Note: Descriptions are shown in the official language in which they were submitted.
Patent Application
of
Jon W. Erickson
for
s OPI`ICAL LEVER ACOUSTIC AND IJLTRASOUND SENSOR
FELD OF T~E INVENTION
The present invention relates generally to diagnostic medical instrumentation and, more
particularly, to ultrasonic transducers used in medical imaging. A primary objective of the
1 Q present invention is to provide a robust, low-cost compact transducer with greatly increased
sensitivity to ultrasound signals. -
BACKGROUND OF THE INVENTION
A typical ultrasonic imaging system makes use of one or more piezoelectric transducers
which act as the source ~actuator) of the ultrasonic beam or signal, and which often also serve
to sense the reflected signal (sensor). An electrical pulse generated by an electronic control
module is converted to an ultrasonic pulse by the transducer/actuator in the probe. The probe
is in contact with the body, and the ultrasonic pulse is transmitted through the probe into the
body. The pulse is then absorbed by body tissues or reflected to different degrees from the
2 o boundaries between body tissues. The reflections reach the transducer/sensor at different
times, which vary with the distance to the tissue boundaries. The reflections also have
different energies, due to the different acoustic impedances of the tissues, as well as absorption
by the intervening tissues. The transducer/sensor converts the reflections into a weak electrical
signal, which contains information that can be processed into an image of the body.
2 5 A great variety of ultrasonic transducers are presently in use or under development.
Shapes and sizes vary widely in order to meet special needs. Focusing by electronic or
mechanical means, or some combination thereof, can be used to produce and sleer a narrow
ultrasonic beam of desired focal length. Likewise, mechanical and electronic focusing can be
used to sense the reflections from a particular direction and distance. Phased transducer arrays
3 o of various configurations have been employed to achieve particular focusing properties, under
electronic control. (The term "phased array" is taken from radar technology, in which the - ~ -
phase relationships of signals from multiple antennae are processed electronically to improve
resolution and sensitivity.) The acquired signal is then converted into an image using analog
or, depending on cost and technological considerations, digital processing.
3 5 &ood resolution of ultrasound images is important for medical applications. Some
limits to resolution are fundamental to the physics of wave propagation (for example, acoustic
shadows and reverberations, and geometric artifacts) and are best dealt with by educating the ~ : -
user, or by appropriate image processing algorithms. Other factors affecting resolution involve ~ ~ ~
tr~msducers and electronic instrumentation (such as axial and lateral resolution, and dynamic
range) and are susceptible to improvement.
Axial resolution can be limited in part by the wavelength of the ultrasonic signal
("ultrasound" simply designates sound waves of a frequency above the audible range, with
s wavelengths of millimeters or less). Absorption of ultrasonic energy by body tissues tends to
restrict the useful depth of f1eld to about 200 wavelengths, due to attenuation of the signal.
Thus resolution can be improved by use of shorter wavelengths (higher frequencies) but this
also implies a shallower depth of field.
For a simple system with a single element and spherical or parabolic focusing, the
I o lateral resolution is limited by the aperture of the transducer. Larger apertures provide greater
resolution but shallower depth of field. The size of the transducer element or elements also can
lirnit the resolution, since the detected signal will be known to originate from a given
transducer but not any particular location on that transducer.
The dynamic range of the instrument determines the useful number of gray scale levels
in the image. Most commercial transducers use piezoelectric crystal elements or other materials
(e.g. piezoelectric polymers such as polyvinylideneflouride) both as actuators which produce
the ultrasonic pulse, and as sensors which detect the reflected signal. The physics and
engineering of piezoelectric sensors are relatively well understood. The sensitivity of a simple
piezoelectric sensor, such as a small block of quartz, can be greatly improved by use of a more
2 o complicated geometry, the "piezoelectric bimorph" shape. The bimorph has been used since
1930 in microphones and phonograph needle assemblies, but various design considerations
such as high cost and fragility preclude its use in ultrasound transducers.
