WO2020238620A1 - 一种婴儿大脑t1加权磁共振成像优化方法 - Google Patents

一种婴儿大脑t1加权磁共振成像优化方法 Download PDF

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WO2020238620A1
WO2020238620A1 PCT/CN2020/089882 CN2020089882W WO2020238620A1 WO 2020238620 A1 WO2020238620 A1 WO 2020238620A1 CN 2020089882 W CN2020089882 W CN 2020089882W WO 2020238620 A1 WO2020238620 A1 WO 2020238620A1
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brain
value
infant
average
month
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吴丹
张祎
刘婷婷
张洪锡
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浙江大学
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Priority to US17/218,172 priority patent/US11474180B2/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/5602—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/0033—Features or image-related aspects of imaging apparatus classified in A61B5/00, e.g. for MRI, optical tomography or impedance tomography apparatus; arrangements of imaging apparatus in a room
    • A61B5/004—Features or image-related aspects of imaging apparatus classified in A61B5/00, e.g. for MRI, optical tomography or impedance tomography apparatus; arrangements of imaging apparatus in a room adapted for image acquisition of a particular organ or body part
    • A61B5/0042—Features or image-related aspects of imaging apparatus classified in A61B5/00, e.g. for MRI, optical tomography or impedance tomography apparatus; arrangements of imaging apparatus in a room adapted for image acquisition of a particular organ or body part for the brain
    • 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
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/40—Detecting, measuring or recording for evaluating the nervous system
    • A61B5/4058—Detecting, measuring or recording for evaluating the nervous system for evaluating the central nervous system
    • A61B5/4064—Evaluating the brain
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/50—NMR imaging systems based on the determination of relaxation times, e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences

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  • This application relates to the field of brain magnetic resonance imaging optimization, in particular to the optimization of T1-weighted imaging of the infant brain.
  • Infant brain magnetic resonance imaging is a safe and universal method for examining infant brain structure, function, and diseases during early development.
  • MRI magnetic resonance imaging
  • WM white matter
  • T1w T1-weighted
  • the contrast of GM is opposite to that of the adult brain.
  • the contrast of older infants is similar to that of adults, and there will be a period of WM and GM signals close to each other before this reversal, which usually occurs at 3-6 months.
  • the present invention proposes a method for optimizing T1w magnetic resonance imaging of the infant brain.
  • This method first collects the T1 and proton density (PD) mapping of the infant’s brain from 0-12 months old, and obtains the average T1 and PD values of the infant’s brain WM and GM, respectively, and based on the WM T1 value and GM T1 value of the infant’s brain Relationship characteristics divide babies into different month age groups. Then, the WM and GM signals at different inversion times (TI) are calculated by Bloch simulation. Subsequently, determine the theoretical optimal TI optimization plan for each month-old group.
  • PD proton density
  • the theoretical optimal TI optimization program is applied to the target infant brain for 3D T1w magnetic resonance imaging.
  • the invention fills the gap in the optimization of brain T1-weighted imaging during the entire infant period, and divides the infants into three-month-old groups according to the relationship between the white matter and gray matter T1 value of the infant’s brain, and then finds the optimal TI optimizations for different month-age groups. Scheme, thereby significantly improving the contrast of T1-weighted imaging of the infant brain.
  • the realization of this method is conducive to the anatomical demarcation of the infant's brain and the detection of diseases, and the method is simple and convenient to be used in routine clinical examinations.
  • a method for optimizing infant brain T1-weighted magnetic resonance imaging which includes the following steps:
  • S1 Collect the T1 and PD mapping of several infant brains at different ages from 0-12 months of age as samples to obtain the average T1 value and average PD value of the brain white matter and gray matter regions of each infant's brain;
  • each step can further provide the following preferred implementations. It should be noted that the technical features in each preferred manner can be combined with each other without conflict. Of course, these preferred manners can also be implemented in other manners that can achieve the same technical effect, and do not constitute a limitation.
  • the calculation method of the average T1 value and the average PD value in the step S1 is as follows:
  • S101 Collect T1 and PD mapping of several infant brains at different months of age from 0-12 months;
  • S102 Delineate the regions of interest in the gray matter of the cerebral cortex and the white matter of the subcortex;
  • S103 For each infant brain, calculate the average T1 value of subcortical white matter as the average T1 value of infant brain white matter; calculate the average PD value of subcortical white matter as the average PD value of infant brain white matter; calculate the average T1 value of cerebral cortex gray matter as the infant brain The average T1 value of gray matter; calculate the average PD value of cerebral cortex gray matter as the average PD value of infant brain gray matter.
  • step S2 the method for grouping infants by month age in step S2 is as follows:
  • the infants are divided into three-month-old groups, and the characteristics of the three groups are as follows:
  • the T1 value of the white matter of the brain of the infant at this month age is higher than the T1 value of the gray matter of the infant’s brain;
  • the T1 value of brain white matter of infants at the age of this month is close to the T1 value of infant brain gray matter
  • the T1 value of the white matter of the brain of the infants of this month was lower than the T1 value of the gray matter of the brain of the infants.
  • the three-month-old group is 0-3 months old (that is, 0 to 90 days after birth), 3-7 months old (that is, 91 to 210 days after birth), and 7-12 months old (that is 211 to 360 days after birth).
