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66 Cards in this Set

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  • Back
What happens to the frequency of the RF pulse in a high magnetic field
it will have to be increased
What happens to the size of the wavelength if the frequency is increased dramatically
it will dramatically increase in size
How big can a wavelength get when using a high strength magnet like (8T)
the size of a human head
What is the B1 ( 1 is a subsscript )field
the RF pulse is called an B1 field ans
What happens to the magnetic field causes (B1) caused by the RF pulse itself when the wavelength is very large
it will create an inhomogenous field (further subdivided into + B1 and - B1) distributed in the tissue
What is the problem with having an inhomogenous field (further subdivided into + B1 and - B1) distributed in the tissue
have a devastating effect on the integrity of the
images and on the safety of the patient.
How is localization of the MR signal accomplished
Localization of theMR signal is
obtained by applying a gradient that produces a
controlled linear spatial variation of the B0 mag-
netic field (z direction), which creates small per-
turbations to the field in three directions (x, y,
and z)
What is the frequency encoding direction
the X- component
What is the phase encoding direction
Y component
What is the typical gradient range
20-80 mT/m
What is the definition of the slew rate
where the slew rate is defined as the maximum gradient divided by the rise time
What is the rise time
The
rise time is how long it takes for the gradient to go
from zero to the maximum value.
What is the slew rates in a gradient
with increased slew rates from 30 to 220 mT/m/msec,
What is the rise time
how long it takes for the gradient to go from zero to the maximum value
Do both the gradient value and slew rates values increase or decrease along the gradient
yes (the gradient strength will and since the slew time is the gradient strength dived by rise time it will increase also)
What is the function of a slice selection gradient
determines the amount of tissue (slice) to be excited by a single RF pulse
What is contained with in a 90 degree RF pulse
A 90o pulse contains a band of frequencies. (bandwith)
What does Fourier Transformation allow to occur
permits signal to be decomposed into a sum of sine waves each of different frequency, phases and amplitudes
How is the MR pusle sequence encoded
In MRI the signal is spatially encoded by changes of phase/frequency which is then unravelled by performing a 2D FT to identify pixel intensities across the image
What is the fourier Transform
The signals that we measure in MRI are a combination of signals from all over the object being imaged. It so happens that any signal (even if you simply make one up and draw a squiggle) is composed of a series of sine waves, each with an individual frequency and amplitude. The Fourier transform allows us to work out what those frequencies and amplitudes are. (That is to say, it converts the signal from the time domain into the frequency domain.) Since we encode the signal with magnetic field gradients which make frequency and (rate of change of) phase relate to position, if we can separate out the frequencies we can say where we should plot the amplitudes on the image.

For any image, use of the Fourier transform allows us to manipulate the data in the frequency domain (k-space), which can be easier, and makes things easier to understand, like in the example of a high pass filter.
What determines the slice thickness
The parameters that
determine the slice thickness are the bandwidth of the RF pulse ( f ) and the gradient strength
across the FOV (Gz),
What determines the amount of tissue to be excited by an RF pulse with a fixed bandwith that is applied
the slice selection gradient
What is the name of the RF pulse called when trying to cause the protons in a slice selection to spin at the same resonance frequency
B1
What does the slice selection gradient do
creates a one to one correspondance between the bandwith of the RF pulse and a narrow slice of tissue that is to be excited.
What happens to the slice of the slice thickness in larger gradients
they become thinner
What is the Gz gradient
slice selection gradient
What is the Gx gradient
frequency encoding or read out gradient
When is the frequency encoding gradient applied
is applied perpendicular to the slice-
selection gradient before and during the echo for-
mation
What happens when the protons during the frequency encoding gradient
The protons are spatially “frequency
encoded” by their characteristic resonant fre-
quency along the x axis
What is the slice selection gradient direction
look at the formula for the frequency
What is the z component of the gradient
the z component
What is the X component of the gradient
frequency encoding
What is the Y component of the gradient
phase encoding
What is the linear variation of the magnetic field in a magnet (accounting for gradients)
Bi= Bo + G x ri

Bo= external magnetic field
G= gradient in chosen direction
ri location
What is an example of the linear dependence in the x direction
What happens to the frequencies because of the changes in magnetic field along the magnet
it will change
What is the formula for the changes of frequency along the x direction (because of the magnetic gradient)
Is the frequency encoding gradient applied perpendicular to the slice selection gradient
yes
What is another name for the frequency encoding gradient
the readout gradient
How does the readout gradient work
In regards to slice selection gradient how is it applied
In this case we perform slice selection along the z-direction: a gradient in this direction is turned on such that it acts symmetrically about the centre of the scanner (the isocentre.)
How does the resonant frequency change moving from the toes to head
In this way the resonant frequency is smaller than w0 towards the patient's feet, unchanged at the isocentre, and greater towards the head.
How is the axial slice acquired in slice selection
simultaneously using a shaped RF pulse containing a finite bandwidth only a section of spins either side of the isocentre is excited into the transverse plane
How is the slice selection size changed
The slice thickness or position can be varied by using different gradient strengths or RF bandwidths.
What is the basic principle of frequency encoding
Once again the centre of the slice remains unaltered but to the left of this point the field and therefore resonant frequency is smaller, to the right it is larger
How are columns of pixels determined in the x directions
Columns of pixels from left-to-right are therefore discriminated in terms of frequency differences
http://www.hull.ac.uk/mri/lectures/gpl_page.html
http://www.hull.ac.uk/mri/lectures/gpl_page.html
What does the gradient slice selection look like
What does the gradient along the slice selection look like
What does the gradient slice selection look like
Once the signal from the slice has been isolated what must be accomplished
Once the signal from the slice has been isolated the remaining two in-plane dimesions need to be encoded (in this case the 'x' and 'y' directions).
How is the frequency encoded gradient created
turning on gradient in the X direction
What is the phase encoding direction
this is the Y direction
Why is the phase encoding gradient needed
It can be shown that a gradient applied in the y-direction to change frequency in this dimension would not be sufficient to uniquely ascribe frequency to each column and row of pixels
What is changed in the phase encoding direction
a number of gradients are needed to create phase changes from row-to-row so that the FT is provided with enough information to fully encode the final image.
What doe the frequency look like in an external magnetic field before the phase encoding gradient is applied
What do the protons look like before the phase encoding gradient is appleied
What do the protons look like when the frequency encoding gradient is applied
When is the phase encoding gradient applied
after slice selection gradient and before frequency encoding gradient
What direction is the phase encoding
the Y which is side to side in patient
When do most arifacts occur
in the phase direction due to the longer acquistion time
How is a phase gradient created
by temporarily applying a gradient in the Y direction and then removing it. Unlike the frequency encoding where the gradient is kept and the frequency is changed through out the image acquistion
What happens onces the phase encoding gradient is removed
Once the gradient is removed the resonant frequency is the same as it was before for all the spins (i.e. w0). However, the spins will now be 'out of phase' with each other
What is accomplished by applying a phase encoding and frequency encoding gradient
By combining the frequency- and phase-en-
coding gradients, each pixel will have a distinct
requency and phase associated with it. This allows creation of an image of the object by using
mathematical methods. By combining all these
steps into a pulse sequence, we can generate an
MR image
How is a MR voxel triangulated using Gss (slice selection) Gro (frequency encoding) and Gpe (phase encoding)
What are the respective TE/TR times for T1 and T2 images
T1-weighted im-ages have a short TR and short TE (eg, 700/20
msec), while T2-weighted images have a long TR
and long TE (eg, 2000/80 msec).