An alternative means of sensing small deflections or increments of motion is the optical - ~ .
Iever. Optical levers have proven to be effective in routine measurements of extremely small
2s deflections, of less than 0.01 nanometer, in atomic force microscopy (AFM). This
measurement strategy can be implemented in robust ultrasound transducers at lo~y cost, with
great flexibility in design.
Accordingly, this patent application presents an acoustic sensor which uses the optical ;
lever principle to amplify ultrasonic signals. The patent application also presents certain
3 o improvements to the optical lever acoustic sensor which use an optical amplification means to
improve the amplification of the sensor. The signal arnplification provided by the basic optical
lever acoustic sensor is dependenE on the geomeEry of the optical lever. Even though the
optical lever arrangement presented is capable of very high signal amplification, there are
practical limitations to the level of amplification possible before the size of the sensor becomes
3 5 unwieldy. The improvements presented overcome these limitations by using the optical
amplification means to amplify acoustic or ultrasonic signals to an even greater degree without
significantly increasing the size of the sensor.
. . . ,.. ... . :. ~ , . ,
SUMMARY OF T~E INVENTION
The primary objective of the present invention is to provide an acoustic and ultrasound
sensor which uses the optical lever principle to amplify ultrasonic signals. The optical lever
acoustic sensor provides increased sensitivity to acoustic signals and an improved signal-to-
noise ratio in a low cost, highly robust transducer.
It is a further objective of the present invention is to provide an optical lever acoustic
and ultrasound sensor with increased sensitivity to acoustic signals. It is an important part of
this objective to provide the desired increase in sensitivity by an optical amplification means
which amplifies the motion of an incident acoustic wave and converts it to an electical signal
for image processing. An optical amplification means is preferred because it is not subject to
the same signal-to-noise lirnitations as the electronic amplifiers typically used in the prior art.
The optical amplification means is also capable of operating in harsh environments with high
levels of electromagnetic interference that would render prior art electronic amplifiers
ineffective, since they amplify the electronic noise as well as the signal.
It is also an objective of the invention to provide a compact acoustic sensor where the
optical amylification means does not significantly increase the overall size of the sensor. At the
same time, it is an objective to provide a highly sensitive acoustic sensor that is both robust and
low cost to manufacture.
In distinct contrast to the piezoelectric transducer/sensors of the prior art, the present
2 o invention proposes the use of an optical lever to detect the ultrasonic reflections. The optical -
lever makes use of a beam of light shining at an oblique angle on a mirrored surface (e.g., a
membrane or piston) in good acoustic contact with the ultrasonic medium. The reflected beam
of light is directed onto a position-sensitive detector. Small movements in the mirrored surface -
result in relatively large changes in the position where the beam of light strikes the detector. -
2 5 The position-sensitive detector is insensitive to fluctuations in the light intensity, which lowers
the overall costs ~especially in arrays of such sensors). The size of the sensor and of the
transducer as a whole can also be reduced considerably, since all the components can be
fabricated with microelectronic techniques.
In addition, the present invention proposes several improvements to the basic design of
3 o the optical lever acoustic sensor which include an optical amplification means for improving the
sensitivity of the sensor. Three different approaches to the optical amplification are disclosed
which can be used separately or in combination with one another.
In the first approach to optical amplification, the vibrating mirror M1 is made part of a
cantilever arrangement that increases the angular deflection of the incident light beam. The
3 5 acoustic energy impinging on the large area of the diaphragm is transferred into the small area
of the post. The post is connected to the cantilevered mirror M1 at a point close to the hinge, in
order to increase the angular deflection for a given increment of vertical deflection, and hence
the overall sensitivity.
In the second approacll to optical amplification, a second, stationary mirror M2 is
positioned approximately parallel to the vibrating mirror surface. The reflected beam of light is
directed by the stationary mirror M2 back onto the vibrating mirror M1, and picks up a second
increment of inforn~ation about the acoustic signal with each increment in angular deflection.
s Even more reflections may be included so as to increase the total number of signal increments,
the total signal in the light beam being proportional to the number of times the light beam has
been reflected from the vibrating surface M1.
