Stroke is the leading cause of mortality and the leading cause of disability in the United States.
Stroke can result from blood flow disruptions to the brain caused by plaques and clots forming on the inner walls of the blood vessels and blocking blood flow through the vessels (thrombotic stroke). Alternatively, the obstruction of blood flow can occur when particles or debris in the bloodstream from another location lodges in a smaller vessel (embolic stroke). One source of this debris is ruptured atherosclerotic plaques that otherwise do not present an immediate risk of arterial blockage. Plaques that are prone to rupture are termed “vulnerable plaques”.
Whether a plaque is vulnerable appears to depend on the internal structure of the plaque. Generally, as a plaque forms in the artery, a calcified layer forms over the softer fatty core. Vulnerable plaques have a thin fibrous cap over the top of a soft lipid pool underlying the cap. Large forces of pulsatile blood flow, for example during strenuous exercise, can break this fibrous cap. Plaques having a thick fibrous cap are less likely to rupture.
Current treatment of arterial plaque focuses on the percent blockage (stenosis) of the carotid artery. When blockage reaches a certain amount, a surgical procedure may be undertaken to remove the plaque from the artery or to widen the artery using a stent.
The present invention attempts to better identify asymptomatic plaques that are prone to rupture releasing clinical emboli into the cerebral blood stream The invention involved two steps. First, the inventors determined that some otherwise asymptomatic plaques nevertheless appeared to be associated with measurable cognitive decline. The inventors believe that these asymptomatic plaques are releasing subclinical emboli. These subclinical emboli are a concern in themselves but also appear to indicate that the plaques are vulnerable to rupture.
Second, the inventors, have determined that these vulnerable, asymptomatic plaques can be distinguished from other asymptomatic plaques earlier and ideally before there is significant cognitive decline, by measurement of the elasticity and mobility of the plaque under the pulsatile force of blood. Plaques that undergo large deformations and thus incur large axial displacements and strains, large lateral displacements and strains and increased shear strains are particularly of interest.
Specifically, the present invention provides apparatus for the characterization of arterial plaque using an imaging system producing images distinguishing plaque and at least a portion of a supporting arterial wall. An electronic computer executes a stored program and receives the acquired images to: (1) isolate movement of the plaque from movement of the supporting arterial wall under a periodic force of pulsatile blood flow; (2) analyze the movement of the plaque to characterize a risk of the plaque rupturing to produce dangerous embolisms; and (3) output an indication of the risk.
It is thus an object of one embodiment of the invention to provide a noninvasive assessment of the vulnerability of plaques, independent of a measurement of stenosis of the blood vessel.
The analysis may determine one or more of: axial strain or displacement in the plaque, lateral strain or displacement of the plaque, and shear in the plaque.
It is thus another object of one embodiment of the invention to provide a set of different measurements that may quantitatively characterize the plaque using standard image data. It is an object of a least one embodiment of the invention to further provide a functionally continuous measurement that allows more sophisticated assessment of risk.
The imaging system may be an ultrasonic imaging system.
It is thus another object of one embodiment of the invention to provide an assessment suitable for use with readily available ultrasonic imaging systems.
The apparatus may isolate movement of the plaque by subtracting out a movement of the supporting arterial wall.
Thus it is an object of one embodiment of the invention to provide a system that may make use of the periodically varying force of blood flow to characterize the plaque in vivo by isolating motion of the plaque from motion of the vessels supporting the plaque.
The apparatus may isolate movement of the plaque by determining movement of the plaque with respect to other portions of the plaque.
It is thus another object of one embodiment of the invention to provide some measurements that are self-referential and thus tend to decrease the effect of movement of the frame of reference.
The electronic computer may provide a display of the images and a cursor for identifying a region of plaque and a region of supporting arterial wall.
It is thus an object of one embodiment of the invention to make use of the expertise of a physician or other healthcare worker to identify the plaque and the arterial wall for analysis.
The apparatus may include a pulse monitoring system, such as an ECG, providing a timing reference to the computer for the analysis.
