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Vibrational (Infrared and Raman)
Spectroscopy
And
NMR, MRI
Dr. Sunil Kumar,
Department of Biotechnology
Post Graduate College,
Sec-11,Chandigarh
Introduction
• Spectroscopy is an analytical technique which
helps determine structure.
• It destroys little or no sample.
• The amount of light absorbed by the sample is
measured as wavelength is varied.
2
Types of Spectroscopy
• Infrared (IR) spectroscopy measures the bond
vibration frequencies in a molecule and is used to
determine the functional group.
• Mass spectrometry (MS) fragments the molecule and
measures the masses.
• Nuclear magnetic resonance (NMR) spectroscopy
detects signals from hydrogen atoms and can be
used to distinguish isomers.
• Ultraviolet (UV) spectroscopy uses electron
transitions to determine bonding patterns. =>
3
The EM Spectrum
The Spectrum and
Molecular Effects
=>
Chapter 12
=>
5
Infrared Spectroscopy
region of infrared that is most useful lies between
2.5-25 mm (4000-400 cm-1)
depends on transitions between vibrational
energy states :stretching and bending
Molecular Excitation Modes
Provides a spectroscopic tool for analyzing molecular components in pigments
stretching
bending
scissoring
twisting
Stretching mode between molecules

1
2 
E  h  ;
Phonon energy
K
 m1  m2 


 m1  m2 
k
Δf=K·ΔX

c
Wave number
K is spring constant
Units K: N/m=kg/s2
Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
=>
Stretching Frequencies
• Frequency decreases with increasing
atomic weight.
• Frequency increases with increasing bond
energy.
=>
Vibrational Modes
Nonlinear molecule with n atoms usually has 3n 6 fundamental vibrational modes.
=>
How much movement occurs in the
vibration of a C-C bond?
stretching vibration
154 pm
10 pm
For a C-C bond with a bond
length of 154 pm, the
variation is about 10 pm.
bending vibration
4o
10 pm
For C-C-C bond angle a
change of 4o is typical.
This moves a carbon atom
about 10 pm.
Fingerprint of Molecule
• Whole-molecule vibrations and bending
vibrations are also quantitized.
• No two molecules will give exactly the same
IR spectrum (except enantiomers).
• Simple stretching: 1600-3500 cm-1.
• Complex vibrations: 600-1400 cm-1, called
the “fingerprint region.”
=>
IR Stretching Frequencies: What Do they
Depend On?
Directly on the strength of the bonding between the two atoms ( ~ k)
Inversely on the reduced mass of the two atoms (v ~ 1/m)
Expect:  will increase with increasing bond strength (bond order) and
decreasing mass
Quantum mechanics: The frequency () depends on the energy gap between vibrational
levels
E = h= hc/(cm-1)
Only the natural frequency will be absorbed
The natural frequency (8.67 x 1013 s-1) is absorbed selectively
An Infrared Spectrometer
=>
Dipole Moment Must Change during for
a vibration to be “IR active”!
• In order to interact
strongly with the EM
radiation, the motion
of the molecule must
be such that the dipole
moment changes.
An infrared spectrum consists of a plot of
absorbance versus frequency or wavenumber
(1/λ)
What type of vibrations would occur
in pentane?
Let’s examine the IR spectrum of pentane.
IR spectrum
C-H
stretching
C-H
bending
Increasing
absorption of IR
radiation
C-C
bending
Increasing wavelength
Increasing wavenumber (energy, frequency)
What can we learn from IR spectroscopy?
• Atoms vibrate with frequencies in the IR range
• Chemical Analysis:
• Match spectra to known databases
– Identifying an unknown compound, Forensics, etc.
• Monitor chemical reactions in-situ
• Structural ideas:
• Can determine what chemical groups are in a specific
compound
• Electronic Information:
• Measure optical conductivity
– Determine if Metal, Insulator, Superconductor,
Semiconductor
– Band Gaps, Drude model
Basic Principles of Vibrational Spectroscopy
Inelastic scattering of light at a molecule
Sir Chandrashekhara Venkata Raman
• November 7, 1888-November 21,
1970
• Won the Noble Prize in 1930 for
Physics
• Discovered the "Raman Effect"
• Besides Discovering the Raman
Effect, He studied extensively in
X-Ray Diffractions, Acoustics,
Optics, Dielectrics, Ultrasonics,
Photo electricity, and colloidal
particles.
