MRI Physics 3: Hardware

1. MRI Physics 3:Hardware Douglas C. Noll Biomedical Engineering University of Michigan

2. MRI Hardware • Recall the three magnetic fields • B0 • B1 • Gx, Gy, Gz • Also, we need pulse control and data acquisition systems

3. Major MRI Components

4. RF Screen Room • Encloses the MRI device • Shields MRI scanner from electromagnetic noise from computers, radio stations, etc. • MRI signals are weak compared to noise sources • Any wires going in/out must be filtered and grounded so they don’t introduce noise • These rooms can also incorporate magnetic shielding to contain the magnetic field.

5. The Main Field • B0 range: 0.2 T to 7 T • Higher fields: • Higher SNR • Slightly longer T1’s, • Poorer RF homogeneity • Higher power deposition • Higher cost • Typically superconducting

6. The Main Field • Safety issues: • Attraction of ferrous objects • Attraction of metallic implants (e.g. cochlear implants, neurostimulators, pacemakers, and poorly designed/manufactured stents, screws, pins, aneurysm clips, etc.) • Attraction of foreign metal objects (metal in eyes, shapnel, ingested ferrous objects). • Affects magnetic switches in pacemakers • Some reports of dizziness, light flashes, unusual tastes, etc. at very high magnetic fields. • The FDA has classified 7 T and under as a “non-significant risk.”

7. RF Fields • RF coils must generate a rotating (or oscillating) magnetic field • Ideally uniform application of B1 • RF coils also detect precessing magnetization • Ideally sensitive mainly to tissue of interest • Uniformity is of secondary importance • Three main types: • Volume Coils • Surface Coils • Arrays Coils (which are basically arrays of surface coils)

8. RF Coils Volume Coils Array Coil

9. RF Coils • Typical Coils • Head Coil (volume birdcage coil) • Body Coil (volume, mostly transmit only) • Surface Coils (e.g. occipital coils for visual system studies, mostly receive only) • Improved SNR vs. volume coils • Head Array (array of surface coils, receive only) • The idea here is that one can receive the benefits of surface coils, but over a larger volume. • Also allows use of parallel MRI technology.

10. RF Fields • Safety issues: • RF heating of the body (FDA limits our power deposition) • Focal RF heating around metals (glasses, jewelry, implants, some tattoos, etc.) can lead to burns • Focal heating near implanted devices e.g. cochlear implants, neurostimulators, pacemakers, and poorly designed/manufactured stents, screws, pins, aneurysm clips, etc.)

11. Gradient Fields • High power amplifiers generate magnetic fields in the same direction as B0, but with variations along x, y and z.

12. Gradients x-gradient (Gx) y-gradient (Gy) z-gradient (Gz)

13. Gradient Coil Designs Z Gradient X,Y Gradient

14. Gradient Fields • Safety issues: • Rapid changes of fields can lead to peripheral nerve stimulation (e.g. like a twitching in the back). FDA guidelines state that stimulation cannot be “painful.” • The gradients are also responsible for the substantial acoustic noise. FDA says it can’t exceed 99 dBA with hearing protection in place.

15. Additional Slides

16. Typical fMRI Protocol • Scout Acquisition • Where am I? • T1-weighted images • To align structure to fMRI maps • T2-weighted images • To rule out pathology • fMRI – T2*-weighted, dynamic • EPI or spiral • High-resolution 3D T1-weighted images • For segmentation of anatomy