Basic Principles of Magnetic Resonance Imaging (MRI)

Introduction

The use of Magnetic Resonance Imaging (MRI) as a diagnostic tool in health care has gained preference in the recent past. This is attributable to the potential and many advantages that it has on hospitals which ensures efficiency in the delivery of healthcare services (Meriles 2006). This paper is therefore an exploration of the basic principles and uses of the MRI with a close discussion of what it is.

Magnetic Resonance Imaging defined

Being a special form of radiology technology, MRI uses magnetism and radio waves with a computer to view the internal parts of the body through image production (Consiglio, 2006). This technology is made possible by the combination of the aforementioned three tools. “MRI is based on the use of inhomogeneous or ‘gradient’ fields, supplementing an, otherwise, uniform invariant magnetic field” (Meriles et.al, 2006, p. 106). Reconstruction of an image is made possible by the relationship of the magnetic resonance and the location during the imaging process.

MRI principle

Both the CT scan and the MRI play great roles in diagnostic examination in the healthcare industry. The only difference is that the CT scan is dependent on the fading of the tissue properties while the MRI relies on the “magnetic spin properties of hydrogen nuclei in tissues and how these nuclei recover after excitation with radio frequency electromagnetic waves” (Faerber, 1995, p. 1). This therefore makes the MRI to be more sensitive especially to the small and insidious differences found in the tissues. It is also sensitive to the tissue chemical states that could be present in a patient.

In the figure shown below, the MRI basic process is well described with some of the main parts such as the magnet, antenna and the gradient being shown. From the figure, it is possible to see the parts of the body that are able to be scanned using the machine.

MRI process
Figure 1 MRI process (Nobel Prize.org, 2003).

The MRI Process

According to Faerber (1995, p. 1), the process of MRI begins with the patient being subjected to a strong magnetic field which is achieved by the patient lying on their back. The created magnetic field normally has a range of 0.3 to 2.0 Tesla. Tesla is the representative unit of a magnetic field. Once created, the magnetic field arrays the body’s hydrogen nuclei into double states of equilibrium. Thereafter, radio frequency waves are executed into the body so that they can stimulate the state of equilibrium. Following the return of the radio frequency waves into their normal state, the stimulated energy is transferred into the body in the form of radio frequency energy. The resultant energy is diagnosed by special energy coils which in turn make it possible to detect the flow from excitation to relaxation state. The period of relaxation is normally measured by the use of parameters T1 and T2. The aforementioned parameters depend on the tissues found in the human body for instance, swellings, tumour, and water among other body elements. Consequently, “images can be produced with proper spatial localization of these signals as diagnostic decisions are made” (Faerber, 1995, p. 1).

Schematic Diagram of MRI imaging
Figure 2: Schematic Diagram of MRI imaging (Oxford Centre for Functional of the Brain, n.d).

MRI Parameters (T1, T2, PD weighted)

In an expansion on Faerber’s accounts, Jacobson (2008) explains that as the protons take up the T1 relaxation array, they often release some impact and rate of energy. In the process, a record in the form of spatially localized signals is made by a coil. It is now the function of the computer to make an analysis of the signals produced so as to cast anatomic images. During the image projection, the image brightness is dependent on the radio frequency pulse as well as the gradient of the wavelengths. These two are responsible for determining the density and characteristics of the tissues. However, in order to generate specific pulses the computer program has its wave forms controlled. These pulses are what later settle as images.

The image resolution is basically determined by T1, T2, or proton density due to the fact that the tissues of the body are manifested in diverse ways. An emphasis on this is made by Jacobson (2008) who notes that a tissue such as the fat will seem to be bright and with high signal intensity when used with T1 while the same will be reflected as dark images when used with T2 images. On the same note, tissues such as water and other fluids of the body are reflected as dark images in T1 while they appear bright on the T2 weight. From this, it can be concluded that T1 is best suited to project the soft masses such as fat among other tissues while T2 will best project fluids such as water and other abnormalities. Nevertheless, both the T1 and T2 weighted images serve to produce complementary information thus both function to help in detection of abnormalities that could be present in the body (Jacobson, 2008).

The figures shown below compare the T1, T2, and PD images.

