Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/06—Measuring blood flow
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/08—Clinical applications
  • A61B8/0891—Clinical applications for diagnosis of blood vessels
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/44—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device
  • A61B8/4483—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device characterised by features of the ultrasound transducer
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/48—Diagnostic techniques
  • A61B8/483—Diagnostic techniques involving the acquisition of a 3D volume of data
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/48—Diagnostic techniques
  • A61B8/488—Diagnostic techniques involving Doppler signals
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/52—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/5215—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves involving processing of medical diagnostic data
  • A61B8/5223—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves involving processing of medical diagnostic data for extracting a diagnostic or physiological parameter from medical diagnostic data
• • G—PHYSICS
  • G16—INFORMATION AND COMMUNICATION TECHNOLOGY [ICT] SPECIALLY ADAPTED FOR SPECIFIC APPLICATION FIELDS
  • G16H—HEALTHCARE INFORMATICS, i.e. INFORMATION AND COMMUNICATION TECHNOLOGY [ICT] SPECIALLY ADAPTED FOR THE HANDLING OR PROCESSING OF MEDICAL OR HEALTHCARE DATA
  • G16H50/00—ICT specially adapted for medical diagnosis, medical simulation or medical data mining; ICT specially adapted for detecting, monitoring or modelling epidemics or pandemics
  • G16H50/30—ICT specially adapted for medical diagnosis, medical simulation or medical data mining; ICT specially adapted for detecting, monitoring or modelling epidemics or pandemics for calculating health indices; for individual health risk assessment

Definitions

• This invention relates to medical diagnostic ultrasound systems and, in particular, to a non-imaging Doppler probe system for automated screening of carotid stenosis .
• Stroke is the third leading cause of death worldwide. According to the World Health Organization, stroke accounted for more than 5.5 million deaths in 2002 with about 50% of those deaths occurring in China and India. Although the incidence is declining in developed countries, stroke nevertheless accounts for a mortality of 163,000 in the United States. A
• the carotid artery bifurcation where the common carotid artery (CCA) branches into the internal (ICA) and external (ECA) carotid arteries, is a common site of atherosclerotic disease.
• CCA common carotid artery
• ECA external carotid artery
• carotid atherosclerosis and mortality associated with stroke is an increasing problem in the developing world.
• a carotid artery screening device that can be used in low resource settings would address this growing problem by offering several features. First, there is often a lack of skilled personnel in these settings, and so the device should be able to be used with minimal training and instruction. Second, the device should be
• the device should be able to classify the degree of stenosis in the carotid arteries in a highly automated way without the use of high-end duplex ultrasound scanners that may not be available in the rural setting.
• a diagnostic ultrasound system for carotid artery diagnosis includes a simple, non-imaging Doppler ultrasound probe.
• the probe has a two dimensional array of transducer elements with a low count of elements of relatively large size which can cover an area of the carotid artery at its bifurcation.
• the large sized elements are operated independently with no phasing, thereby reducing the cost of the Doppler system.
• the probe and system of the present invention can produce a representation of carotid blood flow in two or three dimensions and can assemble an extended view of the flow by matching segments of the carotid flow as the probe is moved over the vessel.
• FIGURE 1 illustrates in block diagram form an ultrasound system constructed in accordance with the principles of the present invention.
• FIGURE 2 illustrates the size and dimensions of the elements of an array transducer of a probe of the present invention.
• FIGURE 2a illustrates the angling of the transducer elements of the array of FIGURE 2 for good Doppler reception.
• FIGURE 3 illustrates a probe placement display of an ultrasound system of the present invention which guides an operator in proper probe placement over the carotid artery.
• FIGURE 4 illustrates the operation of a Doppler probe of the present invention in discrete rows of elements .
• FIGURE 5 illustrates a two dimensional vessel map produced by an ultrasound system of the present invention .
• FIGURE 6 illustrates a three dimensional vessel map produced by an ultrasound system of the present invention .
• FIGURE 7 illustrates the progressive assembly of a three dimensional vessel map produce by scanning with a probe and system of the present invention.
• FIGURE 8 illustrates spectral Doppler displays of flow conditions encountered in the carotid artery.
• FIGURE 9 is a flow chart of the automated
• FIGURE 10 is a diagram of the basic steps of flow analysis in accordance with the present invention.
