AU2011204834A1 - Ultrasound transducer improvement - Google Patents

Abstract

An ultrasound transducer probe including a motor adapted to move a transducer assembly with respect to an enclosure wherein there is a coupling layer adapted to 5 couple the transducer output to the acoustic window and hence to a body to be imaged, wherein a surface of the transducer assembly is fixed to the coupling layer and a surface of the acoustic window is fixed to the coupling layer.

Description

1 TITLE ULTRASOUND TRANSDUCER IMPROVEMENT TECHNICAL FIELD The present invention relates to a real time medical ultrasound imaging system. In 5 particular it relates to an embodiment of such a system employing an array transducer having a motor drive. BACKGROUND ART Ultrasound is a non invasive technique for generating image scans of interior body organs. There are a number of types of real time ultrasound systems supporting a 10 wide variety of ultrasound transducers. These systems can be divided into electronic systems such as phased array, curvilinear array, and linear array transducers which employ fully electronic techniques for beam forming and for directing the ultrasonic beam; and mechanical systems where a transducer or a transducer array is moved mechanically to direct the beam. 15 Mechanical scanners are traditionally the simplest and least expensive types of real time imaging systems. These systems utilize one or more piezoelectric crystals which transmit the sensing ultrasound signal, and receive the echoes returned from the body being imaged. To be effective, the ultrasound signal has significant directionality and may be described as an ultrasound beam. An electromagnetic 20 motor is employed to move the crystal in a repetitive manner in order for the beam to cover an area to be imaged. The motor may be of any type, depending upon the movement characteristics required. Devices using stepping motors, DC motors and linear motors are known. In general, mechanical ultrasound scanners employ one of two techniques for 25 moving the beam and generating an image. The first technique is the rotating wheel transducer where one or more crystals are rotated through 360' such that a beam emitted from the crystal would sweep out a circle. A sector of that circle constitutes the area to be imaged. The ultrasound signal is only transmitted and received while that sector is being swept out. 30 The second type of mechanical ultrasound scanner employs an oscillating transducer where a single crystal is moved back and forth by an electromagnetic motor such that the ultrasound beam emitted by the transducer sweeps out the region of interest.

2 Motor driven systems have an inherent design conflict in that the surface of the transducer which is in contact with the patient - often called the acoustic window should preferably be able to be held stationary with respect the patient's skin. However, the transducer itself must be kept moving in order to sweep the scanning 5 ultrasound beam. Accordingly, the probe unit of such a system consists of a transducer enclosure which is gripped by the operator, the enclosure having an integral acoustic window. Within the enclosure the transducer is mounted upon a pivotable or rotatable support, which is moved by the motor. A gap is provided between the transducer and the acoustic window in order to allow relative movement. 10 A further requirement for any ultrasound system is that the acoustic impedance of the transducer should be matched to the acoustic impedance of the body to be imaged in order to achieve maximum power transfer to and from the transducer from and to the body being imaged. In particular, step changes in acoustic impedance should be avoided, since these will provide significant reflections, masking the reflections from 15 features within the body. Accordingly, the material of the acoustic window will be well matched to the body to be imaged. This leaves the acoustic properties of the gap to be determined. Air is not a good acoustic match for the human body and is excluded as far as possible from path of the ultrasound transmission and reception. In order to achieve this, the gap between 20 the transducer and the acoustic window is filled with a liquid having acoustic properties similar to those of the acoustic window. The transducer is able to be moved by the motor through this liquid, while maintaining acoustic coupling to the acoustic window. The liquid introduces the problem of protecting the motor from ingress of liquid. This 25 is achieved by providing a membrane or barrier between the transducer assembly and the motor. This approach has significant problem. A seal must be provided in the membrane which allows the motive force of the motor to be provided to the transducer assembly, for example via a rotating shaft, without the liquid reaching the motor or otherwise being lost. The loss of liquid must be prevented, since 30 replenishment or replacement of the liquid in service is practically impossible. Electronic systems which do not use motors, including phased array, curvilinear, and linear transducers overcome many of the problems with mechanical systems, including image registration, ability to perform Doppler imaging, vibration and noise.

3 However, electronic systems have other shortcomings. They are more expensive to manufacture, have relatively high power consumption, and are relatively larger because, to achieve satisfactory performance, they include a large number of transducer elements and associated electronic channels. 5 DISCLOSURE OF THE INVENTION A transducer array with few elements, set relatively far apart gives good image width for less cost, but sacrifices the number of scanlines in the image. Moving the transducer allows the relatively wide gap between the transducer elements to be "filled in" with scanlines. 10 The transducer array moves, while the acoustic window, in contact with the patient for medical ultrasound, remains stationary with respect to the patient and the hand held probe unit. Acoustic and physical coupling between the assembly including the transducer array and the acoustic window is preferably maintained. The invention may be said to lie in an ultrasound transducer probe including an 15 enclosure enclosing a transducer assembly and supporting an acoustic window fixed with respect to the enclosure. There is a motor which is able to move the transducer assembly with respect to the enclosure There is a coupling layer which couples the transducer to the acoustic window, physically and/or acoustically wherein a surface of the transducer assembly is fixed to the coupling layer and a surface of the acoustic 20 window is fixed to the coupling layer. In preference, the acoustic impedance of the coupling layer is substantially the same as the impedance of the acoustic window. In preference, the coupling layer includes a flexible wall member enclosing a liquid. In the alternative, the coupling layer includes an elastic wall member enclosing a 25 liquid. In the alternative, the coupling layer may be an elastic member, without a liquid component. In a further embodiment, the invention may be said to lie in an ultrasound transducer probe as described, wherein in use, relative movement between a surface of the 30 transducer assembly and a surface of the coupling layer is less than the travel of the transducer assembly, and relative movement between a surface of the acoustic 4 window and a surface of the coupling layer is less than the travel of the transducer assembly. In an embodiment, the transducer array includes only one transducer. BRIEF DESCRIPTION OF THE DRAWINGS 5 Figure 1 shows an ultrasound scanning apparatus incorporating the instant invention. Figure 2 shows a diagrammatic representation of an ultrasound transducer array according to the present invention. Figure 3 shows a diagrammatic representation of the motion of the ultrasound transducer slider of Figure 2. 10 Figure 4 is a cross-section view of a preferred scan head embodiment. Figure 5 shows a diagrammatic representation of an embodiment of a coupling layer. Figure 6 shows a further diagrammatic representation of an embodiment of a coupling layer. Figure 7 shows a further diagrammatic representation of an embodiment of a 15 coupling layer where the coupling layer a solid or gel. Figure 8 shows a diagrammatic representation of a curvilinear transducer with a coupling layer. Figure 9 shows a system block diagram of an ultrasound probe unit scan head having a curvilinear shaped transducer and including a USB interface connection to 20 a host display. Figure 10 shows a system block diagram of an ultrasound probe unit scan head having a single channel ultrasound element and including a USB interface connection to a host display. Figure 11 shows a system block diagram of an ultrasound probe unit scan head 25 having a curvilinear shaped transducer and including a gyroscope and a USB interface connection to a host display. Figure 12 shows a system block diagram of an alternative embodiment of the ultrasound probe unit scan head including a wireless interface to a host display. Figure 13 shows a system block diagram of an ultrasound probe unit scan head 30 having an integrated display.

