Alle hoorcolleges uitgewerkt van de CIPT minor deel 2. Ze zijn gesorteerd per week en onderwerp. er staan plaatjes bij en duidelijke uitleg. Aangezien het een open tentamen is, is het belangrijk dat je de stof goed begrijpt. deze samenvatting is het enige wat ik nodig had om te leren.
cijfer: 8...
Week 10: Neuroimaging
MRI contrast
THE MR image does not exist → an unlimited number of variations are possible.
The basis: PD (proton density) = always present in every MRI.
MRI basis:
● T1 and T2 refer to the time between the magnetic pulse and the image taken. These
differences are used to detect different structures on the MRI. They are tissue
properties.
○ T1: fat = light, water = dark.
○ T2: water tissues appear lighter compared to fat tissues. Suppose the grey
matter is lighter than the white matter → T2.
● The most important choices are TE and TR
○ TR: repetition time: short TR gives better T1
○ TE: echo time: long TE gives better T2
Basis of no questions asked, just for understanding
● Signal and image formation: put a patient into a big magnet. The protons (H+) align
with the magnetic field.
● When you apply radio waves (RF pulse) at a good frequency you can change the
orientation of the spins as they absorb energy. After you turn the radio waves off,
they return to equilibrium.
Contrast formation:
● MRI creates a magnetic field, forcing the H+ to align with eachother (picture on the
left). Then, there there is an RF causing the H+ to align against their gradient
(picture on the right). After this citation, the H+ moves back to equilibirium. This
takes different time between the tissues, and that is what you measure.
○ Relaxation of longitudinal component → Mz → M0 = T1 = spin-lattice
○ Relaxation of transverse component → Mxz → 0 = T2 = spin-spin
○ T1 relaxation: LOG increasing curve at different spinning points. Contrast =
(Mz-Mz) of T1 of different tissues. Saturation spins at t=0. At some point,
there is a maximum difference of contrast. Relaxation time:
, ○ T2 relaxation: LOG decreasing at different
spinning points. Excitation spins at t=0. With an
Contrast versus TR & TE: increasing echo time, there is an increasing
contrast difference.
When do you use which contrast:
T1 weighted T2 weighted PD weighted
anatomy, good SNR/unit pathology anatomy/MS lesions
time
additional information tumour, oedema, lesions additional information
pathology (MS) pathology.
Spin-echo/gradient echo:
● Gradient echo (GRE) = T2* contrast = T2 + the effect of the local perturbation of the
magnetic field. This T2* depends on TE
○ Micro-bleeds (iron or calcium) can be seen in gradient echo, T2* weighted.
● Spin echo (SE) or Hahn-echo = refocus macroscopical dephasing. Only spin echo
can provide real T2 contrast. The amount of T2 depends on TE.
Magnetisation preparation pulses:
● We can work with contrast manipulation. Fluid attenuated inversion recovery =
FLAIR.
○ FLAIR = removes the signal of CSF by suppression, resulting in a T2 image
with dark CSF.
○ This is used to identify MS lesions.
● Contrast agents: used to create a better MRI picture.
○ Results in a reduction of T1 and T2 of the water protons → the contrast agent
itself does not produce the MR signal.
■ A shortening of T1 → signal enhancement.
■ High concentration of contrast agent → signal reduction in T2
shortening.
○ Shortening of the T1 →increase the signal of T1-weighted images. Often
used are Gd, Mn ions and iron particles.
, ■ Gd-DTPA molecule: Gd itself is toxic, but the DTPA shell makes it not
toxic to the body. IV injection. Extracellular agent. No BBB passage.
90 min T1/2.
Diffusion MRI
Basics:
● Brownian motion = the random movement of particles in the fluid.
● Factors that influence: temperature and viscosity.
● The diffusion coefficient also depends on the surrounding molecules.
○ Water in a liquid environment (like CSF) → have free diffusion.
○ Water in tissue → diffusion is hindered → smaller diffusion coefficient. At the
same time → smaller distance.
● Diffusion sensitising gradients = short additional magnetic field, varying with the
position.
○ Fixed water molecules = spin feel the same magnetic field, no effect of
gradients.
○ Mobiele water molecules = diffusing through CSF or extracellular space.
Spins feel different fields, resulting in loss of signal. The loss is larger when
the diffusion is faster and when the gradients are stronger.
○ B-value = the strength of diffusion weighting.
○ The apparent diffusion coefficient (ADC) = magnitude of diffusion of water
molecules inside the tissue.
○ DWI = measuring Brownian motion of water molecules within a tissue or
voxel.
○ Occlusion of artery --> high signal on DWI and low on ADC --> faillure of the
Na/K pump --> cell swelling --> cytoktoxi oedema --> restricted diffusion.
■ With an increasing b-value → decreasing ADC
■ ADC changes during development.
● Traveling along/parallel to axons is also called L1 or
axial diffusion → longitudinal along the optic nerve for
example. (D//).
● Strongly hindered perpendicular to nerve direction,
also called L23 or radial diffusivity → transverse
sections of the optic nerve. (D|).
● D (diffusion) is a tensor; you need at least 6 gradient directions to estimate D.
● Echo-planer imaging (EPI) = fast sequence, with a short TE as possible and always
T2 weighted.
● FA = 0 means isoptic = equally
restricted to all directions.
, FA is a measure of WM integrity:
FA = fractional anisotropy. Used to measure connectivity in the brain. Can be derived from
DTI.
● In MS there is demyelination → in the lesion there is a decreased FA.
● An increase of FA → better WM integrity.
● Colouring the FA can indicate the direction.
● Can be combined with a V1→ vector along where the diffusion is the highest.
● Dark bands in healthy volunteers → FA lower in WM → crossing fibres
Tractography = nerve tracts can be visualised using diffusion. This is only the possibility of a
direction.
● Clinical application: when the tumour is pressing the healthy tissue aside, you can
determine the tract. Possible to follow the healthy tissue.
○ Main motor fibres.
Neuroradiology
Computed tomography: CT
Pros:
● Fast (1 min), available 47/7
● God contrast for bone/air/CSF/fat/blood/calcification
● Easy to detect haemorrhage, fractures, ischemia.
○ Modality of choice in the acute setting especially trauma and stroke.
Cons:
● X-ray exposure
● Low diagnostic accuracy detection of small lesions
● Not very useful for characterization of brain lesions
● Beam-hardening artefacts = edges of an object to appear brighter than the centre,
even if the material is the same. → skull base and posterior fossa.
Houndsfield units (HU) = scaling of attenuation of various tissues/materials.
● Water = 0 by definition
● Bone = >1000
● Iodine contrast = 50-150
● Gray matter = 37
● White matter 32
● Fat = - 40
● Air = - 1000 by definition
● Hypodens: appear darker on a CT. Like air, fat, and water
● Hyperdens: appear lighter on a CT. Like blood, calcification and IV contrast
○ Contrast enhancement in
CT: intravenous iodinated
contrast (thicken the blood)
(hyperdense).
○ Enhance visibility/
discrimination in vascular
structures, tumours and
infections.
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