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Magnetic resonance imaging

Magnetic resonance imaging:

Magnetic resonance imaging is a non-invasive (radiation is not involve) modality in which image of internal anatomical structure are obtained using a high strength external magnetic film. Earlier, it was known as nuclear magnetic resonance (NMR) but the term nuclear was replaced with magnetic as it sounded very heavy and frightful and these after NMR was renamed as Magnetic resonance imaging (MRI). MRI also known as the spectroscopy as the MRI image gives different colors to different types of body tissues (like healthy tissues) have a different MR signal than the unhealthy ones.

Similarly, fluid, soft tissues, air, fat etc. shows different MR densities. MRI was invented by Sir Edward Purcell in1956 in collaboration with Sir Felix Bloch who provided the complex mathematical algorithm. They jointly shared the noble prize for physics in 1971. The first clinical application of MRI was done at Mayo’s clinic of USA. The basic postulates of MRI tell that “the free hydrogen nuclei or protons [H+] in our body behave as a tiny magnet”. More than 90% of cell plasma of human cell is weather and such as there are abundant amount of protons in out body and the free hydrogen nuclei or proton produce magnetic signal.

According to the quantum theory of motion these hydrogen nuclei has a positive charge on them and they rotate around their own air due to intrinsic angular momentum. According to basic principles of electromagnetism any moving charged particle i.e. current produces a magnetic field around its surroundings. Since protons are moving charged and behaves as tiny magnet. However these protons are randomly scattered in our body and they do not have a particular direction of alignment. There is no fixed direction of spinning also some protons spin in anti-clock wise direction; some protons are aligned in positive direction and some in negative therefore although this spinning proton produce magnetism, they cancel out the magnetic effect of each due to opposite direction of spinning or alignment.

Therefore the net magnetization is zero. However when the body of a patient is put into a strong magnetic field as applied in MRI (high frequency magnetic field of about 22megaHz) the protons align them only in two directions that is parallel or anti-parallel to the main magnetic field. Protons in parallel direction are in higher energy state and those in anti-parallel direction are in lower energy state. Both sided protons produce magnetic effect so produce cancelled out due to opposite directions. However there are always some extra proton aligned in parallel direction in comparison to total number of protons in anti-parallel direction and it is because of these extra protons in parallel direction that MR signals are produced.

As against one million protons in anti-parallel direction there are 6 protons in parallel direction and because of these extra protons the MR signals are produced. When main magnetic field is applied the protons in parallel direction spin around their own axis due to intrinsic angular momentum and also around the axis of the main magnetic field due to the electrostatic force of attraction. A point comes where all the protons in parallel direction are aligned along the axis of the main magnetic field. When the external magnetic field is terminated or switched off the protons try to get into their natural position in this process they release the magnetic energy they absorbed during excitation around the surrounding tissues. This is known as relaxation. The magnetic energy released during relaxation is released as magnetic signals which are interrupted and interpreted by the radiofrequency coils. These signals are then transferred into ADC (analog to digital converter) which converts the magnetic signals which are displayed on the monitor in the form of thin slices.

Advantages of MRI:

  1. The biggest advantage of MRI is that it is a non-invasive imaging modality and doesn’t have any radiation effect. Therefore it can be safely used for all categories of patients.
  2. No apparent biological effects of MRI have been noted in clinical applications except body tissues heating in some cases. The magnetic field up to 8 Tesla considered to receive biologically whereas, in MRI we are not using more than 3 Tesla magnetic fields in MRI.
  3. MRI gives excellent soft tissue contrast resolution and idea for diagnosis of brain parenchyma, spinal nerves and stenosis, infractions, synovial joints etc.
  4. MRI is very useful in diagnosis of chronic and diffuse brain parenchymal diseases.
  5. MRI can differentiate the grey matter and white matter of the brain parenchyma and useful in evaluation of complete central nervous system.
  6. It gives excellent diagnostic information about degenerated changes of the spine and inter-vertebral disc disease like prolapsed displaced, scoliosis/kyphosis lumbar spondylolisthesis.

Disadvantages of MRI:

  1. MRI unit is space occupying.
  2. MRI unit is very heavy and may weight in tons. Therefore it is difficult to install, dismantle or reinstall.
  3. Contrast enhanced MRI is even costlier.
  4. MRI produces air images artifact and not suitable for the image of bones.
  5. Some of the patients may develop claustrophobia during MRI examination.
  6. The hammering sound in MRI during MRI examination can produce disturbance and fear to patient.
  7. MRI is not suitable for acute/emergency cases.
  8. MRI is contraindicated for the patients with artificial pacemaker as the lithium oxide battery of the pacemaker may be dislodge to magnetic effect of MRI.
  9. It is also contraindicated for the patient having cochlear implantations artificial dentures, femoral implantations, nailing, splints, ocular implants etc.
  10. MRI is also not suitable for patient having projectiles, sharpeners, bullets or ingested foreign bodies and it may increase the intensity of injury.
  11. Patient with tattoos embossed in their bodies are contraindicated.

