A study of the impact of x-ray tube performance on angiography system imaging efficiency.

This work compared the impact of x-ray tube performance and automatic dose rate control (ADRC) parameter selection on system imaging efficiency of two Siemens angiography systems: a Siemens Megalix x-ray tube installed on an Artis Zee system (denoted 'MEGALIX') and a newer generation Gigalix x-ray tube installed on an Artis Q (denoted 'GIGALIX'). A method was used that accounted for two potential sources of bias in this comparison: differences in radiation output between the x-ray tubes and differences between the x-ray detectors on the two systems. First, ADRC x-ray factors (tube voltage, tube current, pulse length, focus size, spectral prefilter) and radiation output were recorded as a function of poly(methyl) methacrylate (PMMA) thickness on the MEGALIX unit. These factors were then applied manually on the GIGALIX system and incident air kerma rate (IAKR) and signal difference to noise ratio (SDNR) were measured. Second, the ADRC on the GIGALIX system was used to give the x-ray factors and both IAKR and SDNR relevant to the GIGALIX based system directly. This method enabled the SDNR to be measured from images acquired on the same x-ray detector. SDNR and IAKR were measured on both systems using a PMMA phantom covering thicknesses from 6 cm to 40 cm. A small 0.3 mm iron insert was used to measure SDNR, which was then multiplied by modulation transfer function based weighting factors for focal spot blurring and motion blurring. These factors were evaluated for an object motion of 25 mm s-1 and at a spatial frequency of 1.4 mm-1 in the object plane, relevant to interventional cardiology, giving a spatial frequency dependent SDNR(u). In the second phase of the study, a technical figure of merit (FOM) was used to express imaging performance of both systems, calculated as SDNR2(u)/IAKR. Averaged over all phantom thicknesses, the FOM of the GIGALIX-based system was 42% and 73% higher compared to that of the MEGALIX based system, for fluoroscopy and acquisition mode respectively. The results indicate that increased x-ray tube power and smaller foci can improve overall system efficiency and reduce doses.

X-ray image quality and system exposure parameters for a hybrid Angio-MR system.

This study investigates the exposure parameters and required x-ray tube output when performing neurological procedures on a hybrid Angio-MR concept system proposed by Siemens Healthineers. The x-ray part of this system uses a longer source to detector distance than conventional (C-arm) systems and will have a fixed amount of filtration. Additionally, as the x-ray source is situated inside a magnetic field, the focal spot size and shape may be slightly distorted. In order to compare the Angio-MR system to a typical C-arm system, the exposure parameters of 60 thrombectomy procedures, performed in our hospital over the course of one year, were investigated in detail and a set of median values was determined. An analytical simulation platform was then developed to calculate the required tube voltage, tube current and pulse length to reach similar spatial frequency dependent signal difference to noise ratio (SDNR(u)) values as a conventional C-arm angiography system. These simulations were performed for a variety of focal spot sizes for the Angio-MR system. Results show that a standard current x-ray tube has sufficient power to reach similar SDNR(u) values as obtained in a conventional system if the focal spot size between both systems is comparable.

Radiation protection of operators and patients in a hybrid Angio-MR suite.

This work investigates the patient eye lens dose and x-ray scatter to the operator expected for a proposed hybrid Angio-MR concept. Two geometries were simulated for comparative assessment: a standard C-arm device for neuro-angiography applications and an innovative hybrid Angio-MR system concept, proposed by Siemens Healthineers. The latter concept is based on an over-couch x-ray tube and a detector inside an MRI system, with the aim of allowing combined, simultaneous MRI and x-ray imaging for procedures such as neurovascular interventions (including x-ray fluoroscopy and angiography imaging, 3D imaging, diffusion, and perfusion). To calculate the scattered radiation dose to the physician, Monte Carlo simulations were performed. Dose estimates of simplified models of the brain and eyes of both the patient and the physician and of the physician's torso and legs have been calculated. A number of parameters were varied in the simulation including x-ray spectrum, field of view (FOV), x-ray tube angulation, presence of shielding material and position of the physician. Additionally, 3D dose distributions were calculated in the vertical and horizontal planes in both setups. The patient eye lens dose was also calculated using a detailed voxel phantom and measured by means of thermoluminescent dosimeters (TLDs) to obtain a more accurate estimate. Assuming the same number of x-rays and the same size of the irradiated area on the patient's head, the results show a significant decrease in the scattered radiation to the physician for the Angio-MR system, while large increases, depending on setup, are expected to patient eye lens dose.

Implementation of a spatio-temporal figure of merit for new automatic dose rate control regimes in dynamic x-ray imaging.

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.

Evaluation of automatic dose rate control for flat panel imaging using a spatial frequency domain figure of merit.

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-1 and at a spatial frequency of 1.4 mm-1 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.