Novel CR chemistry cuts dose, sharpens image

January 9, 2006

Manufacturers have for years tweaked the lasers and optics that record radiographic data from plates. But only recently have they substantially reformulated the plates themselves. Recent developments shown at the 2005 RSNA meeting may give computed radiography an edge over digital radiography.

Manufacturers have for years tweaked the lasers and optics that record radiographic data from plates. But only recently have they substantially reformulated the plates themselves. Recent developments shown at the 2005 RSNA meeting may give computed radiography an edge over digital radiography.

Agfa and Fujifilm have each created new plate chemistries, fashioning a new type of storage phosphor into novel shapes that enhance performance. Agfa crafts its storage phosphor into needles, while Fujifilm uses columns.

Agfa's new DirectriX plate technology is built into its DX-S product line, shown at the RSNA meeting but released in mid-October at the Journeés Françaises de Radiologie 2005 conference (DI SCAN 10/24/05). This phosphor technology provides diagnostic performance comparable to previous CR plates but requires only half the radiation exposure, according to a study presented at the RSNA meeting.

Researchers at the University of Munich led by Dr. Markus Koerner compared plain chest x-rays obtained with conventional powder-based phosphor plates at standard dose with follow-up studies performed with Agfa's needle phosphor technology in the same patients. The study involved 24 ICU patients. Koerner documented a 50% radiation dose reduction in studies done with needle-phosphor plates.

The shape of the phosphor allows Agfa to use a thicker coating on the plate, which improves x-ray absorption. This boosts the overall detective quantum efficiency (DQE) and may increase radiographic efficiency of the newly developed plates. The needle shape of the phosphor may also contribute to efficiency.

After exposure to x-rays, storage phosphor plates are fed into a CR reader, where they are stimulated by laser. Phosphor needles absorb and then emit light in a specific direction, whereas powder phosphor emits light in all directions. The preferential emission of light achieved with needle phosphor may account for the increased efficiency of the newly developed plates. It also increases the sharpness of the images. The needles make the plates easier to erase, according to the company.

The phosphor-based detector developed by Fujifilm uses a columnar fiber crystal structure. The advanced phosphor plate, which will be built into Fuji's upright and table configurations later this year, pending FDA clearance, increases DQE, according to the company.

The two companies fashioned the novel imaging plates from a new type of phosphor called cesium bromide. A cousin to this phosphor, cesium iodide, is widely used in image intensifiers and as the scintillator in DR detectors using amorphous silicon. Cesium iodide, however, is not a storage phosphor. When struck by x-rays, it emits light immediately.

Cesium bromide represents the best of both worlds: It is a highly efficient producer of light that stores x-ray energy rather than immediately translating it into photons.

The plate reformulations are critical to the improved performance of their respective systems, but both Agfa and Konica Minolta went beyond chemistry in the development of their new products.

Agfa developed an optical system that reads the plates line by line rather than point by point, as is usually done in CR. This linear system, called ScanHead, uses a string of laser diodes to stimulate the phosphor and a matched line of CCDs to record the emitted light.

Fuji has developed similarly advanced optics, called HD LineScan, a subsystem that also reads the plate line by line. The technology uses multiple laser diodes coupled with high-capacity CCDs to capture whole lines of data. The company has also modified the imaging plate to include a thicker than usual coating of phosphor so as to accommodate faster scanning and erasure.