Structure of the Cone-beam CT

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The structure of a simple cone-beam CT used for imaging of small (3-7cm in diameter) objects is shown on the Figure 1. The two main components, the x-ray source and the detector are located on a common framework (so called gantry) in opposite to each other. During the acquisition both rotate around a common axis of rotation in order to scan the object.

Figure 1. Structure of a simple cone-beam CT A practical implementation of a cone-beam CT system. 1, baseplate of the imaging system mounted on a rotating gantry; 2, microfocus X-ray source; 3, flat panel detector; 4, x-ray source controller electronics; 5, aluminium filter; 6, pre-patient collimator; 7, place of the object, the object can be moved through the circular opening into the field of view of the x-ray detector. The axis of rotation is located in the centre of the opening.

X-ray source

 
Depending on their application area x-ray sources vary in structure. Systems used for imaging small animals or ex-vivo samples are equipped with microfocus x-ray tube, where the focal spot size is typically falling in the 1-10µm range. These sources has low power (<100W) and usually integrated with the high voltage power supply into a common chassis. Their anode material is often tungsten; they have continuous duty cycle and conventional air cooling, therefore they are compact in size and light weighted.
X-ray tubes with higher power (1-10kW) are used for dental applications or for providing CT based attenuation correction in a PET or SPECT system. These sources usually have rotating anode and their focal spot size is eventually bigger.
The parameters of the x-ray source are also varying based on the application. The tube voltage range goes form 40kVp (small animals, soft tissue samples) to 80-100kVp (human application), while the tube current is usually limited by the highest possible power at the given tube voltage (100µA – 10mA). Some practical examples of x-ray source parameters are given in Table 1.

Table 1 Comparison of x-ray tubes used in cone-beam CT systems (examples)

X-ray detector

 
The x-ray detector can be analogue (like the image intensifier) or digital, which is often referred as flat panel detector. The latter can be further divided into two groups based on the detection principle they employ. The direct conversion type directly transforms the x-ray photons to electrical signal (e.g. amorphous Se), while the indirect conversion type devices has a scintillator crystal, which generates visible light pulses upon the absorption of the x-ray photons, which can be detected by optically coupled photo-sensors.
The imaging performance of the detectors is determined by its two major components. In case of the scintillator crystal the main characteristics are the material (e.g. Gd₂O₂S – Gadolinium-oxysulfide or CsI – caesium-iodide), thickness and structure (e.g. amorphous or columnar). For the imaging sensor the dominant features are its type (e.g. CMOS photodiode matrix or CCD) and the quality of the optical coupling to the scintillator (e.g. direct or with optical fibres). Some typical parameters of x-ray detectors used in cone-beam CT systems are shown in Table 2.

Table 2. Comparison of parameters of the X-ray detectors used in Cone-beam CT (examples)

Mechanical filters

 
Similarly to the standard x-ray and CT systems various mechanical filters are used also in cone-beam CT systems to modify the spectra of the x-ray beam. The material of the filters is usually aluminium (Al), silver (Ag) or cerium (Ce) and their thickness goes from some tenth of a millimetre to some millimetre.
The aim of using filters is to remove the low energy part of the continuous spectrum of the x-ray source. These components are completely absorbed by the object, therefore they do not contribute to the detected signal, while they increase the dose to the object. Moreover after filtering the continuous spectrum of the x-ray source is getting closer to a spectrum with a single dominant energy, therefore it limits the beam hardening effect (see Beam-hardening correction in Section).

Collimators

 
Again similarly to other x-ray systems collimators are used to shape the x-ray beam in cone-beam CT. They limit the illumination of the object to the area which is projected to the active surface of detector, therefore reducing the dose to the object by eliminating the useless radiation.

Coordinate system of the cone-beam CT

 
The coordinate system of the cone-beam CT systems is shown on Figure 2.
The x-rays (represented by dotted lines) are originated from the focal spot of the x-ray tube (point S on the right), which is considered as an ideal point. The flat x-ray detector is located on the left at a D distance from the focal spot (focus-detector distance, FDD). During the acquisition the detector and the x-ray source are rotating together around the axis of rotation (AOR) located at a d distance from the focal spot (focus-axis distance, FAD). The z axis of the orthogonal coordinate system is usually parallel to the axis of rotation.

Figure 2 The coordinate system of the cone-beam CT system.

 
Due to the imaging geometry the x-rays coming from the focal spot are divergent. Therefore the image of the object placed around the axis of rotation is magnified on the active surface of the detector (Figure 2, grey area). The magnification in the axis of rotation (n) is equal to the ratio of the source - detector distance (D) to source - axis of rotation (d) distance:

 
If the source and the detector are mounted on a common framework – therefore the distance between them is constant – and this framework is moved relative to the axis of rotation in a way that the source getting closer to it, then even small movement can effectively increase the magnification, because D > d :

 
The field of view (FOV) of the system is defined as the size of the projection of the active area of the detector at the axis of rotation. The detectors are usually rectangular, therefore the size of the field of view can be defined in two directions: in the direction parallel to the axis of rotation it is called axial field of view (AFOV), while in the direction perpendicular to that it is referred as transaxial field of view (TFOV). The field of view inversely proportional to the magnification, thus higher the magnification smaller the region of the object projected to one imaging pixel of the detector, therefore the achievable spatial resolution is also higher.