Here I discussed about the concept,types, types of images obtained, the advantages and disadvantages of MRI shortly...anyone who wants to know about MRI just go through it. I just prepared it in very simple language for the convenience of the readers all over the world. Thank you.

2. What’s that?
►a noninvasive diagnostic technique that
produces computerized images of internal
body tissues and is based on nuclear
magnetic resonance of atoms within the
body induced by the application of radio
waves

3. MRI is a very close relative of NMR, which allows
clinicians to obtain chemical and physical
information about certain molecules. In the
1970’s the name was changed from NMRI to MRI
due to the negative connotations associated with
the word “nuclear”. Many patients thought that
the exam would expose them to radiation.

4. We all are made up of elements
►94 elements occur naturally on earth.
►Human body is built of only 26 elements.
►Oxygen, hydrogen, carbon, nitrogen
elements constitute 96 % of human body
mass.
►Human body is mainly composed of fat and
water, which makes the human body
composed of about 63% hydrogen.

5. Why hydrogen?
►Simplest element with atomic number of 1
and atomic weight of 1
►When in ionic state (H+), it is nothing but
a proton.
►Proton is not only positively charged, but
also has magnetic spin (wobble)!
►MRI utilizes this magnetic spin property of
protons of hydrogen to elicit images!!

6. A Single Proton
+
+
+
There is electric charge
on the surface of the proton,
thus creating a small current
loop and generating magnetic
moment m.
The proton also has
mass which generates
an
angular momentum
J when it is spinning.
J
m
Thus proton “magnet” differs from the magnetic bar in that it
also possesses angular momentum caused by spinning.

7. SO WE ARE MAGNETS!!!!
REALLY!!

8. But why we can’t act like
magnets?
► The protons (i.e.
Hydrogen ions) in
body are spinning in a
hap hazard fashion,
and cancel all the
magnetism. That is our
natural state!
► We need to discipline
them first, how?

9. We need a big magnet from
outside!
►Magnetic field strength: 0.3 – 7 T (30,000
times more than earth’s magnetic field
i.e.0.05mT).
► Average field strength – 1.5 T
►Open magnet – less field strength, less
claustrophobic
►Closed magnet – more field strength,
claustrophobic

10. Proton alignment
► Compass aligns with
the earth
► In a similar fashion,
► Our body protons
(hydrogen) align with
this external magnetic
field.
► But it will not align
perfectly with the
external magnetic
field.
► Why?

11. Magnetic Principles
 The spinning protons wobble or “precess” about
that axis of the external Bo field at the precessional
Larmor or resonance frequency.
 Magnetic resonance imaging frequency
n = g Bo
where g is the gyromagnetic ratio
The resonance frequency n of a spin is proportional
to the magnetic field, Bo.

13. A schematic representation of the major
systems on a magnetic resonance imager

14. How does this make IMAGING possible?
• How know where in object EM energy being measured comes from?
By the use of gradient coils

15. GRADIENT COILS
• Gradient coils are used to produce deliberate variations in
the main magnetic field
• It gives an additional magnetic field to the main magnetic
field.
• There are usually three sets of gradient coils, one for each
direction.
• The variation in the magnetic field permits localization of
image slices as well as phase encoding and frequency
encoding.
• The set of gradient coils for the z axis are Helmholtz pairs,
and for the x and y axis are paired saddle coils.

17.  Gradient coils which will create gradient magnetic fields
 Generate field with gradation in field strength with only
a narrow band at 1.5 Tesla
 Only hydrogen atoms within that narrow band respond
to 64 MHz pulse
 So, response must come from atoms within 1.5 T
portion of field
 Keep moving position of 1.5 T band to localize source
of responses to repeated rf pulses (“slices”)
 Narrowness of band determines granularity of
localization of response

21. Resonance
 So, when the RF pulse we are applying equals the natural
frequency of the protons:
– When pulse frequency = precession frequency, nuclei
resonate to it
– Pushes all of the spinning nuclei into phase with one another
– Amplitude of wobble of the whole magnetic field generated
by the spinning nuclei increases (= spin axis is pushed farther
out)
– How far spin axis moves (= flip angle) depends on rf pulse
intensity and duration

