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66 Cards in this Set
- Front
- Back
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What happens to the frequency of the RF pulse in a high magnetic field
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it will have to be increased
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What happens to the size of the wavelength if the frequency is increased dramatically
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it will dramatically increase in size
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How big can a wavelength get when using a high strength magnet like (8T)
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the size of a human head
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What is the B1 ( 1 is a subsscript )field
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the RF pulse is called an B1 field ans
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What happens to the magnetic field causes (B1) caused by the RF pulse itself when the wavelength is very large
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it will create an inhomogenous field (further subdivided into + B1 and - B1) distributed in the tissue
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What is the problem with having an inhomogenous field (further subdivided into + B1 and - B1) distributed in the tissue
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have a devastating effect on the integrity of the
images and on the safety of the patient. |
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How is localization of the MR signal accomplished
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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) |
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What is the frequency encoding direction
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the X- component
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What is the phase encoding direction
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Y component
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What is the typical gradient range
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20-80 mT/m
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What is the definition of the slew rate
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where the slew rate is defined as the maximum gradient divided by the rise time
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What is the rise time
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The
rise time is how long it takes for the gradient to go from zero to the maximum value. |
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What is the slew rates in a gradient
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with increased slew rates from 30 to 220 mT/m/msec,
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What is the rise time
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how long it takes for the gradient to go from zero to the maximum value
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Do both the gradient value and slew rates values increase or decrease along the gradient
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yes (the gradient strength will and since the slew time is the gradient strength dived by rise time it will increase also)
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What is the function of a slice selection gradient
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determines the amount of tissue (slice) to be excited by a single RF pulse
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What is contained with in a 90 degree RF pulse
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A 90o pulse contains a band of frequencies. (bandwith)
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What does Fourier Transformation allow to occur
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permits signal to be decomposed into a sum of sine waves each of different frequency, phases and amplitudes
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How is the MR pusle sequence encoded
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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
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What is the fourier Transform
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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. |
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What determines the slice thickness
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The parameters that
determine the slice thickness are the bandwidth of the RF pulse ( f ) and the gradient strength across the FOV (Gz), |
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What determines the amount of tissue to be excited by an RF pulse with a fixed bandwith that is applied
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the slice selection gradient
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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
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B1
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What does the slice selection gradient do
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creates a one to one correspondance between the bandwith of the RF pulse and a narrow slice of tissue that is to be excited.
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What happens to the slice of the slice thickness in larger gradients
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they become thinner
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What is the Gz gradient
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slice selection gradient
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What is the Gx gradient
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frequency encoding or read out gradient
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When is the frequency encoding gradient applied
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is applied perpendicular to the slice-
selection gradient before and during the echo for- mation |
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What happens when the protons during the frequency encoding gradient
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The protons are spatially “frequency
encoded” by their characteristic resonant fre- quency along the x axis |
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What is the slice selection gradient direction
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look at the formula for the frequency
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What is the z component of the gradient
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the z component
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What is the X component of the gradient
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frequency encoding
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What is the Y component of the gradient
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phase encoding
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What is the linear variation of the magnetic field in a magnet (accounting for gradients)
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Bi= Bo + G x ri
Bo= external magnetic field G= gradient in chosen direction ri location |
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What is an example of the linear dependence in the x direction
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What happens to the frequencies because of the changes in magnetic field along the magnet
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it will change
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What is the formula for the changes of frequency along the x direction (because of the magnetic gradient)
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Is the frequency encoding gradient applied perpendicular to the slice selection gradient
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yes
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What is another name for the frequency encoding gradient
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the readout gradient
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How does the readout gradient work
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In regards to slice selection gradient how is it applied
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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.)
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How does the resonant frequency change moving from the toes to head
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In this way the resonant frequency is smaller than w0 towards the patient's feet, unchanged at the isocentre, and greater towards the head.
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How is the axial slice acquired in slice selection
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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
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How is the slice selection size changed
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The slice thickness or position can be varied by using different gradient strengths or RF bandwidths.
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What is the basic principle of frequency encoding
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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
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How are columns of pixels determined in the x directions
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Columns of pixels from left-to-right are therefore discriminated in terms of frequency differences
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http://www.hull.ac.uk/mri/lectures/gpl_page.html
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http://www.hull.ac.uk/mri/lectures/gpl_page.html
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What does the gradient slice selection look like
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What does the gradient along the slice selection look like
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What does the gradient slice selection look like
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Once the signal from the slice has been isolated what must be accomplished
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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).
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How is the frequency encoded gradient created
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turning on gradient in the X direction
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What is the phase encoding direction
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this is the Y direction
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Why is the phase encoding gradient needed
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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
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What is changed in the phase encoding direction
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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.
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What doe the frequency look like in an external magnetic field before the phase encoding gradient is applied
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What do the protons look like before the phase encoding gradient is appleied
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What do the protons look like when the frequency encoding gradient is applied
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When is the phase encoding gradient applied
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after slice selection gradient and before frequency encoding gradient
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What direction is the phase encoding
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the Y which is side to side in patient
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When do most arifacts occur
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in the phase direction due to the longer acquistion time
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How is a phase gradient created
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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
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What happens onces the phase encoding gradient is removed
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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
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What is accomplished by applying a phase encoding and frequency encoding gradient
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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 |
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How is a MR voxel triangulated using Gss (slice selection) Gro (frequency encoding) and Gpe (phase encoding)
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What are the respective TE/TR times for T1 and T2 images
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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). |
