Principle of MRI.pptx

Principle of MRI (Physics)
Dr Musa WA
Resident
Radiology Dept., FMCK
26-01-2023
1

Outline
• Introduction
• Magnetism: Tiny Magnets vs Main Magnets
• MRI Machine
• Signal Generation On MRI
• MRI Pulse sequences.
• Spatial Encoding
• MRI Artifacts.
• Safety in MRI
• Reference
2

Introduction
DEFINITION:
• Magnetic Resonance Imaging (MRI) is an imaging modality based on
an interaction between transmitted radiofrequency (RF) waves and
protons (hydrogen nuclei) in human body under the influence of a
strong magnetic field.
3

Introduction
• The human has abundance of hydrogen (H) atoms – H2O, protein and
fat.
• H nuclei have small magnetic field due to their +ve charge and
rotatory movement called spin.
• This magnetic field can be influenced by an external magnetic field
and then radio waves to generate MR signals.
• MR signal are converted to visual via a computational process called
fourier transform.
• The MR signals emitted varies according to the tissues from which the
signals emanate → contrast between tissues.
4

Magnetism – Tiny Magnets
Hydrogen nuclei (protons) as a magnet
• Protons are +vely charged and have rotatory movement called spin.
• Any moving charge generates current which in turn give rise to a
small magnetic field around it.
• So every spinning proton has a small magnetic field around it, also
called magnetic dipole moment.
5

• Magnetic characteristics of the nucleus are also described by the nuclear
magnetic moment, represented as a vector indicating magnitude and direction.
• This is determined through the pairing of the constituent protons (P) and
neutrons (N).
• If P + N in the nucleus is even then the nuclear magnetic moment is essentially
zero.
• However, if N is even and P is odd, or N is odd and P is even, the resultant non-
integer nuclear spin generates a nuclear magnetic moment.
6

• If the mass number A (P + N) is odd, the nuclear spin, I, is a multiple
of ‘1/2 (‘1/2;, 3/2, 5/2, 7/2).
• If the mass number A and the atomic number Z (protons) are both
even, I is 0 .
• If the mass number A is even but the atomic number Z is odd, I is a
whole number (I, 2, 3, 4, or 5)
7

Magnetism – Main Magnets
Three types of main magnet are used in MRI:
1. Permanent Magnet.
2. Resistive Electromagnet.
3. Superconducting Electromagnet.
9

Magnetism – Large Magnets
PERMANENT MAGNET
• Consists of two opposing flat faced highly magnetised pole pieces fixed to
an iron frame
• It is bulky and can weigh up to 80 tonnes but modern scanners weigh as
low as 10 tonnes.
• Requires no electricity to generate external magnetic field but cannot be
shut down which is a major disadvantage in an emergency situation.
• Has low magnetic field strength of typically up to 0.3 Tesla (usually
between 0.1 and 0.3 Tesla).
• It is expensive to buy but the cheapest to run in terms of maintenance cost.
• Direction of external magnetic field is anteroposterior with the patient
lying supine
10

RESISTIVE ELECTROMAGNET
• Direct current coils are wound with copper or aluminium strips.
• It is an air cored magnet.
• An electric current is passed through the coils of wire which then behave
as a solenoid.
• It is cooled with water.
• It is the cheapest and smallest type.
• It weighs about 2 tonnes
• Magnetic field strength is limited by heating to 0.5 Tesla.
• Direction of external magnetic field is head to toe with the patient lying
supine.
11

SUPERCONDUCTING ELECTROMAGNET
• Usually comprises of direct current solenoid (coil) made of fine filaments of Niobium
Titanium ( NbTi ) alloy conductors embedded in copper matrix.
• Niobium Tin (Nb3Sn) conductors are used when the magnetic field experienced by the
conductor is in excess of 9 Tesla mainly for laboratory experiments.
• Many filaments of the coil are set in a copper matrix and cooled with liquid Helium
(Boiling point 4.20 K or – 2690 C) because it generates plenty of heat.
• Evaporation of liquid helium is reduced by surrounding the tank (cryostat) with liquid
Nitrogen (Boiling point 770 K or – 1960 C).
• The space around the tanks are insulated by vacuum.
• The machine is large and expensive weighing up to 6 tonnes.
• Direction of the external magnetic field is head to toe with the patient lying supine.
12