In the third approach to optical amplification, the effective moment of the optical lever
is increased within a small volume by the use of additional stationary n!lirrors M3 and M4. The
0 stationary mirrors are introduced to increase the length of the path which the reflected beam of
light must follow before arriving at the position-sensitive detector. This increases the relative
movement of the light beam on the surface of the position-sensitive detector and, therefore, the
overall sensitivity of the sensor.
Furthermore, the following improvements are made to the sensor so that it can take
s fuller advantage of optical amplification methods just described. The incident beam of light is
focused by a lens between the light source and the vibrating mirror M1, so that the focal point
is in the plane of the position-sensitive detector. The smaller spot size and greater intensity of
the incident light offers the potential for greater detector sensitivity.
Under some conditions, it may be desirable to direct two or more beams of light at a
2 o single vibrating surface M1. One example is when the light beams are chopped so phase- .
Iocked loops can reduce the signal bandwidth, and increase the signal-to-noise ratio. Two or
rnore frequencies may be sampled simultaneously by directing two or more independently
chopped light beams onto a single surface M 1. Electronic cross-talk may be minimized by the
use of a separate position-sensitive detector for each beam.
2 5 Small movements in the mirrored surface M1 result in relatively large changes in the
position where the beam of light strikes the detector. The position-sensitive detector is
insensitive to fluctuations in the light intensity, which lowers the overall costs (especially in
arrays of such sensors). The size of the sensor and of the transducer as a whole can also be
reduced considerably, since all the components can be fabricated ~,vith microelectronic
3 o techniques. Other objects and advantages of the invention will no doubt occur to those skilled
in the art upon reading and understanding the following detailed description along with the
accompanying drawings.
BREF DESCRIPTION OF THE D~AWINGS
3 5 Figure 1 shows a perspective schematic of the optical lever ultrasound sensor.
Figure 2 shows a perspective schematic of a sensor array using the optical leverultrasound sensor.
Figure 3 shows a cutaway perspective of an ultrasound sensor element having a solid
reflective membrane.
Figure 4 shows a cutaway perspective of an ultrasound sensor element having a
polymer reflective membrane.
Figure S shows a cutaway perspective of an ultrasound sensor element having a
cantilever with a light reflective surface.
Figure 6 shows a cutaway view of an ultrasound sensor element having a cantilevered
light reflective surface which gives additional sensitivity by increasing the angular deflection
for a given increment of acoustic pressure.
Figure 7 shows a multiple-bounce arrangennent in which the light beam is redirected
onto the vibrating mirror surface M1 by a second, stationary rnirror M2.
0 Figure 8 shows the use of two additional stationary mirrors M3 and M4 to "fold" a
long lever arm into a more compact volume.
Figure 9 shows two independent light beams striking a single vibrating surface, and ~ -
being detected by two independent position-sensitive detectors.
I s DESCRI~ION OF THE PRl~FERRED ~MBODIMENTS
Figure 1 shows a schematic view of an optical lever ultrasound sensor built in .accordance with the present invention. A light source 11 is used to generate a narrow beam of :
collimated light 15 which is directed toward a reflective surface 13 at an acute angle to the
surface. In the preferred embodiment, the light source 11 is a laser light source and a single
mode optical fiber 12 directs the beam of light 15 onto the reflective surface 13. Alternatively,
a source of collimated light other than a laser may be coupled to the optical fiber 12, or a laser
light source, for instance an integrated AlGaAs/GaAs diode laser, may be used to direct a beam
of light 15 directly onto the reflective surface 13 without the use of an optical fiber 12.
