Inside the MRI scanner, there is a powerful magnetic field that interacts with protons in the body. The scanner produces radiofrequency pulses which further interact with protons, causing them to give off a signal that is detected by the scanner and transformed into images. MRI sequences such as T1-weighted, T2-weighted, FLAIR, DWI, and images with gadolinium contrast are analyzed and compared to the clinical history to identify abnormalities and arrive at a diagnosis. Diffusion tensor imaging can also be used to detect and characterize ischemic brain injuries.

1. MRI interpretation
X-ray and CT images can be considered to be a map of density of tissues in the body; white
areas on X-ray and CT images represent high density structures.
Bright areas on an MRI image represent high ‘signal’ given off by protons in the body during
the scanning process.
White areas on an X-ray or CT image = high density
White areas on an MRI image = high signal

2. 1. MRI produces detailed images of many body parts but is not always the best imaging modality
2. A wide range of different MRI images can be produced to help answer specific clinical questions
[Project] [Stroke]
Neurophysiological and neuroimaging biomarkers are preferred over clinical behaviors for long-term
prognosis. DTI should be done after 30 days of stroke for use in prediction of recovery
3. A systematic approach is required for image interpretation
4. The successful application of MRI depends on the clinical question in mind, and the body part to be imaged.
5. MRI provides exquisite images of body parts that do not move, such as the brain, and anatomical structures
that can be kept still, such as parts of the musculoskeletal system.
6. Each set of images produced takes several minutes to obtain. MRI is not suitable for patient who are unable
or unwilling to remain motionless.
7. With some applications of MRI, drugs may be given to help reduce movement and improve image quality

3. 1. Inside the MRI scanner there is a powerful magnetic field
2. The magnetic field interacts with protons in the body
3. The scanner produces radiofrequency pulses which further interact with protons in the body
4. Protons give off 'signal' which is detected by the scanner
5. When the patient enters the magnet, all the protons (hydrogen ions) within their body become aligned with
the magnetic field. The scanner produces radiofrequency pulses which further interact with protons causing
them to give off ‘signal’. This signal is detected by the scanner and transformed into images.
Conventional MRI is more sensitive and specific than CT for the detection of ACUTE ISCHEMIC BRAIN
INFARCTS, within the first few hours after the onset of symptoms
MRI is also superior to CT in detecting ischemic infarcts of the posterior cranial fossa, because MR images are
not degraded by osseous structures as is the case with CT.
MRI is unable to detect HEMORRHAGE IN THE EARLY HOURS after a stroke as the oxyhemoglobin
predominates in the fresh bleed because it is a nonparamagnetic substance.
Acute stroke patients are attached to monitoring equipment which are incompatible with strong magnetic fields
preclude early MRI examination. Acute stroke may uncooperative due to long scan times

4. MRI signal production = To produce 'signal', the MRI scanner interacts with protons in the body.
Randomly orientated protons => aligned with the powerful magnetic field in the bore of
the scanner
Radiofrequency pulse is applied Signal creation
‘excitation’ of protons => Protons are aligned at an
angle to the magnetic field
The radiofrequency pulses also cause the protons to spin
in phase with each other creating 'resonance'
Milliseconds after removal of each radiofrequency pulse
the excited protons 'relax', giving off radiofrequency
signal which is detected by the scanner

5. Two types of relaxation occur –
1. Realignment of protons with the magnetic field How Does MRI Work- - Nuffield Health GIF.mp4
2. Dephasing of spinning protons (loss of resonance)
Two types of signal can be detected
T1 signal relates to the speed of realignment with the magnetic field – the more quickly the protons realign the
greater the T1 signal
T2 signal relates to the speed of proton spin dephasing – the slower the dephasing the greater the T2 signal
The scanner produced a rapidly repeating sequence of
radiofrequency pulses which causes 'excitation' and
'resonance' of protons.
As each radiofrequency pulse is removed, the protons
‘relax’ to realign with the magnetic field, and as they do
so they give off radiofrequency 'signal' which is detected
by the scanner and transformed into an image.