MRI Machine
COMPONENTS
1. Magnet mainly superconducting magnet
• It is responsible for the main permanent external magnetic field (B0)
• Majority of electromagnets create a magnetic field strength of 1.5 Tesla (T);
newer machines, 3T or even 7T.
• 1 Tesla = 10,000 gauss and the Earth's magnetic field is approx. 0.5 gauss
2. Shim coil
• Lie just inside of the outer main magnet and are used to fine tune the main
magnetic field to ensure it is as uniform as possible.
14

MRI Machine
COMPONENTS (Contd.)
3. Gradient coils
• There are three sets of gradient coils orientated in the x, y and z axes.
• Used to alter the gradient of the magnetic field ( "Spatial Encoding").
• Are switched on and off rapidly, in 1 ms or less → the loud noise.
4. RF (radiofrequency) coils
• are tuned to a particular frequency.
• Produce a magnetic field at right angles (XY plane) to the main magnetic
field.
• Also receive the MR signals being produced.
• The coils are placed as close to the imaged part as possible to max signal
15

MRI Machine
COMPONENTS (Contd.)
4. RF (radiofrequency) coils
• Types
1. Standard body coil (transmit and receive)
2. Head coil (transmit and receive)
3. Surface (or local) coils (receive only)
4. Phased array coils
5. Transmit phased array coils
16

MRI Machine
TYPES
• Open or Closed based on shape of main magnet
• Closed
• Closed narrow tunnel (open cylinder)
• used in conventional units
• Open
• Large bore or c–shaped magnets
• For claustrophobic Pt
• Limitations: magnets are weaker (0.1 – 0.3T); some limitation in anatomic and
spatial resolution
19

Signal Generation On MRI - Precession
• As earlier mentioned H nuclei are tiny magnets spinning about their
axes.
• When a person (full of H nuclei as started earlier) is placed in the MRI
machine, H nuclei undergo precession due to the external magnetic
field (B0) from the main magnet conventionally applied in the long
axis (Z) of the patient.
• Precession means the H nuclei are also made to spin about the axis of
the magnetic field while still spinning about their axes.
22

• The frequency of this spinning is
the precessional/ Larmor
/rotational frequency.
• The precessional frequency is
calculated by the Larmor
Equation, F = K x B0
23

• The nuclei precess in the Z-axis along the applied magnetic field.
• Most will precess aligned with it (the low energy state) but a few will
precess in the opposite direction (the high energy state).
• However, the majority will be aligned, creating a net longitudinal
magnetisation (Mz) in the Z axis direction.
25

Signal Generation On MRI - Transverse
Magnetisation
• The longitudinal magnetisation cannot be measured.
• To create an MRI signal, the magnetisation needs to "flip" usually to
90°.
• To flip the magnetisation, a rapidly oscillating magnetic field at 90° to
B0 is applied (B1 / radiofrequency pulse / RF pulse).
• This flips the net magnetisation into a transverse plane (Mxy).
28

Signal Generation On MRI - Transverse
Magnetisation
• In order to do this, the B1 magnetic field needs to oscillate at the
same frequency as the precessing nuclei, the resonant frequency, as
this ensures the most efficient transference of energy to the nuclei.
• Remember, this is 42 MHz for a 1 Tesla scanner, or 63 MHz for a 1.5
Tesla scanner.
• NB: RF pulse is a radio wave on the EM spectrum. (Your FM radio
works at a frequency of 88 to 108 MHz). The Faraday Cage prevent
interference by external RF.
30

Signal Generation On MRI - Relaxation
• As soon as the RF is switched off, the transverse magnetisation begins
to disappear and the nuclei relax back to their resting state of net
longitudinal magnetisation (B0, large Mz).
• This happens via two mechanisms and forms the basis for the T1 and
T2 signals.
1. Spin-Lattice Relaxation or T1 Recovery
2. Spin-Spin Relaxation or T2 Decay
• These two processes are occurring at the same time but are
completely independent.
32

1. SPIN-LATTICE RELAXATION OR T1 RECOVERY
• As the nuclei precess in the transverse plane they are jostled by the
surrounding molecules (i.e. the surrounding lattice) giving up their
energy to these molecules.
• As they do so they return to the longitudinal magnetisation (Mz)
exponentially.
• This is called Spin- Lattice or Longitudinal Relaxation.
• The rate at which this happens is governed by the time constant T1.
33

