MRI PHYSICS ARE DESCRIBED IN DETAILIN THIS PPT. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. MRI PHYSICS
MR. ROHIT BANSAL
ASSISTANT PROFESSOR (RADIOPHYSICS)
MAMC,AGROHA

2. There are essentially two ways of explaining the fundamentals of MRI:
classically and via quantum mechanics.
Classical theory (accredited to Sir Isaac Newton and often called
Newtonian theory) provides a mechanical view of how the universe (and
therefore how MRI) works. Using classical theory, MRI is explained using
the concepts of mass, spin, and angular momentum on a large or bulk
scale.
Quantum theory (accredited to several individuals including Max Planck,
Albert Einstein, and Paul Dirac) operates at a much smaller, subatomic
scale and refers to the energy levels of protons, neutrons, and electrons.
Although classical theory is often used to describe physical principles on a
large scale and quantum theory on a subatomic level, there is evidence
that all physical principles are explained using quantum concepts.
However, for our purposes, this chapter mainly relies on classical
perspectives because they are generally easier to understand. Quantum
theory is only used to provide more detail when required.
Basic Principle of MRI

3. All things are made of atoms. Atoms are organized into
molecules, which are two or more atoms arranged together.
The most abundant atom in the human body is hydrogen, but
there are other elements such as oxygen, carbon, and
nitrogen. Hydrogen is most commonly found in molecules of
water (where two hydrogen atoms are arranged with one
oxygen atom; H2O) and fat (where hydrogen atoms are
arranged with carbon and oxygen atoms; the number of each
depends on the type of fat).
The atom consists of a central nucleus and orbiting electrons.
The nucleus is very small, one millionth of a billionth of the
total volume of an atom, but it contains all the atom’s mass.
This mass comes mainly from particles called nucleons, which
are subdivided into protons and neutrons.
Atomic Structure

4. Atoms are characterized in two ways.
• The atomic number is the sum of the protons in the
nucleus.This number gives an atom its chemical identity.
• The mass number or atomic weight is the sum of the
protons and neutrons in the nucleus.
The number of neutrons and protons in a nucleus is usually
balanced so that the mass number is an even number. In
some atoms, however, there are slightly more or fewer
neutrons than protons. Atoms of elements with the same
number of protons but a different number of neutrons are
called isotopes.

5. Motion in the atom
Three types of motion are present within the atom:
• Electrons spinning on their own axis
• Electrons orbiting the nucleus
•The nucleus itself spinning about its own axis.
The principles of MRI rely on the spinning motion of specific
nuclei present in biological tissues.
There are a limited number of spin values depending on the
atomic and mass numbers.
A nucleus has no spin if it has an even atomic and mass
number, e.g., six protons and six neutrons, mass number 12.
In nuclei that have an even mass number caused by an even
number of protons and neutrons, half of the nucleons spin in
one direction and half in the other. The forces of rotation
cancel out, and the nucleus itself has no net spin.

6. However, in nuclei with an odd number of protons, an odd
number of neutrons, or an odd number of both protons and
neutrons, the spin directions are not equal and opposite, so
the nucleus itself has a net spin or angular momentum.
only nuclei with an odd mass number or atomic weight are
used.These are known as MR-active nuclei.

7. MR-active nuclei are characterized by their tendency to align their axis of
rotation to an applied magnetic field. This occurs because they have
angular momentum or spin and, as they contain positively charged
protons, they possess an electrical charge.
The law of electromagnetic induction (determined by Michael Faraday
in 1833) refers to the connection between electric and magnetic fields and
motion. Faraday’s law determines that a moving electric field produces a
magnetic field and vice versa. MR-active nuclei have a net electrical charge
(electric field) and are spinning (motion), and, therefore, automatically
acquire a magnetic field.
In classical theory, this magnetic field is denoted by a magnetic
moment. The magnetic moment of each nucleus has vector properties,
i.e., it has size (or magnitude) and direction. The total magnetic moment of
the nucleus is the vector sum of all the magnetic moments of protons in
the nucleus.
MR-Active Nuclei

8. Important examples of MR-active nuclei, together with
their mass numbers are listed below:
• 1H (hydrogen)
• 13C (carbon)
• 15N (nitrogen)
• 17O (oxygen)
• 19F (fluorine)
• 23Na (sodium)

