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79 Cards in this Set
- Front
- Back
What does diffusion MR imaging look at
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the micromovement of molecules
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What is it called when a molecule moves in all directions
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isotropic
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What is it called when a molecules movement is confined to a given direction
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anisotropic
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Why is DWI mostly used to analyze the brain
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Most applications of the method are restricted to brain since DWI is intrinsically sensitive to movement; making application to the heart, and abdomen extremely challenging.
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What is a problem with analyzing the spinal cord with DWI
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pulsation artifact
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Are water molecules isotropic or anotropic in gray matter and CSF
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isotropic
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Are water molecules isotropic or anotropic in white matter
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anotropic
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What is diffusion
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Diffusion is a mass transport process arising in nature. It results in molecular or particle mixing without requiring bulk motion.
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What is the distance that a molecule will move
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The diffusion coefficient (DC) is the constant of proportionality between the root-mean-square displacement of a water molecule and the square root of time during which the motion occurred
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What is the root square mean displacement
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it quantifies the fraction of particles that will traverse a certain distance within a particular time frame.
x^2 = 2DΔ where x is mean squared displacement. |
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What happens to the DC when there is a boundary surrounding a tissue
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many tissues show a decrease in the DC when boundaries of varying permeability restrict free diffusion
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What does the ADC measure
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the restriction caused by the permeability of certain boundaries
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What does it mean if the ADC is low
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there is a high level of restriction
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What influences ADC
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The ADC is influenced by the translational movement of water that occurs in the extracellular space via swelling or increased cellularity
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What are some obstacles that water molecules hit inside the body
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cell membranes, proteins, macromolecules, fibers…
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What is Diffusion imaging examining
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Extracellular fluid. Diffusion data provides indirect information about the structure surrounding these water molecules.
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What is an example of water molecules in the body that have free motion
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csf
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What are examples of water molecules that will have restricted isotropic motion
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abscess, tumor at high cell density
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What is meant by restricted anisotropic diffusion
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Certain structured tissues create obstacles that orientate the motion of the water molecules (tendency to displace themselves in one or several particular directions). Diffusion is only restricted in certain spatial directions.
Example: nerve fibers (organization in bundles of axons running in parallel, with concentric layers of myelin restricting transversal diffusion) |
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What does DW MRI aim to look at
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DW-MRI aims at highlighting the differences in water molecule mobility, irrespective of their direction of displacement.
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What does DTI aim to look at
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Diffusion tensor MRI, on the other hand, studies the directions of water molecule motion to determine, for example, whether or not they diffuse in all directions (fractional anisotropy), or attempts to render the direction of a particular diffusion (which can be applied to indirectly reconstituting the nerve fiber trajectory).
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f
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w
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The aim of these diffusion-weighted sequences is to obtain images whose contrast is influenced by the differences in water molecule mobility.
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yes
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What does SE-EPI mean
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Spin Echo ultrafast Echo Planar Imaging
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How is the diffusion imaging done
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This is done by adding diffusion gradients during the preparatory phase of an imaging sequence
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Is SE-EPI T2 weighted
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yes, usually
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Is there a special gradient used in DWI
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yes, diffusion gradients
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What do diffusion gradients look like
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What does the gradient (on the right) look like during the first during the first gradient before the 180 rephasing
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Notice that a specific area of water molecules is being analyzed.
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What does the gradient on the right look like after the second diffusion gradient (meant right on the other one too)
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the gradient is the same for both
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What happens to the water moleucles that are stuck in the middle and unable to move
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the spins of the immobile water molecules between the applications of the two gradients are dephased by the first gradient and rephased by the second.
NOTE THE RESTRICTED SIGNAL IS DARK |
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What is the signal characteristics of when there is restricted diffusion
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restricted diffusion is bright
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What are the 3 types of types of freedom of motion that diffusion imaging looks at
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free diffusion
restricted isotropic diffusion restricted anisotropic diffusion |
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When is the diffusion gradient done
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the preparatory phase of SE-EPI sequence
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What is the point of having 2 different gradients for diffusion imaging
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The basic premise behind the addition of these “diffusion-weighting” gradients is that water molecules that are stationary during the application of the pulse sequence will be unaffected by the diffusion gradients. However, water molecules that move will experience a given amount of phase twist from the first gradient, but will have moved to a different location in physical space by the time the second gradient is applied. As a result, the phase twist provided by the first gradient will not be reversed by the second gradient resulting in a signal loss for those water molecules.
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What are the characteristics of diffusion that we should know about
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Basic elements of a DWI pulse sequence. The diffusion-weighting gradients are of amplitude G, the duration of the gradients is d and the time between the start of the first gradient and the second gradient is Δ.
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Basic elements of a DWI pulse sequence. The diffusion-weighting gradients are of amplitude G, the duration of the gradients is d and the time between the start of the first gradient and the second gradient is Δ.
