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Physics of Magnetic Resonance Imaging
The body is largely composed of water molecules. Each water molecule has two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments (the measure of its tendency to align with a magnetic field) of some of these protons changes, and aligns with the direction of the field.

Magnetism is a property of matter that is a result of the orbiting electrons in atoms. The orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic angular momentum called 'spin'.

Magnetic field strengths are measured in units of gauss (G) and Tesla (T). One Tesla is equal to 10,000 gauss. The earth's magnetic field is about 0.5 gauss. The strength of electromagnets used to pick up cars in junk yards is about the field strength of MRI machines (1.5-2.0T).

The echo time refers to time between the application of radiofrequency excitation pulse and the peak of the signal induced in the coil. It's measured in milliseconds. The amount of T2 relaxation is controlled by TE.

The repetition time or (TR) is the time from the application of an excitation pulse to the application of the next pulse. It determines how much longitudinal magnetization recovers between each pulse. It's measured in milliseconds.

The Bo in MRI refers to the main magnetic field and is measured in Tesla. The majority of MRI systems in clinical use are between 1.5T and 3T. Altering the field strength will affect the Larmour frequency at which the protons precess.

Resonance and Radiofrequency

Protons in a magnetic field have a microscopic magnetisation and act like tiny toy tops that wobble as they spin.The rate of the wobbling or precession is the resonance or Larmor frequency. In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of proton nuclei aligned with the main magnetic field B0 as counter aligned. The aligned position is slightly favored, as the nucleus is at a lower energy in this position. For every one-million nuclei, there is about one extra aligned with the B0 field as opposed to the field. This results in a net or macroscopic magnetisation pointing in the direction of the main magnetic field. Exposure of individual nuclei to radiofrequency (RF) radiation (B1 field) at the Larmor frequency causes nuclei in the lower energy state to jump into the higher energy state.

On a macroscopic level, exposure of an object or person to RF radiation at the Larmor frequency, causes the net magnetisation to spiral away from the B0 field. In the rotating frame of reference, the net magnetisation vector rotate from a longitudinal position a distance proportional to the time length of the RF pulse. After a certain length of time, the net magnetization vector rotates 90 degrees and lies in the transverse or x-y plane. It is in this position that the net magnetisation can be detected on MRI. The angle that the net magnetisation vector rotates is commonly called the 'flip' or 'tip' angle. At angles greater than or less than 90 degrees there will still be a small component of the magnetisation that will be in the x-y plane, and therefore be detected.


MRI image contrast is influenced by several characteristics of tissues and other materials including: T1, T2 and T2* relaxation as well as spin density, susceptibility effects and flow effects.

Relaxation is the process in which spins release the energy received from a radiofrequency pulse.

T1 and T2 relaxation rates affect the SNR in an image. Improvement in the SNR is seen when the TR is increased significantly to about 3-5 T1 times. Changing the TR time will also affect the T1 weighting of the image and the acquisition time. T1 weighting occurs in a short TR spin echo sequence because of incomplete recovery of longitudinal magnetization.

The T1 relaxation time, also known as the spin-lattice relaxation time, is a measure of how quickly the net magnetisation vector (NMV) recovers to its ground state in the direction of B0. The return of excited nuclei from the high energy state to the low energy or ground state is associated with loss of energy to the surrounding nuclei. Nuclear magnetic resonance was originally used to examine solids in the form of lattices, hence the name "spin-lattice" relaxation. Two other forms of relaxation are the T2 relaxation time (spin-spin relaxation) and T2* relaxation.

Now for all of you science and/or math geeks out there, and you know who you are, the length of the net magnetisation vector for a spin echo sequence is given by the following equation:

Mt = Mmax (1-e-t/T1)

Where Mt is the magnetisation at time = t, the time after the 90o pulse, Mmax is the maximum magnetisation at full recovery.

Another term that you may hear is the T1 relaxation rate. This is merely the reciprocal of the T1 time (1/T1). T1 relaxation is fastest when the motion of the nucleus (rotations and translations or "tumbling rate") matches that of the Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic field strength that specifies the Larmor frequency. Higher magnetic fields are associated with longer T1 times.

