Magnetic Resonance Imaging

MAGNETIC RESONANCE
IMAGING
Sailakshmi P
MSc Radiation Physics
Department of Physics

CONTENTS
 Introduction
 History
 Theory and Physics
 Instrumentation
 Artifacts
 Risks & Benefits

INTRODUCTION
 MRI is a rapidly changing and growing powerful imaging modality.
 It is very close relative of NMR, which allows clinicians to obtain chemical and
physical information about certain molecules in multiple planes.
 In the 1970’s the name was changed from NMRI to MRI due to the negative
connotations associated with the word “nuclear”. Many patients thought that the
exam would expose them to radiation. But MRI not involving the use of ionizing
radiation.

HISTORY
 In 1952 Felix Bloch and Edward Purcell
were discovered the concepts
surrounding NMR/MRI.
 During the time between 1950-1970,
the idea was used for chemical and
physical analysis of molecules.

History . . . . . . . .
 In 1971 Raymond Damadian discovered
that NMR could be used in the detection
of diseases.
 In 1977, Damadian did his first scan on a
human, his assistant, Larry Minkoff.

History . . . . . .
 Damadian’s first prototype was called
“Indomitable”, due to criticism and
the seven years that it took to
complete.
 In 1978, Damadian established a new
corporation called FONAR, which
introduced the first commercial MRI
scanner in 1980.

History . . . . . . . .
 MRI machines have come a long way
since Indomitable. It took up to five
hours to get an image.
 In 1992, functional magnetic
resonance imaging (fMRI) was
discovered, Which allowed clinicians
to see various regions of the brain,
their functions, and their specific
locations.

THEORY
• Magnetic resonance (MR) is based upon the interaction between an external
magnetic field and a nucleus that possesses spin.
• Nuclei containing an odd number of protons and or neutrons have a characteristic
motion or precession.
• Every element in the periodic table except argon and cerium has at least one
naturally occurring isotope that possesses spin.

Theory…………….
Magnetization properties
• Magnetism is a fundamental property of matter, it is generated by moving
charges, usually electrons.
• Magnetic susceptibility describes the extent to which a material becomes
magnetized when placed in a magnetic field.
• The induced internal magnetization can oppose the external magnetic field and
lower the local magnetic field surrounding the material. On the other hand, the
internal magnetization can form in the same direction as the applied magnetic
field and increase the local magnetic field.

Fig 1 : Precession of a nuclei

• The magnetic field strength, B, (also called the magnetic flux density) can be
conceptualized as the number of magnetic lines of force per unit area.
• The magnetic field drops off with the square of the distance.
• The SI unit for B is the tesla (T)
• The earth's magnetic field is about 1/20,000 T (0.5 gauss (G))
• 1 T = 10,000 G.
• The magnets in use today in MRI are in the 0.5-Tesla to 3.0-Tesla range, or 5,000
to 30,000 gauss.

• The H nucleus, consisting of a single proton, is a natural choice for probing the body
using MR techniques.
• It has a spin of ½ and the most abundant isotope of hydrogen is protium. It is very
sensitive to the magnetic field due to its large value for g (42.58).
• Finally, the body is composed of tissues that contain primarily water and fat, both of
which contain hydrogen.

• If the tissue is placed inside a magnetic field Bo, the individual protons begin to rotate,
or precess, about the magnetic field.
• The protons are tilted slightly away from the axis of the magnetic field, but the axis of
rotation is parallel to Bo.
• This precession occurs because of the interaction of the magnetic field with the moving
positive charge of the nucleus.
• The rate or frequency of precession is proportional to the strength of the magnetic field
and is expressed by Larmor equation.

The precessional frequency and the external magnetic
field are related by the Larmor Equation.
ωo = γ/ 2π B0
ωo Larmor precessional frequency
γ Gyromagnetic ratio for that nucleus
B0 Applied magnetic field

Fig 2: B0 is defined to be oriented
in the z direction of a Cartesian
coordinate system; the axis of
precession is also the z axis. The
motion of each proton can be
described by a unique set of x, y
(perpendicular to B0), and z
(parallel to B0) coordinates. The
perpendicular, or transverse,
coordinates are nonzero and vary
with time as the proton precesses,
but the z coordinate is constant
with time .

