Ukufana kweMRI

Ukufana kwentsimi yemagnethi (homogeneity), okwaziwa ngokuba yi-magnetic field uniformity, kubhekisa kubunikazi bentsimi yemagnethi ngaphakathi komda othile wevolumu, oko kukuthi, nokuba imigca yentsimi yemagnethi kulo lonke indawo yeyunithi iyafana na. Umthamo othile apha uhlala uyindawo engqukuva. Iyunithi yokufana kwentsimi yemagnethi yi-ppm (inxalenye ngesigidi), oko kukuthi, umahluko phakathi kwamandla amakhulu entsimi kunye namandla amancinci entsimi yemagnethi kwindawo ethile eyahlulwe ngamandla aphakathi entsimi ephindaphindwe ngesigidi esinye.

I-MRI ifuna inqanaba eliphezulu lokufana kwentsimi yamagnetic, nto leyo emisela isisombululo sendawo kunye nomlinganiselo wesignali-kwingxolo yomfanekiso kuluhlu lwemifanekiso. Ukufana okungalunganga kwentsimi yamagnetic kuya kwenza umfanekiso ungabonakali kwaye ugqwethekile. Ukufana kwentsimi yamagnetic kugqitywa kuyilo lwemagnethi ngokwayo kunye nokusingqongileyo kwangaphandle. Okukhona indawo yokufota yemagnethi inkulu, kokukhona ukufana kwentsimi yamagnetic kunokubakho okuphantsi. Ukuzinza kwentsimi yamagnetic sisalathisi sokulinganisa inqanaba lokuhamba kobunzulu bentsimi yamagnetic ngokuhamba kwexesha. Ngexesha lolandelelwano lwemifanekiso, ukushukuma kobunzulu bentsimi yamagnetic kuya kuchaphazela isigaba sesignali ye-echo ephindaphindwayo, okubangela ukuphambuka komfanekiso kunye nokwehla komlinganiselo wesignali-kwingxolo. Ukuzinza kwentsimi yamagnetic kunxulumene ngokusondeleyo nohlobo lwemagnethi kunye nomgangatho woyilo.

Imigaqo yomgangatho wokulingana kwentsimi yamagnetic inxulumene nobukhulu kunye nemilo yendawo yokulinganisa ethathiweyo, kwaye ngokubanzi isebenzisa indawo engqukuva enobubanzi obuthile kunye neziko lemagnet njengoluhlu lokulinganisa. Ngokwesiqhelo, ukumelwa kokulingana kwentsimi yamagnetic kwimeko yendawo ethile yokulinganisa, uluhlu lotshintsho lobunzulu bentsimi yamagnetic kwindawo enikiweyo (ixabiso le-ppm), oko kukuthi, isigidi esinye samandla amakhulu entsimi yamagnetic (ppm) njengeyunithi yokuphambuka ukuya ekuchazeni ngokwenani, ngokwesiqhelo le yunithi yokuphambuka ibizwa ngokuba yi-ppm, ebizwa ngokuba yi-absolute value representation. Umzekelo, ukufana kwentsimi yamagnetic ngaphakathi kwesilinda yonke yokujonga ukuskena yi-5ppm; Ukufana kwentsimi yamagnetic kwindawo yesphere ye-40cm kunye ne-50cm concentric kunye neziko lemagnet yi-1ppm kunye ne-2ppm, ngokwahlukeneyo. Ingabonakaliswa ngolu hlobo: ukufana kwentsimi yamagnetic kwindawo yecube yesentimitha nganye yecubic kwindawo yesampuli ephantsi kovavanyo yi-0.01ppm. Nokuba umgangatho unjani na, phantsi kwengcamango yokuba ubungakanani besphere yokulinganisa buyafana, okukhona ixabiso le-ppm lincinci libonisa ukuba i-magnetic field iyafana ngcono.

