FIELD OF THE INVENTION
[0001] The present invention relates to a drive method for cMUT cells, particularly in the
context of harmonic ultrasound imaging.
BACKGROUND OF THE INVENTION
[0002] Capacitive micromachined ultrasonic transducers (cMUTs) generally combine mechanical
and electronic components in very small packages. The mechanical and electronic components
operate together to transform mechanical energy into electrical energy and vice versa.
Because cMUTs are typically very small and have both mechanical and electrical parts,
they are commonly referred to as micro-electronic mechanical systems ("MEMS") devices.
cMUTs, due to their small size, can be used in numerous applications in many different
technical fields, including medical device technology.
[0003] One application for cMUTs within the medical device field is imaging soft tissue.
Tissue harmonic imaging has become important in medical ultrasound imaging because
it provides unique information about the imaged tissue. In harmonic imaging, ultrasonic
energy is transmitted from an imaging array to tissue at a center frequency (f0) during
transmission. This ultrasonic energy interacts with the tissue in a nonlinear fashion,
especially at high amplitude levels, and ultrasound energy at higher harmonics of
the input frequency, such as 2f0, are generated. These harmonic signals are then received
by the imaging array, and an image is formed. To achieve a good signal-to-noise ratio
during harmonic imaging, ultrasonic transducers in the imaging array would preferably
be sensitive around both the fundamental frequency f0 and the first harmonic frequency
2fo.
[0004] Conventional ultrasonic transducers are not capable of performing in such a manner.
For example, piezoelectric transducers are not suitable for harmonic imaging applications
because these transducers tend to be efficient only at a fundamental frequency (f0)
and its odd harmonics (3f0, 5f0, etc.). To compensate for the odd harmonic efficiencies
of piezoelectric transducers, the transducer is typically damped, and several matching
layers are used to create a broad band (~90% fractional bandwidth) transducer. This
approach, however, requires a trade-off between sensitivity and bandwidth since significant
energy is lost due to the backing and matching layers. Additionally, conventional
piezoelectric transducers and fabrication methods do not enable device manufacturers
to control or adjust the vibration harmonics of conventional piezoelectric transducers.
[0005] cMUT transducers are suitable for use for the purpose of harmonic imaging.
These can be operated to utilize multiple vibration modes of the cMUT membrane and
permit adjustable vibration modes and/or controllable vibration harmonics. Harmonic
imaging cMUTs are designed to achieve higher sensitivity over a wide bandwidth and
adapted to exploit multiple vibration modes of a cMUT membrane.
[0006] Thus, for harmonic mode imaging, the excitation vibration of the cMUT is at a lower
frequency than in the receive mode, since in the receive mode, the cMUT samples a
multiple of the frequency in the excitation mode, for image formation. This means
that an increase in receive sensitivity at higher frequencies should also lead to
an increased performance.
[0007] It is known that increasing the bias of a cMUT element increases the sensitivity
for higher frequencies.
[0008] However, there is a maximum field, and corresponding maximum voltage that can be
applied, above which the dielectric layers in the device break down. This is known
as the breakdown voltage. The combination of the bias voltage and the RF voltage must
not exceed this breakdown voltage. Indeed, in practice, the cMUT is typically operated
well below the breakdown voltage to avoid tunnelling of electrons through the dielectric.
For example, for a cMUT with breakdown field at ~7-9 MV/cm, a typical upper operating
limit might be set at a voltage which corresponds to a field of around 4.5-5 MV/cm
(where voltage = field
∗ total thickness of the cMUT dielectric). A cMUT may typically include at least one
dielectric layer between the lower electrode and the cMUT cavity and another dielectric
layer between the upper electrode and the cavity. Breakdown voltage is dependent on
the dielectric thickness, equal to the total thickness of all dielectric layers between
the electrodes. The breakdown voltage may typically be in the range of 150-200 V,
alternatively in the range of 70-100 V, alternatively in the range of 60-80 V.
SUMMARY OF THE INVENTION
[0009] The invention is defined by the claims.
[0010] According to examples in accordance with an aspect of the invention, there is provided
a method for driving a cMUT device with a drive cycle comprising a transmit period
and a receive period. The method comprises: in the transmit period, driving a cMUT
element of the cMUT device with a first bias voltage and an RF voltage; and in the
receive period, driving the cMUT element with a second bias voltage, and without an
RF voltage. The second bias voltage is higher than the first bias voltage, and the
combined RF voltage and the first bias voltage are such as to cause the cMUT element
to operate in a collapsed mode during the transmit period. The second bias voltage
is such as to cause the cMUT element to operate in collapsed mode during the receive
period.
[0011] Thus, the concept proposed by the inventors is to vary the bias voltage between the
transmit and receive cycle phases. After generation of the ultrasound wave in the
transmit period, the bias voltage can be safely increased during the receive period
due to the removal of the applied RF voltage. In this way, it is possible to improve
sensitivity in receive period. The bias voltage can then be lowered again before the
next generation of the US wave in the next transmit period.
[0012] Thus, it is proposed to use a separation in time principle for adjusting the bias
voltage level. Importantly however, it is proposed to configure the bias and RF voltage
levels such the cMUT stays in collapse mode at all times, during both transmit and
receive periods. This circumvents known problems in state-of-the-art technology, wherein
artefacts can occur, and also reduces reaction time of the membranes, as the motion
required is lower.