The reflected light beam 16 from the reflective surface 13 stIikes a position-sensitive
2 s light detector (PSD) 17, which generates a signal indicative of the position at which the beam
of light 16 strikes the PSD 17. The reflective surface 13 is coupled to a membrane 10 which
moves in reaction to an incident ultrasonic wave 14. When the membrane 10 is at rest, the
reflected light beam 16 strikes somewhere near the center of the PSD 17. The small
movements of the reflective surface 13 due to the incorning ultrasonic wave 14, result in large
3 0 movements of the position at which the reflected light 16 strikes the PSD 17. The PSD 17 is
sensitive to movements of greater than 5 nm in the location of the spot of light on it. The
deflection of the reflective surface 13 is thus amplified by this optical lever, the amplif1cation
being determined by the distance of the PSD 17 from the reflective sensor surface 13.
The output of the PSD 17 is a voltage signal which varies in proportion to the position
3 s of the light spot on the PSD surface, which in turn is proportional to the amplitude of the
vibrations of the reflective sensor surface 13, and to the amplitude of the ultrasonic pressure
wave 14. The signal has a very low level of noise due to the measurement process or strategy.
The PSD 17 output is processed by the imaging electronics 18, either as a singleelement or as one channel of an array of sensors. The leading edge of the incident pulse may
be used in such an array to electronically focus on the position of the echo source. This
positional information is then used to build up an image of the objects or tissue interfaces
responsible for the echoes.
Figure 2 shows one manner of constructing an array of ultrasound sensors using the
principle of the optical lever. A light source 11, preferably a laser light source, generates a
collimated beam of light which is coupled to a bundle of optical fibers 19. Each of the optical
fibers 12 within the bundle 19 directs a narrow beam of light 22 onto sne of the reflective
sensor elements 13 within an array of sensors 20. Each beam of reflected light 23 strikes one
of the PSD elements 17 within a PSD array 21. The PSD array 21 may be made from a
o number of separate PSD elements 17, or a large scale integrated array of detectors may be
manufactured on a single chip.
Each set of one optical fiber 12, one reflective sensor element 13 and one PSD element
17 is analogous to the single sensor shown in Figure 1. Thus a number of sensors can be
integrated together to form a linear array, a square aIray or other desired geometries of sensor
arrays.
Figure 3 shows one preferred embodiment for the reflective sensor elements for asingle ultrasound sensor or an array of ultrasound sensors. A substrate 24, which may be a
metal, ceramic, polymer or other material, is etched or machined to form a thin membrane 26.
The extent of the membrane ~6 determines the aperture 25 of the sensor. A reflective surface
2 0 13 is coupled to the back of the membrane 26. The reflective surface 13 may be simply the
polished rear surface of the membrane 26, or the membrane 26 may be metalized to provide a
reflective surface 13.
The material of the substrate 24 and the membrane 26 may be chosen so that it has the
p~oper combination of density and stiffness to match the acoustic impedance of the acoustic
2 s medium to be imaged. Alternately, other well known techniques, such as quarter wave
matching layers, can be used to provide good acoustic coupling. The space behi~d the
membrane 26 may be filed with a damp;ng material to prevent excessive ringing of the sensor.
Figure 4 shows another preferred embodiment of the reflective sensor element. Anaperture 29 is formed in a substrate 27 by etching, machining or other methods. A membrane
3 o 28, which is a thin layer of metal, polymer or other material, is placed over the aperture 29. A
reflective surface 13 is formed on the back of the membrane 28, for instance, by polishing or
metalizadon. The material of the membrane 28 may be chosen to match the acoustic impedance
of the imaging medium. An advantage of this design is that the substrate material 27 may be
chosen solely for its structural properties since it does not need to have the same ac~ustic
3 5 properties as the membrane 28. Again a damping material may be added to prevent excessive
ringing in the sensor.
Figure S shows a third preferred embodiment of the reflective sensor element that
combines a membrane 31 with a cantilever 33. An aperture 32 is formed in a substrate 30. A
cantilever 33 mounted on the substrate 30 contacts the membrane 31 near the middle of the
aperture 32 by means of a stylus 34 or other coupling link. A reflective surface 13 is formed
on the back of the cantilever 33.
