Principles of MRI. Practical Issues in MRI T2 decay. Tissue is a combination of. Results are complex. Started talking about off-resonance

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1 Projec Principles of MRI Lecure 18 EE225E / BIO265 Insrucor: Miki Lusig UC Berkeley, EECS No eams Oral presenaion (20min) Or, Repor as a wikepedia enry Level of presenaion -- assume exbook level of knowledge Grading based on accuracy, clariy and normalized by scope Work: Implemenaion Review of advanced opics/papers - could be from your own research Pick a opic, wrie a shor proposal (1-2paragraphs) Send me by March 03/ Las Time Pracical Issues in MRI T2 decay Map decay o k-space resul in arifacs Image weighing blurring in he readou Sared alking abou off-resonance Heerogeneous Tissue Tissue is a combinaion of Chemical shif Suscepibiliy Geomery Resuls are complex Ineresing cases: Blood (fmri) Lungs Trabecular bone Iron in brain / liver 3 4

2 Effec on Imaging Magniude Phase Geomeric Blurring All depend on spaial scale we look a and acquisiion sraegy Off Resonance Effec on FID Simple specroscopy experimen On resonance RF A/D Signal decays wih T2 e /T Off Resonance Effec on FID Simple specroscopy experimen Wih Suscepibiliy variaion, FID is: RF A/D Why? Off Resonance Effec on FID The field is: B(~r) = B 0 + E(~r) uniform error-field Error is spaial and specral In he roaing frame w0 ~B = E(~r)ˆk! E (~r) = E(~r) f E (~r) = E(~r) 2 7 8

3 Off Resonance Effec on FID Off - Resonance Effecs on FID m x,y (~r, ) =m xy (~r, 0)e i! E(~r) e Received signal is: s() = Z ~R m xy (~r, 0)e i! E(~r) e wih ime, phase dispersion causes signal cancellaion and loss. T 2(~r) T 2(~r) d~r magneizaion dephasing relaxaion 9 10 Off Resonance Effecs on FID Off Resonance Effecs on FID 11 12

4 Off Resonance Effecs on FID Example fmri T2*: No a good model for Large-Scale variaions (dephasing near sinuses) T2*: Is a good model for small scale disribued variaions (dephasing near capilaries) Off -Resonance Effecs on Imaging Off -Resonance Effecs on Imaging k x () neglecing T2 and subsiuing for : m xy (~r, ) =m xy (~r, 0)e i! E(~r) kx() 2 G x +TE e i2 k x()x d~r Tex k x = G x ( TE) ) = k x 2 2 G x + TE =TE Or, m xy (~r, ) =m xy (~r, 0)e i! E(~r)TE e i2 k x() x+! E (~r) Gx d~r The x-verse magneizaion is: phase/dephasing displacemen m xy (~r, ) =m xy (~r, 0)e i! E(~r) e T 2 (~r) e i2 k x()x d~r 15 16

5 Off -Resonance Effecs on Imaging mxy (~r, ) = mxy (~r, 0)e i!e (~ r )T E phase/dephasing Examples: e i2 kx ()! (~ r) x+ EGx signal loss d~r displacemen shape An on-resonance spin a posiion: x0 = x +!E (~r) Gx readou direcion? Produces he same signal as a spin a x, wih off-resonance! Fa on Gradien Echo Images waer and fa are in phase Examples: Phase vs Echo Time ou of phase waer and lipids in phase Example: Chemical Shif waer and lipids ou-of-phase x0 = x +!E (~r) Gx Increasing Gx reduces chemical shif arifac 19 20

6 Example: Le, A: x = x 0 2 G x =1 E(~r) =3! E (~r) =2 200 x =! E(~r) G x = x =1mm ppm KHz/cm Hz Hz 2 1 KHz/cm =0.2cm his is a shif of wo pixels! Effecs in Spin-Warp Off-Resonance Modes spaial disorions ( few pixels) Relaively benign arifacs Reduce arifacs wih large Gx Chemical-Shif Fa shif of 1.5T Fa image is displaced from Waer In pracice F/W shif limied o ~2pixels 2 pixels shif are wo cycles of linear phase across k-space 2 cyc 9.1 ms 220 Hz Off-Resonance in EPI Example: Lipids phase accumulaion poin-spread funcion Off-Resonance in EPI poin-spread funcion simulaion Leg 0.44cyc 1ms 1ms 0.44cyc 0.44cyc 1ms 96ms 42.24cyc 1ms 0.44cyc 23 24

7 Off-Resonance in Spiral phase accumulaion poin-spread funcion Off-Resonance in Spiral Readou 2ms 5ms 13ms Spiral scan wih linear off-resonance real imag 2 ms 0.88cyc 25 26