Statistical Parametric Mapping

1. Statistical Parametric Mapping Lecture 3 - Chapter 5 Hardware for functional MRI Textbook: Functional MRI an introduction to methods, Peter Jezzard, Paul Matthews, and Stephen Smith Many thanks to those that share their MRI slides online

2. N N S S The Magnetic Field • Ferrous Bar Similar to Bar Magnet • Torque – align • Force – toward poles Mostly Force Mostly Torque

3. Nature of Forces Around Magnet • Ferromagnetic materials mostly • Depend on shape of object (longer is worse) • Increase rapidly with approach to magnet (depends on B0 spatial gradient) • Increase approximately with square of B0(3T vs 1.5T) • Depends on type of magnet (open, self-shielded, etc.) (depends on B0 spatial gradient)

4. fMRI Basic Requirements EPI T2* demands Relative to physiological noise Rapid Imaging fast high-strength gradients wide bandwidth transceiver Stable System systematic drift small noise small High Signal Levels high field strength magnet RF coil design

5. Magnetic Susceptibility Net B = B0 + B • B is proportional to both field strength (H) and susceptibility (). In air B = 0. • Macroscopic changes of B induced at different locations result in spatial gradients in B that can be significant for EPI. • For many parts of the brain the macroscopic susceptibility gradient is small so Larmor frequencies are similar. • For areas where the macroscopic susceptibility gradient is large (e.g. near tissue air interfaces) Larmor frequencies of nearby voxels also changes greatly. • Microscopic changes in susceptibility due to BOLD effect can be masked when near areas in brain with large changes in macroscopic susceptibility.

7. EPI style BOLD fMRI -advantages and disadvantages - • Fast • Resolve hemodynamic changes, whole head coverage in 3 seconds or less. • Freeze subject motion (k-space encode of slice in <50ms). • Encodes full k-space image without RF signal reset compared to non-EPI imaging (phase errors accumulate). • Susceptibility weighted • Want good signal from microscopic dephasing due to BOLD induced susceptibility. • Interference from macroscopic dephasing due to large extent changes in susceptibility.

8. BOLD is microscopic susceptibility Problems With Macroscopic Susceptibility Gradients Signal Dropout... Distortions… All susceptibility effects increase with Bo!! Wald, Toronto 2005

9. ky kx Image Encoding for EPI All lines in one shot… • Fast (high BW ) in kx. • Slow (low BW) in ky. • No “reset by RF”, so phase errors accumulate. • Fast (~10 slices per second) for ~2 mm res. • Physiological fluctuations modulate overall intensity • Readouts alternating polarity. • All k-space NOT treated equally. dt=0.005ms dt=0.5ms Wald, Toronto 2005

10. Temporal Sampling is Asymmetric in EPI 100x longer in phase direction ky • k-space errors due to susceptibility are small in kx direction because of short time sampling intervals. • but can be significant in ky encode direction (100x longer here). Note frequency gradient from point 1 to point 2 1 2 kx Dj = Du t dephasing leads to signal loss frequency map Wald, Toronto 2005

11. t exp(-t/T2*) Gx Image encoding strategies: EPI All k-space not treated equally: T2* filtering across k-space increases point-spread function. • T2* shortens as B0 increases • Limit total readout time to 2T2* • increase readout gradient • receiver BW increases Wald, Toronto 2005

12. ky ky kx kx Gx Gx Gy Gy EPI and Spiral Scanning of k-space EPI Spiral Gx and Gy 90 degrees phase difference for sprial Interpolated to regular kx and ky spacing. Wald, Toronto 2005

13. EPISpirals Susceptibility: distortion, blurring, dephasing dephasing Eddy currents: ghosts blurring k = 0 is sampled: 1/2 through beginning Corners of kspace: yes no Gradient demands: very high pretty high Wald, Toronto 2005

14. 4.5 4 3.5 3 2.5 normalized SNR 2 1.5 1 0.5 0 0 1 2 3 4 5 6 7 8 9 10 B0, Tesla Normalized SNR vs. Magnetic Strength • TAD - total readout time • Time fore single Kx (SE,GRE) TAD << T2* • Time for full K-space (EPI) TAD ~ T2* • TAD intermediate for others (FSE, TSE) Figure 5.3 from textbook.

