WITH CONTROLLABLE OUTPUT IMPEDANCE

This invention relates to medical diagnostic ultrasound systems and, in particular, to ultrasound probes with microbeamformers .

Ultrasonic array transducers use beamformers to transmit, receive and appropriately delay and sum the ultrasonic echo signals received from elements of the transducer array. The delays are chosen in

consideration of the direction (steering) and focus depth of the beams to be formed by the beamformer. After the signals received from each element have been properly delayed by a channel of the beamformer, the delayed signals are combined to form a beam of properly steered and focused coherent echo signals. During ultrasonic beam transmission, the time of actuation of individual elements is the complement of the receive delay, steering and focusing the transmit beam. The choice of delays is known to be

determinable from the geometry of the array elements and of the image field being interrogated by the beams .

In a traditional ultrasound system the array transducer is located in a probe which is placed against the body of the patient during imaging and contains some electronic components such as tuning elements, switches, and amplification devices. The delaying and signal combining is performed by the beamformer which is contained in the ultrasound system mainframe, to which the probe is connected by a cable.

The foregoing system architecture for an array transducer and a beamformer suffices quite well for most one dimensional (ID) transducer arrays, where the number of transducer elements and the number of beamformer channels are approximately the same. When the number of transducer elements exceeds the number of beamformer channels, multiplexing is generally employed and only a subset of the total number of elements of the transducer can be connected to the beamformer at any point in time. The number of elements in a ID array can range from less than one hundred to several hundred and the typical beamformer has 128 beamformer channels. This system

architecture solution became untenable with the advent of two dimensional (2D) array transducers for two and three dimensional (3D) imaging. That is because 2D array transducers steer and focus beams in both azimuth and elevation over a volumetric region.

The number of transducer elements needed for this beam formation is usually in the thousands. The crux of the problem then becomes the cable that connects the probe to the system mainframe where the

beamformer is located. A cable of several thousand conductors of even the finest conductive filaments becomes thick and unwieldy, making manipulation of the probe cumbersome if not impossible.

A solution to this problem is to perform at least some of the beamforming in the probe itself, as described in US Pat. 5,229,933 (Larson, III) . In the ultrasound system shown in this patent, the

beamforming is partitioned between the probe and the system mainframe. Initial beamforming of groups of elements is done in the probe by microcircuitry known as a microbeamformer, where partially beamformed sums are produced. These partially beamformed sums, being fewer in number than the number of transducer

elements, are coupled to the system mainframe through a cable of reasonable dimensions, where the beamforming process is completed and the final beam produced. The partial beamforming in the probe is done by what Larson, III refers to as intragroup processors, in a microbeamformer in the form of microelectronics attached to the array transducer.

See also US Pat. 5,997,479 (Savord et al . ) ; US Pat. 6,013,032 (Savord); US Pat. 6,126,602 (Savord et al . ) ; and US Pat. 6,375,617 (Fraser) . The thousands of connections between the thousands of elements of the transducer array and the microbeamformer is done at the tiny dimensions of the microcircuitry and the array pitch, while the many fewer cable connections between the microbeamformer and the beamformer of the system mainframe are done by more conventional cable technologies. Various planar and curved array formats can be used with microbeamformers such as the curved arrays shown in US Pat. 7,821,180 (Kunkel, III) and US Pat. 7,927,280 (Davidsen) .

Microbeamformers can also be used with one

dimensional arrays and with 2D arrays operated as one dimensional arrays. See, e.g., US Pat. 7,037,264 (Poland) .

