MRI Report

Contents

1. Introduction2. Physics of MRI

3. Applications4. The MRI equipment5. A Comparison: MRI versus CT 6. Safety7. References

IntroductionMagnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.

An MRI scanner is a tube in which the patient lies inside and is surrounded by a large, powerful magnet where the magnetic field is used to align the magnetization of some atomic nuclei 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 scanner—and this information is recorded to construct an image of the scanned area of the body. Magnetic field gradients cause nuclei at different locations to rotate at different speeds. By using gradients in different directions 2D images or 3D volumes can be obtained in any arbitrary orientation.

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 does not use ionizing radiation.

Physics of MRI

The principle behind the use of MRI machines is that they make use of the fact that body tissue contains lots of water (and hence protons) which gets aligned in a large magnetic field. A moving electric charge, be it positive or negative, produces a magnetic field. The faster it moves or the larger the charge, the larger the magnetic field it produces. Some of the basic properties of a simple proton include mass, a positive electric charge and spin. Granted, a proton does not have a very large electric charge, but it does spin very fast and, therefore, does produce a small, but noticeable, magnetic field. Water is the biggest source of protons in the body, followed by fat. Normally, the direction that these tiny magnets point in is randomly distributed.

Just as a compass aligns with the earth's magnetic field, a spinning proton placed near (or within) a large external magnetic field (called Bø) will align with the external field. Unfortunately, it is not quite so simple. At the atomic level, some of the protons align with the field and some actually align against the field cancelling each other out. A slight excess will align with the field so that the net result is an alignment with the external field.

A complete explanation of why the protons align both with and against the external magnetic field would require a study of quantum mechanics. Suffice it to say that both alignments are

possible, but the one with the field is a lower energy state. The protons are continually oscillating back and forth between the two states but at any given instant, and with a large enough sample, there will be a very slight majority aligned with the field. The larger the external Bø field, the greater the difference in energy levels and the larger the excess number aligned with the field.

At 1.5 Tesla, for every 2 million protons, there are 9 more protons aligned with the field than there are aligned against the field.

1.1) Choice of the nuclei :

The nucleus has two properties that enable the working of MRI, spin and charge. They are made up of neutrons and protons. Both of the nucleons have ½ spin and the protons have charges. As we know that pair of spins tend to cancel, so only atoms with an odd number of protons or neutrons have net spin.

Thus elements suitable for this purpose are for example 1H, 13C, 19F, 23Na, 31P.

1.2) Hydrogen atoms are best for MRI

Biological tissues predominantly contain 12 C, 16O, 1H, 14N. Of all these atoms hydrogen is the only major species that is MR sensitive. Moreover Hydrogen is the most abundant in the body as the body is predominantly made up of water(~85%); and the majority of the Hydrogen is in water(H2O). So ,essentially all MRI is proton(Hydrogen) imaging.

1.3) A single Proton

There is electric charge on the surface of a proton, thus creating a small current loop and generating a magnetic field µ. The proton also has mass which generates an angular momentum J when it is spinning. Thus proton “magnet” differs from the bar magnet in that it possesses angular momentum caused by spinning.

The magnetic moment µ and the angular momentum J lie along the spin axis. The relation between the two is given by:

µ=γ J

Here γ is the gyromagnetic ratio and is constant for a nucleus.

1.4) Larmor frequency and Precession

Just as a spinning top wobbles about its axis so do spinning protons wobble, or precess, about the axis of the external Bø field. The frequency of the precession is directly proportional to the strength of the magnetic field.

It is defined by the Larmor equation:

Where wø (omega zero) is known as either the precessional, Larmor or resonance frequency. At the magnetic field strengths used in clinical MRI systems, .05 to 2 Tesla, the resonance frequency of hydrogen ranges from 2.13 MHz to 85 MHz.

