MR

MR=Magnetisk Resonans

Magnetisk resonans skanning anvender et kraftigt magnetfelt angivet i Tesla (T) til at få protoner i kroppen til at spinne i magnetfeltets plan i stedet for den normale helt frie (tilfældige) orientering af protonernes spin.

Der er ingen stråling involveret i MR-skanninger. MR-skanninger blev opfundet i 1970'erne.

--Jannick Brennum 1. apr 2018, 07:57 (UTC)

Schematic of construction of a cylindrical superconducting MR scanner. 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, protons (hydrogen atoms) in tissues containing water molecules create a signal that is processed to form an image of the body. First, energy from an oscillating magnetic field temporarily is 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.[4] 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. 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 1.5 T, commercial systems are available between 0.2 and 7 T. 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.[5] Recently, MRI has been demonstrated also at ultra-low fields, i.e., in the microtesla-to-millitesla range, where sufficient signal quality is made possible by prepolarization (on the order of 10-100 mT) and by measuring the Larmor precession fields at about 100 microtesla with highly sensitive superconducting quantum interference devices (SQUIDs).[6][7][8] T1 and T2[edit] Further information: Relaxation (NMR)

Effects of TR and TE on MR signal

Examples of T1 weighted, T2 weighted and PD weighted MRI scans 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 for 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: Signal T1-weighted T2-weighted High Fat[9][10] Subacute hemorrhage[10] Melanin[10] Protein-rich fluid[10] Slowly flowing blood[10] Paramagnetic substances, such as gadolinium, manganese, copper[10] Cortical pseudolaminar necrosis[10] More water content,[9] as in edema, tumor, infarction, inflammation and infection[10] Extracellularly located methemoglobin in subacute hemorrhage[10] Inter- mediate Gray matter darker than white matter[11] White matter darker than grey matter[11] Low Bone[9] Urine CSF Air[9] More water content,[9] as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage[10] Low proton density as in calcification[10] Bone[9] Air[9] Fat[9] Low proton density, as in calcification and fibrosis[10] Paramagnetic material, such as deoxyhemoglobin, intracelullar methemoglobin, iron, ferritin, hemosiderin, melanin[10] Protein-rich fluid[10] Diagnostics[edit] Usage by organ or system[edit]

Patient being positioned for MR study of the head and abdomen. MRI has a wide range of applications in medical diagnosis and more than 25,000 scanners are estimated to be in use worldwide.[12] MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is uncertain.[13] 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.[14] Neuroimaging[edit] Main article: MRI of brain and brain stem See also: Neuroimaging

MRI image of white matter tracts MRI is the investigative tool of choice for neurological cancers, as it has better resolution than CT and offers better visualization of the posterior fossa. 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, and epilepsy.[15] Since many images are taken milliseconds apart, it shows how the brain responds to different stimuli, enabling researchers to study both the functional and structural brain abnormalities in psychological disorders.[16] MRI also is used in guided stereotactic surgery and radiosurgery for treatment of intracranial tumors, arteriovenous malformations, and other surgically treatable conditions using a device known as the N-localizer.[17][18][19] Cardiovascular[edit] Main article: Cardiac magnetic resonance imaging

MR angiogram in congenital heart disease 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.[20] Musculoskeletal[edit] Applications in the musculoskeletal system include spinal imaging, assessment of joint disease, and soft tissue tumors.[21] Liver and gastrointestinal[edit] 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.[22][23][24][25] Angiography[edit]