[0013] Working in the collapse mode is advantageous compared to non-collapse mode. The cMUT
has higher transmit pressure in the collapse mode. To obtain high transmit pressure
in non-collapse mode, it would be necessary to operate the cMUT at a bias voltage
close to the collapse point. The resulting device would then show then a more non-linear
behavior which, for purposes of harmonic imaging, is highly disadvantageous. Furthermore,
switching between collapse and non-collapse mode leads to wear and reduces lifetime
(reliability) of the cMUT. Acoustic artefacts can also occur when the membrane moves
out of collapse and into collapse state due to the sudden change in capacitance, which
translates into artefacts in the signals that need to be filtered out. Therefore staying
in collapse mode avoids the need to filter these artefacts and increases device lifetime.
[0014] The way of addressing the issue of receive sensitivity in the state of the art is
to use dynamic gain control, where the level of amplification of the signal is adjusted
as a function of time in the receive phase of the ultrasound probe. Embodiments of
the present invention thus provide an additional and/or alternative way to increase
the receive signal, prior to any amplification. This therefore provides a new way
to improve performance. Moreover, boosting the receive signal prior to amplification
is also beneficial for signal-to-noise-ratio (SNR).
[0015] As discussed, the proposed method finds particularly advantageous application for
harmonic ultrasound imaging, for example using a cMUT adapted for harmonic ultrasound
imaging. The drive cycle may be a harmonic imaging cycle. However, the general principle
can be applied to a cMUT of any type for harmonic or non-harmonic imaging since in
all cases an increase in receive sensitivity is achieved,
[0016] The cMUT device comprises one or more cMUT transducer elements.
[0017] For avoidance of doubt, in the context of the present application, the RF voltage
means an alternating voltage. The bias voltage means a DC voltage.
[0018] The sum of the RF voltage and the first bias voltage, and separately the second bias
voltage, should each at all times not exceed a pre-defined maximum voltage, representing
a breakdown voltage of the cMUT element. In practice, the voltages in the two modes
may be configured such that they are kept below an upper limit which is a defined
margin below the breakdown voltage. For example, for a cMUT with a breakdown field
of ~7-9 MV/cm, a typical upper operating limit might be set at a voltage corresponding
to a field of around 4.5-5 MV/cm (where breakdown field [V/cm] = breakdown voltage
/ total thickness of the dielectric layers of the cMUT element). Thus, preferably
the maximum voltage in both transmit and receive periods is kept below a pre-defined
upper limit which is lower than the breakdown voltage and which is chosen as a manufacturing
choice depending on the lifetime requirements of the cMUT device.
[0019] The cMUT operates in collapse mode when the voltage applied to it (the combined first
bias and RF voltage, or the second bias) exceeds the collapse voltage for the cMUT
element. In preferred embodiments, the bias voltage in both the transmit and receive
periods is set above the collapse voltage. This is beneficial for the lifetime of
the cMUT transducer.
[0020] According to some embodiments, the difference between the second bias voltage and
the first bias voltage may be equal to a voltage amplitude of the RF voltage. This
means that the step up in the bias voltage exactly matches the size of the RF voltage.
If for example the RF+bias in the transmit mode is at or close to the maximum operational
voltage, then this feature ensures that the maximum possible bias voltage increase
is attained in the receive mode without exceeding the maximum operation voltage.
[0021] In some embodiments, the method further comprises sampling the cMUT element during
the receive period to obtain a receive signal.
[0022] In some embodiments, a transition from the transmit period to the receive period
of the imaging cycle comprises a ramp-up of the first bias voltage to the second bias
voltage, according to a first ramping function. In some embodiments, a transition
from the receive period to the transmit period of the cycle comprises a ramp-down
of the second bias voltage to the first bias voltage according to a second ramping
function.
[0023] In some embodiments, the first and second ramping function are controllable.
[0024] In some embodiments, each of the first and second ramping functions is a smooth linear
function.
[0025] In some embodiments, the method further comprises sampling the cMUT element during
the receive period to obtain a receive signal, and wherein said sampling comprises
sampling only between the end of the ramp-up of the bias voltage and the beginning
of the ramp-down of the bias voltage.
[0026] In some embodiments, the method further comprises obtaining an indication of one
or more target acoustic frequencies to be sampled during the receive period and determining
a value of the second bias voltage in dependence upon the one or more target acoustic
frequencies. In other words, it is proposed according to this set of embodiments to
tune the bias voltage in accordance with a harmonic frequency to be measured. This
may for example make use of a pre-defined mapping function or lookup table which relates
target frequencies to optimal bias voltages for sampling those frequencies.
[0027] In some embodiments, the method comprises determining the one or more target acoustic
frequencies, and wherein the one or more target acoustic frequencies are each harmonics
of the frequency of the RF voltage applied during the transmit period. In some embodiments,
one or more target acoustic frequencies include a third harmonic of the frequency
of the RF voltage applied during the transmit period.
[0028] The invention can also be embodied in hardware form.
[0029] In particular, another aspect of the invention is a cMUT apparatus, comprising: a
cMUT element; and drive electronics, adapted to drive a cMUT device with a drive cycle
comprising a transmit period and a receive period. The drive electronics are adapted
to: in the transmit period, drive the cMUT element with a first bias voltage and an
RF voltage; in the receive period, drive the cMUT element with a second bias voltage,
and without an RF voltage; wherein the second bias voltage is higher than the first
bias voltage; and wherein the combined RF voltage and first bias voltage cause the
cMUT element to operate in a collapsed mode during the transmit period, and wherein
the second bias voltage causes the cMUT element to operate in collapsed mode during
the receive period.
[0030] The apparatus may further comprise signal sampling electronics adapted to sample
the cMUT element during the receive period to obtain a receive signal.
