3D Neutron Imaging for Medicine

To take pictures of the body, medical professionals conventionally use x rays, magnetic fields (MRI), ultrasound, and in some cases, radioactive isotopes (PET scans). At this week's annual meeting of the American Association of Physicists in Medicine in Pittsburgh, Duke University researchers presented the first 3D pictures (of an inorganic test object) from a new technique that employs neutrons.

Why use neutrons for medical imaging? Compared to other particles, neutrons are highly penetrating, and therefore can image deeply buried body structures that cannot be reached by other probes. In addition, neutrons can easily identify almost every naturally occurring chemical element in the body.

Called Neutron Stimulated Emission Computed Tomography (NSECT), the technique involves illuminating the body with fast neutrons (those with energies between 1 and 10 MeV). The neutrons cause the nuclei of atoms and molecules in the body to emit gamma-ray photons with distinctive energies that depend on the specific chemical identities of the atoms and molecules to which the nuclei belong. The only two elements that won't show up on an NSECT scan are the lightest elements: helium, which emits gammas at 25 MeV, and hydrogen, which has no excited nuclear states and therefore does not emit gammas.

At the AAPM meeting, Carey Floyd (cef@deckard.duhs.duke.edu) presented the first 3D images ever reconstructed from the emission of characteristic gamma rays stimulated by fast neutrons. The images, of an iron-copper sample, demonstrate the technique's ability to completely distinguish between the iron and copper that made up the object.

With further development, NSECT could potentially diagnose breast cancer early by looking for differences in the concentration of trace elements that are known to exist between benign and malignant tissue. Neutrons could identify cancer by the way it changes concentrations of chemical elements in tissue long before the cancer has begun to cause the anatomical changes (such as the formation of dense tumors or microcalcifications) that are detected by conventional methods.

While an individual neutron is more damaging to the body than a single x ray of equal energy, the researchers' preliminary calculations indicate that an accurate test for breast cancer could be performed at a dose similar to that of a current mammography examination. As an intermediate step towards this goal, the group next plans to develop a prototype system that can image the distribution of iron in the liver in order to diagnose hemochromatosis (iron overload in the liver) without the need for a biopsy. (Meeting Paper WE-D-315-6; also see lay-language paper with pictures.)