Thesis Overview

• Composed of two main parts
  • Short mid-field $B_0$ magnets
  • Magnetic Particle Imaging (MPI) for functional neuroimaging

Part 1: Short Mid-Field $B_0$ Magnets

• Aim: Develop shorter and more open magnets to reduce claustrophobia and enhance patient comfort
• Key Topics:
  • Extended scaling relations for superconducting MRI $B_0$ magnets
  • Hybrid designs for shortening MRI magnets

Importance of Short Magnets

• Patients often suffer from claustrophobia leading to
  • Cancelled scans
  • Increased costs for medical facilities
• Mid-field magnets are more compatible with metallic implants compared to conventional higher-field systems (1.5T & 3T)
• Lower installation costs and operational costs for hospitals

Components of MRI Systems:

1. Static Viscera Field: Determines the Larmor frequency and magnetizes tissue spins.
2. Gradient Coils
3. Radio Coils

• Focus on $B_0$ magnet design, emphasizing homogeneity= $\frac{max - min}{average}$
• Perfect homogeneity = 0, ideally close to 1 PPM deviation in clinical settings, but real-world deviations reach about 500 PPM.

Design Approaches

• Types of magnets: Superconducting, resistive, and permanent magnets
• Main questions:
  • What are the limits on superconducting designs?
  • Can hybrid designs using all three magnet types provide shorter configurations?

Optimization Frameworks for Magnet Design

• Scaling relation: Optimal magnet length $L_{opt}$ relates to diameter $d$ and imaging region size $DSV$
• Extended parameters via optimization studies allow for new configurations

Aims of Part 1

1. Extended Scaling Relations:

   • Expanded parameter space improves flexibility in magnet design
   • Example scaling relations compare properties for spherical vs. ellipsoidal regions of interest.

2. Hybrid Designs:

   • Integration of superconducting windings with rare earth materials leads to greater field homogeneity
   • Experimental observations indicate a possible 20% size reduction for mid-field MRI systems

Part 2: Magnetic Particle Imaging for Functional Neuroimaging

• Focus on measuring local intensity changes in the brain based on activation patterns
• Dual objectives:
  • Apply functional MPI in human studies via primate models
  • Measure peripheral nerve stimulation safety during MPI operations

Functional Neuroimaging Overview

• Measures the cerebral blood volume (CBV) responding to stimuli, to identify brain areas activated by tasks
• Critical differences between fMRI and MPI:
  • fMRI depends on physiological responses, MPI directly measures blood volume changes, potentially offering clearer results

Tools for Functional Neuroimaging

• Traditional methods (e.g., fMRI) struggle with physiological noise and limited sensitivity in clinical diagnostics
• MPI advantages:
  • High contrast-to-noise ratio due to lack of background signals
  • Non-ionizing and safe for human use

Principles of MPI

• Involves injecting superparamagnetic iron oxide nanoparticles (SPIONs)
• AC magnetic fields cause SPIONS to fluctuate magnetization, producing measurable signals via receive coils

Human Scale MPI Scanner

• Features
  • 7mm spatial resolution, high-speed imaging (2 images in 10 seconds)
  • Mechanical and electrical system details

Experimental Protocols for MPI

• Tests conducted on primates to validate feasibility in humans:
  • Injection of tracer, followed by hypercapnia experiments
  • Data processing exercises: stabilization of noisy signals, generating activation maps showing brain regions responding to stimulus with success rates of up to 6 times better than fMRI

Safety Measurements

• Concerns regarding peripheral nerve stimulation from time-varying fields
• Development of thresholds for stimulation at multiple frequencies to ensure safety
• Comparative study identifies deviations from established models when operating above 10 kHz

Conclusion

• MPI presents a promising functional neuroimaging tool with direct blood volume measurement capabilities
• Future improvements necessary for spion tracers, including safety and performance optimizations.
• Need for thorough investigations of nerve stimulation implications at high frequencies.

Future Directions

• Investigate advanced imaging techniques for better clinical utility.
• Ensure regulatory approval for clinical tracers.
• Ongoing assessment of the MPI system's safety characteristics for human usage.