Magnetic Resonance Imaging (MRI) uses strong magnetic field and radio frequency signals making it a good tool for tissue (bone) examination to acquire its images and is best suited for soft (non – calcified) tissue exams.
In terms of medical technology, it was originally named as Nuclear magnetic resonance imaging (NMRI), but in view of avoiding the negative connotations of the word nuclear it was renamed as Magnetic Resonance Imaging (MRI).
Magnetic Resonance Imaging Defination
Magnetic Resonance Imaging (MRI) is defined as a tool to acquire detailed images of the tissues and organs within the body using strong magnetic field and radio frequency signals.
How MRI works?
Medical Magnetic Resonance Imaging (MRI) basically relies on the relaxation properties of excited hydrogen nuclei in water.
When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei with non zero spin numbers (essentially, an unpaired proton or neutron) within the tissue all align in one of the two opposite directions parallel to the magnetic field or anti parallel.
An excess of only one in a million nuclei align themselves with the magnetic field since the thermal energy far exceeds the difference between the parallel and antiparallel states. The bulk collection of nuclei can be portioned into a set whose sum spin are aligned parallel and set whose sum spin are anti parallel.
The magnetic dipole moment of the nuclei then processes around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moment process is called the Larmor frequency.
The tissue is then briefly exposed to pulses of electromagnetic energy (RF Pulses) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high energy state. The frequency of the pulses is governed by the Larmor equations.
In order to selectively image different voxels (volume picture elements) of the subject, orthogonal magnetic gradients are applied. Although it relatively common to apply gradients in the principal axes of a patient (so that the patient is imaged in x, y and z from head to toe). MRI allows completely flexible orientations for images.
All spatial encoding is obtained by applying magnetic field gradients which encode position within the phase of the signal. In either case, a 2D or 3D matrix of spatially encoded phases is acquired, and these data represent the spatial frequencies of the images object. Images can be formed from the acquired data by the use of discrete Fourier transform (DFT).
MRI uses
Uses of MRI are huge in the world of medical science, basically used to examine the inside of the human body in depth.
In critical practise, MRI is used to distinguish pathologic tissue (such as brain tumour) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range.
Many more MRI uses are as follows which will further extend in the days to come:
- MRI is used to detect cysts, tumours and other pathologic tissues in different parts of the body.
- MRI is used to detect or find the abnormalities or injuries of the joints in the body.
- MRI is more generally used for anomalies of the spinal cord and brain.
- MRI is also used to detect certain diseases in liver, abdominal or for certain types of heart problems.
- Breast cancer in women can also be detected using magnetic resonance imaging scanning.
Difference between MRI and CT
A CT scanner uses ionizing radiation, x-rays, to acquire its images, making it a good tool for dense tissue (bone) exams. MRI, on the other hand, uses radio frequency signals to acquire its images, and is best suited for soft (non-calcified) tissue exams.
While CT gives a good spatial resolution (the ability to differentiate two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the tendency to distinguish the differences between two arbitrarily similar but not identical tissues).
The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.
MRI side effects
One advantage of an MRI scan is that it is harmless to the patient. However, some of the side effects of MRI are such as headaches, nausea, and allergy to the contract material, itchy eyes, burning or pain at the point of injection or sometimes even feel uncomfortable when undergoing MRI scan.
How long does an MRI take
MRI scan takes around 15 minutes to one hour depending upon which part of the body being images and upon the type of MRI required to produce the information. Whereas the time required to produce a test report depends upon the urgency and complexity of the test that means whether previously you have x-rays or other imaging that needs to be compare with the new report to analyse the progress of your report.
Specialized MRI scans
Diffusion MRI
Diffusion Magnetic Resonance Imaging measures the diffusion of water molecules in biological tissues. In isotropic medium water molecules naturally move randomly according to the Brownian motion. In biological tissues however, the diffusion may be anisotropic for example, a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. Therefore, the molecules will move principally along the axis of the neural fibre. If we know that molecules in a particular voxels diffuse principally in one direction we can make the assumption that the majority of the fibres in this area are going parallel to that direction.
Finally, it has been proposed that diffusion MRI may be able to detect minute changes in extracellular water diffusion and therefore, could be used as a tool for FMRI.
Magnetic Resonance Angiography (MRA)
Magnetic Resonance Angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for stenosis (abnormal narrowing) or an aneuysms (vessel wall dilatations, at the risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and adnominal aorta, the renal arteries and the legs (called a “run-off”). A variety of techniques can be used to regenerate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as “flow-related enhancement”) (e.g., 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood which has recently moved in to the lane.