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CSH059 Imaging > MRI > Flashcards

MRI Flashcards

1
Q

Types of Magnets

A

Closed (most common) (1.5 or 3T)

Open (low field) (0.4 T) (can be used on very large patients, or those claustrophobic)

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2
Q

What is the benefit of using a higher T?

A

More signal
Better resolution
Improved Image Quality

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3
Q

Elements within the Receiver Coil

A
  • One receiver coil might have several elements within the coil (8, 15, 16, 20, 30, 32, 64)
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4
Q

Types of Closed Receiver Coils

A

Head/Neck Coil
Knee Coil
Foot Coil

Generally, quite rigid

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5
Q

Types of Flexible Receiver Coils

A

Chest Coil
Flex Coil
Body Coil

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6
Q

How to change contrast of MRI image

A

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

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7
Q

T1 and T2 - Dark and Bright

A

T1
- Bright: White matter brighter than gray matter
- Dark: Fluid

T2:
- Bright: White matter darker than gray matter, fluid

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8
Q

Is T1 or T2 more beneficial in viewing fluid based pathology

A

T2 -> Fluid is bright on T2

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9
Q

T1 relaxation rate

A

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)

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10
Q

T2 relaxation rate

A

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

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11
Q

T2 star relaxation rate

A

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

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12
Q

Why use MRI in RT

A
  • 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
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13
Q

Why not use MRI in RT

A
  • 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)
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14
Q

How to incorporate MRI alongside CT in RT workflow

A

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
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15
Q

If doing both, should MR or CT be performed first?

A

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

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16
Q

Positioning considerations for Positioning equipment

A

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

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17
Q

Fiducial markers in MR

A

Most common gold (has signal void on MR) (edge can be hard to determine)

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18
Q

Choice of MR protocol

A

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.

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19
Q

How does volume alter MR quality

A
  • 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
20
Q

What causes distortions in MRI

A

B0 homogeneity

Magnetic susceptibility
* Air-tissue interfaces
* Tissue-bone interfaces
* Metal implants

Chemical shift artefacts
Distance from the isocentre
Pulse sequence and parameters

21
Q

How to reduce distortion in MRI

A
  • 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
22
Q

Effect of Motion on MR images

A

Need to account for movement
* Can cause artefacts in the image if you don’t

23
Q

How to image a moving area

A

If you want to image a moving area (heart, lung,
prostate) you need to:
* Trigger with the movement
* Image rapidly to avoid it

24
Q

How can you gate with MRI (respiratory)

A
  • 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
25
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
26
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)
27
T2 weighting benefit in oncology
High water content means high signal Cancer’s have low signal if they have dense cellularity (high grade)
28
T1 weighting benefit in oncology
Visualise haemorrhage’s
29
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
30
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
31
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
32
T1/T2 mapping of cancers benefits
* Separate gliomas and metastases * Separating high and low grade prostate cancers (Sensitivity 83%)
33
What is isotropic diffusion
random motion of water particles, without restriction
34
What is anisotropic diffusion
motion of water particles are restricted to one direction of movement
35
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
36
What to expect in DCE imaging
* Expect more aggressive tumours to have quicker uptake curves * Higher grades, faster uptake
37
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.
38
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
39
Benefits of MR Linac
Real-time adaptive radiation therapy delivery
40
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)
41
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
42
RT sites where MR is most useful
Head and Neck (e.g., brain) Bone Lung Pancreas Prostate Cervix Rectum
43
FLAIR
Fluid Attenuated inversion recovery Nulls signal from CSF and other fluid filled structures
44
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
45
Advantages of portable low field MRI
Rolls to bedside Less claustrophobic Childrens parents can sit with them safely Faster turnaround
46
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

CSH059 Imaging (8 decks)

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