Magnetic Resonance Imaging and Its Benefits

1.The Study Sr. Secondary School Badi, Udaipur Project to be submitted for the partial fulfillment of CBSE, Class XII, Practical Examination 2006-07 Magnetic Resonance Imaging Submitted by:Kartik Gupta Submitted to:Mr. Mukesh Shrimali

Name:Kartik Gupta

Fathers Name:Lt Col Jayant Gupta

Name of the School:The Study Senior SecondarySchool, Udaipur

CBSE Roll Number: 1228536

CBSE Registration Number:A/05/03732/059693

Address:12-B, Pologround, Saheli Marg, Udaipur

Mode of Project:Single

This project was made under the able guidance of Mr. Mukesh Shrimali. It was through his untiring efforts that I have been able to present my work with such clarity and precision.

Magnetic resonance imaging (MRI), formerly referred to as magnetic resonance tomography (MRT) or nuclear magnetic resonance (NMR), is a method used to visualize the inside of living organisms as well as to detect the composition of geological structures. It is primarily used to demonstrate pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging. MRI has also found many novel applications outside of the medical and biological fields.

NMR studies a magnetic nucleus, like that of a hydrogen atom (protium being the most receptive isotope at natural abundance) by aligning it with a very powerful external magnetic field and perturbing this alignment using an electromagnetic field. The response to the field by perturbing is what is exploited in nuclear magnetic resonance spectroscopy and magnetic resonance imaging.

Credit of Discovery

Background

A Typical MRI Machine

MRI: Basic Theoretical Working

Imaging using an MRI(Sequential & Graphical Display)

The Physical Aspect Of MRI

A Diagrammatic Representation of the Physical Aspect of MRI

Applications of MRI in Medicine

Some Diagnostic Images

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance.

The original name for the medical technology is nuclear magnetic resonance imaging ( NMRI ), but the wordnuclearis almost universally dropped.

Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the magnetic property of an atom's nucleus.

All nuclei that contain odd numbers of nucleons and some that contain even numbers of nucleons have an intrinsic magnetic moment. The most often-used nuclei are hydrogen-1 and carbon-13, although certain isotopes of many other elements nuclei can also be observed.

NMR studies a magnetic nucleus, like that of a hydrogen atom (protium being the most receptive isotope at natural abundance) by aligning it with a very powerful external magnetic field and perturbing this alignment using an electromagnetic field. The response to the field by perturbing is what is exploited in nuclear magnetic resonance spectroscopy and magnetic resonance imaging.

NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about a molecule. It is the only technique that can provide detailed information on the exact three-dimensional structure of biological molecules in solution. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary quantum computers.

Like X ray, MRI is based on a discovery in the physic lab: when the nuclei of hydrogen atoms--single protons, all spinning randomly--are caught suddenly in a strong magnetic field, they tend to line up like so many compass needles.

If the protons are then hit with a short, precisely tuned burst of radio waves, they will momentarily flip around.

Then, in the process of returning to their original orientation, they resound with a brief radio signal of their own.

The intensity of this emission reflects the number of protons in a particular "slice" of matter.

Step One : A tomographic image is taken. A tomographic imageis an image of a thin slice through an object.

Step Two:The signal in a Voxel (Volume Element) is mapped to the intensity of a Pixel (The smallest element of a digital picture).

Nuclear spin and magnets

Electrons, neutrons and protons, the three particles which constitute an atom, have an intrinsic property called spin. This spin is defined by the fourth quantum number for any given wave function obtained by solving relativistic form of the Schrdinger equation (SE). It represents a general property of particles which we can describe using the properties of electrons. Electrons flowing around a coil generate a magnetic field in a given direction; this property is what makes electric motors work. In much the same way electrons in atoms circulate around the nucleus, generating a magnetic field. This generated field has an angular momentum associated with it. It so turns out that there is also an angular momentum with the electron particle itself, denoted the spin, and this gives rise to the spin quantum number,m s .

Spin angular momentum is quantized and can take different integer or half-integer values depending on what system is under study. If we solve the relativistic SE for the electron we get the values + and -. Since the Pauli principle states that no two fermions can have the same quantum number, it is why only two electrons, paired antiparallel (one with positive spin and one negative with negative spin), can appear in a single atomic orbital.

Like the electron, protons and neutrons also have a spin angular momentum which can take values of + and . In the atomic nucleus, protons can pair with other antiparallel protons much in the same way that electrons pair in a chemical bond. Neutrons do the same. Paired particles, with one positive and one negative spin, thus have a net spin of zero "0". We can see that a nucleus with unpaired protons and neutrons will have an overall spin, with the number unpaired contributing to the overall nuclear spin quantum number,I . When this is larger than zero, a nucleus will have a spin angular momentum and an associated magnetic moment, , dependent on the direction of the spin.It is this magnetic moment that we manipulate in modern NMR experiments .

It is worth noting here that nuclei can have more than one unpaired proton and one unpaired neutron, much in the same way that electronic structures in transition metals can have many unpaired spins. For example 27Al has an overall spin I=5/2.

A technique related to nuclear magnetic resonance is electron spin resonance that exploits the spin of electrons instead of nuclei. The principles are otherwise similar.

Values of spin angular momentum

The spin angular momentum of a nucleus can take ranges from + Ito Iin integral steps. This value is known as the magnetic quantum number,m . For any given nucleus, there is a total (2 I +1) angular momentum states. Spin angular momentum is a vector quantity. Thezcomponent of which, denotedI z , is quantized:

I z=mh /2 wherehis Planck's constant.