6. Tissue differentiation - Fat v water
Protons in the body realign and dephase with varying rapidity depending on the tissue type
Detecting the signal after different time intervals allows different tissue types to be highlighted
High T1 signal High T2 signal
Protons in fat realign quickly with high energy and
produce high T1 signal – this phenomenon is exploited to
produce 'T1-weighted' images which highlight fat in
tissues of the body
Protons in water dephase slowly – this
phenomenon is exploited to produce 'T2-
weighted' images which highlight water in
tissues of the body
T1 v T2 images
T1 images – 1 tissue type is bright – FAT
T2 images – 2 tissue types are bright – FAT and WATER

7. Manipulating the repetition time (TR) between the application of radiofrequency pulses and Echo time (TE)
between the radiofrequency pulse and the recording of a signal (i.e., echo) produced by the tissue, can emphasize
the proton density, T1 relaxation time, or T2 relaxation time features of any tissue
TR and TE are expressed in milliseconds.
The most commonly used technique is spin echo = > short TR and TE will emphasize the T1 relaxation time of a
tissue, the so-called T1-weighted image.
T1 weighted => if TR is less than 1,000 milliseconds (ms) and TE is less than 30 ms (e.g., TR = 500 ms, TE = 20
ms)
T2-weighted = > TR longer than 1,500 ms and a TE greater than 60 ms (e.g., TR = 2,000 ms, TE = 85 ms)
Proton density images are obtained with a long TR and a short TE (e.g., TR = 2,000 ms, TE = 20
ms).

8. T1 T2

9. T1 T2
T1-weighted image – Anatomy (spine)
T1 images can be thought of as a map of proton energy within
fatty tissues of the body
Fatty tissues include subcutaneous fat (SC fat) and bone marrow
of the vertebral bodies
Cerebrospinal fluid (CSF) contains no fat – so it appears black on
T1-weighted images
T2-weighted image – Anatomy (spine)
T2 images are a map of proton energy within fatty AND water-
based tissues of the body
Fatty tissue is distinguished from water-based tissue by
comparing with the T1 images – anything that is bright on the
T2 images but dark on the T1 images is fluid-based tissue
For example, the CSF is white on this T2 image and dark on the
T1 image above because it is free fluid and contains no fat
Note that the bone cortex is black – it gives off no signal on either T1 or T2 images because it contains no free
protons

10. MRI images of the brain demonstrate grey and white matter structures
Note that on this image the grey matter of the cerebral cortex is brighter than the white matter

11. MRI allows detailed analysis of musculoskeletal body parts
Bone marrow is clearly visible – in the femur and tibia in this case
Bone cortex is less clearly visible on MRI and is often better seen with CT scans or plain X-rays

12. Inside the MRI scanner there is a powerful magnetic field
The magnetic field interacts with protons in the body
The scanner produces radiofrequency pulses which further interact with protons in the body
Protons give off 'signal' which is detected by the scanner

13. MRI sequences
Compare the fat-sensitive with the water-sensitive images looking for abnormal signal
Look at the fat sensitive T1 images which often provide good anatomical detail of the area being studied
Compare with the water-sensitive images – such as the T2-weighted or STIR images

14. STIR image [Short Tau Inversion Recovery] or Fat suppression and only water
STIR image – Spondylodiscitis
STIR (Short Tau Inversion Recovery) images are highly
water-sensitive and the timing of the pulse sequence used
acts to suppress signal coming from fatty tissues – so ONLY
WATER is bright
A combination of standard T1 images and STIR images can
be compared to determine the amount of fat or water within a
body part
In these MRI images abnormal signal is seen in the vertebral bodies
and intervertebral disc
Abnormal low signal on the T1 image and abnormal high signal on
the STIR image – indicates abnormal fluid [Fat should have been
there but it’s replaced by water]
These are typical appearances of spondylodiscitis (also known as
discitis)

15. FLAIR (FLUID ATTENUATED INVERSION RECOVERY) images
FLAIR images – Multiple sclerosis
The signal from free fluid – such as cerebrospinal fluid (CSF) in the ventricles – is suppressed (compare with the T2 image)
High signal seen on these images indicates a pathological process such as infection, tumour, or areas of demyelination – as in this
patient with multiple sclerosis

16. T2*(gradient echo)
T2*(gradient echo) images – Haemangioma
T2* images (pronounced ‘T2 star’ – also known as ‘gradient echo’ images) can be used to highlight the presence of blood products –
such as in this cerebral haemangioma (arrows)

17. Clinical
Relate your findings to the clinical features and the specific clinical question
Both of these images show an area of abnormal high signal within the grey and white matter of the brain
Patient 1: gradually worsening headaches and seizures – diagnosis = brain tumour
Patient 2: sudden onset left hemiplegia – diagnosis = acute cerebral infarct
Although the MRI appearances provide information regarding the position and size of the areas of abnormality, it is the different
clinical histories which provide the strongest clues to the diagnosis in both cases

