MRI - 2 - Figurn 188 Photograph showing the observation...

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Unformatted text preview: Figurn 188 Photograph showing the observation window of a radio-frequency shielded mom (Courteoy of G Medical Systems, Milwaukes, Wisconsin). E Electromugnetic radiation of 0-1 00 M Hz is reduced by 100 dB inside the magnet area. 1 Transm~tter Coil &Gradients ,1 : Frequency Synthesis +- I 185 Block diagram of a nuqnetic resonance imager. coil I I I ) ) I IB )I F ium 186 (a) Superconducting magnet (1 .O-meter bore) showing its external cryostot (Courtesy of IGC, Guilderlond, New York). (b) Drowing of the cross section of o similar magnet showing the positions of the winding, cryogens, and dewon. .amp ue s! 3 pue 'ies 19 iue!perS e s! 9 ' s l e u h Bu!pnpmredns se llem se e ~ ~ Au8w 8u! pesn e d4 pouelos-qlds e 1 ~ r 0 ieu6ew u!ew ew s! v ~uresk 3 UMN ue p m laque3 1 i p weure6wre ~m!sArld qi (q) e :)eu6ew eyis!ser p!ouelos-ulds W v (8) :qd~rBauoi MN ue 10 43iews v 11-6 o rnhj 4 U w ( 4) 1 103 k u a n b a ~ jO I ~ D U \ ' 1 ~ 6 !Is eY jo u o W P puo WN r o) E! l- 941 ' JQL'O- '1 ' Z / q = J Z / I = P / I ' ,OZl = B ale IW P ' J E L ' Z = '1 slalecuered u6!sep ~eru!ldo ul (3) w e '(q) 'p)UI ' I!& 11-w e arll pa11e3 ! IP s!41 'smoue s arll 40 ~ 6 u epue u o r w ~aw Aci ~ e m ws! WWJIP z 1 !~ ! u! PW s ~ e u aw d uerp k e u 'IF lua!pe~6euo!m!pz v (3) .IW e A e ospe s! w w 'IF I UB(PW~IWW~!PA t d4 m v (q) .I!W k p g aui se pelleo ueuo s! w rlm ' 1 9i w p e ~ 6euor!)ael!px utl (a) 9 16 . mh l w T* XlV Woter content of fat-free normal human tissue Tissue Water contenl (%) XYZ coordinate rotates atalrequency Skdetal muscle M:ocardium L iva 79 80 71 81 Kidmy Brain white matter Brain gray matter Me Femur cortex .l ' 84 72 56 Figure 175 Application of an external RF magnetic field 81, perpendicular to the dc magnetic field &, tips the magnetization away from the Z axis toward the X Y plane. In the laboratory frame of reference (a] the nutated magnetization precesses 1 about the axes of both the & and 8 magnetic fields. The motion i s represented by a spiral path from the Z axis to the X Y plane. In the rotating frame of reference (b) ~ LICcoordinate system rotates about the Z axis at the Larmor frequency. In this representation, the nutated magnetization i s represented by a simple rotation onto the X Y plane. RF Pulse F i n 177 Free-induction decay signal. I l 7b Induced voltage in a coil aligned along the Y axis as a function ~f tirnc Consider the rectangular pulse signal a s shown in Figure 4 1 ( ) Applying eq. ( . 1 ,we find that the Fourier transform of .7a. 46) this signal is e-'J dt =-2sin a TI 0) (4.68) as sketched in Figure 4 1 ( ) .7b. (bl transform. 4 .- - -+ dB/dz ,0 Frequency (kHz) (c) Selective R F (Hz) reqhncy ( kHz) F Figura 192 Slice selection: (a) A cylinder of water placed in a uniform magnetic field. (b) A cylinder placed in a linear magnetic-field gradient. (c) When a cylinder of water is placed in a linear magnetic-field gradient and irradiated with a selective RF pulse, only those nuclei residing in the selected slice are excited. ...
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