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MRI Physics Module 4
Encoding and image formation
50
Physics
Professional
09/30/2012

Term
 Why do we need gradient functions to help us create an image?
Definition
 Because ALL the hydrogen atoms are precessing at the same rate!
Term
 What does the gradient do, on a very basic level?
Definition
 Alters the precessional frequency of hydrogen nuclei. (slightly)
Term
Definition
 Slice selectionPhase encodingFrequency encoding
Term
 what type of field do gradients create?
Definition
 secondary or B1
Term
 In what direction does the Z gradient alter the Bo?
Definition
Term
 In what direction does the Y gradient alter the Bo?
Definition
 Anterior to posterior
Term
 In what direction does the X gradient alter the Bo?
Definition
 Left to right
Term
 On average, a gradient alters the main magnetic field by _____ G/cm
Definition
 1 G/cm
Term
 positive gradients _______ (inc/dec) the main magnetic field, negative gradients ________ it.
Definition
 increase, decrease
Term
 What is the acronym to remember the order of the coils?
Definition
 Pete (patient) Rose (RF) Goes (gradient) to South (shim) Milwaukee (main)
Term
 In the gradient echo pulse sequence, gradients are used to ______ the nuclei
Definition
 rephase
Term
 the rise/fall time is the time it takes the gradients to reach the _____ and then return to _____
Definition
 peak, zero
Term
 the rise/fall time of a gradient is a measure of _____ over time
Definition
 strength
Term
 rise fall time is usually expressed in what units?
Definition
 mT/s (milliTesla/sec)
Term
 another name for rise/fall time is _______
Definition
 ring down time
Term
Definition
 maximum
Term
 most scanners have a peak gradient strength of ______ mT/m, but FSE and EPI scans require higher strengths (20-40 mT/m)
Definition
 15 mT/m
Term
 For gradient applications, the rate which is a combination of rise/fall time and peak strength is called ____ ____
Definition
 slew rate
Term
 slew rate is measured in what units?
Definition
 T/meter/sec
Term
 typical slew rates range from ____ to ____ T/m/sec
Definition
 10 to 100 T/m/sec
Term
 which slices are selected by the three gradients:
Definition
 Gz slices head to foot - so axialGy slices A to P - so coronalGx slices left to right - so sagittal
Term
 Slice thickness is controlled by gradient _____ and transmit ______
Definition
Term
 increasing gradient amplitude will ______ slice thickness
Definition
 decrease
Term
 decreasing gradient amplitude will _______ slice thickness
Definition
 increase
Term
 increasing transmit bandwith will _____ slice thickness
Definition
 increase
Term
 decreasing transmit bandwith will _____ slice thickness
Definition
 decrease
Term
 what is the transmit bandwith?
Definition
 the frequency of the RF signal
Term
 phase encoding uses the gradient that is parallel to the ______ axis of the anatomy being imaged
Definition
 short
Term
 Which gradients do phase encoding for sagittal images?
Definition
 Gy
Term
 Which gradients do phase encoding for coronal images?
Definition
 Gx
Term
Definition
 Gx
Term
 Which gradients do phase encoding for axial BODY images?
Definition
 Gy
Term
 Frequency encoding uses the gradient that is parallel to the _____ axis of the anatomy being imaged.
Definition
 long
Term
 The frequency encoding is done when _______
Definition
Term
Definition
Term
 the steepness of the gradient amplitude is determined by the ______
Definition
 FOV
Term
 Small FOV means _____ (steep/shallow) F. E. gradient
Definition
 steep
Term
 large FOV means ______ (steep shallow) F. E. gradient
Definition
 shallow
Term
 Frequency encoding gradient used for coronal images
Definition
 Gz
Term
 Frequency encoding gradient used for sagittal images
Definition
 Gz
Term
 Frequency encoding gradient used for axial images of the BODY
Definition
 Gx
Term
 Frequency encoding gradient used for axial images of the BRAIN
Definition
 Gy
Term
 the duration of frequency encoding during the "read out" time is called ____________
Definition
 sampling time
Term
 currently, up to _____ frequencies can be sampled.
Definition
 512
Term
 the range of frequencies sampled during the frequency encoding is called the _________ ________
Definition
Term
 this is the mathematical link between sampling time, sampling rate, and receive bandwidth
Definition
 Nyquist theorem
Term
 The Nyquist theorem states that for accurate reproduction, the signal must be sampled at least ____ times
Definition
 2
Term
 When is slice select on?
Definition
 during 90 and 180 pulses
Term
 when is frequency encoding on?
Definition
 during the MR signal or echo
Term
 when is phase encoding on?
Definition
 just prior to the 180 pulse
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