Magnetic Resonance Imaging: Types, Characteristics, And Applications

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MRI or magnetic resonance imaging is known as attest which uses pulses of radio energy and magnetic field to produce pictures of different organs inside a human body, it also expresses the structures of internal organs in a pictorial form.

MRI is done for a variety reasons. It can be used to find the internal problems that are giving trouble inside a human body.  Problems such as tumours, internal bleeding, internal injury, diseases in blood vessels – all these can be found with the help of MRI scan. The MRI scan is done for different parts of human body such as head, chest, abdomen, pelvis, blood vessels, bones and joints and spines.

Proton density or PD images

Concentration is the common characteristic of a tissue that can easily be imaged. Concentration of a tissue is also known as the density of protons. Here the tissue is magnetized and density of proton is obtained. Those tissues which are possessing rich amount of protons will produce a strong signal and their appearance will also be bright.

While the MRI procedure is going on, the magnetization of tissue is cycled. First it flipped into a condition that is unstable and then it starts to recover. The process of recovering is called the relaxation. The time taken from magnetization to relaxation is not same for all tissues. It is different for different types of tissues. Normal tissues and the pathologic tissues can be distinguished with the help of this relaxation time.

Each and every tissues are characterized by two distinct relaxation times such as T1 and T2. Image can be produced in only one of these relaxation times and the relaxation time in which the image can be made is known as the predominant source of that particular tissue. The image can be produced by blending or mixing of these different characteristics. But the image thus created will not be of good quality. An image that is made of only one characteristic will be of high quality and also be of heavily weighted. If an image is described as T1 weighted image, it means that the predominant source of that particular image is T1, but it does not mean that T1 is the only source of that image as there are possibility of contamination or distortion from the other two characteristics such as T2 and PD

From the image we can make this table followed by the graph made by its values:

From the above table, we can understand a clear idea about the Magnetic Resonance Image. Here, it can be seen that there are 6 ROI i.e. the Region of Interest. The area of the region of interest is changed each time. The different areas are 0.20 cm2, 0.22 cm2, 0.19 cm2, 0.19 cm2, 0.22 cm2 and 0.22 cm2. In our experiment the absolute bias is taken as fixed parameter whereas the signal intensity is a variable parameter. The T2 relaxation time is then recorded and it is done in msec. T2 relaxation means the progressive dephasing of dipoles that are spinning and those spinning dipoles are following the ninety degree pulse which can be seen in the spin-echo sequence that is occurred because of certain tissue-particular characteristics, preliminarily those that affect the movement rate of protons and maximum of which can be found in the molecules of water. This is also known as the spin-spin relaxation. Mean and the Standard deviation of theses values are calculated. The duration of echo that i.e. the echo time is also recorded and it is too recorded in msec. The mean and the standard deviation is then again calculated as shown in the table. The graph is shown below:

Image types and characteristics of tissue

In the following MR image it easily be observed that an MR image is capable of displaying the different characteristics of body fluids and different tissues. There are few physical characteristics that can be seen from the figure.

Figure 1

RF signal that is emitted by a tissue is the first thing that one see. In tissue where the density of proton is high are shown by bright areas. In the tissue there is an area that is dark, it means that it does not produce any signal. These two are the two extreme levels and between these two there are different range of signal intensities

Magnetization can be defined as the condition inside the tissue which produces the RF signal. In MRI magnetization is that element that produces the RF signal from inside the tissue that can be displayed in the image. Therefore it can be said that a MR image is the pictorial form of the magnetization.

In the above figure we can see the image of protons which is known to be the nucleus of a hydrogen atom. This is why the MRI is also known as proton imaging.

The magnetization that produces the RF signals origins from these protons which are known to be small magnets available inside the tissue. They are also called magnetic nuclei. These so called small magnets are also nuclei of some definite atoms which contain a special magnetic property that is known to be magnetic moment.

Hydrogen is only element that has been found yet to have adequate magnetic nuclei concentration that are capable of producing good images. It is known to all to all that the nucleus of a hydrogen atom is also a single proton. Hence it can be said that the image produced by MR is the image of a single proton. If the tissue containing hydrogen is located in a strong magnetic field, at that moment few protons go to the same direction that the magnetic field occurs. If a tissue does not contain enough amount of protons inside it, it will not be visible in the MR image.

PD (Proton Density)

Proton density is one of those characteristics that effects on tissue magnetization directly and as a result it also effects the RF signal and the brightness of the image. The magnetization is caused by the amount of proton density. If the proton density is low, the magnetization will also be low and if the proton density is high, the magnetisation will also be high. On the other hand, brightness occurs at those areas of a tissue where proton density is higher and darkness occurs where the density of protons within the tissue is lower.

If the protocol is set to prepare a T1 weighted image, the tissues that possess short values of T1 will make dominant magnetization that will become the highest and that area will become the brightest in that particular image.

If the protocol is set to prepare a T1 weighted image, the tissues that possess long values of T1 will make dominant magnetization that will become the highest and that area will become the brightest in that particular image.

An MR Image

The figure below (MR image) describes the spatial characteristics. Basically MRI is the tomographic imaging process, though in angiography, the complete anatomical volume is displayed in one single image. It includes factors such as slice orientation, number of slices and the structure inside each and every slice.