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What is the duration of a gradient
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d
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What determines the amount of diffusion weighting that is applied during a pulse sequence
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the amplitude
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What happens to the spins of the water molecules from the first gradient
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dephased
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What happens to the spins of the water molecules from the second gradient
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rephased
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What happens to the signal if the water molecule stays in the same location b
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bright
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If the water molecules migrates what happens to the signal of the water molecule from a particular voxel
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it is dark
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How do you determine if there is restricted anisotropic diffusion
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by applying gradients at 90 degrees to eachother
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What happens if there is a lot of fast moving water molecules
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Taking the entire population of water molecules in a voxel, the faster the water molecules diffuse, the more dephased they will be and the weaker the recorded signa
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What amplitude is chosen for slow diffusion
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lower amplitude.
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What is the b-factor
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This is the degree of diffusion weighting of the sequence
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What are 3 characteristics that determine the diffusion weighting (b-factor)
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gradient amplitude
application time time between 2 gradients |
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What does a low ADC value indicate
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high degree of restriction
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How many directions must diffusion gradients be applied
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in atleast 3 directions
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How is ADC determined
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Two diffusion sequences with different b-factors can be used to quantitatively measure the degree of molecular mobility, by calculating the apparent diffusion coefficient (ADC).
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Once ADC values for numerous voxels are obtained what is done
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they are put in to an ADC map/image (like the one that we see at our PACS station)
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So if the ADC value is low what is the signal that we will see
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An ADC hyposignal thus corresponds to a restriction in diffusion.
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When we do imaging how many DWI do we obtain
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3 (i think)
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From the information derived from the DWI images what do are we able to calculate
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ADC
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Within each DWI we obtain how many spatial directions are the gradients applied
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3 (X, Y and Z)
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What signal intensity corresponds to restricted diffusion on diffusion weighted images
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bright
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What is a common B-values used in clinical practice (B-values are the degree of diffusion weighting)
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B- 1000
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What if wanted an imaging that is not diffusion weighted
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B=0
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What is a problem with DWI
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there is going to be a T2 component so this means that that there may be brightness from restriction or brightness from T2 effect
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How does an ADC help the T2 shine through effect
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this does not and will be dark if there is restricted diffusion
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The stronger the b factor the greater the diffusion. What does that mean exactly
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The stronger the gradients, the longer the gradients are applied and the more time between gradients, the greater the b-factor.
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How is the ADC calculated
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twin acquisitions with different b-factors, the ADC can be calculated.
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This is what a diffusion weighted sequence really looks like
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What are signal characteristics that are seen in DWI
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What tissue is most commonly analyzed with DTI
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The microarchitecture specific to nerve tissues causes diffusion anisotropy in the white matter of the brain: water molecule diffusion preferably follows the direction of the fibers and is restricted perpendicularly to the fibers.
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What is the most common application of DWI
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stroke
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How does an acute stroke restrict fluid
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In acute cerebral ischemia, if the cerebral blood flow is lowered, the cell membrane ion pump fails and excess sodium enters the cell, which is followed by a net movement of water from the extracellular to intracellular compartment resulting in cytotoxic edema. Relative to the extracellular space, diffusion of the intracellular water molecules is restricted by intracellular organelles that comprise parts of the cytoarchitecture and the cell membranes
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What are the signal characteristics of DWI and ADC in stroke
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The restricted diffusion results in a decreased ADC and increased signal intensity on diffusion-weighted images
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What is a major difference between DWI and DTI
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the amounts of angles that diffusion gradient is is aquired
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How many spatial directions is the diffusion gradient applied in DWI
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usually 3
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How many direction is the diffusion gradient applied in DTI
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It is done by performing diffusion-weighted acquisitions in at least 6 directions (and far more in angular high resolution imaging), it is possible to extract the diffusion tensor which synthesizes all the data
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Visual representation of how DTI is acquired in many directions
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What is parameters are measured in DTI
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anisotropic coefficient
preferred direction restriction in space |
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What are the signal characteristics of fractional anisotropy
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null if there is isotropic movement
increased signal if there is anisotropic movement |
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What are 3 post processing images that can be obtained from DTI
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Fraction anisotropy map
Main diffusion direction Fiber Tracking |
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What is a limitation of DTI
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precision of direction of movement
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What are new models for DTI that are coming down the pike
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Multitensor model (several diffusion tensors coexisting in a voxel)
Q-ball (S-space): requires a large number of acquisitions in different directions, with a constant b-factor Q-space (Diffusion spectrum imaging) (figure 13.11): the ultimate technique, able to describe fiber crossings, but requiring a high number of acquisitions (129 to 515 !) in different directions and with different b-factors to sample the diffusion equivalent of a 3D k-space |