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FOUNDATIONS OF MRI

FOUNDATIONS OF MAGNETIC RESONANCE IMAGING

This is not official material but shall be posted in short spurs as I navigate my way around the complex yet fascinating world of MRI, hence there is no guarantee on the correctness of the material. For official purposes you can refer some material like The physics of Clinical MRI
taught through Images, From picture to proton, How MRI works, for a better and deeper insight of the topics I will try to cover here. Any kind of misspelling, errors, or in case the extension of any section is needed, can be mailed to me :)

” MRI is no rocket science, but I like it” - Anonymous

How would you impress a stranger you meet at a party with your intelligence? You might claim to be a brain surgeon or a rocket scientist. Well Magnetic Resonance (MR) is not rocket science, it’s better. MR involves an amazing combination of advanced science and engineering,
including the use of superconductivity, cryogenics, quantum physics, digital and computer technology – and all within the radiology
department of your local hospital.

What is the fuss all about?

Magnetic resonance imaging at the core relies on the properties of sensitivity and the presence of water molecules which make up to 70-80% of our body mass. The properties and amount of water in the tissue can dramatically change and effect the results and properties of water,
hence MRI is an a very important diagnostic tool for imaging. MRI helps not only identify the anatomical and the organ functioning but also
in the way the brain works and thinks specifically.

In the earlier days when Raymond Vahan Damadian (born March 16, 1936) an American physician, medical practitioner, and invented the MRI Machine
via his work on sodium and potassium in living cells leading him to his first experiments with nuclear magnetic resonance (NMR) which caused him to first propose the MR body scanner in 1969. Damadian invented an apparatus and method to use NMR safely and accurately to scan the human body, a method now well known as magnetic resonance imaging (MRI).

MEDICAL IMAGING FUNDAMENTALS:

The branch of radiology traces back to centuries when Roentgen accidentally discovered the X-rays in 1895. About at the same time the Curies were
discovering radioactive materials which then lead to the advent of nuclear medicine as another branch of imaging. We then witnessed the advent of fluorescent materials, computed radiography and the times when the simple way for imaging was to put your hand in the way of the beam which is an idea frowned and deemed unacceptable today. The next real breakthrough was the advent of the Computed Tomography or commonly called as CT by
Hounsfield and Cormack paving them the Nobel Prize for medicine and physiology in 1979.It was now possible to visualize slice by slice with
effective spatial resolution of the person’s anatomy and to detect any sort of abnormality, trauma, etc.

In nuclear medicine a similar evolution was occurring, from the development of gamma camera by Anger in 1958 to tomographic imaging in the form of Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), which is ongoing today. PET’s clinical use is increasing, particularly in detecting metastases in oncology.Its ability to image minute concentrations of metabolites is unique and makes it a powerful research tool in an etiology of disease and the effects of drugs. Ultrasound was developed in the 1950s following the development of SONAR in World War II and was unique in involving no ionizing radiation and offering the possibility of safe, non-invasive imaging. Its ability to image in real time and its sensitivity to flow, through the Doppler effect, have been key factors in its widespread role in obstetrics, cardiology, abdominal investigations and vascular imaging, real-time biopsy guidance and minimally invasive surgery. At the same time, developments in cryogenics, or the study of very low temperatures, made the development of whole-body superconducting magnets possible.

Damadian and his colleagues at the State University of New York, starved of mainstream research funding, went so far as to design and build their own
superconducting magnet operating in their Brooklyn laboratory, and the first human body image by NMR is attributed to them. Due to problems of low signal and high sensitivity to motion, body MR did not really take off until the 1990s. The key factors were the development of fast imaging techniques, particularly gradient echo,and phased array coil technology. The 1990s also saw the coming of age of earlier developments, namely cardiac MRI and Echo Planar Imaging (EPI). EPI, which is the fastest and one of the most cutting-edge methods, was actually one of the first imaging methods to be proposed, by Sir Peter Mansfield. EPI is now extensively used in neurological imaging through functional MRI (fMRI) and diffusion imaging.

THE BASIC STUFF

The first new day at work, the feeling that I resonate is getting acquainted just with the basic new stuff (even though it’s a little too overwhelming) to start with. The setup of MRI is a little too complicated at first but eases down just like everything in life eventually does! The following are just some of the useful instructions to take attention of:

a. Magnetic safety

b. Clear instructions and policies on the MR unit

c. Patient cooperation

The MR accommodation typically comprises of the following:

a. The patient waiting areas, reception, toilers, anesthesia room, recovery room.

b.MR system: the MRI scanner room (magnet/examination room), computer/technical room and operator’s console/host computer, dedicated storage areas: trolley bay, general store, resuscitation trolley bay, cleaner’s store.

c. The MRI scanner room, or magnet room or examination room is a restricted access or controlled access area. The MR system is actually
distributed between three of the rooms in the suite: the magnet room which houses the magnet and coils, an air-conditioned technical (computer)room which is full of supporting electronics and electrical plant, and the control room which contains the MR console.

THE MAGNET: The magnet is the heart of the MR unit. The size of an MR system is expressed in terms of its operating magnetic field strength. The scientific name
of field strength is magnetic flux density or induction, and its unit is the tesla (T). You may also come across the gauss (G) as a measure of field strength. One Tesla equals 10 000 gausses, i.e., 1 G equals 0.1 mT (milli Tesla). The Earth’s magnetic field is approximately 0.05 mT (0.5 G). Hydrogen atoms have a nuclear spin and associated with the nuclear spin is the magnetic moment. An externally applied magnetic field causes the atoms
to align in the direction of the applied magnetic field.

The principal types of magnets used in MRI are:

a. Superconducting magnets – typically with fields of 1.5 or 3 T; b. Permanent magnets – capable of sustaining fields up to to about 0.3 T electromagnets – capable of fields up to about 0.6 T.

