Abstract:
G.E. DOCKET NUMBER 15-DS-00536A system and method for measuring a position of an imaging element located within a scanhead of an imaging probe, such as transesophageal ultrasound probe, is provided. The imaging probe may be used in a medical imaging system and/or a three-dimensional imaging system. The probe includes an articulating portion having a scanhead. The scanhead includes an imaging element, such as a transducer, and a position sensor positioned within the scanhead. Preferably, the position sensor is connected to the imaging element via an axle. Therefore, the rotation of the position sensor is synchronized to the rotation of the imaging element. The location of the position sensor within the imaging element provides accurate measurement of the position of the imaging element. The position sensor preferably includes a code disk having apertures and a system of light emitters and detectors. As the code disk rotates in synchronization with the imaging element, the pattern of detection of light through the apertures measures the position of the imaging element. Various alternative position sensors, such as potentiometers, may be utilized within the scanhead of the probe. The probe also includes a control handle having imaging and articulation controls.

Description:
BACKGROUND OF INVENTION 
     The preferred embodiment of the present invention generally relates to improvements in an internal imaging probe, and more particularly relates to a transesophageal ultrasound probe with an imaging element position sensor positioned within the scanhead of the probe to detect the position of an imaging element located within the scanhead. 
     Various medical conditions affect internal organs and structures. Efficient diagnosis and treatment of these conditions typically require a physician to directly observe a patients internal organs and structures. For example, diagnosis of various heart ailments often requires a cardiologist to directly observe affected areas of a patients heart. Instead of more intrusive surgical techniques, ultrasound imaging is often utilized to directly observe images of a patients internal organs and structures. 
     Transesophageal Echocardiography (TEE) is one approach to observing a patients heart through the use of an ultrasound transducer. TEE typically includes a probe, a processing unit, and a monitor. The probe is connected to the processing unit which in turn is connected to the monitor. In operation, the processing unit sends a triggering signal to the probe. The probe then emits ultrasonic signals into the patients heart. The probe then detects echoes of the previously emitted ultrasonic signals. Then, the probe sends the detected signals to the processing unit which converts the signals into images. The images are then displayed on the monitor. The probe typically includes a semi-flexible endoscope that includes a transducer located near the end of the endoscope. Typically, the transducer is a piezoelectric transducer having 48 to 96 piezoelectric elements. 
     Typically, during TEE, the endoscope is introduced into the mouth of a patient and positioned in the patients esophagus. The endoscope is then positioned so that the transducer is in a position to facilitate heart imaging. That is, the endoscope is positioned so that the heart or other internal structure to be imaged is in the direction of view of the transducer. Typically, the transducer sends ultrasonic signals through the esophageal wall; the ultrasonic signals come into contact with the heart or other internal structures. The transducer then receives the ultrasonic signals as the ultrasonic signals bounce back from various points within the internal structures of the patient. The transducer then sends the received signals back through the endoscope typically via wiring. After the signals travel through the endoscope, the signals enter the processing unit, typically via wires connecting the endoscope to the processing unit. 
     Occasionally, the transducer may be rotated about an axis perpendicular to its imaging surface. The transducer may be rotated to change the imaging scan-plane during the imaging process. That is, the transducer may be rotated to image the internal structure from a horizontal scan-plane or a vertical scan-plane (and all positions in between). Typically, the transducer may be rotated 90° in either direction from its normal position. 
     The position, or orientation, of the transducer is typically measured by a position sensor, such as a potentiometer, located within the control handle of the probe. A mechanical transfer mechanism connects the position sensor located in the control handle to the transducer located in the scanhead. For example, the transducer may be connected to the position sensor via a flexible axle or shaft. Thus, the transducer and the position sensor are typically separated by a significant distance. The separation of the transducer and the position sensor may cause errors in the position measurement. For example, mechanical imperfections, such as slack, spring tension, mechanical hysteresis, or dead zones, may occur due to the extended mechanical distance between the transducer in the scanhead and the position sensor in the control handle. The mechanical imperfections may lead to inaccurate position measurement. The position measurement inaccuracies may lead a physician, or other operator of the probe, to believe that the physician is viewing an internal structure from a scan plane other than the scan plane actually being viewed. For example, the position sensor may measure the position of the transducer at a position 33° from the normal orientation of the transducer when the correct measurement is 30° from the normal orientation. Typically, the position of the transducer measured by the position sensor in the control handle is then displayed on the monitor of the imaging system. Consequently, the physician may misdiagnose and/or mistreat the patient who is being imaged if the deviation is great enough, for example a 10° deviation. Further, smaller errors and deviations, such as a deviation between 3°-5°, typically cause inaccuracies when two-dimensional images are combined to form three-dimensional images. 
