In this section, we describe the main components of a “traditional” Anger camera: a gamma camera with analog signal processing (Figure 12.4).

1. The collimator is a sheet or disc made of lead, containing (mostly parallel) holes. Radiation arriving from the patient’s body can get across it only along the holes (i. e., in a perpendicular direction); otherwise, the septa of the holes will absorb it. As a result, the “image” of a point source will be a small spot on the crystal. The spatial resolution of the camera is primarily determined by the collimator.

2. The special scintillation detector is generally a thallium-activated sodium iodide [NaI(Tl)] monocrystal, which is rectangular or circular in most cases. For imaging the 141 keV gamma radiation of Tc-99m, the crystal thickness is around 9 mm. Gamma photons hitting the crystal will produce light in a process called “scintillation.”

3. There are many (19—100) photomultiplier tubes (PMTs) attached to the crystal. Each of them responds to the light, and those closer to its source produce larger electric signals than the distant ones.

4. The output signals of all PMTs are forwarded to the matrix (or Anger) circuit.

5. It calculates the X and Y coordinates, and a Z signal proportional to the energy of the original gamma particle.

6. The three signals (X, Y, and Z) are interfaced to a differential discriminator.

7. The discriminator selects only the signals in a specified narrow (15—20%) energy window; in this way, Compton-scattered radiation can be partly removed from the image.

8. In older cameras, a persistent scope helps to position the patient.

9. Analog—digital converters (ADCs) will form numbers from the X and Y signals.

10. A computer will form a digital image from the signals. Each picture element (pixel) contains the number of photons detected in the corresponding small (square) area of the detector. We then display, process, and store these digital images.