Ex-Search TI Optimization Pipeline

The Ex-Search module provides a high-performance, exhaustive search approach for Temporal Interference (TI) simulations. The system features unified logging, multiple EEG net support, flexible leadfield management, and optimized computational efficiency.

Overview

Ex-Search implements a true exhaustive search approach for Temporal Interference (TI) optimization, systematically evaluating all possible electrode combinations within user-defined constraints. Unlike sampling-based methods, ex-search guarantees finding the globally optimal montage configuration.

Key Features:

• True Exhaustive Search: Evaluates all N⁴ electrode combinations × current ratios (where N = electrodes per channel)
• Multiple EEG Nets: Support for high-density (10:10) and 10-20 nets with automatic co-registration
• ROI Analysis: Spherical ROI definition with configurable radius specification (default 3mm)
• Current Ratio Optimization: Systematic testing of current ratios respecting channel limits
• High-Performance Processing: Memory-efficient in-memory calculations with real-time progress tracking
• Comprehensive Metrics: TImax, TImean, Focality analysis with automatic visualization

Search Modes

Ex-Search supports two electrode assignment strategies:

Bucketed Mode (Original)

Electrodes are pre-assigned to specific channels:

• E1+: Electrodes for positive channel 1
• E1-: Electrodes for negative channel 1
• E2+: Electrodes for positive channel 2
• E2-: Electrodes for negative channel 2

Combinations: N₁ × N₂ × N₃ × N₄ (where N = electrodes per bucket)

Pooled Mode (New)

All electrodes are pooled together and can be assigned to any channel position, with the constraint that each electrode is used only once per montage.

Combinations: C(N,4) × 4! (where N = total electrodes, C = combinations)

Trade-off: Larger search space and longer compute time but with absolute certainty to find optimal solution with given electrode space.

User Interface

The interface provides controls for:

• Subject Selection: Choose from available subjects with automatic leadfield scanning
• Leadfield Management: View existing leadfields, create new ones, and show electrode configurations
• ROI Configuration: Add target regions with coordinates and manage multiple ROIs
• Electrode Setup: Configure E1+, E1-, E2+, E2- with support for both GSN and 10-20 formats
• Execution Control: Run optimization with real-time progress tracking

Supported EEG Nets

Ex-Search automatically detects and supports multiple EEG electrode configurations:
(EEG nets autoamtically co-registered during pre-processing)

EGI 10-20 Okamoto 2004 electrode configuration - widely used standard with 32 electrodes

GSN-HydroCel 256 electrode configuration - high-density net for precise targeting

Example Results:

Pipeline Workflow

1. EEG Net Selection

[INFO] Scanning available EEG nets for subject 101...
  1. EGI10-10_Cutini_2011.csv
  2. EGI10-10_UI_Jurak_2007.csv
  3. EGI10-20_Okamoto_2004.csv
  4. GSN-HydroCel-185.csv        # Default selection
  5. GSN-HydroCel-256.csv
  6. easycap_BC_TMS64_X21.csv

2. Leadfield Management

• Intelligent Detection: Automatic scanning of existing leadfields with HDF5 validation
• Flexible Creation: Generate leadfields for any supported EEG net with automated naming
• Performance Optimization: Efficient loading of large matrices (2-20GB) with memory monitoring

3. Current Ratio Optimization

The optimization systematically tests current ratios respecting channel limits:

For total_current=2.0mA, step=0.2mA, limit=1.6mA:
  (1.6, 0.4), (1.4, 0.6), (1.2, 0.8), (1.0, 1.0),
  (0.8, 1.2), (0.6, 1.4), (0.4, 1.6)

4. Exhaustive Search Algorithm

• Electrode Combinations: N⁴ combinations where N is electrodes per channel group
• Current Ratios: Systematic testing across user-defined current steps
• Total Combinations: electrode_combinations × current_ratios
• In-Memory Processing: No intermediate mesh files, direct field extraction
• Progress Tracking: Real-time monitoring with ETA calculations

5. Analysis & Visualization Pipeline

• Run Time: From minutes to hours depending on leadfield size and electrode combinations
• Metrics Calculation: TImax_ROI, TImean_ROI, TImean_GM, Focality (TImean_ROI/TImean_GM)
• Visualization: Automatic histogram generation (TImax, TImean, Focality distributions)
• Output Formats: JSON results, CSV summaries, PNG histograms

Technical Implementation

Optimized Processing Architecture

The ex-search implementation features several performance optimizations:

• Memory-Efficient Design: In-memory field calculations eliminate intermediate file I/O
• Itertools Integration: Uses itertools.product() for efficient combination generation instead of nested loops

Algorithm Efficiency

for processed, (e1_plus, e1_minus, e2_plus, e2_minus, current_ratio) in \
        enumerate(product(E1_plus, E1_minus, E2_plus, E2_minus, ratios), 1):
    # Process combination

Performance Characteristics

• Scalability: Handles large electrode combinations (1000+ montages) efficiently
• Memory Usage: Constant memory footprint regardless of combination count
• Progress Tracking: Real-time ETA calculation with rate monitoring
• Error Resilience: Individual combination failures don’t halt entire optimization