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23 Cards in this Set
- Front
- Back
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patient coordinates: Z gradient |
head to feet |
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What gradient is applied for a sagital slice? |
X-gradient |
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patient coordinates: Y gradient |
anterior to posterior |
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patient coordinates: X |
right to left |
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What gradient is applied for a transverse slice? |
Z-gradient (head to foot) |
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What gradient is applied for a coronal slice? |
Y-gradient |
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Slice selection |
turning on a gradient at the same time as the RF pulse, we can selectively excite protons in a given slice by applying an RF pulse precisely at the resonant frequency of the protons located at the position of the slice we wish to image |
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What does the frequency encoding gradient do? |
linerally varies the magnetic field and therefore the precessional frequency of protons as each echo is being read
* also know as "the read out gradient" *
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A positive change in frequency followed by an equal negative change in frequency will do what? |
have no effect. the changes cancel each other out. |
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If frequency direction is vertical..... |
protons at the top precess at a different frequency than those on bottom |
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if frequency direction is horizontal... |
protons on left precess at a diffrent frequency than on right of image |
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A wide range of frequencies will select what? |
A thick slice |
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Chemical shift artifact always appears in what direction? |
frequency encoding |
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Every time a gradient is turned on there is a build up of what? |
phase |
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If the frequency of two systems is different what happens to the phase difference? |
The difference will increase linerally over time |
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A narrow range of frequencies will select what? |
a thin slice |
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What are the logical gradients? |
Phase encoding = Y Frequency ecoding = x Slice select = Z
* remember alphabetically (x,y,z) = (f,p,s) |
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Equation to find out the effects of the gradient on preccessional frequency |
f=(y/2tt) x B0 + (DxG / 1000)
f = precessional frequency (in MHz) (y/2tt) = gyromagnetic ratio (in MHz) B0= main field strength D=distance from center G= gradient strength
+ (plus) or - (minus) to match direction |
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Equation to find the effect of a gradient on a magnetic field formula |
B= B0 + (DxG / 1000)
B= final magnetic field strength B0= main magnetic field strength D= distance from isocenter G= gradient strength
+/- to match diection |
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which of the following is not balanced for phase?
Slice select Frequency Read out Phase encoding |
phase encoding
you want to have a difference. Deliberate amount of phase |
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What is contained within a single spin-echo?
anatomical info from a specific line in the image?
Many frequencies?
anatomical info from all images of scan?
Many phases? |
many frequencies
A single spin-echo is recorded during the application of the readout, or frequency encoding gradient. As a result the MR echo contains many frequency components which serves to spatially encode the signal along the frequency encoding direction |
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The phase encoding gradient must be turned on during what event? |
prior to echo collection but exact timing is not crucial.
A unique amount of phase encoding must be applied during each TR period but there is no particular event during which the phase gradient must be turned on. |
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The frequency encoding gradient must be turned on during what event? |
readout of the echo
to creat a spectrum of presessional frequencies within protons which varies linearly with distance along that gradent while the echo is being recorded |
