mri principles 01
TRANSCRIPT
-
8/9/2019 MRI Principles 01
1/48
Medical Imaging Systems
Magnetic Resonance
Imaging
D. Asemani
April 2012
Part 01
General View
-
8/9/2019 MRI Principles 01
2/48
2D. Asemani Magnetic Resonance Imaging
Diagnostic imaging modalities
2. ultrasound
1. standard plane-view x-rays
billions
no ionizing radiation
most frequently used
ionizing radiation
computed tomography (CT) scanning
display of pulse-echoes backscattered from tissues
second most frequently used
-
8/9/2019 MRI Principles 01
3/48
3D. Asemani Magnetic Resonance Imaging
3. Magnetic resonance imaging (MRI)
detailed anatomic information
without using ionizing radiation
sensing the spin of their atoms
most abstract and complicated
technically
precise anatomical capability
often used for presurgery planning
for cancer detection
images of brain activity in response to
various stimuli
successfully to medical imaging of the body because
of its high water content
60% of the body by weight
exceptional soft-tissue contrast
now supplanting many conventional invasive procedures
most important imaging sequences
Diagnostic imaging modalities
-
8/9/2019 MRI Principles 01
4/48
4D. Asemani Magnetic Resonance Imaging
A complex molecule is placed in a strong, highly uniform magnetic field. Electronicshielding produces microscopic field variations within the molecule so that geometrically
isolated nuclei rotate about distinct fields.
Each distinct magnetic environment produces:
a peak in the spectra of the received signal.
relative size of the spectral peaks : ratio of nuclei in each magnetic environment
NMR spectrum : extremely useful for elucidating molecular structure
NMR signal from a human is due predominantly to water protons
NMR signal is simply proportional to the volume of the water
key innovation for MRI:
impose spatial variations on the magnetic field to distinguish spins by their location.
Diagnostic imaging modalities3. Magnetic resonance imaging (MRI)
Origin: NMR - Nuclear Magnetic Resonance
NMR has been used for decades in chemistry
a magnetic field gradient each region of the volume to oscillate at a distinct frequency
-
8/9/2019 MRI Principles 01
5/48
-
8/9/2019 MRI Principles 01
6/48
6D. Asemani Magnetic Resonance Imaging
Principle:
protons of the nuclei of hydrogen atoms subjected to radio frequency
pulses in a strong magnetic field. The protons get thereby “excited” tohigher energy level. Protons also get “relaxed” to the lower energy level
on the switching off radio frequency pulses. The protons emit radio
frequency signals when they move from “excited” to “relaxed” state.
These radio signals can be detected by a receiver and a computer canfurther process the output into an image
In body tissues; protons of hydrogen are most abundant as
hydrogen atoms of water molecules (H of H2O).
MRI image shows difference in the water content anddistribution in various body tissues.
Diagnostic imaging modalities3. Magnetic resonance imaging (MRI)
Each different type of tissues within the same region can be easily
distinguished
-
8/9/2019 MRI Principles 01
7/48
7D. Asemani Magnetic Resonance Imaging
A 1990 study : principal applications for MRI are examinations of the head (40%),
spine (33%), bone and joints (17%), and the body (10%).
The percentage of bone and joint studies was growing in 1990.
typical imaging studies range from 1 to 10 minutes
new fast imaging techniques acquire images in less than 50 ms.
MRI research :
fundamental tradeoffs between resolution, imaging time, and signal-to-noise ratio
(SNR).
Most commonly protons (1H) are imaged, although carbon (13 C), phosphorous (31P),
sodium ( 23Na),and fluorine (19F) are also of significant interest.
Typical field strengths for imaging range between 0.2 and 1.5 T,
although spectroscopic and functional imaging work is often performed with higher field
strengths.
