In general

Nuclear medical diagnosis is based on images of the human body similar to the process in X-ray diagnostics. The main difference is that the radiation originates inside the body. Its source is a very small amount of radioactive substance that is injected into the bloodstream of the patient. During the radioactive decay of such substances they emit gamma rays, i.e. extremely "hard" X-rays that are measured outside the body. Based on the intensity of the measured signals the distribution of the radioactive material can be mapped. Therefore the principle of nuclear medicine can provide information on the function of organs.

Like in X-ray diagnostics there are two different types of images: the projection image in form of scintigrams and the tomographic images as with computer tomography

Radioactive elements for nuclear medicine

Condition for the acquisition of nuclear images is that the radioactive radiation produced inside the body can be detected on its outside. For this reason it must be gamma radiation since only this type of radiation can penetrate the tissue and is not absorbed by body tissue as is the case with alpha and beta rays.

Radiopharmceuticals

Necessary for the examination are suitable radioactive substances, so-called radiopharmaceuticals. Radioactive elements are produced in the cyclotron, a particle accelerator in which stable atoms are bombarded by protons of high kinetic energy. The radioactive substances are then processed and converted to pharmaceuticals in specialized chemical laboratories.

Because of the short half-life period of such substances the generator and the chemical laboratory must be located at the site where the examination is to take place. Therefore, standard examinations are carried out with other elements that can be produced in radio nuclide generators which may be situated at remote locations

The gamma camera

In order to generate a scintigram, i.e. an image of the distribution of a radioactive substance in the body, a single scintillation detector was applied at the beginning of nuclear imaging early in the 1950's. The detector was moved stepwise across the patient and the image was created dot by dot. This extremely slow procedure was replaced by the gamma camera invented by H.O. A. Anger in the 1950's.

Its design was based on a large-size crystal behind which an array of photomultipliers (light amplifiers) were arranged. This camera provided projection images. The information about the distribution in depth was lost, however. A collimator with a large number of holes arranged in parallel determines a certain direction of projection. A proton impinging on the scintilatbayer04or at a certain point produces a light flash which is recorded by several multipliers. The intensity of each individual signal depends on the distance from the point of impact and is analyzed by the computer. Thus, the point of impact can be exactly localized and assigned to a specific pixel on the monitor. The accuracy depends on the number of photomultipliers used; today, their number amounts to more than 100.

One of the main applications of the scintigraphic method is the evaluation of the thyroid gland by using a substance that behaves in a similar way to iodine. If the scintigram shows reduced activity it may be an indication of a pathological process

Tomographic image: SPECT

In analogy to computer tomography, nuclear medicine also offers the opportunity to produce 3-dimensional images. The method is called SPECT (Single Photon Emission Computer Topmography) and was developed by David E. Kuhl.

It is today's standard imaging procedure in nuclear medicine.

The gamma cameras are moved around the patient in a circular pattern. Usually three cameras are employed for optimum utilization of the radiation. The distribution of radio activity is measured, but the absorption of the radiation in the body necessitates some corrections. A common use of SPECT is the scintigraphy of the cardiac muscle which enables the detection of impaired blood flow through the heart muscle.

Tomographic imaging with positrons: PET

The Positron Emission Tomography uses the phenomenon of positron cancellation in order to measure the exact concentration of radiopharmaceuticals and thus to derive information on the function of the organs. The PET consists of a ring arrangement of several hundred detectors.

With a decay of one positron (positively charged anti-electron emitted by an atom nucleus after artificial radioactive stimulation) two gamma quanta (photons in the range of gamma radiation) of reverse directions are generated. Therefore, the decay must happen on the connecting line between two detectors. This allows the computer to calculate the projections in the same way as with SPECT.

PET offers the following advantages: no collimators are required. This results in about a thousandfold increase in sensitivity. The resolution is much higher. Based on the simultaneous detection of two quanta the signal measured by PET is independent of the absorption in the body tissue. Therefore it is possible to make an absolute measurement of the concentration of radiopharmaceuticals.

PET is predominantly used for determining the glucose metabolism. Chemically marked glucose is injected into the blood and the body reacts in its usual way and deposits the glucose in body regions of increased glucose consumption. Many tumors consume higher amounts of glucose than healthy tissue. PET images show tumors and make it possible to determine their degree of malignancy.