Magnetocardiogram (MCG) 10. Biomagnetism. Current Dipole Magnetic Field and MCG. Current and Magnetic Density. Magnetic Source and Measurement

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1 0. Biomgnetism Mgnetocrdiogrm (MCG) Biomgnetic sources Induction coil Fluxgte mgnetometer SQUID mgnetometer Superconducting Quntum Interference Device Mgnetoencephlogrph Mgnetocrdiogrph Bule nd McFee, 963 MCG ECG Coil: 2,000,000 turns e. To mplifier Current nd Mgnetic Density Right-hnd rule Mgnetic field t point P (femto Tesl order) 0 n jq jv j A B cos 4 0 I B cos cos Current Dipole Mgnetic Field nd MCG P (c) MCG mesured t point A when P follows locus of (b) during crdic cycle I A N j n q v j j j geometry of mgnetic field produced by moving chrges such s chloride, potssium nd sodium ions in solid tube () Mgnetic field distribution in x-y plne, due to current dipole vector P. (b) Locus of P during crdic cycle Ambient & Biologicl Mgnetism Mgnetic Source nd Mesurement Biomedicl Informtion Technology Lb

2 Biomgnetic Signls the erth sttic mgnetic field is on the order of 0-5 to 0-6 T. mgnetic signl strength from biologicl orgns is only mgnitude of 0-0 T. Biomgnetism mesurement demnds specil techniques. Mgnetic Field Sensors nd Properties strength nd/or direction Gussmeter = A mgnetometer whose scle is grduted in Guss or kguss, nd usully mesures only the intensity of the mgnetic field. Guss = Mxwell/cm 2 = 0 4 tesl (00 μt). Mgnetic Field Inside nd Outside Coil When n electric current psses through long, hollow coil of wire there will be strong mgnetic field inside the coil nd weker field outside it. The lines of the mgnetic field pttern run through the coil, spred out from the end, nd go round the outside nd in t the other end. Where the field is strongest, the lines re most closely crowded. Induction Coil in Chnging Mgnetic Field d e N dt induced current flows in the coil When mgnet rpidly principle of electromgnetic induction When mgnet rpidly pproches the coil, the mgnetic flux through the coil will chnge, n induced EMF (electromotive force) will cuse current to flow in the coil. The induced current will flow in the direction so s to oppose the chnge tht produces it. When the North pole of the br moves towrds the coil, the induced current will flow in n nti-clockwise direction to produce mgnetic field (North pole) which opposes the motion of the mgnet (to repel the incoming mgnet's North pole). The induced current will exist only when there is chnge in the mgnetic flux. When the br mgnet stops moving, there is no chnge in flux nd the induced current will dispper. Principle of Fluxgte Mgnetometer.c. excittion current excittion coil Excittion nd induced potentils Detection of Asymmetric Component When wveform is symmetric in both polrities, the second hrmonic component is zero. pickup coil mgnetic core The mplitude of the 2nd hrmonics is used to detect the symmetric component tht is proportionl to the externl field strength, Ho. Sensor configurtion When the externl field Ho is zero, the induced voltge is symmetric in both polrities. If Ho is not zero, symmetric component ppers in induced voltge. Biomedicl Informtion Technology Lb 2