[0031] In some embodiments, a transition from the transmit period to the receive period
of the imaging cycle comprises a ramp-up of the first bias voltage to the second bias
voltage, according to a first ramping function; and wherein a transition from the
receive period to the transmit period of the cycle comprises a ramp-down of the second
bias voltage to the first bias voltage according to a second ramping function.
[0032] In particular, further aspects of the invention relate to an ultrasound probe, comprising
the cMUT apparatus as defined above, and to an ultrasound imaging system, comprising
such ultrasound probe.
[0033] These and other aspects of the invention will be apparent from and elucidated with
reference to the embodiment(s) described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] For a better understanding of the invention, and to show more clearly how it may
be carried into effect, reference will now be made, by way of example only, to the
accompanying drawings, in which:
Fig. 1 shows the structure of a cMUT element;
Fig. 2 shows the example cMUT element operated in collapse mode;
Fig. 3 outlines steps of an example method in accordance with one or more embodiments
of the invention;
Fig. 4 shows components of an example apparatus in accordance with one or more embodiments;
Figs. 5-8 illustrate example voltage signal characteristics during transmit and receive
periods according to one or more embodiments; and
Fig. 9 shows an example ultrasound imaging system with receive or sampling electronics
and drive electronics.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0035] The invention will be described with reference to the Figures.
[0036] It should be understood that the detailed description and specific examples, while
indicating exemplary embodiments of the apparatus, systems and methods, are intended
for purposes of illustration only and are not intended to limit the scope of the invention.
These and other features, aspects, and advantages of the apparatus, systems and methods
of the present invention will become better understood from the following description,
appended claims, and accompanying drawings. It should be understood that the Figures
are merely schematic and are not drawn to scale. It should also be understood that
the same reference numerals are used throughout the Figures to indicate the same or
similar parts.
[0037] The invention provides a method for improving receive sensitivity of a cMUT transducer
element by dynamically adjusting the bias volage between the transmit and receive
phases of the drive cycle while keeping the cMUT in collapsed operation mode at all
times. The bias voltage is increased in receive mode to increase sensitivity.
[0038] To aid understanding, some brief background explanation will be provided regarding
collapse mode operation of cMUT elements.
[0039] As described in the paper
Micromachined Ultrasonic Transducers, IEEE Trans UFFC, Vol. 50, No. 9 (2003), for a conventional capacitive micromachined ultrasonic transducer (cMUT) to be
operated in collapsed mode, the flexible membrane of the cMUT is typically excited
with a voltage that causes part of the membrane to collapse onto the corresponding
cMUT substrate. Subsequent reduction of the voltage applied to the membrane to a certain
threshold voltage, commonly characterized as the cMUT 'snapback voltage', will typically
cause the membrane to lift upward from the substrate, and to return to an equilibrium
position. By contrast, to the extent the voltage applied to a previously collapsed
membrane is kept above the snapback voltage, a fairly linear and efficient output
of the device typically can be achieved.
[0040] A conventional cMUT structure is shown in Fig. 1. More particularly, Fig. 1 shows
a cMUT 100 in schematic cross section including a substrate 102 in which a pocket
or cavity 104 is formed, and a flexible membrane 106 mounted to the substrate 102
across the cavity 104. A first electrode 112 is positioned atop the membrane 106,
and a second electrode 114 is positioned beneath the cavity. A first dielectric layer
122 may be disposed between the first electrode 112 and the cavity 104. A second dielectric
layer 124 is disposed between the second electrode 114 and the cavity 104. This therefore
forms an upper layer stack suspended above the cavity 104 comprising the membrane
106 disposed atop the first electrode 112, disposed atop the first dielectric layer
122 and a second layer stack at a base of the cavity 104 comprising the second dielectric
layer 124 disposed atop the second electrode 114.
[0041] The total dielectric thickness of the cMUT is equal to the sum of the thicknesses
of the dielectric layers. The breakdown voltage means the voltage which leads to breakdown
of these dielectric layers. At the breakdown voltage point, current starts to flow
from electrode 112 to 114, effectively destroying the capacitor structure and resulting
in the structure behaving as a resistor. As a result, the device heats up quickly
and can burn through.
[0042] In circumstances in which the bias voltage applied between the electrodes is set
at a relatively low voltage, or at zero volts, the cMUT 100 will typically exhibit
a gap within the cavity 104 between the flexible membrane 106 and the substrate 102.
[0043] Referring now to Fig. 2, in operation, upon a voltage bias applied between the first
112 and second 114 electrode being increased a sufficient amount from the relatively
low or zero level associated with the configuration of the cMUT 100 shown in Fig.
1, the flexible membrane 106 will tend to collapse downward into the cavity 104 and
toward the substrate 102. Such collapse of the flexible membrane 106 can substantially
eliminate the gap (Fig. 1) between the flexible membrane 106 and the substrate 102,
such that a downward- facing surface 200 of the upper layer stack 106, 112, 122 is
at least temporarily placed in physical contact with a corresponding upward-facing
surface 202 of the lower layer stack 114, 124. This collapsed condition of the flexible
membrane 106 with respect the substrate 102, once achieved, may be maintained by the
continuous application across the flexible membrane 106 and the substrate 102 of a
voltage in excess of a certain minimum level, commonly referred to as the collapse
voltage or the snapback voltage.
[0044] Embodiments of the present invention have particularly powerful application within
the context of harmonic ultrasound imaging. As explained earlier in this document,
for harmonic ultrasound imaging, it is necessary for the cMUT transducer to sense
echo waves at multiples of the transmit frequency, i.e. higher harmonics of the transmit
frequency. This requires high sensitivity to higher frequencies. Collapsed mode operation
(by maintaining the bias voltage above the collapse voltage) increases the sensitivity
of the cMUT to higher frequencies without the need to increase the bias voltage too
close to operational limits. In other words, in order to achieve higher receive sensitivity
in non-collapse made, it is necessary to operate the cMUT in a regime close to the
dielectric breakdown voltage, which leads to a more non-linear receive behavior, which
is less optimal particularly for harmonic imaging.
