Diffusion Tensor Imaging Flashcards
What is DTI?
DTI technique was first introduced by Peter Basser in 1994. It is an improved version of conventional MRI wherein signals are solely generated from the movement of water molecules. The term ‘diffusion’ denotes random thermal motion of water molecules. In other words, DTI uses the diffusion of water as a probe to determine the anatomy of a brain network, which basically provides information on static anatomy that is not influenced by brain functions.
What does diffusion tensor describe
he magnitude, the degree of anisotropy, and the orientation of diffusion anisotropy
What can be obtained using diffusion anisotropy and the prinicipal diffusion directions?
Estimates of white matter connectivity patterns in the brain from white matter connectivity
What was the application of diffusion tensor to describe anisotropic diffusion behaviour introduced by ?
Basser et al
What happens in the model of DTI?
Diffusion is described by a multivariate normal distribution which describes the covariance of diffusion placements in 3D normalised by diffusion time.The diagonal elements (Dii > 0) are the diffusion variances along the x, y and z axes, and the off-diagonal elements are the covariance terms and are symmetric about the diagonal (Dij = Dji). Diagonalization of the diffusion tensor yields the eigenvalues (l1, l2, l3) and corresponding eigenvectors (ê1, ê2, ê3) of the diffusion tensor, which describe the directions and apparent diffusivities along the axes of principle diffusion.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2041910/
How can the diffusion tensor be visualised?
using an ellipsoid with the eigenvectors defining the directions of the principle axes and the ellipsoidal radii defined by the eigenvalues
When is diffusion considered isotropic?
When the eigenvalues are nearly equal (e.g., l1 > l2 > l3).
When is the diffusion tensor considered anisotropic?
when the eigenvalues are significantly different in magnitude (e.g., l1 > l2 > l3).
What may the eigenvalue magnitudes be affected by?
changes in local tissue microstructure with many types of tissue injury, disease or normal physiological changes (i.e., aging). Thus, the diffusion tensor is a sensitive probe for characterizing both normal and abnormal tissue microstructure.
What is required to measure the full diffusion tensor?
A minimum of six non-collinear diffusion encoding directions are required to measure the full diffusion tensorThe selection of tensor encoding directions is critical for accurate and unbiased assessment of diffusion tensor measures.
What are the artefacts observed in DTI?
- B0 inhomogeneities
- Subject motion
- T2 shine/mask through
- Eddy currents
- Physiological noise
- Eddy currents and B0 inhomogeneities are particularly important for diffusion MRI and subject motion
What is B0 inhomogeneities?
- We need inhomogeneity of the static magnetic field
- Use controlled magnetic field inhomogeneity to encode the position of the water molecule and then to decode it
- Happens independently on the specific read out you do
- Include a B0 map acquisition during your scan
- It should be noted that in order to correct for EPI distortion, additional data need to be acquired, either additional images for b0 mapping (Jezzard and Balaban, 1995) or a structural image with proper contrast to be used as a target for registration
- b0 mapping may be more robust in characterizing deformations in regions with large susceptibility variations.
What is subject motion?
- Disrupts the contrast of the image
- The motion encodes between directions
- The main consequence of motion is severe ghosting in the images
- With single-shot diffusion MRI, the main consequence of motion is image misregistration, which will corrupt diffusion-derived quantities computed on a voxel-by-voxel basis from different DWIs
- Motion-related misregistration artifacts have more heterogeneous consequences than eddy current misregistration artifacts
- In some cases, they may result in regions of spurious anisotropy at the periphery of the brain that are easy to identify on anisotropy maps or DEC maps.
- In other cases, they may produce a widespread increase or decrease in anisotropy and abnormalities in the measured diffusivity
- The effects of subject motion may have different manifestations depending on the type of motion. The effects of an isolated abrupt movement of the head would be different from those of a progressive small drift in the head position during the scan
What is T2 shine through?
- Lesions that restrict diffusion (strokes, abscesses etc) lower the ADC and appear bright
- Substances with unrestricted diffusion and high ADCs (like cerebrospinal fluid) appear dark
- Trace DW images are both diffusion- and T2-weighted
- Lesions that have very long T2-values may appear bright even though they do not restrict diffusion T2’ shine through
- Whenever a bright lesion is encountered on a Trace DW image, the ADC map should be inspected to look for a corresponding area of low signal (restricted diffusion)
- Such lesions will appear very bright on conventional T2-weighted images serving as a further confirmatory finding
- ADC maps help dis-entangle artefacts from actual tissue property
What happens if T2 mask is too low?
Get signal drop
What is signal drop
If you take a collection of diffusion and take the log and compote slope the ADC tells you the tissue underlying is not just free water – there is actual tissue
• If the T2 is too slow/low – single signal drop that doesn’t exist
What are eddy currents?
- When gradients are rapidly switched on and off, currents are induced in various conducting parts of the magnet, such as the cryostat shields, the main magnet windings, as well as shim-, gradient-, and radiofrequency (RF)-coils
- These currents, known as eddy currents (EC), will act against the field changes producing them and will change the effective gradient fields that are played out from the desired ones
- The magnitudes of these currents vary, and they decay on different time scales, ranging from milliseconds to seconds, and with different amplitudes depending on where they originate from in the magnet as well as the symmetries in magnet structure
- When integrated over the duration of the EPI readout, these ECs are big enough to affect both the trajectory and k-space phase during the slow k-space traversal along the phase-encoding direction• The image artifacts arising from eddy currents depend on the pulse sequence parameters and the spectrum of EC decay rates
What are the two features of eddy currents?