Diagnostic imaging modalities3. Magnetic resonance imaging (MRI)
both gradient and receiver coil hardware innovations
-
8/9/2019 MRI Principles 01
8/48
8D. Asemani Magnetic Resonance Imaging
Magnetic resonance imaging (MRI)
Introduction
hydrogen atoms in water (H2O) and fat make up approximately
60% of the body by weight
a proton in the nucleus of each hydrogen atom
nucleus spins a small magnetic field or moment is created
When hydrogen is placed in a large static magnetic field, the magnetic moment
of the atom spins around it like a tiny gyroscope at the Larmor frequency, which
is a unique property of the material.
-
8/9/2019 MRI Principles 01
9/48
9D. Asemani Magnetic Resonance Imaging
a radio frequency rotating field in a plane perpendicular to the static
field is needed
frequency of this field : identical to the Larmor frequency
once the atom is excited, the applied field is shut off and the
original magnetic moment decays to equilibrium and emits a signal
longitudinal magnetization constant, T1, :
more sensitive to the thermal properties of tissue
transversal magnetization relaxation constant, T2, :
affected by the local field inhomogeneities
T1 weighted images are used most often
Magnetic resonance imaging (MRI)Introduction
-
8/9/2019 MRI Principles 01
10/48
10D. Asemani Magnetic Resonance Imaging
MRI finds widespread application:
detection of disease and surgical planning
highly detailed representations of internal anatomy
(called parameterized images ) cosiderable skill is involved in adjusting theinstrument to obtain images that emphasize
different types of tissue contrast, thediscrimination among different organ types and
between healthy and pathological tissues.
examine most of the body, including :
brain, abdomen, heart, large vessels, breast, bones, as well as soft
tissue, joints, cartilage, muscle, and the head and neck
for both children and adults
for detecting cancer pathologies, tumors, and hemorrhaging
Magnetic resonance imaging (MRI)Introduction
-
8/9/2019 MRI Principles 01
11/48
11D. Asemani Magnetic Resonance Imaging
early precedent to MRI : nuclear magnetic resonance (NMR),
determining composition of materials through unique frequency
shifts associated with different chemical compounds
, and soon, detailed spectral information from phosphorus, carbon, and hydrogen
nuclei were obtained. Specialized magnets were designed to accommodate
parts of the body for study
The nuclear magnetization is very weak;
the ratio of the induced magnetization to the applied fields is only 4×10 –9
Magnetic resonance imaging (MRI)Introduction
biological NMR experiments
were underway
-
8/9/2019 MRI Principles 01
12/48
12D. Asemani Magnetic Resonance Imaging
Isidor Rabi
Nobel Prize for Physics in 1944
invention of the atomic and molecular beam magnetic resonancemethod of observing atomic spectra
Columbia University
magnetic resonance (NMR)
History
Felix Bloch,
Stanford University
first successful nuclear magnetic resonance
(NMR) experiment 1946 independently by two
scientists in the US:
Edward Purcell,
Harvard University
Nobel Prize for Physics in 1951
1946: atomic nuclei absorb and re-
emit radio frequency energy
Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
13/48
13D. Asemani Magnetic Resonance Imaging
In 1973, Paul Lauterbur
NMR pioneer at the State University of New York
first NMR image
On July 3, 1977, nearly five hours after the start of the first MRI test, the
first human scan was made as the first MRI prototype
1973: Lauterbur suggests NMR could be used to
form images
Raymond Damadian,physician and experimenter working atBrooklyn's Downstate Medical Center
hydrogen signal in cancerous tissue is different from that of
healthy tissue because tumors contain more water
medical practitioner
first MR (Magnetic Resonance) Scanning Machine
1977: clinical MRI scanner patented
History Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
14/48
14D. Asemani Magnetic Resonance Imaging
fMRI
-1990: Ogawa observes BOLD effect with T2*blood vessels became more visible as blood oxygen
decreased
1991: Belliveau observes first functional images using a contrast agent
1992: Ogawa & Kwong publish first functional images using BOLD signal
-1977: Mansfield proposes echo-planar imaging (EPI) to acquire images faster
a mathematical model to analyze signals from within the human body in response to a
strong magnetic field, as well as a very fast imaging method
Lauterbur and Mansfield shared the 2003 Nobel prize for
medicine for their MRI discoveries
History Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
15/48
15D. Asemani Magnetic Resonance Imaging
Magnetism and electromagnetism
magnetic susceptibility of a substance : ability of external magnetic fields to
affect the nuclei of a particular atom, and is related to the electron configurations
of that atom
nucleus of an atom
paired electrons
unpaired electrons
external magnetic field
more protected
More affected
magnetic susceptibility
Paramagnetism
Diamagnetism
ferromagnetism
Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
16/48
16D. Asemani Magnetic Resonance Imaging
Paramagnetism
unpaired electronssmall magnetic field about themselves known as the
magnetic moment
presence of an external magnetic field
align with the direction
of the field magnetic moments add
together positive way
local increase in the magnetic field
Example: oxygen
Magnetism and electromagnetism Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
17/48
17D. Asemani Magnetic Resonance Imaging
Diamagnetism show no net magnetic moment as the electron currentscaused by their motions add to zero
external magnetic field show a small magnetic moment that
opposes the applied field
slightly repelled by the magnetic field
negative magnetic susceptibilities
example:
water and inert gases
Magnetism and electromagnetism Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
18/48
18D. Asemani Magnetic Resonance Imaging
Ferromagnetism
contact with a magnetic field strong attraction and alignment
•retains its magnetization
permanent magnetsExample: iron
Magnets :bipolar
two poles, north and south
magnetic lines of flux
Like poles repel and opposite poles attract
Magnetism and electromagnetism Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
19/48
19D. Asemani Magnetic Resonance Imaging
: strength of the magnetic field
units
gauss (G)
kilogauss (kG)
tesla (T)
number of lines per unit area is called the magnetic flux density
B
Magnetism and electromagnetism Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
20/48
20D. Asemani Magnetic Resonance Imaging
Electromagnetism
Magnetic fields are generated by moving charges (electrical current)
direction clockwise or counter-clockwise with respect to the direction of flow of the current
Ampere’s law or Fleming’s Right hand rule
magnitude and direction of the magnetic field due to a current
changing magnetic fields generate electric currents
induced electric current
a closed circuit
Faraday’s law of induction electromotive force (emf ) in the circuit
Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
21/48
21D. Asemani Magnetic Resonance Imaging
laws of electromagnetic induction : induced emf
proportional to the rate of change of magnetic field and the area of the circuit
in a direction so that it opposes the change in magnetic field which causes it (Lenz’s law).
motion of electrically charged particles results in a magnetic
force orthogonal to the direction of motion
protons (nuclear constituent of atom) have a
property of angular momentum known as spin
Electromagnetism Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
22/48
-
8/9/2019 MRI Principles 01
23/48
23D. Asemani Magnetic Resonance Imaging
a stable atom:number of negatively charged electrons equals the number of positively charged protons
Atoms with a deficit or excess number of electrons are called ions.
Motion within the atom
Negatively charged electrons spin on their own axisNegatively charged electrons orbit the nucleus
nucleus spins on its own axis
Each type of motion produces a magnetic field
In MR we are concerned with the motion of the nucleus
Atomic structureMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
24/48
24D. Asemani Magnetic Resonance Imaging
MR active nuclei
Protons and neutrons spin about their own axes within the nucleus
direction of spin is random
even mass numberspins cancel each other out so the nucleus
has no net spin.
odd mass number spins do not cancel each other out and the
nucleus spins
a moving unbalanced charge induces amagnetic field around itself
Atomic structureMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
25/48
25D. Asemani Magnetic Resonance Imaging
Nuclei with an odd number of protons : MR active tiny bar magnets
all have odd mass numbers
Hydrogen 1, Carbon 13, Fluorine 19, Phosphorous 31, Nitrogen 15, Oxygen 17, Sodium 23
hydrogen nucleus is the MR active nucleus used in MRI
a single proton (atomic number 1).
• it is abundant in the human body (e.g. in fat and water);
• its solitary proton gives it a large magnetic moment.