[0045] Embodiments of the present invention facilitate the provision of harmonic imaging
operation with improved receive sensitivity at the harmonic frequencies of the central
base frequency.
[0046] Embodiments of the invention are based on the insight of using time separation for
adjusting the bias voltage level of the cMUT element(s) comprised in the cMUT device
during the transmit/receive drive cycle, so as to provide a dynamic bias voltage control,
in which, after the generation of the transmit ultrasound wave, the bias voltage is
increased with the value of the RF voltage applied, in order to improve sensitivity
in the receive phase for the higher (harmonic) frequencies, and then lowered again
before the next transmit event. During the entire cycle, the cMUT element(s) stays
in collapse mode at all times.
[0047] Embodiments of the invention are based on the insight that the bias voltage can be
safely increased during the receive phase because the additional RF voltage applied
during the transmit phase is not needed during the receive phase. This leaves scope
to increase the bias voltage without risking exceeding the safe upper operating limit
for total applied voltage, which is typically some fraction, e.g. between 50-80% of
the breakdown voltage of the transducer.
[0048] Fig. 3 outlines in block diagram form steps of an example method according to one
or more embodiments. The steps will be recited in summary, before being explained
further in the form of example embodiments.
[0049] Provided is a method 10 for driving a cMUT device comprising one or more cMUT elements
with a drive cycle comprising a transmit period 12 and a receive period 16. The method
comprises, in the transmit period 12, driving a cMUT element of the cMUT device with
a first bias voltage 20 and an RF voltage 22. The method comprises, in the receive
period 16, driving the cMUT element with a second bias voltage 26, and without an
RF voltage 28. The second bias voltage 26 is higher than the first bias voltage 20.
The combined RF voltage 22 and the first bias voltage 20 are such as to cause the
cMUT element to operate in a collapsed mode (at all times) during the transmit period,
and the second bias voltage 26 is such as to cause the cMUT element to operate in
collapsed mode during the receive period.
[0050] As noted above, the method can also be embodied in hardware form.
[0051] With reference to Fig. 4, another aspect of the invention is a cMUT apparatus 30.
The cMUT apparatus comprises a cMUT device comprising at least one cMUT element 32.
The apparatus further comprises drive electronics 34, adapted to drive a cMUT element
with a drive cycle comprising a transmit period and a receive period. The drive electronics
are adapted to: in the transmit period, drive the cMUT element with a first bias voltage
and an RF voltage; in the receive period, drive the cMUT element with a second bias
voltage, and without an RF voltage; wherein the second bias voltage is higher than
the first bias voltage; and wherein the combined RF voltage and first bias voltage
cause the cMUT element to operate in a collapsed mode during the transmit period,
and wherein the second bias voltage causes the cMUT element to operate in collapsed
mode during the receive period.
[0052] With regards to the drive electronics, these are arranged to drive the at least one
cMUT transducer element (either directly or via a microbeamformer) during the transmission
mode. The drive electronics may further include a transmit/receive (T/R) switch for
switching the at least one cMUT element from transmit to receive mode.
[0053] In some embodiments, the apparatus may further comprise signal sampling electronics
adapted to sample the cMUT element during the receive period to obtain a receive signal.
[0054] With regards to the cMUT element itself, the structure of such an element is well
known and has already been described above with reference to Fig. 1 and Fig. 2. For
operation, additionally a pair of electrodes is provided, one applied to the membrane
and another coupled atop the substrate, or just beneath the cavity. In transmit mode,
the bias and the RF voltages are applied together across the two electrodes. In receive
mode, the bias voltage is applied across the two electrodes.
[0055] Standard downstream processing electronics and software to generate an image may
also be provided according to some embodiments. For example, a processing device may
be provided configured to process the received signals from the cMUT to generate a
harmonic image dataset.
[0056] An important feature of the proposed concept is to dynamically adjust the bias voltage
between the transmit and receive phase, but in such a way that in both phases the
cMUT is operating in collapse mode. Collapse mode operation will be known to the skilled
person in this field, and it has already been described above with reference to Fig.
2.
[0057] Fig. 5 illustrates voltage characteristics during transmit and receive periods of
the drive cycle according to at least one set of embodiments of the present invention.
The transmit and receive period timings respectively are indicated by duty cycle waveforms
56, 58 in Fig. 5.
[0058] As indicated, the bias voltage 54 is increased during the receive period, transitioning
from a first bias voltage in the transmit period to a second (higher) bias voltage
in the receive period. It is then decreased again to the first bias voltage once the
receive period has ended, ready for the next transmit period.
[0059] The bias voltage in both the transmit and receive periods is such as to cause the
cMUT elements to operate in collapse mode, i.e. it exceeds the collapse voltage for
the cMUT. The collapse voltage can be easily identified for any cMUT since it is the
minimum applied bias voltage at which the membrane switches into its collapsed state,
as discussed above with reference to Fig. 2.
[0060] Preferably, and as illustrated in the example of Fig. 5, a transition from the transmit
period to the receive period of the drive cycle comprises a ramp-up of the first bias
voltage to the second bias voltage, according to a first ramping function 62. Preferably,
a transition from the receive period to the transmit period of the drive cycle comprises
a ramp-down of the second bias voltage to the first bias voltage according to a second
ramping function 64. Using a ramping function rather than a step change avoids the
change in bias voltage causing generation of a transmit pulse, which is not the intended
effect.