22. Turn off the radio
 Once we turn off the rf pulse:
 The original magnetization begins to recover
(T1)
 The excessive spin begins to dephase (T2)

23. Types of Imaging
►T1-weighted (spin-lattice relaxation)
►T2-weighted (spin-spin relaxation)
►Proton density (more protons, more signal)

25. T1 Weighted
 T1 relaxation is the recovery of the
longitudinal magnetization of the spinning
protons (Mz).
 The higher the Mz at the time of applying
the 90˚ RF pulse the greater the transverse
signal (Mxy).
 The TR (Time to Repetition) is what
determines the length of the time between
RF pulses.(amount of time between
successive pulse sequences applied to the
same slice)

26.  The longer the TR
 The longer the time to the next 90˚ RF pulse
 The more the Mz will have had to recover
 The higher the transverse signal when the 90˚ RF
pulse is applied.
 i.e.it is the TR that determines the T1 signal
 The time constant, T1 is a measure of the time it
takes for the nuclei to reach 63% of its original Mz.
 Hydrogen nuclei in different molecules have different
T1s. Those with short T1 will recover their Mz quicker
than those with a long T1

27. T1-weighted contrast

28. T2 Weighted
 T2 decay is the decay of the transverse
magnetization(Mxy) after application of 90˚ RF pulse
 The longer the time after the 90˚ RF pulse, the more
the Mxy decays and the smaller the transverse
signal.
 TE is the ‘Time to Echo”(time between the delivery of
the RF pulse and the receipt of the echo signal.)
 If we leave a long TE we give more time for the
Mxy(T2 signal) to decay and we get a smaller signal

29.  The longer the TE
 the longer the time allowed for Mxy to decay
 the smaller the transverse (T2) signal.
 It is the TE that determines the T2 signal
 The time constant, T2, is the time it takes for the
hydrogen nuclei to decay to 27% of its excited
Mxy.
 Hydrogen nuclei in different molecules have
different T2s.
 Those with a short T2 will take a shorter time to
decay than those with a long T2.

30. T2-weighted contrast

32. Proton Density
 Unlike T1 and T2 weighted images, proton
density(PD) does not display the magnetic
characteristics of the hydrogen nuclei but the
number of nuclei in the area being imaged.
 To get a PD weighted image we want to minimize
the contribution of both T1 and T2 contrast.
 T1 minimized with a long TR- large signal and
small T1 contrast
 T2 minimized with a short TE- large signal and
small T2 contrast

35. CLOSED

36. OPEN

37. UPRIGHT

39. Functional MRI (fMRI)
• measures brain activity by detecting changes
associated with blood flow.
• This technique relies on the fact that cerebral
blood flow and neuronal activation are
coupled
• When an area of the brain is in use, blood flow
to that region also increases

41. DIFFERENCE BETWEEN MRI AND FMRI
MRI FMRI
• Views anatomical
structure
• Focuses on protons in
hydrogen nuclei
• High spatial resolution
• Utilized for
experimental purposes
• Views metabolic
function
• Calculates oxygen
levels
• Long-distance
resolution
• Utilized for diagnostic
purposes

42. Fracture

43. Avascular necrosis

44. Knee
Coronal Sagittal

45. Spine in Sagittal Plane

47. Advantages of MRI
1. No ionizing radiation & no short/long-term
effects demonstrated
2. Particularly useful for soft tissues, such as
ligaments and cartilage, organs like brain
3. Better contrast resolution & tissue
discrimination
4. Various sequences to play with to
characterize the abnormal tissue
5. Many details without I.V contrast

48. Disadvantages of MRI
►Relies on soft tissues and not able to view
the bony structures such as in CT imaging
►Very expensive
►Larger treatment time( 20-90 mins)
►MRI for Claustrophobic patients can be very
disconcerting
►Machine makes tremendous noise

49. Fixed field magnet is always on!