2014 M.S. Cohen All Rights Reserved Mscohen@g.ucla - Brain Mapping PDF Free Download

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2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

IMAGE QUALITY / ARTIFACTS 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

SYRINGOMYELIASurgery is usually recommended forsyringomyelia patients.The main goal of surgery is to providemore space for the cerebellum (Chiarimalformation) at the base of the skull andupper neck, without entering the brain orspinal cord. This results in flattening ordisappearance of the primary cavity.If a tumor is causing syringomyelia,removal of the tumor is the treatment ofchoice and almost always eliminates thesyrinx.Source http://gait.aidi.udel.edu/res695/homepage/pd ortho/educate/clincase/syrsco.htm 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

Truncating theFourier Data Resultsin Distortions (edgeringing) of HighSpatial Frequencies 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

This is equivalent tosampling only aportion of the rawdata. 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

TRUNCATED RAW DATAMR ImageActualObject 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

THE SYRINGOMYELIA EPIDEMICM. Cohen & D. Baird. Perspective on Science 7:231, 1999 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

WHAT IS THE ACTUALRESOLUTION OF MRI?Original DataMR ImageSingle pixel “activation” 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

THE ACTUAL RESOLUTION facts/MRArtifacts.html 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

AN “EQUATION” IN RESOLUTIONBecause MR is an emission modality thetemporal resolution, spatial resolution andcontrast are inter-dependent:Signal kB0 (voxel size) imaging time contrastwhere B0 is the field strength. 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CONTRAST TO NOISE RATIO 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CNR VS. RESOLUTIONNoise freeImaging time 1XImaging time 2XImaging time 4X64 X 64128 X 128Minimum Imaging Time256 X 256 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CNR VS. RESOLUTION256 X 256Signal/Noise Ratio HeldConstant128 X 12864 X 64Imaging time 16XImaging time 4XImaging time 1X 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CNR VS. RESOLUTIONNoise free16 averages4 averages1 average64 X 64128 X 128256 X 256 2014 M.S. Cohen all rights reservedmscohen@ucla.eduImaging Time HeldConstant

BANDWIDTH AND READOUT Position is encoded by FREQUENCY Bandwidth refers to the Frequency Difference fromthe center of the image to its edge:Frequency per pixel 12* Bandwidth readout durationnumber of pixels Bandwidth decreases with readout duration:Bandwidth number of pixels2 * readout duration 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

BANDWIDTH AND SNRDecreasing the Bandwidth Improves SNR:Imaging Time is INCREASED and high frequency noise ityNoiseFrequency 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

BANDWIDTHBW 8kHzBW 4kHzTE 11-14TR 500 2014 M.S. Cohen all rights reservedNEX 1Thick 3mmMatrix 256x256mscohen@ucla.eduBW 16kHz

BANDWIDTHBW 4kHzTE 11-14NEX 1Thick 3mmTR 500Matrix 256x256? 2014 M.S. Cohen all rights reservedmscohen@ucla.eduBW 8kHzBW 16kHz

THE ORIGIN OF CHEMICAL SHIFTIn water, electrons move fromHydrogen towards Oxygen.Electrons in lipid are shared equallybetween Hydrogen and OxygenThis exposes the Proton to a slightlyhigher magnetic field.WaterLipidResonance FrequenciesHigher Frequency 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CHEMICAL SHIFT ARTIFACTHigher FrequencyIf the frequency width of each pixel is less than the frequencydifference between water and lipid,then water and lipid will appear in separate pixels 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CHEMICAL SHIFTThe Fat-Water chemical shift is about 3.5 ppm or:Which is:with a 32 kHz readout75 Hz @ 0.5 Tesla 1 pixel150 Hz @ 1.0 Tesla 1 pixel220 Hz @ 1.5 Tesla 1 pixel440 Hz @ 3.0 Tesla 3.5 pixelsdWaterFatAmplitudefrequencyLowering the Bandwidth/pixel increases theChemical Shift in pixels 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

SHAPE AND BANDWIDTH 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

DISTORTION INCREASES WITHFIELD STRENGTHVariation in sample magnetization of is proportional to field strength.High Field images lose more signal from field inhomogeneity 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

BLURRING FROM T2* DECAYT2* 80 msT2* 10 ms 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

APODIZATION FROM LONGREADOUTSPhantom 2014 M.S. Cohen all rights reservedReadout 2T2*Readout 4T2*Exercise in Afternoon Labmscohen@ucla.edu