The microbeamformers shown in the above patents operate by forming partially delayed sum signals from contiguous transducer element groups referred to as

"patches." The signals received by all of the elements of a patch are appropriately individually delayed by the microbeamformer, then combined into a partial sum signal. Generally the patches are formed of small two-dimensional groups of elements, such as a 4x6 group or an 8x10 group of elements. This works well for phased array operation during 3D volume scanning, enabling real time scanning of the volume. However, the time required to scan a sizeable

volumetric region of the body can be lengthy, as time is required to transmit and receive all of the beams needed to form the volume image. The scanning time can be reduced by multiline acquisition, whereby multiple spatially distinct receive lines are formed from the echo signals of a single transmit event. To form tightly spaced multilines, it is necessary differently delay and sum closely spaced distinct receive signals. This objective can be limited, however, by the size of the patches of transducer elements, a problem which is overcome by the use of smaller patch sizes. The partial sum signals from smaller patches, however, can be of lesser currents or voltages than the partial sum signals of larger patches. This leads to a quantization problem when the partial sum signals are digitized. Efforts to equalize the partial sum signals can result in saturation of amplifiers by over-current or over- voltage conditions which can clip peak signals, or use of more or less than the full dynamic range of an analog to digital converter when the signals are digitized, resulting in loss of resolution.

Accordingly it is desirable to be able to accommodate array transducers operating with different patch sizes without causing signal clipping and still using the full dynamic range of digitization circuits.

In accordance with the principles of the present invention, a diagnostic ultrasound system is

described with an array transducer probe having a microbeamformer and coupled to an ultrasound system mainframe. The microbeamformer transmits and

receives signals from different size patches of transducer elements, with the signals received by an element being processed by a microchannel of the microbeamformer . A partial sum signal is produced by combining the outputs of the microchannels of elements of a patch, and partial sum signals of a plurality of patches are coupled by a probe cable to a mainframe ultrasound system for further processing. To balance the partial sum signals for different size patches, each microchannel has a controllable output impedance which is set in consideration of the signal and noise level that each microchannel contributes to a combined patch partial sum signal.

In the drawings:

FIGURE 1 illustrates in block diagram form an ultrasonic imaging system constructed in accordance with the principles of the present invention.

FIGURE 2 illustrates a transducer array

configured in NxlO patches which is to be used for multiline beam formation.

FIGURE 3 illustrates the transducer array of FIGURE 2 reconfigured into smaller patch sizes for improved multiline beam formation.

FIGURE 4 illustrates in block diagram form a microchannel of a probe microbeamformer constructed in accordance with the principles of the present invention .

FIGURE 5 illustrates the structure of a variable output impedance suitable for use in the

microbeamformer microchannel of FIGURE 4.

FIGURE 6 illustrates the input of an ultrasound system which is coupled by a probe cable to the microchannel of FIGURE 4.

FIGURE 7 illustrates a capacitive electronic signal storage device used to sample and delay signals from a transducer element in a

microbeamformer microchannel.

FIGURE 8 illustrates in detailed block diagram form an ultrasonic imaging system constructed in accordance with the principles of the present invention .

Referring first to FIGURE 1, an ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form. A probe 10 has a two dimensional array

transducer 12 which may be planar or curved as shown in this example. The elements of the array are coupled to a microbeamformer 14 located in the probe behind the transducer array. A microbeamformer is an integrated circuit located in the probe with

beamforming channels coupled to elements of the 2D array transducer 12. The microbeamformer applies timed transmit pulses to elements of each patch of the array to transmit beams in the desired directions and to the desired focal points in the image field in front of the array. The profile of the transmit beams in the elevation dimension can exhibit a point focus, a plane wave, or any intermediate beam

profile. Echoes returned by cells and tissue from the transmitted beams are received by the array elements and coupled to channels of the

microbeamformer 14 where they are individually delayed. The delayed signals from a contiguous patch of transducer elements are combined to form a partial sum signal for the patch. In an analog

microbeamformer implementation, combining is done by coupling the delayed signals from the elements of the patch to a common bus, obviating the need for summing circuits. The bus of each patch is coupled to a conductor of a cable 16, which conducts the partial sum patch signals to the system mainframe. In the system mainframe analog partial sum signals are digitized and coupled to channels of a system

beamformer 22, which appropriately delays each partial sum signal. The delayed partial sum signals are then combined to form a coherent steered and focused receive beam. System beamformers are well known in the art and may comprise electronic hardware components, hardware controlled by software, or a microprocessor executing beamforming algorithms. In the case of a digital beamformer the beamformer includes A/D converters which convert analog signals from the microbeamformer into sampled digital echo data. The beamformer generally will include one or more microprocessors, shift registers, and or digital or analog memories to process the echo data into coherent echo signal data. Delays are effected by various means such as by the time of sampling of received signals, the write/read interval of data temporarily stored in memory, or by the length or clock rate of a shift register as described in US Pat. 4,173,007 (McKeighen et al . ) The beam signals from the image field are processed by a signal and image processor 24 to produce 2D or 3D images for display on an image display 30. The signal and image processor may comprise electronic hardware

components, hardware controlled by software, or a microprocessor executing image processing algorithms. It generally will also include specialized hardware or software which processes received echo data into image data for images of a desired display format such as a scan converter.