  • the method for determining the theoretical optimal TI optimization scheme in step S3 is as follows:
  • S301 Use Bloch to simulate the 3D T1-weighted MPRAGE sequence for the average T1 value and average PD value of the brain white matter and gray matter regions of the brains of infants in each month-old group; during simulation, fix the inversion pulse ⁇ and the excitation pulses in the MPRAGE sequence Number N, excitation pulse flip angle ⁇ , echo interval ⁇ , and delay time TD, while changing the inversion time TI; Calculate the signal intensity s 1 of the first readout pulse in the voxel as:
  • M 0 is the initial magnetization vector
  • N is the number of single excitations along the layer selection coding direction
  • ⁇ ⁇ cos ⁇
  • T1 is the average T1 value of brain white matter or brain gray matter of all samples in the month-age group to be calculated
  • TR TI+N ⁇ +TD
  • ; relative contrast (
  • S WM is the average signal intensity of white matter voxels calculated according to the formula in S301
  • S GM is the average signal intensity of gray matter voxels calculated according to the formula in S301
  • the theoretical optimal TI optimization scheme adopted in the step S4 is as follows:
  • Another object of the present invention is to provide a method for optimizing T1-weighted magnetic resonance imaging of the infant’s brain during actual use. The specific steps are
  • the TI is set to 700 for 3D T1-weighted MRI -800ms; if the monthly age is 3-7 months (ie 91-210 days after birth), the TI is set to 400-500ms and 600-700ms for 3D T1-weighted magnetic resonance imaging, and the two sets of images obtained under TI Subtract to obtain a contrast-optimized image; if the monthly age is 7-12 months (ie 211-360 days after birth), the TI is set to 600-700ms when performing 3D T1-weighted MRI;
  • the present invention has the following characteristics: for the first time, the present invention realizes the optimization of imaging of the brain T1w in the entire infant period. First, the present invention divides the infant period into three-month-old groups according to the relationship between the white matter of the infant brain and the gray matter T1 value, and focuses on the optimization of the imaging of the infant brain of different ages by changing the TI. This method has the advantages of easy implementation, short acquisition time, and high isotropic resolution, and is suitable for clinical routine scanning.
  • the present invention proposes a dual TIs scanning method, that is, two sets of images with opposite contrast are collected on both sides of the TI inversion point, and the two sets of images are subtracted to To achieve the purpose of enhancing image contrast.
  • the present invention uses RC as the main evaluation criterion of image contrast, instead of AC or AC derivatives used in other methods. Because it can be found from the definition that the size of AC depends on the signal strength of the image itself, and RC can more truly reflect the contrast of the image.
  • the present invention uses a mirror-symmetric k-space trajectory and non-linear coding, because the T1w contrast produced by linear phase coding depends on the number of phase coding steps and local Fourier coefficients, so the versatility is poor.
  • Figure 1 is a flow chart of the optimization method for T1w magnetic resonance imaging of the infant brain.
  • Figure 2 is the use of Bloch simulation to calculate the WM and GM T1 relaxation time and WM/GM contrast of the brains of infants in the 3-month-old group.
  • Figure 3 is the optimized result of 3D T1w imaging of the neonatal brain.
  • Figure 4 shows the optimized results of 3D T1w imaging of the brains of infants aged 3-7 months.
  • Figure 5 is the optimized result of 3D T1w imaging of the brain of infants aged 7-12 months.
  • the steps of the method for optimizing T1-weighted magnetic resonance imaging of the infant brain are as follows:
  • ROIs regions of interest
  • the T1 value of the white matter of the brain of the infant at the age of this month was significantly higher than the T1 value of the gray matter of the infant’s brain
  • the T1 value of brain white matter of infants at the age of this month is close to the T1 value of infant brain gray matter
  • the T1 value of brain white matter of infants at the age of this month was significantly lower than that of infant brain gray matter.
  • a preferred way of dividing the three-month-old group in the present invention is:
  • the infants 211 to 360 days after birth are divided into the third group, namely 7-12 months old.
  • the average T1 value and the average PD value of the brain white matter and gray matter regions of the samples contained in each month-age group need to be arithmetic averaged, as the brain white matter and brain white matter of the infants in the month-age group Average T1 and PD values of gray matter regions.
  • the average T1 value of the brain white matter of the infants in this month age group is the arithmetic mean of the average T1 values of all the samples in the month age group; the average PD value of the brain white matter of the infants in the month age group is in the month age group
  • the average PD value of infant brain gray matter is the arithmetic mean of the average PD value of infant brain gray matter of all samples in this month-age group.
  • the lateral signal of the i-th readout pulse is
  • T1 is the average T1 value of the brain white matter or brain gray matter of the infants in the month-age group to be calculated (determined according to the method described in S2 above).
  • Equation (1) can simplify equation (3).
  • the signal intensity s 1 of the first readout pulse in the voxel is calculated as:
  • the value of T1 is the average T1 value of the brain white matter of the infants in the month age group, and S WM is the signal calculated at this time according to equation (3) Intensity s 1 ;
  • the value of T1 is the average T1 value of the brain gray matter of the infant’s brain in the month age group, and S GM is at this time according to equation (3) The calculated signal strength s 1 .
  • S302 Calculate the image contrast under different TIs for the different month age groups set in S2.
  • the image contrast is divided into absolute contrast and relative contrast. among them:
  • the specific determination method is different for the three month-age groups, among which:
  • high relative contrast performance means that the relative contrast may not be optimal, but not too low, and should be greater than a certain threshold (the threshold can be set according to actual needs).
  • the two sets of different TI images with the highest absolute contrast and opposite white matter/gray matter (WM/GM) contrast performance are collected on both sides of the TI inversion point.
