Ultrasound scanning apparatus.

Background of the Invention

Systems that create two-dimensional images of structures within a body under examination by launching pulses of ultrasonic carrier waves into it along different radial paths and converting any echoes of the pulses into corresponding electrical waves have been commercially available for a number of years. The images thus formed have an intensity that is proportional to the amplitude of the echoes.

In an article entitled "Ultrasonic Characterization of Myocardium" in the September/October 1985 issue of <u style="single">Progress in Cardiovascular Diseases</u>, the quantitative relationship between the power of the backscatter of ultrasonic pressure waves and the condition of myocardium is discussed. Ischemic injury is of particular interest because its early detection would provide a basis for determining in real-time whether or not a patient has had a heart attack as well as its extent, whereas present techniques for simply determining whether or not a heart attack has occurred often require several days of analysis. Although not discussed in the article, it is possible that other abnormal tissue conditions could be revealed by similar analysis. Whereas the article suggests the desirability of forming a two-dimensional image having intensity corresponding to the powers of the intensity of the echoes, no viable way of achieving this objective is described. The article does suggest that the variations in brightness of the images formed in response to the amplitude of echoes, as in the available apparatus briefly described above, can give some indication of the condition of the tissue, but the variations due to such phenomenon are so small that little reliance can be placed on them.

In an article entitled "A Real-Time Integrated Backscatter Measurement System for Quantitative Cardiac Tissue Characterisation", published in the January 1986 issue of <u style="single">I.E.E.E.Transactions</u><u style="single">on</u><u style="single">Ultrasonic,</u><u style="single">Ferroelectrics and</u><u style="single">Frequency</u><u style="single">Control</u> Vol.UFFC-33, No.1, an apparatus is described for deriving a signal corresponding to the power of backscatter from a pulse of ultrasonic carrier waves travelling along a single path. The apparatus described therein was wholly devised by two of the inventors of this invention. It converted the electrical carrier waves produced by a transducer in response to echoes impinging thereon to pressure waves which were applied to a cadmium sulphide crystal. The Cds produced an output corresponding to the power of the backscatter at all ranges. One of the limitations of this apparatus was its small dynamic range about 20 db. No method of forming a two-dimensional image was described.

In accordance with the invention, there is provided apparatus for deriving signals indicating a condition of tissue within a volume of a body in real time, comprising scanner means (2) for launching pulses of supersonic pressure waves along a plurality of known paths into a body under examination and receiving backscatter echoes from locations along said plurality of known paths, converter means (6) for detecting the power of each successive pulse of supersonic waves scattered including means for converting each successive pulse of backscatter echoes reflected from matter within a volume of said body into electrical signals, and means for deriving directly from said signals a pulsed signal proportional to the integrated backscatter from said volume of the body, indicative of the condition of the tissue within it, wherein said pulsed signal is formed by means for deriving amplitude signals proportional to the amplitudes of components of said electrical signals in either the time or frequency domain, means coupled to said latter means for deriving the squares of the amplitude signals, and means for summing said squares, and wherein the pulsed signal is fed to video processing means (10) where it is converted to a video signal which is displayed as a real time video output (12).

Using this invention, an accurate two-dimensional image of the power of backscatter may be formed in which a dynamic range of about 60 db is attained. Unprocessed electrical carrier waves produced by a transducer in response to backscatter or intermediate carrier waves derived therefrom are applied to a circuit for deriving a signal corresponding to the integration of the power of backscatter within a small range such as 2 mm. that runs along the path of the launched pulses. The circuit may derive the signal in either the frequency or time domain with adjustable integration ranges of frequency or time respectively. The signal may then directly modulate the intensity of a cathode ray beam so as to form an image corresponding in intensity to the power of the backscatter or it may be processed by a scan converter before modulation of the cathode ray beam.

In view of the fact that the signal corresponds to power, it is proportional to the square of the amplitude of the electrical carrier waves or electrical waves of intermediate frequency derived therefrom so that it is important that their amplitude be correct.

It is well known that the amplitude of echoes of launched pressure waves decreases with range so that it has been customary for apparatus forming an image of the amplitudes of echoes to employ a timed gain control circuit that increases the amplitude of the electrical carrier waves supplied by the transducer in response to the echoes received after each launched pulse.

Whereas this is satisfactory for images merely displaying the amplitudes of echoes, a gain control circuit used in a preferred form of this invention also provides a gain that changes less with time when the electrical waves result from echoes from blood than when the electrical waves result from echoes from myocardial tissue.

Greater accuracy is achieved if the gain control circuit causes zero gain during a period when ring-down of the transducer occurs following the transmit excitation. Additional accuracy is achieved if the gain control circuit compensates for echoes being received from the chest wall of the patient using a correction factor related to the chest wall attenuation as opposed to one related to myocardial tissue. In accordance with a further refinement, the gain control circuit introduces a component that reduces the gain as the range of echoes approaches the range where the launched beam is focussed and increases the gain thereafter. This is sometimes referred to as "diffraction correction". A still further refinement causes the gain control to compensate for the fact that the frequencies of which a launched pulse is comprised are attenuated by a factor proportional to frequency as the pulse travels through the body. This is sometimes referred to as "beam softening".