• FIGURE 11 illustrates a Doppler spectrum with the upper and lower extremes of its envelope traced.
• An ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form.
• An ultrasound probe 10 contains a transducer array 12 of transducer elements which transmit ultrasound waves into the body and receive returning echo signals for Doppler processing. Control and timing of ultrasound transmission and reception is provided by a beamformer controller 16 which controls system beamformer 14.
• the beamformer 14 is not a conventional delay-and-sum beamformer as the transducer array is not operated in a phased manner. Instead, each element is separately actuated to transmit ultrasound waves directly into the body from its front surface and receive reflections from the waves individually.
• the beamformer controls the timing of successive transmit-receive intervals (the pulse repetition interval or PRI ) by each
• a quadrature bandpass filter 18 processes the echo signals into quadrature I and Q components. The separate components are used by a Doppler angle estimator 20 to estimate the phase or frequency shift of a Doppler signal at the depths in front of each transducer element where Doppler
• Doppler power may be estimated.
• frequencies or intensities at depths in front of each transducer which are produced by the Doppler angle estimator 20 can be mapped directly to velocity values of flow or Doppler power at those depth locations.
• This Doppler data is coupled to a flow image processor 30 which spatially processes the data into a two or three dimensional image format, in which the velocity values are color- or intensity-coded.
• This spatial Doppler vessel map is processed by a display processor 36 and displayed on display 52 to illustrate the locations in the anatomy beneath the array transducer where flow is taking place and the velocity and direction of that flow by the color coding or Doppler power by intensity modulation.
• Doppler data is also coupled to a spectral Doppler processor 32 which produces a spectral analysis of the Doppler data for peak systolic velocity and turbulence analysis as described below.
• a Doppler angle processor 40 is also coupled to a spectral Doppler processor 40
• a graphics processor 34 is provided to overlay vessel map coordinates and other graphical data such as patient name on the displayed image.
• a volume renderer 24 performs volume rendering of three dimensional vessel map data for the production of three dimensional vessel maps as described below. The entire system is operated by user controls 50.
• FIGURE 2 illustrates a transducer array 12 for a probe 10 of the present invention.
• This embodiment is an array for a probe with a total of 80 elements, each measuring 3 mm by 3 mm arranged in a two dimensional matrix containing 10 rows by 8 columns. In such a configuration, the area of coverage is 30 mm by 24 mm.
• Transducer arrays with this small number of elements and elements of this relatively large individual size are easier to dice than fine-pitched array elements, making this array simple to fabricate with high yields and hence relatively inexpensive to make.
• the factors to consider in selecting the number of element include coverage, resolution, and a number which provides acceptable accuracy as compared to a standard
• ultrasound probe Element sizes ranging from 3mm by 3mm to 6mm by 6mm have been found to be acceptable for unfocused elements used with nominal depths of the carotid artery. An array size of around 40mm has been found suitable for covering the branches of the carotid artery. As FIGURE 2 illustrates, the probe is intended to be placed against the neck of a patient over the carotid artery. When the probe aperture is
• the transducer will cover an appreciable area of the carotid artery with the common carotid artery (CCA) at one end and the internal (ICA) and external (ECA) branches at the other end.
• CCA common carotid artery
• ICA internal
• ECA external
• the elements in the probe are placed at an angle to provide a Doppler angle of 30-60 degrees.
• no Doppler signal can be obtained when the direction of blood flow is orthogonal to the direction of the ultrasound waves, and maximal signals are obtained when the wave are directed in line with the flow.
• the elements of the array 12 were aimed straight into the body, little or no Doppler signals would be realized from the carotid flow immediately beneath the array.
• the elements of the array 12 are angled at an angle of 30-60 degrees away from normal as illustrated in FIGURE 2a. A tilt of around 30 degrees has been found to yield acceptable results. Additional angulation can be provided by tilting the probe in relation to the skin surface during scanning.
• This drawing shows the end elements 12-1, 12-2, 12-3 and 12-4 of the first four rows of elements canted at the selected Doppler angle and retained in position by an overlying layer of
• transducer lens material layer 11 which is generally a polymeric rubber-like material such as RTV.