MR active nuclei
Atomic structureMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
26/48
26D. Asemani Magnetic Resonance Imaging
Alignment and precession
Alignment
In a normal environment the magnetic
moments of MR active nuclei point in a
random direction
no overall magnetic effect
placed in an external magnetic field
magnetic moments line up with
the magnetic field flux linesalignment
using two theories: the classical theory and the quantum theory
Magnetic resonance imaging (MRI)
In “field free” space
randomly oriented
-
8/9/2019 MRI Principles 01
27/48
27D. Asemani Magnetic Resonance Imaging
classical theory
uses the direction of the magnetic moments to illustrate alignment
Parallel alignment alignment of magnetic moments in the samedirection as the main field
Anti-parallel alignment alignment of magnetic moments in the oppositedirection to the main field
At room temperature there are always more nuclei with their magnetic moments aligned
parallel to the main field than aligned antiparallel.
net magnetization vector or NMV
balance between the parallel andantiparallel magnetic moments
Magnetic resonance imaging (MRI)Alignment
Alignment
-
8/9/2019 MRI Principles 01
28/48
28D. Asemani Magnetic Resonance Imaging
quantum theory
uses the energy level of the nuclei to illustrate alignment
certain factors that determine whether the magnetic moment of a nucleus
aligns in the parallel direction or the antiparallel direction
magnitude or strength of the external magnetic field,
termed B0 in tesla (T);
energy level of the nucleus
magnetic moments of hydrogen nuclei align in the presence of an external magnetic
field in the following two energy states or populations:
Spin up Spin down
nuclei have low energy
do not have enough energy to oppose
the main field
parallel
nuclei have high energy and have enough
energy to oppose the main field
antiparallel
Magnetic resonance imaging (MRI)Alignment
-
8/9/2019 MRI Principles 01
29/48
29D. Asemani Magnetic Resonance Imaging
Inside magnetic field
oriented with or against B0M = net magnetization
M
Applied Magnetic
Field (B0)
In “field free” space
randomly oriented
there is a small difference (10:1 million) in the number of protons in the low and
high energy states – with more in the low state leading to a net magnetization
(M)
quantum theory
Magnetic resonance imaging (MRI)Alignment
-
8/9/2019 MRI Principles 01
30/48
30D. Asemani Magnetic Resonance Imaging
magnetic moments of the nuclei actually align at an angle to B0 due to the force of repulsion
between B0 and the magnetic moments
Hydrogen can only have two energy states
– high or low
magnetic moments of hydrogen can only align in the parallel or
antiparallel directions
magnetic moments of hydrogen cannot orientate themselves in any
other direction
temperature of the sample being imaged is an important factor that
determines whether a nucleus is in the high or low energy population
In clinical imaging we discount thermal effects
quantum theory
Magnetic resonance imaging (MRI)Alignment
-
8/9/2019 MRI Principles 01
31/48
31D. Asemani Magnetic Resonance Imaging
At any one moment in time there are a greater proportion of nuclei with
their magnetic moments aligned with the field than against it
excess aligned with B0 produces a net magnetic effect called the NMVwhich aligns with the main magnetic field
As the magnitude of the external magnetic field increases, more of the magnetic moments of
the nuclei line up in the parallel direction because the amount of energy they must possess to
oppose the field and line up antiparallel to the stronger magnetic field is increased
NMV gets larger
static nuclear moment is far too weak to be measured when it is aligned
with the strong static magnetic field
Physicists in the 1940s developed resonance techniques that permit thisweak moment to be measured
key idea : to measure the moment while it oscillates in a plane
perpendicular to the static field
Magnetic resonance imaging (MRI)Alignment
-
8/9/2019 MRI Principles 01
32/48
32D. Asemani Magnetic Resonance Imaging
First one must tip the moment away from the static field.
When perpendicular to the static field, the moment feels a torque
proportional to the strength of the static magnetic field
The torque always points perpendicular to the magnetization and causes the
spins to oscillate or precess in a plane perpendicular to the static field.
