1 / 30

Electromagnetic Spectrum

PET – positron emission tomography. Electromagnetic Spectrum. PET – positron emission tomography. Inject Patient with Radioactive Drug. Late 1960’s Drug travels to targeted site Drug emits (  +) positrons (basically a positively charged electron)

zaide
Download Presentation

Electromagnetic Spectrum

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PET – positron emission tomography Electromagnetic Spectrum

  2. PET – positron emission tomography Inject Patient with Radioactive Drug Late 1960’s Drug travels to targeted site Drug emits (+) positrons (basically a positively charged electron) FDG - Fluorodeoxyglucose (most common drug) (F18 – + emitter – two hour half-life) goes to metabolically active sites (tumor) Advantage functional imaging Disadvantage some ionizing radiation low resolution (4mm x 4mm x 4mm) need to make/buy FDG (cyclotron)

  3. PET – positron emission tomography • β+ decay, positron travels several mm and collides with an electron • produce a pair of annihilation photons (511kev, 180o) • simultaneous detection 180o apart

  4. PET/CT - together PET (Xray) CT Pet & CT PET

  5. Hemodynamic & Metabolic Imaging (C15 O2 by inhalation or H215 O i.v.) to measure cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate for oxygen (CMRO2) using the Kety-Schmidt model derived from the Costly and difficult to perform in emergency settings-reserved mainly for select patients and for clinical research C15 F18 Coles et al., JCBFM 29:965-975, 2009

  6. Back to MRI Where were we?

  7. Model of Head Coil Excite Listen RF B0

  8. Excitation – axis’s are Primed Reference Frame (axis's always primed … sometimes viewed from the lab frame … sometimes viewed from the primed frame) Excitation magnetic Field produced by EM radiation White vector – collective magnetic moment for one voxel Blue vector – excitation magnetic field B1 – generates tipping torque Thanks S. Cohen http://www.youtube.com/watch?v=KtWnmFg-u5g

  9. (probably choose w=w0=γB0) Excitation magnetic Field produced by EM radiation Some math to describe motion lab reference frame – Bloch Equations Probably want the rotating reference frame to rotate at w=w0=γB0 One simple solution would just be precession with Myαcos (w0t) and Mxα sin (w0t)

  10. Lab Reference Frame

  11. Excitation magnetic Field produced by EM radiation Some math to describe motion lab reference frame – Bloch Equations Exponential decay with solutions (quick glance) Myor Mxαexp (-t/T2)

  12. Lab Reference Frame Model of Head Coil Voltage proportional to flux change WHY loose signal?

  13. Some math to describe motion lab reference frame – Bloch Equations Probably want the rotating reference frame to rotate at w=w0=γB0 Note – this is equation in your book with w0 = γB0 It is in the rotating reference frame!! In rotating reference frame RF (B1 applied to x’ axis) rotates magnetic moment M around X axis We’ve seen these solutions already! Mz’αcos (w1t) and My’α sin (w1t)

  14. Excitation – axis’s are Primed Reference Frame (axis's always primed … sometimes viewed from the lab frame … sometimes viewed from the primed frame) FLIP Angle White vector – collective magnetic moment for one voxel Blue vector – excitation magnetic field B1 – generates tipping torque Thanks S. Cohen http://www.youtube.com/watch?v=KtWnmFg-u5g

  15. T2decay ……….. Shouldn’t it be T2* decay? T2* decay of Transverse Magnetization (time = T2*= 33% of signal remains) exp (-t/T2*) ‘ Dephasing

  16. Transverse magnetization decay due to magnetic field fluctuations In time or space T2* transverse decay due to both T2 and T2’ effects T2’ transverse decay part due to inhomogieties that are that are due to local field inhomogenieties (constant in time) – bad news – we can fix T2 transverse decay due to inhomogieties that are specific to tissue types (varies with time) (spin-spin interaction) – good news – we can't “fix” 1/T2* = 1/T2’ + 1/T2 signal strength = M0(exp(-t/T2*)) Starting magnetization after 90 pulse

  17. Think briefly about T2’ “bad” starting field B0 ( extreme example) usually we correct for T2’ = does not show up in image air voxel (glass of water) B0 too large (precess faster) B0 ok in (~3ms) the signal is destroyed due to T2* T2 image T2* image

  18. Think about T2 (usually more interesting) SIGNAL exp (-t/T2) fat CSF (watery) T2~ 60ms 800ms T2*~ 5ms 10ms (note T2* is magnet dependent) exp (-t/T2*) TIME 63 MHz (1.5T Trio)

  19. What causes T2 transverse magnetization decay (spin-spin)? Varies with time as hydrogen moves around, rotation, vibration, translation. slow fast in (~100ms) the signal is destroyed due to T2

  20. Water – vibrations states of water (1013 Hz) and fast tumbling rate make this effect “weak” b/c effect gets averaged out = real long T2 Water protons around “stuff” – slows tumbling and translations make T2 effect more evident = long T2 Fat – protons exposed to neighboring protons on fat and neighboring fat for “long” time – T2 effect is more noticeable = shorter T2 Proteins – protons exposed to neighboring protons on protein for “very long” time - strong T2 effect = short T2 Solids – protons exposed to neighboring protons for “very very long time” - T2 too short for imaging (T1 is really long as well) note: long, very long, very very long, etc does not mean they are stationary

  21. How to make a T2 or T2*contrast image? Listen to protons here! Voxel in tissue #2 Voxel in tissue #1 Jack Pot to being bright on T2 image

  22. T2-Weighting (SE) • CSF (fluid) bright • GM gray • WM dark

  23. Some math to describe motion lab reference frame – Bloch Equations Probably want the rotating reference frame to rotate at w=w0=γB0 Note – this is equation in your book with w0 = γB0 It is in the rotating reference frame!! In rotating reference frame RF (B1 applied to x’ axis) rotates magnetic moment M around X axis Exponential recovery!! Mzα [1-exp (-t/T1)]

  24. Rotating Frame ‘ ‘ ‘ ‘

  25. Making an image with T1 contrast Listen to protons here! Voxel in tissue #2 Voxel in tissue #1 Jack Pot to being bright on T2 image

  26. T1-Weighting (SE) • CSF (fluid) dark • WM bright • GM gray

  27. How does the longitudinal magnetization recover (T1)? Two basic water types in our body Bound (rotates close Larmor frequency) Free (rotates a bit too fast) Jack Pot to being bright on T1 image tumbling velocity Just right (almost) Too slow Too fast

  28. Tickle a nucleus at the Larmor frequency for fastest signal recovery!! http://www.revisemri.com/questions/misc/shorter_t1_tissues

  29. At a main field of 1.5 T Note : T1s get larger at 3T b/c Lamor frequency farther from rotational freq. of molecules

More Related