MRI Machine - Main, Gradient and RF Coils/ Magnets | MRI Physics Course | Radiology Physics Course#2

Summary

This educational talk delves into the intricacies of MRI technology, focusing on the machine's components and their roles in creating MRI images. It explains how the main magnetic field is generated by the main coil, using superconductors like niobium titanium to maintain a strong, homogeneous field. Shims are introduced to refine field uniformity. Gradient coils are highlighted for their ability to spatially localize signals by altering magnetic field strength along different axes. The lecture also covers the radio frequency coil's function in exciting hydrogen protons, which is essential for signal detection and image formation. The talk promises further exploration into nuclear magnetic resonance in subsequent sessions.

Takeaways

• 🧲 The MRI machine is composed of multiple layers of magnets, each contributing to the creation of the magnetic field necessary for imaging.
• 🔌 The main coil generates the primary magnetic field (B naught) along the longitudinal axis (Z-axis) of the patient, which is crucial for aligning hydrogen atoms.
• 💡 The strength of the main magnetic field is determined by the number of wire coils and the current flowing through them, with superconductors like niobium-titanium alloys used to maintain high current without resistance.
• ❄️ Liquid helium is circulated around the coils to keep them at temperatures below 4 Kelvin, ensuring the superconductivity necessary for strong magnetic fields.
• 🛠 Shims, both passive (ferromagnetic metals) and active (with their own electric supply), are used to adjust the main magnetic field, making it as homogeneous as possible for accurate imaging.
• 🧲 Gradient coils apply a gradient to the magnetic field, allowing for spatial localization of signals within the MRI by manipulating the field strength along different axes.
• 🌀 The gradient coils create differential magnetic field strengths, which in turn affect the precessional frequencies of hydrogen protons, enabling spatial encoding of the MRI signal.
• 📡 The radio frequency (RF) coil generates a magnetic field perpendicular to the main field, resonating with hydrogen protons at specific frequencies to selectively excite them.
• 🔄 The RF pulse causes hydrogen protons to move from the longitudinal plane into the transverse plane, which is essential for signal measurement and image generation.
• 🔎 The MRI process relies on the principles of nuclear magnetic resonance (NMR), where hydrogen atoms' alignment and precession within the magnetic field are manipulated to produce an image.

Q & A

What is the main purpose of the main coil in an MRI machine?

The main coil in an MRI machine is responsible for generating the B naught or the main magnetic field along the longitudinal or Z axis, which is essential for aligning hydrogen atoms and creating the MRI signal.

How does the strength of the main magnetic field in an MRI machine depend on the number of coils and current?

The strength of the main magnetic field is dependent on the number of coils of wire and the amount of current running through that wire. More coils and increased current result in a stronger magnetic field.

What is the role of superconductors in MRI machines, and why are they necessary?

Superconductors are necessary in MRI machines to generate sufficient current for a strong magnetic field without resistance. Materials like niobium titanium alloys are used as they exhibit superconductivity at low temperatures, typically below 4 degrees Kelvin, allowing large currents to flow without resistance.

Why is liquid helium used in MRI machines, and what happens if the temperature rises above 4 degrees Kelvin?

Liquid helium is used to maintain the temperature of the superconducting coils below 4 degrees Kelvin, which is necessary for superconductivity. If the temperature rises above this critical point, the superconductivity is lost, leading to increased resistance and heat generation, which can cause the liquid helium to expand and potentially lead to a quenching process where the helium is released as a gas.

What are shims in MRI imaging and how do they help in creating a homogeneous magnetic field?

Shims in MRI imaging are used to manipulate the main magnetic field and make it as homogeneous as possible. They can be passive, made of ferromagnetic materials, or active, with their own electric supply, and are used to correct for imperfections in the magnetic field.

How do gradient coils contribute to spatial localization in MRI imaging?

Gradient coils apply a gradient to the magnetic field, making it stronger or weaker along different axes (X, Y, Z). This differential magnetic field strength allows for spatial encoding of the signals, which is crucial for localizing the source of the MRI signal within the body.

What is the isocenter in the context of MRI imaging?

The isocenter in MRI imaging refers to the point in the magnetic field where the magnetic field strength is equal to the background B naught magnetic field. It is the region where the magnetic field is most homogeneous and不受 gradient influence.

What is the function of the radio frequency (RF) coil in an MRI machine?

The radio frequency coil generates an alternating magnetic field perpendicular to the main magnetic field. It selects specific hydrogen protons that are precessing at a set frequency and flips them into the transverse plane, allowing the MRI machine to measure the signal and generate images.

How does the radio frequency pulse affect hydrogen protons in an MRI scan?

The radio frequency pulse matches the precessional frequency of hydrogen protons, causing them to gain energy, fan out, and come into phase. This results in a flip angle, which moves the net magnetization vector from the longitudinal plane to the transverse plane, enabling signal measurement.

What is slice selection in MRI imaging, and how does it relate to the radio frequency pulse?

Slice selection in MRI imaging is the process of isolating a specific plane within the patient to acquire image data. It is achieved using the radio frequency pulse, which is tuned to match the precessional frequency of hydrogen protons in the desired slice, allowing for the selective excitation of these protons.