basic imaging of the cns Flashcards
what are some of the landmarks which help us identify which part of the brain you’re looking at?
lateral fissure, central sulcus, tentorium cerebelli
what are supratentorial structures?
structures above the tentorium
what are infratentorial structures? name some
below the tentorium. E.g. cerebellum, brainstem, pons and medulla
what does the frontal lobe contain?
motor cortex (post-central gyrus) pre-frontal cortex
what does the motor cortex control?
direct control of voluntary muscles via the brainstem and cord
what do neurons of the motor cortex give off?
o Neurons give off myelinated axons which condense and become closely packed together to form corticospinal tracts. Run through white matter tracts (internal capsule) through midbrain + decussates at cranio-cervical junction
what do strokes affecting the medial aspect of the motor cortex affect?
the lower limb
what do strokes affecting the lateral aspect of the motor cortex affect?
the face/upper limb is more affected
what does the parietal lobe do?
integration of sensory information
what does the parietal lobe do?
- Primary somatosensory cortex (Post Central Gyrus)
- Visuospatial Processing
- Complex Sensory Processing
what is the most common view used in CT scans?
axial
what effect do lesions to the frontal lobe have?
motor effects
what effect do lesions to the parietal lobe have?
sensory effects
what are the functions of the temporal lobe?
Auditory perception, language, speech, memory, emotional response
what does the occipital lobe contain?
primary visual cortex
how do the lens of the eye look in an MRI and why?
very dark
doesnt contain a lot of free water
what are the ventricles?
communicating cavities in the brain filled with CSF. 2 lateral ventricles, one third ventricle and one fourth ventricle.
what is another name for the interventricular foramen?
foramina of monro
what does the interventricular foramen connect?
connect 3rd ventricle to lateral ventricles
what happens when the interventricular foramen is blocked?
lateral ventricles to blow up –> hydrocephaly
what lesion commonly leads to the interventricular foramen becoming blocked?
colloid cyst
what is the foramen of magendie?
foramen which drains CSF from 4th ventricle into the cisterna magna
what are the foramina of luschka?
two foramina that link the 4th ventricle to the cerebellopontine cistern.
what does the cerebral aqueduct connect?
connects third and fourth ventricles
what does the 4th ventricle look like on an MRI?
horseshoe
what are the basal ganglia and where are they found?
Deep grey matter structures. Situated at the base of the forebrain –> group of subcortical nuclei
what commonly causes haemorrhages of the basal ganglia?
hypertension
where are the thalami?
Large gray matter structure in the caudal part of the forebrain
what do the thalami do?
Relays sensory and motor signals to the cerebral cortex and is involved in regulating consciousness.
what is the internal capsule and where is it found?
white matter tract. Inferomedial part of each hemisphere. Fibres to and from C. cortex.
what does the corticospinal tract do?
carries motor information from primary motor cortex to lower motor neurons.
what is the function of the cerebellum?
Motor control, coordination, posture/balance, equilibrium
where is the cerebellum?
beneath the tentorium
what blood vessel supplies the cerebellum?
basilar artery
how does the basilar artery present on an MRI?
black hole
what makes up the brainstem?
midbrain, pons and medulla
cranial nerve nuclei 3-12 and the white matter tracts
how often are x-rays used for the skull? in what situations are they used?
rarely
Depressed skull # or penetrating injury only
what type of imaging is used in acute situations?
CT
explain how a CT is carried out?
Person placed in machine that consists of a rotating X-ray source: travels in a helical motion to image a full body to the array of detectors that lie underneath
how is attentuation measured in a CT?
Houndsfield Units
how do different mediums appear on a CT and what are their houndsfield units?
o Air – very black (-1000) o Fat – black (-100) o Water – dark grey (0) o Brain – grey (+40) o Blood – white (+90) o I Contrast – white (+100) o Bone – very white (+1000)
why are CTs used in acute trauma?
v sensitive to trauma and blood
how does an actute haemorrhage look on a CT?
white
how does an acute extradural hematoma look on a CT?
lens shaped
what happens in an acute extradural haematoma?
