Patent Application
20100191113
Embodiments of the invention relate to ultrasound imaging, and more particularly to acoustic radiation force impulse (ARFI) imaging.
Tissue stiffness has been shown to be a marker of disease. For example, some cancerous tissues are stiffer than the normal surrounding tissues. Treatments for certain conditions, such as ablation, also create stiffer regions of tissue. Acoustic radiation force impulse imaging refers to a method of pushing tissue with a relatively long and high intensity acoustic pulse and then tracking the displacement of the tissue. The ARFI imaging method gives information about the stiffness of the tissue.
The long and high intensity pulses used in ARFI can create thermal issues in the imaging system as well as in the object that is being imaged. Typically, the heating created during ARFI can be divided into transducer heating and tissue heating.
A portion of the electrical energy used to excite the crystals/ceramics or other materials in the transducer is lost in the form of heat, thereby causing transducer heating. In ARFI, transducer heating is a concern because of the high amplitude and long duration pulses required. The International Electrotechnical Commision (IEC) requires that the temperature of an ultrasound probe surface which contacts the patient never exceed 43° C. (IEC 60601-1). Generally, the transducer heating can be reduced by modifying the lens material or design, by including thermal management features in the acoustic stack, and through the use of active cooling devices.
In contrast, tissue heating related to ARFI pushing pulses is a more difficult problem to address. Although the rise in temperature inside a body being imaged can be monitored through ultrasound based and MRI based methods for remote temperature sensing, these are cumbersome, unreliable, or expensive. Typically, temperature rise in the body needs to be estimated based on models and assumptions. Even if the temperature can be monitored, there is little that can be done to remove the heat. Therefore, ways of reducing the amount of energy deposited in the tissue are required.
In one embodiment, an ultrasound imaging method is provided. The method includes identifying a plurality of locations within a region of interest, delivering a pulse sequence to two or more of the plurality of locations in a determined order, wherein the pulse sequence comprises a pushing pulse, and a tracking pulse, and applying a motion correction sequence to each of the plurality of locations where the pulse sequence is delivered.
In another embodiment, an ultrasound imaging system is provided. The system includes a transducer array configured to deliver an ARFI pulse sequence to a plurality of locations in a region of interest, wherein the ARFI pulse sequence comprises a tracking pulse, and a pushing pulse, a controller for controlling delivery of the ARFI pulse sequences to the plurality of locations in a determined order, or for controlling delivery of a motion correction sequence, and applying the motion correction sequence to each of the plurality of locations where the pulse sequence is delivered, and a signal processing unit for processing received data from the plurality of locations in response to the plurality of ARFI pulse sequences, and the motion correction sequence.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In certain embodiments, an ultrasound imaging method and system are provided. The imaging method of the present technique may facilitate reduction of heat in a region of interest (ROI) that is being imaged. The method includes identifying a plurality of locations within a region of interest, delivering a pulse sequence to two or more of the plurality of locations in a determined order. The plurality of locations may be selected manually or by employing an automated algorithm. The ROI is composed of a series of vectors or beams. As used herein, vectors refer to locations of pushing and tracking that are used to make an ARFI image. Typically, in ultrasound imaging, the ROI is repeatedly interrogated by firing the same group of vectors multiple times and displaying the results as a series of images that change over time. Usually, the locations of the vectors are held constant from frame to frame. As used herein, a frame refers to the collection of vectors that makes up an ROI that are fired at a similar time. In one embodiment, the plurality of locations may exist in a single frame. The order in which the vectors are fired is chosen to minimize the heat delivered, however, the same locations are used from frame to frame. In another embodiment, the plurality of locations may be inter-grid locations to facilitate reduction of thermal dose. In this embodiment, not only is the order of the vectors different, but the locations of the vectors may also vary from frame to frame. For example, the second frame of firing may fire vectors at locations that would be between the locations of the firings in a first frame. This allows, the location of peak energy deposition to be changed from frame to frame.
