*2.1.1. The Anger gamma camera*

The principle of radiation detection is based on the interaction of these radiations with the matter. When a gamma photon enters in interaction with a detector material, it loses its energy mainly in the form of ionizations or excitations. The excited atoms return to their ground state through the emission of secondary low energy gamma photons. The incident gamma photon can be partially or totally absorbed (photoelectric effect). In the first case, the energy loss is accompanied by a deviation of the photon (Compton scattering). The photon loses "memory" of its initial place of issue. So the photoelectric effect is the right phenomenon which must be considered when we interest to the gamma-ray emission site.

their energy) characteristic of the detected gamma-rays. Detection time (acquisition) should be sufficient to obtain good counting statistics. The theoretical gamma-rays spectrum reaching the crystal is a line spectrum; the spectrum is continuous (Figure 2). The spectrum includes the total energy peak corresponding to gamma directly emitted by the radioactive source without any interaction before reaching the crystal and a background of lower energies due to the partial absorption of gamma by Compton scattering. Compton scattering in the path of the photon is changed making it impossible to locate its transmitter site. It is therefore necessary to take into account only the events corresponding to the photoelectric interactions at the level of the crystal with the total emission energy. This is achieved by the intermediate of a "window"

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5

for selecting the double-threshold energy (pulse height analyzer).

**Figure 1.** Main components of Gamma-camera.

**Figure 2.** Gamma-rays spectrum at the level of the crystal detector (ideal (top) and real (bottom) cases).

The width of the peak of total absorption depends essentially of the random statistical fluctuations of the gain of the PMT. The width at half maximum ΔE relative to an average

In the gamma camera, the detection medium is historically a NaI scintillation crystal typically doped with thallium. This crystal is able to emit light especially through a fluorescence process after the excitation of its molecules by a charged particle (electron). The density of NaI is 3.67 g/cm3 and its atomic number 50. Its time of scintillation (fluorescence) is 230 nm and the maximum light emission is at 4150 Angstroms wave length. Its refractive index is 1.85, and it is relatively transparent to its own light; about 30% of emitted light is transmitted to the detection chain [1]. The energy resolution can reach 7-8% at 1 MeV and the constant time of their pulse is equal to ~10-7 sec. The detection efficiency of NaI is quite large, of the order of 40 photons/keV. Indeed, gamma-ray energy of 100 keV transferring all its energy in the crystal results in the creation of approximately 4000 fluorescence light photons. These photons are collected by the photocathode of a photomultiplier tube (Figure 1).

For the detection of the secondary light photons generated in the crystal by the interaction with the incident gamma radiations, a photomultiplier tube (PMT) located behind the scintillator is used (Figure 1). At the level of the PMT photocathode, each light photon is converted to electrons. These electrons are then accelerated and multiplied by ten dynodes polarized by a gradually increasing voltage, and finally collected by an anode placed at the other side of the PMT where they give birth to an electrical impulse. This pulse has an amplitude proportional to the energy of the detected gamma-ray.

The output signal is amplified by the PMT. Its amplitude is measured, digitized and stored. Numerical analysis enables to obtain a spectrum (number of photons detected as a function of Principles and Applications of Nuclear Medical Imaging: A Survey on Recent Developments http://dx.doi.org/10.5772/54884 5

**Figure 1.** Main components of Gamma-camera.

medical imaging. The survey is limited to developments for hospitals, mainly within the

4 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

In addition to conventional gamma scintigraphic imaging, the two major nuclear imaging techniques developed are Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SCECT). Both imaging modalities are now standard in the major

The principle of radiation detection is based on the interaction of these radiations with the matter. When a gamma photon enters in interaction with a detector material, it loses its energy mainly in the form of ionizations or excitations. The excited atoms return to their ground state through the emission of secondary low energy gamma photons. The incident gamma photon can be partially or totally absorbed (photoelectric effect). In the first case, the energy loss is accompanied by a deviation of the photon (Compton scattering). The photon loses "memory" of its initial place of issue. So the photoelectric effect is the right phenomenon which must be

In the gamma camera, the detection medium is historically a NaI scintillation crystal typically doped with thallium. This crystal is able to emit light especially through a fluorescence process after the excitation of its molecules by a charged particle (electron). The density of NaI is 3.67 g/cm3 and its atomic number 50. Its time of scintillation (fluorescence) is 230 nm and the maximum light emission is at 4150 Angstroms wave length. Its refractive index is 1.85, and it is relatively transparent to its own light; about 30% of emitted light is transmitted to the detection chain [1]. The energy resolution can reach 7-8% at 1 MeV and the constant time of their pulse is equal to ~10-7 sec. The detection efficiency of NaI is quite large, of the order of 40 photons/keV. Indeed, gamma-ray energy of 100 keV transferring all its energy in the crystal results in the creation of approximately 4000 fluorescence light photons. These photons are

For the detection of the secondary light photons generated in the crystal by the interaction with the incident gamma radiations, a photomultiplier tube (PMT) located behind the scintillator is used (Figure 1). At the level of the PMT photocathode, each light photon is converted to electrons. These electrons are then accelerated and multiplied by ten dynodes polarized by a gradually increasing voltage, and finally collected by an anode placed at the other side of the PMT where they give birth to an electrical impulse. This pulse has an amplitude proportional

The output signal is amplified by the PMT. Its amplitude is measured, digitized and stored. Numerical analysis enables to obtain a spectrum (number of photons detected as a function of

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nuclear medicine services.

*2.1.1. The Anger gamma camera*

**2.1. The conventional scintigraphic imaging**

considered when we interest to the gamma-ray emission site.

collected by the photocathode of a photomultiplier tube (Figure 1).

to the energy of the detected gamma-ray.

