Photostimulable phosphor plate

A photostimulable phosphor (PSP) plate is a reusable, cordless intraoral receptor. It stores the X-ray exposure as electrons trapped in a phosphor layer, then gives that energy back as light when a laser scanner reads it, producing a digital radiograph. Bright light erases the plate for reuse. Light that reaches it before scanning removes signal.

Synonyms: PSP plate; phosphor plate; storage phosphor plate; storage phosphor receptor; PSP receptor

What is a photostimulable phosphor plate and how does it capture an image?

The plate is a polyester base coated with europium-activated barium fluorohalide crystals [2]. When the X-ray beam reaches it, electrons in the phosphor are excited and stay trapped in the crystal lattice [1]. That pattern of trapped charge is the latent image. You can't see anything yet. The plate holds an analogue latent image, and it becomes digital only during scanning [1].

In the scanner, a laser releases the trapped electrons. The phosphor gives off light in proportion to the radiation each point absorbed, and that light is collected and digitised into the radiograph [1][2].

Scanning doesn't wipe the plate completely. The leftover energy is cleared by a short exposure to bright, full-spectrum light, and the plate is repackaged and exposed again [1]. The same storage-phosphor principle runs computed radiography (CR), which Fuji introduced commercially in 1981. Intraoral plates are close relatives of the imaging plates used for CR in medical radiology [1][2].

Keep one idea from this section in mind for the rest of the page. Between exposure and scan, the image exists only as stored charge. Anything that releases that charge early takes part of the image with it.

How does a PSP plate compare with film and with a solid-state sensor?

The basic difference from a solid-state sensor is where the image goes after exposure.

Against film. Laboratory studies found digital intraoral systems as accurate as film for caries detection when a good-quality image is obtained [11]. That review dates from 2000. A 2010 systematic review adds two cautions about going digital: dose may not come down, and retakes and errors may go up [10]. The US radiation-control guidance lists no significant time saving for PSP over film [1].

Comfort. Sensors are generally reported as less comfortable than PSP plates or film [7][10]. The one clinical head-to-head in this set, on 58 patients, found no significant comfort difference between CMOS and PSP, with mean scores of 4.57 and 4.48 [12].

Image quality and speed. Here the studies disagree, and the disagreement follows the design:

• In a clinical study on patients, CMOS was faster and its images were rated higher than PSP [12].
• In a student study on a training replica, PSP images were of higher quality than CCD, and CCD was not significantly faster than PSP [13].
• In a student study on a phantom, PSP and CMOS were similar in quality, and PSP took 1.43 ± 0.28 min per radiograph against 1.52 ± 0.61 min for CMOS [14].

Sensor type (CCD or CMOS), who took the image and whether it was a phantom or a patient all differ between these studies. None of them settles which receptor gives the better image in general practice.

How do scanning delay and light exposure fade the image?

Two different things fade a PSP image.

Waiting to scan. The latent image decays between exposure and readout. Trapped electrons drift back to their ground state over time [2][18], and the introduction to one phantom study advises processing plates quickly for that reason [2]. One study's introduction cites crystals losing 23% of their trapped electrons after 30 minutes and 30% after 1 hour [2]. That is a secondary figure for an unspecified crystal, so read it as the direction of travel, not a rate for your scanner.

In a phantom study on Soredex Digora plates kept in their cover and stored under 150 lux light or in a dark room, density fell with delay in all storage conditions, dark room included, and fell significantly on plates scanned 10 minutes or more after exposure. The authors note this may not be detected clinically, and spatial resolution did not change significantly [2]. On 72 extracted molars imaged with DIGORA plates, occlusal caries detection was not significantly worse after a 10- or 30-minute delay, but was significantly worse after 60 and 120 minutes. The authors advise not delaying beyond 30 minutes on that system [9].

