Magnetic resonance imaging (MRI) is a medical imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body. MRI is based on nuclear magnetic resonance, which uses magnetic fields to detect atomic nuclei within tissues from different angles in order to form cross-sectional images of internal structures. The document discusses the physics principles behind MRI, including how hydrogen protons are aligned by magnetic fields and how their signal can be localized to different regions of the body. It also covers differences between T1-weighted and T2-weighted MRI sequences, common artifacts that can appear on images, and advantages and risks of MRI compared to other imaging techniques like X-ray and CT scans.

2. NMRI or MRI?
Magnetic resonance imaging
. (MRI) is an imaging technique used primarily in medical settings to produce high quality
images of the inside of the human body.
* MRI is based on the principles of nuclear magnetic resonance (NMR).
• NMR is a spectroscopic technique used by scientists to obtain microscopic chemical and
physical information about molecules.
• The technique was called magnetic resonance imaging rather than nuclear magnetic
resonance imaging (NMRI).
•MRI started out as a tomographic imaging technique, that is it produced an image of
the NMR signal in a thin slice through the human body.
•MRI has advanced beyond a tomographic imaging technique to a volume imaging
technique.
•The slice is said to be composed of several volume elements or voxels.

3. •The volume of a voxel is approximately 3 mm3.
•The magnetic resonance image is composed of several picture elements called pixels.
•The intensity of a pixel is proportional to the NMR signal intensity of the contents of the
corresponding volume element or voxel of the object being imaged.
Felix Bloch slice
3 mm3

10. = electron
= neutron
= proton

11. X-RAY
Show differences in electron density
Small inherent contrast in soft tissue
Many photons means good S/N ratio
Good geometric resolution
Planar ( projections )
Can be used for tomography ( CT-scan)
Uses ionizing radiation

12. NMR – IMAGING (MRI)
No ionising radiation
Large inherent contrast in soft tissue
Can demonstrate both anatomy and function
Good geometrical resolution
Expensive
Restricted acces to patient during exam

13. NMR/MRI
Nuclear: any nucleus with a non-zero nuclear spin: 1H, 31P, 13C, 19F
Magnetic: uses an external magnetic field to interact with nuclear
magnetic moments
Resonance: exploit resonance to perturb nuclear spins
Imaging: vary the external field as a function of position to localize
signals

14. PHYSICS OF PROTONS.
• Motion of electrically charged particles results in a magnetic force
orthogonal to the direction of motion
• Protons (nuclear constituent of atom) have a property of angular
momentum known as spin
Angular momentum (spin) of a proton.

15. PROTONS ALIGNING WITHIN A MAGNETIC FIELD
In “field free” space
randomly oriented
• when placed in a magnetic field (B0; e.g., our MRI machines) protons will either align
with the magnetic field or orthogonal to it (process of reaching magnetic equilibrium)
• there is a small difference (10:1 million) in the number of protons in the low and high
energy states – with more in the low state leading to a net magnetization (M)
Inside magnetic field
oriented with or against B0
M = net magnetization
M
Applied Magnetic
Field (B0)

16. A top that is spinning slightly off the vertical axis is precessing about the vertical axis.

17. A hydrogen atom precesses about a magnetic field.

18. All of the hydrogen protons will align with the magnetic field in one direction or the other. The
vast majority cancel each other out, but, as shown here, in any sample there is one or two
"extra" protons.

19. * The MRI machine applies an RF (radio frequency) pulse that is specific only to
hydrogen.
* The system directs the pulse toward the area of the body we want to examine.
* The pulse causes the protons in that area to absorb the energy required to make
them spin, or precess, in a different direction. This is the "resonance" part of MRI.
* The RF pulse forces them (only the one or two extra unmatched protons per
million) to spin at a particular frequency, in a particular direction.
* The specific frequency of resonance is called the Larmour frequency.
* It is calculated based on the particular tissue being imaged and the strength of
the main magnetic field.
How MRI Works

