AU2011100208A4

AU2011100208A4 - Operator movement compensated scanning method and apparatus

Info

Publication number: AU2011100208A4
Application number: AU2011100208A
Authority: AU
Inventors: Andrew Medlin; Andrew Niemiec
Original assignee: Signostics Ltd
Current assignee: Signostics Ltd
Priority date: 2010-03-05
Filing date: 2011-02-22
Publication date: 2011-03-24
Anticipated expiration: 2019-02-22

Description

IP0331P AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT Invention Title: Operator movement compensated scanning method and apparatus Name of Applicant: Signostics Limited Address for Service: 40-46 West Thebarton Road Thebarton, S.A. 5031 2 TITLE OPERATOR MOVEMENT COMPENSATED SCANNING METHOD AND APPARATUS TECHNICAL FIELD 5 The present invention relates to the field of ultrasound scanning, in particular medical ultrasound scanning where there is a hand held probe unit in which an ultrasound transducer or transducers are contained. BACKGROUND ART The use of ultrasound scanning of patients for medical diagnostic purposes 10 dates to the mid-20th century. An ultrasound transducer is used to project a beam of ultrasound energy into a patient. The same or another transducer detects the echoes returned from features within the body. These echoes, called a scanline, are then converted to a form suitable for recording or display. When a series of scanlines, spaced angularly apart, are acquired rapidly and 15 displayed on a display screen, the familiar B-mode sector scan is achieved. This is an arc of a circle, wherein the brightness of each pixel of the display is proportional to the magnitude of the ultrasound echo received from the corresponding point in the body being imaged. A method from the prior art for collecting the required series of scanlines which 20 are spaced angularly apart was to provide a single transducer in a handpiece attached to a data processing and display unit by an articulated arm. The articulated arm included means at each joint for tracking the movement of the joint. Tracking the position of each joint allowed the position and orientation of the handpiece, and hence of the transducer, to be known at all times. An 25 operator would place the handpiece against the body of a patient, and sweep the handpiece in an arc to obtain the required set of angularly spaced scanlines. A further development of this method was to place a motor in the handpiece. This motor moved the transducer, relative to the handpiece. The transducer 30 rotated about an axis parallel to the surface of the body to be scanned. Means were provided within the handpiece for accurately determining the position of the transducer relative to the handpiece as the transducer moved. In use, an 3 operator placed the handpiece against the body of a patient, and held the handpiece still. The rotating transducer was activated to cause the ultrasound beam emitted by the transducer to sweep out a sector of the body. All the scanlines obtained in a single sector sweep were displayed to form a static B 5 mode image. An example of an ultrasound device using a motor is disclosed in U.S 6,126,608. Since the relative position of the transducer was now known, it was no longer necessary for the exact position and orientation of the handpiece to be known, and the articulated arm was no longer required. 10 A further method for collecting the required angularly separated scanlines came with the advent of transducer arrays consisting of a number of piezo-electric crystals where the transmitting pulse can be delayed in sequence to each crystal and thus effect an electronic means to steer the ultrasound beam. An operator uses the system in the same way as for the motor driven transducer. 15 The steered beam sweeps out a sector to produce the static B-mode scan, without the need for a motor to move the transducer. Examples of these so called 'phased array' transducer ultrasound units are disclosed in U.S 3,918,024 and US 4,154,113. In each case, the ultrasound transducer is contained within a probe unit which, 20 in general, is not rigidly connected to the main display and processing apparatus. This means that the probe is no longer held mechanically in a fixed position. The probe unit is held in the user's hand, and positioned for use directly by the user. When a fixed probe position is required, this can be achieved only by a user's best efforts. 25 In practice, a probe unit cannot be kept completely motionless by a user, and the resulting movement results in degradation of the imaging achieved by the ultrasound device. DISCLOSURE OF THE INVENTION In one form of this invention there is proposed an ultrasound scanning 30 apparatus of a type having a hand held probe unit adapted to output frames of B-mode ultrasound data composed of scanlines having a fixed relationship with respect to the position of the probe unit, wherein the probe unit includes a 4 sensor adapted to sense the position and/or orientation of the probe unit and to output this information as sensor data; the apparatus including a processor adapted to receive the scanlines and to format them for display on a display device; 5 a processor adapted to receive the scanlines and the orientation data and to process these in such a manner as to adjust the relationship of at least some successive scanlines and/or at least some successive frames such that the effect of a significant proportion of unintended movement of the probe unit is removed from the displayed image. 10 In preference, the hand held probe unit isonifies a sector of a circle within the body of a patient, while the sensor detects small, unintended movements of the probe which we term jitter, which are inevitable with a hand held probe. The processor then processes the output from the sensor along with the scanline data, in order to at least partly compensate for the unintended 15 movement. In an embodiment, the adjustment is rotation and/or translation of scanlines relative to other scanlines within a frame. In an embodiment, the adjustment is the rotation and/or translation of a frame with respect to previously displayed frames. 20 A variety of sensor types may be employed. In preference, the sensor is an inertial sensor. This may be one or more gyroscopes or accelerometers or a combination of the two. A gyroscope will most easily detect unintended rotation of the probe, while an accelerometer will most easily detect unintended translation of the probe 25 across the surface of the body of the patient being scanned. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of an idealised ultrasound unit of the prior art. Figure 2 is schematic view of a practical ultrasound unit of the prior art, showing the effect of operator movement. 30 Figure 3 is a schematic representation of an ultrasound device of the invention.

