~round of the Inventlon The pre~ent ~nvention relates in general to ultrasound apparatu~
for metlcal dlagnostlc lmaging, and more speclflcally to measurement and dlsplay of differentlal bacXscatter cro-s sectlon per unlt volume of insonlfled tlssue.
~ coust~c parameters useful in medlcal ultrasound include reflectlon at dl~contlnultles, volume backscatter coefficient, absorption coefficient and Doppler frequoncr sh~ft. Ultrasonic measuremont of these paramoters provide~ a basis for varlous tlssue characterizations and for constructln~ ima8es. Although reflection imagin8 (using extrlns~c measurements) has histor~cally been the principal basis for dla~nostic ultrasound, it is reallzed that intrin~ic parameters are useful for t~ssue localizatlon and characterization and for detectlon of pathology.
15Back~cattor coefflclent has been lnvestlgated for lts utlllty in imasing and characterizatlon and has been found to prov~de ~ dia~nostlcally useful quantitative data. However, tlssue pathology ; i~ not alw~ys readily spparent from absolute moasuremonts of the backscatter coefflcient. For example, it is somotimes requlred to ; 20 perform filtering of broadband backscatter data to flnd frequency dependencies of tho backscatter coefflc~ent to ldentlfy pathologlcal tissue.
The success of a dia~nostic modallty in providin8 useful infonmatlon depends on an ablllty to interrogate tlssue and acqulre acou~tic data with a maxlmum s~gnal-to-nolse ratlo (S~ O . However, 4(?9 specular reflections, signal correlations, inhome~eneous attenuation, and other factors result in a ~enerally low SUR in ultrasound backscatter meaSuremQnt.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for collectin~ quantitative backscatter data.
It is another ob;ect to improve tissue characterization and diagnosis by means of quantitative backscatter data.
It is another object to improve the signal-to-noise ratio in measurements of intrinsic acoustic parameters for ultrasound.
It is still another object of the invention to detect localized variations in bacXscatter intensity and dependencies in an ob~ect.
SummarY of the Invention These and other ob~ects are achieved in an ultrasound system for measurin~ differential backscatter across a selected volume of tissue or organ and for detecting temporal and frequency dependencies of backscatter in order to distin~ulsh between normal and diseased tissue.
By way of example, sufficient contrast exists between backscatter measurements of normal and diseased myocardial tlssues to allow detection and ima8in8 by ultrasound. Infarcted tlssue exhibits increased backscatter where the local amount of collagen is increased and also exhibits increased signal attenuat~on related to ~5 local collagen concentration and depletion of creative kinase, relatlve to normal myocardial tissues. Severely ischemic myocardium (e.g., 80 percent reduction of local blood flow) results in about a dB increase in bacXYcatter but in a decrease in attenuation.
Furthermore, the cyclic variation present in the backscatter intensi~y of normal myocardium which has a minimum near occurrence of end systole and has a maximum near occurrence of end diastole appears weakened in ischemic tissue. Thus, ima8in~ of the differential backscatter cross section per unit volume of insonified tissue of a heart and its temporal and/or frequency dependence .~. ~ ., ,, , ~, resultin8 from various tissue populations enables detection of various patholo~ical areas.
The signal-to-noise ratio of backscatter measurements in the present inventlon is improved by employing narrowband interro~ating signals to reduce statistical variations in backscatter intensity and to improve tissue regionalization. Use of narrowband si~nals also eliminates the need for ran8e 8atin8 in signal processing. S~R
is further improved by obtaining and combinin8 substantially uncorrelated measurements of a tar8et volume without moving the vector angle by repetitively firin8 narrowband bursts of varyin~
longth or cycles.
In another aspect of the invention, measurements are obtained usln~ a plural~ty of sequentlal narrowband interrogating bursts of d~fferent fundamental frequencies to characterize tissue in terms of a combinatlon of scatter contribution from cellular and parenchymal tissue archltectures as a further means of tissue discrimination.
Thus, frequency dependencies of the backscatter coefficient are employed to infer contrasts in tissue structure according to an empirically derived model.
Brief DescriDtion of the Drawin~s The novel features of the invention are set forth with particularity in the appended claim~. The inventlon itself, however, both as to its organization and method of operation, together with further ob~ects and advantages thereof, may best be understood by reference to the following description taken in con~unctlon with the accompanying drawings in which:
FIGURE 1 is a block dia8ram of an ultrasonic system for practicing the present invention.
FIGURE 2 shows the acquisitlon of an ultrasonic ima~e.