[0061] With regards to particular values for the first bias voltage, second bias voltage
and RF voltage, these can be configured according to preferences or requirements of
the particular hardware and the particular application, so long as the constraints
already discussed are met.
[0062] A further constraint which typically should be met is that the applied voltage(s)
in both transmit and receive mode should not exceed an upper operation limit, which
is usually chosen as some fraction, e.g. between 50-80%, of the breakdown voltage.
[0063] The breakdown voltage is a voltage above which the dielectric layers in the device
break down. This voltage level can be readily tested for any cMUT element by incrementally
increasing the applied voltage, V, while simultaneously monitoring current, I. Plotting
an IV curve allows determination of the voltage value at which breakdown (destruction)
of the device occurs. This is identifiable as a voltage point at which a sudden inflection
point in the IV curve occurs. In particular, it can be identified as a voltage at
which the current increases significantly (i.e. at a faster rate than in the preceding
IV curve). Before breakdown occurs, the tunneling regime can also be identified.
[0064] The combination of the bias voltage and the RF voltage (if applied) must at all times
not exceed this breakdown voltage. Since the RF voltage cycles between an upper and
lower amplitude, it is more precise to say the sum of the maximum amplitude of the
RF voltage plus the first bias voltage should not exceed the breakdown voltage, and
the second bias voltage alone should not exceed the breakdown voltage.
[0065] In fact, in practice, it is preferable that the cMUT is operated well below the breakdown
voltage to avoid tunnelling of electrons through the dielectric. For example, for
a cMUT with breakdown voltage of 170-200 V, a typical upper operating limit might
be set at around 150V-180V. However, the particular selection of the upper operating
limit, as a fraction of the breakdown voltage, can be a manufacturing choice; it represents
a balancing between device lifetime (lower upper voltage limit leads to longer lifetime)
and device sensitivity (higher upper voltage level leads to increased sensitivity).
[0066] The frequency to which a cMUT element is sensitive during the receive mode is a function
of the applied bias voltage, with increased bias voltage leading to an increase in
the frequency to which the cMUT is sensitive in the receive mode.
[0067] For harmonic imaging, the imaging principle relies on sensing the higher harmonics
of the transmit center frequency. Therefore, the higher the target harmonic, the higher
must be the bias voltage in the receive mode relative to the bias voltage in the transmit
mode; i.e. the higher must be the difference between the first and second bias voltages
discussed above.
[0068] Following this logic, in some embodiments, to further increase the difference between
the transmit frequency and the frequency to which the cMUT is sensitive during receive
mode, the first bias voltage (during the transmit period) can be decreased further
to increase the difference between transmit center frequency and the receive frequency
sensitivity. This is illustrated in Fig. 6, which shows that the first bias voltage,
during the transmit period is reduced compared to the example shown in Fig. 5. This
therefore provides a way to increase the difference between the transmit and receive
frequencies without further increasing the bias voltage in the receive mode (which
might risk exceeding the maximum operating level). This however comes as the cost
of slightly reducing the transmit pressure due to the lower bias voltage in the transmit
period. Thus, this is an optional variable which can be optimized according to manufacturing
preferences.
[0069] To illustrate the concept, and without limiting the general scope of the invention,
by way of one example, a lower limit of a suitable cMUT device transmit center frequency
might be about 1.8 MHz. This means that with increased receive sensitivity, the third
harmonic of the center transmit frequency might be detected.
[0070] To illustrate further, an example might be considered in which the combination of
the first bias voltage and the RF voltage during transmit phase is (at the maximum
of the RF cycle) 180 Volts. Any combination of bias voltage level and RF voltage amplitude
can be chosen, as long as the bias voltage level during the transmit phase is larger
than the collapse voltage (e.g. in the order of around 60 volts).
[0071] For example, some illustrative combinations would, for this particular example being
considered, include the following:
Transmit bias 140 V, RF 40 V, receive bias 180 V.
Transmit bias 80 V, RF 100V, receive bias 180 V.
Transmit bias 120 V, RF 40 V, receive bias 180 V.
[0072] By way of example, in the above cases, the breakdown voltage might be around 200V,
so that the total applied voltage in all cases is some margin below the breakdown
voltage.
[0073] Optionally, the difference between the second bias voltage and the first bias voltage
might be equal to a (maximum) voltage amplitude of the RF voltage. This means that
the step up in the bias voltage exactly matches the size of the RF voltage at the
maximum point of its RF cycle. If for example the RF+bias in the transmit mode is
at or close to the maximum operational voltage, then this feature ensures that the
maximum possible bias voltage increase is attained in the receive mode without exceeding
the maximum operation voltage.
[0074] As noted above, the transition from the transmit period to the receive period of
the drive cycle can comprises a ramp-up of the first bias voltage to the second bias
voltage, according to a first ramping function 62 and a transition from the receive
period to the transmit period of the drive cycle can comprise a ramp-down of the second
bias voltage to the first bias voltage according to a second ramping function 64.
[0075] In some embodiments, and as illustrated in Fig. 7, the timing of the receive period
58 can be adjusted so that the receive period only begins once the ramp-up 62 has
finished and the receive period ends before the ramp-down begins 64. This therefore
avoids disturbances to the receive electronics which might be caused by the ramping
phases. The adjusted timings of the receive period are indicated in the circled regions
in Fig. 7. The solid lines indicate the timings of the receive period before adjustment
is made, and the dotted lines indicate the proposed adjustment to the timing, so that
the receive period starts after the ramp-up has finished and ends before the ramp-down
starts.