  • the image subtraction method enhances the image contrast.
  • a preferred method of the theoretically optimal TI optimization solution finally determined by the three-month-old group in the present invention is:
  • T1 measurement results show: 3 months ago, the T1 value of subcortical WM was higher than cortical GM; 3-7 months WM was close to the GM T1 value; 6 months later, WM T1 value was lower than GM .
  • babies can be divided into 0-3 months old group (ie 0 ⁇ 90 days after birth), 3-7 months old group (ie 91 ⁇ 210 days after birth) and 7-12 months old group Group (ie 211 to 360 days after birth).
  • TI 500ms shows positive low contrast (S WM >S GM )
  • TI 700ms shows low confidence.
  • the noise ratio, TI 800ms and TI 1000ms show negative medium contrast (S WM >S GM ), which is consistent with the simulation results.
  • Image segmentation based on multi-atlas for TI 800ms and TI 1000ms it can be found by naked eyes that the segmentation accuracy of TI 800ms image is higher than that of TI 1000ms , and the segmentation performance between subcortical WM and cortical GM is particularly obvious (Figure 3B) Arrows and boxes).
  • the contrast between TI 500ms image and TI 700ms image is opposite, and the subtraction image (TI 700ms -TI 500ms ) can enhance the image contrast, which is also consistent with the simulation results.
  • Figure 4 shows that the RC of the forebrain is low and the signal changes during 5.7-5.8 months, while the RC of the hindbrain is low but exhibits a positive contrast and the contrast increases after 5.7 months.
  • the WM/GM contrast can be enhanced by the method of image subtraction (TI 700ms -TI 500ms ).
  • the T1w image contrast is close to that of adults and significantly higher than the other two groups.
  • This group tested TI at 700, 800 and 1000ms respectively.
  • the ROI of the anterior and posterior cortex GM and the subcortical WM is drawn to calculate the image contrast.
  • Figure 5 shows that compared to TI 800ms and TI 1000ms , the RC of the TI 700ms image is higher, which is also consistent with the simulation results, and the contrast of the front and rear brain regions increases with age.
  • a method for optimizing infant brain T1-weighted magnetic resonance imaging can be provided.
  • the method is as follows:

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Abstract

