12-09-23 – Neuroimaging (look at slides for cross-sectional images) Flashcards
Learning outcomes
- Explain, simply, the basic principles to generate images by CT and MRI
- Compare and contrast the CT and MRI with regard to advantages, disadvantages, indications and contraindications
- Explain the principles of neuro MRI acquisition and interpretation
- Demonstrate the “ABCS” system for interpreting head CTs and MRI
- Identify the key anatomical features seen in normal head CTs and MRI
How often are x-rays currently used in neuroimaging?
- X-rays don’t have any role/a very small roll in neuroimaging at the moment
What are 3 uses of brain imaging techniques?
What do certain scans show?
What are 3 factors to consider regarding neuroimaging?
- 3 uses of brain imaging techniques:
1) Locating cysts and tumours
2) Finding oedema and haemorrhage
3) Identifying the effects of a stroke - Certain scans show the structure, some function, and some both
- 3 factors to consider regarding neuroimaging:
1) Availability
2) Practicality and cost
3) Side effects
Orientation. Images in what planes are needed to generate an image of the brain?
- Orientation
- Images in the sagittal, transverse, and coronal plana are needed to generate an image of the brain
Computerized tomography (CT) scans.
What do CT scans use to generate images of the body?
What are 2 advantages of CT scans?
What is a disadvantage of CT scans?
- Computerized tomography (CT) scans.
- CT scans use X-rays (only in the axial plane) and a computer to create detailed images of the body
- 2 advantages of CT scans:
1) Fast
* Takes about 0.3 seconds to image a single slice
* Up to 64 slices per rotation with 64 slice CT scanner
2) Widely available
- Disadvantage of CT scans - Ionizing radiation
How is radiation dose measured?
What is the head CT radiation dose?
What is radiation risk factor?
What should we consider in the follow up for a tumour?
What is photon-counting CT?
- Radiation is measured in Sievert (Sv) unit
- CT head radiation dose ≈ 200 x CXR (chest x-ray) dose
- Radiation risk factor = total lifetime risk of radiation-induced fatal cancer for the general population (0.05% per mSv (1 in 20000))
- Consider follow-up scan to assess a tumour
- Photon-counting CT is a new advancement in CT that uses far lower doses of radiation, but it is very rare and is currently not available in the UK
What planes are images acquired in for CT scans?
What is radiological convention?
What is the radiological convention for CT scans?
- Images in CT scans used to be acquired in the Orbitomeatal plane, now in the supraorbitomeatal plane (line from the top of the orbit to the External acoustic meatus (EAM))
- This change is due to radiation affecting the lens of the eye in the orbitomeatal plane
- Images are acquired in the axial plane but can be reconstructed
- Radiological convention describes the orientation of scans, so that we know where structures are located
- The radiological convention of CT scans are axial slices viewed ‘as if from the foot of the bed’ - Right side of image is the Left side of the patient and vice versa
What scale are CT scans?
How are CT scans generated? What is HU?
What tissues is the CT not good at displaying?
Why is this?
- CT scans are grey scale x-ray images, meaning they range from very bright (white), to very dark (black)
- CT scans are generated by absorption, with each type of tissue differing in absorption of x-rays, which each correspond to a shade of grey on the image
- The Hounsfield unit (HU) is a relative quantitative measurement of radio density used by radiologists in the interpretation of CT scans
- The CT scan has a narrow range for soft tissues, leading to poor differentiation
- This is because there is little difference between the highest HU value of white mater and the lowest HU value for grey mater
Magnetic resonance imaging (MRI).
How common is MRI in neurology?
What does clinical MRI rely on?
Why is this?
What does MRI use to produce images?
What effect can the RF pulse used in MRI cause?
- Magnetic resonance imaging (MRI) –
- MRI is the 2nd most common radiological technique for neurology
- Clinical MRI relies on hydrogen
- This is because:
1) Has a single proton in its atom
2) Found in abundance in the body good source of signal - MRI uses a combination of magnetic fields and radio waves to produce images
- The RF (radiofrequency) pulse used in MRI is non-ionizing but may cause heating effect
What planes can MRI images be acquired in?
How are axial and coronal images viewed?
What is the orientation for these planes?
What is the line used for standardisation in MRI scans?
- MRI images can be acquired in 3 planes (including oblique)
- Axial images: Viewed `as if from the foot of the bed’
- Right side of image is the Left side of the patient and vice versa*
- Coronal images: Viewed `as if you are looking at the face of the patient’
- Right side of image is the Left side of the patient and vice versa*
- The line used for standardisation is the AC-PC line (anterior-posterior commissure
What is needed in MRI scans to differentiate between tissues?
What 3 properties of tissues affect contrast?
How can these bs used to contract tissues?
What is a pulse (MR) sequence?
- Contrast between different tissues/structures/pathologies is needed for MRI scans
- 3 properties of tissues affect contrast:
1) T1 time (T1 recovery)
2) T2 time (T2 decay)
3) Proton density (PD) - MRI images can be T1-, T2-, or PD weighted in order to create a well contrasted image for a particular tissue, and in order to do this, we need a pulse sequence
- A series of RF and magnetic field applications is called a pulse (MR) sequence
What are T1- and T2- weighted MRI images good for?
What do different structures appear like on these images?