It is thus another object of one embodiment of the invention to permit synchronization of the measurements of plaque with the timing of arterial blood flow for advanced statistical processing.
When the imaging system is an ultrasound imaging system, the electronic computer may further analyze scatterer size in characterizing the risk of a plaque rupturing.
It is thus another object of one embodiment of the invention to permit additional ultrasonic measurements to be used to characterize and distinguish between vulnerable and stable plaques.
These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.
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The ultrasonic transducer 14 may provide radio frequency ultrasonic data to an RF signal processor 22 within the processing unit 12. The RF signal processor 22 provides filtering, envelope extraction (for B-mode imaging), frequency demodulation (for Doppler shift imaging), and other processing techniques well-known in the art of ultrasonic imaging. The ECG electrodes 16, in turn, provide cardiac signals to ECG interface circuitry 24 in the processing unit 12 which may extract cardiac cycle timing information. The RF signal processor 22 and ECG interface circuitry 24, in turn, may communicate with a processor 26 having a stored program 28 implementing standard image formation algorithms to provide an output image 30 on the display screen 18. The processor 26 may also include the program 28 of the present invention, as will be described, to output quantitative risk data 32 on the display screen 18.
The ultrasound signal processing unit 12, for example, may be a Siemens Antares ultrasound system providing an ultrasound research interface package that provides access to radio frequency ultrasonics data. In this case, the processor 26 may be implemented with a freestanding computer providing off-line processing. The ultrasonic transducer 14 may be, for example, a Siemens VFX 13-5 multi-D linear array transducer, or a VFX 9-4 linear array transducer.
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At succeeding process block 46, one or more additional cursors 48 may be placed on the vessel walls 47, or tissue closely coupled thereto, to provide a reference frame for motion correction.
The program 28 per process block 54 may then process additional image frames 42 and may automatically reposition the cursors 44 and 48 to follow the plaque 45 and vessel wall 47, respectively. This tracking of motion of the tissue may be done by means of a local correlation between the data circumscribed by the cursors 44 and 48 in each given image frame 42 with the data in the next image frame 42′ to establish any movement of the circumscribed material. The cursors 44 and 48 are then repositioned in the next image frame 42′ to the point of highest correlation. By this process, a trajectory of the tissue motion through each image frame 42 may be extracted.
Alternatively, the cursors 44 and 48 may be manually repositioned in each frame 42′ by the physician. The automatic or manual positioning of the cursors 44 and 48 may be aided by Doppler velocity data that may readily distinguish blood flow from occluding material, and that may highlight the point of occlusion by the resulting local high blood velocity
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Techniques for determining strain from the ultrasonic data are described in U.S. Pat. No. 7,166,075, entitled: “Elastographic imaging of in vivo soft tissue”; U.S. Pat. No. 6,749,571, entitled: “Method and apparatus for cardiac elastography” and U.S. patent applications: 2007/0083113, entitled: “High resolution elastography using two step strain estimation”; 2005/0165309, entitled: “Ultrasonic elastography providing axial, orthogonal, and shear strain”; 2004/0243001, entitled: “Parametric ultrasound imaging using angular compounding”; 2004/0215075, entitled: “Ultrasonic elastography with angular compounding”; 2004/0210136, entitled: “Method and apparatus for imaging the cervix and uterine wall”, all naming the first inventor and hereby incorporated by reference.
Each of these strain measurements characterizes the flexibility of the plaque 45 under the pulsating force of flowing blood. The measurements are repeated for each pair of successive image data frames 42, 42′ and thus generate a set of time series waveforms 68, 70, and 72. Time series 68 provides variation in lateral strain 60, time series 74 provides variation in axial strain 64, and time series 72 provides variation in shear strain 66, each for a variety of points in the plaque 45.
