Magnetic Resonance Imaging


Breaking down the article:

  1. Background knowledge
  2. MRI principles
  3. Relaxation times
  4. Image Formation
  5. Technical aspects of MRI
  6. Clinical uses
  7. Safety

Background Knowledge:

Nuclear Magnetic Resonance.

Many hospitals possess an instrument called body scanner. This is an NMR spectrometer in which a patient can be placed so as to lie inside a large magnet. In a period no longer than 20 minutes this machine obtains images called magnetic resonance images of soft tissues in any number of planes.

Protons in water, carbohydrates, proteins and lipids give different signals. Different parts of the body possess protons in different environments (chemical environment = protons in the proximity, no more than the length of a chemical bond, of the studied proton) therefore give different signals.

There is no damage to the tissues during the process, so customers can be examined regularly.

SPIN.

The basis of this analytical technique, NMR, is that atomic nuclei can be though of as tiny magnets. Atomic nuclei spin about an axis. The combination of charge and spin gives rise to a magnetic moment, i.e., the nucleus behaves to some extent like a small bar magnet. The nuclear spin is quantised therefore the magnetic moment is quantised. For a proton, 1H, the spin of the quantum number = ½. Other nuclei which contain an odd number of protons or neutrons or both also have a spin quantum number = ½. Examples are 13C, 15N, 19F, and 31P. Two very common nuclei 12C and 16O have zero spin and zero magnetic moment – they are invisible in NMR spectrometry.

NMR is most widely used to identify 1H nuclei.

spin.bmp

nmrlev1

The spin states of a certain nucleus have equal energy in the absence of a magnetic field. If a magnetic field is applied, the spin states are no longer of equal energy. The magnetic moments of different nuclei may either align or oppose the applied magnetic field – the result is to split each spin state into two energy levels (The pictures above).

The nuclei with spins that are aligned with the applied field can absorb a certain amount of energy (E absorbed) and change their orientation from with the field to against the field. They are described as flipping.

E absorbed = h*v ; where h= Plank’s constant and v= radiofrequency

The field required to bring protons into resonance (make them flip) is compared with that of a standard solution and expressed as the chemical shift.

(See A-Level Chemistry, Fourth Edition, E.N. Ramsden, 2000, pp. 699-700 for more information).

Magnetic Resonance Imaging Principles:

  • The first MRI scan (of a human chest) was performed by Damadian in 1977.

As we saw before, each atom with an odd number of nucleons exhibits a certain magnetic moment, μ, where the magnetic moment is equal to current intensity in a loop * area delimited by the loop.

There is a direct proportionality between the magnetic moment of nucleus, μ, and its angular momentum S:

μ = γ * S; where γ= gyromagnetic ratio.

  • In the absence of the external magnetic field, the NM moments are randomly oriented ant their vector sum per volume is zero (vector of magnetisation = 0).
  • However, in the presence of the external magnetic field the vectors of magnetic moments of nuclei will start to circumscribe the surface of a cone with its axis directed parallel to vector B(vector B = direction of the external magnetic field). The frequency of this procession motion (I think they mean angular velocity by this frequency thing), the Larmor procession, is given by the formula:

ω = γ*B

  • The processing nuclei can absorb a quantum of electromagnetic radiation of frequency = Larmor procession, and jump into a higher energetic state. After having been promoted to a higher energy state, the nuclei will give off energy to the surrounding and return to its original energy level.
  • The energy that has been given off will yield the NMR signal – the chemical shift.
This is how an NRM spectrum looks like. PPM = Parts Per Milion. It is not necessary to understand what this picture is.

Relaxation Times:

  1. T1, the longitudinal relaxation time or the spin-lattice relaxation, is a measure of how quickly the net magnetisation vector recovers to its ground state in the direction of B(Only about 63% of the nuclei are monitored when gaining data).
  2. T2, the transversal relaxation time, refers to the progressive dephasing of spinning dipoles following the 90° pulse. It is two to ten times shorter than T1.

(Radiopedia.org, Dr. Bruno di Muzio and Dr. Jeremy Jones)

The principle of image formation:

The phenomenon of nuclear magnetic resonance can be induced and measured in two ways:

  • The external magnetic field of constant B is used to look for the frequency of the electromagnetic oscillations able to cause nuclear resonance
  • We use the frequency of the electromagnetic oscillations at which resonance happens to identify the value of B.

Information about the location of the hydrogen atoms can be obtained by adding a calibrated gradient field across the region of the sample as shown in the bottom sketch above. With an increasing magnetic field as you move to the right across the sample, the spin-flip energy and therefore the frequency of the emitted signal increases from left to right. When excited by an RF transmitter, the emitted signal contains different frequencies for the two proton concentration areas. These frequencies can be separated by means of the Fourier transform.

(http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/mri.html)

Technical Aspects of MRI:

  • It is necessary to work with magnetic fields which range from 0.1 to 3.0 Tesla (T). The best image quality is obtain when using values of approx. 1.0T.
  • Such values cannot be achieved by permanent magnets(max 0.3T), so electromagnets are required.
  • Electromagnets must be cooled, to prevent them from overheating.
  • MRI machines consume tremendous amounts of electric current, and therefore usage is very expensive.
  • It is possible to use superconductors – which require cooling with liquid helium- to achieve better resolutions. These will be even more expensive.

Clinical Uses of MRI:

  • The MRI scan is a spatial reconstruction of resonating nuclei density.
  • MRI has no biological effects – at least none have been observed – and can therefore be used as many times as it is necessary.
  • The MRI scans will produce cross-sectional images of organs and internal structures in the body.

Using MRI scans, physicians can diagnose or monitor treatments for a variety of medical conditions, including:

  • Abnormalities of the brain and spinal cord
  • Tumors, cysts, and other abnormalities in various parts of the body
  • Injuries or abnormalities of the joints
  • Certain types of heart problems
  • Diseases of the liver and other abdominal organs
  • Causes of pelvic pain in women (e.g. fibroids, endometriosis)
  • Suspected uterine abnormalities in women undergoing evaluation for infertility

Safety:

There are no known harmful side-effects associated with temporary exposure to the strong magnetic field used by MRI scanners. However, there are important safety concerns to consider before performing or undergoing an MRI scan:

  • The magnet may cause pacemakers, artificial limbs, and other implanted medical devices that contain metal to malfunction or heat up during the exam.
  • Any loose metal object may cause damage or injury if it gets pulled toward the magnet.
  • If a contrast agent is used, there is a slight risk of an allergic reaction. MRI contrast agents can cause problems in patients with significant kidney disease.
  • Dyes from tattoos or tattooed eyeliner can cause skin or eye irritation.
  • Medication patches can cause a skin burn.
  • The wire leads used to monitor an electrocardiogram (ECG) trace or respiration during a scan must be placed carefully to avoid causing a skin burn.
  • Prolonged exposure to radio waves during the scan could lead to slight warming of the body.

(http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/ucm200086.htm)

Notes Compiled by Andrei Cociug.

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