[0076] In other words, in some embodiments, the method further comprises sampling the cMUT
element during the receive period to obtain a receive signal, and wherein said sampling
comprises sampling only between the end of the ramp-up of the bias voltage and the
beginning of the ramp-down of the bias voltage.
[0077] Additionally, in some embodiments, the shape and timing of the ramp-up 62 and ramp-down
64 functions of the bias voltage change can be modified, as indicated in Fig. 8. A
shallower ramp-up and ramp-down function (illustrated by the dotted lines in Fig.
8) might reduce electronic and ultrasonic effects. In particular, a steep ramp-up
or ramp-down may cause transmission of ultrasound which is not the intended effect
in adjusting the bias-voltage for the receive phase.
[0078] In other words, in some embodiments, the first 62 and second 64 ramping function
are controllable.
[0079] In some embodiments, each of the first 62 and second 64 ramping functions is a smooth
linear function. The slope or gradient of the ramp up and/or ramp down functions could
be adjustable. Other shapes of function however can also be used.
[0080] As discussed above, in some embodiments, the method further comprises sampling the
cMUT element during the receive period to obtain a receive signal.
[0081] One particularly advantageous application for embodiments of the invention is for
harmonic imaging.
[0082] To optimize the method and apparatus for harmonic imaging, in some embodiments, the
method may optionally further comprise obtaining an indication of one or more target
acoustic frequencies to be sampled during the receive period and determining a value
of the second bias voltage in dependence upon the one or more target acoustic frequencies.
[0083] In other words, the method may include a step of tuning the bias voltage in accordance
with a harmonic frequency to be measured. This may for example make use of a pre-defined
mapping function or lookup table which relates target frequencies to optimal bias
voltages for sampling those frequencies.
[0084] The method could further include a step of determining or identifying the one or
more target acoustic frequencies to be measured, and wherein the one or more target
acoustic frequencies are each harmonics of the frequency of the RF voltage applied
during the transmit period. In other words, if the central transmit frequency is known
(e.g. this may be identified from a register entry in a processor register), then
the target acoustic frequencies can be determined as those frequencies which are pre-defined
harmonics, e.g. first or second or third harmonics, of the central transmit frequency.
[0085] In some advantageous embodiments, the one or more target acoustic frequencies may
include a third harmonic of the frequency of the RF voltage applied during the transmit
period.
[0086] Certain embodiments employ use of drive electronics and/or receive or sampling electronics.
By way of further, more detailed explanation, the general operation of an exemplary
ultrasound imaging system which includes the drive electronics, receive/sampling electronics
and also image forming components will now be described, with reference to Fig. 9.
[0087] The system 302 comprises an ultrasound probe, in particular an array transducer probe
304, which has a transducer array 306 for transmitting ultrasound waves and receiving
echo information. The transducer array 306 comprises cMUT transducers. In this example,
the transducer array 306 is a two-dimensional array of transducers 308 capable of
scanning either a 2D plane or a three-dimensional volume of a region of interest.
In another example, the transducer array may be a 1D array.
[0088] The transducer array 306 is coupled to a microbeamformer 312 which controls reception
of signals by the transducer elements. Microbeamformers are capable of at least partial
beamforming of the signals received by sub-arrays, generally referred to as "groups"
or "patches", of transducers as described in
US Patents 5,997,479 (Savord et al.),
6,013,032 (Savord), and
6,623,432 (Powers et al.).
[0089] It should be noted that the microbeamformer is in general entirely optional. Further,
the system includes a transmit/receive (T/R) switch 316, which the microbeamformer
312 can be coupled to and which switches the array between transmission and reception
modes, and protects the main beamformer 320 from high energy transmit signals in the
case where a microbeamformer is not used and the transducer array is operated directly
by the main system beamformer. The transmission of ultrasound beams from the transducer
array 306 is directed by a transducer controller 318 coupled to the microbeamformer
by the T/R switch 316 and a main transmission beamformer (not shown), which can receive
input from the user's operation of the user interface or control panel 338. The controller
318 can include transmission circuitry arranged to drive the transducer elements of
the array 306 (either directly or via a microbeamformer) during the transmission mode.
[0090] The function of the control panel 338 in this example system may be facilitated by
an ultrasound controller unit according to an embodiment of the invention.
[0091] In a typical line-by-line imaging sequence, the beamforming system within the probe
may operate as follows. During transmission, the beamformer (which may be the microbeamformer
or the main system beamformer depending upon the implementation) activates the transducer
array, or a sub-aperture of the transducer array. The sub-aperture may be a one-dimensional
line of transducers or a two dimensional patch of transducers within the larger array.
In transmit mode, the focusing and steering of the ultrasound beam generated by the
array, or a sub-aperture of the array, are controlled as described below.
[0092] Upon receiving the backscattered echo signals from the subject, the received signals
undergo receive beamforming (as described below), in order to align the received signals,
and, in the case where a sub-aperture is being used, the sub-aperture is then shifted,
for example by one transducer element. The shifted sub-aperture is then activated
and the process repeated until all of the transducer elements of the transducer array
have been activated.
[0093] For each line (or sub-aperture), the total received signal, used to form an associated
line of the final ultrasound image, will be a sum of the voltage signals measured
by the transducer elements of the given sub-aperture during the receive period. The
resulting line signals, following the beamforming process below, are typically referred
to as radio frequency (RF) data. Each line signal (RF data set) generated by the various
sub-apertures then undergoes additional processing to generate the lines of the final
ultrasound image. The change in amplitude of the line signal with time will contribute
to the change in brightness of the ultrasound image with depth, wherein a high amplitude
peak will correspond to a bright pixel (or collection of pixels) in the final image.