**2. Principles of nuclear medical imaging and image analysis**

their energy) characteristic of the detected gamma-rays. Detection time (acquisition) should be sufficient to obtain good counting statistics. The theoretical gamma-rays spectrum reaching the crystal is a line spectrum; the spectrum is continuous (Figure 2). The spectrum includes the total energy peak corresponding to gamma directly emitted by the radioactive source without any interaction before reaching the crystal and a background of lower energies due to the partial absorption of gamma by Compton scattering. Compton scattering in the path of the photon is changed making it impossible to locate its transmitter site. It is therefore necessary to take into account only the events corresponding to the photoelectric interactions at the level of the crystal with the total emission energy. This is achieved by the intermediate of a "window" for selecting the double-threshold energy (pulse height analyzer).

**Figure 2.** Gamma-rays spectrum at the level of the crystal detector (ideal (top) and real (bottom) cases).

The width of the peak of total absorption depends essentially of the random statistical fluctuations of the gain of the PMT. The width at half maximum ΔE relative to an average energy E0 defines the energy resolution ΔE/E0. The energy resolution of PMT is about 10% at 140 keV (emission peak of technetium-99m). The pulses selected by the pulse analyzer (maximum intensity) are directed to a time scaling circuit having a time integrator which then delivers a count rate in counts per second (cps). This count rate can be correlated to the real activity of the source after a number of corrections taking into account in particular the geometric efficiency and the detection performance of the detection chain. For very high source activity, the detector response is no longer linear so that a number of events are not taken into account. The lapse of time in which these events are lost (not counted by the detector) is called the dead time. In practice, it is usual, to work under conditions such that the detection dead time correction is not necessary (medium activity source).

The scintillation Gamma-camera was used originally for planer projection imaging is mainly

Principles and Applications of Nuclear Medical Imaging: A Survey on Recent Developments

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7

The scintigraphic image corresponds to the projection of the distribution of radioactivity on the crystal detector. Gamma rays cannot be focused using lenses as in the case of light. The use of a special kind of collimator can permit just to one direction gamma rays to reach the crystal, the most common being perpendicular to the crystal. A collimator is a wafer usually lead wherein cylindrical or conical holes are drilled along a system axes determined. Gamma-ray where the path does not borrow these directions is absorbed by the collimator before reaching the crystal. The partition (wall) separating two adjacent holes i called "septa". The thickness of lead is calculated to cause an attenuation of at least 95% of the energy of the photons passing through the septa. The most commonly used collimator is the parallel holes. It retains the dimensions of the image. For non-parallel collimators, the dimensions of the image depend on the geometrical disposition and the divergence or convergence nature of the collimator. This leads to a geometric distortion must be taken into account. The efficiency of a collimator is the fraction of radiation passing through the collimator (without any interaction), reaching the crystal and effectively participating in the image formation. The collimator resolution corre‐ sponds to the accuracy of the image formed in the detector. Resolution improves with increasing thickness of the septa at the expense of collimator efficiency. A good compromise is to find the realization of a collimator performance depends on the intrinsic characteristics

The *γ*-camera crystals are generally composed of NaI(Tl). Features that make this crystal desirable include high mass density and atomic number (Z), thereby effectively stopping *γ* photons, and high efficiency of light output [3, 4]. The most important characteristics of the crystal that must be ensured are: 1) high detection efficiency, 2) high energy resolution, 3). low decay constant time and a light refraction index close to the glass one. Most current cameras incorporate large (50 cm×60 cm) rectangular detectors. While expensive, the larger field of view results in increased efficiency. In early designs, crystals were often 0.5 inches thick, which was well-suited for high energy *γ* photons. In more recent implementations of the *γ*-camera, crystals only 3/8-inch or 1/4-inch thick are used, which is more than adequate for stopping the predominantly low-energy photons in common use today and which also results in superior

Their role is to convert light energy emitted by the crystal to an electrical signal that can be exploited in electronic circuits [3, 5]. This is achieved by the combination of several elements, placed in a vacuum to allow the flow of electrons. The first element, placed in contact with the crystal is the photocathode, metal foil on which the light photons are able to extract electrons. These electrons are attracted to the first dynode by the application of a high voltage between

composed by the following components:

of the detector and the use we want to make [2].

*2.1.1.2. The scintillator crystal*

intrinsic spatial resolution.

*2.1.1.3. The photomultipliers tubes*

*2.1.1.1. The collimator*

The Anger gamma scintillation camera (Figure 3) uses the information provided by the amplitude of the electrical pulse not only to measure the energy of the detected radiation, but also to locate in the space the emission site of this radiation.

The camera developed by Anger in 1953 has a crystal of sodium iodide (NaI) thallium activated. It can take single crystal of large dimensions, up to 60x50 cm2 with a thickness ranging from 1/4 inch to 1 inch [1]. These crystals are fragile and are highly sensitive to shocks and moisture. The surface of the crystal is covered with a large number of PMTs (between 50 and 100). When scintillation occurs, the sum of the output signals of all the MPTs provides the energy lost in the volume of the scintillator (Z coordinate). The large number of PMTs ensures the collection of maximum light. Moreover, the amplitude of the output signal of PMT varies with the distance between the centre of the photocathode and the place where the scintilaltion is produced is in the crystal. The amplitude distribution of the output pulses of the PMT then provides the location information (X and Y coordinates) by means of a computer listing. For each photon interacting with the detector is thus obtained location coordinates (X and Y) and a value of the energy given or lost in the crystal (Z coordinate). An amplitude analysis allows selecting only the photon energy characteristic of the radionuclide used (eg. 140 keV for 99mTc) having lost all their energy in the crystal (photoelectric peak).

**Figure 3.** Gamma-camera called also Anger camera.

The scintillation Gamma-camera was used originally for planer projection imaging is mainly composed by the following components:
