SEM MATH 0285I
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This 98 page Class Notes was uploaded by Kaylin Wehner on Friday September 4, 2015. The Class Notes belongs to MATH 0285I at University of California - Los Angeles taught by Staff in Fall. Since its upload, it has received 89 views. For similar materials see /class/177826/math-0285i-university-of-california-los-angeles in Mathematics (M) at University of California - Los Angeles.
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Date Created: 09/04/15
UCLA Brain Mapping Center UCLA Brain Mapping Center 39 aJc k Inhomogeneous Magnetic Fields Within Voxels Result in Spin Dephasing and Signal Loss in Gradient Echo S quences Capillary UCLA Brain Mapping Center M RI Preview explores intensitg variations in MR signal intensitg variations re ect venous 02 m I quotOLA NiNDS imaging Markers O F Epiiepfogenesis UCLA Bram Mapping Center r V r r h h k K r v quotF r 39 y y y K V lt w r Baseline ff Kwong et ah PNAS 1992 UCLAJBW MapWRision 7 Image series with visual stimulation 75 v 39 06 4 Correlation UCLA Brain 3 Mapping Center I 120 1530 Time seconds 2210 a x nu t n sags 120 Time seconds UCLA Brain Mapping Center PHYSICS UCLA Brain Mapping Center PROTON SPIN and MAGNETIC MOMENT Free Protons Transition to Equilibrium ParallelAntiparallel state transitions require quantized energy input iii39 Zero Field Field Applied T1 and TR 1 Magnetization signal 05 2 3 Time seconds T1 The Characteristic Time for Longitudinal Relaxation Protons in Applied Field Due to their angular O momentum Protons precess in the magnetic field Proton Responses to Applied Magnetic Field 39 Spin Alignment Along Net Applied Field spins align parallel or antiparallel to the applied field Precession About the Magnetic Field at a precession frequency of y X B lenown as the Larmor frequency Spin Alignment Occurs at the Rate T1 the Resonance Phenomenon Main Magnetic Precession Angle About RF Field Field Axis HF Field Axis When an RF pulse is applied at the Larmor frequency the proton Will precess about the aXis of the RF pulse Off Resonance Excitation The effective B1 field can be calculated as Bleff B1eff 2 Bo 39 WWI 31X y The 2 corn onent of B1eff increases With the difference etween ooy and B0 Offresonance excitation causes relatively little effect as B1eff is predominantly about 2 An RF Pulse Converts Longitudinal Magnetization t0 Signal Longitudin Magnetizat on 9 F Pulse MR Signal Precession Out Of Phase Precession Receiver f MAWmm NMR Signal T2 The Characteristic Time for Transverse Decay T2 and TE 1 Time milliseconds way my D 1 S O I l H E 1 I 0 U H0 l Wu TZWeighted EPI scan Liver Mets Echo Time TlWtd Partial Saturation Sequence Sequence of 90 Pulses NMR Signal Effects of TR density weighted TES H m Mam Contrast TR and TE Proton Density T2Welghted T1 Weighted TE Contrast TR and TE D e n T2 r x39 quot y 1333 MR F ormulm Contrast Summary EChO etrT1eteT2 pis the proton density k represents instrument effects The Bloch Equation dMdt M x B M0 MZT1 1le MyT2 Hahn Spin Echo quotL quilfiiiitfm n d C t h Wa C W 2 T R T f O S t a E E 3D T1 Images TE 32 TR 144 124 slices 125 mm thick 1 NEX Flip Angle 20 T1 500 saw 39 ra i UC A B i apping Divisi M Sample Data Set normal Fast Spin Echo 3 mm S ices 3D IRSPGR TE 32 TII 7mg 4 1 h 4339 quotW vw uw fun Contrast to Noise Ratio GrayWhite Inga1m w U UI U r n quotma H W m t x 4 The Larmor Relation 84 Frequency Megahertz 42 I 10 Magnetic Field Tesla Magnetic Field Gradients Position Imaging System Components U j gtIIIIIIIIIIIIIIIIIgt x x n O 1 t C C 1 C S C C h S Frequency Selective Excitation gx Spin Isochromats Phase relative Phase relative to center line to center line 0 60 88888 0 88888 0 Eeeem Z Equivalent Strategies in kspaceTlt Gradient Gradient L Sampbs TTTTTTTTTTTTTTTT TDEi mawenLygi LE sampms T T T T T T T T T T T T T T T T r T m w T T 4 T t m m r r 1 7 a Gradlent SamprT T T T T T T T T T T T T T T T Ignoring e ects of signal decay and sample motion I K Gradlent Preencodlng W Signal l Gradient Time gt K x Samples W TTTTTTTTEITTTTTTTTTTTTTTTTTTTTTTT W I I TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT Cohen Interleaved Spatial Encoding n 1 L Gradient 2 mnmnm nnnnmn mmmm Points indicated in ll 39139 39i39 I II I White are affected by Gradient 1 but NOT by Gradlent 2 Points indicated in yellow are affected by both gradients In Conventional MRI Sampling is Apportioned Into Brief Episodes Separated by 21 TR Period Spatial Encoding in a Pulse Sequence tr RF Aquot Aquot W orb J IJ rIJ Grad l I I l I A2 A2 A2 Grad 2 Samples Gradient Echo Sequence RF SHce Select