The determined order for which the pulse sequences are to be delivered may be selected based on a cost function, for example, which may be designed to minimize the total amount of heat or thermal dose for a given location. The pulse sequence may be delivered to a particular location one or more times. In embodiments where the pulse sequence is delivered to the locations only once, the pulse sequence may include a reference pulse, a pushing pulse and a tracking pulse. Whereas, in embodiments where the pulse sequences are delivered to the plurality of locations two or more times, the different pulse sequences may or may not contain the reference pulse. In one embodiment where the pulse sequences do not include the reference pulse, the reference pulse may be delivered to the location initially when the pulse sequence is being delivered for the first time to the location, and the subsequent pulse sequences may be delivered without any reference pulse. In another embodiment where the pulse sequences comprise the reference pulse, the reference pulse may be delivered with each pulse sequence.
In certain embodiments, a motion correction sequence may be applied to each of the plurality of locations where the pulse sequence is delivered. The motion correction sequence takes into account any involuntary motion of the imaging object, such as a patient, or the transducer probe, or the person (such as the sonographer or the physician) performing the imaging. The transducer array may be a one-dimensional, or a two-dimensional array. The motion correction sequence may be delivered between the pulse sequences. The motion correction sequence may be delivered immediately before or after delivering a pulse sequence to a particular location. In one embodiment, the motion correction sequence may include a B-mode sequence. The B-mode sequence may be a complete B-mode sequence or a partial B-mode sequence, or a combination of a complete and a partial B-mode sequence.
Subsequently, a partial B-mode sequence represented by the three vectors 20 may be delivered in and around the same location 18 where the first pulse sequence 16 is delivered. The image obtained from the partial B-mode sequence, such as the sequence 20, may be correlated with the image obtained from the original frame B-mode pulse sequence 10 to determine the location of the pulse sequence, such as the pulse sequence 16 in real space. Also, all the subsequent firings of the pulse sequences may be aligned with the original frame B-mode image. By determining the location of the pulse sequence in the real space, the subsequent firing locations of the pulse sequences may be corrected for the newly characterized motion. In addition, an algorithm may be applied to create an image (for example, a two dimensional image) by interpolating to a grid (for example, a two dimensional grid) or sector given the known real space location of the pulse sequences. Generally, the pulse sequence (pushing pulse and a tracking pulse) is a long series of firings, and the additional partial B-mode sequence fired immediately before or after the pulse sequence utilizes only a fraction of time used by the pulse sequence.
In one embodiment, the size of the partial B-mode is selected based on the determined level of a thermal dose, the imaging time, or the motion of tissues in at least one of the plurality of locations. As used herein, the term “size of the partial B-mode” refers to the lateral width of the partial B-mode image and the term “density of the partial B-mode” refers to the number of vectors in the partial B-mode. The size and density of the partial B-mode sequence may be selected based on a number of factors. For example, a large partial B-mode sequence provides more data for correlation than a small partial B-mode sequence resulting in better motion correction. However, as the partial B-mode sequence gets larger there is less space to slide the window of the region of interest in the original B-mode sequence, hence, the range of motion is reduced. Also, as the size of the partial B-mode sequence gets larger the amount of time taken to collect the data increases. In addition, heating from the partial B-mode transmissions becomes larger. Additionally, if the motion of the ROI is not rigid, as the size of the partial B-mode sequence gets larger and a comparison is made against a now warped version of the original B-mode, a poor correlation is obtained. If the motion is a simple translation of the entire field of view, then correlation processing will track the motion well. However, if the motion is more complex and different parts of the tissue move by different amounts or in different directions, then the correlation processing is less effective. A smaller partial B-mode needs the motion to be constant over a smaller region and thus is less affected by the overall non-rigid motion.
Next, a second pulse sequence represented by the vector 22 may be delivered at a second location 24 that is shifted by a distance 28 from the desired location 30. This shift in the desired location 30 and the actual location 24 may be caused due to the inadvertent shift 26 in the position of the transducer probe 12, for example.