5 Figure 14 is a block diagram of an FPGA timing and control unit of the system of Figure 1. Figure 15 is a block diagram of the DSP software of the system of Figure 1. Figure 16a, 16b, and 16c illustrate the scan conversion requirements for different 5 transducer shapes which may be incorporated in apparatus of the present invention. Figure 17 illustrates a Doppler processing window. BEST MODE FOR CARRYING OUT THE INVENTION Now referring to Figure 1, there is shown a view of an ultrasound scan apparatus 300. This includes a display unit 301, and a probe unit 302. These are connected by 10 communications cable 303. The display unit has a display 304. The display screen may be a touch screen allowing a user to control the functionality of the display unit and the probe unit. In the illustrated embodiment, control members 306 are provided on the display unit, in the form of push buttons and a scrollwheel. Control members 305 are provided on the probe unit. Either of both of these control members may be 15 absent. The probe unit 302 includes or is connected to a probe unit scan head 308. The scan head 308 includes a transducer which may be, without limitation, an array transducer having multiple transducer elements, a single element transducer or an array of separate, individual transducers. 20 The scan head 308 includes an acoustic window 307 which contacts the patient during scanning. This acoustic window is has an acoustic impedance which is well matched to the acoustic impedance of the body to be imaged. It is desirable to provide the best possible acoustic coupling between the transducer elements and the body to be imaged in order to achieve the best power tranfer for acoustic enrgy 25 into and out of the body. In use, the probe unit is held against the body of a patient adjacent to the internal part of the body which is to be imaged, with the acoustic window 307 in contact with the patient's skin. Electronics in the probe unit stimulate the emission of ultrasound energy from the transducer. This beam is reflected back to the transducer as echoes 30 by the features to be imaged. The transducer receives these echoes which are amplified and converted to digital scanline data.

6 A motor moves the transducer such that the ultrasound beam or beams sweep out an area to be imaged. Electronics for control of the motor are provided in the probe unit. The motor may be a linear or a rotary motor. In a preferred embodiment, the linear motor is a linear ultrasonic motor. 5 In a further embodiment, the rotary motor is an ultrasonic motor. In an embodiment, the motor is an electromagnetic motor. The term "ultrasonic motor" is used throughout this specification. Other terms may be used for devices having the same principle of operation but varying in size, configuration and/or application. These terms include, without limitation, piezomotor, 10 piezoelectric actuator, piezoactuator, and ultrasonic actuator. The term "ultrasonic motor" as used in this specification covers all of these and any other possible terminology which may be used to describe ultrasonically driven moving devices which may be used to perform the invention. Figure 2 shows a diagrammatic representation of an ultrasound transducer of the 15 current invention. There is a transducer slider 501 which supports ultrasound transducers 502-505. Each transducer is able to transmit ultrasound energy and to receive the resultant echoes from a body to be imaged to gather one scanline 506 509. There may be as few as one transducer crystals, with the maximum number being limited only by practical considerations of cost, size and complexity. The 20 transducers form a sparse array, defined as an array of transducers wherein the distance between adjacent transducers or transducer elements is greater than the minimum separation of adjacent scanlines required to produce an ultrasound image of a desirable resolution. In use the slider is driven in a reciprocating fashion such that the beams from the 25 transducers cover an area to be imaged. The slider may be driven in said reciprocating motion by any convenient means. The scanlines produced by adjacent ultrasound transducers 502-505 are separated by a distance f. The slider is moved, continuously or in steps, the distance f. The transducers 30 transmit and receive ultrasound energy whilst moving or at each step in order to 7 receive a scanline which is a series of echo intensity values returned from features at various depths along a line running into the body to be imaged. Figure 3 shows the effect of moving the transducer slider. The series of dotted line diagrams of the transducer slider 501 represent the movement of the ultrasound 5 slider through the distance f. Each of the transducers is driven to transmit and receive a series of scanlines a,b,c ...,n. The combination of these n scanlines for all of the transducers produces a scanset which covers the distance S. The distance S is the width of the B mode ultrasound image which may be produced by the probe unit. 10 All of the scanlines captured during a single traverse of the transducer of the distance f form a scanset. The scanlines in each scanset are processed and displayed on display screen 304 in the same spatial relationship as they were acquired by the transducer. This produces an ultrasound image of the area to be imaged. Each scanset corresponds to a single frame ultrasound image. The distance 15 moved by the transducer slider between successive firings of the same transducer, for example between the acquisition of scanlines a and b and between the acquisition of scanlines b and c is sufficiently small to provide the desired image resolution. Post-processing may be applied to enhance the image in a variety of ways. 20 The slider is moved in a reciprocating fashion. A new scanset is obtained on each pass over the area to be imaged. Scansets are displayed sequentially as they are received to form a real time display. In prior art linear array transducer ultrasound probes, the lateral distance between scanlines is proportional to the linear distance between successive transducer 25 elements in the array. To provide a higher resolution image, without sacrificing image coverage; or to provide a wider image coverage without sacrificing resolution, requires more scanlines and hence more transducer elements. Providing additional elements is expensive. The larger transducer is itself more expensive, but the additional wiring 30 and electronics required to access and control the additional elements (usually called channels) is also expensive.

