BASIC PHYSICAL PRINCIPLES OF GAMMA CAMERA IMAGING

Gama kamera je uređaj koji se najčešće koristi u slikovnoj dijagnostici nuklearne medicine. Izumio ju je 1953. godine Hal Anger te je od tada znatno poboljšana. Gama kamera zapravo je uređaj koji ima mogućnost detektiranja izuzetno male količine radioaktivnosti u tijelu ispitanika čime se dobiva dvodimenzionalni scintigram. Sastoji se od dva osnovna dijela, kolimatora i detektora. Kolimator ima sposobnost apsorbiranja gama zraka koje nam nisu pogodne za dobivanje scintigrama. Gama zrake koje prođu kolimator, dolaze do detektora. Detektor se sastoji od velikog scintilacijskog kristala te od niza fotomultiplikatora. Scintilacijski kristal u interakciji sa gama zrakama stvara fotone vidljive svjetlosti koji se svjetlovodima vode u fotomultiplikatorske cijevi. Fotomultiplikator određuje točno mjesto scintilacije što pomaže u nastanku slike i fotone vidljive svjetlosti pretvara u slab električni impuls. Električni impuls provodi se do računala gdje se stvara slika, odnosno scintigram. Kako bi dobili kvalitetan scintigram u obzir moramo uzeti i parametre snimanja, tj. prostornu rezoluciju, osjetljivost, ujednačenost i dr. Prostorna rezolucija pokazuje sposobnost gama kamere da razlikuje dva bliska objekta. Obrnuto je razmjerna osjetljivosti gama kamere te je potrebno uspostaviti ravnotežu između tih parametra. scintigrama. Osjetljivost se definira kao broj impulsa koje detektor detektira za svaku jedinicu aktivnosti. Pored ta dva parametra važna je i jednolikost jer se zbog raznih utjecaja slika može pogoršati. U kliničkoj praksi najčešće se koriste gama kamere s jednom glavom i rotirajućim postoljem čime se omogućuje tomografsko snimanje. Osim njih često se koriste i gama kamere sa dvije glave koje omogućuju istovremeni pregled pacijenta iz različitih pogleda. Svaki uređaj ima svoje prednosti i mane pa ih tako ima i gama kamera. Najvažnija prednost gama kamere je mogućnost otkrivanja patoloških procesa, a najveća mana je korištenje ionizirajućih zraka tijekom pregleda gama kamerom. Veliku ulogu u otkrivanju patoloških procesa imaju radioizotopi. Najčešći radioizotop u dijagnostici gama kamerom je tehnecij-99m.The gamma camera is the device most commonly used in nuclear imaging diagnostics. It was invented in 1953 by Hal Anger and has been greatly improved ever since. The gamma camera is a device capable of detecting extremely small amount of radioactivity in the body of a subject, thus producing a two-dimensional scintigram. It consists of two basic parts, a collimator and a detector. The collimator has the ability to absorb gamma rays that are not suitable for scintigrams. The gamma rays that pass the collimator reach the detector. The detector consists of a large scintillation crystal and an array of photomultiplier tubes. The scintillation in interaction with gamma rays produces photons of visible light that are guided by optical fibers into photomultiplier tubes. Photomultiplier tubes determine the exact location of scintillation, which helps making an image, and they convert light photons into a weak electrical pulse. Electrical pulse is transmitted to the computer where the image or scintigram is created. In order to obtain a quality scintigram, we must also consider performance parameters. These are spatial resolution, sensitivity and uniformity. The spatial resolution shows the gamma camera's ability to distinguish between two objects. It is inversely proportional to the sensitivity of the gamma camera, and it is necessary to balance these scintigram parameters. Sensitivity is defined as the number of detected pulses that the detector detects for each unit of activity. Another important parameter is uniformity because various effects can make the image worse. In clinical practice, single-headed gamma cameras with a rotating stand are most commonly used because they enable tomography imaging. Two-headed cameras are also often used to allow the patient to be viewed from different angles at the same time. Each device has its own advantages and disadvantages, so does a gamma camera. The most important advantage of the gamma camera is the ability to detect pathological processes, and the biggest disadvantage is the use of ionizing rays during gamma camera examination. Radioisotopes play a major role in detecting pathological processes. The most common radioisotope in gamma camera diagnostics is technetium-99m