Gyromagnetic ratio for useful elements in magnetic resonance
Nucleus γ/2π
1H 42.58
13C 10.7
17O 5.8
19F 40.0
23Na 11.3
31P 17.2

Fig 3 : (a) I n the absence of a strong magnetic field, hydrogen nuclei are
randomly aligned .(b) When the strong magnetic field, Bo ,is applied, the
hydrogen nuclei precess about the direction of the field .

Fig 3 : (a) The RF pulse, Brf, causes the net magnetic moment of the nuclei , M , to tilt away
from Bo . (b) When the RF pulse stops, the nuclei return to equilibrium such that M is again
parallel to Bo . During realignment, the nuclei lose energy and a measurable RF signal .

RELAXATION
• Relaxation is the process by which the protons release the energy that they
absorbed from the rf pulse.
• Relaxation is a fundamental process in MR, as essential as energy absorption, and
provides the primary mechanism for image contrast.
• During relaxation, the protons release this energy and return to their original
configuration.

There are two types of relaxation process
• T1 Relaxation
• T2 Relaxation
FID : The response of the net magnetization M to an rf pulse as a function of time is
known as the free induction decay or FID.
• The Fourier transformation is used to convert the digital version of the MR signal
(FID) from a function of time to a function of frequency.

T1 RELAXATION
• The relaxation time T1 is the time required for the z component of M to return to
63% of its original value following an excitation pulse.
• It is also known as the spin-lattice relaxation time or longitudinal relaxation .
• Here, Mo is parallel to Bo at equilibrium and that energy absorption will rotate Mo
into the transverse plane. T1 relaxation provides the mechanism by which the
protons give up their energy to return to their original orientation.

Fig 5 : T1 relaxation curve. The change of Mz/ M0 with time t follows an exponential growth
process

T2 RELAXATION
• The relaxation time T2 is the time required for the transverse component of M to decay to
37% of its initial value via irreversible processes.
• It is also known as the spin-spin relaxation time or transverse relaxation time. Spin-spin
relaxation refers to the energy transfer from an excited proton to another nearby proton.
• The absorbed energy remains as spin excitation rather than being transferred to the
surroundings as in T1 relaxation.
• This proton-proton energy transfer can occur many times as long as the protons are in
close proximity and remain at the same Wo.

Fig 6 : Net magnetization M is oriented parallel to B0 prior to pulse (1). Following a 90º rf pulse, the protons
initially precess in phase in the transverse plane (2). Due to inter- and intramolecular interactions, the protons
begin to precess at different frequencies (dashed arrow = faster; dotted arrow = slower) and become
asynchronous with each other (3). As more time elapses (4,5), the transverse coherence becomes smaller until
there is complete randomness of the transverse components and no coherence (6).

Fig 6: Plot of relative Mxy component. The change in Mxy/Mxy max with time follows an
exponential decay process. The time constant for this process is the spin-spin
relaxation time T2 and is the time when MXY has decayed to 37% of its original value. This
dephasing time T2 is always less than or equal to T1.

Spin echo…………
• The initial 90º rf pulse rotates Mo into the transverse plane. During the time t,
proton dephasing will occur through T2* relaxation processes.
• Application of the 180º rf pulse causes the protons to reverse their phases relative
to the resonant frequency.
• If time t elapses again, then the protons will regain their transverse coherence.
This reformation of phase coherence induces another signal in the receiver coil,
known as a spin echo.

CONTRAST WEIGHTING
• Contrast in an image is proportional to the difference in signal intensity between
adjacent pixels in the image, corresponding to two different voxels in the patient.
• It depends on spin(proton) density, T1 and T2 relaxations.
• TR (Time of Repetition) & TE (Time of Echo) are pulse sequence controls on the
MRI machine.

T1 WEIGHTING & T2 WEIGHTING
• TR: short
• TE: short
• fluid: dark
• fat: bright
TR: long
TE: long
fluid: bright
fat: intermediate-bright

• TR: long
• TE: short
• fluid: bright
• fat: bright
• Overall signal: high
SPIN(PROTON) DENSITY WEGHTING

INSTRUMENTATION
The major components in MRI are :
 Computer system
 Magnet system
 Gradient system
 Radio-frequency system

Fig 8: Block diagram of an MRI system.