Word of Caution:

The main field usually points horizontally along the bore (the opening where the patient goes). For superconducting and permanent magnets, the magnetic field is always present; electromagnets are electrically powered and can have their field switched off; however, it is safer to assume that it is always on. Superconducting magnets require liquid helium as a cryogenic cooling fluid. A sudden loss of superconductivity results in a magnet quench were the windings heat up, the field collapses in less than one minute and large amounts of helium boil off as gas. Accidental quenches are a rare occurrence in modern systems. In an emergency a quench can be initiated deliberately. In normal operation, small amounts of helium ‘boil off’ and are released into the atmosphere outside. The helium level is usually maintained by the manufacturer’s service personnel.

Radiofrequency coils

The RF coils are the components that allow the transmittance of the RF pulses that help us generate the MR signals, these are incorporated in
the MR unit usually called the body coils. For extremities or imaging one region of the body, transmitter coils are used sometimes. The MR signals are detected by the receiver coils and are generally are available for the spine, neck, knee, wrist, shoulder, breast, Temporo-Mandibular Joint (TMJ), abdomen, and also peripheral vascular and other general-purpose flexible coils. You can actually use any coil to obtain an image provided it encompasses the anatomical region of interest, but specialist coils, which fit closer and are smaller, usually do a better job. Since the receiver do the job of receiving the signal which forms the basis of the image, they must give the respect they deserve.

IMAGING GRADIENTS:

One of the most crucial elements of MR is the localization of the signal in a region of interest in the body which is possible by varying the magnetic field gradient the magnitude of the gradient magnetic field is in the region of tens of mT, much smaller than the main Bo field. There is one set of
gradient coils for each direction, x, y, z, built in the bore of the magnet. The gradients are applied repeatedly in a carefully controlled pulse sequence.

WILL I EVER FEEL ANYTHING??

Now the fundamental question one often more or less encounters is what does the MR beam comprise of, is it ionizing similar to CT, X-RAY, etc or is it non ionizing which is deemed as “safe”, will I encounter any bio effects post MR? MRI typically uses non ionizing radiation unlike other modalities and there is no evidence that suggests that it can lead to any potentially harmful
disease. The varying gradients used for the localization of signal may lead to peripheral nerve stimulation which might be a little comfortable but
something that should cause worry. One of the major effects that one does observe is the specific absorption rate commonly called the SAR which the
system does dully care off, SAR implies the heating of the tissue, the MR system is built in a way that when one enters the specificities of the patient, depending on the parameters the system accepts or rejects to give a go ahead for the scan. The typical allowable limit for the tissue to get heated up is 1 degree. SECOND WORD OF CAUTION

BURNING ISSUES

-There is a small risk of patients receiving burns through the coupling of RF energy into wires or cables, such as those used for Electrocardiogram (ECG) triggering, that are touching the patient. Care must be exercised in ensuring that cables are not formed into loops, that dry flame-retardant pads are
placed between cables and the patient and that any unnecessary cables are removed from the patient prior to imaging.

MAGENTIC FIELD STRENGTH:

To be preventative and very cautious is the first piece of sane thing to do when dealing with diagnostic equipment. For safety purposes it will always be assumed that the magnetic field will always be on, the fringe field will always be present beyond the physical main magnet, the intensity thus decreases as we move away from the magnet, but the static field gradient is always present. Any kind of ferromagnetic object can prove to be lethal as it can be pulled inside the bore with a very strong force, hence before any personnel or patient enters the MR scan room, he/she is advised to get rid of ant metallic objects, implants. In the modern MR Machines, the stray fields change very rapidly hence in case one is not very careful about the pre check, by the time one realizes it might be too late!!

SAFETY SECOND-ADDITIONAL REQUIREMENTS

Implanted ferromagnetic items such as vascular aneurysm clips may also experience these forces and torques. There has been at least one reported death Of a patient scanned with a ferromagnetic aneurysm clip that moved, rupturing the blood vessel, as they were moved into the magnet. Similar hazards arise with patients who may have metallic foreign bodies located in high-risk areas such as the eye. Alternatively, the function of Active Implanted Medical
Devices (AIMDs) such as pacemakers or cochlear implants may be severely impaired by the static magnetic field and persons with pacemakers are normally excluded from the 0.5 mT fringe field. The same rules apply to any pieces of medical equipment that may also need to be taken into the room; for example , a pulse oximeter for monitoring a sedated patient.

CONTRAINDICATONS FOR MRI:

Conventional cardiac pacemakers or implanted cardiac defibrillators, abandoned cardiac leads and cochlear implants. MRI examinations require particular caution in the following cases: Patients with:

 Implanted surgical clips or other potentially ferromagnetic material, particularly in the brain, patients with AIMDs, e.g., neuro-stimulators, MR conditional cardiac placements, ingested endoscopic cameras;

 Engagement in occupations or activities that may have caused the accidental lodging of ferromagnetic materials, e.g., metal- workers, or anyone who may have embedded metal fragments from military duties;

 Neonates and infants, for whom data establishing safety is lacking;

 Tattoos, including permanent eye-liner;

 Compromised thermoregulatory systems, e.g., neonates, low-birth-weight infants, certain cancer patients

 Prosthetic heart valves

 Pregnant patients: although no MRI effects have been found on embryos and fetal MRI is performed in specialist centers, many units still avoid
scanning pregnant women during the first trimester. The unknown risk to the fetus must be assessed.

References

Donald W. McRobbie , Elizabeth A. Moore , Martin J. Graves , Martin R. Prince Amato, From Picture to Proton,Oxford, UK: Cambridge University Press American National Cancer Institute,NCI, 2021,Available[https://www.cancer.gov/] (Accessed:Jun 16,2021).