8 An array with few elements, set relatively far apart gives good image width for less cost, but sacrifices the number of scanlines in the image. Moving the transducer as described allows the relatively wide gap between the transducer elements to be "filled in" with scanlines, thereby providing similar performance and image size to 5 systems with more channels, at greatly reduced cost and complexity. The scanline density is not restricted by the physical size of the transducer elements or the spacing or number of elements. Conventional linear array systems have transducers with a large number of elements, each cut in a rectangular shape. This generates an acoustic beamshape 10 with a non-symmetrical pattern. It is desirable to use circular crystals, where the beamshape is circular and symmetrical. Since high end systems may have 1024 or more elements, this is not practical for conventional linear array systems. Using a sparse array of individual transducers makes the use of circular transducers in a linear array practical. 15 In order to be displayed as part of a scan image, in the spatial relationship in which the scanlines were received, each scanline must include, or be associated with, data indicating the relative location of the transducer element which received the scanline data at the time it was received. This can be done by any means known in the art. A method based on knowledge of the rotational or linear position of the motor may be 20 used, or the position of the slider may be directly monitored by a device such as a linear encoder. This data may be transmitted to the display unit as part of the scanline data; as a separate data stream; or the information may be inferred and calculated by a processor in the display unit. The image captured by the linear sparse array of Figure 3 has a maximum width of 25 S, being the width of the transducer slider plus the distance travelled by the slider. This can be increased by increasing the width of the slider 501, but this increases cost. Further, a larger slider requires a larger probe unit, which is undesirable form an ease of use perspective. The received scanlines are processed for display as an ultrasound image having a 30 form factor similar to the well known sector of a circle ultrasound image. The transducer slider is reciprocated to achieve real time coverage of the area being imaged. Substantially all of the scanlines captured during one reciprocation are displayed to form an image frame. Successive display of the frames as the scanlines 9 are received gives a real time display. Real time frame rate on the order of 20 frames per second are achieved. Higher frame rates are possible. Figure 4 shows of an embodiment of the invention wherein the slider of the ultrasonic motor is separate from the transducer array. There is a scan head 700 which 5 includes an ultrasonic motor 710, consisting of excitation electrodes 704, vibratory driver 705 and slider 708 held by a frame and guide rails. The slider includes a friction layer 709 which facilitates drive friction between the vibratory driver 705 and the slider 708. The motor 710 is attached to a circuit board (PCB) 701, which is positioned and supported within the scan head 700 by locating guides 702. The PCB 10 includes or has attached to it, ultrasonic motor control circuitry 706. This control circuitry supplies signals to the excitation electrodes 704 in order to drive the motor 710. Attached to the slider is a transducer assembly 723 made up of individual transducers 707embedded in or otherwise held in a fixed spatial relationship by a 15 support structure. The transducers making up the array may be of any suitable type. The transducer assembly is mechanically connected to the slider 708 of the motor. This arrangement simplifies the assembly and manufacturer of the system, as the main PCB is mechanically connected to the motor and ultrasound transducers, and the motor can operate independently of the enclosure. 20 The transducers are electrically connected to circuitry on the PCB 701 which excites the transducers to produce ultrasonic output and receives and processes the electrical signals returned from the transducers. In the illustrated embodiment of Figure 4, the transducer array is an array of eight piezo-electric transducers. In other embodiments, other numbers and other types of 25 transducer elements may be employed. In particular embodiments, capacitive micro machined ultrasonic transducers (CMUTS) or other MEMS based transducers may be used. The transducer assembly 723 is connected to the acoustic window via the lateral movement coupling layer 722. This coupling layer 722 may be any structure which 30 connects the transducer assembly with the acoustic window, substantially eliminating any air gap, whilst allowing the required amount of lateral movement between the two structures. It has an appropriate acoustic impedance to facilitate the coupling of 10 acoustic signals from the transducers to and from the acoustic window, and hence to and from the body to be imaged. In the illustrated embodiment of Figure 4, the coupling layer includes a membrane 721, surrounding and defining a reservoir 720. The reservoir contains a lubricant 5 liquid or semi-liquid, in this case, silicone oil. The lubricant allows the sides of the reservoir to move with respect to each other, and also exhibits appropriate acoustic properties to achieve the acoustic coupling required of the coupling layer. Note that none of the drawings of the coupling layer is to scale. In particular, the thickness of the coupling layer has been exaggerated to show detail. 10 In use, at least a portion of one surface of the coupling layer is substantially stationary with respect to the transducer assembly, whilst at least a portion of an opposite surface is substantially stationary with respect to the acoustic window. The motor moves the transducer assembly such that lateral movement occurs between the transducer assembly and the acoustic window. The opposite surfaces of 15 the coupling layer are able to move with respect to each other, lubricated by the lubricant in the reservoir. The coupling layer fills at least the gap between any operating transducer and the acoustic window at all times, providing acoustic coupling between the transducers and the acoustic window. In a preferred embodiment the stationary state between the coupling layer surfaces 20 and respectively, the acoustic window and the transducer assembly is achieved by bonding the respective surfaces of the coupling layer to the transducer assembly and the acoustic window. In other embodiments, one or both of the surfaces may be held stationary by frictional engagement. The action of the coupling layer id illustrated in Figure 5. There is a transducer 25 assembly 551, having embedded within it individual transducers 552. This assembly is coupled to an acoustic window 553 via a coupling layer 555. These elements are supported and encased by enclosure 554, which forms part of a probe unit of the type of the probe unit 302 shown in Figure 1. The acoustic window is made of a material with an acoustic impedance similar to that 30 of the mammalian body. Such materials are known in the art. One such material is polypropylene, but others may be employed. In use, gel is used to acoustically couple the acoustic window to the body to be imaged.