Principles of Raman Spectroscopy
Molecular excitations are associated with vibrations or rotation of molecules which
correlate with low frequency modes. Raman spectroscopy relies on the interaction of
monochromatic light produced by a laser (in the infrared to near ultraviolet range)
exciting an electron from its molecular bonding configuration with subsequent deexcitation to lower vibrational (rotational) excitation mode.
Emitted radiation from the deexcitation is shifted in energy
(frequency, wavelength) with
respect to laser light energy.
Challenge is to filter weak
Raman transitions from
strong Rayleigh scattering
transition signals.
Raman Principle
When a sample is irradiated with monochromatic light
(e. g. with laser light using discrete lines between 300 and 1064 nm)
Different physical phenomena happen:
1. The main part of the incident light is transmitted without interaction
2. Roughly 10-4 part of the light is elastically scattered
with no energy absorption --->Rayleigh line
3. A part of the incident light may be absorbed by electronic transitions
and is emitted as ---> Flourescence
4. Only 10-8 of the incident light is inelastically scattered and interacts
with the sample .
The energy is partly absorbed ---> Raman spectrum
Raman spectrometer. Monochromatic light is passed
through a concentrated sample. Scattered light (dashed lines) is
collected with the aid of a curved mirror. This is then diffracted
through a monochromator which allows the identification of frequencies
due to both Rayleigh and Raman scattering. Note that the
lower mirror reflects transmitted light back through the sample to
double the intensity of scattered light.
But dominant process is elastic scattering:
Rayleigh scattering
 — 
—  
Photon in
Photon out
No vibration
No vibration
If incident photon energy E; vibration energy v, then
in terms of energy, photon out has energy:
E-v Stokes scattering
E+v anti-Stokes scattering
E Rayleigh scattering
Why Raman?
• In Raman spectroscopy, by
varying the frequency of the
radiation, a spectrum can be
produced, showing the intensity
of the exiting radiation for each
frequency. This spectrum will
show which frequencies of
radiation have been absorbed by
the molecule to raise it to higher
vibrational energy states.
What Exactly Is Being Measured?
METHANE
When Light hits a
sample, It is Excited,
and is forced to vibrate
and move. It is these
vibrations which we are
measuring.
Stokes vs. Anti Stokes
•
•
•
•
Atoms are at a certain energy
level at any given time.
As a laser light hits the atom, it is
excited and reaches a higher level
of energy, and then is brought
back down.
If an atom is at a given energy
level, it can be excited then fall
below the original level.
Anti-stokes spectrum are mirror
spectrums of Stokes Raman
Spectrums
Using Stokes/Peaks to Determine
Vibrations
This is a Raman Spectrum of Methane, along with RS, and IR. Each peak denotes a different
vibration. The highest peak implies the strongest vibration. The smallest peak, the last one,
shows the weakest of all the vibrations. In short, each vibration implies a different
movement: these different movements imply different structures.
Advantages of FT-Raman compared to FT-IR
•
Little or no sample preparation
• High sensitivity for symmetric molecules, non
polar bondings and homonuclear chains
• Only low interferences of water and glass
(measurement of aqueous solutions in
glass vials)
• Raman lines are usually narrower
• Low frequency information to 50 cm-1
Nuclear Magnetic
Resonance Spectroscopy
And
Magnetic Resonance Imaging
Introduction to NMR Spectroscopy
• Nuclear magnetic resonance spectroscopy is a powerful analytical technique
used to characterize organic molecules by identifying carbon-hydrogen
frameworks within molecules.
• Two common types of NMR spectroscopy are used to characterize organic
structure: 1H NMR is used to determine the type and number of H atoms in a
molecule; 13C NMR is used to determine the type of carbon atoms in the
molecule.
• The source of energy in NMR is radio waves which have long wavelengths, and
thus low energy and frequency.
• When low-energy radio waves interact with a molecule, they can change the
nuclear spins of some elements, including 1H and 13C.
38
Nuclear Spin
• A nucleus with an odd atomic number or an
odd mass number has a nuclear spin.
• The spinning charged nucleus generates a
magnetic field.
=>
External Magnetic Field
When placed in an external field, spinning protons
act like bar magnets.