FIGURE 3 illustrates the transmission bandwidth employed in the present invention.
FIGURE 4 is a block dia8ram showing the mid-processor of FIG. l in ~reater detail.
.,.,,~.~,.......
~ h~ 9 15UL02971 Detsiled Descri~tion of the Invention Referrin8 now to FIG. 1 a receive/transmit transducer 10 comprisin~ a plurality of transducer elements in an array is coupled to a front-end processor 11 which includes a pulser 12 and a demodulator and si~nal processor 13. The ultrasound system further includes a mid-processor 15, display processor 16 and a display 17.
Pulser 12 energizes transducer array 10 to insonify along a vector angle in an object. In a receive mode, signals datected by transducer array 10 are coupled to demodulator and signal processor 13 which operates in a known manner to provide a su~med in-phase signal I and summed quadrature signal Q which are phase insensitive.
hit-proces~or 15 determines the back~catter intensity based on signals I and Q. The intensity or other bacXscatter feature is provided to a display processor 16 for presentation on display 17 in any desired format. Other backscatter parameters having diagnostic utility include temporal changes in backscatter intensity (e.g., difference between systole and diastole for cardiac tissue) which can be measured and displayed. Another example is the frequency dependence of the backscatter coefficient which is used to detect variation in tissue structure, as will be described later.
In a preferred embodiment of the invention, tissue features such as lesions can be detected in a tar8et organ by separating various tissue populations accordin~ to localized differential backscatter coefficient. The backscatter coefficient at each resolved point, taken at one or more frequencies, provides the data used to detect le~ion~ and provide secondary information to confirm diagnoses or ~`~ locate a lesion for follow-up exams or treatment, for example.
Ultrasonic interro~ation of an organ is represented in ~IG. 2.
Transducer 10 transmits ultra~onic energy toward a heart 20.
Transmitted ultrasonic waves 21 interact with heart 20 to produce backscattered waves 22. In one preferred embodiment of the present invention, a typical B-mode ima8e of heart 20 is presented on display 17. Thereafter, localized differential backscatter :
:
1~5~09 information is collected for the same ima8e area and is overlaid on the B-mode image (by adding various colors, for example~ so that a lesion 23 can be detected in heart 20. Alternatively, a two-dimensional (2D) B-mode image corresponding only to localized backscatter coefficients can be employed.
In biologlcal tissue, the actual scatterers that cause back~catter and their separation are not known. If a small tissue volume of scatterers (such as l cc) is investigated, independent measurements of backscatter cross section (i.e., backscatter intensity) of the tissue sample are sub~ect to lar~e statistical variations because of tissue structures. This variation in independent measurements of a single tar8et area make~ it difficult or impossible to separate tissue populations based on backscatter intensity. According to the present inventlon, it is found that use of a narrowband frequency for the transmitted ultrasonic energy will select a larger scatterer volume to give consistent intensity measurements and better signal-to-noise performance.
The improved signal-to-noise performance of the present narrowband systems results from a reduction in backscatter intensity variations because of the larger volume of scatterers (scatter number) associated with each backscatter pixel ~i.e., tar8et volume). nean number of scatterers per unit volume follows a Poisson distribution, such that the variation in backscatter intensity varies as the square root of the mean number of scatterers in the effectlve scatterer volume. Thus, an effective scatterer volume containing lO scatters provides standard deviation in intensity measurements of about 30 percent. An increased effective scatterer volume containing lO0 scatterers improves intensity standard deviation to 10 percent. Therefore, signal-to-noise and the ability to separate tissue populations are improved according to the narrowband system of the present invention.
The narrowband transmission of the present invention has the further advantage of improved lateral and depth regionalization by reducing frequency dependencies and interactions present with a 4~9 15uL o 2 9 7 1 wideband system. The wideband effects caused by interference between component frequencies are avoided in the present invention.
Turning now to EIG. 3, an exemplary frequency spectrum for the narrowband transmission of the present invention is shown. A
narrowband signal is centered on a fundamental frequency fO. The ability to define a narrowband energy transmission depends on the passband characteristics of the pul~ing system and the number of cycles of the fundamental frequency in an interro~ating burst. In the present invention, the bandwidth is sufficiently narrow so that all of the backscattered ener~y can be considered as coming from the fundamental frequency. As shown in FIG. 3, a -10 dB fractional bandwidth less than about 20 percent is employed. Further, a preferred fractional bandwidth of between 10 and 20 percent provides excellent narrowbant results.