• the thickness of the RTV lens can be uniform across the array transducer, or can be tapered in thickness to provide some or all of the tilt for setting the nominal Doppler angle.
• the Doppler angle at which the elements are angled can be entered into the ultrasound system automatically from the probe's personality chip or manually from the user controls to condition the system to account for the proper angle correction to be used for the Doppler angle of the probe elements.
• the system may desirably also include a Doppler angle feedback indicator controlled by an algorithm that processes the strength of the Doppler return signals and detects unfavorable Doppler angles, which suggests that the user manually reposition the probe at a more favorable angle.
• the elements of the array transducer are excited with excitation pulses to work in the pulsed wave mode.
• the pulsed wave mode it is possible to provide depth resolution of the Doppler signals.
• an element size of 3 mm by 3 mm generally provides enough resolution to sample and obtain the velocity in a narrow region of the high speed jet produced in carotid arteries with stenosis.
• the elements of the non-phased transducer array 12 are fired simultaneously or independently in a customizable firing sequence under control by the beamformer 14.
• Operation of a probe and system of the present invention proceeds as follows .
• a sequence of elements (a small sub-section of the entire probe) is fired simultaneously.
• the sample volume for Doppler flow interrogation in front of each element is increased sequentially to cover a range of depths.
• 4 rows could be activated simultaneously, e.g., rows #1, #5, #8 and #12 as shown in this drawing.
• the Doppler spectrum is acquired continuously from each of the 8 elements in each of these rows .
• the flow image processor 30 computes a sum-plot (integration of Doppler power in a specified frequency band, e.g. 300 Hz to 1500 Hz) .
• transducer element 12-25 is not over a portion of the carotid artery and will sense no flow (will produce a negligible power Doppler signal.)
• Transducer element 12-61 is directly above the carotid artery and will produce a significant power Doppler signal.
• the flow image processor 30 can then produce a grid-like map of the location and position of the carotid artery branches beneath the probe as indicated in FIGURES 4 and 5.
• Each box in an 8 by 12 display grid is filled in with a brightness or color when a significant Doppler signal is detected by a corresponding transducer element, and is not illuminated when substantially no Doppler signal is detected.
• the third, fourth, seventh and eighth boxes in the first row of the grid are illuminated in response to the detection of flow in the ICA and ECA by those elements in that row which are over those carotid branches.
• a similar result is obtained from elements 34, 35, 38 and 39 in row 5.
• Boxes corresponding to elements 59-63 of row 8 are illuminated because they detected flow in the CCA below the carotid bifurcation, as is also the case for boxes corresponding to elements 92-94 in row 12.
• FIGURE 4 illustrates, this will present a simple two dimensional grid vessel map of the flow in the carotid artery beneath the array transducer.
• a similar result can also be obtained in the depth dimension as shown in FIGURE 5.
• This shows the results from depth-gating the Doppler signal samples from six discrete depths below the elements in the top and bottom rows of the array 12.
• the transverse maps to the right of the array illustration shows faintly illuminated blocks at six depths where the second, third, seventh and eighth elements in the top row have detected flow in the ICA and the ECA.
• the lower map shows a wider, more illuminated set of depth blocks where the center three elements of the bottom row of elements have detected the stronger flow in the CCA which is beneath that row of elements.
• the values of these vertical depth block in each column can be combined in various ways to obtain a display value for a box in the longitudinal display shown to the left of the transverse maps, if desired .
• An object of an implementation of the present invention is to provide real-time feedback to the user such that the user is guided in placing the probe on the neck so that it covers the bifurcation of the carotid artery. Since this is a non-image based system, feedback can be given to the user through a set of indicators on the screen that guides the user to move the probe in the appropriate direction.
• the ideal placement of the probe is when the flow image detector detects a single vessel branch (CCA) at rows 8 & 12 of the grid display and two separate vessel branches (ICA and ECA) in rows 1 & 5 of the grid as shown in FIGURE 4. The elements in these rows are continuous fired and the flow image processor computes the accumulated Doppler power of the signals received by each element in real time.
