Medical diagnostic ultrasound system and method for automated triggered intervals

A region of interest in the body is imaged using at least two different trigger intervals between images. Imaging automatically switches from one trigger interval to another in response to a user command, such as depressing a button. This automation avoids cumbersome manual changes of the trigger intervals. Perfusion is measured in a shorter time in this way, reducing the effects of breathing and transducer movement. Variation of the trigger intervals allows for a convenient determination of perfusion. For example, the trigger intervals are varied from one heart cycle to two heart cycles and then to other integer numbers of heart cycles.

BACKGROUND
 This invention relates to a medical diagnostic ultrasound system and method
 for automated triggering, such as for use in perfusion imaging or studies.
 In particular, automated triggering is used for determining perfusion
 using contrast agents.
 Various methods of imaging contrast agents have been proposed for measuring
 perfusion or perfusion-related parameters. For example, a method is
 described by Dr. Wei in "Quantification of Myocardial Blood Flow with
 Ultrasound-Induced Destruction of Microbubbles Administered as a Constant
 Venous Infusion", Circulation, volume 97, pp. 473-487, 1998. Using an ECG
 signal, images of microbubbles are generated by an ultrasound system at
 fixed trigger intervals. As each image is generated after a trigger
 interval, the microbubbles are destroyed. The amount of contrast agent
 that reflows into a region of interest during the trigger interval as
 represented on the subsequent image is measured. A fixed trigger interval
 is then manually changed and the measurement is repeated. Based on these
 measurements, a re-flow curve showing the amount of contrast agent flowing
 into a region as a function of the trigger interval is plotted. Due to the
 manual change of the trigger interval, an unnecessarily long amount of
 time is required to acquire the images and measure the amount of contrast
 agent re-flow.
 Other triggering schemes for imaging contrast agents are known, such as
 shown in U.S. Pat. No. 5,833,613. An ultrasonic transmission is triggered
 off of an ECG signal to destroy contrast agent. Within the same heartbeat,
 an image is generated at a certain time after transmission of the
 destructive ultrasonic energy. The time between the transmission for
 destruction and transmissions for imaging is varied within the heartbeat.
 Other triggering schemes are shown for example in U.S. Pat. No. 5,686,310.
 To assist a clinician during trigger imaging of contrast agent, images
 generated using low-power transmission may be interleaved with the trigger
 or destructive transmissions. The images allow the user to maintain the
 transducer in the correct position relative to the region of interest with
 reduced destruction of the contrast agents. Such imaging is taught, for
 example, in U.S. Pat. No. 6,110,120 (U.S. application Ser. No.
 08/838,919), filed Apr. 11, 1997.
 BRIEF SUMMARY
 The present invention is defined by the following claims, and nothing in
 this section should be taken as a limitation on those claims. By way of
 introduction, the preferred embodiments described below include a method
 and system for triggering to determine perfusion in a body. A region of
 interest in the body is imaged using at least two different trigger
 intervals between images. Imaging automatically switches from one trigger
 interval to another in response to a user command, such as depressing a
 button. This automation avoids cumbersome manual changes of the trigger
 intervals. Perfusion is measured in a shorter time in this way, reducing
 the effects of breathing and transducer movement.
 Variation of the trigger intervals allows for a convenient determination of
 perfusion. For example, the trigger intervals are varied from one heart
 cycle to two heart cycles and then to other integer numbers of heart
 cycles.
 In one aspect, a medical diagnostic ultrasound system for triggering to
 determine perfusion in a body is provided. A triggering device, responsive
 to a periodic signal source, is operable to provide trigger signals. A
 transmitter transmits pulses in response to the trigger signals. A user
 interface is operable to receive triggering input, where the triggering
 device automatically switches from a first predetermined interval between
 trigger signals to a second, different predetermined interval in response
 to the triggering input. A method corresponding to the system described
 above is also provided.
 In a second aspect, a medical diagnostic ultrasound system for transmitting
 pulses to determine perfusion in a body during an imaging session is
 provided. A triggering device, responsive to an ECG signal source, is
 operable to provide triggering signals. A transmitter transmits pulses in
 response to the trigger signals. During the imaging session, the
 triggering device automatically switches from a first predetermined
 integer heart cycle interval between trigger signals to a second
 predetermined integer heart cycle interval, where one of the first and
 second predetermined integer heart cycle intervals is a greater number of
 heart cycles than the other.
 Further aspects and advantages of the invention are discussed below in
 conjunction with the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Imaging with different trigger intervals allows determination of perfusion.
