Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/88—Sonar systems specially adapted for specific applications
  • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
  • G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
  • G01S15/8909—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration
  • G01S15/8915—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration using a transducer array
  • G01S15/892—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using a static transducer configuration using a transducer array the array being curvilinear
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/523—Details of pulse systems
  • G01S7/526—Receivers
  • G01S7/53—Means for transforming coordinates or for evaluating data, e.g. using computers

Definitions

• the invention relates to a method of imaging objects with a curved surface by means of coherent wave motion.
• the method comprises applying a transmis ⁇ sion signal to a transducer defining a curved transmis ⁇ sion surface of a predetermined shape, and controlling the amplification and delay of the signal to be applied to the transducer so that a wide unfocused beam is di- rected to the object; receiving echo from the object; and reconstructing an image of the object through com ⁇ putation on the basis of the received echo informa ⁇ tion.
• the method of the invention is applied primarily in medical ultrasound imaging but it is likewise appli- cable in other ultrasound imaging, e.g., in non-destruc ⁇ tive testing (NDT) of tubes, rods, pillars or the like cylindrical bodies.
• NDT non-destruc ⁇ tive testing
• the method is also applicable in imaging systems utilizing other coherent waves, such as optical or seismic waves or acoustic and radio waves. Even though merely ultrasound imaging is referred to below, the scope
• Ultrasound imaging has a great variety of ap ⁇ plications in hospitals and health centers as well as in the industry.
• the most widely used imaging technique, the B-scan uses a focused ultrasound beam for scanning the object.
• the lateral resolution of the method, how ⁇ ever, is not satisfactory, and various methods have been developed for improving the B-scan method.
• UHB imaging better resolu ⁇ tion can be achieved both in lateral and longitudinal direction by combining the holography and B-scan tech ⁇ niques (Ref. 1).
• the so called compound B-scanning is also used which superim- poses several B-scan images around the object.
• the disadvantages of this method include the complex transducer configuration, long scanning time, and required high computation capacity.
• the resolution of the compound B-scanning has been improved by using image computation typical of tomography (Ref. 2).
• Computerized ultrasound tomography providing a lateral image of the object similarly as in compound B-scanning, requires a complicated transducer configura- tion and its computation method requires high computer capacity.
• a further disadvantage of the tomography method is that the computation of a sector image is more difficult than the computation of an image repre ⁇ senting a full circle.
• the resolution of the method is not either the best possible (Ref. 3).
• a so called co ⁇ herent reflection tomography (Ref. 4) has been devel ⁇ oped, and a method utilizing a large-beam transducer (Ref. 5) has been developed for simplifying the trans- ducer configuration.
• the object of the present invention is to provide a novel imaging method by means of which a decisive im ⁇ provement is achieved with respect to the above-men ⁇ tioned disadvantages.
• the image reconstruction step comprises (i) transforma ⁇ tion of a curved geometry to a rectangular geometry; (ii) computing the image in the rectangular geometry using reconstruction algorithms developed for linear transducers; and (iii) returning to the curved geometry.
• the basic idea of the invention is to utilize, in image reconstruction with a transducer having a curved transmission surface, reconstruction algorithms developed for a linear transducer, the differences caused by the different basic geometries of the systems being compensated for by carrying out a transformation.
• the method can be used only in cases where the curved surface is such in shape that the differences caused by different basic geometries are wavefront-angle dependent in the rectangular system.
• the major advantages of the method according to the invention include simple transducer configuration, rapid computation algorithm, improved resolution, and easy reconstruction of sector images with lower computer capacity.
• the computation time required for the recon ⁇ struction process of a circular image by means of the method according to the invention is substantially equal to the computation time in a linear imaging method when using the wavefront backward algorithm.
• Figure 1 is a schematical view of a conventional linear transducer system for one-dimensional image re- construction
• Figures 2 and 3 illustrates transformation from a sector geometry to a rectangular geometry, whereby Figure 2 illustrates the image reconstruction geometry in a transducer having a sector-shaped transmission surface, and Figure 3 illustrates the transformed image reconstruction geometry in an imaging system utilizing a linear transducer; and
• Figure 4 illustrates the basic geometry in a system in which an individual transducer moves along a circular path.
• Figure 1 shows, so as to facilitate the under ⁇ standing of the description of the method of the inven ⁇ tion, an imaging system utilizing a linear transducer for the reconstruction of a one-dimensional image.
