CT Perfusion
A rapid bolus of contrast is delivered.
The time/denisity curve for each pixel is analyzed.
CBV Cerebral Blood Volume, this is the area under the signal-time curve (positive enhancement integral) as the bolus travels through the tissue. For the boards, a decrease correlates with infarct (perfusion study version of diffusion series). In practice (consensus at ASNR 2012) it is less reliable.
MTT Mean Transit Time this is the width (so called “full width at half maximum”) of the signal-time curve. MTT is increased in infarct and ischemia. This is most useful for calculating the CBF
CBF Cerebral Blood Flow is calculated using the “central volume principle”
CBF=CBV/MTT
TTP Time to Peak is the time to the peak of the signal-time curve. As one would predict, with an infarct or ischemia this is increased as well.
so:
CBV and CBF matched defects are the “core” infarct
CBF defect without CBV defect is a mismatch and represents salvageable “penumbra”
Inservice Question (2009)
Q: Which of the following would you expect to find on CT perfusion 1 hour post acute
middle cerebral artery occlusion in the affected territory?
A:
A. Decreased time to peak (TTP)
B. Increased cerebral blood volume (CBV)
C. Decreased cerebral blood flow (CBF)
D. Decreased mean transit time (MTT)
A. Decreased time to peak (TTP) TTP should increase with ischemia
B. Increased cerebral blood volume (CBV) CBV should decrease with infarct
C. Decreased cerebral blood flow (CBF) CBF decreases with both ischemia/infarct
D. Decreased mean transit time (MTT) MTT may increase with ischemia while if there is really reduced flow, the MTT could decrease, this is definitely not a good choice.
MR Perfusion
The theory is essentially the same with one critical difference. Gadolinium causes paradoxical signal loss so a "negative enhancement integral is assessed.
Q: What does concentrated gadolinium (e.g. scanning the bottle or the appearance of dense contrast layering in the bladder look like on MR?
A: Gadolinium shortens both T1 and T2. At high concentrations, the T2 shortening effects predominate causing signal loss on T1 and T2 images.
In theory you should be able to determine if there is a penumbra, In practice the CBV map may give a false impression of the infarct
Reference:
Google Elster, Gadolinium, MRI Great article on the bladder by a Neuroradiologist!
http://radiology.rsna.org/content/174/2/379.long
Radiation Safety
Perfusion CT was in the news (NYT 7/30/2010)
http://www.nytimes.com/2010/08/01/health/01radiation.html?pagewanted=all
200-300 rads (or 2-3 Gray) can cause hair loss/skin reddening
References
Case-in-Point (matched defect)
http://3s.acr.org/CIP/CaseView.aspx?CaseId=aqNjG6YzWbo=
Radiographics (Dr. Libfeld 9/24/12)
Chpnet.org resources
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