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8/2/2019 Immuno Assignment
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INTRODUCTION
Magnetic resonance imaging (MRI) produces high quality images of the body in cross section
and in three-dimension. It detects the effects of induced changes in the nuclei of specific
elements within the body and is particularly useful for the imaging of soft tissues, providing
greater contrast between different types of soft tissue than computerised tomography (CT). It is
the technique of choice for many neurological, cardiovascular, oncological and musculoskeletal
conditions. An MRI machine uses a powerful magnetic field to align the magnetization of some
atoms in the body, and radio frequency fields to systematically alter the alignment of this
magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the
scannerand this information is recorded to construct an image of the scanned area of the
body.[1]:36
Strong magnetic field gradients cause nuclei at different locations to rotate at different
speeds. 3-D spatial information can be obtained by providing gradients in each direction.
MRI provides good contrast between the different soft tissues of the body, which makes it
especially useful in imaging the brain, muscles, the heart, and cancers compared with other
medical imaging techniques such as computed tomography (CT) or X-rays. Unlike CT scans or
traditional X-rays, MRI uses no ionizing radiation.
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HISTORY
In the 1950s, Herman Carr reported on the creation of a one-dimensional MR image. Paul
Lauterbur expanded on Carr's technique and developed a way to generate the first MRI images,
in 2D and 3D, using gradients. In 1973, Lauterbur published the first nuclear magnetic resonance
image and the first cross-sectional image of a living mouse was published in January 1974.
Nuclear magnetic resonance imaging is a relatively new technology first developed at the
University of Nottingham, England. Peter Mansfield, a physicist and professor at the university,
then developed a mathematical technique that would allow scans to take seconds rather than
hours and produce clearer images than Lauterbur had.
Raymond Damadian's "Apparatus and method for detecting cancer in tissue."
In a 1971 paper in the journal Science, Dr. Raymond Damadian, an Armenian-American
physician, scientist, and professor at the Downstate Medical Center State University of New
York(SUNY), reported that tumors and normal tissue can be distinguished in vivo by nuclear
magnetic resonance ("NMR"). He suggested that these differences could be used to diagnose
cancer, though later research would find that these differences, while real, are too variable for
diagnostic purposes. Damadian's initial methods were flawed for practical use, relying on a
point-by-point scan of the entire body and using relaxation rates, which turned out to not be an
effective indicator of cancerous tissue.
While researching the analytical properties of magnetic resonance, Damadian created the world's
first magnetic resonance imaging machine in 1972. He filed the first patent for an MRI machine,
U.S. patent #3,789,832 on March 17, 1972, which was later issued to him on February 5, 1974.
As the National Science Foundation notes, "The patent included the idea of using NMR to 'scan'
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the human body to locate cancerous tissue." However, it did not describe a method for generating
pictures from such a scan or precisely how such a scan might be done. Damadian along with
Larry Minkoff and Michael Goldsmith, subsequently went on to perform the first MRI body scan
of a human being on July 3, 1977. These studies performed on humans were published in 1977.
In recording the history of MRI, Mattson and Simon (1996) credit Damadian with describing the
concept of whole-body NMR scanning, as well as discovering the NMR tissue relaxation
differences that made this feasible.
2003 Nobel Prize
Reflecting the fundamental importance and applicability of MRI in medicine, Paul Lauterbur of
the University of Illinois at Urbana-Champaign and Sir Peter Mansfield of the University of
Nottingham were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries
concerning magnetic resonance imaging". The Nobel citation acknowledged Lauterbur's insight
of using magnetic field gradients to determine spatial localization, a discovery that allowed rapid
acquisition of 2D images. Mansfield was credited with introducing the mathematical formalism
and developing techniques for efficient gradient utilization and fast imaging. The actual research
that won the prize was done almost 30 years before, while Paul Lauterbur was at Stony Brook
University in New York.
The award was vigorously protested by Raymond Vahan Damadian, founder of FONAR
Corporation, who claimed that he invented the MRI and that Lauterbur and Mansfield had
merely refined the technology. An ad hoc group, called "The Friends of Raymond Damadian",
took out full-page advertisements in theNew York TimesandThe Washington Postentitled "The
Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the
Nobel Prize. Also, even earlier, in the Soviet Union, Vladislav Ivanov filed (in 1960) a document
with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic
Resonance Imaging device, although this was not approved until the 1970s. In a letter toPhysics
Today, Herman Carr pointed out his own even earlier use of field gradients for one-dimensional
MR imaging.
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THE PHYSICS OF MAGNETIC RESONANCE IMAGING
MRI relies on the fact that some atoms within the human body possess an odd unpaired proton.
The proton nucleus of the hydrogen atom is one of the most abundant examples, being a major
constituent of water. It responds particularly well to the application of an external magnetic field
and is therefore one of the simplest atom to use for MRI. Another example is phosphorus, which
as a component of adenosine triphosphate, allows for many metabolic processes to be studied.
These nuclei possess a spin that results in a local magnetic field because of their charge, allowing
them to act like small magnets. The alignment of these nuclei is usually random (Figure A),
however when a strong electromagnetic field is applied to the body they align themselves with
that field. (Figure B)
These nuclei can be turned out of alignment with the magnetic field by applying brief bursts of
radiofrequency energy, creating an electromagnetic field perpendicular to the first magnetic
field. When the electromagnetic field is removed, the radio-frequency energy taken up by the
nuclei is released slowly as they relax back into alignment. The rate at which realignment takes
place depends on the type of nucleus, or element being measured, and thus the emitted signal
depends on the molecular properties of the tissue.1 This low radiofrequency radiation that is
emitted induces an electrical signal within a set of three orthogonal gradient coils in the MRI
machine. They are positioned in the transverse (X and Y) and longitudinal (Z) planes allowing
for encoding of spatial information. The detected signals are therefore able to form a three-
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dimensional image of the body. It is these gradient coils that are rapidly turned on and off during
an MRI study that is responsible for the loud banging noises
Different tissues within the body have different relaxation rates. T refers to the relaxation time
constant, and images may be T1 weighted (generated a few milliseconds after the
electromagnetic field is removed) or T2 weighted (generated later than T1), depending on the
characteristics of the tissue you wish to look at. Nuclei in hydrogen take a long time to decay to
their original position, so fluid will appear dark (minimal signal) in a T1 weighted (early) image
(Figure 1), but white in the later T2 image as the signal appears.3 (Figure 2)
Because the signal that makes up the final MR image is very weak, any external radiofrequency
sources can greatly interfere with its detection by the gradient coils. To prevent this the MRI
machine is contained within a radiofrequency shield called a Faraday cage. This is built into the
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fabric of the MR room. To allow infusion lines or monitoring cables to enter the MR room, a
hollow brass tube or waveguide is built into the Faraday cage passing through into the control
room.
The Magnetic field
MRI requires strong magnetic fields between 0.2 and 3.0 Tesla that are generated by
superconductors. To minimise the electrical resistance of the superconducting coils, they are
immersed in liquid helium and cooled to below 4.2 Kelvin.
1 Tesla = 10 000 Gauss (Earths magnetic field = 0.5 1.0 Gauss)
= 1 weber/m2
The magnetic field strength falls away exponentially from the magnet. A safety line is usually
demarcated at the level of 0.5mTesla (5 Gauss) within which pacemakers will malfunction, and
therefore unscreened personnel should not enter (see hazards section below). A second line is
demarcated at 50 Gauss within which a significant attractive force will be encountered on all
ferromagnetic objects, which risk becoming dangerous projectiles. Such items include gas
cylinders, needles, watches, floor cleaners and patient trolleys. Within this line anaesthetic
infusion pumps (or any electronic or mechanical equipment) may fail due to the effects of the
magnetic field. While these lines of demarcation are often referred to theoretically, in practice
many MRI units are simply divided into a safe zone outside the scanner, and the controlled
hazardous zone within the MR examination room.
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INDICATIONS FOR THE USE OF MAGNETIC
RESONANCE IMAGING
MRI is usually the preferred imaging technique in the following cases:
Posterior fossa and infratentorial pathology
Sinus and orbit pathology, sensorineural hearing loss and cranial nerve pathology
Cerebral inflammatory disease including encephalitis, myelitis and meningitis
Brain abscess
Acute ischaemic strokes
Spinal cord soft tissue pathology including congenital, traumatic, neoplastic and vascular
abnormalities and disc pathology
Demyelinisation and the myelopathies
Airway malformations
Vascular malformations
Liver vascular pathology
Joint soft tissue pathology
CT scanning remains more useful for bony pathology, chest examinations, intracranial
haemorrhage and abdominal and pelvic applications
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Specialized MRI scans
Diffusion MRI
Diffusion MRI measures the diffusion of water molecules in biological tissues. In an isotropicmedium (inside a glass of water for example), water molecules naturally move randomly
according to turbulence and Brownian motion. In biological tissues however, where the
Reynolds number is low enough for flows to be laminar, the diffusion may be anisotropic. For
example, a molecule inside the axon of a neuron has a low probability of crossing the myelin
membrane. Therefore the molecule moves principally along the axis of the neural fiber. If it is
known that molecules in a particular voxel diffuse principally in one direction, the assumption
can be made that the majority of the fibers in this area are going parallel to that direction.
The recent development ofdiffusion tensor imaging (DTI) enables diffusion to be measured in
multiple directions and the fractional anisotropy in each direction to be calculated for each voxel.
This enables researchers to make brain maps of fiber directions to examine the connectivity of
different regions in the brain (using tractography) or to examine areas of neural degeneration and
Magnetization Transfer MRI
Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water
protons to hydration water protons in NMR and MRI.
In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of
water molecules, free (bulk) and hydration (bound), are found. Free water protons have faster
average rotational frequency and hence less fixed water molecules that may cause local field in
homogeneity. Because of this uniformity, most free water protons have resonance frequency
lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). This
also results in slower transverse magnetization dephasing and hence longer T2. Conversely,
hydration water molecules are slowed down by interaction with solute molecules and hence
create field inhomogeneities that lead to wider resonance frequency spectrum.
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T1rho MRI
T1 (T1rho): Molecules have a kinetic energy that is a function of the temperature and is
expressed as translational and rotational motions, and by collisions between molecules. The
moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect
over a long time-scale may be zero. However, depending on the time-scale, the interactions
between the dipoles do not always average away. At the slowest extreme the interaction time is
effectively infinite and occurs where there are large, stationary field disturbances (e.g. a metallic
implant). In this case the loss of coherence is described as a "static dephasing". T2* is a measure
of the loss of coherence in an ensemble of spins that include all interactions (including static
dephasing). T2 is a measure of the loss of coherence that excludes static dephasing, using an RF
pulse to reverse the slowest types of dipolar interaction. There is in fact a continuum ofinteraction time-scales in a given biological sample and the properties of the refocusing RF pulse
can be tuned to refocus more than just static dephasing. In general, the rate of decay of an
ensemble of spins is a function of the interaction times and also the power of the RF pulse. This
type of decay, occurring under the influence of RF, is known as T1. It is similar to T2 decay but
with some slower dipolar interactions refocused as well as the static interactions, hence T1T2 .
Fluid Attenuated Inversion Recovery (Flair)
Fluid Attenuated Inversion Recovery (FLAIR) is an inversion-recovery pulse sequence used to
null signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal
fluid (CSF) so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis
(MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and
excitation pulses), the signal from any particular tissue can be suppressed.
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
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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 signal is gathered in the plane immediately superior to the
excitation planethus imaging the venous blood that recently moved from the excited plane.
Magnetic resonance gated intracranial CSF dynamics (MR-GILD)
Magnetic resonance gated intracranial cerebrospinal fluid (CSF) or liquor dynamics (MR-GILD)
technique is an MR sequence based on bipolar gradient pulse used to demonstrate CSF pulsatile
flow in ventricles, cisterns, aqueduct of Sylvius and entire intracranial CSF pathway. It is a
method for analyzing CSF circulatory system dynamics in patients with CSF obstructive lesions
such as normal pressure hydrocephalus. It also allows visualization of both arterial and venous
pulsatile blood flow in vessels without use of contrast agents.
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 correspond to different
molecular arrangements of the isotope being "excited". 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).
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Real-time MRI
Real-time MRI of a human heart at a resolution of 50 ms
Real-time MRI refers to the continuous monitoring (filming) of moving objects in real time.
While many different strategies have been developed over the past two decades, 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.
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 a MRI scanner are used to guide
minimally invasive procedures. Of course, such procedures must be done without any
ferromagnetic instruments.
A specialized growing subset of interventional MRI is that of intraoperative MRI in which the
MRI is used in the surgical process. Some specialized MRI systems have been developed that
allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical
procedure is temporarily interrupted so that MR images can be acquired to verify the success of
the procedure or guide subsequent surgical work.
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Current density imaging
Current density imaging (CDI) endeavors to use the phase information from images to
reconstruct current densities within a subject. Current density imaging works because electrical
currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during
an imaging sequence.
Magnetic resonance guided focused ultrasound
In MRgFUS therapy, ultrasound beams are focused on a tissueguided and controlled using MR
thermal imagingand 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.
Multinuclear imaging
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, 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.17
O and19
F can be administered in sufficient
quantities in liquid form (e.g.17
O-water) that hyperpolarization is not a necessity.
Multinuclear imaging is primarily a research technique at present. However, potential
applications include functional imaging and imaging of organs poorly seen on1H MRI (e.g.
lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized3He can be used to
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image the distribution of air spaces within the lungs. Injectable solutions containing13
C or
stabilized bubbles of hyperpolarized129
Xe have been studied as contrast agents for angiography
and perfusion imaging.31
P can potentially provide information on bone density and structure, as
well as functional imaging of the brain.
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HAZARDS AND SAFETY CONSIDERATIONS FOR
PATIENTS AND STAFF
IN THE MRI UNIT
1. The presence of a strong magnetic field
The strong magnetic field poses by far the most important hazard related to anaesthesia and care
of patients requiring MRI. These powerful magnetic fields are able to exert large forces on any
ferromagnetic materials in close proximity. They may also induce currents in metallic objects
causing local heating and may interfere with monitoring equipment. Conversely, ferromagnetic
objects and electrical fields in the vicinity of the magnet will degrade the quality of the MR
images produced. The safety aspects related to ferromagnetic objects as projectiles, implants,
foreign bodies and as equipment will be discussed in further detail below. The human body is
conductive and movement of the body within the magnetic field will induce weak electrical
currents within the tissues. Movement of blood around the body will also result in the generation
of electric potentials and current. These currents can cause symptoms such as nausea and vertigo
as a result of excitation of the semicircular canals of the inner ear, or flashing lights due to their
effects on the retina. The patient as well as the staff positioning a patient in the scanner and
moving within the immediate vicinity of the magnet bore may occasionally notice these effects.
There is currently no evidence that long-term repeated exposure to strong magnetic fields has a
harmful effect on the human body, however current recommendations suggest that a time
weighted average of 200mT over any 8-hour period should not be exceeded by healthcare
personnel. Ideally all staff should vacate the MRI examination room whilst the scan is in
progress.
2. Ferromagnetic objects and the projectile effect
The attractive forces between the magnet and all ferromagnetic objects increase significantly as
such objects are brought closer to the magnet. All ferromagnetic items brought within the 50
Gauss line will be subject to movement and may be rapidly accelerated into the magnetic field.
Objects that are not fixed down therefore risk becoming dangerous projectiles and may cause
injury to anyone in their path, as well as damage to equipment, and interference with the MR
image generated.
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All staff should be fully aware of the dangers of metal objects in the scanner, and before entering
the controlled area of the examination room need to remove all ferromagnetic, metallic or
conducting materials from their person. Magnetised items such as credit cards and mobile phone
SIM cards are at risk of being damaged by proximity to the magnetic field. Before entering the
examination room with an anaesthetised patient a careful inspection for metallic objects should
be made. Items that typically might contain metals include needles, watches and jewellery,
pagers, stethoscopes, anaesthetic gas cylinders, metallic trolleys, ECG electrodes, transdermal
drug patches (GTN) and ventilator systems. After hours, floor polishers are particularly common
projectiles.
3. Implants and foreign bodies
Ferromagnetic materials may also be present inside the body and are subject to similar forces
that can cause them to move or malfunction with potentially fatal consequences. Implanted
ferromagnetic objects may also heat up significantly during the MR examination causing local
tissue damage. Absolute contraindications to MRI include cochlear implants, intra-ocular
metallic foreign bodies or shrapnel, or ferromagnetic arterial or aneurysm clips particularly
neurovascular. Patients with cardiac pacemakers or implanted defibrillators must never undergo
an MRI scan since these will malfunction within the Gauss line. Most modern patient implants,
including metal prostheses, are non-ferromagnetic. General surgical clips, artificial heart valves
and sternal wires are usually deemed safe since they are fixed by fibrous tissue. Nonetheless, no
patient should ever enter an MR examination room if there is any doubt about the safely of an
implanted device or foreign body. All patients therefore need to be screened prior to the MRI
scan for the presence of metallic implants. This is the responsibility of the radiology staff,
however all staff working within the MRI unit should be aware of these risks. The same
precautions regarding foreign bodies and implanted devices apply to all hospital staff that work
near the MRI scanner. Usually a standard screening questionnaire or metalcheck will suffice,
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however X-rays may be used to search for metal implants if any doubt exists. The compatibility
of any implanted devices with the MR scanner may be confirmed online via websites such as
www.mrisafety.com
4. Equipment and monitoring issues
All anaesthetic equipment and monitoring in the MR room should be MRI compatible. An
important distinction exists between equipment that is designated MRI safe and that which is
designated MRIcompatible. MRI safe implies that a piece of equipment will not pose a danger
to patients and staff if it enters an MRI examination room, but does not guarantee that it will
function correctly or avoid degrading the image quality. MRI compatible equipment is both safe
to enter the MR examination room and will operate normally within that environment without
interference to the MR scanner. It is reasonable to deduce that all anaesthetic equipment used
within the MRI examination room should therefore be MRI compatible. Where non-compatible
equipment is used within the magnetic field they may pose serious hazards to the patient they
may become projectile, cause burns if heated cables come in contact with the patient, or they
may malfunction. In the past ferrous anaesthetic machines remained in the MR control room and
anaesthetic breathing systems such as the co-axial Mapleson D or Bain circuit extended through
the waveguide ports to the patient. MR compatible anaesthetic machines, ventilators and
vaporisers are now available from most manufacturers and should be used instead. Anaesthetic
breathing systems including the circle system, Bain circuit or Ayres T piece for children have all
been used successfully. Piped gasses with back up cylinders made of a non-ferrous metal such as
aluminium should be available. Although MR compatible equipment is likely to be more fragile
and costly it is essential that minimal monitoring standards for routine anaesthesia are complied
with. Suppliers often provide basic MR compatible monitors as part of the system. The
anaesthetist must be aware of the fact that MR can interfere with accurate monitoring and
monitors can similarly interfere with the MRI. The changing gradient fields and radiofrequency
currents used for MRI can induce currents in monitoring leads. These can cause burns to the
patient as well as interference with the monitoring. Fibre-optic or carbon fibre cabling should
avoid this problem, however care must still be taken to avoid coiling of cables within the
scanner, and padding should be placed between all leads and the patients skin. The converse
applies too, and monitors should not emit radiofrequencies that might interfere with the image
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quality. Mains power supply should be isolated or filtered, or battery power used. Batteries are
ferromagnetic, and if used, the relevant equipment should be firmly secured. Monitoring screens
should be present in the MR control room to allow for remote monitoring of the patient so that
the anaesthetist can leave the MR examination room. Monitoring cables can be passed through
the waveguide ports to facilitate this. All alarms should be visual because of the noise made by
the MR scanner, and the view of the monitor, anaesthetic machine and patient should be
unobstructed at all times. Electrocardiogram (ECG) monitoring cannot occur with standard
electrodes and MR compatible electrodes are needed. They should be placed in a narrow triangle
on thepatients chest, and leads should be braided and short (15cm). Currents induced by blood
flow through the transverse aorta will interfere with the ECG signal causing artefact in the ST-T
complexes which mimics hyperkalaemia. Pulse oximeter cables should be insulated and placed
as far from the scanner as possible. Finger burns have been reported with standard non
compatible pulse oximeters. Non-invasive blood pressure monitoring is possible if connectors
are changed to plastic, and invasive pressure monitoring is possible if the pressure transducer
cabling is passed through the waveguides. MR compatible pressure transducers are available.
Capnography and monitoring of airway pressures and gasses requires a longer sample tubing
than is routine, this results in approximately a 20 second delay, which the anaesthetist should
take into consideration. Infusion pumps may fail if close to the magnet where the magnetic field
strength exceeds 50 Gauss.
5. Restricted access of the environment
The MR scanner is designed to place the patient in the centre of the magnetic field within the
bore of the magnet. As a result the patient is effectively enclosed within a narrow tube to which
access is extremely limited. Newer designs include open C shaped magnets that are less
claustrophobic for the awake patient and allow improved access, however these are only suitable
for limited investigations as they do not allow such detailed investigations and the duration of the
scans is longer. Not only is the patient access restricted, but also the MRI suite itself is an
environment in which only suitably trained staff should be working. It is often located at a
distance from the hospitals theatre facilities making readily available backup and assistance less
likely.
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6. High level acoustic noise
Noise levels above the safe level of 85 decibels can be produced during MRI due to the rapid
switching of the gradient coils. The exact magnitude of this noise depends on the sequence of
images being collected and the strength of the magnetic field. Staff working in MRI units should
protect themselves by remaining in the MR control room during sequence acquisition, or by
wearing earplugs should they need to remain in the examination room. All patients should be
given ear protection, regardless of if they are awake or anaesthetised. The anaesthetist should be
aware that high ambient noise levels may mask normal auditory alerts such as monitor alarms or
sound the sound of partial airway obstruction, so vigilance and attention to visual cues is
essential.
7. Scavenging of anaesthetic gasses
Volatile anaesthetic agents and nitrous oxide may be used for general anaesthesia in MR units.
MR compatible scavenging systems are available and these gases should therefore be scavenged
in the usual way to comply with the local regulations for these substances. (Control of
Substances Hazardous to Health or COSHH regulations in the United Kingdom)
8. Quenching of superconducting magnets
The coils used in MR magnets need to be kept cold in order to maintain superconductivity. This
is achieved by immersing them in liquid coolants or cryogens, liquid helium being the most
commonly used in modern MRI units. Quenching is a process involving the rapid boil-off of the
cryogen that causes an immediate loss of superconductivity. This may occur spontaneously as a
system error during installation, services and power ups, or may be deliberately induced in order
to shutdown the magnetic field. If this happens, the magnetic field will be lost and a large
volume of helium gas will be produced. This is normally vented to the outside atmosphere
through a quench pipe. In the event of damage to the quench pipe, the build-up of helium within
the scanning room could potentially lead to asphyxiation. Oxygen sensors must be present in the
scanning room to alert the staff in the control room to a hypoxic environment. All Staff working
in an MRI unit should be aware of the emergency procedures for quenching.
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9. Hazards of MRI during pregnancy
The MRI unit may pose hazards to the developing foetus, including exposure to strong magnetic
fields, high noise levels and unscavenged anaesthetic gases. Although limited evidence exists, in
the United Kingdom it is currently recommended that pregnant women should ideally not be
scanned during the first trimester of pregnancy. Pregnant staff working within the MRI unit
should be advised of the risks posed by this environment, and given the option of not entering the
inner controlled area during their first trimester.
10. Use of contrast agents
The most commonly used intravenous MR contrast agent is gadolinium dimeglumine (Gd-DTPA
or Magnevist). It is used to increase the signal intensity on T1 weighted scans and reduce the
signal intensity on T2 weighted scans. It is often used in contrast-enhanced MR angiography and
to help identify tumours. Since it does not normally cross the blood brain barrier it may be used
to demonstrate areas where it has broken down and to delineate intracranial pathology. Gd-
DTPA is used in doses of 0.2 ml/kg and has minor side effects including nausea, vomiting an
pain on injection. There are rare complications of gadolinium called nephrogenic systemi fibrosis
or nephrogenic fibrosing dermopathy, seen in association with renal impairment; all patients
should have an assessment of renal function before MRI, either by history or by urea and
creatinine assay. There has been one incidence of anaphylactoid reaction reported.
11. Maintenance of body temperature
A theoretical problem during sedation or anaesthesia of infants and neonates for MRI is the
maintenance of body temperature within this cooled environment. Passive heat loss should be
prevented by minimizing exposure and by returning the infant to a warm environment as soon as
possible. Recent studies examining the effect of MRI on body core temperature in sedated
infants and children have suggested that this problem is not as significant as once thought.Radiofrequency radiation produced by the MR scanner and absorbed by the patient causes an
increase in body temperature,suggesting that active heating is unnecessary and may in fact cause
hyperthermia. This rise intemperature was more profound in 3 T than in 1.5 T examinations.
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PATIENT MANAGEMENT FOR MAGNETIC
RESONANCE IMAGING
Magnetic resonance imaging requires a patient to lie still in a noisy and restricted space forprolonged periods of time. By far the greater majority of patients should be able to achieve this
without the intervention of an anaesthetist. Understandably this may not be possible for certain
groups of patients, particularly young children. All cases referred for general anaesthesia should
be evaluated and have the risks of anaesthesia weighed against the benefits of the investigation.
Not all patients require general anaesthesia. For example, with regards to infants and children,
other management strategies may be commonly utilized:
Behavioural techniques, including reassurance, communication through informative booklets,
videos and visits to the unit, rehearsal of scans and the skills of play specialists.
Natural sleep techniques, including the feed and wrap method for neonates, and sleep
deprivation prior to a scan for toddlers.
Sedation techniques, lead by specialist and experienced nurse lead sedation services have been
shown to be both highly successful and very safe.
The following groups are more likely to require general anaesthesia:
Infants and children
Patients with learning difficulties
Patients with certain seizure or movement disorders
Patients with claustrophobia
Critically ill patients
Patients undergoing neurological examinations, particularly where raised intracranial pressure
is a concern (sedation is contraindicated as it can be potentially dangerous)
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CONDUCT OF GENERAL ANAESTHESIA FOR MRI
All patients for MRI should be pre-assessed by their anaesthetist and starvation guidelines should
be the same as for any general anaesthetic. A metal check must be performed by the radiology
staff prior to induction of anaesthesia. The choice of anaesthesia technique depends on factors
such as the length of the scan, the age of the child, associated co-morbidities such as raised
intracranial pressure, or the need for a breath hold as for cardiac MRI scans. Small infants
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discharged from the recovery or ward area once they meet the normal discharge criteria for day
case procedures.
CONSENT FOR ANAESTHESIA FOR MRI
Unlike anaesthesia for invasive surgical procedures where the consent for anaesthesia is implied
by the act of consenting for the surgery, and contrary to routine MRI where written consent is not
required, the consent for MRI under general anaesthesia remains a complex issue. In order to
obtain truly informed consent input should ideally be provided from the referring clinician who
has requested the investigation, the radiologist who is performing the scan, and the anaesthetist
responsible for the general anaesthesia. Recent review of this issue has suggested that it is the
referring clinician who is best suited to explain the intended benefits, side effects and risks of the
procedure, and thus to obtain written consent. Nevertheless it remains incumbent upon the
responsible anaesthetist to review the anaesthetic plan and risks with the patient prior to the
procedure.
EMERGENCIES IN THE MRI SUITE
Owing to the presence of a strong magnetic field and the risk of projectiles, as well as the
restricted access imposed by the MRI scanner, it is impossible to manage emergencies and
resuscitation within the scanning room and Gauss line. In the event of an emergency the patient
should be removed from the magnetic field as quickly as possible and transferred to the
induction room, which should be close to the scanner and will contain the necessary anaesthetic
and resuscitation equipment and drugs. The resuscitation team should know not to enter the
Gauss line of the inner controlled area, and in the event of an emergency should be directed by
radiology staff to the induction/resuscitation room.
INTENSIVE CARE PATIENTS REQUIRING MRI
MRI of critically ill adults and children is becoming both an important diagnostic and prognostic
tool. These patients require special expertise, planning and time to be safely examined by MR.12
One notable challenge includes the multiple drug infusions that these patients might require,
inotropic therapy being of particular concern. All unnecessary infusions should be discontinued.
Those that are required should be infused through extensions of adequate length, which can be
passed through the waveguide to a pump remaining in the MR control room. Non-compatible
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MR syringe drivers used within the MR examination room may deliver incorrect drug doses with
significant patient safety dangers. If MR compatible infusion pumps exist they may be used in
the MR examination room, however the anaesthetist may need to remain in the room if the
pumps rate needs to be changed. Critically ill patients also require a higher standard of
monitoring. All monitoring equipment should be changed to MR compatible versions within the
anaesthetic induction room before entering the MR examination room. Arterial pressure
transducers can be passed through the waveguide if not compatible.Pulmonary artery catheters
with conductive wires in contact with heart muscle and epicardial pacing wires pose a theoretical
risk of micro-shock; these should be removed prior to the examination. Central venous catheters
pose no risk to the patient. All in-dwelling catheters should be disconnected from electrical
connections and external accessories before entering the MR examination room. Lastly, care
should be taken to ensure that no surgical interventions undertaken have left the patient with
internal metal work. Often where the patients h istory is vague, a pre-MR X-ray screen might be
required. Many tracheostomy tubes are not MR compatible and will need to be changed prior to
the examination. The pilot balloons of cuffed tracheal tubes may contain a small ferromagnetic
spring that will need to be taped securely away from the area being scanned.
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CONCLUSION
MRI is now a routine investigation, and as the demand for MRI scans increases so will the needfor general anaesthesia in this environment and for MRI scans of more challenging patients. New
scanning techniques are being developed in the areas of orthopaedic soft tissue imaging and
dynamic cardiac imaging. Operating theatres and intensive care units incorporating open MRI
scanners are being developed and introduced. Scanners that permit access to the patient allow for
perioperative scanning. This is an area of anaesthetic practice that will grow in the future, and in
order to maintain the current levels of patient care and safety, all anaesthetists should remain
familiar with the challenges posed by this unique environment.
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REFERENCES
1. Davis PD, Kenny GNC. Basic Physics and Measurement in Anaesthesia, Fifth Edition.ButterworthHeinemann, 2002; 26971
2. Peden CJ, Twigg SJ. Anaesthesia for magnetic resonance imaging. Continuing Education inAnaesthesia, Critical Care and Pain. 2003; 3: 97101
3. Bricker S. The Anaesthesia Science Viva book, First edition. Greenwich Medical Media Ltd,
2004; 25657
4. Association of Anaesthetists of Great Britain and Ireland. Provision of anaesthetic services inmagnetic resonance units. May 2002. Website: www.aagbi.com
5. Roth JL, Nugent m et al. Patient monitoring during Magnetic resonance imaging.
Anaesthesiology. 1985; 62: 8083
6. Taber KH, Thompson J et al. Invasive pressure monitoring of patients during magneticresonance imaging. Canadian Journal of Anaesthesia. 1993; 40: 10925
7. Sesay M, Tauzin-Fin P et al. Audibility of anaesthesia alarms during magnetic resonance
imaging: should we be alarmed?European Journal of Anaesthesiology. 2009; 26: 117122
8. Machata AM, Willschke H et al. Effect of brain magnetic resonance imaging on body coretemperature in sedated infants and children.British Journal of Anaesthesia. 2009;
9. Sury MRJ, Harker H et al. The management of infants and children for painless imaging.
Clinical Radiology. 2005; 60: 731741
10. Sury MRJ, Hatch DJ et al. Development of a nurse-led sedation service for paediatricmagnetic resonance imaging. The Lancet. 1999; 353:166771
11. Wellesly H, Chong WK, Segar P. Who should obtain written consent for magnetic resonance
imaging under general anesthesia? Pediatric Anesthesia. 2009; 19: 96163
12. Tobin JR, Spurrier EA, Wetzel RC. Anaesthesia for critically ill children during MagneticResonance Imaging.British Journal of Anaesthesia. 1992; 69: 48286
13. Kampen J, Tonner PH, Scholz J. Patient safety during anaesthesia for magnetic resonanceimaging.European Journal of Anaesthesiology. 2004; 21: 32035
14. Odegard KC, DiNardo JA et al. Anaesthesia considerations for cardiac MRI in infants andsmall children. Paediatric Anaesthesia. 2004
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MAGNETIC RESONANCE IMAGING
MADE BY:
MUKUL ATTRI
SEC-S
9013
SUBMITTED TO:
Ms. Mallela Martha prem latha
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CONTENTS
INTRODUCTION HISTORY THE PHYSICS OF MAGNETIC RESONANCE IMAGING THE INDICATIONS FOR THE USE OF RESONANCE
IMAGING
SPECIALIZED MRI SCANS HAZARDS AND SAFETY CONSIDERATIONS FOR
PATIENTS AND STAFF IN THE MRI UNIT
PATIENT MANAGEMENT FOR MAGNETIC RESONANCEIMAGING
CONCLUSION REFERENCES