11 On the other side of the acoustic window, the coupling layer 555 provides acoustic coupling between the acoustic window and the transducers of the transducer assembly. The acoustic impedance of the PZT transducers usually used for ultrasound is a poor 5 match for the human body. Consequently, the transducer assembly will usually include a matching layer or layers which bring the output impedance of the transducer assembly closer to that of the body to be imaged. It is possible for some or all of this impedance matching function to be performed by the coupling layer. In a preferred embodiment, the coupling layer has an impedance which is closely 10 matched to that of the acoustic window. In alternative embodiments, the impedance of the coupling layer may be closely matched to the output impedance of the transducer assembly. In further embodiments, the impedance of the coupling layer may be of a value which provides an impedance match between the transducer assembly output impedance and the impedance of the acoustic window. 15 In use, the transducer assembly 551 moves in a reciprocating fashion to cover the area to be imaged with scanlines. The coupling layer provides the necessary freedom of movement to allow for this movement. In the embodiment shown in Figure 5, the coupling layer 555 includes a membrane 556 which forms a closed surface defining an internal reservoir 557. The transducer 20 assembly is bonded to the "upper" side 560 of the coupling layer, along the whole length of the transducer assembly. The acoustic window is bonded to the "lower" side 561 of the coupling layer, along the whole length of the acoustic window. Figure 5a shows the transducer assembly in the middle of its travel, while Figures 5b and 5c show the extremes of that travel. In use, the movement of the transducer 25 assembly causes the upper side 560 of the coupling layer to move with respect to the lower side 561 of the coupling layer. This deformation is facilitated by the lubricant liquid within the reservoir 557 which is formed within the coupling layer. This liquid substantially fills the reservoir and moves with the movement of the sides of the coupling layer to ensure that the characteristics of the coupling layer remain constant 30 during deformation. As can be seen in Figure 5b, the coupling layer deforms to include a portion 562 which lies within the probe enclosure 554 beyond the acoustic layer. When the transducer assembly is at the other extreme of travel, as shown in Figure 5c, the 12 deformation causes another portion 563 of the coupling layer to lie beyond the acoustic layer. Alternatively, as shown in Figure 6, the coupling layer may be bonded to the acoustic window only for part of the length of the window. There is a transducer assembly 5 651, including transducers 652. The coupling layer 653 is bonded to the acoustic window 654 only over the length of the bond 655. Figure 6a shows the transducer assembly at the mid-point of travel, while Figures 6b and 6c show the two extremes of the transducer assembly travel. As can be seen in Figure 6, it is not necessary for the coupling layer to cover the full 10 used length of the acoustic window. It is sufficient for the coupling layer to cover the space between the active transducer and the acoustic layer. The coupling layer of Figure 6 deforms in the same way as that shown in Figure 5 in response to the movement of the transducer assembly. When the transducer assembly is at one extreme of travel, as shown in Figure 6b, an area 660 of the 15 acoustic window is not covered by the acoustic layer. At the other extreme of travel, the coupling layer has deformed to cover the area 660, leaving the area 661 of the acoustic window exposed. This does not cause problems with impedance matching, since there is not an active transducer using the areas of the acoustic window for transmission or reception of ultrasound signals at the time when those areas are not 20 covered by the coupling layer. In a further embodiment shown diagrammatically in Figure 7, the coupling layer consists of a flexible material and does not include an internal lubricant reservoir. There is a transducer assembly 751 including transducers 752. The transducer assembly is bonded to coupling layer 755, which is further bonded to acoustic 25 window 753. Probe unit enclosure 754, encloses and supports these components. Coupling layer 755 is of a material which may be a gel material, which is able to be deformed by stretching in response to the movement of the transducer assembly. Figure 7a shows the transducer assembly at the mid-point of travel, while Figures 7b and 7c show the two extremes of the transducer assembly travel. As shown in Figure 30 7b and 7c, when the transducer assembly moves to an extreme of travel, the solid or gel coupling layer deforms by stretching to accommodate the movement, remaining fixed to the transducer assembly and the acoustic window, and continuing to occupy 13 the space between any active transducer and the acoustic window, excluding air and maintain the acoustic coupling between the assembly and the window. Further embodiments (not shown) combine the operational modes shown in Figures 5, 6 and 7. The combination of a deformable reservoir and a flexible membrane 5 material is used to ensure that the coupling layer occupies the space between any active transducer and the acoustic window at all times during the travel of a transducer assembly. The slider and the associated transducer assembly, for any embodiment, may be driven by a rotary motor, which in a preferred embodiment is an ultrasonic motor. 10 In further embodiments the transducer may be implemented such that the array is moved in a curvilinear pattern. This is illustrated diagrammatically in Figure 8. Figure 8 shows a diagrammatic representation of the invention applied to a curvilinear scan head form factor. There is a transducer slider assembly 851 including one or more transducers. The 15 illustrated embodiment has four transducers 852. The transducer slider assembly is shaped as the arc of a circle. Each transducer is able to transmit and received ultrasound energy in order to produce a single ultrasound scanline 871, transmitting and receiving through the acoustic window 853. The transducer slider is incorporated in and forms part of an ultrasound probe unit as illustrated in Figure 1. This probe 20 unit supports acoustic window 853 which, in use, contacts the body to be imaged. In use, the transducer slider is rotated about an axis through the centre of the circle of which the transducer forms an arc. This may be by use of a pivot arm with a pivot point at said centre. Alternatively the transducer slider may be guided by arc-shaped guides. Other means to constrain the slider to move along the path of the arc of the 25 circle may be employed. The acoustic window remains stationary with respect to the probe unit, while the transducer slider moves. There is a coupling layer 855 which permits the relative movement, while maintaining acoustic coupling between the acoustic window and the transducer assembly. 30 Figure 8b shows the rotated transducer slider 881 with transducers 882 producing scanlines 891, superimposed on the original position of the slider 851. As can be seen, the coupling layer 885 has deformed to allow the relative movement, while 14 continuing to occupy the space between the transducer assembly and the acoustic window, excluding air and maintaining acoustic coupling between the transducer assembly and the acoustic window. The placement of the transducers on the outside of a circle means that the overall 5 area covered by the ultrasound beams is a sector of a circle. This allows broader coverage than the width of the transducer probe unit. The greater the curvature of the slider the greater is the amount by which the width of the scanned area can exceed the width of the probe unit. Either a rotary or linear motor may be used to drive the slider. For a linear motor, the 10 slider and guides may form a linear motor, or a separate linear motor may be provided. Where the radius of curvature of the slider segment is sufficiently small, a rotary motor may move the slider by directly driving a rotor arm. In alternative embodiments, the coupling layer may form the acoustic window. This requires the coupling layer material to have physical characteristics suitable for 15 repeated patient contact, as well as the required acoustic and lateral movement coupling characteristics. In use, the motor moves the transducer array slider across the width of the area to be scanned in a reciprocating fashion. Each transducer element is fired in turn. Each firing results in a single scanline of data being collected. The delay between firing 20 each transducer element should be at least sufficient to allow for the return of echo from that depth within the body being imaged where it is desired for an image to be obtained. The firing process repeats. Generally, each transducer element will be fired many times in the time it takes for a single reciprocation of the slider. Preferably the firing is timed such that on each successive reciprocation, the firing of each 25 transducer element is in substantially the same places with respect to the probe unit body. Preferably, the delay between successive firings of a transducer element is related to the width of the beam from the transducer elements such that the entire length of the area being scanned is covered by a focused beam. 30 In an alternative embodiment, the system operates by firing each transducer element individually with the transducer slider at rest to acquire a set of scanlines. The motor then moves the slider a step, with the slider coming to a halt before the transducer 15 elements are again fired acquiring a second set of scanlines. This continues until the distance f is covered. The process repeats. The size of the step is determined to provide the required scanline density over the area to be imaged. It may be varied in use in order to optimise characteristics of the 5 scan system such as fame rate or motor speed. All of the acquired scanlines for a single full traverse of the slider are then combined to form one frame of a real-time ultrasound display. In order to be displayed as part of a scan image, each scanline must include, or be associated with, data indicating the relative location of the transducer element which received the scanline data at 10 the time it was received. This can be done by any means known in the art, for example, the position of the slider may be directly monitored by a device such as a linear encoder. The number of steps used is determined by the scanline density required and the magnitude of the transducer element separation f. 15 The number of transducer elements used can vary depending on a number of factors including coverage area, motor speed, scanline density desired and motor response time. All of the scanlines acquired in one full transverse movement of the transducer constitute a single frame for display. This means that the firing rate of the transducer 20 as a whole is determined as the product of the number of elements, the frame rate and the number of scanlines required to cover the distance between the elements at the desired density. Other element firing regimes are possible. Where this can be done without undue interference between the scanlines, more than one element may be fired at a time. 25 Without limitation, a means of achieving this is for the transmit pulses to be coded, for example with Barker, Golay, or Gold codes, such that the correlation between pulses from separate transducers is extremely low. In a further embodiment, the slider length is less than the image width S, by an amount greater than the separation of the transducer elements f. It may be much 30 less. The scan head and rails cover the full image width. The slider is driven in incremental steps to cover the required image width. In this embodiment, the 16 elements of the transducer array may be arranged at such a separation that adjacent elements give the desired scanline density. In this case, the slider may move an amount equal to its full length between each full firing of the array. In the most general case, the transducer may consist of a single element. This is not 5 the currently preferred embodiment since one of the limitations of ultrasonic motors is that they have low linear and rotational speeds. This poses a constraint on the speed the ultrasonic crystal can scan the region of interest and hence limits the systems frame rate. Thus using an array incorporating more than one crystal which decreases the commuting distance of the crystals and reduces the motor speed requirements is 10 currently preferred, but further improvements in motor design may make this alternative practical. Pulsed Wave Doppler imaging is achieved by electronically stimulating a transducer element to produce a pulse of ultrasound and then to listen for echoes before another pulse is generated. This is done rapidly at a single location, and the variable 15 frequency shift in the returned echoes is used to detect movement in the feature being imaged, for example blood flow. These modes are not possible with motor driven transducer systems of the prior art, since they cannot be held stationary nor rapidly and accurately repositioned. This can be achieved by the apparatus of the current invention because ultrasonic 20 motors have excellent response time and can be positioned extremely accurately, with errors in the nanometre range. This allows the transducer to be completely stationary at a known position when sending and receiving ultrasound pulses. Color Doppler combines real time imaging with Doppler imaging. Real time images are displayed for the full image area at the same time as Doppler information is 25 displayed for one or more scanlines. This is not possible for any prior art system with a moving transducer, since Doppler requires that the transmitting element be stationary in a known position and real time scanning requires that the transducer be moved. In an embodiment providing color Doppler functionality, the transducer array is 30 controlled to move to produce a real time moving image on a display for a user. The user is able to select an area of the displayed image for acquisition of color Doppler information. A selected element or elements of the transducer, while located above the selected area, fires the required number of pulses and receives the necessary 17 echoes to acquire the movement information, being the color Doppler frequency shift data. The transducer array is stationary whilst the Doppler information is acquired. The real time scan operation then continues, with the transducer being moved by the ultrasonic motor. Real time image scanlines are acquired whilst the transducer array 5 is moving. When transducer has moved a selected distance, the transducer stops and the selected element again fires multiple pulses, acquiring Doppler information. This continues until a full real time image frame has been acquired, along with Doppler information for a subset of that frame. The process then repeats to acquire a real time image display. 10 The color Doppler and real time information are combined into a single display, as is known in the art. The fast response and excellent positioning accuracy of ultrasonic motors allows the transducer to come to complete stop, fire the required number of ultrasound pulses, listen to the echoes, and then move to the next line while simultaneously performing 15 real time imaging. Generally the real time frame rate will be reduced while color Doppler is in use. Embodiments using ultrasonic motors may also use a curvilinear array where the linear slider is replaced with a curvilinear slider with N transducer elements mounted on it or machined as part of the slider. The rails and the acoustic window would also 20 be manufactured in a curvilinear construction. Ultrasonic motors are readily manufactured in different shapes and sizes. In embodiments where the slider itself is part of the motor, no special arrangement or shaping of the slider is necessary to allow it be driven by an external motor. In alternative embodiments, a linear ultrasonic motor separate from the curvilinear 25 slider may be used to move a curvilinear slider through an arc of a circle. The slider may be driven through a full circle in a continuous revolution. In this case, the transducer would be active only when it was over the body to be scanned. This is especially relevant for micro curvilinear shaped scan heads, which have a relatively tight radius of curvature so as to enable imaging between ribs. Two or more 30 transducers may be provided, each having a different operating frequency, allowing simultaneous or separate scanning at different frequencies. Doppler imaging would be possible as described for other embodiments.