Build- up of blood occurs between the dura mater and skull. As the haematoma expands, it peels the dura away off the bone and presses in on the brain
what is coning?
when blood presses down from the hematoma through the foramen magnum and down the brainstem
what happens in a left sided subdural hematoma?
collection of blood below the inner layer of the dura but external to the brain and arachnoid membrane. Falx cerebri is shifted to the side from the midline.
what is a hypertensive bleed?
weakening of vessels due to high blood pressure: causes them to leak.
what is a subarachnoid haemorrhage?
bleeding into the subarachnoid space normally due to a rupture of an aneurysm.
why does a subarachnoid haemorrhage lead to hydrocephalus?
o Blood in the subarachnoid space stops the resorption of CSF
what is an amyloid angiopathy and where is it found?
superficial, found in the cortex/grey matter
what people get amyloid angiopathies?
elderly patients get it
how can aneurysms be detected?
CT scans
how can CT scans be used to find aneurysms?
• Blood vessels filled with contrast medium so it shows up on a CT (contains iodine bc its absorbed by x-rays). 100ml of contrast injected into a vein with a pump and wait for it to go up to the brain. As soon as it enters the brain, a CT is taken (so only arteries show up, not veins)
if there are many aneurysms, how do you know which one bled?
the largest one bled
what are the advantages of CT scans?
Excellent Bone Detail Good for blood and calcium observation Limited soft tissue detail Vascular/perfusion in 3D CT guided biopsy
what are the disadvantages of a CT scan?
X-Ray dose (can be dangerous)
Limited soft tissue resolution (can also be bad).
what magnetic field do mris use?
constant 1.5 Tesla static magnetic field
how is the radiofrequency field pulse applied in mris?
via a coil
how is an image created in MRIs?
o Every water proton acts as a bar magnet.
o H+ line up with the magnetic field (which runs along the main Z axis of the body). H+ ions spin.
o When the RF is turned off, H+ stop spinning and emit an RF back to the coil and an image is created.
what magnetic items are not allowed in the room during an mri?
pacemakers, metal hip replacements, shrapnels from warzones, cochlear implants, aneurysm clips (the ones in the last 10 years are safe to put in)
what pulse sequences are used in MRIs?
T1 and T2
how does CSF look in a T1 weighted MRI?
black
how does CSF look in a T2 weighted MRI?
white
what is an advantage of T1W?
allows for better anatomy
what is a disadvantage of T1W?
less sensitive to pathology than T2W
what substance can T1W identify?
fat
what contrast agent is useful in T1 weighted?
gadolinium
what pathologies is T2 weighted sensitive to?
can see the demyelination of axons occurring in MS; tumours, inflammation, infection, ischaemia, scarring
what are characteristics of T2W?
High sensitivity, low specificity.
how does bacterial meningitis look on imaging?
not normally seen on imaging
how does TB meningitis look on imaging?
severe inflammation / thickening of meninges around the basal cisterns
what does diffusion weighted imaging look at?
looks at water diffusion
why is diffusion weighted imaging used in acute stroke treatment?
as soon as an area of brain loses its brain perfusion, the cells become hypoxic and water is trapped inside them
where are white matter tracts found?
- Connecting the two hemispheres
- Connecting the brain to the spine
- Connecting adjacent parts of the brain
what are advantages of MRI?
Excellent characterization of lesions
Imaging in any place (axial, coronal, sagittal)
No radiation
what are disadvantages of MRI?
Prolonged scanning time
Strong magnetic field
give reasons for CT being better than MRI?
Quick Easily available 1st line Acute trauma, stroke and blood Perfusion Radiation
give reasons for MRI being better than CT?
Time consuming Multiplanar Strong magnetic field. No radiation Functional Spectroscopic
