GB1572617A

GB1572617A - Sound transducer head

Info

Publication number: GB1572617A
Application number: GB1164777A
Authority: GB
Original assignee: Kretztechnik AG
Current assignee: GE Medical Systems Kretztechnik GmbH and Co oHG
Priority date: 1976-04-08
Filing date: 1977-03-18
Publication date: 1980-07-30
Also published as: AT348797B; ATA256776A; DE2709925C2; DE2709925A1

Description

(54) SOUND TRANSDUCER HEAD (71) We, KRETZTECHNIK GESELLSCHAFT M.B.H., a company of the Republic of Austria, of Frankenburg, Austria, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to a transducer head producing periodic mechanical vibrations of a medium, that is to say a sound transducer head for use in examinations generally carried out with projected and reflected ultrasonic pulses, comprising a generator zone which has the shape of a circle or cirular ring and is concentrically surrounded by one or more generator zones, which sound transducer head permits of a selection of the distance from said sound transducer head to a high-resolution zone for which said sound transducer zone has a maximum lateral resolving power.

A normal sound transducer head has a plane generator for projecting a sound beam which has initially the same diameter as the generator and diverges as its distance from the sound head increases so that in an object being examined the sound beam passes through larger areas at a large depth than close to the sound transducer head.

As a result, the lateral resolving power decreases as the depth increases. For this reason, signals which represent echoes that have originated at a large depth in the object under examination will be distorted so that the corresponding portions of section surface pictures or A display pictures resulting from the display of said signals will be blurred and the distinctness and accuracy of the display decrease as the depth increases. Whereas the attenuation of the sound can be compensated in that the pulses are amplified in dependence on their time of travel, this will not improve the lateral resolving power.

It has already been attempted to improve the lateral resolving power by electronic means in that the received signals are stored, data successively obtained from the same location in the object are compared, and those data are selected which result in an improved resolution. That method is expensive and requires specially equipped appliances.

It is known that a sound transducer head can be focussed to produce a convergent sound beam so that the lateral resolution will be improved. Focussing can be accomplished in two basic ways, namely, by the use of spherically curved generators or of sound lenses or combinations thereof or by a division of the generator into a plurality of component generators which in most cases have the configuration of concentric circles or circular rings. During operation all these component generators are excited simultaneously but the excitations of adjacent component generators are displaced in phase. This phase displacement may be effected, e.g., by delay lines connected between the component generators. As a result of the phase displacement, the differences between the times of travel of the sound waves from different component generators to a selected focus are compensated.When the focus to which the sound beam converges is at a predetermined distance from the sound transducer head, the lateral resolving power and sensitivity of the sound transducer head will be improved for the focal zone but behind the same the sound beam will have a much larger divergence than an unfocussed sound beam so that the resolving power and sensitivity for the region succeeding the focal zone will be greatly decreased.

So far, it has always been believed that the depth of that zone of the object for which the sound transducer head has the highest sensitivity and in which the sound beam has the smallest cross-section can be changed only when the sound transducer head comprises a large number of component generators, which are excited and rendered receptive with a phase displacement and that such change in depth requires a change in the phase displacement between adjacent generators in accordance with a predetermined or preselectable programme in such a manner that the focus and with it the focal zone are moved relative to the sound transducer head.This operation requires most expensive equipment and the phase-shifting means must be arranged in the equipment for controlling the sound transducer head rather than in the head itself and the leads to the component generators must be shielded from each other.

It has been believed so far that the distance from the sound transducer head to the zone in which the head has a maximum sensitivity cannot be changed where focussed sound transducer heads are employed which comprise curved generators.

For this reason, such sound transducer heads have previously been used only where the increased resolution was actually required only in the focal zone.

Sound transducer heads comprising two or more generators which in most cases are concentrically arranged and have the configuration of a circle or a circular ring have been used also for other purposes than for focusing by a phase-displaced excitation. For instance, it is known to provide two such generators and to use one generator for transmitting and the other for receiving. In a special design the annular generator is spherically curved to project a sound beam which converges throughout the region in which the object is to be examined to the extent of the sensitivity lobe of the inner generator. This arrangement is used with the intenion to compensate by the focusing the increase in sound absorption at a larger depth..It has also been proposed to use only one of the two generators for an examination using projected and reflected pulses whereas the other generator is operated at a greatly different frequency and serves only to monitor the coupling between the sound transducer head and the object under examination.

