A search for improved technique factors in paediatric fluoroscopy

Abstract

A Monte Carlo computational model of a fluoroscopic imaging chain was used for deriving optimal technique factors for paediatric fluoroscopy. The optimal technique was defined as the one that minimizes the absorbed dose (or dose rate) in the patient with a constraint of constant image quality. Image quality was assessed for the task of detecting a detail in the image of a patient-simulating phantom, and was expressed in terms of the ideal observer's signal-to-noise ratio (SNR) for static images and in terms of the accumulating rate of the square of SNR for dynamic imaging. The entrance air kerma (or air kerma rate) and the mean absorbed dose (or dose rate) in the phantom quantified radiation detriment. The calculations were made for homogeneous phantoms simulating newborn, 3-, 10- and 15-year-old patients, barium and iodine contrast material details, several x-ray spectra, and for imaging with or without an antiscatter grid. The image receptor was modelled as a CsI x-ray image intensifier (XRII). For the task of detecting low- or moderate-contrast iodine details, the optimal spectrum can be obtained by using an x-ray tube potential near 50 kV and filtering the x-ray beam heavily. The optimal tube potential is near 60 kV for low- or moderate-contrast barium details, and 80-100 kV for high-contrast details. The low-potential spectra above require a high tube load, but this should be acceptable in paediatric fluoroscopy. A reasonable choice of filtration is the use of an additional 0.25 mm Cu, or a suitable K-edge filter. No increase in the optimal tube potential was found as phantom thickness increased. With the constraint of constant low-contrast detail detectability, the mean absorbed doses obtained with the above spectra are approximately 50% lower than those obtained with the reference conditions of 70 kV and 2.7 mm Al filter. For the smallest patient and x-ray field size, not using a grid was slightly more dose-efficient than using a grid, but when the patient size and field size were increased a fibre interspaced grid resulted in lower doses than imaging without a grid. For a 15-year-old patient the mean absorbed doses were up to 40% lower with this grid than without the grid.

Implementation of a channelized Hotelling observer model to assess image quality of x-ray angiography systems

“…It is suggested that the minimum number of images used should be at least 10 times the number of independent channels to provide a reliable estimate of the covariance matrix. 19 With 96 unique channels, at least 960 images are required to meet this recommendation. The frame rate of the system was set to 30 fps and four series of 10 s were saved.…”

“…6,10,11 Additionally, application of linear observer models to fluoroscopic and angiographic imaging systems has been investigated. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] However, the vast majority of the reports feature investigations on simulated data. [13][14][15][16][17]19,[21][22][23][24][25] Although simulated data have significant merits for system design, data acquired from real imaging systems are required to assess real-world system performance.…”

Image Quality and Dose

“…The assessment of image quality, including the aspect of minimising radiation exposure, is often carried out with the help of the concept of an image-quality figure (Gajewski and Reiss 1974;Thijssen et al 1989;Jennings et al 1993;Aichinger et al 1994;Tapiovaara et al 1999), which is sometimes derived from the SNR. Some examples are given in the following paragraphs.…”

Optimisation of Image Quality and Dose

“…Tapiovaara et al have used the IQF to determine improved technique factors for paediatric fluoroscopy. The optimum technique was defined as the one that minimises the absorbed dose in the patient with a constraint of constant image quality (Tapiovaara et al 1999). …”