1. SPIN-LATTICE RELAXATION OR T1 RECOVERY (contd.)
• T1 is the time it takes for Mz to recover to 63% of its maximum
value.
• T1 depends on the surrounding molecules and lattice.
• T1 is always longer than T2 (except water in which T1 = T2).
• Effects on T1
• Fat and protein: short T1.
• Water: long T1.
• Bone / calcium / metal: very long T1.
34

2. SPIN-SPIN RELAXATION OR T2 DECAY
• Once the RF pulse is stopped, the magnetic properties of each nuclei
alter the local magnetic field and cause some to precess faster and
some slower.
• Gradually the nuclei lose their coherence and the net transverse
magnetisation reduces to zero.
• The rate it does so is exponential and named the "Free Induction
Decay".
36

2. SPIN-SPIN RELAXATION OR T2 DECAY (contd.)
• The rate at which the transverse magnetisation is lost is determined
by the magnetic interaction between the spins and is called the spin-
spin or transverse decay.
• The time constant of this fall-off is called the T2.
• T2 is the time it takes for the transverse magnetisation to decay to
37% of its value (i.e. loses 63% of its maximum signal)
• T2 depends on the local magnetic field.
38

2. SPIN-SPIN RELAXATION OR T2 DECAY (contd.)
• Effects on T2
• Bone / calcium / metal: short T2.
• Fat: Long T2.
• Water: Very long T2.
40

T2* OR FREE INDUCTION DECAY
• T2 decay described earlier is the exponential curve of transverse
decay in the ideal world.
• In the real world, the transverse decay is much quicker – T2* decay
• Due to local and external magnetic field inhomogeneities.
• The spin-echo sequence deletes the effect of local field
inhomogeneities → only T2 effect is recorded.
41

MRI Pulse Sequences
• An MRI pulse sequence is a programmed set of changing magnetic
gradients.
• Each one is aimed at increasing the tissue contrast of interest.
• They are commands given by the computer to the MRI scanner to:
I. Select a slice(s)
II. Wait for a controlled period(s) for relaxation
III. Precess (Rephase) the protons before signal measurement
IV. Encode and measure the signal
43

MRI Pulse Sequences
• The basic sequences are:
1. Spin echo – SE
2. Gradient (recalled) echo – GRE
• Others:
3. Inversion recovery
4. Diffusion weighted
5. Saturation recovery
6. Echo-planar pulse
7. Spiral pulse
• Multiple sequences needed to adequately evaluate a tissue = MRI
Protocol
44

MRI Pulse Sequences - Spin Echo Sequence
• Aims to remove the effects of the static field (T2*) inhomogeneities
but leave the tissue characteristic T2 effect.
• Step 1:
• A 90° RF pulse is applied.
• All proton vectors precess in phase and the Mxy signal is at its maximum.
• Step 2:
• As Mxy signal decays rapidly (the T2* or free induction decay) some proton
vectors are fast (leaders) and some are slow (laggers)as they dephase.
45

SPIN ECHO SEQUENCE
• Step 3:
• After a time (t) a 180° RF pulse is applied.
• This is a pulse that is applied twice as long as the 90° pulse in the transverse
plane.
• All proton vectors are turned through 180°.
• The laggers become leaders and vice-versa.
46

SPIN ECHO SEQUENCE
• Step 4:
• After the same amount of time (t) the proton vectors are again in phase, the
Mxy signal is at its peak.
• This is the echo and this is the signal that is measured.
• The decay in the signal from the original 90° to the echo is due to the tissue
characteristic T2 effect with the effect of magnetic field inhomogeneities
minimised.
• The time at which this echo is produced is the TE (time to echo).
• It is produced at exactly 2t, t being the time at which the 180° is applied (i.e.
the 180° is applied at TE/2).
48

SPIN ECHO SEQUENCE
• Step 5:
• One cycle has now been completed.
• This cycle is repeated hundreds of times in the sequence.
• The time to the next cycle is TR (time to repetition).
49

