General
- More easily available than MRI.
- MRI still more sensitive in detecting early parenchymal changes
Indication
SAH associated vasospams
- Arterial territories with MTT and CTA suggestion of vasospasm are carefully assessed for a decrease in cortical CBF values.
- If spasms are present, → conventional angiogram for possible endovascular treatment.
- Pros
- As sensitive as, and more specific than performing Doppler alone,
- Allows to obviate unnecessary invasive angiograms in selected lower risk patients
Tumours
- Tumours has increased angiogenic activity and neovascularization that results in increased blood volume and hyperpermeability related to the immature vessels.
- Measurements of CBV and permeability surface product area (PS), a measure of microvascular permeability, were obtained from PCT show PS to be predictive of pathologic grade and to correlate with tumor mitotic activity
- Pros (over MRI)
- Has linear relationship between contrast agent concentration and attenuation changes,
- The lack of sensitivity to flow,
- The high spatial resolution, and
- The absence of susceptibility artefacts.
- Cons (compared with MR, for evaluation of the microvasculature)
- Ionizing radiation
- Potential for adverse reaction to the contrast agent,
- Limited anatomic coverage are limitations of CT
Cerebrovascular reserve
- Chronic cerebral ischemia related to underlying carotid artery stenotic lesions
Stroke
- Enables differentiation of salvageable ischaemic brain tissue (the penumbra) from the irrevocably damaged infarcted brain (the infarct core).
- Infarct core
- Irreversible and dead tissue.
- Defined as an area with
- Prolonged MTT or Tmax,
- Markedly decreased CBF and
- Markedly reduced CBV
- Note, that if one uses CBF alone to visually assess core size, it is easy to overestimate infarct core, as the penumbra often has reduced CBF also. So, even though some automated processes used CBF to define the core, CBV is a safer parameter if 'eye-balling' the scan.
- Ischaemic penumbra (Surrounds the infarct core)
- Prolonged MTT or Tmax
- Moderately decreased CBF and
- Near-normal or even increased CBV (due to autoregulatory vasodilatation)
3 parameters typically used in determining these two areas are
- Mean transit time (MTT) or time to peak (TTP) of the deconvolved tissue residue function (Tmax) 3
- Cerebral blood flow (CBF)
- Cerebral blood volume (CBV)
Normal perfusion parameters
- Grey matter
- MTT: 4 s
- CBF: 60 mL/100 g/min
- CBV: 4 mL/100 g
- White matter
- MTT: 4.8 s
- CBF: 25 mL/100 g/min
- CBV: 2 mL/100 g
How is it done?
- ??? Give contrast and do multiple CT scan over time to detect where the contrast has been to and how long it stays for
- It is in relation to the basal ganglia flow
Clinical
Normal patient
- (A) CT-C
- (B) CBF
- (C) CBV
- (D) MTT
- Blue is lower values
- Red is higher values
Stroke right side
(A) CT-C: some hypointensity in the posterior regions but nothing significant
(B) CBF: reduced CBF in the right hemisphere at front-parieto-occipital region
(C) CBV: reduced blood volume → the whole Rt MCA is a core
(D) MTT: increased transit time.
Stroke left parieto-occipital
(A) CT-C: nothing significant
(B) CBF: reduced CBF in the left parieto-occipital region
(C) CBV: no abnormalities → showing it is a penumbra
(D) MTT: increased transit time → showing it is damaged area
Vasospasm
(A) A1 segment of the right anterior cerebral artery is hypoplastic (black arrow)
(C) significant vasospasm of the M1 and M2 segments of the right middle cerebral artery (black arrow).
(B) CT perfusion
LU: CT'-'C: can see that there is Diffuse modified fisher 3 grade
RU: CBF LT fronto paretial region has dec. Blood flow → ischaemia
LL: CBV there is inc. Volume of blood over the Lt caudate → reversible
RL: MTT there is reduced transit time in the Lt fronto parietal region
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