3 Vcquier Fluxgte Mgnetometer If the externl field is zero, the induced fields in the two coils hve the sme mplitude in opposite polrity, but when n externl field Ho is given, the time of trnsience in polrities is shifted, nd the voltge induced in the pickup coil, which is proportionl to the time derivtive of the mgnetic flux pssing through the coil, hs pulstile components so tht the pulse width is proportionl to the externl field strength. pickup coil Mgnetopneumogrph System In order to void the effect of nonuniform mbient sttic mgnetic field, the mgnetometer is fixed to the spce, nd the subject lies down on the pltform mde of nonmgnetic mterils is scnned mechniclly in the plne. Two identicl cores hve sme number of turns but opposite direction of winding, nd re connected in series. () () Sensor configurtion (b) Opertion when sttic externl field is pplied Helth checkup for lung prticle ccumultion in fctory workers Mgnetopneumogrphy Mgnetiztion nd detection of mgnetic prticles in the lung During the presence of mgnetic field ( few to 20 sec), the dust prticles become mgnetized nd re ligned with the direction of the externl mgnetic field. The rte t which the dust prticles lign with the field is dependent on the mgnitude. Hll probe (T) Flux gte sensors (nt) SQUID (ft) superconductor Josephson Junction wek link superconductor () () mgnetiztion procedure () (b) detection of mgnetic prticles in the lung very thin (<3 nm) film of metl oxide (e.g., Al 2 O 3 ) insultor sndwiched between two superconducting conductors. Superconductors hve zero resistnce below their criticl temperture. Once current strts to flow in it, the current will continue to flow even if left lone. Super-current i.e. current tht flows indefinitely long without ny voltge pplied crosses Josephson junction (JJ) two superconductors coupled by wek link thin insulting brrier, short section of nonsuperconducting metl, or physicl constriction tht wekens the superconductivity t the point of contct Josephson Effect () Josephson Junction configurtion I 0 (b) Nonliner current-voltge chrcteristics Wek link is resistive brrier tht wekens the superconductivity t the point of contct. Electron tunneling hppens between two superconducting regions tht re seprted by the wek link. A current cn penetrte the resistive brrier without cusing voltge drop. When the current is lower thn the criticl current I 0, the voltge cross the junction is zero. However, when the current is incresed beyond I 0, the voltge cross the junction ppers bruptly The bsic equtions governing the dynmics of the Josephson effect Typicl I-V Chrcteristic Josephson or wek-link current-phse reltion t I Ic sin t I c : criticl current of the junction V(t), I(t) nd Φ re the voltge, current nd phse difference cross the Josephson junction K J is Josephson constnt, its inverse Φ 0 is the mgnetic flux quntum I DC Josephson effect superconducting phse evolution eqution V t K J t t Non- Josephson effect V Biomedicl Informtion Technology Lb 3

4 DC nd AC Josephson Effect DC Josephson effect I t Ic sin t A DC current crossing from the insultor in the bsence of ny externl electromgnetic field, owing to tunneling. This DC current is proportionl to the sine of the phse difference cross the insultor, nd my tke vlues between -I C nd I C. AC Josephson effect When fixed voltge V DC crosses the junctions, the phse will vry linerly with time, nd the current will be n AC current with mplitude I C nd frequency (2e/ħ)V DC. This mens Josephson junction cn ct s perfect voltge-to-frequency converter. 2 f ev DC Superconducting QUntum Interference Device SQUID Device A SQUID is low-noise ultr-high sensitive mgnetic field-to-voltge trnsducer for mesuring extremely subtle mgnetic fields, bsed on superconducting loops contining Josephson junctions. The current I enters nd splits into the two pths, I nd I b. A Josephson junction on ech pth seprtes the two superconducting regions. Josephson junction mgnetic flux Electricl Schemtic of SQUID bis current When the flux Φ threds the SQUID, the voltge V proportionl to tht flux is produced. Mgnetic flux quntum, physicl constnt, independent of the underlying superconductor mteril Φ Dependent I-V Property ħ: Plnck constnt q: elementry chrge Φo=ħ/2q= x0-5 Wb criticl current Josephson junction current vs. voltge for SQUID. The voltge is periodic function of mgnetic flux. Periodic voltge responds to flux through SQUID. The voltge vries s periodic function of Φ/Φ 0. The periodicity is equl to one flux quntum, Φ 0 Bsic SQUID I B >Io Two terminls re inputs for dc bis current I B. Two terminls monitor the output voltge Vo. SQUID ring Flux-locked SQUID By djusting I B, the SQUID's operting point is locted t one of the open circles in the Vo vs. Φ/Φo plot By using feedbck null mode of opertion, the SQUID is given lrge liner dynmic rnge; the flux-locked SQUID output is now Vc = KΦi. SQUID ring Detection coil Detection coil boiling liquid helium Cryogenic enclosure The voltge Vo is integrted, mplified, nd used to control V-I converter G M. The current I F in the feedbck coil produces flux equl but opposite to the input flux Φi, nullifying Vo. Biomedicl Informtion Technology Lb 4