MRI Pulse Sequences - T1 Weighted Image
• Uses short TR and short TE.
• TR < 800 ms is used (range: 300 - 800 ms).
• Typically a short TE <30 ms is employed.
• Flip angle is 90o.
• Image contrast is mainly due to proton density and T1 relaxation
properties of imaged tissues.
• The shorter the T 1 of imaged tissues, the stronger the signal and
brighter the pixel
• Fat and fatty bone marrow appear bright.
• CSF and water appear dark .
52

MRI Pulse Sequences – T2 Weighted Image
• Uses long TR and long TE.
• TR >2000 ms is used.
• TE >80 ms is employed (range: 90 - 140ms).
• Flip angle is 90o.
• Image contrast is mainly due to proton density and T2 relaxation
properties of imaged tissues.
• The longer the T2 of imaged tissues, the stronger the signal and
brighter the pixel.
• Fat and fatty bone marrow appear bright
• CSF and water appear brighter.
53

54
Fat is bright on both T1 and T2. Water is bright on T2, dark on T1. Cortical bone, Blood vessels and air all appear dark on T1 and T2.
White mater is white-ish and grey mater is grey on T1, reversed on T2.

MRI Pulse Sequences - Proton Density
Weighted Image
• Uses long TR and short TE.
• TR > 1000 ms is used (range: 1000 - 3000 ms).
• Short TE of 30 ms is employed.
• Flip angle is 900.
• Image contrast is mainly due to proton density properties of imaged tissues.
• The higher the proton density of tissues, the stronger the signal and brighter the
image
• The longer is TR the greater the contrast between tissues of different proton
densities.
• CSF, synovial fluid, fat and other tissues that have high proton density appear
bright .
55

MRI Pulse Sequences - Gradient Recalled Echo
(GRE) 1
• Uses pulse of reduced strength compared to SE sequence.
• The magnetic vector is tipped through a smaller angle than 90° thus allowing for
a short TR.
• A –ve gradient pulse dephases the spins and a +ve gradient pulse rephases them.
• Unlike SE sequence, GRE does NOT eliminate static field inhomogeneities , hence
the acquired image is T2* weighted ; it also does NOT compensate for magnetic
susceptibility.
• In GRE, instead of employing 180°RF pulse as is done in SE sequences, the
gradient field is reversed to refocus spins that are out of focus, thus rephasing is
achieved by means of a gradient echo.
• Images acquired by GRE may be T2* weighted, T1 weighted or intermediate
weighted (Long TR & Short TE), depending on the tissue that is being examined.
57

MRI Pulse Sequences - Gradient Recalled Echo
(GRE) 2
• It gives a significant reduction in scan time.
• Small flip angles are employed making it possible to have very short
repetition time (TR).
• An important application of GRE is its ability to perform 3D imaging
which is achieved because of higher speed of scanning.
• The purpose of GRE technique is to increase the scanning speed.
• In GRE pulse sequence moving blood appears bright which has
useful application in magnetic resonance angiography (TOF).
• In GRE sequence CSF appears bright.
• Echoplanar imaging(EPI) is an extremely fast form of GRE.
58

MRI Pulse Sequences - Echo Planar Imaging
(EPI)
• EPI is an extremely fast form of GRE. (may use only GEs or SE combined with Ges)
• It is the fastest MRI technique available.
• There are two types of EPI namely: single shot EPI and multishot EPI.
• Single shot EPI requires modification of existing hardware by way of installation of
high performance gradients to enable rapid on and off switching of the gradient
magnets.
• Compared to EPI, other fast scanning techniques e.g. FSE and GRE can be
achieved by software upgrade on the machine alone.
• Extremely fast computers are required to enable fast digital manipulations and
signal processing.
• EPI is useful for the following techniques: diffusion weighted imaging (DWI),
diffusion tensor imaging (DTI), perfusion imaging (PI), functional MRI ( fMRI ) and
real time cardiac imaging.
61

MRI Pulse Sequences - Perfusion Imaging
• Uses paramagnetic contrast agent to measure the rate that blood
flows into the capillary bed of tissues.
• Gadolinium diethylene triamine penta-acetic acid (Gd DTPA) is the IV
contrast medium
• Area of uptake is brighter on T1 weighted image.
• It is called a positive contrast agent.
• Effect on T1 is greater than on T2. (shortens T1)
• Hence, T1 weighting is usually used after Gd DTPA injection.
62

