Immuno Assignment

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
http://www.mrisafety.com/http://www.mrisafety.com/


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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
http://www.aagbi.com/http://www.aagbi.com/


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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

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