MRI Flashcards
types of magnets
closed (mostly) - 1.5T or 3T
open systems are available (low field) - less than 0.4T
why MRI in RT
- improved visualisation in MRI can help delineate treatment areas
- non invasive and better for repeated scanning
- better for paediatric patients
- some artefacts in CT (dental fillings) are better on MR
- moving organs can be visualised
why NOT MRI in RT
- MRI contraindications
- certain implants are not compatible with MRI (pacemakers, aneurysm clips)
- claustrophobia
- certain implants, while safe for the patient, cause problems in the image
- Gd contrast agent - allergic reactions, NSF
- patient needs to be changed into scrubs and remove external metallic devices (jewellery, belt)
if MR and CT are both required prior to treatment, which one first?
MR bore is more restrictive - if done first patient positioning will be compatible across both scanners
- same head tilt
- reduced the need for deformable registrations
if CT has occurred first - use body markings for positioning
- be aware of ink - some cause signal loss on MR
MR quality - volume (too large and too small)
too large - distortion increases, scan time increases
too small - missed target or image registration fails
make sure target volume is in isocentre to reduce distortion
distortions in MRI arise from
B0 homogeneity
magnetic susceptibility
- air tissue artefacts
- tissue bone interfaces
- metal implants
chemical shift artefacts
distance from the isocentre
pulse sequence and parameters
application examples of MR and RT
brain
spine
lung
pancreas
prostate
cervix
rectum
musculoskeletal
hybrid systems - PET/MRI
- simultaneous PET and MR
- high resolution, dynamic MR with molecular sensitivity of PET
PET/MRI issues
attenuation compensation, motion detection and correction, partial volume correction
PET/MRI advantages
- lower radiation dose compared to PET/CT
- great for follow up or paediatric scanning
PET/MRI clinical uses
- hypoxia/RT treatment
- metastases - improving diagnosis and treatment
- head and neck RT planning
- neuroinflammation - repeated head injury
- dementia/Alzgiemer’s - amyloid/tau/cholinergic function
- epilepsy
- prostate cancer
MRI/Linac
MRI plus linear accelerator
- radiation beams targeting tumours in real time
- see where the radiation is going as it happens
- maximise dose to tumour and protect surrounding tissue
- particularly good for moving tumours
- breathing can move the tumour 2cm
- prostate, lung, liver, pancreas and abdomen
- head and neck - swallowing
benefits of high tesla (T)
more signal, better resolution = increased image quality
Give some examples of closed receiver coils
head and neck coils
knee coil
foot coil
Give some examples of flexible receiver coils
chest, flex, body coils and spinal array
Explain how contrast changes an image
Contrast can change the degree of excitation, types of signal echo, changing time can change the contrast between different tissue types (pulse sequence)
In T1 relaxation list in order from shortest to longest relaxation rates for grey matter, white matter, liver, spinal cord and blood
Liver, Spinal cord, White matter, Grey matter, Blood
What are some MR considerations?
Signal is relative to distance away from the receiver coils so immobilization devices should lower signal to background ratio (SBR).
Fiducial makers such as gold will be void and have no signal.
Smaller voxels will reduce SNR (longer scan time can improve SNR for small voxel)
What affects MR quality?
Volume size - too large increase distortion, scan time increases
too small cause image registration fails and missing targets, say close to isocenter. For 2D images scan with no slice gaps and 3D scan the shortest axis
How is motion accounted for?
Image rapidly or trigger with the movement.
Gating - respiratory & cardiac
benefits of MR in oncology
Ability to map tissue characteristics, microstructure information looking at diffusion of water, fiber tracking in the brain, tissue perfusion and angiography, temperature mapping, metabolic mapping with spectroscopy
What does T2 images show?
higher water content means high signals cancer have high density = low water content = low signal
How do diffusion images work?
measures the microstructure by looking at the movement of water.
Cancer has high density - image will appear brighter due to restricted water. Can be used for white matter compression in brain. Diffusion is dependent upon temperature therapy monitoring
What is magnetic resonance spectroscopy?
measure of metabolites in-vivo. Choline and creatine concentrations can increase in tumours with grade; citrate concentration decrease - time consuming
What is T1/T2 is good for?
Separating gliomas and metastases, separating high- and low-grade prostate cancer
pros of MRI in RT
non ionising
good soft tissue delineation
contrast agent less likely to cause allergy
cons of MRI in RT
expensive
contraindications
time consuming
noisy and large magnetic field
MR planning not efficient
the purpose of flex bridge
plastic bridges to positioning the flex coils
placing on the patient directly can cause the tissue to deform due to the weight –> we want accurate soft tissue for RT planning
as signal is relative to distance, the image quality wont be as good compared to coils being on the patient
further away from the coil –> low SNR
