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

Noll MRI Hardware Recall the three magnetic fieldsRecall the three magnetic fields –B 0 –B 1 –G x, G y, G z Also, we need pulse control and data acquisition systemsAlso, we need pulse control and data acquisition systems

Noll Major MRI Components

Noll RF Screen Room Encloses the MRI deviceEncloses the MRI device Shields MRI scanner from electromagnetic noise from computers, radio stations, etc.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.These rooms can also incorporate magnetic shielding to contain the magnetic field.

Noll The Main Field B 0 range: 0.2 T to 7 TB 0 range: 0.2 T to 7 T Higher fields:Higher fields: –Higher SNR –Slightly longer T1’s, –Poorer RF homogeneity –Higher power deposition –Higher cost Typically superconductingTypically superconducting

Noll The Main Field Safety issues: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.”

Noll RF Fields RF coils must generate a rotating (or oscillating) magnetic fieldRF coils must generate a rotating (or oscillating) magnetic field –Ideally uniform application of B 1 RF coils also detect precessing magnetizationRF coils also detect precessing magnetization –Ideally sensitive mainly to tissue of interest –Uniformity is of secondary importance Three main types:Three main types: –Volume Coils –Surface Coils –Arrays Coils (which are basically arrays of surface coils)

Noll RF Coils Array Coil Volume Coils

Noll RF Coils Typical CoilsTypical 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 coilsImproved 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.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.Also allows use of parallel MRI technology.

Noll RF Fields Safety issues: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.)

Noll Gradient Fields High power amplifiers generate magnetic fields in the same direction as B 0, but with variations along x, y and z.High power amplifiers generate magnetic fields in the same direction as B 0, but with variations along x, y and z.

Noll Gradients x-gradient (G x ) y-gradient (G y ) z-gradient (G z )

Noll Gradient Coil Designs Z Gradient X,Y Gradient

Noll Gradient Fields Safety issues: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.

Noll Additional Slides

Noll Typical fMRI Protocol Scout AcquisitionScout Acquisition –Where am I? T1-weighted imagesT1-weighted images –To align structure to fMRI maps T2-weighted imagesT2-weighted images –To rule out pathology fMRI – T2*-weighted, dynamicfMRI – T2*-weighted, dynamic –EPI or spiral High-resolution 3D T1-weighted imagesHigh-resolution 3D T1-weighted images –For segmentation of anatomy