Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-10-18
2002-12-24
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S443000
Reexamination Certificate
active
06497660
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrasound imaging device, and more particularly, to an ultrasound imaging device having a transducer, with a poling polarity, to emit ultrasound waves at an object to generate an image of the object, and which prevents depoling of the transducer. Generally, the object to be imaged is an organ of the human body.
2. Description of the Related Art
Ultrasound imaging devices utilize transducers which transform electrical energy into ultrasound and visa-versa. Such transducers are commonly used for non-destructive and non-invasive testing, such as for the examination of internal organs. Transducers, and more particularly, piezoelectric transducers, are often made of ceramic or a crystalline structure. These structures are expensive to grow and even more expensive to integrate with the necessary mechanical and electrical components required for ultrasound imaging. Medical ultrasound probes cost $10,000.00 and up.
Piezoelectric transducer are polarized. That is, the fine structure of the unit cells are oriented with an electric field at high temperatures during the manufacturing process thereof. As a result, when the voltage across the transducer is changed, the various unit cells in the crystal structure strain to produce a net displacement of the material, so as to emit a pressure wave at ultrasonic frequencies. Thus, as part of the normal manufacturing process, the ceramic or crystal structure is polarized. This transducer material process is called poling and produces a material that is called poled. If the material is not polarized, the transducer generally will not work, and the transducer is not piezoelectric anymore as it will not emit ultrasound when the voltage across the same is changed because the various unit cells in the structure behave randomly and the internal strains produce no external displacement. In effect, the various groups of unit cells in the volume of the material cancel each other out when you apply the voltage. The polarization of these materials is chiefly influenced by three variables: voltage; temperature; and time.
Historically, what has been done is to transmit a square pulse which goes from 0 volts and goes negative, perhaps down to −170 volts. The magnitude of the voltage is adjustable, but it always goes negative, so if you pole the transducer with a negative DC source, then when an ultrasonic imaging device transmits, there is merely a repeating of what was done when the transducer was manufactured, and the poling will be maintained.
More recently, it has been found advantageous to preferentially receive and display echo signals that arise because of the non linear properties of tissues or contrast agents. It is not desirable at times to use the simple square pulse. More complicated pulses are advantageous, and in particular, the simplest replacement pulse would be a square wave consisting of a negative going pulse immediately followed by an equal and opposite positive going pulse—or a positive going pulse followed by an equal and opposite negative going pulse. Such a simple pulse is called bipolar. However, any pulse with both positive and negative excursions is bipolar.
One motivation to use bipolar waveforms is to reduce the amount of waveforms exhibiting second harmonic of the fundamental frequency of the desired transmitted. It is ideal to transmit with no second harmonics, and then receive the second harmonic, so that the second harmonic that is received is solely caused by an object to be imaged or the tissue non-linearities in a patient. Another non-linear strategy is to sequentially transmit a first pulse, then receive and store data about the echoes thereof, then transmit an second inverted pulse, and then receive and add data about the echoes thereof to the data previously stored. The sum is zero if the ultrasound medium and target are linear any non zero values are due to non-linearities. Since a bipolar waveform with equal plus and minus excursions, as mentioned above, has two forms—one way to implement the pulse inversion strategy is to alternate between them.
There are other advantages of using bipolar waveforms other than non-linear imaging. For example, with a bipolar waveform having equal plus and minus excursions there are no audible noises that come out of the ultrasound imaging transducer, so that a patient cannot hear any sounds from the ultrasound imaging device at low frequencies. Particularly, if the ultrasound imaging device is to be applied to a patient's head, it is desirable that the patient should not hear the ultrasound wave generated to perform the imaging.
With bipolar pulses, a voltage, opposite from the voltage by which it was manufactured, is momentarily put across the ceramic. This may cause problems, such as depoling. Certainly, at a high enough voltage for a long enough time, and alternatively, also at a high enough temperature, problems occur in that the ceramic becomes depoled. This has not proven to be a significant concern with older lower-frequency transducers that use coarse elements. However, higher-frequency transducers are desired and are currently developed. As a transducer is designed to operate at higher and higher frequencies, thinner and thinner elements are required in the transducer and for a given voltage, that means that the electric field in the transducer gets higher and higher, so that the above-noted problems will only be aggravated. Additionally, different kinds of ceramic are being developed and may be extremely sensitive to depoling.
Transducer materials exhibit capacitor-like characteristics. The manufacturing operation involves applying a DC voltage to this capacitor-like structure at some specified temperature, so that each one of the unit cells in the material has a contributing dipole moment. It has a plus charge and a minus charge that are very close together, and if a field is applied and the temperature is raised, these dipoles will align with this applied external field. Then, when the transducer is cooled down and the external field is removed, the alignment remains, so that the transducer functions as a piezoelectric element. Manufacturing processes seek to align the polarity of the unit cells of the ceramic or crystal structure in a desired direction such that their strains are additive. Any external event which causes them to become randomly aligned, or less aligned would be considered a depoling event. Any external force acting on the transducer which causes it to lose its polarization causes the transducer to become less effective, and that is undesirable.
As noted previously, if the transducer is heated to a high temperature, or if a voltage opposite to the poling voltage is applied, depoling may occur. If either the heating or the application of the opposite voltage is performed for a long period of time, the depoling effect would be worse. Further, higher frequency transducer designs with thinner piezoelectrics increase the probability of depoling.
FIG. 1
shows a conventional ultrasound imaging device. A bipolar transmitter
110
generates a bipolar signal centered around 0 volts. The bipolar signal varies between +Xv and −Xv as shown in
FIG. 2. A
transmitter/receiver (T/R) switch
120
selectively connects a transducer
130
with the bipolar transmitter
110
during a transmit cycle and to a receiver
140
during a receive cycle, so that the receiver
140
is not damaged by the high voltage of the bipolar transmitter
110
. During the receive cycle, any noise that is being emitted from the bipolar transmitter
110
is not seen by the receiver
140
. Also, the signal to the receiver
140
is not shunted by the transmitter
110
.
During the transmit cycle, the T/R switch
120
enables the transducer element
130
to receive the bipolar signal generated by the bipolar transmitter
110
so that the transducer
130
generates an ultrasound wave. During the receive cycle, the transducer
130
receives a reflected ultrasound wave back from the object being
Dillman Richard F
Miller David G
Tykulsky Alexander
Jaworski Francis J.
Koninklijke Philips Electronics , N.V.
Vodopia John
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