MRI Intro Flashcards
What are the two theories used to describe the basic principles of MRI?
Quantum physics as well as Classical physics.
Define Atoms and molecules.
Atom:
All things are made of atoms.
An atom is a unit of matter that consists of a dense central nucleus and is surrounded by a cloud of negatively charged electrons.
Molecules:
Atoms are arranged in molecules.
“A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong covalent chemical bonds.
Define covalent bond.
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms and other covalent bonds.”
Define nucleon.
Nucleon:
A collective name for two particles subdivided into protons and neutrons. The mass number comes from nucleons.
Define Proton, Neutron and Electron.
Proton: It is a subatomic particle that has a positive charge. It is found in the nucleus of each atom along with neutrons it is also known as the hydrogen ion.
Neutron: It is a subatomic particle that has a no net charge. It is found in the nucleus of each atom along with protons.
Electron: It is a subatomic particle that carries a negative electrical charge and spins around the nucleus.
Isotopes are:
Mass number is:
Isotopes: Atoms in the same element having different mass numbers.
Mass number: The sum of the number of neutrons and protons in an atomic nucleus.
Ions:
Ion: “Is an atom or molecule in which the total number of electrons it not equal to the total number of protons, giving it a net positive or negative electrical charge” Atoms that are electrically unstable and leads to an emission of energy (radioactivity).
Why is the mass number important in MRI?
MRI active nuclei are nuclei with an odd mass number, where the neutrons are slightly more or less than the number of protons and the spin directions are not equal and opposite.
The nucleus in those cases has a net spin or angular momentum.
They are characterized by their tendency to align their axis to the applied magnetic field.
What conditions are necessary for resonance in MRI?
Radiofrequency of exactly the Larmor frequency of hydrogen must be applied.
What is the difference between phase and frequency?
analogy
Frequency and Phase the watch analogy.
Frequency is: The time it takes the little hand to make one revolution of the watch face i.e. 12 hours. It moves around the face of the watch every 12 hours.
Phase is: The degrees of the little hand of the watch or the radians e.g. 1 o’clock, 2 o’clock, which corresponds to its position on the face of the watch.
Phase and frequency is connected. The phase of the little hand depends on the frequency. It will tell the correct time if the watch is not fast or too slow.
Another way to say it is that frequency is the change of phase over distance.
Why is hydrogen used in MRI and not the other MRI active nuclei?
It is abundant in the human body.
It contains one proton that gives it a relatively large magnetic moment.
Describe the three motions present within the atom.
1) Electrons spinning on their own axis.
2) Electrons orbiting around the nucleus.
3) Nucleus spins on its own axis.
What is the Larmor equation and what do the symbol’s represent?
The Lamor equation governs the precessional frequency, it states:
ῳ = Bo x γ
ῳ = Precessional frequency Bo = Magnetic field strength of the magnet γ = Gyro-magnetic ratio
Precessional frequency is therefore often called the Larmor frequency.
What does the gyro-magnetic ratio expresses and what is the gyro-magnetic ratio of hydrogen?
Gyro-magnetic ratio is constant and expresses the relationship between the angular momentum and the magnetic moment of each MR active nucleus. It is expressed as the precessional frequency of a specific MR active nucleus at 1T and therefore the unit of gyro-magnetic ratio is MHz/T.
The gyro-magnetic ration of hydrogen is 42.57 MHz/T.
Other MR active nuclei have different Gyro-magnetic ratios and therefore different precessional frequencies at the same field strength.
What is resonance?
“Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.”
When this process happens to a nucleus, it gains energy from the external force. The frequency delivered must be exactly the same as the as the precessional frequency of the nucleus otherwise resonance does not occur.
The energy (precessional frequency of hydrogen) at all field strengths in clinical MRI corresponds to (RF) radio frequency band of the electromagnetic spectrum.
The results of resonance?
Magnetism in the transverse plane that is in phase and Presecces at the Larmor frequency.
