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MRI CAMRT Exam Questions with Complete Solutions| Already Passed
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MRI CAMRT Exam Questions withComplete Solutions| Already Passed
As Bo increases what happens to the difference in energy between high and low energy states
- ✔increases - more spins aligned to Bo means larger NMV which means more signal
Larmour frequency - ✔The precessional or resonant frequency (w0) of a nuclear spin when
it is placed in an external magnetic field.
It is related to the magnetic field (Bₒ) by the Larmor equation:
Wₒ=ϒ*Bₒ
Unit of precessional frequency - ✔Megahertz (MHz)
1MHz is one million cycles or rotations per second
Processional frequencies of hydrogen at 0.5T 1T and 1.5T - ✔0.5T- 21.29MHz (42.58x1.5)
1.0T- 42.57MHz (42.58x1)
1.5T- 63.87MHz (42.58x0.5)
Gyromagnetic ratio of hydrogen - ✔42.57 MHz/T
Define phase, in phase, and out of phase - ✔position of magnetic moments on
their precessional path
In phase/coherent - at the same place on the path
,Out of phase/incoherent - at different places on path
Where does mri fall on the electromagnetic spectrum - ✔the radio frequency (RF) band
Define resonance - ✔A phenomenon that occurs when two objects naturally vibrate at
the same frequency
How is resonance achieved in MRI - ✔by applying RF pulse at the frequency of hydrogen
What happens when the RF pulse is switched off - ✔The NMV is again influenced by B0 and
it tries to realign with it. Hydrogen nuclei lose energy and dephase
Free Induction Decay (FID) - ✔The signal induced in a receiver coil immediately following
the RF excitation pulse, loss of signal due to relaxation
Define TR - ✔The time from the initiation of one RF pulse starting a sequence, to the next
Define TE - ✔Time from the application of the RF pulse to the peak of the signal induced in
the coil
Examples of intrinsic contrast parameters - ✔T1, T2, PD, flow, ADC
Examples of extrinsic contrast parameters - ✔TR
TE
Flip angle
TI
Turbo factor/echo train length
b value.
, Explain T1 recovery - ✔Caused by hydrogen nuclei giving up their energy to the
surrounding environment (spin-lattice transfer), NMV realigns to Bo
T1 recovery time - ✔time taken for 63% of the longitudinal magnetization to recover
Explain T2 decay - ✔Caused by magnetic fields of neighbouring nuclei interacting with
each other (spin-spin relaxation) the transverse magnetization dephases
T2 decay time - ✔time it takes for 63% of transverse magnetization to decay, leaving 37%
still in phase
T1 times of water and fat - ✔Water- 2500ms
Fat - 200ms
T2 times of water and fat - ✔Water-2500ms
Fat-100ms
Proton density of a tissue - ✔is the number of mobile hydrogen protons per unit volume
of that tissue.
2 factors that T1 and T2 relaxation depend on - ✔-if molecular tumbling rate matches
the larmour frequency of hydrogen = good energy transfer
-if molecules are closely packed together
Explain why fat and water appear different on images - ✔-Fat has molecules that are
closely packed together and tumbling rate is slow which better matches the larmour
frequency so NMV recovers fast and and magnetic moments dephase quickly (T1 and T2
times are short)
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