Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (21), 3192 - 3194
A Gentle Probe for Acids in the Body
A gadolinium compound could help doctors
with tumor diagnosis and sports
scientists with their research
Looking into the body without having to open it up has long been a dream come true for doctors; refined techniques such as magnetic resonance imaging (MRI) deliver more and more information about the heart and kidneys, even without a scalpel. The acid-content of blood may soon also be determined painlessly, right at the diseased spot, without a blood sample. A discovery by Dean Sherry, who does research in the USA at the University of Texas in Dallas, at least lets one hope this will be the case. He and his coworkers have found a substance that changes the contrast of "magnetic resonance images" depending on the acidity of the blood.
Chemists and doctors describe the acidity of blood in terms of its pH value, which indicates the concentration of acids in a liquid. Blood also contains acids; normally, the pH of this life-giving liquid lies in the neutral range, but it changes with physical exertion, when the body produces more acids. Tumors also can give themselves away when their strongly accelerated metabolism causes the pH-value to deviate from the norm.
How can an MRI determine the acid concentration in the body? Greatly simplified, these instruments measure how many water molecules are contained in tissue. To do this, they measure how much energy is needed to flip the magnetic atomic hydrogen nuclei of water, which are aligned in a strong external magnetic field, against the direction of the field. Aside from this "flipping" process, an MRI can also register how quickly the flipped atomic nucleus swings back to its original position.
This is where Sherry and his colleagues employ a gadolinium compound: the metal atom at the center of the molecule likes to surround itself with water molecules, and causes the hydrogen atoms in those water molecules to return to their starting position in the magnetic field more quickly. By surrounding the gadolinium with a special shell made of other molecules, Sherry and his coworkers can cause the water molecules to be held longer and the water hydrogens to exchange with bulk water hydrogens in proportion to the acidity of the solution the researchers are examining. The result is that the less acid present, the slower the water hydrogen nuclei flip back, and the darker the corresponding pixel on the monitor gets. Depending on the character of the molecules that make up the shell, Sherry can even tune his contrast-agent to different pH ranges.