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Hospital Healthcare Europe
Hospital Healthcare Europe

Designing a DR system using the ALARA philosophy

Paolo Labrini
1 January, 2008  

Paolo Labrini
Project Leader
Italray srl

When the Italray research team started the development of the X-Frame DR family of digital ­radiography (DR) systems, from the very beginning one of the main goals was to follow the ALARA (as low as reasonably achievable) dose management philosophy and to take advantage of the dose reduction opportunities that solid-state flat-panel detectors provide in many ways. The ALARA philosophy was followed throughout the imaging chain: from image acquisition to postprocessing, from patient and user interfaces to workflow enhancement and to end-user training.

Flat-panel detector with high sensitivity
The flat-panel detector employed in Italray DR systems (Trixell Pixium 4600) guarantees an ­extremely high sensitivity and detection quantum efficiency (DQE). This is of course the starting point for the entire dose reduction strategy. In practical terms this translates in an equivalent-detector film speed greater­ than 800, and this means that, compared with the equivalent film speed of computed radiography ­systems, which is ­typically around 200, the dose is potentially reduced by a factor of four thanks to the sole flat-panel detector.

Pendulum moving-grid system (InstaFocus)
The majority of DR systems on the market today employ, for a number of technical and practical ­reasons, fixed high-density antiscatter grid systems (up to 200 lp/inch). Unfortunately, this high strip density means that, in addition to the scatter radiation, the grid will also absorb a substantial amount of nonscatter radiation (ie, radiation that carries diagnostic information), thus unavoidably increasing the required dose. Furthermore, high-density grids have a fairly strict focal distance tolerance, and this forces the operator to change grid when switching between different examinations. Italray DR systems, however, employ a low-density moving-grid system, which not only guarantees a lower absorption rate, and hence a lower dose, but also makes it possible to use the same grid for virtually all focal distances. In addition to this, thanks to the special pendulum grid movement, the grid is always 100% in focus (with the traditional linear movement the grid is 100% in focus only in the centre), thus guaranteeing a higher image quality and a lower unwanted absorption that translates in an additional geometrical dose reduction of up to 18%. Also, using a carbon-interspaced grid provides a further 40% dose reduction compared with traditional Al-interspaced grids.

Anatomical programme organisation
All X-ray parameters (eg, kV, mA, ms) and collimation values (using an automatic collimator) are organised in predefined anatomical programmes, with four different patient sizes (including paediatric, with a special collimation mode for pediatric chest examinations). The anatomical programmes are the result of a substantial laboratory and real-world experience and have benefited from the crucial feedback of ­customers and reference sites; the main goal is always to achieve diagnostic image quality with the ­minimum dose necessary. X-ray parameters, together with dose-area product readings, are always stored in the image DICOM header for data analysis and review purposes, and a validation tool is always available to verify dose levels in selectable regions of interest. The system is equipped with an automatic exposure control system, totally integrated within the anatomical programmes, which is calibrated to cut X-ray emission when the detector has received the minimum necessary dose for a diagnostic-quality image.

Postprocessing: everest-X
The everest-X image-processing engine employs a hierarchic image enhancement strategy that is ­precisely tuned for each individual examination and projection, and substantially increases image ­quality and latitude. The main everest-X objective is image quality, but by enhancing image clarity and contrast, the employed algorithms also manage to compensate for lower doses (mAs) and yield diagnostic image quality at even lower dose levels.

Workflow: first is good
The system graphical user interface (GUI) and operation scheme have been designed after a thorough ­analysis of the typical X-ray department workflow, thus allowing operators to focus on the ­examination and not on the system itself. Furthermore, thanks to the detector’s very high dynamic range and image-­processing engine, diagnostic image quality is consistently guaranteed, with very high X-ray ­parameters ­tolerance. This means that retakes are virtually eliminated, which means a further average dose ­reduction.

Dose creep and the necessity of customer support
Paradoxically, the very high detector dynamic range is the cause of a phenomenon known in literature as “dose creep”: since detector overexposure is almost impossible, if images are taken with higher than ­necessary doses, the main result is a reduction of quantum noise, which means that images are “­prettier” to look at while providing no extra diagnostic content. So, in some instances, it has been found that operators tended to drift towards higher dose levels with the impression of gaining extra diagnostic content. In reality there was no extra diagnostic content, and the main result was only a higher patient dose. This phenomenon needs to be addressed with a comprehensive customer training programme and by ­working together with key reference sites to avoid unnecessarily high patient doses.

All the mentioned elements collectively contribute to a systematic dose reduction. It is, however, of ­crucial importance to support end-users with thorough training and support in order to take ­maximum advantage of these new technologies and fine tune the system to perfectly match the local X-ray ­department’s needs.


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