Magnetic resonance imaging (MRI)Alignment
-
8/9/2019 MRI Principles 01
33/48
-
8/9/2019 MRI Principles 01
34/48
34D. Asemani Magnetic Resonance Imaging
gyromagnetic ratio : precessional frequency of a specific
nucleus at 1 T and therefore has units of MHz/T
precessional frequency is proportional to the strength of the external
magnetic field
precessional frequencies of hydrogen (gyromagnetic
ratio 42.57 MHz/T) commonly found in clinical MRI are:
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
PrecessionMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
35/48
35D. Asemani Magnetic Resonance Imaging
Out of phase or incoherent : magnetic moments
of hydrogen are at different places on the
precessional path
In phase or coherent : magnetic moments of hydrogen are
at the same place on the precessional path
What is actually aligned with the B0 is the axis around which the proton precesses
the decay of precession (i.e., it is the rate of precession out of alignment with B0together with the proton density of the tissue concerned that is crucial in MRI)
nuclei not aligned but still precessing
in the same direction.all nuclei aligned and precessing
in the same direction.
Precession Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
36/48
36D. Asemani Magnetic Resonance Imaging
Resonance and signal generation
Resonance an energy transition that occurs when an object is subjectedto 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
hydrogen nuclei to resonate (receive energy from the RF pulseMR active nuclei do not resonate because their gyromagnetic ratios are
different from that of hydrogen
Larmor equation: their precessional frequency is different and therefore
they only resonate if RF at their specific precessional frequency is applied
Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
37/48
37D. Asemani Magnetic Resonance Imaging
Two things happen at resonance:
energy absorption and phase coherence.
Energy absorption
The hydrogen nuclei absorb energy from the RF pulse (excitation pulse)
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
Resonance and signal generationMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
38/48
38D. Asemani Magnetic Resonance Imaging
Phase coherence
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
hydrogen nuclei do not move
nuclei are not flipped onto their sides in the transverse plane and neither
are their magnetic moment
Only the magnetic moments of the nuclei move, either aligning with or
against B0. This is because hydrogen can have only two energy states,
high or low. It is the NMV that lies in the transverse plane not the
magnetic moments, nor the nuclei themselves
Resonance and signal generationMagnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
39/48
39D. Asemani Magnetic Resonance Imaging
RF Excitation
• protons can flip between low and high energy states (i.e., flip
between being aligned with or against B0)
• to do so the energy transfer must be of a precise amount
and must be facilitated by another force (e.g., other protonsor molecules)
• in MRI, RF (radio frequency) pulses are used to excite the
RF field – the Swing analogy – tipping the net magnetization
out of alignment with B0
Resonance and signal generationMagnetic resonance imaging (MRI)
M ti i i (MRI)
-
8/9/2019 MRI Principles 01
40/48
40D. Asemani Magnetic Resonance Imaging
It can be shown :
a rotating RF field introduces a fictitious field in the Z direction of strength ω/γ
By tuning the frequency of the RF field to ω0 , we effectively delete the B0 field.
RF slowly nutates the magnetization away from the z axis. The Larmor relation still
holds in this “rotating frame,” so the frequency of the nutation is γB1,where B 1 is the
amplitude of the RF field
Since the coils receive x and y (transverse) components of induction, thesignal is maximized by tipping the spins completely into the transverse plane
This is accomplished by a π/2 RF pulse, which requires γB1τ=π/2, where τ
is the duration of the RF pulse.
Another useful RF pulse rotates spins by π radians. This can be
used to invert spins. It also can be used to refocus transverse
spins that have dephased due to B0 field inhomogeneity.
spin echo
widely used in imaging
Resonance and signal generationMagnetic resonance imaging (MRI)
M ti i i (MRI)
-
8/9/2019 MRI Principles 01
41/48
41D. Asemani Magnetic Resonance Imaging
MR signal
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.