18. Abnormal MRI signal
Check for abnormalities of MRI signal
Determine the nature of the signal change – abnormal fat or
fluid?
Note the anatomical location, size and shape of the abnormality
The combination of standard T1 images (fat sensitive) and STIR
images (water sensitive) can be compared to determine the
amount of fat and water within a body part
In this pair of images, the high signal mass seen on the T1
image is dark on the STIR image – confirming it contains fat
and no water
These are typical signal characteristics of a lipoma

19. Contrast agents
The pre-gadolinium image shows only an indistinct area of
abnormality in the left cerebral hemisphere
The post-gadolinium image of the brain shows a very well-
defined area of enhancement – in this case due to a malignant
brain tumour
Pre and post-gadolinium T1-weighted images are compared in order to assess 'enhancement' of tissues.
Abnormal tissue, such as inflamed or cancerous tissue, is often more vascular than surrounding tissue and so 'enhances', appearing
brighter on post-gadolinium images
 Gadolinium is the most common contrast agent used for
MRI – it can be given intravenously or injected directly
into a body part
 Abnormal tissue may enhance more than surrounding
normal tissue following intravenous gadolinium
 Abnormal tissue may also retain gadolinium longer than
normal tissue

20. Direct Gadolinium

21. DIFFUSION WEIGHTED IMAGING
DWI is most usefully applied to the problem of early detection of
BRAIN INFARCTION. DWI can detect ischemia within minutes
The diffusion coefficient of ischemic brain tissue rapidly decreases
within minutes of onset of tissue ischemia
Standard T2-weighted MRI examination can detect brain
ischemia/infarction as early as 3 hours after onset
Diffusion is a physical property of molecules whereby they move randomly via Brownian motion and spread out through a medium in
accordance with their thermal energy. The rate of diffusion for a particular molecule in a particular environment is measured by the
diffusion coefficient.
Molecular diffusion is influenced by concentration (i.e., chemical) gradients, as well as by mechanical tissue structure that can confer a
directional (“anisotropic”) component. For example, in the muscle or in cerebral white matter, anisotropic tissue structure creates a
preferred pathway for water diffusion parallel to muscle or nerve fibers, and the direction of diffusion is largely uniform across each
imaged voxel.
Diffusion of polarized molecules may also be influenced by an electrical potential gradient. In Diffusion imaging, imaging is performed
in the amplitudes of the bipolar magnetic field gradients are increased greatly so as to be able to image the relatively small distances and
slower velocities associated with molecular diffusion as opposed to blood flow.
Rapid image acquisition techniques such as echoplanar imaging are used to limit the influence of motion artifact.
The direct MR signal cannot differentiate between diffusion-related motion in extracellular fluid, blood flow, perfusion, and tissue
pulsation–related motion. Thus, what is being imaged is not actually a true tissue diffusion coefficient, but rather an ADC. Since
diffusion-weighted images are strongly T2-weighted because of the long probe times of the magnetic field gradients, a calculation can be
performed to separate T2 relaxation effects from diffusion related changes in the signal and derive the ADC. A map of ADCs can then
be calculated for specific regions of interest from the diffusion-weighted image.

22. Diffusion weighted imaging – Infarct (brain)
Diffusion Weighted Imaging (DWI) and Apparent Diffusion Coefficient (ADC) images are viewed together
Areas of high signal on the DWI images and low signal on the ADC images indicate 'restricted diffusion' - an indicator of a
pathological process of cell death such as infarction, cancer, or abscess formation
Restricted diffusion in a wedge-shaped region of the brain (arrow) is a characteristic finding of a recent cerebral infarct
These images also show smaller areas of restricted diffusion due to recent lacunar infarcts (arrowheads)
Question is why should we even bother to see these images, we can see as and
when they come. Every activity you join with academic is psycho-motor
activity learning by doing. Unless you see the easy ones, you will never attend
to things of your interest !
The deeper understanding comes when you think on simple things.

