Principles of Magnetic Resonance
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Applications in the musculoskeletal system include spinal imaging , assessment of joint disease, and soft tissue tumors. Hepatobiliary MR is used to detect and characterize lesions of the liver , pancreas , and bile ducts. Focal or diffuse disorders of the liver may be evaluated using diffusion-weighted , opposed-phase imaging, and dynamic contrast enhancement sequences. Extracellular contrast agents are used widely in liver MRI and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging.
Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography MRCP. Functional imaging of the pancreas is performed following administration of secretin. MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors. MR-colonography may play a role in the detection of large polyps in patients at increased risk of colorectal cancer.
Magnetic resonance angiography MRA generates pictures of the arteries to evaluate them for stenosis abnormal narrowing or aneurysms vessel wall dilatations, at risk of rupture. MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs called a "run-off". A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent gadolinium or using a technique known as "flow-related enhancement" e.
Techniques involving phase accumulation known as phase contrast angiography can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography MRV is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while the signal is gathered in the plane immediately superior to the excitation plane—thus imaging the venous blood that recently moved from the excited plane.
MRI for imaging anatomical structures or blood flow do not require contrast agents as the varying properties of the tissues or blood provide natural contrasts. However, for more specific types of imaging, exogenous contrast agents may be given intravenously , orally , or intra-articularly. Anaphylactoid reactions are rare, occurring in approx.
The FDA also called for increased patient education and requiring gadolinium contrast vendors to conduct additional animal and clinical studies to assess the safety of these agents. The most frequently linked is gadodiamide , but other agents have been linked too. In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.
An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance. Magnetic resonance spectroscopy MRS is used to measure the levels of different metabolites in body tissues, which can be achieved through a variety of single voxel or imaging-based techniques. This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,  and to provide information on tumor metabolism. Magnetic resonance spectroscopic imaging MRSI combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient.
The spatial resolution is much lower limited by the available SNR , but the spectra in each voxel contains information about many metabolites.
Magnetic Resonance - Basic Principles
Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths 3 T and above. However, recent compressed sensing -based software algorithms e. Real-time MRI refers to the continuous imaging of moving objects such as the heart in real time. This gives a temporal resolution of 20—30 ms for images with an in-plane resolution of 1. Real-time MRI is likely to add important information on diseases of the heart and the joints, and in many cases may make MRI examinations easier and more comfortable for patients, especially for the patients who cannot hold their breathings or who have arrhythmia.
The lack of harmful effects on the patient and the operator make MRI well-suited for interventional radiology , where the images produced by an MRI scanner guide minimally invasive procedures. Such procedures use no ferromagnetic instruments. Some specialized MRI systems allow imaging concurrent with the surgical procedure.
More typically, the surgical procedure is temporarily interrupted so that MRI can assess the success of the procedure or guide subsequent surgical work. In guided therapy, high-intensity focused ultrasound HIFU beams are focused on a tissue, that are controlled using MR thermal imaging. This technology can achieve precise ablation of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for the precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area.
This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment. Hydrogen has the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. However, any nucleus with a net nuclear spin could potentially be imaged with MRI.
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Such nuclei include helium -3, lithium -7, carbon , fluorine , oxygen , sodium , phosphorus and xenon Gaseous isotopes such as 3 He or Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. Moreover, the nucleus of any atom that has a net nuclear spin and that is bonded to a hydrogen atom could potentially be imaged via heteronuclear magnetization transfer MRI that would image the high-gyromagnetic-ratio hydrogen nucleus instead of the low-gyromagnetic-ratio nucleus that is bonded to the hydrogen atom.
Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1 H MRI e. Inhaled hyperpolarized 3 He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13 C or stabilized bubbles of hyperpolarized Xe have been studied as contrast agents for angiography and perfusion imaging.
Multinuclear imaging holds the potential to chart the distribution of lithium in the human brain, this element finding use as an important drug for those with conditions such as bipolar disorder. MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though.
Magnetic resonance imaging
This problem stems from the fact that the population difference between the nuclear spin states is very small at room temperature. For example, at 1. Improvements to increase MR sensitivity include increasing magnetic field strength, and hyperpolarization via optical pumping or dynamic nuclear polarization. There are also a variety of 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.
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To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. To enhance the sensitivity of the contrast agents, these targeting moieties are usually linked to high payload MRI contrast agents or MRI contrast agents with high relaxivities.
In the UK, the price of a clinical 1. Pre-polarizing MRI PMRI systems using resistive electromagnets have shown promise as a low-cost alternative and have specific advantages for joint imaging near metal implants; however, they are likely unsuitable for routine whole-body or neuroimaging applications.
MRI scanners have become significant sources of revenue for healthcare providers in the US. This is because of favorable reimbursement rates from insurers and federal government programs. This covers three basic scans including one with an intravenous contrast agent as well as a consultation with the technician and a written report to the patient's physician.
MRI is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error. Magnetic resonance imaging in pregnancy appears to be safe at least during the second and third trimesters if done without contrast agents. MRI uses powerful magnets and can therefore cause magnetic materials to move at great speeds posing risk. Deaths have occurred. Medical societies issue guidelines for when physicians should use MRI on patients and recommend against overuse.
MRI can detect health problems or confirm a diagnosis, but medical societies often recommend that MRI not be the first procedure for creating a plan to diagnose or manage a patient's complaint. A common case is to use MRI to seek a cause of low back pain ; the American College of Physicians , for example, recommends against this procedure as unlikely to result in a positive outcome for the patient.
An MRI artifact is a visual artifact , that is, an anomaly during visual representation. Many different artifacts can occur during magnetic resonance imaging MRI , some affecting the diagnostic quality, while others may be confused with pathology. Artifacts can be classified as patient-related, signal processing-dependent and hardware machine -related.
MRI is used industrially mainly for routine analysis of chemicals. The nuclear magnetic resonance technique is also used, for example, to measure the ratio between water and fat in foods, monitoring of flow of corrosive fluids in pipes, or to study molecular structures such as catalysts.
In , Paul Lauterbur applied magnetic field gradients in all three dimensions and a back-projection technique to create NMR images.
GS02 1032 Principles of Magnetic Resonance Imaging
He published the first images of two tubes of water in in the journal Nature , followed by the picture of a living animal, a clam, and in by the image of the thoracic cavity of a mouse. Lauterbur called his imaging method zeugmatography, a term which was later replaced by N MR imaging. From Wikipedia, the free encyclopedia. For other uses, see MRI disambiguation.