Control of ultrasound system parameters such as probe selection, beam steering and focusing, and signal and image processing is done under control of a system controller 26 which is coupled to various modules of the system. The system controller may be formed by ASIC circuits or microprocessor circuitry and software data storage devices such as RAMs, ROMs, or disk drives. In the case of the probe 10 some of this control information is provided to the

microbeamformer from the system mainframe over data lines of the cable 16, conditioning the

microbeamformer for operation of the 2D array as required for the particular scanning procedure. The user controls these operating parameters by means of a control panel 20. This basic ultrasound system block diagram illustrates the partitioning of beamformation between the microbeamformer, which performs beamforming of the signals from a patch of elements, and the system beamformer which completes the beamformation process by combining the partial sum signals from the patches.

FIGURE 2 illustrates a portion of 2D array transducer 12 which is configured in eight element by ten element patches. For ease of illustration only the length dimension in the azimuth plane is shown. The array of FIGURE 2 may alternatively be viewed as a one dimensional array configured in ten-element patches. In the drawing alternate patches are shaded. The number of elements of each patch is indicated above the respective patch. The array transducer 12 may be operated to transmit a main beam 50 from the array and receive echo signals with each element of the array. The signals from the

individual elements of a patch are each delayed in microchannels 18 of the microbeamformer 14 and the delayed signals are combined at a node 28 at the microchannel output to form a partial sum signal PSi, PS ₂ of the patch for the main beam. In FIGURE 2 the microchannel symbols are of different sizes to indicate the relative length of its delay time. The partial sum signals PSi, PS ₂ are then coupled through the probe cable 16 with other patch signals (not shown) to the system beamformer 22 where the partial sum signals are delayed relative to each other, then combined to form a main beam signal.

Suppose that it is further desired to

simultaneously form additional beams from the

received signals, shown as multilines 52 and 54 on each side of the main beam 50. To form simultaneous multilines in the system beamformer it is necessary to delay and sum the partial sum signals differently for each multiline, depending on the location of each multiline. However, in this example it is seen that the multilines are very close to the main beam and within the dimensions of the same patch of the array, the patch indicated by the bracket above the array. The problem is that there is only a single partial sum signal from this patch, which represents one particular focusing of the elements of the patch, that required for the main beam 50. The partial sum signal cannot be undone and parsed into separate sub- signals which are needed to differently focus the multilines. This is a particular problem for echoes received from the near field, where the receive aperture is small and may be no larger than a patch. Consequently, the multilines 52 and 54 can only be focused the same as the main beam 50 in the near field, causing the resulting image to appear blocky with poor definition in the near field.

A solution to this problem is shown in FIGURE 3. This is to redefine the patches as smaller sized patches, particularly in the region where multilines 52 and 54 are to be formed on either side of the main beam 50. In this example this central portion of the array 12 has been configured with four 2 by N element patches. On each side of these patches is a pair of 3 by N patches, and outward from them are pairs of five by N patches, with the other visible lateral patches being ten by N patches. The microchannels for three different size patches are shown in detail in FIGURE 3. A two-element patch has two

microchannels 18a coupled at their outputs to a partial sum node 28a, a three-element patch has three microchannels 18b coupled at their outputs to a partial sum node 28b, and a five-element patch has five microchannels 18c coupled at their outputs to a partial sum node 28c. The partial sum signals produced at the summing nodes, including partial sum signals PS ₃ , PS ₄ , and PS5, are coupled by conductors of the probe cable 16 to the main system beamformer, where the beamformation process is completed and the multilines formed. The numerous partial sum signals from the small central patches on either side of the main beam center can be relatively delayed differently in the formation of multilines 52 and 54, steering and focusing the multilines more precisely on either side of the main beam, which is itself a product of different delays of the same partial sum signals.

