Dual-frequency ultrasonic array transducer and method of harmonic imaging

• US 6,540,683 B1
• Filed: 09/14/2001
• Issued: 04/01/2003
• Est. Priority Date: 09/14/2001
• Status: Expired due to Fees

• Current Assignee: Gregory Sharat Lin
• Original Assignee: Gregory Sharat Lin
• Inventors: Lin, Gregory Sharat
• Primary Examiner(s): Jaworski, Francis J.
• Application Number:
  US09/953,740
• Publication Number:
  US 20030055337A1
• Time in Patent Office: 564 Days
• Field of Search: 600/437, 600/443, 600/447, 600/459, 600/458, 310/334-336
• US Class Current: 600/447
• CPC Class Codes: A61B 8/0833 involving detecting or locating foreign bodies or organic structures
• CPC Class Codes: A61B 8/481 involving the use of contrast agent, e.g. microbubbles introduced into the bloodstream analysis of echo signal exploiting properties of a contrast enhancer in devices for short-range imaging with acoustic waves G01S7/52039
• CPC Class Codes: B06B 1/0622 on one surface
• CPC Class Codes: G01S 15/8915 using a transducer array
• CPC Class Codes: G01S 7/52038 involving non-linear properties of the propagation medium or of the reflective target
• CPC Class Codes: G01S 7/52074 Composite displays, e.g. split-screen displays; Combination of multiple images or of images and alphanumeric tabular information

Abstract

An ultrasonic transducer, method, and system are disclosed for performing ultrasonic harmonic imaging in a medium or a living body. The ultrasonic transducer consists of a linear array of alternating long and short elements. A first set of transducer elements is for transmitting and receiving at a fundamental frequency, and a second set of transducer elements is for receiving second harmonic or subharmonic echoes, each set operating at their respective center frequencies. This dual-frequency ultrasonic transducer is coupled to an ultrasound system wherein transmit beamforming is done at the fundamental frequency, and receive beamforming is done at the second harmonic or subharmonic frequency. When receive beamforming at the fundamental frequency is added, the method enables parallel fundamental, harmonic, compound, and difference imaging. These methods may be utilized to improve ultrasonic harmonic imaging of hard-to-image patients by optimizing the transmission of fundamental-frequency ultrasound beams and the receiving of second harmonic or subharmonic echoes, while minimizing harmonic distortion and signal losses.

Claims

1. A dual-frequency ultrasonic array transducer comprising:

a first set of piezoelectric elements operable at their center resonant frequency to transmit ultrasonic pulses at a fundamental frequency;

a second set of piezoelectric elements operable at their center resonant frequency to receive ultrasonic pulses at a second frequency which is either twice the fundamental frequency (harmonic) or half the fundamental frequency (primary subharmonic), wherein said first set of piezoelectric elements and said second set of piezoelectric elements are positioned in a linear alternating sequence;

an acoustic-damping backing layer;

electrical contacts bonded to the front and back of each piezoelectric element;

multiple acoustic impedance matching layers of one-quarter wavelength of the higher said center resonant frequency, wherein the number of said acoustic impedance matching layers is equal to or twice the ratio of the higher of said center resonant frequency to the lower said center resonant frequency; and

an acoustic lens.

2. A dual-frequency ultrasonic transducer of claim 1 wherein a standoff pad is attached to said acoustic lens.

3. A dual-frequency ultrasonic transducer of claim 1 wherein:

said first set of piezoelectric elements are operable at their said center resonant frequency to transmit and receive ultrasonic pulses at said fundamental frequency; and

said ultrasonic pulses received at said fundamental frequency and ultrasonic pulses received by said second set of piezoelectric elements at said second frequency are simultaneously processed by a beamformer and scan converter for simultaneous and/or compound image viewing.

4. A dual-frequency ultrasonic transducer of claim 1 wherein:

a transmitter/receiver coupled to said dual-frequency ultrasonic transducer is operable to transmit focused beams at one said center resonant frequency and receive focused beams at the other said center resonant frequency;

a transmit beamformer coupled to said transmitter/receiver is operable to focus transmit pulses by applying appropriate delays to all channels across any given active transmit aperture;

a receive beamformer coupled to said transmitter/receiver is operable to focus on echoes received from a sequence of focal points within said medium or body by applying appropriate delays to all channels in any given active receive aperture;

a scan converter coupled to said receive beamformer is operable to convert the geometric coordinate system of the ultrasound vectors into Cartesian coordinates (raster display format); and

a display subsystem and monitor coupled to said scan converter is operable to display an ultrasound image.

5. A dual-frequency ultrasonic transducer of claim 4 wherein:

said receive beamformer is further operable to independently focus on echoes received at both said center resonant frequencies;

said scan converter is further operable to independently convert the geometric coordinate system of the ultrasound vectors received at both said center frequencies into Cartesian coordinates (raster display format); and

a frame processor coupled to said scan converter is operable to independently assemble a fundamental-frequency image, a second harmonic or subharmonic image, a compound image being a combination of said fundamental-frequency image and said second harmonic or subharmonic image, and/or a difference image being a subtraction of said fundamental-frequency image from said second harmonic or subharmonic image.

