Introduction
This topic covers how digital X-ray images are produced, how their quality is determined, and how the resulting images are stored, sent between devices and read on screen. It also explains the DICOM standard that makes image exchange possible and the PACS systems that manage images in a hospital. Knowing these elements together is what examiners expect when you describe a modern radiology department.
Principles of digital image acquisition
In digital radiography the X-ray beam that has passed through the patient is captured by an electronic detector instead of photographic film. The detector converts the transmitted radiation into an electric signal, which is digitised by an analogue-to-digital converter and stored as a matrix of pixels. Two main technologies are used: computed radiography (CR) and digital radiography (DR).
Computed radiography (CR)
- A reusable photostimulable phosphor imaging plate (commonly europium-activated barium fluorohalide) is placed in a standard cassette and exposed like conventional film.
- Absorbed X-ray energy is stored as a latent image in trapped electrons within the phosphor.
- The plate is read in a separate scanner, where a laser stimulates the phosphor and the emitted light is collected and digitised.
- The plate is then erased by strong light and reused. The cassette system keeps the existing workflow, but reading takes time and the plate is not an immediate detector.
Digital radiography (DR) with flat-panel detectors
- Indirect conversion: a scintillator (usually caesium iodide or gadolinium oxysulfide) converts X-rays into visible light, which is detected by an array of photodiodes with thin-film transistors.
- Direct conversion: a photoconductor such as amorphous selenium converts X-ray photons directly into electric charge, which is collected by the transistor array.
- The image is available within seconds and can be viewed immediately, which is important for emergency and bedside work.
- Before display, the raw signal undergoes offset and gain correction and correction of defective pixels, followed by image processing such as contrast adjustment and noise reduction.