If an electromagnetic radio frequency (RF) pulse is applied at the resonance (Larmor, precession, wobble) frequency, then the protons can absorb that energy. At the quantum level, a single proton jumps to a higher energy state. At the macro or classical level, to an observer in the external laboratory frame of reference, the magnetization vector, Mø, (roughly 6 million billion protons) spirals down towards the XY plane. If you could somehow jump aboard Mø, just like a

merry-go-round, the laboratory would be rotating around you. In this rotating frame of reference, Mø would seem to smoothly tip down. The tip angle, alpha, is a function of the strength and duration of the RF pulse.

Once the RF transmitter is turned off three things begin to happen simultaneously. 1. The absorbed RF energy is retransmitted (at the resonance frequency). 2. The excited spins begin to return to the original Mz orientation. (T1 recovery to thermal equilibrium).3. Initially in phase, the excited protons begin to dephase (T2 and T2* relaxation).

Once Mz (a magnetization vector) has been tipped away from the Z axis, the vector will continue to precess around the external Bø field at the resonance frequency wø. A rotating magnetic field produces electromagnetic radiation. Since wø is in the radio frequency portion of the electromagnetic spectrum the rotating vector is said to give off RF waves.

So, just like phosphorescent paint glows in the dark, the absorbed RF energy is now being retransmitted, thereby producing the NMR signal.

1.5) Mz recovery via T1 relaxation

The process of giving off RF energy occurs as the spins go from a high energy state to a low energy state, realigning with Bø. The RF emission is the net result of the Z component (Mz) of the magnetization recovering back to Mø. Not all of the energy given off is detectable as an RF pulse. Some of the energy goes to heating up the surrounding tissue, referred to as the lattice. In a global, or rather, universal sense, this system can be divided into the spins, and the rest of the universe, or a very large lattice. This type of spin-lattice interaction is the result of the excited system returning to thermal equilibrium. In the classical description, the Mz component begins to grow at the expense of the Mxy component.

The time course whereby the system returns to thermal equilibrium, or Mz grows to Mø, is mathematically described by an exponential curve. This recovery rate is characterized by the time constant T1, which is unique to every tissue. As will be discussed in detail later, this uniqueness in Mz recovery rates is what enables MRI to differentiate between different types of tissue. At a time t=T1 after the excitation pulse, 63.2% of the magnetization has recovered alignment with Bø.

T1 Recovery Curve

Mz = Mø * ( 1 - e-t/T1 )

In general, T1 values are longer at higher field strengths.

1.6) Mxy Spreads Out via T2 Relaxation

When the spins are first tilted down to the XY plane, they are all in phase. Think of a playground with a million swings. If all of the children are going up and down together, at exactly the same rate, then they are swinging in phase. Assuming that all the children are pumping their legs with the same force and frequency, then they will stay in phase. But if one child stops pumping for a few seconds and another child pumps a little harder or a little faster, then they will start to get out of synch with everyone else. The same type of thing happens to the spins. For reasons that we will go into soon, some protons spin a little faster while others spin a little slower. Very quickly, they get out of phase relative to some reference, (usually the spins at the center of the magnet.)

How fast a proton wobbles or precesses depends on the magnetic field that it experiences. An isolated proton, far from any other proton (or electron) only feels (is affected by) the main Bø field. As protons (or spins) move together (due to random motion for example), their magnetic fields begin to interact. If the field from one proton increases the field that the second proton feels then the second proton will precess at a slightly faster rate. If the first field opposes the main field then the second proton will precess more slowly. As soon as the spins move farther apart their fields no longer interact and they both return to the original frequency but at different phases! This type of interaction is called spin-spin interaction. These temporary, random interactions cause a cumulative loss of phase across the excited spins resulting in an overall loss of signal.

Similar to T1 relaxation, the signal decay resulting from transverse or spin-spin relaxation is described mathematically by an exponential curve, identical in concept to radioactive decay with

a half-life measured in tens of msecs. The value T2 is the time after excitation when the signal amplitude has been reduced to 36.8% of its original value (or has lost 63.2%. - This is the opposite of T1 where 63.2% of Mz is recovered in one T1 period.) The value of T2 is unique for every kind of tissue and is determined primarily by its chemical environment with little relation to field strength. In chapter 5, we will discuss in more detail how these unique T2 values are used to produce different types of image contrast.