MRI Parameters (T1, T2, PD weighted)
Figure 3. (Wang, n.d, p. 46).
    1. PD-weighted
    2. T2 weighted
    3. T1 weighted

Parts of the MRI

According to Topol & Califf (2007, p. 898) the MRI scanner is known to have five main parts which entail the magnet, antenna, transmitter, receiver and the computer. The magnet part of the MRI is made up of a high field strengthened magnet to aid in obtaining optimum signal-to-noise ratio. On the other hand, the transmitter is responsible for the execution of radio frequency transmissions to the antenna which then sends out RF to the patient. The antenna then receives returning transmissions. Another part, the coil is normally placed on the body surface of the patient depending on the part of the body being examined. Lastly, the receiver functions to detect signals set from the coil and thereafter amplify them to make it easy for the computer to process (Topol & Califf, 2007, p. 898).

Types of magnets in MRI systems

The MRI system normally uses magnets of three types which are the permanent, resistive and superconductive magnets. Just as mentioned earlier, the Tesla or rather ‘T’ is the unit of measure of a magnetic field. The magnetic field serves to generate a net magnetization of hydrogen protons required for the MRI (Consiglio, 2006, p. 5). In a simultaneous execution, the static magnetic force- torque and translational force act on the fixed magnetic objects. However, these forces pose a threat to the patient if embedded on the patient’s body. This is as a result of the projectile effect which generates kinetic energy on the object thus rendering it fatal to people within the trajectory (Consiglo, 2006, p. 5).

Basis of MRI

The basis of MRI can be traced back to the Nuclear Magnetic Resonance whereby there is an “absorption and emission of energy by atomic nuclei in the presence of an externally applied magnetic field” (McKie & Brittenden, 2005, pp. 13-14). The role of the hydrogen nuclei is to generate the image that will be viewed on the screen of the computer. It is because of this that the hydrogen proton nuclei are in high concentration in water and fat molecules. On getting into the machine, the patient has energy release in the form of radio frequency pulses which correspond to the hydrogen atoms frequency in a bid to absorb energy pulses. On application of the radio frequencies energy is again released as magnetic resonance by the hydrogen atoms. The magnetic resonance released is what causes a stimulation of low voltage on the receiver coil near the patient (McKie & Brittenden, 2005, pp. 13-14).

In his discussion on MRI, Van Geuns, et.al (2003, p. 149) points out that the quantum mechanics are very significant in the imaging process. The nuclei spins that surround the axes function as little magnets. Apart from hydrogen, some other forms of nuclei such as carbon-13, sodium and phosphorus are of great essentiality in the process of clinical imaging. The minute magnets are well disseminated in space thus cancelling out on each other. All the same, submission of the patient to a strong external magnetic field causes the nuclei to either adapt an orientation parallel or anti-parallel to the external field (Van Geuns, et.al., 2003, p. 149). Of the two different orientations, the parallel one is most preferred given its low state of energy. In addition to this, it ensures successful imaging through production of high resolution images, large field of views, as well as being insensitive to field in homogeneities and motion. The above elements are what make MRI a difficult task to undertake. The fact that motion leads to the generation of artifacts from disturbances such as blood flow, cardiac wall movement, respiratory action or peristalsis makes it a challenge. This has however been overcame by the recent advancements in the MRI machine (Constable, 2003, p. 1).

MRI Variations

With reference to Jacobson (2008), the MRI is multivariate. In circumstances where the signal intensity is closely related to water diffusion in tissues the diffusion-weighted MRI is observed. This helps in the detection of ischemia and infarctions. On the other hand, the echo planar imaging is a very fast technique and is used in the functional imaging of the brain and the heart. Additionally, the gradient echo imaging is carried on through pulse sequence, and later on used in the attainment of moving blood images and CSF. Another variation, the perfusion MRI is used in the assessment of relative flow of the cerebral blood as well as in ischemia detection in cases of stroke imaging (Jacobson, 2008).

As for the case of quantitative MRI, biophysical importance is estimated from “a collection of MR signals that are related to one another through a function of one or more experimentally controlled variables “(Koay, et.al., 2009, p. 108). It is thus notable that higher degree of sensitivity is achievable using MRI given its improvement in spatial and temporal resolution.

Functional MRI (fMRI)

The functional MRI is blood-oxygenation-level dependent (BOLD), and is the most “commonly employed method for non-invasive localization of activity in the brain in cognitive psychology investigations” (Sutton, et.al., 2008, p. 33). Functional MRIs abbreviated as (fMRI) serve to measure the ‘function’ and brain activity. It is through BOLD that the fMRI is able to clearly differentiate the susceptibility of magnetism in oxygenated and deoxygenated haemoglobin in microcirculation through the brain pathways. Reckoning the possible changes in metabolism, perfusion and blood velocity there could be an effect on the neural function. Therefore, those kinds of images are better imaged using the fMRI based on blood oxygenation and magnetic susceptibility. It should however be noted that oxygenation and magnetic susceptibility do not have close correlation. The magnetic field on the object being tested is what corresponds to the intensity of the signal and the MRI spatial focus. The availability of protons in fluids also causes the alignment of the strong electromagnetic field which is capable of being manipulated for brain activity identification (Sutton, et.al., 2008, p. 33).

The history of fMRI can be traced back to 1990 when Owaga with the help of his colleagues tried to manifest that images from an MRI machine were in a position to independently reflect information on BOLD. It was thus concluded that BOLD has the ability to diagnose haemodynamic changes that react during neuronal activation. As a result, this approach has been applied in pharmacological MRI in a bid to check the effect of stimulants on brain haemodynamic (Marzola, et.al., 2003, p. 165).

Contrast Agents

It has been noted that, contrast agents may be used so as to “highlight vascular structures and to help characterize inflammation and tumours” (Jacobson, 2008). The common agents used are the gadolinium derivatives which have magnetic characteristics that affect proton relaxation joints thus resonance imaging.

Water diffusion and magnetic field gradients

MRI has been noticed to be sensitive to water self-diffusion. This makes the gradients of the magnetic field to experience sequential shifts on coherence loss as well as a decrease in signal intensity. It is via DW-MRI, that there is “a unique form of MR contrast that enables the diffusional motion of water molecules to be measured (Bozzali & Cherubini, 2007, p. 970). “Consequently, the interactions between tissue water and cellular structures provide information on size, shape, orientation and geometry of brain structures”. (Bozzali & Cherubini, 2007, p. 970). Water sensitivity is essential in the judgement of neuro-degeneration in dementia as it is depicted in the diffusion coefficient. This sensitivity is also the rationale behind the brain pathways basing the assumption that equal sensitivity and algorithms are relevant to segment specific tracts (Bozzali & Cherubini, 2007, p. 970).

According to Consiglio (2006, p. 6) the gradient fields of the MRI play great roles in signal localization of tissues. The system is endowed with three gradients with each of them being responsible current flow. This occurs through the distinguished wire loops installed on one of the gradient coils. In the process of MRI, the gradient fields play the role of emitting electrical currents to the muscles and nerves which act as conductors in the body of the patient. It is as result of these gradients that the patient may experience muscle contractions, sensations on the skin or even cardiac arrhythmias. Forces and vibrations on the gradient system are what bring about the noises produced during the MRI process. This is attributed to the change in electrical currents to forces and vibrations which produce noise (Consiglio, 2006, p. 5).

Conclusion

From the above discussion it is clear that, Magnetic resonance imaging is a form of special radiology technology that helps in image detection with the help of radio frequency waves. The MRI process is based on the principle of magnetism of the hydrogen nuclei of tissues. It has also been noted that the MRI has five main parts which function together to produce desired images. The array is created by the magnet and the proton nuclei of the body tissues. A major part of the MRI is the transmitter which functions to send frequencies from the patient to the antenna. Consequently, the antenna sends back the transmissions. The coil is another important part which helps in accumulation of information that has been generated by the aforementioned parts. It is through this complex process that information can be accurately executed concerning body tissues.

References

Bozzali, M., & Cherubini, A. (2007). Diffusion tensor MRI to investigate dementias: a brief review. Magnetic Resonance Imaging, 25(6), 969–977. Web.

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NerdyRoo. (2022, March 28). Basic Principles of Magnetic Resonance Imaging (MRI). Retrieved from https://nerdyroo.com/basic-principles-of-magnetic-resonance-imaging-mri/

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