KINESIOL 1Y03 Lecture Notes - Lecture 16: Ionizing Radiation, Electrical Resistance And Conductance, Teratology
23 Jun 2018
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Introduction to MRI
- MRI DETECTS THE HYDROGEN NUCLEUS USING STATIC AND RF MAGNETIC FIELDS
- MAGNETIC FIELD GRADIENT PULSES LOCALISE THE SIGNAL TO PRODUCE THE IMAGE
- IMAGE CONTRAST DEPENDS ON T1 AND T2 RELAXATION TIMES AND MOLECULAR
DIFFUSION
- MAGNETS ARE GENERALLY 1.5-3T, BUT TECHNOLOGY IS DEVELOPING UPWARDS
- THERE ARE MINOR VIO-EFFECTS ACCUTELY BUT ONLY HAZARD POSED BY ACTIVE IMLPANTS
AND FERROMAGNETIC OBJECTS IN SUITE
The nucleus of hydrogen is a single proton which has angular momentum, thus creating a
small magnetic moment (µ).
When a magnetic force is paplied, it continues spinning but also rotates around the
magnetic field. The frequency of this = Larmor frequency (MHz) = γ x field (T) where gamma
= the gyro-magnetic ratio, which is 42
In the field, protons can either spin ‘wuith the fie;’ or ‘aginast it’. This leads to it being in two
rotations: high energy or low energy, with the difference between the two = static energy.
The opposite spins on the protons creates many vectors which you can simplify and call ‘M’
using vector maths
When a patient is in an MRI scanner, they are being magnetised – you are forcing the
magnetic moment of their tissue to move away from its axis, which creates a currency in a
coil, read as the MR signal.
The RF pulse must be applied at the Larmor frequency in order for the magnetisation of
tissue
There are different types of image which can be created depending on the signal used: a
proton-density image (PD), T1-weighted image and T2-weighted image.
Spin echo
= standard means of MRI with most reliable image quality
Has long TR
Longer scan times (can be speeded up by fast/turbo spin echo - fastest type is echo planar
imaging [EPI])
Spin echo is a means to reverse dephasing of T2 (signal lost as energy runs out as you leave
it)
Time between the RF pulse and the echo = TE = controls the T2 weighting
Time between RF pulses = TR = controls the T1 weighting
T1 = longitudinal = a measure of how long the RF energy stays in tissues (a long time for
fluids as molecules further apart). Hence oedema/increased vascularisation causes RF
energy to stay in tissue longer
T2 = transverse = measure of how long the signal remains (how long the echo reverberates)
find more resources at oneclass.com
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Document Summary
Mri detects the hydrogen nucleus using static and rf magnetic fields. Magnetic field gradient pulses localise the signal to produce the image. Image contrast depends on t1 and t2 relaxation times and molecular. Magnets are generally 1. 5-3t, but technology is developing upwards. There are minor vio-effects accutely but only hazard posed by active imlpants. The nucleus of hydrogen is a single proton which has angular momentum, thus creating a small magnetic moment ( ). When a magnetic force is paplied, it continues spinning but also rotates around the magnetic field. The frequency of this = larmor frequency (mhz) = x field (t) where gamma. In the field, protons can either spin wuith the fie;" or aginast it". This leads to it being in two rotations: high energy or low energy, with the difference between the two = static energy. The opposite spins on the protons creates many vectors which you can simplify and call m" using vector maths.
