Acquisition and display of ultrasonic images from sequentially orientated image planes

Abstract: An ultrasonic imaging system is disclosed which produces a sequence of images of planes of a subject including both image and spatial positional information of the image plane. In one embodiment the positional information is developed from a plurality of accelerometers located within a scanhead. The second integrals of the acceleration signals are used to determine positional information of the image plane. In a second embodiment a tranmitter transmits a magnetic field and a receiver attached to the scanhead detects the position of the scanhead in relation to the transmitted magnetic field. Spatially related images are displayed by displaying one image plane in the plane of the display and a second image plane projected in relation thereto. Either of the displayed planes may be displayed in outline form, and the outline may be modulated to depict depth.

Description: This invention relates to the acquisition and display of ultrasonic images from sequential image planes of a subject, such as a spatially consecutive sequence of images from parallel image planes. This invention finds one of its applications in the presentation of two dimensional image information in manner which gives the effect of three dimensions. Two dimensional ultrasonic images are traditionally a collection of sequentially oriented individual lines taken over a planar region of a is subject. This is because the ultrasonic scanhead which transmits and receives acoustic information is designed to "look,' along only one line or vector at a time. The scanhead gathers information over a depth of field from a given direction, then through mechanical or electrical steering of the scanhead transducer the scanhead "looks" along a sequence of such vectors or directions in the image plane. The scan converter will then store and orient all of these image lines in their proper planar orientation, whichis conventionally either parallel or radial. The assembly of all of the lines is the basis of a two dimensional image of the subject in the plane being scanned for display. Such conventional two dimensional ultrasonic images depict image information in two directions. The direction along the length of each vector or line is depth into the object or patient being scanned. Thesecond direction from line to line, is a lateral direction which is approximately parallel to the scanning surface. An ultrasonic scanner according to the preamble of claim 1 is already known of US 4,092,867. This scanner has a housing, and a phased array transducer disposed within the housing and mounted for movement in a predetermined path therewithin. Magnetic means are provided for energizing the transducer and receiving signals therefrom. The scanner is servo-controlled and provided with position sensing means for determining the position of the transducer within the predetermined path, and signal processing means for receiving feedback signals form said position sensing means and adjusting said transducer position responsive to said feedback signals. The ultrasonic scanner disclosed by the cited document has means for moving the phased array transducer inside the housing or scanhead. So, in said ultrasonic scanner, the scanhead is not moved. Besides, inside the scanhead, the phased array is only submitted to rotational movements. The present invention is given by an ultrasonic imaging system according to claim 1. In accordance with a preferred embodiment of the present invention, a sequence of conventional two dimensional images are acquired and displayed in a second lateral direction from one conventional image plane to another. The display of two dimensional images in their sequential order in this second lateral direction results in an effective three dimensional presentation of the image information. one such effective presentation is made by displaying the sequence of images in rapid succession. The display of images may continuously show the images in the same sequence, or may display them cyclically from first to last, then last back to first. In one embodiment of the present invention, a perception of depth in the second lateral direction or dimension is enhanced by displaying the content of a number of sequential images simultaneously, wherein contributions of individual image planes are weighted as a function of position in the sequence. When acquiring ultrasonic image sequences for use in accordance with the foregoing or other techniques of three dimensional presentation, it is frequently useful or necessary to know the position of each image plane relative to the others. Such positional information allows the combined planes to be appropriately weighted, for instance, and is often needed to enable the graphical reconstruction of three dimensional surfaces within the subject being imaged. Attempts in the prior art to generate such positional information have usually been cumbersome, such as the articulating arm of prior art B-arm scanners, or have been restraining when employed in combination with the conventional hand-held scanhead. For instance one prior art technique generated repetitive sharp sounds from the scanhead by means of a spark gap. The sounds were detected by a number of audio detectors located around the scanhead