23. Acute stroke produces an electrolyte imbalance, which
causes water molecules to rush into the intracellular
compartment, where free random motion is no longer
possible and therefore falling into a state of restricted
diffusion. DW images reflect restricted diffusion as a
signal increase, which corresponds with a signal drop
in its accompanying sequence, the apparent diffusion
coefficient (ADC) map. The combination of increased
signal in the DW images and decreased signal in the
ADC map is compatible with an infarct in the
appropriate clinical setting, as other entities such as
viscous abscesses and dense masses such as
lymphomas can have a similar restricted diffusion
pattern (Fig. 5-79A and B). One of the key features of
DWI of acute cerebral ischemia is that it becomes
positive as soon as 30 minutes after the insult and can
remain positive for 5 days or more
Ischemic infarct
Restricted diffusion is shown as increased
signal intensity in the diffusion-weighted image
Restricted diffusion is shown as with
corresponding signal drop in the ADC map

24. ISCHEMIC STROKE
Goal: To see edema of an early infarct [the first few hours after the onset of symptoms.]
Sequence T1 - weighted
FLAIR (fluid-
attenuated inversion
recovery)
T2-weighted images
Signal Low signal intensity High signal intensity High signal intensity
Conventional MRI is more sensitive and specific than CT for the detection of acute
ischemic brain infarcts, within the first few hours after the onset of symptoms.
Axial proton density (PD) sequence. There is increased signal intensity (arrow)
due to restricted diffusion characteristic of an infarct

25. MRI is more sensitive than CT at detecting lacunar infarcts, which
are small infarcts of less than 1.5 cm typically located in the basal
ganglia and periventricular areas and at the brainstem
MRI, these infarcts are shown as multiple hyperintense areas (arrows).

26. Intra cranial bleed following ischemic stroke
Nonenhanced CT is highly sensitive in detecting intracranial bleeds,
which, in the setting of an ischemic stroke, represents hemorrhagic
transformation.
In MR imaging, T2*-weighted gradient-echo sequences depict areas
of hemorrhage as focal regions of low signal intensity, secondary to
a phenomenon known as blooming
Axial gradient echo T2* image shows an irregular area of signal
drop with surrounding high–signal-intensity edema at the left superior parietal lobe
(arrow), compatible with a hemorrhagic stroke.
DIFFUSION TENSOR IMAGING

27. Diffusion tensor imaging (DTI) comprises a group of techniques
where calculated eigenvalues (λ1, λ2, and λ3) and eigenvectors (ε1,
ε2, and ε3) are used to create images reflecting various diffusion
properties of a tissue. "Regular" diffusion-weighted (DW) imaging
produces images based on only the sum or average of the
eigenvalues. The sum of the eigenvalues (λ1+λ2+λ3) is called the
trace, while their average (= trace/3) is called the mean diffusivity
or apparent diffusion coefficient (ADC).
Three diffusion tensor imaging techniques in common use are
- The fractional anisotropy map
- The principal diffusion direction map
- Fiber-tracking maps

28. DIFFUSION TENSOR IMAGING
Three diffusion tensor imaging techniques in common use are the fractional anisotropy map, the principal diffusion direction map, and
fiber-tracking maps
Fractional Anisotropy Map
Fractional anisotropy (FA) is an index for
the amount of diffusion asymmetry within
a voxel, defined in terms of its
eigenvalues:
The value of FA varies between 0 and 1. For perfect isotropic diffusion, λ1 = λ2
= λ3, the diffusion ellipsoid is a sphere, and FA = 0. With progressive diffusion
anisotropy, the eigenvalues become more unequal, the ellipsoid becomes more
elongated, and the FA → 1. The FA map is a gray-scale display of FA values
across the image. Brighter areas are more anisotropic than darker areas.
Fractional Anisotropy map

29. DIFFUSION TENSOR IMAGING
Three diffusion tensor imaging techniques in common use are the fractional anisotropy map, the principal diffusion direction map, and
fiber-tracking maps
Principal Diffusion Direction Map
This is a map that assigns colors to voxels based on a combination of anisotropy
and direction. It is also called the colored fractional anisotropy map, fiber
direction map or diffusion texture map. The color assignment is arbitrary, but
the typical convention is to have the orientation of the principal eigenvector (ε1)
control hue and fractional anisotropy (FA) control brightness. Specifically, if ε1
makes angles α, β, and γ with respect to the to the laboratory x-, y-, and z-axes,
the color scheme might be apportioned in the ratios:
Red = FA • cos α
Green = FA • cos β
Blue = FA • cos γ

30. DIFFUSION TENSOR IMAGING
Three diffusion tensor imaging techniques in common use are the fractional anisotropy map, the principal diffusion direction map, and
fiber-tracking maps
Fiber Tracking Map
Axonal tracts are commonly mapped using a deterministic method known as
FACT (fiber assignment by continuous tracking). In this method the user
selects "seed voxels" in a certain area of the brain and automated software
computes fiber trajectories in and out of that area. This is accomplished by
following the primary eigenvector (ε1) in each voxel until it encounters a
neighboring voxel, at which time the trajectory is changed to point in the
direction of the new eigenvector.

