Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form anatomic images and physiological processes of the body in both health and disease. The MRI scanner uses strong magnetic fields, electric field gradients, and radio waves to produce images of organs in the body. MRI does not involve X-rays or the use of ionizing radiation, which differentiates it from CT scan or CAT. Magnetic resonance imaging is a nuclear magnetic resonance medical application (NMR). NMR can also be used for imaging in other NMR applications such as NMR spectroscopy.
While the dangers of X-rays are now well controlled in most medical contexts, MRI may still be seen as a better choice than CT scans. MRI is widely used in hospitals and clinics for medical diagnosis, disease staging and follow-up without exposing the body to radiation. However, MRI can often produce different diagnostic information than CT. There may be risks and discomfort associated with MRI scans. Compared to CT scans, MRI scans usually take longer and more loudly, and they usually require a subject to enter a narrow limited tube. In addition, people with some medical implants or other non-removable metals in the body may not be able to undergo MRI examination safely.
MRI was originally called 'NMRI' (nuclear magnetic resonance imaging) and was a form of NMR, although the use of 'nuclear' in the acronym was dropped to avoid negative associations with the word. The nucleus of a particular atom is capable of absorbing and emitting radio frequency energy when placed in an external magnetic field. In clinical and MRI research, the hydrogen atom is most often used to produce detectable radio frequency signals received by the antenna near the anatomy examined. Hydrogen atoms are naturally abundant in humans and other biological organisms, especially in water and fats. For this reason, most MRI scans basically map the location of water and fat in the body. Radio waves generate nuclear spin transition energy, and magnetic field gradients localize signals in space. By varying the pulse sequence parameters, different contrasts can be generated between tissues based on the relaxation properties of the hydrogen atoms in them.
Since its development in the 1970s and 1980s, MRI has proven to be a highly versatile imaging technique. While MRI is most prominently used in diagnostic treatment and biomedical research, MRI can also be used to form images of non-living objects. MRI scans are capable of generating a variety of chemical and physical data, in addition to detailed spatial images. The continuous increase in MRI demand in health systems has led to concerns about cost effectiveness and overdiagnosis.
Video Magnetic resonance imaging
Mechanism
Construction and physics
To conduct the research, the person is positioned in an MRI scanner that forms a strong magnetic field around the area to be imaged. In most medical applications, protons (hydrogen atoms) in tissues containing water molecules create signals that are processed to form body images. First, the energy of the oscillating magnetic field is temporarily applied to the patient at the right resonance frequency. Excited hydrogen atoms emit radio frequency signals, as measured by the receiving coil. Radio signals can be made to encode position information by varying the main magnetic field using the gradient coil. As these scrolls are quickly switched on and off, they create repetitive repetitive sounds from an MRI scan. The contrast between different networks is determined by the rate at which the excited atoms return to the equilibrium state. Exogenous contrast agents may be given to the person to make the picture clearer.
The main component of the MRI scanner is the main magnet, which polarizes the sample, the shim coil to correct the shift in the homogeneity of the main magnetic field, the gradient system used to locally alter MR signals and RF systems. samples and detects the resulting NMR signal. The entire system is controlled by one or more computers.
MRI requires a strong and uniform magnetic field. The magnetic field strength is measured in the test - and while the majority of systems operate at 1.5 T, commercial systems are available between 0.2 and 7 T. Most clinical magnets are superconducting magnets, requiring liquid helium. Lower field strength can be achieved with a permanent magnet, which is often used on an "open" MRI scanner for claustrophobic patients. Recently, MRI has been shown also in the ultra-low plane, ie, in the microtesla-to-millitesla range, where sufficient signal quality is possible by prepolarization (on the order of 10-100 mT) and by measuring the Larmor precession plane of about 100 microtesla with a highly sensitive superconducting quantum interference device (SQUID).
