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what is frequency?
the rate that an event repeats itself
what is phase?
refers to whether certain events are synchronized with each other
if the frequency of two systems is the same,
there can still be different amounts of phase between them
what happens to the amount of phase difference if the frequency of two systems remains constant?
the amount of phase difference between them will remain constant
if the frequency of each of two systems is different
the amount of phase difference between them will accumulate linearly over time
a positive change in frequency followed by an equal negative change in frequency will have what type of effect on the phase?
no net effect
list patient coordinates. for ex. X,Y,Z and their directions
- X: left to right
- Y: Anterior to posterior
- Z: head to feet
what are magnetic field gradients?
are a hardware component of the system, consisting of many coils of wire which, when current is passed through them, are capable of varying the main magnetic field
which gradient is turned on if we want to vary the field from the patient's head to their feet? how about from the pt's left to right or anterior posterior?
- head to feet- z gradient
- left to right- x gradient
- anterior posterior- y gradient
how do you calculate the magnetic field at any point along an applied gradient?
+ (D x G /1000)
- B=final magnetic field strength (in Tesla)
- = main magnetic strength (in Tesla)
- D= distance from magnet center (in meters)
- G= gradient strength (in mT/m)
In a 1.5 Tesla magnet, what is the strongest magnetic field that can be measured 1/2 meter from the center of the magnet when an 8 mT/m gradient field is applied? Hint the strongest field occurs on the positive side of the gradient field
- B= + (D x G/1000) Tesla
- = 1.5 + (+0.5 x 8 / 1000) Tesla
- = 1.5 + 0.004 Tesla
- = 1.504 Tesla
If a patient is placed in a perfectly homogenous 1.0 Tesla magnet, what is the strength of the magnetic field at isocenter when a 4 mT/m gradient is applied along the z-direction? how about one meter in the positive direction? how about 1m in the negative direction?
isocenter = 1.0 Tesla (it's always the same at isocenter)
1m in positive direction = 1.0 T + 0.004T = 1.004T
1m in negative direction= 1.0T - 0.004T = 0.996T
what effects do gradients have to the precessional frequencies of net magnetization?
remember from the larmor equation that the net magnetization precesses at a frequency determined by the magnetic field experienced by the protons. Because the gradients varies the main magnetic field along the direction it is applied, we are directly varying the precessional frequency of the net magnetization by applying a magnetic gradient. Under these conditions, the precessional frequency of the net magnetization corresponds to a specific location on the axis along which the gradient is applied
what is the larmor equation? what is the equation used to calculate the magnetic field at any point along an applied gradient?
according to the principle of resonance, the transfer of energy occurs only if
the frequency of the transmitting system is matched to that of the receiving system
what must happen to selectively excite protons in a given slice?
turning on a gradient at the same time as the RF pulse. The RF pulse is transmitted precisely at the resonant frequency of the protons located at the position of the slice we wish to image after a gradient is turned on
which slice gradient is turned on if you want to obtain an axial slice?
the slice thickness can be specified by what?
- by the transmitter bandwidth or range of frequencies
- the slope of the gradient applied
how can you achieve a thick or thin slice?
thin slice: narrow range of frequencies and/or steep or higher gradient slope
thick slice: wider range of frequencies and/or shallow or lower gradient slope
Refer to MRI in practice book for explanation of phase encoding gradient