MRI Physics (Phys 352A)

Transcription

1 MRI Physics (Phys 352A) Manus J. Donahue: Department of Radiology, Neurology, Physics, and Psychiatry Office: Vanderbilt University Institute of Imaging Science (VUIIS) AAA-3115 Web site: Special thanks to Karla Miller Oxford University MRI Physics Graduate Course

2 Looking Inside the Body (c. 1632) Rembrandt

3 Looking Inside the Body (2013) Can see whole body in a few minutes Tissue structure, function, blood flow, chemistry, etc. Major developments in physics + diagnosis + physiology 7 Tesla MRI

4 Brief History of Magnetic Resonance Imaging (MRI)

5 MRI Physics A bit about me: Faculty member in Dept. of Radiology, but also Physics, Neurology and Psychiatry My work focuses on developing and implementing new MRI methodologies for understanding brain function in health and disease Post-doctoral fellow: Clinical Neurology Graduate school: Biophysics Undergraduate: Physics and Philosophy MRI transcends many disciplines (e.g., Physics, Biophysics, Medicine, Neurology, etc.) This course is designed to not just allow you to understand basic MRI methods, but to understand how MRI can be used to address biologically relevant questions

6 MRI Physics Schedule Introduction Math review Image formation Pulse sequences Image contrast Hardware Fast imaging Review + Exam 1 MRI Physics Principles Structural clinical imaging Functional MRI Chemical imaging (MRS/CEST) Diffusion imaging Emerging sequences and high field (i.e., > 3 Tesla) Clinical MRI in cerebrovascular disease, neurological disorders, dementia, and outside the brain Projects Review + Exam 2 Applications

7 Guest Lecturers Jay Moore Pulses + coils Chuck Nockowski Hardware + tour Seth Smith Chemical imaging + MRI outside the body

8 Expected Background Course will focus on understanding MRI principles from a users perspective, therefore extensive knowledge of mathematics is not required I will review most of what you need to know I am much more concerned that you understand how images are generated, where contrast comes from, and how new contrast can be obtained rather than that you can calculate Fourier transforms, spin density matrices, coherence pathways, etc. by hand Expected background: Introductory college calculus and physics (mechanics and electricity and magnetism) Knowledge of quantum mechanics, differential equations, and Fourier transforms is helpful but not required We will also cover clinical and neuroscience applications. Therefore knowledge of basic human physiology is helpful but not required

9 Grading Homework (2/mo): 25% Midterm Exam: 25% Final Exam: 25% Project/Presentation: 25% Exams will be based heavily on homework and lectures i.e., if you keep up with homework you will do well on exams Project/presentation: focus on an advanced topic of interest and present a written report (10-20 pgs/double spaced) and 15 min class presentation. Undergrads: Project grade may replace single exam (if lower) Grads: Project grade may not replace single exam

10 Project/Presentation Pick a special topic of interest to you: Clinical (e.g., cancer, stroke, Alzheimer s disease) Technical (e.g., parallel imaging, k-space, novel acquisition) Hardware (e.g., coils, gradients, radiofrequency transmission) NOTE: if you are actively working on an imaging research project (Ph.D., etc.) you must choose something different from your thesis topic! I must approve all topics: just or talk to me. Prepare written report pgs; double-spaced; font=12 pt; margins=1 Present summary to class 15 min (~10 slides)

11 Office Hours / Expectations Office hours: After class or by appointment mj.donahue@vanderbilt.edu VUIIS; 3 rd Floor, AAA-3115 Expectations: - I want everyone to do well in this class - e.g., learn a lot and get an A - Exams/grading designed not to be tricky, but to ensure knowledge of covered material. - If you come to class and do the work, you should be happy with your grade

12 MRI Physics Why take a course in Magnetic Resonance Imaging (MRI)? Unlike other imaging modalities (e.g., X-ray, PET, SPECT, CT, etc.) whose contrast depends on injected material and hardware, MRI contrast depends more broadly on changes to sequence parameters: - Many different aspects (e.g., structure, function, etc.) can be visualized if you understand basic principles!