Magnetic resonance angiography Main article: Magnetic resonance angiography 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.g., 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane (see also FLASH MRI). 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.[26] Contrast agents[edit] Main article: MRI contrast agent 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.[4] The most commonly used intravenous contrast agents are based on chelates of gadolinium.[27] 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%.[28] 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.[29] 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, which may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too.[30] 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.[31][32] In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.[33][34] Recently, a new contrast agent named gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: this has the theoretical benefit of a dual excretion path.[35] Sequences[edit] Main article: MRI sequences An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance.[36] The T1 and T2 weighting can also be described as MRI sequences. Overview table edit This table does not include uncommon and experimental sequences. Group Sequence Abbr. Physics Main clinical distinctions Example Spin echo T1 weighted T1 Measuring spin–lattice relaxation by using a short repetition time (TR) and echo time (TE) Lower signal for more water content, [9]as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage [10] High signal for fat[9][10] High signal for paramagnetic substances, such as MRI contrast agents[10] Standard foundation and comparison for other sequences. T1-weighted-MRI.png T2 weighted T2 Measuring spin–spin relaxation by using long TR and TE times. Higher signal for more water content.[9] Low signal for fat.[9] Low signal for paramagnetic substances.[10] Standard foundation and comparison for other sequences. Normal axial T2-weighted MR image of the brain.jpg Proton density weighted PD Long TR (to reduce T1) and short TE (to minimize T2)[37] Joint disease and injury.[38] High signal from meniscus tears[39] (pictured) Proton density MRI of a grade 2 medial meniscal tear.jpg Gradient echo Steady-state free precession SSFP Maintenance of a steady, residual transverse magnetisation over successive cycles.[40] Creation of cardiac MRI videos (pictured).[40] Four chamber cardiovascular magnetic resonance imaging.gif Inversion recovery Short tau inversion recovery STIR Fat suppression by setting an inversion time where the signal of fat is zero.[41] High signal in edema, such as in more severe stress fracture.[42] Shin splints pictured: Shinsplint-mri (crop).jpg Fluid attenuated inversion recovery FLAIR Fluid suppression by setting an inversion time that nulls fluids. High signal in lacunar infarction, multiple sclerosis (MS) plaques, subarachnoid haemorrhage and meningitis (pictured).[43] FLAIR MRI of meningitis.jpg Double inversion recovery DIR Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.[44] High signal of multiple sclerosis plaques (pictured).[44] Axial DIR MRI of a brain with multiple sclerosis lesions.jpg Diffusion weighted (DWI) Conventional DWI Measure of Brownian motion of water molecules.[45] High signal within minutes of cerebral infarction (pictured).[46] Cerebral infarction after 4 hours on DWI MRI.jpg Apparent diffusion coefficient ADC Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.[47] Low signal minutes after cerebral infarction (pictured).[48] Cerebral infarction after 4 hours on ADC MRI.jpg Diffusion tensor DTI Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.[49] Evaluating white matter deformation by tumors[49] Reduced fractional anisotropy may indicate dementia[50] White Matter Connections Obtained with MRI Tractography.png Perfusion weighted (PWI) Dynamic susceptibility contrast DSC Gadolinium contrast is injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.[51] In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (pictured).[52] Tmax by MRI perfusion in cerebral artery occlusion.jpg Dynamic contrast enhanced DCE Measuring shortening of the spin–lattice relaxation (T1) induced by a gadolinium contrast bolus.[53] Arterial spin labelling ASL Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest.[54] It does not need gadolinium contrast.[55] Functional MRI (fMRI) Blood-oxygen-level dependent imaging BOLD Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.[56] Localizing highly active brain areas before surgery.[57] 1206 FMRI.jpg Magnetic resonance angiography (MRA) and venography Time-of-flight TOF Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation. Detection of aneurysm, stenosis or dissection.[58] Mra-mip.jpg Phase-contrast MRA PC-MRA Two gradients with equal magnitude but opposite direction are used to encode a phase shift, which is proportional to the velocity of spins.[59] Detection of aneurysm, stenosis or dissection (pictured).[58] Vastly undersampled Isotropic Projection Reconstruction (VIPR) Phase Contrast (PC) sequence MRI of arterial dissections.jpg (VIPR) Susceptibility weighted SWI Sensitive for blood and calcium, by a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to exploit magnetic susceptibility differences between tissues. Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium.[60] Susceptibility weighted imaging (SWI) in diffuse axonal injury.jpg Other specialized configurations[edit] Magnetic resonance spectroscopy[edit] Main articles: In vivo magnetic resonance spectroscopy and Nuclear magnetic resonance spectroscopy Magnetic resonance spectroscopy (MRS) is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that corresponds to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,[61] and to provide information on tumor metabolism.[62] 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).[citation needed] Real-time MRI[edit] File:Real-time MRI - Thorax.ogv Real-time MRI of a human heart at a resolution of 50 ms Main article: Real-time MRI Real-time MRI refers to the continuous monitoring ("filming") of moving objects in real time. While many different strategies have been developed since the early 2000s, a recent development reported a real-time MRI technique based on radial FLASH and iterative reconstruction that yields a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. The new method promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.[63] Interventional MRI[edit] Main article: Interventional MRI 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 must be done with no ferromagnetic instruments.[citation needed] A specialized growing subset of interventional MRI is intraoperative MRI, in which doctors use an MRI in surgery. Some specialized MRI systems allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MRI can verify the success of the procedure or guide subsequent surgical work.[citation needed] Magnetic resonance guided focused ultrasound[edit] In MRgFUS therapy, ultrasound beams are focused on a tissue—guided and controlled using MR thermal imaging—and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C (150 °F), completely destroying it. This technology can achieve precise ablation of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for 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.[64] Multinuclear imaging[edit] Hydrogen is 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-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O and 19F can be administered in sufficient quantities in liquid form (e.g. 17O-water) that hyperpolarization is not a necessity.[citation needed] Using helium or xenon has the advantage of reduced background noise, and therefore increased contrast for the image itself, because these elements are not normally present in biological tissues.[65] 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.[66] In principle, hetereonuclear magnetization transfer MRI could be used to detect the presence or absence of specific chemical bonds.[67][68] Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g., lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain. 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.[citation needed] Molecular imaging by MRI[edit] Main article: Molecular imaging 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. First, MRI has a sensitivity of around 10−3 mol/L to 10−5 mol/L, which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the population difference between the nuclear spin states is very small at room temperature. For example, at 1.5 teslas, a typical field strength for clinical MRI, the difference between high and low energy states is approximately 9 molecules per 2 million. 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.[citation needed] 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.[69] A new class of gene targeting MR contrast agents (CA) has been introduced to show gene action of unique mRNA and gene transcription factor proteins.[70][71] This new CA can trace cells with unique mRNA, microRNA and virus; tissue response to inflammation in living brains.[72] The MR reports change in gene expression with positive correlation to TaqMan analysis, optical and electron microscopy.[73]