MRI Pulse Sequences – Diffusion Weighted
Imaging
• Requires the combination of echo planar imaging (Fast GRE) sequences with two
large gradient pulses applied after each RF excitation.
• Diffusion gradients must be strong and applied in the X,Y and Z directions to
obtain DWI.
• Tissue water with normal diffusion attenuates MRI signal.
• Tissue water with abnormal diffusion gives high MRI signal
• Helps to detect compromise of the integrity of cell membranes which affects
diffusion.
• Useful to examine damaged tissues because the resulting oedema gives abnormal
diffusion e.g. in thrombotic infarct in the brain or traumatic injury.
• It measures the apparent diffusion coefficient (ADC) of normal and diseased
tissue.
63

MRI Pulse Sequences – Inversion Recovery
• In IR sequence, a 1800 RF pulse is applied first ; then there is a waiting
period of time (the inversion time TI); after this a 900 pulse is applied.
• Uses long TR and short TE.
• Usually TR = 1000 ms and TE = 25 ms.
• Time consuming technique especially at higher field strengths which
make T1 long.
• Gives good grey/white matter discrimination.
• Useful for imaging demyelinating diseases of the brain such as
multiple sclerosis.
66

MRI Pulse Sequences – Short TI (Tau) Inversion
Recovery (STIR)
• N.B. TI stands for Time to Inversion.
• STIR sequences use short inversion recovery time (TI = ln(2)·T1fat)
• Used for fat suppression b/c in SE sequence bright signal from fat may
obscure contrasts in other tissues.
• IR sequence is first used to remove signal from fat with an initial 1800
pulse followed by a 900 pulse.
• STIR is useful to suppress fat in musculoskeletal imaging and for
diseased tissue that contains abnormal fat.
• STIR sequence is employed for fat suppression in machines with low
field strength magnets (< 0.3 Tesla).
67

MRI Pulse Sequences – Fat Saturation (FAT-
SAT)
• Used for fat suppression because bright signal from fat may obscure
contrasts in other tissues.
• It employs a high field magnet >1Tesla to achieve fat suppression.
• It can be applied in either SE or GRE techniques.
• It takes advantage of the different resonance frequencies that exist
b/w fat and water molecules.
• As the magnetic field strength increases, the differences in resonance
frequencies b/w fat and water also increases.
69

MRI Pulse Sequences – Fluid Attenuated
Inversion Recovery (FLAIR)
• It is an IR technique that annuls fluid.
• It uses a long inversion time
• Used in the brain to suppress CSF in SE T2 Weighted images.
• CSF suppression brings out periventricular hyperintense lesions e.g.
multiple sclerosis plaques.
• In FLAIR GRE T2* Weighted images CSF appears bright.
70

71
NB: FLAIR images
appear similar to T2
weighted images with
grey matter brighter
than white matter but
CSF is dark instead of
bright.

MRI Pulse Sequences – Chemical Shift Imaging
• The resonant frequency of a proton is affected by its chemical
environment.
• Spinning protons are affected by magnetic fields of atomic electrons
in nearby atoms.
• Measurement of the effect of chemical environment on a proton i.e.
chemical shift ” gives information about the molecular structure of an
atom
• Difference in resonance (precession) frequencies of the proton from
the influence of valence electrons in H-O bond in water and H-C bond
in lipids is harnessed to produce separate images of water and fat.
72

Spatial Encoding
• Unlike in CT and plain films in which localisation of the signal is simple
(an x-ray beam travels through the material and where it hits the
receptor is the physical location of what it has passed through).
• MRI is much more complicated.
• With MRI the signal is localised in the 3D space by manipulating the
magnetic properties of the nuclei in a predictable way.
• The signals are then returned with a particular frequency and phase
and these are slotted into their respective locations.
• The brightness of the pixel is the amplitude of the signal returned.
73

Spatial Encoding
• 3 steps involved in identifying
where in a 3D location a signal is
arising from:
1. Slice selected along z-axis
2. Segment of slice along x-axis
selected by frequency encoding
3. Part of segment along y-axis
selected by phase encoding
74

Spatial Encoding - Slice Selection
1. A magnetic field gradient is applied in the z-axis.
2. The Larmor frequencies of the nuclei vary along the z-axis.
3. An RF pulse with a frequency matching the Larmor frequency of
the nuclei we want to select is applied.
4. In this way, a slice along the z-axis is selected (correlates with an
axial slice of the patient).
5. The phases of the nuclei are reset by reversing the gradients
75

