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CSB063 EXAM QUESTIONS AND VERIFIED ANSWERS
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CSB063 EXAM QUESTIONS ANDVERIFIED ANSWERS
What are the 3 main components of the MRI system and their purpose? - Answer 1.
Super conducting magnet / coil - creates magnetic field (B0)
2. Gradient coils - three magnetic field gradient coils that apply a magnetic field on top
of B0 which adds or subtracts magnetism. Alters and localises MR signal in X,Y,Z
3. Radiofrequency transmit coils - emits RF waves which generate time varying
magnetic field (B1)
Shimming - works to creating a homogenous magnetic field by adding or subtracting
magnetic field permanently
Shielding - active. Great coil of wire outside the MRI with opposite polarity of the main
magnetic field to hold the magnetic field in and stopping external objects from being
affected
What is the magnet range in tesla? - Answer 0.3-7T
What are the desired aspects of a magnet? - Answer Cost, homogeneity and strength
What are the different types of magnets? - Answer Permeant (not very strong)
Electromagnetic (needs electricity)
Superconducting magnet
What is a superconducting magnet (B0) made out of? - Answer A superconducting alloy
such as Niobium Titanium
How is the superconducting magnet cooled down and why? - Answer With a cryogen
such as liquid Helium to remain superconductive and avoid friction
Gives a perpetual electric current
No external electricity needed (Only to maintain cryogen in liquid state)
Define quench? - Answer Quench is the phenomenon where resistance builds in the
superconducting magnet which should be 0 resistance coil wires so that the current is
continuous --> loss of superconductivity in conductivity --> friction --> heat and so
cryogen gases starts to boil as it changes state --> quench
How does precession occur in the body? - Answer All protons in the human body have a
magnetic moment spinning randomly which align to B0 as the patient enters a magnetic
field.
,Hydrogen has protons and electrons which act as a north and south magnet which align
to the magnet's south and north BUT due to unstable weight and instability of particle,
we have a precession movement
Define precession? - Answer The speed at which the proton's magnetic moment spins
around its axis
The frequency of precession of hydrogen atoms in an MRI is proportional to what? -
Answer Magnet strength
Stronger magnetic field = faster precession
What is rotating frame of reference? - Answer Rotating frame of reference - we were to
rotate at the precession frequency, we can almost halt the proton movement and view it
still in two planes
What are the energy levels of hydrogen? - Answer Spin-up and spin-down where spin
down = high energy as it goes against magnetic field
What is the number of spin-down dependent on in an MRI scan? - Answer B0 strength
How many magnetic moments of all hydrogen? - Answer One as any spin-ups and
spin-downs will cancel each other
Summary of precession? - Answer Charged nucleus --> angular momentum (spin) due to
unbalanced mass between protons and neutrons) --> magnetic moment like a magnet -->
precession --> larmor frequency
How much hydrogen (%) in human body and H20? - Answer 62
65
What are the different types of radiofrequency coils? - Answer Receiver (Faraday's law)
and transmitter coil
What is Faraday's law? - Answer Wherein a changing/time varying magnetic field
induces a voltage in a nearby conductor
How does a radiofrequency receiver coil receive its signal? - Answer Transverse
magnetisation perpendicular to receiver RF coil will generate alternating voltage
(signal) --> a change in magnetic flux
How does a radiofrequency transmitter coil transmit energy? - Answer An
electromagnetic wave is given with the same frequency as the Larmor frequency of
hydrogen
This deflects the longitudinal energy into the transverse
What is the deflection angle determined by? - Answer The RF transmitter coils power
and duration
,What are the two things that occur from an RF pulse? - Answer Energy transfer to spins
so some spin0ups will spin-down
All magnetisation moves into one direction - transverse magnetisation
Define relaxation in MRI? - Answer The reverse of excitation
The spins realigning themselves to B0
What are the two types of relaxation in MRI? - Answer Longitudinal magnetisation regain
- T1 relaxation - spin-lattice relaxation
Transverse magnetisation decay (dephasing) - T2 relaxation
Why does dephasing occur (T2 relaxation)? - Answer 1. Spin-spin interaction - molecular
level, molecules bump into each other as they are moving and pass on energy
2. Any separate magnetic field from the patient i.e. implants or from inhomogeneity of
the magnet - FIXED by the 180 flip
What is the relaxation or decay rate dependent on? - Answer The tissue and its particles
What is the T2 relaxation time? - Answer The time for 63% of transverse magnetisation
to decay due to spin-spin interaction
What is the difference between decaying and dephasing? - Answer Decay should only
be occurring ideally due to spin-spin
Dephasing is realistically from patient implants or inhomogeneity in magnet
Decay = transverse to longitudinal
Dephase = spin around axis
What is it called when we get T2 relaxation or decaying not due to spin-spin? - Answer
T2* relaxation or free induction decay
Due to inhomogeneity
Define T1 relaxation time? - Answer The time taken for 63% of longitudinal
magnetisation to be regain/recover
• The time it takes for the spins to give the energy they obtained from the RF pulse back
to the surrounding lattice in order to go back to their equilibrium state
IT IS A CONSTANT
What are the reasons to T1 and T2 relaxation? - Answer Spin-lattice interaction
Spin-spin interaction (intrinsic) or free induction decay / field inhomogeneity (extrinsic)
• T1>>T2>T2*
, What are echoes? - Answer Used in order to acquire a signal and are a result of phase
refocusing
What is the spin-echo procedure? - Answer 90RF excitation pulse followed by 180
refocussing echo to beat dephasing
When the 90° RF pulse is removed, a free induction decay signal (FID) is produced. T2*
dephasing occurs almost immediately, and the signal decays. A 180° RF pulse is then
used to compensate for this dephasing (Figure 2.16).
What relaxation does TE and TR control? - Answer TE - T2
TR - T1
What is TE? - Answer The time from the application of the RF pulse to the peak of the
signal induced in the coil (echo) and is also measured in ms
TE controls T2
Why does TE control T2? - Answer The TE determines how much decay of trans- verse
magnetization is allowed to occur. TE thus controls the amount of T2 relaxation that has
occurred when the signal is read.
What is TR? - Answer Repetition time
Time from the application of one RF pulse to the application of the next RF pulse (ms)
• Short TR = less longitudinal magnetisation recovery = less transverse magnetisation in
the next repetition
• Long TR = more longitudinal magnetisation recovery = more transverse magnetisation
In the next repetition
Why does TR control T1? - Answer The TR determines the amount of longitudinal
relaxation that is allowed to occur between the end of one RF pulse and the application
of the next. TR thus determines the amount of T1 relaxation that has occurred when the
signal is read.
What is signal intensity determined by? - Answer Proton density
T1 constant
T2 constant
What are the three scanning parameters to signal intensity? - Answer TR
TE
Flip angle
How many Gauss in;
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