Brief Description of the Drawings

<ul id="ul0001" list-style="none"><li>FIGURE 1 is a general block diagram of apparatus for producing a two-dimensional image of the power of echoes in a supersonic system in accordance with this invention;</li><li>FIGURES 1A, 1B and 1C are block diagrams of respectively different circuits that operate in the time domain to derive the signal representative of the power in the echoes;</li><li>FIGURES 1D and 1E are block diagrams of respectively different circuits that operate in the frequency domain to derive the signal representative of power in the echoes;</li></ul>

Detailed Description of the Invention

In FIGURE 1, a scanner 2 contains a transducer means not shown for sequentially launching pulses of supersonic waves and converting any received echoes into corresponding electrical waves. A timed gain control circuit 4 varies the amplitudes of the electrical waves and applies them to a power detector 6 that may be as illustrated in any one of FIGURES 1A through 1E. After being operated on by an output mapping circuit 8 that determines the brightness or color corresponding to the various amplitudes of the signal, and after suitable video processing has been accomplished by a video processor 10, the signal is applied so as to control the intensity or color of the image formed on a cathode ray tube 12.

In the following description of the various figures of the drawings, components having like functions have the same designations.

Reference is now made to a power detector of FIGURE 1A that operates in a time domain. The electrical signals corresponding to echoes impinging on the transducer contained in the scanner 2 are passed through a normalizing filter 13 having an amplitude/frequency characteristic largely determined by the transducer. They are then sampled by an A/D device 14, and its digital output is applied to the input of a ROM 16 that derives a digital signal representing the square of the amplitude of each sample. The digital word at the output of the ROM 16 is applied to one input of a summer 18 and to an input of an adjustable delay 20 that may be a FIFO shift register. An arithmetic logic unit 22 is coupled to the output of the summer 18 and to the output of the delay 20 so as to subtract the output of the delay 20 from the output of the summer 18, and the output of the logic unit 22 is connected to another input of the summer 18 as well as to an output mapping circuit 8 , which is generally a ROM.

As each supersonic pulse is launched, all the devices in FIGURE 1A are reset. Thus, if there are n steps in the FIFO delay 20, its output is zero for the first n samples of the A/D device 14, and the output of the logic unit 22 changes with each sample. When sample n+1 is taken, the output of the ROM 16 for the first sample appears at the output of the adjustable delay 20 and is subtracted from the old sum plus the square of sample n+1 so that the output of the logic unit 22 becomes the sum of the squares of the samples between 2 and n+1. Thus, the range represented by the signal at the output of the logic unit 22 is for n samples and equals the old sum of n-1 squared samples plus the squared value of the new sample.

In FIGURE 1B, the output of the ROM 16 is directly connected to the input of the logic unit 22 to which the output of the summer 18 was connected in FIGURE 1A, one input of a summer 24 is connected to the output of the logic unit 22, and the other input of the summer 24 is connected to its output. The mapping device 8 is connected to the output of the summer 24. Thus, the output of the logic unit 22 is the difference between the square of the current sample and the square of the n^th previous sample so that the signal at the output of the summer 24 is the same as the signal at the output of the logic unit 22 of FIGURE 1A.

In FIGURE 1C, the ROM 16 for deriving the squares of the samples is eliminated, a portion of the address input of a different ROM 26 is connected to the output of the adjustable delay 20, and the remaining address bits of the ROM 26 are connected directly to the output of the A/D device 14. The bits at the output of the A/D device 14 and the bits at the output of the adjustable delay 20 combine to address a place in the ROM 26 where the difference of the squares is stored. The summer 24 derives a running sum of the squares of n samples as the signal for display.

The circuit illustrated in FIGURE 1D operates in the frequency domain. The output of the A/D device 14 is connected to the input of a Fast Fourier Transform 28, and its output in turn is connected to a ROM 30 that derives the sum of the squares of the real and imaginary components for each frequency from the FFT 28. Since everything including noise is sampled by the A/D device 14, the output of the ROM 30 would include everything from DC to the Nyquist limit. An integrator 34 is provided that integrates over the usable bandwidth of the apparatus to provide the signal that is applied to the mapping means 8 to control the brightness and/or color of the image.

As described, the signal would be the integration of the squares of samples over some number of the frequency bins in the FFT and the FFT and the integrator 34 would have to be reset by a timer 36 after each range is completed. The timer 36 is started by a start-scan signal provided by the scanner 2 of FIGURE 1. In order to provide overlapping rather than contiguous ranges, an FFT 28', a ROM 30' and an integrator 34' are connected as shown by the dashed lines, and the FFT 28' is started at some time during the range covered by the components shown in solid lines so as to cover a second range overlapping the first. A separate FFT, ROM and integrator would have to be supplied for each successive overlapping range, up to the number required to allow the first FFT, ROM and integrator to be available for another computation.

The circuit of FIGURE 1E also operates in a frequency domain. The unprocessed electrical waves are applied to a spectrum analyzer 40 which may be a bank of filters, each of which is followed by a squaring circuit. The output of the analyzer 40 is normalized by means (Not shown). An integrator 34 integrates the normalized output over the usable bandwidth.

Whereas most supersonic equipment uses the same transducer for launching pulses and receiving echoes, the term "backscatter" is used but, as used herein, that term is meant to apply to scatter in any direction. Thus, the means for launching pulses may be at one point on the body and the means for receiving the echoes may be at another point because backscatter or scatter goes in all direction. It is only necessary to process the scatter or backscatter from locations along a plurality of known paths.

In operating the invention, it makes no difference whether you add the squares of the amplitudes of n time samples or of n frequency samples for a given volume. Both will be proportional to the integrated backscatter from the volume.

Equally, it has not been stated from where the volume for which the signal proportional to the integrated backscatter has been derived. One could derive the signal for volumes at equal or different distances along different radii. If the distances are the same, the volumes would form a portion of an annulus. In any event the signals for a number of different volumes can be derived and stored in display bit map and scanned in any manner from the bit map by means for producing an image.