WO2017113178A1 - 一种聚焦超声位移成像方法及装置 - Google Patents

一种聚焦超声位移成像方法及装置 Download PDF

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WO2017113178A1
WO2017113178A1 PCT/CN2015/099844 CN2015099844W WO2017113178A1 WO 2017113178 A1 WO2017113178 A1 WO 2017113178A1 CN 2015099844 W CN2015099844 W CN 2015099844W WO 2017113178 A1 WO2017113178 A1 WO 2017113178A1
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displacement
pulse
focused ultrasound
end time
phase
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PCT/CN2015/099844
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English (en)
French (fr)
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郑海荣
刘新
乔阳紫
邹超
孟德
帖长军
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深圳先进技术研究院
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Priority to CN201580001259.0A priority Critical patent/CN107205718B/zh
Priority to PCT/CN2015/099844 priority patent/WO2017113178A1/zh
Publication of WO2017113178A1 publication Critical patent/WO2017113178A1/zh

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves

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  • the present invention relates to the field of displacement imaging technology, and in particular, to a focused ultrasound displacement imaging method and apparatus.
  • High intensity focused ultrasound can selectively concentrate energy in the deep target tissue of the body, and the damage to surrounding normal tissue is small, which has gradually become an important means of tumor treatment. Focus positioning during high-intensity focused ultrasound therapy, especially when there is uneven media on the ultrasound pathway, is the key to the safety and effectiveness of the treatment.
  • Magnetic resonance focus localization methods can be mainly divided into two categories: magnetic resonance temperature imaging and magnetic resonance acoustic radiation force displacement detection. The following are introduced separately:
  • Magnetic resonance temperature imaging uses high-intensity focused ultrasound to detect focal position changes in temperature caused by local tissue heat production.
  • the unique temperature measurement function of magnetic resonance imaging can obtain a temperature change distribution map in a space, and the position with the highest temperature change can be regarded as the actual focus of high intensity focused ultrasound.
  • This method requires a large power to be applied to cause local tissue temperature changes, and there is a certain safety hazard.
  • temperature imaging itself is also subject to interference caused by factors such as motion and magnetic susceptibility artifacts.
  • the magnetic resonance acoustic radiation force displacement detection mainly uses pulsed (millisecond) ultrasonic radiation force to make a small deformation (micron level) of the local focus of the biological tissue, and then uses the motion coding gradient to convert the displacement caused by the tissue under the action of ultrasound into The phase change of the magnetic resonance image is used to quantify the small displacement in the tissue by detecting the phase change of the tissue.
  • Magnetic resonance acoustic radiation imaging has high detection sensitivity, usually only requires a small ultrasonic power, less energy deposition in the tissue, high positioning accuracy, and has gradually developed into an important high-intensity focused ultrasound focus positioning. method.
  • the currently used magnetic resonance displacement detection sequences mainly include:
  • the magnetic resonance acoustic radiation detection method first proposed by McDannold et al. uses this scanning sequence. Because of the one-dimensional line scanning method, the sequence is not sensitive to motion, but the image signal-to-noise ratio is relatively low, and the image sensitivity needs to be positive. Comparing the two results of the coding gradient and the negative encoding gradient, when collecting N phase encoding lines, 2N TR acquisition time is required.
  • the signal-to-noise ratio of the sequence image is significantly improved compared with the one-dimensional linear scan sequence, but the two-dimensional scanning method requires a relatively long scan time and is sensitive to motion. In the body, tissue movement (breathing, heartbeat, cerebrospinal fluid flow) is almost inevitable.
  • the image signal-to-noise ratio of the one-dimensional linear scan sequence is low and the resolution is limited.
  • the scan time of the two-dimensional spin echo sequence is relatively long, which cannot meet the requirements of fast detection; the single-shot spin echo fast imaging sequence often With obvious image distortion, this distortion directly affects the accuracy of positioning.
  • the invention provides a method and a device for focusing ultrasonic displacement imaging, so as to at least solve the problem that the image distortion of the magnetic resonance acoustic radiation force displacement detection is more obvious in the prior art.
  • a focused ultrasonic displacement imaging method includes: adding a displacement coding gradient before and after a 180° refocusing pulse of a segment readout plane echo sequence, according to the displacement
  • the encoding gradient determines a pulse start and stop time point of the focused ultrasound; controlling the focused ultrasound to operate according to the pulse start and stop time points, and acquiring two sets of phase maps; wherein the displacement coding gradients of the two sets of phase maps are opposite in polarity;
  • the phase difference and displacement quantitative formula of the graph generates a displacement distribution field map; wherein the displacement maximum value in the displacement distribution field map is a focus position; and the displacement distribution field map is three-dimensionally imaged.
  • the displacement coding gradient is a bipolar repetition displacement coding gradient
  • determining a pulse start and end time point of the focused ultrasound according to the displacement coding gradient comprises: arranging a bipolar repetition displacement before the 180° refocusing pulse The latter half-pole start time of the code gradient is determined as the pulse start time point, and the first half-pole end time of the bipolar repeat displacement coding gradient after the 180° back-focus pulse is determined as the pulse end time point.
  • the displacement coding gradient is a unipolar displacement coding gradient
  • determining a pulse start and end time point of the focused ultrasound according to the displacement coding gradient comprises: unipolar displacement coding gradient after the 180° refocusing pulse The start and end time is determined as the pulse start and stop time point.
  • the displacement encoding gradient is a bipolar reverse displacement encoding gradient
  • determining a pulse start and end time point of the focused ultrasound according to the displacement encoding gradient comprises: inverting the bipolar before the 180° refocusing pulse
  • the first half-pole start and end time of the displacement encoding gradient is determined as the start and end time point of the first pulse
  • the first half-polar start and end time of the bipolar reverse-shift coding gradient after the 180° re-accumulation pulse is determined as the start and end time of the second pulse
  • the pulse of focused ultrasound comprises the first pulse and the second pulse.
  • the method further includes: subtracting the two sets of phase maps to obtain a phase change value ⁇ d ; performing magnetic field correction on the two sets of phase maps, and subtracting two sets of phase maps after the magnetic field correction to obtain image phase changes
  • the value ⁇ B0 is obtained by subtracting the phase change value ⁇ B0 from the phase change value ⁇ d to obtain the phase difference.
  • three-dimensional imaging the displacement distribution field map includes: superimposing the displacement distribution field pattern perpendicular to and parallel to the sound wave direction of the focused ultrasound, and performing the superimposed displacement distribution field map Three-dimensional imaging.
  • a focused ultrasonic displacement imaging apparatus comprising: a time determination module for adding a displacement before and after a 180° refocusing pulse of the segment readout plane echo sequence a coding gradient, determining a pulse start and stop time point of the focused ultrasound according to the displacement coding gradient; a phase map generation module, configured to control the focused ultrasound to work according to the pulse start and stop time points, and acquire two sets of phase maps; wherein, two groups The displacement coding gradient of the phase diagram has the opposite polarity; the displacement distribution field map generation module is configured to generate a displacement distribution field map according to the phase difference and the displacement quantitative formula of the two sets of phase maps; wherein the displacement maximum value in the displacement distribution field map is a focus position; an imaging module for performing three-dimensional imaging of the displacement distribution field map.
  • the invention is based on a readout segmented SE-EPI (RS-EPI) to realize magnetic resonance acoustic radiation force displacement imaging, and has a relatively high image signal to noise ratio compared to a one-dimensional linear scan; Rotating wave sequence, scanning speed is fast; relative to single-shot plane echo sequence, image distortion is small, in the case of fast focus feedback It has great application potential in use. Thereby, the detection time and resolution are effectively improved, the magnetic susceptibility artifacts and image distortion are reduced, and the high-intensity focused ultrasound focus is accurately positioned.
  • RS-EPI readout segmented SE-EPI
  • FIG. 1 is a flow chart of a focused ultrasound displacement imaging method in accordance with an embodiment of the present invention
  • FIG. 2 is a timing diagram of a magnetic resonance acoustic radiation force displacement detection sequence based on segmented readout plane echoes, in accordance with an embodiment of the present invention
  • FIG. 3 is an operational timing diagram of a unipolar displacement coding gradient and a high intensity focused ultrasound, in accordance with an embodiment of the present invention
  • FIG. 4 is an operational timing diagram of a bipolar reverse shift coding gradient and high intensity focused ultrasound, in accordance with an embodiment of the present invention
  • FIG. 5 is a schematic structural view of a focused ultrasonic displacement imaging apparatus according to an embodiment of the present invention.
  • FIG. 1 is a flowchart of a focused ultrasound displacement imaging method according to an embodiment of the present invention. As shown in FIG. 1, the method includes the following steps (step S102 - step S108) :
  • Step S102 adding a displacement coding gradient before and after the 180° refocusing pulse of the segment readout plane echo sequence, and determining a pulse start and end time point of the focused ultrasound according to the displacement coding gradient;
  • Step S104 controlling the focused ultrasound to work according to the start and end time of the pulse, and acquiring two sets of phase maps; wherein the displacement coding gradients of the two sets of phase maps are opposite in polarity;
  • Step S106 generating a displacement distribution field map according to the phase difference and the displacement quantitative formula of the two sets of phase maps; wherein the displacement maximum value in the displacement distribution field map is a focus position;
  • Step S108 performing three-dimensional imaging on the displacement distribution field map.
  • the magnetic resonance acoustic radiation force displacement imaging is realized based on the segmentation readout plane echo, thereby effectively improving the detection time and resolution, reducing the magnetic susceptibility artifact and image distortion, and realizing the high-intensity focused ultrasound focus precise positioning.
  • the displacement encoding gradient may be in various forms, such as a bipolar repeat displacement encoding gradient, a unipolar displacement encoding gradient, a bipolar inverse displacement encoding gradient, and the like.
  • other forms of displacement coding gradients may be used as long as the pulse start and end time points of the focused ultrasound can be determined according to the displacement coding gradient. The specific process of determining the start and end time of the pulse of the focused ultrasound is described below for the above three displacement coding gradients.
  • FIG. 2 is a timing diagram of a magnetic resonance acoustic radiation force displacement detection sequence based on a segmented readout plane echo, as shown in FIG. 2, a 180° refocusing pulse in a segmented readout plane echo sequence, in accordance with an embodiment of the present invention.
  • the first half-pole end time of the displacement coding gradient is determined as the pulse termination time point.
  • the high-intensity focused ultrasound works according to the above-mentioned pulse start time point and pulse end time point, and repeats the work twice, and acquires two sets of phase maps. It should be noted that the polarities of the bipolar repeated displacement coding gradients of the two sets of phase diagrams are opposite.
  • the solid line in Fig. 2 indicates the polarity of the bipolar repeated displacement coding gradient in the generation of the first set of phase diagrams, and the dotted line indicates the second The polarity of the bipolar repeat displacement coding gradient when the group phase map is generated is opposite in polarity.
  • FIG. 3 is a timing chart of operation of a unipolar displacement coding gradient and a high intensity focused ultrasound according to an embodiment of the present invention.
  • a single table is added before and after the 180° convergence pulse of the segment readout plane echo sequence.
  • the pole displacement coding gradient determines the start and end time of the unipolar displacement coding gradient after the 180° refocusing pulse as the pulse start and end time point.
  • the high-intensity focused ultrasound works according to the above-mentioned pulse start and stop time points, repeats the work twice, and acquires two sets of phase maps. It should be noted that the polarities of the unipolar displacement coding gradients of the two sets of phase diagrams are opposite.
  • a bipolar reverse displacement coding gradient, the focused ultrasound pulse is divided into two segments: a first pulse and a second pulse, the first half of the start and end times of the bipolar reverse displacement coding gradient before the 180° backscatter pulse is determined as the first At the start and end time of the pulse, the first half-polar start and end time of the bipolar reverse displacement coding gradient after the 180° refocusing pulse is determined as the start and end time of the second pulse.
  • the high-intensity focused ultrasound operates according to the start and end time points of the first pulse and the second pulse described above, and repeats the work twice to acquire two sets of phase maps. It should be noted that the polarities of the bipolar reverse displacement coding gradients of the two sets of phase diagrams are opposite.
  • phase difference between the two sets of phase diagrams is calculated by the following process:
  • phase change value ⁇ d that is, a phase change caused by the displacement
  • the magnetic field correction process is as follows: It is assumed that the pixel phase of the non-moving region around the focal region (referred to as the focal region) is slowly and smoothly changed, and the phase satisfies the polynomial model, and the least square method is used, and the non-displaced focal region can be obtained by the fitting formula.
  • the initial phase is fitted and the fit formula is: among them, For the phase at the coordinate point (x, y), W n (m) is the fitting coefficient and N is the fitting order.
  • the image phase change value ⁇ B0 caused by the background unevenness can be obtained;
  • phase difference between the two sets of phase maps is calculated.
  • the magnetic field correction of the phase map improves the accuracy of the phase difference calculation, improves the sensitivity of the phase map to motion, and further ensures the subsequent generation of the displacement distribution field map. Precision.
  • the displacement quantitative formula is used to quantify the displacement, and the displacement distribution field map is generated.
  • the position corresponding to the maximum displacement point in the displacement distribution field map is the high intensity focused ultrasound focus.
  • the displacement distribution field map After generating the displacement distribution field map, it can be three-dimensionally imaged, specifically, the displacement distribution field map perpendicular to and parallel to the sound wave direction of the focused ultrasound is superimposed, and the superposed displacement distribution field map is three-dimensionally imaged. Thereby, the displacement distribution field map can be displayed in a three-dimensional imaging effect, which is convenient for real-time observation.
  • TR 200ms
  • TE 36ms
  • bandwidth 391Hz / Pixel
  • layer thickness 5mm
  • resolution 2.2 * 1.6mm2.
  • the image of the single-shot spin echo sequence is severely distorted, and the Nyquist ghost also appears.
  • the image distortion and artifacts in the image of the segment readout plane echo sequence are well suppressed.
  • the image signal-to-noise ratios of the segment readout plane echo sequence, the single-shot fast spin echo sequence and the two-dimensional spin echo sequence are 42.6, 19.2, and 71.6, respectively.
  • the acquisition time of a single image is 3.4s, 0.2. s, 15s.
  • the imaging results of the segmented readout plane echo sequence have higher signal-to-noise ratio and smaller image distortion; compared with the two-dimensional spin echo sequence, the segmentation readout plane returns The acquisition time of the wave sequence is approximately equivalent to 1/5.
  • the image distortion of the single-shot spin echo sequence is severe and the signal-to-noise ratio is low, image acquisition of a large FOV (Field of View) is not applicable.