and position determined by triangulation. It is accordingly desirable to be able to provide positional information about the image plane of the scanhead without interfering with the clinician's ability to use the scanhead in the conventional manner. In accordance with the present invention positional information about the image plane of the scanhead is derived by incorporating a plurality of accelerometers as a part of the scanhead. The accelerometers generate electrical signals representative of acceleration along predetermined axes when the scanhead is moved. By integrating these acceleration values the relative coordinates of the scanhead can be determined in three dimensional space, then a translation made to the orientation of the image plane extending from the scanhead. Preferably, six accelerometers are used. In the drawings: FIGURE 1 illustrates in block diagram form an ultrasonic imaging system adapted to acquire a sequence of spatially related images; FIGURE 2 illustrates a sequence of spatially related images of a hollow ball; FIGURE 3 illustrates the ordering of the sequence of images of FIGURE 2 for display in a three dimensional presentation; FIGURE 4 illustrates a scanhead axis about which a plurality of accelerometers are arrayed to produce signals for scanhead position determination; FIGURE 5 is a perspective view of a scanhead containing accelerometers for position determination; FIGURE 5a is a detailed view of the mounting of accelerometers for the scanhead of FIGURE 5; A system for acquiring ultrasonic image information and presenting such image information in a three dimensional image format is shown in FIGURE 1. A linear array scanhead 10 transmits pulses of ultrasonic energy and receives echoes returned from the subject being imaged. The returning echoes are converted to electrical signals by the transducer array in the scanhead and applied to a beamformer 12, which combines the electrical signals to form a coherent vector of ultrasonic image information. The vector information is applied to a scan converter and memory 14, which forms an array of two dimensional image information which is displayed on a display 40. As the subject is being scanned and real-time images are appearing on the display, a particular image can be frozen on the screen by pressing the freeze button 16. If the user desires to save the frozen image, a save button 22 is pressed and the displayed image is stored in an image memory 20. After a number of images are viewed and stored in the image memory 20 in this manner, the sequence of images may be reviewed by pressing a load button 36 to load the sequence of images saved by the image memory into a cineloop(R) frame store 30. The cineloop frame store has a capacity of 160 image frames, and retains the loaded images in an ordered sequence. By adjusting the sequence control 34 the user determines how the sequence of images is to be displayed: from first through last repetitively, from last to first repetitively, or sequencing from the first image to the last image, then back from the last to the first in a continuous loop. The rate control 32 enables the user to determine the rate at which the frames are displayed, from a slow rate to a fast rate. The cineloop frame store thus plays back the stored images in the desired sequence for review on the display 40. As an example of the scanning techniques which may be performed by this arrangement, consider the scanning of a hollow ball, setting aside for the moment considerations of the ultrasonic transmission medium. The hollow ball is scanned by initially locating the scanhead to scan a plane which is adjacent to the ball. The scanhead is then moved normal to this plane in increments of 1 mm. At each incremental position an image of the plane being scanned is frozen on the display and stored in the image memory. The first image, acquired as the scan plane just begins to intersect the edge of the ball, is shown as frame F 1 in FIGURE 2. At the next incremental position of the scanhead a cross-sectional image as shown in frame F 2 is acquired and saved. Subsequent increments in the position of the scanhead and the scan plane produce further "slices" of the ball, as shown by frames F 3 and F 4 in FIGURE 2. This scanning and image acquisition process continues until the scan plane passes the opposite side of the ball. The sequence of two dimensional images thereby acquired and stored in the image memory is shown in FIGURE 3, beginning with image frame F1 and continuing through the last frame F n . The sequence of images is then loaded into the cineloop frame store 30. The display sequence 34 and rate 32 adjustments are set for the desired display rate and sequence and the sequence of images is played back on the display 40. When these sequential, parallel oriented images are played back on the display, the viewer is given a three-dimensional presentation of the hollow ball. In accordance with the principles of the present invention the three-dimensional appearance of the image sequence may be enhanced by weighting the image content of the frames. A preferred weighting technique is to give greater weight to the frames which are in front, that is, image slices which are closer to the viewer, than frames which are in the rear. This presents the near field of the three dimensional