     While the transducer typically images an internal structure in two dimensions, the two-dimensional images may be recorded and combined to produce three-dimensional images. In order to produce three-dimensional images, the transducer is typically rotated through various radial angles thereby imaging various scan-planes. The images from the various scan-planes are recorded and combined using corresponding recorded position measurements. However, inaccuracies in position measurement may skew the resulting three-dimensional images. Further, accurate position measurements are necessary to produce the desired accurate three-dimensional images. 
     Therefore, a need exists for a more accurate system and method for measuring the position of an imaging element, such as a transducer, within an imaging probe. Specifically, a need exists for an imaging system that provides more accurate measurement of the position of a transducer within a transesophageal ultrasound probe. Additionally, a need exists for an imaging system that provides more accurate measurement of the position of a transducer within an imaging probe to assist in producing accurate three-dimensional images. 
     SUMMARY OF INVENTION 
     The present invention relates to an imaging probe, such as a transesophageal ultrasound probe, for use in a medical imaging system and/or a three-dimensional imaging system. The probe includes an articulating portion having a scanhead. The scanhead includes an imaging element, such as a transducer, and a position sensor positioned within the scanhead. Preferably, the imaging element is connected to the position sensor via an axle. Therefore, the imaging element and the position sensor rotate in the same direction and at the same rate as one another. That is, the rotation of the imaging element and the position sensor is synchronized. The location of the position sensor within the scanhead provides accurate measurement of the position of the imaging element. 
     The position sensor preferably includes a code disk having apertures and a system of light emitters and detectors. As the code disk rotates in synchronization with the imaging element, the pattern of detection of light through the apertures measures the position of the imaging element. Various alternative position sensors, such as potentiometers, may be utilized with the imaging element. The probe also includes a control handle having imaging and articulation controls. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a transesophageal ultrasound probe according to a preferred embodiment of the present invention. 
     FIG. 2 illustrates a side articulating view of the transesophageal ultrasound probe of FIG. 1 according to a preferred embodiment of the present invention. 
     FIG. 3 illustrates a top articulating view of the transesophageal ultrasound probe of FIG. 1 according to a preferred embodiment of the present invention. 
     FIG. 4 illustrates an internal view of the scanhead of the transesophageal ultrasound probe of FIG. 1 according to a preferred embodiment of the present invention. 
     FIG. 5 illustrates a position sensor coupling of the position sensor disk to the imaging element according to a preferred embodiment of the present invention. 
     FIG. 6 illustrates a close view of the position sensor of FIG. 4 according to a preferred embodiment of the present invention. 
     FIG. 7 illustrates a flow chart according to a preferred embodiment of the present invention. 
     FIG. 8 illustrates a timing diagram of the position sensing process according to a preferred embodiment of the present invention. 
     FIG. 9 illustrates a magnetic sensor coupling of a magnetic position sensor to the imaging element according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a transesophageal ultrasound probe  100  according to a preferred embodiment of the present invention. The probe  100  includes a probe shaft  110 , a control handle  130 , and a system cable  150 . The probe shaft  110  includes an articulating portion  115 . The articulating portion  115  includes an articulation section  126 , and a scanhead  120  having an imaging element window  124 . The control handle  130  includes imaging controls  134 . The imaging controls  134  include an upper deflection control wheel  136 , an upper brake  137 , a lower deflection control wheel  138 , a lower brake  139  and scan plane push buttons  140 . 