9. The hydrogen nucleus
The isotope of hydrogen called protium is the most commonly used MR-active
nucleus in MRI.
It has a mass and atomic number of 1, so the nucleus consists of a single proton
and has no neutrons.
It is used because hydrogen is very abundant in the human body and because
the solitary proton gives it a relatively large magnetic moment. These
characteristics mean that the maximum amount of available magnetization in
the body is utilized.
Faraday’s law of electromagnetic induction states that a magnetic field is
created by a charged moving particle (that creates an electric field). The
protium nucleus contains one positively charged proton that spins, i.e., it
moves. Therefore, the nucleus has a magnetic field induced around it and acts
as a small magnet.
The magnet of each hydrogen nucleus has a north and a south pole of equal
strength. The north/south axis of each nucleus is represented by a magnetic
moment and is used in classical theory.
In diagram, the magnetic moment is shown by an arrow. The length of the
arrow represents the magnitude of the magnetic moment or the strength of
the magnetic field that surrounds the nucleus. The direction of the arrow
denotes the direction of alignment of the magnetic moment.

11. In the absence of an applied magnetic field, the magnetic moments of
hydrogen nuclei are randomly orientated and produce no overall magnetic
effect. However, when placed in a strong static external magnetic field the
magnetic moments of hydrogen nuclei orientate with this magnetic field. This
is called alignment. Alignment is best described using classical and quantum
theories as follows:
Classical theory uses the direction of the magnetic moments of spins
(hydrogen nuclei) to illustrate alignment.
• Parallel alignment: Alignment of magnetic moments in the same direction
as the main B0 field (also referred to as spin-up).
• Anti-parallel alignment: Alignment of magnetic moments in the opposite
direction to the main B0 field (also referred to as spin-down). After alignment,
there are always more spins with their magnetic moments aligned parallel
than anti-parallel. The net magnetism of the patient (termed the net magnetic
vector, NMV) is therefore aligned parallel to the main B0 field in the
longitudinal plane or z-axis.
Alignment

13. Quantum theory uses the energy level of the spins (or hydrogen nuclei)
to illustrate alignment. Protons of hydrogen nuclei couple with the external
magnetic field B0 (termed Zeeman interaction) and cause a discrete
number of energy states. For hydrogen nuclei, there are only two possible
energy states:
• Low-energy nuclei do not have enough energy to oppose the main B0
field. These are nuclei that align their magnetic moments parallel or spin-
up to the main B0 field in the classical description.
• High-energy nuclei do have enough energy to oppose the main B0 field.
These are nuclei that align their magnetic moments anti-parallel or spin-
down to the main B0 field in the classical description.

15. The net magnetization vector in MRI is the summation of all the magnetic
moments of the individual hydrogen nuclei.
In the absence of an external magnetic field, the individual magnetic
moments are randomly oriented and since they are in opposition, the net
magnetization vector is considered to be zero.
If hydrogen nuclei are placed within a strong external magnetic field, they
become aligned within the field in one of two directions parallel to the direction
of the field.
In MRI, the main magnetic field is termed B0.
aligned in the direction of B0 (parallel)
aligned in the opposite direction of B0 (anti-parallel)
Parallel and anti-parallel hydrogen nuclei have equal but opposite magnetic
moments and cancel each other out. However, there are always slightly more
hydrogen nuclei parallel to B0 and this slight difference is termed the NMV (net
magnetization vector).
Net Magnetization Vector (NMV)

17. Each hydrogen nucleus spins on its axis. The influence of B0 produces an
additional spin or wobble of the magnetic moments of hydrogen around B0.
This secondary spin is called precession and causes the magnetic moments
to circle around B0. The course they take is called the precessional path,
and the speed at which they precess around B0 is called the precessional
frequency.
The Larmor equation is used to calculate the frequency or speed of
precession for a specific nucleus in a specific magnetic field strength. The
Larmor equation is stated as follows.
W0 = B0 × γ
Where the precessional frequency is denoted by W0, the strength of the
external magnetic field is expressed in tesla and denoted by the symbol B0,
the gyromagnetic ratio is the precessional frequency of a specific nucleus at
1 T and therefore has units of MHz/T. It is denoted by the symbol γ. As it is a
constant of proportionality the precessional frequency is proportional to
the strength of the external magnetic field.
Precession