Spatial Encoding – Frequency Encoding
1. Dephase gradient applied along x-axis.
2. Rephase read-out gradient applied along x-axis.
3. Gradient echo signal received (combination of all signals along the
x-axis).
4. Fourier transfer/transformation (FT) applied to combined signals.
5. Signals separated out by frequency:
i. Each frequency relates to location along x-axis
ii. Each frequencies amplitude measured to give brightness
78

Spatial Encoding – Phase Encoding
1. Phase-encoding gradient applied along y-axis.
2. The frequencies of each segment in the column are now different.
3. Gradient switched off
4. The frequencies return to the frequency of that column (as determined by the frequency-
encoding gradient)
5. However, they are now out of phase.
6. The amplitude of the signals is plotted.
7. The cycle is repeated with different strengths of phase encoding gradients producing different
phase shifts each time.
8. The plot of the amplitude with each phase shift forms a wave with a particular frequency.
9. Each area in the column will have a phase-shift wave with a different frequency depending
upon its area along the y-axis
10. Fourier transform is applied to separate out the frequencies and slot them into their position
along the y-axis.
80

Spatial Encoding – K Space
• The wavelength signals acquired are stored in K-space.
• Each column contains data acquired from 1 frequency encoding step.
• Each row contains data acquired from phase-encoding steps.
• Any point in K-space contributes to the whole image.
• Any image pixel is derived from the whole of K-space.
• High-frequency signals (edges) are stored in the periphery.
• Low-frequency signals (contrast) are stored in the centre.
82

Computer System Of An MRI Scanner
• The signal time display (which are normally seen on an oscilloscope)
is converted to a signal frequency display by a mathematical process
called Fourier Transform
• By means of slice selection and slice encoding, the same type of
signal data as provided by detectors of a CT scanner can be obtained.
• An image can therefore be constructed using similar algorithms to
those developed for CT scanning
84

Computer System Of An MRI Scanner
• Operator and viewing consoles for soft copy image.
• Hard copy image production system which consists of either film
camera (obsolete) or laser printer.
• Image archiving with magnetic tapes (obsolete) or discs or external
hard drives.
85

MRI Artefacts
• Motion Artefacts
• Movement of patient’s body.
• Heart Motion.
• Breathing movement.
• Aliasing Artefact
• Occurs when field of view (FOV) is too small hence part of the patient which is
excited overlaps the chosen FOV.
• Truncation or Ringing ” Artefact
• Describes parallel lines which appear at high contrast interfaces e.g. between fat and
muscle or between CSF and spinal cord.
• Central Line or Zipper ” Artefact
• Occurs across the middle of the image and is due to RF leaking from the transmitter
to the receiver coils.
86

MRI Artefacts (Contd.)
Susceptibility Artefacts
• Produced at interface between tissues that have magnetic
susceptibility (e.g. fat or muscle) and tissues that are not susceptible
to magnetization.
• Example:
• air in paranasal sinuses or lungs.
• Haemoglobin concentration inside haematoma after a bleed b/c haemoglobin
contains iron which is ferromagnetic.
87

Safety in MRI
• Does not use ionizing radiation.
• No known side effects.
• Patients with the following medical devices (unless they are MRI compliant)
and foreign bodies should not be subjected to MRI scanning:
a. Cardiac pacemakers.
b. Metallic surgical clips especially for aneurysm.
c. Metallic prosthesis.
d. Shrapnel.
e. Bullet.
• Patients with first trimester pregnancy should not be subjected to MRI
scanning.
91

Conclusion
• The knowledge of the Principle behind MRI is essential for the in-
depth understanding of how MR images are obtained which
ultimately will aid recognition of both normal anatomy, pathology and
artifacts.
92

References
• Bushberg J. T., Seibert J. A. The essential Physics of Medical Imaging,
3rd Edition. 2002.
• Abdulla S., Clarke C. FRCR Physics Notes, 2nd Edition. 2019.
• Prof Donald Nzeh. NPMCN Virtual Lecture On The Physical Principles
Of Magnetic Resonance Imaging. 2022.
• Elmira Hassanzadeh. MRI pulse sequences. 2021. Radiopedia.
https://radiopaedia.org/articles/mri-pulse-sequences-1
93

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