Imaging the brain Flashcards
How can we image the brain?
CT and MRI
*can also use functional imaging, plain films and ultrasound
Why would you use a CT?
*4 scenarios and what the CT would be able to identify
- Acute stroke: exclude a bleed
- Trauma: to determine whether there is a fracture or an extradural, subdural, subarachnoid,intracerebral bleed
- Abnormal neurological signs/symptoms: identifies whether there is a space occupying lesion (i.e primary brain tumour, metastasis, abscess, etc)
- Acute headache: identifies a subarachnoid hemorrhage
Name scenarios where you would perform an MRI
- Acute stroke
- Suspected demyelination disease (i.e MS)
- Space occupying lesions: with contrast can identify the size, nature and extent of surrounding brain involvement
- Post operative tumour surgery
- Pituitary gland: identify abnormalities and relation to surrounding structures (like optic chiasma)
- Epilepsy: to identify if there is an underlying structural cause
- Congenital abnormalities of the brain
- Vascular abnormalities
How does dense tissue and less dense tissue appear on a CT? Provide an example of each
Dense: more X rays absorbed so less reaches the detector - appears white, i.e bone
Less dense: less X rays absorbed so more reaches detector - appears black, i.e air
What is a Hounsfield unit? What HU values correspond with air, water and bone?
A measure of the density of a structure on a CT (depends on how much of the X ray beam is absorbed)
*more dense = more positive HU
Air: -1000 HU
Water: 0 HU
Bone: +1000 HU
What are the Hounsfield units (HUs) for the lung, fat and soft tissue (including blood)
Lung: -500 HU
Fat: -50 HU
Soft tissue and blood: +50 HU
How do the HU values for CSF, white matter, grey matter and clotted blood change?
CSF: slightly more dense than water so HU +15
Grey matter (+40) more dense than white (+25)
Clotted blood ~+80
What HU values must a structure have to appear as white or black? What HU range produces the spectrum of black-white?
White: +80
Black: -5
Spectrum black-white: 0-+70
Simply describe the structure of an MRI
A large magnet tube that produces a magnetic field which is suspended in liquid helium
Gradient coils are within that produce a smaller magnetic field and can focus/alter the large magnetic field
Radio frequency coils: fires radio frequencies/waves at the patient
Describe (as simply as possible) how an MRI works
*including a definition of T1 and T2
Body is filled with H20 and H+ acts as a magnet, the protons align with the MRI’s magnetic field
MRI fires radio waves at the body which tips the protons out of alignment - so now the proton emits its own radio waves which are collected by the MRI. The longer the proton emits a radio frequency the brighter the image, different tissues emit radio frequencies for different amounts of time so the tissue can be differentiated
Eventually the proton stops emitting radio waves and goes back into the magnetic field alignment
Can either be T1 or T2 weighted
T1: the time it takes do go back into magnetic field alignment
T2: the time it takes to emit radio waves
What appearances are associated with a long and short T2 time and why? Provide an example for either ends of the spectrum.
Long T2 time means image will be white, like CSF
Short T2 time means image will be black, like bone
Which T1 values create bright and dark images and why?
What does T1 primarily provide?
Primarily provides the most anatomically-relevant images
The quicker the protons align with the magnetic field the brighter the image, therefore
Long T1: dark
Short T1: bright
Compare the appearance of fluid, white and great matter in T1 and T2
T1:
- fluid (in CSF and orbits) is dark, this is because it takes a long time to go back into magnetic field alignment
- Grey matter is darker than white matter
T2:
- fluid is bright (as it takes a long time to emit radio waves)
- White matter is darker than grey