19. The precessional frequencies of hydrogen (gyromagnetic ratio
42.57 MHz/T) commonly found in clinical MRI are:
• 21.285 MHz at 0.5T
• 42.57 MHz at 1T
• 63.86 MHz at 1.5T
The precessional frequency corresponds to the range of
frequencies in the electromagnetic spectrum of radio waves.
Therefore, hydrogen precesses at a low frequency and energy. At
equilibrium the magnetic moments of the nuclei are out of phase
with each other. Phase refers to the position of the magnetic
moments on their circular precessional path.
• Out of phase or incoherent means that the magnetic moments of
hydrogen are at different places on the precessional path.
• In phase or coherent means that the magnetic moments of
hydrogen are at the same place on the precessional path.

20. Resonance is an energy transition that occurs when an object is
subjected to a frequency the same as its own. In MR, resonance is
induced by applying a radiofrequency (RF) pulse:
• At the same frequency as the precessing hydrogen nuclei.
• At 90° to B0.
This causes the hydrogen nuclei to resonate (receive energy from
the RF pulse) whereas other MR active nuclei do not resonate
because their gyromagnetic ratios are different from that of
hydrogen. Owing to the Larmor equation their precessional
frequency is different and therefore they only resonate if RF at their
specific precessional frequency is applied. Two things happen at
resonance: energy absorption and phase coherence.
Resonance

21. The hydrogen nuclei absorb energy from the RF pulse
(excitation pulse). The absorption of applied RF energy at 90°
to B0 causes an increase in the number of high energy, spin up
nuclei.
If just the right amount of energy is applied the number of
nuclei in the spin up position equals the number in the spin
down position.
As a result, the NMV (which represents the balance between
spin up and spin down nuclei) lies in the transverse plane as
the net magnetization lies between the two energy states.
As the NMV has been moved through 90° from B0, it has a
flip or tip angle of 90°.
Energy Absorption

23. The magnetic moments of the nuclei move into phase with each other. As
the magnetic moments are in phase both in the spin up and spin down
positions and the spin up nuclei are in phase with the spin down nuclei, the
net effect is one of precession so the NMV precesses in the transverse plane
at the Larmor frequency.
Phase Coherence

24. A receiver coil is situated in the transverse plane. As the NMV rotates around
the transverse plane as a result of resonance, it passes across the receiver coil
inducing a voltage in it.This voltage is the MR signal.
When the RF excitation pulse is switched off, the NMV is influenced only by
B0, and it tries to realign with it. To do so, the hydrogen nuclei lose energy given
to them by the RF excitation pulse. The process by which hydrogen loses this
energy is called relaxation.
As relaxation occurs, the NMV returns to realign with B0 because some of the
high-energy nuclei return to the low-energy population and therefore align
their magnetic moments in the spin-up direction.
At the same time, but independently, the magnetic moments of hydrogen
lose coherency due to dephasing. These occur because of inhomogeneities in
the B0 field and due to interactions between spins in the patient’s tissue. As the
magnitude of transverse coherent magnetization decreases, so does the
magnitude of the voltage induced in the receiver coil. The induction of decaying
voltage is called the free induction decay (FID) signal. This is because spins
freely precess influenced only by B0, signal decays with time, and magnetic
moments of the spins induce a current in the receiver coil.
The MR Signal

25. All clinical diagnostic images must demonstrate contrast
between normal anatomical features and between anatomy
and pathology. If there is no contrast, it is impossible to
identify anatomy or detect abnormalities within the body.
One of the main advantages of MRI compared to other
imaging modalities is its excellent soft tissue discrimination.
The contrast characteristics of each image depend on many
variables, and it is important that the mechanisms that affect
image contrast in MRI are clearly understood.
Image Weighting and Contrast

26. An image has contrast if there are areas of high signal (white on the
image), as well as areas of low signal (dark on the image).
Some areas have an intermediate signal (shades of grey in-between
white and black).
The NMV can be separated into the individual vectors of the tissues
present in the patient such as fat, cerebro-spinal fluid (CSF) and muscle.
 A tissue has a high signal (white) if it has a large transverse component
of magnetization. If there is a large component of transverse
magnetization, the amplitude of the magnetization received by the coil is
large, and the signal induced in the coil is also large.
A tissue gives a low signal (black), if it has a small transverse component
of magnetization. If there is a small component of transverse
magnetization, the amplitude of the magnetization received by the coil is
small, and the signal induced in the coil is also small.
A tissue gives an intermediate signal (grey), if it has a medium
transverse component of magnetization.