  • FOV Field of View
  • the distortion is improved, the signal-to-noise ratio is improved, and it is not limited by the image FOV. Therefore, the displacement field distribution perpendicular to and parallel to the direction of focused ultrasound propagation can be simultaneously acquired. And by moving the position of the imaging plane, multi-layer scanning is performed, and finally a three-dimensional high-intensity focused ultrasound focal region image is obtained.
  • an embodiment of the present invention further provides a focused ultrasonic displacement imaging apparatus, which can be used to implement the method described in the above embodiments, as described in the following embodiments. Since the principle of solving the problem by the focused ultrasonic displacement imaging device is similar to that of the focused ultrasonic displacement imaging method, the implementation of the focused ultrasonic displacement imaging device can be referred to the implementation of the focused ultrasonic displacement imaging method, and the repeated description will not be repeated.
  • the term "unit” or A "module” can implement a combination of software and/or hardware for a predetermined function.
  • the apparatus described in the following embodiments is preferably implemented in software, hardware, or a combination of software and hardware, is also possible and contemplated.
  • FIG. 5 is a schematic structural diagram of a focused ultrasound displacement imaging apparatus according to an embodiment of the present invention. As shown in FIG. 5, the apparatus includes: a time determination module 10, a phase diagram generation module 20, a displacement distribution field map generation module 30, and an imaging module 40. The structure will be specifically described below.
  • the time determining module 10 is configured to add a displacement encoding gradient before and after the 180° refocusing pulse of the segment readout plane echo sequence, and determine a pulse start and end time point of the focused ultrasound according to the displacement encoding gradient;
  • the phase map generating module 20 is connected to the time determining module 10 for controlling the focused ultrasound to operate according to the pulse start and stop time points, and acquiring two sets of phase maps; wherein the displacement coding gradients of the two sets of phase maps are opposite in polarity;
  • the displacement distribution field map generating module 30 is connected to the phase map generating module 20, and is configured to generate a displacement distribution field map according to the phase difference and the displacement quantitative formula of the two sets of phase maps; wherein the displacement maximum value in the displacement distribution field map is the focus position ;
