MRI uses strong magnetic fields and radio waves to produce diagnostic images of the body. There are three main types of MRI magnets - permanent magnets which use iron alloys, resistive magnets which require large power supplies, and superconductive magnets which use niobium alloys cooled with liquid helium. Safety is crucial with MRI due to the strong magnetic fields which can pull metal objects or cause interference. Different pulse sequences such as T1-weighted, T2-weighted, and FLAIR are used to visualize tissues and abnormalities in the brain.

1. MAGNETIC RESONANCE
IMAGING

2.  MRI is an imaging modality that uses non-ionizing
radiation to create useful diagnostic images. MRI was
initially called Nuclear Magnetic Resonance Imaging
after its early use for chemical analysis.

3. HISTORY

5. Necessary Equipment
Magnet Gradient Coil RF Coil
RF Coil
4T magnet
gradient coil
(inside)

6. MAGNET
1 Tesla (T) = 10,000 Gauss
Earth’s magnetic field = 0.5 Gauss
4 Tesla = 4 x 10,000  0.5 = 80,000X Earth’s
magnetic field
Continuously on

7. MAGNETS
Permanent MRI magnets:
 Bar magnet that has been twisted into a "C" shape where the two
poles are close together and parallel.
 Up to 30 tons of iron may be needed.
 Magnets can be made with alloys such as neodymium.

8.  Resistive (air core) MRI magnets: It operate at
room temperature using standard conductors such as copper in
the shape of a solenoid or or Helmholtz pair coil.
 inexpensive to make but require a large constant flow of
current while magnetized and imaging.
 The operating costs are high because of the large power
requirements.

9. Superconductive MRI magnets :
 Solenoid shaped coil made of alloys such as niobium/titanium
or niobium/tin surrounded by copper.
 These alloys has zero resistance to electrical current when
cooled down to about 10o Kelvin.
 The coil is kept below this temperature with liquid He.
 Copper acts as an insulator at low temperatures compare to the
zero resistance of the alloy.
 The current continues in the closed loop of the coil for years
without significant decline.
 Copper protects the alloy in case of quench.

11. MAGNET SAFETY
The whopping strength of the magnet makes safety essential.
Things fly – Even big things!

12.  Anyone going near the magnet – subjects, staff and visitors – must be
thoroughly screened:
 Subjects must have no metal in their bodies:
• pacemaker
• aneurysm clips
• metal implants (e.g., cochlear implants)
• intrauterine devices (IUDs)
• some dental work (fillings okay)
 Subjects must remove metal from their bodies
• jewellary, watch, piercings
• coins, etc.
• wallet
• any metal that may distort the field (e.g., underwire bra)
 Subjects must be given ear plugs (acoustic noise can reach 120 dB)
Subject Safety
This subject was wearing a hair band with a ~2 mm
copper clamp. Left: with hair band. Right: without.

13. PRINCIPLE
Hydrogen atom: MR sensitive
Abundent

15. RADIOFREQUENCY
 Radio frequency (RF) is any of the electromagnetic wave
frequencies that lie in the range extending from around 3 kHz to
300 GHz.
 Exposure of an object or person to RF radiation at the Larmor
frequency, causes the net magnetisation to spiral away from the B0
field.
 In the rotating frame of reference, the net magnetisation vector
rotate from a longitudinal position a distance proportional to the
time length of the RF pulse.

16. LARMOR FREQUENCY
ω = γB
Where ω is the Larmor frequency in MHz , γ is the gyromagnetic
ratio in MHz/Tesla and B is the strength of the static magnetic
field in Tesla.

18.  After a certain length of time, the net magnetization vector
rotates 90 degrees and lies in the transverse or x-y plane. It is
in this position that the net magnetisation can be detected on
MRI.

19. (X axis)
(Z axis)
(Y axis)

20. PROTONS WHICH GIVE
MAGNETIC POTENTIAL

21. Magnetic field
Subject in magnetic field
Longitudinal
magnatization

22. RADIOFREQUENCY IMPULSE
Longitudinal
magnetization
Transverse
magnetization

24. AFTER RF IS STOPED

25. T2 IMAGING
T2* decay/ relaxation refers to an exponential decrease in Mxy (i.e.
signal strength) following the initial excitation pulse as a function of
time constant T2*.

27. T1 IMAGING
The T1 relaxation time, also known as the spin-lattice relaxation time,
is a measure of how quickly the net magnetisation vector (NMV)
recovers to its ground state in the direction of B0.

29. PROTON DENSITY T2 WEIGHTED
T1 WEIGHTED
Short Long
TE
TR
Long
Short
TE(echo time) is time between the application of radiofrequency
excitation pulse and the peak of the signal induced in the coil.
TR(repetition) is the time from the application of an excitation
pulse to the application of the next pulse.

30. BRAIN IMAGING SEQUENCE
 T1 weighted
 T2 weighted
 FLAIR
 GRE
 DIFFUSION

31.  T1 weighted: T1 weighted image (also referred to as T1WI or "spin-
lattice" relaxation time) is one of the basic pulse sequences in MRI
and demonstrates differences in the T1 relaxation times of tissues.
 T2 weighted: T2 weighted image (also referred to as T2WI or “spin-
spin” relaxation time)
 Fluid-attenuated inversion recovery (FLAIR): It is used in brain
imaging to suppress cerebrospinal fluid (CSF) effects on the image, so
as to bring out the periventricular hyper intense lesion. (Part of T2)
 Gradient Recalled Echo (GRE): The sequence of GRE T2WI
requires high uniformity of the magnetic field, independent of the
paramagnetic substance or diamagnetic substance. With changes in the
uniformity of the magnetic field, the sequence is sensitive for
detection and can improve the detection rate of small lesions.
 Diffusion Weighted Imaging (DWI), the intensity of each image
element reflects the best estimate of the rate of water diffusion at that
location.

32. IMAGES
CSF DARK IN T1 CSF WHITE IN T2

33. T1 image T2 image Flair
GRE image Diffusion

34. GRE IMAGE

35. BLEED

36. CEREBRAL ABSCESS
T1 T2 Diffusion

37. GLIOBLASTOMA
T1 T2 T1 contrast Flair contrast

38. FUNCTIONAL MRI

39. TRACTOGRAPY

40. SPECTROSCOPY