T2 Decay Curve

Mxy = Mø * e-t/T2

After the RF transmitter is turned off, the protons immediately begin to re-radiate the absorbed energy. If nothing is affecting the homogeneity of the magnetic field all of the protons will be spinning at the same resonance frequency. The initial amplitude of the signal is determined by the portion of the magnetization vector (Mø) that has been tipped onto the XY plane. This, in turn, is determined by the sine of the flip angle, a. The maximum signal is obtained when the flip angle is 90°. (Remember, sin(0°) = 0, sin(90°) = 1.0) The signal unaffected by any gradient is known as a Free Induction Decay (FID). The time constant that determines the rate of decay is called T2. An FID has no positional information.

1.7) T2 * Relaxation

In the real world, the NMR signal decays faster than T2 would predict. Pure T2 decay is a function of completely random interactions between spins. The assumption is that the main external Bø field is absolutely homogeneous. In reality, there are many factors creating imperfections in the homogeneity of a magnetic field. The main magnet itself will have flaws in its manufacture. Every tissue has a different magnetic susceptibility which distorts the field at tissue borders, particularly at air/tissue interfaces. Patients may have some type of metal on or in them (dental work, clips, staples, etc.). The sum total of all of these random and fixed effects is called T2* (pronounced T - Two star).

T2* Decay

T2 relaxation comes from random causes while T2* comes from a combination of both random and fixed causes.

ApplicationsBasic MRI scans

1) T1-weighted MRI: T1-weighted scans refer to a set of standard scans that depict differences in the spin-lattice (or T1) relaxation time of various tissues within the body. T1

weighted images can be acquired using either spin-echo or gradient-echo sequences. T1-weighted contrast can be increased with the application of an inversion recovery RF pulse. Gradient-echo based T1-weighted sequences can be acquired very rapidly because of their ability to use short inter-pulse repetition times (TR). T1-weighted sequences are often collected before and after infusion of T1-shortening MRI contrast agents. In the brain T1-weighted scans provide appreciable contrast between gray and white matter. In the body, T1 weighted scans work well for differentiating fat from water - with water appearing darker and fat brighter.

2) T2-weighted MRI: T2-weighted scans are another basic type. Like the T1-weighted scan, fat is differentiated from water - but in this case fat shows darker, and water lighter. For example, in the case of cerebral and spinal study, the CSF (cerebrospinal fluid) will be lighter in T2-weighted images. These scans are therefore particularly well suited to imaging edema, with long TE and long TR. Because the spin echo sequence is less susceptible to inhomogeneities in the magnetic field, these images have long been a clinical workhorse.

3) T2*-weighted MRI : T2* (pronounced "T 2 star") weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T*2. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.

4) Spin density weighted MRI : Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo or sometimes a gradient echo sequence, with short TE and long TR.

Specialized MRI scans

Diffusion MRI : Diffusion MRI measures the diffusion of water molecules in biological tissues. Clinically, diffusion MRI is useful for the diagnoses of conditions (e.g., stroke) or neurological disorders (e.g., Multiple Sclerosis), and helps better understand the connectivity of white matter axons in the central nervous system

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.

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").

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.

Functional MRI : Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2–3 seconds). Increases in neural activity cause changes in the MR signal via T*2 changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity.

Real-time MRI : Real-time MRI refers to the continuous monitoring (“filming”) of moving objects in real time.

The MRI equipmentThe MRI equipment consists of following components:

The magnet generates the magnetic field.

Shim coils make the magnetic field homogeneous.

Radio frequency coils transmit the radio signal into the body part being imaged.

Receiver coils detect the returning radio signals.

Gradient coils provide spatial localization of the signals.

Shielding coils produce a magnetic field that cancels the field from primary coils in regions where it is not desired.

The computer reconstructs the signals into the image.

The MRI scanner room is shielded by a faraday shield.

Different cooling systems cool the magnet, the scanner room and the technique room.