=>
Two Energy States
The magnetic fields of
the spinning nuclei
will align either with
the external field, or
against the field.
A photon with the right
amount of energy can
be absorbed and
cause the spinning
proton to flip.
=>
The energy difference between these two nuclear spin states
corresponds to the low frequency RF region of the
electromagnetic spectrum
Nuclear Spins in B0
– for 1H and 13C, only two orientations are allowed.
Nuclear Spin in B0
– The energy difference between allowed spin states
increases linearly with applied field strength.
– Values shown here are for 1H nuclei.
Nuclear Magnetic Resonance
• If the precessing nucleus is irradiated with
electromagnetic radiation of the same
frequency as the rate of precession,
– the two frequencies couple
– energy is absorbed
– the nuclear spin is flipped from spin state +1/2
(with the applied field) to -1/2 (against the applied
field).
Nuclear Magnetic Resonance
– (a) Precession and (b) after absorption of
electromagnetic radiation.
Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is the
absorption of energy by a precessing nucleus and the
resulting “flip” of its nuclear spin from a lower energy
state to a higher energy state.
• The precessing spins induce an oscillating magnetic
field that is recorded as a signal by the instrument.
– Signal: A recording in an NMR spectrum of a nuclear
magnetic resonance.
In order to undergo an NMR transition at a particular value of v, a specific
set of circumstances called the resonance condition needs to exist
The resonance condition. Exposure of the nucleus to radio frequency
radiation sets up a magnetic field (of field strength B1, shown in grey) which
has a frequency of oscillation. The resonance condition occurs when this
frequency equals the Larmor frequency of the spin magnetic moment.
Transition between spin states only occurs at the resonance condition.
Nuclear Magnetic Resonance
– If we were dealing with 1H nuclei isolated from all other
atoms and electrons, any combination of applied field and
radiation that produces a signal for one 1H would produce
a signal for all 1H. The same is true of 13C nuclei.
– Hydrogens in organic molecules, however, are not isolated
from all other atoms. They are surrounded by electrons,
which are caused to circulate by the presence of the
applied field.
– The circulation of electrons around a nucleus in an applied
field is called diamagnetic current and the nuclear
shielding resulting from it is called diamagnetic shielding.
E and Magnet Strength
• Energy difference is proportional to the
magnetic field strength.
• E = h =  h B0
2
• Gyromagnetic ratio, , is a constant for each
nucleus (26,753 s-1gauss-1 for H).
• In a 14,092 gauss field, a 60 MHz photon is
required to flip a proton.
• Low energy, radio frequency.
=>
Nuclear Magnetic Resonance Spectroscopy
Introduction to NMR Spectroscopy
• Protons in different environments absorb at slightly
different frequencies, so they are distinguishable by NMR.
• The frequency at which a particular proton absorbs is
determined by its electronic environment.
• The size of the magnetic field generated by the electrons
around a proton determines where it absorbs.
• Modern NMR spectrometers use a constant magnetic field
strength B0, and then a narrow range of frequencies is
applied to achieve the resonance of all protons.
• Only nuclei that contain odd mass numbers (such as 1H, 13C,
19F and 31P) or odd atomic numbers (such as 2H and 14N) give
rise to NMR signals.
Magnetic Shielding
• If all protons absorbed the same amount of
energy in a given magnetic field, not much
information could be obtained.
• But protons are surrounded by electrons
that shield them from the external field.
• Circulating electrons create an induced
magnetic field that opposes the external
magnetic field.
=>
Shielded Protons
Magnetic field strength must be increased for
a shielded proton to flip at the same
frequency.
=>
Protons in a Molecule
Depending on their chemical environment,
protons in a molecule are shielded by
different amounts.
=>
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—The Spectrum
• An NMR spectrum is a plot of the intensity of a peak against its chemical
shift, measured in parts per million (ppm).
54
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—The Spectrum
•
•
•
•
NMR absorptions generally appear as sharp peaks.
Increasing chemical shift is plotted from left to right.
Most protons absorb between 0-10 ppm.
The terms “upfield” and “downfield” describe the relative location of peaks.
Upfield means to the right. Downfield means to the left.
• NMR absorptions are measured relative to the position of a reference peak
at 0 ppm on the d scale due to tetramethylsilane (TMS). TMS is a volatile
inert compound that gives a single peak upfield from typical NMR
absorptions.
