Ultrasonic probe, system and method for two-dimensional imaging or three-dimensional reconstruction

An ultrasonic probe that includes at least two ultrasonic arrays and allows three dimensional images to be constructed of the region examined by the probe in a precise and facile manner.

FIELD OF THE INVENTION 
This invention relates to an ultrasonic probe, system and method for 
acquiring two-dimensional image information and relative positional 
information to allow subsequent three dimensional reconstruction utilizing 
an ultrasonic probe that has at least two ultrasonic transducer arrays 
mounted thereon which generate differing image formats. 
BACKGROUND OF THE INVENTION 
Probes that incorporate more than one transducer array are known. For 
example, a 1994 model of the Toshiba biplane endocavity transducer 
incorporates linear imaging elements and axial imaging elements to offer a 
choice of views without transducer repositioning. B&K Medical Model 8558 
bi-plane imaging transducer incorporates a linear ultrasound array and a 
110.degree. convex ultrasound array to allow for switching from 
longitudinal to transverse imaging and vice versa. In addition, the Acuson 
ER7B endorectal biplane transducer integrates a 128 element radial phased 
array which provides 150.degree. of radial phased array coverage with a 
separate longitudinal array of 128 elements. These known probes allow 
different two dimensional views to be obtained from the different arrays. 
Attempts have been made to construct three-dimensional images using a probe 
with a linear array by collecting multiple two dimensional image data 
frames along with relative positional information among the image data 
frames so that these image frames could be subsequently assembled into a 
three dimensional volume to form the desired three dimensional 
reconstruction. The relative positional information was acquired by 
externally rotating the probe while trying to maintain angular control. 
Such manual techniques are slow and cumbersome and therefore have many 
drawbacks. Thus, these probes have not been successfully used to construct 
three dimensional images. 
Thus, it is desirable to provide an ultrasonic probe that allows three 
dimensional images to be constructed of the region examined by the probe 
in a precise and facile manner. 
SUMMARY OF THE INVENTION 
According to a first aspect of the present invention there is provided a 
method for registering image information acquired from an interior region 
of a patient. The method includes the steps of: 
(a) inserting an ultrasonic probe having a body having a longitudinal axis, 
a circumference and a distal end region, a first ultrasound array disposed 
in the distal end region of the body and a second ultrasound array 
disposed in the distal end region of the body into a patient to image an 
interior region of the patient; 
(b) acquiring image data with the first ultrasound array; 
(c) acquiring tracking data with the second ultrasound array; 
(d) repeating steps (b) and (c) after moving the ultrasonic probe along a 
direction having a component of motion in the tracking plane; 
(e) automatically determining the component of motion based on a comparison 
of the tracking data acquired in steps (c) and (d); and 
(f) automatically using the component of motion determined in step (e) to 
register the first image information acquired in step (d) with the first 
image information acquired in step (b). 
According to a second aspect of the present invention there is provided a 
method for registering image information acquired from an interior region 
of a patient. The method includes the steps of: 
(a) inserting an ultrasonic probe having a body having a longitudinal axis, 
a circumference and a distal end region, a first ultrasound array disposed 
in the distal region of the body and a second ultrasound array disposed 
around the circumference of the distal end region of the body into a 
patient to image an interior region of the patient; 
(b) acquiring first two-dimensional image data in an image plane with the 
first ultrasound array; 
(c) acquiring tracking data in a tracking plane oriented at a non-zero 
angle with respect to the image plane with the second ultrasound array; 
(d) repeating steps (b) and (c) after moving the ultrasonic probe along a 
direction having a component of motion in the tracking plane; 
(e) automatically determining the component of motion based on a comparison 
of the tracking data acquired in steps (c) and (d); and 
(f) automatically using the component of motion determined in step (e) to 
register the first image information acquired in step (d) with the first 
image information acquired in step (b). 
According to a third aspect of the present invention there is provided a 
probe including a body having a longitudinal axis, a circumference, and a 
distal end region; a linear phased array disposed in the distal end region 
of the body; and a radial phased array disposed 360.degree. around the 
circumference of the body. 
