Compared with CT scans, MRI scans typically take longer and are louder, and they usually need the subject to enter a narrow, confining tube. In addition, people with some medical implants or other non-removable metal inside the body may be unable to undergo an MRI examination safely. MRI was originally called NMRI nuclear magnetic resonance imaging , but "nuclear" was dropped to avoid negative associations.
In clinical and research MRI, hydrogen atoms are most often used to generate a detectable radio-frequency signal that is received by antennas close to the anatomy being examined. Hydrogen atoms are naturally abundant in people and other biological organisms, particularly in water and fat. For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the signal in space. By varying the parameters of the pulse sequence , different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein.
Since its development in the s and s, MRI has proven to be a versatile imaging technique. While MRI is most prominently used in diagnostic medicine and biomedical research, it also may be used to form images of non-living objects.
MRI scans are capable of producing a variety of chemical and physical data, in addition to detailed spatial images. The sustained increase in demand for MRI within health systems has led to concerns about cost effectiveness and overdiagnosis. To perform a study, the person is positioned within an MRI scanner that forms a strong magnetic field around the area to be imaged. In most medical applications, hydrogen nuclei, which consist solely of a proton , that are in tissues create a signal that is processed to form an image of the body. First, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency.
The excited hydrogen atoms emit a radio frequency signal, which is measured by a receiving coil. The radio signal may be made to encode position information by varying the main magnetic field using gradient coils. As these coils are rapidly switched on and off they create the characteristic repetitive noise of an MRI scan. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Exogenous contrast agents may be given to the person to make the image clearer. The major components of an MRI scanner are the main magnet , which polarizes the sample, the shim coils for correcting shifts in the homogeneity of the main magnetic field, the gradient system which is used to localize the MR signal and the RF system, which excites the sample and detects the resulting NMR signal.
The whole system is controlled by one or more computers.enter
Application of Aqueous Proton T2-Weighted NMR Imaging in Material Characterization
MRI requires a magnetic field that is both strong and uniform. The field strength of the magnet is measured in teslas — and while the majority of systems operate at 1. Most clinical magnets are superconducting magnets, which require liquid helium.
Lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients. Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 spin-lattice ; that is, magnetization in the same direction as the static magnetic field and T2 spin-spin ; transverse to the static magnetic field. To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time TR. This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general obtaining morphological information, as well as for post-contrast imaging.
To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time TE. This image weighting 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 tissues turn out as follows:. MRI has a wide range of applications in medical diagnosis and more than 25, scanners are estimated to be in use worldwide.
MRI is the investigation of choice in the preoperative staging of rectal and prostate cancer and, has a role in the diagnosis, staging, and follow-up of other tumors. MRI is the investigative tool of choice for neurological cancers over CT, as it offers better visualization of the posterior cranial fossa , containing the brainstem and the cerebellum. The contrast provided between grey and white matter makes MRI the best choice for many conditions of the central nervous system , including demyelinating diseases , dementia , cerebrovascular disease , infectious diseases , Alzheimer's disease and epilepsy.
Cardiac MRI is complementary to other imaging techniques, such as echocardiography , cardiac CT , and nuclear medicine. Its applications include assessment of myocardial ischemia and viability , cardiomyopathies , myocarditis , iron overload , vascular diseases, and congenital heart disease. 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.
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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. 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.
OSA | Petrophysical applications of NMR imaging
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. 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.
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.
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.