Universitt ErlangenNrnberg Institut fr Organische Chemie Titel Magnetic

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

Universität Erlangen-Nürnberg Institut für Organische Chemie Bauer/Imaging 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 that you 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 Power. Point 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.

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

Universität Erlangen-Nürnberg Institut für Organische Chemie Bauer/Imaging Lauterbur 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", Nature 242, 190 -191 (1973)

Universität Erlangen-Nürnberg Lauterbur faksimile Institut für Organische Chemie Bauer/Imaging 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", Nature 242, 190 -191 (1973)

Universität Erlangen-Nürnberg Princ. I - head homog. Institut für Organische Chemie Imaging Principles I: Homogeneous Magnetic Field Fundamental NMR equation: = /2 B 0 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 Bauer/Imaging

Universität Erlangen-Nürnberg Princ. II - gradient Institut für Organische Chemie Bauer/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

Universität Erlangen-Nürnberg Princ. III - backproj Institut für Organische Chemie Bauer/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

Bauer/Imaging Universität Erlangen-Nürnberg Princ. IV - sequence 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.

Universität Erlangen-Nürnberg Princ. V - storage Institut für Organische Chemie Bauer/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"

Bauer/Imaging Universität Erlangen-Nürnberg Princ. VI - slice select 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 B 0) 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?

Bauer/Imaging Universität Erlangen-Nürnberg Princ. VII - rectang. pulse 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!

Universität Erlangen-Nürnberg Princ. VIII - poor slice Institut für Organische Chemie Bauer/Imaging Principles VIII: Poor Slice Selection by a Rectangular Pulse A rectangular 90 o-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?

Bauer/Imaging Universität Erlangen-Nürnberg Princ. IX - sinc pulse 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.

Universität Erlangen-Nürnberg Princ. X - backproj. scheme Institut für Organische Chemie Bauer/Imaging Principles X: Pulse Sequence for Backprojection Pulse sequence which may be employed in real life for backprojection purposes. Note the selective 90 o 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. . .

Universität Erlangen-Nürnberg FT-Imag. I - phase coherent Institut für Organische Chemie Bauer/Imaging 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 90 o 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)

Universität Erlangen-Nürnberg FT-Imag. II - phase difference Institut für Organische Chemie Bauer/Imaging 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: = (B 0 + x. Gx) = 0 + x. Gx This is similar to the frequency encoding process described earlier.

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

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

Universität Erlangen-Nürnberg FT-Imag. V - phase enc. casc. Institut für Organische Chemie Bauer/Imaging 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

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

Bauer/Imaging Universität Erlangen-Nürnberg FT-Imag. VII - mag. phase dur. 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

Bauer/Imaging Universität Erlangen-Nürnberg FT-Imag. VIII - mag. phase aft. 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

Universität Erlangen-Nürnberg FT-Imag. IX - mag. frq. encode Institut für Organische Chemie Bauer/Imaging FT-Imaging IX: Magnetization During Frequency Encoding Gradient Gf 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

Universität Erlangen-Nürnberg Sig. Proc. I - two voxels Institut für Organische Chemie Bauer/Imaging 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 presentation is more convenient. This doesn't change anything of the principles!

Universität Erlangen-Nürnberg Sig. Proc. II - FID matrix Institut für Organische Chemie Bauer/Imaging 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!

Universität Erlangen-Nürnberg Sig. Proc. III - first Fourier Institut für Organische Chemie Bauer/Imaging 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

Universität Erlangen-Nürnberg Sig. Proc. IV - vertical slices Institut für Organische Chemie Bauer/Imaging 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!

Universität Erlangen-Nürnberg Sig. Proc. V - second Fourier Institut für Organische Chemie Bauer/Imaging 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

Universität Erlangen-Nürnberg Sig. Proc. VI - "contour plot" Institut für Organische Chemie Bauer/Imaging 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"

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

Bauer/Imaging Universität Erlangen-Nürnberg Sig. Proc. VIII - resolution 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

Universität Erlangen-Nürnberg Sig. Proc. IX - resolution Institut für Organische Chemie Bauer/Imaging 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 T 2*!

Bauer/Imaging Universität Erlangen-Nürnberg Sig. Proc. X - resolution Institut für Organische Chemie Signal Processing X: Image Resolution Short T 2* Therefore, the pixel size should be chosen to be approximately equal to ( Gx T 2*)-1 Long T 2* 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 T 2* in the upper picture the resolution is limited

Bauer/Imaging Universität Erlangen-Nürnberg Tech. I - multislice 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 TR => why not excite one slice after another in an interleaved way?

Universität Erlangen-Nürnberg Tech. II - multislice Institut für Organische Chemie Bauer/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

Universität Erlangen-Nürnberg Tech. III - spin echo Institut für Organische Chemie Bauer/Imaging Techniques III Spin-Echo Imaging Some tissues have similar T 1 but different T 2. We know from the "Modern NMR Methods" lecture that the spin echo sequence introduces a T 2 -dependence on the signal amplitude => "T 2 -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. . .

Universität Erlangen-Nürnberg Tech. IV - spin echo examp. Institut für Organische Chemie Imaging Techniques IV T 2 -Weighted Images TE = 20 ms TR = 1000 ms TE = 80 ms TR = 2000 ms Bauer/Imaging

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

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

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

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

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

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

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

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

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

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

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

Bauer/Imaging Modern MRTomograph (Siemens Magnetom)

Bauer/Imaging Latest development in MRI (2004): mobile tomograph for application, e. g. , in brain surgery „Pole. Star N 20“ Mobile MRT (Medtronic Inc. , Germany) => high resolution pictures during surgery within 12 sec.

Bauer/Imaging Sir Peter Mansfield *1933 in London Revolutionary idea 1977: „echo planar imaging“ => allows MRI body scanning in much shorter time! Nobel Prize (together with Lauterbur) 2003

Bauer/Imaging Universität Erlangen-Nürnberg Tech. V - echo planar im. Institut für Organische Chemie Imaging Techniques V Echo Planar Imaging (Functional MRI) Peter Mansfield Nobel Prize 2003 Echo planar imaging is the fastest method today. It may produce 15 -30 images per second (!), hence "real time" videos can be obtained. All images of a slice are recorded within one period of the repetition time TR! A series of phase encoding gradients after the spin echo 180 o-pulse with interleaved signal sampling and simultaneous frequency encoding pulse alteration yields the image of a complete slice within fractions of a second

Bauer/Imaging Universität Erlangen-Nürnberg Tech. VI - echo planar im. Institut für Organische Chemie Imaging Techniques VI Echo Planar Imaging (Functional MRI) "Real time movie" of human brain during cognitive process obtained by echo planar imaging

Bauer/Imaging Raymond Damadian Claims to have been overlooked the Nobel Prize comittee and to be the „true inventor of the MRI principle“ => definitely not true!

Bauer/Imaging Full page advertisings posted by R. Damadian in Washington Post Los Angeles Times New York Times Die Zeit & others complaining about his having been overlooked by the Nobel Prize commitee $ 80. 000 each!!

Bauer/Imaging Materialprüfung mit Hilfe der "NMR-MOUSE" (MObile Universal Surface Explorer) Prof. Blümich, Aachen

Bauer/Imaging Bildgebung mittels "NMR-MOUSE" Prof. Blümich, Aachen