Introduction au cours

Fund BioImag 201312-112: MRI contrast mechanismsWhat is the mechanism of T2* weighted MRI ? BOLD fMRIHow are spin echoes generated ?What are the standard contrast MR sequences ? T1 ,T2 and proton-density weighted MRIBy which mechanism do contrast agents act ?After this course youare capable of describing the biophysical basis of BOLD contrastUnderstand the mechanism of spin echo generation Know the three contrasts that can be generated by the spin echo imaging sequence and how the timing parameters are optimized for each contrastUnderstand why the same tissue appear bright on T2 weighted images and dark on T1 weighted imagesUnderstand the mechanism by which the two principal contrast agent mechanisms lead to signal increase or decrease.Fund BioImag 201312-2 MRI: One magnet, many contrast mechanismsFLAIR:T2 and T1 weighted (inversion recovery CSF-nulled) [TI=ln2T1(CSF)]T2-weighted[TE=T2(CSF)]

T1-weightedT2-weighted

Proton density-weightedExamples of proton density, T1, and T2-weighted images, from the Whole Brain Atlas site at Harvard. Note fluid appearance in all images.Large cyst (just cerebrospinal fluid)

Multiple sclerosisT1-weighted[TR=T1(GM)]T1-weightedT2-weightedFund BioImag 201312-3T2*-Weighted ImagesVenography Designed to make venous blood (rich in deoxy-hemoglobin) darker than normal tissue (=reduce magnetization in blood)

HypoxiaNormoxia

BOLD fMRI=functional MRI3Fund BioImag 201312-4Another view on spatial encoding with MRILets give it another try (compare w. Lesson 11)SliceSelect (Gz)Freq.Encode (G)RF1. Excitation3. Frequency encoding

(For G along x)S(t2) and M(x) are linked by FTM(w2) = FT{S(t2)}= M(x)Linear combination of e.g. Gx and Gy: G=(Gx,Gy)=cosfG,sinfGM(x) is Radon transform measured along the direction of the Gradient G

TEMeasure Radon Transforms along f (as in CT) every TR s (T1 relaxation) ( sinogram, projection reconstruction, see also central slice theorem)PhaseEncode (Gy)DGyt2

nDGyt=GynDtGyDttDefine t1=nDt: S(t1,t2)

M(w1,w2)=M(y,x)

Phase encoding is just frequency encoding in a 2nd time dimension2. Phase encoding2D FT4Fund BioImag 201312-512-1. What is the contrast in gradient echo imaging ?T2* weighting static dephasingSliceSelect (Gz)Freq.Encode (Gx)RFSignalaTEPhaseEncode (Gy)Static field imperfection

Mxy summed over voxel

Empirically:NB. T2>T2*

NB. gDB(r)TE=gnDB(r)[TE/n]Fund BioImag 201312-6

Deoxy-hemoglobin : paramagnetic oxy-Hb : diamagnetic

B0oxyRBCdeoxyRBCWhat is the Biophysical basis of T2* changes ?Blood Oxygenation Level Dependent (BOLD)

Magnetic susceptibility c: magnetic field in object depends on object propertiescparamagnetism (attracting force)zDe-Oxygenated capillarycoxygenated capillaryoxy ~ -0.3deoxy ~ 1.6DB0 in tissueDB0 in tissueT2* increases with decreased tissue deHb concentrationdepends on venous architecture in the imaging voxel

Fund BioImag 201312-7What does Blood oxygen level dependent (BOLD) contrast measure ?deHb content

Brain physiology: O2 consumption increases less than Flow during thinkingWhat is the consequence? Saturation=%oxy-Hb(deHB=100%-saturation)

steady-state hemodynamic response cerebral blood flow (CBF) : dHb : BOLD cerebral blood volume (CBV) : dHb : BOLD

venuolesarteriolescapillary bedarteryvein1-2 cm

50 m0510152025secondsfMRI signal (T2*) -5051.01.52.02.5% signal changeFund BioImag 201312-8

T2*-weightedImageAverageDifferenceImageStatisticalSignificanceImageThresholdedStatisticalImageOverlay onAnatomicImageHow is brain function imaged using functional MRI (fMRI) ? Brain Activation AnalysisTime series

taskfMRI signalOFFON

Statistical analyses (lots)8Fund BioImag 201312-9

How do magnetic susceptibility differences affect T2* ?(B0 imperfections, e.g. air-tissue interface, implants)Ear canalsinusT2*-weighted image Gradient echo (TE~T2*)How to minimize these effects? Signal of gradient echo ~ e-TE/T2*Solution I: Minimize TEGradient echo (TE