According to a fourth aspect of the present invention there is provided a 
system including a probe having a body having a longitudinal axis, a 
circumference, and a distal end region, a linear phased array disposed in 
the distal end region of the body, and a radial phased array disposed 
360.degree. around the circumference of the body; and a transmit 
beamformer and a receive beamformer coupled to each of the linear phased 
array and the first radial phased array.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIG. 1 is a schematic view of a distal portion of a probe 10 that includes 
a body 12, which preferably is in the form of a rigid shaft having a 
longitudinal axis L and a circumference C. The body 12 has a distal end 
region 18 which includes at least two ultrasonic transducer arrays that 
generate different image formats when excited as will be described in 
greater detail hereinafter. The probe 10 may have various configurations 
for various uses. For example, the probe 10 may be an endorectal probe, an 
endovaginal probe or a transesophageal probe. The particular shape of the 
probe 10 will be dictated by its use and FIG. 1 is merely intended to 
represent the distal end portion of the probe 10 which typically is a 
cylindrical shaft. The present invention, however, is not limited to such 
a configuration. A lens or acoustic window (not shown) may cover the 
emitting faces of the transducer arrays, however, it has not been shown 
for clarity purposes. 
In a preferred embodiment, the body 12 is preferably constructed of 
RADEL.TM. available from Amoco Polymers of Atlanta, Ga. Other high impact 
thermoplastics preferentially having substantial chemical resistance may 
be used. The body 12 preferably has a length ranging from about 10 cm to 
about 20 cm and a diameter ranging from about 10 mm to about 20 mm. 
A first ultrasonic transducer array 20 ("first array 20") and a second 
ultrasonic transducer array 22 ("second array 22") are provided in the 
distal end region 18 of the probe 10. In a preferred embodiment, the first 
array 20 is a linear phased array and the second array 22 is a radial 
phased array. In a preferred embodiment, the radial phased array is an 
annular array. When the annular array 22 is excited all of the emitted 
acoustic lines have a common origin lying at the center of the annular 
array 22. An annular array 22 is used to obtain a 360 degree scan. A 360 
degree scan, however, is not always necessary for every application. In 
particular, in another preferred embodiment shown in FIG. 2 the radial 
array 22' may be formed by a curved linear phased array which does not 
form an entire annulus and only provides a partial radial scan. In another 
preferred embodiment shown in FIG. 3, the radial array 22" may be formed 
by a substantially planar linear phased array which provides a partial 
radial scan. A radial array, as that term is used in the present 
invention, is any array that generates a scan in a plane perpendicular to 
the longitudinal axis L of the probe when the array is excited. If the 
radial array is formed by a linear or curved linear phased array the scan 
obtained may be linear, sector or VECTOR.TM. format. A particular example 
is the Acuson V510B bi-plane transesophagael probe which includes two 
planar linear phased arrays operated in a VECTOR.TM. format to collect 
longitudinal and radial plane image data. The linear phased array 20 
generates a sector, VECTOR.TM., or linear format image plane 24 upon 
excitation as illustrated in FIG. 1. 
Linear phased array 20 is formed by a plurality of ultrasonic transducer 
elements 30 that are sequentially arranged along the longitudinal axis L 
of the body 12. The azimuth of the array 20 extends parallel with the 
longitudinal axis L of the body 12. In a preferred embodiment, the linear 
phased array 20 is formed by 128 transducer elements having an elevation 
dimension extending into the FIGS. 1-3 of about 5 mm and are spaced on a 
0.3 or 0.4 mm pitch. The linear phased array 20 can be of conventional 
form, such as a flat linear phased array with a cylindrical elevation 
focusing lens. Alternately, the array 20 can be generally flat, but the 
transducer elements can be curved in elevation to focus. In this case, a 
non-refractive filler such as a polyurethane can be used since a focusing 
lens is no longer required. All imaging modes including B mode, color 
Doppler, color Doppler energy and the like are supported. The linear 
phased array 20 may include more or less than 128 elements and may have a 
different pitch and elevation. 
Radial phased ultrasonic transducer array 22 in FIG. 1 is formed by a 
plurality of transducer elements 32 sequentially arranged 
circumferentially so that it is preferably concentric with the 
circumference of the body 12. In a preferred embodiment, the radial phased 
array 22 is formed by 128 elements having an elevation dimension of 5 mm 
spaced on a 0.2 mm pitch. In another preferred embodiment, the radial 
phased array 22 is formed by 256 elements having an elevation dimension of 
5 mm spaced on a 0.25 mm pitch to form an annulus and provide a 
360.degree. scan. The annular array may be formed by wrapping a flat 
transducer array that has been partially diced into a circle around a 
support. Alternatively, the radial phased array 22 may be formed by fewer 
elements and, thus, provide less than a 360.degree. scan. Of course, if 
the radial phased array 22 has the format shown in FIGS. 2 or 3, the 
number of elements, pitch, and elevation may be different. 
As is well known in the art, conventional ultrasound transducers are 
typically constructed of piezoelectric material, such as PZT. In a 
preferred embodiment, the piezoelectric material for arrays 20 and 22 is 
preferably 3203HD sold by Motorola Ceramics of Albuquerque, N. Mex. 
Preferably, each transducer element includes two matching layers. The 
matching layer adjacent to the PZT is an epoxy loaded with alumina or 
lithium aluminum silicate and/or metal power such as tungsten preferably 
325 mesh and possesses an acoustic impedance of approximately 8-10 MRayls. 
The second matching layer--further from the PZT--is preferably an unfilled 
epoxy possessing an impedance of approximately 2.5 MRayls. The arrays 20 
and 22 are constructed using well known techniques which involve 
laminating the matching layers, an electroded slab of PZT and a flexible 
circuit onto a thin backing block substrate. Since a very high acoustic 
loss is desired, it may be preferable to form the backing block from 
polymeric particles which have been fused to form a macroscopically rigid 
structure having remnant tortuous permeability, as described in U.S. Pat. 
No. 5,297,553, assigned to the assignee of this invention. Once the 
structure has been laminated, individual elements are defined by dicing 
through the matching layers, PZT and partially into the backing block as 
is well known. Thereafter, the substrate can be bent to its final shape. 
The ultrasonic probe 10 according to the preferred embodiments shown in 
FIGS. 1-3 can be used to reconstruct three dimensional images. More 
particularly, in a preferred embodiment, one array is used as an imaging 
array and the other array is used as a tracking array. For example, if the 
radial phased array 22, 22', 22" is used as the imaging array and the 
linear phased array 20 is used as the tracking array, multiple two 
dimensional image data sets are accumulated from the radial phased array 
as the probe is pushed or pulled through a region of interest. The linear 
phased array is used for collecting frame-to-frame tracking data by 
feature tracking between successive frames using, for example, the sum of 
absolute differences technique. In this way, the longitudinal displacement 
between successive radial phased scans is obtained and sufficient locating 
data is acquired to allow the multiple two dimensional image data sets to 
be assembled into a three dimensional volume. Alternatively, if the linear 
phased array 20 is used as the imaging array and the radial phased array 
22, 22', 22" is used as the tracking array, multiple two dimensional image 
data sets are acquired using the linear phased array. The probe is rotated 
and the radial phased array acquires multiple data sets which are analyzed 
to determine the extent of rotation between frames. This provides enough 
locating information to allow the multiple two dimensional image data sets 
to be assembled into a three dimensional volume. Alternatively, both 
arrays 20 and 22 may be used as tracking arrays. Image reconstruction 
techniques are described in greater detail in U.S. patent application Ser. 
No. 08/407,498, entitled "Multiple Ultrasound Image Registration System, 
Method and Transducer," concurrently filed herewith (Attorney Docket No. 
5050/183) which is a continuation-in-part of U.S. patent application Ser. 
No. 08/621,561, filed Mar. 25, 1996, which is a continuation-in-part of 
provisional patent application Serial No. 60/012,578 filed Feb. 29, 1996, 
all of which are assigned to the assignee of the present invention and all 
of which are hereby incorporated herein by reference. 
FIG. 4 is a block diagram of an ultrasonic imaging system according to a 
preferred embodiment of the present invention. The following discussion 
will first present a system overview, and then a detailed description of 
select components of the system. 
