最新magnetic resonance imaging (mri) - basics磁共振成像（mri-基础-药学医学精品资料.ppt

《最新magnetic resonance imaging (mri) - basics磁共振成像（mri-基础-药学医学精品资料.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、最新MAGNETICRESONANCEIMAGING(MRI)-BASICS磁共振成像（MRI）-基础-药学医学精品资料MRIMagneticFieldsCont’dAfterafewsecondsinamagneticfield,thespinaxesofasmallfractionoftherelevantnucleialignwitheachother=AxesallwobbleinsamewayStrengthofthemagneticfielddetermineshowbigthefractionisSpinaxesarealigned,b

2、utnotprecessinginphaseatthispointAlignmentofspinaxisofnucleicausesthewholemagneticfieldgeneratedbythespinningnucleitoprecessaroundtheaxisofstaticmagneticfieldResonanceAnEMpulsewithanarrowrangeofradiofrequencies(rfpulse)isappliedtothealignednucleiinthemagneticfieldWhenpulsefrequ

3、ency=precessionfrequency,nucleiresonatetoitPushesallofthespinningnucleiintophasewithoneanotherAmplitudeofwobbleofthewholemagneticfieldgeneratedbythespinningnucleiincreases(=spinaxisispushedfartherout)Howfarspinaxismoves(=flipangle)dependsonrfpulseintensityanddurationFixedfieldm

4、agnetisalwayson!StructuralMRIAnatomicalscansgenerallymeasurehydrogenatomsinwaterSincedifferentkindsoftissuehavedifferentproportionsofwaterTypicalanatomicalscanvoxelgranularity=1x1x7mmFunctionalMRI(fMRI)Substancemeasuredishemoglobin(iron)inbloodBloodflowincreasestoactivebrainreg

5、ionsIncreasesmorethanisusuallyneededSoratioofde-oxygenatedtooxygenatedblooddecreasesOxygenated&de-oxygenatedhemoglobinresponddifferentlytomagneticfieldandrfpulsesThus,candetectwhereratioofoxygenatedtode-oxygenatedbloodchangesduring/aftersomeeventTakes6-9secondsfortheresponsetop

6、eakFastestreliablydetectablepre-peakresponsesofar=2-4secSignalstrengthchangeverysmall–generallylessthan1%changefMRICont’dSpatialresolution:UltimatelimitprobablyspatialspecificityofthecirculatorysystemWorsethanstructuralMRITypicalfunctionalscanvoxels=3x3x7mmTemporalresolution:IF

7、bloodflowiswhat’smeasured,nevergoingtobefasterthansecondsWorkingondetectingthebriefinitialdecreaseinoxygenatedbloodprecedingincreasedbloodflowWorkingonimagingothersubstancesSubtractiveLogicMostofthebrainisactiveduringmosteventsTrytoisolateregionsthatarespecifictosomeaspectofthe

8、eventofinterestSo,construct2conditionsthatyoubelieveha