Light on an uncovered plate. This works in seconds. In a step-wedge study on two systems, VistaScan and Express, exposing plates to ambient light for 5 to 90 seconds before read-out increased brightness and contrast and reduced contrast-to-noise and signal-to-noise ratios. The longer the exposure, the worse the damage [3]. When observers judged skull radiographs from the same two systems, rejections rose mostly from about 10 seconds, and the authors propose 5 seconds as a safe limit for those systems [4]. On three systems (Express, VistaScan Mini and CS 7600), up to 10 seconds of ambient light did not compromise detection of simulated internal root resorption [5]. That result covers one diagnostic task, not every task.

You'll sometimes read that phosphor plates tolerate room light. For an uncovered plate, the two studies on VistaScan and Express plates say otherwise: damage starts within seconds [3][4]. The plates measured over minutes in the Digora phantom study were kept in their cover, under 150 lux light or in a dark room, while they waited [2].

What it looks like. The whole radiograph comes out too bright, or density is uneven where only part of the plate saw light. Fluctuating signal loss adds noise [7]. In a series of 50,000 intraoral radiographs from one institution, delayed-scanning artefacts were the most common scanning error, at 145 images, 39.9% of scanning errors [7]. A separate classification study found fading was the most common artefact in its ambient-light group, at 44.1% [8]. The higher the light intensity and the longer the exposure, the more plate information is lost [7].

How it differs from film fog. Radiographic fog runs the other way. On film, fog is unwanted density, measured as base-plus-fog density on film that was processed without exposure [16], and darkroom safelights and light leaks are common sources [15]. Light reaching an exposed PSP plate removes stored signal instead, so the radiograph comes out brighter [3]. Film fog adds density. PSP light damage takes it away.

How is light fade different from mechanical plate damage?

They are separate failure modes, and artefact classifications file them separately. One study of 5,000 intraoral images, 1,822 of which carried artefacts, sorted them into six groups by cause. Ambient-light errors are one group. Plate artefacts from physical deformation and contamination are another, and scanner artefacts a third [17]. An earlier classification also files ambient-light, plate and scanner artefacts in separate groups [8].

Fade is signal that went missing from this exposure. It shows up as brightness and noise across the image, or across the part of the plate that saw light [7]. The plate itself is erased and reused afterwards. Rough handling, heat and prolonged bright light can damage a plate, though [1].

Scratches are damage to the plate, and plates can pick them up in clinical use [10]. In the 50,000-radiograph series, scratches made up 19.6% of plate errors [7]. How a scratch looks on the radiograph, and why it stays, is covered in sensor dropout and scratched plate artifact.

White radiopaque lines have a third cause. In the same series, 136 images (37.2% of scanning errors) carried white lines attributed to dust or dirt on the scanner's opening or rollers [7].

What it's confused with

How to be sure

• Scan promptly. Density fell significantly on plates scanned 10 minutes or more after exposure in one phantom study [2], and caries detection held to 30 minutes but not 60 in another [9]. Both figures come from Soredex DIGORA plates.
• Keep exposed plates covered until they're in the scanner. The guidance says ambient light reaching an exposed plate degrades the image [1]. Uncovered, two studies on VistaScan and Express plates measured damage in seconds [3][4].
• Don't count on software to rescue a light-struck image. Enhancement restored 23.48% of brightness on VistaScan and below 1% on Express [6]. Light exposure can mean a retake and an extra exposure for the patient [6].

The available guidance is qualitative [1]. The seconds-and-minutes figures above are study findings on specific systems, not professional limits.

Common errors

• Treating PSP plates as insensitive to room light. In two studies on VistaScan and Express plates, uncovered plates lost image quality within seconds [3][4].
• Reading a bright, noisy PSP radiograph as fog. Fog is added density on film. Light damage on a PSP plate goes the other way [3][15][16].
• Blaming the plate for a white line. Dust or dirt in the scanner is a common cause [7].
• Counting on software enhancement. How much enhancement recovers depends on the system, and on both systems tested it recovered only part of the damage [6].