ENK
Goo : MRI:
Magnetic Resonance Imaging: is a way of seeing inside people and animals. How is this relevant to us?
Erasmus : Magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce detailed images from any part of the body.
For imaging purposes the hydrogen nucleus (a single proton) is used. It is abundant in water and fat in the human body. Also, Single protons are easier to flip magnetically than electrons.
Though electrons are a lot lighter they orbit much further from the nucleus (e.g. the proton of a hydrogen atom in water). So when we calculate momentum: Mass x Velocity (movement at a distance), protons have a much smaller Magnetic Dipole Moment than electrons. This means they are easier to flip: requiring less energy and less powerful magnetic fields to create alignment with the magnetic fields.
The hydrogen proton spins on its axis, with a north-south pole, perhaps akin to a bar magnet. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned.
When the body is placed in a strong magnetic field, such as an MRI scanner, the protons' axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner.
MRI Scanner
Kinkajou : MRI scanners come in different field strengths, usually between 0.5 and 1.5 tesla. When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the element sought (hydrogen in this case) and the strength of the magnetic field.
Erasmus : The RF frequency required is called the Lamor Frequency and can be calculated using:
W = YB. Where W is the Larmor frequency in MHz, Y is the gyromagnetic ratio in MHz/tesla and B is the strength of the static magnetic field in tesla.
Note that the gyromagnetic ratio is defined in different ways by different authors. In this case a useful, simplified version is shown representing the Armor frequency when B0=1.
Erasmus : Follow the Link to see how the variables affect the RF Frequency required to spin Protons. Link
* The gyromagnetic ratio (MHz/T) for a few commonly measured or imaged isotopes are :
o H-1 42.58
o F-19 40.05
o Na-23 11.26
| Larmour Frequency Calculator for | |||||||||
| Various Nuclei in Nuclear Magnetic Resonance | |||||||||
| Nucleus: | 
|
|
|
|
|||||
| 13C | 14N | 15N | 17O | ||||||
| 23Na | 31P | 129Xe | |||||||
| Magnet Field Strength: | (T) | ||||||||
| Theoretical Gyromagnetic Ratio: | 0 [MHz/T] | ||||||||
| Actual Gyromagnetic Ratio: | 0 [MHz/T] | ||||||||
| Theoretical Larmour Frequency: | 0 [MHz] | ||||||||
| Actual Larmour Frequency: | 0 [MHz] | ||||||||
Calculations are for a uniform magnetic field at the given strength.
Data for actual numbers obtained from:
-M A Bernstein, K F King and X J Zhou (2004). Handbook of MRI Pulse Sequences. San Diego: Elsevier Academic Press. p. 960. ISBN 0-1209-2861-2.
-R C Weast, M J Astle, ed (1982). Handbook of Chemistry and Physics. Boca Raton: CRC Press. p. E66. ISBN 0-8493-0463-6.
Erasmus : Specific Quantised energy levels for RF must be attained to align the protons in the magnetic field.
The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied.
When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted. It is this signal which is used to create the MR images. Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up.
Multiple transmitted radiofrequency pulses can be used in sequence to emphasise particular tissues or abnormalities. A different emphasis occurs because different tissues relax at different rates when the transmitted radiofrequency pulse is switched off.
The time taken for the protons to fully relax is measured in two ways. The first is the time taken for the magnetic vector to return to its resting state and the second is the time needed for the axial spin to return to its resting state. The first is called T1 relaxation; the second is called T2 relaxation.
Kinkajou : Pacemakers, metal clips, and metal valves can be dangerous in MRI scanners because of potential movement within a magnetic field. Metal joint prostheses are less of a problem, although there may be some distortion of the image close to the metal. MRI departments always check for implanted metal and can advise on their safety.
Kinkajou : The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. In this respect it behaves like a small bar magnet. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned.
When the body is placed in a strong magnetic field, such as an MRI scanner, the protons' axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner.
Kinkajou :
Why are electrons not influenced when the magnetic field is applied (in NMR)?
Erasmus : The electron is not affected by NMR because the magnetic dipole moment of the electron is by far greater than a proton (x658 stronger), due to the greater mass of the proton (x1836) but especially the much smaller effective size of the proton.
In electron spin resonance experiments the electron is affected by the magnetic field, a phenomenon called Zeeman splitting.
Kinkajou : Tell about the Zeeman Effect in Hydrogen
Erasmus : When an external magnetic field is applied, sharp spectral lines like the n=3 > 2 transition of hydrogen split into multiple closely spaced lines. This splitting is attributed to the interaction between the magnetic field and the magnetic dipole moment associated with the orbital angular momentum. In the absence of the magnetic field, the hydrogen energies depend only upon the principal quantum number n, and the emissions occur at a single wavelength.
The Team: Erasmus met Dr Xxxxx and over a number of years grew to develop a belief in the problems of our world.
Erasmus is an old dog, but he reckons he still can do new tricks. He’s been around, and up and down a few dry creek beds in his lifetime .Born into servitude at the Daisy Hill Puppy Farm, Erasmus quickly realised the world could be a hostile and dangerous place. Puppies disappeared daily, never to be seen again.
Kinkajou: Features in our web sites as an adventurer but he is equally accomplished in the art of reasoning and has often followed the comments of Erasmus to their logical conclusion. His belief in the truth leads him to always "tell it true". This is the world as it can be seen and this is the real world.
Dr Xxxxx. This good doctor actually represents a diversity of opinions held by a small cabal of medically aware people.
Dr AXxxxx. Simply best described as a cold inhuman bastard, Erasmus says we are indeed lucky to have this paranoid psychopath on our side. In spite of his antecedents, his perspective is valuable for us to use to understand our world. Dr AXxxxx’s past background is in taking charge and being master at the helm of the ship of destiny. Control is the ultimate goal. Nothing must stand in the way of control. Domination not cooperation. Suspect your enemies and kill them first, preferably after extracting all useful information. Your destiny is yours to make and to take. Vengeance is a heady mix to this paranoic murderer.
The Commandant: Head of our security detail, the Commandant believes in doing the right thing and always considering safe options. However, his view of what is right is coloured by association with Erasmus, Kinkajou and the doctors Xxxxx.
Frobisher & Beethoven: These guys are our bullywot "security mooks", Frobisher is the literate one while Beethoven provides just more raw muscle power. While Officious and Intolerant of transgressions, they none the less have a sensitive and fragile side to their natures, an aspect their personality totally at odds with their aggressive and fierce appearance.
Goo is a simple and humble mammal.(Actually a marsupial numbat really). His ability to change the world is limited by his meagre intellect and his short life span. Furthermore, as a member of an endangered species, he has to face the prospect of personal death as well as species extinction. He could one day be the last of his kind.