Why noise during process of MRI

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publisher: Gordon

Issue Time: Aug 6,2012

Summary

Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.

Before disclosing the secret of noise when MRI. Let's learn some basic knowledge of MRI.

Structure and Characteristics of Magnetic Resonance Magnet

There’re 4 types of MRI based on different structure: Permanent magnet type, normally conductive type, superconducting type and hybrid type.

Type	Description	Advantages	Disadvantages
Permanent magnet type	The main materials are aluminum nickel cobalt, ferrite and rare earth cobalt, which are composed of multiple pieces of material. The arrangement of the magnet block must not only meet the requirements of forming a certain imaging space, but also make its magnetic field uniformity as high as possible.	low cost, field strength can reach 0.3T, low power consumption, low maintenance cost, can be installed in a relatively small room, open for intervention.	Low magnetic field strength, poor uniformity, and susceptibility to external factors (especially temperature).
Normally conductive type	The normally-conducting magnet is designed according to the principle that the current generates a magnetic field, and its coil is usually wound with copper wire. Because copper has a certain resistivity, the magnet made of this kind of coil is called an impedance magnet.	Simple structure and low cost, easy to manufacture and install, can reduce the radius or increase the coil current to improve the field strength of the magnet.	The power consumption is large, the stability is poor, a perfect circulating water cooling device is needed, the uniformity is poor, and it is greatly affected by the environment.
Superconducting type	Superconducting magnets make use of the characteristics of superconductors with zero resistance at absolute zero degrees, and generate strong magnetic fields through powerful currents on very small wires.	High field strength, good stability and uniformity, magnetic field strength can be adjusted, and the magnetic field can be turned off if necessary.	The superconducting coil must be immersed in the sealed liquid nitrogen dewar to work, the technology is complex, and the cost is high.
Hybrid type	A magnet constructed using two or more magnet technologies. Common combinations of permanent magnet and normally conductive magnets.	It can produce higher field strength, overcoming the disadvantages of permanent magnets being unstable, bulky and normally conducting magnets with large power consumption.	The structure is complicated, low-temperature containers need to be installed, and the cost is also high.

How strong magnetic field of MRI?

At present, the highest medical superconducting magnetic resonance field strength is 4.5T, which is mainly used for scientific research. The highest value used for routine inspection is 3.0T, which is absolutely safe under this field strength. If the field strength is high, too much heat generated by the human body receiving the radio frequency pulse will cause harm to the human body. The highest field strength as I know in magnetic resonance is 9.4T, which is mainly used in animal experiments and will not be used in routine medical examinations.

Why noise during MRI process

There are many sources of noise in MRI, but the most important one is the noise associated with the gradient coil.

So what is the gradient magnetic field:

Magnetic resonance imaging requires spatial positioning, and the main method of spatial positioning is to superimpose three linear gradient magnetic fields Gx, Gy, Gz based on the

main magnetic field B0. The three-path gradient field can perform layer selection coding, frequency coding and phase coding in three directions in space.

During the MRI process, the gradient magnetic field needs to be turned on and off alternately (usually a cycle of a few seconds to a few hundred milliseconds, which means dozens of switching times per second).

The gradient coil is located inside the main magnetic field B0. Due to the current passing through the coil, the wire in the coil is subjected to the Lorentz force.

When the current in the gradient coil changes sharply, the force on the coil changes accordingly, which causes the high-frequency vibration of the coil and hits the frame that fixes it to generate high-frequency noise. This is the main source of resonance "noise".

Different MRI sequences produce different noises, and the differences are also great. Watch a video to understand the noise of different sequences (DTI-FLARE-GRADIENT)

The pulses cause not just the desired changes but undesired vibrations of the gradient coils, resulting in the banging heard during an M.R.I. examination.

as stronger magnets result in stronger vibrations, the higher the field strength of the M.R.I. scanner, measured in tesla, the louder the banging

In a three-tesla system, a strength common in clinical practice, he said, these sounds may be as loud as 125 decibels, equivalent to a rock concert or a balloon popping near the ear.