5 Figure 4 is a schematic representation of a further embodiment of an ultrasound device of the invention BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 is a schematic representation of an idealised ultrasound device of the 5 prior art. There is provided an ultrasound probe unit 100 including an ultrasound transducer 101which is adapted to transmit ultrasound energy into a scan target 102 and to receive ultrasound energy reflected by the target. The ultrasound energy sweeps out a sector of a circle 103 within the body to be scanned. The sweeping may be by mechanical movement of the transducer, or 10 the transducer may be a phased transducer array allowing the beam to be swept electronically. Ultrasonic pulses are fired at different angles through the surface of a scan target, as the scanner sweeps though an angle of total, being the total sweep of arc 103. The resultant echoes are captured as a series of scanlines 104, each 15 of which consists of a series of echo intensity values p, and a transducer angle which may be expressed as an angle, 0, between the central axis and each scanline. The scanline data is then fed to a scan conversion processor 105 which converts the intensity and angle data to a raster image of the familiar B-mode 20 ultrasound scan 109 for display on a display screen 106. Since e is accurately known, the scan conversion process is able to generate an image from any particular scan sweep which accurately shows the geometry of a reflective object 107 in the scan target 102. The accurately depicted image 108 appears in the B-mode display 109. There are a number of ways this can 25 be done; one is disclosed in Australian provisional patent application AU2007904743. For this to work correctly, and for the image 108 to be undistorted, the position and orientation of the probe must remain fixed relative to the scan target throughout each 'sweep' of the scan - i.e., that the probe is held still. This is 30 implicitly or explicitly assumed in the use of this method in the prior art.

6 Since the probe is hand held, this assumption is incorrect. Any user, however careful or well trained, will introduce some degree of movement or jitter to the probe. Figure 2 shows a schematic representation of a practical device of the prior art. 5 In this simple example, in use, user hand jitter causes the probe to be tilted, at a uniform rate, through an angle 201 of Cptotal during a scan sweep. It can be seen that the actual scanlines 204 are in different positions to where they would have if the probe had been still. The 'probe still' positions of the scanlines 104, differ from the actual scanline positions 204 by an angle which 10 varies up to Cptotal. The angle swept by the scan is larger than that of the 'probe still' sweep 103, total, by an angle 201 of Cptotal. The scanline data is passed to the scan conversion process 105 for processing and display as B-mode ultrasound scan 209. The scan conversion process has 15 no information as to the true angle which the probe has swept, that being an angle, relative to the scan target, of Etota + (Ptotal. Hence the larger sweep is effectively 'compressed' into the display window expecting a sweep of angle total, The image 208 of the reflective object 107 is distorted, as shown in Figure 2. 20 For this example, it is assumed here that the probe sweeps from left to right, in the same direction as the intra-scan tilt; if the tilt were in the opposite direction to the sweep, a 'stretching' distortion would be observed in the image instead. Other movements of the users hand will result in other distortion patterns, which need not be regular. 25 An ultrasound scan apparatus of the present invention is shown schematically in Figure 3. There is provided an ultrasound probe unit 100 including an ultrasound transducer 101which is adapted to transmit ultrasound energy into a scan target 102 and to receive ultrasound energy reflected by the target. 30 The transducer may be mechanically moved, or the transducer may be a phased transducer array allowing the beam to be swept electronically.