object more strongly than the the image frames of the far field, where near and far refer to the sequential direction. The progressive weighting from the front of the object to the back may be scaled linearly or exponentially depending upon the desired effect. The weighted frames may be viewed in several different ways. One way is to play the sequence of weighted frames in rapid succession. This would present the sequence in a "swim through" mode or a "swim back and forth" mode as described in U.S. Patent 4,297,009. A second way is to stack, or overlay the weighted frames into a single composite frame. A third way is to perform a corresponding pixel-by-pixel summation process of the weighted frames, then divide the result by the number of images in the sequence. An effective implementation of the second technique is to weight the last frame, frame F n in FIGURE 3, by 40%, and the next adjacent frame F n-1 by 60%. The two weighted frames are combined into a composite frame, 40% of which is contributed by the most distant frame and 60% of which is contributed by the next adjacent frame. This composite frame is then weighted by 40% and the next adjacent frame F n-2 is weighted by 60%. The weighted composite frame and the weighted frame F n-2 are combined to form a new composite frame. This weighting and combining process is continued, progressing to the nearest frame F 1 , resulting in a final composite frame with the near field more greatly weighted than the far field. It may be appreciated that the weighting progression may be made as liner or nonlinear as desired; for example, a constant weighting of 50% to each frame and composite would result in a substantially linear weighting progression. It has been found that at a net weighting of 5% the content of an image makes no appreciable contribution to the composite image. Hence, an effective presentation has been made by deleting up to one-third of the far field frames, then progressively weighting the remaining two-thirds of the frames from the far to the near field. A number of mechanisms have been devised and proposed to provide the positional information of ultrasonic image planes. For instance, the well-known B-arm scanning system with a linear array transducer connected at the end of the articulating B-arm with potentiometer resolvers at the joints could be used to acquire a sequence of substantially parallel image frames. However, the sequence of images need not be parallel to present the three-dimensional effect. For instance, the scanhead could be rocked to acquire a sequence of images at sequential angular increments, or the scanhead could be rotated about the center of the face to acquire a sequence of image planes which pass through a common axis. The B-arm mechanism however is sizeable, often awkward, and lacks the portability and ease of use of current hand-held scanheads. Accordingly it would be preferable to provide a means for position determination in a hand-held scanhead. One such preferred device for acquiring a sequence of images at known positional increments is illustrated in FIGURES 4 and 5. The bar 70 in FIGURE 4 represents a central axis of a scanhead, such as a horizonal axis 70' of scanhead 10' of FIGURE 5. Located inside the scanhead case 24 and oriented as indicated by acceleration vector arrows 1-6 in FIGURE 4 are six accelerometers. A preferred accelerometer is type AMD-CK/0A2, available from Access Sensors SA. These accelerometers are capable of resolving very small acceleration forces, down to one milliGravity (mG). Six of these accelerometers positioned as indicated in FIGURE 4, with 5 parallel to the axis, 4 and 6 normal to 5 and to each other, 2 and 3 at the other end of the axis and parallel to 4 and 6, and 1 offset from and parallel to 4, enable the resolution of any change in orientation of the scanhead, be it a change in cartesian coordinates x, y, and z, or a rotational change in roll, pitch, or yaw. The acceleration signals produced by the six scanhead accelerometers are applied by means of the scanhead cable 72 to an increment calculator 80 in the system processor as shown in FIGURE 1. In the construction of a scanhead the axis 70' passes through the centers of two rigid bodies such as aluminum cubes 70a and 70b. The aluminum cubes are mounted inside the scanhead case 24. The lower portion 74 of the scanhead houses the ultrasonic transducer, which transmits and receives ultrasonic energy through an acoustic window 76. Attached to certain faces of the aluminum cubes 70a and 70b are accelerometers 2', 3', 4', 5', and 6', which detect acceleration in directions corresponding to the respective unprimed numbered vectors of FIGURE 4. An enlarged view of cube 70b is shown in FIGURE 5a, with its attached accelerometers 4', 5', and 6'. As this FIGURE 5a indicates, the three vectors 4, 5, and 6 all coincide at the center of the cube. Wires (not shown) from the individual accelerometers lead to the scanhead cable 72 and the increment calculator 80. FIGURE 5 also shows that the accelerometer 1' is spaced apart from the aluminum cubes and their axis 70' inside the scanhead, where the separate accelerometer 1' is in an orientation to sense rotation about the axis 70'. Positional information of the scanhead is computed from the six acceleration signals as follows. The signals from the