In this design, the acoustic impedance of the sensor is determined by the combined
mass of the cantilever and the membrane, and the combined stiffness of the cantilever and the
s membrane. This allows additional flexibility in the design of the sensor for matching
impedance and for tuning the sensitivity of the sensor. The cantilever can also be used to
linearize the pressure response of the sensor. If the response of the membrane sensor by itself
does not obey Hooke's law, a cantilever with the desired force constant may be added to
improve the sensor's linearity.
o Figures 6-8 show embodiments of the present invention which use an optical -
amplification means to increase the amplification of the optical lever acoustic sensor. Each of
the improvements which will be described can be used ~n combination with any of the sensor
embodimentsdescribed above.
Figure 6 illustrates the present invention using a first approach to optical amplification
of the acoustic signal. This approach uses an arrangement of the cantilever which offers more
sensitivity at relatively little cost. In analogy to the leverage provided by the malleus and stapes
of the human ear, this mechanical arrangement concentrates the acoustic energy impinging on
the large mernbrane 42 into the smaller area of the post 44. The top of the post 44 pushes up
the cantilevered mirror 46 at a location close to its hinge 48. This forms a class three lever
2 0 which maximizes the angular deflection of the reflective surface for a given amplitude of
movement in the membrane. The increased angular deflection of the reflective surface results
in greater relative movement of the reflected light beam on the PSD which increases the overall
amplification of the sensor. The design is compatible with contemporary silicon/silicon
dioxide micromachine technology, and permits impedance matching at particular frequency
2 s ranges of interest.
In this design, the acoustic impedance of the sensor is determined by the combined ~ -
mass of the cantilever and the membrane, and the combined stiffness of the cantilever 50 and ~
the membrane 42. This allows additional flexibility in the design of the sensor for matching ~ ~ ~ q
impedance and for tuning the sensitivity of the sensor. The cantilever can also be used to
3 0 linearize the pressure response of the sensor. If the response of the membrane sensor by itself
does not obey Hooke's law, a cantilever with the desired force constant may be added to
improve the sensor's linearity.
Figure 7 shows an embodiment of the present invention which uses a multiple bounce
approach to optical amplification of the acoustic signal. The light beam (focused onto the
3 5 detector by lens Ll) strikes the vibrating mirror surface M1, and then is reflected onto a
stationary mirror M2. The light beam is reflected from M2 back onto the vibrating surface M1
again. Each reflection from M1 adds an increment of signal to the light beam, in the forrn of
angular deflection, so that after two such reflection or bounces the amplitude of the angular
signal is doubled. With high-reflectivity mirrors, many such bounces can be achieved with
.. ,. .,. . . : . .. .
relatively little loss of light intensity. The greater mgular deflection of the incident light beam
for a given amplitude of movement of the vibrating mirror surface M1 results in greater relative
movement of the reflected light beam 16 on the PSD 17 which increases the overall
amplification of the sensor.
s The PSD 17 is only sensitive to the position of the light beam, not the intensity, so
some loss in the intensity of the light beam due to the multiple reflections can be tolerated
without affecting the overall sensitivity of the acoustic sensor. The mirrors may be metal films
(such as Cr, Cu, Ag, or ~u) which can achieve reflectivities of about g9.4%. After 100
bounces with 99.4% reflectivity the light beam intensity would be 54.78% of the original
o value, which is quite acceptable. If greater numbers of bounces are recluired to achieve the
desired amplification, multilayer films (such as of alternating Si and SiO2 layers with quarter- .
wavelength matching) may be used to increase the reflectivity to about 99.93%. After 100~)
bounces with 99.4% reflectivity the light beam intensity is reduced to about 0.24% of the
original value, while with 99.93% reflectivity it retains approximately 49.6% of the original "
value. Moreover, scattering and absorption are reduced by using multilayer films.