27. The factors that affect image contrast in diagnostic imaging
are usually divided into two categories.
• Intrinsic contrast parameters are those that cannot be
changed because they are inherent to the body’s tissues.
• Extrinsic contrast parameters are those that can be
changed because they are under our control.
For example, in radiography, intrinsic contrast parameters
include the density of structures through which the X-ray
beams passes and is attenuated by, while extrinsic contrast
parameters include the exposure factors. These determine
image contrast in a radiograph, but exposure factors can be
changed, whereas tissue density cannot. In MRI, there are
several parameters in each group.

28. Intrinsic contrast parameters are as follows:
•T1 recovery time
•T2 decay time
• Proton density (PD)
• Flow
• Apparent diffusion coefficient (ADC).
All these are inherent to the body’s tissues and cannot be changed.
Extrinsic contrast parameters are as follows:
•TR
•TE
• Flip angle
•TI
•Turbo factor/echo train length
• b value.
These are all selected in the scan protocol.

29. Repetition time (TR) is the length of the relaxation period between two excitation
pulses and is therefore crucial forT1 contrast.
When TR is long, more excited spins rotate back into the z-plane and contribute to
the regrowth of longitudinal magnetization. The more longitudinal magnetization
can be excited with the next RF pulse, the larger the MR signal that can be collected.
If a short repetition time (less than about 600 msec) is selected, image contrast is
strongly affected byT1.
Under this condition, tissues with a short T1 relax quickly and give a large signal
after the next RF pulse (and hence appear bright on the image). Tissues with a long
T1, on the other hand, undergo only little relaxation between two RF pulses and
hence less longitudinal magnetization is available when the next excitation pulse is
applied.
These tissues therefore emit fewer signals than tissues with a short T1 and appear
dark. An image acquired with a short TR is T1-weighted because it contains mostly T1
information. If a fairly long repetition time (typically over 1500 msec) is selected, all
tissues including those with a long T1 have enough time to return to equilibrium and
hence they all give similar signals.
As a result, there is less T1 weighting because the effect of T1 on image contrast is
only small.
Repetition Time (TR) And T1 Weighting

30. Thus, by selecting the repetition time, we can control the
degree ofT1 weighting of the resulting MR image:
ShortTR → strongT1 weighting
LongTR → lowT1 weighting
The relationship between the MR signal of a tissue and its
appearance onT1-weighted images is as follows:
Tissues with a short T1 appear bright because they regain
most of their longitudinal magnetization during the TR interval
and thus produce a stronger MR signal.
Tissues with a long T1 appear dark because they do not
regain much of their longitudinal magnetization during the TR
interval and thus produce a weaker MR signal.

32. The echo time determines the influence ofT2 on image contrast.
T2 is in the range of several hundred milliseconds and therefore much shorter
thanT1.
If a short echo time is used (less than about 30 msec), the signal differences
between tissues are small because T2 relaxation has only just started and there has
only been little signal decay at the time of echo collection. The resulting image has
lowT2 weighting. I
f a longer echo time in the range of the T2 times of tissues (over about 60 msec) is
used, the tissues are depicted with different signal intensities on the resulting MR
image: tissues with a short T2 having lost most of their signal appear dark on the
image while tissues with a long T2 still produce a stronger signal and thus appear
bright.
This is why, for instance, cerebrospinal fluid (CSF) with its longer T2 (like water) is
brighter onT2-weighted images compared with brain tissue.
By selecting an echo time (TE), the operator can control the degree of T2
weighting of the resulting MR image:
ShortTE → lowT2 weighting
LongTE → strongT2 weighting.
Echo Time (TE) And T2 Weighting

34. The composition of fat and water
All substances possess molecules that are constantly in motion. This
molecular motion is made up of rotational and transitional movements. The
faster the molecular motion, the more difficult it is for a substance to release
energy to its surroundings.
• Fat comprises hydrogen atoms linked to carbon that make up large
molecules. The large molecules in fat have a slow rate of molecular motion
due to inertia of the large molecules. They also have a low inherent energy
which means they are able to absorb energy efficiently.
• Water comprises hydrogen atoms linked to oxygen. It consists of small
molecules with little inertia that have a high rate of molecular motion. They
have a high inherent energy which means they are not able to absorb energy
efficiently. Because of these differences, tissues that contain fat and water
produce different image contrast. This is because there are different
relaxation rates in each tissue.