     The probe shaft  110  is fixed to the control handle  130  which is in turn connected to the system cable  150 . The articulating portion  115  of the probe shaft ranges from the distal end of the probe shaft  110  to a position approximately 3 inches towards the control handle  130 . The articulating portion  115  includes the scanhead  120  connected to the articulation section  126 . The scanhead  120  includes an acoustical imaging element window  124 . The imaging element window  124  provides an imaging element (not shown), such as a transducer (not shown), imaging access to structures outside of the probe  100 . 
     The probe shaft  110  is connected to the control handle  130 . The control handle  130  includes imaging controls  134  that are positioned on the control handle  130 . The side of the control handle  130  includes scan-plane push buttons for controlling the positioning of the imaging element. The top of the control handle  130  supports the lower brake  139  and the lower deflection control wheel  138 . The lower deflection control wheel  138  is positioned underneath the upper deflection control wheel  136  and the upper brake  137 . 
     The proximal end of the control handle  130  is connected to the system cable  150 . The system cable  150  includes wiring (not shown). The wiring extends throughout the system cable  150  into the control handle  130  where it connects to the imaging element within the scanhead  120  of the probe  130  via a signal track (not shown). The system cable  150  is in turn connected to a processing unit (not shown). The imaging element is connected to the processing unit via wiring that extends through the scanhead  120  and throughout the length of the body of the probe  100 . The wiring in the probe  100  is then connected via the system cable  150  to the processing unit. The processing unit is then connected via wiring to a monitor (not shown) for displaying images. 
     FIG. 2 illustrates a side articulating view  200  of the transesophageal ultrasound probe  100  of FIG. 1 according to a preferred embodiment of the present invention. The side articulating view  200  includes the probe shaft  110 , the control handle  130 , and the system cable  150 . The probe shaft  110  includes the articulating portion  115 . The articulating portion  115  includes the scanhead  120  having the imaging element window  124  and the articulation section  126 . The articulating portion  115  is shown in a non-articulated position  205 , an upward articulated portion  210 , and a downward articulated portion  220 . The control handle  130  includes the imaging controls  134 . The imaging controls  134  include the upper deflection control wheel  136 , the upper brake  137 , the lower deflection control wheel  138 , the lower brake  139  and the scan plane push buttons  140  (not shown in FIG.  2 ). 
     FIG. 3 illustrates a top articulating view  300  of the transesophageal ultrasound  100  probe of FIG. 1 according to a preferred embodiment of the present invention. The top articulating view  300  includes the probe shaft  110 , the control handle  130 , and the system cable  150 . The probe shaft  110  includes the articulating portion  115 . The articulating portion  115  includes the scanhead  120  having the imaging element window  124  and the articulation section  126 . The articulating portion  115  is shown in a non-articulated position  305 , a first laterally articulated portion  310 , and a second laterally articulated portion  320 . The control handle  130  includes the imaging controls  134 . The imaging controls  134  include the upper deflection control wheel  136 , the upper brake  137 , the lower deflection control wheel  138 , the lower brake  139  and the scan plane push buttons  140 . 
     In operation, the probe shaft  110  of the probe  100  is introduced into the esophagus of a patient via the patients mouth. The probe shaft  110  is then positioned via the control handle  130  so that an internal structure to be imaged may be imaged by the imaging element through the imaging element window  124 . During imaging, the articulating portion  115  of the probe shaft  110  may be articulated in order to provide easier imaging access, or to image internal structures from different angles and perspectives. The articulating portion  115  may be positioned in an upward articulated position  210  or lower articulated position  220  by turning the lower deflection control wheel  138 . The articulating portion  115  may be articulated via the lower deflection control wheel  138  ranging from 120° upward to 40° downward and all positions in between. The lower brake  139  may be engaged to lock the articulating portion  115  in an upward or downward articulated position. 
     Similarly, the articulating portion  115  may be positioned throughout a lateral range. The articulating portion may be positioned in a first laterally articulated portion  310  or a second laterally articulated portion  320  by turning the upper deflection control wheel  136 . The articulating portion  115  may be articulated via the upper deflection control wheel  137  ranging from 40° in a first lateral deflection, such as left, to 40° in a second lateral deflection, such as right, and all points in between. The upper brake  137  may be engaged to lock the articulating portion  115  in a laterally articulated position. 
