An ultrasonic transducer probe having a mechanically steerable ultrasonic beam

Abstract: An ultrasonic transducer probe (200) having a mechanically steerable acoustic element (230) includes a linear motor having a stationary magnet assembly (202) and a coil (206) bidirectionally linearly movable with respect to the magnet assembly. A mechanical arrangement formed of a pair of spaced apart, rotatably mounted pulleys (222, 224) and flexible belt (210) connecting the pulleys couples the coil and the ultrasonic transducer or acoustic element (230). The transducer element is mounted for rotation with at least one of the pulleys so that linear motion of the motor coil is converted to rotative motion of the transducer whereby the transducer and the ultrasonic beam (234) it emits is caused to sweep through a selected angular sector (10). A position sensor formed of a supplemental stationary magnetic material (216) and a position coil (220) mounted for movement with the motor coil is utilized for detecting the linear position of the coil and, correspondingly, the angular orientation or position of the transducer element. The pulley (250) carrying the transducer may have a non-circular periphery.

Description: This invention relates to an ultrasonic transducer probe having a mechanically steered acoustic element so that the ultrasonic beam can be steered within a sector of a plane. The probe is primarily intended to be used for ultrasonic imaging of biological tissue structures together with blood velocity measurements and imaging of blood flow. There exist in the market several similar devices for similar applications. The problem with present devices is that it is difficult to do simultaneous imaging and Doppler measurements. Using the Missing Signal Estimator method, as described in US-A-4,559,952, it is possible to obtain for practical purposes simultaneous 2-Dimensional (2D) amplitude imaging and Doppler blood velocity measurements along a defined beam direction, if the transducer can be stopped very quickly from its sweeping motion during imaging to do the Doppler measurement along the defined beam direction, and then be rapidly accelerated after the measurement to perform a new imaging sweep. The present invention provides a design for achieving rapid acceleration of beam direction movement so that for practical purposes simultaneous imaging and Doppler measurement can be attained. The rapid acceleration achieved is also useful for combined 2D tissue imaging and 2D flow measurements, as described in British Patent Specification No. 2,142,142. A transducer with a mechanically steered acoustic element is disclosed in US-A-4,421,118. This transducer comprises a transducer head assembly containing a movable holder having a transducer element coupled to a stationary base by a mechanical linkage. A motor adapted to provide linear reciprocating motion is coupled to the mechanical linkage for causing the element holder and element, when driven by the motor, to undergo reciprocating swinging motion for transmitting ultrasonic energy to and receiving echo signals from an object in front of the transducer in a sector scanning manner. The mechanical linkage in this known arrangement, however, has serious drawbacks which are particularly related to the problem of obtaining a rapid acceleration of the angular sweep. Reference is also directed to US-A-4,385,521 which discloses an ultrasonic transducer arrangement with a pivotally mounted transducer element coupled to a belt drive mechanism. The transducer arrangement of this Patent is primarily for scanning applications, and the rotational position of the transducer element is monitored as it moves through its scanning cycle to ensure that the movement is reversed when it reaches its limit in either direction. The aim of the present invention is the provision of an ultrasonic probe design which provides for rapid acceleration of the transducer so that the periods of switching the beam direction can be kept sufficiently small as to be practically negligible, with tolerable vibration of and power consumption by the transducer. It also seeks to achieve smooth motion of the beam which is important, especially for flow imaging, to avoid high Doppler shifts from tissue. The present invention is directed at a probe for sweeping an ultrasonic beam within a prescribed angular sector which comprises a housing, a mechanically steerable, rotable, ultrasonic transducer element within said housing, driven by a linear motor having a magnet and an electrical winding via a flexible linear-motion transmission means which is supported between and entrained about a pair of spaced-apart pulleys to one of which the transducer element is coupled, and between which the motor is mounted, and electrical position sensor means arranged for sensing the angular position of the ultrasonic transducer element. According to the invention, the position sensor means is mounted on the linear motor, the motor and position sensor means together comprising a body having a fixed assembly incorporating the magnet at one end and a fixed section comprising a fixed portion of the position sensor means, and a linearly-movable assembly attached to the linear-motion transmission means and substantially axially co-extensive with and surrounding at least in part the fixed assembly. The linearly-movable assembly has the electrical winding located at one end adjacent the magnet and a movable portion of the position sensor means adjacent the other end so as to coincide in position with the fixed portion of the position sensor means, and the linear motor and position sensor means are disposed between the two pulleys on a common axis which is perpendicular to the axes of the pulleys. The assembly of the transducer element, two pulleys, linear motor and position sensor means may be contained within a sealed housing comprising a single enclosure filled with an acoustic-coupling fluid. The design of a probe according to the invention facilitates rapid acceleration of the beam direction so that complex sweep sequences can be obtained as, for example, is described hereinafter. A Missing Signal Estimator, in accordance with U.S.