 An ultrasound system automatically acquires images separated by these
 different trigger intervals. For example, two images separated by
 one-heartbeat intervals are acquired, followed by two images separated by
 two heartbeat intervals. Switching between the trigger intervals is
 performed automatically. For example, the system is programmed to switch
 between intervals after a certain number of images are acquired or
 automatically switches in response to user input of triggering control
 information, such as the depression of a button. The user then reviews the
 acquired images, and/or the system calculates a perfusion parameter from
 the acquired information.
 Automatic switching allows for different trigger intervals during one
 imaging session without suspending imaging to manually change the trigger
 interval. Thus, simple and quick determination of a perfusion parameter is
 provided. The perfusion parameter may be more accurate due to the use of
 multiple integer heart cycles for the triggering intervals. Depending on
 the trigger intervals used, a patient may hold their breath while all of
 the information for determining the perfusion parameter is acquired,
 minimizing relative transducer and region of interest movement.
 Furthermore, such techniques allow for repeatability and easier
 quantification determination by an ultrasound system or other
 quantification software.
 Turning now to the drawings, FIG. 1 is a block diagram of one preferred
 embodiment of a medical diagnostic ultrasound imaging system 10 that
 incorporates a presently preferred embodiment of this invention. The
 system 10 includes a periodic signal source generator 12, a trigger device
 14, a user interface 16, a transmit beamformer 18, a transducer 20, a
 receive beamformer 22, a signal processor 24, a display 26, and CINE
 memory 28. Additional or fewer components may be used in the system 10.
 For example, the system 10 may not include the CINE memory 28. Both analog
 and digital systems are suitable. Ultrasound systems marketed by Acuson
 Corporation under the trade names 128 XP, Aspen and Sequoia are capable of
 being modified to implement this invention. The Sequoia ultrasound imaging
 system is described for example in the following patents, assigned to the
 assignee of the present invention: U.S. Pat. Nos. 5,549,111, 5,551,433,
 5,555,534, 5,570,691, 5,581,517, and 5,617,862, the disclosures of which
 are herein incorporated by reference. Ultrasound systems manufactured by
 others may also be adapted to implement this invention.
 The periodic signal source generator 12 comprises hardware and/or software
 for generating a periodic signal, such as an ECG device which recognizes
 the R wave or other portions of an ECG signal, a system clock or timer, a
 device for monitoring the breathing cycle, or other devices for monitoring
 periodic functions. In alternative embodiments, the periodic signals
 source generator 12 includes hardware and/or software for monitoring a
 combination of signals, such as monitoring the heart cycle relative to the
 respiration cycle. Periodic signal source generator 12 outputs a signal
 representing a portion of a period, or a signal representing the variance
 of a parameter throughout the period.
 The trigger device 14 responds to the output of the periodic signal source
 generator 12 and optionally responds to the user interface 16. The trigger
 device 14 comprises a processor, a digital signal processor, an ASIC,
 dedicated hardware or other devices for monitoring the output of the
 periodic signal source and responsibly generating trigger signals. In one
 preferred embodiment, the trigger device 14 comprises a transmit
 beamformer controller, such as disclosed in U.S. Pat. Nos. 5,581,517 and
 5,675,554. As another example, the trigger device 14 comprises a
 controller of the system 10, such as a general processor operating
 pursuant to software control.
 The trigger signals generated by the trigger device 14 comprise a control
 instruction, a change in a signal, a pulse, or an another signal
 indicating the beginning of a sequence to transmit and receive acoustic
 energy. The trigger signals are separated by intervals. Preferably, the
 intervals are predetermined. As used herein, predetermined is intended
 broadly to include intervals determined as a function of other inputs
 during imaging, intervals programmed before imaging, and other intervals
 that are not just a function of slight physiological changes, such as the
 natural variance of the heart cycle.
 The trigger device 14 is operable to generate trigger signals separated by
 at least two different intervals. Preferably, at least three different
 trigger intervals are provided, such as a 1-cycle interval, a 2-cycle
 interval, a 4-cycle interval, and an 8-cycle interval. Other schemes where
 at least two different intervals are provided may be used. In one
 preferred embodiment, the difference between the two intervals is an
 integer function of the heart cycle, such as one interval being one heart
 cycle and a second interval being two heart cycles. Alternatively, the
 intervals are related as a function of a fraction of a periodic cycle. In
 one preferred embodiment, the different intervals are timed to cause a
 positive increment between trigger signals (e.g. shortest to longest).
