医学影像学课件：MRIbasics

WhatisMRI?MRI-MagneticResonanceImagingExcellentsofttissuecontrastwithhighresolutionDisplayofseveralimagesandobliquecutsNoionizingradiationTheorycomplicatedHardware：Mainmagnet

主磁体永磁型磁体常导型磁体超导型磁体Gradientsystem

梯度系统：用于扫描层面的空间定位，梯度线圈形成微弱的梯度磁场与主磁场重叠，可根据梯度差别确定层面位置。RFsystem射频系统：用来发射射频脉冲，使质子吸收能量产生共振，在驰豫过程中产生MR信号，并进行接收的一种装置。Coils发射和接受线圈Controlsystem

：包括梯度场、射频场的发生和控制，MR信号接收和控制等部分组成，起承上启下的作用。Imageprocessingsoftware

：每部分都要求配备大容量的计算机和高分辨的模/数转换器，以完成数据采集、图像处理、图像显示等任务。MRsystem

磁体内径Bo

磁体线圈

匀场线圈梯度线圈

射频线圈射频线圈

匀场线圈磁体线圈梯度线圈MRI(MagneticResonanceImaging)Basicphysicalprinciples

MagneticField（磁场）RadioFrequencyWave（射频脉冲）Protonresonancesignal（氢质子共振发出微弱信号）receivethesignalandtransformintoimage（接收信号转换为图像）Radiofrequencypulsetoperturbsteady-stateprotonmagnetization向稳态的原子核发射射频脉冲Transient,smallradiosignalemitted原子核发射信号Spatialencodingwithmagneticfieldgradients梯度磁场进行空间编码ImagemapofMRsignalstrength产生不同MR信号的图像Spinisanintrinsicpropertyofatomicparticles.Whatarethecharacteristicsofnuclei?

Precessionhappenswhenanexternalmagneticfield(B0)applies.

1H:

g=

42.58MHz/T

Itistheresultof

Spin+B0

ThefrequencyofprecessionisdeterminedbyLarmorEquation:

w=gB0

ApplyRadioFrequencythatmustbeatorneartheresonant(Precessional)frequency.RemovetheRFenergy.Electricalsignalisinducedinthecoilbecausemagnetizationvectorsreturntoequilibrium.

Howtocommunicatewiththenuclei?ProtonrelaxationDifferenttissueshavetheirownvalueofT1&T2DifferentT1&T2meandifferentsignalamplitude(振幅)Howtodistinguishdifferenttissues?Timeconstant:T1&T2&WhyT1constantsdifferent?自旋－晶格驰豫（spin-latticerelaxation）高能级的氢原子降至低能级时，将能量释放给周围的晶格的过程WhyT2constantsdifferent?自旋－自旋驰豫（spin-spinrelaxation）各个氢原子小磁场间相互影响致矢量散相位的过程T1andT2constantofdifferentcompositioninthehumanbodyT1andT2constantofdifferentcompositioninthehumanbodyHowtogettheprotondensity,