13 Unhealthy brain Example: Brain Tumor Necrosis/tumor Border FLAIR T1 1/CBV Total Tumor Segmented Tumor T1+Contrast GBM AO LG GBM: Glioblastoma multiforme; AO: Anaplastic oligodendroglioma; LG: Low-grade

14 Why Learn MRI Physics Knowledge of MRI physics will help you: Address a relevant biological question e.g., How can I measure tissue volume, blood flow, glutamate, etc.? Identify the relevant parameters Bandwidth, resolution, echo time (TE), etc.? Evaluate data quality Measure distortion and signal-to-noise ratio

15 MRI Physics: All About Spin Protons (e.g., hydrogen): consist of charged particles with spin Spin is a quantum mechanical, intrinsic property. Classical analogue: rotation about an axis Charge + non-zero spin = magnetic moment Source of (most) MRI signal: protons in water H 1 What do we know about magnetic objects? What to we know about rotating objects?

16 Principles of MRI Compass Magnetically sensitive pointer Gyroscope Spinning wheel + disk

17 Compass Oscillation Lars G. Hanson v=1orpcnvsa4o

18 Magnetic Resonance Think about the compass Compass needle oscillates about magnetic field before stopping This oscillation has a well-defined frequency ( resonance frequency )

19 Magnetization Excitation and Relaxation Excitation: An additional magnetic field (B1) can deflect the compass needle This deflection can be maximized by choosing the new field to be the same as the resonance frequency Relaxation: After B1 field is removed, the magnetic oscillations decay with a welldefined time constant

20 Polarization What direction does a compass point? In absence of magnetic field, compass needle is randomly oriented In presence of magnetic field (e.g., Earth), needle has slight tendency to align with magnetic field.

21 Spin Magnetization + Spin Think about gyroscope Gravity tilts a spinning object Because of spin, the axis precesses instead of tilting Spin + Gravity = Precession

22 What About Protons? Can we use what we know about: Magnets Spinning Objects To understand Hydrogen proton in water molecule? H1

23 Yes: Protons are Tiny Magnets H 1 Magnetic Field (B 0 ) = 0 Tesla With no magnetic field, the spin associated with hydrogen nuclei are randomly oriented

24 Protons in Magnetic Field H 1 H 1 H 1 H 1 H 1 H 1 H 1 H 1 Magnetic Field (B 0 ) > 0 Tesla M With magnetic field, spins along (slightly) with the field, creating a net magnetic moment (M)

25 Coordinate System B 0 The direction of the main field (B 0 ) defines the coordinate system Longitudinal axis: Parallel to B 0 Transverse axis: Perpendicular to B 0 x z M y MRI experiments involve reorienting M relative to B 0 and studying behavior of M

26 Magnetic Resonance H 1 ω = γb 0 ω = Larmor frequency γ = gyromagnetic ratio B 0 = static field strength MRI and NMR detects magnetization (M) Usually we detect protons on water In principle can detect other elements with spin ( 13 C, 19 F, 31 P, etc.) Magnetization has characteristic resonance frequency: Larmor frequency (ω) For water protons, ω is in (or just above) radiofrequency (RF) range: MHz

27 MR Excitation B 0 z M y B 0 z M y B 1 x x Excitation: Additional field (B 1 ) tips magnetization (M) away from main field (B 0 ) if this new field is applied at resonance frequency (ω)

28 Relaxation: Turn Off B1 Field B 0 z B 0 z M y B 1 M y x x Relaxation: Excited magnetization reverts to orientation before B 1 introduced Described by time constant T1 (longitudinal plane) Described by time constant T2 (transverse plane)

29 Magnetization Precesses about Main Magnetic Field (B0) B 0 Movie courtesy of William Overall

30 What Happens After We Turn Off the RF Excitation Pulse? B 0 M z returns to alignment with main magnetic field - T1 describes this time Movie courtesy of William Overall

31 What Happens After We Turn Off the RF Excitation Pulse? B 0 M x,y dephases in transverse plane T 2 describes this time Movie courtesy of William Overall

32 Signal Detection B 0 Changing magnetic field introduces a current in a wire Precessing magnetization detected with a coil tuned to the appropriate frequency Important: can only detect components in transverse (x-y) plane Movie courtesy of William Overall