Figure 8 illustrates the third approach to optical amplification of the acoustic signal by
increasing the 'ilever arm" of the optical lever. As mentioned previously, the amplification of
the sensor is partiLally determined by the length of the "lever arm", which is the distallce of the
PSD 17 from the reflective sensor surface 13. If sensor size were of no concern, the
2 o sensitivity of the acoustic sensor could be increased by simply making this distance larger. In
practice, however, this approach would eventually make the overall size of the sensor very
unwieldy. In order to overcome the problem of size, this embodiment of the invention
provides two stationary mirrors M3 and M4 which are used to "fold" a long lever arm into a
more compact volume. Doubling the path length the reflected light beam travels between the
2 s reflective sensor surface 13 and the PSD 17 doubles the relative movement of the light beam on
the PSD, thereby doubling the amplification provided by the optical lever. As in ~he previous
embodiment, with highly reflective mirrors the amplification of the sensor can be increased
tremendously by multiple reflections of the light beam to increase the length of the lever arm.
Figure 9 shows two independent light beams 64, 66 striking a single vibrating surface
3 o 68, and being detected by two independent position-sensitive detectors 60, 62. This illustrates
the use of multiple independent phase-locked loops, each chopping a light beam at a unique
frequency, to measure a given frequency component of the acoustic excitation. The signal-to-
noise ratio can be improved significantly by decreasing the bandwidth of the detection
circuitry. The comparison of two (or more) measurements at different frequencies can be used
3 5 to eliminate certain kinds of artifacts in ultrasonic images. Finally, ~wo (or more) detection
frequencies are of great utility in the Doppler or color Doppler modes of medical ultrasonic
imaging. Implementation of separate phase-locked loops in the sensor hardware may better
achieve the optimal results, of narrow bandwidths at multiple frequencies.
It is worth noting that, while Figure 9 shows how multiple light beams can be directed
at different azimuthal angles onto a vibrating mirror, it is also possible to use multiple light
beams of different wavelengths on a vibrating grating to achieve an analogous result in
reciprocal space.
S
TECHNICAL DISCUSSION
SENSlTIVITY.
A typical piezoelectric sensor may have a sensitivity, measured in units of power per
area, on the order of 10-7 Watt cm-2. When operating at a recommended biological threshold
o limit of about 10-2 Watt cm-2, signal attenuation due to absorption by b;ological tissue limits
the depth of view to about 200 wavelengths. For a 3 MHz signal, a 10 cm depth corresponds
to a loss of about 5 orders of magnitude in signal strength. -
In contrast, an optical lever sensor can detect signals of less than 10-l8 Watt cm-2.
(This corresponds to a routine situation in AFM involving a deflection of 0.01 nm against a
force constant of 2 Newton m-l, measured in less than 10-3 second.) Thus an initial signal of
10-2 Watt cm-2 will in theory still be detectable even after it has been attenuated by 16 orders of
magnitude. With the additional improvements to the optical lever acoustic sensor described
above, the practical limits of sensor sensitivity can be pushed even closer to the theoretical
limit, in cost effective ways. - '
2 o The increased sensitivity (with respect to convendonal piezoelectric transducers) can be
used in several different ways. The size of the sensor may be reduced, which may have
advantages in terms of image resolution (both axial and lateral). The dynamic range of the
acquired signal may be increased, which can be used to improve image quality. The power of
the initial signal may be decreased, which may be a consideration for examination of certain -
2 5 kinds of biological tissue (e.g. eyes, embryos). Shorter wavelengths of ultrasound may be
used while still viewing depths of at least 10 cm, which would improve axial reso,lution.
DYNAMIC RANGE.
In practice, it is convenient to limit the dynamic range to 12 orders of magnitude or
less. The practical constraints on dynamic range are the amplitude of the deflection produced ;~
3 0 by the ultrasound excitation of the membrane, diaphragm, or piston; and the size of the
position-sensitive detector (PSD). A nearly linear response of the vibrating surface to the
excitation is desirable, and this will constrain the acceptable amplitude.
