How to Make a Superconducting MRI Magnet

Question 1

Magnet

Answer 1

Magnet coils and former placed in a helium vessel. A thermal shield for radiation protection. A vacuum chamber. Suspension chamber to suspend this in the middle.
-269 oC
Large forces on a single coil
Field uniformity dependent on a very small tolerance
A large amount of stored energy in a magnet
Materials are 4000x more sensitive to heat due to the low temperature
Repeatability

Question 2

Capacity Problems

Answer 2

Improve productivity
Space not just labour hours
Product performance improvement (less time spent testing)

Expand operations (new factory, extensions)
Hire temporary local space

Using the global supply chain
Coils and complete magnets from China

Question 3

Manufacture

Answer 3

Wind the coils (20-50km with diameters of 1.5-2m diameter

Pot the coils in epoxy resin - Coils are wound with a set number of turns and layers to give a predictable and repeatable magnetic field

Assemble the coils

Check the homogeneity at Room Temp

Join magnet
Fit the magnet to the cryostat
Pressure test the cryostat
Chill magnet to 4K
Run magnet to field
Test magnet and cryostat
Ship magnet: time to dry is 20-28 days

Question 4

Magnets Without Al Formers

Answer 4

It seems like you’re discussing the design and considerations for magnets without aluminum formers, likely for applications such as in high-energy physics research or medical imaging systems. Here’s a breakdown of the points you’ve mentioned:

1. Reduced Mass and Cost: Eliminating aluminum formers from the magnet design can reduce the overall mass of the magnet system, which may lead to lower transportation costs and material expenses. Additionally, the absence of aluminum formers could potentially reduce manufacturing costs.
2. No Relative Movement Between Coils: Without aluminum formers, there may be less relative movement between the coils of the magnet upon energization. This can contribute to improved performance and stability of the magnet system, particularly in applications where precise magnetic field control is critical.
3. Compressive Stresses Transferred to Innermost Coils: In a magnet without aluminum formers, compressive stresses may be transferred directly to the innermost coils of the magnet. It’s essential to design the coils and support structure to withstand these stresses without compromising performance or integrity.
4. Thermal Stresses Induced at Cooldown: During cooldown cycles, thermal stresses may be induced in the magnet system due to differential thermal expansion and contraction of materials. Proper thermal management and design considerations are crucial to mitigate the risk of thermal stress-induced damage.
5. Transport of Spacer/Coil Sets: Transporting spacer and coil sets for magnets without aluminum formers may require special considerations, particularly regarding temperature control. The glass transition temperature (Tg) of materials used in the construction of the magnet components should be considered to prevent thermal damage during transportation, especially in regions with high average temperatures.
6. Attachment/Restraint of Outer Coils: Without aluminum formers, alternative methods for attaching and restraining outer coils must be implemented. This could involve the use of structural supports, clamps, or adhesives to ensure the coils remain securely in place during operation and transportation.

Overall, designing magnets without aluminum formers requires careful consideration of structural integrity, thermal management, and performance requirements. While eliminating aluminum formers may offer advantages in terms of reduced mass and cost, it also presents unique challenges that must be addressed through thoughtful design and engineering solutions.

Question 5

Common Failure Mode

Answer 5

Quench: wire stops superconducting - becomes resistive and 1,000 litres liquid He suddenly boils away

Electrical short

Homogeneity (field is not what we want)

Decay: full field not maintained between service intervals

Cryogenic: the system will require frequent topping up of liquid helium
Poor vacuum
Thermal short
Poor connection to refrigeration

In MRI magnets, common failure modes can vary depending on the specific design, construction, and operating conditions of the magnet system. However, several general failure modes are commonly observed in MRI magnets, particularly in superconducting magnets. Here are some common failure modes:

1. Quench: A quench occurs when part of the superconducting magnet coil transitions from the superconducting state to a resistive state, leading to the generation of heat and loss of superconductivity. Quenches can be triggered by factors such as mechanical disturbances, electrical faults, or thermal gradients. If not properly managed, quenches can lead to local heating, thermal stress, and potentially damage to the magnet coils.
2. Mechanical Stress: Mechanical stress can result from factors such as vibration, mechanical shock, or thermal expansion and contraction. Over time, mechanical stress can lead to fatigue, deformation, or even mechanical failure of structural components, such as aluminum formers or support structures.
3. Cryogenic System Failure: The cryogenic system, which maintains the superconducting coils at cryogenic temperatures, is critical for the operation of MRI magnets. Failure of components such as cryocoolers, cryogenic pumps, or cryogenic lines can lead to loss of cooling and overheating of the magnet coils, potentially causing damage to the superconducting material.
4. Electrical Faults: Electrical faults, such as short circuits, open circuits, or insulation breakdown, can occur within the magnet system, leading to localized heating, arcing, or damage to electrical components. Electrical faults can be caused by factors such as insulation degradation, mechanical damage, or improper wiring.
5. Coolant Leakage: MRI magnets rely on cryogenic fluids, such as liquid helium or liquid nitrogen, for cooling. Coolant leakage can occur due to factors such as mechanical damage, material degradation, or faulty seals. Coolant leakage can lead to loss of cooling, pressure buildup, and potential damage to surrounding components.
6. Quench Back Propagation: In some cases, a quench in one part of the magnet coil can propagate to other regions of the coil, leading to a larger-scale quench event. Quench back propagation can be caused by factors such as insufficient quench detection and protection systems or inadequate thermal insulation between coil segments.
7. Material Degradation: Over time, materials used in MRI magnets, such as superconducting wire, insulation materials, or structural components, may degrade due to factors such as thermal cycling, mechanical stress, or radiation exposure. Material degradation can lead to reduced performance, increased susceptibility to failure, or shortened operational lifespan of the magnet system.

Overall, mitigating the risk of these common failure modes requires careful design, robust construction, regular maintenance, and effective monitoring and diagnostic systems to detect and address potential issues before they escalate into critical failures. Additionally, proper training and procedures for magnet operation and emergency response are essential for ensuring the safety and reliability of MRI magnets.

Question 6

Helium

Answer 6

He is expensive and increasingly difficult to source – it is a diminishing global resource SMT has an extensive recovery and recycling system – 3 liquefiers on-site

Gas generated in the magnet flows to recovery

Low-pressure storage in bags in the roof draws down to medium pressure and then high-pressure storage

Gas re-liquefied for dispensing to new magnets

Question 7

Safety

Answer 7

Strong magnetic field

Extreme cold

Very heavy magnets

Strong acids

Question 8

Future

Answer 8

Greater range of fields

Warm superconductors

Cheaper systems- for low-tech applications

Minimise He

Dry system