 Each trigger signal is used to initiate acquisition of one or more images.
 For example, two or more trigger signals separated by an interval
 representing one heart cycle are consecutively generated, resulting in
 sequential acquisition of two or more respective images separated by one
 heart cycle each. The trigger device 14 then automatically switches to
 generating trigger signals separated by a different interval, such as two
 heart cycles. Any of various triggering schemes, such as generating a
 different number of trigger signals separated by each of the at least two
 different (e.g. three trigger signals separated by one heart cycle,
 followed by two trigger signals separated by two heart cycles). One
 interval may be used a single or multiple times, such as separating
 trigger signals at the beginning of a sequence by one heart cycle and at
 the end of a sequence by one heart cycle.
 The trigger device 14 is also responsive to the user interface 16. As used
 herein, responsive to is intended broadly to include any situation where a
 first element alters its operation in response to a signal generated by a
 second element, whether directly or indirectly. Thus, the first element is
 said to be responsive to the second when the first element responds
 directly to an output signal of the second element. Similarly, the first
 element is responsive to the second when intermediate elements or
 processors alter or modify a signal of the second element before it is
 applied as an input to the first element.
 The user interface 16 comprises a keyboard, a mouse, soft screen key
 display, and associated hardware and software, a track ball, dedicated
 buttons, a voice activation system or other devices for receiving input
 from the user. In one preferred embodiment, the trigger device 14 is
 programmed in response to input from the user interface 16. For example,
 various aspects of the triggering scheme are programmed by the user,
 including one or more of the different intervals, one or more portions of
 the periodic signal to count or otherwise use for initiating the trigger
 signals, the order in which the different intervals are used, and the
 number of triggering signals separated by each interval, and other aspects
 of the triggering scheme. The user programs these various aspects by
 selecting a parameter for each individual aspect or by selecting from two
 or more triggering schemes that include all the aspects of the scheme. In
 alternative embodiments, the system 10 is pre-programmed through software
 programming or hardware devices to implement one particular scheme, and
 the user merely selects triggered imaging for perfusion studies.
 During an imaging session, the user interface 16 generates trigger control
 signals in one embodiment. The trigger control signals indicate a change
 in the triggering scheme, such as changing from one interval to a second
 interval between trigger signals. In response, the trigger device 14
 automatically switches from generating trigger signals separated by a
 first interval to trigger signals separated by a second interval. By
 depressing a button or other device on user interface 16, the user
 indicates the desire to change the predetermined intervals.
 In alternative embodiments, the triggering device 14 automatically switches
 from one interval to another interval without the triggering control input
 from the user interface 16. In yet other embodiments, the triggering
 device 14 is programmed to automatically switch between intervals without
 input from the user interface 16, but may still receive triggering control
 input from the user interface 16 to override that programming, or
 triggering control input is used for automatically switching between one
 subset of intervals and pre-programming of the triggering device 14 is
 used for switching in a second different subset of intervals. Regardless
 of the mechanism for automatically switching between intervals, the
 triggering device 14 automatically switches between predetermined
 intervals without manual selection of the interval during the imaging
 session. As used herein, imaging session comprises the transmission and
 reception of acoustic energy for imaging without manual alteration of
 imaging parameters, including predetermined intervals between images. The
 user does not have to stop acquisition of images in order to change the
 interval between trigger signals.
 In response to the trigger signals, the transmit beamformer 18 generates
 transmit waveforms for imaging. The transmit beamformer 18 comprises a
 signal generator, a pulse generator, a pulse shaper, a filter, or other
 dedicated hardware for generating transmit waveforms. In one preferred
 embodiment, the transmit beamformer 18 comprises the transmit beamformer
 disclosed in U.S. Pat. No. 5,675,554, the disclosure of which is herein
 incorporated by reference. Other transmit beamformers such as transmitters
 on commercially available ultrasound systems may be used. The transmit
 beamformer 18 includes digital components, analog components, or
 combination thereof.