Should larger amplitudes be acceptable for the vibrating surface in a given
implementation, it may be useful to adjust the sensitivity. Range switching is accomplished
3 s relatively easily, by shortening the lever arm, or moving the PSD closer to the point of
reflection. A typical commercially available PSD is about S mm in length and can distinguish
positions of incident light that are separated by more than about 5 nm. This gives a dynarnic
range of about 6 orders of magnitude in amplitude, or about 12 orders of magnitude in
intensity. If the distance to the point of reflection is shortened by a factor of 100, the
`" 10
sensitivity will be less, but signals 100 times larger in amplitude (or 10,000 in intensity) may
be measured.
The improvements may permit better or more convenient utilization of the dynamicrange. For example, the mirrors M3 and M4 in figure 4 may be adjusted to alter the lever arm.
s The number of bounces involving mirror M2 in figure 3 may be altered by changing the
incident angle of the light beam, or the position of mirror M2 relative to the vibrating rnirror
M1 .
RESOLUTION.
When a sensor is smaller in size than the wavelength of the detected signal, the phase
10 of the signal becomes an important parameter in determining the resolution. Pulsewidth or the
duration of the excitation may be of less concern. For example, the small size and great
sensitivity of the sensor can be used to detect the phase of the wave and identify the leading
edge, rather than the entire pulse. If the arriving edge detection is very efficient, the axial
resolution may be limited only by the lateral solid angle subtended by the sensor.
Lateral resolution also may be enhanced by the small size of the sensor. Most present
designs do not detect where on a given sensor element the incident ultrasound wave impinges.
Therefore, the lateral resolution is limited not only by the distance between sensor elements,
but by the size of each element.
l~IERMAL AND OTHER NOISE.
2 o Thermal and other energy fluctuations will provide a background of vibrations in the
ultrasound frequency range, for which the probability can be readily estimated. Well known
techniques exist for addressing this problem, such as moving the signal to a part of the
frequency domain which is lower in noise, or the use of a lock-in amplifier.
The use of multiple beams, phase-locked loops, independent chopping and detection r
2s can reduce ~he sensitivity of the system to such therrnal fluctuations. Comparison of signals
obtained at two or more frequencies can be used to reduce artifacts in images acquired by ;
ultrasound. Separate phase-locked loops may be optimized in hardware, to give better results
in Doppler or color Doppler ultrasound imaging.
LINEAR AND SQUARE ARRAYS.
3 o This measurement strategy lends itself to high-yield, low-cost manufacture. In most ~ ~
implementations a separate actuator and sensor is required, instead of the single transducer. ~ -
- However, the low cost should compensate for the separation of functions. Moreover, the
separation of functions itself should permit the use of cheaper materials that need not serve ~ ~
both as actuator and sensor. ~ :
3 5 The sensor elements can be scaled over a wide range of sizes. Arrays of such elements `
can be used in electronic focusing. Generally linear arrays have proved adequate in medical
imaging, since two dimensions suffice for most present diagnostic purposes. Square or two-
dimensional arrays are also possible, giving rise to the possibility of three-dimensional
ultrasonic imaging.
It should be noted that the various improvements such as the reconfigured cantilever,
the multiple-bounce design, or the use of two or more incident beams on a single vibrating
surface, can be incorporated into arrays.
THE REFLECTIVE SURFACE.
s The reflective surface must be in good acoustic contact with the ultrasonic medium, and
should be displaced similarly by waves of similar amplitude. The simplest response function
is linear. For example, the surface response will obey Hooke's law (F = k-~) if the force
opposing displacement is proportional to the magnitude of the displacement. The displacement
due to the incident ultrasonic wave or pulse mwst a~so be quickly damped, in order to avoid
subsequent ringing or spurious signal. -
A stable force constant can be achieved in various implementations. Examples include
silicon or polymer membranes or diaphragms, solid or fluid pistons, and micromachined
springs or cantilevers.
