Page 285 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 269

blood, sperm, etc), in transplantation of organs (kidney, tissue grafting, etc), in detection
of genetic diseases, etc. The importance of DNA-analysis is constantly growing.
The sequence of the DNA analysis is as follows. (i) The cell walls are broken up by
osmosis etc. and the doublestranded DNA is denatured to pieces of singlestranded DNA.
The molecules may be concentrated by centrifugation. (ii) By exposing this DNA to
restriction enzymes the DNA nucleotide chain is sectioned further into smaller fragments;
different restriction enzymes cut the DNA molecule at different positions. The fragments
are exposed to radioactively labelled compounds (e.g. containing 14C or 32p) which
selectively attach to the different fragments. Alternatively to (i) and (ii), the DNA is
directly labeled e.g. by 32p in a cloning process; the clones will then contain 32p at its P-
sites also after it has been split by restriction enzymes. (iii) When the DNA samples are
exposed to electrophoresis in a suitable gel (agarose, polyacrylamide, etc), the fragments
distribute themselves along the potential direction according to their migration velocities.
Usually 10 - 40 samples (each treated by a different restriction enzyme) are run
simultaneously. (iv) After the electrophoresis, a photographic plate is placed on the gel and
exposed to the radioactive fragments, producing, after development, a pattern of spots or
bands on the plate. (v) The band pattern is analyzeA to reveal its information about the
individual. The technique has many similarities with those demonstrated in Figures 9.7 and
9.8.

9.5.2. Transmission computer tomography (TCT)

Conventional "x-ray pictures" show the morphological structure of the internal organs.
The technique is used with x-ray or -y-ray sources; e.g. Ra was extensively used during
World War I in operations at the front where electricity was lacking. To improve the
contrast in the photographic pictures, dense media like barium sulfate or iodine compounds
are administered to the patient; this method is said to be invasive and can be painful to the
patient.
Though this technique is still widely in use, the 1970s saw a large step forward in medical
imaging referred to as computerized tomography (CT), developed by Cormack and
Hounsfield (Nobel laureates in 1979) and others. Though the radiation source is the same,
the photographic plate is replaced by one or several radiation sensitive detectors, a digital
computer and a display ("TV monitor"). The radiation source and detector array are moved
(scanned) in relation to the patient, see Figure 9.13 A. This technique requires computer
software to handle the reconstruction arithmetic neeAed to provide an image on the screen
from the observed changes in count rates by the detectors (due to different absorbancies of
the organs) at different geometries. Tomography means "slice'; the technique shows slices
through the body on the screen. The resolution of a TCT-scan is usually - 1 mm, allowing
quite exact pictures, e.g. of a tumor. Since nowadays only X-ray tubes are used as radiation
source, the technique is handled by radiologists at the hospital "X-ray department".
Transmission tomography can be compared to a silent movie: one can see what happens
physically with the organs. Emission tomography (next section) uses internally administered
radioactive isotopes, which provides explanation to why the organs function as they do.
Thus, to extend the comparison, computerized emission tomography becomes more like a
medical sound movie of the patient.