MRI Hardware and Magnetism

Hardware Needed

• The hardware needed to produce an MRI image.

MRI Scanners

• A powerful magnet creates a magnetic field over a 50-60 cm spherical volume
• A shim system improves the homogeneity of the magnetic field.
• A gradient system creates linear slopes in field strength in any direction
• An RF transmission system generates and transmits pulses of electromagnetic radiation
• A set of RF receiver coils detect signals from the patient
• A computer system allows input of parameters and display images
• A computer subsystem coordinates RF pulses and gradients, reconstructs images, and stores them

Magnetism

• Magnetic susceptibility effects include diamagnetism, paramagnetism, superparamagnetism, and ferromagnetism.
• Diamagnetic substances include hydrogen, helium, water, carbon, and metals like copper, mercury, gold, and silver.
• Diamagnetic substances show mild negative effects on the local magnetic field within the nucleus.
• Lines of flux diverge around a diamagnetic substance.
• Paramagnetic substances include gadolinium chelates.
• Paramagnetic substances have a positive effect on the local magnetic field and magnetic flux converges towards a paramagnetic object.
• Paramagnetic elements include calcium, oxygen, and metals like aluminum, titanium, and platinum
• Superparamagnetic substances such as iron oxides have large magnetic moments.
• Superparamagnetic substances have a positive susceptibility (greater than paramagnetic substances) and create large disruptive changes in local magnetic fields.
• Ferromagnetic substances include iron, nickel, cobalt, and gadolinium that have high positive susceptibilities.
• Ferromagnetic substances acquire large magnetic moments when placed in a magnetic field and retains this magnetization even when the external field is removed.
• The flux lines of an external magnetic field are powerfully distorted by a ferromagnetic object that causes geometric distortion of images in MRI patients with ferromagnetic implants.

Magnet Equipment

• The external applied field is represented by static field (Bo).
• Magnets purpose is to magnify tissue and strength is measured at tesla(T) at the isocenter.
• 1 T = 10 000 Guass (G)
• The current clinical strength range is 0.2T – 3.0T
• The Earth's magnetic field = 0.5 G with 3 T-system = 30 000 G, which equals 60 000 times stronger than earth's magnetic field.
• Higher strength size equals higher tissue magnetization and higher available signal which means greater flexibility in acquisition techniques

Three Types of Magnets

• Permanent Magnets are called 'Bricks' of natural magnetic material that are referred to as open magnets.
• Magnetic field exists above and below with vertical field direction along the patient's y-direction.
• Permanent magnets are supported by an Iron frame
• Field strength range: 0.01 T-1T
• Resistive electromagnets (Electromagnets) are created by an electrical field, that passes direct current through a coil of wires as solenoid electromagnets.
• Solenoid magnets are lighter than permanent magnets but have low field strength.
• Requires a power supply, can be turned on and off.
• Field strength: 0.2 T – 0.7 T with a required cooling system
• Superconducting Electromagnet (open-bore systems) consist of a coil that has been made superconductive by 'submerging' the coils into cryogenic liquid with liquid helium (He).
• Temperature of liquid helium: 4 Kelvin = -452 F (-269.15°C) with other cryogen.
• Superconducting Elecrtromagnets have a field strengthsof – 0.2T, 1.0T, 1.5T, 3.0T for clinical use or 7T and above for research, involving safety issues like Fringe fields, quench.
• Fully-sealed systems do not require a vent pipe and is around 900kg lighter and reduces the construction costs.
• Modern MRI scanners are unlikely to require a helium refill during their operational lifetime using helium recondensing or recycling which reduces helium loss to a negligible amount Current research explores superconductive magnets operate at higher temperatures using alloys such as yttrium barium copper oxide rather than niobium or titanium

Extremity Systems

• Extremity scanners are designed to scan limbs and are smaller in size than their whole-body counterparts with the size of a domestic washing machine, having a narrow aperture in the center to accommodate an arm or leg.
• Slightly larger models are the size of a fluoroscopy unit and may be angled to allow weight-bearing views of the spine, hips, and knees.
• The magnetic field is typically generated by permanent magnets and is therefore restricted to below 1 T.

Ramping a Magnet

• Bringing an MRI magnet to required field strength involves using known as a “persistent switch”
• The parallel windings is a superconductor with a heating coil inside.
• When heated, the terminals of the solenoid are connected to an external power supply to power up the magnet.
• At required field strength, heater is turned off
• Persistent switch becomes superconductive creating a closed loop of superconductive wire and effectively bypasses the external power supply.
• Flowing electrons show preference for nonresistive circuit.
• Ramping occurs when process is energizing an MRI solenoid.
• During ramping down the heater is reactivated on the persistent switch that diverts the current from the solenoids through a resistor to dissipate the energy.
• If a large ferromagnetic object becomes lodged inside the MRI scanner, the technique is used to safely remove it in a the non-emergency event where there is no danger to life or limb.

Overview of the hardware components needed for MRI, including magnets, shims, gradient systems, and RF coils. Also covers magnetic susceptibility effects like diamagnetism and paramagnetism with examples.