5 AC-modulted Flux-locked SQUID Detection coil I B is set close to the criticl current. The biomgnetic field is picked up by the detection coil nd inductively coupled to the SQUID ring. SQUID ring When the current in the SQUID ring chnges, the flux through the ring chnges nd induces signl in nother coil. The signl is mplified nd fed bck to the ring so tht the flux chnge is compensted for nd the ring current is mintined close to the criticl current. Consequently, the strength of the biomgnetic field is detected s the mount of the feedbck. () mgnetometer (b) symmetricl st -order Vrious Pickup Coils ΔBz=Δz d) (c) symmetricl st -order (e) prllel plnr off-digonl ΔBz=Δx nd ΔBz=Δy (d) seril plnr off-digonl (f) symmetricl 2 nd -order Δ 2 Bz=Δz 2 HF AC superimposes on DC flux in its superconducting ring. (g) symmetricl 2 nd -order (h) plnr 2 nd -order Δ 2 Bz=Δx 2, Δ 2 Bz=Δy 2 (i) symmetricl 3 rd -order st -order Grdiometer SQUID 2 nd -order Grdiometer SQUID Mgnetic strength is inversely proportionl to the distnce 3 between the coil nd the field source. If two coils with opposite winding directions re seprted by distnce, uniform fields re neglected while chnging fields re enhnced. It responds to 2 B/ z 2, nd is better in discriminting brin fields ginst nonbrin, or bckground mgnetic noise. Two pickup coils wound in opposition nd spced δz prt so tht the Φ coupled to the SQUID ring is proportionl to δbz nd responds to B/ z Bz: skull surfce mgnetic field Brin Mgnetic Field By mesuring the brin mgnetic fields creted by the electric current flowing within the neurons, vrious brin functions re evluted with millimeter sptil ccurcy. MEG Mesurement The circle of constnt B of rdius provides more flux to the grdiometer coil closest to the skull, hence produces SQUID output. Flux cutting both coils eqully, such s from the Erth's mgnetic field, produces no output. There cn be millions of current dipoles ctive t once, nd becuse of the wy the cortex is folded, some cn be prllel to the surfce of the skull, nd others my be directed rdilly. B from the rdil dipoles will only be sensed wekly with SQUID coils tht re prllel to the skull surfce. Biomedicl Informtion Technology Lb 5

6 SQUID Grdiometer Probe 48-sensor rry SQUIDs nd their pickup coils re cooled by liquid helium. Sensor nd Helmet An rry of SQUIDs re configured in helmet-like structure tht closely surrounds the subject's hed. Brin Mgnetic Mesurement Both spontneous brin function nd evoked brin ctivity cn be mesured. The outputs loclize the source of the brin ctivity within millimetres, nd re overlid on MRI. Spontneous MEG rtifcts produced by blinking, sccdes, biting nd crdic cycle. six positions two orthogonl directions of ech position () Corticl 275-chnnel MEG System Evoked MEG Mgnetic Source Imging MSI = locliztion of the mgnetic source using biomgnetic mesurement nd MRI (b) (b) Somtosensory-evoked MEG recorded by () (DC to 300 Hz bndwidth, 628 verges nd 3 rd order grdiometer) Biomedicl Informtion Technology Lb 6

7 Source Locliztion in Epilepsy MCG-recording 62ch twin Dewr (2 3ch) biomgnetometer (Philips). MEG locliztion superimposed on surfce-rendered MRI imges: Blue, subdurl electrodes; Yellow, interictl locliztions; Red, ictl locliztions; Green, spred pttern. D. Elishiv The double-dewr construction llows mesurements to be performed over both hemispheres of the hed for mgnetoencephlogrm, such s uditory nd somtosensory-evoked responses, presurgicl mpping, fetl MEG, spontneous brin ctivity, or over the chest for mgnetocrdiogrm. MCG in Helthy Subject The different mesures re bsed on the morphology nd the time intervl of the MCG signl, the reconstruction of mgnetic field mps, the estimtion of current density nd current source. Fetl MEG An rry of SQUIDs is rrnged for optiml coverge of the mother s nterior bdominl surfce, from the perineum to the top of the uterus () Signl-verged trces from 6-chnnel prethorcic cquisition (b) Mgnetic field mp t Q onset schemticlly superimposed on n MRI projector for the opticl positioning system Bby MEG 76 detection coils (dimeter 6 mm) with sensor coverge re of 300 cm 2. Sptil resolution is four times greter thn tht of existing dult wholehed MEG sensors. MCG Mesurement System liquid nitrogen Biomedicl Informtion Technology Lb 7

8 Mgneticlly Shielded Room Alloy (Ni 80%) with exceptionlly high mgnetic permebility, very low coercive force, very low core losses, nd low remnence Mobile SQUID System DC mgnetic field of the erth is close to 50 ut, which is bout million times greter thn the mgnetic field generted by the hert. The brin signls re bout two orders of mgnitude lower thn tht of the hert. The noise in lbortory buildings is of the mgnitude -0 nt/hz /2 t Hz. Prt of the noise cn be eliminted by using grdiometer coils in the SQUID equipment, such s 2 nd or 3 rd order grdiometers. Superconducting Mgnetic Shield It consists of cylinder 65 cm in dimeter nd 60 cm in length. The cylindricl superconductor ws kept below 70 K by circulting helium gs. prtly covered superconducting shield for whole hed SQUID system to reduce the surrounding noise. Biomedicl Informtion Technology Lb 8