36. After the RF excitation pulse has been applied and resonance and the
desired flip angle achieved, the RF pulse is removed. The signal induced in the
receiver coil begins to decrease. This is because the coherent component of
NMV in the transverse plane, which is passing across the receiver coil, begins
to decrease. The amplitude of the voltage induced in the receiver coil therefore
decreases.This is called free induction decay (FID).
The NMV in the transverse plane decreases due to relaxation processes and
field inhomogeneities.
The magnetization in each tissue relaxes at different rates. This is one of the
factors that create image contrast. The withdrawal of the RF produces several
effects.
• Nuclei emit energy absorbed from the RF pulse through a process known as
spin lattice energy transfer and shift their magnetic moments from the high
energy state to the low energy state. The NMV recovers and realigns to B0.
This relaxation process is called T1 recovery.
• Nuclei lose precessional coherence or dephase and the NMV decays in the
transverse plane.The dephasing relaxation process is called T2 decay.
Relaxation Process

37. T1 recovery is caused by the exchange of energy from nuclei to
their surrounding environment or lattice. It is called spin lattice
energy transfer. As the nuclei dissipate their energy their magnetic
moments relax or return to B0, i.e., they regain their longitudinal
magnetization. The rate at which this occurs is an exponential
process and it occurs at different rates in different tissues.
The T1 time of a particular tissue is an intrinsic contrast parameter
that is inherent to the tissue being imaged. It is defined as the time it
takes for 63% of the longitudinal magnetization to recover. The
period of time during which this occurs is the time between one
excitation pulse and the next or the TR. The TR therefore determines
how muchT1 recovery occurs in a particular tissue.
T1 Recovery

38. T1 recovery in fat
• T1 relaxation occurs as a result of nuclei exchanging the energy given to
them by the RF pulse to their surrounding environment. The efficiency of this
process determines theT1 time of the tissue in which they are situated.
• Owing to the fact that fat is able to absorb energy quickly, the T1 time of fat
is very short, i.e., nuclei dispose of their energy to the surrounding fat tissue
and return to B0 in a short time.

39. T1 recovery in water
• Water is very inefficient at receiving energy from nuclei. The T1 time of
water is therefore quite long, i.e., nuclei take a lot longer to dispose of their
energy to the surrounding water tissue and return to B0.

40. The TR controls how much of the NMV in fat or water has recovered before
the application of the next RF pulse. Short TRs do not permit full longitudinal
recovery in either fat or water so that there are different longitudinal
components in fat and water.
These different longitudinal components are converted to different
transverse components after the next excitation pulse has been applied. As
the NMV does not recover completely to the positive longitudinal axis, they
are pushed beyond the transverse plane by the succeeding 90° RF pulse. This
is called saturation. When saturation occurs, there is a contrast difference
between fat and water due to differences inT1 recovery using shortTRs.
Long TRs allow full recovery of the longitudinal components in fat and
water. There is no difference in the magnitude of their longitudinal
components. There is no contrast difference between fat and water due to
differences in T1 recovery when using long TRs. Any differences seen in
contrast are due to differences in the number of protons or proton density of
each tissue. The proton density of a particular tissue is an intrinsic contrast
parameter and is therefore inherent to the tissue being imaged.

41. TypicalT1 recovery times of brain tissue at 1T.
Tissue T1 recovery time (ms)
Water 2500
Fat 200
CSF 2000
White matter 50

42. T2 decay is caused by the exchange of energy from one nucleus to
another. It is called spin–spin energy transfer.
It occurs as a result of the intrinsic magnetic fields of the nuclei
interacting with each other. This energy exchange produces a loss of
phase coherence or dephasing, and results in decay of the NMV in the
transverse plane. It is an exponential process and occurs at different
rates in different tissues.
The T2 decay time of a particular tissue is an intrinsic contrast
parameter and is inherent to the tissue being imaged. It is the time it
takes for 63% of the transverse magnetization to be lost due to
dephasing, i.e., transverse magnetization is reduced to 63% of its
original value (37% remains).
The period of time over which this occurs is the time between the
excitation pulse and the MR signal or the TE. The TE therefore
determines how muchT2 decay occurs in a particular tissue.
T2 Decay