- They occur at the boundary – they are hyperintensity and then they deform the brain
- Deformation is different from the magnetic field deformation
What happens if you have a varying magnetic field across magnetic substance?
It generates on the surface electrical currents
what do Eddy currents generate?
Magnetic field that counteract with diffusion gradients
Why is the actual artefact nature caused by short-term ECs more complicated?
the involvement of multiple exponentially decaying currents of different time constants and amplitude, which together both affect the k-space trajectory and induce variable-rate phase accruals during the readout that do not have a simple corresponding shape in the image domain
How can you mitigate EC currents?
- Calibration methods, performed on phantoms prior to the examination
- Modifications of the diffusion-weighted EPI sequence
- Post-processing methods, using only the DTI data itself
What are physiological noise (e.g. cardiac pulsation)?
- Pulsation of the fluids in the brain
- Compensate by cardiac triggering
- Trigger acquisition by cardiac rhythm – see patterns in the signal intensity going up and down that mirror cardiac frequency Enzmann and Pelc (1992) studied brain motion during the cardiac cycle using a pulse oxymeter on a finger and found that brain motion is relatively small during the first 25% of the cardiac cycle
- Typical manifestations were severe signal dropouts in the periventricular areas in the center of the brain and thalamic regions
- the main effect of cardiac pulsation in the DWIs is exaggerated signal loss in certain regions
- Signal dropouts affect mainly images acquired at the peak of the systolic phase - regions known to have large velocity variations during the cardiac cycle
What are the applications of tractography?
- Diffusivity and anisotropy useful biomarkers
- Diffusivity in stroke
- Anisotropy in, for example, dementia or multiple sclerosis
- Cognitive ability
What is streamline tractography?
- Collect the DTI data
- Measure the diffusion tensor
- Diagnoses it and find the main direction
- Plot the main directions in each voxel and look at row of first neighbours
- It is not precise
- Tractography is used a lot in surgery planning specifically for brain tumour surgery
What are limitations of tractography?
- No quantitative information
- Complex white-matter architecture (branching, crossing)
- False positives and false negatives
What is probabilistic tractography?
- Model uncertainity of fiber orientations
- Repeat:
- Sample orientations
- Track a streamline - Count streamline in each voxel
What is microstructure imaging?
Diffusivity and anisotropy non-specific.
Radius, density, permeability, orientation distribution, all affect dispersion.
Direct measurements
Better biomarkers
Better connectivity
What artefact are important for diffusion MRI?
- Eddy currents
- B0 inhomogeneities
- Subject motion
B0 injomogeneities
- We need inhomogeneity of the static magnetic field
- Use controlled magnetic field inhomogeneity to encode position of the water molecules and then to decode it
- Locate water molecules in the wrong place
Why is subject motion a problem?
It disrupts the contrast of the image
The motion occurs between directions
Can correct is posteriorly but not completely - it depends on the gravity of the motion
When is T2 shine through not problematic?
If you process the image and perform the diagnosis/evaluation on the processed image
Hyper-intensity suggests more restricted water
Artefact - high T2 signal that propagates into the diffusion image
What does the quantitative map of the ADC help with?
Disentangle artefacts from actual tissue properties
What is T2 mask?
If the T2 is too low - there is a signal drop - in ADC map you don’t have a signal drop
Signal drop that doesn’t exist
Where does Eddy currents occur?
Majority at the boundary
What are the two features of Eddy currents?
- They mostly occur at the boundary
- They are hyper intensity
- They deform the brain
Deformation depends on the diffusion gradients - one direction shrinks the brain in one
What is Eddy currents?
Tissue is paramagnetic - when you have a varying magnetic field across a paramagnetic substance - it generates on the surface spurous currents (electrical currents) which are the Eddy currents
What does the Eddy currents generate?
Spurous magnetic fields that counteract with diffusion gradients
They are strong if you use high b values
The centre of the image is not affected much by it - surface currents
What is physiological noise?
Pulsation of the fluids in the brain
Compensate for it by cardiac triggering - trigger acquisition with cardiac rhythm - see pattern in signal intensity that mirror cardiac frequency
High b value - 3000-5000
What can T2 give?
Information about micro molecular content
What can T1 give ?
Detailed map about myelin
What is tractography used for?
Surgical planning
Brain tumour surgery
What is Trace-DW image directly and inversely related to?
Directly - T2
Inversely - ADC
What is T2- blackout?
Lesions with very short T2 (or T2*) value reduce signal intensity in the DW image, potentially masking or destroying its diffusion sensitivity
Why does the T2*-weighted b0 image have an extremely low signal?
Due to the paramagnetic effects of intracellular deoxyhaemoglobin
What is the DW image strongly dependent on?
Diffusion but also reflects underlying T2, spin density and T1 effects
What happens when the intrinsic T2 of a lesion is long?
T2 effects may spill over into the DW image making it appear bright and thus mimicking the restricted diffusion
What happens when T2 (or T2*) is very short?
The opposite occurs, with resulting decrease in signal intensity on the DW image (“T2-blackout”)
