10.1 CR vs DR Systems

Two Roads to a Digital Image

Radiographic imaging has moved almost entirely off film. Every modern department captures the latent image electronically using one of two receptor families: computed radiography (CR) and digital radiography (DR). Both replace the film-screen cassette with a device that converts absorbed x-ray energy into a digital signal, but they do it in fundamentally different ways. The ARRT Image Production domain expects you to know the physical mechanism, the workflow, and the practical trade-offs of each.

CR is often called cassette-based digital imaging because it preserves the familiar film-room workflow: expose a plate in a cassette, then carry it to a reader. DR is cassette-less (or, for portable wireless panels, functionally self-reading). Understanding how a photostimulable phosphor stores a latent image versus how a flat-panel detector reads one out in real time is the single most testable idea in this section.

Computed Radiography (CR)

CR uses an imaging plate coated with a photostimulable storage phosphor (PSP) - typically a barium fluorohalide doped with europium (for example, barium fluorobromide). When x-rays strike the plate, electrons in the phosphor are raised to higher-energy traps, where they are held as a stored latent image. The plate produces no visible picture on its own; the energy simply sits in the trapped electrons until the plate is processed.

The exposed plate is carried to a CR reader (plate reader). Inside, a finely focused helium-neon or solid-state laser scans the plate line by line. The laser light frees the trapped electrons, which drop back to their ground state and emit visible light. This released light is photostimulated luminescence (PSL). A light guide collects the PSL, a photomultiplier tube (PMT) amplifies it, and an analog-to-digital converter turns the varying light intensity into pixel values. Higher x-ray exposure at a point means more trapped electrons, more PSL, and a brighter digital signal there.

After reading, the plate passes under a bright erasure lamp that floods it with intense light, emptying any residual trapped electrons so the plate can be reused. Skipping or shortening this erase cycle leaves a faint ghost of the prior image - a classic CR artifact. Because one reader can service many plates, CR is inexpensive to deploy and drops into existing Bucky trays and mobile units, which is why it dominated the first wave of digital conversion.

The trade-off is workflow speed and dose efficiency. Each plate must be physically transported to a reader, and CR generally has lower detective quantum efficiency (DQE) than DR, meaning it needs somewhat more exposure to produce equivalent image quality.

Digital Radiography (DR)

DR uses a flat-panel detector that reads itself out electronically within seconds - no plate, no separate reader. The image appears at the acquisition workstation almost immediately, which is why DR dominates high-volume rooms and trauma settings. DR panels come in two designs: indirect and direct.

Indirect-Conversion DR

An indirect-conversion detector converts x-rays in two steps. First, a scintillator - commonly cesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S) - absorbs the x-ray photon and emits visible light (this is the indirect step: x-ray to light). That light then strikes a photodiode array built on a thin-film transistor (TFT) active-matrix layer, where it is converted to electrical charge and read out pixel by pixel. Cesium iodide is often grown in needle-like crystals that channel light toward the photodiodes, limiting lateral light spread and preserving spatial resolution.

Direct-Conversion DR

A direct-conversion detector skips the light step. A photoconductor - almost always amorphous selenium (a-Se) - absorbs the x-ray and converts its energy directly into electrical charge. An applied voltage draws that charge straight down to the underlying TFT array with very little lateral spreading, giving direct systems excellent inherent sharpness. There is no scintillator and no intermediate visible light.

CR vs DR at a Glance

Common Traps and Clinical Points

Several points recur on the exam. First, CR is not real-time - it always requires a reader, so a wireless flat panel is DR, not CR. Second, the photostimulable phosphor is the defining CR feature; if a question describes a laser releasing stored energy from a plate, it is CR. Third, amorphous selenium equals direct DR and a scintillator (CsI or GdOS) equals indirect DR; ARRT loves to swap these. Fourth, both CR and DR have wide exposure latitude, so under- and over-exposures both still render a diagnostic-looking image after processing - which is exactly what makes the exposure monitoring covered in the next section so important. Finally, CR plates are prone to moire grid-line aliasing when a stationary grid frequency interacts with the laser scan; using an appropriate grid frequency or orientation avoids it.