Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the soft tissues of the human body.

1. MRI of brain: Basics
Dr. Aminur Rahman
FCPS(Med), MD(Neuro) ,FINR (Switzerland), MACP (USA)
Fellow Interventional Neuroradiology (Thailand)
Assistant Professor
Department of Neurology
Sir Salimullah Medical College

2. Introduction
• Magnetic resonance imaging (MRI) is an imaging
technique used primarily in medical settings to
produce high quality images of the soft tissues of
the human body.

3. Introduction contd..
• Magnetic resonance imaging (MRI) is based on the
magnetic characteristics of the imaged tissue.
• It involves creation of tissue magnetization (which
can then be manipulated in several ways) and
detection of tissue magnetization as revealed by
signal intensity.

4. Brief history of MRI
• Rabi et al 1st observed NMR phenomenon in 1939.
• Bloch detected strong proton signal from H in 1946
and later on won Nobel prize in early 50s.
• Jasper Jakson produced 1st MR signal from a live
animal in 1967.
• Lauterbur in 1974 produced 1st image of live
animal by adding magnetic gradient. He won the
noble prize for physics in the year 2003.

5. Indications for cranial MRI
1. Vascular (Ischemic and hemorrhagic stroke, AVM,
aneurysm, Venous thrombosis)
2. Infection (abscess, cerebritis, encephalitis,
meningitis)
3. Inflammatory/Demyelinating Lesions (multiple
sclerosis, sarcoidosis, etc.)
4. Tumor (primary CNS and metastatic)
5. Trauma (epidural hematoma, subdural hematoma,
contusion)
6. Hydrocephalus

6. Contraindications of MRI
1. Implanted devices and other metallic devices
a) Pacemakers and other implanted electronic devices
b) Aneurysm clips and other magnetizable materials
c) Cochlear implants
d) Some artificial heart valves
2. Intraocular metallic foreign bodies
a) Screening CT of the orbits if history suggests possible
metallic foreign body in the eye.
3. Unstable patients (most resuscitation equipment
cannot be brought into the scanning room).
4. Other relative contraindication – severe agitation or
claustrophobia (may require anesthesia assistance)

7. Limitations of MRI
1. Subject to motion artifact
2. Inferior to CT in detecting acute hemorrhage
3. Inferior to CT in detection of bony injury
4. Requires prolonged acquisition time for many
images

8. Advantages of MRI of brain:
(compared to CT scan of brain)
1. High quality soft tissue delineation
2. Better views of posterior fossa and temporal
lobes
3. No ionizing radiation
4. Non invasive
5. Can detect old hemorrhage and inflammatory
lesions
6. Can detect acute infarct within 30 mints
7. Can directly scan any plane, e.g. Coronal,
Sagittal, Axial, Oblique.

9. Disadvantages of MRI of brain:
1. Limited slice thickness – 2–3 mm with 3
Tesla; 3-5 mm with 1.5 Tesla (cf. CT – 1 mm).
2. Bone imaging limited to display of marrow.
3. Claustrophobia.
4. Cannot use with pacemaker or ferromagnetic
implant.

10. M R I
??
Basics of Magnetic Resonance Imaging

11. MRI ---> NMR
N ----> NUCLEAR ( unpaired protons )
M ----> MAGNETISM ( M D M )
R ----> RESONANCE ( state of energy transfer )

12. Sequence of Events in MRI ( M R R )
* M agnetic Field
* Radio - Frequency Pulse
* Relaxation
MRI
M
R
R

13. Magnet
• Protons spin in a
magnetic field (2.5kHz,
150,000 rpms, in the
Earth’s field)
• The stronger the
magnet, the stronger
the signal and the faster
the rotation
• Magnet field strength is
measured in Tesla(T) or
Gauss(G) (1T =
10,000G)

14. Magnet Types
A. Permanent (ferromagnetic
magnet)
B. Resistive (science class-
nail/wire/battery)
C. Superconductive (super
cooled with cryogens, e.g.
liquid helium)

15. Types of magnetic field
A. Low to Mid field ( 0.3 - 0.4 T or
below 1 T)
Usually Permanent Magnets
B. Low field 0.2 and below
Permanent or Resistive Magnets
C. Superconducting Magnet
High field 1 T to 3 T or above

16. Resistive Magnet
A magnet whose magnetic field originates from
current flowing through an ordinary electrical
conductor.