MAGNET SYSTEM
In MRI different types of magnets are used, they are :
 Permanent magnet
 Resistive magnet and
 Superconducting magnet

GRADIENT SYSTEM
Gradient system consist of three types of gradient coils :
• Gradient coils are used to systematically vary the magnetic field by producing
additional linear electro magnetic fields, thus making slice selection and spatial
information possible.
• As we have three dimension in space ,there are three sets of gradient coils (x ,
y & z). That are the cause of noise during a MR examination.
• Typical MRI scanner generates 110 decibels of noise, certain MRI scanners
could get up to 118 decibels at their loudest point.

Fig 9 : Three types of gradient coils

RADIO-FREQUENCY SYSTEM
(RF COILS)
• RF coils are used for transmitting RF energy to the tissue of interest and to receive the
induced RF signal from the tissue of interest.
• They are placed concentric to each other and act as an antenna of the MR system. There are
coils specifically designed for brain , breast and other body organs.
• RF signal is generated by a transmit RF coils and applied to an area of interest and output
signal is picked up by the RF receive coil and transmitted to an RF amplifier for
reconstruction of images.
• Faraday’s cage

ARTIFACTS
• MRI artifacts are varied and numerous and some effect the quality of the MRI exam
while others do not affect the diagnostic quality but may be confused with pathology.
• When encountering an unfamiliar artifact , it is useful to systematically examine general
features of the artifact to try and understand the type of artifact and how to negate it if
needed. These features include:
 type of sequence (e.g. fast spin echo, or gradient or volumetric)
 direction of phase and frequency
 fat or fluid attenuation
 presence of anatomy outside the image field
 presence of metallic foreign bodies

Artifacts ……………………………
Many artifacts have a characteristic appearance and with experience they can be readily identified.
 MRI artifacts are classified into three broad areas-those :
Based on the machine
 MR hardware and room shielding.
 MR software.
Based on the patient
 Patient and physiologic motion.
 Tissue heterogeneity and foreign bodies.
Based on signal processing
 Fourier transform and Nyquist sampling theorem.

Based on the machine :
MR hardware and room shielding
• Herringbone artifact
• Moire fringes
• Zebra stripes
• Central point artifact
• RF overflow artifacts
• Inhomogeneity artifacts
• Inhomogeneity artifacts
MR software
• Slice-overlap artifact (cross-talk artifact)
• Cross excitation

Based on the machine:
Herringbone Artifact
• Also called as crisscross
artifact or corduroy artifact. It appears as
a fabric of herring bone . Artifact is
scattered all over the image in a single
slice or multiple slices .
Causes:
• electromagnetic spikes by gradient coils
• fluctuating power supply
• RF pulse discrepancies

Moire fringes
• It is an interference pattern most commonly
seen when doing gradient echo images with the
body coil.
• Because of lack of perfect homogeneity of the
main magnetic field from one side of the body
to the other, aliasing of one side of the body to
the other results in superimposition of signals
of different phases that alternatively add and
cancel. This causes the banding appearance and
is similar to the effect of looking though two
screen windows.

Central point artifact
• It is a focal dot of increased signal in the
center of an image. It is caused by a
constant offset of the DC voltage in the
receiver. After Fourier transformation,
this constant offset gives the bright dot in
the center of the image as shown in the
diagram.
• The axial MRI image of the head shows a
central point artifact projecting in the
pons in the center of the image.
Correction and prevention
• Repeating the sequence may get rid of the
artifact.
• Maintain constant temperature in scanner
and equipment room for receiver
amplifiers.
• Software to estimate DC offset and adjust
the data in k-space.
• Call service engineer for recalibration.

RF overflow artifacts
• cause a nonuniform, washed-out
appearance to an image.
• occurs when the signal received by the
scanner from the patient is too intense to
be accurately digitized by the analog-to-
digital converter.
• Autoprescanning usually adjusts the
receiver gain to prevent this from
occurring but if the artifact still occurs, the
receiver gain can be decreased manually .
Post-processing methods also exist but
may be time consuming .