  • the imaging module 40 is coupled to the displacement distribution field map generation module 30 for three-dimensional imaging of the displacement distribution field map. Specifically, the displacement distribution field map perpendicular to and parallel to the sound wave direction of the focused ultrasound is superimposed, and the superimposed displacement distribution field map is three-dimensionally imaged, so that the displacement distribution field map can be displayed by the three-dimensional imaging effect, which is convenient for real-time observation.
  • the magnetic resonance acoustic radiation force displacement imaging is realized based on the segmentation readout plane echo, thereby effectively improving the detection time and resolution, reducing the magnetic susceptibility artifact and image distortion, and realizing the high-intensity focused ultrasound focus precise positioning.
  • the present invention realizes magnetic resonance acoustic radiation force displacement detection, high intensity focused ultrasound focus positioning, and three-dimensional high intensity focused ultrasound focal length imaging based on the segmented readout plane echo sequence.
  • the energy deposition in the tissue is small, and only a millisecond-level high-intensity focused ultrasonic pulse is applied in the collection process, which is not easy to cause temperature rise in the tissue, and the detection method is safe and accurate; the collection speed is fast, subject to image resolution and back
  • the limitation of the wave time is small, and the image signal-to-noise ratio is high; it is not easily affected by the magnetic field inhomogeneity, and the image distortion is small.

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Abstract

一种聚焦超声位移成像方法及装置，其中，该方法包括：在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据位移编码梯度确定聚焦超声的脉冲起止时间点（S102）；控制聚焦超声按照脉冲起止时间点工作，采集得到两组相位图（S104）；根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，位移分布场图中位移最大值为焦点位置（S106）；对位移分布场图进行三维成像（S108）。该方法基于分段读出平面回波实现磁共振声辐射力位移成像，具有较高的图像信噪比，扫描速度快，图像畸变小。有效提高检测时间和分辨率，减少磁化率伪影及图像畸变，实现高强度聚焦超声焦点精准定位。

Description

一种聚焦超声位移成像方法及装置 技术领域
本发明涉及位移成像技术领域，尤其涉及一种聚焦超声位移成像方法及装置。
背景技术
高强度聚焦超声(High intensity focused ultrasound，HIFU)可选择性的将能量汇聚于体内深部靶区组织，而对周围正常组织的损伤较小，已逐渐成为肿瘤治疗的一种重要手段。高强度聚焦超声治疗过程中的焦点定位，尤其当超声通路上存在不均匀介质时的焦点定位，是影响治疗安全性及有效性的关键。
磁共振焦点定位方法主要可分为两类：磁共振温度成像和磁共振声辐射力位移检测。下面分别进行介绍：
1、磁共振温度成像：
磁共振温度成像利用高强度聚焦超声在局部组织产热引起的温度变化来检测焦点位置。磁共振成像特有的温度测量功能可得到一幅空间中的温度变化分布图，温度变化最高的位置即可认定为高强度聚焦超声实际焦点。该方法需要施加一个较大的功率以引起局部组织温度变化，存在一定的安全性隐患，此外温度成像本身也受到运动和磁化率伪影等因素的干扰存在误差。
2、磁共振声辐射力位移检测：
高强度聚焦超声作用于靶向组织时，生物组织焦点局部区域受声辐射力的作用会产生微米尺度位移，利用磁共振运动敏感梯度，将该位移在磁共振图像上进行编码显示的方法，称为磁共振声辐射力位移成像(Magnetic resonance acoustic radiation force imaging，MR-ARFI)。磁共振声辐射力位移检测主要是利用脉冲式(毫秒级)超声辐射力使得生物组织焦点局部区域产生微小形变(微米级)，然后利用运动编码梯度，将组织在超声作用下引起的位移转换为磁共振图像相位变化，通过检测组织的相位变化实现对组织内微小位移的定量。