• the grid blocks which are illuminated in response to significant
• Doppler signal returns inform the system of the placement adjustment needed to obtain good probe placement. If the placement is good, the "OK"
• the User Mapping buttons at the bottom of the guidance display tell the user the current mode of operation, either probe placement or vessel mapping. If the user is in the Vessel Mapping mode and wants to readjust the positioning of the probe over the carotid artery, the user taps the Probe placement display button to return the system to that mode and the Probe placement button responds by illuminating. Since approximately 3 seconds worth of Doppler data is required for a single iteration, a guidance indication can be determined and updated in a time of 3 seconds. The maximum time to achieve acceptable placement of the probe over the carotid artery depends on the number of iterations required to successfully place the probe in the proper position for carotid artery stenosis detection.
• the next step is to obtain ultrasound Doppler data from all the elements in the probe array and map the vessels in accordance with the elements which detect flow.
• the accumulated Doppler power is computed as described above in conjunction with FIGURE 5 for the depths of interest to generate a 3D representation of the carotid vessels.
• the 3D representation is produced from the depth values detected by all of the elements and processed by the volume renderer 24 into a 3D display map. As shown in FIGURE 6, a 3D display map 60 of the bifurcated vessel is displayed.
• Doppler data is acquired from all of the elements to extract the peak systolic velocity which can aid in classification of a stenosis. Additionally if desired a Doppler spectrum can be extracted at a suspected point of stenosis to further aid the diagnosis as described below.
• the probe 10 When the probe 10 has a small aperture, it may not be possible to acquire a sufficiently sized portion of the carotid, its branches and bifurcation in a single acquisition. In such a case multiple volume
• acquisition is employed to acquire segments of the carotid artery in sub-areas or sub-volumes by movement of the probe along the neck, then stitching the smaller segments together to form the desired vessel map.
• each subvolume may have a column of voxels with flow velocity values of 0, 3, 5, 3, and 0 cm/sec.
• the subvolumes are aligned on the basis of this identity of sequential voxel values.
• Other Doppler spectral characteristic instead of or in addition to velocity can be used to make the match. This stitching is performed purely based on the signal characteristics because relative position of the probe with reference to the neck is not easy to obtain.
• Peak systolic velocity is a clinically accepted diagnostic indicator to grade stenosis according to the following consensus criteria : Degree of stenosis PSV (cm/s)
• the probe 10 Since the probe 10 is non imaging, it is difficult for the user to understand how much of the artery cluster under investigation has been covered by a sweep of the probe. It is difficult to interpret from the reconstruction of any of the typical sampled sub- volumes, e.g., (a) (b) or (c) of FIGURE 7, whether the artery structure has been covered with anatomical cues from a single volume. Therefore, the data from
• the probe 10 is placed in a location LI resulting in acquisition of sample volume VI.
• the probe is manually moved to location L2, adjacent to LI such that the resulting new acquisition sample volume V2 is partially overlapped with VI.
• Each volume in this example is a three dimensional matrix containing returned ultrasound echo data.
• further sample volumes Vn are acquired in the same manner, such that the vessel structure of interest is fully covered. Since this is a non-imaging approach, it may not be feasible to know the completion of vessel map reconstruction until the end of the process .
• signal in the volume is identified by highlighting the different segments in the 3D space of the vessel as shown in FIGURE 7.
• FIGURE 7(b) containing the bifurcation point of the carotid artery, using anatomical information from the reconstruction. It also gives an indication of where to search for matching signal data, e.g., the overlap between FIGURE 7(b) and FIGURE 7(c) can only be where there are two distinct vessels in FIGURE 7 (b) .
• the data of all the good spectra from the top most row of each column of one sample volume is extracted in order, and correlated with the data of each of the rows of the subsequent (adjacent) volume.
• a spike in correlation at row k indicates that all the data from row k to the last row of the probe are in the
• the overlapping area is assigned to the volume where this area has more analyzable spectra. In the event that the number of such spectra is equal, the
• overlapping region can be assigned to either volume.
• FS ⁇ 1 T lj (iii) where c is the number of columns.
• the start of the overlap region is obtained by cross-correlating the row signal matrix with FS as follows:
• Blood flow in the carotid artery can be laminar or turbulent.
• Laminar flow is uniform, with all blood cells moving at relatively the same speed and in the same direction.
• Turbulent flow is characterized by random chaotic swirling, in which blood cells within the vessel are moving in many directions but typically have a net forward flow.