-A-4,599,952 may be used to generate a Doppler substitute signal based on Doppler measurements in the intervals when the transducer stands still. This Doppler substitute signal can be utilized in the periods during which 2D tissue or flow imaging is performed, so that an apparently simultaneous imaging and Doppler measurement is obtained. The invention will now be described by way of example and with reference to the accompanying drawings wherein: Figures 1a and 1b diagrammatically and graphically illustrate by way of example a composite angular sweep of an ultrasonic transducer with rapid jumps in beam direction for combined tissue imaging, flow imaging and blood velocity measurements; Figure 2 is a simplified longitudinal cross section of a preferred embodiment of an ultrasonic transducer probe having a mechanically steerable ultrasonic beam and constructed in accordance with the present invention; and Figure 3 diagrammatically illustrates an alternative embodiment of a transducer steering arrangement in a probe according to the invention and incorporating a pulley wheel having an angle dependent radius. Fig. 1a illustrates a sector 10 of a plane within which an ultrasonic transducer beam is sweeping. A heavy black line 12 indicates the then current beam direction; the angular position of the beam is measured from the sector center line 14 to the beam direction 12, as indicated by arrow 16. Fig. 1b charts, by way of typical example, the changing angular position of the sweeping ultrasonic beam over a time interval. Thus, Fig. 1b illustrates the following discernable beam sweep segments: 1. a first sector sweep 101 of the beam for performing pulse echo amplitude imaging of biological tissue (∼20 msec); 2. a rapid change 102 of beam direction (∼5 msec or less) preparatory to a second sector sweep 103 beginning at a decreased angular position for performing pulse echo Doppler flow imaging (∼40 msec); 3. another rapid change 104 of beam direction (∼5 msec or less) to a stationary beam angular position 105 for performing either pulsed or continuous wave Doppler blood velocity measurements; and 4. yet another rapid change 106 of beam direction to a predetermined angular position for initiating a new sequence of beam sweeps 101 to 105. It is intended that the ultrasonic transducer probe of the present invention meet two design criteria deemed important to its contemplated utility. The first criteria is that the probe provide for rapid acceleration of the beam direction so as to minimize switching time between its anticipated modes of operation -- i.e. 2-dimensional structure imaging, 2-dimensional flow imaging and Doppler blood velocity measurements. Second, it is desired to provide for constant sweep velocity of the ultrasonic beam (i.e. no high frequency vibrations) to thereby avoid high Doppler shifts in the signals from tissue and thus artifacts in the flow image. In attaining performance meeting this second criteria, it is important to avoid the use of gear transmission arrangements -- such for example as those incorporating bevel gears, or rack and pinion mechanisms -- as such arrangements can cause or introduce vibrations unless carefully fabricated and assembled, thereby increasing the cost of the probe. It is therefore preferred to employ either a drive motor, where the acoustic transducer is directly mounted to the moving element of the motor (as either an integral part of the moving element or connected for example through a rod), or a pulley system or belt type transmission arrangement between the motor and the acoustic element. For the rapid beam accelerations intended it is important to utilize an efficient electric motor providing a large force with minimum electric losses. Toward this end, the magnetic field should be concentrated in a narrow air gap. In a particularly advantageous arrangement the motor is separate from the acoustic transducer so that the motor and transducer can be individually configured and shaped for optimum performance. It is equally important to keep the mass of moving parts small, which is readily achievable by employing a motor design having a moving coil and a fixed permanent magnet with a narrow airgap for generating a strong stationary magnetic field. Such an arrangement employing a moving coil can be attained with either a linear or a rotary motor design. To maintain a cylindrical transducer shape of relatively small diameter that fits conveniently into the user's hand, like a pen or pencil, and also utilizes a pulley or belt-type transmission arrangement, a linear motor with a pulley transmission is preferred as depicted in Fig. 2. For rapid acceleration, the disclosed pulley system of the invention provides substantial, advantages over the mechanical linkage used to transfer linear to rotary motion in the arrangement of U.S. Patent No. 4,421,118 (Dow). The range or extension of linear motion, and thus the mass of moving parts employed, can be further reduced by utilizing a pulley wheel having an angle dependent -- as opposed to a constant -- radius as hereinafter discussed. The currently preferred embodiment of the transducer probe 200 of the invention shall now be described with reference to Fig. 2. As there shown, a cylindrical magnet 202 with a magnetic field iron circuit 204 generates a strong magnetic field across an annular airgap 205. A cylindrical electric (motor) coil 206, through which an electric current is passed for generating an electromagnetic force along its cylindrical axis, is disposed for axial bidirectional movement through airgap 205. Motor coil 206 is mounted to an assembly 208, and assembly 208 is in turn connected to a flexible pulling element 210 at attachment portion 212 of assembly 208. Motor coil 206, together with its integrally secured assembly 208, is accordingly linearly movable axially through the airgap 205, its movement being guided by a fixed shaft assembly 213 extending axially rearwardly from magnetic field iron circuit 204 through an opening (not shown) defined in the forward portion of assembly 208, and formed of portions or sections 214, 216 and 218. Portions 214 and 218 of this shaft assembly 213 may be constructed of a noncritical