磁共振声辐射力成像具有很高的检测灵敏度，通常只需要一个很小的超声功率，在组织内形成的能量沉积少，定位精度高，已逐渐发展成为一种重要的高强度聚焦超声焦点定位方法。
目前常用的磁共振位移检测序列主要包括：
1)一维线性扫描序列(1D line scan)：
McDannold等人最早提出的磁共振声辐射检测方法使用的就是这种扫描序列，由于采用一维线扫描方法，该序列对运动不敏感，但图像信噪比较低，为增加图像灵敏度需正向编码梯度和负向编码梯度两组结果对比，采集N条相位编码线时，需2N个TR采集时间。
2)二维自旋回波序列(2D spin echo,2DSE)：
该序列图像信噪比比一维线性扫描序列有显著提高，但二维扫描方法所需扫描时间相对较长，对运动敏感。而在身体应用时，组织运动(呼吸、心跳、脑脊液流动)几乎是不可避免的。
3)单次激发自旋回波快速成像序列(Single shot SE-EPI，ss-EPI)：
2011年Pauly等人首次提出将单次激发回波平面成像应用于声辐射力成像，该成像采集速度快，但图像分辨率有限，图像TE较长，导致图像信噪比低，且常伴有非常明显的图像畸变。
由此可知，一维线性扫描序列的图像信噪比较低且分辨率有限；二维自旋回波序列的扫描时间相对较长，不能满足快速检测需求；单次激发自旋回波快速成像序列常伴有明显的图像畸变，这种畸变会直接影响定位的准确性。
针对磁共振声辐射力位移检测的上述问题，目前尚未提出有效的解决方案。
发明内容
本发明提供了一种聚焦超声位移成像方法及装置，以至少解决现有技术中磁共振声辐射力位移检测图像畸变较明显的问题。
根据本发明的一个方面，提供了一种聚焦超声位移成像方法，其中，该方法包括：在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点；控制所述聚焦超声按照所述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，所述位移分布场图中位移最大值为焦点位置；对所述位移分布场图进行三维成像。
一实施例中，所述位移编码梯度是双极重复位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：将所述180°回聚脉冲之前的双极重复位移编 码梯度的后半极开始时间确定为脉冲起始时间点，将所述180°回聚脉冲之后的双极重复位移编码梯度的前半极结束时间确定为脉冲终止时间点。
一实施例中，所述位移编码梯度是单极位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：将所述180°回聚脉冲之后的单极位移编码梯度的起止时间确定为所述脉冲起止时间点。
一实施例中，所述位移编码梯度是双极反向位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：将所述180°回聚脉冲之前的双极反向位移编码梯度的前半极起止时间确定为第一脉冲的起止时间点，将所述180°回聚脉冲之后的双极反向位移编码梯度的前半极起止时间确定为第二脉冲的起止时间点；其中，所述聚焦超声的脉冲包括所述第一脉冲和第二脉冲。
一实施例中，还包括：将所述两组相位图相减得到相位变化值Δφd；对所述两组相位图进行磁场校正，将磁场校正之后的两组相位图相减得到图像相位变化值ΔφB0；将所述相位变化值Δφd减去所述图像相位变化值ΔφB0，得到所述相位差。
一实施例中，所述位移定量公式是：
Figure PCTCN2015099844-appb-000001
其中，Δφ是相位差，γ是磁旋比，γ＝42.576MHz/T，Ge是所述位移编码梯度的强度，τ是所述位移编码梯度的时长，Δx表示位移。
一实施例中，对所述位移分布场图进行三维成像，包括：将垂直于及平行于所述聚焦超声的声波方向上的所述位移分布场图叠加，对叠加后的位移分布场图进行三维成像。
根据本发明的另一个方面，提供了一种聚焦超声位移成像装置，其中，该装置包括：时间确定模块，用于在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点；相位图生成模块，用于控制所述聚焦超声按照所述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；位移分布场图生成模块，用于根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，所述位移分布场图中位移最大值为焦点位置；成像模块，用于对所述位移分布场图进行三维成像。
本发明基于分段读出平面回波(Readout segmented SE-EPI，RS-EPI)，实现磁共振声辐射力位移成像，相对一维线性扫描，具有较高的图像信噪比；相对二维自旋回波序列，扫描速度快；相对单次激发平面回波序列，图像畸变小，在需要快速焦点反馈的应 用中具有较大的应用潜力。从而有效提高检测时间和分辨率，减少磁化率伪影及图像畸变，实现高强度聚焦超声焦点精准定位。
附图说明
为了更清楚地说明本发明实施例中的技术方案，下面将对实施例描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。在附图中：
图1是根据本发明实施例的聚焦超声位移成像方法的流程图；
图2是根据本发明实施例的基于分段读出平面回波的磁共振声辐射力位移检测序列时序图；
图3是根据本发明实施例的单极位移编码梯度和高强度聚焦超声的工作时序图；
图4是根据本发明实施例的双极反向位移编码梯度和高强度聚焦超声的工作时序图；
图5是根据本发明实施例的聚焦超声位移成像装置的结构示意图。
具体实施方式
为使本发明实施例的目的、技术方案和优点更加清楚明白，下面结合附图对本发明实施例做进一步详细说明。在此，本发明的示意性实施例及其说明用于解释本发明，但并不作为对本发明的限定。
本发明实施例提供了一种聚焦超声位移成像方法，图1是根据本发明实施例的聚焦超声位移成像方法的流程图，如图1所示，该方法包括以下步骤(步骤S102-步骤S108)：
步骤S102，在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据上述位移编码梯度确定聚焦超声的脉冲起止时间点；
步骤S104，控制聚焦超声按照上述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；
步骤S106，根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，上述位移分布场图中位移最大值为焦点位置；
步骤S108，对上述位移分布场图进行三维成像。