     Additionally, the articulating portion  115  may be articulated in an upward direction and a lateral direction at the same time. Further, the articulating portion  115  may be articulated in a downward direction and a lateral direction at the same time. Alternatively, the probe  100  may not include lateral articulation capabilities. 
     During imaging, the imaging element within the scanhead  120  may be rotated about an axis perpendicular to the imaging element window  124 . Preferably, the imaging element is a piezoelectric transducer including 48 to 96 piezoelectric elements. As the imaging element is rotated, the scan plane of the imaging element changes. For example, if the imaging element is initially set to image a vertical scan plane, the imaging element may be rotated 90° to image a horizontal scan plane. A position sensor (not shown) positioned within the scanhead  120  and preferably connected to the imaging element, or to another structure that is attached to the imaging element, measures the position, or orientation, of the imaging element. The position sensor then relays the position measurement to the processing unit via wires within the probe  100 . The processing unit then displays the position measurement on the monitor. The position measurement displayed on the monitor instructs a physician operating the probe as to the orientation of the image displayed on the monitor. That is, the physician is able to determine the scan plane at which he/she is viewing a patients internal structure. For example, the physician is able to determine whether the physician is viewing a patients internal structure from a horizontal or vertical scan-plane. Additionally, the position measurement may be recorded by the processing unit. 
     As further described below, in a preferred embodiment of the present invention, the position sensor is located in the scanhead  120  rather than in the control handle  130 . The location of the position sensor in the scanhead  120  provides more accurate position measurement. That is, because the position sensor is attached to the imaging element, the imperfections associated with significant distance between the imaging element and the position sensor are alleviated. Therefore, because slack, dead zone, spring tension, mechanical hysteresis and other mechanical phenomena that result in inaccurate position measurement are alleviated, the position measurement is more accurate. Accurate position measurement provides accurate position resolution of the two-dimensional images. 
     The two-dimensional images may be combined to form accurate three-dimensional images. Forming accurate three-dimensional images via recorded two-dimensional images requires accurate position measurement of the two-dimensional images. Because the position sensor is located in the scanhead  120 , the accuracy of the position measurements of the two-dimensional images is increased. The two-dimensional images may be recorded within the processing unit. The processing unit may then combine the recorded two-dimensional images via referencing the position measurements of the two-dimensional images to form three-dimensional images. The resulting three-dimensional images formed from the recorded two-dimensional images are more accurate because the recorded positions of the two-dimensional images are accurate. 
     After imaging is complete, the articulating portion  115  is returned to the non-articulated positions  205 ,  305 . The probe shaft  110  is then removed from the esophagus of the patient. 
     FIG. 4 illustrates an internal view  400  of the scanhead  120  of the transesophageal ultrasound probe  100  of FIG. 1 according to a preferred embodiment of the present invention. The internal view  400  includes the scanhead frame  420 , the lower section frame  406 , a mechanical support frame  416 , a flexible drive shaft  412 , a worm screw shaft  419  having a worm screw head  422 , and a signal track  445 . The scanhead frame  420  includes a distal portion of the worm screw shaft  419  and the worm screw head  422  connected to the worm screw shaft  419 . The scanhead frame  420  also includes a rotation wheel  426 , an imaging element  436 , an imaging element driven cog wheel  438 , a bi-directional mounting  440 , a track passage  446  and a position sensor disk  435 . The rotation wheel  426  includes lateral cogs  428  and longitudinal cogs  430 . The lower section frame  406  includes the flexible drive shaft  412  and a proximal portion  405  of the signal track  445 . 
     The scanhead frame  420  is connected to the lower section frame  406  via the mechanical support frame  416 . The signal track  445  provides power and signal passage between the imaging element  436  and the processing unit. The proximal portion  405  of the signal track  445  connects to the processing unit via miniature coaxial cables (not shown). The signal track  445  provides electrical connections and signals between the imaging element  436  and the processing unit. The signal track  445  passes through the track passage  446  into the mechanical support frame  416 . The signal track  445  passes through the mechanical support frame  416  via an opening (not shown) in the mechanical support frame  416 . 
     The flexible drive shaft  412  extends from the control handle  130  through the probe shaft  110  and into the mechanical support frame  416 . The worm screw shaft  419  connects to the flexible drive shaft  412  at the mechanical support frame  416 . The worm screw shaft  419  extends into the scanhead frame  420 . The mechanical support frame  416  allows passage of the worm screw shaft  419  via an opening (not shown). The worm screw head  422  operatively engages the lateral cogs  428  of the rotation wheel  426 . The longitudinal cogs  430  of the rotation wheel  426  operatively engage the imaging element driven cog wheel  438 . The imaging element driven cog wheel  438  may be a ring attached to the perimeter of the imaging element  436 , or the imaging element driven cog wheel  438  may be included as part of the body of the imaging element  436 . 
     The imaging element  436  is positioned on the bi-directional mounting  440 . The bi-directional mounting  440  includes an opening (not shown) formed within the bi-directional mounting  440  that allows for the passage of an axle, as further described below with reference to FIG. 5, that connects the imaging element  440  to the position sensor disk  435 . 
     The position sensor disk  435  may be connected to the imaging element in a variety of ways. Preferably, the imaging element  436  attaches to the position sensor disk  435  via the axle. That is, the axle extends through the imaging element driven cog wheel  438 , to connect the imaging element  436  to the position sensor disk  435 . Preferably, the position sensor disk  435  is directly fastened to the imaging element  436 . Alternatively, the axle may be fastened to the imaging element driven cog wheel  438 . 
     FIG. 5 illustrates a position sensor coupling  500  of the position sensor disk  435  to the imaging element  436  according to a preferred embodiment of the present invention. The position sensor coupling includes the imaging element  436 , the imaging element driven cog wheel  438 , the position sensor disk  435 , a first light emitter  524 , a second light emitter  528 , a first light detector  514 , a second light detector  518 , a number of apertures  530  radially positioned around the position sensor disk  435 , and an axle  510 . The axle  510  connects the center of the imaging element  435  to the center of the position sensor  436 . 
     Referring again to FIG. 4, in operation, the scan plane push buttons  140  of FIGS. 1-3 are engaged to rotate the imaging element  436 . The scan plane push buttons  140  are connected to a motor (not shown) located in the control handle  130 , the probe shaft  110 , or at a position within the articulating portion  115 . The motor is connected to a proximal portion of the flexible drive shaft  412  which is in turn connected to the worm screw shaft  419 . As the motor is engaged, the motor axially rotates the flexible drive shaft  412 . The rotation of the flexible drive shaft  412  causes worm screw shaft  419  to rotate. Because the worm screw head  422  is connected to the worm screw shaft  419 , the rotation of the worm screw shaft  419  causes the worm screw head  422  to rotate in the same manner. 
     As the worm screw head  422  rotates, the worm screw head  422  operatively engages the lateral cogs  428  of the rotation wheel  426  causing the rotation wheel  426  to rotate in response to the rotation of the worm screw head  422 . Because the lateral cogs  428  and the longitudinal cogs  430  are located on the rotation wheel  426 , the rotation of the lateral cogs  428  operatively engages the longitudinal cogs  430  to rotate the longitudinal cogs  430 . The rotation of the longitudinal cogs  430  operatively engages the imaging element driven cog wheel  438  to rotate the imaging element driven cog wheel  438  in the opposite direction as that of the rotation wheel  426 . The rotation of the imaging element driven cog wheel  438  in turn causes the imaging element  436  to rotate in the same direction as that of the imaging element driven cog wheel  438 . Because the position sensor disk  435  is axially connected to the imaging element  436  via the axle  510 , the position sensor disk  435  axially rotates at the same rate, and in the same direction, as the imaging element  436 . That is, the rotation of the imaging element  436  is synchronized to the rotation of the position sensor disk  435 . As further described below, the position sensor including the position sensor disk  435  accurately measures the position of the imaging element  436 . The position sensor including the position sensor disk  435  relays the position of the imaging element to the processing unit via wiring or through the signal track  445 . 
     Alternatively, various types of engaging members may be used to rotate the imaging element  436 . For example, a semi-flexible rotating axle may be used in place of the flexible drive shaft  412 . Additionally, a series of push/pull wires may be used to control the rotation of the imaging element  436 . 
     FIG. 6 illustrates a close view  600  of the position sensor according to a preferred embodiment of the present invention. Preferably the position sensor is an optical sensor that includes the position sensor disk  435 , the first light detector  514 , the second light detector  518 , the first light emitter  524  and the second light emitter  528 . Preferably, the position sensor disk  435  is a code disk. The position sensor disk  435  includes the apertures  530  radially positioned around the position sensor disk  435 . The first and second light emitters  524 ,  528  and the first and second light detectors  514 ,  518  preferably are formed into a single integrated structure (not shown). The structure is preferably attached to the interior of the probe  100 . Preferably, the structure is shaped to allow the position sensor disk  435  to pass between the first and second light emitters  524 ,  528  and the first and second light detectors  514 ,  518  as shown. 
     The first light detector  514  is aligned with the first light emitter  524 . The first light emitter  524  continuously emits light that is detected by the first light detector  514 . The second light detector  518  is aligned with the second light emitter  528 . The second light emitter  528  continuously emits light that is detected by the second light detector  518 . The position sensor disk  435  is opaque and thus blocks light. For example, when an aperture  530  of the position sensor disk  435  is positioned between the first light detector  514  and the first light emitter  524 , light passes through the aperture  530  and the first light detector  514  detects light. However, if the position sensor disk  435  is rotated so that no aperture  530  is positioned between the first light emitter  524  and the first light detector  514 , the position sensor disk  435  blocks the light emitted by the first light emitter  524  and thus the first light detector  524  does not detect light. 
     The apertures  530  are offset relative to the first and second light emitters  524 ,  528  and the first and second light detectors  514 ,  518 . That is, when the first light detector  514  detects light, the second light detector  518  detects only a portion of light emitted from the second light emitter  528 , or does not detect light at all. Also, when the second light detector  518  detects light, the first light detector  514  detects only a portion of light emitted from the first light emitter  524 , or does not detect light at all. Thus, as further describe below with respect to FIG. 8, as the position sensor disk  435  rotates, the signals of the first and second light detectors  514 ,  518  may be compared to determine the direction and speed of the rotation of the position sensor disk  435 . 
     The direction of the rotation of the position sensor disk  435 , such as a code disk, may be determined by the sequence of the signals from the first and second light detectors  514 ,  518 . The speed of the position sensor disk  435  may be determined by the frequency of transitions detected by the first and second light detectors  514 ,  518 . Therefore, as the position sensor disk  435  rotates, the magnitude and direction of the movement of the position sensor  435  may be measured through the light signals received by the first and second light detectors  514 ,  518 . Because the position sensor disk  435  rotates at the same rate and in the same direction as the imaging element  436 , the rotation of the position sensor disk  435  matches, that is, is synchronized to, the rotation of the imaging element  436 . Therefore, the rotation of the imaging element  436  may be determined from the rotation of the position sensor disk  435 . 
     FIG. 8 illustrates a timing diagram  800  of the position sensing process according to a preferred embodiment of the present invention. The timing diagram  800  includes a rotation diagram  805  and a transition pulse diagram  806 . The rotation diagram  805  includes the position sensor disk  435 , such as a code disk, the first light emitter  524 , the first light detector  514 , the second light emitter  528 , the second light detector  518 , the apertures  530 , and an arrow  810  defining the direction of rotation of the position sensor disk  435 . The transition pulse diagram  806  includes a first transition pulse sequence  820  corresponding to the first light detector  514 , a second transition pulse sequence  830  corresponding to the second light detector  518 , a reference time-line  840 , and an arrow  850  corresponding to advancing time. 
     The first transition pulse sequence  820  includes logical high pulses, for example logical high pulse  822 , high-to-low transitions, for example high-to-low transition  823 , low-to-high transitions, for example low-to-high transition  825 , and logical low pulses, for example logical low pulse  824 . The second transition pulse sequence  830  includes logical low pulses, for example logical low pulse  832 , low-to-high transitions, for example low-to-high transition  833 , high-to-low transitions, for example high-to-low transition  835 , and logical high pulses, for example logical high pulse  834 . The specific instant in time shown in the rotation diagram  805  is represented in the transition pulse diagram  806  by the reference time-line  840 . 
