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- Naveed Akhtar, MD
- Robert Blanch, MD
- Kristine Dettloff, MD
- Tony Lawrence, MD
- Aaron Lewis, MD
- Kay North, DO
- Daryl Pinedo, MD
- Julia Prescott-Focht, DO
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- The authors have nothing to disclose.
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- Upon completing this electronic exhibit, the reader should be able to:
- Describe the basic technique for performing perfusion imaging
- Define MTT, CBF, CBV, and TTP
- Interpret basic CT brain perfusion image
- Assess the extent of penumbra for clinical salvageability
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- Non-contrast CT head may show no findings to only very subtle findings
in a patient who is within the therapeutic window (with IV t-PA, IA
t-PA, or interventional radiology). We hope to identify those who would
benefit from early and aggressive therapy by identifying “at risk” brain
tissue.
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- Perfusion imaging is based on the theory that brain tissue can be
underperfused and still remain viable. This is called the ischemic
penumbra. This is the tissue that may go on to infarction if perfusion
is not restored. However, if perfusion is restored in adequate time, the
brain tissue may be saved and disability in the patient kept to a
minimum.
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- At our institution, CT perfusion is performed on a Philips Brilliance 40
CT scanner
- 40cc of Omni 350 is injected at 3cc/sec
- Immediate imaging of eight contiguous 5mm axial slices begins at the
thalamus and extends rostrally
- Image acquisition occurs every 2 seconds over 1 minute
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- Performed by the technologist:
- Midline is defined
- Location of the arterial input function (AFI) region of interest and
the venous input are identified.
- The anterior cerebral artery1 and straight sinus are
automatically identified by the computer
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- The technologist selects three parenchymal regions of interest in the
right hemisphere at three axial levels (regions 1-3)
- Computer mirrors these regions of interest in the left hemisphere
(regions 4-6)
- Caveat: interoperator variability when defining ROIs for CBV, CBF, and
MTT analysis may limit the use of quantitative criteria during
interpretation2
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- There are four main parameters to assess when looking at CT perfusion
imaging:
- Cerebral Blood Volume (CBV)
- Cerebral Blood Flow (CBF)
- Mean Transit Time (MTT)
- Time To Peak (TTP)
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- Indicates how much blood is flowing through the brain in a set period of
time
- Units are milliliters of blood per 100 grams brain tissue per minute
- Normal is between 50-100 ml/100g/min
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- The volume of blood within a specific volume of tissue
- Units are milliliters of blood per 100 grams brain tissue
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- Time required for maximal signal intensity change after infusion of a
contrast bolus
- Measured in seconds
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- Mean time for blood to enter and leave a given tissue
- Measured in seconds
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- When describing change in signal intensity as a function of time (or TTP
map)3:
- The slope of signal intensity change is an approximation of CBF
- The full width of the signal intensity change curve at half of maximum
value is MTT
- The area under the curve is proportional to the CBV
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- Usually fairly constant through auto-regulation
- When systemic blood pressure increases the cerebral vessels constrict
- When systemic blood pressure decreases the cerebral vessels dilate
- CBF will decrease only when vessels are maximally dilated and the
pressure continues to drop
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- Normal is 50-100 ml/100 g/minute
- <20 ml/100 g/minute will begin to develop neurologic failure but may
still be reversible
- <10-15 ml/100 g/minute will begin to develop cell death or
nonreversible injury
- CBF is reduced in both infarct tissue and ischemic tissue or penumbra
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- CBV will increase when the vessels are maximally dilated (remember this
occurs when pressure drops—the vessels dilate to try to maintain CBF)
- CBV will decrease when auto regulation no longer functions (this occurs
when the tissue is infarcted)
- When CBF cannot be maintained and CBV is at maximum, this is considered
to be the penumbra
- When CBF cannot be maintained and CBV decreases, this is considered to
be the infarcted tissue
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- Even minimal disruption in blood supply can cause increases in MTT and
TTP
- Ischemia, infarction, and vessel stenosis or vasospasm can cause
increase in MTT and TTP
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- We can do perfusion imaging before and after acetazolamide
administration.
- This is based on the theory that at risk areas have maximally dilated
vessels already (maximum CBV so CBF is maintained). When acetazolamide
is given, the pressure will drop and therefore CBV stays the same and
CBF will also decrease.
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- By comparing the before and after images we can identify “at risk”
tissue. We can then search for the stenosis using angiography and
hopefully repair it before and stroke occurs. In cases of known
stenosis, this will help quantify how significant a lesion may be and
help us in the decision making of what may be the proper intervention
for the patient (which may range from aspirin to stenting to
endarterectomy).