本实施例基于分段读出平面回波，实现磁共振声辐射力位移成像，从而有效提高检测时间和分辨率，减少磁化率伪影及图像畸变，实现高强度聚焦超声焦点精准定位。
一实施例中，位移编码梯度可以是多种形式，例如：双极重复位移编码梯度、单极位移编码梯度、双极反向位移编码梯度等。当然也可以是其他形式的位移编码梯度，只要能够根据位移编码梯度确定好聚焦超声的脉冲起止时间点即可。下面分别针对上述三种位移编码梯度，介绍确定聚焦超声的脉冲起止时间点的具体过程。
1)根据双极重复位移编码梯度确定聚焦超声的脉冲起止时间点
图2是根据本发明实施例的基于分段读出平面回波的磁共振声辐射力位移检测序列时序图，如图2所示，在分段读出平面回波序列的180°回聚脉冲前后各添加一个双极重复位移编码梯度，将180°回聚脉冲之前的双极重复位移编码梯度的后半极开始时间确定为脉冲起始时间点，将180°回聚脉冲之后的双极重复位移编码梯度的前半极结束时间确定为脉冲终止时间点。
高强度聚焦超声根据上述脉冲起始时间点和脉冲终止时间点进行工作，重复工作两次，采集得到两组相位图。需要注意的是，两组相位图的双极重复位移编码梯度的极性是相反的，图2中实线表示第一组相位图生成时双极重复位移编码梯度的极性，虚线表示第二组相位图生成时双极重复位移编码梯度的极性，二者极性相反。
2)根据单极位移编码梯度确定聚焦超声的脉冲起止时间点
图3是根据本发明实施例的单极位移编码梯度和高强度聚焦超声的工作时序图，如图3所示，在分段读出平面回波序列的180°回聚脉冲前后各添加一个单极位移编码梯度，将180°回聚脉冲之后的单极位移编码梯度的起止时间确定为脉冲起止时间点。高强度聚焦超声根据上述脉冲起止时间点进行工作，重复工作两次，采集得到两组相位图。需要注意的是，两组相位图的单极位移编码梯度的极性是相反的。
3)根据双极反向位移编码梯度确定聚焦超声的脉冲起止时间点
图4是根据本发明实施例的双极反向位移编码梯度和高强度聚焦超声的工作时序图，如图4所示，在分段读出平面回波序列的180°回聚脉冲前后各添加一个双极反向位移编码梯度，聚焦超声的脉冲分为两段：第一脉冲和第二脉冲，将180°回聚脉冲之前的双极反向位移编码梯度的前半极起止时间确定为第一脉冲的起止时间点，将180°回聚脉冲之后的双极反向位移编码梯度的前半极起止时间确定为第二脉冲的起止时间点。
高强度聚焦超声根据上述第一脉冲和第二脉冲的起止时间点进行工作，重复工作两次，采集得到两组相位图。需要注意的是，两组相位图的双极反向位移编码梯度的极性是相反的。
一实施例中，在得到两组相位图之后，需要根据两组相位图的相位差和位移定量公式生成位移分布场图，将位移分布场图中位移最大值确定为焦点位置，从而实现准确对焦。
两组相位图的相位差通过以下过程计算得到：
1)将两组相位图相减得到相位变化值Δφd，即由位移引起的相位变化；
2)对两组相位图进行磁场校正，将磁场校正之后的两组相位图相减得到图像相位变化值ΔφB0；
具体地，由于相位图中可能存在背景场不均匀性引起的图像相位变化，因此需要对相位图进行磁场矫正。磁场校正过程如下：假设焦点区域(简称焦域)周围的未运动区域的像素相位是缓慢平滑变化的，其相位满足多项式模型，则利用最小二乘法，通过拟合公式可对无位移时焦域处的初始相位进行拟合，拟合公式为：
Figure PCTCN2015099844-appb-000002
其中，
Figure PCTCN2015099844-appb-000003
为坐标点(x,y)处的相位，Wn(m)为拟合系数，N为拟合阶数。将磁场校正之后的两组相位图相减，可得到背景不均匀性引起的图像相位变化值ΔφB0；
3)将上述相位变化值Δφd减去上述图像相位变化值ΔφB0，得到上述相位差，即Δφ＝Δφd-ΔφB0。
通过上述过程计算得到两组相位图的相位差，通过对相位图的磁场校正提高了相位差的计算精确度，提高了相位图对于运动的敏感度，进一步保证了后续生成的位移分布场图的精准度。
在计算出两组相位图的相位差之后，利用位移定量公式进行位移定量，生成位移分布场图，位移分布场图中的位移最大点对应的位置即为高强度聚焦超声焦点。上述位移定量公式是：
Figure PCTCN2015099844-appb-000004
其中，Δφ是施加强度和时间相同、极性相反的运动编码梯度对应的两组相位图的相位差，γ是磁旋比(γ＝42.576MHz/T)，Ge是位移编码梯度的 强度，τ是位移编码梯度的时长，Δx表示位移。基于此，实现了聚焦超声焦点的精准定位，提高运用聚焦超声进行治疗的安全性及有效性。
在生成位移分布场图之后，可对其进行三维成像，具体地，是将垂直于及平行于聚焦超声的声波方向上的位移分布场图叠加，对叠加后的位移分布场图进行三维成像，从而能够以三维成像效果显示位移分布场图，便于实时观测。
为了验证本发明技术方案的效果，在西门子3T磁共振成像系统(Siemens TIM Trio,Erlangen，German)上进行实验。本实验将分段读出平面回波序列与二维自旋回波序列、单次激发自旋回波序列进行比较，采集过程中所使用的运动编码梯度强度为32mT/m，双极重复运动编码梯度时长10ms。三组序列采用的成像参数如下所示：
分段读出平面回波序列：TR＝200ms，TE＝36ms，带宽＝1149Hz/Pixel，层厚＝5mm，分辨率2.2*1.6mm2，EPI factor＝9；
单次激发平面回波序列：TR＝200ms，TE＝69ms，带宽＝1698Hz/Pixel，层厚＝5mm，分辨率＝2.2*1.6mm2；
二维自旋回波序列：TR＝200ms，TE＝36ms，带宽＝391Hz/Pixel，层厚＝5mm，分辨率2.2*1.6mm2。
根据实验结果，单次激发自旋回波序列的图像存在严重畸变，还出现奈奎斯特鬼影，分段读出平面回波序列的图像中图像畸变及伪影得到了很好的抑制。分段读出平面回波序列、单次激发快速自旋回波序列、二维自旋回波序列的图像信噪比分别为：42.6，19.2，71.6，单幅图像采集时间分别为：3.4s，0.2s，15s。相比单次激发自旋回波序列，分段读出平面回波序列的成像结果具有较高的信噪比和较小的图像畸变；相比二维自旋回波序列，分段读出平面回波序列的采集时间大约相当于其1/5。
由于单次激发自旋回波序列的图像畸变严重，信噪比低，因此不适用大FOV(Field of View，成像区域)的图像采集。但改用分段读出平面回波序列之后，改善了畸变情况，提高了信噪比，不太受图像FOV的限制，因此可同时采集垂直于及平行于聚焦超声传播方向的位移场分布图，并可通过移动成像平面的位置，进行多层扫描，最终叠加得到三维高强度聚焦超声焦域图像。
