Introduction to Magnetic Resonance Imaging (MRI)

1. Titel Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Magnetic Resonance Imaging MRI Walter Bauer

2. Introduct Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Introductory Remarks This lecture is an excerpt of an excellent article on the Internet, authored by Joseph P. Hornak, Rochester Institute of Technology, Rochester, New York. The web address is www.cis.rit.edu/htbooks/mri/ Only the very fundamentals of MRI can be covered in the present lecture. It is strongly recommended thatyou look up the web pages in order to extend your knowledge and your understanding of MRI. The basics of NMR (spin physics, Fourier transform, pulsed field gradients etc.) are omitted here; these topics have already been covered in the lecture "Modern NMR Methods". The slides are made available to the general public on the Internet as a .pdf-file as well as a PowerPoint Presentation (.ppt) which contains a variety of animated gifs. The slides are constantly being updated, The most recent version is indicated by the file name: „imaging_lecture_ddmmyy" where ddmmyy indicates day, month, year.

3. Example pictures Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Here are some example pictures which demonstrate today's powers of MRI. Note that MRI is largely complementary to X-ray and CT

4. Lauterbur Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie MRI: How it all began... Paul C. Lauterbur *1929 in Sidney, OH (USA), †2007 in Urbana, IL (USA) State University of New York Stony Brook; until his death Director of the Biomedical Magnetic Resonance Laboratory of the University of Illinois at Urbana-Champaign Heineken Prize 1989 Nobel Prize 2003 Idea: usage of magnetic field gradients for spatial localization of spins; "NMR-zeugmatography" (after greek zeugma = yoke), meaning that two magnetic fields are "joined"  P.C. Lauterbur, "Image Formation by Induced Local Interaction: Examples Employing Magnetic Resonance", Nature242, 190-191 (1973)

5. Lauterbur faksimile Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie MRI: How it all began... Faksimile from Lauterbur's first paper on "Zeugmatography" P.C. Lauterbur, "Image Formation by Induced Local Interaction: Examples Employing Magnetic Resonance", Nature242, 190-191 (1973)

6. Princ. I - head homog. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles I: Homogeneous Magnetic Field Fundamental NMR equation:  =/2 B0 Head with three distinct regions where there is hydrogen spin density, placed into homogeneous magnetic field => identical Larmor frequencies for all spins in absence of field gradient => one signal in NMR spectrum

7. Princ. II - gradient Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles II: Magnetic Field Gradient and Frequency Encoding Same head placed into inhomogeneous magnetic field with horizontal linear field gradient => spins at different vertical positions have identical Larmor frequencies => spins at different horizontal positions have different Larmor frequencies => spatial resolution of spins in horizontal direction => frequency encoding linear gradient G

8. Princ. III - backproj Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles III: Backprojection Imaging Application of frequency encoding gradient Gf at different angles  between 0 <  < 3600 by using linear combinations of x and y gradients: Gy = Gf Sin  Gx = Gf Cos  y x

9. Princ. IV - sequence Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles IV: Backprojection Pulse Sequence Pulse sequence which might be principally employed for backprojection. The rotation of the gradient in the x,y-plane is accomplished by a linear combination of Gx and Gy The z-gradient is used for "slice selection". This will be explained below.

10. Princ. V - storage Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles V: Backprojection Imaging - Storage in Memory After recording of spectra with gradients at different angles  data can be backprojected. After suppression of background intensity => signal can be seen => "inverse Radon transform"

11. Princ. VI - slice select Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles VI: Slice Selection In order to obtain a two-dimensional picture out of a three-dimensional object we need to select a slice, in the same way as we employ thin slices when looking through a microscope. This can be achieved by applying a field gradient Gz in z-direction (the direction of the main field B0) during the application of the 900-pulse. This is the explanation for the Gz-gradient employed two slides above: the gradient creates different Larmor frequencies in z-direction, and the pulse excites only those spins with the "correct" frequency! However: a rectangular 900-pulse is not ideal for slice selection. Why?

12. Princ. VII - rectang. pulse Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles VII: Rectangular Pulse This is the excitation profile of a rectangular pulse: a "sinc"- function sinc(x) = sin(x)/x Apart from the center lobe there are many unwanted side lobes which excite spins in unwanted areas of the object!

