Split Coil Metal Detector for MRI Implant Localization
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face challenges in scanning patients with implants due to RF eddy current-induced local SAR, as existing techniques are inadequate for large conductive structures and often require precise implant location to avoid excitation, limiting the effectiveness of MRI scans for patients with certain medical devices.
Innovation Solution
A metal detector system using split coils to create a reduced field zone with a time-varying magnetic field, allowing for the detection and localization of metallic objects within the imaging zone by moving the field zone in a predetermined pattern and measuring electrical data to determine the object's location, which can then modify the MRI pulse sequence to reduce RF fields at the object's location.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If patients with metallic implants are scanned using MRI system, then clinical images can be obtained, but RF eddy currents are induced in the implants causing local SAR and safety issues
Solution Approach 1:
The metal detector performs preliminary detection of metallic implants before the MRI scan is initiated. By using split coils to create a time-varying magnetic field and detecting eddy currents in metallic objects, the system identifies implant locations in advance, allowing the MRI sequence to be modified beforehand to avoid harmful RF heating
Solution Approach 2:
The split coil metal detector acts as an intermediary system between the patient and the MRI scanner. It provides implant location information that mediates the interaction between RF fields and metallic implants, enabling the MRI system to adjust its transmit fields to minimize eddy current heating while maintaining imaging capability
2Object-affected harmful factors
If linear polarized transmit field is used instead of circular polarized, then eddy currents in pacemakers are reduced, but the technique does not help much for big conductive structures
Solution Approach 1:
The detection system uses multiple split coils that can be independently controlled to create time-varying magnetic fields. By segmenting the detection approach into multiple coil elements with independent control, the system can adapt to detect different types of metallic implants (small pacemakers, large orthopedic implants, etc.) with varying geometries and conductivity
Solution Approach 2:
The system employs dynamic magnetic field generation through time-varying currents in the split coils. The magnetic field is not static but varies over time, allowing the detection scheme to be adapted for different implant types by adjusting frequency, amplitude, and coil activation patterns, providing versatility across different implant geometries
3Object-affected harmful factors
If preliminary magnetic resonance data is used to locate implants, then RF transmit field can be reduced at implant location, but the method requires additional preliminary scanning and data processing
Solution Approach 1:
The system replaces the conventional approach of using preliminary MRI data (which requires complex image processing and reconstruction) with a dedicated metal detector using split coils. This substitution uses simple electromagnetic induction principles to directly detect metallic objects, eliminating the need for time-consuming MRI preprocessing while achieving the same goal of implant localization
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables safe and effective MRI scanning by accurately detecting and localizing metallic objects, reducing eddy currents and improving imaging safety by modifying the RF field to minimize interference and ensure patient safety.
Implementation Method 1
a first coil (102) for generating a time-varying magnetic field along a first direction within a measurement zone (120)
Implementation Method 2
RF eddy current induced local SAR
Data Source
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AI summary
The invention provides for a metal detector (100, 300) with at least a first coil (102) for generating a first magnetic field (108) along a first direction (119). The first coil is a split coil with a first (104) and a second (106) portion (104). A coil power supply (110) separately supplying time varying electrical power to the coil portions. At least one electrical sensor (116, 118) measures electrical data (136) descriptive of the electrical power supplied to at least the first coil portion and the second coil portion. The coils are controlled such as to move a field-free region in a predetermined pattern within a measurement zone. If metal is detected, the pattern is modified for refining localisation of the metallic object.