T1 weighted images can be obtained using an inversion recovery sequence or by setting short TR (<750ms) and TE (<40ms) values in conventional spin echo sequences. In gradient echo sequences, T1WI can be obtained by using flip angles over 50o and setting the TE to less than 15 ms.

T2 relaxation refers to the progressive dephasing of spinning dipoles following the 90° pulse as seen in a spin-echo sequence due to tissue-particular characteristics, primarily those that affect the rate of movement of protons, most of which are found in water molecules. This is alternatively known as spin-spin relaxation.

Acoustic Noise

Position information can then be recovered from the resulting signal by the use of a Fourier transform. These fields are created by passing electric currents through specially-wound solenoids, known as gradient coils. 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. Since these coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise (clicking or beeping)that is heard during operation. This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 120 decibels (dB) (equivalent to a jet engine at take-off).

As a reference, 120 dB is the threshold of loudness causing sensation in the human ear canal — tickling, and 140 dB is the threshold of ear pain. Since decibel is a logarithmic measurement, a 10 dB increase equates to a 10-fold increase in intensity — which, in acoustics, is roughly equal to a doubling of loudness.

The use of ear protection is essential for anyone inside the MRI scanner room during the examination.

Contrast Agents

The most commonly used intravenous contrast agents are based on a chemical compound of Gadolinium (gd). 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 approximately 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-enhancement.

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.

Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors, or inflammation in the case of multiple sclerosis (MS). Contrast agents are typically used for MS patients to help determine if there is any active disease progression. Unlike computed tomography (CT), MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, spinal cord, muscle, connective tissue and most tumors.


Death and injuries have occurred from projectiles created by the magnetic field, although few compared to the millions of examinations administered. MRI makes use of powerful magnetic fields that, though not known to cause direct biological damage, can interfere with metallic and electromechanical devices. Additional (small) risks are presented by the RF systems, components or elements of the MRI system's operation, elements of the scanning procedure and medications that may be administered to facilitate MRI imaging.

There are many steps that the MRI patient and referring physician can take to help reduce the remaining risks, including providing a full, accurate and thorough medical history to the MRI provider.

Implants and Foreign Bodies

Pacemakers are generally considered an absolute contraindication towards MRI scanning, though highly specialized protocols have been developed to permit scanning of select pacing devices. Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Other electronic implants have varying contraindications, depending upon scanner technology, and implant properties, scanning protocols and anatomy being imaged.

In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself.

Ferromagnetic foreign bodies such as metal fragments, or metallic implants such as surgical prostheses and aneurysm clips are also potential risks, and safety aspects need to be considered on an individual basis. 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, thermal injury from Rf induction heating of the object, or failure of an implanted device. These issues are especially problematic when dealing with the eye. Most MRI centers require an orbital x-ray to be performed on anyone suspected of having metal fragments in their eyes, something not uncommon in metalworking.

Because of its non-ferromagnetic nature and poor electrical conductivity, titanium and its alloys are useful for long term implants and surgical instruments intended for use in image-guided surgery. In particular, not only is titanium safe from movement from the magnetic field, but artifacts around the implant are less frequent and less severe than with more ferromagnetic materials such as stainless steel. Artifacts from metal frequently appear as regions of empty space around the implant that are frequently called a "black-hole artifact". For example, a 3 mm titanium alloy coronary stent may appear as a 5 mm diameter region of empty space on MRI, whereas around a stainless steel stent, the artifact may extend for 10–20 mm or more.


No known 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. 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's typically 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.

Claustrophobia and Discomfort

Due to the construction of some MRI scanners, they can be potentially unpleasant to lie in. Older models of closed bore MRI systems feature 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 an hour for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an MRI scan without management. Modern scanners may have larger bores and scan times are shorter. This means that claustrophobia is less of an issue, and many patients now find MRI an innocuous and easily tolerated procedure.

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.