Cylindrical Transducer Ultrasonic Scanner

1. Field of the Invention

This invention relates to ultrasound imaging systems.

2. Description of the Prior Art

Ultrasound imaging systems are attractive for medical applications because they permit imaging of internal structures of the body without the use of harmful forms of radiation. Numerous proposals have been made for such systems and many are in actual use. Although the systems vary widely in detail, most known systems basically utilize an ultrasonic transducer or bank of transducers to beam pulses of ultrasound into a structure to be imaged and to receive reflections of those pulses. The beamed pulses are directed through the structure to be imaged in a suitable scanning pattern by mechanical and/or electronic means.

A major problem with such systems is lack of resolution in lateral directions. Some improvement can be obtained utilizing large aperture transducers which are electronically or acoustically focused but this improvement is at the expense of a very shallow depth of field. Consequently sophisticated means are required to maintain focus throughout a scan, and the operation of such means tends to limit the rate of scan and thus increase the time taken to generate an image. Even with such means, resolution is often inadequate for reliable detection of small anomalies in the structure being imaged.

An example of a known type of B-scanning ultrasonic system is disclosed in U.S. Pat. No. 4,014,207, issued Mar. 29, 1977, to Meyer, et al., for SECTOR SCANNING ULTRASONIC INSPECTION APPARATUS.

I have now found that excellent lateral resolution can be maintained over a substantial depth of field by utilizing separate transducers for transmission and reception of ultrasound pulses utilized for imaging a structure, one transducer being an actual or simulated cylindrical or part-cylindrical transducer having a line focus, and the other being aimed along said line focus of the said one transducer. The line of focus is moved relative to the structure being imaged so as to provide a desired scan of the latter.

The invention is described further with reference to the exemplary embodiments shown in the accompanying drawings, in which:

FIG. 1 is a diagrammatic part sectional, part schematic drawing of one embodiment of the invention, intended for breast scanning;

FIGS. 2A and 3A are diagrams illustrating how the apparatus of FIG. 1 can be utilized in two different modes to scan a breast;

FIGS. 2B and 3B illustrate the types of image plot obtained in the two modes; and

FIG. 4 schematically illustrates a modified embodiment of the invention which can be scanned electronically in a vertical plane.

Referring to FIG. 1, an aluminum trough 2 has, bonded to its surface by means of an epoxy resin, a sheet 4 of piezoelectric synthetic plastic, in this instance polyvinylidene fluoride sheet, thirty microns in thickness, available from Kreha Corp., of New York, USA. The aluminum trough 2 acts as a back electrode whilst the upper surface of the plastic 4 is metallized to form a front electrode of the part cylindrical transducer formed by the trough. The characteristics of such a transducer are well suited to 1-10 MHz frequencies commonly employed in medical ultrasound imaging. The transducer trough 2 has a radius of 14.0 cm, a length of 20 cm, and subtends an angle of 120.degree..

A pillar 8 mounted at one end of the trough carries a second transducer 10, which may be a conventional disc transducer capable of operating at the desired operating frequency. It need not be highly directional although it should be aligned with the axis of the trough 2, which is also the focus of the transducer formed by the sheet 4. The transducer 10 is connected to a receiver 12, whilst the transducer formed by a sheet 4 is energized by high frequency pulses gated from a transmitter 14 generating an energizing potential at, typically, 3-5 MHz at 10-200 volts peak to peak.

The trough 2 is mounted on a housing 16 supported by a fluid-tight bearing assembly 18 of an inner housing 20 which contains a stepping motor 22 and a sine-cosine potentiometer 24. The motor is operative to rotate the trough 2 about the axis 26, while the potentiometer 24 signals its angular position.

The inner housing 20 is supported by a platform 28 which can be moved laterally in a frame 30 by means of a drive screw 32 which can be rotated by a further stepping motor 34. Further hydraulic or screw operated means (not shown) may be provided to lift or lower the frame 30 bodily within a tank of water (not shown) in which the assembly is submerged to a depth such that the level 36 of the water is above the transducer 10. The water forms an ultrasonic coupling medium between a patient's breast to be imaged and the transducers 4 and 10. A thin plastic membrane may be provided to protect the patient from direct contact with the water and a suitable supporting couch (not shown) is of course provided to enable the patient to assume a suitable position relative to the apparatus, in accordance with known practice for ultrasonic breast imaging systems.

The apparatus is capable of operation in two different scanning modes. In a first mode, as illustrated in FIGS. 2A and 2B, the motor 22 is utilized to rotate the trough 2 and thus the line of focus 38 of the transducer 4 about the axis 26, so that the line 38 intersects a breast 40 being examined diametrically at successive angularly incremented locations 42 as the trough is stepped by motor 22, as shown in FIG. 2B. In the second mode, the line of focus intersects the breast 40 at successive locations 44 laterally displaced by motor 34 and in a common plane as shown in FIG. 3B. A mode control 46 determines which motor 22 or 34 is stepped by a motor pulser 48. The motor pulser 48 also applies pulses to a digital/analog converter 50 whose output is applied to a time base unit 52 comprising X and Y ramp generators. The time base unit also receives an input from a timing unit 54 which applies gating pulses to the transmitter 14. When the apparatus is operating in the second mode, the output from the D/A converter 50 is processed by the time base unit 52 to provide an X signal proportional to the lateral displacement of the platform 28. The input from the timing unit is utilized to generate a sawtooth waveform generating a Y-scan at the repetition rate of the pulses applied to the transmitter, but subject to a delay such that the scan coincides with the reception by receiver 12 of signals from the transducer 4 scattered by a breast being imaged and picked up by the transducer 10. The signals from the receiver 12 are processed in a signal processing unit 56, in which they may be subjected to known noise reduction and signal enhancement techniques. The processed signals are applied to a display and storage unit, for example a storage oscilloscope in which the beam is Z-modulated by the received signal and deflected on the X and Y axes to provide an image of the type shown in FIG. 3B.

