Magnetic resonance imaging (MRI) is an imaging technique used for the examination of anatomical structures, biological functions, and body fluids. The magnetic resonance imaging (MRI) uses the body’s natural magnetic properties to yield detailed images from any part of the body. For imaging, the hydrogen nucleus (a single proton) is used as it is abundant in water and fat. Since the 1980s, magnetic resonance imaging (MRI) has become a powerful tool in musculoskeletal diagnosis, staging, and monitoring, with a wide array of potential applications. The MRI allows noninvasive and dynamic evaluation of organs, muscles, and cartilage in multiple planes (2-D or 3-D). In MRI no ionizing radiation is used; therefore, it is essential in the diagnosis and monitoring of a variety of musculoskeletal and neuromuscular conditions in the human body.
Anatomical tissue- and sequence-dependent parameters control the quality of the images generated by magnetic resonance imaging. The specificity of the MRI to manipulate these interactions allow delineating different tissue contrasts and pathological conditions from the same anatomy. The magnetic resonance imaging provides excellent soft tissue contrast and is ideal for the examination of striatal muscle volume, architecture, composition, and shape. The signal characteristics of soft aqueous tissue could provide not only anatomical but also functional information such as perfusion, diffusion, magnetic susceptibility, and metabolism. Therefore, the MRI is widely used for the examination of the brain, spinal cord, nerves, as well as muscles, ligaments, and tendons (Berger, 2002).
Magnetic resonance imaging utilizes the body’s magnetic field to create images. Static magnetic field makes certain nuclei (including the proton, 1H) sensitive to the oscillating magnetic fields and resonates them in a synchronized manner. The hydrogen nucleus is used which is present in water molecules, and therefore in all body tissues. The hydrogen nuclei act like compass needles partially aligned by the strong magnetic field in the scanner. The nuclei are rotated using radio waves and subsequently oscillated in the magnetic field while coming back to equilibrium. They emit a radio signal during this, which is detected by the antennas (coils) and is used for making detailed images of body tissues. The frequency of these magnetic oscillations at which the nuclei resonate is proportional to the static field strength. The application of a linear magnetic field gradient makes the magnetic field stronger on one side of the object as compared to the other. The resonance frequency can determine the position of the resonating nuclei. The signals from nuclei resonating at different frequencies would immediately cancel each other out. Specific methods are needed to refocus these signals for detection. To position the object in the three dimensions of space, three field gradients are required. The relaxation properties of the nuclei could provide additional information for the diagnosis (McMahon., Cowin., & Galloway., 2011).
The magnetic resonance imaging system consists of major functional components that work together to produce images. To acquire the images the patient’s body is placed in a strong magnetic field with specific direction at each point. The direction of the magnetic field is used as a reference for the direction of the tissue magnetization. The strength of the magnetic field is determined by the type and design of the magnet used. In MRI, the superconducting magnets are used to produce strong magnetic fields. Whereas, the resistive and permanent magnets are limited to relatively weak magnetic fields. Homogeneity of the field is required for efficient imaging; it is reduced by magnetically susceptible materials that interfere with the field. Shimming is the process of homogenizing the magnetic field. This is achieved by adding passive shims when a magnet is installed. There are three gradient coils oriented to produce gradients in the three orthogonal directions. Gradients perform several functions in the image acquisition process. The MRI process also consists of an exchange of radio frequency (RF) pulses and signals between the machine and the patient’s body. This is done through the RF coils which serve as the antenna for pulse transmission and signal reception. The imaging area is enclosed in a conductive metal to block external RF interference. The RF images are acquired and assessed in the computer to determine the characteristics of the images (Sprawls, 2000).
Patient set-up and equipment
- Interview the patient to ensure that there are no implants such as cardiac pacemaker containing conducting or ferromagnetic materials that could interfere with the image quality. Make sure to have emergency equipment present near the patient if he/she is suffering from any serious health problem.
- Obtain the consent of the patient if the research protocol is planned.
- Remove all the jewelry and change the clothes of the patient to eliminate any metal that could be found in clothing.
- Remove all the makeup to avoid local tissue heating and image artifacts.
- Inform the patient about the procedure.
- Lay down the patient on the scanning table. Set up the monitoring equipment.
- Position the patient’s head in the 8-channel head coil. Constrain the patient’s head with pads to reduce any motion.
- Place a triangular-shaped support or pillow under the knees of the patient.
- ON the patient fan and give the patient a sheet or lightweight blanket.
- Level the centering light at the level of the eyebrows, and then advance the patient into the center of the magnet.
- Guide the patient to remain still during the scans. Sedatives might be given if the patient could not stay still during the procedure.
- To capture the major vessels of the head in anterior projection, run the phase-contrast localizer scan.
- Acquire ASSET calibration that provides a low-spatial-resolution sensitivity map for the coil elements of the 8-channel receive-only head coil.
- Obtain sub-volume maximum intensity projections (MIPs) by running the interactive vascular imaging (IVI) post-processing program.
- Examine the images after the scan to check whether any more are required.
Applications of magnetic resonance imaging in particulate systems (Sederman., Gladden., & Mantle, 2007)
Magnetic resonance imaging (MRI) is a well-established technique not only for the disease diagnosis, staging, and treatment monitoring but also in the field of engineering and materials science. The MRI scans are particularly useful in the area of particulate systems and granular material flows. MRI is used to decipher the particle distribution and motion with in situ measurements. The imaging of particulate systems with MRI has been used to identify the particles that have a hard outer shell and solid particles in motion but have liquid-like centers. The experiments were performed on the rotating, gas-, and vibro-fluidized beds where poppy/millet seed mixture was examined for signal intensity. The high-resolution 3-D images of particle distribution were obtained to measure and quantify the segregation as the process develops. A much less sharp interface was found within the bed, and a thin cylinder of poppy seeds remained along the center of the bed. In gas fluidized beds, the local velocity distributions could be measured by the minimization of velocity-encoding times and echo times. In vibro-fluidized beds, it was observed that further adjustment is required because of the higher velocities and accelerations. It was concluded that the magnetic resonance imaging could also be used for velocity imaging in vertical fluidized beds and phase-resolved velocity distributions within vertical vibro-fluidized beds in rotating bed segregation.
