CR method lowers radiation dose, improves image quality

By Erik L. Ridley, staff writer

July 26, 2011 -- Computed radiography (CR) detector technology that uses a storage phosphor plate with needle-shaped crystals can yield lower radiation dose and better image quality in knee imaging than conventional storage phosphor plate methods, according to research published in the August 2011 issue of the American Journal of Roentgenology.

In a study involving 30 patients with knee pain, researchers from the Medical University of Vienna found that the needle-structured image plate produced higher-quality images at 75% of the radiation dose, compared with a conventional powder-structured image plate. And they believe even more radiation dose savings may be possible.

"Our results indicate that in knee radiography the new [needle-structured image plate] storage phosphor system offers a potential dose reduction of up to 50%, depending on the anatomic structure being imaged and the clinical question," wrote the team led by Dr. Michael Gruber. "Compared with a conventional [powder-structured] phosphor screen system, a [needle-structured image plate] storage phosphor system has higher [detective quantum efficiency], resulting in superior depiction of high- and low-contrast structures even when images are obtained with 50% of the reference dose."

Current CR systems frequently use storage phosphor plates with photostimulable crystals in a powdery inordinate structure. However, CR readers with powder-structured image plates suffer from lower spatial resolution compared to conventional film-screen systems, according to the researchers.

One approach that aims to improve upon this weakness is a needle-structured image plate, which modifies the structure of the photostimulable phosphor crystals to arrange them in an array of needles; these needles act as light guides and reduce light scatter in the phosphor screen. A needle phosphor also yields quantum efficiency gains, owing to the increased thickness and elimination of the binder, according to the authors.

Seeking to assess the potential for radiation dose reductions in knee imaging from the use of the needle-structured image plate detector technology, the study team obtained anteroposterior knee radiographs on a reader (DX-S, Agfa HealthCare) using a needle-structured plate and also a system (ADC Compact Plus, Agfa) employing a powder-structured plate in 30 consecutive patients (AJR, August 2011, Vol. 197:2, pp. W318-W323).

Images were produced using the powder-structured plates at a standard dose (8 mAs, measured dose = 300 µGy). The needle-structured image plate was used to produce images at the standard dose, as well as at 75% (6.3 mAs, measured dose = 235 µGy) and 50% (4 mAs, measured dose = 154 µGy) of the standard dose.

All images were postprocessed using the Musica2 (Agfa) software, and they were presented in random order in a side-by-side fashion on an Impax PACS (Agfa). Asked to determine their rating scale for each image comparatively, six observers independently assessed the visibility of specific anatomic structures using a 10-point rating scale, ranging from 10 (very high image quality, optimal for diagnosis) to 1 (very poor image quality, not acceptable for diagnosis).

The structures included those in relatively low-attenuation areas of the knee region such as the transition from cortical to trabecular bone at the distal metaphysis of the femur and the bone structure in the proximal epiphyses of the tibia. Those in relatively high-attenuation areas included soft tissue at the upper part of the images at the middle third of the femur, the patella, and the transition from cortical to trabecular bone at the upper margin of the images.

Three of the six observers were board-certified radiologists with seven, 10, and 12 years of experience, respectively, in musculoskeletal radiography, while the other three were residents with three, four, and five years of experience, respectively, in digital musculoskeletal radiography, according to the researchers.

The following mean rating scores were obtained for evaluation of anatomical landmarks:

  • Powder-structured image plate (standard dose): 6.01
  • Needle-structured image plate (standard dose): 6.97
  • Needle-structured image plate (75% dose): 6.48
  • Needle-structured image plate (50% dose): 5.47

"When obtained at the same dose, [needle-structured image plate] images outperformed [powder-structured image plate] images with regard to delineation of all anatomic landmarks regardless of whether the landmarks were located in a high- or low-attenuation area," the authors wrote. "The [needle-structured image plate] images obtained at about 75% of the standard dose were also ranked as being superior to standard-dose [powder-structured image plate] images for the delineation of all anatomic landmarks except the transition from cortical to trabecular bone of the femur at the upper margin of the images and the patella."

Later in a second reading session, readers scored the visibility of preselected abnormalities using a 10-point scale ranging from 10 (very good) to 1 (poor).

Mean rating scores for evaluation of abnormalities were as follows:

  • Powder-structured image plate (standard dose): 6.30
  • Needle-structured image plate (standard dose): 7.16
  • Needle-structured image plate (75% dose): 6.70
  • Needle-structured image plate (50% dose): 5.83

"For the entire group of abnormalities, [needle-structured image plate] images were superior to [powder-structured image plate] images at the same dose level and at about 75% dose and were inferior to [powder-structured image plate] images when obtained at halfdose," the authors wrote.

The researchers noted that further comparison studies are needed to evaluate the performance of the new detector system and its potential to reduce radiography dose in other areas of the body, such as the spine or chest.

Copyright © 2011

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