accelerometers is continuously sampled at a predetermined sampling rate and analyzed. Since the acceleration signals provide relational data and the position of the scanhead is arbitrarily determined by the user, some reference must be established for the positional information. This is accomplished by initializing the position of the scanhead when it is at rest. When the scanhead is at rest the only acceleration force being experienced by the accelerometers is gravitational acceleration. Hence the increment calculator will check for initialization by verifying that the accelerations remain constant and that the magnitude of the acceleration vector is equal to gravitational acceleration (9.8 m/sec 2 ). At the starting rest position the x,y,z coordinate system of the scanhead is established as the origin point for all subsequent calculations. As the scanhead is moved during scanning to acquire a sequences of ultrasonic images, the accelerometers experience acceleration forces and their output signals are continually sampled to produce a continuous stream of position data. The output signals are combined in the following manner to determine linear and angular acceleration characteristics of the scanhead and correspondingly of the image plane: d 2 x/dt 2 = -a 4 - g x d 2 y/dt 2 = a 6 - g y d 2 z/dt 2 = a 5 - g z and d 2 roll/dt 2 = (a 3 - a 6 )/D 1 d 2 pitch/dt 2 = (a 2 - a4)/D 1 d 2 yaw/dt 2 = (-a 1 + a4)/D2 where X,Y, and Z are taken in relation to the coordinates shown adjacent FIGURES 5 and 5a; D 1 is the distance between the centers of the two aluminum cubes 70a and 70b; D 2 is the distance between the center of accelerometer 1' and the center of cube 70b; the "a" terms are acceleration values of the accelerometers of FIGURES 5 and 5a in correspondence to the subscript numerals; and g x , g y , and g z are the three vector components of gravitational acceleration. As the second differential notation of the three expressions indicates, the second integral of these expressions taken as a function of the signal sampling rate (time) will yield the three translational and rotational characteristics of position in relation to the starting rest position. The scanhead of FIGURE 5 with the accelerometer position determination arrangement is used to acquire a sequence of image frames for three-dimensional presentation in the following manner, with reference to FIGURE 1. The user first adjusts the Set Increment control 82 on the imaging system to determine the increments at which images in the sequence are to be acquired. The increments could be every 1 mm for linear movement of the scanhead, or every 10° for rotational movement of the scanhead, or every n milliseconds, for instance. The user then positions the scanhead at its starting rest position. The user presses the Initialization button 84 to cause the system to initialize position as discussed above. Increments then proceed from this initial position. The user then proceeds to scan by moving the scanhead in the desired manner. The movement of the scanhead is detected by the accelerometers and acceleration is resolved to position. As the desired positional or time increments are attained as determined by the Increment Calculator 80, the Save button 22 is automatically triggered to save an image frame at each increment. When the scanhead comes to a stop the sequence acquisition is terminated unless acquisition is manually terminated by the user. The image memory now contains a sequence of images obtained at the desired increments, which may then be displayed as desired in a three-dimensional presentation.

Claims: An ultrasonic imaging system for providing planar image information, said system including means for providing spatially identified planar images comprising: a scanhead (10, 10') including means for ultrasonically scanning a plane of a subject; and means for determining the spatial location of said plane, including accelerometer means (1'-6') attached to said scanhead (10, 10') for sensing movement of said scanhead (10, 10') and means for processing acceleration signals produced by said accelerometer means (1'-6') to determine positional information of said plane, said processing means utilizing said acceleration signals to calculate angular accelerations of said scanhead (10, 10'), characterized in that said acceleration signals are utilized by said processing means to also calculate linear accelerations of said scanhead (10, 10'). The ultrasonic imaging system of Claim 1, wherein said acceleration signals are compensated for gravitational acceleration. The ultrasonic imaging system of one of Claims 1 or 2, wherein said positional information is produced by the second integral of said acceleration signals. The ultrasonic imaging system of one of Claims 1 to 3, wherein said accelerometer means (1'-6') includes a plurality of accelerometers located within the housing (24) of said scanhead (10, 10'). The ultrasonic imaging system of Claim 4, wherein said accelerometers (1'-6') are positioned to detect orthogonally orientated vectors of acceleration.

Inventors: AUGUSTINE LARRY J, DAIGLE RONALD E, KELLER PHILIP, AUGUSTINE, LARRY J., DAIGLE, RONALD E., KELLER, PHILIP
Applicants: ADVANCED TECH LAB, ADVANCED TECHNOLOGY LABORATORIES, INC.
Application year: 1991
Main classification: A61B8
IPC codes: G01S15, A61B8