• Doppler spectra of laminar flow appear as broadening of the spectral line and filling of the spectral window.
• a turbulent flow pattern appears as spectral broadening with components below the baseline.
• Spectral broadening is a term applied to waveform alterations that are representative of turbulent flow. It is defined as the increased distribution of the frequencies present in a Doppler spectrum as illustrated by the Doppler flow
• the main way of distinguishing these two cases of turbulence is through the measurement of the peak systolic velocity (PSV) .
• PSV peak systolic velocity
• the presence of stenosis is usually characterized by an elevated PSV accompanying turbulence.
• the criteria for dangerous levels of plaque are generally related to percentage of stenosis; also, the degree of stenosis is determined by the peak velocity of the blood as follows:
• Detecting the Doppler shift caused by increased velocity can determine whether there is dangerous plaque present.
• the present invention uses this information to detect the segments of interest without any imaging of the carotid artery. Further, the present invention provides an automated method to characterize the turbulence present in the carotid cluster with
• Spectral Doppler ultrasound velocimetry involves the systematic analysis of the spectrum of frequencies that constitute the Doppler signal.
• the Doppler signal obtained for clinical use is composed of a range of frequencies with varying amplitude content. Therefore, systematic processing is required before the Doppler shift frequencies are computed.
• the systematic process of FIGURE 9 estimates the two most important parameters to be estimated for stenosis evaluation, the PSV and turbulence .
• Doppler signals are acquired from the volumetric region of the carotid artery as described above and the Doppler signals is pre-processed (74) to remove unwanted noisy signal components.
• the vessel localization step 76 identifies whether the acquired Doppler signals contain vessel information (CCA, ICA, ECA or bifurcation point) or not. This is done with the help of a predefined power threshold for the
• Doppler power The power in the time domain (the sum of the square of all the values divided by signal length) is computed for all of the sample volumes of the carotid artery. Then the average power is computed from all the sample volumes. Finally the sample volume that exhibits the highest power above the average value is considered as a segment having vessel information.
• the spectral profile extraction at step 82 involves two steps: (i) computing a spectrogram using the spectral Doppler processor 32 and (ii) processing the spectrogram (smoothing, thresholding, noise removal, etc.,) to obtain a good spectral profile.
• the spectrogram is obtained using a Fast Fourier Transform (FFT) based power spectrum with a 20 millisecond
• step 88 To estimate PSV in step 88, first a 3D vessel map is created in step 78 from the identified vessel information as described previously. From the vessel map the Doppler angle is estimated in step 86 by piecewise linear fit of the geometrical centroids of the carotid vessel structure obtained from the vessel map. The maximum Doppler shift frequency is also computed from the estimated spectral profile.
• the turbulence is characterized by the spectral broadening which is normally seen in the presence of high flow velocity at the branching of a vessel or in small-diameter vessels.
• Spectral broadening can be estimated by estimating the bandwidth of the spectrum, also referred to as the spectral width.
• Turbulence is characterized by estimating the spectral width and therefore extracting both an upper and a lower trace of the envelope of the spectrogram.
• the turbulence was estimated by using the area between the upper and lower spectral traces as a proportion of the area under the upper spectral trace as described by FIGURE 10.
• This drawing 5 illustrates the computation steps involved and an illustration of a traced Doppler spectrogram is shown in FIGURE 11.
• the concluding determinations of the method of FIGURE 9 are to report the finding of the carotid bifurcation point (92), indication of stenosis (90), or a normal carotid (96) to the user.
• turbulence estimate of spectral width SW from step 84 and the PSV estimate from step 88 are made by comparing the turbulence estimate of spectral width SW from step 84 and the PSV estimate from step 88 to a turbulence threshold T s and a velocity threshold T p in comparison steps 98a, 98b, and 98c.
• a nominal velocity threshold T p is 125 cm/sec, and a nominal turbulence criteria is 0.5. The results of these comparisons are then presented to the user on the display screen.
• an implementation of the present invention provides a method for detecting
• inventive technique makes use of the non-imaging two dimensional array Doppler probe described above and the method of vessel localization previously described, together with Doppler angle estimation for detecting a stenosis and estimating its severity.
• Major elements of the invention include computing the turbulence at various points in the carotid structure and

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