Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-16
2001-08-21
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06277077
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ultrasound transducers. In particular, the invention relates to ultrasound transducers mounted on catheters that include ultrasound dampening regions to improve the performance thereof.
2. Description of the Related Art
For certain types of minimally invasive medical procedures, endoscopic visualization of the treatment site within the body is unavailable or does not assist the clinician in guiding the needed medical devices to the treatment site.
Examples of such procedures are those used to diagnose and treat supra-ventricular tachycardia (SVT), atrial fibrillation (AF), atrial flutter (AFL) and ventricular tachycardia (VT). SVT, AFL, AF and VT are conditions in the heart which cause abnormal electrical signals to be generated, causing irregular beating of the heart.
A procedure for diagnosing and treating SVT or VT involves measuring the electrical activity of the heart using an electrophysiology (EP) catheter introduced into the heart via the patient's vasculature. The catheter carries mapping electrodes which are positioned within the heart and used to measure electrical activity. The position of the catheter within the heart is ascertained using fluoroscopic images. The mapping electrodes measure the electrical activity of the heart at the position of the catheter. A map is created by correlating locations in the heart determined by viewing the position of the catheter with the fluoroscope. The physician uses the map to identify the region of the endocardium which he believes to be the source of the abnormal electrical activity. An ablation catheter is then inserted through the patient's vasculature and into the heart where it is used to ablate the region identified by the physician.
To treat AF, an ablation catheter is maneuvered into the right or left atrium where it is used to create elongated ablation lesions in the heart.
An improvement over fluoroscopy is a display system using a fixed coordinate system for determining the relative locations of medical devices within the body. Such a display system using a fixed coordinate system can avoid the tracking errors inherent in fluoroscopic imaging that can make it difficult to guide medical devices to the desired locations within the body. Ultrasound can be used to track medical devices relative to a fixed internal coordinate system. Such an ultrasound tracking system uses at least four ultrasound transducers.
An ultrasound tracking system can be based on the time difference measured from the time an ultrasound pulse is transmitted by one transducer to the time it is received by another transducer. Given the velocity of sound in tissue and blood of approximately 1570 m/sec, the distance between the transmitter and receiver can be calculated. This process of distance measurement with sound is called sonomicrometry. Distance measurements between multiple transducers are used to triangulate the positions of the transducers in a three-dimensional coordinate system. A minimum of four transducers create a three-dimensional coordinate system, with three transducers defining a plane and the fourth defining a position above or below the plane. Additional transducers may be used for redundancy. Once the coordinate system is established, additional transducers on medical devices may be used to calculate the locations of the devices relative to the coordinate system.
FIG. 1
shows an example of transmit and receive waveforms for a typical sonomicrometry procedure. Transmission waveform
26
is a pulse initiated at time t
1
by a transmitting transducer. Reception waveform
27
corresponds to the voltage generated by a receiving transducer that intercepts the transmit pulse. The time t
2
is the time at which the reception waveform crosses a detection threshold
28
. The time difference between t
1
and t
2
may then be used to calculate the distance between the transmitter and the receiver.
The detection threshold
28
is needed to filter out detected signals that are too small to have been generated by a measurement pulse. Such signals could result from crosstalk with signals on nearby wires or from random noise.
For optimal performance of a sonomicrometer system, transducers should exhibit fully isotropic operation, that is, the same transmit or receive (or both) generation or detection sensitivity in all directions. Isotropic operation allows measurement of the same distance amount regardless of the orientation of the transducers. Accurate distance measurement is desirable for measurements within the human body, especially within the heart or other vital organs, both for accurate placement of the sensing devices and for accurate location of areas for monitoring or surgery.
Sonometrics Corp. of London, Ontario, Canada has developed a sonomicrometer system based on a family of small transducers which might be sewn onto living tissue. These transducers typically consist of either small, flat squares of piezoelectric ceramic
30
(
FIG. 2
) in a spherical bead of epoxy (not shown) or small cylinders of piezoelectric ceramic
34
(
FIG. 3
) in a spherical bead of epoxy (not shown).
Piezoelectric ceramics convert electrical energy into vibrational energy, and vice versa. The vibration results from the piezoelectric ceramic expanding in one direction, which causes it to contract in another direction. For the square transducer
30
, expansion by the top and bottom (thickness) causes the sides to contract. For the cylindrical transducer
34
, expansion by the top and bottom (length) causes the thickness and the circumference to contract. Each direction of expansion/contraction is termed a vibrational mode.
The expansion and contraction causes an ultrasound signal to be emitted from each surface of the transducer when transmitting. Similarly, a received ultrasound signal causes a receiving transducer to expand and contract, generating electricity.
The epoxy bead serves as a lens to focus the ultrasound signal.
The frequency of expansion and contraction is determined by the size of the transducer. The speed of sound in the piezoelectric ceramic is about 4000 m/sec. This speed is equal to the wavelength times the frequency, and the wavelength is twice the direction of expansion/contraction. For example, for the square transducer
30
, a length and width d of 0.052 inches relate to a frequency of 1.5 MHz. For the cylindrical transducer
34
, the wavelength of the circumferential vibration is the average circumference, and the wavelength of the length vibration is twice the length, so a length of 0.052 inches and a circumference of 0.105 inches relate to a frequency of 1.5 MHz.
Line
40
in
FIGS. 2 and 3
passes through the center of the transducer in a direction parallel to a longitudinal axis of a catheter (not shown) on which either of the transuducers
30
or
34
may be mounted. Line
38
indicates a direction perpendicular to line
40
, where line
38
passes through the center of the transducer. The angle &THgr; denotes the angle from line
38
in the direction of line
40
.
Unfortunately, the transducers shown in
FIGS. 2 and 3
are anisotropic because the physical geometry of the transducer contributes to the radiation emitted and received. For example, referring to
FIG. 3
, line
38
lies in a radial plane perpendicular to line
40
through the center of transducer
34
. The cylindrical symmetry of transducer
34
suggests that the emissions or receptions in this plane would also be symmetric. However, for measurements made at an angle &THgr; as the angle approaches line
40
(an angle of 90 degrees), defining a longitudinal plane through the center of the cylinder, anisotropy in emissions results as the radiation shifts from emission off the cylindrical side to emission off the flat end
44
of the transducer. Similarly, referring to
FIG. 2
, anisotropy results from interference between the sides and the top.
Furthermore, anisotropy results from the physical dimensions of a receiving transducer at the same angles. This is because rec
Brisken Axel F.
Willis N. Parker
Cardiac Pathways Corporation
Jaworski Francis J.
Limbach & Limbach LLP
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