Magnetic Resonance Imaging: The Diffusion and ADC Imaging way
This document provides an overview of magnetic resonance imaging (MRI) diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping. It discusses the history and physics behind DWI/ADC, how images are formed, and what tissue contrast represents restricted versus increased diffusion. Examples of clinical applications are given for evaluating brain lesions, tumors, and monitoring treatment response. DWI/ADC is shown to be more sensitive than conventional MRI for acute stroke and useful for distinguishing abscesses from tumors. Quantitative ADC mapping can help assess chemotherapy response in cancers like osteosarcoma. Case examples in human and veterinary patients demonstrate the appearance of various pathologies with DWI
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2. Outline
Basics (very) of MRI-DWI and ADC
physics
Image contrast formation
DWI in brain lesions and oncology
Cases
Human cases (best examples, biopsy)
Vet medicine cases (not confirmed, wlel most
of them)
3. Conventional Magnetic Resonance: Magnetic Nuclei.
Image contrast based on proton behavior in the
tissue
No magnetic field
Applied magnetic field
•Net Magnetization Vector (M)
•Increases as Bo Increases
•High field strength – better signal
•Natural precessional frequency
•It varies with field strength (ω0=B0 x γ)
4. Net magnetization vector
This net magnetization
becomes the source of our MR signal and is
used to produce MR images.
Absorption of RF energy.
5. Longitudinal relaxation or T1
relaxation
As energy is absorbed from the RF pulse, the net magnetization
rotates away from the longitudinal direction. The amount of rotation
(termed the flip angle) depends on the strength and duration of the
RF pulse.
Longitudinal relaxation or T1 relaxation: The rate at
which this longitudinal magnetization grows back is
different for protons associated with different tissues
Fundamental source of contrast in T1-weighted images.
7. During the RF pulse, the protons begin to precess
together (they become “in phase”).
Immediately after the 90° RF pulse, the protons are still
in phase but begin to dephase due to several effects
8. Different tissues have different values of T2 and
dephase at different rates.
Different tissues have different rates of T2
relaxation.
T2-weighted contrast.
Image is obtained at a time when the relaxation
curves are widely separated
T2-weighted contrast will be maximized.
9. How the image is acquired in
Conventional MRI : Pulse sequences/ the
TR’s and TE’s
Pulse
sequences
Spin echo
Inversion
recovery
T1, T2, PD
FLAIR
STIR
Gradient echo
T1 and T2
Saturation
sequence
Fat Sat
Water Sat
T1 and T2
10.
11. MRI Diffusion-Weighted Imaging:
a little bit of history first (by knowing our history we
will built a better future!)
History of Diffusion of molecules
In 1827, Robert Brown used a microscope to observe the
continuous random motion of pollen grains suspended in
water. Then A. Einstein published paper explaining that
pollen was being moved by water molecules in motion
This movement, later named Brownian motion in his honor,
is the cause of molecular diffusion.
Diffusion of molecules in a liquid medium
Molecules in a liquid that is contained within a small cavity will
diffuse randomly until they encounter the walls: diffusion will be
unrestricted.
Diffusion becomes increasingly restricted as the molecules
encounter the walls
12. •Denis Le Bihan et al described how the “microscopic random
translational motion” of molecules in fluid could be used to
obtain physiologic information:
1. The water molecules found in tissues are either intracellular, extracellular, or
in a vessel.
1. Diffusion of intracellular water molecules is impeded by organelles and the
cell membrane.
2. Diffusion of extracellular water is affected by the cellularity of the tissue,
tissue tracts, and the boundaries of the tissue compartments
MRI Diffusion-Weighted Imaging:
a little bit of history first
Normal liver Liver tumor
13. •In 1965 first description of DWI
•In 1986 diffusion MRI : diagnostic tool for neurologic disorders.
•Denis Le Bihan et al described how the “microscopic random
translational motion” of molecules in fluid could be used to
obtain physiologic information:
MRI Diffusion-Weighted Imaging:
a little bit of history first
Normal liver Liver tumor
14. MRI Diffusion-Weighted Imaging:
a little bit of history first
In 1990, Michael Moseley et al published
an article on early detection of regional
cerebral ischemia in cats and compared
routine T2-weighted MRI, DWI, and
magnetic resonance spectroscopy
15. MR Diffusion weighted images
Signal is based on motion of water
molecules
Molecular motion leads to loss of
signal
A strong MRI signal comes from
tissues with stationary molecules
The Apparent Diffusion Coefficient
(ADC) can be measured to obtain a
quantitative evaluation
16. Pulsed Gradient in Diffusion-weighted MR
Imaging for image formation
Two strong gradient pulses are used/applied that allow controlled
diffusion weighting, according to the following equation:
The degree of diffusion-weighting applied
is indicated by the b-value (measured in s
mm−2).
