Diagnosis

The refinement of X-ray techniques for the visualisation of the chest and lungs has played a major part in conquering tuberculosis. The development of safe contrast media, which are visible on radiographs because they absorb either more or less Xradiation than the surrounding tissue, have resulted in improved and new techniques for the examination of the heart, veins and arteries, kidneys, and other internal systems and organs. Radiography of the breast, mammography, is a valuable addition to the techniques of cancer detection with each centre evolving the technique best suited to its needs and available equipment. This examination, using low kilovoltages, has proved useful in assisting the diagnosis of tumours where clinical diagnosis is uncertain and in detecting unsuspected tumours in a supposedly normal breast.

Improvements in equipment and techniques have led to positive identification of many other types of abnormalities and tumours. In addition, X-ray localisation has made possible a more exact definition of the tumour volume and its relation to adjacent structures and this has resulted in more effective radiation treatment should this be required.

Modern medicine is becoming increasingly dependent on diagnostic techniques using minute amounts of radioactive material, known as tracer doses, for the identification of diseases and for the study of the course of disease after treatment. Any radionuclide' may serve as a true tracer for the element with which it is isotopic since its behaviour can be studied by measurements of its emitted radiations. Similarly, if a radioactive atom is incorporated into a molecule a "labelled" compound results whose physical or chemical reactions, may be observed in the same manner. For this reason they have found wide applications in tracing metabolic processes —the physical and chemical processes by which the living body is maintained and energy made available for various forms of work—and for the measurement of blood flow, blood volume, excretion, and other important parameters or functions.

In the diagnostic use of radioisotopes it is essential that no harmful concentration of the radioactive material occurs. Isotopes of high radiation energy which are retained in the body for long periods of time and which concentrate in small organs give the highest doses. A judicious choice of radioisotopes can often result in a radiation dose smaller than that from a conventional X-ray examination.

The capacity of the thyroid gland to concentrate iodine rom the blood has long been recognised. The use of radioactive iodine for measuring this capacity has become a standard clinical procedure and is the commonest application of radioisotopes in the field of clinical medicine both in New Zealand and overseas. When radioiodine is administered to the patient, gamma rays capable of external detection can be readily counted and the degree of thyroid activity measured. A more recently developed test which represents a significant advance in the search for a simple and reliable measurement of thyroid function is carried out by adding a resin impregnated with radio-iodine to a sample of blood taken from the patient. This test, which causes no radiation exposure to the patient, is significant in the presence of unrelated non-thyroidal factors which are known to complicate the interpretations of a conventional test finding.

Evaluation of the kidney function by radioisotope procedures is performed using a compound labelled with radio-iodine. Distinctive excretion patterns in normal and abnormal kidney states are obtained by external counting of radioactivity over each kidney. This method provides a simple, fast, and accurate means of evaluating kidney function, As the compound is excreted rapidly and exclusively through the kidneys, radiation exposure to the patient is minimised. This test often provides more diagnostic data than any one of the conventional kidney function tests.

Radio - lodinated human serum albumen is an ideal tracer substance and has proved to be of value in a wide variety of clinical procedures. It is derived from natural plasma protein and mixes readily with normal blood constituents. It can also be detected and measured easily and is excreted promptly following metabolic breakdown of the albumen, It is used in the determination of blood and plasma volumes and enables the physician to know the quantity of blood which has to be transfused to restore the blood volume to normal after an incident causing blood loss. Labelled albumen has also been used for the determination of blood circulation and heart output.

Although radioisotopes are not routinely used during pregnancy, certain clinical conditions warrant their administration. Localisation of the placental site is often of such importance as to justify a low level exposure of the foetus. The use of labelled albumen is an alternative to X-ray procedures for this examination and it generally results in a lower radiation exposure both to mother and foetus.

Before any procedure involving radio-iodinated serum albumen, a special substance is administered to minimise thyroid uptake of radio-iodine.

The measurement of the total red cell volume of a patient can be readily performed using radioactive chromium by a method known as the isotope dilution technique. By this procedure the patient's own red cells or those of a compatible donor are labelled, the total amount of radioactivity injected is measured, and after complete mixing has taken place the radioactivity per millilltre of red cells is measured. The total red cell volume may then be calculated. Radioactive chromium is also used in the study of red cell survival and in the detection of bleeding in the stomach.

If it is desired to determine how well a patient's liver is functioning, a small amount of radio-iodinated dye is injected into a vein in the arm. The passage of radioactive dye through the body is followed with an external detecting, apparatus similar to that used for thyroid tests. The radioactive dye accumulates in the normal liver and is then excreted through the gall bladder and bile ducts into the intestines. The manner in which the dye passes through the body indicates the degree of liver function. It also helps the physician to determine the type of liver disorder present, if any, and whether there is any obstruction.

The use of vitamin B12 labelled with radioactive cobalt is a well-established practice for the diagnosis of pernicious anaemia. Pernicious anaemia results when the stomach fails to secrete, into the gastric fluids, a biochemical known as "intrinsic factor." The entire body of a normal adult contains only a few milligrams of vitamin B12 which must combine with the (intrinsic factor before it is absorbed. The percentage of labelled vitamin B12 excreted in the faeces or urine is measured after administering an oral dose to the patient and this indicates the extent of intestinal absorption of vitamin B12 and confirms the presence or absence of intrinsic factor in the stomach.

Radioactive iron is one of the most versatile and most interesting of the radioisotopes available for medical diagnostic and research procedures. Radioisotope tracer techniques employing radioactive iron provide a comprehensive picture of the mech anism of iron metabolism and of the influence of disease, processes upon this mechanism. By means of radio-iron, one can measure the rate of absorption of iron from the intestine, the demand for iron by the bone marrow, the efficiency of the marrow in synthesising new red blood corpuscles, and the survival of labelled cells. Radioactive iron is also used for the diagnosis of many types of anaemic conditions.