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
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Relaxation Theory & Measurement - T1 & T294
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Image Registration36
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Analysis of Quantitative MRI54
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Physiology & Contrast Mechanisms for mapping brain activity58
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Bolus MR/CT37
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Magnetisation Transfer Theory, Measurement and Analysis60
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Arterial Spin Labelling59
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Experimental Design & Data Acquisition for Mapping Brain Activity (Block Design)93
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The General Linear Model and Single-Subject & Group Analyses45
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Resting State & Event-Related fMRI37
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Voxel-Based Morphometry82
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Principles of ROI volumetry26
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An Overview of Medical Image Segmentation33
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Connectivity: Dynamic Causal Modelling (DCM) for fMRI67
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Diffusion-Weighted Imaging (DWI)89
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Diffusion Tensor Imaging48
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Analysis, Tractography and Applications in Diffusion Imaging47
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Study Design: From group comparisons to individual predictions24
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Multi-Modal Imaging - Motivation, Synchronicity, Instrumentation & Data Quality Issues34
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Data Analysis - MRI/fMRI/DTI23
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Fusion, Co-Registration, Technical Issues & Limitations32
