Essay: Magnetic resonance imaging / MRI

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  • Published: June 25, 2021*
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  • Magnetic resonance imaging / MRI
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A magnetic resonance imaging also commonly known as MRI is a large tube which has powerful magnets. It’s a type of scan which uses magnetic fields and radio waves to produce images of the inside of the body.

How happens inside the body, during a MRI.

A MRI is created by powerful magnets and radio waves. The way this is done is as your body is made up from many hydrogen atoms which are magnetic. The body’s hydrogen protons can be like the earth spinning on its axes. The difference is that the protons spin on their axes randomly aligned (refer to figure 1). (Berger, 2016)

(Figure 1- Hydrogen protons before going into a MRI) (Sharma, 2014)

When you go into the MRI tube the magnetic field that is created allows the atoms to align with the field. The nuclei of atoms which have an unequal number make the protons and neutrons spin in different directions. The charge particles which are constantly moving then create a magnetic field.

When the body is then placed inside this magnetic field the nuclei irons align themselves with a magnetic field. Some of the nuclei will be facing the magnetic field while some will be facing the opposite directions which will need higher energy. They will then be blasted by a pulse radio frequency and the nuclei will have enough energy to turn the other way. The pulse radio frequency causes the nuclei with the same frequency to resonate and absorb energy. This process is known as larmor frequency which is in the radio frequency range (figure 2). (Berger, 2016)

(Weishaupt, Köchli and Marincek, 2008)

(Figure 2 – Hydrogen protons after being blasted by a pulse radio frequency) (Sharma, 2014)

When the pulse ends the nuclei will eventually return to its equilibrium state (known as relaxation state). The signal will then be picked up by the radio frequency coil and sent to the computer. Once this is done the protons will go back to normal and emit a radio signal of their own.
(Berger, 2016)

How is a MRI created and the principles behind it?

Each component of a MRI is made separately and then brought together. The components are made around the magnet. The magnet is the most important component. Other components such as the gradient system which helps the MR/RF system.

Magnet

As there magnet is the most important component in the MRI scanner they are made from different materials and they are many different types of magnets. The strength of magnets is measured in teslas and has a field strength range in 0.1-3.0T. The more strength the magnet has depends on the type of the magnet and the straightness of the magnetic lines within the center. This is more known as homogeneity. The three types of magnet are;

A resistive electromagnet is a solenoid which has copper wire around it. It’s not the best as it’s cheap to run but stability and control isn’t very good also it uses a lot of electricity which makes it more expensive during operation.

A permanent magnet which is made from steel alloys can be used. This type of magnet is large and takes up a lot of space. These don’t cost a lot to use but the problem with this is that it is not that powerful only reaching maximum of 0.4T. Also the magnetic field can’t be turned off.

The most common type of MRI magnet is the superconducting electromagnet. They was this is made is by niobium titanium alloy being cooled by a liquid helium to -269°C. These have very high field strengths with stability. These are expensive to make but the results of the image outweigh the cost. The materials that are used are made into wires which can withstand high magnetic fields. Usually they would have many filaments in a copper matrix. This matrix will give more stability to the magnet and will also help when superconducting state is lost.

The only way to cool the superconducting magnets is through the liquid helium. (Rohlf, 1994)

Gradients

The gradient system has the job of locating the tissue signals, with three gradients in place at x, y and z directions and produced by gradient coils. Gradient coils are made from flow of current from an amplifier and are in each different direction of the scanner. Gradient system is measured in mTm-1 or G cm-1. Usually in a MRI scanner the maximum would be 10-15 mTm-1. Larger maximum gradient strength allows “thinner slices and smaller images to be obtained without changing parameters”. (Brown and Semelka, 1999)

The time it will take for the gradient t work is not instantly. It usually takes 0.5-1.0 msec for the gradient to reach its final value.
The gradient needs to be performing at 100% and the duty cycle of the amplifier is a big part of it. The duty cycle tells us how fast the amplifier responds to a pulse sequence. Another factor that can impact on the gradient pulses is eddy currents. Eddy currents are electric fields which are produced by time varying magnetic fields. It’s right in the centre of the body coil and cryoshield (the innermost point of the magnet cryostat). The currents can generate a magnetic field which clash with the original gradient pulse. As eddy currents decay with time and can cause the field’s homogeneity and the corresponding frequencies to change. You can resolve this by making sure you predistort the gradient pulse, so you know the gradient pulse in the magnet is the preferred one. (Brown and Semelka, 1999)

Radiofrequency system

The rf transmitter system is used for exciting the protons. It has 4 main components which is a frequency synthesizer, a digital envelope of rf frequencies, high power amplifier and a coil or antenna. The rf system is broadcasted to the patients and has a carrier frequency and a range of bandwidth frequencies. They would be a frequency synthesizer that would produce the center/carrier frequency that we be broadcasted to patients. To reduce the echo artifact which is caused by pulse imperfections the pulse sequence will need to alternate the phase of excitation pulse by 180o.The rf envelope will have complex data points (usually 512 number). Usually the digitals points will be converted to analogue signal before them getting mixed with the carrier frequency. The narrow bandwidth or nonselective frequency is used to show us the resonant frequency of the patient. The role of the rf amplifier is to make a lot of power in order to excite the protons. It usually is a tube like and the amplifiers power is usually 10 kW. The last component of the rf system is the transmitter coil. Every MR has a transmitter coil (saddle design coil) so it can broadcast the rf signal. The role of this is to produce uniform rf penetration and to generate an effective B1 field perpendicular to B0. The coils can usually be manually adjusted and often are in order to have the maximum efficiency. (Brown and Semelka, 1999)

Each of the components of the MRI are assembled together and placed into an appropriate frame (Figure 3). Assembly can take place at the plant or on-site, where the system will be used. In either case, the nature of the magnet typically requires special handling precautions, such as transporting it in an air-suspended vehicle.

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