Imaging
- 5 questions → 30 min per question
Contents
MRI-contrast principles ........................................................................................................................... 2
MR imaging principles........................................................................................................................... 19
Flow effects, flow imaging, angiography .............................................................................................. 57
Diffusion ................................................................................................................................................ 73
Contrast agents and ME ........................................................................................................................ 88
Functional MRI .................................................................................................................................... 108
Perfusion Bolus Tracking ..................................................................................................................... 132
Perfusion ASL ...................................................................................................................................... 150
Molecular imaging: use of radiotracers in NS ..................................................................................... 170
Intro to CT and MRI ............................................................................................................................. 191
Neuroradiological anatomy of the brain............................................................................................. 197
Neuroradiological anatomy of the spine and spinal cord ................................................................... 204
MR-angiography principles and application ....................................................................................... 206
Diffusion weighted imaging ................................................................................................................ 212
MR-spectroscopy: principles and application ..................................................................................... 216
Future of spinal imaging: plain films ................................................................................................... 219
Update on spine imaging: the way to move forward ......................................................................... 227
Imaging in stroke: CT-perfusion .......................................................................................................... 234
Introduction to functional MRI ........................................................................................................... 241
Neuro-angiography ............................................................................................................................. 245
CT-angiography ................................................................................................................................... 254
General knowledge ............................................................................................................................. 275
Aims
1
, MRI-contrast principles
Good animal model: transgenic animal (rat, mouse, bird,…) eg mouse only lives for 2 years, so it is
easy to follow the full progression of the disease. Birds are used for to study neuroplasticity (see bird
song – fMRI).
Different diagnostical techniques
- With EM waves (different frequencies)
o X-ray imaging such as RX and computerized tomography
→ CT uses electromagnetic waves which have a low wavelength and a high
frequency (high energy). This high energy may be kind of worrying because this may
ionize DNA and be harmful for the patient
o Radiowaves: MRI/spectroscopy
o Gamma-wave = nuclear medicine such as PET, SPECT
- Without EM waves: ultrasound, not often used on brain because it is very hard to pass
through skull (only maybe on very young babies because of the fontanelle).
Spectrum of EM waves
E = hf
- Energy (E)
- Planck’s constant (h) = 6,63*10-34 Js
- Frequency (f)
The higher the frequency, the higher the energy (the more
radioprotection becomes an issue = more safety is necessary).
MRI uses energy of radio-waves, is not dangerous. There is no
radiation. Contrary, CT is dangerous and can not be repeated
daily.
c = f λ (c = 3108 m/s)
Introduction of MRI
- MRI: provides info about the amount of protons (the hydrogen atoms in a water molecule)
- Proton density = hydrogen nuclei density + interactions of these nuclei with surrounding
molecules (due to tissue specific relaxation times T1,
T2,…)
- More protons = more signal
Example: T2-weighted image (=water-image) of hydrocephalus
(enlarged ventricles – these are coloured)
- T2-image shows fluid in models
- Clearly shows that there is more liquid in knock-out (KO)
model compared to WT
Advantages MRI
- Provides different contrasts that you can measure to provide specific information. Different
measures can be obtained (proton density, T1-weighted image, T2 weighted image)
- Use radiofrequency waves, which have a high wavelength, but low frequency (low energy) –
not harmful
- You can follow the progress of the disease because it is in vivo
2
,Pixel vs voxel
Another advantage of MRI → 3D images.
- 2D picture: pixels with different grey level intensities that vary along the image
→ the more pixels, the more detailed the image (the higher the resolutionà
- 3D picture (such as a body): voxels = volume elements specifying the signal intensity
1H-MR images
What kind of info do MR images provide:
- Proton density + interactions of these nuclei with surrounding molecules (T1, T2, T2*)
- Free diffusion water nuclei (diffusion coefficient)
- Tissue perfusion (tumor, cerebral blood flow)
- Blood volume, blood flow
- Angiogram: visualisation of blood vessels (with or without contrast agents)
- Activated brain regions – fMRI eg differences in activity in the primary and
secondary auditory region depending on what the bird hears (white noise,
Bach, conspecific bird song). Because when you learn a sound, it becomes
crystallized (memorized) in the brain → different activity in brain.