18 One of the drawbacks of ultrasonic motors is the limited lifetime due to the frictional drive. A spring like mechanism is required to maintain the force between the stator and the slider/rotor. Contact areas are required between the stator and the slider to provide the frictional drive, and these contact surfaces wear. This wear increases the 5 separation between the contact surfaces, reducing the effectiveness of the drive. This problem can be mitigated by using a piezoelectric actuator as the spring like mechanism to provide the force between the stator and slider/rotor. The force imposed by the actuator is proportional to a DC voltage across the actuator. This DC voltage in effect pushes the piezoelectric driver closer into the frictional coating and 10 hence the coupling force between the stator and the slider is maintained throughout the life time of the motor. The magnitude of the DC voltage can be controlled so that it is increased with the motor's operating life to compensate for the layer wear. Ultrasonic motor efficiency is insensitive to size and hence they are superior in the mini-motor area. This allows for the construction of devices of small size and low 15 weight. Ultrasonic motors do not produce the electromagnetic interference which is inherent in the operation of electromagnetic motors. This is an advantage for ultrasound systems where the electrical signals being received are inherently of low power and susceptible to interference. This has a further advantage for hand held ultrasound 20 systems in reducing the need for shielding between the electronic components and the motor, which reduces size, weight and cost. In order to produce ultrasound energy for scanning, the ultrasound transducer elements are stimulated by a high voltage power supply. This high power supply would generally supply power at voltage magnitudes on the order of tens or 25 hundreds of volts. The ultrasonic elements which form the ultrasonic motors in those embodiments which employ ultrasonic motors are also driven by high voltage power supplies which may have similar output characteristics. In further embodiments, the ultrasound transducer elements and the ultrasonic motor are driven from a common power supply or power supplies. This results in lower costs. 30 In Figure 9, there is shown an architecture of an embodiment of the present invention. The block diagram shows a probe unit, including a scan head 901 and a probe unit body 903. There may also be a host unit (not illustrated) providing a 19 display and at least part of a user interface, connected to the system using a USB interface 920. Small size and weight are desirable for the probe unit to facilitate hand held use. In an embodiment, the size of the probe unit is about 15cmx8cmx3cm. In a further 5 embodiment, the weight of the probe unit is 500g or less. In a preferred embodiment the size of the probe unit is about 1 lcmx6cmx2cm and the weight is about 200g. The scan head 901 includes transducers 906, arranged as a transducer array on a slider element 905. The slider element is driven by ultrasonic motor 908. The position of the slider 905 is monitored by position encoder 907. 10 The transducers 906 are connected to multiplexer 909. Electrical signals to excite the transducers to produce ultrasound output, and the electrical signals from the transducers in response to received echoes, are passed through the multiplexer 909. The multiplexer may be provided within the probe body 903, or omitted, at the expense of providing more connections between the scan head 901 and the probe 15 unit body 903. User control of the ultrasound scanning system is via a user interface which may be provided wholly on the host unit, wholly on the probe unit, or split or duplicated between the probe unit and the host unit. In the illustrated embodiment there is a User Control Panel 902 incorporated in the probe unit, said Panel including a freeze 20 button to start and stop scanning, a set of buttons providing increment, decrement and select functionality, and a "back" button. These buttons are used for basic control of the system, including without limitation, some or all of start and stop scan, depth adjustments, gain adjustments, operating mode selections, and preset selections as well as other basic settings. The user control panel 902 sends data 25 identifying the state of the buttons to a micro-Controller 930, which in this embodiment is part of a combined microcontroller/DSP device such as the OMAP3525 Applications Processor from Texas Instruments. The microcontroller monitors the state of the control panel and provides appropriate commands to control the appropriate electronic circuitry to allow the user to control the ultrasound system. 30 Where this requires a graphical user interface, this is displayed on the host unit. The microcontroller 930 contains a set of parameters for controlling the operation of the device during scanning. At initial power up, a default set of parameters are created, which can then be modified by the user before or during scanning. The 20 parameters include without limitation, the operating frequency, the active scanning mode, the gain curve, the scanning depth, the Doppler gate depth and angle if required, color Doppler ranges if required, power Doppler ranges if required, and M mode pulse repetition rate. The scanning modes available may include, without 5 limitation any or all of B-mode, M-mode, and modes available using Pulse Wave Doppler, including color Doppler, power Doppler and spectral Doppler, and Duplex Doppler. The microcontroller passes the relevant parameters to the Digital Signal Processor (DSP) 931 and Field Programmable Gate Array (FPGA) 932 when scanning is 10 activated or the parameters change. When a user commences a scan, either by pressing a button on the control panel or by activating a control on the host, the microcontroller sends a command to the DSP and Field Programmable Gate Array (FPGA) to activate scanning, along with any updated configuration parameters. The DSP is configured to receive and process 15 ultrasound data according to the parameters, which may include parameters concerning Doppler processing. The microcontroller and the FPGA together provide the functionality to control the scan head ultrasonic motor. The ultrasonic motor position encoder 908 produces a value proportional to the position of the ultrasonic motor at any point in time. This 20 position is saved with a timestamp in a register in the FPGA, and the microcontroller reads this information to calculate a velocity. The velocity is compared with the desired velocity. The motor control unit 910 is instructed to alter the voltage and frequency of the drive signal applied to the ultrasonic motor 908 in order to achieve the desired velocity. 25 Figure 14 illustrates the FPGA functionality. The FPGA receives and decodes the output of the ultrasonic motor position encoder 907, and provides the information to the microcontroller in order for it to recalculate the ultrasonic motor drive signals. The FPGA includes a timing state machine 145 which uses the decoded position information to determine the appropriate time to generate the next ultrasound pulse 30 sequence to be output from the transducers, which is achieved by driving the transmit pulse controller 144. Transmit pulse controller 144 generates control signals corresponding to the type of scan line required to be generated. For a B-mode scan line, a single pulse is 21 generated at the imaging frequency at the voltages provided by the High Voltage power supply (HV Supply) 934. These voltages are typically up to +-100V. For Doppler a sequence of several pulses, typically eight pulses, at the Doppler imaging frequency is generated. These Doppler pulses are typically of longer duration than B 5 mode pulses. The multiplexer is preconfigured before the Tx Pulser 933 fires to steer the transmit pulse to the appropriate circular transducer crystal. The transducer crystal 906 generates an ultrasonic waveform in response to the electrical pulse, which is transmitted into a body to be imaged. The ultrasonic waveform is reflected by changes in acoustic impedance, producing 10 echoes which are received back at the transducer crystal 906 at lower pressure. The crystal converts this lower pressure waveform into an electrical signal, which is directed through the multiplexer to the input protection circuitry 935. The input protection circuitry protects the sensitive low noise amplified (LNA) 936 from the high voltage transmit pulse while letting through a low voltage receive signal. Several 15 input protection circuits are known in the prior art. The low noise amplifier (LNA) provides amplification of the small receive signal, while adding little noise to the output signal. The LNA is typically single ended, and generates a differential output voltage for feed into a time gain amplifier (TGA) 937. The TGA provides further amplification with the amplification provided being 20 dependent on time. Ultrasound signals attenuate as they propagate through tissue, and the TGA compensates for this attenuation by increasing the gain dependent on the time from the beginning of the received pulse, which is proportional to the depth of the echo reflection. The output of the time gain amplifier is filtered to remove as much noise as possible 25 and to prevent aliasing. Typically a bandpass filter 938 is used. The output of the filter is input to the analog to digital converter 939. A sampling frequency of at least 4x the imaging frequency is preferred. The analog to digital output is input to a first in first out memory (FIFO) 147 in the FPGA 932. The FPGA adds some header information to each complete scan line, including the type of scan line (for example, 30 B-mode or Doppler), the motor positional encoder input if required, and a time count. This information may be used at a later stage in processing dependent on the configuration parameters.

