Magnetic Resonance Imaging

Magnetic field gradients

Gradient echo

Gradient Amplitude Time M xy T 2 * envelope Equal areas

Maxwell Pair Net B field from coils Local fields Current paths B o a3 a Maxwell Pair for dz dB z

Straight wires B o y z Increases B o Reduces B o Straight wires for dy dB z

Conventional Waveform Minimum slew rate waveforms Gradient field: Acoustic noise One of the biggest challenges at high field Acoustic noise –Ear defenders and foam to reduce bone conduction –Gradient system decoupled from bed (and room) –Modification of gradient waveforms –Novel gradient coil designs

Gradients can be combined to produce gradients in arbitrary directions

Slice selection using gradients Sample x x1x1 x2x2  (x 1 )  (x 2 ) BzBz

Slice selection

Imaging in gradients Sample x Frequency Signal (spectrum) x1x1 x2x2 B(x 1 ) B(x 2 ) BzBz 11 22 PROJECTION

Projection Reconstruction

Basic reconstruction X-ray beam is attenuated forming a projection

Projections are collected from various directions

The projection tells you that the beam was attenuated with equal probability anywhere along its path

Projection Reconstruction Diffusion imaging Radial Spin Echo and TSE Scan PR Low sensitivity to motion Low sensitivity to susceptibility artifacts

Projection Reconstruction Sodium Imaging Human Skeletal Muscle: Sodium MR Imaging and Quantification- Potential Applications in Exercise and Disease Constantinides, Gillen, Boada, Pomper, Bottomley, Radiology

K-space Image

K space Units: rad/ m Increasing spatial frequency

Projection Reconstruction

With MRI we can control how we sample k-space Time RF G slice G phase G read kyky kxkx Spin-warp pulse sequence Spin-warp k-space trajectory A B C ABC n n Signal

Phase encoding Phase encoding Readout gradient Initial phase during readout depends on phase encoding amplitude

Phase encoding Initial phase during readout depends on phase encoding amplitude Phase encoding Readout gradient

With MRI we can control how we sample k-space Time RF G slice G phase G read kyky kxkx Spin-warp pulse sequence Spin-warp k-space trajectory A B C ABC n n Signal

Spin warp imaging Wicklow, Washington University, St. Louis

Roger Ordidge, UCL

Ultrashort TE- Tendons

Motion artefacts Steve Keevil UMDS Anesthetic

EPI

Diffusion EPI

Diffusion

EPI- fetus Fetal Brain Placenta Fetal Lung Fetal Liver

EPI- gut

EPI - fMRI Ludman, Mason, O’Donoghue, Morris, Nottingham Cochlear Stimulation