31. Susceptibility-Weighted Imaging
Uses
3D gradient-echo sequence
High-resolution images based on local tissue magnetic
susceptibility and on BOLD effects
Magnetic susceptibility is related to loss of signal in voxels with
magnetic field nonuniformities, due to greater T2* decay.
SWI was originally intended for sub-millimeter cerebral vein
imaging without the use of contrast agents, based on the fact that
deoxygenated venous blood produces more magnetic field
inhomogeneities than oxygenized arterial blood.
TBI
Hemorrhagic disorders
Vascular malformations
Cerebral infarctions
Tumors
Neurodegenerative disorders associated with intracranial
calcifications
Neurodegenerative disorders associated with iron deposition.
When adding SWI sequences, in addition to diffusion-weighted and perfusion-weighted sequences, to the standard MR protocol, a
more complete understanding of the disease process in question is accomplished, particularly when dealing with neurovascular
and neurodegenerative disorders

32. Susceptibility-Weighted Imaging
Axial MRI scans of multiple traumatic cerebral microbleeds in a patient with alcoholism and repetitive falls.
A. A conventional spin-echo T2-weighted (T2w) image depicts the posttraumatic gliosis in the right frontal pole but shows no
microbleeds.
B. A conventional intermediate-echo T2*-weighted 3.0-T image depicts some evidence of hemosiderin and bleeding.
C. Susceptibility-weighted imaging (SWI) findings show diffuse microbleeds and vascular damage in the frontal lobes (arrows)
and show the veins because of their inherent deoxyhemoglobin content.

33. Susceptibility-Weighted Imaging
Images show the effect of the application of susceptibility-weighted imaging (SWI) scan processing on standard two-dimensional
(2D) T2*-weighted gradient-echo (GRE) imaging
The resulting SWI contrast-enhanced image improves the detectability of microbleeds (middle) on the basis of the same data set.
Microbleeds are better visible after SWI post-processing (arrows); some can only be seen after SWI postprocessing (arrowheads).

34. SUSCEPTIBILITY-WEIGHTED ANGIOGRAPHY (SWAN)
Appearance of veins and other structures with susceptibility in nonphase-processed versus phase-processed susceptibility-weighted
imaging (SWI) data.
A, C = > A standard, multiecho, susceptibility-enhanced, susceptibility-weighted angiography (SWAN) sequence without SWI phase
postprocessing.
B, D = > The post hoc application of SWI postprocessing to the exact same dataset changes the visual impression of the basal ganglia,
cortical, and venous contrast

35. WHITE MATTER DISEASE OF BRAIN
Although the T1-weighted MR images are usually normal, the FLAIR and T2-weighted images demonstrate MS plaques as high–signal
intensity areas. These are most frequently seen in the periventricular white matter, especially around the atrium and the tips of the
anterior and posterior horns of the lateral ventricles (Fig. 5-93A and B). The high–signal-intensity plaques also can be seen in other
white matter areas of the cerebral hemispheres, the brainstem, and even the upper spinal cord. When these lesions are seen in patients
younger than 40 years of age, they tend to be relatively specific for MS (84). In patients more than 50 years of age, the MRI findings of
MS are similar to findings in some aging brains, and correlation with the clinical findings helps establish the diagnosis. Recent
T2-weighted (A) and FLAIR (B) MRI demonstrates the periventricular demyelinating plaques of MS as hyperintense areas (arrows)
adjacent to the anterior horns and atria of the lateral ventricles.

36. WHITE MATTER DISEASE OF BRAIN
MS plaques that involve damage to the blood-brain barrier frequently enhance with the use of IV gadolinium-DTPA