33 Magnetic Resonance Summary Polarization: In external magnetic field (B 0 ), protons align to create net magnetization (M) Excitation: RF pulse tips magnetization away from B 0 Precession: Excited magnetization rotates about B 0 Detection: Magnetization induces a current in a correctly tuned coil close to the object Relaxation: Magnetization returns to alignment with B 0, causing signal decay

34 Making an Image G B 0 ω = γb 0 ω = γ(b 0 +G) Need to differentiate protons at different locations Add a second, spatially varying magnetic field (G) Gradient field at each location is either parallel or antiparallel to B 0 Therefore, the gradient field either adds or subtracts from the main magnetic field

35 High Frequency Fast Precession Magnetic Gradients ω = γb 0 B G 0 Low Frequency Slow Precession ω = γ(b 0 +G) Spins at each position have different frequency RF coil hears all of the spins at once Differentiate material at different positions by selectively listening to only a certain frequency

36 MRI Scanner Hardware GE Philips Siemens Major Vendors (Humans): General Electric (American) Siemens (German) Philips (Dutch)

37 MRI Hardware: Three Important Coils Magnet (B 0 =1.5T, 3.0T, 7.0T) RF Coil Gradient Coil The MRI scanner consists of coils that generate the different fields: Main static field (B0) Transient field (gradients) Transmit field (RF)

38 Coils Magnet Larry Wald MGH RF Gradient

39 First Human MRI Scanner

40 Flexibility of MRI: Structural Imaging 3.0 T (1.0 mm) 7.0 T (0.7 mm) Axial Sagittal

41 Flexibility of MRI: Vascular Imaging Structural imaging: visualization of blood water in vessels

42 Flexibility of MRI: Visualization of the Vessel Wall and Plaque Circle of Willis Structural Vessel Wall imaging: visualization of blood water in vessels R L R L

43 Flexibility of MRI: Functional Imaging 9 0 Z- sta4s4c

44 Flexibility of MRI: Functional BOLD Reac4vity Scan 1 imaging (Pre- Surgery) in the clinic BOLD Reac4vity Scan 2 (Six months post- surgery) BOLD Reac4vity Scan 3 (Twelve months post- surgery) 0 12 Z- sta4s4c

45 Flexibility of MRI: Measuring Functional Changes Based on Changes in Cerebral Blood Flow (CBF) FIG 1 A CBFw P R (a) RPI L (b) (c)

46 Beyond Water Imaging: Interactions between Proteins, Iron and Water

47 Beyond Water Imaging: Interactions between Amide Protons and Water

48 Technical Issues Global Shim + 1 st order Dynamic Slice 3 Slice 3 Global Shim Only Slice 15 Saikat Sengupta

49 Improving Detection Strategies spin-system density matrix metabolite(k) = H1 H3 H5 basis selection initial conditions p(k) = Σω j ϕ ><ϕ j j j H2 H4 H6 Hamiltonian Precision H (ti ) search algorithm H(ti)= N CRLB analysis quantification of mutual information S 1 (t) S k (t) signal detection Kevin Waddell

50 Research in MRI To be successful in research need: Good ideas (significant, innovative, etc.) Ask questions (and find answers) to problems with public health/interest relevance Note: > 300,000 papers using traditional BOLD fmri Many more with structural imaging (MRI, CT, X- ray, etc.) The ability to use new methods to address new or old questions greatly increases research potential!

51 MRI Physics Schedule Introduction Image formation Pulse sequences Hardware Image contrast Fast imaging MRI Physics Principles Structural clinical imaging Functional MRI Chemical imaging (MRS/CEST) Diffusion imaging Emerging sequences Review Applications

52 Office Hours / Expectations Office hours: After class or by appointment mj.donahue@vanderbilt.edu VUIIS; 3 rd Floor, AAA-3115 Expectations: - I want everyone to do well in this class - e.g., learn a lot and get an A - Exams/grading designed not to be tricky, but to test knowledge of covered material. - If you come to class and do the work, you should be happy with your grade.