Chapter 2: The physics behind MRI
Study online at https://quizlet.com/_fai15f
The phenomenon that nuclei with a magnetic moment absorb
Nuclear magnetic resonance (NMR) energy if the oscillation frequency of a magnetic field matches the
Larmor frequency of the nuclei.
The frequency of the spin of a nucleus. It differs between different
elements and depends linearly on magnetic field strength.
=> for hydrogen, it is 63.76 MHz at 1.5T, 127.7 MHz at 3.0T, and
298.0 MHz at 7.0T. These frequencies are in the range of radio
Larmor frequency waves.
When the magnetic field is changing periodically, thus oscillating,
it will make the nuclei absorb energy from the field if the oscillation
Resonance frequency
frequency matches the Larmor frequency of the nuclei. This is also
referred to as the resonance frequency.
A magnetic field that is applied for a very short time (pulse) and
Radio frequency (RF) pulse that oscillates in a frequency that is in the same part of the
frequency spectrum as radio waves.
The angle in which spins are flipped by the radio frequency (RF)
Flip angle
pulse
First, the spins get in phase, meaning that they are in the same
position on the cycle when they spin around. Second, the spin
changes direction and will be flipped in the direction of the oscil-
lating field. This flip goes together with an increase in energy state
(the nucleus absorbs energy). The angle of this flip depends on
the difference in angle between the static magnetic field and the
oscillating field.
When the oscillating radio frequency field is no longer applied,
then these two effects gradually disappear. Each of these effects
has a certain time constant, which determines the time it takes
before the effect is diminished by a certain amount. The smaller
the time constant, the faster the effect changes over time.
First, the nuclei get out of phase again: dephasing.
Second, the nuclei realign with the static magnetic field by flipping
back in the direction of this static field. Each time a nucleus
realigns, it emits energy that by itself is a very small signal in the
Effects of oscillating on the spin of the affected atoms
radio frequency range. These small signals integrate over all the
nuclei that realign, and the resulting signal is stronger the less
dephasing has happened. If too much dephasing has happened,
then the signals of different nuclei will largely cancel out and no
net signal will remain.
The absorption of energy by biological tissue is coupled with a
slight increase in temperature. The larger the tissue, the more
energy it can absorb without a significant increase in temperature.
SAR
This is taken into account in a safety index known as the specific
absorption rate (SAR). The parameters and safety settings used
with scanners for human imaging take this index into account.
1/7
, Chapter 2: The physics behind MRI
Study online at https://quizlet.com/_fai15f
To calculate these safety margins => include the weight of a
participant
Larmor frequency depends on field strength. If field strength differs
across space according to a particular gradient, then nuclei will
have a Larmor frequency that depends on their position in this
static field gradient. For example, if the gradient increases from
left to right, then nuclei on the left will have a slower Larmor
frequency compared with nuclei on the right. The application of
a radio frequency (RF) wave will then only affect those nuclei that
How is the dependence of Larmor frequency used to obtain 2D/3D are in a spatial position where the static field gradient gives them
images? a Larmor frequency that matches the frequency of the RF wave.
To use this phenomenon to obtain two-dimensional (2D) and even
three-dimensional (3D) images, scientists apply gradients of field
strength on top of the overall stationary field. For the most part,
these gradients are linear (more accurately: one tries to make
them as linear as possible). The gradients are often applied for
only short periods of time, but when applied they are stationary
(they do not oscillate).
A slice-selection gradient is applied during the RF pulse and
allows the selection of one slice by matching its net magnetic
field strength to the strength needed to be excited by the radio
frequency (RF) pulse. Often slices are excited in an interleaved
manner (interleaved slice acquisition) to minimize cumulative ef-
fects due to cross-slice excitation (an RF pulse will partially excite
neighboring slices as well).
Slice-selection gradient
This gradient is applied after the RF pulse. The phase-encoding
gradient affects the resonance frequency, resulting in a phase shift
that is different for nuclei at different positions along the gradient.
The PE gradient will change the spin resonance frequencies of
the excited nuclei, causing differences in phase depending on
where nuclei are along the PE gradient. When the PE gradient is
removed, the resonance frequencies will be the same again but
the differences in phase will persist.
Phase-encoding (PE) gradient => All nuclei at a certain position in the PE gradient (a row
perpendicular to the gradient) will have the same phase, thus the
phase is informative about where the nuclei are.
The frequency-encoding gradient affects the resonance frequency
during readout, resulting in a resonance frequency that depends
on the position of nuclei along the gradient. So, gradient is turned
on during data acquisition and is for that reason also referred to
Frequency-encoding gradient as the "readout gradient."
=> All nuclei at a certain position in this gradient will have a
particular resonance frequency that deviates from nuclei at other
positions. For this reason, the resonance frequency at the time of
acquisition is informative about where these nuclei are.
