10.3 Diffuse optical tomography and functional imaging

Diffuse optical tomography uses near-infrared light to peek inside our bodies. It's like having X-ray vision, but safer and better for seeing blood flow and oxygen levels. This tech can map brain activity, spot breast tumors, and keep tabs on patients in critical care.

The magic happens when light travels through tissue, getting absorbed and scattered along the way. By measuring how the light changes, we can create 3D images of what's going on inside. It's non-invasive and gives real-time info on tissue health and function.

Principles of Diffuse Optical Tomography

Near-Infrared Light Propagation in Tissue

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• Near-infrared spectroscopy utilizes light in the 650-900 nm wavelength range
• Light in this range penetrates deeper into biological tissues compared to visible light
• Penetration depth ranges from several millimeters to a few centimeters depending on tissue type
• Near-infrared light interacts with tissue through absorption and scattering processes
• Absorption primarily occurs due to hemoglobin, water, and lipids in tissue
• Scattering results from refractive index mismatches at cellular and subcellular structures

Tissue Optical Properties and Light Propagation Models

• Tissue scattering dominates over absorption in the near-infrared region
• Scattering coefficient (μs) quantifies the number of scattering events per unit length
• Typical values for μs in soft tissues range from 10 to 100 cm^-1
• Absorption coefficient (μa) measures the probability of photon absorption per unit path length
• μa values for most soft tissues in the near-infrared range from 0.1 to 1 cm^-1
• Diffusion approximation simplifies light propagation modeling in highly scattering media
• Assumes light transport can be described by a diffusion equation
• Valid when scattering dominates over absorption (μs >> μa)
• Provides a computationally efficient method for modeling photon transport in tissue

Measurement Techniques and Instrumentation

• Continuous wave (CW) systems use constant intensity light sources
• Measure changes in light intensity after passing through tissue
• Time-domain systems employ ultra-short light pulses (picoseconds)
• Measure temporal distribution of photons (time-of-flight)
• Frequency-domain systems utilize intensity-modulated light sources
• Measure amplitude attenuation and phase shift of detected light
• Detectors include photomultiplier tubes, avalanche photodiodes, and CCD cameras
• Multiple source-detector pairs arranged on the tissue surface enable 3D imaging

Reconstruction and Imaging

Image Reconstruction Algorithms and Techniques

• Linear reconstruction methods assume small perturbations in optical properties
• Utilize sensitivity matrices to relate changes in measurements to optical property changes
• Non-linear reconstruction methods iteratively solve the forward and inverse problems
• Forward problem calculates light propagation for given optical properties
• Inverse problem estimates optical properties from measured data
• Regularization techniques address ill-posedness of the inverse problem
• Tikhonov regularization adds a penalty term to the objective function
• Total variation regularization promotes piecewise constant solutions
• Iterative algorithms (conjugate gradient, Gauss-Newton) optimize the reconstruction process
• Multispectral reconstruction incorporates data from multiple wavelengths simultaneously

Oxygen Saturation Mapping and Functional Imaging

• Oxygen saturation mapping measures spatial distribution of tissue oxygenation
• Utilizes spectral differences between oxy- and deoxyhemoglobin
• Typically uses measurements at two or more wavelengths
• Calculates relative concentrations of oxy- and deoxyhemoglobin
• Oxygen saturation (SO2) computed as ratio of oxyhemoglobin to total hemoglobin
• Functional imaging tracks changes in hemodynamics and metabolism over time
• Measures variations in oxy- and deoxyhemoglobin concentrations
• Can detect local changes in blood flow and oxygen consumption
• Temporal resolution ranges from seconds to minutes depending on the system
• Spatial resolution typically 5-10 mm for deep tissue imaging

Clinical Applications

Functional Brain Imaging and Neurological Disorders

• Functional brain imaging measures cortical activation patterns
• Detects local changes in cerebral blood flow and oxygenation
• Applications include studying cognitive processes and language development
• Can be used to assess brain function in infants and children
• Advantages over fMRI include portability and tolerance of subject movement
• Neurological disorder assessment includes stroke and traumatic brain injury
• Monitors cerebral oxygenation and blood flow in critical care settings
• Potential for early detection of ischemia and guiding therapeutic interventions
• Limitations include lower spatial resolution compared to fMRI
• Depth sensitivity restricted to outer cortical regions

Breast Cancer Detection and Characterization

• Breast cancer detection utilizes differences in optical properties between healthy and tumor tissue
• Tumors typically exhibit increased blood volume and metabolism
• Higher concentrations of hemoglobin and altered scattering properties
• Can detect tumors as small as 5-10 mm in diameter
• Combines with other imaging modalities (X-ray mammography, ultrasound) for improved diagnosis
• Potential for monitoring response to neoadjuvant chemotherapy
• Tracks changes in tumor vascularity and metabolism during treatment
• Non-invasive and does not use ionizing radiation
• Challenges include high false-positive rates and limited sensitivity for deep tumors
• Ongoing research focuses on improving specificity and depth sensitivity

Hemodynamic Monitoring in Critical Care

• Hemodynamic monitoring assesses tissue perfusion and oxygenation
• Applications in intensive care units and during surgery
• Measures regional tissue oxygenation in organs (brain, muscle, abdominal viscera)
• Can detect early signs of shock and guide fluid resuscitation
• Monitors cerebral oxygenation during cardiac surgery and carotid endarterectomy
• Assesses peripheral perfusion in patients with sepsis or trauma
• Advantages include continuous, non-invasive monitoring at the bedside
• Limitations include motion artifacts and variability in probe placement
• Research ongoing to develop wearable devices for long-term monitoring
• Integration with other physiological monitors for comprehensive patient assessment