My spin on MRI: The basics of MRI physics and image formation.

1. My spin on MRI: The basics of MRI physics and image formation. Jonathan Dyke, Ph.D. Assistant Research Professor of Physics in Radiology Citigroup Biomedical Imaging Center Weill Cornell Medical College Sackler Institute for Developmental Psychobiology Summer Lecture Series July 7, 2009

2. Now not all nuclei are “MRI active”.. Which of the following could produce an MRI image? Only those with an odd number of protons and neutrons. 1H 11C 13N 18F 19F 31P • Which isotopes at the right are radioactive?

3. Does an MRI scanner produce radiation? The MRI signal is generated by receiving radiofrequency photons that return to their lower energy state.

4. A hydrogen atom (whether bound in water or lipid) acts as a small magnet due to the spinning of the positively charged _______. proton

5. Protons from what compounds comprise an MRI signal? What percentage of your body is composed of water? What percentage of your body is composed of fat? A) 40%-50%, B) 50%-60%, C) 60%-70%, D) 70%-80%

6. When placed in a “very” strong magnetic field, • a “slightly” greater number of atoms • align in the preferred direction with the magnetic field. Vs. • “very”: The earth’s magnetic field is ~ 0.3 Gauss. • The scanner at our facility is 30,000 Gauss. • “slightly”: only 5:1,000,000 spins align! • Note: 1 Tesla = 10,000 Gauss (Metric)

7. Typical Magnetic Field Map of a Clinical 3T MRI What effects will be felt by a pacemaker, credit cards, earrings, IPAD or cell phone?

8. The MRI scanner is always on!! A magnetic field is present 24/7!!

9. MRI Safety • Implants and foreign bodies • Projectile or missile effect • Radio frequency energy • Peripheral nerve stimulation (PNS) • Acoustic noise • Cryogens • Contrast agents • Pregnancy • Claustrophobia and discomfort -“Cheap” Earrings - Tattoo Ink > Rock concert @ the gardens. - “Quench” - Nephrogenic Systemic Fibrosis - No X-rays/Gd crosses placenta.

10. How does resonance come into play in MRI? A typical tuning fork produces a frequency of 400 Hertz, while a scan from Sackler was actually resonating at 127, 503, 172 Hertz. • A tuning fork produces sound waves at a single frequency that may be detected by objects that are of lengths related to multiples of the wavelength.

11. Larmor Equation: w=gB • w=Precessional Frequency • g= Gyromagnetic Ratio • B=Magnetic Field Strength • (42.57 MHz/Tesla * 3.0 Tesla = 127.5 MHz) • What field strength does my favorite • FM Classic Rock station transmit at? • Radio waves are transmitted at an angle of 90˚ • into the body at the Larmor frequency. • This imparts energy to the nuclei to achieve “resonance” • The additional energy in turn rotates the nuclei • out of alignment with the main field.

12. 3.0 Tesla GE MRI Scanner Y X Z Coil

13. “Magneto” Faraday’s Law of Induction states that a voltage is created by a changing magnetic flux. (1831) Was it easier back then to get a law named after you?

14. How do the motion of these two objects differ? “Rotation” vs. “Precession” • It is the precession of the nuclei that creates the • changing magnetic field needed to produce a signal.

15. What kind of signal is actually received by the scanner? • The frequency & phase information in time from the Free Induction Decay “FID” are transformed • into the frequency domain. (NMR 1946) • A Fourier series can represent any function • as a sum of sines and cosines. (1822)

16. Typical NMR signal after Fourier transformation. Can you identify the peaks? How about concentration? H2O (4.7ppm) Lipids CH2 (1.3ppm) Lipids CH3 (0.9ppm)

17. Where were all of these metabolic peaks hiding? What price is paid in detecting these signals?

18. Damadian’s Design for a Clinical MRI Scanner - 1974

19. Basic MRI Hardware Block Diagram How many of you have had an MRI? What’s it like?

20. How loud is loud? Fast imaging sequences such as EPI/Spiral used in functional neuroimaging (fMRI) can play upwards of 100+ decibels inside the bore of the scanner.

