General
- Brownian Movement from high concentration to low
- Random movement of water and small molecule due to thermal collision
- Water molecules are distributed between the intracellular and extracellular compartments in a ratio of approximately 3:1 (the exact proportion depends upon the organ of interest) → The balance between these two pools is maintained by energy-dependent Na-K ion pumps → When these pumps fail (as may happen with toxic or hypoxic-ischemic insults) → cellular swelling and/or rupture may occur with dramatic changes in water fractions and diffusivity
- Extracellular water diffuses somewhat more freely than intracellular water → Many tissues contain highly asymmetric structures, such nerve or muscle fiber bundles
- Diffusion anisotropy
- The preferential diffusion of water molecules in certain directions more than others.
- Diffusion times are often prolonged in many non-acute and chronic diseases.
- Remotely injured tissues contain evidence of cellular destruction, including disrupted cytoarchitecture, necrosis, or microcystic degeneration. These injured tissues to have larger extracellular spaces become more "water-like".
- In addition to having prolonged diffusion coefficients, they also frequently show elevations of their T1 and T2 times for the same reasons.
- Marked reduction in tissue diffusion occurs much less commonly than diffusion prolongation, but when it occurs it can have a dramatic effect on the diffusion-weighted MR images, rendering the abnormal areas "light-bulb bright".
- Isotropic material
- Diffusion is same in every direction
- Characterized by a single diffusion coefficient
- Anisotropic material
- Diffusion varies with direction
- Characterized by diffusion tensor, a 3x3 matrix
Process
- DWI is done by using gradient of magnetic field in different plane followed by a 90° RF pulse then 180- pulse.
- If a NMR is static → the phase difference will be zero
- If a NMR is in motion → there will be a phase difference
- You then generate an image of the brain in each plane
- You need at least three planes
- The more planes the better the quality of the image
- Tractography uses 20 planes (DTI)
- You combined the plane into a diffusion weighted image (DWI)
- DWI = T2 + Apparent Diffusion Coefficient map (ADC)
- Methods
- Stejskal and Tanner
- Paired pulsed gradients
- Cause signal loss from diffusing (but not stationary) spins
- Trace and ADC images
- Trace DWI: a set of images created by averaging source images sensitive to diffusion in different directions
- ADC Map: a set of images derived from the Trace DWI where T2 effects have been removed
- Frequently viewed
- The phases of stationary spins are unaffected by the DG pair since any phase accumulation from the first gradient lobe is reversed by the second. Diffusing spins, however, move into different locations between the first and second lobes, falling out of phase and losing signal.
- Sequence of calculating ADC map
- The DW pulse sequence is first run with the DG's turned off or set to a very low value. This generates a set of b0 ("b-zero") images that are T2-weighted and will serve as a baseline for later calculated maps.
- The DW sequence is then run with the DG's turned on individually or in combination and at various strengths. This produces DW source images sensitized to diffusion in multiple different directions.
- The DW source images are combined to produce a set of Trace DW images, the first-line images used for clinical diagnosis.
- An Apparent Diffusion Coefficient (ADC) map is then calculated using the data from the b0 and source images. The ADC map is used to clarify abnormalities seen on the trace images
DWI b-Value
- An operator-selected parameter that defines gradient strength and duration
- It determines the degree of diffusion weighting
- No optimal value, typically 0-1000 s/mm²
- A factor that reflects the strength and timing of the gradients used to generate diffusion-weighted images. The higher the b-value, the stronger the diffusion effects
- In the INS they show as part of the DWI
- B0 image
- Typical diffusion-weighted imaging protocol begins with production of a baseline b0 image obtained with all diffusion-sensitizing gradients turned off. Immediately thereafter, diffusion gradients are applied individually and in various combinations to produce a set of source images sensitized to diffusion along various directions. At least three sets of source images must be obtained.
- B1000 image
- DWI pulse sequences and consists of two strong gradient pulses of magnitude (G) and duration (δ), separated by time interval (Δ). The formula for b, specific to this particular implementation only, is shown in the diagram right.
- The b-value depends on the strength, duration, and spacing of these pulsed gradients. A larger b-value is achieved with increasing the gradient amplitude and duration and by widening the interval between gradient pulses.
What it looks like for a stroke patient
- Stroke has restricted diffusion = low ADC
- DWI displays strength of MR signal when diffusion-sensitizing gradients are applied — Stroke = little dephasing = bright
- ADC map displays of ADC values — Stroke = low ADC = dark
ADC
- Represents the magnitude of water diffusion inside voxels.
- Is called apparent because it is an image produce after removing effects due to
- Blood flow
- Breathing motion
- Other artefactual diffusion
- You get a ADC image by mathematically removing the T2 image.
- That is why ADC is so pixelated, since the maths removes T2 in each pixel.
- ADC is the measured or observed diffusion coefficient obtained from an experiment
- ADC reflects not only true diffusion, but depends on spatial orientation, microscopic perfusion, bulk tissue motion, and pulse sequence timing
T2 shine-through
- Lesions that have very long T2-values may appear bright even though they do not restrict diffusion. This phenomenon, illustrated below, is known as "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 also appear very bright on conventional T2-weighted images serving as a further confirmatory finding.