15. SNR Total SNR vs Thermal SNR Data from 1.5T (triangles) and 3.0T systems (squares) Physiological Signal - S Thermal and system noise - 0 Physiological sources - p Total noise -  1. 3. 2. Figure 5.4 from textbook.

16. Exciter XMTR Synthesizer T/R RF switch RCVR Preamp Coil Network Host A/P RAM Amps Gx, Gy, Gz Shim coils Shim Pulse driver Synthesizer, A/P programmer XMTR, RCVR, T/R Gradient coils Schematic of MRI System Same or different transmit and receive coil. A/P - Array Processor RF, Shim, Gradient Coils inside magnet All but Host, RAM, and A/P in equipment room Figure 5.1b from textbook.

17. RF Coil Uniformity and SNR Surface coil/head coil comparison 1400 17 cm spherical phantom b 1200 SNR 2 SNR 1000 800 600 1 (1) (2) 400 4 3 200 0 0 50 100 150 200 250 distance, mm (3) (4) B1 directions indicated by color arrows. (1) two surface coils on opposite sides in phase. (2) two surface coils out of phase. (3) single surface coil on right side. (largest SNR) (4) head coil. (most uniform SNR) Figure 5.7 from textbook.

18. Surface Coils

19. Z (b) Worst  (c) Acceptable (a) Best Surface Coil Orientations B0 Surface coils are like loops for detecting B1 which is precessing about B0 which is parallel to the z-axis • Best orientation is with plane of coil perpendicular to B0 which for the brain in normal orientation leads to following as best sites • Left or right side • Anterior of posterior Figure 5.8 from textbook.

20. Tissue Heating During RF Transmit • Concerns are total body and localized heating • Not practical to monitor increase in temperature except in phantoms • Specific Absorption Rate (SAR) used to estimate temperature increase • 1 SAR = 1 W/kg • 1 SAR would increase temperature of an insulated slab by ~ 1 C/hr • SAR also used in monitoring RF for cell phones

21. Scanner Software Estimates SAR • Runs a calibration routine • Determines energy for RF pulses • Adds up energy from all RF pulses per TR and divides by TR • Divides by tissue weight to get total body or regional SAR • Requires height and weight for algorithm • If limits are exceeded operator must alter pulse sequence

22. RF - FDA Limits • Integrated SAR limits • Head SAR = 60 W-min/kg • Trunk SAR = 120 W-min/kg • Extremeties SAR = 180 W-min/kg • SAR rates • Head (38° C) SAR=3.2 W/kg • Trunk (39° C) SAR =8 W/kg • Extremities (40° C) SAR =12 W/kg • Other • Infants, pregnancy, cardiocirculatory or cerebral vascular impairment (1.5 W/kg)

23. SAR Pulse Sequence Impact • Minimal for EPI acquisition (1-2 RF pulses per plane) • Higher for 3D anatomical scan GRE (1 RF pulse per kx reradout) and short TRs. • High for T1W spin echo (one 90º and one 180º RF pulse per kx line) with slice geometry same as GRE • Within pulse sequence effects • Increasing TR without increasing # of RF pulses reduces SAR • Reducing number of slices per TR (in multislice SE) • Partial Fourier imaging reduces number of phase encodes with RF for each (in multislice SE)

24. b a Z Z X or Y X or Y Transverse (Gx, Gy) Longitudinal (Gz) Figure 5.5 Need strong gradients and shortened readout time to keep TAD in range. Figure 5.6 from textbook.

25. Current and Gradient Pulse Shape c a • a. gradient current supplied (short rise time induces eddy currents) • b. eddy currents oppose changing field w/o compensation • c. gradient current supplied with eddy current compensation • d. potential field vs time with eddy current compensation d b Jerry Allison.