A further improvement in the SNR received for each tar8et volume is achieved by the present lnvention by repetitively firin~
narrowband bursts of varying length to acquire backscattered data for each tar8et volume. For example, narrowband performance occurs in a system employing a fundamental frequency in the ran~e of several megahertz when interrogating bursts of 3 or more cycles are used. A first interrogating burst of 3 cycles can be averaged with a subsequent burst length of S cycles at the same tar8et volume.
The varied burst length alters the receive amplitude statlstics, such that a sub~tantially independent estimate of the backscatter intensity is obtalned without movin~ the interrogatin~ vector angle. Each independent measurement i8 partially uncorrelated so as to improve the SNR of the si~nal avera~e. The change in burst length or cycles slightly alters the spectral content of the ~; interrogating beam, a~ well as changes the volume of scatterer thatcontrlbute to the si~nal. In a preferred embodiment, separate interro~atlng bursts of 7, 9 and 11 cycles are emyloyed at a fundamental frequency of 2.5 HHz to obtain three partially ; uncorrelated measurements which are equivalent to about 2.5 ; independent samples of the tar8et volume. ~t a fundamental 4~9 frequency of 3.6 ~Hz, burst lengths of lO, 13, and 16 cycles have been employed with good results.
In another aspect of the invention, backscatter intensity measurements that provide independent, uncorrelated estimates are obtained at a plurality of fundamental frequencies in separate, sequentlal measurements using narrowband transmission. Thus, one or more interrogatlons are performed at a first fundamental frequency and may include bursts of varying cycles. Other measurements obtained at a second fundamental frequency are combined with the measurements at the first fundamental frequency to generate a value for the backscatter intensity.
Backscatter measurements at a plurality of narrowband frequencies are further employed in the invention to find the frequency dependence of backscatter for a tar8et area of tissue.
Thl8 frequency dependence enables characterization of tissue in terms of the relative bacXscatter contribution from cellular and parenchymal scatterer populations. Backscatter coefficient N as a function of a frequency f can be modeled accordin~ to the equation:
N(f) Wlf W2f where Wl and W2 are empirically derived factors thought to represent Mie region scattering ob~ects and Rayleigh-l~Xe scattering ob~ects within tissue, respectively.
When N~f) is measured at two frequencies (e.g., 2.5 HHz and 3.6 MHz), two equations are obtained and solutions for Wl and W2 can be found using matrix mathematics. For the case with fl = 2.5 MHz and f2 = 3.6 HHz, the solutions are:
Wl = N(2.5 NHz)D - N~3.6 MHz)B and W2 = N(3.6 HHz)A - N(2.5 MHz)C
where A = 2.5, B = 3.6, C = 39, and D = 168.
~954~39 The two main tissue populations (i.e., cellular and parenchymal) are thus distin~uishable by the determination of Wl and W2, and images correspondin~ to values of Wl and/or W2 provide a representation of tissue architecture.
Turning now to FIG. 4, a preferred confi~uration of mid-processor 15 for implementin8 the improvements of the present invention is shown. The phase insensitive sum } and sum ~ si~nals are provided from front-end processor 11 to down sampler and ma~nitude detector 30 which performs a reduction in samples through a rectangular window convolution. The down-sampled I and Q signals are preferably squared and then summed in down sampler and magnitude detector 30. The output of detector 30 is filtered by a digital filter 31 to narrow the bandwidth of the received intensitr information. A buffer 32 receives filtered backscatter measurements corre8pondins to bursts having a fundamental frequency fl. When a predetermined number of measurements (e.g., 3) have been stored in buffer 32, the measurements are normalized (i.e., averaged) in normalizing circuit 33. In the instance where measurements are taken at only one fundamental frequency, the averaged backscatter measurement from normalizing circuit 33 is passed through math circuit 34 unchanged to display processor 16. Information is then presented on display 17 either as an ima~e, ima8e overlay, or as an ima8e histogram, as desired.
In the case where measurements are obtained at a plurality of fundamental frequencies in order to find frequency dependencies Wl or W2, measurements obtained at a second fundamental frequency f2 are sent to a buffer 35. A plurality of f2 measurements are averaged in normalizin~ circuit 36. The normalized estimates from normalizing circuit 33 and normalizin~ circuit 36 are then processed in math circuit 34. Math circuit- 34 acts as a feature extractor employing weighting coefficients A to D as discussed above, which are determined by the fundamental frequencies and stored in a look-up table, for example.