According to one aspect of the present invention we provide a sound transducer head comprising a generator zone which has the shape of a circle and is concentrically surrounded by one or more generator -zones, which sound transducer head permits of a selection of the distance from said sound transducer head to a high-resolution zone for which said sound transducer zone has a maximum lateral resolving power, characterized in that the sound transducer head has a fixed focal length and the generator zones are connected to separate leads and are excitable individually, jointly or in groups, preferably in phase with each other, so that the depth position in the object of a high resolution zone, in which a sound beam projected by said sound transducer head has the smallest effective crosssection and in which a sound transducer head has a maximum lateral resolving power, can be selected by a selection of the area of the sound-emitting and soundreceiving surface of the sound transducer head.

According to another aspect of the present invention we provide ultrasonic examination equipment comprising said sound transducer head characterized in that the equipment comprises a selector switch for selectively connecting individual generator zones of said sound transducer head or groups of said generator zones or all of said generator zones to a control pulse generator and an echo signal receiver of said equipment.

The invention is based on the entirely new recognition that the depth position of the high-resolution zone depends not only on the focal length of the sound transducer head but also on the area of its soundemitting and sound-receiving surface, i.e., on the effective diameter of the generator.

That recognition is contrary to the previous belief that the position of said zone depends only on the focal length. For this reason the generator diameter and the generator area have not been taken into account in all known formulae for the calculation of sound lenses or curved generators with respect to the high-resolution zone.

In a preferred embodiment, the widths of the annular zones differ from each other and from the radius of the circular central generator, if the width of an annular zone is defined as the difference between its outer and inner radii. The position of the high-resolution zone can be changed by a change of the connections of the generator zones. Such change of connections may be effected even during an examination of an object, e.g., in accordance with a cyclic program, so that all depth zones of the object can be examined with an improved lateral resolution.

Further details of the invention and equipment in which the sound transducer head according to the invention can be employed will be described more in detail with reference to the accompanying drawing, in which the subject matter of the invention is illustrated by way of example.

Figure 1 is a diagrammatic view showing the sound beam of a focussed sound transducer head.

Figure 2 is a side elevation showing a sound transducer head having generators shown in a longitudinal sectional view.

Figure 3 is a sectional view taken on the section station III-III in Figure 2.

Figure 4 is a simplified circuit diagram of a circuit arrangement for use with the sound transducer head shown in Figures 2 and 3.

Figure 5 is a longitudinal sectional view showing the generator of another sound transducer head and Figure 6 is a block circuit diagram of equipment provided with the sound transducer head.

In a conventional focussed ultrasonic transducer head as shown in Figure 1, a sound lens 2 is arranged in front of a plane generator 1. Alternatively, a curved generator with or without a succeeding lens may be used. The effective beam zone 3 of the sound beam is represented by the area which is hatched in the longitudinal sectional view. This effective beam zone is defined as the zone from which echoes originated in said zone can be received and converted into electric echo signals by said sound transducer head, and the effective cross-section of the beam is defined as the cross-section of said effective beam zone. If the sound transducer head comprising the generator 1 were not focussed, the sound beam would have the contour which is represented by the dotted lines 4.That portion 5 of the effective beam zone of the focussed sound beam which has a cross-section that is smaller than a predetermined fraction of the cross-section of the unfocussed sound beam is described as a high-resolution zone, for which the sound transducer head has the maximum lateral resolving power. The position and extent of that zone 5 is influenced by the focal length of the sound lens or of a spherical generator but is influenced not only by that focal length so that the highresolution zone does not agree with the focal zone but is also influenced by the diameter or effective area of the generator which is used. In the sound transducer head, that effective diameter can be changed so that the high-resolution zone 5, in which the sound beam has the smallest effective cross-section, can be shifted along the sound beam.