After a short period of time the
RF pulse is removed
The signal induced in the receiver coil begins to decrease
amplitude of the voltage induced in the receiver coil
therefore decreases. This is called free induction decay
(FID):
„free‟ : absence of the RF pulse
“induction decay‟ : the decay of the induced signal in the receiver coil
Magnetic resonance imaging (MRI)
MR i l M ti i i (MRI)
-
8/9/2019 MRI Principles 01
42/48
42D. Asemani Magnetic Resonance Imaging
signals produced during relaxation (move from higher energy to lower
energy) is dependent on:
density of hydrogen the velocity of flowing fluid through the tissue
the rate at which the excited nucleus are relaxed
relaxation parameters are marked T1 and T2
T1 and T2 : depend on the physical properties of the tissues when exposed to a series of pulses at predetermined time intervals
Different tissues have different T 1 and T 2 properties based on the response of
their hydrogen nuclei to radio frequency pulses in the strong magnetic field
These differential properties are made use of by setting equipment parameters (TR
and TE ) in order to generate images either based on T1 or T2 properties of the tissues.
MR signal Magnetic resonance imaging (MRI)
MR i l M ti i i (MRI)
-
8/9/2019 MRI Principles 01
43/48
43
D. Asemani Magnetic Resonance Imaging
images of the tissues are known as either T1 or T2 weighted.
TR : the time to repeat RF pulses while
TE : time to receive echo i.e., time interval between application of pulse and listeningof the signal.
signal intensity pertains to the brightness of signal generated by specific tissue:
The tissues that are bright (white) : hyperintense
darker signal tissues : hypointense.
The tissues which are in between bright and dark : isointense.
MR signal Magnetic resonance imaging (MRI)
MR i l Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
44/48
44
D. Asemani Magnetic Resonance Imaging
fat : bright on T 1 weighted images and less bright on T 2
weighted images
water : dark on T 1 weighted images and bright on T 2 weighted
images
gas : dark on T 2 weighted images
MR signal Magnetic resonance imaging (MRI)
C t t h i Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
45/48
45
D. Asemani Magnetic Resonance Imaging
Contrast mechanisms
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).
intermediate signal (shades of grey in-between white and black).
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
A tissue gives a low signal (black), if it has a small transverse
component of magnetization
A tissue gives an intermediate signal (grey), if it has a
medium transverse component of magnetization
Magnetic resonance imaging (MRI)
Contrast mechanisms Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
46/48
46
D. Asemani Magnetic Resonance Imaging
Image contrast is controlled by extrinsic contrast parameters (those that are
controlled by the system operator). These include :
• Repetition time (TR).
time from the application of one RF pulse
to the application of the nextmilliseconds (ms).
affects the length of a relaxation period
after the application of one RF excitationpulse to the beginning of the next
Echo time (TE). This is the time between an RF excitation
pulse and the collection of the signal
affects the length of the relaxation period after the
removal of an RF excitation pulse and the peak of
the signal received in the receiver coilmilliseconds (ms).
Contrast mechanisms Magnetic resonance imaging (MRI)
Contrast mechanisms Magnetic resonance imaging (MRI)
-
8/9/2019 MRI Principles 01
47/48
47
D. Asemani Magnetic Resonance Imaging
• Flip angle. This is the angle through which the NMV is moved as a result of aRF excitation pulse.
• Turbo-factor or echo train length (ETL/TF)
• Time from inversion (TI)
• ‘b’ value
intrinsic contrast mechanisms :
do not come under the operators control
• T1 recovery
• T2 decay
• proton density• flow
• apparent diffusion coefficient (ADC).
Contrast mechanisms Magnetic resonance imaging (MRI)
extrinsic contrast parameters :
-
8/9/2019 MRI Principles 01
48/48
48
MRI At A Glance, Catherine Westbrook, 2002 By Blackwell Science Ltd,
Introduction To Biomedical Engineering, John D. Enderle, 2005, Elsevier
Fundamentals of Biomedical Engineering, G.S. Sahney, New Age
International P.
The Biomedical Engineering HandBook, Second Edition. Ed. , Joseph D.
Bronzino, Boca Raton: CRC Press LLC, 2000
Selected References: