Magnetic resonance imaging

Magnetic resonance imaging (MRI, also briefly MR; tomography of ancient Greek τομή tome, cut ' and γράφειν graphein, write ') is an imaging method that is mainly used in medical diagnosis for imaging the structure and function of tissues and organs in the body. It is based physically on the principles of nuclear magnetic resonance ( NMR), in particular the field gradient NMR, and is therefore also known as magnetic resonance imaging ( colloquially sometimes shortened to nuclear spin ). The well -to-find abbreviation MRI comes from the English name Magnetic Resonance Imaging.

Take the MRT, you can create cross-sectional images of the human (or animal) body, which allow an assessment of the organs and many pathologic organ changes. It is based on - in a magnetic resonance imaging system (short form: MRI scanner, MRI machine ) generated - very strong magnetic fields and alternating magnetic fields in the radio frequency range (usually hydrogen nuclei / protons) are resonantly excited in the body with which certain atomic nuclei, which in a receiver circuit, an electric signal is induced. Thus, since the observed object " itself radiates ", subject to the MRT is not the physical law for the resolution of optical instruments, by which the wavelength of the radiation used must be the smaller, the higher the resolution required. On MRI, with wavelengths in the meter range ( low-energy radio waves) object points can be resolved in the submillimeter range. Are an essential basis for the image contrast different relaxation times of various tissues. In addition, also contributes to the different content of hydrogen atoms in various tissues (eg muscle, bone ) to the image contrast at.

In no case equipment X-rays or other ionizing radiation is produced or used. However, the effects of alternating magnetic fields on living tissues are not fully understood.

• 5.1 Advantages of magnetic resonance imaging
• 5.2 Disadvantages of MRI
• 5.3 artifacts
• 5.4 Contraindications
• 5.5 List of abbreviations commonly used MR sequences

Procedures and systems

Numerous specific MRI techniques have been developed out of position and shape of organs and information about their microstructure and to be able to function (especially their blood ) represent, for example:

• Real-time MRI to cinematic representation of moving joints or organs ( eg heart ),
• The magnetic resonance angiography (MRA) for the representation of the vessels,
• Functional magnetic resonance imaging (fMRI or fMRI ) of the brain,
• The perfusion MRI for the investigation of tissue perfusion,
• The diffusion or diffusion tensor imaging ( DTI) for a virtual reconstruction of nerve fiber connections,
• MR elastography.

After construction, a distinction is closed MRI systems with short or long tunnel and open MRI systems ( oMRT ) with C- arm or side open tunnel. Closed tunnel systems provide comparatively better image data, open MRI systems, however, allow access to patients under MRI control.

Another distinguishing factor is the type of magnetic field generation. For low field strengths up to 0.5 Tesla permanent magnets or conventional electromagnets are used for higher field strengths, however, superconducting magnetic coils.

Historical development

The MRI was developed as NMR imaging from 1973 mainly by Paul C. Lauterbur significant contributions of Sir Peter Mansfield. They received in 2003 shared the Nobel Prize in Physiology or Medicine.

Paul Lauterbur (USA) had two basic ideas that made an imaging based on the NMR possible. First succeeded with field gradient NMR, i.e., the introduction of magnetic gradient fields in the conventional NMR experiment to map the NMR signals given spatial regions of an extended sample ( spatial encoding ). Second, he proposed a method in which by rotation of the ortskodierenden magnetic field in successive experiments different location codes (projections ) of the object under examination were obtained, from which then using the filtered back projection (English filtered back projection ) an image of the object under examination could be calculated. In 1973 publicized result shows a two-dimensional illustration of two filled with normal water tubes in an environment of heavy water.

For a practical use of this discovery special apparatus innovations were required. Bruker Karlsruhe, Germany, at that time the only commercial manufacturer of the necessary " pulsed NMR spectrometer " in the early 1960s in a group led by physicists Bertold Bludgeon and Manfred Holz " quartz-controlled " NMR pulse spectrometer was developed, for example, of Mansfield could be used for basic experiments. Mansfield then developed from 1974 onwards mathematical method to convert the signals quickly into image information, and techniques for slice-selective excitation. He also led in 1977 to the use of extremely fast switching of the gradient a (EPI = echo planar imaging ). This was an image acquisition in well under a second are possible ( "snapshot " technique ), but must be bought to date with smears in picture quality. Mansfield is also due to the introduction of magnetically shielded gradient coils. In his last active years, he was looking for solutions to reduce the significant noise exposure for the patient by the extremely rapid gradient switching.

More important for the widespread clinical use of MRI contributions come from German research laboratories. In Freiburg developed Jürgen Hennig and employees a variant of the spin- echo MRI, now under the abbreviations RARE (Rapid Acquisition with Relaxation Enhancement ), FSE ( Fast Spin Echo) or TSE ( turbo spin echo ) is known to the early 1980s. It takes place because of their sensitivity to pathological tissue structures and their metrological efficiency general use. 1985 succeeded Axel Haase, Jens Frahm, Dieter Matthaei and in Göttingen with the invention of the fast-image method, FLASH, a fundamental breakthrough in MRI. The FLASH technique reduced the former measurement times by up to two orders of magnitude ( factor of 100 ) without substantial loss of image quality. The method also allows continuous, sequential measurements in dynamic equilibrium, as well as completely new clinical investigations such as recordings from the abdominal cavity with bated breath, dynamic recordings synchronized with the ECG heart films, three-dimensional images of complex anatomical structures, vascular images with MR angiography and today functional mapping of the cortex with particularly high resolution. This paved the way from the mid-1980s free for a broad application of MRI in medical diagnostics.

Controversial is the contribution of Raymond Damadian ( USA), who in 1974 applied for a U.S. patent on the application of NMR for cancer. Although the patent described no method for imaging, but only a point measurement, but Damadian successfully fought with another Patent (Multi- layer multi-angle measurements, eg for MRI examinations of the spine) more than 100 million U.S. dollars of the various MRI manufacturers. His original NMR scanner, which produced no pictures, was never used clinically and its cancer detection method for allegedly found is not free from doubt. It is based on differences in the NMR relaxation times of healthy tissue and tumor tissue. This of Damadian in 1971 published observation was confirmed though in principle, but had to be relativized then later the effect that the differences do not apply throughout. Differing relaxation times of the fabric are neither necessary nor sufficient for the presence of tumor tissue in the subject. Damadian was in the awarding of the Nobel Prize for NMR imaging (MRI, Eng. MRI ) is not involved, while he publicly protested violently.

Physics

Abridged version

This section describes the principle of MRI simplistic and not necessarily complete. For a more precise description, see the next sections.

The method is based on the atomic nuclei are excited to a specific movement in the tissue examined by a combination of static and high-frequency magnetic fields targeted phase-synchronously and then give a measurable signal in the form of an alternating voltage until the motion is resolved. This motion is called Larmor precession and is mechanically analogous to observe at a toy top, if its axis of rotation is not perpendicular, but the perpendicular around a precession performs ( see right ). Both for the excitation and observation of the signal is a resonant condition to meet, by means of which it is possible by means of an inhomogeneous static magnetic fields to determine the location of the precessing nuclei.

Some atomic nuclei (such as hydrogen nuclei ) in the molecules of the tissue to be tested have an intrinsic angular momentum ( MRI ) and are thereby magnetically. These cores form after application of a strong static magnetic field, a small longitudinal magnetization in the direction of the static field ( paramagnetism ). Through a short time -scale additional high-frequency alternating field in the radio frequency range, this magnetization can be from the direction of the static field deflect ( tilt ), ie partially or completely ( saturation) in a transverse magnetization convert. The transverse magnetization begins to precess around the immediate field direction of the static magnetic field, i.e., the magnetization direction is rotated (see figure to the precession ). Such precessional motion of the magnetization induced tissue such as the rotation of the magnets in the dynamo coil in a (receiver circuit), an electric voltage and can thus be detected. Its amplitude is proportional to the transverse magnetization.

After switching off the high-frequency alternating field increases, the transverse magnetization ( again ), which spins so directed back parallel to the static magnetic field from. For this so-called relaxation they need a characteristic decay time. This depends on the chemical compound and the molecular environment in which the precessing hydrogen nucleus is located. Therefore, the different tissue types differ characteristically in their signal, leading to different signal strengths ( magnitudes ) results in the resulting image.

Basics

The physical basis of magnetic resonance imaging (MRI ) is the nuclear magnetic resonance ( engl. nuclear magnetic resonance, NMR). Here one uses the fact that the atomic nuclei of hydrogen ( protons ) have an intrinsic angular momentum (spin ) and a magnetic dipole moment. Also, some other atomic nuclei have spin and will get a magnetic moment. ( An atomic nucleus can be simplified from the standpoint of classical physics as a spherical top are considered the angular momentum and a magnetic dipole moment, the cause of its angular momentum classically but can not be described correctly.)

If such a core placed in a static magnetic field, its energy is at its lowest when the magnetic dipole moment is aligned parallel to the field. All other nuclei acts a torque which attempts to rotate the direction of the magnetic moment in the direction of the magnetic field. Because of the intrinsic angular momentum of the nucleus, and conservation of angular momentum results from the precession, i.e., the orientation of the angular momentum of the core rotates without changing the angle of attack to the direction of the applied magnetic field.

By the thermal energy of the kernels at normal temperatures, the dipole moments almost completely random (isotropic ) are aligned; there is only a very small excess of atomic nuclei (corresponding to the Boltzmann distribution ) whose dipole moments are aligned in the direction of the static magnetic field. Only this slight excess causes measurable magnetization in the direction of the outside of the outer static field ( the longitudinal magnetization in the longitudinal direction ).

The precession of the nuclear spins occurs at the Larmor frequency. It depends on the strength of the external magnetic field and the observed nucleus for protons at 1 Tesla is 42.58 MHz, ie in the FM radio band. A high frequency auxiliary field perpendicular to the static magnetic field, ie in the transverse plane, swings and whose frequency is the Larmor frequency in resonance, directs all cores phase synchronism from their current position to the static field. The macroscopic magnetization is tilted from the direction of the static field, it produces a transverse magnetization, which can be equal to the initial longitudinal magnetization just a maximum at the correct exposure time of the alternating field ( saturation).

In a measuring coil is induced by the rotating transverse magnetization, an alternating voltage. Their frequency is the Larmor frequency, which depends on a static gradient of the place, its amplitude indicates the strength of the transverse magnetization, which in turn from the exact sequence ( sequence) of pulses, the location and the type of tissue is dependent.

The target of the MR imaging is the generation of tomographic images ( arbitrary orientation ) of the spatial distribution of the transverse magnetization.

Spin -lattice relaxation ( T1 longitudinal relaxation )

Is through a magnetic alternating field of appropriate frequency, intensity and duration of the magnetization as in the longitudinal direction ( z-direction) has been tilted so that they precess in the xy plane, the first longitudinal magnetization is zero. You then turn off the alternating field, the equilibrium state with only longitudinal magnetization, ie lower energy, rebuild begins. Cause of this spin -lattice relaxation is the effect of fluctuating interference to the moments of the individual cores, which are caused by adjacent atoms, which in turn are in thermal equilibrium with the surrounding area, which is called for historic reasons as " grid ". That is, the magnetization is directed back along the static field of the energy goes from the nuclei of the atoms into the lattice. This alignment is exponential:

Wherein the magnitude of the magnetization in the direction of the equilibrium state. The constant indicates what state the system out of equilibrium is located at the beginning of the relaxation process (eg: saturation, : inversion). The time until the Z- component approximately 63% of its initial value is reached again, is called the spin- lattice relaxation time or period.

The times in pure, low-viscosity fluids such as water are usually in the range of a few seconds. Higher viscosity liquids (e.g. oils) or water in the structured systems, such as gels, porous materials, or fabric generally have shorter times. In highly ordered solids, however, very long relaxation times are found that can possibly be in the range of hours. However, such materials do not play a role because of the short times in solids for conventional magnetic resonance imaging. Typical values ​​for the human tissue lying between a few seconds for body fluids such as blood or cerebrospinal fluid ( CSF ) and about 100 ms for body fat ( for example, is the time of CSF at 1.5 Tesla for about 4 seconds, the time of the gray matter about 1.2 seconds ).

Spin-spin relaxation ( T2 Querrelaxationzeit )

The transverse magnetization of a spin ensemble is divided now, similar to the component increases, due to interaction with neighboring atoms. There is, however, the so-called spin-spin interaction is responsible for the dephasing. The decay can be again represented by an exponential function, but with a different time constant:

Often the transverse magnetization in the xy-plane decreases much more quickly than can be explained by the spin-spin interaction. The reason is that is averaged at a MR image on a volume element in which the external magnetic field is not constant ( but inhomogeneous ). Shift after removal of the RF signal, the phase of the precession of the nuclei with each other and the xy - components of the individual nuclear spins diverge.

Measurement sequence, spatial coding, image building

For better understanding, here the principle of the simplest, so-called spin-echo sequence is briefly outlined, invented in 1950 by Erwin Hahn. A " sequence " (or " pulse sequence " ) in this context is a combination of radio frequency pulses and magnetic gradient fields, a specific frequency and intensity, which are widely used in every second, turned on and off in a predetermined order.

At the beginning is a high frequency pulse of the appropriate frequency ( Larmor frequency ), the so-called 90 ° excitation pulse. By this, the magnetization by 90 ° transversely deflected to the outer magnetic field. She begins to revolve around the original axis. Like a spinning top, which is triggered is called this movement precession.

The resultant high-frequency signal can be measured outside of the body. It decreases exponentially because the proton spin from the " clock " get ( " dephasing " ) and increasingly overlap destructive. Time decayed after 63 % of the signal is called a relaxation time ( spin-spin relaxation). This time depends on the chemical environment of the hydrogen; it is different for each type of tissue. Tumor tissue for example, has usually a longer time than normal muscle tissue. A - weighted measurement represents the tumor about dar. brighter than its surroundings

By a suitable 180 ° rephasing RF pulse can cause a part of the dephasing is made ​​( dephasing by time-invariant magnetic field inhomogeneities ) at the time of measurement to reverse, so that more spins are in the same phase. Then the signal strength is not dependent on the relaxation time, but only on the relaxation time, which is based on a non - reversible effect. Depending on the parameters, the signal sequence In addition, the so-called relaxation time ( spin-lattice relaxation) dependent, which is a measure of the rate at which the initial longitudinal alignment of the spins is adjusted again to the external magnetic field. The time is also tissue specific, but usually significantly (5 × to 20 × ) longer than the time. The time of water is, for example, 2.5 seconds. - weighted sequences allow measurement because of the stronger signal a better spatial resolution but a lower tissue contrast than - weighted images.

A clinical MRI investigation generally includes always - and - weighted image series in at least two spatial levels.

In order to assign the signals of the individual volume elements ( voxels ), is generated with linear position-dependent magnetic fields ( gradient fields ) spatial encoding. This takes advantage of that for a given particle, the Larmor frequency of the magnetic flux density depends (the stronger the field component perpendicular to the direction of the Teilchendrehimpulses, the higher the Larmor frequency ):

• A gradient is achieved at the excitation and ensures that only a single layer of the body has the appropriate Larmor frequency, so only the spins of this layer are deflected ( slice ).
• A second gradient transverse to the first is turned on shortly after the excitation, and causes dephasing of the spins in a controlled such that in each image line, the precession of the spins of a different phase position ( phase coding gradient ).
• The third gradient is switched during the measurement at right angles to the other two; He ensures that the spins of each image column have a different precession, so send another Larmor frequency ( readout gradient, frequency coding ).

All three gradients together thus cause a coding of the signal in three planes of space. The received signal associated with a particular layer of the body and contains a combination of frequency and phase coding, which can convert into a two dimensional view of the computer with a Fourier transformation.

Strength of the magnetic

The strength of the magnetic field directly affects the signal quality of the measured data because the signal -to-noise ratio is approximately proportional to the field strength. Therefore there since the dawn of MRI a trend towards higher field strengths, which requires the use of frozen superconducting magnets. Thus, the cost and the technical complexity at higher field strengths increase significantly. Especially with magnets with large openings for the study of human develop rapidly inhomogeneous and difficult producible fields at higher field strengths.

Low-field devices with 0.2-0.5 Tesla are now only sporadically in use. Usually, the magnetic field for diagnostic purposes today has a thickness of at least 1-1.5 Tesla. Since around 2006 high-field devices are increasingly being set up with a field strength of 3 Tesla with new acquisitions. If 3 Tesla exceeded, the subjects can be only very slowly moved into the magnet, otherwise it may cause dizziness and nausea as a result of the resulting eddy currents in the brain.

Even higher field strengths ( ultra-high field systems) are currently used in human medicine only for research purposes, but not for routine examinations. The following institutions operate in the German-speaking world at present MRI systems for human studies with a field strength of 7 Tesla or more:

• The Leibniz Institute for Neurobiology ( Neurobiology ) in Magdeburg ( 7 Tesla for head examinations; since 2005)
• The Erwin L. Hahn Institute for Magnetic Resonance of the Universities of Duisburg -Essen and Radboud (Nijmegen ) (7 -Tesla whole-body MRI, since 2006)
• The Institute for Biomedical Engineering ( IBT ) of ETH Zurich (7 -Tesla whole-body MRI, since 2006)
• The Max Planck Institute for Biological Cybernetics in Tübingen ( 9.4- Tesla system for head examinations, since 2007)
• The General Hospital of Vienna as part of the Medical University of Vienna ( 7-Tesla whole-body MRI, since 2008)
• The Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig ( 7 Tesla for head examinations; since 2008)
• The German Cancer Research Center in Heidelberg ( 7-Tesla whole-body MRI, since 2008)
• The Max Delbrück Center for Molecular Medicine in Berlin ( 7-Tesla whole-body MRI, usability from 2009 )
• Forschungszentrum Jülich ( 9.4 Tesla MR-PET hybrid system for head examinations, since April 2009).

A whole-body MRI with an even higher field strength of 11.7 Tesla is being developed at the CEA center Neurospin in Saclay (France).

Experimental Systems

In the physical, chemical and biomedical research high- field devices for samples and small animals up to 21 Tesla are common. The opening of these devices is provided with a diameter of a few centimeters, but substantially smaller than that of the aforementioned systems. With such a high- field scanner, for example, age determinations of objects can be performed, which are not chemically or radiologically possible.

Image assessment

The signal strength of the voxel is mapped encoded in gray values. As it depends on many parameters (such as the magnetic field strength ), there is no standard for the signal values ​​of certain tissues and not a defined unit, comparable to Hounsfield units in computer tomography. The MR console displays only arbitrary (arbitrary ) units, which are diagnostically not directly usable. The image interpretation relies instead on the overall contrast, the respective weights (synonym weighting) of the measurement sequence, and the signal differences between known and unknown tissues. The findings, therefore, is in the description of a lesion not from "light" or "dark" spoken, but of hyperintense signal for rich, bright and hypointense on hypointense, dark.

Depending on the weighting of the various tissues in a characteristic intensity distribution come to represent. In the T1 - weighted fat appears hyperintense ( high signal intensity, light ) and hence fatty / rich tissue ( eg, bone marrow ). This weighting is therefore well suited to the anatomical representation of organ structures and especially after administration of contrast agent ( gadolinium ) for better delineation of unknown structures (eg, tumor). In T2-weighted stationary liquids appear hyperintense so that fluid-filled structures appear hyperintense and so bright ( eg, subarachnoid ). As a result, this weighting is suitable for the representation of effusions and edema, as well as, for example, the delineation of cysts compared with solid tumors. In radiographs, especially in the special X-ray technique of computed tomography ( CT), the terms are hyperdense and hypodense used to describe the relative degree of blackness in contrast.

In the voxel-based morphometric MR images are algorithmically processed in order to determine objective parameters and statistically analyzed. These methods are particularly suitable for use to detect the size of certain brain structures for the examination of the human brain.

Properties

Advantages of magnetic resonance imaging

An advantage of MRI over other imaging techniques is the better soft tissue contrast. It results from the difference of the fat and water content of different tissue types. Thereby, the method does not require any harmful ionizing radiation. A further improvement is obtained by recording two series, with and without administration of contrast agents, as inflammation or vital tumor tissue are better recognized, for example, through more intensive whitening.

New, faster absorption methods allow for scanning single -section images in a fraction of a second and thus provide a true real-time MRI, which replace the previous attempts based on the conventional fluoroscopy. Thus, movements of organs, for example, shown or the position of medical instruments are monitored during surgery ( interventional radiology). For imaging of the beating heart (pictured left) synchronized measurements are used so far with an ECG, where data are combined from several cardiac cycles to complete pictures. Newer approaches for real-time MRI promise, however, a direct cardiac imaging without ECG synchronization, and during free breathing with a temporal resolution of up to 20 milliseconds.

It is also essential the lack of radiation exposure, which is why this method is preferably used in studies of infants and children as well as during pregnancy compared with the CT.

Disadvantages of MRI

• The resolution is in standard clinical systems by technical factors, in particular the limited field strength, limited to about one millimeter. In the research area spatial resolutions of less than 0.02 mm can be achieved.
• Metal on or in the body can cause side effects and interference. Existing metallic foreign body (eg, iron splinter in the eye or brain) can even be dangerous, so a MRI scan in such patients may be contraindicated due to relocation or heating during the investigation. Modern metal implants such as titanium and even steel alloys are dependent on the composition of paramagnetic or diamagnetic and are therefore usually is not a problem in MRI
• For MRI of 1.5 Tesla is known that they are safe for amalgam fillings. However, Turkish scientists show that novel MRI with field strengths of 3 Tesla and more are not completely devoid of effects on the marginal leakage of amalgam fillings.
• Electrical equipment can be damaged in the magnet. Use a cardiac pacemaker and similar devices could therefore has not yet been investigated. Recent studies show that under certain conditions, these persons can be examined. This is happening only in some larger centers and not as a routine diagnosis, because the pacemaker manufacturer does not guarantee yet that their devices an MRI scan indemnify each other ( for device and patient).
• Fast moving organs like the heart can be represented with the most available devices have reduced quality or require motion compensation by temporal multiple scanning. Through the development of advanced multi-channel systems but these studies are no longer a problem and keep more and more popularity in clinical routine diagnostics.
• The investigation is often time-consuming compared to other imaging techniques.
• The lime content of bony structures can not be quantified due to the used field densities under routine conditions, as bone tissue contains little water and little fat. Bone diseases such as inflammation or tumors are, however, often can be seen better due to the increased blood flow and the associated water content than with X-ray or CT scan.
• Very rarely can occur an allergic reaction to the contrast agent, MR contrast agents are much better -tolerated than iodine-containing X-ray contrast agent. Lately, however, sporadically contrast- induced nephrogenic systemic fibrosis are observed.
• Due to the extremely fast switching of the magnetic fields occurs during recording sometimes loud noises; depending on the chosen sequence is heard an intermittent chirping, knocking, buzzing, rattling or sawing; the repetition frequencies of the image acquisition can extend into the kHz range.
• The high power consumption for the direct cooling, air conditioning and ventilation system. This is in operation and at 40-100 kilowatts in standby mode or standby mode at about 10 kW, some components such as the vacuum pump, the cooling of the superconducting coil and the parts of the electronic control system also when not in use the system can not be switched off to obtain superconductivity.
• Due to the small diameter of the tube, into which the patient is moved, there may be Beklemmungs and anxiety. Meanwhile, there are also devices having a slightly larger tunnel opening of 75 cm ( instead of 60 cm). There are also special open devices, which do have a slightly inferior field homogeneity, but also the physician to grant access, for example for MRI-guided biopsies.

Artifacts

Compared to computed Artifacts ( picture interference ) frequently and interfere with the image quality usually more. Typical MRI artifacts are:

• Motion and flow artifacts
• Refolding artifacts (object is outside the field of view ( " Field of View", FOV), but still within the receiver coil )
• Chemical -shift artifacts ( due to different precession frequencies of fat and water protons)
• Canceling distortion and artifacts ( due to local magnetic field inhomogeneities ), a so-called susceptibility (these may however be used, for example, to diagnose bleeding in the brain)
• Edge artifacts ( in the range of tissue transitions with strongly varying signal)
• Line artifacts (high frequency leaks )
• Artifacts caused by external noise sources in the room such as Perfusors and anesthesia machines older design ( even if they are relatively far away from the magnet ); they often present themselves as strip is in the phase
• Artifacts as a result of any radio transmitting equipment, such as 433 MHz transmitter of the ISM band and Bluetooth devices

Contraindications

• Pacemaker and defibrillator systems can be damaged by the investigation or through interactions with the magnetic fields of the MRI for patient harm. Thus, the contact surfaces of the implanted electrodes can heat, magnetic parts of the implant could move or the system may be disturbed in its function completely. Some manufacturers of such implants have become conditioned developed MRI -compatible systems that have been approved in the European Union, the USA and Japan. Many pacemakers and ICD systems are now observed in controlled clinical trials.
• Metal flakes or vascular clips of ferromagnetic material in an unfavorable position (eg in the eye or in the brain)
• Temporary vena cava filter
• First trimester (= 1st - 13th week) of pregnancy ( relative contraindication )
• Cochlear implant (Some cochlear implants MRI by following exactly most of the manufacturer of cochlear implant instructions is possible. For example, certain MRI machines or field strengths should be used and the cochlear implant are fixed in the head with an additional pressure bandage / backed up. )
• Implanted insulin pumps ( external pumps must be stored for examination )
• Large or loop-like tattoo in the study area ( metal-containing pigments which may heat up and cause burns to Grade II )
• Claustrophobia ( = " claustrophobia " ) ( relative contraindication, examination under sedation or anesthesia possible)
• Larger, non-removable piercings from magnetic materials

List of abbreviations commonly used MR sequences

Study period in a magnetic resonance tomography

The duration of an MRI scan depends on the examined body section, the clinical problem and the equipment used. The commonly performed examination of the head typically takes 10-30 minutes, a lumbar spine examination usually about 20 minutes. The higher the desired level of detail resolution, the longer the time to induce beating investigation. Frequently, two group shots are created, first one without contrast, followed by contrast medium.

Scan time must be taken into account in the selection of the diagnostic method. The ability of a patient to lie still for the required time may be limited individually and disease- dependent. For MRI examination of infants and young children sedation or general anesthesia is usually required.

Recent developments promise to significantly shorten the scan time by the simultaneous recording of the MR signal with several receiving coils, so that less than one second are possible in extreme cases, exposure times.

Spatial encoding

For spatial encoding of the image information to the main magnetic field additional gradient fields are superimposed ( in the x-, y -and z- direction). Gradient field means that, for example,

• The magnetic field in the header area is significantly weaker than in the abdominal region or vice versa
• The magnetic field on the left ear is stronger than the right ear
• The magnetic field at the back of the head is stronger than at the end

About this used gradient coils that produce these changes, strong magnetic fields are assembled and disassembled within milliseconds. The resulting electromagnetic forces pull so strong at the coil anchors that loud knocking or thumping, creaking, humming or even squeaky noises depending on the frequency of the Magnetfeldauf and termination, which vary according to road sequence. The machine almost works like a loudspeaker: A strong magnet is surrounded by current-carrying coils. Patients usually a hearing protection or headphones with music is therefore placed in the investigation. This has to be completely metal-free. In some studies, especially in the head region, the hearing protection to avoid hearing damage in the form of acoustic trauma is imperative. Most of the patients, the noise is, however, very well tolerated, since they are sufficiently reduced by the hearing protection. Some patients sleep during prolonged investigations even relaxed. This also applies to patients who indicate prior to the examination fear of loud noises.

Cost of a Magnetic resonance ( Germany )

The prices for an MRI directed in Germany by the rate of doctors and are depending on the organ and effort investigating 140-1200 euros. The statutory health insurance program for people insured by the Uniform Value Scale, which defines much lower prices ( 90-125 euros ). Special procedures ( cardiac MRI, whole-body examinations, vascular representations, breast MRI) of the statutory insurance paid only partially or not at all, for example because the benefit of the investigation is not yet occupied or because the side effects in the form of mal- and overdiagnosis are too high. The construction costs are sometimes so high, according to radiologists that the devices can be operated with mixed calculations and additional private offerings.

2009 in Germany were around 5.89 million people, at least one magnetic resonance imaging. The deputy chairman of the Barmer GEK, Rolf -Ulrich Schlenker, announced in January 2011, the estimated total annual cost of computed tomography ( CT) and MRI scans with 1.76 billion euros.

Photo Gallery

Section, the nose is through a man's head to the left; Animated version of several sagittal section planes

MRI of the human heart, four-chamber view; Animated version

MRI of the human heart, sagittal view; Animated version

MRI of the left ankle with Achillessehnenödem above the calcaneus

Open MRI Scanner at the Department of Diagnostic Radiology of the University Hospital Magdeburg

Manufacturer of MRI equipment

• Bruker ( high-field research tomograph )
• Esaote ( low-field scanner with permanent magnet for the extremities diagnostics)
• Fonar Corp.. , Melville, N. Y.
• GE Healthcare
• Hitachi Medical Systems
• PhiHealth ( partnership with Cerner Corporation)
• Philips GmbH Germany
• Siemens Healthcare Sector
• Toshiba
• Varian, Inc. (from 2011 Agilent)

Data format

For the storage and archiving of the results of medical imaging techniques, the DICOM standard has become widely accepted. Often, the patient is given a disk (eg CD -ROM or DVD-ROM) with its own sectional images after the examination, which he then passes on to the treating physician. Often these images are not converted to a more common graphics format such as JPEG so that the patient requires a separate viewer for viewing. Often such included on the disk, which may also offer additional features such as surveys or magnifying tools in addition to the representation of DICOM images.