Each of these time series 68, 70, and the 72 has a regular period tracking the cardiac cycle and thus the individual cardiac cycles may be “ensemble averaged” by aligning successive cardiac cycles with each other and performing a point by point averaging of corresponding points within each cardiac cycle. This statistical processing and/or other statistical processing techniques, indicated at process block 78 of
At process block 80, extracted parameters from the statistically processed time series 68, 70, and 72 may be applied to risk data, for example contained in a lookup table, to equate these extracted parameters to the risk presented to the patient by the plaque 45. Generally, for example, the peak-to-trough strain variation 75 may be determined for time series 68 for each of these measurements to provide three different views of the elasticity of the plaque 45. The present inventors have determined this extracted parameter of peak-to-trough strain variation 75 (maximum accumulated axial strain) applied to axial strain will range from 18% for softer plaques to 7% for calcified plaques.
The inventors have linked extracted parameters to the risk presented by the underlying plaque 45 from a study population of individuals who have been tested for cognitive decline and who have had ultrasonic measurements of the elasticity of their plaques 45. In the preferred embodiment, this study population was presented with a brief but reliable assessment of the domains of cognition including immediate memory (word list learning, paragraph recall), visuospatial ability (construct a complex figure, spatial orientation), language (confrontation naming, semantic fluency), attention (digit span forward, digit symbol substitution), delayed memory (word list, paragraph recall, complex figure), and a summary measure of overall cognitive performance (RBANS (Repeatable Battery for the Assessment of Neuropsychological Status) Total).
The inventors have determined that there is a significant correlation between peak-to-trough lateral strain variation 75 and the RBANS total performance (−0.79, p=0.02) with higher strain associated with poorer cognitive performance. In addition there are significant associations between these strain types and immediate memory (−0.793 p=0.03) and delayed memory (−0.88 p=0.009).
In the present invention cognitive studies of a standard population are used to generate a multidimensional risk function embodied, for example, in a lookup table being part of program 28 and relating one or more of these strain measurements to cognitive decline. This lookup table is used at process block 80 to provide a continuous range of risk values, for example, tracking the measured cognitive decline of the standard population. This risk value may be output on the display screen 18 for example as a number or in the form of a chart, for example a bar chart placing the risk within a range and optionally relating it to individual reference populations by age or the like. A threshold 90 with respect to this risk may be established to indicate when corrective procedures should be undertaken based on current and evolving standards of medical care.
This analysis of process block 80 may also make use of other data that may be obtained with respect to the plaque 45 and that may help improve the correlation including for example, scatterer size, and attenuation. Scatterer size indicates the diameter of acoustic scatterers and is particularly useful in an ultrasound imaging system and may be determined by an analysis of the spectra of the underlying radio frequency ultrasound data according to methods known in the art. Generally plaques 45 that are calcified and thus may provide more resistance to rupture tend to have smaller scatterer sizes (Faran: 120˜180 μm) whereas plaques 45 without calcification (softer plaques) have larger scatterer sizes (Faran: 280˜470 μm).
Similarly, attenuation as a function of frequency may be used to further characterize the plaques 45 with calcified regions showing increased attenuation with frequency (1.4˜2.5 db/cm/MHz) whereas plaques 45 having higher risk of rupture have lower attenuation 0.3˜1.4 db/cm/MHz).
It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.
While in the preferred embodiment, ultrasonic imaging is used for deducing strain, it will be understood to those of ordinary skill in the art that other imaging modalities may also be used including, for example, x-ray CT or MRI imaging. The use of these modalities for the measurement of tissue elastic properties is disclosed in U.S. Pat. No. 6,037,774, entitled: “Inertial driver device for MR elastography”; U.S. Pat. No. 6,862,468, entitled: “Systems and methods for magnetic resonance imaging elastography”, and U.S. Pat. No. 7,257,244 entitled: “Elastography imaging modalities for characterizing properties of tissue”; all in corporate it by reference. It will be further understood that although cognitive decline was used to establish the benchmark against which vulnerable plaques are classified, a similar classification could be derived from risk of vascular cognitive dementia, stroke or Alzheimer's disease with additional studies.
This invention was made with United States government support awarded by the following agencies: NIH EB003853. The United States has certain rights in this invention.