Magnetic Resonance Imaging of the Human Body: Principles, Techniques, and Applications

MRI is a non-invasive diagnostic tool

The 2003 Nobel Prize in Medicine was awarded jointly to Sir Peter Mansfield and Paul Lauterbur for their contribution to the development of magnetic resonance imaging (MRI).  However, Raymond Vahan Damadian 1977 was the first to perform a full-body scan of a human being to diagnose cancer.

The principles of magnetic resonance imaging (MRI) are based on the fundamentals of nuclear magnetic resonance (NMR), which is used to obtain structural and physical information on chemical compounds. This magnetic resonance imaging (MRI) spectroscopic technique is based on the absorption and emission of energy from the electromagnetic spectrum in the radiofrequency range (20 kHz to 300 GHz).

Spatial variations in the phase generate the images. These are dependent on the radiofrequency energy,  which is absorbed and emitted by the imaged object. Several biologically active nuclei can produce magnetic resonance images, including hydrogen, oxygen-16, oxygen-17, fluorine-19, sodium-23 and phosphorus-31.

The human body contains 63% hydrogen atoms from its fat and water components. Each hydrogen nuclei has a characteristic NMR signal, enabling clinical magnetic resonance imaging to generate pictures.

One way to understand this is to imagine the protons behaving like tiny bar magnets with the associated north and south poles lying within the domain of the magnetic field.  However, the magnetic moment of a single proton is tiny and undetectable, whereas the orientation of the protons is random in the setting without an external magnetic field.

An external magnetic field must be applied to allow the protons to become aligned in order to produce the magnetic resonance imaging (MRI) signal. This results in an increase in a measurable magnetic moment in the direction of the external magnetic field. Therefore, by applying a sequence of radiofrequency pulses, various images can be created based on the change in the signal from the hydrogen atoms in different types of human tissue.

Several MRI systems are currently used in medical imaging. Regarding whole-body clinical scanners with field strengths of up to 3.0 Tesla, it is important to mention that most MRI scanners have a 1.5-Tesla superconducting magnet. To put this magnetic field strength into context, it is 30,000 times stronger than that of the Earth.

MRI scans produce detailed images of the body’s interior

A complete MRI scan can take up to 75 minutes, depending on the procedure involved. These may include scanning the brain, heart, spine, liver and muscles.  MRI data can produce exceptional structural images of the brain and spinal cord.

These images are formed from a series of slices which can be viewed from:

  • The front to the back (coronal) and from top to bottom (axial).
  • From one side of the body to the other (sagittal).

These images are generated in 3 planes and analysed by the radiologist, who will provide a clinical opinion.

MRI techniques are useful due to their higher soft-tissue contrast resolution, especially in imaging the internal joints and the central nervous system, including conditions that involve inflammatory responses.

Magnetic Resonance Imaging: Advantages and Limitations in Clinical Diagnosis

  • MRI and Ultrasound imaging involve no ionising radiation.
  • Resolution of soft tissue using contrast agents.
  • High-resolution imaging, including multiplanar imaging abilities.

The duration of an MRI image is regarded as a significant disadvantage, and this continues even with faster-computed tomography using multislice CT technology. To circumvent this, newer imaging techniques, such as parallel imaging, have been developed to generate a faster pulse sequence and, therefore, higher field strength systems. The most basic pulse sequences include T1-weighted and T2-weighted sequences to highlight the differences in the signals of various soft tissues.

  • T1-weighted sequences are suitable for the evaluation of various anatomic structures.
  • Tissues that allow a high signal (bright) and T1-weighted images include fat, blood, proteinaceous fluid, melanin and the contrast agent gadolinium.
  • T2-weighted sequences are useful for the identification of pathologic processes.
  • Higher T2-weighted images are obtained with joint and cerebrospinal fluids and cysts.
  • Pathologic state causes an increase in an extracellular fluid resulting from infection or inflammation.

Several advanced medical imaging techniques are based on MRI and include:

  • Functional imaging of the brain.
  • Diffusion-weighted imaging.
  • Magnetic resonance angiography (MRA).
  • MR spectroscopy.
  • Chemical shift imaging (fat suppression).

Several of the above techniques are especially useful in brain imaging and include:

  • MRA in the time-of-flight or phase contrast mode, in addition to diffusion-weighted imaging, is helpful in detecting and characterising ischemic insults in the brain.
  • MRS uses changes in tissue chemical composition to differentiate necrosis from normal brain matter caused by a tumour.

In musculoskeletal imaging, MR arthrography is a technique that facilitates the depiction of internal derangements in joints.

  • Indirect arthrography involves the intravenous administration of gadolinium to be diffused into the joint.
  • Direct arthrography involves injecting a dilute gadolinium solution into the joint. This allows the joint to be enlarged to evaluate ligaments, cartilage, and synovial proliferation.

MR Arthrography: Techniques, Applications, and Clinical Outcomes

  • contouring the labral ligamentous abnormalities in the shoulder. This technique can differentiate damage to the rotator cuff, labral tears in the hip, and the collateral ligament of the elbow.
  • Meniscectomy of the knee, especially in detecting recurrent or residual meniscal tears.
  • Investigate perforations of the ligaments and triangular fibrocartilage in the wrist.
  • The stability of osteochondral lesions associated with the articular surface of joints.

MR arthrography uses T1-weighted images to convey the T1 shortening effects of gadolinium resulting from fat saturation.  The T2-weighted sequence in at least one plane is also necessary to detect cysts and oedema in other soft tissues and bone marrow.

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