~54~)9 In a particular implementation of the invention, it is possible to detect 4 dB shifts in mean backscatter levels between 1 cc target cells within a heart. The system achieves sufficient specificity and sensitivity by combining lO independent samples per target cell. Intensity measurements are easily obtained at 5 mm intervals (either vector angle or depth). Using 3 separate burst lengths (equivalent to 2.5 independent measurements) at each backscatter pixel, it is easy to achieve the needed lO samples within the 1 cc target volume. Uncertainty in the value obtained for mean backscatter cross section in the tar8et cell is reduced to one-third of the uncertainty if only one sample was used. Thus, it is possible to separate tissue populations based on differential backscatter inten~ity.
The fore~oing inventlon has provided a medical diagnostic modality having hi8h sensitiv~ty and specificity in detecting differential backscatter tissue populations. She use of a narrowband driving frequency or frequencies increases the volume of scatterers that contribute to a signal at each depth. The larger scattering volume produces better si6nal-to-noise measurements of the backscatter cross section in a tar8et volume. By employin8 different interrogatin6 burst lengths, partial independence of scatter estimates for a specific vector angle and depth are achieved. Sequential use of plural narrowband fundamental frequencies further improves the signal-to-noise performance and provides a means for separating tissue populations based on frequency dependence of the backscatter coefficient. The invention allows detection of localized variations in backscatter cross section per unit volume in an object, but is liXewise applicable to detection of other intrinsic acoustic parameters by ultrasound.
~hile preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variation~, changes, and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordin61y, it is .
intended that the appended claim~ cover all such variations as fall within the spirit and scope of the invention.
BacX~round of the Inventlon The pre~ent ~nvention relates in general to ultrasound apparatu~
for metlcal dlagnostlc lmaging, and more speclflcally to measurement and dlsplay of differentlal bacXscatter cro-s sectlon per unlt volume of insonlfled tlssue.
~ coust~c parameters useful in medlcal ultrasound include reflectlon at dl~contlnultles, volume backscatter coefficient, absorption coefficient and Doppler frequoncr sh~ft. Ultrasonic measuremont of these paramoters provide~ a basis for varlous tlssue characterizations and for constructln~ ima8es. Although reflection imagin8 (using extrlns~c measurements) has histor~cally been the principal basis for dla~nostic ultrasound, it is reallzed that intrin~ic parameters are useful for t~ssue localizatlon and characterization and for detectlon of pathology.
15Back~cattor coefflclent has been lnvestlgated for lts utlllty in imasing and characterizatlon and has been found to prov~de ~ dia~nostlcally useful quantitative data. However, tlssue pathology ; i~ not alw~ys readily spparent from absolute moasuremonts of the backscatter coefflcient. For example, it is somotimes requlred to ; 20 perform filtering of broadband backscatter data to flnd frequency dependencies of tho backscatter coefflc~ent to ldentlfy pathologlcal tissue.
The success of a dia~nostic modallty in providin8 useful infonmatlon depends on an ablllty to interrogate tlssue and acqulre acou~tic data with a maxlmum s~gnal-to-nolse ratlo (S~ O . However, 4(?9 specular reflections, signal correlations, inhome~eneous attenuation, and other factors result in a ~enerally low SUR in ultrasound backscatter meaSuremQnt.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for collectin~ quantitative backscatter data.
It is another ob;ect to improve tissue characterization and diagnosis by means of quantitative backscatter data.
It is another object to improve the signal-to-noise ratio in measurements of intrinsic acoustic parameters for ultrasound.
It is still another object of the invention to detect localized variations in bacXscatter intensity and dependencies in an ob~ect.
SummarY of the Invention These and other ob~ects are achieved in an ultrasound system for measurin~ differential backscatter across a selected volume of tissue or organ and for detecting temporal and frequency dependencies of backscatter in order to distin~ulsh between normal and diseased tissue.
By way of example, sufficient contrast exists between backscatter measurements of normal and diseased myocardial tlssues to allow detection and ima8in8 by ultrasound. Infarcted tlssue exhibits increased backscatter where the local amount of collagen is increased and also exhibits increased signal attenuat~on related to ~5 local collagen concentration and depletion of creative kinase, relatlve to normal myocardial tissues. Severely ischemic myocardium (e.g., 80 percent reduction of local blood flow) results in about a dB increase in bacXYcatter but in a decrease in attenuation.
