Clinical Instrumentation

Gammacamera:

        It is a device that  allows obtaining images of the biodistribution of radiotracers administered to patients. Built with a sodium iodide crystal activated with thallium in contact with a variable number of  photomultiplier tubes protected on the back and side by a lead shield that prevents the entry of photons originating outside the object to study. Studies performed may be static, dynamic and tomographic with gamma or positron emitters.

        The crystal thickness is usually 1/4 or 3/8 inch (6.35 mm and 9.52 mm, respectively). The former has better resolution than the latter, for 99mTc, low efficiency for 131I and 67Ga, 360 keV and 300 keV, respectively. The best spatial resolution of thin crystals is due to the fact that low-energy gamma rays are scattered less in them. Thus, only reach the photomultiplier, those ones related to the specific area of impact on such crystals. The crystal shape can be circular or rectangular, the latter has a larger field of view, with advantages especially for bone studies.

        Leaning on the detector is the collimator, interchangeable element, which is chosen according to the needs of the study. Is made of lead or tungsten and is crossed by a series of holes distributed according to various geometric patterns. Lead between holes, are called septa. The collimator limits the field of view of the detector and improves the detection system performance.

        There are different collimators, the choice of which depends on the study to do: we must take into account the energy of the isotope administered (collimators are for low, medium and high energy) and the area of the  body to assess. In the case of small organs (thyroid, carpal, tarsus, femoral head) it is recommended to use the pinhole collimator, a convergent single hole, which has very high resolution, though low sensitivity and magnifies the image. In general, the most frequently used collimator are with parallel holes. It is important to remember that there is a compromise between spatial resolution and sensitivity in the collimators, the relationship is inverse, i.e. when one of the parameters increases the other decreases.

            Currently, it is taking place the introduction of new devices to overcome some limitations of the Anger gammacamera. They are esentially based on new detectors and colimators. Cadmium, Zinc and Tellurum are semiconductor detectors, which do not need photomultiplier tube since the gamma energy absorved is directly transformed into an electric impulse. The energy resolution is better than that NAI crystal, as can be seen for 4 radionuclides commonly used in nuclear medicine, in the figure below. Note that the spikes are narrower and better defined and significant less scatter.

            They have greater sensitivity, so imaging may take less time with increasing quality. Tungsten collimator with square holes fits better with the detectors and so spatial resolution is also improved. Equipment size may be reduced and the patient can be scanned more comfortably.

Characteristic parameters

        Energy Resolution is the parameter that describes the possibility for the system to separate two energies as different peaks . It is determined from the width at half height (FWHM) of the photopeak in the energy spectrum as a percentage. Spatial Resolution is the shortest distance between two radioactive sources that can be seen as separate foci. It has two components: intrinsic and extrinsic. The first  is determined in a crystal not collimated, the second is for the whole system, and determined by use with a collimator on, being therefore the result of assessing jointly the intrinsic resolution and that of the system. Response uniformity: Evaluates the difference in the response of the detector at different points in the field of view. There are two ways to calculate the field uniformity: Integral Uniformity (IU), when considering all pixels in the visual field and Differential Uniformity (DU) if it considers the average variation within 5 pixels in the field of view. Both measurements are crucial for assessing whether the instrument is able to conduct clinical studies. Temporal resolution: After receiving an event, the detector should return to its baseline state before starting to process another one. During that brief period (dead time) it can not detect. Spatial linearity: Correction of non-uniformities produced by mismatch between the position of the signal and the actual location of the radioactive event.
 

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