Magnetic Resonance Imaging (MRI) is a technique to get images of human tissues. MRI machines, which were first developed in 1945, have evolved a lot over the years. With high-end computers being used for efficient scans and the non-invasive nature of the imaging, MRI devices have become valuable diagnostic machinery in the medical industry.
How does an MRI device work?
An MRI device produces a very strong, constant magnetic field with the help of powerful magnets. This magnetic field forces the protons in the body of the patient to align with it. Then, a radio frequency current is passed through the patient’s body. As a result, the protons in the body lose their equilibrium. When the current is stopped, the protons try to realign with the magnetic field after some time and release some energy in the process. The energy released is detected by the sensors in the MRI. The amount of time it takes for the protons to realign and the amount of energy released in doing so can help identify the different types of tissues. The protons in the body, when realign with the magnetic field, emit a signal which is used to create visual images on the computer screen.
Important components of an MRI device.
The basic component of an MRI include:
- A large and powerful external magnet
- Gradient Coils
- Radiofrequency radiation equipment
- A computer
- A radiofrequency shield
- Power supply
- NMR (Nuclear Magnetic Resonance) probe
The Magnet:
The magnet used in MRI devices is a superconducting electromagnet. Usually, the electromagnet is made using a coil, a power supply, and a cooling system. The coils are made by wrapping conductive wire in a large loop. The wires are usually made from a niobium-titanium alloy embedded in copper. Several coils are used together to create the required magnetic field.
To reduce the temperature and make the coils superconductive, the coils are immersed in liquid helium. The container of liquid helium is surrounded by two more containers with other coolants to prevent overheating. This whole assembly is suspended in a vacuum-sealed enclosure using thin rods. A power supply is switched ON only when the MRI needs to be used. The magnet is attached to a sliding table on which the patient is laid down for an MRI scan.
Gradient Coils:
There are three gradient coils in a typical MRI system. They are the resistant-type of electromagnetic coils made from copper or aluminium. They have shields to prevent eddy current interference. The size of the coils determines the width of the opening in the MRI device where the sliding table with the patient on it is placed. The opening should be big enough for the sliding table to comfortably go inside without causing severe discomfort to claustrophobic patients. However, the bigger the opening is, the more is the electricity required. So, the gradient coils should be of the optimum size.
Radio Frequency System:
The components of the RF system are attached to the RF coils which are made from the same material as the gradient coils. The coils have transmitter and receiver coils which are set up in the same way as the main magnet, but they can create an oscillating magnetic field. The coils are attached to a power supply and are used to pass an RF current through the patient’s body. The coil could be placed on the patient’s body, or it can be placed inside the gradient coils.
Computer:
The computer needs to have the relevant software installed to receive and send signals to the MRI device. The user interface needs to be attached to the computer along with the Fourier transformer, a signal convertor, and an amplifier. A separate display device can also be used and a laser printer to print out the scan reports of the patient.
Assembling the MRI device:
The MRI machinery is assembled at the site or it can be assembled and transported from the manufacturing unit. The whole system needs stringent quality control and regular inspections. Ruggedly built reliable power and data connectors are needed to ensure that the system is working efficiently.
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