55
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—The Spectrum
• The chemical shift of the x axis gives the position of an NMR
signal, measured in ppm, according to the following equation:
•
•
By reporting the NMR absorption as a fraction of the NMR
operating frequency, we get units, ppm, that are independent of
the spectrometer.
Four different features of a 1H NMR spectrum provide
information about a compound’s structure:
a. Number of signals
b. Position of signals
c. Intensity of signals.
56
d. Spin-spin splitting of signals.
NMR Signals
• The number of signals shows how many
different kinds of protons are present.
• The location of the signals shows how
shielded or deshielded the proton is.
• The intensity of the signal shows the number
of protons of that type.
• Signal splitting shows the number of protons
on adjacent atoms.
=>
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Number of Signals
• The number of NMR signals equals the number of different types
of protons in a compound.
• Protons in different environments give different NMR signals.
• Equivalent protons give the same NMR signal.
• To determine equivalent protons in cycloalkanes and alkenes,
always draw all bonds to hydrogen.
58
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Position of Signals
• In the vicinity of the nucleus, the magnetic field generated by the
circulating electron decreases the external magnetic field that
the proton “feels”.
• Since the electron experiences a lower magnetic field strength, it
needs a lower frequency to achieve resonance. Lower frequency is
to the right in an NMR spectrum, toward a lower chemical shift,
so shielding shifts the absorption upfield.
59
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Position of Signals
• The less shielded the nucleus becomes, the more of the applied magnetic
field (B0) it feels.
• This deshielded nucleus experiences a higher magnetic field strength, to it
needs a higher frequency to achieve resonance.
• Higher frequency is to the left in an NMR spectrum, toward higher chemical
shift—so deshielding shifts an absorption downfield.
• Protons near electronegative atoms are deshielded, so they absorb
downfield.
60
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Position of Signals
61
Chemical Shift
• Chemical shift depends on the (1)
electronegativity of nearby atoms, (2)
hybridization of adjacent atoms, and (3)
diamagnetic effects from adjacent pi bonds.
• Electronegativity
Electron eg- Chemical
CH3 -X
ativity of X
Shift ()
CH3 F
CH3 OH
CH3 Cl
CH3 Br
CH3 I
4.0
3.5
3.1
2.8
2.5
4.26
3.47
3.05
2.68
2.16
(CH3 ) 4 C
(CH3 ) 4 Si
2.1
1.8
0.86
0.00
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Chemical Shift Values
• In a magnetic field, the six  electrons in benzene circulate around the
ring creating a ring current.
• The magnetic field induced by these moving electrons reinforces the
applied magnetic field in the vicinity of the protons.
• The protons thus feel a stronger magnetic field and a higher
frequency is needed for resonance. Thus they are deshielded and
absorb downfield.
64
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Chemical Shift Values
• In a magnetic field, the loosely held  electrons of the double bond create a
magnetic field that reinforces the applied field in the vicinity of the protons.
• The protons now feel a stronger magnetic field, and require a higher frequency for
resonance. Thus the protons are deshielded and the absorption is downfield.
65
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Chemical Shift Values
• In a magnetic field, the  electrons of a carbon-carbon triple bond are
induced to circulate, but in this case the induced magnetic field
opposes the applied magnetic field (B0).
• Thus, the proton feels a weaker magnetic field, so a lower frequency
is needed for resonance. The nucleus is shielded and the absorption
is upfield.
66
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Chemical Shift Values
67
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Intensity of Signals
• The area under an NMR signal is proportional to the number of absorbing
protons.
• An NMR spectrometer automatically integrates the area under the peaks, and
prints out a stepped curve (integral) on the spectrum.
• The height of each step is proportional to the area under the peak, which in turn is
proportional to the number of absorbing protons.
• Modern NMR spectrometers automatically calculate and plot the value of each
integral in arbitrary units.
• The ratio of integrals to one another gives the ratio of absorbing protons in a
spectrum. Note that this gives a ratio, and not the absolute number, of absorbing
protons.
