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Non-destructive techniques have a wide range of applications. These include surface crack and flaw detection, internal flaws and discontinuities in forgings, rolled products, welds, castings, aircraft components etc. It is also used in the testing of parts of the automotive industry, in power plants, construction structures, industrial plants, railways etc.
INTRODUCTION TO MAGNETIC RESONANCE IMAGING
Magnetic Resonance Imaging is a non-destructive technique used mainly to visualize the internal structure of the human body in detail. MRI scanning is advantageous in the fact that it can provide good contrast between the different kinds of soft tissues in the body which helps immensely in the scanning the brain, heart, muscles and various kinds of cancers as compared to other medical scanning techniques
BASIC PRINCIPLE OF ITS FUNCTIONING
Magnetic resonance imaging is an imaging modality which is used to construct pictures of the Nuclear Magnetic resonance (NMR) signal from the hydrogen atoms in an object. In medical MRI, radiologists are most interested in looking at the NMR signal from water and fat, the major hydrogen containing components of the human body.
The principle behind all magnetic resonance imaging is the resonance equation, which shows that the resonance frequency Î½ of a spin is proportional to the magnetic field, Bo, it is experiencing. The constant of proportionality being Î³, which is the gyromagnetic ratio.
Assume that a human head contains only three small distinct regions where there is hydrogen spin density. In reality the entire head would contain signal. When these regions of spin are experiencing the same general magnetic field strength, there is only one peak in the NMR spectrum.
If each of the regions of spin was to experience a unique magnetic field we would be able to image their positions. A gradient in the magnetic field allows us to achieve this. A magnetic field gradient is a variation in the magnetic field with respect to position. A one-dimensional magnetic field gradient is a variation with respect to one direction, while a two-dimensional gradient is a variation with respect to two. The most useful type of gradient in magnetic resonance imaging is a one- dimensional linear magnetic field gradient. A one-dimensional magnetic field gradient along the x axis in a magnetic field, Bo, indicates that the magnetic field is increasing in the x direction. Here the length of the vectors represent the magnitude of the magnetic field.
The point in the center of the magnet where is called the isocenter of the magnet. The magnetic field at the isocenter is Bo and the resonant frequency is Î½o. If a linear magnetic field gradient is applied to our hypothetical head with three spin containing regions, the three regions experience different magnetic fields.
The result is an NMR spectrum with more than one signal. The amplitude of the signal is proportional to the number of spins in a plane perpendicular to the gradient. This procedure is called frequency encoding and causes the resonance frequency to be proportional to the position of the spin.
This principle forms the basis behind all magnetic resonance imaging.
SPECIALISED MRI SCANS
Specialised MRI scans are used when more intensive techniques are required . Each specialised MRI deals specifically with a particular problem.
Some of the specialised MRI scans are :
Magnetization transfer MRI
Fluid attenuated inversion recovery
Magnetic resonance angiography
Magnetic resonance gated intracranial CSF dynamics (MR-GILD)
Magnetic resonance spectroscopy
Radiation therapy simulation
Current density imaging
Magnetic resonance guided focused ultrasound
Susceptibility weighted imaging (SWI)
DIFFUSION MRI :
The diffusion of water molecules are measured by the diffusion MRI in biological tissues . Majorly the diagnoses of many common conditions can be done using the diffusion MRI. Apart from common conditions like stroke , neurological conditions can also be detected using this MRI. For example , Multiple sclerosis can be diagnosed . The connection between the white matter axons which are present in the central nervous system can be understood better . There is a turbulence and Brownian motion in water molecules when they move randomly . This is a typical characteristic in isotropic medium while there is an anisotropic diffusion when there is low Reynolds number which makes the flow laminar .
Diffusion tensor imaging which allows multiple direction measurements and fractional anisotropy which can be calculated in each direction for each voxel are the most recent development.
MAGNETIZATION TRANSFER MRI :
Transferring longitudinal magnetization from protons in free water to hydration water protons in MRI and NMR is called magnetization transfer. The bulk and the hydration or bound water molecules are the two types of water molecules found in protein solutions when MRI of molecular solutions are taken . The average rotational frequency is quick for free water protons which results in the fixed water molecules to be less which may result in local field inhomogeneity.63 MHz is the normal proton resonance frequency around which the free water protons have the resonance frequency due to this uniformity .Slower dephasing of transverse vibrations occurs i.e. longer T2. Alternatively the interaction of the solute molecules slows down the hydration water molecules . A field of inhomogeneities which leads to a broader resonance frequency spectrum is created. Motional averaging is that causes the moving dipoles to disturb the magnetic field surrounding it such that the average field is zero . On the contrary there are conditions in which the average field is not zero generally seen in proteins. A rise in residual dipolar coupling caused by a spatial pattern in the magnetic field occurs due to this .
Kinetic energy as a function of temperature are present in T1rho molecules which are showed as rotational and translation motions. It is also shown as molecular collisions . The magnetic field disturbance are due to the moving dipoles but still are extremely fast such that long time average effect is zero. Based on the time scale, the interactions change and are not always average. Infinite interaction time is achieved at the slowest extreme. At this condition the stationary field conditions are large. Static dephasing is a condition which occurs when there is a loss of coherence. This is measured as T2. It excludes static dephasing with an RF pulse by which the slowest types of dipolar interactions are reversed. Pulses other than RF pulse can also be focused in a biological sample. Rate of decay is a function of poor and interaction time. With the influence of RF pulse , it is known as T1rho which is identical to T2 decay but with slower dipolar interactions which are refocused along with static interactions.
FLUID ATTENUATED INVERSION RECOVERY ( FLAIR ):
The signals from the fluids are made to null by the inversion recovery pulse sequence. It is used to supress the cerebrospinal fluid in brain imaging so that the periventricular hyper intense lesions are brought out. E.g. : Multiple sclerosis plaques. Suppression of the signal of a particular tissue is done by choosing the inversion time carefully .
MAGNETIC RESONANCE ANGIOGRAPHY:
Pictures of the arteries are captured and generated to detect stenosis and aneurysms. The legs , renal arteries , abnormal aorta and thoracic aorta , neck , brain arteries are evaluated by the MRA. Flow related enhancement like 2D and 3D time of flight sequences or administration of paramagnetic contrast agent techniques are used to get pictures . Here the blood on the plane is where the signal is maximum. Accurate flow velocity maps are generated using phase accumulation techniques. Imaging of veins is done using magnetic resonance venography. Inferior excitation of the tissue which gathers the signal in the plane superior to the excitation plane which results in the imaging of the venous blood which was shifted from the excited plane
MAGNETIC RESONANCE FATED INTRACRANIAL CSF DYNAMICS (MR GOLD):
The intracranial cerebrospinal fluid through the ventricles, aqueduct of slyvius , cisterns and the pathway of intracranial CSF pathway is demonstrated according to bipolar gradient pulse. This technique is called magnetic resonance gated intracranial cerebrospinal fluid or simply liquid dynamics. Some patients have normal pressure hydrocephalus which is a CSF obstructive lesion, which is analysed from the circulatory system dynamics of the patients.
MAGNETIC RESONANCE SPECTROSCOPY:
The body tissues contains different metabolites whose levels can be measured using this technique. The "excited" isotopes have different molecular arrangements which is produced by the MR signal . It is mainly used to diagnose specific metabolic disorders , affecting the brain and are a good source of information for tumor metabolism.
For every change in the neural activity in the brain , there are signal changes which are detected by the functional MRI. These are low resolution scans but take place at a rapid rate like every 2-3 seconds. The phenomenon of causing changes in the signal due to increase in neural activity is called BOLD or blood oxygen level department effect. The BOLD effect permits high resolution 3D maps of certain venous vasculature inside the neural tissue.
REAL TIME MRI:
Monitoring the moving objects continuously in real time . Many strategies have been generated but finally it was based on radial FLASH and iterative reconstruction whose resolution is around 20 - 30 milliseconds. The recent developments promises to give more information about the diseases of the joints and the heart.
To have reduced harmful effects on the operator as well as the patient , this MRI can be used as the images are produced by a scanner which uses minimal invasive techniques . No ferromagnetic instruments can be used in this to make it less harmful. A subset of this method is an intraoperative MRI which is mainly used in surgical procedures.
RADIATION THERAPY SIMULATION:
The preparation for radiation therapy whose tumours are located in the body can be detected by this . The major reason is that it has really good imaging of the soft tissues. The patient has to be placed in a specific body position which can be reproducible and later scanned. The exact postion , orientation and shape of the tumour is computed and also correction for any spatial distortion present in the system. The radiation therapy has to be triangulated precisely . For this the patient is marked with points or tattooed in combination with the body position.
CURRENT DENSITY IMAGING:
The current densities within a subject can be reconstructed by using phase information from the images obtained. During an imaging sequence , the phase of the magnetic dipoles are affected by the magnetic fields generated by the electrical currents. This is how this imaging works. Whole body MRI are highlighted because X Ray and CT manifestations are shown , there may already be unfavourable prognosis.
MAGNETIC RESONANCE GUIDED FOCUSED ULTRASOUND:
MR thermal imaging controls the ultrasound beams which are majorly focused on tissues in this therapy which in turn rises the temperature to more than 65 degrees by deposition of significant amount of energy. It destroys it completely. Precise ablation of the damages or infected tissue can be achieved by this. The ultrasound can be focused precisely since a 3D view of the target tissue is for by the MRI. A real time , thermal image of the treated area is got which allows the physician to have sufficient thermal ablation within the tissue.
Spatially resolved maps or the images of various chemicals present in the body can be detected using this imaging . The chemicals may be natural chemicals or those that are injected or induced in the body but detectable. The most frequently imaged nucleus is the hydrogen cause of its abundance in the body. It mainly includes functional imaging and those of the organs. Though imaging is poorly seen on bones and lungs , phosphorous can be used. Phosphorous can also be used to get imaging of the brain.
SUSCEPTIBILITY WEIGHTED IMAGING (SWI):
Susceptibility weighted imaging is a little different from T1 ,T2 and spin density. It exploits the differences of susceptibility between tissues by using full velocity compensated , three dimensional gradient echo scan . The specialised form of data acquisition produces an image with enhanced contrast magnitude. Sensitive to venous blood, haemorrhage and iron storage. It is used to detect neurovascular diseases and brain trauma.
APPLICATIONS OF MRI AND USE IN SCIENTIFIC STUDIES
The main applications of MRI's are in the field of medicine. It helps detect problems in the body which cannot be seen by using other scanning methods such as the x-ray, CT scan or an ultrasound.
An MRI of the chest can tell us about the heart, the coronary blood vessels and the valves. It can show if the heart or lung are damaged and can also check for lung or breast cancer.
An MRI can be used to look at blood vessels and the flow of blood through them is called magnetic resonance angiography (MRA). It can detect problems of the arteries and veins, such as an aneurysm, a blocked blood vessel, or the torn lining of a blood vessel.
MRI can be used to find problems in organs and structures in the belly, such as the liver, gallbladder, pancreas, kidneys, and bladder. It is used to find tumors, bleeding, infection, and blockage. In women, it can look at the uterus and ovaries. In men, it looks at the prostate.
MRI can check for problems of the bones and joints, such as arthritis, problems with the temporomandibular joint, bone marrow problems, bone tumors, cartilage problems, torn ligaments or tendons, or infection. MRI may also be used to tell if a bone is broken when X-ray results are not clear. MRI is done more commonly than other tests to check for some bone and joint problems.
MRI can check the discs and nerves of the spine for conditions such as spinal stenosis, disc bulges, and spinal tumors.
MRI's have been used in the imaging study of human cartilage glycosaminoglycan concentration. This was done to study degenerative cartilage diseases because it was very difficult to study the disease progression and therapeutic efficacy. The reason for it being so difficult to study was because there was no non-destructive technique to visualize the distribution of functionally important macromolecules in living cartilage. The MRI helped in giving the opportunity to image directly the concentration of GAG, a major and critically important macromolecule in human cartilage thereby helping in study of the progression of the disease.
High field MRI's are used to examine the porcine knee meniscal tissue structure and meniscal tears. High-resolution 3D MR imaging allowed visualization of internal architecture of the meniscus and disruption to the internal structural network. This technique thus has tremendous potential in the field of functional cartilage/meniscus biomechanics and bio tribology.
In one particular study MRI was used as a non-destructive technique to study the preparation of catalyst extrudates.
In another study carried out at a psychiatric institute in London used functional magnetic resonance imaging to map generic brain activation.
The National Institute of Standards and Technology is working on a project to develop scanned probe microscopy and micro electromechanical systems for nanometer-scale magnetic measurements in support of the magnetic data storage industry with help from work on a magnetic resonance spectrometer on a chip to achieve magnetic resonance imaging resolution of 1 nanometer on ferromagnetic thin films.
The Alberta Research Council makes use of MRI technology in the petroleum industry to investigate a wide range of parameters and applications.
PRECAUTIONARY MEASURES TO BE TAKEN
MRI scanning can have several risks to it, especially when used frequently on a patient. These risks include exposure to radio waves and powerful magnetic fields, cases of claustrophobia and also the MRI contrast agents have their own associated risks.
There are certain types of medical implants that are considered contraindications for MRI examinations, while others may be acceptable for patients under high specific MRI conditions. Patients are therefore always asked to provide complete information about all their implants before entering the room for an MRI scan. Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. To reduce these risks, implants are increasingly being developed to make them able to be safely scanned, and specialized protocols have been developed to permit the safe scanning of selected implants and pacing devices. Cardiovascular stents are considered safe, however.
Ferromagnetic foreign bodies such as shell fragments, or metallic implants such as surgical prostheses and ferromagnetic aneurysm clips are also potential risks. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field or thermal injury from radio-frequency induction heating of the object.
Titanium and its alloys are safe from movement from the magnetic field.
There is a classification system for implants that can be allowed into the MRI scanning machines. Its classified into three categories :-
MR-Safe - The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure.
MR-Conditional - A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1.5 tesla' or 'safe in magnetic fields below 500 gauss in strength').
MR-Unsafe - Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room
The high magnetic field intensity causes accidents in which the ferromagnetic substance is attracted to the center of the magnet. This has caused a large number of accidents in which people have died. Thus patients have to be checked for ferromagnetic objects on them before being taken for the scan.
The rapid switching on and off of the high magnetic field gradient also causes nerve stimulation specially in the more high powered magnets.
Danger caused by contrast agents-
The most commonly used intravenous contrast agents are based on chelates of gadolinium. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03-0.1%. Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses-this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT.
Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too. Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.
Thus it has been seen that Magnetic Resonance Imaging is probably one of the best things to have happened in the medical profession. It is one of the best options when it comes to having to take a look at what is going on inside a persons body. A huge advantage of this scan is that it allows you to take an image in any plane which the CT scan cannot. It is highly useful in diagnosing several medical conditions accurately and at an early stage such as cancers, brain and pituitary tumors, visualization of torn ligaments in the knee, ankle and wrist especially for sportsmen, evaluating tumors present in the bones and also herniated discs in the spine. MRI's also aid doctors perform brain surgery, especially functional MRI's. It is gaining great significance in the industry too and is being used in several scientific studies recently.
MRI's though having so many advantages also can cause serious problems due to over exposure. Legislation was thus formed to limit the exposure workers face while working with the MRI's. Field strength limits are given, which may not be exceeded. An employer may be committing a criminal offense by allowing a worker to exceed the mentioned exposure limit, but that is only if this directive is followed in that particular country or state. The introduction of the Directive has brought to surface an existing potential issue with occupational exposures to these intense MRI fields. There are at present very limited data on the number or types of MRI practice that might lead to exposures in levels excess of the Directive.