20. LARMOR FREQUENCY
Larmor equation
f = B0
 = 42.58 MHz/T
At 1.5T, f = 63.76 MHz
At 4T, f = 170.3 MHz
Field Strength (Tesla)
Resonance
Frequency for 1H
170.3
63.8
1.5 4.0
• The energy difference between the high (oriented with B0) and low (oriented against
B0) energy protons is measurable and is expressed in the Larmor equation

21. RF pulses
* These are usually applied through a coil.
* MRI machines come with many different coils designed for different parts of the body: knees,
shoulders, wrists, heads, necks and so on.
* These coils usually conform to the contour of the body part being imaged, or at least reside very close
to it during the exam.
* At approximately the same time, the three gradient magnets jump into the act.
* They are arranged in such a manner inside the main magnet that when they are turned on
and off very rapidly in a specific manner.
* They alter the main magnetic field on a very local level.
What this means is that we can pick exactly which area we want a picture of.

22. In MRI we speak of "slices." Think of a loaf of bread with slices as thin as a few millimeters
-- the slices in MRI are that precise.
We can "slice" any part of the body in any direction, giving us a huge advantage over any
other imaging modality. That also means that you don't have to move for the machine to get
an image from a different direction -- the machine can manipulate everything with the
gradient magnets.

24. When the RF pulse is turned off, the hydrogen protons begin to slowly
(relatively speaking) return to their natural alignment within the magnetic
field and release their excess stored energy.
When they do this, they give off a signal that the coil now picks up and
sends to the computer system.
What the system receives is mathematical data that is converted, through
the use of a Fourier transform, into a picture that we can put on film.
That is the "imaging" part of MRI.

38. Mx (RE) My (IM)

39. CauseArtifact
Failure of the RF detection circuitryRF Offset and
Quadrature Ghost
Failure of the RF shieldingRF Noise
Metal object distorting the Bo fieldBo Inhomogeneity
Failure in a magnetic field gradientGradient
Objects in the FOV with a higher or lower
magnetic susceptibility
Susceptibility
Failure or normal operation of RF coil, and
metal in the anatomy
RF Inhomogeneity
Movement of the imaged object during the
sequence
Motion
Movement of body fluids during the sequenceFlow
Large Bo and chemical shift difference between
tissues
Chemical Shift
Large voxel sizePartial Volume
Improperly chosen field of viewWrap Around
Small image matrix and sharp signal
discontinuities in an image
Gibbs Ringing
IMAGE ARTIFACTS

40. LONGITUDINAL RELAXATIONAND T1.
• temperature influences the number of collisions (and hence the
rate at which protons flip between low and high energy states)
• so magnetic equilibrium (M0), or the rate at which a body placed
inside B0 becomes magnetized depends on temperature – this is
known as longitudinal relaxation
• the T1-weighted image (usually used for anatomical images)
measures the rate at which the object placed in B0 (the
unsuspecting subject in our case) goes from a non-magnetized to
a magnetized state – the longitudinal relaxation
• different types of molecules (and by extension tissue) approach
M0 at different rates allowing us to differentiate things like white
and grey matter – we creep close towards the image!!!

41. The time constant which
describes how MZ returns to its
equilibrium value is called the
spin lattice relaxation time (T1).
The equation governing this
behavior as a function of the
time t after its displacement is:
Mz = Mo ( 1 - e-t/T1 )
T1 RELAXATION

42. T2 RELAXATION
The time constant which describes the return to equilibrium of
the transverse magnetization, MXY, is called the spin-spin
relaxation time, T2.
MXY =MXYo e-t/T2

43. T1 AND T2
• T1 measures the longitudinal relaxation (along B0) – or the rate at
which the subject (and the various different constituents of that
subject) reaches magnetic equilibrium
• T2 measures the transverse relaxation (along B1) – or the rate of
decay of the signal after an RF pulse is delivered
• T1 – recovery to state of magnetic equilibrium
• T2 – rate of decay after excitation
Tissue T2 decay times (in 1.5 T magnet)
white matter 70 msec
grey matter 90 msec
CSF 400 msec