Thus, the main beam 50 and its lateral multilines 52 and 54 will be well defined in the resultant image, particularly in the near field.

This use of different size patches can give rise to a problem, however, which is that the output signal and noise levels of each partial sum (patch signal) can vary significantly from one patch to another, depending on the patch size. This is because a large patch produces an output which is the sum of a large number of microchannel outputs, whereas a small patch produces an output which is the sum of a small number of microchannel outputs. One solution to this would be to include an analog gain stage before the ADC (analog to digital converter) that digitizes each partial sum signal. However, the analog gain for all channels is generally set by a global TGC gain stage for all channels. This global gain will not equalize the signal and noise of the individual partial sum signals, however. A

selectable gain control in each microchannel would boost the voltage gain in the output buffers of microchannels that are part of smaller patches so as to increase the signal and noise that eventually is summed in the partial sum signal. A problem with this approach is that the analog signal path within a microchannel is ideally using the full available dynamic range of the output buffer on each

microchannel. Boosting the gain on a few of them will lead to saturation of the output buffers, causing clipping of peak signals. This further problem could be averted by reducing the TGC gain in microchannels where the output gain is boosted.

Reducing the TGC gain to keep selected high-gain microchannels from saturating implies that other microchannels contributing to larger patch size partial sums are not fully using the dynamic range available in the microchannel, which limits

sensitivity .

It is an object of the present invention to provide greater latitude in the use of variable transducer patch sizes in the probe, and to be able to couple all of their partial sum signals to the system beamformer without signal degradation.

In accordance with the principles of the present invention, a microchannel which accommodates use in different patch sizes without signal degradation is shown in block diagram form In FIGURE 4.

Microchannel 18 is one of "U" microchannels in a group of microchannels in a microbeamformer 14, which has "B" groups of microchannels for the many elements of a 2D array transducer. As mentioned above, prior to a scanning procedure control data is loaded into the microbeamformer 14 from the mainframe system to condition the probe to scan as desired by a user. In the illustrated implementation this data is coupled over the probe cable as serial data, which is loaded into digital Shift Register and Logic circuitry 52 and a shift register for a TGC Controller 56. For ultrasound transmission some of this data is used for transmit (Tx) control, which commands a high voltage

(HV) transmitter 54 in the microchannel to drive a transducer element 42 to produce the desired transmit waveform or pulse. During transmission a T/R switch is open as shown in the drawing. Following transmit, echoes are received by element 42 and the T/R switch is closed to couple received signals to a TGC

amplifier 68. The gain of the TGC amplifier is varied with depth under control of TGC Controller 56. The output signals of the TGC amplifier are delayed by a delay time dT in a delay circuit 60 as

determined by the data in a Focus register. The delayed signals are coupled by an amplifier 74 to a cable driver 78. In accordance with the present invention the microchannel output signal produced by the cable driver 78 is coupled through a variable output impedance Outimp. The output signal is directed by a multiplexer Outmux to a desired output node 28, where it is summed with the output signals of the other microchannels of the patch. The summed signals are applied to a conductor ARX of the probe cable 16, which couples the signals to the mainframe ultrasound system.

The output impedance Outimp of a microchannel is set in accordance with the number of microchannels of the patch, the number of outputs summed to form the partial sum signal. The passage of an output signal through the output impedance converts it to a current signal, which is summed with other current signals to form a partial sum signal which in turn is coupled over the probe cable and applied to an input

impedance of the receiving component in the mainframe system, generally a preamplifier or analog to digital converter. Thus, the signal at the system input is a function of the fixed input impedance and the

selectable output impedances of the patch

microbeamformers . Selection of a microchannel output impedance is done in consideration of the size of the patch, with a lower output impedance used for smaller size patches (fewer microchannel outputs combined) . The control of the output impedances can maintain the same noise level among partial sum signals produced by different size patches.

