Imaging of the CNS Flashcards
Which form of imaging has the highest spatial resolution?
Radiography
Which imaging modality has the highest contrast resolution?
MRI
What structure provides contrast within the vertebral canal?
The epidural fat
When may MRI be more beneficial for the imaging of bone than CT?
Infiltrative disease
What is the difference in MPR image acquisition in CT and MRI?
- With CT, sagittal and dorsal planes are reformatted/reconstructed after acquisition of transverse images
- With MRI, images for each anatomical plane are obtained using seperate acquisition
The spatial resolution of MRI sagittal and dorsal images may therefore be better than CT
What modality is usually considered the first line of imaging in trauma? Why?
What is the exception to this rule?
- CT as is it very good for identifying haemorrhage and fractures
- The exception may be in cases of TBI as MRI findings can be used to prognosticate
What are the commonly used contrast agents for CT and MRI?
- CT: iodinated contrast
- MRI: gadolinium-based contrast
What are the terms (-suffix) used to describe the level of brightness in radiography, CT and MRI?
- Radiographs = opacity
- CT = attenuation or denstiy
- MRI = intensity
What are the Houndsfield units of air, fat, water, brain, acute to subacute clotted blood, mineral and bone, metal?
List some causes of hypoattenuation on CT scan
- Cystic or fluid-filled
- Necrosis
- Oedema
- Fattu infiltration
- Gas
List some caused of hyperattenuation in CT
- Haemorrhage
- Mineral
- Metal
- Densely cellular/fibrotic
What produces the signals in MRI?
Mobile hydrogen atoms within the tissue
Define pulse sequencing
A series of timed events by which a radiofrequency pulse is used to creaste a signal
Which are the only pulse sequences upon which all others are built?
- Spin echo (considered the work-horse of clinical MRI and is used to produce T1W, T2W and proton density-weighted images
- Gradient echo
What is FLAIR?
Why is it useful?
- Fluid-attenuated inversion recovery - suppresses the signal from fluid
- Give the ability to distinguish pure fluid structures (nulled signal) from solid, but hihgh-water content lesions such as oedema within tissue (high signal)
- Improved conspicuity of hyperintense lesions compared with spin echo T2W images
WHat are STIR sequences?
Why is it useful?
- Short Tau Inverstion Recovery - supresses fat signalling
- Allows assessment of high water contect fluid or soft tissues against a background of suppressed fat
- Useful for vertebral and paravertebral soft tissue pathology
Whar are T2* sequences used for?
Useful for identifying haemrrhage or blood clots
What sequence is particularly useful for radiation planning?
spoiled gradient echo
What lesions is fat suppresion useful for?
- Lesions within fatty cancellous bone
- Lesions involving the meninges
- Nerve sheath tumours
- To discriminate between other structures which may be hyperintense on T1W and T2W images
What MRI sequences can perform angiography without the need for IV contrast?
- Time-of-flight (for rapis arterial flow)
- Phase-contract MRA (slow venous or CSF flow)
What is MRI spectroscopy?
A physiological MR technique that evaluates brain chemistry and metabolism
List the structures with high signal intensity on T1W and T2W images
What are the dose ranges for IV contrast for CT and MRI?
- Iodinated contrast 400 - 800mg iodine/kg
- Gadolinium - 0.1-0.15mmol/kg
What is normally enhanced on post-contract imaging of the CNS?
What are two options for imaging after contrast injection?
- Normally enhanced: Meninges, choroid plexus, pituitary gland
- Can obtain images immediately after injection to evaluate vasculater and after a delay of a few minutes to evaluate distribution or leakage
How does vasogenic oedema appear on CT, T1W and T2W MRI?
- CT: hypoattenuating
- T1W - hypointense
- T2W - hyperintense
Typically migrates alson the white matter tracts
What are are minimum sequences recommened to be acquired for brain MRI?
- T1W before and after gadolinium contrast
- T2W
- T2 FLAIR
- T2*
What are come age-expected normal varients on brain MRI?
- Enlargement of ventricles and subarachnoid space with advancing age
- Signal intensity reversal of white and grey matter in neonates (until about 16wk when myelination has progresses)
- Appearance of bones in neonates
What are some secondary cahnges associated with alteration in CSF flow?
- Ventriculomegaly
- Syringohydromyelia
- Spinal cord (presyrinx) oedema
List the three forms of secondary hydrocephalus
- Noncommunicating (obstruction of flow)
- Communicating (decreased resorption by arachnoid villu or increased production
- Compensatory (loss of brain parenchyma)
List some ways of categorising primary pathology of the brain
- Intra vs extraparenchymal
- Solitary, multifocal or diffuse
- Symmetric or random distribution
What is the mnemonic for general categories of neuro disorders?
MIIND
- malformations
- inflammation
- injury
- neoplasia
- degenerative
What are the recommended volumes for CT myelogram
- Using nonionic contrast media (iohexol or iopamidol) administer 0.45ml/kg (full spine) or 0.3ml/kg (regional)
- NO MORE than 8ml total (seizures)
What are the main lesion localisation within the vertebral column?
- Extradural
- Intradural-extramedullary
- Intramedullary