17. Superconducting Magnet
A magnet whose magnetic field originates
from current flowing through a
superconductor. Such a magnet must be
enclosed in a cryostat (cold chamber).

19. • Passive shielding is nothing more than constructing a thick wall of steel around the
MRI system on all sides. The large amount of iron in the steel walls keeps the
magnetic fields from penetrating outside the wall. If the MRI is located on the
ground level with no floor below (sited on grade), then the shielding below may not
needed. Likewise, if the ceiling opens to the sky, shielding may not be needed
above the magnet, provided that there is no risk of someone with a pacemaker or
aneurysm clip walking on the roof over the magnet.
• A wall of steel can be very heavy, adding to the already high total weight of the
system. In addition, constructing this type of magnet shield takes time and in some
cases could delay the process of installation of the MRI system.
• A second kind of passive shimming is also available. In this type of shielding, six bars
of steel and two end caps, each weighing 5 tons are mounted to the outer casing of
the magnet. This type of shielding weighs much more than any other type, but can
be very beneficial when the space required to site the magnet is smaller than what
can be accommodated by the wall shielding.

20. Basics of MRI
• The basis of MRI is the directional magnetic
field, or moment, associated with charged
particles in motion.
• Nuclei containing an odd number of protons
and/or neutrons have a characteristic motion
or precession. Because nuclei are charged
particles, this precession produces a small
magnetic moment.

21. Basics of MRI…
• When a human body is placed in a large
magnetic field, many of the free hydrogen
nuclei align themselves with the direction of
the magnetic field. The nuclei precess about
the magnetic field direction like gyroscopes.
This behavior is termed Larmor precession.

22. Magnetic Resonance Imaging
• Magnetic Resonance is enhanced absorption of
energy by the nuclei of atoms (having odd atomic
number) within an external magnetic field when
they are exposed to RF energy at a specific
frequency (called Larmour frequency or resonance
frequency).

23. Physical Principles of MRI
An MR system consists of the following components:
1) A large magnet to generate the magnetic field,
2) Shim coils to make the magnetic field as
homogeneous as possible,
3) A radiofrequency (RF) coil to transmit a radio signal
into the body part being imaged,
4) A receiver coil to detect the returning radio signals,
5) Gradient coils to provide spatial localization of the
signals, and
6) A computer to reconstruct the radio signals into the
final image.

24. Magnetic resonance imaging -
Technique
• Placing the patient in a powerful magnetic field,
causing endogenous proton (commonly hydrogen
protons) of tissues & CSF to align in longitudinal
orientation .
• Energy in the form of radiofrequency (Rf) waves
(at right angles to alignment of protons) of a
specific frequency introduced by coils placed next
to the body part of interest. The protons resonate
and spin, then revert to their normal alignment.

25. Magnetic resonance imaging -
Technique
• The energy state of the hydrogen protons is
transiently excited by Rf, which is
administered at a frequency specific for the
field strength of the magnet.
• The subsequent return to equilibrium energy
state (relaxation) of the protons results in a
release of Rf energy (the echo), which is
detected by the coils that delivered the Rf
pulses.

26. Basic Principles of MRI Techniques

27. MRI sequences
A. T1, T2 Relaxation Times
B. Inversion recovery Phases:
1.FLAIR,
2. STIR,
C. Diffusion-weighted imaging (DWI)
1.DWI(Uses T2)
2. Apparent diffusion coefficient (ADC)
D. Other sequences:
1. Diffusion Tensor Imaging (DTI),
2. Perfusion-Weighted MRI (PWI)
3. Susceptibility-Weighted Imaging (SWI) etc.
E. Flow sensitive
1. MR angiography
2. MR venography
F. Miscellaneous
1. MR spectroscopy
2. MR perfusion
3.Functional MRI
4. Tractography

28. MRI sequence in brain
Sequences TR
(msec)
TE
(msec)
A. T1-Weighted (Short TR and TE) 500 14
B.T2-Weighted (Long TR and TE) 4000 90
C. FLAIR (Very long TR and TE) 9000 114
 Repetition Time (TR) is the amount of time between two RF pulse sequences
applied to the same slice.
 Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt
of the echo signal.

