Current automatic dose rate controls (ADRCs) of dynamic x-ray imaging systems adjust their acquisition parameters in response to changes in patient thickness in order to achieve a constant signal level in the image receptor. This work compares a 3 parameter (3P) ADRC control to a more flexible 5-parameter (5P) method to meet this goal. A phantom composed of 15 composite poly(methyl) methacrylate (PMMA)/aluminium (Al) plates was imaged on a Siemens Artis Q dynamic system using standard 3P and 5P ADRC techniques. Phantom thickness covered a water equivalent thickness (WET) range of 2.5 cm to 37.5 cm. Acquisition parameter settings (tube potential, tube current, pulse length, copper filtration and focus size) and phantom entrance air kerma rate (EAKR) were recorded as the thickness changed. Signal difference to noise ratio (SDNR) was measured using a 0.3 mm iron insert centred in the PMMA stack, positioned at the system isocentre. SDNR was then multiplied by modulation transfer function (MTF) based correction factors for focal spot penumbral blurring and motion blurring, to give a spatial frequency dependent parameter, SDNR(u). These MTF correction factors were evaluated for an object motion of 25 mm s and at a spatial frequency of 1.4 mm in the object plane, typical for cardiac imaging. The figure of merit (FOM) was calculated as SDNR(u)²/EAKR for the two ADRC regimes. Using 5P versus 3P technique showed clear improvements over all thicknesses. Averaged over clinically relevant adult WET values (20 cm-37.5 cm), EAKR was reduced by 13% and 27% for fluoroscopy and acquisition modes, respectively, while the SDNR(u) based FOM increased by 16% and 34% for fluoroscopy and acquisition. In conclusion, the generalized FOM, taking into account the influence of focus size and object motion, showed benefit in terms of image quality and patient dose for the 5-parameter control over 3-parameter method for the ADRC programming of dynamic x-ray imaging systems.

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This work describes a new approach to automatic dose rate control (ADRC) for dynamic x-ray imaging, utilizing a spatial frequency weighted signal difference to noise ratio (SDNR(u)). Three aspects of ADRC programming using SDNR(u), which contains information on the target material, velocity and size, were examined. First, whether SDNR(u) can be held constant at the requested level over some patient thickness range for five materials relevant to interventional imaging (iron, gadolinium, platinum, bismuth, and tantalum). Second, the efficiency of the new ADRC was compared to the current settings using a figure of merit (FOM), defined as SDNR(u)²/reference air kerma rate for iron and platinum, over a range of simulated patient thicknesses. Third, the ability of the new ADRC to optimize exposure parameters for iron, iodine, gadolinium, tantalum, platinum and bismuth was examined. A phantom of 20 mm PMMA and 2 mm Al sheets was used to simulate patient equivalent thicknesses between 25 mm to 375 mm. The relevant metal foil targets were placed at the phantom centre and imaged on a Siemens Artis Q cardio-angiography system. SDNR(u) and reference air kerma were measured, along with the FOM for the relevant conditions. The optimal exposure factor study was made for patient equivalent thicknesses of 100 mm, 200 mm and 300 mm. The new ADRC regulation held SDNR(u) constant versus phantom thickness within 5%, for the five materials studied. FOM increase compared to the current regulation used on the Artis Q ranged between 18% and 296% (averaged over all thicknesses), and depended on acquisition mode and material. Material optimization via the new ADRC increased FOM by 68%, 165%, 164% and 32% for gadolinium, tantalum, platinum and bismuth respectively, corresponding to potential dose savings of 40%, 62%, 62% and 24% for the same target SDNR(u). An SDNR(u) driven approach to the ADRC logic of dynamic imaging systems is a viable alternative to current programming, with a resulting improvement in imaging efficiency and corresponding dose reduction.

A substantial 77% reduction of DAP values and 64% and 66% reduction in organ dose on ovaries and uterus, respectively, was demonstrated with the new imaging platform, while maintaining optimal imaging quality and efficacy.

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