Membranes or diaphragms designate thin, usually circular and planar bodies fastened at
l s the periphery to a thicker support. Often the material itself opposes motion out of the plane of
the resting surface, although another force constant may be imposed te.g. the cantilever in
Figure 2). An air-fluid interface by itself provides a simple reflective surface in which surface
tension opposes displacement, but also presents many design problems incompatible with a
wide variety of sensor applications. Membranes and diaphragms made of solids such as
2 o silicon, or polymers of various kinds, are, however, the preferred choice in most applications.
Pistons designate either solids or fluids (liquids or gases) which move along the axis of
a cylindrical cavity in response to the ultrasonic wave. Problems of friction would seem to be
more readily overcome with fluid pistons, such as ferromagnetic liquids. The movement of the
piston is typically opposed by a force proportional to the displacement, for example due to
2 s compression of à solid spring or a volume of gas. -
The above examples serve simply to illustrate ways to design or fabricate a reflective
surface with a reproducible and sensitive response to ultrasonic excitation. :
CONCLUSION, RAMIFICATIONS, AND SCOPE ~ ~ -
3 0 The present invention can be incorporated in various alternatives to the embodiments
described above.
Both analog and digital signal processing can be used with virtually no changes from
current imaging technology. This allows full use of the great art and ingenuity presently
achieved in ultrasound signal processing, to deliver the maximum diagnostic value in medical
3 5 care.
SENSITIVITY. Very low noise is integral to the design. The optical lever in effect
acts as an amplifier with a high gain and low noise. Even higher sensitivity can be achieved
when the opticai lever is combined with the optical amplification methods which have been
described.
:; ,
RESOLUTION. High axial resolution is possible, perhaps even with longer
wavelengths of ultrasound. Sensor elements smaller than the wavelength can be used, which
should permit reliable measurement of phase.
Similarly small sensor elements can aid in improving lateral resolution, by increasing
the precision with which the signal coordinates are determined.
ROBUST. Optical levers have already proven to be a robust measurement strategy.
LOW COST. The cost is low, and suitable for arrays and wide range of designs ~e.g.
catheter or invasive as well as non-invasive sensing). A single laser source can be used for an
entire array of sensors, with a suitable number of optical fibers.
0 The ultrasound source or transducer/actuator can be made up of less expensive
piezoelectric materials, since these do not need to play a dual role as transducer/sensors as
well.
SUITED TO MINIATURIZATION AND MASS-PRODUCl'ION. The sensor design
involves design elements which are compatible with planar microfabrication technology, and
which may be incorporated to further reduce the size of the sensors and actuators.
LOW-POWER. The sensor design requires only low power levels and thus is well-
suited to use in portable ultrasound units. The great sensitivity of the sensor requires less
power in the ultrasound source as well. The power needed to drive the ultrasound source or
transducer/actuators can be reduced, due to the sensitivity of the transducer/sensors.
2 o DYNAMIC RANGE. The greater sensitivity and lower noise of the design confer an ~ -~
increased dynatnic range. This can be used to deliver better image clarity, with its attendant
clinical diagnostic values.
OTHER APPLICATIONS. Though specifically conceived for use as an ultrasound
transducer, the optical lever acoustic sensor of the present invention is also suitable for use as a
2 s microphone or hydrophone in the ultrasonic or audible range. With proper calibration, the
present invention would also be useful as a pressure transducer for measurement of static or
dynamic fluid pressure.
- ~- :--
While the foregoing description contains many specific details, these should not be
3 o construed as limitations on the scope of the invention, but rather as an exempli~1cation of some
of its preferred embodiments. Many other variations are possible and will no doubt occur to
others upon reading and understanding the preceding description. Accordingly, the scope of
the invention should be determined, not by the embodiment illustrated, but by the appended
claims and their legal equivalents.
