MRI Flashcards
Types of Magnets
Closed (most common) (1.5 or 3T)
Open (low field) (0.4 T) (can be used on very large patients, or those claustrophobic)
What is the benefit of using a higher T?
More signal
Better resolution
Improved Image Quality
Elements within the Receiver Coil
- One receiver coil might have several elements within the coil (8, 15, 16, 20, 30, 32, 64)
Types of Closed Receiver Coils
Head/Neck Coil
Knee Coil
Foot Coil
Generally, quite rigid
Types of Flexible Receiver Coils
Chest Coil
Flex Coil
Body Coil
How to change contrast of MRI image
Can change
* Degree of excitation
* Type of signal echo (gradient/spin/steady state)
* Changing the timing can change the contrast between different tissue types (to create different pulse sequences)
For each sequence, you can change other things like
* Whether fat is visible or not
* Resolution and orientation of the images
T1 and T2 - Dark and Bright
T1
- Bright: White matter brighter than gray matter
- Dark: Fluid
T2:
- Bright: White matter darker than gray matter, fluid
Is T1 or T2 more beneficial in viewing fluid based pathology
T2 -> Fluid is bright on T2
T1 relaxation rate
The higher the mobility, the longer its T1 relaxation time
Short to Long:
Fat -> Tissue & Water -> CSF
Utilise short TR to allow contrast between structures (fat and fluid)
T2 relaxation rate
Constricted tissues, where spins are likely to bump together or collide, means shorter T2 times.
- Changing the time between excitation and reading the signal, can change the T2 contrast
Short to Long:
- Bone -> cartilage -> liver -> heart -> kidney -> white matter -> SC -> gray matter -> blood
T2 star relaxation rate
Always faster than T2
Differing types of material sitting close together (air-bone tissue) can cause local field changes
Good to see iron and calcification
Why use MRI in RT
- CT is still an essential part of Radiation therapy planning, but the improved visualisation in MRI can
help delineate treatment areas - MRI is 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 use MRI in RT
- Certain implants are not compatible with MRI
(pacemakers, aneurysm clips etc) - 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, etc)
How to incorporate MRI alongside CT in RT workflow
Both MR and CT prior to treatment
* Ideally on the same day/same unit – one venous access for contrast
- MR – Can be used Tumour delineation
- CT – Can be used for Dose calculation/planning
- MR segmentation (if possible)
- Use of improved soft tissue contrast of MR should reduce the systematic error in RT planning
If doing both, should MR or CT be performed first?
MR bore is more restrictive – if done first patient positioning will be compatible across both scanners.
* Same head tilt
* Reduces 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
Positioning considerations for Positioning equipment
Flex Coil
* Plastic bridge will be used to position flex coil around patient
* Plastic bridge ensures coil does not deform the patient skin
Mask
- cannot contain metal
Fiducial markers in MR
Most common gold (has signal void on MR) (edge can be hard to determine)
Choice of MR protocol
Use the most signal as appropriate
Accuracy required for RT planning
* Smaller voxels means less signal
* Results in lower SNR
* Or longer scan time, if you use averaging to bring
the SNR back up.