22 The DSP reads the scan lines out of the FPGA FIFO, and performs appropriate processing on the data depending on the configuration parameters. Figure 10 is a configuration of the system which does not have real-time ultrasound capability, but uses a lower cost single channel system where the ultrasound beam is 5 stationary with respect to the system. A single transducer 1001 is provided, which is in a fixed relationship to the probe unit body. There is also provided a gyroscope 1002. Images created by moving the probe unit in an arc with the point of contact of the scan head 1003 with the patient being substantially constant while the transducer is pulsed. This will generate a sequence of scan lines which together make up a 10 sector image. The gyroscope provides information as to the relative position of each scan line, enabling the scan lines to be assembled into an image. Figure 11 illustrates a further embodiment of the invention, where the scan head 1101 contains a motor 908 and a gyroscope 1103. The gyroscope is oriented so it provides angular measurements perpendicular to the image plane of the B-mode 15 image which is produced from the transducer array 1102. By providing the gyroscope, the system is able to construct 3D images from a sequence of 2D image scans, and determine volumes. In use, a series of B-mode images are made as for the system of Figure 9. The probe unit is moved in an arc perpendicular to the image plane of the B-mode images. The operation is otherwise similar to the system as 20 described in Figure 9. Referring to Figure 14, The FPGA timing state machine 145 interfaces to the gyroscope in embodiments where a gyroscope is provided. The gyroscope angular measurement is combined with the ultrasound scan line information in a first in first out (FIFO) memory 147, and read by the digital signal processor (DSP). Where a 25 single transducer is used, the gyroscope information is used to assemble the scan lines to form a sector image. Where a transducer array is used, the gyroscope information is used to assemble B-mode sector images into a representation of a volume which may also be used for any applications which require 3D volume calculations. 30 Figure 12 illustrates an alternative embodiment of the invention, whereby the interface to the host display and control system is via a wireless interface. There is provided a wireless interface module 1202 controlled by the microcontroller 930. An antenna 1201 is provided for signal transmission. This interface provides 23 communication to and from the host unit. No USB interface is required for communication with the host unit, although one may be provided for access to third party hardware. In a presently preferred embodiment the wireless protocol used is WiFi Direct. 5 Figure 13 illustrates an alternative embodiment of the invention, wherein there is no host unit. There is provided a display 1301 which is integral with the probe unit. There is also provided an enhanced user control module 1302. This includes the functionality of the control panel of previously described embodiments, but further includes functionality allowing for full control of the system in the absence of a host 10 unit. The control module may include a graphical user interface displayed on the display 1301. The display may be a touchscreen, able to provide input to the user interface. The operation of the DSP in different embodiments will vary. Where a host control and display system is connected, some processing and control functions and the 15 display function may be performed by the host unit. Where no host is used, all functions are performed by the probe unit, increasing the load on the DSP. Limitations on the division of functionality between a host and the probe unit come from the processing power of the host and the bandwidth available for transmission between the host and the probe unit. In general, the implementation and subdivision 20 of algorithms is designed to minimise the host processing requirements and the bandwidth required for transmission. Figure 15 illustrates the algorithms for processing combined B-mode/Doppler, with a division of the tasks between the ultrasound probe unit and the host display system. The first step in the B-Mode processing chain is to filter the digitised incoming rf scan 25 line data using an FIR bandpass filter. The filter is adjusted depending on the transducer connected to the system and the imaging frequency. For example, for a 3MHz imaging frequency a bandpass filter of 1 to 5MHz could be used. For an 8MHz imaging frequency a bandpass filter of 4.5 to 11.5MHz could be used. Following filtering, the envelope of the rf scan line data is generated. The preferred method is 30 to use a Hilbert transform to generate the in-phase (1) and quadrature (Q) components of the rf scan line. The final envelope is generated by summing the squares of the I and Q components, and taking the square root of the result.