- T1-weighted image: good for anatomy and contrast:
- Water (CSF) is dark, grey matter is dark grey, white matter is off-white, fat is white
- T2-weighted image: good for identifying pathology – inflammation, oedema
- Water is white, white matter is darker than grey matter (‘WW2’ – Water is White on T2)
T1- weighted MRI images
T2- weighted MRI images
How is MR angiography conducted?
What type of angiography doesn’t require contrast agent?
Why is this?
What else is contrast agent used for?
What is the role of positive and negative contrast agents?
What are examples of each?
What can happen to positive contrast agents?
- MR Angiography can be with or without contrast agent
- Inflow angiography (aka Time-of-flight) and phase-contrast angiography does not require contrast agent
- Can over-exaggerate stenoses, underestimate true lumen size
- Contrast agents are not only for angiography but also to enhance contrast between tissues
- Positive contrast agents produce an increase in signal intensity in affected tissues e.g Gadolinium (chelated)
- Potential to build up in the body
- Negative contrast agents produce a decrease in signal intensity in affected tissues - Contain iron oxide (coated)
What is Diffusion weighted imaging (DWI)?
What are 4 uses of DWI?
- Diffusion weighted imaging (DWI) is a derivative of MRI that relies on diffusion of water molecules
- 4 Uses of DWI:
1) Areas of cerebral infarction have decreased diffusion, which results in increased signal intensity in DWI
2) Standard imaging for early (within mins) detection of ischaemia/infarct/stroke,
3) Differentiation of brain tumours
4) Intracranial infections
What type of process is Apparent diffusion coefficient (ADC)?
What is it a measure of?
How is ADC commonly calculated?
How does ADC contrast compare to DWI contrast?
- Apparent diffusion coefficient (ADC) is a complementary process of DWI
- ADC is a Is a measure of the magnitude of diffusion of water molecules within tissue
- ADC is Commonly calculated using diffusion-weighted imaging (DWI)
- ADC values are calculated automatically by the software and then displayed as a parametric map
- The image contrast on ADC maps is opposite to that of DWI
What is the role of Inversion recovery (IR, STIR, FLAIR) in MRI scans?
What tissues does this affect?
What is this helpful for?
- Inversion recovery (IR, STIR, FLAIR) nullifies the signal from certain tissue types based on their inversion timings
- Tissues such as fat and CSF do not appear bright signals with IR
- This is helpful to identify the pathology
What type of technique is tractography?
Why is this useful?
- Tractography is a 3D reconstruction technique to show/assess axons/tracts using data related to diffusion of water collected by diffusion MRI
- This is a good technique for neurosurgeons, as it shows how they should approach and how conservative they should be e.g can see if their approach will affect the pathway between Wernicke and Broca’s areas will be affected
What happens during functional MRIs (fMRI)?
What is it used for?
How does it do this?
- During functional MRIs (fMRI), the patient is asked to perform certain activities and the areas of the brain responsible for these actions lights up
- fMRI is used to obtain functional information by “visualising” cortical activity
- This is done by detecting subtle alteration in blood flow or blood oxygenation (deoxy/oxyhaemoglobin) in response to stimuli or actions
- This can be detected due to the haem in blood, which is magnetic
What are SPECT and PET scans?
How do they work?
How do both of these scans compare?
- SPECT and PET scans are perfusion techniques that require intravenous contrast agent
- They show the metabolic or biochemical function by measuring radiotracer uptake (usually glucose) by the tissues (cancer, infection, coronary artery disease, brain disorders)
- PET offers better spatial resolution, higher diagnostic accuracy and lower patient dosimetry than SPECT but it is very expensive
What are we considered about in A - Adequacy, Alignment, Artefact?
What are 4 sources of artefacts?
- A - Adequacy, Alignment, Artefact:
- Adequacy and alignment - Can we see the brain from top to bottom
- 4 Sources of artefacts:
1) Motion – confused patient, children
2) Beam hardening – posterior fossa particularly
3) Medical – metal clips (e.g for aneurysm), intraventricular shunts
4) Aliasing – caused by improper parameters
MRI artefacts
B – bones. What is the bone window on CT scan?
How does MRI show bones?
- B – bones
- The bone window on CT scans are the parameters needed (e.g voltage, time) in order to show bones rather than soft tissues
- MRI shows bones indirectly, as they don’t have water
- MRI highlights the structures around bones in order to make the bones visible e.g fat
B – blood
B – blood
B – Brain.
What are the 3 things we are looking at for the brain in neurological scans?
- B – Brain
- 3 things we are looking at for the brain in neurological:
1) Mass
2) Poor grey-white matter differentiation
3) Loss of sulci / atrophy
C – CSF filled spaced.
What are 2 examples of CSF-filled locations we look at for pathologies on scans?
- C – CSF filled spaced:
- 2 examples of CSF-filled locations we look at for pathologies on scans:
1) Subarachnoid haemorrhage – blood collects in cisterns
2) Obstructive hydrocephalus – blockage in ventricular system leads to enlargement of lateral ventricles
S – Subcutaneous & Soft tissues.
What 4 subcutaneous/soft tissue areas are we checking on neurological scans?
- S – Subcutaneous & Soft tissues
- 4 subcutaneous/soft tissue areas are we checking on neurological scans:
1) Skin/subcutaneous lesions e.g masses underneath skin
2) Orbit e.g masses on eye
3) Sinuses e.g sinusitis
4) Nasopharynx & oropharynx