FIGURE 5 shows the structure of the variable output impedance circuit 100 used in an integrated circuit implementation of the present invention. The circuit is formed of a number of resistors R ₀ - 7 which can be selectably connected in series or parallel or a combination thereof. Microchannel output signals from the cable driver 78 are coupled to the top of the resistive network, and the bottom of the variable impedance circuit is coupled to a conductor of the probe cable. In the specific example of FIGURE 4, this is done by way of the output multiplexer,

Outmux. The output impedance of circuit 100 always includes the resistance of resistor R ₇ which is not switched. The other seven resistors R ₀ -R6 can be switched into and out of a parallel configuration with resistor R ₇ by closing and opening MOSFET switches in series with the resistors, as shown by switches 70 and 76. The selection of resistors is done by use of a three-bit decoder 102. The three- bit input address to the decoder is decoded to define the levels of the seven outputs of the decoder, which in turn are applied to the control gates of the

MOSFET switches. The three-bit address is loaded into the address register 104 from the Shift Register & Logic circuitry 52 during conditioning of the microchannel for a scanning procedure. Each output line of the decoder 102 is coupled to control a gate of one of the seven MOSFET switches 70-76, with a "1" bit turning on a MOSFET (closing the switch) and a "0" bit turning off a MOSFET (opening the switch.) Thus, the bit pattern of the decoder output

determines which of resistors R0- 6 are coupled in parallel with resistor R ₇ . The more resistors used, the lower the output impedance of the microchannel, and the greater the current of the partial sum signal applied to the input impedance of the mainframe system. If the patch size is increased, fewer parallel resistors are used in the microchannels , maintaining the same relative sum signal current at the system input.

FIGURE 6 illustrates a typical input for a mainframe ultrasound system. In this example a conductor 16 of the probe cable is coupled to the input of a preamplifier 82. The preamplifier has an inherent input impedance, and the partial sum current on conductor 16 is applied to the preamplifier input and dropped across the input impedance. The

amplified partial sum signal is applied to an analog to digital converter (ADC) 84, which digitizes the partial sum signal. The partial sum signal is coupled to the system beamformer 22 where it is used with the other partial sum signals from the probe to produce a fully beamformed signal. FIGURE 7 illustrates an integrated circuit implementation of the delay circuit 60 . The circuit 60 is a capacitive circuit which samples the signal produced by a transducer element 42 , stores the sample on a capacitor 62 of the circuit then, at a later time which defines the intended delay, the sample is read from the capacitor. The signal delayed in this manner is then available for further processing as shown by the subsequent circuit

elements in FIGURE 4 . The time that a signal is stored on a capacitor 62 i , 62 ₂ , ... 62 _M is determined by the operation of a write controller 64 and a read controller 66. The write controller is a pointer which determines the closure of one of switches 65 i , 652, ··· 65 _M , the brief closing of which samples the signal of transducer 42 at the output of buffer amplifier 68 and stores the sample on a capacitor. After a switch has "written" one sample to a

capacitor, the write controller then closes another switch 65 to store another sample of the signal on another capacitor 62 . The write controller thus stores in rapid succession a plurality of samples of the signals received by transducer element 42 during its reception interval. The frequency with which samples are acquired exceeds the Nyquist rate for the received frequency band, and is usually well in excess of this rate. The read controller 66 operates in a similar manner to read the stored signal samples after they have been stored on the capacitors for the desired delay period. The read controller closes one of switches 67 , coupling a stored signal sample to an output buffer 74 from which it is available for further processing. In a rapid succession a sequence of the sampled signals are read from capacitors 62 and the now-delayed samples are forwarded for further processing by the microchannel .