24 Emyetope = 112 +Q There are a number of algorithms for generating Hilbert Transforms. The preferred embodiment is using an FIR approximation. As part of the envelope generation, the scan line can be downsampled or decimated. 5 Downsampling by a factor of 2-4 is possible, depending on the scan conversion algorithm used. In the preferred embodiment, the scan line is downsampled by a factor of 4 with scan conversion using bilinear interpolation. An alternative is to downsample by a factor of 2 and use a less computationally intensive interpolation algorithm, such as one which computes pixel intensity from the average of sample 10 points inside the pixel area, and interpolates for other pixels between adjacent pixels. Following downsampling, the scan line is compressed from the analog to digital word size into an 8 bit word size to map the signal into the desired grey scale levels for display. The downsampled scan lines are then converted to a rectangular image display 15 through scan conversion. The scan conversion is performed in 2 stages. The first stage is converting the image into a compressed rectangular array with high resolution, preferably with pixel resolution of less than half the axial resolution of the ultrasound pulse. A common scan conversion algorithm is a 2x2 bilinear interpolation, mapping the scan line points from a polar co-ordinate system to a 20 rectangular co-ordinate system. Referring to Figure 16a there is shown an idealised scan from a phased array transducer. The scan area 1602 can be seen as a sector of a circle, with origin 1601. In practice, phased array transducer is not a point source, but it is small compared to the depth of scan. The location of each scanline 1603 can be characterised in polar co-ordinates by a length rand an angle e. 25 Referring to Figure 16b, there is shown a sector scan 1604 for a curvilinear array with a radius of curvature R and a width W. The scan is a truncated sector of a circle. Each scanline 1605 can be characterised by polar co-ordinates of a length R+r and an angle 6. Figure 16c shows an idealised scan 1607 for a linear array, with scanline 1608. The 30 characterising co-ordinates are linear, but in general will not correspond to the linear co-ordinate system desired for display.