Furthermore, the cyclic variation present in the backscatter intensi~y of normal myocardium which has a minimum near occurrence of end systole and has a maximum near occurrence of end diastole appears weakened in ischemic tissue. Thus, ima8in~ of the differential backscatter cross section per unit volume of insonified tissue of a heart and its temporal and/or frequency dependence .~. ~ ., ,, , ~, resultin8 from various tissue populations enables detection of various patholo~ical areas.
The signal-to-noise ratio of backscatter measurements in the present inventlon is improved by employing narrowband interro~ating signals to reduce statistical variations in backscatter intensity and to improve tissue regionalization. Use of narrowband si~nals also eliminates the need for ran8e 8atin8 in signal processing. S~R
is further improved by obtaining and combinin8 substantially uncorrelated measurements of a tar8et volume without moving the vector angle by repetitively firin8 narrowband bursts of varyin~
longth or cycles.
In another aspect of the invention, measurements are obtained usln~ a plural~ty of sequentlal narrowband interrogating bursts of d~fferent fundamental frequencies to characterize tissue in terms of a combinatlon of scatter contribution from cellular and parenchymal tissue archltectures as a further means of tissue discrimination.
Thus, frequency dependencies of the backscatter coefficient are employed to infer contrasts in tissue structure according to an empirically derived model.
Brief DescriDtion of the Drawin~s The novel features of the invention are set forth with particularity in the appended claim~. The inventlon itself, however, both as to its organization and method of operation, together with further ob~ects and advantages thereof, may best be understood by reference to the following description taken in con~unctlon with the accompanying drawings in which:
FIGURE 1 is a block dia8ram of an ultrasonic system for practicing the present invention.
FIGURE 2 shows the acquisitlon of an ultrasonic ima~e.
FIGURE 3 illustrates the transmission bandwidth employed in the present invention.
FIGURE 4 is a block dia8ram showing the mid-processor of FIG. l in ~reater detail.
.,.,,~.~,.......
~ h~ 9 15UL02971 Detsiled Descri~tion of the Invention Referrin8 now to FIG. 1 a receive/transmit transducer 10 comprisin~ a plurality of transducer elements in an array is coupled to a front-end processor 11 which includes a pulser 12 and a demodulator and si~nal processor 13. The ultrasound system further includes a mid-processor 15, display processor 16 and a display 17.
Pulser 12 energizes transducer array 10 to insonify along a vector angle in an object. In a receive mode, signals datected by transducer array 10 are coupled to demodulator and signal processor 13 which operates in a known manner to provide a su~med in-phase signal I and summed quadrature signal Q which are phase insensitive.
hit-proces~or 15 determines the back~catter intensity based on signals I and Q. The intensity or other bacXscatter feature is provided to a display processor 16 for presentation on display 17 in any desired format. Other backscatter parameters having diagnostic utility include temporal changes in backscatter intensity (e.g., difference between systole and diastole for cardiac tissue) which can be measured and displayed. Another example is the frequency dependence of the backscatter coefficient which is used to detect variation in tissue structure, as will be described later.
In a preferred embodiment of the invention, tissue features such as lesions can be detected in a tar8et organ by separating various tissue populations accordin~ to localized differential backscatter coefficient. The backscatter coefficient at each resolved point, taken at one or more frequencies, provides the data used to detect le~ion~ and provide secondary information to confirm diagnoses or ~`~ locate a lesion for follow-up exams or treatment, for example.
Ultrasonic interro~ation of an organ is represented in ~IG. 2.
Transducer 10 transmits ultra~onic energy toward a heart 20.
Transmitted ultrasonic waves 21 interact with heart 20 to produce backscattered waves 22. In one preferred embodiment of the present invention, a typical B-mode ima8e of heart 20 is presented on display 17. Thereafter, localized differential backscatter :
:
1~5~09 information is collected for the same ima8e area and is overlaid on the B-mode image (by adding various colors, for example~ so that a lesion 23 can be detected in heart 20. Alternatively, a two-dimensional (2D) B-mode image corresponding only to localized backscatter coefficients can be employed.
In biologlcal tissue, the actual scatterers that cause back~catter and their separation are not known. If a small tissue volume of scatterers (such as l cc) is investigated, independent measurements of backscatter cross section (i.e., backscatter intensity) of the tissue sample are sub~ect to lar~e statistical variations because of tissue structures. This variation in independent measurements of a single tar8et area make~ it difficult or impossible to separate tissue populations based on backscatter intensity. According to the present inventlon, it is found that use of a narrowband frequency for the transmitted ultrasonic energy will select a larger scatterer volume to give consistent intensity measurements and better signal-to-noise performance.