A detailed block diagram of an ultrasound system constructed in accordance with the principles of the present invention is shown in FIGURE 8. An

ultrasound probe 10 includes a two dimensional array transducer 12 which transmits electronically steered and focused beams over a planar or volumetric region and receives echo signals in response to each

transmit beam. The elements of the transducer array are coupled to microchannels of a microbeamformer

(yBF) 14 where the signals from the elements of each patch of the array are delayed by the microchannels and combined with selectable impedances at

microchannel outputs to form a partial sum signal for each patch. Suitable two dimensional arrays are described in U.S. Patent 6,419,633 (Robinson et al . ) and in U.S. Patent 6,368,281 (Solomon et al . )

Microbeamformers are described in U.S. Patents

5,997,479 (Savord et al . ) and 6,013,032 (Savord) . The transmit beam and receive beam processing

characteristics of the probe are controlled by data from a beamformer controller 26, which causes the apodized aperture elements of the array to emit a focused beam of the desired breadth in a desired direction through a region of interest for imaging.

When transmit pulses are coupled directly to a transducer probe by the beamformer controller the system beamformer is protected from high voltages by a transmit/receive switch 18. The received partial sum signals from the microbeamformer 14 are coupled to the system beamformer 22. The partially

beamformed echo signals from the microbeamformer are delayed and summed in the system beamformer to form fully beamformed single or multiple receive beams in response to a transmit beam. A suitable beamformer for this purpose is described in US Pat. 8,137,272 (Cooley et al . )

The receive beams formed by the beamformer 22 are coupled to a signal processor 24a which performs functions such as filtering and quadrature

demodulation. The echo signals of the processed receive beams are coupled to a Doppler processor 28 and/or a B mode processor 24. The Doppler processor 28 processes the echo information into Doppler power or velocity information. For B mode imaging the receive beam echoes are envelope detected and the signals logarithmically compressed to a suitable dynamic range by the B mode processor 24. The echo signals from a volumetric region are processed to form a 3D image dataset by a 3D image processor 32.

The 3D image data may be processed for display in several ways. One way is to produce multiple 2D planes of the volume. This is described in U.S.

patent 6,443,896 (Detmer) . Such planar images of a volumetric region are produced by a multi-planar reformatter 34. The three dimensional image data may also be rendered to form a perspective or kinetic parallax 3D display by a volume renderer 36. The resulting images, which may be B mode, Doppler or both as described in US patent 5,720,291 (Schwartz), are coupled to a display processor 38, from which they are displayed on an image display 40. User control of the beamformer controller 26 and other functions of the ultrasound system are provided through a user interface or control panel 20.

One skilled in the art will appreciate that the controllable output impedances of the microbeamformer microchannels can be used to control the apodization of the received signal. More resistors can be coupled in parallel to produce lower output impedances and thus higher signal levels at the center of the receive aperture, and fewer resistors producing higher output impedances and lower signal levels at the lateral extremes of the receive

aperture can be implemented using the principles of the present invention, for instance.

It should be noted that the various embodiments described above and illustrated by the exemplary ultrasound system of FIGURES 1 and 8 may be

implemented in hardware, software or a combination thereof. The various embodiments and/or components, for example, the modules, or components and

controllers therein, also may be implemented as part of one or more computers or microprocessors. The computer or processor may include a computing device, an input device, a display unit and an interface, for example, for accessing the Internet. The computer or processor may include a microprocessor. The

microprocessor may be connected to a communication bus, for example, to access a PACS system. The computer or processor may also include a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM) . The computer or processor further may include a storage device, which may be a hard disk drive or a removable storage drive such as a floppy disk drive, optical disk drive, solid-state thumb drive, and the like. The storage device may also be other similar means for loading computer programs or other instructions into the computer or processor.

As used herein, the term "computer" or "module" or "processor" may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set computers (RISC) , ASICs, logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of these terms.

The computer or processor executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also store data or other

information as desired or needed. The storage element may be in the form of an information source or a physical memory element within a processing machine .

The set of instructions may include various commands that instruct the computer or processor as a processing machine to perform specific operations such as the methods and processes of the various embodiments of the invention. The set of instructions may be in the form of a software program. The software may be in various forms such as system software or application software and which may be embodied as a tangible and non-transitory computer readable medium. Further, the software may be in the form of a collection of separate programs or modules, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to operator commands, or in response to results of previous processing, or in response to a request made by another processing machine.

Furthermore, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. 112, sixth paragraph, unless and until such claim limitations expressly use the phrase "means for" followed by a statement of function devoid of further structure.