Image Acquisition

      Image Acquisition

Technology overview

There are a number of technologies used for digital imaging in planar radiography. They can be divided into CR and DDR.

CR is, in first appearance, similar to the use of a film/screen system. The CR plate is in a cassette, which will fit the table and vertical Bucky trays and can be used with mobile equipment.
The plate is then scanned in a reading system similar in size to a daylight processor. This therefore makes the change to digital radiography easier.

A DDR system entails more changes in X-ray couch and vertical Bucky design and often changes to the X-ray tube assembly. Unlike the removable CR cassette, the DR plate or detector is fully integrated into the exposure equipment. The patient is radio-graphed and the image appears on the acquisition workstation in a few seconds. Here, the image can be optimized and then sent for reporting or repeated if necessary.

Computed radiography technology

The active phosphor layer of a CR plate usually comprises a layer of europium-doped barium fluoro-bromide, which is coated on to a semi-rigid or flexible polyester base. X-ray photons are absorbed by the phosphor layer, and the phosphor electrons become ‘excited’ and are raised to a higher energy level, where they can stay trapped in a semi-stable higher-energy state. The trapped
electrons represent a latent image in the phosphor plate in the form of ‘stored energy’. The stored energy can be released by adding energy to the trapped electrons. This is done by stimulation with a laser beam. The trapped electrons then ‘escape’ from the traps to fall back to their equilibrium state. As they fall back, the electrons release energy in the form of light. This phenomenon is otherwise known as photostimulable luminescence (PSL). The emitted light intensity is proportional to the original X-ray intensity. The light energy is detected and the signal is digitized. These data are processed digitally to produce a visible diagnostic image on a monitor. The phosphor plate is then ‘erased’ with
a bright white light to remove any remaining trapped electrons, and the plate is then ready for the next examination.

Digital radiography technologies

The main detector technologies used in digital radiography are:

• X-ray scintillator bonded to a read-out array (amorphous silicon photo-diode/thin-film transistor (TFT) array) or coupled to a charge-coupled device (CCD);
• X-ray detector of amorphous selenium bonded to a TFT readout array.

Both types can be constructed in the form of a flat panel.

Scintillator detector

The X-ray detector is normally a scintillator of thallium-doped CsI(Tl) crystals, although other phosphors such as Gd2O2S are also used. The scintillator converts the X-rays into a light output. The CsI has a columnar crystal structure that guides the light to the read-out device, which allows the CsI to be thicker than a phosphor powder without significantly increasing unsharpness. As with phosphors in film cassettes, thinner powder phosphors (such as Gd2O2S) will have lower unsharpness. Gd2O2S phosphors are thinner than CsI scintillators, but they have higher conversion efficiency.

Scintillators are usually coupled directly to an amorphous silicon
photodiode TFT flat-panel read-out array. The light from the scintillator is converted into electrical charge in a photodiode array, which stores the charge until it is read out from each of the pixels. These are commonly referred to as amorphous silicon systems.

Charge-coupled device

The light output from the scintillator detector can be read out by a CCD camera. The CCD is generally smaller than the phosphor, and so it is usually coupled using a lens or fibre-optic bundles. Demagnification may be necessary, and this can result in a loss
of sensitivity if the demagnification is high.

Amorphous selenium/thin-film transistor flat-panel detector

The detector consists of a layer of amorphous selenium with a matrix of electrodes on each face. The X-ray energy produces electron-hole pairs in the selenium layer, which are attracted towards the electrodes by an electric field. The charge is collected and read out using a TFT array. The resolution of this type of detector is better than that using a phosphor due to the absence of light scattering.

Scanning technology

An alternative detection method for covering the full image area is to use slot-scanning technology. A linear array of detectors scans across the patient in conjunction with a narrow-fan X-ray beam. This method may result in good scatter rejection and

contrast differentiation, but it has a number of disadvantages, including a long exposure time and high tube loading. Also, the alignment of the scanning radiation beam and the detectors requires tight mechanical tolerances and mechanical stability of
the scanning mechanism.