68
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Spin-Spin Splitting
• Consider the spectrum below:
69
Spin-Spin Splitting in 1H NMR Spectra
• Peaks are often split into multiple peaks due to magnetic
interactions between nonequivalent protons on adjacent carbons,
The process is called spin-spin splitting
• The splitting is into one more peak than the number of H’s on the
adjacent carbon(s), This is the “n+1 rule”
• The relative intensities are in proportion of a binomial distribution
given by Pascal’s Triangle
• The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 =
quartet, 5=pentet, 6=hextet, 7=heptet…..)
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
singlet
doublet
triplet
quartet
pentet
hextet
heptet
Rules for Spin-Spin Splitting
•
Equivalent protons do not split each other
•
Protons that are farther than two carbon atoms apart do not
split each other
Origins of Signal Splitting
• Signal coupling: An interaction in which the nuclear
spins of adjacent atoms influence each other and
lead to the splitting of NMR signals.
• Coupling constant (J): The separation on an NMR
spectrum (in hertz) between adjacent peaks in a
multiplet.
– A quantitative measure of the spin-spin coupling
with adjacent nuclei.
Origins of Signal Splitting
• Illustration of spin-spin coupling that gives rise to
signal splitting in 1H-NMR spectra.
The Origin of 1H NMR—Spin-Spin Splitting
Let us now consider how a triplet arises:
•
When placed in an applied magnetic field (B0), the adjacent
protons Ha and Hb can each be aligned with () or against () B0.
•
Thus, the absorbing proton feels three slightly different
magnetic fields—one slightly larger than B0(ab). one slightly
smaller than B0(ab) and one the same strength as B0 (ab).
74
The Origin of 1H NMR—Spin-Spin Splitting
•
Because the absorbing proton feels three different magnetic
fields, it absorbs at three different frequencies in the NMR
spectrum, thus splitting a single absorption into a triplet.
•
Because there are two different ways to align one proton with B0,
and one proton against B0—that is, ab and ab—the middle peak
of the triplet is twice as intense as the two outer peaks, making
the ratio of the areas under the three peaks 1:2:1.
•
Two adjacent protons split an NMR signal into a triplet.
•
When two protons split each other, they are said to be coupled.
•
The spacing between peaks in a split NMR signal, measured by the
J value, is equal for coupled protons.
75
The Origin of 1H NMR—Spin-Spin Splitting
76
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Structure Determination
77
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Structure Determination
78
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Structure Determination
79
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Structure Determination
80
Magnetic Resonance Imaging
(MRI)
Introduction
• What is MRI?
– Magnetic resonance imaging (MRI) is a spectroscopic
imaging technique used in medical settings to produce
images of the inside of the human body.
– MRI is based on the principles of nuclear magnetic
resonance (NMR), which is a spectroscopic technique used
to obtain microscopic chemical and physical data about
molecules
– In 1977 the first MRI exam was performed on a human
being. It took 5 hours to produce one image.
Introduction
• How Does it Work?
– The magnetic resonance imaging is accomplished through
the absorption and emission of energy of the radio
frequency (RF) range of the electromagnetic spectrum.
The Components:
• A magnet which produces a very powerful uniform magnetic
field.
• Gradient Magnets which are much lower in strength.
• Equipment to transmit radio frequency (RF).
• A very powerful computer system, which translates the signals
transmitted by the coils.
The Magnet
• The most important component of the MRI scanner
is the magnet:
– The magnets currently used in scanners today are in the .5tesla to 2.0-tesla range (5,000 to 20,000-gauss).
Higher values are used for research.
– Earth magnetic field: 0.5-gauss
The Magnet (cont.)
• There are three types of magnets used in MRI systems:
– Resistive magnets
– Permanent magnets
– Super conducting magnets (the most commonly used
type in MRI scanners).
• In addition to the main magnet, the MRI machine also
contains three gradient magnets. These magnets have a
much lower magnetic field and are used to create a
variable field.
The Technology
• How Does It All Work?
• Spin:
– The atoms that compose the human body have a
property known as spin (a fundamental property of all
atoms in nature like mass or charge).
– Spin can be thought of as a small magnetic field and
can be given a + or – sign and a mathematical value of
multiples of ½.
– Components of an atom such as protons, electrons and
neutrons all have spin.
The Technology (cont.)
• Spin (cont.):
– Protons and neutron spins
are known as nuclear
spins.
– An unpaired component
has a spin of ½ and two
particles with opposite
spins cancel one another.
– In NMR it is the unpaired
nuclear spins that produce
a signal in a magnetic field.