- DW images are both T2- and diffusion-weighted
- Long T2 lesions can increase DWI signal mimicking restricted diffusion
- Clarified by reviewing ADC images
- Rounded left parietal lesion (a glioma) shows moderate brightness on Trace DW image. This is not due to restricted diffusion, however, as the lesion is also bright on the ADC map (implying increased diffusivity). The T2-weighted SE image confirms the brightness on the Trace image is a T2 "shine-through" effect.
- Lesions can lead to shine through
- Post stoke vasogenic oedema
- Epidermoid cyst
T2 Blackouts
- T2-blackout is the opposite of T2- shine-through
- Lesions with very short T2 (T2*) reduce signal on DWI
- ADC calculation may be erroneous
- An example of T2-blackout is shown below for a subacute hematoma.
- The T2*-weighted b0 image has extremely low signal due to the paramagnetic effects of intracellular deoxyhemoglobin.
- The trace DW image also has a dark center, but this is primarily due to blackout from the T2 (T2*) effects.
- The ADC map shows a scattered mix of bright and dark pixels within the hematoma, demonstrating the computational hazards of estimating ADC values in such situations.
DWI has a major role in the following clinical situations
- Early identification of ischaemic stroke
- Differentiation of acute from chronic stroke
- Differentiation of acute stroke from other stroke mimics
- Differentiation of epidermoid cyst from an arachnoid cyst
- Differentiation of abscess from necrotic tumours
- Assessment of cortical lesions in Creutzfeldt-Jakob disease (CJD)
- Differentiation of herpes encephalitis from diffuse temporal gliomas
- Assessment of the extent of diffuse axonal injury
- Grading of diffuse gliomas and meningiomas
- Assessment of active demyelination
- Grading of prostate lesions (see PIRADS)
- Differentiation between cholesteatoma and otitis media
Clinical features
Category | Examples | Diagnostic features |
Vascular | Infarction (venous or arterial), diffuse hypoxic injury, posterior reversible encephalopathy (PRES) | Cerebral Infarction Restricted diffusion typically occurs within 30-120 minutes after a cerebral infarction, returning to normal by 10-14 days. The principal mechanisms are thought to be • Increase in intracellular water. With cell death or insufficient intracellular energy metabolism → Failure of Na/K ATP pump → cannot maintain ionic gradients → swelling (cytotoxic edema) • Reduction in extracellular space. Cellular swelling → reduce extracellular pathways. • Fragmentation of cellular components. cell death → release of cellular components (membranes, mitochondria, endoplasmic reticulum, proteins, etc) → inc. in both intracellular and extracellular viscosity. |
Neoplastic | Lymphoma, xanthogranuloma of choroid plexus, medulloblastoma, malignant glioma, malignant meningioma, primitive neuroectodermal tumor (PNET), atypical teratoid-rhabdoid tumor, metastases | Neoplasm • Most neoplasms do not restrict diffusion or only change it only mildly. • A few interesting tumors, including lymphomas and some highly malignant gliomas, may show significant restriction of diffusion (bright on DWI dark on ADC) • The ADC values of tumors is inversely correlated with cellularity → Cell walls form barriers to the diffusion of water molecule → more cells, the more barriers → If the packing density is very efficient, then the amount of extracellular space (where diffusion is normally freer) is also reduced. • Uniform small round or hexagonal cells (as seen in lymphomas, medulloblastomas, and PNETs) pack more efficiently into the same volume than larger, irregular cells. So we would expect their ADC values to be correspondingly smaller. |
ㅤ | • Special as although it restricts. It produces high intensity on T2 and DWI but on ADC map it produces ISOINTENSITY rather than low intensity. This is differentiating feature from abscess or lymphoma | |
Infectious | Abscess, empyema, meningoencephalitis (herpes), CJD | Bacterial abscesses and empyema • Demonstrate restricted diffusion, and DWI has proved useful in distinguishing abscesses from necrotic tumors, resolving hematomas, and other fluid-filled cavities. • The restricted diffusion in abscesses is thought to be due to the presence of viscous fluid containing cellular debris, bacteria, inflammatory cells, and mucoid proteins • As treated abscesses mature, central liquefaction occurs and T2-"shine-through" gradually replaces the ADC-driven high signal on DW images. |
Traumatic | Hematoma, diffuse axonal injury (DAI), Wallerian degeneration, status epilepticus, contusion | |
Toxic/Metabolic | Carbon monoxide (CO), drugs (heroin, vigabatrin, carbamazepine, methotrexate), hypoglycemia, hyperglycemia, Wernicke's, congenital biochemical disorders (phenylketonuria, glutaric aciduria, urea cycle defects, maple syrup urine disease, Canavan's, many others) | • Release of excitotoxic amino acid neurotransmitters, especially glutamate. → inc. Na and Ca influx → cell swelling (can restrict diffusion) • If cell dies → release more excitatory neurotransmitters + inc. viscosity → inc. restriction • Intramyelin oedema: if fluid collects between layers myelin → fluid cant move → inc. Restriction |
Demyelinating | Acute disseminated encephalomyelitis (ADEM), osmotic demyelination, multiple sclerosis, delayed post-anoxic encephalopathy, Marchiava-Bignami |
Images
- Lymphoma
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