Figure 2

In MRI, slice selection can be defined as the selection of spins on a platform through the object. Its principle can be explained by the resonance equation. An examination will contain at least a pair of bad or contiguous slices. The slices can be placed on any plane of the patient’s body.

Voxel is the matrix form in which each and every slice are subdivided into columns and rows. The quality of image depends on the size of the voxel. Each and every voxels are known to be an independent source of RF signal or can produce the RF signal.

Image pixel or picture element is the element that is formed by dividing the image into row and columns. The image pixel denotes the corresponding voxel. The brightness of pixel can be found by RF signal intensity. 

While starting a MRI, the following steps must not be avoided and the reason is described below:

The factors are:

Efficient use of the hardware (power amplifier, coil, gradients).

Image resolution, shape and registration.

Slice selection.

Signal to noise ratio (SNR).

Solution:

The efficiency of a power amplifier is important in order to have a good quality of MRI. The amplification must be so pure that a high quality image can be produced. The pure amplification is also essential in order to gain stability and linearity.

A coil can be defined as an object that contains one or many loops of wire that are conductive that is looped surrounding the core of the coil. In MRI coils help to create a magnetic field. It can also detect if there is any change in the magnetic field. The magnetic field changes due to the variable voltage induced in those conductive wire. A coil is called a perfect coil if it creates a uniform magnetic field but significant amount of radiation.

Gradients can be defined as loops of conductive wire on a cylindrical shell which is contained just inside the bore of a MR scanner. At the time of passing current through these gradient coils, a secondary magnetic field is automatically created. This secondary magnetic field distorts the primary magnetic field slightly and it does this in a predictable manner.

Therefore, The main function of gradients is allowing the spatial encoding of MR signal.

Resolution can be defined as the capability of human eyes that can distinguish a structure from other. In case of MRI, the resolution is fixed by pixel number. The small pathologies are diagnosed by increasing the resolution. If the pixel number is increased, the resolution will also increases. Hence, it can be said that Resolution is directly proportional to the pixel number.

SNR tells one about the quality of image or how grainy the image is. Signal to Noise Ratio or SNR is directly proportional to the size of pixel or it can be said that it is inversely proportional to the basic resolution. Hence it can be said that if the base resolution is increased, the pixel size will be automatically decreased and SNR will also be decreased.

In MRI, slice selection can be defined as the selection of spins on a platform through the object. Its principle can be explained by the resonance equation. An examination will contain at least a pair of bad or contiguous slices. The slices can be placed on any plane of the patient’s body.

Hence all these effects the quality of the image produced by MRI. These steps must to be omitted. But if these are omitted, a result of getting a poor quality image may occur.

You will notice that the bore of the magnet is open. Unlike a clinical scanner the instrument is not in an RF-shielded room. What are the possible consequences of these two facts, and if there are any what is the cause?  Answer briefly.

Solution:

Unlike a clinical scanner if the instrument is not in an RF-shielded room, the possible consequences are

1.      The MR signal will get distorted and hence accurate result cannot be obtained.

2.      The function of nearby medical devices that are being used will be interfered. Hence, actual result will not be obtained.

These two consequences will occur because the following functions of RF-shielded room is missing:

 1) RF – shielded room prevents extraneous electromagnetic radiation so that it cannot contaminate or distort the signal of MR.

2) RF – shielded room prevents electromagnetic radiation generated by the MR scanner so that it cannot cause any interference in nearby devices that are being used.

Conclusion:

Hence, it can be concluded that magnetic resonance imaging or MRI is beneficial in various ways and the use of this process along with the machines used i.e. the hardware are not too expensive. MRI helps us to know our different internal problems so that we can try to cure the problem.

References

Analogic.com, (2015). RF Power Amplifiers, Broad Band Amplifiers for MRI - Analogic. [online] Available at: https://www.analogic.com/products-medical-magnetic-resonance-imaging-rf-amps.htm [Accessed 22 Dec. 2015].

Cis.rit.edu, (2015). CHAPTER-6. [online] Available at: https://www.cis.rit.edu/htbooks/mri/chap-6/chap-6.htm [Accessed 22 Dec. 2015].

Microwavejournal.com, (1999). An RF Power Amplifier for Whole-body MRI Scanner Applications. [online] Available at: https://www.microwavejournal.com/articles/2551-an-rf-power-amplifier-for-whole-body-mri-scanner-applications [Accessed 22 Dec. 2015].

Mrimaster.com, (2015). MRI resolution and image quality | how to manipulate mri scan parameters. [online] Available at: https://mrimaster.com/index.4.html [Accessed 22 Dec. 2015].

Mritutor.org, (2015). SNR. [online] Available at: https://www.mritutor.org/mritutor/snr.htm [Accessed 22 Dec. 2015].

Sprawls.org, (2015). Magnetic Resonance Image Characteristics. [online] Available at: https://www.sprawls.org/mripmt/MRI01/index.html [Accessed 22 Dec. 2015].

WebMD, (2015). Magnetic Resonance Imaging (MRI). [online] Available at: https://www.webmd.com/a-to-z-guides/magnetic-resonance-imaging-mri?page=5 [Accessed 22 Dec. 2015].