18.2. Imaging Flashcards
What are some different methods of imaging the brain in vivo?
- Positron Emission Tomography (PET)
- Magnetic Resonance Imaging (MRI), Spectroscopy (MRS) and Functional (fMRI)
- X-ray Computed Tomography (CT)
- Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (TDCS) - non-invasive procedure
- Magneto and Electro - Encephalography (MEG/EEG/ERP)
What different features of the brain can we look at through imaging?
- Anatomy
- Blood flow or perfusion
- Brain metabolism
- Receptor density
- White matter tracts
- Brain biochemistry
- Essentially: Strucure and Function
Summarise how different imaging methods range from covering anatomy to biology, and what the relative scales of resolution are.
What are the features of MRI?
[EXTRA?]
- Magnetic Resonance Imaging
- Invented in the early 1970’ s
- Uses the established technique of nuclear magnetic resonance (NMR) to generate images of the human body
- NMR is an old term for MRI
- Does not involve ionising radiation
- Images can be sensitised to physiology
- e.g. Blood flow, free water diffusion, blood oxygenation levels.
- As these features can differ between different brain regions, it can give an insight into their properties
- Nobel Prize for Medicine 2003 Sir Peter Mansfield (English physicist) for inventing MRI
- MRI measures signals from protons in water and fat
- How the protons react to the magnetic field informs us about their surrounding environment, so different signals can be perceived and associated with different structures
- High water concentration (55M) allows for small pixels (~1mm^3 - fairly high resolution)
What is the resolution of MRI and what allows this?
- Small pixels of ~1mm^3 are allowed
- This is through the high water concentration of the body (55M)
What are some measurements enabled by MRI?
- Morphometry (lesion size, cartilage volume)
- Relaxometry (T1, T2)
- Pathology (tumour, MS lesions, joint damage)
- Perfusion (tumour, stroke, depression)
- Flow (angiography, cardiac output, velocity)
- Diffusion (stroke treatment, white matter disorders)
- Hyper-polarized gas (Lung)
- functional MRI (focal injury, psychiatric, pain)
- Spectroscopy (pathology, metabolism)
What are some applications for MRI?
How are MR signals generated?
[EXTRA]
- When protons are placed in a magnetic field they oscillate
- The frequency at which they oscillate depends on the strength of the magnetic field
- 3-5 Tesla scale used in hospitals, but the larger the magnetic field, the higher the resolution of the image (as it will ensure that more protons change energy states/oscillate, therefore a larger signal is generated - 7 Tesla MRI machine in the lab)
- Unpaired spin in protons allows them to interact with the magnetic field and oscillate
- Protons are capable of absorbing energy if exposed to electromagnetic energy at the frequency of oscillation (radiofrequency)
- The energy put in is non-ionising, and only flips the spins of the protons so that they now oppose the magnetic field
- After they absorb energy, the nuclei release or reradiate this energy so that they return to their initial state of equilibrium (as they always tend towards the lowest energy state)
- The energy is provided in a pulsing manner
- This re-radiation or transmission of energy by the nuclei as they return to their initial state is what is observed as the MRI signal and can be converted into an image
- As the proton changes from one energy state to another, the vectors of its amplitude change (one decreasing, one increasing) - it is these two different vectors that are the ‘relaxation times’ that give T1 and T2 signals on MRI scans
- This return of nuclei to their equilibrium state not instantaneous – occurs over time
- The strength of the MRI signal depends primarily on three parameters:
- Density of protons in a tissue: The greater the density of protons, the larger the signal will be
- T1 relaxation time
- T2 relaxation time
- THESE PARAMETERS FORM THE BASIS OF T1-WEIGHTED, T2-WEIGHTED AND PROTON DENSITY WEIGHTED MRI
- The contrast between brain tissues is dependent upon how these 3 parameters differ between tissues
- For most “soft” tissues in the body, the proton density is very homogeneous and therefore does not contribute in a major way to signal differences seen in a image
- However, T1 and T2 can be dramatically different for different soft tissues, and these parameters are responsible for the major contrast between soft tissues.
- T1 and T2 are strongly influenced by the viscosity or rigidity of a tissue
- Generally speaking, the greater the viscosity and rigidity, the smaller the value for T1 and T2
- Contrast is important for a good image, so when targeting particular structures it can be beneficial to consider whether T1 or T2 weighting will provide a better contrast
- It is possible to manipulate the MR signal by changing the way in which the nuclei are initially subjected to electromagnetic energy
- This manipulation can change the dependence of the observed signal on the three parameters: proton density, T1 and T2
- Hence, one has a number of different MR imaging techniques (“weightings”) to choose from, which accentuate some properties and not others
![](https://s3.amazonaws.com/brainscape-prod/system/cm/341/650/318/a_image_thumb.jpg?1617544592)
What features are bright and which are dark on a T1 weighted MRI image?