 The transmit beamformer 18 generates transmit waveforms centered at a
 fundamental frequency. The spectral shape, bandwidth and/or transmit
 power, as well as other characteristics of the transmit waveform, are
 controlled by the transmit beamformer 18. For example, for reception of
 signals at harmonics of the fundamental frequency, the transmit waveforms
 are generated to minimize or eliminate energy at or near the harmonic of
 the fundamental frequency. In this embodiment, the energy level at the
 harmonic of the fundamental frequency for each transmit waveform is
 preferably at least 6 dB, more preferably at least 12 or 20 dB, and most
 preferably at least 30 dB below the energy level of the fundamental
 frequency. As another example, a narrow bandwidth is selected for
 destruction of contrast agents. Likewise, higher powers are used for
 greater destruction of contrast agents, and conversely, lower power is
 used to reduce destruction of contrast agents. For example, a low power
 transmit wave is generated for imaging between triggered images that
 destroy contrast agent. As another example, high power, narrow bandwidth
 transmit waveforms are generated to destroy contrast agents without
 responsive generation of an image. See for example, U.S. application Ser.
 No. 09/348246, filed Jul. 2, 1999 for Contrast Agent Imaging With
 Destruction Pulses in Diagnostic Medical Ultrasound, the disclosure of
 which is herein incorporated by reference. Transmit beamformers 18 capable
 of controlling none, fewer or additional characteristics of the transmit
 waveform may be used.
 The transmit beamformer 18 immediately begins generation of the transmit
 waveform in response to the trigger signal. In alternative embodiments,
 the transmit beamformer 18 begins a count down or otherwise implements a
 delay from reception of the trigger signal to output of the transmit
 waveform.
 The transmit waveforms are provided to the transducer 20. The transducer 20
 generates acoustic energy in response to the transmit waveforms. The
 transducer 20 also converts reflected ultrasonic energy into electrical
 signals. The electrical signals are provided to the receive beamformer 22.
 The receive beamformer 22 comprises buffers, summers, filters, ASICs,
 digital signal processors, processors, and other devices for delaying and
 summing the various signals from the transducer 20. In one preferred
 embodiment, the receive beamformer 22 comprises the beamformer disclosed
 in U.S. Pat. No. 5,555,534. The receive beamformers 22 used on commercial
 systems may be used. The receive beamformer 22 generates data representing
 a line through a region of interest, such as in phase and quadrature (I/Q)
 or RF data.
 In one embodiment, the receive beamformer 22 includes filters for
 selectively filtering out one of the fundamental transmit frequency or
 harmonics of the fundamental transmit frequency. For harmonic imaging, the
 filter removes or minimizes energies associated with the fundamental
 frequency bandwidth. Preferably, signals associated with a harmonic
 frequency, such as the second harmonic, are used for further processing.
 In another embodiment, energy at fundamental frequencies is used for
 further processing, regardless of any filtering.
 The signals output by the receive beamformer 22 are provided to the signal
 processor 24. The signal processor 24 comprises one or more digital signal
 processors, general processors, ASICs, or other dedicated hardware for
 detecting information from the received signals. For example, a Doppler
 processor and/or B-mode processor as well as optional spatial and temporal
 filters are provided for detecting and filtering the data. The signal
 processor 24 generates acoustic data by detection in any of these various
 modes. As used herein, acoustic data may also include data output by the
 receive beamformer or data at various stages of processing prior to and
 after detection, such as data prior to scan conversion or image data after
 scan conversion (e.g., ultrasound images).
 In one preferred embodiment, the signal processor 24 includes one or more
 processors, buffers, adders, multipliers, or other dedicated hardware for
 comparing acoustic data for different images. For example, B-mode
 intensities or Doppler energies above a threshold level are summed or
 averaged for each image. Based on a change in the sum or average as a
 function of the interval between trigger signals, a parameter representing
 perfusion is calculated. Comparison is also provided by plotting the sums
 or averages as a function of the trigger interval or the display 26. Other
 functions and data used for determining the quantities may be used to
 generate indicia of perfusion, such as calculations disclosed by Wei as
 discussed above.
 Another means for comparing acoustic data to indicate perfusion is the
 display 26. The display 26 comprises a monitor, LCD or other imaging
 device and a scan converter or other processors for generating an image
 from the acoustic data. For comparison, each image associated with a
 trigger signal is displayed sequentially. Subjectively, an amount of
 perfusion of the contrast agent into tissue structures is indicated in
 each image. Differences between the images acquired in response to
 different trigger intervals also indicate perfusion characteristics.
 In alternative embodiments, two or more images acquired at different times
 are displayed simultaneously. The user subjectively determines an amount
 of perfusion or other perfusion characteristic by comparing the two
 images, such as an image associated with a single heart cycle interval as
 compared to an image associated with a two heart cycle interval. In one
 embodiment, all the images associated with a same triggering interval are
 averaged and displayed adjacent to an average of images associated with
 different intervals. Other imaging schemes, including sequential or
 simultaneous display of B-mode, M-mode, Doppler or other display modes or
 combinations thereof may be used.
