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.
Why has the spin echo sequence proven effective?
1) The echo occurs a considerable time after the RF pulse (i.e TE is quite large)
Giving enough time to electronically prepare the external coil, which is used for RF pulse generation and detection of subsequent FIDs.
2) Both T_1 and T_2 weighted contrast can be displayed by selection of appropriate values of TE and TR
How are resistive copper coils placed surrounding the main magnet, to produce the x,y,z gradients?
The z-gradient is produced by wrapping a resistive coil around the outside main magnet
The x/y-gradient are produced by saddling the coils around the inner cylindrical surface of the magnet
What is the net effect at the mid point between the x/y gradient coils?
the net effect at midpoint between the coils is neutral as the current through the coils flows in opposite directions
What is the result of the interaction between the gradient fields and the main gradient field?
a force is produced on the gradient coil, causing a vibration. A loud sound is produced each time a gradient is activated during a scan.
What is the first purpose of the Radiofrequency coil?
produce a RF pulse which causes the net magnetisation M to move (i.e in the spin echo pulse sequence)
What is the second purpose of the radiofrequency coil?
Detect the precessing M in the sample, which induces a current to flow in the coil
Why is only a single coil tuned to the Larmor frequency used as a RF coil?
The frequencies of the pulse and the resulting signal (RF pulse vs signal detected after the spin echo pulse sequence) are the same
How to maximise the strength of the signal detected?
The RF coils are placed as close as possible to the part of the body being scanned.
What are the 3 main components of an MRI scanner?
the main magnet
the gradient coils
the RF coils
electronic computer system to control the coils according to the desired pulse sequence
What does it mean to have a higher field strength?
better signal to noise ratio, higher spatial resolution, faster scan times
What is one of the requirements for MRI magnets?
produce low field inhomogeneity so that the non instrinsic relaxation time is high enough to be measured by the electronic system
What is another of the requirements for MRI magnets?
stable enough so that the value of the original magnetic field doesn’t fluctuate significantly
What is the issue of having such a high resistance coil in the main wire?
produce a lot of heat dissipationw
What happens when a lot of heat is dissipated?
Expansion, and temporal instability
Solution to reduce heat dissipation, and effectively temporal instability and expansion
Use superconducting wires, which have 0 resistance at liquid helium temperatures (which is what is used to fill in the vaccum containing the solenoid)
how to improve the field homogeneity?
using several solenoids with a lot of turns
How are the solenoids contained?
Each contained in a solid stainless steel housing filled with liquid helium. The steel housing is surrounded by vaccum shields, which are cooled by liquid nitrogen.
in summary, stainless steel housing filled with liquid helium, surrounded by vaccum shielding, cooled with cheap nitrogen
What is inside the main magnet bore?
a set of resistive coils through which current is passed to adjust, or shim the magnetic field to ensure homogeneity
How do relaxation times vary?
related to tissue physical and chemical properties
In a T_2 weighted image, what happens if the T_2 time is short or long?
Short : low signal
Long: high signal
In a T_1 weighted image, what happens if the T_1 time is short or long?
Short: high signal
Long: low signal
Why is the signal high in a T_2 weighted image if the T_2 value is long?
Larger echo amplitude
Why is the signal high in T_1 weighted image if the T_1 duration is very short?
The M_z component will have recovered prior to the 90 degree pulse
What are the major applications of MRI?
Brain: diagnosis of acute and chronic neurological diseases
Liver: tumours
Musculoskeletal: cartilage degeneration
Cardiac: myocardial infarcts
How is x-ray angiography different to MRI angiography?
MRI angiography doesn’t require contrast agents
What fact about pulse sequences does MRI angiography exploit?
The fact that the value of the maximum vertical momentum vector within the selected slice is much smaller than the thermal equilibrium value of M_0. Blood flowing within the slice will enter when the vertical component of moment is equal to the thermal equilibrium value of the magnetic moment.
What happens to the resulting T_1 value of the in-flowing blood when the magnetic moment vector in the z axis = thermal equilibrium magnetic moment vector?
The time of longitudinal relaxation of the in-flowing blood appears shorter.
How can flowing blood be differentiated from stationary tissue?
Using a T_1 weighted pulse sequence, as the value of T_1 naturally appears to be shorter when M_z = M_0
What does paramagnetic mean?
The deoxyhaemoglobin molecules set up a local field in the same direction as the external B_0 field.
What is the result of Hb molecules setting up a local magnetic field in the same direction as the external B_0 field?
it produces greater variation in the local magnetic field, which causes faster dephasing. As a result, the values of T_2 and T_2* decrease locally.
How is the fact that the values of T_2 and T_2 * decreasing locally in fMRI useful?
It is useful to visualise changes in blood oygenation resulting from brain activity. Neural activity in the brain’s grey matter causes localised changes in blood flow, causing the concentration of oxyhaemoglobin to increase and concentration of deoxyhaemoglobin to decrease. In the areas where the brain activity is greater, the value of T_2 will be higher, producing a T_2 weighted image, which will be brighter.
What is the Blood oxygen dependent effect?
Where pulse sequences are used to reveal local differences in the T_2 and T_2 *
Uses of fMRI
Study the brain’s response to cognitive tasks and brain functions such as speech, language and sensory processing
Disadvantages of fMRI
signals produced as very weak and require significant averaging to provide a sufficient SNR.
Usual display format of fMRI
In colour on top of a greyscale anatomical image
Why are contrast agents applied?
To improve visualisation of certain tissues
What are paramagnetic contrast agents?
Contrast agents which improve the visualisation of certain tissues by creating a local magnetic field in the direction of the main external field B_0
How are contrast agents introduced into the body?
Injected intravenously shortly before the scan
What do contrast agents usually do once introduced intravenously into the body right before the scan?
distribute within tumours
What is the entire process from introduction to excretion of the contrast agents?
Injected intravenously into the body, and distributed within a tumour, then absorbed and excreted by the kidneys.
What other function, apart from visualising tumours, are contrast agents also used for?
study blood vessels in diseases such as cardiovascular disease
