Multiple frequency doppler ultrasound probe

A method of measuring blood flow through a blood vessel is provided using a single quasi-continuous mode probe that can support multiple frequencies without increasing the probe tip size. A plurality of elements are provided in the probe tip. Each element emits ultrasound waves using a long pulsed signal with each element having a different resonant frequency. Each element also receives ultrasound energy in a continuous mode. A selector is manually controlled by a practitioner to select the active element. The output may take a variety of forms. For example, the output may be printed, displayed, recorded to a memory, and/or played through a speaker or headset.

FIELD OF THE INVENTION

The subject of the disclosure relates generally to ultrasound probes. More specifically, the disclosure relates to medical Doppler ultrasound probes that can operate at multiple frequencies.

BACKGROUND

To measure blood flow, a hand held probe is typically used to transmit a beam of ultrasonic energy through body tissue to a target blood vessel. Blood cells flowing through the blood vessel scatter the ultrasonic energy in many directions. A portion of the transmitted ultrasonic energy is reflected back to the probe, which receives and processes the reflected energy. In accordance with the well known Doppler phenomenon, the frequency of the received signal is different than that of the source signal due to the velocity (magnitude and direction) of the blood cells. Movement toward the probe compresses the wavelength of the reflected wave, causing an increase in the frequency. Movement away from the probe lengthens the wavelength of the reflected wave, resulting in a decrease in the frequency. This difference between the emitted and received frequencies is known as the Doppler shift. Thus, the speed and direction of blood flow within a blood vessel can be measured in a noninvasive manner using ultrasound emissions and the measured shift in frequency of the received signal. Similarly, a heartbeat, such as a fetal heartbeat, can be measured using ultrasound emissions.

With a continuous-wave (CW) Doppler ultrasound probe, a piezoelectric crystal or element contained inside the probe tip continuously transmits an ultrasonic beam that is reflected by the circulating red blood cells. A separate crystal in the tip continuously receives the reflected sound waves. The transmit and receive crystals are often made from a circular element that has been cut down the middle into two semi-circle shaped elements. The two semi-circles are fixed side by side inside the probe tip with a slight angle to each other to form an intersection of the beam patterns in a patient. Alternate arrangements include using two side-by-side square crystals or a central disk surrounded by an annular ring element. Processing is done on the received signal to extract the Doppler shift frequency. Simplicity of design, ease of use, and low power consumption make CW Doppler the typical choice for small battery powered applications. Also, sensitivity of CW Doppler is typically high because damping of the crystals is not required as known to those skilled in the art

The useful operating frequency range for Doppler ultrasound probes is typically 2-10 megahertz (MHz). The required depth of penetration in body tissue determines the operating frequency based on well-known attenuation effects as a function of frequency. A lower probe frequency provides deeper penetration of the body tissue. Thus, in the medical field, probes having frequencies from about 2 to about 3 MHz may be used to detect deep blood flow, fetal blood flow, or intracranial blood flow due to their deeper penetration of body tissue. Probes having frequencies from about 4 to about 5 MHz may be used to detect vascular blood flow, for example, in the neck, arms, or legs. Probes having frequencies from about 8 to about 10 MHz may be used to detect blood flow in vessels near the skin or in intraoperative applications.

The transmitting piezoelectric crystal is electrically stimulated to produce an ultrasound signal at a specific frequency, for example 2, 3, 4, 5, 8, etc. MHz. The crystal has geometrical and material characteristics that define a specific resonant frequency. CW crystals are typically used undamped with a narrow bandwidth and high Q factor. Operating the undamped crystal at its resonant frequency creates the most efficient ultrasound transmitter and requires the lowest energy power source. Conversely, an undamped receiving crystal is most efficient at producing a voltage when deformed by pressure at or near its resonant frequency. An efficient receive crystal reduces ultrasound exposure risks by allowing lower ultrasound energy to be transmitted into tissue. To change the operating frequency during use, for example from 2 to 3 MHz or from 5 to 8 MHz, a CW ultrasound probe is typically replaced with a probe designed for the desired frequency. Alternatively, the probe can be designed with damped or backed crystals to provide a wider bandwidth of operation and multiple frequencies, but with reduced efficiency due to the wider bandwidth. Additional crystals can be mounted in the probe. For example, two 5 MHZ and two 8 MHz crystals can be mounted in the probe tip. However, the resulting increase in the size of the probe tip make it potentially awkward for a practitioner to use. Thus, a practitioner must carry and manually switch between multiple probes, accept use of a probe having a reduced sensitivity and high transmit power, or use a bulky probe including multiple crystals to provide blood flow measurements at multiple frequencies. What is needed therefore is a system that provides multiple frequencies selectable for optimal signal acquisition in a single probe without reduced sensitivity or loss of Doppler signal. What is further needed is a system that provides the multiple frequencies with little or no increase in the size of the probe tip.