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- There are multiple indications for CT Perfusion, which include the
following at our institution:
- Stroke evaluation
- Assessment of vasospasm severity and progression
- ECIC bypass evaluation and follow-up
- Prior to performing intracranial stenting procedures and afterward to
evaluate for stent function
- Tumor evaluation (although not routinely used at our institution)
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- There is low sensitivity for disease outside of limited area scanned.
- MTT is directly calculated by using a deconvolution algorithm, which
could vary among vendors (i.e. GE, Philips, Toshiba, Siemens).
- Appropriate venous output location is important:
- Corrects arterial input function for partial volume averaging effects.
- Its peak enhancement value correlates with signal-to-noise ratio of CBF
and CBV maps.
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- Xenon CT/PET CBF normal and disease threshold values are used in CT
perfusion. Equivalency has not been validated.
- Variation created by location of arterial input ROI.
- Uncertainty in cases with
diffuse atherosclerotic disease and vasospasm.
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- CBF and CBV are both decreased in infarcted tissue.
- CBF is decreased and CBV stays the same in ischemic tissue (penumbra).
- MTT/TTP are increased in ischemic and infarcted tissue.
- Find the CBF/CBV or MTT/CBV mismatch to determine penumbra.
- Identify the penumbra so perfusion can be restored and tissue viability
maintained.
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- Choose the answer that best describes Cerebral Blood Flow (CBF):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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37
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- Choose the answer that best describes Cerebral Blood Flow (CBF):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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38
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- Choose the answer that best describes Cerebral Blood Volume (CBV):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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39
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- Choose the answer that best describes Cerebral Blood Volume (CBV):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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40
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- Choose the answer that best describes Time to Peak (TTP):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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41
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- Choose the answer that best describes Time to Peak (TTP):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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- Choose the answer that best describes Mean Transit Time (MTT):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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43
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- Choose the answer that best describes Mean Transit Time (MTT):
- The volume of blood within a specific volume of tissue
- Indicates how much blood is flowing through the brain in a set period
of time
- Mean time for blood to enter and leave a given tissue
- Time required for maximal signal intensity change after infusion of a
contrast bolus
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- What steps must the technologist perform?
- Locate the ventricles and select bilateral regions of interest
- Locate the anterior cerebral artery and straight sinus
- Define the midline and locate arterial and venous input
- Manually time the contrast bolus injection
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45
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- What steps must the technologist perform?
- Locate the ventricles and select bilateral regions of interest
- Locate the anterior cerebral artery and straight sinus
- Define the midline and locate arterial and venous input
- Manually time the contrast bolus injection
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- Locate the area of abnormal perfusion
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47
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- Locate the area of abnormal perfusion
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48
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- These findings indicate a large area of infarct (true/false)
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49
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- These findings indicate a large area of infarct (true/false)
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50
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- Wintermark M, Lau BC, Chien J, Arora S. The anterior cerebral artery is
an appropriate arterial input function for perfusion-CT processing in
patients with acute stroke. Neuroradiology. December 2007.
- Fiorella D, Heiserman J, Prenger E, Partovi S. Assessment of the
reproducibility of postprocessing dynamic CT perfusion data. Am J
Neuroradiol. January 2004;25(1):97-107
- Gonzalez, RG. Imaging-Guided Acute Ishemic Stroke Therapy: From “Time is Brain” to “Physiology is
Brain”. Am J Neuroradiol. April 2006;27:728-735.
- Wiesmann M. The Basics of the Method and Interpreting Images: CT
Perfusion of the Brain. Visions Journal.
September 2006. Available at:
http://www.visions-journal.com/GB-2006-V09/S06-08.pdf
- Choksi v, Quint DJ, Maly-sundgren P, Hoeffner E. Imaging of Acute
Stroke. Applied Radiology Online.
February 2005. Available at:
http://www.appliedradiology.com/articles/Article.asp?ID=1158&IssueID=138&ThreadID=
- Hanes TS. CT Angiography and Perfusion in Stroke. Applied Radiology
Online. June 2002. Available at:
http://www.appliedradiology.com/articles/Article.asp?ID=667&IssueID=88&ThreadID=
- Kim N, Ortiz O. Perfusion CT in the Assessment of Cerbrovascular
Disease. Applied Radiology Online.
April 2004. Available at:
http://www.appliedradiology.com/articles/Article.asp?ID=992&IssueID=126&ThreadID=
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Notes
