MRI uses strong magnetic fields and radio waves to produce detailed images of the inside of the body without using ionizing radiation. It works by aligning hydrogen atoms in the body when placed in a magnetic field and using radio waves to stimulate them, causing them to emit signals detected by a scanner to form an image. The document discusses the history of MRI's development, the basic principles including how protons are used to generate signals, the components of an MRI machine, and the different types of sequences and images that can be produced.

1. Principles of MRI
By :
Dr. Muhannad M. Hadi Al-Mukhtar
Final stage - Anaesthesia and Intensive Care
Iraqi Board for Medical Specializations
Medical City Baghdad Hospital
December 2017
Supervised by :
Dr. Ayad Abbas Salman
Chairman of scientific council of
Anesthesia and intensive care
Iraqi Board for Medical Specialities

2. History
• MRI was initially called Nuclear Magnetic Resonance
Imaging after its early use for chemical analysis.
• The "Nuclear" was dropped off about 35 years ago
because of fears that people would think there was
something radioactive involved, which there is not.

3. History
• The magnetic resonance phenomenon has been known
since the 1940s.
• Initially employed to determine the structures of
molecules.
• It was discovered by Felix Bloch and Edward Purcell, two
American scientists, who were awarded the Nobel Prize in
physics in 1952 for their discovery.

4. History
- The use of NMR to produce
2D images was accomplished
by Paul Lauterbur , and Sir
Peter Mansfield who imaged
the fingers of a research
student, Dr Andrew Maudsley
in 1976.
- In 2003, Paul C. Lauterbur
and Sir Peter Mansfield were
awarded the Nobel Prize in
medicine for their contribution
to the development of MRI for
medical purposes.
Paul Lauterbur Sir Peter Mansfield

5. Definition
• MRI (Magnetic Resonance Imaging) is an imaging
modality based on an interaction between transmitted
radiofrequency (RF) waves and hydrogen nuclei in human
body under the influence of a strong magnetic field.

6. Principles of MRI
• Simply stated, MRI is based on measurements of energy
emitted from hydrogen nuclei following their stimulation by
radio-frequency signals.
• The energy emitted varies according to the tissues from
which the signals emanate.
• This allow MRI to distinguish between different tissues.

7. • Almost 99% of the mass of the human body is made up of
six elements: oxygen, carbon, hydrogen, nitrogen,
calcium, and phosphorus.
• Only about 0.85% is composed of another five elements:
potassium, sulfur, sodium, chlorine, and magnesium.
• All 11 are necessary for life.

8. How do protons help in MR
imaging?
• Protons are positively charged and have rotatory
movement called Spin.
• Any moving charge generates current.
• Every current has a small magnetic field around it.
• So every spinning proton has a small magnetic field
around it, also called magnetic dipole moment.

9. Why Proton only?
• Other substances can also be utilized for MR imaging.
• The requirements are that their nuclei should have spin and
should have odd number of protons within them. Hence
theoretically 13C, 19F, 23Na, 31P can be used for MR imaging.
• Hydrogen atom has only one proton.
• Hence H + ion is equivalent to a proton.
• Hydrogen ions are present in abundance in body water.
• H+ gives best and most intense signal among all nuclei.

10. 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.
• MRI utilizes this magnetic spin property of protons of
hydrogen to elicit images.

11. Elements of MRI

12. PRIMARY MAGNETIC FIELD
PRIMARY MAGNETIC FIELD
GRADIENT COILS
GRADIENT COILS
RF COILS
RF COILS
COMPUTER RF DETECTORCOMPUTERIMAGE

13. Elements of MRI

14. Magnetic Field
• measured by Tesla (T).
• Clinical MRI (1.5 - 3 )Tesla
• Earth Magnetic field = 0.00003 T
• 1 Tesla = 20,000 times the strength of the earth’s
magnetic field.

16. Gradient Coils
• Three gradient coils, one for each of the orthogonal planes, are located
within the core of the MRI unit.
• Alter primary magnetic field.
• Responsible for loud noises of MRI.
• Allow spatial encoding for MRI images in the X,Y, and Z axis i.e
localization.
• Z gradient runs along the Long axis to produce Axial images.
• Y gradient runs along the Vertical axis to produce Coronal images.
• X gradient runs along the Horizontal axis to produce Sagittal images

18. Z
X
Y

19. Radio-frequency Coils
The RF coils serve two purposes
1. Transmitting the RF pulses that alter the alignment of the protons
2. Receiving the signals emitted from the protons

20. Steps to get MR images
• 1. Placing the patient in the magnet.
• 2. Sending Radiofrequency (RF) pulse by coil
• 3. The radiowave is turned off
• 4. The patient’s body emits a signal
• 5. Receiving signals from the patient by coil
• 6. Transformation of signals into image by complex
processing in the computers.

21. Protons in Human Body
• Normally the protons in human body (outside the magnetic
field) move randomly in any direction.
• When external magnetic field is applied, i.e. patient is
placed in the magnet, these randomly moving protons
align (i.e. their magnetic moment align) and spin in the
direction of external magnetic field.
• Some of them align parallel and others anti-parallel to the
external magnetic field.

22. Human protons and
Magnetic Field
• When a proton aligns along
external magnetic field, not only it
rotates around itself (called spin)
but also its axis of rotation moves
forming a ‘cone’. This movement of
the axis of rotation of a proton is
called as precession.
• The number of precessions of a
proton per second is called
precession frequency. It is
measured in Hertz.
• Precession frequency is directly
proportional to strength of external
magnetic field.
on frequency of the hydrogen proton at 1, 1.5 and 3 Tesla is roughly 42, 64 and 128 MHz res

24. Protons before and After
applying Magnetic Field

26. Basic four steps of MR
imaging include:
• 1. Patient is placed in the magnet—
• All randomly moving protons in patent’s body align and
precess along the external magnetic field. Longitudinal
magnetization is formed long the Z-axis.