13. Princ. VIII - poor slice Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles VIII: Poor Slice Selection by a Rectangular Pulse A rectangular 90o-pulse with its "sinc" excitation profile hits also spins outside the wanted area, leading to artifacts. Solution: make the pulse frequency selective! How can this be achieved?

14. Princ. IX - sinc pulse Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles IX: Sinc-Shaped Excitation Pulse Excitation profile of a sinc-shaped pulse: a rectangle, and thus no unwanted excitation at side-lobes => this is the pulsed employed in MRI for slice selection! The longer the pulse, the more frequency selective it is.

15. Princ. X - backproj. scheme Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Principles X: Pulse Sequence for Backprojection Pulse sequence which may be employed in real life for backprojection purposes. Note the selective 90o sinc pulse as compared to the rectangular pulse in the analogous sequence shown above: the sinc pulse has a rectangular excitation profile In summary, the backprojection technique is quite easy to understand. However: the backprojection technique is NOT "state of the art" since long. Other and better techniques are being used today...

16. FT-Imag. I - phase coherent Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging I: Phase Encoding Gradient Modern imaging sequences employ a phase encoding gradient, along with the already familiar frequency encoding gradients and the slice selection gradient. Here's the working principle. Imagine three areas with spins. After the 90o excitation pulse, in the absence of a gradient the spins will precess in the x,y-plane with the same speed (the same Larmor frequency) and in phase (they are phase coherent)

17. FT-Imag. II - phase difference Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging II: Phase Encoding Gradient When a gradient is applied along the x-direction, the three vectors will precess with a freqeuncy given by the resonance equation:  =  (B0 + xGx) = 0 + xGx This is similar to the frequency encoding process described earlier.

18. FT-Imag. III - phase angle Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging III: Phase Encoding Gradient Now, when the phase encoding gradient is switched off, the spins have again identical Larmor frequencies, however, with non- identical phase angles , according to their position in x-direction

19. FT-Imag. IV - time diagram Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging VI: Time Diagram Time diagram for FT tomographic imaging with slice selection, phase encoding and frequency encoding.

20. FT-Imag. V - phase enc. casc. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging V: Phase Encoding Cascade The complete sequence is repeated many times (typically 128 or 256) where the strength of the phase encoding gradient is varied step by step. This yields a matrix of signal FIDs, in the very same way as we learned from the principles of two-dimensional NMR! The processing of such a data matrix will be described below

21. FT-Imag. VI - mag. after slice Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging VI: Magnetization After Slice Selection After the slice selection pulse has been applied, the magnetization vectors of a 3x3 voxel slice look like this: all vectors have the same Larmor frequencies and the same phase

22. FT-Imag. VII - mag. phase dur. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging VII: Magnetization During Phase Gradient When the phase encoding gradient is switched on, the vectors precess with different speeds in the direction of the gradient G

23. FT-Imag. VIII - mag. phase aft. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging VIII: Magnetization After Phase Gradient When the phase encoding gradient is switched off, the vectors precess with identical speeds, however, with phase differences according to the length and the strength of the phase encoding gradient

24. FT-Imag. IX - mag. frq. encode Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie FT-Imaging IX: Magnetization During Frequency Encoding Gradient When the frequency encoding gradient is switched on (at the same time as signal detection starts) the spins precess with different speeds along the axis of the frequency encoding gradient and with identical speed along the axis of the former phase encoding gradient, however, with the introduced phase differences Gf

25. Sig.Proc. I - two voxels Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing I: Two Voxels with Net Magnetization Think of a slice (selected by the initial slice selection gradient) where there are two voxels with net magnetization Note: as compared to the previous slides, the phase encoding and the frequency encoding axes are now interchanged. Don't get confused from that, for didactical reasons the following presen- tation is more convenient. This doesn't change anything of the principles!

26. Sig.Proc. II - FID matrix Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing II: Obtained FIDs After a series of spectra has been recorded with varying strengths of the phase-encoding gradient, the resulting FIDs look like this (remember the basic principles of two-dimensional NMR!). Note that in the horizontal direction the FIDs contain the pattern of two overlayed sinusoidals and in the vertical direction there is phase modulation of the FIDs!