2/7
Study online at https://quizlet.com/_fai15f
The phenomenon that nuclei with a magnetic moment absorb
Nuclear magnetic resonance (NMR) energy if the oscillation frequency of a magnetic field matches the
Larmor frequency of the nuclei.
The frequency of the spin of a nucleus. It differs between different
elements and depends linearly on magnetic field strength.
=> for hydrogen, it is 63.76 MHz at 1.5T, 127.7 MHz at 3.0T, and
298.0 MHz at 7.0T. These frequencies are in the range of radio
Larmor frequency waves.
When the magnetic field is changing periodically, thus oscillating,
it will make the nuclei absorb energy from the field if the oscillation
Resonance frequency
frequency matches the Larmor frequency of the nuclei. This is also
referred to as the resonance frequency.
A magnetic field that is applied for a very short time (pulse) and
Radio frequency (RF) pulse that oscillates in a frequency that is in the same part of the
frequency spectrum as radio waves.
The angle in which spins are flipped by the radio frequency (RF)
Flip angle
pulse
First, the spins get in phase, meaning that they are in the same
position on the cycle when they spin around. Second, the spin
changes direction and will be flipped in the direction of the oscil-
lating field. This flip goes together with an increase in energy state
(the nucleus absorbs energy). The angle of this flip depends on
the difference in angle between the static magnetic field and the
oscillating field.
When the oscillating radio frequency field is no longer applied,
then these two effects gradually disappear. Each of these effects
has a certain time constant, which determines the time it takes
before the effect is diminished by a certain amount. The smaller
the time constant, the faster the effect changes over time.
First, the nuclei get out of phase again: dephasing.
Second, the nuclei realign with the static magnetic field by flipping
back in the direction of this static field. Each time a nucleus
realigns, it emits energy that by itself is a very small signal in the
Effects of oscillating on the spin of the affected atoms
radio frequency range. These small signals integrate over all the
nuclei that realign, and the resulting signal is stronger the less
dephasing has happened. If too much dephasing has happened,
then the signals of different nuclei will largely cancel out and no
net signal will remain.
The absorption of energy by biological tissue is coupled with a
slight increase in temperature. The larger the tissue, the more
energy it can absorb without a significant increase in temperature.
SAR
This is taken into account in a safety index known as the specific
absorption rate (SAR). The parameters and safety settings used
with scanners for human imaging take this index into account.
1/7
, Chapter 2: The physics behind MRI
Study online at https://quizlet.com/_fai15f
To calculate these safety margins => include the weight of a
participant
Larmor frequency depends on field strength. If field strength differs
across space according to a particular gradient, then nuclei will
have a Larmor frequency that depends on their position in this
static field gradient. For example, if the gradient increases from
left to right, then nuclei on the left will have a slower Larmor
frequency compared with nuclei on the right. The application of
a radio frequency (RF) wave will then only affect those nuclei that
How is the dependence of Larmor frequency used to obtain 2D/3D are in a spatial position where the static field gradient gives them
images? a Larmor frequency that matches the frequency of the RF wave.
To use this phenomenon to obtain two-dimensional (2D) and even
three-dimensional (3D) images, scientists apply gradients of field
strength on top of the overall stationary field. For the most part,
these gradients are linear (more accurately: one tries to make
them as linear as possible). The gradients are often applied for
only short periods of time, but when applied they are stationary
(they do not oscillate).
A slice-selection gradient is applied during the RF pulse and
allows the selection of one slice by matching its net magnetic
field strength to the strength needed to be excited by the radio
frequency (RF) pulse. Often slices are excited in an interleaved
manner (interleaved slice acquisition) to minimize cumulative ef-
fects due to cross-slice excitation (an RF pulse will partially excite
neighboring slices as well).
Slice-selection gradient
This gradient is applied after the RF pulse. The phase-encoding
gradient affects the resonance frequency, resulting in a phase shift
that is different for nuclei at different positions along the gradient.
The PE gradient will change the spin resonance frequencies of
the excited nuclei, causing differences in phase depending on
where nuclei are along the PE gradient. When the PE gradient is
removed, the resonance frequencies will be the same again but
the differences in phase will persist.
Phase-encoding (PE) gradient => All nuclei at a certain position in the PE gradient (a row
perpendicular to the gradient) will have the same phase, thus the
phase is informative about where the nuclei are.
The frequency-encoding gradient affects the resonance frequency
during readout, resulting in a resonance frequency that depends
on the position of nuclei along the gradient. So, gradient is turned
on during data acquisition and is for that reason also referred to
Frequency-encoding gradient as the "readout gradient."
=> All nuclei at a certain position in this gradient will have a
particular resonance frequency that deviates from nuclei at other
positions. For this reason, the resonance frequency at the time of
acquisition is informative about where these nuclei are.
2/7