The improved signal-to-noise performance of the present narrowband systems results from a reduction in backscatter intensity variations because of the larger volume of scatterers (scatter number) associated with each backscatter pixel ~i.e., tar8et volume). nean number of scatterers per unit volume follows a Poisson distribution, such that the variation in backscatter intensity varies as the square root of the mean number of scatterers in the effectlve scatterer volume. Thus, an effective scatterer volume containing lO scatters provides standard deviation in intensity measurements of about 30 percent. An increased effective scatterer volume containing lO0 scatterers improves intensity standard deviation to 10 percent. Therefore, signal-to-noise and the ability to separate tissue populations are improved according to the narrowband system of the present invention.
The narrowband transmission of the present invention has the further advantage of improved lateral and depth regionalization by reducing frequency dependencies and interactions present with a 4~9 15uL o 2 9 7 1 wideband system. The wideband effects caused by interference between component frequencies are avoided in the present invention.
Turning now to EIG. 3, an exemplary frequency spectrum for the narrowband transmission of the present invention is shown. A
narrowband signal is centered on a fundamental frequency fO. The ability to define a narrowband energy transmission depends on the passband characteristics of the pul~ing system and the number of cycles of the fundamental frequency in an interro~ating burst. In the present invention, the bandwidth is sufficiently narrow so that all of the backscattered ener~y can be considered as coming from the fundamental frequency. As shown in FIG. 3, a -10 dB fractional bandwidth less than about 20 percent is employed. Further, a preferred fractional bandwidth of between 10 and 20 percent provides excellent narrowbant results.
A further improvement in the SNR received for each tar8et volume is achieved by the present lnvention by repetitively firin~
narrowband bursts of varying length to acquire backscattered data for each tar8et volume. For example, narrowband performance occurs in a system employing a fundamental frequency in the ran~e of several megahertz when interrogating bursts of 3 or more cycles are used. A first interrogating burst of 3 cycles can be averaged with a subsequent burst length of S cycles at the same tar8et volume.
The varied burst length alters the receive amplitude statlstics, such that a sub~tantially independent estimate of the backscatter intensity is obtalned without movin~ the interrogatin~ vector angle. Each independent measurement i8 partially uncorrelated so as to improve the SNR of the si~nal avera~e. The change in burst length or cycles slightly alters the spectral content of the ~; interrogating beam, a~ well as changes the volume of scatterer thatcontrlbute to the si~nal. In a preferred embodiment, separate interro~atlng bursts of 7, 9 and 11 cycles are emyloyed at a fundamental frequency of 2.5 HHz to obtain three partially ; uncorrelated measurements which are equivalent to about 2.5 ; independent samples of the tar8et volume. ~t a fundamental 4~9 frequency of 3.6 ~Hz, burst lengths of lO, 13, and 16 cycles have been employed with good results.
In another aspect of the invention, backscatter intensity measurements that provide independent, uncorrelated estimates are obtained at a plurality of fundamental frequencies in separate, sequentlal measurements using narrowband transmission. Thus, one or more interrogatlons are performed at a first fundamental frequency and may include bursts of varying cycles. Other measurements obtained at a second fundamental frequency are combined with the measurements at the first fundamental frequency to generate a value for the backscatter intensity.
Backscatter measurements at a plurality of narrowband frequencies are further employed in the invention to find the frequency dependence of backscatter for a tar8et area of tissue.
Thl8 frequency dependence enables characterization of tissue in terms of the relative bacXscatter contribution from cellular and parenchymal scatterer populations. Backscatter coefficient N as a function of a frequency f can be modeled accordin~ to the equation:
N(f) Wlf W2f where Wl and W2 are empirically derived factors thought to represent Mie region scattering ob~ects and Rayleigh-l~Xe scattering ob~ects within tissue, respectively.
When N~f) is measured at two frequencies (e.g., 2.5 HHz and 3.6 MHz), two equations are obtained and solutions for Wl and W2 can be found using matrix mathematics. For the case with fl = 2.5 MHz and f2 = 3.6 HHz, the solutions are:
Wl = N(2.5 NHz)D - N~3.6 MHz)B and W2 = N(3.6 HHz)A - N(2.5 MHz)C
where A = 2.5, B = 3.6, C = 39, and D = 168.
~954~39 The two main tissue populations (i.e., cellular and parenchymal) are thus distin~uishable by the determination of Wl and W2, and images correspondin~ to values of Wl and/or W2 provide a representation of tissue architecture.