SUMMARY

An exemplary embodiment of the present invention provides a single Doppler ultrasound probe that can operate at multiple frequencies for measuring blood flow without increasing the probe tip size above that of a single frequency CW probe. Multiple elements of differing resonant frequencies are provided in the probe tip. In an exemplary embodiment, each element is a piezoelectric crystal. Each element can transmit ultrasound waves using a pulsed signal and receive ultrasound waves continuously. A switch is manually controlled by a practitioner to select the optimum probe frequency dependent on the application. The Doppler shift output may be presented to the user in a variety of forms. For example, the output may be printed, displayed, recorded to a memory, and/or played through a speaker or headset. The manual selector is operatively connectable to a plurality of elements.

A method of determining blood flow velocity through a blood vessel or a heart rate is provided. Using a selector, a practitioner manually selects an element to provide a measured output. The method includes, but is not limited to, receiving a frequency selection from a manual selector, wherein the frequency selection identifies an element from a plurality of elements; generating a pulsed transmit signal; emitting energy from the identified element toward an object in response to the generated pulsed signal; receiving a reflected signal from the object at the identified element; processing the received signal to determine a characteristic of the object; and outputting the characteristic of the object. For example, the object may be a blood vessel and the characteristic may be a blood flow velocity or a heart rate.

Another exemplary embodiment of the invention includes an ultrasound probe capable of implementing the operations of the method and including a plurality of elements. Another exemplary embodiment of the invention includes an ultrasound system including the ultrasound probe.

DETAILED DESCRIPTION

Ultrasound refers to the use of ultrasonic waves or waves with a frequency over 20 kilohertz (kHz). For use in Doppler medical devices, sound waves are transmitted through body tissues using a probe. The probe is placed directly on top of the skin, which generally has a coupling gel applied to the surface. The sound waves are reflected by different body tissue and blood as “echoes.” Portions of the echoes return to the probe and are processed to determine the Doppler shift frequencies. The results are sent to an output media.

With reference toFIG. 1, a side view of an ultrasound probe20having a manually selectable operating frequency in accordance with the invention is shown. Ultrasound probe20may include a housing22, a probe neck24, and a probe tip26. Housing22houses the electronics for operating ultrasound probe20and is held in the hand of a practitioner. Housing22may have a different size and shape to accommodate different applications and may vary depending on the size and arrangement of the electronics. Probe neck24connects housing22to probe tip26and has a generally tapered exterior surface. The size and shape of probe neck24may vary depending on the relative size and shape between housing22and probe tip26. Probe tip26houses a plurality of elements that each emit and receive pulsed energy at different ultrasound frequencies. In an exemplary embodiment, each element is a piezoelectric crystal. Probe neck24houses electrical wires that connect the elements in probe tip26to the electronics in housing22. Different and additional components may be included with ultrasound probe20. For example, ultrasound probe20may include one or more power source, various connectors, a display, a printer, a speaker, etc. Alternatively, ultrasound probe20may connect with a separate device that houses the additional components and optionally the electronics.

Housing22includes a frequency selection interface30. Frequency selection interface30may be mounted on any side of housing22and may include a manual switch32, a first indicator34, and a second indicator36. Manual switch32may be any type of switch as known to those skilled in the art both now and in the future. Manual switch32provides a mechanism by which the practitioner selects an operating frequency (or active element) of an ultrasound probe20. In an exemplary embodiment, first indicator34is a light emitting diode (LED) that is “on” when the switch selects the element indicated by a first frequency37. In an exemplary embodiment, second indicator36is a light emitting diode (LED) that is “on” when the switch selects the element indicated by a second frequency38. As known to those skilled in the art both now and in the future, other methods for indicating a frequency selection may be implemented with ultrasound probe20. Manual switch32and the indicators provide a means for a practitioner to select from a plurality of elements having unique resonant frequencies. Additional manual selectors are possible including those that allow selection from among three or more elements. For example, to select from three or more elements, a dial or a sliding switch may be used as a manual selector. Frequency selection interface30may also be located on a separate device connected to the probe.

