Layers of PSPs
Photostimulable phosphor (PSP) Imaging plate has six main layers. These layers include the protective, phosphor, reflective, conductive, color, support, and backing layers (Radiology Key (a). The protective layer is a thin clear, thin and tough plastic protecting the phosphor layer. The phosphor layer is the active layer. It is the layer that traps electrons when the film is exposed to radiation. The layer may also contain color to absorb the stimulating light and prevent its spread. The reflective layer sends light forward when released. The layer is black to reduce the spread of the light used for simulation. The conductive layer is responsible for reducing and absorbing static electricity. Newer layers have color layers which are located between the support and the support layer, and it is responsible for stimulating light. However, the emitted light is reflected. The support layer, which is a semi-rigid material provides the imaging sheet with strength. Lastly, the backing layer protects the back of the film.
Sensitivity of PSPs
Imaging plates are more sensitive to radiation than radiographic film because they absorb more low energy radiation. The reason for the high sensitivity of imaging film is that they are covered in photostimulable phosphors (PSP) material. While using PSP films, the patient is exposed in the same way it is done in radiography. The patient is positioned with the body part aligned with the image receptor using the required positioning techniques. During exposure, the radiologist ensures the right combination of distance, milliamperage and kilovoltage. The difference is in the way exposure is recorded. PSP is more sensitive to radiation the beam interacts electrons contained in the imaging plate (Radiology Key (a). The interaction stimulates energy in electrons within the PSP crystals and traps them in the phosphor center. The trapped signal remains for days. These trapped electrons are often few due to the little radiation such that the subsequent exposures are not affected.
CR and DR image Receptors
There are two main types of digital image receptors used in radiography: digital radiography (DR) and computed radiography (CR) image receptors (IR). The difference between the two IRS is in how they are constructed and acquire latent images. Once the raw data is digitalized, and the latent image is acquired, the image display and processing is the same irrespective of the type of IR used. CR image receptors are either fixed or portable. It includes a cassette that holds the imaging plate. The radiation from the patient interacts with the imaging plate.
Consequently, the phosphor absorbs the photon intensities. Some of the absorbed light comes from the visible light; however, the phosphor stores most of the energy in the form of a latent image (Radio Key (b). On the other hand, DR IRs employ a self-scanning readout method and have several x-ray detectors to receive the radiation and convert intensities to proportional electronic signals which are digitized. Compared to CR, which have a two-step acquisition, DR combines image readout and image capture. Therefore, DR images are instantly available after the exposure. Nonetheless, DR receptors are more expensive and fragile compared to CR IRs.
How the Design of a TFT FPD Affects its Performance Measurements
The design of a TFT FPD is crucial in ensuring correct measurements. The design determines the characteristics that will be beneficial in converting analog data to digital—therefore enabling the easy and instantaneous calculation of the light intensities averages of neighboring pixels. Hence it becomes easier to modify image resolutions. Besides the design determines if the users can directly sum up signals coming directly from various pixels located in the same node (Salahuddin et al. 468-469). Therefore allowing users to make light intensities summations of close pixels at the time of reading.
Radio Key (b). “Image Receptors and Image Acquisition”. Radiology Key, 2016, https://radiologykey.com/image-receptors-and-image-acquisition/.
Radiology Key (a). “Photostimulable Phosphor Image Capture”. Radiology Key, 2016, https://radiologykey.com/photostimulable-phosphor-image-capture/. Accessed 13 Oct 2020.
Salahuddin, Nur Sultan, et al. “Design of Thin-Film-Transistor (TFT) arrays using current mirror circuits for Flat Panel Detectors (FPDs).” arXiv preprint arXiv:1105.1407 (2011).