 Simultaneous display for comparison is preferred to assess myocardial
 perfusion, since echo cardiographers are better able to distinguish
 artifacts from perfusion when presented with side-by-side images. Any
 quantities and associated graphs are preferably displayed during the
 imaging session in real time, but may be displayed after the imaging
 session.
 The acoustic data is also stored in the CINE memory 28. The CINE memory 28
 comprises a buffer, RAM, or other memory device operable to re-generate
 the acquired images, such as for later sequential or simultaneous
 generation of images. Alternatively or additionally, a tape, diskette or
 other moveable memory device is provided to store acoustic data or
 generated images for later imaging or quantification.
 Referring to FIG. 2, one embodiment of a process for triggering to
 determine perfusion in a body is shown. In act 40, at least two different
 trigger intervals are programmed into the system 10. The user selects a
 number of periods for each interval and/or a delay for each interval. For
 example, the user selects one or more ECG R waves and a delay sufficient
 to place the trigger signal at the systole portion of the heart cycle. The
 selection is made for each interval, such as selecting a different integer
 number of heart cycles for each interval and a same or a different delay.
 In alternative embodiments, only a delay or only a number of cycles are
 selected. The user also selects the number of acquisitions or images to be
 acquired using any given interval. In alternative embodiments, the system
 10 selects the various intervals and associated parameter using
 pre-programmed information, such as based on a selected imaging
 application. In yet other alternative embodiments, the user selects from
 two or more pre-programmed triggering sequences.
 In act 42, imaging is triggered at a first interval. For example, based on
 an initiation signal (e.g. triggering input) from the user interface 16 or
 a pre-programmed delay after a particular image, a counter counts cycles
 or an amount of time before acquiring a first image. Preferably, the
 amount of time or number of cycles is the same as or longer than the first
 interval. A selected number of images are then acquired separated by the
 first interval. Where the interval is an integer number of cycles, the
 triggering signals are provided for each of those integer number of
 cycles. Preferably, each image or associated acoustic data is stored after
 initiation of the triggering sequence. For example a video or frame
 buffer, CINE memory or a DIMAQ format image is stored.
 In act 44, generating trigger signals at the first interval automatically
 switches to generating trigger signals at the second interval. As
 discussed above, the automatic switching occurs in response to triggering
 control signals from the user interface 16 or pre-programming of the
 system 10. For example, the system 10 automatically switches from one
 interval to another interval after acquiring a pre-determined or selected
 number of images or in response to depression of a button.
 In act 46, acquisition of images and the associated transmissions are
 triggered at a second interval. In one embodiment, the first and second
 intervals are both integer number of heart cycles, such as a first
 interval of one heart cycle and a second interval of two heart cycles. An
 amount or other characteristic is determined as a function of the acoustic
 data or images acquired at the two different trigger signal intervals.
 Referring to FIGS. 1 and 3, one embodiment of a preferred triggering scheme
 is shown. A pulse train 50 representing periodic signals indicative of the
 R wave portion of a heart cycle are provided by the periodic signal source
 12. The pulse train 50 includes a pulse 52 at each R wave.
 Below the pulse train 50 in FIG. 3 are act numbers 1 through 8 indicating
 various acts that occur as part of the triggering scheme. In act 1,
 imaging is ceased for 10 or more heart cycles. Preferably, all ultrasonic
 transmission cease, but low power or other energy that does not destroy or
 minimally affects perfusion of contrast agents may occur. After 10 or more
 heartbeats, ultrasonic energy is transmitted for generating an image in
 act 2. Preferably, the ultrasonic transmissions are associated with a high
 power to destroy contrast agents throughout the region of interest being
 imaged. The transmission and associated image are synchronized to occur at
 a specific time after the R wave pulse 52. The specific time is preferably
 chosen to coincide within systole. Preferably, all triggered transmissions
 and associated images are generated at the same portion of the heartbeat.
 In act 3, the triggering device 14 generates the next trigger signal after
 one heart cycle. In act 4, transmissions and associated images are
 generated. The same or different transmit waveforms may be used as are
 used for transmission in act 2. The resulting image demonstrates the
 amount of perfusion of contrast agent after one heartbeat in the region of
 interest. The trigger device 14 automatically switches to a two heart
 cycle interval and waits for two heart cycles in act 5. In act 6,
 transmissions and associated images are generated, demonstrating perfusion
 after two heart cycles. The trigger device 14 automatically switches to a
 four heart cycle interval in act 7. After waiting four heart cycles,
 transmissions and associated images are generated in act 8. The resulting
 image shows the amount of perfusion after four heart cycles. Preferably,
 all of the transmissions associated with this embodiment are high-power
 transmissions with a high density of scan lines to destructively image
 contrast agents.