material, which is preferably nonmagnetic and nonconducting, while portion 216 comprises a magnetic material, preferably nonconducting, such as a ferrite. Mounted on coil assembly 208, and carried through bidirectional linear movement therewith, is a second coil 220. Thus, as coil assembly 208 moves, so too moves coil 220 peripherally along and over the magnetic portion 216 of shaft assembly 213. The inductance of second coil 220 will accordingly depend on its position relative to magnetic shaft portion 216, and thus on the position of coil assembly 208; coil 220 can therefore be readily employed as a simple position sensor. By feeding an AC current through the second or position coil 220 with a predetermined frequency and amplitude, the voltage over the coil will be proportional to the coil inductance and, therefore, dependent on the position of the coil assembly. In order to avoid eddy currents induced by the current in position coil 220, the materials of shaft assembly 213 should be nonconducting. The constructional material of shaft portion 214 should also be nonmagnetic so as to avoid magnetic interference between the motor coil 206 and the position coil 220. The pulling element 210 extends between and partially about each of spaced apart pulley wheels 222 and 224 which rotate about respective rotatable shafts 226 and 228. By this arrangement the linear motion of the coil assembly 208 is transformed into a rotary motion of the pulley wheels 222, 224. An acoustic transducer 230 is mounted or connected to one of the wheel shafts, as for example shaft 226 as indicated in the figure, for rotation with the respective pulley wheel 222. The entire assemblage is then disposed within a cover 232 filled with a liquid which transmits the ultrasound beam 234 generated by angularly steerable transducer 230 through the front material of the probe 200. As herein described, the moment or lever arm in the transfer from linear to rotary motion is constant with the pulley system disclosed, independent of the angular position of the beam. The linear stroke of the motor coil is accordingly shorter or smaller for a given beginning angle of the sector scan, as compared to systems employing a mechanical linkage rod or arrangement. Moreover, since the lever arm is constant, there is a linear relationship between the linear position of the coil and the angular orientation or position of the beam. It is as a consequence far simpler to employ a position sensor for detecting linear motion of the coil rather than angular motion of the transducer. Of course, the pulling element must be sufficiently inelastic that the resonance frequency of transmission is well above the required bandwidth. A notably simple position sensor can thus be employed as, for example, illustrated in the Figure 2 embodiment; other methods of position sensing, such as bicoil induction, can alternatively be used. To maintain the pulley wheel lever arm where the beam direction is at the outer extensions of the sweep sector, while reducing the linear stroke of the motor coil, a noncircular pulley wheel 250, such as that illustrated in Fig. 3, can be employed in lieu of a constant radius pulley wheel 222 or 224. In such a noncircular wheel 250 the radius varies in angular dependence upon the wheel position providing, by way of example, a larger lever arm at the outer sweep extensions of the beam, where a large momentum is required, and a smaller lever arm at more central beam directions within the sector requiring less linear motion of the motor coil. By this modification a shorter coil can be employed and the mass of moving parts reduced. While there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims: A probe for sweeping an ultrasonic beam within a prescribed angular sector which comprises a housing (200), a mechanically steerable, rotable, ultrasonic transducer element (230) within said housing, driven by a linear motor (202, 204, 206) having a magnet (202) and an electrical winding (206) via a flexible linear-motion transmission means (210) which is supported between and entrained about a pair of spaced-apart pulleys (222, 224) to one of which the transducer element (230) is coupled, and between which the motor is mounted, and electrical position sensor means (216, 220) arranged for sensing the angular position of the ultrasonic transducer element CHARACTERISED IN THAT the said position sensor means is mounted on the linear motor, said motor and position sensor means together comprising a body having a fixed assembly (202, 204, 214) incorporating the magnet (202) at one end and a fixed section (214, 216, 218) comprising a fixed portion (216) of the position sensor means, and a linearly-movable assembly (206, 212, 208) attached to said linear-motion transmission means and substantially axially co-extensive with and surrounding at least in part the fixed assembly, said linearly-movable assembly having the electrical winding (206) located at one end adjacent the magnet and a movable portion (220) of the position sensor means adjacent the other end so as to coincide in position with the fixed portion of the position sensor means, and wherein the linear motor and position sensor means are disposed between the two said pulleys (222 and 224) on a common axis which is perpendicular to the axes of the pulleys. A probe according to Claim 1 wherein the pulley (222) to which the head (230) is attached has a periphery which is non-circular such that the generator of the periphery lies at a radius the length of which is angle-dependent. A probe according to Claim 2 wherein the periphery (250) of the pulley has four equidistant points of inflection joined by equal arcuate portions. A probe according to any preceding Claim wherein the assembly of the transducer element (230), two pulleys (222,224), linear motor (202,204,206) and position sensor means (216,220) is contained within a sealed housing (200) comprising a single enclosure filled with an acoustic-coupling fluid.

Inventors: ANGELSEN BJORN A J, ANGELSEN, BJORN A. J.
Applicants: ANGELSEN BJORN A J, ANGELSEN, BJORN A. J.
Application year: 1987
Main classification: G10K11
IPC codes: G10K11, G01S15, G01S7, A61B8