Zipper artifacts
• where one or more spurious bands of electronic noise
extend perpendicular to the frequency encode direction
and is present in all images of a series.
• There are various causes for zipper artifacts in images.
Most of them are beyond the radiologist immediate
control.
• It can be controlled easily are those that occur when the
door is open during acquisition of images due to RF
entering the scanning room from electronic equipment
and are being picked by the receiver chain of imaging
sub-systems. RF from some radio transmitters will cause
this artifact.

MR Software
Slice-overlap artifact
• Cross talk overlap is a name given to the loss of signal
seen in an image from a multi-angle, multi-slice
acquisition, as is obtained commonly in the lumbar
spine.
• If the slices obtained at different disk spaces are not
parallel, then the slices may overlap. If two levels are
done at the same time, e.g., L4-5 and L5-S1, then the
level acquired second will include spins that have
already been saturated. This causes a band of signal
loss crossing horizontally in your image, usually worst
posteriorly.The dark horizontal bands in the bottom
of the following axial image through the lumbar spine
demonstrates this artifact

Based on the Patient :
Patient and physiologic motion
• phase-encoded motion artifact
• entry slice phenomenon
Tissue heterogeneity and foreign bodies
• black boundary artifact
• magic angle effect
• susceptibility artifact / magnetic
susceptibility artifact
• chemical shift artifact

Patient and physiologic motion
Phase-encoded motion artifact
• It occurs as a result of tissue / fluid moving during the scan and
manifests as ghosting in the direction of phase encoding, usually in
the direction of short axis of the image (i.e left to right on axial or
coronal brains, and anterior to posterior on axial abdomen).
• It may be seen from arterial pulsations, swallowing, breathing,
peristalsis, and physical movement of a patient. When projected
over anatomy it can mimic pathology, and needs to be recognized.
Motion that is random such as the patient moving produces a
smear in the phase direction. Periodic motion such as respiratory
or cardiac/vascular pulsation produces discrete, well defined
ghosts. The spacing between these ghosts is related to the TR and
frequency of the motion.

Black boundary artifact
• Black boundary artifact or India ink artifact is
an artificially created black line located at fat-
water interfaces such as those between muscle
and fat. This results in a sharp delineation of
the muscle-fat boundary that is sometimes
visually appealing but not an anatomical
structure.
• Case here is a coronal image through the upper
body with an echo time of 7 ms. A black line is
seen surrounding the muscles of the shoulder
girdle as well as around the liver.

Magnetic susceptibility artifact
• It refers to a distortion in the MR image
especially seen while imaging metallic
orthopedic hardware or dental work. This
results from local magnetic field
inhomogeneities introduced by the
metallic object into the otherwise
homogeneous external magnetic field B0.
These local magnetic field
inhomogeneities are known as magnetic
susceptibility and are a property of the
object being imaged.

Magic angle effect
• It occurs on sequences with a short TE (less than 32ms).
• It is confined to regions of tightly bound collagen at 54.74°
from the main magnetic field (Bo), and appears hyper
intense, thus potentially being mistaken
for tendonopathy.
• In tightly bound collagens, water molecules are restricted
usually causing very short T2 times, accounting for the
lack of signal. When molecules lie at 54.74° there is
lengthening of T2 times (don't understand why.) with
corresponding increase in signal.
Typical sites include :
• proximal part of the posterior
cruciate ligament (PCL)
• peroneal tendons as they hook
around the lateral malleolus.
• cartilage can also be affected e.g.
femoral condyles
• supraspinatus tendon
• triangular fibrocartilage complex (if
the patient is imaged with the arm
elevated)
• It appears that at 3.0T the effects
are reduced.

chemical shift artifact
• Misregistration is common finding on
some MRI sequences, and used in MRS.
• Chemical shift is due to the differences between
resonance frequencies between fat and water. It
occurs in the frequency encode direction where a
shift in the detected anatomy occurs because fat
resonates at a slightly lower frequency than water.
Essentially it is due to the effect of the electron
cloud to a greater or lesser degree shielding the
nucleus from the external static magnetic field
(Bo).