一种婴儿大脑T1加权磁共振成像优化的方法。首先采集0-12月龄婴儿大脑的T1和PD mapping，得到婴儿大脑白质和灰质的平均T1、PD值，并根据婴儿大脑白质T1值与灰质T1值的关系特点，将婴儿分为三个月龄组。然后，通过Bloch仿真计算3D T1加权图像中婴儿大脑白质和灰质的理论信号强度，根据不同TI下婴儿大脑白质/灰质对比度特点，分别确定每组理论最佳TI优化方案。最后，将理论最佳TI优化方案应用于目标婴儿大脑进行3D T1加权磁共振成像。填补了出生后0-12个月整个婴儿时期大脑T1加权成像优化的空白，并依据婴儿大脑白质与灰质T1值的关系特点将婴儿分为三个月龄组，分别找出不同月龄组的最佳TI优化方案，从而显著提升婴儿大脑T1加权成像对比度。

Description

一种婴儿大脑T1加权磁共振成像优化方法 技术领域
本申请涉及脑磁共振成像优化领域，尤其涉及婴儿大脑T1加权成像优化。
背景技术
婴儿大脑磁共振成像(magnetic resonance imaging，MRI)是用来检查婴儿大脑结构、功能以及早期发育过程中的疾病的一种安全且通用的方法。然而，由于婴儿大脑成像对比度差，且因出生后第一年大脑快速发育导致图像对比度也随之迅速变化，这些都是导致难以对婴儿大脑成像进行解剖分界以及图像自动化分析的原因。由于出生时白质(white matter，WM)髓鞘发育不成熟导致T1弛豫时间延长，新生儿(≤1个月)大脑的T1加权(T1-weighted，T1w)图像中白质与灰质(gray matter，GM)的对比度与成人大脑表现相反。而年龄稍长的婴儿对比度则类似于成人，而在出现这种反转之前会有一段时期WM和GM信号表现接近，这段时期通常出现在3-6个月。
目前一些研究已经实现对新生儿大脑T1w成像的优化。但事实上新生儿大脑WM/GM对比度非常差，理论上使用3T MRI扫描仪进行T1w成像，新生儿WM/GM的对比度最高仅能达到成人的1/3，而在其他时期(1-12月龄)WM/GM对比度甚至比出生后第一个月更差且更复杂，且目前罕有针对1-12月龄婴儿大脑优化成像的研究。
值得一提的是，由于6个月左右婴儿大脑的WM和GM信号几近相等，对该时期的婴儿大脑图像进行分割仍然是个巨大挑战。一些研究利用延长扫描时间、整合多模态数据、深度学习网络等方法能在一定程度上改善分割的结果。然而除了发展先进的图像处理算法以外，通过在采集过程中提升图像对比度也会对后续分析大有裨益。
发明内容
为了打破对整个婴儿时期(0-12月)T1w成像进行优化的空白，本发明提出一种实现婴儿大脑T1w磁共振成像优化的方法。该方法首先采集0-12月大婴儿大脑的T1和质子密度(proton density，PD)mapping，分别得到婴儿大脑WM 和GM的平均T1、PD值，并依据婴儿大脑WM T1值和GM T1值的关系特点将婴儿分为不同的月龄组。接着，通过Bloch仿真计算不同反转时间(inversion time，TI)下的WM和GM信号。随后，确定每个月龄组的理论最佳TI优化方案。最终，将该理论最佳TI优化方案应用于目标婴儿大脑进行3D T1w磁共振成像。本发明填补了整个婴儿时期大脑T1加权成像优化的空白，并依据婴儿大脑白质与灰质T1值的关系特点将婴儿分为三个月龄组，进而分别找出不同月龄组的最佳TI优化方案，从而显著提升婴儿大脑T1加权成像对比度。这种方法的实现有利于婴儿大脑的解剖学分界以及疾病的检测，且方法简便，方便应用于临床常规检查中。
为了达到上述目的，本发明采用以下技术方案予以实现：
一种婴儿大脑T1加权磁共振成像优化的方法，其包括以下步骤：
S1：在0-12月龄中不同月龄分别采集若干婴儿大脑的T1和PD mapping作为样本，得到每个婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值；
S2：根据每个样本中婴儿大脑的白质与灰质T1值关系特点，将婴儿的月龄分为三个月龄组；
S3：根据不同月龄组婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值，通过Bloch仿真分别计算3D T1加权MPRAGE序列产生的婴儿大脑白质和灰质区域的理论信号强度；并根据白质和灰质的对比度，分别确定每个月龄组的理论最佳TI优化方案；
S4：利用所述的理论最佳TI优化方案，对目标婴儿大脑进行3D T1加权磁共振成像。
基于该方案，各步骤还可以进一步提供以下优选的实现方式。需要注意的是，各优选方式中的技术特征在没有冲突的情况下均可进行相互组合。当然这些优选方式也可以通过其他能够实现相同技术效果的方式实现，不构成限制。
作为优选，所述步骤S1中平均T1值和平均PD值计算方法如下：
S101：在0-12月龄中的不同月龄分别采集若干婴儿大脑的T1和PD mapping；
S102：勾画脑皮层灰质和脑皮层下白质的感兴趣区；
S103：针对每个婴儿大脑，计算脑皮层下白质平均T1值作为婴儿大脑白质平均T1值；计算脑皮层下白质平均PD值作为婴儿大脑白质平均PD值；计算 脑皮层灰质平均T1值作为婴儿大脑灰质平均T1值；计算脑皮层灰质平均PD值作为婴儿大脑灰质平均PD值。
作为优选，所述步骤S2中婴儿的月龄分组方法如下：
按照不同月龄段的婴儿大脑白质和灰质T1值关系特点，将婴儿分为三个月龄组，其中三组的所述关系特点分别为：
第一组中，该月龄段婴儿大脑白质T1值高于婴儿大脑灰质T1值；
第二组中，该月龄段婴儿大脑白质T1值接近婴儿大脑灰质T1值；
第三组中，该月龄段婴儿大脑白质T1值低于婴儿大脑灰质T1值。
作为优选，所述的三个月龄组分别为0-3月龄(即出生后0～90天)、3-7月龄(即出生后91～210天)和7-12月龄(即出生后211～360天)。
作为优选，所述步骤S3中理论最佳TI优化方案的确定方法如下：
