Detecting Chemicals Through Bone

MTorrice writes "To understand the brain and its chemical complexities, researchers would like to peer inside the skull and measure neurotransmitters levels as the brain at work. Unfortunately, research methods to measure levels of chemicals in the brain require drilling holes in the skull, and noninvasive imaging techniques, such as MRI, can't detect specific molecules. Now, as a first step toward a new imaging tool, chemists report they can detect molecules hidden behind 3- to 8-mm-thick bone."

Re:It's NUCLEAR magnetic resonance

[Anonymous Coward]

NMR only reports the presence of (certain isotopes of) nuclei. With most biochemicals of interest being made almost entirely of the same four atoms (carbon, hydrogen, oxygen, and nitrogen) there's nothing to tell the MRI which particular large molecule the atoms are part of.

Are you sure you know what you're talking about?

NMR spectroscopy perhaps the most powerful analytic technique in modern organic chemistry -- it works on molecules as simple as hydrocarbons and as complex as proteins. Information is extracted from the fact that the resonant frequency of a hydrogen atom changes slightly depending on the nearby atoms. This has to do with the shielding of the nuclei by electrons. It's really amazing what it can do -- you can detect not only the different functional groups, but also their relative numbers and positions in the molecule.

The relatively small "alphabet" of biomolecules is certainly no barrier to NMR spectroscopy.

The answer to the GP's question, by the way, is that NMR spectroscopy requires a pure substance. If you tried to do NMR spectroscopy on someone's head, you would probably just get the overlapping spectra of every molecule in the head. Not too helpful.

Spectroscopy with MRI

[DrLudicrous]

Disclaimer: I am a physicist who works in MRI. MRI can be used to measure concentrations of certain biochemicals. MRI is sensitive enough to different proton-containing species that the frequency difference between fat and water causes image artifacts that can pose great difficulty. Not all biomolecules are sufficiently concentrated in the brain, or have a spectrum that is unique enough to be measured in vivo. A good example of a brain chemical that can be measured is N-acetyl aspartate (NAA), which has a proton peak at around 2 ppm that doesn't overlap with much else. Magnetic resonance spectroscopy is very difficult, and is most easily accomplished on research scanners operating at 3 tesla or higher. The reason for this is that rather than letting all hydrogen nuclei contribute to one signal that is then spatially located, one must parse what kinds of nuclei (i.e. what their chemical shift is) within each voxel. This not only imposes technical difficulties, but reduces the signal to noise ratio, potentially requiring more signal averaging in order to see sufficient signal above the noise floor.