Magnetic Resonance Imaging Flashcards
Benefits of MRI
a. No additional radiation exposure
b. Excellent tissue contrast and resolution bone better seen on CT) (good grey/white differentiation)
c. Multiplanar (can scan along anatomical structures if needed)
d. Not dependent on operator (sequences are fixed and repeatable)
e. Functional imaging
What is the Lamour Frequency
a. Atoms are continuously precessing within the human body.
b. In normal state, hydrogen atoms are precessing at a rate of 42.58 MHz.
What is an RF coil?
a. Placed around the patient (typically body part which is being scanned)
b. Without RF coil, image quality would be decreased.
MRI image production summary
a. Body consists of water molecules (contain hydrogen atoms) – normally spin spontaneously.
b. In the presence of a magnet – some of the atoms align with the direction of the field, majority oppose the magnetic field.
c. MRI technologist sends RF pulses at the resonant frequency of the protons.
d. Protons spinning at a different frequency absorb the energy of the RF waves – protons flip on their axis.
e. RF wave pulse stops – flipped protons release energy – return to original position.
f. Signal is emitted back to the coil – signal is converted into an electric current.
MRI Image Production - Step 1
i. Atoms spin parallel or anti-parallel to B0 (main magnetic field)
ii. A net magnetisation is created, and a vector is formed.
iii. Vector is in the Z-plane along main magnetic field of hydrogen atoms.
MRI Image Production - Step 2 (RF excitation)
i. Net magnetic field in the z-direction that is precessing at the Lamour frequency (42.58 MHz).
ii. RF pulse applied at that particular frequency – excites hydrogen protons.
iii. Protons gain energy and move into a higher energy state.
iv. Protons move into the Y-direction (flip 90 degrees / Z -> Y plane / longitudinal -> transverse axis)
v. Hydrogen is precessing in phase at a higher energy state.
MRI Image Production - Step 3a (T1 Relaxation)
i. RF pulse is turned off -> transverse magnetisation begins to relax back to longitudinal plane (B0 magnetic field is puling it back)
ii. Longitudinal magnetisation regrows -> transverse magnetisation declines.
iii. Tissues relax back at slightly different times.
iv. Various vectors are created -> leads to grey scale.
MRI Image Production - Step 3b (T2 Relaxation)
i. When longitudinal vector is flipped into transverse plane – all protons are precessing together in phase.
ii. As RF pulse is turned off -> protons start to de-phase at different rates (some will spin faster, some will spin faster and spread out like a fan)
MRI Image Production - Step 4 (Signal)
i. At a set time – coil is set to receive signal (record residual transverse magnetisation)
ii. Induction decay signal is sent to computer.
iii. Raw data (k space) is then saved.
iv. Raw data is converted through Fourier into an image.
Parameters regarding image weighting
a. TR – Repetition time (from one 90-degree RF excitation pulse to the next)
b. TE – Echo time (from the 90-degree RF excitation pulse to when the echo or signal is being received in the coil)
c. IT – Inversion time (from the 180 degree pulse to the 90 degree excitation pulse)
Benefit of MRI and CT in RT workflow
Both MR and CT prior to treatment
i. Ideally on same day/same unit – one venous access for contrast
ii. MR – can be used for tumour delineation
iii. CT – can be used for dose calculation/planning
Examples of MRI applications in RT
Planning
- Can be utilised as sole imaging modality or co-registered with CT (for planning to be performed). This is beneficial as below:
- No ionising radiation
- Good soft tissue resolution (more accurate target delineation - RO)
- Multiplanar (can image along desired anatomical structures)
- Can be beneficial for trauma patients
Treatment
- Afford opportunity for real time tracking / real time planning (accounts for interfraction and intrafraction motion, reduces dose to NTT)
How to image a moving area
If you want to image a moving area (heart, lung, prostate) you need to:
i. Trigger with the movement
ii. Image rapidly to avoid it
Contraindications to MRI use in RT
a. Certain implants are not compatible MRI (pacemakers, aneurysm clips)
b. Claustrophobia
c. Certain implants can cause problems in the image
d. Gd contrast agents – allergic reactions
e. Patient needs to be changed into scrubs and remove external metallic devices (jewellery, belt)
How to reduce distortion in MRI
a. Apply 3D distortion correction in protocol (2D as last resort)
b. Use new sequences with improved imaging around metallic implants
c. Ensure target is as close to the isocentre as possible