S301：针对每个月龄组婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值，利用Bloch仿真3D T1加权MPRAGE序列；仿真时，固定MPRAGE序列中的反转脉冲α、激发脉冲个数N、激发脉冲翻转角θ、回波间隔τ和延迟时间TD，同时变化反转时间TI；计算体素内第一个读出脉冲的信号强度s 1为：
Figure PCTCN2020089882-appb-000001
其中
Figure PCTCN2020089882-appb-000002
式中：M 0为初始磁化矢量，N为沿层选编码方向的单次激发次数，
Figure PCTCN2020089882-appb-000003
Figure PCTCN2020089882-appb-000004
μ＝δ·cosθ；T1为待计算的月龄组中所有样本的大脑白质或大脑灰质的平均T1值；TR＝TI+N·τ+TD；
S302：针对S2中设定的不同月龄组，分别计算在不同TI下的图像对比度，其中绝对对比度＝|S WM|-|S GM|；相对对比度＝(|S WM|-|S GM|)/(|S WM|+|S GM|)；
式中：S WM为根据S301中公式计算得到的白质体素的平均信号强度，S GM为根据S301中公式计算得到的灰质体素的平均信号强度；
S303：根据不同TI下不同月龄组的图像对比度，分别确定每个月龄组的理论最佳TI优化方案，其中：
对于第一组和第三组月龄组，对比θ与TR不变时不同TI下的图像对比度，分别确定每组绝对对比度表现最佳且相对对比度表现较高时对应的TI；
对于第二组月龄组，在TI反转点两侧采集绝对对比度最高且白质/灰质对比度表现相反的两组不同TI图像，通过将两组图像相减的方法增强图像对比度。
作为优选，所述步骤S4中采用的理论最佳TI优化方案如下：
将3D T1加权磁共振成像参数设置为：θ＝10°，TR＝2000ms；
将0-3月龄划分为第一组，进行3D T1加权磁共振成像时TI设置为700-800ms；
将3-7月龄划分为第二组，进行3D T1加权磁共振成像时TI分别设置为400-500ms以及600-700ms，将两组TI下获得的图像相减，以获得对比度优化的图像；
将7-12月龄划分为第三组，进行3D T1加权磁共振成像时TI设置为600-700ms。
本发明的另一目的在于提供一种实际使用时的婴儿大脑T1加权磁共振成像优化的方法，其具体步骤为
1)根据婴儿的当前月龄，选择相应的理论最佳TI优化方案：若月龄为0-3月龄(即出生后0～90天)，进行3D T1加权磁共振成像时TI设置为700-800ms；若月龄为3-7月龄(即出生后91～210天)，进行3D T1加权磁共振成像时TI分别设置为400-500ms以及600-700ms，将两组TI下获得的图像相减，以获得对比度优化的图像；若月龄为7-12月龄(即出生后211～360天)，进行3D T1加权磁共振成像时TI设置为600-700ms；
2)3D T1加权磁共振成像时其他参数设置为：θ＝10°，TR＝2000ms，然后根据确定的参数，对目标婴儿大脑进行3D T1加权磁共振成像，完成共振成像优化。
相对于现有技术，本发明具有以下特点：本发明首次实现对整个婴儿时期的大脑T1w成像优化。首先，本发明按照婴儿大脑白质与灰质T1值的关系特点将婴儿时期划分为三个月龄组，并着重于通过改变TI实现针对不同年龄阶段婴儿大脑的成像优化。该方法具有易于实现、采集时间短、各向同性分辨率高的优点，适合临床常规扫描。
其次，针对对比度最差的3-6月龄的婴儿大脑，本发明提出双TIs扫描方法，即在TI反转点两侧采集对比度相反的两组图像，通过将两组图像相减以达到增强图像对比度的目的。
再次，本发明使用RC作为图像对比度的主要评价标准，而非其他方法中使用的AC或AC衍生物。因为从定义中可以发现，AC的大小依赖于图像本身的信号强度，而RC能更真实地反映图像的对比度。
最后，本发明使用的是镜面对称的k空间轨迹而非线性编码，因为线性相位编码产生的T1w对比度取决于相位编码的步数以及局部傅里叶系数，因而通用性较差。
附图说明
图1是婴儿大脑T1w磁共振成像优化方法的流程图。
图2是使用Bloch仿真分别计算3个月龄组婴儿大脑的WM和GM的T1弛豫时间以及WM/GM对比度。
图3是对新生儿大脑3D T1w成像的优化结果。
图4是对3-7月龄婴儿大脑3D T1w成像的优化结果。
图5是对7-12月龄婴儿大脑3D T1w成像的优化结果。
具体实施方式
下面基于本发明提出的方法结合实施例展示其具体的技术效果，以便本领域技术人员更好地理解本发明的实质。
在本发明的一种较优实现方式中，如图1所示，婴儿大脑T1加权磁共振成像优化的方法步骤如下：
S1：在3T MRI扫描仪上，针对0-12月龄中不同月龄，分别采集多个婴儿大脑的T1和PD mapping作为样本，这些样本需要排除存在任何已知会改变T1w对比度的情况或MRI图像上发现异常的被试。样本应当在0-12月龄的不同月龄段中尽量均匀分布，以减少误差。然后计算每个婴儿大脑的大脑白质(WM)和灰质(GM)区域的平均T1值以及平均PD值，具体计算方法为：
首先对样本图像勾画脑皮层灰质和脑皮层下白质的感兴趣区(ROIs)。然后，针对每个婴儿大脑样本，计算脑皮层下白质平均T1值作为婴儿大脑白质平均T1值；计算脑皮层下白质平均PD值作为婴儿大脑白质平均PD值；计算脑 皮层灰质平均T1值作为婴儿大脑灰质平均T1值；计算脑皮层灰质平均PD值作为婴儿大脑灰质平均PD值。
S2：根据每个样本中婴儿大脑的白质与灰质T1值关系特点，将婴儿的月龄分为三个月龄组。出生后第一年大脑快速发育导致图像对比度也随之迅速变化，随着婴儿脑部的不断发育，不同月龄段的婴儿大脑白质和灰质T1值关系会形成不同的特点，而这些大脑白质和灰质T1值的不同关系特点会影响最终的成像效果，因此需要对其进行区分。在本发明中，可以根据不同的关系特点将婴儿分为三个月龄组，其中三组的关系特点分别为：