26. dB dt dB/dt Effect (more eddy currents) Peripheral Nerve Stimulation • dB/dt --> dE/dt • dt is gradient ramp time • dB/dt largest near ends of gradient coils • spatial gradient of dE/dt important

27. dB/dt / E-Field Characteristics of Stimulation • Not dependent on B0 • Gradients - 40mT/m (larger Bmax for longer coil) • Gradient Coil Differences - strength (increases dB) and length (head vs. body determines site) • Rise Time - shorter rise time means shorter dt and therefore larger dB/dt • Other • Disruption of nearby medical electronic devices • Subject Instructions • Don’t clasp hands - closed circuit, lower threshold • Report tingling, muscle twitching, painful sensations

28. Acoustic Noise Levels Ouch • Earplugs & Headphones • Noise Reduction Rating • 25-30 dB • Combined 5 dB more front row R&R band Tomoyuki et al, Toshiba

29. Acoustic Noise • Lorentz forces acting on gradient coils • Forces & gradient noise level increases with both B0 and gradient strength • Levels for EPI fMRI • Peak 130 dB @ 3T, 110 dB @ 1.5T • Average 90-117 dB(A) • Frequency content varies by sequence • EPI higher average frequency (more read and phase gradients/time) • 3D GRE probably next (short TR) • Spin Echo (depends on TE and TR slices per TR, etc.)

30. Acoustic Noise • Lorentz forces acting on gradient coils • Gradient noise level increases with both B0 and gradient strength • Levels for EPI fMRI • Peak 130 dB @ 3T, 110 dB @ 1.5T • Average 90-117 dB(A) • Frequency content varies by sequence • EPI higher average frequency (more read and phase gradients/time) • 3D GRE probably next (short TR) • Spin Echo (depends on TE and TR slices per TR, etc.)

31. Gradient Noise Management for fMRI • Experimental Designs • Reduce intra- acquisition noise • Reduce inter-acquisition noise • Reduce Noise at source • Hardware changes • Gradient shaping • Passive and Active Noise Reduction • Earplugs, mufflers • Noise reducing headphones Covered in later lectures.

32. Active Noise CancellationHeadphones • Amplitude of sound transmitted to the ear by bone conduction is frequency dependent and maximal at ~2 kHz. • Active noise cancellation systems may be more useful for 1.5T and 2T systems that produce sounds below 1 kHz. • Some 3T scanners produce strong sounds in the 1.5-2.5 kHz frequency range.

33. Larissa Stanberry, U of Washington Additional Equipment

34. Video Projection Approaches LCD Projector, Mirror, & Screen Mirror on RF coil & Screen

35. Stimulus Presentation / Monitoring

36. Additional Equipment E-Prime • Software • Time-Line • Control Stimulus • Monitor Response • Synchronize timing with MRI

37. Additional Equipment • Hardware • Stimulation • Visual • Motor • Auditory • Response • Visual • Motor • Auditory

38. fMRI Personnel • Patient or Volunteer Support • Family • Nurse, physician • MRI Operation • Board Certified Tech • Research Group • PI & collaborators • Associated Equipment Tech • Stimulus presentation, monitoring, etc. • Analysis • PI • Post doc, research assistant, etc. I know this is not following the theme of this chapter, but important.

39. fMRI Study Time 4+ hr (one instance) • New Design • Scanning • Setup • Scans • Take down • Preprocessing • Statistical Analysis 1-1.5 hr/subject 15-20 min 45 min to 1 hr 15 min <2 hr/ subject  variable

40. fMRI Study – Raw Data • Localizer image < 1 MByte • Anatomy image • Same resolution (2562 x 25) > 3 MByte • 3D high resolution (2563) > 30 MByte • Event Related fMRI study • 20 slices/image x 15 images/event x 20 repetitions x 128x128 images ~200 mByte • Reorganizing data into volumes indexed by time ~200 mByte

41. fMRI Study – All Data • Raw Data ~200 mBytes • Motion Correction ~180 mBytes • Other Corrections ~180 mBytes each possibly • Spatial Normalization ~ 30 mBytes • Statistical Analysis • Statistical Parametric Image (128x128x20) < 1 MByte • Statistical Parametric Map (2x SPI) > 1 MByte Total Data per subject can be 0.5-1.0 gBytes

42. Statistical Parametric Mapping Lecture 3 - Chapter 5 Hardware for functional MRI Textbook: Functional MRI an introduction to methods, Peter Jezzard, Paul Matthews, and Stephen Smith Many thanks to those that share their MRI slides online