Magnectic Resonance Imaging III Flashcards
What are the imaging principles?
w=yB
Unique distribution of B must lead to unique distribution of w
What is volume element isolation?
Use of complex (multiple axis) B-fields
B varies uniquely in 3-D
Each element associated with a unique B-field
Systematically excite/record emissions from each element
How can resonance intelligently to our advantage?
Slice selection – 1D
Gradient field
Linear relationship between gradient and position
…consequently a linear relationship between frequency and position (w=yB)
Frequency encoded position
What is the main field of a MRI?
Bo~ 1.5T – typically superconducting (3-10K)
What is the gradient field of a MRI?
typically linear (20mT/m)
What is slice selection?
The slice of interest is isolated by a combination of B and resonant RF Apply B0 Apply spatial gradient Blo -> Bo -> Bhi Excite slice choice (RF - w1) Only B1 slice will resonate Detect RF (w1 = yB1) Signal must have come from the B1 slice
How is the MRI process speeded up?
Flood the sample with many frequencies simultaneously
Record the emitted cacophony of RF and interpret…
…by disassembling the signal into its component parts (Fourier Transform)
Speed up the process
What are the principles o
Any signal can be reconstructed by the addition of sinusoidal oscillations of different frequencies, amplitudes and phases
see equations
What is many frequencies at once uniquely related to?
position
Does excitation use multiple frequencies in reality?
no, but detection does
What happens in the excitation of a single slice?
Single frequency slice selection, followed by… …absence of stimulating RF …but excited slice continues to radiate …and relaxes over 100s of ms Detect the emissions
What does frequency encode?
for spatial information
What is projection reconstruction?
Gradient field slice select
Excite with resonant RF
The instant the RF stops…
…apply an orthogonal gradient B-field to frequency encode the slice
Record the emitted signal
FT to relate amplitude to frequency (ie. position)
This constitutes a single projection
Repeat with rotated field for next projection
(eg. use combined x/y gradients)
Collate all projections and reconstruct
What is phase encoding?
Encode a phase axis first, then…
…encode other axis as frequency (as before)
Multiple repeats & use (double) FT to decode
How is the image createdin an MRI?
The spatially decoded signal is used to create a map of the parameter of interest
Typically mm resolution in slice plane
Slice thickness - several (typically many) mm
Contrast results from differences in the value of that parameter between neighbouring pixels
The image may map proton density, T1, T2 or a combination of these
Creative use of pulse sequences can highlight specific area of interest
