1 Perfusion Imaging Matthias Günther 1,2,3 1 Fraunhofer MEVIS, Institute for Medical Image Computing 2 Faculty 01 (Physics/Electrical Engineering), University Bremen 3 mediri GmbH, Heidelberg, Germany Overview Blood flow and perfusion definition Quantification of perfusion tracer-kinetics: steady-state, bolus-tracking Basics of spatial perfusion measurement techniques Pros and cons of imaging techniques Blood flow and perfusion Perfusion means transport of blood to unit volume of tissue per unit time Supplies cells with oxygen and other nutrients Important parameter for status and activity of tissue A lot of effort was put into measurement What is perfusion?
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Perfusion Imaging
Matthias Günther1,2,3
1 Fraunhofer MEVIS, Institute for Medical Image Computing 2 Faculty 01 (Physics/Electrical Engineering), University Bremen
3 mediri GmbH, Heidelberg, Germany
Overview
Blood flow and perfusiondefinition
Quantification of perfusiontracer-kinetics: steady-state, bolus-tracking
Basics of spatial perfusion measurement techniques
Pros and cons of imaging techniques
Blood flow and perfusion
Perfusion means transport of blood to unit volume of
tissue per unit time
Supplies cells with oxygen and other nutrients
Important parameter for status and activity of tissue
A lot of effort was put into measurement
What is perfusion?
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Background: why measure perfusion?
• All organs are critically dependent on blood supply for homeostasis
• Default in blood supply results in ischemia, or the lack of oxygenation to the organ
• This in turn leads to energy failure, cytotoxic edema, and cell death
Blood flow and perfusion
Macro-vascular: blood flow
volume per time
unit: [ml/min]
Micro-vascular: perfusion
latin: perfusio = to moisten
volume per time per tissue
unit: [ml/min/g]
Perfusion is often incorrectly called blood flow
Perfusion ≠≠≠≠ blood velocity ≠≠≠≠ blood volume!
What is the difference between blood flow and perfusion?
What is perfusion?
Circulation
Perfusion
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Measurement of perfusion
Basic idea of perfusion measurement
1. Use of tracer, either through breathing or intravenous injection
2. Tracer is transported to tissue of interest due to blood flow
3. Local tracer concentration is measure of perfusion
historically older technique, mostly attributed to technical
limitations
• Bolus-tracking (non-equilibrium)
more advanced, more complex modeling possible
Basic idea of perfusion measurement
Measurement of perfusion
Steady-State technique
Trying to reach constant level of tracer in body.
Continuous infusion instead of singular bolus (breathing of gas)
No modeling necessary to estimate relative perfusion, more
quantitative analysis using Kety-Schmidt-model
Limited possibility to generate additional parameter maps
Useful for ”slow” imaging modalities
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Measurement of perfusion
Steady-State technique
Kety-Schmidt model
Kety and Schmidt, Am J Physiology, 1945, 53-66. Kety and Schmidt, J Clin Invest, 1948, 27:476-483.
Historically used to model uptake of nitrous oxide (N2O, “laughing gas”)
Measurement of arterial and venous
concentration
Steady-state:
Ct(t) constant
Ct proportional to volume of tissue
Measurement of perfusion
ρ Partition coefficient for water: proportionality constant for tracer concentration in tissue
and blood
λ decay constant: depending on tracer, can be radioactive (e.g. PET: decay of 15O, ASL: T1-
relaxation of labeled magnetization, DCE: physiological half-life)
Arterial inflow Venous outflow decay
Equilibrium assumption:
Steady-State techniqueModification:
Imaging techniques allow measurement of Ct
Ce is not measured
but at equlibrium:
Measurement of perfusion
Arterial inflow Venous outflow decay
Calculate perfusion:
Needed is: tracer concentration Ca and Ct,
ρ partition coefficient for water,
λ decay constant
Steady-State technique
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Measurement of perfusion
Bolus-tracking technique
Also known as dynamic imaging approach or first-pass technique.
Method:
• Injection of tracer as fast as possible
• Observe passage of tracer bolus (venous outflow, tissue concentration)
• tracer kinetics for non-diffusable tracers (Zierler-Meyer-model)
• non-equilibrium
• time-dependent tracer concentration
Zierler KL. Fed Proc. 1965 Sep-Oct;24(5):1085-91.
Measurement of perfusion
Bolus-tracking technique
Method of evaluation depends on injection speed (length of bolus)Tofts PS,Berkowitz BA. "Measurement of capillary permeability from the Gd enhancement curve: a comparison
of bolus and constant infusion injection methods." Magn Reson Imaging. 1994;12(1):81-91
Measurement of perfusion
Bolus-tracking technique
Venous outflowArterial inflow
tissue
Various models, e.g.:
Single-compartment model
Two-compartment model
Three-compartment model
„tissue“-compartment
microvasculature
tissue
micro-
vasculature
extra-cellular
compartment
intra-cellular
compartment
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Measurement of perfusion
Bolus-tracking technique
Tracer-bolus Convolution with
Response-function tracer-concentration
in tissue
ctissue
time t
Ctissue
ctissuecart
= blood volume
amplitude ~ perfusion
model-free
after deconvolution
width = mean transit time (MTT)
Central volume theorem:
MTT=blood volume / perfusion
after deconvolution
Time to peak (TTP) after deconvolution
slope after deconvolution
Cart
What is perfusion? Parameters …
• The Cerebral Blood Flow (CBF, Ft, f ) can be defined as the steady state
delivery rate of blood to the tissue capillary bed. It is measured in
ml/min/100g
• The Cerebral Blood Volume (CBV) is the amount of blood in the tissue. It is
measured in ml/100g
– In physiology, the regional CBV (rCBV) is defined as the amount of blood in the
capillaries
• The Mean Transit Time (MTT) can be defined as: MTT = CBV / CBF. It is
measured in s.
• The Bolus Arrival Time (BAT) or Arterial Transit Time is the time taken by a
bolus of indicator to reach the tissue, measured in s.
Perfusion imaging approaches
Image-based perfusion estimation:
Necessary:
• tissue concentration of tracer Ct
• arterial concentration of tracer Ca
Difference steady-state and bolus tracking:
steady-state: in principle, only one measurement necessary (at equilibrium)
in practice, slow sampling, since equilibrium is not reached
bolus tracking: time-dependent Ct(t) and Ca(t)
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Overview
Spatial perfusion measurement techniques
• Single Photon Emission Computer Tomography (SPECT)