In the first mode, operation is similar except that the inputs to the time base from the sine-cosine potentiometer 24 and the timing unit 54 are combined to provide X and Y outputs giving a vectorscope type display similar to that shown in FIG. 2B.

In operation, the time unit 54 gates short pulses of high frequency electrical energy from the transmitter 14 to the transducer 4 in which it is converted into a cylindrical wavefront of ultrasound which converges to a sharp focus on the line 38. Scattering of the ultrasound pulse takes place in the breast being imaged, the degree of scattering being dependent on the nature of the tissue causing the scattering. Most of this scattering will occur along the line of focus and components aligned with the line of focus will be picked up by the transducer 10 after a time delay dependent on the distance travelled by the scattered sound energy. Since ultrasound intensity on the line 38 will be very high compared with that elsewhere within the pickup range of the transducer 10, the signals received by that transducer will represent successive soundings along the line of focus as it is scanned through a plane intersecting the tissue being examined. It is found that the resolution and depth of field obtained are substantially better than with conventional techniques when operating at similar frequencies and imaging rates. Typical conditions of operation utilize an ultrasound pulse length of 1 microsecond at a frequency of 3 MHz and a pulse repetition rate of 1 kHz.

In the embodiment so far described, scanning is achieved solely mechanically. However, by utilizing a cylindrical transducer or simulated cylindrical transducer having an electronically variable line of focus, together with a second transducer or set of transducers of sufficient extent to receive ultrasound energy scattered along said line of focus throughout its range of movement, an electronic scan is achieved.

This is illustrated in FIG. 4, in which the film 4 of FIG. 1 is replaced by a series of n parallel strips of film 4A. The transducer 10 is replaced by a line of transducers 10A connected to the receiver 12, preferably with some means to exclude output from transducers 10A which at any particular stage in the scan are not in line with the focus 38. The line of focus 38 of the cylindrical transducer can be varied as shown at 38A, B and C in known manner by pulsing the strips 4A independently. Trigger pulses are derived from a trigger 60 fed through a series of n electronically variable delay lines 62, conveniently implemented by "bucket brigade" charge coupled devices, and these trigger pulses are used to drive n pulsers 64 driving the individual strips 4A to produce ultrasound pulsers. By relatively increasing or decreasing the delay applied to the outer strips as compared to the inner strips, the position of the line of focus can be varied, and by varying the line of focus progressively as pulsing of the strips continues, a scan can be achieved. The signal used to control the delay lines can also be used to provide the Y signal of a display 66, the X signal being derived from trigger pulses from the trigger 60. This electronic scan in one plane can be combined with a mechanical scan in an orthogonal plane to provide three-dimensional imaging of a tissue structure to be examined.

The function of the delay lines 62 is to equalize the combined electrical and acoustic delays occurring between the generation of pulses by the trigger 60, and the arrival of the ultrasound pulses, at the desired line of focus 38. It will thus be appreciated that a similar delay line arrangement can be used between the strips 4A and the receiver 12 if the cylindrical transducer is utilized to receive ultrasound scattered from pulses transmitted by the transducers 10A.

It will be understood that the above embodiments of the invention are described by way of example only. Many variations are possible.

The functions of the cylindrical transducer, and the transducer aligned with the line of focus of the cylindrical transducer may be transposed, so that the latter becomes the transmitting transducer and the former the receiving transducer. The mode chosen is largely a matter of convenience in design and electrical matching.

The cylindrical transducer itself is subject to a wide range of constructional variation. Thus in the embodiment of FIG. 4, the strips 4A could be arranged in a common plane, focussing being achieved entirely by the electronic means described. Focussing of the output of a plane transducer or battery of transducers could also be achieved by use of an acoustic lens so as to simulate a cylindrical transducer.

Whilst in the embodiments described the cylindrical transducer has subtended an effective angle of about 120.degree. at its focus, this angle can of course be varied. However, if the angle is reduced substantially below about 120.degree., the definition of the image achieved also deteriorates substantially. Further increases in the angle substended provide relatively little improvement in image sharpness, whilst making it more difficult to bring a structure to be imaged into an appropriate positional relationship with the transducers.

The piezoelectric plastic film utilized in the exemplary embodiments of the invention has particularly convenient characteristics for this application, but there is of course no reason why other ultrasonic transducer materials, capable of being incorporated into a functional actual or simulated cylindrical transducer, should not be used.

The embodiments described show a transducer 10 or 10A at one end only of the line of focus 38. In some cases, improved results will be obtained by providing transducers at both ends of the line of focus 38, and suitably combining the signals obtained into the final display.

It will also be understood that the invention fundamentally resides in the arrangement of transducers utilized, and that a wide range of possibilities exists as to the means utilized to scan the line focus of the cylindrical transducer through the structure to be imaged, and to construct an image from the signals received.