How does volume alter MR quality
- 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
What causes distortions in MRI
B0 homogeneity
Magnetic susceptibility
* Air-tissue interfaces
* Tissue-bone interfaces
* Metal implants
Chemical shift artefacts
Distance from the isocentre
Pulse sequence and parameters
How to reduce distortion in MRI
- Apply 3D distortion correction in
protocol (2D as last resort) - Use new sequences with improved
imaging around metallic implants - Ensure target is as close to the isocentre as possible
Effect of Motion on MR images
Need to account for movement
* Can cause artefacts in the image if you don’t
How to image a moving area
If you want to image a moving area (heart, lung,
prostate) you need to:
* Trigger with the movement
* Image rapidly to avoid it
How can you gate with MRI (respiratory)
- Small air box that moves with your
chest/stomach. - Can set acquisition to acquire either at
inspiration or expiration - Set a threshold level so it knows when to
acquire
How to gate with MRI (cardiac)
MR-compatible ECG leads
* Acquire at some part of the diastolesystole cycle
* Set a threshold level so it knows when to
acquire (R-wave)
* Set a delay so it knows which part of the
cycle you’d like
Uses of MRI in oncology
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)
Treatment
- Afford opportunity for real time tracking / real time planning (accounts for interfraction and intrafraction motion, reduces dose to NTT)
T2 weighting benefit in oncology
High water content means high signal
Cancer’s have low signal if they have dense cellularity (high grade)
T1 weighting benefit in oncology
Visualise haemorrhage’s
DWI benefit in oncology
Measures the microstructure by looking at the movement of water
Cancers with tightly packed cells will restrict the motion of water
More restrictive the higher the tumour grade
Diffusion weighted images would appear bright for restricted water
In the brain, compression of white matter can be mapped with diffusion
Diffusion is dependent upon temperature – therapy monitoring
dynamic contrast enhancement benefit in oncology
Gadolinium bolus injected, followed by T1-weighted imaging
Signal increases as the contrast agent flows through the vasculature
and tissue.
Increased blood flow with greater malignancy
Leaky capillaries or blood brain barrier is indicative of tumour
Magnetic Resonance
Spectroscopy benefits in oncology
Measure of metabolites in-vivo
Choline and Creatine concentrations can increase in tumours and with grade; Citrate concentrations decrease
Time consuming
T1/T2 mapping of cancers benefits
- Separate gliomas and metastases
- Separating high and low grade prostate cancers (Sensitivity 83%)
What is isotropic diffusion
random motion of water particles, without restriction
What is anisotropic diffusion
motion of water particles are restricted to one direction of movement
Perfusion Imaging process - DCE
- Image before injection, then inject contrast agent (e.g. gadolinium)
- Scan continuously while contrast moves through the body
- T1 weighted images, contrast is bright
- See the “wash-in” and “wash-out” of the contrast
What to expect in DCE imaging
- Expect more aggressive tumours to have quicker uptake curves
- Higher grades, faster uptake
Benefits and disadvantages of PET/MRI
Benefits
* Simultaneous PET and MR
* High resolution, dynamic MR with molecular
sensitivity of PET
* Lower radiation dose compared to PET/CT
* Great for follow up or paediatric scanning
- Issues – attenuation compensation,
motion detection and correction, partial
volume correction.
Clinical uses of PET/MRI
- Hypoxia/radiotherapy treatment
- Metastases – improving diagnosis
and treatment - Head/Neck radiotherapy planning
- Neuroinflammation – repeated
head injury - Dementia/Alzhiemer’s – Amyloid/tau/cholinergic function
- Epilepsy (78-82% sensitivity)
- Prostate cancer
Benefits of MR Linac
Real-time adaptive radiation therapy delivery
Disadvantages of MR Linac
X-rays can be affected by the magnetic field
Often have a lower MRI field to reduce the effects (0.5-1.5T)
Considerations of coils for RT patients
1) Placed around patient (typically body which is being scanned)
2) Without the image quality would be decreased
Body coil can be utilised, but without RF coil, image quality decreases
Good to have minimal distance between coil and body part being scanned –> immobilisation equipment may increase distance -> may decrease SNR
RT sites where MR is most useful
Head and Neck (e.g., brain)
Bone
Lung
Pancreas
Prostate
Cervix
Rectum
FLAIR
Fluid Attenuated inversion recovery
Nulls signal from CSF and other fluid filled structures
What is diffusion
Random thermal motion
Will displace over time
Provide information about microstructure
Iso tropic or anisotropic
Measured as apparent diffusion coefficient or ADC
Advantages of portable low field MRI
Rolls to bedside
Less claustrophobic
Childrens parents can sit with them safely
Faster turnaround
MRI in oncology
Staging and grading of malignancy (check oedema, haemorrhage, tumour reoccurrence, cell density, increased angiogenesis)
Mapping of tissue characteristics - separate gliomas from Mets
Metabolic mapping with spectroscopy
Microstructure information looking at diffusion of water
Temperature mapping