Better MRI equipment and software design along with the latest information technology improves system maintenance and overall communication. Software and digital imaging and communications in medicine (DICOM) compatibility allows to network into hospital databases, helps to modify pulse sequences, data post processing, and archiving via picture archiving and communication system (PACS).

A Comparison : MRI versus CTThe use of X-rays, a type of ionizing radiation, by computed tomography (CT) scanner, to acquire images, make computed tomography a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for soft tissue (although MRI can also be used to acquire images of bones, teeth and even fossils).

CT scans use a high level of ionizing radiation, which alters the DNA of each and every cell of the body part that is irradiated, causes double strand breaks at a rate of 35 double strand breaks per Gray, and removes a portion of the epigenetic markers of the DNA, which regulate the gene expression. This insult is followed by an effort of the cell in attempt to repair the damaged and broken DNA, however, the repair process is not perfect, and faults that are not properly repaired can cause the cell to stray from its original design of operation. The improper operation can manifest in cell death, cancer, and in other puzzling health conditions, as can be expected from an operation, which randomly alter each cell's DNA, and epigenetic markers. A portion of the population possess a flawed DNA repair mechanism, and thus suffer a greater insult due to exposure to radiation. Unlike CT, MRI does not use ionizing radiation, and does not cause double strand breaks to the DNA.

Contrast in CT images is generated purely by X-ray attenuation, while a variety of properties may be used to generate contrast in MR images. By variation of scanning parameters, tissue contrast can be altered to enhance different features in an image (see Applications for more details). Both CT and MR images may be enhanced by the use of contrast agents. Contrast agents for CT contain elements of a high atomic number, relative to tissue, such as iodine or barium, while contrast agents for MRI have paramagnetic properties, such as gadolinium and manganese, used to alter tissue relaxation times.

CT and MRI scanners are able to generate multiple two-dimensional cross-sections (tomographs, or "slices") of tissue and three-dimensional reconstructions. MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial plane (or near axial). The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. For purposes of tumor detection and identification in the brain, MRI is generally superior. However, in the case of solid tumors of the abdomen and chest, CT is often preferred as it suffers less from motion artifacts. Furthermore, CT usually is more widely available, faster, and less expensive. However, CT has the disadvantage of exposing the patient to harmful ionizing radiation.

MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation. However MRI is usually contraindicated if the patient has any type of medical implant, such as vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, etc; metallic foregin bodies such as shapnel or shell fragments; or metallic implants such as surgical prostheses. In these patients, CT scans are considered the safer option.

Safety

A number of features of MRI scanning can give rise to risks.

These include:

Powerful magnetic fields Cryogenic liquids Noise Claustrophobia

In addition, in cases where MRI contrast agents are used, these also typically have associated risks.

Magnetic field

Most forms of medical or biostimulation implants are generally considered contraindications for MRI scanning. These include pacemakers, vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators and capsules retained from capsule endoscopy. Patients are therefore always asked for complete information about all implants before entering the room for an MRI scan. Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions . To reduce such risks, implants are increasingly being developed to make them able to be safely scanned, and specialized protocols have been developed to permit the safe scanning of selected implants and pacing devices. Cardiovascular stents are considered safe, however.

Ferromagnetic foreign bodies such as shell fragments, or metallic implants such as surgical prostheses and aneurysm clips are also potential risks. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field or thermal injury from radio-frequency induction heating of the object.

Peripheral nerve stimulation (PNS)

The rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation. Volunteers report a twitching sensation when exposed to rapidly switched fields, particularly in their extremities. The reason the peripheral nerves are stimulated is that the changing field increases with distance from the center of the gradient coils (which more or less coincides with the center of the magnet). Although PNS was not a problem for the slow, weak gradients used in the early days of MRI, the strong, rapidly switched gradients used in techniques such as EPI, fMRI, diffusion MRI, etc. are indeed capable of inducing PNS. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in field per unit time) or else prove that no PNS is induced for any imaging sequence. As a result of dB/dt limitation, commercial MRI systems cannot use the full rated power of their gradient amplifiers.