The Technology (cont.)
• Human body is mainly composed of fat
and water, which makes the human body
composed of about 63% hydrogen.
• Why Are Protons Important to MRI?
– positively charged
– spin about a central axis
– a moving (spinning) charge creates a
magnetic field.
– the straight arrow (vector) indicates the
direction of the magnetic field.
The Technology (cont.)
• When placed in a large magnetic field,
hydrogen atoms have a strong
tendency to align in the direction of
the magnetic filed
• Inside the bore of the scanner, the
magnetic field runs down the center of
the tube in which the patient is placed,
so the hydrogen protons will line up in
either the direction of the feet or the
head.
• The majority will cancel each other, but
the net number of protons is sufficient
to produce an image.
The Technology (cont.)
• Energy Absorption:
– The MRI machine applies radio
frequency (RF) pulse that is specific to
hydrogen.
– The RF pulses are applied through a coil
that is specific to the part of the body
being scanned.
RF Coil as the Receiver
• After excitation, the
magnetization flips to
being partially or
completely “flipped”.
• The M-vector now
precesses around the Bofield at the Larmor
frequency.
• A coil placed near the
object can detect this
magnetization
Behavior when radio excitation is stopped
• If we stop this RF
excitation, it returns to
equilibrium
(
M z  M 0 1 e
 t / T1
)
T1 = spin-lattice relaxation time
T1 is one source of clinical
info we take advantage of in
MRI.
The Technology (Cont.)
Resonance (cont.)
The gradient magnets are rapidly turned on and off
which alters the main magnetic field.
– The pulse directed to a specific area of the body causes
the protons to absorb energy and spin in different
direction, which is known as resonance
Frequency (Hz) of energy absorption depends on strength of external
magnetic field.
The Technology (cont.)
Larmor Equation
 =  2
0
For hydrogen at 1.5T:
0
  2.675x108 s1T
 1.5T
 63.864MHz
0
0
•
The resonance frequency, 0, is referred to as the Larmor
frequency.
Relaxation and Receiving
Receive Radio Frequency Field
• receiving coil: measure net magnetization (M)
• readout interval (~10-100 ms)
• relaxation: after RF field turned on and off, magnetization
returns to normal
longitudinal magnetization  T1 signal recovers
transverse magnetization  T2 signal decays
Source: Robert Cox’s web slides
T1 and TR
T1 = recovery of longitudinal (B0) magnetization
• used in anatomical images
• ~500-1000 msec (longer with bigger B0)
TR (repetition time) = time to wait after excitation before sampling T1
Source: Mark Cohen’s web slides
The Technology (cont.)
• Imaging:
– When the RF pulse is turned off the hydrogen protons slowly
return to their natural alignment within the magnetic field and
release their excess stored energy. This is known as relaxation.
• What happens to the released energy?
– Released as heat
OR
– Exchanged and absorbed by other protons
OR
– Released as Radio Waves.
The Technology (cont.)
• Measuring the MR Signal:
– the moving proton vector induces a signal in
the RF antenna
– The signal is picked up by a coil and sent to the
computer system.
the received signal is sinusoidal in
nature
– The computer receives mathematical data,
which is converted through the use of a Fourier
transform into an image.
The Image
Physics of MRI
It is an interplay of
• Magnetism
• Resonance
Recap: What Does the Image Represent?
• For every unit volume of tissue, there is a number
of cells, these cells contain water molecules, each
water molecule contain one oxygen and two
hydrogen atoms.
• Each hydrogen atom contains one proton in its
nucleus. Different tissues thus produce different
images based on the amount of their hydrogen
atoms producing a signal
Summary: Magnetic Resonance Principles
• Some atomic nuclei have a property
called “spin”
• Spin gives the nuclei a magnetic
moment
• These moments are randomly oriented
• When the spins are placed in a magnetic
field, they align either with or against
the field.
Summary: Magnetic Resonance
Principles
• The two states are not equal in energy and
therefore not equally populated.
• This results in a net polarization or longitudinal
magnetization
• Transverse magnetization can be created by
“flipping” the spins with a magnetic field applied
in the rotating frame of reference (i.e an RF
pulse)
• After the “flip” the spins return to the
equilibrium condition through the T1 and T2
relaxation mechanisms measured by the RF coil.