A peak appearing near the beginning of the line signal will represent an echo from
a shallow structure, whereas peaks appearing progressively later in the line signal
will represent echoes from structures at increasing depths within the subject.
[0094] One of the functions controlled by the transducer controller 318 is the direction
in which beams are steered and focused. Beams may be steered straight ahead from (orthogonal
to) the transducer array, or at different angles for a wider field of view. The steering
and focusing of the transmit beam may be controlled as a function of transducer element
actuation time.
[0095] Two methods can be distinguished in general ultrasound data acquisition: plane wave
imaging and "beam steered" imaging. The two methods are distinguished by a presence
of the beamforming in the transmission ("beam steered" imaging) and/or reception modes
(plane wave imaging and "beam steered" imaging).
[0096] Looking first to the focusing function, by activating all of the transducer elements
at the same time, the transducer array generates a plane wave that diverges as it
travels through the subject. In this case, the beam of ultrasonic waves remains unfocused.
By introducing a position dependent time delay to the activation of the transducers,
it is possible to cause the wave front of the beam to converge at a desired point,
referred to as the focal zone. The focal zone is defined as the point at which the
lateral beam width is less than half the transmit beam width. In this way, the lateral
resolution of the final ultrasound image is improved.
[0097] For example, if the time delay causes the transducer elements to activate in a series,
beginning with the outermost elements and finishing at the central element(s) of the
transducer array, a focal zone would be formed at a given distance away from the probe,
in line with the central element(s). The distance of the focal zone from the probe
will vary depending on the time delay between each subsequent round of transducer
element activations. After the beam passes the focal zone, it will begin to diverge,
forming the far field imaging region. It should be noted that for focal zones located
close to the transducer array, the ultrasound beam will diverge quickly in the far
field leading to beam width artifacts in the final image. Typically, the near field,
located between the transducer array and the focal zone, shows little detail due to
the large overlap in ultrasound beams. Thus, varying the location of the focal zone
can lead to significant changes in the quality of the final image.
[0098] It should be noted that, in transmit mode, only one focus may be defined unless the
ultrasound image is divided into multiple focal zones (each of which may have a different
transmit focus).
[0099] In addition, upon receiving the echo signals from within the subject, it is possible
to perform the inverse of the above described process in order to perform receive
focusing. In other words, the incoming signals may be received by the transducer elements
and subject to an electronic time delay before being passed into the system for signal
processing. The simplest example of this is referred to as delay-and-sum beamforming.
It is possible to dynamically adjust the receive focusing of the transducer array
as a function of time.
[0100] Looking now to the function of beam steering, through the correct application of
time delays to the transducer elements it is possible to impart a desired angle on
the ultrasound beam as it leaves the transducer array. For example, by activating
a transducer on a first side of the transducer array followed by the remaining transducers
in a sequence ending at the opposite side of the array, the wave front of the beam
will be angled toward the second side. The size of the steering angle relative to
the normal of the transducer array is dependent on the size of the time delay between
subsequent transducer element activations.
[0101] Further, it is possible to focus a steered beam, wherein the total time delay applied
to each transducer element is a sum of both the focusing and steering time delays.
In this case, the transducer array is referred to as a phased array.
[0102] To provide the DC bias voltage for the cMUT transducers, the transducer controller
118 can be coupled to control a DC bias control 345 for the transducer array. The
DC bias control 345 sets DC bias voltage(s) that are applied to the cMUT transducer
elements.
[0103] For each transducer element of the transducer array, analog ultrasound signals, typically
referred to as channel data, enter the system by way of the reception channel. In
the reception channel, partially beamformed signals are produced from the channel
data by the microbeamformer 312 and are then passed to a main receive beamformer 320
where the partially beamformed signals from individual patches of transducers are
combined into a fully beamformed signal, referred to as radio frequency (RF) data.
The beamforming performed at each stage may be carried out as described above, or
may include additional functions. For example, the main beamformer 320 may have 328
channels, each of which receives a partially beamformed signal from a patch of dozens
or hundreds of transducer elements. In this way, the signals received by thousands
of transducers of a transducer array can contribute efficiently to a single beamformed
signal.
[0104] The beamformed reception signals are coupled to a signal processor 322. The signal
processor 122 can process the received echo signals in various ways, such as: band-pass
filtering; decimation; I and Q component separation; and harmonic signal separation,
which acts to separate linear and nonlinear signals so as to enable the identification
of nonlinear (higher harmonics of the fundamental frequency) echo signals returned
from tissue and micro-bubbles. This facilitates for example harmonic imaging. The
signal processor may also perform additional signal enhancement such as speckle reduction,
signal compounding, and noise elimination. The band-pass filter in the signal processor
can be a tracking filter, with its pass band sliding from a higher frequency band
to a lower frequency band as echo signals are received from increasing depths, thereby
rejecting noise at higher frequencies from greater depths that is typically devoid
of anatomical information.
[0105] The beamformers for transmission and for reception are implemented in different hardware
and can have different functions. Of course, the receiver beamformer is designed to
take into account the characteristics of the transmission beamformer. In Fig. 9 only
the receiver beamformers 312, 320 are shown, for simplicity. In the complete system,
there will also be a transmission chain with a transmission micro beamformer, and
a main transmission beamformer.
[0106] The function of the micro beamformer 312 is to provide an initial combination of
signals in order to decrease the number of analog signal paths. This is typically
performed in the analog domain.
[0107] The final beamforming is done in the main beamformer 320 and is typically after digitization.