30. Value of axial section

33. General Rule in MRI reading
• All Infarct, ICSOL, Inflammatory lesion are:
Hypointense in T1;
Hyperintense in T2;
Hypointense in FLAIR.
• Except:
Fat: Hyperintense in T1
Haemorrhage: Mixed
Cystic lesion: T1 Hypointense/T2 Hyper
intense and FLAIR: Hypointense.

34. Gradient-echo (GRE)
• GRE is T2* - based sequence, which is
extremely sensitive to local magnetic field
inhomogeneity and is especially useful for
detection of microhemorrhages, which may
be undetectable by other sequences.
• Microbleeds are usually defined as cerebral
bleeds less than 5-10 mm in size, and they are
now thought to represent microangiopathy
with consequent prognostic implications.

35. •The CT scan shows a hyperdense hemorrhage predominantly in the left frontal lobe. On MRI, the central portion of the
hematoma is isointense to brain parenchyma on the T1-weighted image and hyperintense on the T2-weighted and T2*
gradient echo images, consistent with hemorrhage containing oxyhemoglobin.
On the T2-weighted and T2* gradient echo images (GRE), the periphery of the hemorrhage is hypointense, consistent
with deoxygenation that occurs more rapidly at the borders. On the T2 weighted image, tissue adjacent to and
surrounding the hematoma is hyperintense, consistent with vasogenic edema.

36. Some Common Intensities on T1- and T2-
Weighted MRI Sequences
Note: TR, interval between radiofrequency (Rf) pulses; TE, interval between Rf pulse and signal
reception; CSF, cerebrospinal fluid; T1W and T2W, T1- and T2-weighted.

37. Inversion Recovery Phases:
Fluid attenuation inversion recovery (FLAIR):
• Fluid attenuation inversion recovery (FLAIR) is a
special inversion recovery sequence with long
inversion time (TI) which results in removing
signal from CSF from the resulting images.
• Fluid attenuation inversion recovery (suppress
high intensity of CSF) – High signal for
parenchymal lesion, low signal for CSF.
• The FLAIR sequence is similar to a T2-weighted
image except that the TE and TR times are very
long.

38. Clinical Applications of FLAIR
1. Infarction,
2. Multiple sclerosis (MS) plaques,
3. Subarachnoid haemorrhage,
4. Head trauma and
5. Post-contrast FLAIR images for assessing
leptomeningeal diseases, e.g. meningitis.

39. Short tau inversion recovery (STIR) /
Short T1 inversion recovery
• STIR is an inversion recovery pulse sequence
with specific timing so as to suppress the
signal from fat.
• The easiest way to identify STIR images is to
look for fat and fluid filled space in the body.
• Fluids normally appear bright and fat appear
very dark in a STIR image.

40. STIR Contd…
• Pathological processes normally increase the
water content in tissues.
• Due to the added water component this
results in a signal increase on STIR images.
Consequently pathological processes are
usually bright on STIR images.

41. Diffusion weighted (DWI) MR Imaging
• Diffusion weighted imaging (DWI) is a commonly
performed MRI sequence for evaluation of
acute ischaemic stroke, and is sensitive in the
detection of small and early infarcts.
• Conventional MRI sequences (T1WI, T2WI) may
not demonstrate an infarct for 6 hours and small
infarcts may be hard to appreciate on CT for days,
especially without the benefit of prior imaging.

42. DWI
• DWI is a technique that measures the free
diffusion of water molecules within tissue.
• In acute ischemic stroke, failure of the
sodium-potassium ATPase pump leads to
cellular swelling and reduced intercellular
space, thus limiting the free movement of
water and producing hyperintensity on DWI.

43. DWI Contd..
• It is a relatively low resolution image with the
following appearance:
• Grey matter: intermediate signal intensity (grey)
• White matter: slightly hypointense compared to
grey matter
• CSF: low signal (black)
• Acute pathology (ischaemic stroke, cellular
tumour, pus) usually appears as increased signal
denoting restricted diffusion.

44. Diffusion-weighted MRI (DWI)

45. Apparent Diffusion Coefficient (ADC)
• Apparent diffusion coefficient maps (ADC) are
images representing the actual diffusion values of
the tissue without T2 effects.
• They are relatively low resolution image the
following appearance:
• Grey matter: intermediate signal intensity (grey)
• White matter: slightly hyperintense compared to
grey matter
• CSF: high signal (white)

46. Diffusion Tensor MR imaging (DTI):
• Diffusion tensor imaging (DTI) is an extension
of diffusion weighted imaging (DWI) that
allows data profiling based upon white matter
tract orientation.
• Fiber tractography (FT) is a 3D reconstruction
technique to access neural tracts using data
collected by DTI.

