Lecture 2 - MRI Flashcards
Strength of the static magnetic field is determined by…
the current flow through the coil
Larmor frequency of hydrogen is…
42 MHz/T
TR refers to
the time interval between successive excitation pulses
usually measured in seconds
Spatial encoding within a slice involves…
changing the magnetic field in the y direction and x direction using gradient coils
After RF pulse, magnetic signals from the H atoms are detected in the ___ plane
transverse (x,y) plane
T-1 weighted image is dependent on…
difference in the recovery of magnetic signal in the z direction
T-2 decay results from…
changes in the synchronization of the hydrogen ions as they rotate in the x,y plane
Slice selection involves the combined use of…
z gradient coil and an appropriate radio-frequency (RF) pulse
how to reduce resistance in the coils
superconductive magnets cooled by liquid helium
How do you increase the homogeneity of the magnet?
Increase length and decrease diametre of bore
Increase number of turns in coil
Has hydrogen a positive or negative charge?
positive
What is the speed of rotation (larmour frequency) dependent on?
the strength of the magnetic field
What frequency does the RF pulse have to be to knock the hydrogen atoms into the transverse plane?
Their larmor frequency (42MHz if a 1T magnet)
How many gradient coils are there?
3 - x,y,z
What is a bottom up, interleaved technique?
taking odd slices ascending up the brain, and even numbers on the way down
Which gradient do you manipulate to excite different slices in the axial plane?
z
What is phase encoding?
changing magnetic strength along the y plane
What is frequency encoding?
changing magnetic strength along the x plane
What kind of spatial information does the centre of k space give you?
course spatial information with a low spatial frequency
what kind of spatial information does the outer regions of k space give you?
finer detail spatial information
Where is the overall signal represented in k space?
in the centre
How do you decode k space into image space?
Apply inverse Fourier transformation
what is TE?
time from RF pulse being delivered to image being captured
Proton Density Imaging
Hydrogen Density
TR Long, TE short
Measures the amount of water in different parts of the brain
T1 contrast image
TR intermediate, TE short
T2 contrast
TR long, TE intermediate
Used in functional MRI
Echo-planar imaging
rapidly changing x and y gradients allows quick sampling of k space and gain image of whole brain in three seconds
What are shimming coils?
Reduce the imhomogeneity of the magnet
Account for size and shape of PSs head
Which direction is phase encoding applied to minimize the spread of artifacts into the brain?
In the left-right direction
Where in k space are low and high frequencies? What about orientation?
Low frequencies in the centre
High frequencies in the surround
Different orientations round the clock
K space and echo planar imaging?
Sample k-space in a linear zig-zag trajectory
Spiral imaging and K-space
Sample k-space in a spiral trajectory
What are field map corrections?
Correct for distortions and inhomogeneities in image
Aunt Bessies MRI recipe
Put protons in strong magnet - these protons precess at specific frequency mainly in the longitudinal plane
Transmit RF pulse at larmor frequency - protons absorb this energy and flip to transverse plane
Stop RF transmission - protons return to original state
Longitudinal magnitization increases (T1)
Transverse magnitization decays (T2) due to spin-spin interaction
Measure RF frequency at appropriate point in time
Convert RF represented in k-space to representation of physical space
Equal magnetic fields that point in different directions?
Cancel out
If the magnetic field was increased along the z axis…
the RF frequency to excite the top of
the brain would be different from the
bottom of the brain
If you wanted to select coronal slices, which direction would you vary the magnet in?
Y direction
If you wanted to select sagittal slices, which direction would you vary the magnet in?
X direction
If you wanted to select axial slices, which direction do you vary the magnet?
Z direction
Vertical information in the image
is represented along _____ axis in K-space
x
TE in the order of ms or s
MS
TR in the order of ms or s
S
Which is longer, T1 or T2?
T1 longer (seconds) T2 shorter (milliseconds)
Proton density imaging measures the density of what?
Hydrogen/water
Why do we use a short TE for proton density imaging?
Because we are interested in the amount of signal in the transverse plane of hydrogen atoms
T1 contrast is used for what type of image?
Structural image
shows differences in white matter, grey matter and ventricles
T2 weighted image is used for what type of scans?
Clinical scans and fMRI
T2 contrast: TE and TR?
TR long (not interested in differences of T1) TE intermediate (because contrast in T2 is larger at that point)
Why wouldn’t you use proton density?
For when there is the same amount of hydrogen in 2 different areas
T1 relaxation for ____ matter is quicker than ____ and ____ matter
white
ventricles
grey
If you are interested in T1, is TR or TE more important?
TR (intermediate)
If you are interested in T2, is TR or TE more important?
TE (intermediate)