Different imaging sequences provide anatomical, physiological, functional, or molecular information
(eg migration of labelled stem cells). MRI is mostly used on soft tissue (such as head, neck and spinal
regions of the body), because hydrogen atoms are needed in liquid form. Not very useful for imaging
of bone tissue.
Synopsis of MRI
1. Subject is exposed to a big static magnetic field (0,2 T vet → 0,5 T → 3 T human → 11,5 T
small animal). The magnetic field B0 is created by a superconducting magnet, that consists of
many coils through which a current is passed (rechterhand regel to know the direction of the
field). The higher the current, the higher the magnetic field.
2. Superconductivity: maintaining such a large magnetic field requires a lot of energy, which is
accomplished by superconductivity, or reducing the electrical resistance in the wires to
almost zero. To do this, the wires are continually bathed in liquid helium at 4 Kelvin. This
cold is insulated by a vacuum. While superconductive magnets are expensive, the strong
magnetic field allows for the highest-quality imaging, and superconductivity keeps the
system economical to operate. Once the current is turned off, the magnetic field will always
be on. (DANGEROUS)
3. A dedicated coil (around part of the body that needs to be imaged) transmits radio waves
briefly (2 – 10 ms). The coil is immersed in liquid helium at a very low temperature (4K) so
there is no longer an electrical resistance = superconducting magnet.
The helium is slightly consumed so a refill is necessary. Sometimes there is a compression
pump on the MRI that will do that automatically. Danger of MRI: the magnetic field is always
on and the force is very high.
4. The radio-wave transmitter is then turned off
5. Time-varying magnetic fields are applied (position encoding)
6. Radio-waves re-transmitted by subject are received and read
7. The acquired RF data is converted to an image
Normally magnetic field is horizontal but to make it easier it will be drawn vertically
3
, What is needed for NMR
- Superconducting magnet
- 3 gradient magnets that have a much lower strength (compared to main
magnetic field). The gradient magnets create a variable field, which
allows position encoding to create the image
- Set of coils that transmit RF waves into the patients body. There are
different coils depending in the body part that needs to be imaged.
These coils usually conform to the contour of the body part being
imaged, or at least reside very close to it during the exam. The RF-waves
are transmitted to the body parts and bring the H atoms out of
equilibrium. The body sends RF back which are then converted into data
and an image.
:
Normal situation: atoms consist of a nucleus and electrons. In the nucleus there are positively
charged protons. These protons are constantly spinning around their own axis and they posses a
spin, which is the proton’s intrinsic angular momentum L (which is constant). Only atoms with an
uneven atom number have this angular momentum and can be used for MRI.
NMR: nuclei in a strong constant magnetic field B0 are perturbed by a weak oscillating magnetic field
and respond by producing an EM-signal with a frequency characteristic of the magnetic field of the
nucleus. The resonance frequency is directly proportional to the strength of the magnetic field B0.
The principle of NMR involves 3 sequential steps:
1. The alignment (polarization) the magnetic nuclear spins in an applied, constant magnetic
field B0
2. The perturbation of this alignment of the nuclear spins by a weak oscillating magnetic field =
radio-frequency (RF) pulse. The oscillation frequency required for significant perturbation
depends on B0 and the nuclei of observation
3. The detection of the NMR signal during or after the RF pulse, due to the voltage induced in a
detection coil by precession of the nuclear spins around B0. After an RF pulse, precession
usually occurs with the Larmor frequency and does not involve transitions between spin
states or energy levels
The two magnetic fields are usually perpendicular to each other as this maximizes the signal.
MRI is no longer called NMR (which is not dangerous) because it was confused with nuclear imaging
(which is dangerous).
?
Nucleus needs to have 2 proportions: spins and a charge
Nuclei are made of protons and neutron
- Both have spin ½
- Protons have a charge, neutrons do not
Pairs of spins tend to cancel each other out, so only
atoms with an odd number of protons or neutrons have
spin. Only they have an intrinsic nuclear magnetic
moment and angular momentum.
Hydrogen atoms are mostly used because of their high biological abundance.
Phosphorus is often used for metabolic imaging. Few other nuclei may be used for specialised
imaging purposes (e.g. 13C, 31P).
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