Thursday, June 4, 2009

Physics: MRI a quark based dipole?

In MRI (magnetic resonance imaging) an image is obtained by placing a person in a strong magnetic field. The strong magnetic field causes the proton of a hydrogen atom to align with the field. A radio wave is applied to the person which causes the proton to be knocked out of alignment with the magnetic field. The strong magnetic field causes the proton to return to its original state, and this process of realigning lets off another (much smaller) radio wave, which we listen to. The final image is formed from measurements of the strength of these radio waves that emanate from hydrogen protons in the body.

About 40 years ago particle physicists proposed a 3 quark model of the proton (see figure below) made of two up quarks and one down quark. We know the electric charge on a proton to be 1, and it was discovered that each of the two up quarks had a charge of 2/3 and the one down quark had a charge of -1/3 (making the overall charge of the proton 1). Now some particle physicists have stressed that studying quarks is challenging as they are always unstable. If hydrogen is placed in a very strong magnetic field and we know its proton aligns with the magnetic field, I can’t help but wonder if it manages to align simply because it enters a stable (or somewhat stable since the triad of quarks would probably be spinning) 3 quark state with the two up quarks pointing towards one end of the strong magnetic field and the one down quark pointing in the opposite direction. At present only some substances are known to exhibit this magnetic resonance phenomena (hydrogen, carbon 13, helium 3, etc.) but I can’t help but wonder if the reason that some substances exhibit the phenomena while others do not is because of the configuration of protons (and neutrons) in the nucleus of the atom. The configuration of protons will heavily affect the strong nuclear forces exhibited within the atom – it is possible that these strong forces prevent any of the nucleus’ protons (from those elements that do not exhibit magnetic resonance) from entering a state where MRI based ringing can occur. Perhaps the proximity of the quarks in a neighbouring proton/neutron is what prevents any given proton from ringing (ie. strong forces attract the various quarks thus tying the nucleus together in a way that prevents MRI based ringing at currently used field strengths). Note that those elements that do exhibit magnetic resonance either have just 1 proton (hydrogen with no proton-to-proton strong force interaction) and a bunch of isotopes (irregular configurations of protons and neutrons).

The 3 quark model of the proton:


It would also be interesting to see an experiment whereby a hydrogen atom is kept still (at low temperature) in a strong magnetic field. The nucleus is then examined thoroughly to see whether the proton appears to be in a stable or semi-stable 3 quark model state.