Digital Imaging Systems: Detectors, Processing, and Display

How Digital X-Ray Systems Work

Modern radiographic departments rely almost entirely on two types of digital imaging: computed radiography (CR) and digital radiography (DR). Both replace traditional film-screen combinations, but they capture, process, and display the x-ray image in very different ways. Understanding these differences is essential for producing diagnostic-quality images and for practicing safely.

Computed Radiography: The Imaging Plate

A CR system uses a reusable imaging plate (IP) housed inside a cassette that looks similar to a conventional film cassette. The plate is coated with a photostimulable phosphor (PSP), most commonly barium fluorohalide with europium (BaFBr:Eu²⁺). When x-rays strike the plate, electrons in the phosphor are excited and become trapped in higher-energy states, forming a latent image stored as a pattern of energy within the crystal structure.

To read the image, the cassette is fed into a CR reader. Inside the reader, a focused laser beam scans across the plate row by row. The laser light releases the trapped electrons, which fall back to their ground state and emit visible light — a process called photostimulated luminescence (PSL). The intensity of the emitted light at each point is proportional to the x-ray exposure received at that point. A photomultiplier tube (PMT) converts the light signal into an analog electrical signal, which an analog-to-digital converter (ADC) then transforms into a numerical (digital) value. These values are assembled into a digital matrix — the final image. After reading, the plate is exposed to bright white light to erase any residual signal, making it ready for reuse.

Key safety point: CR plates must be erased before each use. A plate that is not fully erased can carry ghost images (called image lag or ghosting) from a previous exam, potentially obscuring pathology on the new image.

Digital Radiography: Flat-Panel Detectors

DR systems use a flat-panel detector (FPD) built into the table, wall stand, or a portable unit. The FPD converts x-ray energy directly into a digital signal — no cassette handling, no plate reader. There are two conversion pathways:

Indirect conversion is the more common type. X-rays strike a scintillator layer (typically cesium iodide, CsI, or gadolinium oxysulfide). The scintillator converts x-ray energy into visible light, which then strikes an array of photodiodes coupled to thin-film transistors (TFTs). Each photodiode generates an electric charge proportional to the light it received; the TFT array reads out each pixel's charge and sends it to the ADC.

Direct conversion detectors use a layer of amorphous selenium (a-Se) instead of a scintillator. X-rays interact directly with the selenium layer, freeing electron-hole pairs that migrate under an applied electric field to TFT collection electrodes. Because there is no intermediate conversion to light, fewer steps mean less image blur, and spatial resolution can be very high.

In both DR types, the detector reads out the image in seconds and sends it to the PACS workstation — a major workflow advantage over CR.

Image Processing and Display

Once data are digitized, the acquisition workstation applies preprocessing (flat-field correction, bad-pixel masking) and then post-processing (edge enhancement, contrast adjustment, noise reduction). The final image is displayed on a high-resolution diagnostic monitor. Radiographers can adjust window width and window level to optimize the displayed contrast, but these adjustments affect only the display — they do not change the actual dose the patient received.

Safety point: The ability to manipulate the image after acquisition can mask a serious exposure error. An image that is ten times overexposed may look acceptable on screen after rescaling, but the patient received unnecessary radiation. This is why exposure indicators must be checked on every image, independent of how the image looks.

Exposure Indicators: Measuring Receptor Dose

Every digital system generates an exposure indicator (EI) — a numerical value that reflects the amount of radiation that reached the detector after the patient exposure. The International Electrotechnical Commission (IEC) established a standardized framework (IEC 62494-1) that defines:

• EI (Exposure Index): The actual exposure the detector received.
• EI_T (Target Exposure Index): The facility-established target for a given exam.
• DI (Deviation Index): A log-based score comparing EI to EI_T. A DI of 0 means the exposure matched the target. Positive DI indicates overexposure; negative DI indicates underexposure. A DI between −1 and +1 is generally considered acceptable.

Before IEC standardization, manufacturers used proprietary indicators with different names and scales (for example, Fuji used an "S number" where a higher number meant less exposure, the opposite of IEC EI). Always consult the manufacturer's exposure index reference guide for the specific equipment in your facility.

Clinical practice: Check the EI and DI on every image before releasing the patient. If the DI is outside the acceptable range, determine whether technique, positioning, or collimation caused the deviation, then decide whether a repeat exposure is clinically justified — always weighing the diagnostic benefit against additional patient dose.