44. T1 AND TR
T1 = recovery of longitudinal (B0) magnetization after the RF pulse
• used in anatomical images
• ~500-1000 msec (longer with bigger B0)
TR (repetition time) = time to wait after excitation before sampling T1

45. T2 AND TE
T2 = decay of transverse magnetization after RF pulse
TE (time to echo) = time to wait to measure T2 or T2* (after re-focusing
with spin echo)

47. MR-T11 MR-T21 xray-CT2
dense bone dark dark bright
air dark dark dark
fat bright bright dark
water dark bright dark
brain "anatomic"3 interm. interm.
Normal tissue

48. MR-T1 MR-T2
x-ray-
CT
enhancement1
infarct dark bright dark subacute
bleed bright2 bright2 bright no
tumor dark bright dark3 yes
MS plaque dark bright dark4 acute
Abnormal tissue

49. Photo courtesy NASA
An MRI scanner. See more MRI pictures and images .

50. Photo courtesy NASA
In this MRI scan, you can clearly see the shattered fragments of a human wrist broken
from a fall.

51. In this photograph, you can see a fully loaded pallet jack that has been sucked into the bore of an MRI system.

52. Magnetic Resonance and MRI Safety
* The biggest and most important component in an MRI system is the magnet
• The magnet in an MRI system is rated using a unit of measure known
as a tesla.
•Another unit of measure commonly used with magnets is the gauss
)tesla = 10,000 gauss).
* The magnets in use today in MRI are in the 0.5-tesla to 2.0-tesla
range
* More powerful magnets -- up to 60 tesla -- are used in research
* Metal objects can become dangerous projectiles if they are taken into
the scan room.
•Credit cards, bank cards and anything else with magnetic encoding will be
•erased by most MRI systems.

53. Photo courtesy NASA
This image set is comparing a young individual (left) with an athletic male in his 80's (center) and
with a person of similar age having Alzheimer's Disease (right), all imaged at the same level.

54. Advantages
-MRI scanning is one of the safest imaging techniques available.
-MRI scans can be used to produce images of almost any part of the body and can
produce images from all angles. They are especially useful for showing structures
made of soft tissue, such as ligaments and cartilage, and organs such as the brain,
heart and eyes.
-MRI scans have specific advantages over X-rays, as they can:
• Provide greater detail when looking at soft tissue,
• Show the differences between various types of tissue,
• Show swelling and inflammation,
• Show the condition of blood vessels and blood flow, and
• Show both three-dimensional and cross-section images of the body.

55. Disadvantages
-During an MRI scan you may have to remain motionless for up to an hour, which can be
difficult. However, most MRI scans only last between 15-30 minutes.
-MRI scanners can be very noisy, although you will be given headphones to block out the
noise, and in many cases allow you to listen to music.
-Depending on the part of the body being scanned, your head may need to be inside the
MRI scanner tube, and for some people this may feel claustrophobic.
-MRI scanners can be affected by movement, which makes them unsuitable for
investigating problems such as mouth tumors, because coughing or swallowing can make
the scan less clear.
-Bone and calcium do not show up on an MRI scan. This means that tissue calcification, a
feature of a number of diseases such as osteoporosis, cannot be detected using MRI
scanning.

56. Risks
-MRI is considered a very safe procedure. Although the magnetic field created by an
MRI scanner can cause certain types of metal within the body to move, everyone
having an MRI scan is taken through a detailed checklist before their scan to make
sure it is safe for them to have one.
-Apart from the effect on metal within the body, there are no known negative effects
of being exposed to the magnetic field and radio waves of an MRI scan.
-An MRI scan has no known side effects on pregnant women. However, the long-term
effects of strong magnetic fields on a developing baby are not yet known, so pregnant
women are not normally scanned unless absolutely necessary.
-Some MRI scans require the injection of a special dye into the body. Allergic
reactions to this dye are possible, but rare, and can usually be treated immediately.

57. THANK YOU