Turning now to FIG. 4, a preferred confi~uration of mid-processor 15 for implementin8 the improvements of the present invention is shown. The phase insensitive sum } and sum ~ si~nals are provided from front-end processor 11 to down sampler and ma~nitude detector 30 which performs a reduction in samples through a rectangular window convolution. The down-sampled I and Q signals are preferably squared and then summed in down sampler and magnitude detector 30. The output of detector 30 is filtered by a digital filter 31 to narrow the bandwidth of the received intensitr information. A buffer 32 receives filtered backscatter measurements corre8pondins to bursts having a fundamental frequency fl. When a predetermined number of measurements (e.g., 3) have been stored in buffer 32, the measurements are normalized (i.e., averaged) in normalizing circuit 33. In the instance where measurements are taken at only one fundamental frequency, the averaged backscatter measurement from normalizing circuit 33 is passed through math circuit 34 unchanged to display processor 16. Information is then presented on display 17 either as an ima~e, ima8e overlay, or as an ima8e histogram, as desired.
In the case where measurements are obtained at a plurality of fundamental frequencies in order to find frequency dependencies Wl or W2, measurements obtained at a second fundamental frequency f2 are sent to a buffer 35. A plurality of f2 measurements are averaged in normalizin~ circuit 36. The normalized estimates from normalizing circuit 33 and normalizin~ circuit 36 are then processed in math circuit 34. Math circuit- 34 acts as a feature extractor employing weighting coefficients A to D as discussed above, which are determined by the fundamental frequencies and stored in a look-up table, for example.
~54~)9 In a particular implementation of the invention, it is possible to detect 4 dB shifts in mean backscatter levels between 1 cc target cells within a heart. The system achieves sufficient specificity and sensitivity by combining lO independent samples per target cell. Intensity measurements are easily obtained at 5 mm intervals (either vector angle or depth). Using 3 separate burst lengths (equivalent to 2.5 independent measurements) at each backscatter pixel, it is easy to achieve the needed lO samples within the 1 cc target volume. Uncertainty in the value obtained for mean backscatter cross section in the tar8et cell is reduced to one-third of the uncertainty if only one sample was used. Thus, it is possible to separate tissue populations based on differential backscatter inten~ity.
The fore~oing inventlon has provided a medical diagnostic modality having hi8h sensitiv~ty and specificity in detecting differential backscatter tissue populations. She use of a narrowband driving frequency or frequencies increases the volume of scatterers that contribute to a signal at each depth. The larger scattering volume produces better si6nal-to-noise measurements of the backscatter cross section in a tar8et volume. By employin8 different interrogatin6 burst lengths, partial independence of scatter estimates for a specific vector angle and depth are achieved. Sequential use of plural narrowband fundamental frequencies further improves the signal-to-noise performance and provides a means for separating tissue populations based on frequency dependence of the backscatter coefficient. The invention allows detection of localized variations in backscatter cross section per unit volume in an object, but is liXewise applicable to detection of other intrinsic acoustic parameters by ultrasound.
~hile preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variation~, changes, and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordin61y, it is .
intended that the appended claim~ cover all such variations as fall within the spirit and scope of the invention.
Claims (13)
1. An ultrasound apparatus for obtaining a two-dimensional backscatter image comprising:
transducer means for transmitting ultrasound into and receiving ultrasound from an abject to be studied, said transducer means having a fundamentsl frequency:
pulse means coupled to said transducer means for driving said transducer means to emit fundamental ultrasonic waves, said waves having a bandwidth sufficiently narrow to obtain said image without range gating; and processing means coupled to said transducer means for calculating independent measurements of the backscatter intensity for a plurality of target areas in said object directly from echo waves received by said transducer means.
transducer means for transmitting ultrasound into and receiving ultrasound from an abject to be studied, said transducer means having a fundamentsl frequency:
pulse means coupled to said transducer means for driving said transducer means to emit fundamental ultrasonic waves, said waves having a bandwidth sufficiently narrow to obtain said image without range gating; and processing means coupled to said transducer means for calculating independent measurements of the backscatter intensity for a plurality of target areas in said object directly from echo waves received by said transducer means.
2. The apparatus of claim 1 wherein the 10 dB bandwidth of said narrowband waves is in the range of less than about 20 percent relative to said fundamental frequency.
3. The apparatus of claim 1 wherein said processing means is adapted to determine a backscatter intensity corresponding to each target area based on intensity measurements from a plurality of separate narrowband bursts, said bursts being driven by said pulse means and having a plurality of burst lengths.