43. T2 decay in fat and water
T2 relaxation occurs as a result of the spins of adjacent nuclei interacting with
each other and exchanging energy. The efficiency of this process depends on
how closely the molecular motion of the atoms matches the Larmor frequency
and the proximity of other spins.
The Larmor frequency is relatively low and therefore fat is much better at this
energy exchange than water, whose molecular motion is much faster than the
Larmor frequency and whose atoms are closely spaced. Fat’s T2 time is
therefore very short compared with that of water.
The TE controls how much transverse magnetization has been allowed to
decay in fat and water when the signal is read.
Short TEs do not permit full dephasing in either fat or water so their
transverse components are similar. There is little contrast difference between
fat and water due to differences inT2 decay using shortTEs.
Long TEs allow dephasing of the transverse components in fat and water.
There is a contrast difference between fat and water due to differences in T2
decay times when using long TEs. It should be noted that fat and water
represent the extremes in image contrast. Other tissues, such as muscle, grey
matter and white matter have contrast characteristics that fall between fat and
water.

46. T1 contrast
The term T1 contrast means that image contrast is derived from differences
in the T1 recovery times of the tissues rather than any other mechanism. T1
contrast is likely to occur if vectors do not fully recover their longitudinal
magnetization between each RF excitation pulse. It therefore increases if the
TR is short. If the TR is longer than the relaxation times of the tissues, full
recovery occurs in all tissues, and, therefore, it is not possible to produce an
image that demonstrates contrast based on the differences in their T1
recovery times.
The T1 recovery time of fat is much shorter than that of water, so the fat
vector realigns with B0 faster than the water vector. If the TR is shorter than
the total recovery times of the tissues, the longitudinal component of
magnetization of fat is larger than that of water. When the next RF excitation
pulse is applied, it flips the longitudinal components of magnetization of both
fat and water into the transverse plane (assuming a 90° RF excitation pulse is
applied). As there is more longitudinal magnetization in fat before the RF
excitation pulse, there is more transverse magnetization in fat after the RF
excitation pulse. Fat therefore has a high signal and is hyperintense. As there is
less longitudinal magnetization in water before the RF excitation pulse, there
is less transverse magnetization in water after the RF excitation pulse. Water
therefore has a low signal and appears relatively hypointense.

48. T2 contrast
The term T2 contrast means that image contrast is derived from
differences in the T2 decay times of the tissues rather than any other
mechanism.
T2 contrast is likely to occur if vectors dephase and there is a
difference in coherent transverse magnetization in each tissue. It
therefore increases if theTE is long.
Magnetic moments of the hydrogen nuclei dephase at different rates,
so if the TE is long, it is possible to produce an image that demonstrates
differences in their T2 decay times. If the TE is short, then little
dephasing occurs, and therefore it is not possible to produce images
that demonstrate differences inT2 decay times of the tissues.
The T2 decay time of fat is shorter than that of water so the
transverse component of magnetization in fat decays faster than the
transverse component in water. There is more coherent transverse
magnetization in water than in fat. Water, therefore, has a high signal
and is hyperintense, and fat therefore has low signal and is relatively
hypointense on aT2 contrast image.

50. Proton density contrast
Proton density contrast refers to differences in signal intensity between tissues
that are a consequence of their relative number of mobile hydrogen protons per
unit volume. To produce contrast due to differences in the proton densities
between the tissues, the transverse component of magnetization must reflect
these differences. Tissues with a high proton density have a large transverse
component of magnetization (and therefore a high signal) and are hyperintense.
Tissues with a low proton density have a small transverse component of
magnetization (and therefore a low signal) and are relatively hypointense. Proton
density contrast is always present and depends on the patient and the area under
examination. The signal intensity of a tissue therefore depends on its intrinsic
contrast properties.
Three of these are:
• Proton density (PD)
•T1 recovery time
•T2 decay time.
Signal intensity also depends on extrinsic contrast parameters.
Two of these are:
•TR
•TE.