25 In each of the case illustrated in Figure 16, scan conversion is required to convert the acquired scanline data into pixel data for display of the image to a user. For sector shaped images, the compression algorithm reduces the image size by ignoring image area which is not containing actual image data. Several methods are 5 possible to perform this, with one being a simple format where each pixel row contains a header with the starting pixel and number of pixels with valid data. This technique yields a compression ratio of close to 2. Another lossless technique which would provide reasonable results is run-length encoding. LZW encoding, or Huffman based encoding such as png could be used. 10 The high resolution compressed rectangular array is handled differently depending on the system configuration. The array is stored in a local memory, with typically up to 100 compressed frames being stored locally. At the same time the current frame is transmitted from the ultrasound probe to a host display system. The host display system completes the processing by decompressing the image into a high resolution 15 buffer, interpolating from the high resolution buffer to an interim display image buffer, applying any grey scale adjustment, combining with any Doppler image information if required, and finally writing to the display buffer. Figure 15 also illustrates the steps required for processing and displaying Doppler information overlayed on the B-mode image. When generating Doppler information, 20 the system generates a sequence of Doppler pulses with a consistent phase and processes the received set of scan lines. The raw input rf scanlines are quadrature encoded, with the inphase (1) and quadarture (Q) components extracted by multiplying the echo signal by the cosine and sine of the transducer excitation frequency. The I and Q outputs from the quadrature encoder are low pass filtered 25 and decimated or downsampled. A downsampling ratio of four produces satisfactory results, but other factors may be employed. The output of the downsampler is saved into a buffer until a complete set of scan lines is received. Typically a set will be eight scan lines, although the size of the set can be adjusted. The data set is filtered using a Wall filter. The function of the wall filter is to reduce contribution from large, slow 30 moving features such as an abdominal wall. The wall filter in the illustrated embodiment is an FIR type high pass filter. An alternative is to use a state-space formulation IIR filter, which reduces the transient 26 response length of the IIR filter. The state-space initialisation provides satisfactory attenuation using a step initialisation scheme. In the preferred embodiment the filtered set of scan lines is processed using autocorrelation techniques to generate velocity, power, and turbulence information. 5 The Doppler power of the set of scan lines is calculated as follows: I S5 d - 1'1 The correlation between adjacent scan line points is calculated as follows: CdSd..k-*1,'Sdk The velocity estimation is calculated as follows: _ Im( c) 10 Turbulence estimation is calculated as follows: t,= 1 -( where s is the complex representation of a scan line (s = / +j Q) 15 d is the sample at a depth in a scan line. k is the scan line number in a set of scan lines. When the system is configured for power Doppler, only the power estimation is required. The correlation between adjacent scan lines, velocity, and turbulence are not required. When the system is configured for color Doppler, the system calculates 20 velocity, and optionally power and turbulence. The output of the flow and turbulence estimation is scan converted to a rectangular array, of size 200 x 200 x 8 for each flow estimator. The rectangular arrays are compressed using a simple lossless compression algorithm. Figure 17 provides an illustration of a typical Doppler window 1702, where the Doppler processing area is limited with respect to the full B-mode 27 image 1701 size. This improves the compression of the 200x200x8 arrays, as a large proportion of the array is empty. The compressed array are transmitted to the host, with a further interpolation step to the required Doppler image window, and color map conversion. Positive velocities are encoded in red shades, while negative velocities 5 are encoded in blue shades. Turbulence is encoded in green. When power Doppler is selected, only red shades are used to represent the total power of the Doppler signals. An alternative embodiment of the invention is to provide a system which plots power Doppler with different color schemes used depending on whether the direction of 10 flow is positive or negative. The power Doppler is calculated as above, scan converted, and transmitted to the host. In parallel, an efficient method is used to determine the direction of the flow. The full correlation between adjacent scan line points can be used, with the sign of the real and imaginary outputs used for each point to determine a direction. Alternatively, for imaging where sensitivity is not an 15 issue, the correlation to decode direction can be shortened, and use a smaller set of scan lines. Power is optimised by providing a system which has feedback, where a small set of scan lines is used, and if the direction result is unstable (changing rapidly) the number of scan lines is increased to maximum. Then when the result is stable, the system runs a test correlation in parallel on a smaller set of scan lines and 20 if that result is stable moves to using the smaller set. The system therefore optimises the power transmitted into the body, and minimises power consumption. The system supports Cine playback or video recording. For Cine playback, the ultrasound probe stored compressed 500x500x8 B-mode frames into local DSP memory. 100 frames provides around five seconds of automatic Cine recording. 25 When the user instructs the host user interface to scroll back through recorded frames, the DSP transmits the compressed frames up to the host and scan conversion is completed as per normal operation. For video recording, when the user instructs the host to record a segment to video, the DSP activates operations to complete any scan conversion and Doppler combining, and runs a video encoder 30 which records the real-time images to a video format saving directly to memory, preferably at 640x480 pixel resolution. The video file can then be played back by a video decoder streaming data to a frame buffer which is transmitted to the host for display.

28 The system can be configured to operate in M-mode or duplex mode where a B mode image and M-mode image are shown on the same screen. In M-mode or Duplex mode, the DSP reads the M-mode scan lines and interpolates each scan line to a 500x1x8 buffer. The M-mode buffer is transmitted to the host 5 which performs grey scale adjustment and renders the scan line to the display. For duplex mode, two sorts of M-mode are possible. Full duplex is where an M-mode scan line is placed on a realtime B-mode image, and the M-mode pulse repetition rate is the same rate as the B-mode image update rate, typically 20 frames per second. 10 For quasi duplex mode, a high M-mode frame rate is required, such as for looking at rapidly moving heart valves. In this mode, an M-mode line is placed on a B-mode image, and when the select button is pressed the B-mode image is frozen and the M mode graph is rendered to the screen. A quasi duplex gated Doppler mode is also provided. A Doppler line is placed by the 15 user on a B-mode image, and a gate area moved along the line to select the area for Doppler analysis. The gate angle can also be adjusted. When the select button is activated, the B-mode image is frozen, and a spectral Doppler graph - which displays time, frequency and magnitude of the Doppler shift - is plotted. The Doppler signal is processed in a similar way to color Doppler, except the scanlines are 20 repeatedly pulsed only along the selected line (rather than moving across the image). In addition, the user selects a particular depth of interest, and the received echoes are 'range gated' such that only Doppler information from said depth of interest is analysed. This provides a large number of scanlines from the region of interest; frequency analysis is carried out on large groups of these scanlines (say 25 256) at a time, with the preferred method being applying the Fast Fourier Transform (FFT) on the complex representation of a group of scanlines. Each FFT performed provides a frequency spectrum of the Doppler signal shift, and these spectra can be displayed together to generate a time-frequency plot of the flow characteristics at the depth of interest which is often called 'spectral Doppler'. Spectral Doppler enables 30 visualisation of how the velocity and power of blood flow changes with time for the selected region of interest. Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognised that departures can 29 be made within the scope of the invention, which is not to be limited to the details described herein but is to be accorded the full scope of the appended claims so as to embrace any and all equivalent devices and apparatus. 5

Claims (6)

1. An ultrasound transducer probe including an enclosure; a transducer assembly; an acoustic window fixed with respect to the enclosure; 5 a motor adapted to move the transducer assembly with respect to the enclosure; and a coupling layer adapted to couple the transducer to the acoustic window, wherein a surface of the transducer assembly is fixed to the coupling layer and a surface of the acoustic window is fixed to the coupling layer.

2. The probe of claim 1 wherein the acoustic impedance of the coupling layer is 10 substantially the same as the impedance of the acoustic window.

3. The probe of claim 1 wherein the coupling layer includes a flexible wall member enclosing a liquid.

4. The probe of claim 1 wherein the coupling layer includes an elastic wall member enclosing a liquid. 15

5. The probe of claim 1 wherein the coupling layer includes an elastic member.

6. An ultrasound transducer probe including an enclosure; a transducer assembly; an acoustic window fixed with respect to the enclosure; a motor adapted to move the transducer assembly with respect to the enclosure; 20 and a coupling layer adapted to couple the transducer to the acoustic window, wherein in use, relative movement between a surface of the transducer assembly and a surface of the coupling layer is less than the travel of the transducer assembly, and relative movement between a surface of the acoustic window and a surface of the coupling layer is less than the travel of the transducer assembly. 25