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MRI physics

VM 685 - MRI phyics and applications.
by

Stephen Joslyn

on 31 March 2015

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Transcript of MRI physics

Veterinary Use
B
0
B
0
= 1 excess
= 6 excess
B
0
= 6 excess
RF pulse
RF pulse
T2* relaxation
T 1 relaxation
T1w
T2
T2w
H2O
spin lattice
receiver
Contrast enhancement
Gadolinium chelates
enhance T1 and T2 relaxation
paramagnetic
Superparamagnetic Iron Oxide (SPIO, USPIO)
T2 and T2* signal drop
MR Angiography
Non -enhanced and Gadolinium Enhanced
Tanimoto A. Eur J Radiol. 2006
e.g. Gd-DTPA
Small Animal protocols: intra-cranial
to define lesion margins
and increase detection
@ 1T, only 6 per 1,000,000 are in excess

1 drop of water will still have 15 million billion excess protons available.
1T
42.56 MHz
T2w
FLAIR
STIR
T1w
T2*
T1w
T1w +C
T2 sag
Size <12nm
MW <180 da
B
0
Z axis
Z
y
x
Full 180
90
f
RF pulse
180
T2 relaxation / spin spin
T2*
T2w
T2*w
Spatial Resolution and multiplanar imaging
in phase
out of phase
out of phase
in phase
TR = Repetition time. Time between applied pulses.
TE = Echo time. Time to listen for echo pulse back.
T1, T2* and T2 effects are happening in close timing to each other. Over a couple milliseconds.
Weighting
T1w
T2
T2*
42.56 MHz
size: 60-70nm
mw: ~590 da
Magnetic
Resonance
Imaging
Abundance in tissue!
MRI is the utilisation of Nuclear Magnetic Resonance for the formation of medical images depicting internal structures.
There is no ionizing radiation.
Tissue Contrast
Summary:
Large Magnetic Field needed
RF pulse applied to excite protons
Relaxing protons emit RF echo.
Receiver Coil detects this echo at a specified time.
Bright = High Signal = hypERintense
Dark = Low Signal = hypOintense
T2 = Bright fluid & fat
T1 = Bright fat only
Sequences &
Small Animal protocols: spinal
Equine Imaging Protocols
MRI physics
Magnetic fields and Hydrogen Atoms
Excitation and Relaxation
PD
Fat and Water are hyperintense.
Provides best neuro tissue contrast
Only fat is Hyperintense
Provides homogenous soft tissue contrast.
Pre and Post contrast sequences
Highly susceptible to changes in the magnetic field.
(e.g. Metallic composition or gas)
Suppresses free water molecules (CSF in ventricles), but not bound water molecules (Oedema)
Suppresses Fat signal, but not water.
Visualisation of overall proton density. Hydrated tissues such as neuro tissue will be more hyperintense than bone.
45 MHz
50 MHz
55 MHz
40 MHz
35 MHz
58 µT = Earth's Magnetic Field Strength
0.05 T = Fridge magnet.
0.3T = LAC low field magnet.
1.5T - 3T = Typical Medical Strength Scanners
=
Full transcript