51. During the MRI procedure, the gradient coils are adjusted and turned on
and off rapidly, so they alter the main magnetic field. The gradient
produces a thin cross-section of the body tissue. Phase-encoding and
frequency-encoding methods are used to divide each slice into a matrix of
voxels.
Image Acquisition

52. Different types of coils are used for specific body parts. During the
acquisition, the receive coil is positioned at right angles to the main
magnetic field (BO).
The RF signals are emitted by tissues and received up by a coil. The
different slices and voxels of the tissues emitted unique frequency and
phase signals so they can be separated easily during image reconstruction.
The received signal is sinusoidal and has low energy. The signal received
by the coil has low energy that is not enough to convert them into an image
hence; repeated on-off of RF pulses is required.
The Image acquisition cycle is repeated many times. The quality of an
image can be improved by increasing the number of acquisition cycles. The
received signals are then sent to the computer system.
After the acquisition process, a large amount of data is collected and
stored in computer memory. The data represent RF signal intensities
(frequency and phase) and are stored in k space.
Data Acquisition

53. K-Space is an imaginary space which represents a raw data matrix. It
represents stage between reception of signals and image formation.
After acquisition all signals are stored in K-Space in a particular fashion.
This raw data from K-Space is used to reconstruct image by
FourierTransformation.
K-Space has two axes. Horizontal axis represents the phase axis and
is centered in the middle of several horizontal lines. The frequency axis
of K-Space is vertical and is perpendicular to the phase axis.
Signals are filled in K-Space as horizontal lines. The number of lines of
K-Space that are filled is determined by the number of different phase
encoding steps. If 128 different phase encoding steps are selected then
128 lines of K-Space are filled to form an image. In conventional spin-
echo imaging one line of K-Space is filled every TR. If matrix size is 128
× 128 then 128TRs will be required to form an image.
K-Space

54. Array of signals as lines in the K-Space does not correspond with
rows or columns of pixel in the image. It is a complex transformation
of this data into an image where center part of K-Space represents
contrast of the image and the periphery corresponds with resolution
and fine details.
Two halves of K-Space—right and left or upper and lower are
exact symmetric conjugate and represent same information. This
fact is used to manipulate K-Space sampling (partial sampling) to
get same image in less time. In fast imaging like single-shot fast
spin-echo sequence such as HASTE, just more than half of K-Space
is filled. In ultrafast Echo Planar Imaging (EPI) all the K-Space lines
required to form an image are filled within a single TR, thus reducing
the scanning time in seconds.

56. Reconstruction is the mathematical process performed by the
computer. The data of k space will be processed by the
reconstruction algorithm. The special software processes the data
of k space and converts it into an image. The most common
method is the 2-D Fourier transformation. The Fourier
transformation can determine the location of each signal. The data
are mathematically processed by Fourier transformation according
to intensity levels then displayed as shades of gray in a matrix to
produce a final image.
Image Formation

57. When a human body is placed in a strong uniform magnetic field, as a
result, many of the free hydrogen nuclei align themselves with the
direction of the magnetic field.
The nuclei precess about the magnetic field direction. Next, a radio-
frequency pulse is applied, which has a specific frequency similar to the
Precession frequency of hydrogen nuclei. The hydrogen nuclei absorb
energy from the RF pulse and become in a higher energy state, as a result,
the net magnetization vector trip away from Z-axis.
When a radio-frequency pulse is removed, the nuclei realign themselves
and return to equilibrium; it is referred to as relaxation.
When nuclei return to equilibrium from the excited state they release
the absorbed energy by emitting RF energy which is called MR signal. This
signal is referred to as the free induction decay (FID). The FID signals are
received by the RF coils.
MRI Image Formation

58. The signal is picked up by a coil but the emitted signal energy is too low.
Hence, repeated RF pulses are required. During the acquisition, the cycle is
repeated many times by the repeated process of alignment, excitation, and
relaxation of the proton.
The time required for image acquisition is determined by the time TR. During
the image acquisition, the gradients are used. The gradients will be turned on
and off for a specific time for signal acquisition.
The gradient produces a thin cross-section of the body tissue. Phase-encoding
and frequency-encoding methods are used to divide each slice into a matrix of
voxels.
This result in a large amount of data is collected. The received signals are then
sent to the computer system. After the acquisition process, a large amount of
data is collected and stored in computer memory. The data represent RF signal
intensities (frequency and phase) and are stored in k space.
The data of k space will be processed by the reconstruction algorithm. The
special software processes the data of k space and converts it into an image. The
most common method is the 2-D Fourier transformation.
The data are mathematically processed by Fourier transformation according
to intensity levels.
The computer displays reconstructed MR images in a controlled manner.