4. The apparatus of claim 3 wherein the 10 dB bandwidth of each of said bursts is in the range of less than about 20 percent relative to said fundamental frequency.
5. The apparatus of claim 1 wherein said processing means is adapted to determine a backscatter intensity corresponding to each target area based on intensity measurements from a plurality of separate narrowband bursts, said bursts being driven by said pulse means and having a plurality of fundamental frequencies.
6. The apparatus of claim 5 wherein the 10 dB bandwith of each of said bursts is in the range of less than about 20 percent relative to said fundamental frequency.
7. The apparatus of claim 1 wherein said processing means includes means for determining cyclic variations over time of said backscatter intensity for at least one of said target areas.
8. An ultrasound apparatus for measuring intrinsic acoustic parameters comprising:
transducer means for transmitting ultrasound into and receiving ultrasound from an object to be studied:
pulser means coupled to said transducer means for driving said transducer means to exit ultrasonic bursts having variable lengths con extending to predetermined numbers of cycles of the fundamental frequency of each respective burst; and processing means coupled to said transducer means for calculating ultrasound parameters for a plurality of target areas in said object, a respective parameter value corresponding to each target area being based on a plurality of measurements obtained from each target area with different burst lengths.
transducer means for transmitting ultrasound into and receiving ultrasound from an object to be studied:
pulser means coupled to said transducer means for driving said transducer means to exit ultrasonic bursts having variable lengths con extending to predetermined numbers of cycles of the fundamental frequency of each respective burst; and processing means coupled to said transducer means for calculating ultrasound parameters for a plurality of target areas in said object, a respective parameter value corresponding to each target area being based on a plurality of measurements obtained from each target area with different burst lengths.
9. The apparatus of claim 8 wherein said pulser means drives said transducer means at a single fundamental frequency and wherein said processing means is adapted to calculate an average of said measurements.
10. A method for performing ultrasonic measurement of an object, comprising the steps of:
insonifying said object with narrowband ultrasonic energy having a 10 dB bandwidth in the range of less than about 20 percent relative to the fundamental frequency of said ultrasonic energy;
selecting a plurality of target volumes in said object for measurement; and detecting the backscatter intensity for each respective target volume.
insonifying said object with narrowband ultrasonic energy having a 10 dB bandwidth in the range of less than about 20 percent relative to the fundamental frequency of said ultrasonic energy;
selecting a plurality of target volumes in said object for measurement; and detecting the backscatter intensity for each respective target volume.
11. The method of claim 10 wherein said detecting step includes interrogating each target volume with a plurality of bursts of different burst lengths.
12. The method of claim 10 wherein said detecting step includes interrogating each target volume with a plurality of bursts, each burst having a respective fundamental frequency, and wherein said bascatter intensity at each target column is determined from intensity values from said plurality of bursts.
13. A method for ultrasonic examination of an object comprising the steps of:
obtaining a backscatter intensity measurement N from a target area at a first fundamental frequency f1;
obtaining a backscatter intensity measurement N from said target area at a second fundamental frequency f2;
finding a solution for at least one of the factors W1 and W2 according to the equations W1=N(f1)D-N(f2)B and W2=N9(f2)A-N(f1)C
where A,B,C, and D are predetermined constants;
and assigning a factor value to said target area.
obtaining a backscatter intensity measurement N from a target area at a first fundamental frequency f1;
obtaining a backscatter intensity measurement N from said target area at a second fundamental frequency f2;
finding a solution for at least one of the factors W1 and W2 according to the equations W1=N(f1)D-N(f2)B and W2=N9(f2)A-N(f1)C
where A,B,C, and D are predetermined constants;
and assigning a factor value to said target area.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000580098A CA1295409C (en) | 1988-10-13 | 1988-10-13 | Backscatter data collection technique for ultrasound |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000580098A CA1295409C (en) | 1988-10-13 | 1988-10-13 | Backscatter data collection technique for ultrasound |
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| Publication Number | Publication Date |
|---|---|
| CA1295409C true CA1295409C (en) | 1992-02-04 |
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| Application Number | Title | Priority Date | Filing Date |
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| CA000580098A Expired - Fee Related CA1295409C (en) | 1988-10-13 | 1988-10-13 | Backscatter data collection technique for ultrasound |
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| CA (1) | CA1295409C (en) |
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1988
- 1988-10-13 CA CA000580098A patent/CA1295409C/en not_active Expired - Fee Related
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