Readout Spin Echo Sequence RF SHce Select Phase I I Encode Readout I I I I kspace kx f t O t ky Conventional KSpace Trajectory V V V 39Kfrequency EchoPlanar kspace Trajectory kphase I kfrequency Interleaved Spatial Encoding Gradient 1 H Gradient 2 mnmnm nnnnmn mmmm Points indicated in ll 39139 39i39 I II I White are affected by Gradient 1 but NOT by Gradlent 2 Points indicated in yellow are affected by both gradients Gradient Echo EPI Sequence RF lt gt Slice Select Cohen Properties of KSpace Kphase L r 39Kfrequency 39Kfrequency Increasing K values Represent Higher Resolution Finer Grain Sampling Results Kphase in Wider F 0V Re ections Across KO are approximate Complex Conjugates Gradient Coil Characteristics Gradient Strength k i Gradient 00 k z 1 Gausscm I100 Amps Rise Time Current and Voltage a VL dt 1mH 250 Amps 01 msec For 250 Amps in 100 usec VL 2500 Volts Power 2500 Volts X 250 Amps 625 X 1 Watts Equivalent Strategies in kspacegt lt I mmmmmnm I Ignoring e ects of signal decay and sample motion Resonant Gradient 0 Instascan Pulse Sequence 90 ML Phase Encoding n n n n Frequency A J Instascan EPI Block Diagram Gradients Transfer Switch Resonant Readout High Speed Coil Set Image Processmg Display Console Raw Data Symmetry kt kt A V Detector 1 Q kt kt Detector 2 kspace conjugate symmetry For a Stationary Object in a Homogeneous Field Skxa39kY Where SkX ky is the signal at kxkygt Example if Skxky a ib then Skxky a ib Typical Multislice EPI data 6 seconds 1 T39s quot a q Reduced Flip Angle Imaging Outline 39 Determinants of Imaging Time 39 TR Saturation and Image Quality Reduced Flip Angle Techniques FLASH SPGR FISP GRASS Gradient Echoes Applications of Shallow Flip Imaging UltraFast Imaging Determinants of Imaging Time Scan Time Repetition Time TR X Number of Phase Encodes X N EX Averages X Number of 3D Steps TR and Image Quality Reduced TR Yields 39 Decreased Scan Time 39 Increased T 1 Contrast Reduced Useable T2 Contrast Reduced Signal to Noise Ratio Increased Power Deposition 39 Reduced Slice Coverage Signal and Flip Angle Small Flip Angle Large Flip Angle B E 39o 3 0 C 9 K transverse Small and Large Flip Angle Loss of Longitudinal Magnetization l After Small Flip After Large Flip Angle Excitation Angle Excitation Flip Angle and TRTl 1 Relative SNR Large Flip Angles Long TR Short Short Long Proton Density T2Weighted TlWeighted Small Flip Angles Long TR Short Short Long Proton Density T2Weighted Proton Density T2Weighted TR amp Flip Angle Combinations Having Similar Contrast FLASH TR Flip Angle 5000 90 2400 50 1700 40 750 20 250 0 T2 and T2 T2 Transverse Magnetization Decay from SpinSpin Interactions T 2 Transverse Magnetization Decay from Local Magnetic Field Variations Magnetic Susceptibility The Extent to Which a Substance Becomes MAGNETIZED When Placed Within a Magnetic Field Magnetic Susceptibility Air Distort the Magnetic Field n a h t t n C a m D m M t p C C S u S h H W C W b O Hahn Spin Echo Ejtnf lgti fugu FLASH Timing Diagram Mo GSeect I I Spoiler Gradientl GPhase GRead ml 10 msec FISP GRASS Timing Diagram w GSelectl GPhase g GRead 10 msec the FLASH Magnetization Cycle Longitudinal Recovery 3 3F DUISI f0 0W d 3 y ata co ection Spoiling of transverse magnetization The GRASS FISP Magnetization Cycle 1 0 Longitudinal Recovery 0 degree RF pulse and T2 relaxation and data collection 0t degree RF pulse Longitudinal Recovery and data collection and T2 relaxation CEFAST Sequence RF Channel GSelect GPhase SSFP Sequence RF Channel GSelect GPhase SNR vs TRTl Assuming Constant Imaging Time Relative SNR Questions Transform Pairs DC Signal equency Fourier Transform Pairs Amplitude Frequency 1 ms 2 ms Frequency Amplitude Fourier Transform of 21 Square Wave 39114r 1000 2000 3000 4000 5000 6000 Frequency Hz The Origin of Chemical Shift Electrons in lipid are shared equally In water electrons move between Hydrogen and Oxygen from Hydrogen towards Oxygen This exposes the Proton to a slightly higher magnetic field Lipid Resonance Frequencies Higher Frequency Chemical Shift Artifact gt Higher Frequency If the frequency width of each pixel is less than the frequency dz erence between water and lipid then water and lipid Will appear in separate pixels GSelegrerI TE1 TE2 TE3 GReadout m 1 2 3 II 4 II 5 I GPhase A PULSE SEQUENCE CONTROLS Slice Location 0 Contrast Sllce Orientation TR TE TL Flip Angle Slice Thickness Number of Slices DQ173550quot etc Resolution 39 Artifact Correction F0 V and Matrix Saturation Pulses Flow Comp Fat Suppression etc Spatial Pre Saturation Imaging Volume
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