Acoustic noise

Switching of field gradients causes a change in the Lorentz force experienced by the gradient coils, producing minute expansions and contractions of the coil itself. As the switching is typically in the audible frequency range, the resulting vibration produces loud noises (clicking or beeping). This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 120 dB(equivalent to a jet engine at take-off), and therefore appropriate ear protection is essential for anyone inside the MRI scanner room during the examination.

Cryogens

As described in Physics of Magnetic Resonance Imaging, many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids used are non-toxic, their physical properties present specific hazards.

An unintentional shut-down of a superconducting electromagnet, an event known as "quench", involves the rapid boiling of liquid helium from the device. If the rapidly expanding helium cannot be dissipated through an external vent, sometimes referred to as 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.

Oxygen deficiency monitors are usually used as a safety precaution. Liquid helium, the most commonly used cryogen in MRI, undergoes near explosive expansion as it changes from liquid to a gaseous state. The use of an oxygen monitor is important to ensure that oxygen levels are safe for patient/physicians. Rooms built in support of superconducting MRI equipment should be equipped with pressure relief mechanisms and an exhaust fan, in addition to the required quench pipe.

Because a quench results in rapid loss of cryogens from the magnet, recommissioning the magnet is expensive and time-consuming. Spontaneous quenches are uncommon, but a quench may also be triggered by equipment malfunction, improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances.

Contrast agents

The most commonly used intravenous contrast agents are based on chelates of gadolinium. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03–0.1%. Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT.

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic

systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too. Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly. In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. Recently a new contrast agent named gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: this has the theoretical benefit of a dual excretion path.

Pregnancy

No effects of MRI on the fetus have been demonstrated. In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided.

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound and it lacks the ionizing radiation of CT. MRI without contrast agents is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratomas, facilitating open fetal surgery, other fetal interventions, and planning for procedures (such as the EXIT procedure) to safely deliver and treat babies whose defects would otherwise be fatal.

Claustrophobia and discomfort

MRI scans can be unpleasant. Older closed bore MRI systems have a fairly long tube or tunnel. The part of the body being imaged must lie at the center of the magnet, which is at the absolute center of the tunnel. Because scan times on these older scanners may be long (occasionally up to 40 minutes for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an MRI scan without management. Some modern scanners have larger bores (up to 70 cm) and scan times are shorter. This means that claustrophobia could be less of an issue, and additional patients may now find MRI to be a tolerable procedure.

Nervous patients may still find the following strategies helpful:

Advance preparation o visiting the scanner to see the room and practice lying on the tableo visualization techniqueso chemical sedationo general anesthesia

Coping while inside the scanner o having a loved one in the room to hold hand, reassure them, etc.o holding a "panic button"o closing eyes as well as covering them (e.g. washcloth, eye mask)o listening to music on headphones or watching a movie, using mirror-glasses and a

projection screen or via a Head-mounted display, while in the machine.

Many newer MRI systems place a diagonal mirror above the eyes to allow the patient to look down the tunnel rather than at the bore wall immediately above their faces.

Alternative scanner designs, such as open or upright systems, can also be helpful where these are available. Though open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. However, commercial 1.5 tesla open systems have recently become available, providing much better image quality than previous lower field strength open models.

For babies and young children chemical sedation or general anesthesia are the norm, as these subjects cannot be instructed to hold still during the scanning session. Obese patients and pregnant women may find the MRI machine to be a tight fit. Pregnant women may also have difficulty lying on their backs for an hour or more without moving.

References

1) Magnetic Resonance Imaging by Stark and Bradley, second edition ( Basics,Contrast Agents, Instrumentation)

2) Magnetic resonance imaging artifacts: Mechanism and clinical significance by Pusey et al.RadioGraphics, September, 1986

3) http:// simplyphysics.com4) http://hull.ac.uk5) http://yorkshirecancerresearch.com6) www.cis.rit.edu/htbooks/mri

7) http://www.biac.duke.edu/education/courses/fall05/fmri8) http://www.e-radiography.net/mrict9) http://www.mritutor.org10) http://www.spectroscopynow.com