MRI (Mentally Retarded Ignoramus) Flashcards
Explain the NMR phenomenon
When a magnetic field is introduced to atomic nuclei, (H protons) they align with or against the field due to their own magnetic fields
What is Larmor Precession and Larmor Frequency?
Larmor Precession - In the existence of a magnetic field, the aligned nuclei start to rotate around the direction of the mag field
Larmor Frequency - Frequency of the Larmor Precession. Depends on the strength of the mag field and the properties of the nuclei
Explain the Relaxation process
The precession is not stable and will decay over time.
T1 relaxation is logitudinal
T2 relaxation is Transverse
They relaxations generate the signal
What does T1 relaxation refer to?
Recovery of the nuclei’s longitudinal magnetisation (alignment of the nuclei with mag field)
What is T1 relaxation influenced by?
Type of tissue
(provides info on tissue composition and contrast)
What does T2 relaxation refer to?
Decay of transverse magnetisation (perpendicular component of magnetisation) due to interactions with neighboring nuclei / measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field
(provides info on tissue micro-structure and contrast)
what are magnetic field gradients and what are they used for?
its additional magnetic field that is applied but it’s a gradient to suppliment the original mag field.
They produce variations in the mag field strength so the precession frequencies of protons at different spatial positions are intentionally made to differ
It allows for spatial encoding to determine where exactly the signal came from
Explain RF coils
used to transmit and receive the EM energy to manipulate the nuclear magnetisation
the RF coil gives out a pulse that is the same frequency as the Larmor frequency of the specific nuclei
This perturbs (disturbs) the magnetised nuclei from its equilibrium
What are excitation pulses
applied to flip the magnetised nuclei to the transverse plane which is perpendicular to the mag field.
This disturbance starts the NMR signal which is detected
How is the signal detected?
disturbed magnetised nuclei relax and emit NMR signal.
RF coil switches to recieve mode from transmit mode to catch the signal
Detected signal is weak and needs amplification and processing to create an image.
How is the image reconstructed?
raw data is contained in k-space, is collected by sampling the signal using the gradient fields. Fourier Transforms are used to convert the raw k-space data into an image and then inverse fourier transforms are used to get a spatial representation.
Image contrast and pulse sequences
different pulses = different contrasts
sequences control the timing and parameters of the excitation and pulses
This affects T1 and T2 relaxation times and resulting image appearance
(spin-echo, gradient-echo, inversion recovery)
What does changing the TR and TE do?
controls the timing and characteristics of the MRI pulse sequence.
Determines contrast and image acquisition times in scans
Explain TR (Repetition Time)
TR is the time between RF pulses emitted
Its the time taken by the scanner to get data from one excitation to the next
What does TR influence?
Image contrast and overall scan time
Short TR = less time for protons to relax
tissues that have short T1 relaxation times give out more intense signals
long TR time is used for T2-weighted images (sensitive to fluids and fats)
