MRI Flashcards
For any set of fixed values of the gradients, what is the resulting plane?
The plane will exist in a magnetic field at which the field strength, and thus precession frequency ,is constant
If a 90 degree pulse (B1 field), is applied at the frequency at which protons are precessing in the plane defined by the fixed set of gradients,. what happend to the net magnetisation vector M for all protons in that plane?
they are rotated into the transverse plane
what is the effect of the applied B1 pulse to the protons outside of the plane?
as they precess at different frequencies, there is no effect on protons outside of the plane
in practice, what are the real frequencies over which the B1 field oscillated?
it will oscillate over a finite range of frequencies, known as the bandwidth
What does the bandwidth depend on?
the pulse length in units of time
the higher the pulse length the lower the frequency bandwidth
How can different slices be selected?
by changing the gradient or varying the central frequency of the RF pulse
After the RF pulse, what happens to protons on one face of the slice compared to the other face?
the ones in one face will be precessing at a faster pace compared to ones on the other face
How to make the protons on either faces precess at the same rate?
a negative gradient is briefly applied along the same direction
Once a plane of spins has been selected, how can the frequencies and phases of the Larmor precessions within the slice be manipulated?
by application of further gradient fields, so that the signal from each volume element within the slice can be identified more properly (resolve the frequency differences between the protons even within the same plane to identify differences in activity)
What is phase encoding?
One component process to the overall process of generating an image.
The slice gradient is switched off, and a perpendicular gradient is switched on for a short period of time delta t. A perpendiculra gradient (y) is switched on for a short period of time.
If the relaxation processes are ignored, what happens to the magnetisation vector of protons in the selected slice?
They will precess at a rate dependent on the field strength along the y axis
What is the consequence of phase encoding?
a variable shift occurs along the y axis
What happens within the selected slice when the phase encoding gradient is switched off and the frequency encoding gradient is switched on?
the magnetisation vector of protons on the selected slice will precess at a rate dependent on the field strength along the x axis
What is the consequence of frequency encoding?
Within the selected slice, the frequency of the signal is dependent on the position along the x axis
What does the strength of a signal at a particular frequency depend on in frequency encoding?
number of nuclei and how the signal has decayed since the slice was selected due to transverse and longitudinal relaxation effects, for a specific line across the slice (a specific fixed precession frequency)
What does the contrast within each pixel of the image depend on?
The local density of hydrogen nuclei (protons)
The local value of the decay time in the longitudinal direction
The local value of the transverse decay time
What do the relative contributions of the 3 factors affecting the contrast within each pixel of the image depend on?
the timing and ordering of the RF pulses and gradient fields. Also known as the pulse sequence
What happens first in the spin-echo pulse sequence?
1st) 90 deg pulse flips the moment vector into the xy plane. Following the pulse, the xy moment component will begin to decay due to the effects of the non-intrinsic transverse decay time.
Individual protons will precess at slightly different frequencies until the net transverse component of the magnetisation vector returns to 0.
What happens after the second pulse is applied at TE/2?
The net magnetisation vector is rotated by 180 degrees so that the M vector now points in the complete opposite direction.
What is the effect of the 180 degree pulse?
The magnetisation vector components in the xy and z direction are now inverted (completely opposite direction )
In a spin-echo sequence with TR approx = T_1 and TE «_space;T_2, what type of image will be produced?
A T1-weighted image
In a spin-echo sequence with TR»_space; T_1 and TE approx = T_2, what type of image will be produced?
T2- weighted image
In a spin-echo sequence with TR»_space; T_1 and TE «_space;T_2, what type of image will be produced?
proton density image as oth terms with time will become negligible, i.e = 1
How is slice selection gradient applied during the spin-echo sequence?
during the 90 degree pulse, it is applied with a reverse gradient in the direction of the main magnetic field axis
during the 180 degree pulse, it is not applied, as the 180 degree pulse has the same effect of inverting the components of the net magnetisation vector
How is a 2 dimensional image obtained in the spin-echo pulse sequence?
the sequence is repeated for different values of the phase encoding gradient to acquire a 2D image
When is the frequnecy encoding gradient applied in the spin-echo pulse sequence?
applied during the echo and earlier
Why is the frequency encoding gradient applied earlier before the echo?
to ensure that a 0 frequency offset occurs exactly in the middle of the readout time after the RF pulse.