T1 and T2
Each tissue returns to its equilibrium state after excitation by an independent T1 relaxation process (spin-lattice, that is, magnetization in the same direction as a static magnetic field) and T2 (spin-spin across the static magnetic field). To create a T1-weighted image, magnetization is allowed to recover before measuring MR signal by changing the repetition time (TR). The weighting of these images is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing liver focus lesions and generally for obtaining morphological information, as well as for post-contrast imaging. To create a T2-weighted image, the magnetization is left to rot before measuring the MR signal by changing the echo time (TE). The weighting of these images is useful for detecting edema and inflammation, revealing white matter lesions and assessing zonal anatomy in the prostate and uterus.
The standard display of MRI images is to represent fluid characteristics in black and white images, where different networks change as follows:
Maps Magnetic resonance imaging
Diagnostics
Usage by organ or system
MRI has a wide range of applications in medical diagnosis and more than 25,000 scanners are expected to be used worldwide. MRI affects diagnosis and treatment in many specialties although the effects on improved health outcomes are uncertain.
MRI is an investigation of choice at preoperative stages of anal and prostate cancer and, has a role in the diagnosis, staging, and follow-up of other tumors.
Neuroimaging
MRI is the preferred investigative tool for neurological cancer, as it has better resolution than CT and offers better visualization of the posterior fossa. The contrast given between gray and white matter makes MRI the best choice for many central nervous system conditions, including demyelinating diseases, dementia, cerebrovascular disease, infectious diseases, and epilepsy. Because many images are taken separate milliseconds, it shows how the brain responds to different stimuli, allowing researchers to study functional and structural brain abnormalities in psychological disorders. MRI is also used in guided stereotactic surgery and radiosurgery for the treatment of intracranial tumors, arteriovenous malformations, and other conditions that can be treated by surgery using a device known as N-localizer.
Cardiovascular
Cardiac MRI complements other imaging techniques, such as echocardiography, cardiac CT, and nuclear medicine. Its applications include assessment of ischemia and myocardial viability, cardiomyopathy, myocarditis, iron overload, vascular disease, and congenital heart disease.
Musculoskeletal
Applications in the musculoskeletal system include spinal imaging, joint disease assessment, and soft-tissue tumors.
Liver and gastrointestinal
Hepatobiliary MR is used to detect and characterize liver lesions, pancreas, and bile ducts. Focal or diffuse diffusion of the liver can be evaluated using weighted-diffusion, phase-opponent imaging, and dynamic contrast enhancement. Extracellular contrast agents are widely used in liver MRI and newer hepatobilier contrast agents also provide an opportunity to perform functional biliary imaging. Anatomical imaging of the bile duct is achieved by using a weighted T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP). Pancreatic functional imaging is performed after administration of secretion. MR enterography provides a non-invasive assessment of inflammatory bowel disease and small bowel tumors. MR-colonography can play a role in detecting large polyps in patients with an increased risk of colorectal cancer.
Angiography
Magnetic resonance angiography (MRA) produces images of the arteries to evaluate them for stenosis (abnormal narrowing) or aneurysms (dilation of blood vessel blood vessels, risk of rupture). MRA is often used to evaluate the artery of the neck and brain, the thoracic aorta and abdomen, renal arteries, and legs (called "runoff"). Techniques can be used to generate images, such as the administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related improvements" (eg, 2D and 3D time-of-flight sequences), where most of the signals the image is due to the blood just moved to the plane (see also FLASH MRI). Techniques involving phase accumulation (known as angiography contrast phases) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure used for venous images. In this method, the network is now excitedly inferior, while the signal is assembled on the plane immediately superior to the excitation plane - resulting in recent venous blood imaging moving from the excited field.
Contrast Agent
MRI for imaging anatomical structures or blood flow does not require contrast agents because various properties of tissue or blood provide a natural contrast. However, for more specific types of imaging, exogenous contrast agents may be administered intravenously, orally, or intra-articularly. The most commonly used intravenous contrast agent is based on cholates of gadolinium. In general, these agents have been shown to be safer than the iodinated contrast agents used in X-ray or CT radiography. Anaphylactoid reactions are rare, occur approximately. 0.03-0.1%. Interestingly, the incidence of nephrotoxicity is lower, compared to the iodinated agent, when administered at regular doses - this has made MRI contrast scans as an option for patients with renal impairment, who otherwise would not be able to undergo CT with enhanced contrast..
Although gadolinium agents have proven useful for patients with renal impairment, in patients with severe kidney failure who require dialysis there is a rare but serious risk of disease, nephrogenic systemic fibrosis, which may be associated with the use of certain gadolinium-containing agents. The most commonly associated is gadodiamide, but other agents have also been linked. Although the cause-and-effect relationship has not been established, the current guideline in the United States is that dialysis patients should only receive gadolinium agents where important, and that dialysis should be done as soon as possible after scanning to remove the agent from the body immediately. In Europe, where more agents containing gadolinium are available, the classification of agents according to potential risks has been released. Recently, a new contrast agent named gadoxetate, the brand name Eovist (US) or Primovist (EU), has been approved for diagnostic use: it has the theoretical benefits of multiple excretion pathways.
Order
MRI sequence is a special arrangement of radio frequency pulses and gradients, resulting in a particular image display. T1 and T2 scales can also be described as MRI sequences.
Summary table
This table excludes unusual and experimental sequences.
Other custom configurations
Magnetic resonance spectroscopy
Magnetic resonance spectroscopy (MRS) is used to measure different levels of metabolites in body tissues. The MR signal produces a resonance spectrum corresponding to the different molecular settings of the "vibrant" isotope. These signatures are used to diagnose certain metabolic disorders, especially those affecting the brain, and to provide information about tumor metabolism.
Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce local spatial spectra from within the sample or patient. Spatial resolution is much lower (limited by available SNRs), but the spectrum in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR can only be achieved at higher field strength (3 T and above).
Real-time MRI
Real-time MRI refers to ongoing monitoring ("shooting") of real-time moving objects. While many different strategies have been developed since the early 2000s, a recent development reported real-time MRI techniques based on radial FLASH and recurrent reconstruction resulting in a temporal resolution of 20 to 30 milliseconds for images with in-flight resolution of 1, 5 to 2.0 mm. This new method promises to add important information about the disease in the joints and heart. In many cases, MRI examination may become easier and more convenient for the patient.
Interactive MRI
The lack of harmful effects on patients and carriers makes MRI highly suitable for interventional radiology, in which images produced by MRI scanners guide minimally invasive procedures. Such a procedure should be performed without a ferromagnetic instrument.
A special subset that develops from an interventional MRI is an intraoperative MRI, in which doctors use MRI in surgery. Some special MRI systems allow imaging along with surgical procedures. More typically, however, is that the surgical procedure is temporarily impaired so that the MRI can verify the success of the procedure or guide the next surgical work.
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In MRGFUS therapy, ultrasound light is focused on guided and controlled tissue using MR thermal imaging - and due to significant deposition of energy at the focus, the temperature inside the tissue rises to over 65 Ã, Â ° C (150 Â ° F). ), completely destroy it. This technology can achieve the proper ablation of diseased tissue. MR imaging provides a three-dimensional view of the target network, enabling proper ultrasonic energy focusing. MR imaging provides a quantitative, real-time thermal image of the treated area. This allows the physician to ensure that the temperature generated during each ultrasonic energy cycle is sufficient to cause thermal ablation in the desired tissue and if not, to adjust the parameters to ensure effective treatment.
Multinuclear imaging
Hydrogen is the most commonly imaged core in MRI because it is present in biological tissue in great abundance, and because its high gyromagnetic ratio provides a strong signal. However, any nucleus with a clean nuclear spin is potentially imaged with MRI. These nuclei include helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. 23 Na and 31 P is naturally abundant in the body, so it can be imaged directly. Gas isotopes such as 3 He or 129 Xe must be hyperpolarized and then inhaled because their nuclear density is too low to produce useful signals under normal conditions. 17 O and 19 F can be given in sufficient quantities in liquid form (eg 17 O-air) that hyperpolarization is not a requirement. Using helium or xenon has the advantage of background noise reduction, and therefore increases the contrast for the image itself, since these elements are usually absent in biological tissue.
In addition, nuclei that have a clean nuclear spin and which are bonded to a potentially imaged atomic hydrogen by MRI heteronuclear magnetization transfer will describe hydrogen nuclei with a high-gyromagnetic ratio rather than a low-gyromagnetic-ratio-low nucleus attached to a hydrogen atom. In principle, MRI hetereonuclear magnetization transfer can be used to detect the presence or absence of specific chemical bonds.
Multinuklear imaging is primarily a research technique today. However, potential applications include poor functional imaging and organ imaging seen in 1 H MRI (eg, lung and bone) or as alternative contrast agents. Inhalation of hyperpolarized 3 He can be used to describe the distribution of air space inside the lungs. An injection solution containing 13 C or stable hyperpolarizing bubble 129 Xe has been studied as a contrast agent for angiography and perfusion imaging. 31 P has the potential to provide information about bone density and structure, as well as functional brains imaging. Multinuclear imaging holds the potential to map the distribution of lithium in the human brain, finding these elements used as an essential drug for those with conditions such as bipolar disorder.
Molecular imaging by MRI
MRI has the advantage of having very high spatial resolution and is proficient in morphological imaging and functional imaging. MRI does have some disadvantages. First, MRI has a sensitivity of about 10 -3 mol/L up to 10 -5 mol/L, which, compared with other imaging types, can be very restrictive. This problem stems from the fact that the population difference between nuclear spin states is very small at room temperature. For example, at 1.5 teslas, typical field strength for clinical MRI, the difference between high and low energy states is about 9 molecules per 2 million. Improvements to increase MR sensitivity include increasing magnetic field strength, and hyperpolarization through optical pumping or dynamic nuclear polarization. There are also various signal amplification schemes based on chemical exchange that increase sensitivity.
To achieve molecular imaging of disease biomarkers using MRI, targeted MRI contrast agents with high specificity and high relaxivity (sensitivity) are required. To date, much research has been devoted to developing targeted MRI contrast agents targeted to achieve molecular imaging by MRI. Generally, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 afibody, have been applied to achieve targeting. To improve the sensitivity of contrast agents, this targeting group is usually connected to MRI contrast agents or high-contrast MRI agents with high relaxivity. A new class of genes targeting MR contrast agents (CA) have been introduced to show the action of the mRNA gene and the unique factor-protein transcriptional genes. The new CA can track cells with unique mRNA, microRNA, and viruses; tissue response to inflammation of the living brain. The MR report changes in gene expression by positive correlation with TaqMan analysis, optical microscopy and electrons.
Economy
In the UK, the price of a 1.5-tesla clinical MRI scanner is around Ã, Â £ 920,000/ US $ 1.4 million, with lifetime maintenance costs generally equal to the cost purchase. In the Netherlands, the average cost of the MRI scanner is around EUR1 million, with the 7-T MRI being used by UMC Utrecht in December 2007, for EUR7 million. MRI suite construction can cost up to US $ 500.000 /EUR370.000 or more, depending on the scope of the project. MRI pre-polarization systems (PMRIs) using resistive electromagnets have shown promise as a low-cost alternative and have special advantages for joint imaging near metal implants, but are unlikely to be suitable for whole-body applications or routine neuroimaging.
MRI scanners have become a significant source of revenue for healthcare providers in the US. This is because of the favorable reimbursement rates of insurance companies and federal government programs. Replacement insurance is provided in two components, equipment costs for actual performance and operation of MRI scans and professional fees for radiologist's review of images and/or data. In the Northeastern US, equipment costs may be $ 3,500/EUR2,600 and professional fees may be $ 350/EUR260, although the actual costs received by equipment owners and doctors interpret are often significantly less and dependent on prices negotiated with the insurance company or determined by the schedule Medicare costs. For example, the orthopedic surgery group in Illinois collected a $ 1,116/EUR825 fee for MRI knee in 2007, but Medicare replacement in 2007 was only $ 470.91/EUR350. Many insurance companies require preliminary approval of the MRI procedure as a condition for coverage.
In the US, the 2005 Deficit Reduction Act significantly reduces the reimbursement costs paid by the federal insurance program for equipment components from multiple scans, which shift the economic landscape. Many private insurance companies have followed suit.
In the United States, brain MRI with and without contrast billed to Medicare Part B requires, on average, technical payments US $ 403 /EUR300 and separate payments to radiologists US $ 93 /EUR70. In France, the MRI exam costs around EUR150/ US $ 205 . It includes three basic scans including one with an intravenous contrast agent as well as consultation with a technician and a written report to the patient's physician. In Japan, MRI inspection costs (excluding contrast material and film costs) range from US $ 155 /EUR115 to US $ 180 /EUR133, with additional cost radiologist US $ 17 /EUR12.50. In India, the cost of an MRI examination including the cost for the opinion of a radiologist comes to about Rs 3000-4000 (EUR37-49/ US $ 50-60 ), excluding the cost of contrast materials. In the UK the retail price for an MRI scan personally ranges between Ã, Â £ 350 and Ã, Â £ 700 (EUR405-810).
Security
MRI is generally a safe technique, although injuries can occur as a result of failed safety procedures or human error. Contraindications to MRI include most cochlear implants and pacemakers, shrapnel, and foreign metal objects in the eye. MRI safety during the first trimester of pregnancy is uncertain, but may be better than other options. Because MRI does not use ionizing radiation, its use is generally preferred over CT when modalities can produce the same information. In certain cases, MRI is not favored because it may be more expensive, time consuming, and claustrophobia-worsening.
MRI uses a strong magnet and therefore can cause magnetic materials to move at high speeds that pose a risk. Death has taken place.
Overdeliver
The medical community issued guidelines for when doctors should use MRI in patients and recommend excessive use. MRI can detect health problems or confirm the diagnosis, but the medical community often recommends that MRI is not the first procedure for making plans to diagnose or manage patient complaints. A common case is to use MRI to look for causes of low back pain; The American College of Physicians, for example, recommends this procedure is unlikely to produce a positive outcome for patients.
Artifact
MRI artifacts are visual artifacts, namely anomalies during visual representation. Many different artifacts may occur during magnetic resonance imaging (MRI), some affecting diagnostic quality, while others may be confused with pathology. Artifacts may be classified as patient-related, depending on signal processing and associated hardware (machine). Artifacts remain problematic in magnetic resonance imaging (MRI). Some affect the quality of the examination, while others may be confused with pathology.
Non-medical use
MRI is used primarily for industry routine chemical analysis. Nuclear magnetic resonance techniques are also used, for example, to measure the ratio of water and fat in foods, monitoring the flow of corrosive liquids in pipes, or to studying molecular structures such as catalysts.
History
In the late 1970s, physicists Peter Mansfield and Paul Lauterbur, developed techniques related to MRI, such as echo-planar imaging techniques (EPI). Mansfield and Lauterbur were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discovery of magnetic resonance imaging".
See also
References
Further reading
External links
- MRI: Peer Reviewed Critical Introduction. The European Magnetic Resonance Forum (EMRF)/The Round Table Foundation (TRTF); Peter A. Rinck (editor)
- MRI Guided Tour: Introduction to the Public Layer of the National Magnet High Field Laboratory
- Basics of MRI. Physics and the underlying technical aspects . Video
- : What to Expect During Your MRI Test from the Magnetic Resonance Security, Education and Research Institute
- Royal Institution Lecture - MRI: Windows in the Human Body
- THE SHORT HISTORY OF MAGNETIC DISASTER DISPLAYS FROM THE DISPLAY POINT IN EUROPE
- Animal Imaging Database (AIDB)
- The workings of the MRI are described using only the chart
- Real-time MRI video: Biomedizinische NMR Forschungs GmbH.
Source of the article : Wikipedia
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