Magnetic Resonance Imaging

Magnetic field gradients

Z

Xdx

dBzdz

dBz

Gradient echo

dx

dBz

x

Bz(x)

Bo

0

Mxy in rotating frame:

fast slow

t =0

t =T

t =2T

x

T2T

t

Gradient

Gradient echo

GradientAmplitude

Time

Mxy

T2* envelope

Equal areas

Slice selection using gradients

Sample

xx1 x2

(x1)(x2)

Bz dxdBG z

).(

).(

.)(

202

101

0

xGB

xGB

xGBxB

Slice selection

FT

t

FT

Sinc envelope(shown truncated)

o

Carrier (oscillator) Frequency

o

Gradients can be combined to produce gradients in arbitrary directions

Gy

Gx

G

sin

cos

GG

GG

y

x

Imaging in gradients

Sample

x

Frequency

Signal (spectrum)

x1 x2

B(x1)

B(x2)

Bz

1 2

dxdBG z

).(

).(

.)(

202

101

0

xGB

xGB

xGBxB

PROJECTION

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

2

k Units: rad/ m

Increasing spatial frequency

Projection Reconstruction

Time

RF

Gslice

Gread

Signal

ky

kx

pulse sequence k-space trajectory

Typically Gz

Typically G

Repeat for n different values of

With MRI we can control how we sample k-space

Time

RF

Gslice

Gphase

Gread

ky

kx

Spin-warppulse sequence

Spin-warpk-space trajectory

A

B C

A B C

123

n

123

n

Signal

Spin warp imaging

Wicklow, Washington University, St. Louis

Roger Ordidge, UCL

Ultrashort TE- Tendons

Motion artefacts

Steve Keevil UMDS

Anesthetic

EPI

EPI pulse sequence

RF

Gslice

Gphase

Gread

Time

Repeat 128 times

A B C

A

B C

kx

ky

EPI k-space trajectory

Diffusion EPI

Diffusion

Fast spin echo

T2 weighted

Turbo/Fast field echo

T1 weighted

Modulus

Phase

TFE for Phase v T2* contrast

T1 Contrast: MPRAGE & IR-TSE

MPRAGE: 0.5 mm isotropic (11 mins) 384 x 384 x 200

IR-TSE: 0.25 x 0.25 x 1.5 mm3

Numerical Simulations

• PNS is associated with hotspots of electric field (E) or current density (J)

• Get “hot-spots” of |E|/|J| in low/high conductivity tissues.

Gradient safety: Peripheral nerve stimulation

JJ

E

Head-centred coronal slice y = 0

Conventional WaveformConventional Waveform Minimum slew rate waveformsMinimum 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