28. • 2. RF pulse is sent
• The precession frequency of protons should be same as RF pulse
frequency for the exchange of energy to occur between protons
and RF pulse. When RF pulse and protons have the same
frequency protons can pick up some energy from the RF pulse.
This phenomenon is called as “resonance”- the R of MRI.
• Precessing protons pick up energy from RF pulse to go to higher
energy level and precess in phase with each other. This results in
reduction in longitudinal magnetization and formation of
transverse magnetization in X-Y plane.

29. RESONANCE
• Resonance relates to the transfer/exchange of energies
between two systems at a specific frequency.
• It is analogous to talking to someone on your cell phone.
• In magnetic resonance, only protons with the same frequency
as the RF pulse will respond.
• During RF pulse delivery nuclei become excited and then
return to equilibrium.
• During equilibrium, they emit energy in the form of
electromagnetic waves.

30. T1 AND T2
RELAXATION
• When RF pulse is stopped higher energy gained by proton
is retransmitted and hydrogen nuclei relax by two
mechanisms
• T1 or spin lattice relaxation- by which original
magnetization (Mz) begins to recover.
• T2 relaxation or spin spin relaxation - by which
magnetization in X-Y plane decays towards zero in an
exponential fashion. It is due to incoherence of H nuclei.
• T2 values of CNS tissues are shorter than T1 values

31. T1 is defined as the time it takes for the
hydrogen nucleus to recover 63% of its
longitudinal magnetization

32. T2 relaxation time is the time for 63% of the
protons to become dephased owing to
interactions among nearby protons.

33. • 3. MR signal is received
• The transverse magnetization vector precesses in
transverse plane and generates current. This current is
received as signal by the RF coil.
• 4. Image formation—
• MR signal received by the coil is transformed into image
by complex mathematical process such as Fourier
Transformation by computers.

34. EXCITATION AND RELAXATION

36. TE is the time at which the signal is captured.
TR is the time at which the RF pulse is repeated to again displace the protons.

37. TR ( repetition time)
• The time between two excitation pulses is called repetition
time.

38. TE ( time of echo)
• TE (echo time) : It is the time between the excitation pulse
and the echo.

39. TYPES OF MRI
IMAGINES
• T1WI
• T2WI
• FLAIR
• STIR
• DWI
• ADC
• GRE
• MRS
• MT
• Post-Gd images
• MRA
• MRV

40. T1-Weighted Imaging
• characterized by short TR and TE times.
• Thus the signal is caught early, at a time when the
difference in relaxation characteristics for fat and water is
most noticeable and tissues that rapidly recover their
longitudinal magnetization, such as fat, give rise to high
signal intensity (create a bright image).
• When short TE is employed, tissues that are slow to
regain longitudinal magnetization, such as tissues with
high free-water content, render low signal intensity .These
tissues appear dark on T1-weighted images.

41. T1
• TR: short
• TE: short
• fat: bright
• fluid: dark

42. T2
• TR: long
• TE: long
• fat: intermediate-bright
• fluid: bright

43. • Long TR and TE times characterize T2 imaging.
• Because in T2-weighted imaging the signal is measured
late in the decay process, tissues that are most reluctant
to give up energy are selectively imaged.
• Free water is slow to give up its energy and consequently
renders high signal intensity on T2 sequences.
• Fat, which gives up its energy rapidly, gives rise to low
intensity on T2.
T2-Weighted Imaging

44. The Difference Between T1-
and T2-Weighted Imaging
• T1 imaging measures energy from structures such as fat,
which give up energy rapidly, early in the process of
longitudinal remagnetization.
• T1 imaging provides images of good anatomic detail,
displaying the tissues in a fairly balanced manner.
• T2 imaging measures energy late in the decay of
transverse relaxation and selectively images structures
that do not readily give up energy, such as water.
• It is particularly valuable for detecting inflammation

49. Proton Density (PD)
• 1. TR is long (more than 2,000 ms) and TE is short (20 to
30 ms).
• 2. Images are based on measurements of proton density
and are similar in appearance to T1 images but with
greater anatomic detail.

50. Advantages of MRI
• 1. Non-ionising, for they do not use X-ray as medium for imaging.
• 2. Multiplanar imaging is automatically possible, for images in sagittal,
coronal and transverse planes are generated simultaneously.
• 3. Superior contrast in tissue give exquisite anatomical details.
• 4. Certain tissue diagnosis is possible, e.g. lipoma, edema, age of
hemorrhage, etc.
• 5. MR myelogram is created without injection of any contrast medium.
• 6. In tumor imaging, it gives exact anatomical details regarding the
tumor limits, edema limits, vascularity, etc.

51. Disadvantages of MRI
(compared to CT scan)
• 1. It has low sensitivity for calcium, therefore cannot diagnose calcification
clearly.
• 2. It has low sensitivity for acute hemorrhage.
• 3. Scan time is prolonged.
• 4. Contraindications prevent certain patients from entering the MRI
system. Patients with metallic implants like cochlear implant, steel
sutures, pacemakers, etc. are not allowed inside the MR scanner.
• 5. Patients with claustrophobia cannot tolerate the study and some
young children may need anesthesia.
• 6. Intravenous contrast agents may be needed.