Clinical Instrumentation
In this part of the manual practical issues regarding the author's experience in the field will be discussed. The contents are divided in the following: Dose Calibrator, Gamma Probes, Gammacamera, Spect Camera, PET camera and CT. These pieces of equipment are the ones considered essential to conduct diagnostic and therapeutic procedures accurately. Equipments employed in radioprotection are not included in this chapter and will be covered in the Clinical Radioprotection part.
This is a piece of equipment that provides the absolute activity of a radiotracer. It is used to determine the dosage of radiopharmaceuticals in diagnosis and therapy.
It consists of a gas detector, usually an ionization chamber, whose design is such that allows the radioactive material to be positioned inside it for measurement. The ionization chamber is a space full of gas at an appropriate pressure, with two electrodes isolated from each other and connected to a high voltage source and a device for measuring current or voltage. When ionizing radiation passes through the gas, produces ion pairs. Their behavior depends on the nature and pressure of the filling gas and the applied voltage.
According to the latter, the ions produced, can collide and recombine, or they can reach the respective electrodes: negative ions are directed to the anode and positive ions to the cathode. When ion pairs are collected by electrodes, originate a current in the measuring device. This current is converted to digital signals in the detector with an associated electronic circuit. So, they make possible the reading of the activity of the sample directly into: Ci, mCi or MBq.
The current produced in an ionization chamber depends on the type, energy and relative abundance of radiation, so they need appropriate adjustments to the amplification voltage signal because registration for individual radionuclides will be provided in units of activity. For this purpose, the activity meters are fitted with a selector for the radionuclides most commonly used and a potentiometer with a dial calibrated for specific radionuclides used in a particular laboratory, and where each number corresponds to a different radionuclide.
The parameters that characterize the functioning of a calibrator are the following and can be determined by a set of measurements and tested regularly: Background reading (It can reveal the existence of electronic noise and radioactive contamination); Sensitivity (The minimum measurable activity with acceptable accuracy); Stability; Accuracy (It is verified by measuring a gold standard); Precision (Is determined by the variability of repeated measurement of any one sample); Linearity (Proportional linear response in the range of low and high activity levels to use clinically).
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