A partial B-mode sequence represented by the three vectors 32 is then delivered in and around the actual position 24. Next, a third pulse sequence represented by the vector 34 may be then delivered to an actual location 36, which may be shifted by a distance 38 from the desired location 40 due to an involuntary shift 44 in the position of the transducer probe 12, for example. A partial B-mode sequence represented by the three vectors 46 may be delivered in and around the actual location 36.
Subsequently, a fourth pulse sequence represented by the vector 50 may be delivered to a location that may fall outside the frame 14 due to a further shift 52 in the probe position. The shift 52 in the probe position results in a shift 54 of the desired location 56 for delivering the pulse sequence 50. Due to this shift 52 of the probe 12, the partial B-mode sequence 60 may be delivered in and around the actual location. Accordingly, at least a portion of the B-mode sequence 60 may fall outside the frame 14.
In some embodiments, each of the first, second, third and fourth pulse sequences 16, 22, 34 and 50, respectively, include a pushing pulse and a tracking pulse. In other embodiments, the first pulse sequence includes a reference pulse in addition to the pushing pulse and the tracking pulse, whereas, the remaining pulse sequences include only the pushing pulse and the tracking pulse. In some embodiments, all the pulse sequences may include a reference pulse, a pushing pulse and a tracking pulse. The reference pulse may be delivered to detect an initial position of the location, the pushing pulse may be delivered to a location to displace the tissues of the location to a first displaced position, and the tracking pulse may be delivered to a location to detect the first displaced position of the target region. The pushing pulse may be a single pulse or a combination of pulses. Similarly, the tracking pulse may either be a single pulse or a series of pulses.
The delivery of the pulse sequences to a particular location may be separated in time. This separation may be determined by the time required to allow the tissue to settle back into a particular state, which may either be the initial state, or a slightly displaced state. In one embodiment, additional time, also referred to as cooling time, may be added between delivering the pulse sequences to facilitate reduction of tissue heating. Typically, the amplitude and length of the pushing pulse determines the pace at which the tissue heats up. In embodiments, where a single frame of an image is desired, it may be possible to acquire the image without significant heating of the tissue. However, if multiple frames are desired, for example, to track change over time, or to provide averaging ability, then cumulative heating may result. In one embodiment, the cooling time between the individual pulse sequences may be adjusted depending on the number of frames required for the application. For example, if a single or small number of frames is required, the cooling time may be smaller, thereby allowing for faster collection. Whereas, if a large number of frames is required, the cooling time between the pulse sequences may be increased to reduce the cumulative heating effect.
The illustrated embodiment of
Although
Once the displacements have been determined for all of the vectors in the sequence, the scan conversion can interpolate an image in the plane using the actual locations of the vectors, thereby removing any distortion introduced by the motion.
In certain embodiments, a similar method as described in
As mentioned above, in certain embodiments, the scan sequence may be modified to minimize the heating of the tissue in the region of interest. Delivering a pulse sequence repeatedly in the same direction results in increased tissue heating because all of the energy is deposited in the same location. However, delivering pulse sequences at spatially close locations near in time also can lead to increased tissue heating. Therefore, the scan sequence may be chosen to minimize the tissue heating.
At block 148, an order of delivering the pulse sequences to the plurality of locations is determined. The first push location of the determined order may be either chosen randomly, or may be a pushing location from a previous frame, or selected by the operator. The determined order may be based on a cost function that may be evaluated for each of the possible locations where the pulse sequence is to be delivered. The cost function may be designed to minimize the total thermal dose and the peak temperature rise. In one embodiment, the cost function is based on thermal models of the system. At block 150, the pulse sequences are delivered to each of the plurality of locations. The push location which minimizes the cost function (and therefore minimizes the thermal impact) is chosen as the next push location. Optionally, at block 152 based on thermal models or absolute rules, there may be cooling delay may be inserted at any point in the above scan sequence to ensure that the temperature rise is at an acceptable level. For example, if the cost function for the next position is higher than a threshold (i.e. if the thermal dose increases substantially with the next firing) then a cooling delay may be inserted by the algorithm. This process is repeated until the entire region of interest has been placed in the firing order. A cooling delay may be inserted by switching off the transducer probe between delivering two or more pulse sequences for example. At block 152, a motion correction sequence is applied to the plurality of locations. The motion correction sequence may be applied in a similar manner as discussed with regard to
In embodiments where multiple frames are to be imaged, the process illustrated in flow chart 140 is iterated through the subsequent frames. In some of these embodiments, the push locations may be moved to inter-grid locations to help reduce the heating at the peak. This shift can be accounted for in the scan conversion. This movement may reduce the total thermal dose.
In one embodiment, the cost function is based on a finite element model of the thermal spatiotemporal distribution. In this embodiment, the finite element model may model one or more of a transducer field, an ultrasound field, and a thermal distribution created by the ultrasound transmission. In another embodiment, relatively simpler ultrasound field models, which can be calculated more promptly, may be used as inputs to a finite element model which calculates the thermal distribution. The finite element model can model a simple homogenous material, or it can assume a typical configuration such a layer skin, a layer of fat, and a soft tissue layer, or it could be based on a complicated model made from an ultrasound, CT, MRI or other image.
In one embodiment, a simplified model may be used to determine the thermal cost of a firing. In this embodiment, it is assumed that the thermal dose delivered by a push pulse firing has a Gaussian spatial distribution in the lateral dimension. In the present embodiment, a model for the lateral distribution is provided for simplicity. Although the axial and elevational distributions may also be modeled. Assuming that the thermal distribution takes the form provided by Equation 1:
where, S(x) is the spatial variation in the thermal distribution, x is the lateral spatial coordinate, x0 is the later position of the focus of the ultrasound pushing beam, and σ is the characteristic width of the thermal beam. σ is a function of the tissue and a function of the pushing pulse.
It is assumed that the temporal portion of the spatiotemporal distribution is modeled by an exponential decay of the form provided by Equation 2.
where, T(t) is the temporal variation of the thermal distribution, t is the time, and τ is a characteristic decay time which is a function of the tissue.
Further, assuming that the thermal contribution from a particular pushing pulse at a particular location and a given time is the product of the spatial and temporal factors.
D(x,t)=S(x)*T(t) Equation 3
Alternatively, assuming that the total thermal contribution at a particular spatial location and time is given by the sum of thermal contributions of all the previously fired pushing beams.
Given a set of vectors in a ROI, the firing order can be determined as follows. To begin, a first vector to fire is selected. D(x,t) values are calculated for each of the remaining possible vectors using Equation (3), where x is the location of the possible firing, and t is the current firing time. The sum of the D(x,t) values for each of the previously fired pushing vectors is determined, and the vector with the minimum sum is the next vector to be fired. In embodiments, where the sum of the D(x, t) values is greater than a threshold, then a delay may be introduced prior to firing the next pulse sequence.
Next, a determination is made as to which of the possible vectors has the minimum sum of the thermal contributions. The vector with the minimum sum will be the next vector to fire. If the sum is greater than a threshold, then a cooling delay is introduced prior to delivering the pulse sequence at the next location. The cooling delay may be determined such that the sum of D(x,t) for the new value of t is below the threshold value. This process is then repeated until all of the vectors in the ROI have been assigned to fire at a particular time.
The spatial characteristic distance, σ, and the temporal characteristic time, τ, will typically effect the firing order. These values should be determined for the particular tissues and ultrasound beam parameters being used.
In some embodiments, one or more parameters of the pushing pulse or the tracking pulse may be altered from one location to another. In another embodiment, the parameters of the pushing pulse or the tracking pulse may be varied while delivering a subsequent pulse to the same location. In one embodiment, the one or more parameters that may be varied may include amplitude, a peak power, an average power, a length (length of the pushing pulse or the length of the pushing pulse packet), a frequency, a waveform, or combinations thereof. In another embodiment, the pulse repetition frequency (PRF) of the tracking pulse may be varied.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
