Biomedical Engineering Reference
In-Depth Information
potassium iodide to image the circulatory system. High-density iodide provided
significant attenuation to X-ray radiation, causing the blood vessels to appear lighter
on film. a few years later, Heuser utilized another iodinated compound called lipiodol
synthesized in 1901 by the french chemist M. guerbet. lipiodol is a low-viscosity
radio-opaque diagnostic agent formed by the iodination of the fatty acids in poppy-
seed oil and was applied to investigate the uterine cavity and fallopian tubes. Due to
its high density and low toxicity, many iodinated compounds are commonly used
today in X-ray and computer tomography (cT) imaging—a successor of the X-ray
technique. (One of the leading companies of X-ray contrast agents is the guerbet
group established by the son of lipiodol's inventor in 1926.) However, despite sev-
eral decades of continuous efforts to improve X-ray instrumentation and expand X-ray
imaging to soft tissue with contrast agents, diagnosing diseases of internal organs
suffered from unacceptably low contrast. New technologies were desperately needed.
1.3
rise of tHe nuclear imaging tecHniques (1940s-1950s)
Shortly after World War II in 1946, the U.S. congress passed the atomic energy act
that transferred nuclear weapon development and nuclear power management to
civilian, rather than military control. The Oak ridge laboratory in Tennessee was
directed to provide radioisotopes for peaceful purposes, especially for medical appli-
cations. One of the first isotopes made available was
198
au colloid. It was produced
by bombarding gold foil with slow neutrons in a uranium pile and was immediately
(1947) utilized for cancer therapy in patients [5]. Since gold cations are extremely
reactive due to their high reduction potential (au
3+
(aq) + 3e
−
→au(s), +1.50v vs.
NHe), they are incompatible with biological tissues. In contrast, gold colloid is
chemically stable for storage, and the author recalls seeing bottles of colloidal gold
that were several decades old. In addition, gold colloid is biologically inert and has
been known in medicine since the time of Paracelsus [6].
198
au emits radiation consisting of 0.97 Mev beta (β
−
)- and 0.411 Mev gamma
(γ)-rays with a half-life of 2.7 days [7]. The beta radiation from this isotope is
absorbed under several millimeters of tissue rendering its importance for cancer
treatment. The gamma emission that penetrated freely through the body became
important for imaging. Produced colloidal gold nanoparticles were small enough
(3-7 nm) [8] to pass through the pulmonary capillaries (<7 µm) but were accumu-
lating mostly in the liver and spleen [9]. at higher dosages, even bone marrow could
be visualized. The problem with
198
au was its high radiation dosage of 50-100 rad/μci
that limited its clinical utility. In the search for compounds offering better imaging
properties,
99m
Tc-sulfur colloid has been explored. Subsequently, other radioactive
colloids such as
68
ga ferric oxide and
113
In ferric hydroxide have been employed.
With the help of these nanoparticles, untreated leukemia with grossly expanded
marrow compartments was shown to be distinguished from aplastic anemia or mye-
lofibrosis with less than normal activity of marrow [10].
following the acceptance of isotopes in imaging, the 1940s and 1950s witnessed a
rapid development of imaging instrumentation. The diagnostics with radioactive metals
Search WWH ::
Custom Search