17. Pulsed Gradient in Diffusion-weighted MR
Imaging for image formation
Two strong gradient pulses are used/applied that allow controlled
diffusion weighting, according to the following equation:
The degree of diffusion-weighting applied is indicated by the b-
value (measured in s mm−2).
18. Tissues with highly mobile water, such as cerebrospinal fluid (CSF)
(strong diffusion) appear dark due to dephasing part of the
contributing spins.
Hyperintense areas = reduced diffusion
However, the hyperintense lesion on a diffusion-weighted image
may reflect a strong T2 effect (aka: T2 "shine-through" effect)
instead of reduced diffusion.
MRI Diffusion-Weighted Imaging
Image contrast:
1. tissue cellularity
2. integrity of cell membranes
20. The Tricky part: T2 shine through
Areas of restricted diffusion may appear bright
in the DWI sequence: false positive for real
leison
Apparent diffusion coefficient
(ADC) is a measure of the magnitude
of diffusion (of water molecules) within
tissue, and is commonly
clinically calculated using MRI
with diffusion weighted imaging (DWI)
21. The Tricky part:
Areas of increased diffusion:
May appear hyperintense, isointense,or
hypointense on DWI images depending on the
strength of the T2 and diffusion components,
But will appear hyperintense on the ADC
map
22. •To eliminate T2 shine through diffusion coefficient maps
can be calculated.
•A diffusion map can be calculated by combining at least
two diffusion-weighted images that are differently
sensitized to diffusion but remain identical with respect
to the other parameters
MRI DWI vs ADC
ADC image = -1/b ln (DW image/T2W image)
23. DWI/ADC-indications:
Mandatory in all patients referred with a suspicion
of stroke or cerebrovascular disease
• Any cystic lesions (e.g., to differentiate abscess
from necrotic tumor, or epidermoid from arachnoid
cyst)
• Trauma to detect diffuse axonal injury (DAI) and
hemorrhagic lesions; findings on DWI are believed
to correlate closely with outcome
• Brain tumors to assess cell density
• The modus operandi should be: “diffusion
imaging for all neuro-patients”
25. Stroke
Theories for decreased diffusion in acute stroke
1. Failure of Na+/K+ ATPase and other ionic pumps with loss of
ionic gradients across membranes. This leads to a massive shift
of water from the extracellular into the intracellular compartment
(cytotoxic edema)
2. Decrease in the size of the extracellular space due to fluid
shifts and cell swelling with a resultant increase in extracellular
space tortuosity
Increased intracellular viscosity and intracellular space tortuosity
secondary to breakdown of organelles and the cytoskeleton
Increased cell membrane permeability
26. Acute Infarction. Human data
Early (within the first 6 hours after stroke)
CT signs of brain ischemia are subtle and difficult to detect.
On conventional MR images, early (within the first 6 hours after
stroke) morphologic signs (produced by tissue swelling) are
detected in 50% of acute infarctions; however, signal abnormalities
are not detected.
With diffusion-weighted imaging of acute infarction (within the first
6 hours after stroke), 94% sensitivity and 100% specificity have been
reported.
32. MRI Diffusion-Weighted
Imaging in oncology
Measures random motion of water in tissue
Motion decreases in cellular tissue (tumor)
Motion increases in necrosis or apoptosis (treated
tumor)
Can do subjective and quantitative analysis
Quantitative measurement is ADC (apparent diffusion
coefficient)
Does not require contrast
33. Diffusion Weighted MRI w/o ADC: Basic
Image in oncology
Viable tumors
High cell density
Less water motion
Bright (higher signal)
Necrotic tumors
Few membranes
More water motion
Dark
Tumor
Cystic
changes/
necrosis
42. Radiation Tumor response
The value of diffusion-weighted imaging for
monitoring the chemotherapeutic response of
osteosarcoma: a comparison between average
apparent diffusion coefficient and minimum
apparent diffusion coefficient: Human Data
With both the average ADC and the minimum ADC, post-chemotherapy values were
significantly higher than pre-chemotherapy values (P < 0.05).
The patients with a good response had a significantly higher minimum ADC ratio
than those with a poor response (1.01 + or - 0.22 and 0.55 + or - 0.29 respectively,
P < 0.05).
CONCLUSION:
The minimum ADC is useful for evaluating the chemotherapeutic response of
osteosarcoma
50. USUAL APPEARANCE OF
CEREBRAL ABSCESS ON MRI
DWI: Same patient in
previous two slides.
There is marked high
signal intensity in the
abscess corresponding
to restricted diffusion of
water molecules in the
cavity. Note mild
hyperintensity
surrounding the cavity
due to “T2 shine
through” from edema.
