Term
| Why do we need gradient functions to help us create an image? |
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Definition
| Because ALL the hydrogen atoms are precessing at the same rate! |
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Term
| What does the gradient do, on a very basic level? |
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Definition
| Alters the precessional frequency of hydrogen nuclei. (slightly) |
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Term
| What three tasks do we use gradients to perform? |
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Definition
Slice selection
Phase encoding
Frequency encoding |
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Term
| what type of field do gradients create? |
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Definition
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Term
| In what direction does the Z gradient alter the Bo? |
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Definition
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Term
| In what direction does the Y gradient alter the Bo? |
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Definition
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Term
| In what direction does the X gradient alter the Bo? |
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Definition
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Term
| On average, a gradient alters the main magnetic field by _____ G/cm |
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Definition
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Term
| positive gradients _______ (inc/dec) the main magnetic field, negative gradients ________ it. |
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Definition
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Term
| What is the acronym to remember the order of the coils? |
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Definition
| Pete (patient) Rose (RF) Goes (gradient) to South (shim) Milwaukee (main) |
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Term
| In the gradient echo pulse sequence, gradients are used to ______ the nuclei |
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Definition
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Term
| the rise/fall time is the time it takes the gradients to reach the _____ and then return to _____ |
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Definition
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Term
| the rise/fall time of a gradient is a measure of _____ over time |
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Definition
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Term
| rise fall time is usually expressed in what units? |
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Definition
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Term
| another name for rise/fall time is _______ |
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Definition
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Term
| another name for peak gradient strength is ______ gradient stength |
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Definition
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Term
| most scanners have a peak gradient strength of ______ mT/m, but FSE and EPI scans require higher strengths (20-40 mT/m) |
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Definition
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Term
| For gradient applications, the rate which is a combination of rise/fall time and peak strength is called ____ ____ |
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Definition
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Term
| slew rate is measured in what units? |
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Definition
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Term
| typical slew rates range from ____ to ____ T/m/sec |
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Definition
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Term
| which slices are selected by the three gradients: |
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Definition
Gz slices head to foot - so axial
Gy slices A to P - so coronal
Gx slices left to right - so sagittal |
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Term
| Slice thickness is controlled by gradient _____ and transmit ______ |
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Definition
gradient amplitude (or slope)
transmit bandwidth |
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Term
| increasing gradient amplitude will ______ slice thickness |
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Definition
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Term
| decreasing gradient amplitude will _______ slice thickness |
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Definition
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Term
| increasing transmit bandwith will _____ slice thickness |
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Definition
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Term
| decreasing transmit bandwith will _____ slice thickness |
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Definition
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Term
| what is the transmit bandwith? |
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Definition
| the frequency of the RF signal |
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Term
| phase encoding uses the gradient that is parallel to the ______ axis of the anatomy being imaged |
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Definition
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Term
| Which gradients do phase encoding for sagittal images? |
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Definition
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Term
| Which gradients do phase encoding for coronal images? |
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Definition
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Term
| Which gradients do phase encoding for axial HEAD/BRAIN images? |
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Definition
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Term
| Which gradients do phase encoding for axial BODY images? |
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Definition
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Term
| Frequency encoding uses the gradient that is parallel to the _____ axis of the anatomy being imaged. |
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Definition
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Term
| The frequency encoding is done when _______ |
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Definition
| the echo (signal) is received |
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Term
| another name for frequency encoding gradient is ________ gradient |
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Definition
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Term
| the steepness of the gradient amplitude is determined by the ______ |
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Definition
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Term
| Small FOV means _____ (steep/shallow) F. E. gradient |
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Definition
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Term
| large FOV means ______ (steep shallow) F. E. gradient |
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Definition
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Term
| Frequency encoding gradient used for coronal images |
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Definition
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Term
| Frequency encoding gradient used for sagittal images |
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Definition
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Term
| Frequency encoding gradient used for axial images of the BODY |
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Definition
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Term
| Frequency encoding gradient used for axial images of the BRAIN |
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Definition
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Term
| the duration of frequency encoding during the "read out" time is called ____________ |
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Definition
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Term
| currently, up to _____ frequencies can be sampled. |
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Definition
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Term
| the range of frequencies sampled during the frequency encoding is called the _________ ________ |
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Definition
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Term
| this is the mathematical link between sampling time, sampling rate, and receive bandwidth |
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Definition
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Term
| The Nyquist theorem states that for accurate reproduction, the signal must be sampled at least ____ times |
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Definition
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Term
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Definition
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Term
| when is frequency encoding on? |
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Definition
| during the MR signal or echo |
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Term
| when is phase encoding on? |
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Definition
| just prior to the 180 pulse |
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