 Referring to FIG. 4, another preferred embodiment of a triggering scheme is
 shown and includes transmissions for destroying contrast agents without
 generating corresponding images. A pulse train 60 including pulses 62
 representing the occurrence of an R wave is shown. In act 1, imaging and
 associated transmissions are ceased for 10 or more heart cycles (e.g. 10
 or more pulses 62). In act 2, ultrasonic energy is transmitted and
 reflections received to detect and image contrast agents. The
 transmissions for imaging may be low power or high power. The transmission
 is synchronized to occur at a specific time after the R wave, preferably
 chosen to coincide within systole. Other delays or no delay after the
 pulse 62 may be selected. Immediately after transmission and reception for
 imaging, ultrasonic energy is transmitted to destroy any remaining
 contrast agents. Preferably, the destructive transmissions are high power,
 narrow bandwidth signals. In act 3, transmissions or at least
 transmissions substantially destructive of contrast agents are ceased for
 an interval of one heart cycle. In act 4, the transmissions of act 2 are
 repeated in reverse order. The transmission of the destructive beam
 precedes the transmission of the beam for imaging. The resulting image
 represents tissue and substantially no contrast agent. In act 5, the
 triggering device 14 provides trigger signals still separated by one heart
 cycle. In act 6, transmissions associated with imaging are generated and
 then followed by transmissions associated with destruction of contrast
 agents. The resulting image shows the amount of perfusion or re-flow of
 contrast agent after one heart cycle. In act 7, the triggering device 14
 automatically switches to a two heart cycle interval. In act 8,
 transmissions for imaging are followed by transmissions for destruction.
 The resulting image represents perfusion or re-flow of contrast agent
 after two heart beats. In act 9, the triggering device 14 automatically
 switches to providing triggering signals separated by an interval of four
 heart cycles. In act 10, transmissions associated with first imaging and
 secondly with destruction of contrast agents are generated. The resulting
 image represents the amount of reflow or perfusion by contrast agents
 after four heart cycles.
 The images acquired using the schemes of either of FIGS. 3 or 4 are
 preferably simultaneously displayed. In alternative embodiments, more than
 one transmissions and associated multiple images responsive to any single
 interval may be acquired. In other embodiments, the partial cycle delays
 associated with each interval vary as a function of the interval. Thus,
 depending on the interval, an image representing a different portion of
 the heart cycle is created. In yet other alternative embodiments, a
 sequence of images responsive to the same integer number of heart cycles
 separation but with a swept delay to image different portions of the heart
 cycle is used, followed by swept delay after a different number of integer
 heart cycles.
 Triggered or non-triggered additional low power transmissions for imaging
 without or with minimal destruction of contrast agents may be provided.
 Preferably, these transmissions are generated to minimize destruction of
 contrast agent, such as disclosed in U.S. Pat. No. 6,110,120 (Ser. No.
 08/838,919) filed Apr. 11, 1997, the disclosure of which is herein
 incorporated by reference. These low power transmissions can be used to
 maintain image orientation or location during a perfusion assessment
 imaging session. In one embodiment, the system performs low-power,
 fundamental imaging whenever the system is not producing triggered images
 (i.e., locator imaging between triggered images). These orientation or
 location images preferably occur with sufficient frequency to produce real
 time images and are suspended when a triggered image is acquired. The
 operator may react to patient motion in order to maintain a constant image
 plane position and thus improve the accuracy of comparisons between the
 triggered images. Preferably, these low power transmissions are timed to
 occur at specific points during the triggering interval. In one
 embodiment, the low power transmissions are suspended for a period after
 each high-power transmission associated with triggering. In alternative
 embodiments, the system 10 does not generate any additional transmissions
 other than those associated with the triggering signals.
 While the invention has been described above by reference to various
 embodiments, it is to be understood that many changes and modifications
 can be made without departing from the scope of the invention. For
 example, various triggering schemes may be developed through testing and
 as a function of different imaging applications. The techniques described
 herein may be used for such triggering schemes.
 It is therefore intended that the foregoing detailed description be
 understood as an illustration of the presently preferred embodiments, and
 not as a definition of the invention. It is only the following claims,
 including all equivalents, that are intended to define the scope of this
 invention.