6. A method of performing ultrasonic harmonic imaging comprising the steps of:

(a) generating electrical pulse sequences at a fundamental frequency;

(b) applying said electrical pulse sequences to a first set of piezoelectric elements with center resonant frequency equal to said fundamental frequency, thereby converting them into acoustic pulses—

said first set of piezoelectric elements being arranged in alternating positions with respect to a second set of piezoelectric elements in a dual-frequency ultrasonic linear array transducer;

(c) introducing said acoustic pulses into an area of a medium or body to be ultrasonically imaged through multiple acoustic impedance matching layers each of whose thickness is one quarter of the wavelength of the higher of the center resonant frequencies of said first set of piezoelectric elements and said second set of piezoelectric elements, wherein the number of said acoustic impedance matching layers is equal to or twice the ratio of the higher of said center resonant frequencies to the lower of said center resonant frequencies;

(d) receiving echoes at a second harmonic or subharmonic frequency in said second set of piezoelectric elements with center resonant frequency equal to said second harmonic or subharmonic frequency, thereby converting them into a first set of received electrical pulses;

(e) beamforming said first set of received electrical pulses so as to focus on second harmonic or subharmonic echoes originating from a specified depth zone within said medium or said body;

(f) assembling a second harmonic or subharmonic image by scan converting all image vectors comprised of intensities mapped from said first set of received electrical pulses; and

(g) displaying said second harmonic or subharmonic image.

7. A method according to claim 6 further comprising the steps of:

(h) receiving echoes at said fundamental-frequency in said first set of piezoelectric elements, thereby converting them into a second set of received electrical pulses;

(i) beamforming said second set of received electrical pulses so as to focus on fundamental-frequency echoes originating from a specified depth zone with said medium or said body;

(j) assembling a fundamental-frequency image by scan converting all image vectors comprised of intensities mapped from said second set of received electrical pulses; and

(k) displaying said fundamental-frequency image.

8. A method according to claim 7 wherein said fundamental-frequency image and said second harmonic or subharmonic image are summed or averaged together pixel-by-pixel to generate a compound image.

9. A method according to claim 7 wherein said fundamental-frequency image and said second harmonic or subharmonic image are subtracted one from another pixel-by-pixel to produce a difference image.

10. An apparatus comprising:

a first plurality of ultrasonic transducer element means for transmitting acoustic energy at a first set of frequencies into an area of a subject being scanned;

a second plurality of ultrasonic transducer element means for receiving acoustic energy at a second set of frequencies from an area of a subject being scanned, wherein said first plurality of ultrasonic transducer element means is laid out in an alternating sequence with respect to said second plurality of ultrasonic transducer element means;

an acoustic-damping backing layer means;

electrical connection means;

acoustic impedance matching means comprised of multiple layers, each having a thickness equal to one quarter of the wavelength of the higher of the center resonant frequencies of said first plurality of ultrasonic transducer element means and said second plurality of ultrasonic transducer element means, and the number of layers being equal to or twice the ratio of the higher and lower center resonant frequencies of said first plurality of ultrasonic transducer element means and said second plurality of ultrasonic transducer element means; and

an acoustic lens means.

11. An apparatus of claim 10 wherein:

said first plurality of ultrasonic transducer element means have a center frequency equal to a fundamental frequency; and

said second plurality of ultrasonic transducer element means have a center frequency equal to either twice said fundamental frequency (second harmonic) or half said fundamental frequency (primary subharmonic).

12. An apparatus of claim 10 further comprising acoustic stand-off pad means for eliminating near-field acoustic artifacts.

13. An apparatus of claim 10 further comprising:

transmitter means for generating and beamforming transmit pulses at said first set of frequencies to be applied to said first plurality of ultrasonic transducer element means;

a first receiver means for digitizing and beamforming pulses generated by said second plurality of ultrasonic transducer element means received from echoes at said second set of frequencies;

scan converter means for mapping pulses at said second set of frequencies into intensities and arranging the data into a first raster image; and

display means for visualizing said first raster image.

14. An apparatus of claim 13 further comprising:

switching means for switching said first plurality of ultrasonic transducer element means between said transmitter means and a second receiver means;

said second receiver means for digitizing and beamforming pulses generated by said first plurality of ultrasonic transducer element means received from echoes at said first set of frequencies;

scan converter means for mapping pulses at said first set of frequencies into intensities and arranging the data into a second raster image;

compound imaging means for averaging said first raster image and said second raster image;

difference imaging means for subtracting said second raster image from said first raster image; and

display means for visualizing said second raster image, the compound image, and the difference image.