7 Ultrasonic pulses are fired at different angles through the surface of a scan target, as the scanner sweeps though an angle of total, being the total sweep of arc 103. The resultant echoes are captured as a series of scanlines 104, each of which consists of a series of echo intensity values p, and a transducer angle 5 which may be expressed as an angle, 0, between the central axis and each scanline. The apparent arc swept by the ultrasound energy 103 has an angle total. The actual angle swept varies from this because of user hand movement. This variation may be in any direction and at any speed, but for this example it is 10 shown as extending the arc swept by an angle (total, being the angle by which the probe unit is tilted during the scan. It can be seen that the actual scanlines 204 are in different positions to where they would have if the probe had been still. The 'probe still' positions of the scanlines 104, differ from the actual scanline positions 204 by an angle which 15 varies up to (total. The angle swept by the scan is larger than that of the 'probe still' sweep 103, total, by an angle 201 of (total. The scanline data is passed to the scan conversion process 105 for processing and display. 20 The probe unit further includes an orientation sensor which detects the angular movement of the probe unit during a scan. This orientation sensor in this embodiment is a gyroscope. Preferably it is an inertial sensor. In other embodiments it may be an accelerometer, or two or more orthogonally mounted gyroscopes, or two or more orthogonally mounted 25 accelerometers, or any combination of gyroscopes and accelerometers. The use of a sensor adapted to sense movement in more than one plane allows user movement in planes other than the plane of the scan arc to be compensated for. The orientation sensor 301 is continually capturing the angle, cp, called the jitter 30 angle, by which the orientation of the central axis of the probe has changed since the sweep started. The relative angle, 0, of each scanline as well as the jitter angle p at the time the scanline was captured is fed into a jitter 8 compensation module 302, from where the compensated angle, 0', is formed by subtracting (p from e. The time taken for a pulse-echo of a single scanline is very short, so for realistic levels of jitter the change in (p during a single scanline can be neglected. 5 The compensated angle 0' is passed to the scan conversion module 105. This compensated angle is combined with the echo intensity values p to form compensated scanlines. These compensated scanlines are processed for display on display unit 106. The resultant image 309 is a compensated sector image in which the image 308, of the reflector 107 is substantially distortion 10 free. In an alternative embodiment the intra-sweep jitter distortion is compensated for by use of the orientation information from the probe mounted orientation sensor to control the pulse firing rate of a phased array transducer. The delay between successive pulses is advanced or retarded, depending on any 15 orientation changes in the probe, such that the resulting pattern of scanlines in the scan target is the same as that produced if the probe were held still. In this embodiment, the jitter compensation module is not provided. A firing control module is provided, and the orientation sensor data is passed to this module. The required delay modifications are calculated based on the orientation 20 sensor data, and the phased array controlled accordingly. In a further embodiment, jitter compensation is not attempted. The data from the orientation sensor is monitored. When the data indicates that movement beyond an acceptable level has occurred, scanlines for that sweep are discarded and not displayed. 25 The advantages of using an intra-scan jitter compensation system include less user skill required to obtain clear images, as well as power savings and less acoustic radiation exposure if fewer scans are required in a session. In embodiments where the orientation sensor includes multiple sensor elements capable of sensing movement or rotation in multiple directions and 30 planes it is possible to perform intra-scan jitter compensation in multiple planes, compensate for probe rotation, and compensate for intra-sweep lateral slide. In 9 these embodiments, translation in any direction as well as rotation in any plane may be applied to the scanlines in order to correct jitter. This kind of jitter compensation is particularly useful for '4D' (real time 3D) ultrasounds, such as that disclosed in US 6,179,780, where the lower frame 5 rate (compared with real-time B-mode) makes them more vulnerable to movement-related image distortion. In addition, 4D ultrasound devices often use a 2D phased array; these are capable of generating pulses and capturing scanlines not confined to a single plane, so they can more easily compensate for jitter in multiple planes. 10 When used in real time mode, phased array or motor driven ultrasound devices produce a full screen image for each transducer sweep. These images are normally rendered and displayed immediately at the conclusion of each sweep. To give the appearance of a stable image, the operator must hold the probe very still, or else move it slowly and steadily in the desired direction for example 15 when following the abdominal aorta of a patient to check for any signs of an aneurysm. The inevitable shaking of the probe introduced by the hand of even the most careful and well trained operator will cause successive images to not line up properly, resulting in an image which does not appear as a continuous real time 20 moving image, but which has discontinuities or 'jumps'. Intra-scan jitter compensation as disclosed in this specification will not prevent this problem. There is a need to remove the 'shakiness' of images caused by rapid user hand jitter, whilst not impinging on the ability of the user to pan across the scan target with relatively slower movements. 