The resultant magnetic moment of this nucleus is intrinsically connected with its spin angular momentum. In the absence of any external effects the magnetic moment of a spin nucleus lies approximately 52.3 from the angular momentum axis or 127.7 for the opposing spin. This magnetic moment is intrinsically related toIwith a proportionality constant , called the gyromagnetic ratio:

Spin behaviour in a magnetic field

Consider the case of nuclei which have a spin of a half, like 1H, 13C or 19F. The nucleus thus has two possible magnetic moments it could take, often referred to as up or down, + or - , which are also called the spin states and . The energies of each state are degenerate - that is to say that they are the same. The effect is that the number of atoms, theirpopulation , in the up or state is the same as the number of atoms in the state.

If a nucleus is placed in a magnetic field, the angular momentum axis coincides with the field direction. The resultant magnetic momenta, space quantized from the angular momentum axis,no longer have the same energysince one state has a z-component aligned with an external field and are lower in energy (positive I values) and the other opposes the external field and is higher in energy. This causes a population bias toward the lower energy states.

The energy of a magnetic moment when in a magnetic fieldB 0 , the zero subscript is used to distinguish this magnetic field from any other applied field, is the negative scalar product of the vectors:

E =- z B 0

We've already defined z =I z . So placing this in the above equation we get:

E = - mhB 0/ 2

Resonance

The energy gap between our and states is ( hB 0 )/2. We get resonance between the states, therefore equalizing populations, if a radiofrequency is applied with the same energy as the energy difference E between the spin states. The energy of a photon isE =h , whereis its frequency.

E=hB 0 /2

Thus, the frequency of electromagnetic radiation required to produce resonance of a specific nucleus in a fieldBis:

= B 0 /2

It is this frequency that we are concerned with, and detect in NMR. And it is this frequency which describes the sample we are observing. But importantly, it is this resonance that gives rise to the nuclear magnetic resonance spectrum.

Nuclear shielding

It would appear from the above equation that all nuclei of the same nuclide, which have same the gyromagnetic ratio (y), resonate at the same frequency. This is not the case. Since the gyromagnetic ratio of a given nuclide does not change, we conclude that the effect of the external magnetic field is different for different nuclei. Local effects of other nuclei, especially spin-active nuclei, and local electron effects shield each nucleus differently from the main external field.

It was stated that the energy of a spin state is defined by E=- z B 0 . It can be seen that by shielding the strength of the magnetic field, the experienced effect, oreffective magnetic fieldat the nucleus is lower:B effective< B 0 . Thus the energy gap is different, and hence the frequency required to achieve resonance deviates from the expected value.

These differences due to nuclear shielding give rise to many peak frequencies in a nuclear magnetic resonance spectrum. It can be seen why nuclear magnetic resonance is a direct probe of chemical structure.

It is possible for the shielding to change as the orientation of the molecule, this is called chemical shift anisotropy, and can even be used in some types of experiment. Especially if the sample is solid, the external fields of the crystal structure interfere with the nuclear field too much for the spectrum to be of any use. Anisotropy is usually averaged out by spinning the sample. For liquids, anisotropy is often relatively small, but the accuracy is increased by using a pneumatic spinner to rotate the sample. For solids, magic angle spinning at very high angular velocities (20 kHz) is used, since the anisotropy is often very large.

Relaxation

The process called population relaxation refers to nuclei that return to the thermodynamic state in the magnet. This process is also called T 1relaxation, where T 1refers to the mean time for an individual nucleus to return to its equilibrium state. Once the population is relaxed, it can be probed again, since it is in the initial state.

The precessing nuclei can also fall out of alignment with each other (returning the net magnetization vector to a nonprecessing field) and stop producing a signal. This is calledT 2 relaxation . It is possible to be in this state and not have the population difference required to give a net magnetization vector at its thermodynamic state. Because of this,T 1is always larger (slower) thanT 2 . This happens because some of the spins were flipped by the pulse and will remain so until they have undergone population relaxation. In practice, the T 2time is the life time of the observed NMR signal, the free induction decay. In the NMR spectrum, meaning the Fourier transform of the free induction decay, the T 2time defines the width of the NMR signal. Thus, a nucleus having a large T 2time gives rise to a sharp signal, whereas nuclei with shorter T 2times give rise to more broad signals. The length of T 1and T 2is closely related to molecular motion.

In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the chance of malignancy, especially in a fetus.

While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library ofpulse sequencesthat the modern medical MRI scanner includes, each of which is optimized to provideimage contrastbased on the chemical sensitivity of MRI.

For example, with particular values of theecho time(T E ) and therepetition time(T R ), which are basic parameters of image acquisition, a sequence will take on the property of T 2 -weighting. On a T 2 -weighted scan, fat-, water- and fluid-containing tissues are bright (most modern T 2sequences are actuallyfast T 2sequences). Damaged tissue tends to develop edema, which makes a T 2 -weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T 2 -weighted sequence can be converted to a FLAIR (Fluid Light Attenuation Inversion Recovery) sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating diseases, such as multiple sclerosis.

The typical MRI examination consists of 5-20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician.

Magnetic Resonance Imageshowing a vertical (sagittal) cross section through a human head.

Magnetic resonance angiography

A fMRI scan showing regions of activation in orange, including the primary visual cortex

Nuclear Magnetic Resonance is the safest and the most advanced diagnostic imaging instrument available in the international market. Although its reach is limited to top class medical institutions because of it its high price [costing approximately US$1 million per tesla for each unit (common field strength ranges from 0.3 to 3 teslas)] with several hundred thousand dollars per year of upkeep costs], it still remains as one of the most reliable tools of a neurologist.

This project has been made with references made from:

www.wikipedia.com

Encyclopaedia Britannica

Technology

Magnetic Resonance Imaging and Its Benefits