21. Position Magnetic Field Strength So how do we get spatial information? Back to the Larmor equation.. w=gB i.e. 1 Gauss will increase the frequency by 4.3kHz. Typical gradient strengths are 2-5 Gauss/cm.

22. What would the frequency difference be between two objects that are separated by 3cm? • w=gB • = 42.57E6 Hz/Tesla • B = Gz * z • = 0.01 T/m * 0.03 m • w = 12,771 Hertz

23. Conventional 3-Axis MRI Gradient Coil Diagram

24. Slice Selection 1st step is to excite a single slice instead of all space! Frequency To excite a thickness z use: DZ=Dw/gGZ To excite off axis use: w+w0 where w= gGZ DZ

25. Slice Selection How thin a slice could an MRI scanner produce? i.e. Could we perform in-vivo pathology scans?

26. General Electric Spin Echo Pulse Sequence Diagram Readout Read Rewinder Phase Encode Rewinder Phase Slice Select Gradients Slice TE/2 TE/2 90° 180° TR

27. Explaining the spin echo pulse sequence Ready, Set, Go!! Gun starts With 90 deg pulse. Courtesy: Siemens

28. Runners fan out with ability

29. Gun fires again reversing direction of race. [180 deg pulse]

30. The runners then reach the finish line at the same time TE.

31. General Electric Spin Echo Pulse Sequence Diagram Readout Read Rewinder Phase Encode Rewinder Phase Slice Select Gradients Slice TE/2 TE/2 90° 180° TR

32. Now that we have selectively excited a specific slice in space, we then must localize a specific xy-plane. With what pattern is MRI data generally acquired? Why would you choose one over the other?

33. Spatial encoding in x is called “Frequency Encoding”. • The frequency of the signal ~ position on the x-axis. • dx = FOVx/Nx = 1/(g/2p Gxtx) • e.g. A standard brain scan uses a 24 cm FOV • and a 512x512 matrix size on our 3T magnet. • This gives an in-plane resolution of 0.47mm/pixel. • RBW = Nx / tx = 1 /DT • e.g. A 15.63kHz RBW and Gx = 0.3 G/cm would • then apply the x-gradient for 32.8 ms to get • a single line of image “k-space”.

34. Spatial encoding in y is called “Phase Encoding”. • The phase f of the signal ~ position on the y-axis. • dy = FOVy/Npe = =1/(2 g/2p Gyrty) • The phase of a signal is given by: f = w t • To acquire the next line in “k-space”, an additional • phase (gGyy) is applied for a time t. • This is repeated until the entire image space is covered. • It is standard for the time to be fixed and the • gradient amplitude to increase/decrease.

35. Why is a Fourier Transform used? Application of pulses in the “time” domain are transformed into the MRI “frequency” domain.

36. K-space vs. Image Space FT FT http://www.leedscmr.org/images/mritoy.jpg

37. k-space Contribution to Image Properties Center = contrast Periphery = resolution http://www.radinfonet.com/cme/mistretta/traveler1.htm#part1

38. Voila’ - Spin Echo Images

39. How does an MRI scanner differ from a CT scanner? • Radiation, 2) Soft-Tissue Contrast • The intensity on a CT scan is directly related to what? • How much energy does MRI impart? • EMRI=h(g/2p)B0 =0.3 meV vs. ECT~ 25keV T2 CT T1

40. Image Weighting in MRI – * Learning Point * T1W GM=950ms WM=600ms T2W GM=100ms WM=80ms

41. Summary: • Magnetic Resonance Imaging • Soft Tissue Contrast (GM vs. WM, etc.) • High Spatial Resolution ( 1 mm isotropic voxels) • Oblique scanning options • Additional functionality: • Diffusion MRI, Magnetization Transfer MRI • Fluid attenuated inversion recovery (FLAIR) • Angiography, CSF Dynamics, Spectroscopy • Functional MRI, Interventional MRI, Contrast agents • MR guided focused ultrasound, Multinuclear imaging • Susceptibility weighted imaging (SWI)