Introduction and Basics of MRI for Physiotherapy students.

1. MRI
Magnetic Resonance Imaging

2. Magnetic resonance imaging (MRI) is a type of scan
that uses strong magnetic fields and radio waves to produce detailed images of
the inside of the body.
unlike X-rays, MRI scanners create images of the body using a large magnet
and radio waves. No radiation is produced during an MRI exam. Because
ionizing radiation is not used,

3. The first MRI scanner used to image the human body was built in New York in 1977.
The MRI scanner is essentially a giant magnet. The strength of the magnet is
measured in a unit called Tesla (T).
Most MRI scanners used in hospitals and medical research clinics are 1.5 or 3 T.
Putting that in to perspective, the earth’s magnetic field is around 0.00006T.
A 3 T MRI scanner is around 60,000 times stronger than the earth’s magnetic field!

4. MRI uses magnetic fields and radio waves to measures how much water is in
different tissues of the body, maps the location of the water and then uses this
information to generate a detailed image.
The images are so detailed because our bodies are made up of around 65% water,
so we have lots of signal to measure.
The water molecule (H2O) is made up of two hydrogen atoms and one oxygen
atom.
The hydrogen (H) atoms are the part that makes water interesting for MRI, and
what we use to measure the signal from the body when we do an MRI scan.

5. Basic Principles

6. Basic Principles:
The magnet embedded within the MRI scanner can act on these positively charged
hydrogen ions (H+ ions) and cause them to ‘spin’ in an identical manner. By varying
the strength and direction of this magnetic field, we can change the direction of ‘spin’
of the protons, enabling us to build layers of detail.
When the magnet is switched off, the protons will gradually return to their original
state in a process known as precession. Fundamentally, the different tissue types
within the body return at different rates and that allows us to visualise and
differentiate between the different tissues of the body.

9. Let’s Make it Easy!

10. Characteristics

11. Signal T1-weighted T2-weighted
High
•Fat
•Subacute hemorrhage
•Melanin
•Protein-rich fluid
•Slowly flowing blood
•Paramagnetic or diamagnetic substances, such
as gadolinium, manganese, copper
•Cortical pseudolaminar necrosis
•Anatomy
•More water content as
in edema, tumor, infarction, inflammation and
Infection
•Extracellularly located methemoglobin in subacute
hemorrhage
•Fat
•Pathology
Intermediate Gray matter darker than white matter White matter darker than grey matter
Low
•Bone
•Urine
•CSF
•Air
•More water content, as
in edema, tumor, infarction, inflammation, infection,
hyperacute or chronic hemorrhage
•Low proton density as in calcification
•Bone
•Air
•Low proton density, as in calcification and fibrosis
•Paramagnetic material, such as deoxyhemoglobin,
intracellular methemoglobin, iron, ferritin, hemosiderin
, melanin
•Protein-rich fluid

12. Sequences

13. Group Sequence Abbr. Main clinical distinctions
Spin echo
T1 weighted T1
•Lower signal for more water content, as
in edema, tumor, infarction, inflammation, infection, hyperacute or
chronic hemorrhage.
•High signal for fat
•High signal for paramagnetic substances, such as MRI contrast agents
Standard foundation and comparison for other sequences
T2 weighted T2
•Higher signal for more water content
•Low signal for fat − Note that this only applies to standard Spin Echo (SE) sequences
and not the more modern Fast Spin Echo (FSE) sequence (also referred to as Turbo
Spin Echo, TSE), which is the most commonly used technique today. In FSE/TSE, fat
will have a high signal.
•Low signal for paramagnetic substances
Standard foundation and comparison for other sequences
Proton density
weighted
PD •Joint disease and injury. High signal from meniscus tears.

