Biomedical Applications of Emission Tomography - Correction Techniques in Emission Tomography - page 12

of a single cell, a tissue or complex organisms ranging from model organisms,
such as drosophila, to patients. In this context imaging methods cover a broad
spectrum from high resolution microscopic techniques over dedicated small
animal scanners to whole-body human systems in clinical diagnostics.
The field of medical imaging had already been founded when, in 1895, the
German physicist Wilhelm Conrad Rontgen discovered \invisible rays" which
he initially termed X-rays. X-rays in combination with X-ray sensitive films
were immediately applicable for planar imaging in medical diagnostics when
used in a transmission approach. One of the first examples was clinical imag-
ing of the thorax with the patient positioned in front of a film cassette and the
X-ray tube in front of the patient. The rapid spread of X-ray-based imaging
in medicine was especially driven by technological advances such as digital de-
tectors, tomography (computer tomography) and contrast agents, which were
important milestones in the entire field of medical diagnostics. X-ray imag-
ing depicts the density of structures by their specific X-ray absorption; dense
structures such as bones can be excellently imaged, whereas soft tissue con-
trast is limited. X-ray imaging is therefore primarily used for high resolution
morphological imaging; contrast enhancement is used to delineate structures
such as vessels.
With the discovery of radioactive radiation by Henry Becquerel in 1896 a
new biomedical imaging principle was founded, which nowadays has become a
major tool in biomedical imaging. Based on the initial description of radioac-
tivity by Becquerel, George Charles de Hevesy first described in 1923 the
use of radioactive labelling of molecules to trace their fate upon application
into a biological system such as plants, cells, animal models, or even patients.
With the parallel development of gamma detectors as the basis of scintigraphy
replacing the Geiger-Muller-Counters, imaging of the distribution of radioac-
tivity in organisms became a reality. These inventions essentially founded the
field of diagnostic functional and molecular imaging in medicine. One of the
earliest examples of a medical application of scintigraphic imaging was the
first thyroid imaging study of Paul Blanquet in Bordeaux as early as 1951. In
a patient with thyroid nodules Blanquet applied [ 131 I], which, as the \origi-
nal" non-radioactive iodine [ 127 I], is taken up by the thyroid cells and reflects
their molecular metabolic activity. In this case [ 131 I] \traces" the fate of the
physiological iodine in the thyroid gland since biochemically [ 131 I] is identical
to the \cold" [ 127 I], [ 131 I] being an example of an authentic tracer. Today,
[ 131 I] has been replaced by injection of [ 123 I] or [ 99m Tc]O 4 ; however, the prin-
ciple of functional and molecular thyroid diagnostics is unchanged and was
not replaced by any other technique.