27. Sig.Proc. III - first Fourier Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing III: First Fourier Transform of the FIDs The first Fourier transform of all individual FIDs in the frequency encoding direction yields the "blue" and "red" signals at their frequency encoded spatial positions. Note that there is now a phase-/amplitude modulation of the signals in the phase encoding direction

28. Sig.Proc. IV - vertical slices Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing IV: Slices in Vertical Direction When we take vertical slices of the data matrix of the previous slide, the picture looks like this. We have new "FIDs" which, as opposed to the directly detected domain signals, have never been "really" recorded. All this is completely identical to the principles of two-dimensional NMR!

29. Sig.Proc. V - second Fourier Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing V: Second Fourier Transform A second Fourier transform along all slices in the phase encoding direction yields peaks at exactly those locations where the initial voxels had been located => we have imaged our object! This type of double Fourier transform MR imaging is termed "spin warp"imaging

30. Sig.Proc. VI - "contour plot" Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing VI: Image Presentation All that remains is the job of the computer to present the image in an analogous way as in two-dimensional NMR as a "contour plot"

31. Sig.Proc. VII - "intensity plot" Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing VII: Image Presentation The Fourier transformed data is displayed as an image by converting the intensities of thepeaks to intensities of pixels representing the tomographic image.

32. Sig.Proc. VIII - resolution Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing VIII: Image Resolution As familiar from the principles of NMR spectroscopy, the number of data points in each dimension is inversely correlated with the obtainable resolution

33. Sig.Proc. IX - resolution Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing IX: Image Resolution Within the "field of view" (FOV) the number of data points N determines the pixel size. The higher the number of N, the smaller the pixels, and, hence, the resolution. However: a huge number of N is meaningless when the signal has decayed early due to a short effective relaxation time T2*!

34. Sig.Proc. X - resolution Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Signal Processing X: Image Resolution Therefore, the pixel size should be chosen to be approximately equal to ( Gx T2*)-1 Short T2* Both images on the left have been recorded with a pixel size much less than defined by the above expression. Nonetheless, due to the short T2* in the upper picture the resolution is limited Long T2*

35. Tech. I - multislice Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Techniques I Multislice Imaging The basic imaging sequence has one drawback: during most of the repetition time TR nothing happens, the spins in the selected slice simply relax, others haven't been effected anyway => why not excite one slice after another in an interleaved way? TR

36. Tech. II - multislice Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Techniques II Multislice Imaging Solution: take same pulse sequence, however, at each pass vary the frequency of the slice selection RF pulse => different slices are excited sequentially; for a specific slice there is enough time for spins to relax until new excitation sets in

37. Tech. III - spin echo Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Techniques III Spin-Echo Imaging Some tissues have similar T1 but different T2. We know from the "Modern NMR Methods" lecture that the spin echo sequence introduces a T2-dependence on the signal amplitude => "T2-weighted" images may be obtained by variation of the echo time TE. Try to figure out how the sequence works! You should be familiar with all principles at this point...

38. Tech. IV - spin echo examp. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Techniques IV T2-Weighted Images TE = 80 ms TR = 2000 ms TE = 20 ms TR = 1000 ms

39. Hardware I - scheme Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware I Overview Scheme of a MR Tomograph

40. Hardware II - photo Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware II Tomograph Photo

41. Hardware III - cross section Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware III Magnet Cross Section

42. Hardware IV - z-grad coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware IV Anti-Helmholtz coil pair

43. Hardware V - x-grad coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware V

44. Hardware VI - y-grad coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware VI

45. Hardware VII - saddle coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware VII Employed as main RF coil inside magnet

46. Hardware VIII - suf. coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware VIII Employed as "receive-only" coil. Good signal-to-noise ratio

47. Hardware IX - suf. coil photo Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware IX Surface Coil

48. Hardware X - bird cage coil Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware X Employed for imaging of head/brain

49. Hardware XI - birdcage phot. Bauer/Imaging Universität Erlangen-Nürnberg Institut für Organische Chemie Imaging Hardware XI Bird Cage Coil

50. Siemens Magnetom Bauer/Imaging Modern MR- Tomograph (Siemens Magnetom)