14. Gradient
echo (GRE)
Steady-state free
precession
SSFP Creation of cardiac MRI videos
Effective T2
or "T2-star"
T2* Low signal from hemosiderin deposits and hemorrhages.
Susceptibility-weighted SWI
Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or
calcium.
Group Sequence Abbr. Main clinical distinctions

15. Inversion
recovery
Short tau inversion recovery STIR
High signal in edema, such as in more severe stress fracture. Shin
splints pictured:
Fluid-attenuated inversion
recovery
FLAIR
High signal in lacunar infarction, multiple sclerosis (MS)
plaques, subarachnoid haemorrhage and meningitis (pictured).
Double inversion recovery DIR High signal of multiple sclerosis plaques (pictured).
Group Sequence Abbr. Main clinical distinctions

16. Diffusion
weighted (DWI)
Conventional DWI High signal within minutes of cerebral infarction (pictured).
Apparent diffusion
coefficient
ADC Low signal minutes after cerebral infarction (pictured).
Diffusion tensor DTI
•Evaluating white matter deformation by tumors
•Reduced fractional anisotropy may indicate dementia.
Group Sequence Abbr. Main clinical distinctions

17. Group Sequence Abbr. Main clinical distinctions
Perfusion
weighted (PWI)
Dynamic susceptibility
contrast
DSC
•Provides measurements of blood flow
•In cerebral infarction, the infarcted core and the penumbra have decreased
perfusion and delayed contrast arrival (pictured).
Arterial spin labelling ASL
Dynamic contrast
enhanced
DCE
Faster Gd contrast uptake along with other features is suggestive of malignancy
(pictured).[85]

18. Functional
MRI (fMRI)
Blood-oxygen-level
dependent imaging
BOLD
Localizing brain activity from performing an assigned task (e.g. talking, moving
fingers) before surgery, also used in research of cognition.
Magnetic resonance
angiography (MRA)
and venography
Time-of-flight TOF Detection of aneurysm, stenosis, or dissection
Phase-contrast
magnetic resonance
imaging
PC-MRA Detection of aneurysm, stenosis, or dissection
Group Sequence Abbr. Main clinical distinctions

19. Images

20. T1 weighted

21. T2 weighted

22. Proton density weighted

23. Steady-state free
precession

24. "T2-star"

25. Susceptibility-weighted

26. Short tau inversion recovery

27. Fluid-attenuated
inversion recovery

28. Double inversion recovery

29. Conventional DWI

30. Apparent diffusion coefficient

31. Diffusion tensor

32. Dynamic susceptibility contrast
Arterial spin labelling

33. Dynamic contrast
enhanced

34. Blood-oxygen-level
dependent imaging

35. Time-of-flight

36. Phase-contrast
magnetic resonance
imaging

37. What we need!

38. MRI IMAGING SEQUENCES
The most common MRI sequences are T1-weighted and T2-weighted scans.
T1-weighted images are produced by using short TE (Time to echo) and TR (Repetition
Time). The contrast and brightness of the image are predominately determined by T1
properties of tissue.
Conversely, T2-weighted images are produced by using longer TE and TR times. In these
images, the contrast and brightness are predominately determined by the T2 properties of
tissue.
In general, T1- and T2-weighted images can be easily differentiated by looking the CSF.
CSF is dark on T1-weighted imaging and bright on T2-weighted imaging.

39. T1 weighted sequences:
T1 weighted (T1W) sequences are part of almost all MRI protocols and are best thought of
as the most 'anatomical' of images.
(historically the T1W sequence was known as the anatomical sequence), resulting in images that most closely
approximate the appearances of tissues macroscopically, although even this is a gross simplification.
The dominant signal intensities of different tissues are:
• Fluid (e.g. urine, CSF): low signal intensity - Black
• Muscle: intermediate signal intensity - Grey
• Fat : high signal intensity - White
• Brain:
grey matter: intermediate signal intensity (grey)
white matter: hyperintense compared to grey matter (white-ish)

40. i. Contrast enhanced
The most commonly used contrast agents in MRI are gadolinium based.
At the concentrations used, these agents have the effect of causing T1 signal to be
increased (this is sometimes confusingly referred to as T1 shortening).
The contrast is injected intravenously (typically 5-15 mL) and scans are obtained a few
minutes after administration.
Pathological tissues (tumors, areas of inflammation/infection) will demonstrate
accumulation of contrast (mostly due to leaky blood vessels) and therefore appear as
brighter than surrounding tissue.