With reference toFIG. 2, a cross sectional view of probe tip26is shown along axis2-2ofFIG. 1. Probe tip26includes a first element40and a second element42mounted adjacent each other. Probe tip26may also include additional elements. First element40and second element42are shaped in a semi-circle. Alternative shapes are possible with the effect of changing the distribution of energy emitted by the element. With reference toFIG. 3, a cross sectional view of probe tip26is shown along3-3ofFIG. 2. First electrical wires44connect first element40with the electronics housed in housing22. Second electrical wires46connect second element42with the electronics housed in housing22.

First element40and second element42contain a piezoelectric material to generate the ultrasound pulses. First element40and second element42are used both for generating the ultrasound pulses and for receiving the echoes that result from energy reflected back to the element from the target object. When the piezoelectric material is subjected to an electrical voltage, it undergoes a change in dimension depending on the polarity of the voltage. Alternating voltage near the resonant frequency of the element produces ultrasound pressure waves. Conversely, when a reflected pressure wave strikes the piezoelectric material, it causes mechanical deformation of the piezoelectric material which produces an electrical voltage. Piezoelectric materials include natural and synthetic materials such as quartz, ceramics, polymers, etc. Piezoelectric materials can be manufactured in many different shapes and sizes.

Piezoelectric materials typically resonate within narrowly defined frequency ranges. Operating the element undamped at or near the resonant frequency is most efficient, and thus, requires the lowest operating power. First element40and second element42have distinct resonant frequencies selected by the practitioner based on the clinical need. First element40and second element42may combine any pair of resonant frequencies over the ultrasound spectrum. In an exemplary embodiment, the ultrasound spectrum utilized extends from about two megahertz (MHz) to about ten MHz. In a first exemplary embodiment, first element40has a resonant frequency of about five MHz while second element42has a resonant frequency of about eight MHz. In a second exemplary embodiment, first element40has a resonant frequency of about two MHz while second element42has a resonant frequency of about three MHz. The resonant frequencies may be varied depending on the particular embodiment. For example, instead of two MHz, the resonant frequency may be about 2.25 MHz.

In order to use the same element for both transmission and reception of ultrasound energy, a quasi-continuous or long pulse mode of operation is used. The transmit signal is gated on and off at a 50% duty cycle. For simplicity of operation and design, the receive signal is not gated, and thus, return energy from all tissue depths is processed. The pulse repetition frequency (PRF) of the transmit signal is determined by the required operating depth. Because the same element both emits and receives the energy, the transmission time must be coordinated based on the expected depth of the target object (and resulting delay time before the reflection returns back to the element) resulting in a range of possible pulse repetition rate or frequencies (PRFs) based on the frequency selected. Using a frequency range from about two MHz to about ten MHz, an exemplary PRF range extends from about five kHz to about 125 kHz. Using a frequency range from about five MHz to about eight MHz, an exemplary PRF range extends from about 62 kHz to about 63 kHz. Preferably, using a frequency range from about five MHz to about eight MHz, the PRF is 62.5 kHz. Using a frequency range from about two MHz to about three MHz, an exemplary PRF range extends from about five kHz to about six kHz. Preferably, using a frequency range from about two MHz to about three MHz, the PRF is 5.3 kHz.

In alternative embodiments, probe tip26may include a lens mounted in front of first element40and/or second element42to focus or to defocus the energy emitted from either element. For example, a lens formed of plastic material may be arranged in front of first element40and/or second element42to narrow the beam of emitted energy to assist in locating a target blood vessel. Additionally, probe tip26may use a single or multilayered waveplate in front of first element40and second element42to reduce the acoustic impedance mismatch at the probe/tissue interface. In use, first element40and second element42emit through a coupling medium such as a gel that is placed on the surface of the medium to be analyzed. For example, a gel is placed on the skin of a patient, and the probe tip26is placed on top of the gel.

With reference toFIG. 4, electronics50in accordance with an exemplary embodiment are shown. Electronics50include electronic switch52, a first element circuit55, a second element circuit57, a filter84, and an output90. Setting of electronic switch52between a first position54and a second position56is controlled by manual switch32. First position54selects first element circuit55. Second position56selects second element circuit57. In the exemplary embodiment ofFIG. 4, electronic switch52activates only one of first element circuit55or second element circuit57disabling the other circuit55,57. First element circuit55may include a first oscillator circuit60, a first transmit amplifier64, first element40, a first receive circuit72, a first mixer76, and a first amplifier80. Second element circuit57may include a second oscillator circuit62, a second transmit amplifier66, second element42, a second receive circuit74, a second mixer78, and a second amplifier82.

First oscillator circuit60and second oscillator circuit62produce a series of pulses at a pre-selected PRF and center frequency. The center frequency is approximately equal to the resonant frequency of the respective elements40,42. First transmit amplifier64and second transmit amplifier66amplify the high frequency oscillation output of first oscillator circuit60and second oscillator circuit62, respectively, and provide a high frequency voltage to first element40and second element42, respectively, while providing high impedance during receive. The high frequency voltage is converted to ultrasound emitted by first element40or second element42toward an object of interest. For example, the ultrasound energy is emitted toward a blood vessel or a heart.

A portion of the reflected ultrasound is received by first element40or second element42and is converted into electronic signals received at first receive circuit72and second receive circuit74, respectively. First receive circuit72and second receive circuit74provide electrical matching, limiting, and signal gain. The Doppler shift of the reflected signal is detected using first mixer76and/or second mixer78. In first mixer76and second mixer78, the respective received signal is electronically mixed with the high frequency input signal of first oscillator circuit60and second oscillator circuit62, respectively. By mixing the two sound waves, four frequency components are obtained: 1) the frequency of the transmitted signal, 2) the frequency of the reflected signal, 3) the frequency of the sum of the two signals, and 4) the frequency of the difference between the two signals. The difference signal includes the Doppler shift frequency that is proportional to the relative velocity of the target object. In the exemplary embodiment ofFIG. 4, electronic switch52selects either first mixer76or second mixer78to minimize noise from the unused element circuit. First amplifier80and second amplifier82buffer and scale the mixer output as required. Filter84may include a low pass filter to remove higher frequency components created as a result of the pulsed mode of operation and a high pass filter to remove low frequency noise and low frequency Doppler components. In alternative embodiment, a bandpass filter may be used. Output90receives the filtered difference signal.

With reference toFIG. 5, the filtered signal may be output from ultrasound probe20in a variety of forms. Some or all of the various forms may be implemented within housing22of ultrasound probe20. Alternatively, ultrasound probe20may connect to a separate device that includes some or all of the various forms of output media. For example, ultrasound probe20may connect using various wired or wireless media to a separate device. Output structures include, but are not limited to, a speaker94, a headset96, a printer104, a display106, and a memory102. The output signal may be fed to an audio amplifier92that provides its output to speaker94and/or headset96. Simultaneously, the output filtered signal may be provided to analog-to-digital converter (ADC)98which provides digital output data to a processor100, which performs real time buffering and signal processing, manages communications with the user, and executes instructions.

Processor100executes instructions that may be written using one or more programming language, scripting language, assembly language, etc. The instructions may be carried out by a special purpose computer, logic circuits, or hardware circuits. Thus, processor100may be implemented in hardware, firmware, software, or any combination of these methods. The term “execution” is the process of running an application or the carrying out of the operation called for by an instruction. The output data from processor100can be provided to printer104, display106, and/or memory102. The information displayed on display106, recorded on printer104, and stored in memory102can take various forms as known to those skilled in the art both now and in the future.

For example, because not all blood cells in the sample volume are moving at the same speed, a range or spectrum of Doppler shifted frequencies are reflected back to ultrasound probe20. Thus, the signal received at ultrasound probe20may be processed to produce a velocity profile of the blood flow, which varies over the period of a heartbeat to produce a beat-to-beat flow pattern on a display. Color coding may be used to indicate the proportion of blood cells flowing within that particular velocity range. The information displayed on the video screen can be used by a trained observer to determine blood flow characteristics at particular positions within the blood vessel of the individual being tested, and can detect anomalies in such blood flow, for example, the possible presence of a blockage or restriction, or the passage of an embolus through the artery.

As known to those skilled in the art, electronic switch52may be located at a different position within element electronics50. For example, both element circuits55,57may transmit and receive simultaneously, and electronic switch52may select which element circuit drives output90. Using this alternative embodiment, each element circuit includes a separate filter84and only switches the output line connecting to output90.

The foregoing description of exemplary embodiments of the invention have been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. Additional circuits and/or instructions may be added to improve the signal quality, integrated chips may be used to perform multiple or all functions together, etc. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.