基于同一发明构思，本发明实施例中还提供了一种聚焦超声位移成像装置，可以用于实现上述实施例所描述的方法，如下面的实施例所述。由于聚焦超声位移成像装置解决问题的原理与聚焦超声位移成像方法相似，因此聚焦超声位移成像装置的实施可参见聚焦超声位移成像方法的实施，重复之处不再赘述。以下所使用的，术语“单元”或者 “模块”可以实现预定功能的软件和/或硬件的组合。尽管以下实施例所描述的装置较佳地以软件来实现，但是硬件，或者软件和硬件的组合的实现也是可能并被构想的。
图5是根据本发明实施例的聚焦超声位移成像装置的结构示意图，如图5所示，该装置包括：时间确定模块10、相位图生成模块20、位移分布场图生成模块30、成像模块40，下面对该结构进行具体说明。
时间确定模块10，用于在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据上述位移编码梯度确定聚焦超声的脉冲起止时间点；
相位图生成模块20，连接至时间确定模块10，用于控制上述聚焦超声按照上述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；
位移分布场图生成模块30，连接至相位图生成模块20，用于根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，上述位移分布场图中位移最大值为焦点位置；
成像模块40，连接至位移分布场图生成模块30，用于对上述位移分布场图进行三维成像。具体地，将垂直于及平行于聚焦超声的声波方向上的位移分布场图叠加，对叠加后的位移分布场图进行三维成像，从而能够以三维成像效果显示位移分布场图，便于实时观测。
本实施例中各个模块的具体工作过程前面已经进行了详细介绍，在此不再赘述。本实施例基于分段读出平面回波，实现磁共振声辐射力位移成像，从而有效提高检测时间和分辨率，减少磁化率伪影及图像畸变，实现高强度聚焦超声焦点精准定位。
从以上的描述中可知，本发明基于分段读出平面回波序列实现了磁共振声辐射力位移检测、高强度聚焦超声焦点定位、三维高强度聚焦超声焦域成像。应用本发明技术方案，组织内能量沉积小，采集过程中只需毫秒级的高强度聚焦超声脉冲作用，不易引起组织内温升，检测方法安全、准确；采集速度快，受图像分辨率及回波时间的限制小，图像信噪比高；不易受磁场不均匀性影响，图像畸变小。
在本说明书的描述中，参考术语“一个实施例”、“一些实施例”、“示例”、“具体示例”、或“一些示例”等的描述意指结合该实施例或示例描述的具体特征、结构、材料或者特点包含于本发明的至少一个实施例或示例中。在本说明书中，对上述术语的示意性表述不一定指的是相同的实施例或示例。而且，描述的具体特征、结构、材料或者特点可以在任何的一个或多个实施例或示例中以合适的方式结合。
以上所述的具体实施例，对本发明的目的、技术方案和有益效果进行了进一步详细说明，所应理解的是，以上所述仅为本发明的具体实施例而已，并不用于限定本发明的保护范围，凡在本发明的精神和原则之内，所做的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。

Claims (8)

  1. 一种聚焦超声位移成像方法，其特征在于，包括：
    在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点；
    控制所述聚焦超声按照所述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；
    根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，所述位移分布场图中位移最大值为焦点位置；
    对所述位移分布场图进行三维成像。
  2. 根据权利要求1所述的方法，其特征在于，所述位移编码梯度是双极重复位移编码梯度，
    根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：将所述180°回聚脉冲之前的双极重复位移编码梯度的后半极开始时间确定为脉冲起始时间点，将所述180°回聚脉冲之后的双极重复位移编码梯度的前半极结束时间确定为脉冲终止时间点。
  3. 根据权利要求1所述的方法，其特征在于，所述位移编码梯度是单极位移编码梯度，
    根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：将所述180°回聚脉冲之后的单极位移编码梯度的起止时间确定为所述脉冲起止时间点。
  4. 根据权利要求1所述的方法，其特征在于，所述位移编码梯度是双极反向位移编码梯度，
    根据所述位移编码梯度确定聚焦超声的脉冲起止时间点，包括：
    将所述180°回聚脉冲之前的双极反向位移编码梯度的前半极起止时间确定为第一脉冲的起止时间点，将所述180°回聚脉冲之后的双极反向位移编码梯度的前半极起止时间确定为第二脉冲的起止时间点；其中，所述聚焦超声的脉冲包括所述第一脉冲和第二脉冲。
  5. 根据权利要求1所述的方法，其特征在于，还包括：
    将所述两组相位图相减得到相位变化值Δφd；
    对所述两组相位图进行磁场校正，将磁场校正之后的两组相位图相减得到图像相位变化值ΔφB0；
    将所述相位变化值Δφd减去所述图像相位变化值ΔφB0，得到所述相位差。
  6. 根据权利要求5所述的方法，其特征在于，
    所述位移定量公式是：
    Figure PCTCN2015099844-appb-100001
    其中，Δφ是相位差，γ是磁旋比，γ＝42.576MHz/T，Ge是所述位移编码梯度的强度，τ是所述位移编码梯度的时长，Δx表示位移。
  7. 根据权利要求1所述的方法，其特征在于，对所述位移分布场图进行三维成像，包括：
    将垂直于及平行于所述聚焦超声的声波方向上的所述位移分布场图叠加，对叠加后的位移分布场图进行三维成像。
  8. 一种聚焦超声位移成像装置，其特征在于，包括：
    时间确定模块，用于在分段读出平面回波序列的180°回聚脉冲前后各添加一个位移编码梯度，根据所述位移编码梯度确定聚焦超声的脉冲起止时间点；
    相位图生成模块，用于控制所述聚焦超声按照所述脉冲起止时间点工作，采集得到两组相位图；其中，两组相位图的位移编码梯度极性相反；
    位移分布场图生成模块，用于根据两组相位图的相位差和位移定量公式生成位移分布场图；其中，所述位移分布场图中位移最大值为焦点位置；
    成像模块，用于对所述位移分布场图进行三维成像。
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