Based on signal processing:
Fourier transform and Nyquist sampling theorem
• Gibbs artifact / truncation artifact
• zero-fill artifact
• aliasing / wrap around artifact

Gibbs artifact / truncation artifact
• It refers to a series of lines in the MR image parallel to abrupt
and intense changes in the object at this location, such as the
CSF-spinal cord and the skull-brain interface
• The MR image is reconstructed from k-space which is a finite
sampling of the signal subjected to inverse Fourier transform in
order to obtain the final image. At high-contrast boundaries the
Fourier transform corresponds to an infinite number of
frequencies, and since sampling is finite the discrepancy appears
in the image in the form of a series of lines. These can appear in
both phase-encode and frequency-encode direction.
• The more encoding steps, the less intense and narrower the
artifacts.

Zero-fill artifact
• Zero fill artifact is one of many MRI
artifacts and is due to data in the K-space
array missing or set to zero during
scanning. The abrupt change from signal
to no signal results in artifacts in the
images showing alternating bands of
shading and darkness, often in an oblique
direction.
• A spike in k-space as from an electrostatic
spark is another artifact that causes
oblique stripes.

Aliasing / wrap around artifact
• It occurs when the field of view (FOV) is
smaller than the body-part being imaged. The
part of the body that lies beyond the edge of
the FOV is projected on to the other side of
the image.
• This can be corrected, if necessary, by
oversampling the data. In the frequency
direction, this is accomplished by sampling
the signal twice as fast. In the phase direction,
the number of phase-encoding steps must be
increased with a longer study as a result.

Uses
• Using MRI scans, physicians can diagnose or monitor treatments for a variety of
medical conditions, including:
• Abnormalities of the brain and spinal cord
• Tumors, cysts, and other abnormalities in various parts of the body
• Injuries or abnormalities of the joints
• Certain types of heart problems
• Diseases of the liver and other abdominal organs
• Causes of pelvic pain in women (e.g. fibroids, endometriosis)
• Suspected uterine abnormalities in women undergoing evaluation for infertility

Risks/Benefits
• MRI does not use ionizing radiation (high-energy radiation that can potentially
cause damage to DNA, like the x-rays used CT scans).
• Using MRI we get better image contrast of the soft tissue
• There are no known harmful side-effects associated with temporary exposure to
the strong magnetic field used by MRI scanners. However, there are important
safety concerns to consider before performing or undergoing an MRI scan:
• The magnet may cause pacemakers, artificial limbs, and other implanted medical
devices that contain metal to malfunction or heat up during the exam.

• Any loose metal object may cause damage or injury if it gets pulled toward the magnet.
• If a contrast agent is used, there is a slight risk of an allergic reaction. MRI contrast agents
can cause problems in patients with significant kidney disease.
• Dyes from tattoos or tattooed eyeliner can cause skin or eye irritation.
• Medication patches can cause a skin burn.
• The wire leads used to monitor an electrocardiogram (ECG) trace or respiration during a
scan must be placed carefully to avoid causing a skin burn.
• Prolonged exposure to radio waves during the scan could lead to slight warming of the
body.

REFERENCES
• MRI Book by H. H. Schild
• The Essential Physics of Medical Imaging By Jerrold T Bushberg
• MRI Basic Principles And Application 2nd
Edition By Mark A Brown & Richard C. Semelka
• Magnetic Resonance Imaging: Bioeffects and Safety Concerns By N R Jaganathan -
Department of N.M.R., All India Institute of Medical Sciences
• http://www.mr-tip.com/serv1.php?type=mri_safety&p=intro
• Real Whole Body MRI By Mathias Goyen
• http://www.simplyphysics.com/MAIN.HTM
• MRI Safety, Policies and Procedures., PPT By Suny Stony Brook Social Cognitive, and Affective
Neuroscience (SCAN) Center
• http://radiopaedia.org/articles/mri-artifacts
• Wikipedia

Magnetic Resonance Imaging

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (20)

Similaire à Magnetic Resonance Imaging

Similaire à Magnetic Resonance Imaging (20)

Dernier

Dernier (20)

Magnetic Resonance Imaging