MRI.
P.K. Ghatak, M.D.
The magnetic resonance imaging (MRI) is one of the most advanced bioengineering technologies introduced in medical diagnostic armaments.
The first ever use of MRI took place in Britain in 1980, and then in the USA. Then MRI spread across the globe, limited in the countries by their inability to pay for the high price of the machine and non-availability of the infrastructure.
History of Magnetic Resonance:
Protons and Electrons when placed in an Electromagnetic field become excited, either due to absorption or emission of Electromagnetic Radiation. This phenomenon was first observed by the Soviet scientist Y.K. Zavoysky in 1944. The Nuclear Magnetic Resonance was detected by physicists Bloch and Purcell in 1946.
The Nobel Prize in Medicine in 2003 was awarded to Paul Lauterburg, of the University of Illinois and Peter Mansfield of the University of Nottingham, UK, for their discoveries in MRI. However, it must be understood that these two scientists based their work on the works of so many other scientists, and notable among them, are Felix Bloch and Edward Purcell, a pair of Nobel Laureates in Physics in 1952, for their discovery that certain nuclei could absorb and emit radiofrequency energy (Rfw) when placed in a magnetic field.
How MRI came to the medical field:
In 1970, Raymond Damadian discovered normal and cancer cells had different magnetic properties. John Mallard of the University of Aberdeen, Scotland used this principle in producing a full body image in 1970. Subsequently, this machine was used at St. Bartholomew hospital, London from 1983 to 1993. John Millard was given credit for the first ever use of MRI in the medical field.
Basic characteristics of human cells that are utilized in MRI.
Every living cell of the body contains water (H2O). The amount of water in normal cells of different tissues, and specially those cells that are diseased, has different levels of water content.
The hydrogen ion of the water is a Proton. In a strong magnetic field, the protons of the body can be manipulated to line up in one direction. A second magnetic field of different strengths and different orientations is pulsed several times a second. This energy knocks off the orientation of the protons and the protons take up another position. When the second magnetic field is completely switched off, the protons begin to go back to their original orientation and emit energy. The energy emitted by cells is proportional to their water content. This energy is collected and synthesized to generate MRI.
There are two standard images, one is T-1 weight image and the second is a T-2 weighted image. Additional images are the Proton Density (PD) image and Fluid Attenuated Inversion Recovery Image (FLAIR).
T-1 Image:
T-1 image relates to how quickly the nuclei of Hydrogen atoms return to their equilibrium state after being disturbed by an external magnetic pulse. It measures the time taken for the longitudinal magnetization of a tissue to recover. T-1 is also called the Longitudinal Relaxation Time image. Different tissues have different T-1 values, leading to varying rates of recovery and influencing the contrast in MRI images.
T-2 image:
The T-2 image sequences are controlled by the Repetition Time (TR) and Echo Time (TE) values. It measures the signal decay in the transverse planes. It is also known as a transverse relaxation image.
By manipulating the timing of RF pulses and sequences, the T-I and T-2 images are generated. T-1 images show normal anatomy in great detail. The T-2 images highlight the inflamed area and edema.
Proton Density Image:
The tissues with a high concentration of protons generate a strong signal and appear as the brightest image by minimizing the impact of T- and T-2 differences, by using a long TR and a short TE.
FLAIR image:
It is designed to suppress the signal from the cerebrospinal fluid (CSF) and make signals coming from diseased portions of the brain and spinal cord more intense. The fluid appears dark and the pathological parts stand out due to enhanced contrast.
How different parts of brain appears under T-1, T-2, FLAIR and PD views:
|
Part |
T-1 |
T-2 |
FLAIR |
PD |
|
CSF |
Dark |
Intermediate |
Darker |
Dark |
|
Fat |
Bright |
Very bight |
Bright |
Very bright |
|
Air / sinus |
Dark |
Dark |
Dark |
Dark |
|
Blood |
Dark |
Dark |
Dark |
Dark |
|
Blood vessels |
Dark |
Dark |
Dark |
Dark |
|
White Brain Matter |
Intermediate |
Darker than Gray matter |
Intermediate |
Darker than Grey matter |
|
Gary Brain Matter |
Darker than white matter |
Intermediate |
Intermediate |
Intermediate |
|
Bone |
Dark |
Dark |
Dark |
Dark |
|
Bone marrow |
Bright |
Variable |
Dark |
Variable |
|
Brain Stem |
Intermediate |
Intermediate |
Intermediate |
Intermediate |
The MRI images can be synthesized in three planes – Axial, sagittal and Coronal Planes.
Contraindications and precautions for MRI:
The MRI suite is specially built because a very strong magnetic field is generated in this room. Any ferromagnetic metal, free or embedded in the body, will be magnetized and move towards the strong magnetic force.
This is a prime concern for patients having a cardiac packer, internal cardiac defibrillator, and inferior vena cava filters implanted in their bodies. The current manufacturers of these kinds of medical devices and have responded and now manufacture the devices with non–non-ferromagnetic metals.
The patients going for MRI should bring to the attention of the MRI personnel, if they have any of the following devices implanted in the body.
Cardiac pacemaker, implanted pacemaker, vascular shunt, Skull fracture, or craniotomy repaired with metals. Bullet or projectile injury, implanted simulators, deep-brain simulators, and vegas nerve simulators. Stimulators of the urinary bladder, spine, and nerves. Metal replacement of joints. Cochlear implants. Insulin pump and narcotic or any medicine delivery devices. Programmable shunts, aneurysm clips or coils. Venous filters. Continuous glucose monitor. A medication patch is applied to any part of the body. Any other devices attached to the body are not mentioned.
It is important to note that not all of these devices implanted in the body are contraindications for taking MRI images, but the radiologist must know beforehand so that proper precautions are taken during the operation of the MRI machine and also for the correct interpretation of the images.
Actual procedure:
The patient is taken to the waiting room of the MRI suite, provided with a gown and asked to change. Asked to remove jewelry, watches, credit cards and hearing aids, pins, metal hair accessories, pens, pocketknives, nail clippers multitool, and coins.
History of allergy to medication, iodine, shrimp and any IV medication or previous use of IV contrast.
The patient is taken to the scan room. The patient lies on a padded platform, ears are covered by noise-canceling ear pads and eyes are covered by a black eye patch. And provided with a panic button, just in case to call attention of the operator.
The patient is instructed to lie still and not to panic. The machine makes repeated loud bangs during scanning and parts of the machine come within inches of the parts under examination.
There are also open MRI machines.
Duration of scanning:
It takes 45 minutes to an hour to complete a scan.
IV contrasts containing Gadolinium are often used to enhance the tumors or clearly delineate blood vessels or hematoma.
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footnote:
Ferromagnetic metals are Iron, Nickel, Cobalt, Gadolinium, Dysprosium, Terbium, Ferric Stainless Steel, Neodymium, and Iron-Boron Alloy
If you want to see how MRI looks like, copy this link and paste in the browser -
https://www.imaios.com/en/e-anatomy/brain/mri-axial-brain
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