Physics of MRI:

Magnet: Magnet is any material that attracts metals like iron filling and which have two poles.

Magnetism: The property of a bar magnet by virtue of which a magnet shows the property of magnetism i.e. attract like iron, cobalt and nickel particles is known as magnetism.

Properties of magnets:

Magnet attracts iron, cobalt and nickel some of the materials are strongly attracted while some are feebly attracted. Based on this metals are classified as paramagnetic, ferromagnetic and diamagnetic material.

A magnet has two poles namely north and south.

Unlike poles attract each other and like poles repel each other.

When a bar magnet is freely suspended in air it is not called aligned itself align the south pole of the earth.

The magnetic aligned of force originates from north and travel towards south.

The magnetic lines never intersect each other.

The magnetic lines of force expand laterally and concentric at the longitudinal axis.

Types of magnet:

There are various types of naturally available magnets as well as artificially made magnets. Based on this these could be following categories of magnet.

Temporary magnet: Temporary magnets are those which retain the property of magnetism for a shorter period of time and may be demagnetized by various method including heat and pressure. Example: bar magnet.

Permanent magnet: Permanent magnets are those which ones magnetized retain the magnetism for a long period of time and they cannot be easily demagnetized. They have wide industrial applications like transmitters and peri communication devices.

Super conducting or resistive magnets: They are widely used in MRI and they produced very less resistance when kept at a comparative low temperature of -273° K they have easy cool off properties and produced excellent uniform magnetic signals.

Electromagnet: When an iron bar is placed in a current carrying solenoid it attains the property of magnetism and become a magnet because according to the law of electromagnetism a changing field of electric current produces magnetic field around it and iron bar being a ferromagnetic material easily magnetized. However the property of electromagnets retails only up to the time from which current is flowing through the solenoid and keeps on changing its direction.

Hybrid magnet: It is the magnet which has a property of both temporary and permanent magnet is called hybrid magnet. Example: compass

Overview of larmor frequency, torque, resonance and excitation:

The general definition of frequency is number of cycles repeated per second in a wavelength. Smaller the wavelength greater will be the frequency and high frequency wave length have high penetration power.

Larmor frequency: Larmor frequency refers to the rate of precision of spinning nuclei or protons around the axis of the main magnetic field. Higher the magnitude of the main magnetic field higher will be the larmor frequency. Further larmor frequency depends upon the availability of freely moveable hydrogen nuclei inside the organ.

Resonance: When two or more energy state are existing inside a physical system, energy transfer from the system in a higher energy state to the system in lower energy state takes place. This is known as resonance.

Excitation: When the main magnetic field is applied along the long axis of the body of the patient, the hydrogen nuclei align in parallel direction. Main magnetic field absorbs the energy of the main magnetic field and get excited. This is called R/R excitation.

Torque: According to Newton’s law everything tends to be in a position of rest or motion till external force is applied to change its position. Similarly, when an external magnetic field is applied in MRI the hydrogen nuclei aligned along the direction of main magnetic field tend to be in their natural position and keep moving around their own momentum. However, the energy of the external magnetic field is very high which tries to align the spinning nuclei along its own axis. This produces a centrifugal force which is known as torque.

Safety concerns in MRI:

MRI is contraindicated for patients having artificial pacemaker and other metallic implantations and also for patients with bullets sharpeners and projectiles.

Type of coils that we use in MRI:

There is a wide range of MRI coils that used in clinical MRI these coils acts as receiver of the magnetic signal and sometimes as transmitter also. MRI coils are important component of MR unit which receive and intercept the MR signal and helps in producing homogenous magnetic field. The various types of MRI coils are:

  1. Radiofrequency coil: In earlier time the radiofrequency coil were used both as transmitter of the magnetic field as well as the receiver. However, in the modern MRI scanners radio frequency coils are utilized separately for transmission of the magnetic field and for receiving purpose. A number of coils are used in MRI to produce a magnetic environment for uniform distribution of equate magnetic field. Radio Frequency coil is one of them. When used as a transmitter the radio frequency coil produce an oscillating magnetic field i.e. perpendicular to the main magnetic field (Bo) when used as a receiver the radio frequency coil detect the MR signal.
  2. Gradient coil: The gradient coils create a secondary magnetic field apart from the main magnetic field applied in MRI. It slightly destroyed the main magnetic field. It is used in spatial encoding of the produced MR signal. Gradient coils maybe of different types depending upon the safe and function/application like:
  • Figure 8 gradient coil
  • Helmholtz coil
  • Maxwell coil
  • Golay’s coil etc.
  1. Surface coil: Surface coils in MRI are used only for receiving purpose of MR signals. They have a very high signal to noise ratio for the tissues placed adjacent to the coil. The further the tissue from the coil the cell sensitive, it is to the surface coil and it becomes difficult for the surface coil to intercept the MR signals from the tissue. Surface coils are used for superficial and large parts like spine, shoulder, TM joint and some small body parts as well.
  2. Volume coil: The volume coil in the MRI is used in the case of large areas of interest like whole body MR scan. The purpose of using volume coil is to produce a homogenous magnetic field across the entire volume of the scanning. However, the volume coils can also be smaller in size and can be used for smaller but dense body parts like head and extremities. The commonly used volume coils are designed as birdcage and saddle coils.
  3. Shims coil: The shims coil in MRI is used for shimming or removal of homogeneity of magnetic energy they act as a auxiliary magnetic field with respect to the main magnetic field and used before the virtual operation of the super conductive magnets of MRI.
  4. Array coil: the array coils combines with the benefits of smaller coils with those of the larger coils. There are different types of array coil like coublt array coils, phased array coils, isolated array coils etc.
  5. Extremity coil: The extremity coils are used for smaller synovial joints like wrist, ankle, elbow, etc. and also for the scanning of the extremities.
  6. Hybrid coils: The hybrid coils are a fusion of transmitter and receiver coils can be used for smaller anatomical areas like metatarsals and saddle joints.

Spin density: The spin density in MRI refers to the density or number of spinning hydrogen nuclei in a unit area of a specific anatomical organ. The MR signals produced in MRI are widely depend upon the availability of proton density or spin density, higher will be the MR signals produced.

Main magnetic field: The external magnetic field or main magnetic field is a strong magnetic field of a very high frequency ranging from 2-20megaHz. It is applied parallel to the longitudinal axis of the body of the patient along the z-axis and denoted by (Bo). It produces the phenomenon of excitation and resonance when the main magnetic field is turned off magnetic signals is produced from the body tissues according to the spine density of tissues.

RF pulse: The radiofrequency pulse is a secondary magnetic field applied at an angle of 90° with respect to the main magnetic field. It is a short pulse that is applied very near to the Larmor frequency. It is time varying pulse and repeated at a regular interval of time. It is denoted by B1 and important pulse sequence for homogenous distribution of the main magnetic energy applied along the longitudinal axis.

T1 relaxation time:

T1 relaxation time is also known as the spine lattice relaxation time, it is the measure of how quickly the net magnetization vector reaches its original position or the ground state. T1 relaxation time is always greater than the T2 relaxation time. Since the absorbed magnetic energy is released around the surrounding body tissues during relaxation process of MRI.

T2 relaxation time:

T2 relaxation time or lattice-lattice relaxation time (also known as tissue-tissue relaxation time) is the measure of time taken for transverse magnetization vector to decay up to 37% of its original value around x-axis larger the T2 relaxation time brighter the image of MRI.

Signal to noise ratio (SNR): SNR referred to the amount of magnetic energy absorbs by the body tissue to the magnetic signals produce oblique detectors from that part. It depends upon the total number of freely moveable hydrogen nuclei in that part SNR value in case of dense body structures in high giving rise to air image artifact.

Contraindications:

Implants and metal

  • Cerebral aneurysm clips
  • Metallic foreign body in eye
  • Shrapnel, bullets (in critical care)
  • Ocular implants (metal containing)

Electromechanical implants

  • Pacemakers/ICD’s
  • Pacing wires
  • Cochlear implants
  • Neuro-stimulators
  • Hydrocephalus stents

Contrast:

MRI contrast agents are used to increase the visibility of internal anatomical structures in MRI. MR-Gadolinium

The difference material utilized in MR called gadolinium is less inclined to deliver an unfavorably susceptible response than the iodine-based materials utilized for x-beams and CT examining. Rarely, patients are sensitive to gadolinium-based difference materials and experience hives and irritated eyes. Responses ordinarily are gentle and effectively constrained by drug. Serious responses are uncommon.

Nephrogenic foundational fibrosis (NSF), a thickening of the skin, organs and different tissues, is an uncommon complexity in patients with kidney ailment that experience a magnetic resonance with differentiates material. Gadolinium-based differentiation material might be retained in certain patients with serious kidney sickness.

There is proof that little hints of gadolinium might be held in various organs of the body, including the cerebrum, after difference upgraded MRI. While there are no known negative impacts from this, your PCP may consider gadolinium maintenance while choosing a complexity operator. There are various diverse gadolinium-based difference specialists accessible, each with its own wellbeing profile. Choices on which material to utilize might be influenced by the piece of the body being imaged, the expense of the material and different variables. These choices are particularly significant in patients liable to experience different MRI checks with gadolinium-based differentiation material, for example, pediatric patients, malignant growth patients and individuals with numerous scleroses.