continuation of MRI Flashcards
MRI
Relaxation
The phenomenon responsible for most basic MR contrast, characterized by the return of magnetization components to their equilibrium states after perturbation.
What are the Bloch Equations
Empirical equations describing the rate of change of magnetization components (M x, M y, M z) with respect to time (t), influenced by longitudinal (T1) and transverse (T2) relaxation times.
What is Longitudinal Relaxation (T1)
The time constant describing the rate at which the longitudinal magnetization (Mz) recovers to its equilibrium state after perturbation, influenced by spin-lattice interactions.
Transverse Relaxation (T2)
The time constant describing the rate at which the transverse magnetization (M xy) decays due to loss of phase coherence among spins, influencing image contrast.
Relationship Between T1 and T2
Generally, T2 is shorter than T1, indicating faster dephasing of spins compared to the return to equilibrium polarization. T2 can be comparable to T1 in systems with highly mobile molecules (e.g., water), but T2 is significantly shorter than T1 in solids, resulting in poor bone visibility in standard MR images.
Name all three Bloch Equations
dt/dMz = − (Mz −M0)/T1
dt/dMx = −(Mx/T2)+γMyBz
dt/dM = (My/T2) - γMxBz
Longitudinal Relaxation (t1)
The process by which the spin system returns to the Boltzmann equilibrium distribution following absorption of RF energy.
What causes Longitudinal Relaxation
Longitudinal recovery involves the exchange of spin energy with thermal energy of molecules, facilitated by Brownian motion and dipole-dipole interactions.
What is brownian motion
Random motion of molecules, generating fluctuating magnetic fields that cause transitions between spin states and contribute to longitudinal relaxation.
What is dipole-dipole interaction?
Interactions between spins due to the fluctuating magnetic fields generated by neighboring molecules, contributing to longitudinal relaxation.
Why is relaxation inefficient in pure water
due to a small fraction of motions being at the Larmor frequency.
what can influence relaxation time T1
Hydrogen bonding and molecular mobility influence relaxation time (T1), with reduced mobility leading to shorter T1 values.
T1 relaxation time is influenced by temperature, with higher temperatures generally leading to shorter T1 values.
T1 relaxation time is affected by the magnetic field strength, as it determines the Larmor frequency influencing relaxation processes.
Measurement of T1 using Inversion Recovery Sequence
In the inversion recovery sequence, a 180° RF pulse inverts longitudinal magnetization, followed by a 90° pulse to measure magnetization recovery at varying inversion times (TI).
Longitudinal Magnetization Recovery
After inversion, longitudinal magnetization (Mz) recovers towards equilibrium with a time constant T1, allowing measurement of T1 values.
Inversion Time (TI)
The time between the 180° inversion pulse and the subsequent 90° pulse in the inversion recovery sequence, affecting the measured magnetization recovery.
What is Tnull
The time at which longitudinal magnetization is zero during the inversion recovery sequence, influencing the signal measured in the experiment.
FID (Free Induction Decay) Signal
Signal generated in MRI from precessing spins after the application of RF pulses, used to measure longitudinal magnetization recovery.
Boundary Conditions for Longitudinal Relaxation
Initial conditions of the magnetization and its recovery process in the inversion recovery sequence, influencing the measured signal.
What is T1 mapping
Process of generating spatial maps of T1 relaxation times by measuring longitudinal magnetization recovery at different inversion times (TI).
Signal Fitting for T1 Measurement
Process of fitting measured signal values at different inversion times (TI) to the equation describing longitudinal magnetization recovery, yielding T1 values.
What does a 180 degree pulse do
Inverts longitudinal magnetisation.
Longitudinal magnetisation will recover over towards equilibrium distribution w time constant T1.
How to measure longitudinal relaxation using inversion recovery sequence
Apply 180degree pulse to invert longitudinal magnetisation
Mz will recover to equilibrium distribution w time constant T1.
After an inversion time, 90 degree pulse flips longitudinal magnetisation.
Providing a FID proportional to longitudinal magnetisation.
Steps to Measure T1
Flip Mz upside down with 180 pilse
Vary TI with 90degree pulse as it returns to normal
Bt changing 180-90degree pulses you get a sample at different stages
Reaches a null point
Recovers to normal state
Calculate
What happens to the magnetization vector Mxy after a 90° RF pulse in MRI?
The 90° RF pulse tips the net magnetization vector from its equilibrium position along the z-axis into the xy-plane, initiating precession at the Larmor frequency.
How is the Larmor frequency (ωL) calculated?
The Larmor frequency is calculated as ωL=γB, where
γ is the gyromagnetic ratio, and
B is the local magnetic field strength.
Steps of Inversion Recovery sequence
180degree RF pulse inverts longitudinal magnetisation
longitudinal magnetisation recovers towards equilibrium distribution w/ time constant, T1
After an inversion time a 90degree pulse is applied which flips longitudinal magnetisation at that time point to x-y plane
FID w amplitude proportional to longitudinal magnetisation exist at TI
At Tnull longitudinal magnetisation is zero
Vary TI in repeated 180-90 degrees, sample Mz during recovery to allow T1 to be calculated
T1 Time inversion equation
Mz(TI) = M0(1-2exp(-TI/T1)
Why is Tnull important
Applying pulse w acordance with T1 of that tissue will null its intensity.
Spin Echo Sequence
180degree refoccussing pulse flips net magnetisation into x-y plane
180 inversion pulse flips net magnetisation in z-plane; used to measure T1
-90degreex creates coherent transverse magnetisation Mxy = M0
Decay w T2* due to static & varying T2 processes
-spins w high fields spins >w0
-spins w lowfields spins < w0
180degree pulse applied after TE/2; Mxy is reversed
-Hare and tortoise analogy
- Signal we receive is weaker as magnetic field are subject to change
Contast Agents in MRI
-Agent injected into blood stream and transit is observed
-Contrast agent gives info on blood flow, volume, vessel permability
Example of a contrast agent
Gd-DTPA
How contrast agent work
Gd3+ion is paramagnetic
Unpaired electron on Gd has a magnetic moment 1000x Nuclear magnetic moment
therefore enhance relaxation by dipole-dipole interaction
Gd is toxic therefore chelated to DTPA medication
In Gd-DTPA, what is responsible for biological interactions
DTPA
In Gd-DTPA, what is responsible for magnetic effect
Gd
its a bit toxic
Contrast T1 equation
1/ T’1 = 1/T1 +[Gd]r
T’1 = T1 w/ contrast agent
T1 = T1 w/o contrast agent
[Gd] concentration
r1 = relaxvity
How to excite a narrow range of frequencies
What dies changing radiofreq do
What dies changing the signal do
Name Equation
Apply a B-fields which oscillated in z-direction
(Mag freq changes along that direction)
Send radiowaves over the slice
Mix radiowaves w/ another signal (since)
Signal targets the frequency we want to excite
Changing radiofreq changes where in the sample we are looking
Chnaging the mixrd sigbak changes how thick the slice is
deltaw = (gyrometric const. )(Gradient)(deltax)
