MRI: Magnetic Resonance Imaging

Context

Magnetic resonance imaging (MRI) stands as a crucial method to look inside the human body without surgery. 

What is MRI (Magnetic Resonance Imaging)?

• Evolution of MRI Techniques: Its foundational techniques were developed in the early 1970s.
  • In the later part of the decade, Paul Lauterbur and Peter Mansfield made pivotal refinements that enabled its widespread adoption for medical purposes. 
  • For these efforts, they were awarded the medicine Nobel Prize in 2003.
• About: MRI is used to obtain images of soft tissues within the body. Soft tissue is any tissue that hasn’t become harder through calcification. 
  • It is a non-invasive diagnostic procedure widely used to image the brain, the cardiovascular system, the spinal cord and joints, various muscles, the liver, arteries, etc.
• Usage: Its use is important in the observation and treatment of certain cancers, including prostate and rectal cancer, and to track neurological conditions including Alzheimer’s, dementia, epilepsy, and stroke. 
  • Researchers have utilized MRI scans to observe alterations in blood flow, allowing them to infer fluctuations in brain neuron activity. This application of the technique is known as functional MRI (fMRI).

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Working of MRI

Hydrogen Atom Imaging: An MRI procedure reveals an image of a body part using the hydrogen atoms in that part.  A hydrogen atom is simply one proton with one electron around it. These atoms are all spinning, with axes pointing in random directions.  Hydrogen atoms are abundant in fat and water, which are present almost throughout the body.

• MRI Machine Components: An MRI machine has four essential components. 
  • Bore: The machine itself looks like a giant donut. The person whose body is to be scanned is inserted inside the hole in the centre, called the bore. 
    • There is a powerful superconducting magnet inside the donut whose job is to produce a powerful and stable magnetic field around the body. Once the body part to be scanned is at the centre of the bore, the magnetic field is switched on.
  • Magnetic Moment: Each hydrogen atom has a powerful magnetic moment, which means in the presence of a magnetic field, the atom’s spin axis will point along the field’s direction. 
    • The superconducting magnet generates a magnetic field along the central axis of the machine.
    • This causes approximately half of the hydrogen atoms within the targeted area to align in one direction while the other half aligns in the opposite direction. 
    • This alignment is nearly precise, with only a few atoms out of millions remaining unaligned, forming a small population of “excess” atoms oriented in one of the two directions.
  • Radiofrequency Pulse and Atom Excitation: A device that emits a radiofrequency pulse at the part under the scanner. 
    • When the pulse is ‘on’, only the small population of ‘excess’ atoms absorbs the radiation and gets excited. 
    • When the pulse goes ‘off’, these atoms emit the absorbed energy and return to their original, lower energy states. 
    • The frequency of pulse the ‘excess’ atoms have to absorb is called the Larmor frequency. Its value depends on the strength of the magnetic field and the type of tissue in which the atoms are present.
  • Detector:  A detector, receives the emissions and converts them to signals, which are sent to a computer that uses them to recreate two- or three-dimensional images of that part of the body.

Pros of MRI

• Gradient Magnetic Fields for Focused Imaging: Following the activation of the primary strong magnetic field, the MRI machine engages three additional magnets that generate smaller magnetic fields.
  • These fields are weaker than the main field by a factor of approximately 80 or more. These auxiliary fields also possess a gradient, meaning they are not uniform. 
  • They interact with the main field in the targeted area of the scan, emphasizing specific regions for detailed imaging.
• Precision Scanning with Gradient Magnet Sequences: By selectively activating and deactivating the gradient magnets in predetermined sequences, the MRI machine can scan segments as narrow as a few millimeters. 
  • These sequences can also be coordinated to scan various regions of the individual’s body without necessitating movement within the bore.
• Comprehensive Body Imaging Capability: Due to the machine’s construction and the arrangement of magnets within it, an MRI scan can effectively capture images of the body from multiple useful angles and, if necessary, in extremely fine increments.
• Tissue Differentiation: When the “excess” atoms release the absorbed energy to revert to lower energy states, this process occurs over a period known as the T1 relaxation time. 
  • Hydrogen atoms in water exhibit varying T1 values based on the tissue they inhabit. MRI machines capitalize on this discrepancy to represent different tissues in distinct shades of grey. 
  • Additionally, clinicians may inject a contrast agent—usually a gadolinium-based compound—to individuals, reducing the T1 time in certain tissues and enhancing their visibility in MRI scans.
• Safety of MRI Scans and Magnetic Field Effects: Extensive research has explored the impact of strong magnetic fields on the human body. 
  • MRI scans are deemed safe as the magnetic fields dissipate once the scan concludes, and the atoms within the scanned area return to their normal state without enduring any lasting effects. 
  • There is no evidence of long-term harm associated with MRI scans. 

Cons of MRI

• Shortcoming: Due to the strong magnetic fields utilized in MRI scans, individuals with implanted metallic objects such as shrapnel or medical implants like pacemakers may be ineligible for the procedure. 
  • Additionally, even carrying a credit card in their pocket can result in the magnetic strip being erased by the MRI’s magnetic fields.
• Huge Cost: MRI machines incur significant expenses, with costs ranging from several tens of lakhs to a few crores, depending on factors like magnetic field strength and imaging quality specifications. 
  • These expenses are transferred to patients by diagnostic facilities. Depending on clinical needs, individual scans can often exceed Rs 10,000, a substantial amount in India, particularly for uninsured individuals and those needing multiple MRI scans.
• Discomfort of Using the Machine: Although, conveniently, the individual doesn’t need to relocate within the bore for scanning different body parts, they are required to remain still for extended periods, often tens of minutes, until the scan concludes. 
  • Any movement by the individual can distort the resulting image, necessitating a repeat of the scan.
• Heat Dissipation: Producing a magnetic field of 1 tesla or higher, like the main magnet does, is a considerable challenge. 
  • As the non-superconducting materials will dissipate energy as heat, sustaining this setup requires a significant amount of energy, which incurs high costs.
• Noise Generation in MRI Operation: The switching of heavy currents within the machine, particularly when the gradient coils operate sequentially, results in loud noises during operation. 
  • This additional noise can cause discomfort for the individual undergoing the scan.

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