• a transducer array 1 comprising several successive trans- ducer elements 2, N in number, is in a hologram plane HP. The interval between two successive transducer elements is indicated with the reference dx.
• the object O to be imagined is in an object plane OP parallel with the hologram plane. The distance between the planes is indicated with the reference D.
• ⁇ is the wave length
• f x is the spatial frequency
• k 2 ⁇ t/ ⁇
• j is the imaginary unit
• a two-dimensional image of the object slice can be obtained as a stack of several one- dimensional images from different depths.
• the shape of the transducer having curved transmission surface has to be known.
• the curved transmission surface is represented by a sector the quantities of which are changed to correspond to an imaging system utilizing a linear transducer in order that the recon ⁇ struction of the image could be performed by using reconstruction algorithms developed for linear trans ⁇ ducers.
• Figure 2 illustrates a transducer array 1.1 in the form of a circular arc, comprising several (N) successive transducer elements 2, similarly as above.
• the angular interval between two successive transducer elements is d ⁇ .
• the arcs AB and EF have the same center point C, and they are confined between the polar angles ⁇ ⁇ and ⁇ 2 .
• the sector geometry of Figure 2 first has to be changed to the rectangular geometry of Figure 3.
• the transducer arc AB and the object arc EF in the sector are trans ⁇ formed to straight lines A'B' and E'F' in the rectangle.
• the center point C of the sector is also transformed to a straight line C'C".
• the cylindrical wavefront from the center point of the sector becomes a plane wave indicated by the arrows T.
• the following list shows how the rectangle quantities are derived from the sector quantities:
• the expansion coefficient means that the wave ⁇ lengths at different depths after the change of geometry are different in the rectangular system. It is therefore necessary in the phase compensation process to divide the wave length ⁇ in Eq. (14) by the expansion coeffi ⁇ cient ⁇ for a correct reconstruction.
• u h (x h ) is the hologram data
• x h is the abscissa of the transformed rectangular transducer system
• u 1 (x i ) is the image of the object line in the transformed rect- angular system
• F and F" 1 denote the Fourier and inverse Fourier transforms, respectively
• H(f x ) is the transfer function for the rectangular system
• B(f x ) is the compensation function for correcting the geometrical differences as follows B(f ⁇ )
• f sin ⁇ / ⁇ - exp(n ⁇ ) (18) where ⁇ is given in Eq. (1
• the expansion of the sector to a full circle enables the reconstruction of an image covering the entire circumference.
• the image reconstruction process of the invention has been described above in a one-dimensional case, whereby it was assumed that the signal was a cylindrical wavefront coming out from the rotation center.
• the fol- lowing description deals with two-dimensional pulse- echo imaging. In pulse-echo imaging, all the parameters must be doubled due to the round trip of ultrasound. For example, the aperture width is twice that of the transducer and the depth level is also doubled.
• the assumptions of a plane wavefront in a rect ⁇ angular system and a cylindrical wavefront in a sector system do not hold any longer.
• FIG. 4 shows the geometry of the system in a specific case in which an individual transducer H moves along a circular path.
• the distance p between the object O and the transducer H is (Ref. 9) p - [R t 2 +R o _ 2 R t R o cos( ⁇ t - ⁇ 0 )] ⁇ » (19)
• R t and ⁇ t are the polar coordinates of the trans ⁇ ducer position as shown in Figure 4
• R 0 and ⁇ 0 are the polar coordinates of the position of the point object 0.
• the distance appears to be the corresponding depth level of the object in the hologram plane.
• the hologram plane data should be corrected to the right depths corresponding to Eq. (19). This can be carried out in the spatial plane of the hologram prior to the recon ⁇ struction. However, as mentioned above, these processes have to be carried out separately for each point of the image, which in practice results in an unreasonably long computation process. According to the invention, it is preferable to carry out the compensation in the frequency domain of the hologram if the near field curvature distortion is wave-front-angle-dependent. Inserting Eq. (12) in Eq.
• the rearranging operation is carried out on the basis of Eq. (23) by substituting the spectrum f ⁇ for the wavefront angle ⁇ in accordance with Eq. (15).
• R denotes the rearrangement operation ac- cording to the invention.
• the above-described reconstruction process is performed, in which the spectrum is first multiplied with the phase compensation coefficient and with the transfer function according to Eq. (17), whereafter the inverse Fourier transform is carried out line by line.

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Acoustics & Sound (AREA)
• Computer Networks & Wireless Communication (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)
• Holo Graphy (AREA)

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Cited By (1)

Citations (3)

• 1988
  • 1988-03-30 FI FI881503A patent/FI81205C/fi not_active IP Right Cessation
• 1989
  • 1989-03-30 WO PCT/FI1989/000058 patent/WO1989009399A1/en unknown

Patent Citations (3)

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