[0108] The transmission and reception channels use the same transducer array 306 which has
a fixed frequency band. However, the bandwidth that the transmission pulses occupy
can vary depending on the transmission beamforming used. The reception channel can
capture the whole transducer bandwidth (which is the classic approach) or, by using
bandpass processing, it can extract only the bandwidth that contains the desired information
(e.g. the harmonics of the main harmonic).
[0109] The RF signals may then be coupled to a B mode (i.e. brightness mode, or 2D imaging
mode) processor 326 and a Doppler processor 328. The B mode processor 126 performs
amplitude detection on the received ultrasound signal for the imaging of structures
in the body, such as organ tissue and blood vessels. In the case of line-by-line imaging,
each line (beam) is represented by an associated RF signal, the amplitude of which
is used to generate a brightness value to be assigned to a pixel in the B mode image.
The exact location of the pixel within the image is determined by the location of
the associated amplitude measurement along the RF signal and the line (beam) number
of the RF signal. B mode images of such structures may be formed in the harmonic or
fundamental image mode, or a combination of both as described in
US Pat. 6,283,919 (Roundhill et al.) and
US Pat. 6,458,083 (Jago et al.) The Doppler processor 328 processes temporally distinct signals arising from tissue
movement and blood flow for the detection of moving substances, such as the flow of
blood cells in the image field. The Doppler processor 328 typically includes a wall
filter with parameters set to pass or reject echoes returned from selected types of
materials in the body.
[0110] The structural and motion signals produced by the B mode and Doppler processors are
coupled to a scan converter 332 and a multi-planar reformatter 344. The scan converter
332 arranges the echo signals in the spatial relationship from which they were received
in a desired image format. In other words, the scan converter acts to convert the
RF data from a cylindrical coordinate system to a Cartesian coordinate system appropriate
for displaying an ultrasound image on an image display 340. In the case of B mode
imaging, the brightness of pixel at a given coordinate is proportional to the amplitude
of the RF signal received from that location. For instance, the scan converter may
arrange the echo signal into a two dimensional (2D) sector-shaped format, or a pyramidal
three dimensional (3D) image. The scan converter can overlay a B mode structural image
with colors corresponding to motion at points in the image field, where the Doppler-estimated
velocities to produce a given color. The combined B mode structural image and color
Doppler image depicts the motion of tissue and blood flow within the structural image
field. The multi-planar reformatter will convert echoes that are received from points
in a common plane in a volumetric region of the body into an ultrasound image of that
plane, as described in
US Pat. 6,443,896 (Detmer). A volume renderer 342 converts the echo signals of a 3D data set into a projected
3D image as viewed from a given reference point as described in
US Pat. 6,530,885 (Entrekin et al.).
[0111] The 2D or 3D images are coupled from the scan converter 332, multi-planar reformatter
344, and volume renderer 342 to an image processor 330 for further enhancement, buffering
and temporary storage for optional display on an image display 340. The imaging processor
may be adapted to remove certain imaging artifacts from the final ultrasound image,
such as: acoustic shadowing, for example caused by a strong attenuator or refraction;
posterior enhancement, for example caused by a weak attenuator; reverberation artifacts,
for example where highly reflective tissue interfaces are located in close proximity;
and so on. In addition, the image processor may be adapted to handle certain speckle
reduction functions, in order to improve the contrast of the final ultrasound image.
[0112] In addition to being used for imaging, the blood flow values produced by the Doppler
processor 328 and tissue structure information produced by the B mode processor 126
are coupled to a quantification processor 334. The quantification processor produces
measures of different flow conditions such as the volume rate of blood flow in addition
to structural measurements such as the sizes of organs and gestational age. The quantification
processor may receive input from the user control panel 338, such as the point in
the anatomy of an image where a measurement is to be made.
[0113] Output data from the quantification processor is coupled to a graphics processor
336 for the reproduction of measurement graphics and values with the image on the
display 340, and for audio output from the display device 340. The graphics processor
336 can also generate graphic overlays for display with the ultrasound images. These
graphic overlays can contain standard identifying information such as patient name,
date and time of the image, imaging parameters, and the like. For these purposes the
graphics processor receives input from the user interface 338, such as patient name.
The user interface is also coupled to the transmit controller 318 to control the generation
of ultrasound signals from the transducer array 306 and hence the images produced
by the transducer array and the ultrasound imaging system. The transmit control function
of the controller 318 is only one of the functions performed. The controller 318 also
takes account of the mode of operation (given by the user) and the corresponding required
transmitter configuration and band-pass configuration in the receiver analog to digital
converter. The controller 318 can be a state machine with fixed states.
[0114] The user interface is also coupled to the multi-planar reformatter 344 for selection
and control of the planes of multiple multi-planar reformatted (MPR) images which
may be used to perform quantified measures in the image field of the MPR images.
[0115] Variations to the disclosed embodiments can be understood and effected by those skilled
in the art in practicing the claimed invention, from a study of the drawings, the
disclosure and the appended claims. In the claims, the word "comprising" does not
exclude other elements or steps, and the indefinite article "a" or "an" does not exclude
a plurality.
[0116] A single processor or other unit may fulfill the functions of several items recited
in the claims.
[0117] The mere fact that certain measures are recited in mutually different dependent claims
does not indicate that a combination of these measures cannot be used to advantage.
[0118] A computer program may be stored/distributed on a suitable medium, such as an optical
storage medium or a solid-state medium supplied together with or as part of other
hardware, but may also be distributed in other forms, such as via the Internet or
other wired or wireless telecommunication systems.
[0119] If the term "adapted to" is used in the claims or description, it is noted the term
"adapted to" is intended to be equivalent to the term "configured to".
[0120] Any reference signs in the claims should not be construed as limiting the scope.