EPI READOUT DURATIONS1MR SignalT2* signal decay(T2* 45 msec)UCLA 64x1280.5GE Product 64x128UCLA 128x128GE Product 128x128Stanford Spiral 128x12800 2014 M.S. Cohen all rights reserved20mscohen@ucla.edu406080100

MOTION e/MRArtifacts/MRArtifacts.html 2014 M.S. Cohen all rights reservedExercise in Afternoon Labmscohen@ucla.edu

GRADIENTS AND PHASE0T/2 Phase withrespect tocenter– 2014 M.S. Cohen all rights reservedmscohen@ucla.eduT

GRADIENTS AND PHASE2x(t) x0 vt at 2 Tϕ γ G(t)x(t)dt0T/20T γ G(t) x0 vt at 2 dt 02 γ vT 4 23aT 24 2 2014 M.S. Cohen all rights reserved3mscohen@ucla.eduT

MOTION e/MRArtifacts/MRArtifacts.html 2014 M.S. Cohen all rights reservedExercise in Afternoon Labmscohen@ucla.edu

ϕ γ vT 4 23aT 24 2 2014 M.S. Cohen all rights reservedmscohen@ucla.edu3

ALIASINGIf the Sampling Rate isLess Than Twice theSignal Frequency, theApparent Frequency isAmbiguous 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

SATURATIONExercise in Afternoon Lab 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

SATURATION (CLIPPING)Amplifier Limit 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

SIGNAL INHOMOGENEITYGradient Echospgr, tr/te/flip60/6/20 2014 M.S. Cohen all rights reservedSpin Echotr/te 500/1136mscohen@ucla.edu

7 TESLA 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

RF PENETRATIONHigherRF Field&EffectiveFlipAngleLowerThe well-known “skin effect” results ingreater current density (and flip angle)on the surface of conducting objectsthan at their center 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

K-SPACE TRAVERSALStr20-35 msSpiral EPI4-16 msConventional0.25-0.5 msRectilinear EPI80-100 ms 2014 M.S. Cohen all rights reserved40mscohen@ucla.edu

SPIRAL SCANS (128X128) 2014 M.S. Cohen all rights reserved41mscohen@ucla.edu

3 TESLA ECHO PLANAR IMAGES128X25610 of 20 4 mm sections, each acquired in 78 ms.TE 55 msec, 1.5 mm pixels 2014 M.S. Cohen all rights reserved42mscohen@ucla.edu

? 2014 M.S. Cohen all rights reservedExercise in Afternoon Labmscohen@ucla.edu

? 2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

3 ppm-3 ppm 2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

Stimulated /artifacts.html 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

? 2014 M.S. Cohen all rights reservedmscohen@g.ucla.edu

tifacts.html 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

THE WEISSKOFF PLOTThe Expected Standard Deviationof the Mean Signal of a Region overTime Falls with the Square Root ofthe Number of Voxels.0LCoefficient of Variation10-110-21012510ROI Edge Length 2014 M.S. Cohen all rights reservedmscohen@ucla.edu30

THE WEISSKOFF PLOTCoefficient of Variation0.2MeasuredTheoretical0.1Deviations from theTheoretical Curve areEvidence ofCorrelated NoiseRDC 16.6101520ROI Edge LengthRDC (Radius of Decorrelation) is aSingle Point Quantification of theWeisskoff Plot 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

CHARACTERIZE YOUR TOOLSTest Statistics are Effect/VarianceVariance includes:Intrasubject (motion, attention, physiology, fatigue, )Intersubject variance (position, morphology, performance,pathology, physiology, )Experimental Variance (uncontrolled variables, stimulationvariance, )Instrument VarianceSitewise VarianceTrue Random Noise 2014 M.S. Cohen all rights reservedmscohen@ucla.edu

ARTIFACTS IN FMRIBasic MRI Artifacts Motion Shape and bandwidth Aliasing RF Penetration/Uniformity Apodization Signal Voids Chemical Shift Data Spikes K-Space errors (spiral)Time Series Artifacts Signal Drifts Field ChangesData Analysis Global Normalization Motion SmoothingFalse Positives and False Negatives Excess variance Respiration Ratty Image Quality Motion Timing Error Wrong Statistical Test Cardiac Pulsation Over-aggressive Smoothing 2014 M.S. Cohen all rights reservedmscohen@ucla.edu