magnetic resonance imaging scanner

Magnetic Resonance Imaging

The 2003 Nobel prize in medicine was awarded jointly to Sir Peter Mansfield and Paul Lauterbur for their contribution towards the development of magnetic resonance imaging (MRI).  However, Raymond Vahan Damadian in 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 MRI spectroscopic technique is based on the absorption and emission of energy of the electromagnetic spectrum in the radiofrequency range (20 kHz to 300 GHz).

The images are generated by spatial variations in the phase. These are dependant 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 which enables clinical MRI images to be generated.

A way of understanding 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.

In order to produce the MRI signal, an external magnetic field must be applied to allow the protons to become aligned.  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 then be created based on the change in the signal from the hydrogen atoms in different types of human tissue.

Currently, several MRI systems are used in medical imaging. With regards to whole-body clinical scanners which have field strengths of up to 3.0-Tesla; it is important to mention that the majority of MRI scanners have a 1.5-Tesla superconducting magnet. In order to put this magnetic field strength into context, it is 30,000 times stronger than that of the earth.

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); in addition to from the top to the 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 superior soft tissue contrast resolution. MRI is used to image internal joints, the central nervous system including conditions which involve inflammatory responses.

MRI has advantages over other imaging modalities and these include:

  • No ionising radiation is involved.
  • 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 which continues to be so even with faster-Computed Tomography with the usage of multislice CT technology. To circumvent this, newer imaging techniques such as parallel imaging have been developed which have the ability to generate a faster pulse sequence and therefore higher field strength systems.  The most basic of pulse sequences include T1-weighted and T2-weighted sequences to highlight the differences in the signal of various soft tissues.

  • T1-weighted sequences are suitable for 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.
  • Tissues that show a high signal on T2-weighted images include fluid-containing structures such as cysts, joint fluid and cerebrospinal fluid.
  • Pathologic states that cause an increase in extracellular fluid resulting from infection or inflammation.

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

  • magnetic resonance angiography (MRA).
  • diffusion-weighted imaging.
  • chemical shift imaging (fat suppression).
  • functional imaging of the brain.
  • MR spectroscopy (MRS).

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 the detection and characterisation of ischemic insults in the brain.
  • MRS uses the changes in chemical composition in tissues to differentiate necrosis from normal brain matter as a result of a tumour.

In musculoskeletal imaging, MR arthrography is a technique available to facilitate the depiction of internal derangements of joints.

  • Indirect arthrography involves intravenous administration of gadolinium to be diffused into the joint.
  • Direct arthrography is when a dilute gadolinium solution is percutaneously injected into the joint. This is to allow enlargement of a joint for evaluation of ligaments, cartilage and synovial proliferation.

MR arthrography is widely used to evaluate:

  • The shoulder by contouring the labral-ligamentous abnormalities. This technique can differentiate damage to the rotator cuff;  in addition to any labral tears in the hip and the collateral ligament of the elbow.
  • Meniscectomy of the knee especially in the detection of 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 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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