System Overview 
The system 100 includes a beamformer system/signal detector 102 which 
includes both transmit and receive beamformers and is connected via a 
multiplexer/demultiplexer 104 to an ultrasonic probe 10 such as that shown 
in FIG. 1. If both arrays are operating in a conventional mode where the 
active transducer aperture is operated simultaneously in a phased manner, 
then any conventional device--such as the Acuson XP may be used for 
element 102. If the arrays are being operated in a synthetic aperture 
mode, i.e., in which the elements of the array are operated in a 
sequential rather than simultaneous mode, then it is necessary for the 
system to store the receive element signals in a temporary store until all 
of the transmit-receive element combinations have been received. Once all 
the echo signals have been received then the data in the temporary storage 
registers are delayed and summed to produce a beamformed signal. Systems 
for implementing this type of synthetic focusing by temporarily storing 
single channel data until all channel data has been received are well 
known, for example, see Proudian U.S. Pat. No. 4,917,097. The system 
preferably accumulates multiple signals for each transmitter-receiver pair 
so that signal averaging is achieved thereby resulting in an improvement 
in the signal to noise ratio. Alternatively, instead of using a common 
transducer element for both transmitter and receiver a separate receiver 
can be used for each transmitter channel selected. Such a method is 
described by O'Donnell et al. in "Synthetic Phased Array Imaging of 
Coronary Arteries With An Intraluminal Array," Proceedings of the 1995 
IEEE Ultrasonics Symposium, pp. 1251-1254 (1995). Individual elements are 
sequentially used as transmitters. As each element is used as a 
transmitter, separate adjacent elements are used as receivers on a 
sequential basis. In this way, the array can be made to synthesize the 
operation of a conventional large scale phased array scanner but with the 
added advantage that dynamic transmit focusing as well as dynamic receive 
focusing is possible since the individual channel transmit path lengths 
are known uniquely. The low signal to noise ratio of the array elements is 
partially overcome by averaging the successive firings of the same element 
pairs. Preferably, as many averages as possible are used consistent with 
not providing an imaging frame rate which is slower than desired by the 
user. Preferably the array is operated with frequencies in the range of 
about 5 to 10 MHz. If lower frequencies are used, then the linear array 
has less problems with grating lobes. Alternatively, a lower frequency can 
be used when operating steered ultrasonic lines as describe in U.S. Pat. 
No. 5,549,111. When the linear phased array is used to accumulate tracking 
information, the array can be operated at a high frequency, for example, 
10 MHz, since only a relatively small set of data is required in order to 
derive the motion information. 
The beamformer system/signal detector 102 sends excitation signal pulses to 
the arrays 20 and 22 and supplies summed returning echoes to a signal 
detector 102. The beamformer system/signal detector 102 accumulates data 
from the array elements 20 and 22 and forms beamformed acoustic line 
outputs. The output of the beamformer signal detector 102 is supplied to a 
scan converter 124. The scan converter 124 controls an output display 126 
to display preferably the two images generated by the two arrays 20, 22. 
In a preferred embodiment, the output display 126 displays the views 
obtained from the linear phased array 20 and the radial phased array 22 
simultaneously on a split screen. Alternatively, the operator may flip 
back and forth between views. Other display options will be described in 
greater detail hereinafter. 
In addition, scan-converted image information from the scan converter 124 
is stored in a data storage system 128. In this preferred embodiment, the 
data storage system 128 includes two separate storage arrays 130 and 132, 
each storing data for image frames from a respective one of the arrays 20 
and 22. In a preferred embodiment, one array of the probe is used for 
collecting image data that will be used to construct displayed 
representation of the region of interest and other array operates as a 
tracking array. In a preferred embodiment, the linear phased array 20 is 
used to collect image data and the radial phased array 22 is use to 
collect tracking data. Thus, image information from the image array 20 is 
stored as frames of image data in the storage array 130, and tracking 
information from the tracking array 22 is stored as respective frames of 
tracking data in the storage array 132. The frames of data in the storage 
arrays 130 and 132 are all time marked, so that they can be associated 
with one another appropriately. This time marking can take the form of 
real-time clock information or frame number information, for example. 
The frames of image data in the storage array 130 are applied to a computer 
136. It is these frames that are used to form the displayed representation 
of the region of interest. The tracking frames stored in storage array 132 
are not necessarily registered to create a displayed reconstruction of the 
region of interest but are instead used to determine the relative 
positions of individual frames of image data from the image data storage 
array 130. 
In order to estimate movement of the probe 10 between successive frames of 
the image data, the tracking information from the tracking array data 
storage array 132 is supplied to a motion estimator 138. The motion 
estimator 138 compares sequences of frame data from the tracking array 22 
to estimate a component of motion of the probe 10 between the respective 
frames. This estimate of the component of motion is smoothed in logic 140, 
and then applied to a calculator 142 that calculates a vector value 
defining the best estimate of the movement between selected frames of the 
data stored in the image data storage array 130. This vector is then 
applied as another input to the computer 136. 