第一组中，该月龄段婴儿大脑白质T1值明显高于婴儿大脑灰质T1值；
第二组中，该月龄段婴儿大脑白质T1值接近婴儿大脑灰质T1值；
第三组中，该月龄段婴儿大脑白质T1值明显低于婴儿大脑灰质T1值。
根据后续的实施例，本发明中三个月龄组的一种优选划分方式为：
将出生后0～90天的婴儿划分为第一组，即0-3月龄；
将出生后91～210天的婴儿划分为第二组，即3-7月龄；
将出生后211～360天的婴儿划分为第三组，即7-12月龄。
由于上述三个月龄组的婴儿大脑具有不同的特点，因此需要分别针对三组月龄组进行相应的最佳TI优化，从而显著提升每个月龄组的婴儿大脑T1加权成像对比度。
对月龄组进行划分后，需要根据每个月龄组中各自所含有的样本的大脑白质和灰质区域的平均T1值以及平均PD值进行算数平均，作为该月龄组婴儿大脑的大脑白质和灰质区域的平均T1和PD值。对任一月龄组而言：
该月龄组婴儿大脑的大脑白质平均T1值为该月龄组中所有样本的婴儿大脑白质平均T1值的算数平均值；该月龄组婴儿大脑的大脑白质平均PD值为该月龄组中所有样本的婴儿大脑白质平均PD值的算数平均值；该月龄组婴儿大脑的大脑灰质平均T1值为该月龄组中所有样本的婴儿大脑灰质平均T1值的算数平均值；该月龄组婴儿大脑的大脑灰质平均PD值为该月龄组中所有样本的婴儿大脑灰质平均PD值的算数平均值。
S3：根据不同月龄组婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值，通过Bloch仿真分别计算3D T1加权MPRAGE序列产生的婴儿大脑白 质和灰质区域的理论信号强度；并根据白质和灰质的对比度，分别确定每个月龄组的理论最佳TI优化方案。此处，理论最佳TI优化方案的具体确定方法如下：
S301：针对每个月龄组的平均T1值以及平均PD值，利用Bloch仿真3D T1加权MPRAGE序列；仿真时，固定MPRAGE序列中的反转脉冲α、激发脉冲个数N、激发脉冲翻转角θ、回波间隔τ和延迟时间TD，同时变化反转时间TI，以计算不同TI下的信号强度。图像对比度由第一个读出信号决定。
第i个读出脉冲的横向信号为
Figure PCTCN2020089882-appb-000005
其中参数M eq为：
Figure PCTCN2020089882-appb-000006
式中：M 0为初始磁化矢量(M 0与前述计算的WM和GM的平均PD值有关)，N为沿层选编码方向的单次激发次数；λ、
Figure PCTCN2020089882-appb-000007
δ、ρ、μ均为中间参数，
Figure PCTCN2020089882-appb-000008
Figure PCTCN2020089882-appb-000009
μ＝δ·cosθ，总的重复时间TR＝TI+N·τ+TD。T1为待计算的月龄组婴儿大脑的大脑白质或大脑灰质的平均T1值(根据前述S2中描述的方法确定)。
空间编码使用镜像对称的采集方法，图像对比度由第一个读出信号决定，因而方程(1)可以简化方程(3)，计算体素内第一个读出脉冲的信号强度s 1为：
Figure PCTCN2020089882-appb-000010
需要注意的是，上述计算方程(3)中，计算不同的信号值时T1的取值不同，具体来说，对于任一月龄组而言：
当计算该月龄组的白质体素的平均信号S WM时，T1的取值为该月龄组婴儿大脑的大脑白质平均T1值，S WM即为此时根据方程(3)计算得到的信号强度s 1；当计算该月龄组的灰质体素的平均信号S GM时，T1的取值为该月龄组婴儿大脑的大脑灰质平均T1值，S GM即为此时根据方程(3)计算得到的信号强度s 1。
S302：针对S2中设定的不同月龄组，分别计算在不同TI下的图像对比度，图像对比度分为绝对对比度和相对对比度。其中：
绝对对比度(absolute contract，AC)＝|S WM|-|S GM|
相对对比度(relative contract，RC)＝(|S WM|-|S GM|)/(|S WM|+|S GM|)
S303：根据不同TI下不同月龄组的图像对比度，以白质/灰质对比度为标准，分别确定每个月龄组的理论最佳TI优化方案。由于相较于AC，RC受图像本身信号强度影响较小，本发明使用RC作为主要评价标准。另一方面，由于AC与信噪比相关，本发明将AC作为辅助评价标准。
具体确定方法对于三组月龄组均不同，其中：
对于第一组和第三组月龄组，对比θ与TR不变时不同TI下的图像对比度，分别确定每组绝对对比度表现最佳且相对对比度表现较高的TI。此处，相对对比度表现较高是指在相对对比度可以不是最佳的，但不能过低，应当大于一定的阈值(该阈值可以根据实际需要进行设定)。
对于第二组月龄组，由于大脑白质与灰质信号接近，在TI反转点两侧采集绝对对比度最高且白质/灰质(WM/GM)对比度表现相反的两组不同TI图像，通过将两组图像相减的方法增强图像对比度。
根据后续的实施例，本发明中三个月龄组最终确定的理论最佳TI优化方案的一种优选方式为：
将3D T1加权磁共振成像参数设置为：θ＝10°，TR＝2000ms；
将0-3月龄(即出生后0～90天)划分为第一组，进行3D T1加权磁共振成像时TI设置为700-800ms；
将3-7月龄(即出生后91～210天)划分为第二组，进行3D T1加权磁共振成像时TI分别设置为400-500ms以及600-700ms，将两组TI下获得的图像相减(TI＝600-700ms得到的图像减去TI＝400-500ms得到的图像，即TI 600- 700ms-TI 400-500ms)，以获得对比度优化的图像；
将7-12月龄(即出生后211～360天)划分为第三组，进行3D T1加权磁共振成像时TI设置为600-700ms。
S4：利用上述确定的理论最佳TI优化方案，对目标婴儿大脑进行3D T1加权磁共振成像。
