MRI Flashcards
How does MRI differ from CT
- Different Sounds
- Different paremeters (CT parameters are KV and mA
MRI paremeters are TE and TR) - CT measures attenuation
MRI maps hydrogen in the body - MRI can visualise soft tissues better
- No ionising radiation in MRI
MRI?
M - magnetic ( involves magnetic fields)
R - Resonance (involves resonance- a transfer of energy between 2 systems at the same frequency)
I - Imaging
B0
The main magnetic field from the magnet in the MRI machine
Why is net magnetisation vector 0 in a body outside the MRI machine?
The human body has abundant hydrogen, each with a single proton that exhibits spin, causing it to act as a tiny magnet. These protons have a random distribution so cancel each other out causing it to have null net magnetism.
Identify types of MRI units
- static / conventional closed magnet
- open magnet
Operating theatre mri unit and challenges
- anything ferromagnetic cannot be utilised in it - Must use titanium me to avoid magnetic attraction of tool
-Rf can interfere with the monitoring
What is a purpose of the cryogen bath?
- maintains superconductivity by keeping the magnet coils at an extremely low temperature
-by enabling superconductivity, it ensure the generation of a stable,powerful and homogenous magnetic field
- prevents overheating
Different orthogonal gradient coils, what are they?
What is the purpose/function of gradient coils
- Gradient coils add small variations to the magnetic field
- Slice selection
- Particular pulse sequences
- Spatial locate the data once we have measured it in the receiver coil
What field strength would you normally find in practice? What is the significance of field strength
1.5 to 3 Tesla
Significance:
The stronger the magnetic field the:
- better SNR (more clear and detailed images)
- higher spatial resolution
- improved contrast as field strength affects T1 and T2 relaxation times
- faster scans
- more potential for dangerous projectiles to be brought into them
- Photophores
- Nerve stimulation
What can we use MRI for?
- image soft tissue and differentiate between benign and malignant tissue
-multi planar capabilities
-morphology & physiology
-molecular imaging
What does the generation of the MR signal rely on?
MRI signal generation relies on the fact that the human body has abundant hydrogen protons which exhibit a spin (that produce an electromagnetic charge) which can align with B0 to create an NMV and the principle of resonance.
Explain the generation of MRI signal step 1 and 2
Initial State (Outside the MRI Machine):
1 - Outside the MRI machine, the protons in the body have a random orientation, resulting in null net magnetisation.
2- Alignment in the Magnetic Field (B₀):
When the patient is placed in the MRI machine, the strong external magnetic field (B₀) causes the protons to align in two directions:
Parallel (low-energy state).
Antiparallel (high-energy state).
A slight excess of protons aligns parallel, creating a net magnetisation vector (NMV) along the longitudinal axis.
Two magnetic fields now exist: the external magnetic field (B₀) and the protons’ magnetic field.
Explain the generation of MRI signal step 3 and 4
3 - Precession and Larmor Frequency:
The protons precess (spin) around the external magnetic field (B₀) at a frequency known as the Larmor frequency.
The Larmor frequency is directly proportional to the strength of B₀:
Stronger B₀ → Faster Precession.
4- Excitation with RF Energy:
To separate the two magnetic fields, the principle of resonance is applied.
A radiofrequency (RF) pulse at the Larmor frequency is introduced.
This transfers energy to the protons, tipping the net magnetisation vector (NMV) from the longitudinal axis into the transverse plane.
The RF pulse also causes the protons’ spins to synchronise (phase coherence).
Explain the generation of MRI signal step 5 and 6
5 -Relaxation and Signal Induction:
When the RF pulse is turned off, the protons return to their equilibrium state, releasing energy that is captured by the receiver coil.
This involves two processes:
T1 Relaxation (Spin-Lattice):
The recovery of 67% of the NMV along the longitudinal plane as protons realign with B₀.
T2 Relaxation (Spin-Spin):
The decay of 37% of phase coherence in the transverse plane due to interactions between protons.
6 - Signal Conversion:
The receiver coil detects the changing magnetic fields created by the protons as they relax.
These magnetic changes are converted into an electrical signal, which is processed to generate an image.