Kwimeko yesixhobo se-1.5-tMRI, ukuguquguquka kwamandla e-magnetic field okumelwe yiyunithi enye yokuphambuka (1ppm) yi-1.5×10-6T. Ngamanye amazwi, kwinkqubo ye-1.5T, ukufana kwamandla e-magnetic field ye-1ppm kuthetha ukuba i-magnetic field ephambili inokuguquguquka kwamandla e-drift kwe-1.5×10-6T (0.0015mT) ngokusekelwe kwimvelaphi yamandla e-magnetic field ye-1.5T. Ngokucacileyo, kwizixhobo ze-MRI ezinamandla ahlukeneyo e-field, umahluko wokuqina kwamandla e-magnetic field okumelwe yiyunithi nganye yokuphambuka okanye i-ppm wahlukile, kulo mbono, iinkqubo ze-low field zinokuba neemfuno eziphantsi zokufana kwamandla e-magnetic field (jonga iTheyibhile 3-1). Ngolu hlobo, abantu banokusebenzisa umgangatho wokufana ukuthelekisa ngokulula iinkqubo ezinamandla ahlukeneyo e-field, okanye iinkqubo ezahlukeneyo ezinamandla afanayo e-field, ukuze bavavanye ngokungathandabuzekiyo ukusebenza kwe-magnet.

Ngaphambi kokulinganisa ngokuchanekileyo ukulingana kwentsimi yemagnethi, kuyimfuneko ukumisela ngokuchanekileyo iziko lemagnethi, uze ulungiselele isixhobo sokulinganisa ubunzulu bentsimi (iGauss meter) probe kwisphere yesithuba seradiyu ethile, uze ulinganise ubunzulu bentsimi yemagnethi inqaku ngenqaku (indlela yeplane engama-24, indlela yeplane engama-12), uze ekugqibeleni ucubungule idatha ukuze ubale ukulingana kwentsimi yemagnethi ngaphakathi komthamo wonke.

Ukufana kwemagnethi kuya kutshintsha ngokwendalo esingqongileyo. Nokuba imagnethi ifikelele kumgangatho othile (ixabiso eliqinisekisiweyo lomzi-mveliso) ngaphambi kokuba iphume kumzi-mveliso, Nangona kunjalo, emva kokufakwa, ngenxa yempembelelo yezinto ezingqongileyo ezifana nokukhusela ngemagnethi (ukuzikhusela), ukukhusela ngeRF (iingcango kunye neefestile), ipleyiti ye-waveguide (ityhubhu), isakhiwo sentsimbi phakathi kweemagnethi kunye nezixhaso, izixhobo zokuhombisa zokuhombisa, izixhobo zokukhanyisa, iipayipi zomoya, iipayipi zomlilo, iifeni zokukhupha umoya ezingxamisekileyo, izixhobo ezihambayo (kwaneemoto, iilifti) ecaleni kwezakhiwo eziphezulu nezisezantsi, ukufana kwayo kuya kutshintsha. Ke ngoko, ukuba ukufana kuyahlangabezana neemfuno ze-magnetic resonance imaging kufuneka kusekelwe kwiziphumo zokulinganisa zokwenyani ngexesha lokwamkelwa kokugqibela. Ukulinganisa intsimi engasebenziyo kunye nokulinganisa intsimi esebenzayo ye-superconducting coil eyenziwe yinjineli yokufakela yomenzi we-magnetic resonance kumzi-mveliso okanye esibhedlele ngamanyathelo aphambili okuphucula ukufana kwentsimi yemagnethi.

Ukuze kufumaneke imiqondiso eqokelelweyo ngokwendawo kwinkqubo yokuskena, izixhobo ze-MRI nazo kufuneka zibeke ngaphezulu kwe-gradient magnetic field △B ngotshintsho oluqhubekayo nolwandayo ngokusekelwe kwi-main magnetic field B0. Kucingeleka ukuba i-gradient field △B ebekwe kwi-voxel enye kufuneka ibe nkulu kune-magnetic field deviation okanye i-drift fluctuation ebangelwa yi-main magnetic field B0, kungenjalo iya kutshintsha okanye itshabalalise isignali yokuma kwendawo engasentla, nto leyo ekhokelela kwi-artifacts kunye nokunciphisa umgangatho womfanekiso.

Okukhona ukuphambuka kunye nokuguquguquka kwentsimi yemagnethi eveliswa yintsimi yemagnethi ephambili i-B0, kokukhona ukufana kwentsimi yemagnethi kusiba kubi, umgangatho womfanekiso uphantsi, kwaye kunxulumene ngokuthe ngqo nolandelelwano lokucinezelwa kwamafutha (umahluko weresonance frequency phakathi kwamanzi kunye namafutha emzimbeni womntu yi-200Hz kuphela) kunye nempumelelo yokuhlolwa kwe-magnetic resonance spectroscopy (MRS). Ke ngoko, ukufana kwentsimi yemagnethi yenye yezona zinto zibalulekileyo zokulinganisa ukusebenza kwezixhobo ze-MRI.

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