The computer 136 registers selected frames of image data from the image 
data storage array 130 with respect to one another by appropriate use of 
the vectors supplied by the calculator 142. Also, any necessary 
interpolation is done, and the respective frames of image data are stored 
in proper registration with respect to one another in a three-dimensional 
data storage device 144. The computer 136, when operating in a display 
mode, can select appropriate information from the three-dimensional data 
storage device 144 to provide a desired image on the display 146. For 
example, cross sections can be taken in various planes, including a wide 
variety of planes that do not correspond to the planes of the image data. 
Also, surface renderings and segmentation displays can be created if 
desired. 
Common signal conductors can be used between the beamformer/signal detector 
102 and the housing for the probe 10. In the housing, individual signals 
are routed between the signal conductors and the transducer elements of 
the arrays 20 and 22 by high voltage analog switches or multiplexers. 
Various other preferred embodiments of transducer probes are possible and 
within the scope of the present invention. FIG. 5 illustrates the distal 
end region 18' of an ultrasonic probe according to another preferred 
embodiment of the present invention. In this preferred embodiment, a 
second tracking array 200, preferably a radial phased array, is provided 
proximal of the linear phased array 20'. FIG. 6 illustrates a system in 
which a second tracking array 200 is incorporated in the probe. As 
previously described with respect to radial array 22, the second tracking 
array 200 may extend around the entire circumference to obtain a 
360.degree. scan of the probe or it may extend only partial around the 
circumference. When two tracking arrays are used, the ability to 
compensate for impure rotation (where one or both ends of the image array 
20' is linearly translated as well as rotated) is substantially increased. 
Since the tracking arrays are on either side of the image array, and the 
exact geometry of the image data plane with respect to the tracking arrays 
is known, it is possible to interpolate linearly along the image data 
array azimuth axis to calculate the exact pixel translations for all 
points on the image data plane. FIG. 7 illustrates the distal end region 
of an endo vaginal (EV) or endo rectal (ER) probe according to the prior 
art. A tightly curved linear array is provided at the end of the 
cylindrical probe as shown in end view in FIG. 7A. FIG. 8 illustrates the 
distal end region of an EV or ER probe according to a preferred embodiment 
of the present invention. The probe 300 has a tightly curved array similar 
to that shown in FIG. 7 but also includes an imaging array 301 at the 
distal end of the probe except that a first tracking array 302 and 
optionally a second tracking array 304 have been added to the probe 300 to 
facilitate three dimensional reconstruction. In a preferred embodiment, 
array 301 at the distal end of the probe is used as the imaging array and 
the first and second arrays 302 and 304 are used as tracking arrays. The 
first tracking array 302 extends 360 degrees around the shaft of the probe 
while the second tracking array 304 extend only partially around the shaft 
of the probe. Either or both of the tracking arrays may scan 360.degree. 
or less than 360.degree.. In use, the probe is inserted into the rectum or 
vagina and is rotated so that the image array 301 sweeps out a volume and 
the first and second tracking arrays 302 and 304 track motion within a 
plane. While a second tracking array 304 is shown, the probe may be 
provided with only one imaging array and one tracking array. By providing 
the second tracking array, impure rotation of the probe can be accounted 
for as previously discussed. In the preferred embodiment shown in FIG. 8, 
the first tracking array 302 proximal of the imaging array 301 is 
preferably set back a distance of 10 mm and, if a second tracking array 
304 is provided, it is set back about 3 or 4 cm proximal of the first 
tracking array 302. 
Other preferred embodiments may be provided. For example, the Acuson EV7, 
shown diagrammatically in FIG. 9, which has a phased array 400 mounted at 
the distal end of the probe at an angle of about 60 degrees with respect 
to the end of the cylindrical base of the probe and may be modified to 
include at least one tracking array 402 located in the side of the probe, 
for example, to permit motion tracking as the probe is rotated and the 
image array at the end of the probe sweeps out a volume. 
Another preferred embodiment of a probe according to the present invention 
is shown in FIG. 10. In this embodiment, the probe 500 includes an imaging 
array 502 in the distal end of the probe and at least one tracking array 
504 is provided on the side of the probe. The imaging array 502 is 
preferably a combined curved and flat array. With this design, the probe 
must be rotated 360 degrees in order to scan an entire volume whereas the 
probe shown in FIG. 8 only requires the probe to be rotated 180 degrees to 
scan an entire volume. This type of array may be simpler to manufacture 
since the number of elements contained within the end region is minimized 
and hence wiring to the elements is less cramped. The tracking arrays 
should form scans in the radial plane. Preferably the tracking arrays are 
radial in form and scan 360.degree. although they do not absolutely have 
to be annular arrays. 
