Estimating Oxygen Saturation of Blood in Vivo with MR
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Transcript Estimating Oxygen Saturation of Blood in Vivo with MR
Mark Elliott, PhD
Associate Director of CMROI,
Department of Radiology,
University of Pennsylvania School of Medicine, Philadelphia, PA
Overview
CMROI
Mechanisms of functional imaging with NIR
light
Methodology of fNIR
Comparison with and without Difuse Optical
Tomography (DOT)
Slide 2
Methods for Imaging Neural
Activity
metabolic response
electrical activity
- excitatory
- inhibitory
- soma action potential
electrophysiology
CMROI
- ATP tightly regulated
- glucose consumption
- oxygen consumption
FDG PET
H215O PET
hemodynamic response
- blood flow
- blood volume
- blood oxygenation
fNIR
EEG
fMRI
MEG
Perfusion MRI
Slide 3
Vascular Sensitivity of
fMRI and fNIR
Arterial
Venous
II
Intravascular
fNIR
I
Perfusion MRI
II
fMRI
III
IV
I
Extravascular
III
IV
Vessel Size
CMROI
Slide 4
Vascular Response
fMRI vs fNIR
fMRI
Spatial Resolution
8-27 mm3
Temporal Resolution
Slow (1-2 sec)
Measurement parameter
CMROI
Mix of blood volume, blood
flow, and O2 metabolism
fNIR
“Blobs”
1-10 cm3
Fast (50 Hz)
important?
[Hb] and [HbO]
Slide 5
Mechanisms of fNIR:
Overview
fNIR = functional Near InfraRed
Measure changes in infrared light absorption and scattering
Primary source of signal contrast [Hb] and [Hb0]
Biological tissue is highly scattering in NIR window
Primarily used in vivo as a spectroscopic modality
Not used to produce true images
DOT = Diffuse Optical Tomography
Methods for accurate image reconstruction
CMROI
Slide 6
Mechanisms of fNIR:
Absortion of [Hb] and [Hb0]
Water Absorption
• Near infrared “window” ~ 650-900 nm
• Water absorption is mimized
• Hemoglobin species are dominant absorbers
[Hb] & [HbO] Absorption
CMROI
Slide 7
Mechanisms of fNIR:
Beer-Lambert Law
Beer-Lambert law models ballistic photon propagation in absorbing media
Transmittance, T = I/Io
Absorbance, A = -log(I/Io)
Beer-Lambert Law:
A = [X] d
d
Io
where:
d = distance between I0 and I
= absorptivity (M-1 cm-1)
[X] = concentration of absorber (M)
I
solution
CMROI
[X]
Slide 8
Mechanisms of fNIR:
Modified Beer-Lambert Law
Photons travelling through biological
tissue are highly scattered (not
ballistic)
Scattering adds to “pathlength”
travelled by photons
Source
Detector
Detector
d
shallow
deeper
Modified Beer-Lambert Law:
(
Fat
Muscle
Source-detector spacing influences depth penetration
A = -log(I/Io) = [X] d DPF + G
where:
DPF = differential pathlength factor
G = Scattering loss factor (generally unknown)
CMROI
Slide 9
Mechanisms of fNIR:
Measures Changes in [Absorber]
• Scattering factor, G, is unknown
• Absolute concentrations are not derivable
• Can measure changes in [Hb] & [HbO]
• Need baseline assumption or independent measure of [Hb]
Measure [Absorber]
A2–A1 = -log(I2/I1) = [X] d DPF
where:
A2,A1 = absorption measured at two time points
CMROI
Slide 10
fNIR Methodology: Tissue
Penetration
• NIR light penetration into biological tissue allows for surface imaging
• Penetration increases with source light intensity
• Limits on safe levels of source light intensity (~1mW/mm2)
• SNR sqrt(Io)
• Highly sensitive detectors (PMTs) allow 2-6 cm deep probing
CMROI
Slide 11
fNIR Methodology:
Quantitation of Multiple Chromophores
Multiple absorbers ([Hb], [Hb0]) multiple wavelengths
Extension of MBLL to multiple absorbers:
(MBLL):
A1 = (Hb 1[Hb] + HbO1[HbO]) d DPF
A2 = (Hb 2[Hb] + HbO2[HbO]) d DPF
1 2 3
Source illumination is time or frequency
multiplexed at several wavelengths.
CMROI
Slide 12
fNIR Methodology:
Temporal Resolution
Extremely high temporal resolution possible
Practical systems ~ 10 – 100 Hz
fMRI ~ 1-2 Hz
Hemodynamic changes are slow ~ 2-5 sec
Fiber-optic systems for simultaneous fMRI
Fast signal – cell conformation and swelling
Scattering changes > 10 Hz
Extremely low signal
Ellusive to date
from Strangeman Biol Psych 2002
CMROI
Slide 13
fNIR Methodology:
Spatial Localization
Discrete arrays of sources and detectors
# voxels = # sources # detectors
Typical systems 10 – 100 voxels
Poorly localized “blobograms”
Resolution 1-8 cm3
Surface FOV
Compare to low-res fMRI: 64x64x30 217 voxels!
Whole brain coverage
CMROI
from Franceshini, NeuroImage, 2004
Slide 14
fNIR Methodology:
Spatial Localization with DOT
True tomographic methods ~ 10,000 S-D pairs
Flying spot illumation
(r)
CCD detection
Low temporal resolution ~10 – 100 sec / image
ill suited for functional assessment
“Hitting Density”, – poor basis set
undetermined inversion problem
A = (r) (r) dr
= Hb[Hb] + HbO[HbO])
CMROI
from Strangeman Biol Psych 2002
Slide 15
fNIR Methodology:
MBLL vs DOT
CMROI
Many fNIR implementations report [Hb]
changes from individual S-D pairs w/o
attempt at DOT
DPF in MBLL calculated from uniform
background absorption and scattering.
Focal changes not properly modelled.
“MBLL and DOT results did not agree in
terms of absolute magnitudes, relative
magnitudes, or even the relative sign for
changes in [HbO] and [Hb].” (Boas,
NeuroImage, 2001)
Slide 16
Spatial Maps of HRF Metrics:
TTP Maps
CMROI
Slide 17
fMRI: Mental Chronometry
ADC compartmentalization resolves events separated by 125ms.
TTP Map
1 second right fovea & auditory delay
CMROI
Slide 18