The sound transducer head shown in Figures 2 and 3 comprises a circular generator 1, which is concentrically surrounded by an annular generator la. The two generators are electrically disconnected from each other at least with respect to one pole and are adhered to the sound lens 2. A damping member 6 is attached to the rear face of the generators 1, la. Because the generators must be electrically disconnected, the damping member 6 and/or the lens 2 must consist of electrically non-conducting material or must be separated from the generators 1, la by an insulating layer.

When only the central generator 1 is excited, the position of the zone 5 will depend on the focal length of the lens 2 and on the effective diameter r1 of that generator.

When both generators are connected in parallel and are excited, the position of the zone 5 will be controlled by the lens and by the outside diameter r of the annular generator and the sound transducer head will operate like one comprising a generator having the diameter 7'3.

When only the annular generator la is excited, a focussed sound beam will also be projected and the position of the highresolution zone will depend on the difference rrrl between the outside and inside diameters of the ring. If r1 and r2 are properly chosen, the sound transducer head can be operated to have a highest lateral resolving power in either of three zones, which suitably overlap at their ends. The desired mode of operation can be selected by a simple switch.

A suitable circuit arrangement is shown in Figure 4. One pole of each of the generators 1 and la is grounded by a lead 10. A selector switch 8 is connected to a control and processing unit for ultrasonic examination by a line 7 for transmitting control pulses to and echo-representing pulses from the sound transducer head.

Depending on the position of the selector switch 8, each or both of the two generators 1, la can be connected to the control and processing unit via the selector switch 8 and the line 7. The lead connected to the corresponding electrode of each of the generators 1, la is also connected to one end of a coil 9 or 9a, the other end of which is also grounded. Each of the coils 9, 9a and the capacitance of the associated generator constitute a resonant electric circuit, which can be tuned to the mechanical natural frequency of the respective generator 1 or la to ensure a high efficiency. As separate coils are employed, this requirement can be met even when the capacitances of the two generators differ because the resonant circuits can be tuned independently of each other. The switch 8 may consist of a manually operated mechanical switch or of a sequentially controlled electronic switch.

When both systems are connected in parallel, they will operate satisfactorily when they vibrate at the same frequency.

Whereas this is ensured by the electrical coupling, the reliability with which the above-mentioned requirement is fulfilled can be improved by a mechanical coupling.

In this case a common generator- body is provided and the manufacture of the sound transducer head is further simplified.

An arrangement of the last-mentioned kind is shown in Figure 5 and contrary to Figures 2 to 4 does not comprise separately made and mechanically separate generators but a common generator member 1 which has the diameter r2 and is provided on both sides with junction electrodes 11, 1 la, and 13. The rear face of the generator 1 is formed with a concentric annular groove 12 by which the generator is also divided into separate zones in part of its depth. The electrodes on the rear face of the generator consist of a central electrode 11 and an annular electrode 1 la. There is a mechanical coupling between the annular and inner zones and the absolute value of the coupling coefficient can be determined by the selection of the depth of the annular groove 12. The electrode 13 is suitably grounded.Additional annular grooves could also be provided adjacent to the electrode 13. The entire generator has a mechanical natural frequency which is the same for both generator systems. Where separate systems are employed, they should also have the same mechanical natural frequencies but this can be accomplished only with measurements and trimming procedures.

A plurality of concentric annular generators may be provided. In that case the widths of the annular zones suitably differ from each other and from the radius of the circular generator, if the width of an annular zone is defined as the difference between its outer and inner radii. In such arrangement each of these annular generators will define in operation a high-resolution zone at a different depth.

The position of the high-resolution zone in which the sound beam has the smallest effective cross-section can be adjusted to that depth zone of an object which is most important for the evaluation. If the depth extent of the zone which is to be examined exceeds the depth extent of a single highresolution zone, and use of only a single high-resolution zone would involve a falloff in sharpness in other zones. When equip- ment provided with a sound transducer head according to the invention comprises a special selector switch, a picture can be displayed in several strips, each of which is associated with one of the high-resolution zones which can be selected. A suitable circuit arrangement is shown in Figure 6.

