Innovative Ultrasound Plane-wave Flow Imaging Method For Carotid Artery

Carotid atherosclerosis(CA)is a common disease that threatens human health.Around 20%of all strokes are caused by CA and thus,the early diagnosis and treatment of CA is rather necessary.Due to the key role of hemodynamics in the formation and diagnosis of CA,measur-ing the blood flow velocity accurately and precisely is of significant importance.Benefited by its noninvasive,multi-mode,and real-time imaging nature,medical ultrasound is the preferred option for measuring the blood flow velocity.However,conventional ultrasound blood flow sensing techniques,e.g.,color flow imaging(CFI),can obtain only the flow velocity component along the ultrasound beam axis and may introduce significant bias.A novel ultrasound tech-nique,plane wave(PW)blood vector velocity imaging,has been developed to measure the 2-or 3-D flow vector at a high frame rate.Since this technique can remedy the limitations of conven-tional CFI,it has become a big research topic in the field of medical ultrasound.However,the PW technique still has the following limitations:(1)The non-focus nature of PW brings a high-sidelobe-level point spread function(PSF),which introduces interferences between blood scatterers and degrades the accuracy of flow vector estimation.(2)The compli-cated emission sequence commonly used in PW will cause severe reverberation clutter and re-quires a high-throughput signal processing ultrasound system.(3)Clutter filtering will result in corrupted blood velocity vector estimates.(4)The gray-scale images of soft tissues obtained by PW have only limited quality.Thus,the PW technique is still not used widely for carotid artery disease diagnosis.In this thesis,the performance of the PW technique is improved according to the following three steps:(1)a single unsteered PW(SUPW)emission scheme is used instead of the dual-or multi-steered PW emission scheme,(2)a regularized least-squares(RLS)algorithm is devel-oped to reconstruct the flow vector,and(3)an adaptive beamformer is proposed to improve the quality of the gray-scale images.Simulations and experiments are conducted and the results show that the accuracy of velocity vector sensing,and the resolution and contrast ratio(CR)of the tissue structure image are all improved.In addition,a preliminary clinical study is conducted to validate the proposed PW flow vector imaging method.Detailed contents of this thesis are listed as follows:(1)Single unsteered plane wave blood vector velocity imaging for carotid artery.According to the theoretical investigation of the PSF induced by dual steered PW(DSPW)emission,an SUPW emission scheme is designed to reduce the influences of sidelobe level and reverbera-tions on blood velocity estimation.Directional beamform is used to obtain the flow velocity components in different directions and the corresponding vector velocity is derived by RSL re-construction.The estimation error of the proposed SUPW method was examined using simula-tions of a 9-mm-radius rotating cylindrical phantom with an angular speed of 20 rad/s.The median of root-mean-square errors(RMSEs)of the lateral velocity component estimates for the proposed SUPW method is 0.025 m/s,compared with 0.051 m/s for the DSPW scheme.The medians of RMSEs of the axial velocity component estimates are 0.005 and 0.008 m/s,for the proposed SUPW method and the DSPW scheme,respectively.In vivo experiments show that the proposed SUPW method can reveal the complex fluid field(e.g.,vortex)in a health carotid ar-tery bifurcation better than the DSPW scheme.(2)Single unsteered plane wave gray-scale tissue imaging.To improve the quality of SUPW gray-scale images for carotid artery,an image former composed of minimum variance(MV)adaptive beamform and post-filtering is designed.Sub-array beamformed signals are used to determine post filters to improve the contrast ratio(CR)of the artery.Simulations of point scatterers and cyst phantom demonstrate that,compared with conventional eigen-space based(ESB)beamformer,the proposed method can reduce the main lobe width by 15%and improve the CR by 10 dB.In vivo experiment on carotid artery shows that,compared with ESB beam-former,the proposed method can improve the CR of the vessel region by at least 4 dB.(3)Plane wave ultrasound imaging experiment platform design.To validate the proposed SUPW carotid artery imaging technique,an experimental platform based on a commercial ul-trasound scanner and a multi-channel data acquisition module is designed.To fit the limitations of system processing power and to map the scanned region of PW in real time,an efficient ul-trasound image former is designed.Experiment results show that the frame rate of the platform is as high as 40 fps,which can meet the requirement of the PW experiments.(4)SUPW carotid artery imaging preliminary clinical study.Twenty two carotid arteries were scanned by using the proposed innovative PW imaging scheme with the experiment plat-form.Four typical cases are selected according to the structures of the arteries.Experiments re-veal that the proposed SUPW method can provide the images of complex flows in carotid artery,and new information in the field of hemodynamics maybe revealed.