25 Figure 4 shows a schematic representation of an apparatus and method for compensation for this inter-frame jitter. There is provided an ultrasound probe unit 100 including an ultrasound transducer 101 which is adapted to transmit ultrasound energy into a scan target 102 and to receive ultrasound energy reflected by the target. 30 The ultrasound energy sweeps out an arc within the body to be scanned 102. The sweeping may be by mechanical movement of the transducer, or the transducer may be a phased transducer array allowing the beam to be swept 10 electronically. It is also possible for the transducer to include an array of transmitting elements, allowing the entire arc to be isonified simultaneously. In an embodiment, the transducer may sweep out a three dimensional volume rather than a two dimensional arc. 5 The transducer repeatedly sweeps the target. The scan arcs are shown successively on a display to give a moving, real time image of the scan target. The user attempts to keep the probe unit still, but in practice, the probe unit rotates by an angle <p from any arbitrarily chosen start point. The effect of this is that successively displayed images do not perfectly align, giving a jitter to the 10 moving image. Such jitter is distracting to a user, and increases user fatigue when watched for an extended period. It is advantageous for the display to be adjusted to remove the effect of this unwanted movement. There is provided an orientation sensor 301, as described in the previous 15 description of embodiments. This produces a data stream giving the change in orientation of the probe unit. It is the effect of this change which is to be removed. For each successive sweep, the average change 401 in the central axis angle, (Pav, relative to that of the previous sweep is calculated. This may be done by 20 any convenient method. The midpoint of the two extremities of the central axis during each sweep may be compared, or the position of central axis half way through successive sweeps may be compared. This produces raw orientation data which can be described by a plot of relative probe orientation against sweep number 402 25 (frame number). The raw orientation plot 402 shown in Figure 4 shows the relative orientation data produced when slow, intentional probe movement orientation change is corrupted with faster, unintended jitter. This orientation data is then passed to high pass filter 403 where it is filtered to remove the low frequency component, so that the remaining signal is largely 30 composed of the high frequency 'jitter'. This filter may be implemented in any convenient form known in the art. It may be an IIR (Infinite Impulse Response) or FIR (Finite Impulse response) filter. It is advantageous to design the filter in 11 such a way that it has a zero phase shift for high frequencies; as this will enable the effect of jitter to be immediately removed in the same sweep it was encountered. The hi-pass filtered, jitter only signal 404 is passed to image rotation module 5 405. The filtered signal gives a value of (Pay for each sweep which corresponds to a frame of the moving image. The rotation module rotates each frame to be displayed by the corresponding (Pav of the filtered jitter signal. This has the effect of stabilising the final image, by compensating for the 10 shakiness caused by small, high frequency jitter on successive images, but slower movements (such as panning around a patient) are still shown properly. The image rotation module implements one of numerous image processing techniques known to those skilled in the art, such as the use of a rotation matrix. 15 The rotated frames are then displayed successively on display 106, giving a smooth moving image 406. In this embodiment, the rotation is applied to the image produced from the scan conversion just prior to display, but it could also be incorporated into the scan conversion phase itself. 20 In embodiments where the orientation sensor includes multiple sensor elements capable of sensing movement or rotation in multiple directions and planes it is possible to perform intra-scan jitter compensation in multiple planes, compensate for probe rotation, and compensate for inter-sweep lateral slide. In these embodiments, translation in any direction as well as rotation in any plane 25 may be applied to the frames in order to correct jitter. This kind of jitter compensation is particularly useful for '4D' (real time 3D) ultrasounds, such as that disclosed in US 6,179,780, where the lower frame rate (compared with real-time B-mode) makes them more vulnerable to movement-related image distortion. 30 Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognised 12 that departures can 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.

Claims

1. An ultrasound scanning apparatus of a type having a hand held probe unit adapted to output frames of B-mode ultrasound data composed of scanlines having a fixed relationship with respect to the position of the 5 probe unit, wherein the probe unit includes a sensor adapted to sense the position and/or orientation of the probe unit and to output this information as sensor data; the apparatus including a processor adapted to receive the scanlines and to format them for display on a display device, groups a scanlines forming 10 frames being displayed together and successive frames being displayed to form a moving image; a processor adapted to receive the scanlines and the orientation data and to process these in such a manner as to adjust the displayed spatial relationship of at least some successive scanlines and/or at least some 15 successive frames such that the effect of a significant proportion of unintended movement of the probe unit is removed from the displayed image.

2. The apparatus of claim 1 wherein the sensor includes at least one of a gyroscope and an accelerometer. 20

3. The apparatus of any one of the preceding claims wherein the probe unit includes a phased array transducer.

4. The apparatus of any one of the preceding claims wherein the adjustment is rotation and/or translation of scanlines relative to other scanlines within a frame. 25

5. The apparatus of any one of the preceding claims wherein the adjustment is the rotation and/or translation of a frame with respect to previously displayed frames.