下面基于上述方法，结合实施例对其技术效果进行展示，以便本领域技术人员更好地理解本发明的实质。
实施例
对上述婴儿大脑T1加权磁共振成像优化方法进行测试。首先使用飞利浦3T MRI扫描仪(Achieva；Philips Healthcare,Best,The Netherlands)，采集57名正常发育婴儿大脑的“MIX”序列图像，利用比值以及最小二乘法求解自旋回波(spin-echo，SE)和反转恢复(inversion recovery，IR)信号方程，分别得到T1和PD maps，并以侧脑室PD值为标准对大脑PD值进行归一化。
在T1maps上手工勾画感兴趣区(regions of interest，ROI)，包括脑皮层灰质(GM)和脑皮层下白质(WM)，并分别针对每个婴儿大脑，计算脑皮层下白质平均T1值作为婴儿大脑白质平均T1值；计算脑皮层下白质平均PD值作为婴儿大脑白质平均PD值；计算脑皮层灰质平均T1值作为婴儿大脑灰质平均T1值；计算脑皮层灰质平均PD值作为婴儿大脑灰质平均PD值。
T1测量结果(图2A)显示：3个月前，皮层下WM的T1值高于皮层GM；3-7个月WM与GM的T1值接近；6个月后，WM的T1值低于GM。在这种规律的基础上，可以将婴儿分为0-3月龄组(即出生后0～90天)、3-7月龄组(即出生后91～210天)和7-12月龄组(即出生后211～360天)。根据每个月龄组中所有样本的大脑白质和灰质区域的平均T1值以及平均PD值进行算数平均，计算该月龄组婴儿大脑的大脑白质/灰质区域的平均T1和PD值，并利用Bloch仿真计算各月龄组婴儿大脑的大脑WM和GM的理论信号强度，结果如下：
1)0-3月龄组婴儿大脑在TI＝602ms左右出现RC对比度反转(图2B)，表现为反转点两侧出现明显的正负对比。此外，在对比度反转点处AC接近于零，这是由于反转点接近T1零点(WM＝610ms，GM＝596ms)，因而所有信号都接近于零。因此，最佳TI的选择不仅应追求高RC还应考虑到AC(反映信噪比)，如当TI＝800ms(图2B中的竖直虚线)时，RC≈-0.1，AC约为最大AC的一半。
2)由于3-7月龄组婴儿大脑WM和GM的T1弛豫时间高度相似(1342±104ms和1354±55ms)，该组RC和AC均低于其他组(图2C，注意y轴比例不同于图2B和2D)。对于此阶段，选择在对比度反转点两侧采集双TIs图像，例如TI＝500ms和700ms能得到两幅对比度相反、同时RC和AC较高的图像，通过将两幅图像相减能增强WM和GM之间的差异。
3)7-12月龄组婴儿大脑的RC、AC曲线与0-3月龄组相反(图2D)，且对 比度反转点向低TI方向移动。同样地，最佳TI的选择应同时考虑RC和AC，例如当TI＝700ms时，RC≈0.1，AC约为最大AC的2/3。
另外采集一组0-12月龄婴儿大脑的3D MPRAGE图像，视野为180mm x 180mm x 120mm，轴位扫描，矩阵为180x 180x 120，TR/TE＝2000/3.7ms，τ＝8ms，，空间编码使用镜像对称的采集方法，反转恢复脉冲，α＝180°，θ＝10°，N＝120(沿层选编码方向单次激发次数)，2倍SENSE(加速度与相位编码方向一致)，扫描时间为3.07分钟。基于Bloch仿真结果(图2)，分别测试了：1)设定TI＝500，800和1000ms分别对6个足月新生儿大脑进行扫描；2)设定TI＝500和700ms分别对7个3-7月龄的婴儿大脑进行扫描；3)设定TI＝700，800和1000ms分别对5个7-12月龄的婴儿大脑进行扫描。
图3A显示，在新生儿大脑中，T1w图像对比度会在TI＝500ms到TI＝1000ms范围内发生改变，如TI 500ms表现为正的低对比度(S WM>S GM)，TI 700ms表现出低信噪比，TI 800ms和TI 1000ms表现为负的中等对比度(S WM>S GM)，这与仿真结果一致。对TI 800ms和TI 1000ms进行基于多图谱的图像分割，通过肉眼即可发现TI 800ms图像的分割准确度较TI 1000ms更高，对皮层下WM与皮层GM之间的分割表现尤为明显(图3B中的箭头和方框)。以TI 800ms图像GM和WM的分割结果为基础，TI 500ms、TI 800ms和TI 1000ms图像的RC和AC表现出统计学差异。Post-hoc t检验证实TI 800ms较TI 1000ms图像RC更高(p<0.001,n＝6)，而AC则无统计学差异(图3C)。
在3-7月龄婴儿组中，TI 500ms图像与TI 700ms图像对比度相反，减法图像(TI 700ms-TI 500ms)能增强图像对比度，这也与仿真结果一致。图4显示前脑的RC低且5.7-5.8月期间信号发生转变，而后脑的RC低但表现为正的对比度且在5.7个月后对比度增强。相较于单个TI图像，通过图像相减(TI 700ms-TI 500ms)的方法能够增强WM/GM对比度。
对于7-12月龄婴儿，T1w图像对比度接近成人且明显高于另外两组。该组分别对TI为700、800和1000ms进行了测试。同样的，通过勾画前、后皮层GM和皮层下WM的ROI以计算图像对比度。图5显示相较于TI 800ms和TI 1000ms，TI 700ms图像的RC更高，这也与仿真结果一致，且前后脑区的对比度均随年龄增大而增大。
综上所述，基于本实施例中的结构结果，可以提供一种婴儿大脑T1加权磁 共振成像优化的方法，其做法为：
根据婴儿的当前月龄，选择相应的理论最佳TI优化方案：