[IMPORTANT]
Bright on a T1 weighted image:
- Fat
- Subacute hemorrhage
- Melanin
- Protein-rich fluid
- Slowly flowing blood
- Paramagnetic substances: gadolinium, manganese, copper
- Calcification (rarely)
- Laminar necrosis of cerebral infarction
Dark on a T1 weighted image:
- Increased water, as in edema, tumor, infarction, inflammation, infection, hemorrhage (hyperacute or chronic)
- Low proton density, calcification
- Flow void
Which features are bright and which are dark on a T2 weighted MRI image?
[IMPORTANT]
Bright on T2 weighted image:
- Increased water, as in edema, tumor, infarction, inflammation, infection, subdural collection
- Methemoglobin (extracellular) in subacute hemorrhage
Dark on T2 weighted image:
- Low proton density, calcification, fibrous tissue
- Paramagnetic substances: deoxyhemoglobin, methemoglobin (intracellular), iron, ferritin, hemosiderin, melanin
- Protein-rich fluid
- Flow void
What are some simple tissue characteristics when being imaged in MRI T1, MRI T2 and X-ray/CT for normal tissue?
[IMPORTANT]
What are some simple tissue characteristics when being imaged in MRI T1, MRI T2 and X-ray/CT for abnormal tissue?
[IMPORTANT]
What are some issues with imaging pathology using MRI?
- The relationship between the signal change, current underlying pathology and clinical status
- This means that it can be difficult to predict so it is frequently best to take a range of weightings and observe what results are obtained
- Conventional MRIs can also be slow in showing some pathology - for example, MRI imaging is frequently not overly beneficial during the acute phase of a stroke
What is the pathology shown in these images?
- NB that if relaxation agents (contrast agents) such as gadolinium are used, there are huge changes in local relaxation parameters at specific areas of interest/abnormal tissue
- E.g. if there is a BBB leak, gadolinium will exit the brain and affect the local conditions
- Due to its unpaired spin, Gd will distort the magnetic field at its location and change how surrounding protons relax
What is the best weighting to view MS lesions?
[EXTRA]
- T2 weighted images provide the best contrast to detect white matter tract lesions
What can be seen in this scan?
[EXTRA? Possible MCQ]
- Sudden onset = likely stroke
- If similar presentations but slower onset, consider degeneration or tumour
What can be seen in this scan?
[EXTRA? Possible MCQ]
- NB MR is not great at showing strokes in the acute phase
- This lead to the development of diffusion-weighted imaging, which looks at the diffusion of water within the brain and is thought to be more sensitive
What can be seen in this scan?
[EXTRA? Possible MCQ]
- Anaplastic astrocytoma
- Left parietal lesion, found at biopsy to be an anaplastic astrocytoma
- The T2-weighted image shows poorly circumscribed mass with core of mixed high and low signal, a rounded periphery of higher signal, and a component of infiltrating edema, appearing as somewhat lower signal
- Tumour appears to centered in white matter underlying the left post-central gyrus
What can be seen in this scan?
[EXTRA? Possible MCQ]
- Anaplastic astrocytoma (Gd contrast)
- The core of lesion contains elements which enhance with gadolinium
- Note extension of enhancing portion of the mass to right hemisphere
- T1-weighted image - highest signal comes from water whose relaxation time shortened due to proximity to injected contrast agent gadolinium-DTPA
- Such signal generally seen from water which is either intravascular or within regions of breakdown of the blood-brain barrier
How can radioligands be used to visualise the brain (e.g. BBB breakdown or tumours)?
[EXTRA?]
- If radioligand is used (e.g. Thalium-201, a potassium analogue), it can ‘leak’ into regions of the brain in which there are active tumour growth
- Multimodal imaging (e.g. in combo with MRI) can improve localisation of lesions
- Note on the image: two large foci of red-coloured activity in the sites of 201-Tl uptake
What do these scans show?
(Pathology on left scan, right scan is an age-matched control)
- Can see shrinkage of sulci in left image, lots of (white) CSF filling the gap
- Age matched control still shows a small amount of shrinkage, but this is normal/associated with the ageing process