In the equipment, a pulse generator 14 supplies control pulses via a line 19, a selector switch 8, and a multipole line 20 to a sound transducer head 18 to excite the same. The position of the selector switch 8 will determine which of the generator systems of the sound transducer heads are excited and will thus control the position of the high-resolution zone 5. The echo pulses which are generated in response to the projected sound pulses are received and converted into electric signals, which are delivered via lead 20, selector switch 8, lead 19 and a lead 21 to an amplifier 15 and are amplified by the latter.

The amplified echo signals are delivered via a lead 22 to an electronic selector switch 16, which at a predetermined, adjustable timeafter the projection of each pulse is closed for a short time. The time for which the switch 16 is closed to transmit the signals is selected so that the switch transmits only signals from the high resolution zone which has been selected and blocks all signals which arrive earlier or later. To that end, the switch is triggered by the pulse generator 14 via lead 23 and is supplied from the selector switch 8 via lead 24 with control signals which control the gating times of the switch 16 in depend ence on the position of the high-resolution zone, which is determined by the selector switch 8.The signals which have been transmitted are delivered via a lead 25 to a display or storage unit 17, which may con sist of a picture storage tube, an electronic storage device provided with addressing means or the like. The unit may constitute -an A display unit, as shown, or a section surface display unit. In the latter case the system must comprise a section surfaceselecting mechanism and an addressing device and if a storage device is provided, an address selector will be provided too, so that each signal which has been trans mitted can be displayed in the section sur face picture at a location which is associated with the location at which the echo represented by the signal has originated.

For A and B display, the examination may comprise a first scanning cycle, during which those signals which represent echoes that have originated in the selected high resolution zone are displayed in a certain part of the picture or display area. When the position of switch 8 has been changed, another scanning cycle is performed and the resulting signals are displayed in an adjacent striplike area. This operation is repeated until the picture or display has been completed. If the selector switch 8 is an electronic switch which has a sampling cycle frequency that is high compared to the scanning cycle frequency the sound transducer head can be switched to all high-resolution zones which are available during a single scanning cycle so that the entire picture is displayed during a single scanning cycle and the interruptions which result between the examinations of adjacent zones owing to the operation of the selector switch are so small that they are not perceived or not disturbing.

Claims

WHAT WE CLAIM IS: -

1. A sound transducer head comprising a generator zone which has the shape of a circle and is concentrically surrounded by one or more generator zones, which sound transducer head permits of a selection of the distance from said sound transducer head to a high-resolution zone for which said sound transducer zone has a maximum lateral resolving power, characterized in that the sound transducer head has a fixed focal length and the generator zones are connected to separate leads and are excitable individually, jointly, or in groups, preferably in phase with each other, so that the depth position in the object of a high-resolution zone, in which a sound beam projected by said sound transducer head has the smallest effective cross-section and in which the sound transducer head has a maximum lateral resolving power, can be selected by a selection of the area of the sound-emitting and sound-receiving surface of the sound transducer head.

2. The sound transducer head according to Claim 1, characterized in that said generator zones comprise a circular central generator zone and a plurality of annular generator zones and the width of said annular generator zones differ from each other and from the radius of said central generator zone, if the width of an annular generator zone, is defined as the difference between its outer and inner radii.

3. The sound transducer head according to Claim 1 or Claim 2, in which said generator zones are mechanically coupled.

4. A sound transducer head for ultrasonic examinations, substantially as described hereinbefore.

5. Ultransonic examination equipment comprising a sound transducer head according to any one of Claims 1 to 4, characterized in that the equipment comprises a selector switch for selectively connecting individual generator zones of said sound transducer head or groups of said generator zones or all of said generator zones to a control pulse generator and an echo signal receiver of said equipment.

6. Ultrasonic examination equipment according to Claim 5, characterized in that an electronic switch of the equipment is so controlled by the selector switch that only those echo signals which represent echoes that have originated in said highresolution zone are transmitted by said electronic switch to a display or storage unit of the equipment

7. Ultrasonic examination equipment according to Claim 6, characterized in that the selector switch consists of an electronic switch and is operated to connect said generator zones to said control pulse generator in combinations which result in different depth positions of said highresolution zone and at a sampling cycle frequency which is higher than the scanning cycle frequency of the scanning operation performed by said sound transducer head.

8. Ultrasonic examination equipment, substantially as described hereinbefore and as shown in the figures of the accompanying drawings.