若月龄为0-3月龄(即出生后0～90天)，进行3D T1加权磁共振成像时TI设置为700-800ms；若月龄为3-7月龄(即出生后91～210天)，进行3D T1加权磁共振成像时TI分别设置为400-500ms以及600-700ms，将两组TI下获得的图像相减(TI 600- 700ms-TI 400-500ms)，以获得对比度优化的图像；若月龄为7-12月龄(即出生后211～360天)，进行3D T1加权磁共振成像时TI设置为600-700ms。3D T1加权磁共振成像时其他参数设置为：θ＝10°，TR＝2000ms，根据确定的参数，对目标婴儿大脑进行3D T1加权磁共振成像，完成共振成像优化。
需要指出的是，以上所述的实施例只是本发明的一种较佳的方案，然其并非用以限制本发明。有关技术领域的普通技术人员，在不脱离本发明的精神和范围的情况下，还可以做出各种变化和变型。因此凡采取等同替换或等效变换的方式所获得的技术方案，均落在本发明的保护范围内。

Claims (6)

  1. 一种婴儿大脑T1加权磁共振成像优化的方法，其特征在于，包括以下步骤：
    S1：在0-12月龄中不同月龄分别采集若干婴儿大脑的T1和PD mapping作为样本，得到每个婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值；
    S2：根据每个样本中婴儿大脑的白质与灰质T1值关系特点，将婴儿的月龄分为三个月龄组；
    S3：根据不同月龄组婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值，通过Bloch仿真分别计算3D T1加权MPRAGE序列产生的婴儿大脑白质和灰质区域的理论信号强度；并根据白质和灰质的对比度，分别确定每个月龄组的理论最佳TI优化方案；
    S4：利用所述的理论最佳TI优化方案，对目标婴儿大脑进行3D T1加权磁共振成像。
  2. 根据权利要求1所述的婴儿大脑T1加权磁共振成像优化的方法，其特征在于，所述步骤S1中平均T1值和平均PD值计算方法如下：
    S101：在0-12月龄中的不同月龄分别采集若干婴儿大脑的T1和PD mapping；
    S102：勾画脑皮层灰质和脑皮层下白质的感兴趣区；
    S103：针对每个婴儿大脑，计算脑皮层下白质平均T1值作为婴儿大脑白质平均T1值；计算脑皮层下白质平均PD值作为婴儿大脑白质平均PD值；计算脑皮层灰质平均T1值作为婴儿大脑灰质平均T1值；计算脑皮层灰质平均PD值作为婴儿大脑灰质平均PD值。
  3. 根据权利要求1所述的婴儿大脑T1加权磁共振成像优化的方法，其特征在于，所述步骤S2中婴儿的月龄分组方法如下：
    按照不同月龄段的婴儿大脑白质和灰质T1值关系特点，将婴儿分为三个月龄组，其中三组的所述关系特点分别为：
    第一组中，该月龄段婴儿大脑白质T1值高于婴儿大脑灰质T1值；
    第二组中，该月龄段婴儿大脑白质T1值接近婴儿大脑灰质T1值；
    第三组中，该月龄段婴儿大脑白质T1值低于婴儿大脑灰质T1值。
  4. 根据权利要求1所述的婴儿大脑T1加权磁共振成像优化的方法，其特征在于，所述的三个月龄组分别为0-3月龄、3-7月龄和7-12月龄。
  5. 根据权利要求4所述的婴儿大脑T1加权磁共振成像优化的方法，其特征在于，所述步骤S3中理论最佳TI优化方案的确定方法如下：
    S301：针对每个月龄组婴儿大脑的大脑白质和灰质区域的平均T1值以及平均PD值，利用Bloch仿真3D T1加权MPRAGE序列；仿真时，固定MPRAGE序列中的反转脉冲α、激发脉冲个数N、激发脉冲翻转角θ、回波间隔τ和延迟时间TD，变化反转时间TI；计算体素内第一个读出脉冲的信号强度s 1为：
    Figure PCTCN2020089882-appb-100001
    其中
    Figure PCTCN2020089882-appb-100002
    式中：M 0为初始磁化矢量，N为沿层选编码方向的单次激发次数，
    Figure PCTCN2020089882-appb-100003
    Figure PCTCN2020089882-appb-100004
    μ＝δ·cosθ；T1为待计算的月龄组中所有样本的大脑白质或大脑灰质的平均T1值；TR＝TI+N·τ+TD；
    S302：针对S2中设定的不同月龄组，分别计算在不同TI下的图像对比度，其中绝对对比度＝|S WM|-|S GM|；相对对比度＝(|S WM|-|S GM|)/(|S WM|+|S GM|)；
    式中：S WM为根据S301中公式计算得到的白质体素的平均信号强度，S GM为根据S301中公式计算得到的灰质体素的平均信号强度；
    S303：根据不同TI下不同月龄组的图像对比度，分别确定每个月龄组的理论最佳TI优化方案，其中：
    对于第一组和第三组月龄组，对比θ与TR不变时不同TI下的图像对比度，分别确定每组绝对对比度表现最佳且相对对比度表现较高时对应的TI；
    对于第二组月龄组，在TI反转点两侧采集绝对对比度最高且白质/灰质对比度表现相反的两组不同TI图像，通过将两组图像相减的方法增强图像对比度。
  6. 根据权利要求1所述的婴儿大脑T1加权磁共振成像优化的方法，其特征在于，所述步骤S4中采用的理论最佳TI优化方案如下：
    将3D T1加权磁共振成像参数设置为：θ＝10°，TR＝2000ms；
    将0-3月龄划分为第一组，进行3D T1加权磁共振成像时TI设置为700-800ms；
    将3-7月龄划分为第二组，进行3D T1加权磁共振成像时TI分别设置为400-500ms以及600-700ms，将两组TI下获得的图像相减，以获得对比度优化的图像；
    将7-12月龄划分为第三组，进行3D T1加权磁共振成像时TI设置为600-700ms。
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