Angular Motion Detection 
With respect to the radial arrays described previously, the output of the 
beamformer are polar in format. For measuring rotational motion rather 
than Cartesian motion, it is simpler to retain the acoustic line data in 
polar format, i.e., not scan converted. Typically, the beamformer outputs 
lines are detected to form unipolar signals and are scan converted to 
digital quantities. FIG. 11 illustrates how a subset of beam data appears 
in reality , i.e. scan converted into Cartesian coordinates. It is much 
simpler, however, to unwrap the axial display shown in FIG. 11, i.e. do 
not scan convert it. FIG. 12 illustrates how this data is unwrapped to 
form the straight polar case. The increment between successive beam lines 
is simply their angular separation, for example, 5 degrees. With respect 
to detecting the motion of pixel values from Line 1 to Line 1' etc., it is 
evident that by using polar coordinates the correct answer for rotation is 
arrived at more simply. In this case the lines are spaced 5 degrees apart 
and the detected motion from Line 1 to Line 1' is approximately two-thirds 
of 5 degrees. 
Display Options 
Since one is to able collect image data from both arrays and use one or 
both sets for tracking motion of the other plane described previously, 
various display options exist. 
FIG. 13 illustrates a display generated by the linear phased array. The 
angle of probe rotation with respect to some user defined arbitrary 
starting point has been measured. This angle is an indication of the 
relative angular direction of the image frame produced by the linear 
phased array and may be displayed as a circular icon as shown in FIG. 13 
and/or a numeric output as is also displayed. The circular icon assumes 
that the user defined origin is at the top of the circle (for example)and 
the angular rotation of the probe with respect to this position is shown 
by an arrow suitably angled with respect to the starting point, i.e., the 
top of the circle. Software for displaying such icons is well within the 
scope of those skilled in the art. 
FIG. 14 illustrates a display generated by the radial phased array. The 
radial display is presented and depth of penetration as detected by motion 
sensed from the linear array is also displayed. Again, the reference point 
for the start of motion detection is arbitrary and the user should have 
the option of resetting it by, for example, selection of a key on a 
keyboard. An icon display for the detected depth relative to the last 
resecting of the depth measurement is also shown in FIG. 14. Preferably 
the icon is in the form of a ruler like object with an arrow pointing to 
the current position. Optionally, a numeric display indicating millimeters 
of penetration is also provided. 
FIG. 15 illustrates a display of images formed both the linear phased array 
and the radial phased array. In the embodiment shown in FIG. 15, both the 
radial and linear array images are displayed each having tick marks 
indicating a scale in either mm or cm. Preferably, the scan converter sets 
the millimeter scales to be equal in dimension in both displays. 
Displaying multiple ultrasound images is relatively well known, for 
example, simultaneous B-Mode and M-Mode. In this case, an angle display is 
also provided which indicates the present position of the linear array 
image with respect to the last resetting of the angle measurement. 
FIG. 16 illustrates a display formed by both the linear phased array and 
the radial phased array. In this preferred embodiment, the radial image 
display is rotated according to the detected rotation angle such that the 
display rotation completely compensates for rotation of the physical 
device. Thus, the image appears to remain static though the image is 
moving with respect to the array. If the system detects that an arbitrary 
object has moved 20 degrees anticlockwise, the system signals the scan 
converter to rotate the image 20 degrees clockwise to compensate. The 
concept of the detecting image motion and altering the display to correct 
for it is described in considerable detail in Bamber U.S. Pat. No. 
5,538,004. 
If desired, the probe can include an absolute sensor incorporated in its 
distal end region for position, orientation, or both, such as a magnetic 
sensor or an accelerometer. The sensor 19 may be used to supplement or 
back up the motion detection approach and may be of the types described in 
Keller U.S. Pat. No. 5,353,354 or one of the smaller sensors manufactured 
by Biosense, Inc. of Setauket, N.Y. 
While this invention has been shown and described in connection with the 
preferred embodiments, it is apparent that certain changes and 
modifications, in addition to those mentioned above, may be made from the 
basic features of the present invention. Accordingly, it is the intention 
of the Applicant to protect all variations and modifications within the 
true spirit and valid scope of the present invention.