47. Diffusion Tensor MR imaging (DTI):
• Colour coding:
• Red for fibres crossing from left to right
• Green for fibres traversing in anteroposterior
direction
• Blue for fibres going from superior to inferior

48. Clinical applications of DTI
• Assess the deformation of white matter by
tumors - deviation, infiltration, destruction of
white matter
• Delineate the anatomy of immature brains
• Pre-surgical planning
• Alzheimer disease - detection of early disease
• Schizophrenia
• Focal cortical dysplasia
• Multiple sclerosis - plaque assessment

49. Figure: 1.81 Diffusion Tensor Imaging, a type of neuroimaging that builds for monitor brain’s
white matter. White-matter fibers (called axons) allow communication between brain regions and
between the brain and the spinal cord. In this image, the axons are colored according to
orientation. Fibers running between superior to inferior are blue, those between right and left
are red, and those running between the brain's interior and exterior are green.

50. Perfusion-Weighted MR Imaging
(PWI)
• Perfusion weighted MR imaging is a term used to
denote a variety of MRI techniques able to give
insights into the perfusion of tissues by blood.
• There are different techniques of detecting perfusion
parameters with the use of MRI. The most common
techniques are:
1. Dynamic susceptibility contrast imaging (DSC-MRI)
and
2. Arterial spin labeling (ASL).

51. Clinical Applications of PWI
• The main role of perfusion MR imaging is in
evaluation of ischaemic conditions (e.g. acute
cerebral infarction to determine ischaemic
penumbra and moya-moya disease to identify
vascular reserve).
• To indentify neoplasms (e.g. identify highest
grade component of diffuse astrocytomas,
help distinguish glioblastomas from cerebral
metastases) and neurodegenerative diseases.

52. Figure: Diffusion weighted (DWI) & Perfusion-weighted MRI (PWI) before and
after treatment. Penumbra model demonstrates infarct in red and penumbra
in green, incorporating the concept of autoregulation.

53. Susceptibility Weighted MR Imaging
(SWI)
• Susceptibility weighted imaging (SWI) is
an MRI sequence which is particularly
sensitive to compounds which distort the local
magnetic field and as such make it useful in
detecting blood products, calcium etc.
• High resolution 3D gradient Echo sequences,
where the tissues of higher susceptibility
show signal loss (blood products, calcium etc).

54. Clinical applications of SWI:
1. Detection of intracranial haemorrhage, especially
small haemorrhage.
2. Delineation of small vessels, particularly veins is
exquisite.
3. Evaluation of traumatic brain injuries, coagulopathic
and haemorrhagic brain disorders.
4. Evaluation of neoplasm, cerebral infarction, vascular
malformations.

55. Figure: Susceptibility weighted imaging (SWI) showing the posterior fossa (A) and at the skull
convexity (B) demonstrate multiple foci of low signal intensity on the subcortical white matter
and cerebellum suggesting microbleeds and linear low signal intensity on the surface of the
frontal cortex, compatible with subarachnoid hemorrhage.

56. Functional MRI (fMRI)
• The oxygenated state of hemoglobin influences the T2
relaxation time of perfused brain.
• A mismatch between the supply of oxygenated blood and
oxygen utilization in activated areas produces an increase
in venous oxygen content within post capillary venules
causing signal change due to Blood Oxygenation Level
Dependent (BOLD) contrast.
• fMRI of the brain localizes regions of activity in the brain
following task (Cognitive & motor task) activation.

57. fMRI Contd…
• Neuronal activity elicits a slight increase in the delivery
of oxygenated blood flow to a specific region of
activated brain.
• This results in an alteration in the balance of
oxyhemoglobin and deoxyhemoglobin, which yields a
2–3% increase in signal intensity within veins and local
capillaries.
• Functional MRI has, in many instances, superseded PET
as a tool to study blood flow and oxygenation changes
in normal and abnormal states, because of superior
anatomical resolution and the ability to perform serial
studies safely.

58. Functional MRI (fMRI)

59. Thank you