     The first light detector  514  emits a logical high pulse when the first light detector  514  detects light. The first light detector  514  emits a logical low pulse when the first light detector  514  does not detect light. Transitions occur at a logical cut-off, such as 50% of the maximum amplitude of the light emitted by the first light detector  514 . For example, a low-to-high transition occurs when the first light detector  514  emits a logical low pulse and then detects 50% or more of the maximum amplitude of the light emitted by the first light emitter  524 . A high-to-low transition occurs when the first light detector  514  emits a logical high pulse and then detects less than 50% of the maximum amplitude of the light emitted by the first light emitter  524 . 
     Similarly, the second light detector  518  emits a logical high pulse when the second light detector  518  detects light. The second light detector  518  emits a logical low pulse when the first second detector  518  does not detect light. Transitions occur at a logical cut-off, such as 50% of the maximum amplitude of the light emitted by the second light detector  518 . For example, a low-to-high transition occurs when the second light detector  518  emits a logical high pulse and then detects 50% or more of the maximum amplitude of the light emitted by the second light emitter  528 . A high-to-low transition occurs when the first light detector emits a logical high pulse and then detects less than 50% of the maximum amplitude of the light emitted by the second light emitter  528 . 
     The direction of movement of the position sensor disk  435  determines the sequence of transitions of the first and second light detectors  514 ,  518 . As the position sensor disk  435  moves in the direction denoted by the arrow  810 , a low-to-high transition pulse of the second transition pulse sequence  830  precedes a high-to-low transition of the first transition pulse sequence  820 . The sequence of transitions is registered by the position sensor in order to determine the direction of rotation of the position sensor disk  435 . Conversely, as the position sensor  435  rotates in a direction opposite that of the arrow  810 , a low-to-high transition of the first transition pulse sequence  820  precedes a high-to-low transition pulse of the second transition pulse sequence  830 . Therefore, as the sequence of transitions is registered, the direction of rotation of the position sensor disk  435  may be determined. 
     Alternatively, the optical system may be reflective rather than aperture based. That is, the position sensor disk  435  may include reflectors, instead of apertures  530 , radially positioned around the position sensor disk  435  and separated by non-reflective surfaces. If the position sensor disk  435  includes reflectors, the first and second light emitters  524 ,  528  and the first and second light detectors  514 ,  518  may be located on the same side of the position sensor disk  435 . The first and second light emitters  524 ,  528  and first and second light detectors  514 ,  518  may be angled such that the first and second light emitters  524 ,  528  emit light that reflects from the reflectors on the position sensor disk  435 . However, no light is reflected when the first and second light emitters  524 ,  528  emit light that contacts a non-reflective surface. For example, the first and second light emitters  524 ,  528  may be positioned to emit light that strikes the passing reflectors at a 45° angle of incidence. Therefore, the first and second light detectors  514 ,  518 , may be positioned to detect the light reflected at a corresponding 45° angle of reflection. 
     FIG. 9 illustrates a magnetic sensor coupling  900  of a magnetic position sensor to the imaging element  436  according to an alternative embodiment of the present invention. The magnetic sensor coupling  900  includes the imaging element  436 , an axle  912 , a magnet  910 , a first magnetic field detector  920  and a second magnetic field detector  930 . The imaging element  436  is connected to the magnet  910  via the axle  912 . Preferably, the first magnetic field detector  920  and the second magnetic field detector  930  are Hall detectors. Additionally, preferably the first magnetic field detector  920  and the second magnetic field detector  930  are angled 90° relative to one another and are positioned underneath the magnet  910 . The first magnetic field detector  920  and the second magnetic field detector  930  are attached to the interior of the scanhead  120 . The magnetic field of the magnet  910  is a non-uniform magnetic field. That is, one side of the magnet has a different magnetic polarity than the other side of the magnet. 