37. Comparison of MR Imaging Methods
MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
T1-
weighted
TR: short (<1,000 ms)
TE: short (<30 ms)
Spin echo technique
Tl contrast. Fluid is dark. Fat
is white.
Brain parenchyma is gray.
Tumour and oedema are gray
or dark.
Ligament and tendon are
dark
Methaemoglobin
(haemoglobin breakdown
product >7 d) has high
signal intensity
Best spatial resolution for
con-
nective tissue anatomy and
bone marrow and trabecular
anatomy
Myelin distribution and
mucoid
degeneration of tendons/
menisci. Used for all post-
contrast imaging—e.g., with
gadolinium
Identify subacute to chronic
haemorrhage
Poor tissue edema contrast
resolution. Poor sensitivity
to pathologic lesions since
most lesions involve
increased tissue
or edema fluid and Tl is not
sensitive to the appearance
of tissue fluid associated
with
acute inflammatory reaction
or pathologic tissue changes
MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
T2-
weighted
TR: long (>l,500 ms)
TE: long (>60 ms)
E.g., FSE technique
T2 contrast. Fluid is white.
Fat is variable. Brain
parenchyma is gray.
Tumor and edema are
white (hyperintense).
Ligament and tendon
are dark
Regions with high free water
content have high signal
intensity—e.g., CSF, edema,
nucleus pulposus, synovial
fluid, abscess, and
hyperacute hemorrhage (<1
d). Sensitive to pathologic
appearance of fluid
in parenchymal tissue
Difficult-to-see high-
intensity lesions adjacent
to CSF or other free fluid
regions—CSF has high
signal intensity, as do T2
lesions; therefore, tissue
lesions at CSF interface
are obscured
MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations

38. STIR
TR: long (>4,000 ms)
TE: short (<50 ms)
Tl: short values to null out
fatty
tissue
Very sensitive to tissue
edema
Water content in soft
tissue (e.g., nerve and
muscle) produces a high
signal intensity. Fat is
suppressed
MR neurography
Muscle pathology imaging
Pathologic fracture, bone
edema,
ligamentous or tendinous
injury
imaging
Poor spatial resolution but
suitable to large body
parts such as the limbs
and trunk or pelvis
MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
FLAIR
TR: long (e.g., 9,000 ms)
TE: long (e.g., 130 ms)
Heavily T2-weighted
Tl adjusted to null out free
fluid signal (e.g., 2,200 ms)
Heavily T2-weighted with
bulk water suppression, but
without extracellular
fluid/tissue oedema
suppression
White matter T2 lesions seen
more clearly than with T2-
weighted Applied to MS,
DAI/TBl, Lyme, HIV, brain
infarction/ischemia
CSF pulsation artifact
Blood flow artifact
Requires careful adjustment
of Tl, especially when CSF
is abnormal
MBA
Phase-contrast imaging;
bipolar
MFG
Flowing spins
Blood flow; angiography.
Non-invasive screening for
cerebrovascular anomalies
(e.g., cerebral aneurysms)
Longer study time Lower
spatial resolution compared
with x-ray
angiography

39. MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
DWI
EPIA
TR: long (e.g., 10,000 ms)
Heavily T2-weighted
Phase-contrast imaging with
large - amplitude bipolar
MFGs
Spin echo
Diffusing spins. To look at
"restricted" or "anisotropic"
diffusion, a diffusion
"tensor" of directionally
specific diffusion rates is
computed for each voxel.
DTI can be used to compute
pathways of white fiber tracts
(i.e., diffusion tensor
tractography) and focal white
matter disruption
Ischemia; acute stroke; TBI;
DAI
Held to be exquisitely and
nearly
immediately sensitive to the
effects of ischemia on the
brain tissue due to focal
reduction in diffusion of
extracellular water in the
ischemic region.
H2o moves into intracellular
space due to metabolic
failure of ATP-dependent
membrane-based Na-K pump
Tissue perfusion assessment
Motion artifact: MR
measurement cannot
differentiate diffusion from
local blood flow or tissue
pulsation; T2-weighted due
to long probe time.
Can correct for effect of T2
weighting by calculating
map of ADC values
MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
PWI
TR: short
TE: short
Images taken while contrast
medium (e.g., gadolinium) is
power - injected
intravenously at a fixed rate
Appearance of blood-
conducted paramagnetic
contrast medium in different
brain regions
Involves dye injection using
specialized equipment to
control rate of infusion

40. MR
Method
Excitation-Emission
Sequence
What ls imaged? Applications Limitations
fMRI-
BOLD
EPIA; T2-weighted; multiple
image averaging to extract
deoxyhemoglobin signal
Spatially localized tissue
decrease in
deoxyhemoglobin in regions
of functionally related
metabolic activation
Local tissue activation in
functional brain activation /
physiologic studies
Must average images to
extract small deoxyhemo-
globin signal
Limited temporal resolution
Subject must be cooperative
and able to continuously
perform the activation task
MS plaques that involve damage to the blood-brain barrier frequently enhance with the use of IV gadolinium-DTPA