Imaging in pediatric oncology (Manias et al., 2017)
Magnetic resonance imaging is central to the detection and management of solid tumors in children. Conventional magnetic resonance imaging (MRI) serves as a standard imaging modality for tumors of the central nervous system (CNS), limbs, and abdomen. It provides the clinicians with appropriate structural detail but imparts limited information about tumor type, aggressiveness, metastasis, and treatment response. In addition to the conventional magnetic resonance imaging, the magnetic resonance spectroscopy, diffusion, and perfusion-weighted imaging, probe tissue properties are also used for the characterization of metabolites and the assessment of their structure. The MRI is increasingly improving the understanding of childhood tumors in situ. Clinically, magnetic resonance imaging could improve non-invasive diagnosis and early treatment monitoring, as well as provide ample information about biomarkers for disease prognosis.
Detection of cardiovascular thrombi in children (Dominguez., Yoo., Seed., & Grosse-Wortmann., 2018)
Cardiovascular thrombosis is a rare condition in children but predisposing conditions such as indwelling vascular catheters, tumors, aneurysms, ventricular dysfunction, or after surgery cause it. Clots can form in the cardiac chambers, veins, arteries, or inside conduits. Detection of thrombi has become easier and feasible with a variety of magnetic resonance imaging (MRI) techniques, including contrast-enhanced MR angiography. MRI is useful in detecting thrombus particularly in areas of difficult visualization such as atrial appendages, the ventricular apices, coronary arteries, and distal pulmonary arteries. Magnetic resonance imaging provides information about arterial wall integrity and inflammation. The specificity and non-invasiveness of the magnetic resonance imaging benefit the clinicians with the diagnosis of congenital heart disease, vasculitis, cardiomyopathy, and many other conditions.
Applications in clinical neurology and psychiatry (Mitterschiffthaler et al. 2006)
Magnetic resonance imaging has been rapidly used as a tool for studying brain function. The development of robust methods for data acquisition, paradigm design and data analysis techniques of functional magnetic resonance imaging (fMRI) has enabled the researchers to study normal brain function. The fMRI has made the diagnosis of three less well-documented psychiatric disorders: attention deficit hyperactivity disorder (ADHD), depression, and obsessive-compulsive disorder (OCD) feasible. It is also being used as an important imaging technique to differentiate between genes and phenotypes, and therefore, contribute to a better understanding of the pathophysiology of major neuropsychiatric diseases. The magnetic resonance imaging is a valuable imaging technique for the objective assessment of therapeutic intervention and treatment response in neurological disorders.
- Keep the metallic objects out of the room where the machine is operating as the powerful magnetic field of the MRI system can attract ferromagnetic objects and cause them to move with great force.
- Remove all the belongings and jewelry from the patient before starting the procedure.
- Special care has to be taken for patients with surgical implants or medication pumps.
- Guide the patient carefully as there is a risk of fingers being trapped between the patient table and top board.
- Provide the subjects with hearing protection and audio diversions.
- Special training and guidance should be given to the patient and the technicians before starting the procedure.
Strengths and limitations
- The magnetic resonance imaging does not involve exposure to radiation, so it can be safely used in people who are vulnerable to the effects of radiation, such as babies and pregnant women.
- The MRI is useful for the visualization of soft tissue structures, such as ligaments and cartilage, and delicate organs such as the heart, brain, and eyes.
- The MRI can yield information about the blood flow through certain organs and blood vessels, allowing the detection of the problems with blood circulation, such as blockages.
- MRI scans have specific advantages over X-rays, as they can show swelling and inflammation and present both the three-dimensional and cross-section images of the body.
- The loud noise and enclosed space could make some people feel claustrophobic having an MRI scan.
- MRI procedures could be affected by movement, making them unsuitable for investigating mouth tumors.
- The magnetic resonance imaging could not show tissue calcifications or osteoporosis.
- Sederman., L. F. Gladden., & Mantle, M. D. (2007). Application of magnetic resonance imaging techniques to particulate systems. Advanced Powder Technol, 18(1), 23-28.
- Manias., S. K. Gill., L. MacPherson., K. Foster., A. Oates., & Peet., C. A. (2017). Magnetic resonance imaging based functional imaging in paediatric oncology. Eur J Cancer, 72, 251-265.
- Berger, A. (2002). Magnetic resonance imaging. BMJ, 324(7328), 35.
- , S. J. Yoo., M. Seed., & Grosse-Wortmann., L. (2018). Magnetic resonance imaging of cardiovascular thrombi in children. Pediatr Radiol, 48(5), 722-731.
- K. McMahon., G. Cowin., & Galloway., G. (2011). Magnetic resonance imaging: the underlying principles. J Orthop Sports Phys Ther, 41(11), 806-19.
- A. Bernstein., & Huston., J. (2008). Head and Neck MRA at 3.0T. Current Protocols in Magnetic Resonance Imaging, A7.8.1-A7.8.16.
- Sprawls, P. (2000). Magnetic Resonance Imaging Principles, Methods, and Techniques. Wisconsin: Emory university.
- M. Mitterschiffthaler., U. Ettinger., A. M. Mehta., D. Mataix-Cols., & Williams, S. C. (2006). Applications of functional magnetic resonance imaging in psychiatry. J Magn Reson Imaging, 23(6), 851-61.