     In operation, the magnetic field detectors  920 ,  930  sense the magnetic field from the magnet  910 . The output from the magnetic field detectors  920 ,  930  is proportional to the magnetic flux on the surface of the magnetic field detectors  920 ,  930 . Therefore, the output from the first magnetic field detector  920  is minimal when the magnetic field is parallel to the surface of the first magnetic field detector  920 ; and the output from the second magnetic field detector  930  is minimal when the magnetic field is parallel to the surface of the second magnetic field detector  930 . Thus, as the imaging element  436  rotates, the magnets magnetic field that is perceived by the magnetic field detectors  920 ,  930 , and the output from the magnetic field detectors  920 ,  930  change. When the output from the first magnetic field detector  920  is at a maximum value, the output from the second magnetic field detector  930  is at a minimum value, and vice versa. Alternatively, one magnetic field detector may be used instead of two magnetic field detectors. Two magnetic field detectors, however, provide better resolution and accuracy. 
     Also, alternatively, the position sensor may be a potentiometer. Typically, a potentiometer is calibrated to a normal position. When the position of the potentiometer is changed, the potentiometer translates the change in position through an electrical resistance value. Consequently, the rotation of the imaging element causes the resistance value of the potentiometer to change. The resistance value is then relayed to the processing unit. 
     Additionally, the position sensor may be an inductive position sensor. The inductive position sensor may include a fixed coil and a coil attached to the imaging element  436 . For example, the fixed coil may be fixed to an interior portion of the scanhead  120 . As the imaging element coil moves, the inductance between the coils changes. Deviations from a preset calibrated inductance may then be used by a detector to measure the position of the imaging element. 
     Further, the position sensor may be a capacitive position sensor. The position sensor disk  435  may include one or more conductive plates fixed within the scanhead  120  and a number of conductive plates radially positioned to the position sensor disk  435 . As the position sensor disk  435  rotates, the plates of the position sensor disk  435  rotate. The capacitance between the fixed plates and the plates on the position sensor disk  435  changes. Deviations from a preset calibrated capacitance may then be detected by a voltage detector and used to measure the position of the imaging element. 
     FIG. 7 illustrates a flow chart  700  of the imaging process according to a preferred embodiment of the present invention. At step  710 , a physician begins the imaging process by introducing the scanhead  120  located on the articulating portion  115  of the probe shaft  110  into the esophagus of a patient. The articulating portion  115  of the probe  100  may be articulated during imaging. At step  720 , the physician engages the probe  100  to image an internal structure of a patient. 
     The orientation of the imaging element  436  is measured with the position sensor  435  in the scanhead  120  of the probe  100  at step  730 . Preferably, the position sensor  435  is connected onto the axle  510  which in turn connects to the imaging element  436  or the imaging element driven cog wheel  438 . The imaging element driven cog wheel  438 , the axle  510  and the position sensor  435  are located within the scanhead  120  as further described above. 
     At step  740 , the physician may rotate the imaging element  436  to view the internal structure from a different scan plane. At step  750 , the orientation of the imaging element  436  is again measured via the position sensor  435 . After imaging is complete, the physician removes the scanhead  120  of the probe shaft  110 , and the probe shaft  110  from the esophagus of the patient at step  760 . 
     Thus, the present invention provides an improved system and method for measuring the position of an imaging element, such as a transducer, within an imaging probe. Particularly, the present invention provides an improved system and method for accurately measuring the position of a piezoelectric transducer within a transesophageal ultrasound probe. Accurate measurement of the position of the imaging element is achieved via locating a position sensor within the scanhead of the probe where the measurement of the imaging element may be determined without the mechanical imperfections associated with prior art probes. 
     Locating the position sensor  435  within the scanhead  120  of the imaging element  436  provides more accurate measurement of the position and orientation of the imaging element  436 . Accurate measurement of the position of the imaging element  436  facilitates more accurate diagnosis and treatment. Additionally, the resultant images may be combined via referencing position measurements to form accurate three-dimensional images and/or illustrations. The accurate measurement of the position of the imaging element  436  enables three-dimensional imaging because accurate position measurements are required to combine the recorded images into a single image. 
     While particular elements, embodiments and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.