Microdose
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Microdose refers to a category of radiology modalities providing very low radiation dose. These modalities are typically based on a photon-counting detector for capturing the image.
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[edit] Photon-counting Detector
A photon-counting detector utilizes the discrete nature of X-rays, as it counts individual photons when they are captured in a detector medium. This is in contrast to “pseudo-digital” detectors that integrates photon energy over time, thus in practice measuring an analog signal that in turn is digitized. While the conversion steps from analog to digital signal in a traditional detector introduces thermal noise, quantization errors and loss of information during signal processing, a truly digital detector suffers from none of these weaknesses. A photon-counting detector is capable of capturing close to all information in the signal. This way it is possible to lower the radiation dose to a minimum while preserving or even improving image quality.
[edit] Uses
There are today very few commercially available Microdose modalities. The fist to hit the market was a mammography system from Medical imaging supplier Sectra, mainly targeted for intense breast screening programs. Breast examinations is one of the most demanding with respect to image quality, as early detection of cancerous changes relies on finding micro-calcifications in the breast tissue. These tiny structures can be perceived by a trained eye, but only as far as the image quality allows. At the same time, the significance of low dose comes particularly apparent in screening programs, as a large part of a population is targeted.
The Sectra system uses silicon as detector medium, where a large array of very thin silicon sticks constitutes the pixels in the photon-counting detector. Another mammography system under development by XCounter uses noble gas contained in thin glass tubes as detector medium.
[edit] References
- M. Lundqvist, B. Cederström,V. Chmill, M. Danielsson and B. Hasegawa, “Evaluation of a photon-counting x-ray imaging system”, IEEE Transactions on Nuclear Science, Vol 48 (4), pp. 1524-1529, 2001
- M. Danielsson, H. Bornefalk, B. Cederström, V. Chmill, B. Hasegawa, M. Lundqvist, D. Nygren and T. Tabár, “Dose-efficient system for digital mammography”, Proc. SPIE, Physics of Medical Imaging, vol. 3977, pp. 239-249 San Diego, 2000
- M. Lundqvist, M. Danielsson, B. Cederström, V. Chmill, A. Chuntonov, and M. Åslund, “Measurements on a full-field digital mammography system with a photon counting crystalline silicon detector”, Proc. SPIE, Physics of Medical Imaging, vol. 5030, pp. 547-552, San Diego, 2003
- B. Hemdal, L. Herrnsdorf, I. Andersson, G. Bengtsson, B. Heddson, M. Olsson, “Average glandular dose in routine mammography screening using a Sectra MicroDose Mammography unit”, PMID: 15933152, PubMed, 2005
