Introduction to MRI (Magnetic Resonance Imaging)

MRI, short for Magnetic Resonance Imaging, is a non-invasive imaging device that generates three-dimensional images of human anatomy. It was invented by Paul C. Lauterbur, who based his logic on a mechanism that stores spatial data via magnetic gradients to generate NMR (Nuclear Magnetic Resonance) signals, the first image was produced in 1970 and its first image on a human subject was not until 1977. Doctors have become reliant on MRI to detect diseases through detecting tumours and disordered structures, as well as monitoring treatment processes. Compared to CT scans (computed tomography), MRI scans are more advantageous for creating a more detailed and improved image of soft tissues.¹  

Low-field MRI, similar to MRI, uses a lower magnetic field than normal, ranging from  0.25 Tesla (T) to 1 Tesla (T), whereas regular MRI uses 0.2 T to 7 T. MRI systems with low fields are far more useful than those with high fields in certain scenarios. From a medical perspective, they are used for diagnosing a range of musculoskeletal conditions, like cartilage lesions. Furthermore, low-field MRI systems may have faster scan times and need less power. For healthcare facilities trying to maximise their imaging capabilities, this can result in decreased operating costs.^2,3

How does low-field MRI work?

Using the body's inherent magnetic properties, MRI can create detailed images of any portion of the body. The single proton hydrogen nucleus is employed for imaging because it is abundant in fat and water. The Earth is a planet that spins on its axis and has a north-south pole; the hydrogen proton is comparable to the Earth. It functions similarly to a tiny bar magnet in this regard. Normally, the axes of these "bar magnets" made of hydrogen protons spin randomly within the body. The axes of all protons coincide when the body is placed under a powerful magnetic field. An example would be in an MRI scanner. A magnetic vector oriented parallel to the MRI scanner's axis is produced by this consistent alignment. There are several field strengths available for MRI scanners, typically ranging from 0.5 T to 1.5 T. The magnetic vector is deflected when more energy is provided to the magnetic field as a radio wave. The element being sought (in this case, hydrogen) and the strength of the magnetic field determines the radio wave frequency (RF) at which the hydrogen nuclei resonate.⁴

Sequential use of multiple transmitted radiofrequency pulses might be employed to highlight certain tissues or problems. When the transmitted radiofrequency pulse is turned off, various tissues relax at different speeds, resulting in a distinct emphasis. There are two methods to gauge how long it takes for the protons to completely relax. Two different times need to pass before the magnetic vector and axial spin return to their resting states: the first is the time it takes for the magnetic vector to return to its resting state. T1 relaxation refers to the first state, while T2 relaxation is the subsequent state. Thus, an MR examination consists of a sequence of pulses. Different tissues can be distinguished from one another based on their distinct relaxation durations, such as fat and water.⁴ 

For example, the signal from fat will be eliminated by employing a "fat suppression" pulse sequence, leaving only the signal from any anomalies present. MRI is useful in a way such that most diseases show up as an increase in water content. Comparing image quality between low and high-field MRI, low-field MIR tends to produce a poorer image quality due to a reduced signal-to-noise ratio.^4,5

Advantages and disadvantages of low-field MRI

Advantages

The advantages of low-field MRI include:^3,6

• Reduced cost - low-field MRIs are often less expensive to produce, buy, install and maintain. It also has improved accessibility to clinical services, or research, and reduced imprint. Moreover, there is no need for a dedicated shielded chamber because magnets and other components are lighter and smaller
• Lower usage of power - overall, only electronics and gradients require electricity for permanent magnets; these can be powered by batteries, generators, or standard outlets
• Elevated user experience - it is produced as a C-shaped, with a single side, a broader bore and a vertical orientation. It also improves user interaction with the MRI due to the reduction of claustrophobia for the user. Additionally, it can be used for imaging in children and for surgery 
• Safety - low-field MRIs are often more secure which ultimately reduces danger from metallic projectiles. There is also a reduction in the specific absorption rate and heating of the device

Disadvantages

• Lower resolution - this is ultimately caused by a reduction in signal, reduced sharpness, decreased field of view, as well as, a longer time to produce the needed scans^3,6
• Relaxivity differences- there is diminished grey/white contrast which can make it harder to spot tumours or other abnormalities at times^3,6 

Applications of low-field MRI 

Low-field MRI, like regular MRI, has a lot of applications, but it also provides a more niche representation. Low-field MRI devices are useful for diagnosing diseases due to their ability to generate anatomical imaging. They are useful in the diagnosis of many different illnesses because of their ability to see soft tissues, bones, joints, and organs. Initially, it is employed in musculoskeletal imaging to assess joint damage. Additionally, it is employed in neurological imaging to evaluate abnormalities of the brain and spinal cord. Moreover, low-field MRI systems are less expensive and provide less of a safety risk than high-field systems, they are useful in research settings. Examples include cerebral activity mapping, and brain function research, by radiologists using functional magnetic resonance imaging (fMRI).⁶

Future of low-field MRI

Current scientists and engineers are working towards developing multiple different brain-computer interfaces, also known as hybrid BCIs to produce the best performances. An example would be to combine computed tomography (CT), and positron emission tomography (PET), with low-field MRI to obtain a more thorough diagnostic picture. This could be done to combat the limitation of lower resolutions.⁷ 

Summary

Low-field magnetic resonance imaging is a significant development in medical imaging. Although its image quality might not be as proficient as that of high-field MRI systems, its affordability, improved accessibility and safety, make it a good choice for a variety of clinical and research applications. These systems, which use lower magnetic fields, offer crucial diagnostic capabilities, especially for neurological and musculoskeletal imaging, without the expensive infrastructure and high running costs associated with high-field MRI. Its practical benefits are further highlighted by the enhanced patient experience, particularly for individuals who require paediatric imaging or have claustrophobia. With the development of technology, the existing limits of low-field MRI could be overcome by merging it with other imaging modalities such as CT and PET, thereby providing even more comprehensive diagnostic options.

In conclusion, low-field MRI is necessary specifically for healthcare due to its importance in diagnosing and treating disease, alongside striking a balance between cost, safety, and efficiency. Its expanding uses and continuous technical advancements should increase its usefulness, and guarantee, that it stays a vital instrument for research and healthcare.

FAQs

Can low-field MRI be used in emergencies? 

Yes, low-field MRI can be used in emergencies, particularly because of its faster scan times and reduced need for extensive infrastructure.

How much does it cost for a scan?

In the UK, the average cost of a private magnetic resonance imaging scan ranges from £200 to £1,500.

How is low-field MRI cheaper?

Low-field MRIs are less expensive to maintain compared to high-field MRI systems. This is because they do not require enormous machines like regular MRIs do. They ultimately have specialised shielding, as well as requiring a lot less power. This results in lower energy use and lower operational costs.