41. ii. Fat suppression
Fat suppression (or attenuation or saturation) is a tweak performed on many T1
weighted sequences, to suppress the bright signal from fat. This is performed most
commonly in two scenarios:
Firstly, and most commonly, after the administration of gadolinium contrast. This has
the advantage of making enhancing tissue easier to appreciate.
Secondly, if you think that some particular tissue is fatty and want to prove it, showing
that it becomes dark on fat suppressed sequences is handy.

42. T2 weighted sequences:
T2 weighted (T2W) sequences are part of almost all MRI protocols.
Without modification the dominant signal intensities of different tissues are:
• Fluid (e.g. urine, CSF): high signal intensity (white)
• Muscle: intermediate signal intensity (grey)
• Fat: high signal intensity (white)
• Brain:
grey matter: intermediate signal intensity (grey)
white matter: hypointense compared to grey matter (dark-ish)

43. 1. Fat suppressed
In many instances one wants to detect edema in soft tissues which often have significant
components of fat.
As such suppressing the signal from fat allows fluid, which is of high signal, to stand out.
This can be achieved in a number of ways (e.g. chemical fat saturation or STIR) but the end
result is the same.
2. Fluid attenuated
Similarly in the brain, we often want to detect parenchymal edema without the glaring high
signal from CSF. To do this we suppress CSF.
This sequence is called FLAIR. Importantly, at first glance FLAIR images appear similar to T1
(CSF is dark). The best way to tell the two apart is to look at the grey-white matter.
T1 sequences will have grey matter being darker than white matter.
T2 weighted sequences, whether fluid attenuated or not, will have white matter being darker
than grey matter.

44. Diffusion weighted imaging assess the ease with which water molecules move around within
a tissue (mostly representing fluid within the extracellular space) and gives insight into
cellularity (e.g. tumors), cell swelling (e.g. ischemia) and edema.
1. DWI
Acute pathology (ischemic stroke, cellular tumor, pus) usually appears as increased signal denoting restricted diffusion
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)
• fat: little signal due to paucity of water
• other soft tissues: intermediate signal intensity (grey)
because there is a component of the image derived from T2 signal, some tissues that are bright on T2 will
appear bright on DWI images without there being an abnormal restricted diffusion. This phenomenon is known
as T2 shine through.
Diffusion weighted sequences:

45. 2. ADC
Acute pathology (ischemic stroke, cellular tumor, pus) usually appears as decreased signal denoting restricted diffusion.
Apparent diffusion coefficient maps (ADC) are images representing the actual diffusion
values of the tissue without T2 effects.
They appear basically as grayscale inverted DWI images.
They are relatively low resolution images with the following appearances:
• grey matter: intermediate signal intensity (grey)
• white matter: slightly hyperintense compared to grey matter
• CSF: high signal (white)
• fat: little signal due to paucity of water
• other soft tissues: intermediate signal intensity (grey)

46. Comparison of
T1 T2 Flair

47. Comparison of T1 vs. T1 with Gadolinium

48. Comparison of Flair vs. Diffusion-weighted

49. References

50. 1. https://case.edu/med/neurology/NR/MRI%20Basics.htm
2. https://radiopaedia.org/articles/mri-sequences-overview
3. https://www.mayoclinic.org/tests-procedures/mri/about/pac-
20384768#:~:text=Magnetic%20resonance%20imaging%20(MRI)
%20is,large%2C%20tube%2Dshaped%20magnets.
4. https://en.wikipedia.org/wiki/Magnetic_resonance_imaging
5. https://www.radiologyinfo.org/en/info/bodymr
References

51. THANK YOU…
Presented by: Dinu Dixon
MPT - NEUROLOGY

52. The human body is largely made of
water molecules, which are comprised of
hydrogen and oxygen atoms.
At the center of each atom lies an even
smaller particle called a proton, which
serves as a magnet and is sensitive to
any magnetic field.
H
H
O

53. The strong magnetic field created by
the MRI scanner causes the atoms in
your body to align in the same
direction.
How Does it Work???
Normally, the atoms in the body
are randomly arranged,

54. Radio waves are then sent from the
MRI machine and move these atoms out
of the original position.
As the radio waves are turned off, the
atoms tries to return to their original
position and send back radio signals.
These signals are received by a computer and converted into an
image of the part of the body being examined. This image appears
on a viewing monitor.

55. Is that clear???