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This work aims to construct a method to objectively evaluate CT image quality when new clinical protocol performances must be compared with a standard reference. We compare iterative reconstruction in the image space with filtered back projection reconstruction and accurately quantify the dose reduction. The comparison strategy accounts for both physical and clinical image qualities that are evaluated using a standard metric. The quasi-ideal observer metric is also explored to verify its reportedly high correlation with perceived image quality. Water or spatial resolution phantom images are used to characterise the physical image quality using the classic metrics in the Fourier domain by calculating the modulation transfer functions and noise power spectra (NPS). The clinical-image quality is evaluated with a 4-alternative forced-choice test. The human observers are asked to detect a positive image that contains a simulated lesion in a background image. Then, the same positive images are characterised with the quasi-ideal observer metric, which calculates the non-prewhitening matched filter signal-to-noise ratio (SNRNPWMF). Iterative reconstruction strongly reduces the image noise, but the NPS are slightly shifted to lower frequencies, which gives the images a coarse graininess. Compared with the reference FBP protocol for abdomen exams, the highest dose reduction is 40% if the standard metric is used and 30% if the SNRNPWMF metric is used. The detectability test results achieve a better correlation with SNRNPWMF than with the standard metric. The identified Fourier metric is a useful descriptor of human quality perception and can be used for future protocol optimisation.

Purpose RF Multi-coil arrays is a well known MRI technology aimed at increasing signal to noise ratio (SNR) over large volumes. However when the coil number increase and coil elements become very small there is a lack of combined signal uniformity, particularly for the head coils, and the real benefit should be evaluated. This work compares quantitatively a 20 and 64 RX head multicoil array performance both for phantoms and in vivo measurements. Methods Measurements were performed on 3T Siemens Skyra system. A spherical homogeneous phantom was scanned with both RX coils using a high resolution T1-3D-MPrage protocol optimized for in vivo morphological studies and repeated with different acceleration factors (AF from 0 to 8). Phantom volumes were analyzed offline with a bespoke imagej plugin that calculates quality indices, such as SNR and Normalized Absolute Average Difference (NAAD), over the entire volume, extending to 3D the standard 2D evaluation proposed by NEMA. The same acquisitions were performed in vivo, scanning 10 healthy volunteers and analyzing data offline in terms of SNR of small sub-cortical tissues located in the central brain volumes, where the worst coil performance is expected. Results Results from phantom measurements showed no significant differences, in terms of uniformity, for the coils performance. NAAD didn’t change with acceleration factor (NAAD = 0,84). SNR (see Fig. 1) is about the same for unaccelerated and AF = 8 images, these last due to the very low SNR values, but it becomes higher for 64ch coil with AF = 2,4,6, due to a lower noise increase rate. In vivo measurements present a significant difference between SNR from the two coils, with better performances of the 64ch coil for all the segmented tissues when an AF of 4 and 6 is used and for some of the segmented tissues with AF = 2. Conclusions increasing coil numbers can be useful in morphological protocols only when high acceleration factors, up to 6, are used.

Purpose The AIFM working group on MR intercomparison has proposed a quality assurance protocol for MRI diffusion weighted imaging (DWI). The aim of this study is to test the accuracy of apparent diffusion coefficient (ADC) evaluation at the isocentre of the scanner. Methods Seventeen centres and twenty-six MRI scanners (20/6; 1.5 T/3.0 T) were enrolled in this study. A custom spherical water phantom (14 cm diameter, filled with doped aqueous solution) specifically designed for DWI QA was provided to all participants. ADC value was certified as in [1] . Phantom was placed at isocentre of scanner. 5 mm thick axial slices at the isocentre were acquired, using three DWI sequences (b-value varying form 0 s/mm2 to 1000 s/mm2 at 200 s/mm2 steps), differing in gradient diffusion direction. For each diffusion direction, ADC image was generated with monoexponential fit and corrected for temperature. Analysis was performed with a custom Matlab software code. Average ADC was calculated for each diffusion direction ( ADC x , ADC y , ADC z ) in a 2 × 2 cm2 ROI at the centre of the phantom. A chi-squared consistency test was performed on the three ADC values. The mean value ( ADC ref ) was compared with the certified ADC C value. Results Differences within ADC x,y,z are less than 1.5%, for 21 scanners. For seven scanners ADC x,y,z were not consistent among themselves. ( ADC ref / ADC C - 1 ) vary from +10% to −7% (mean value −1%). Only in four scanners | ( ADC ref / ADC C - 1 ) | is larger than 5%. Conclusions The proposed protocol and multipurpose phantom provide a simple ADC quality assurance procedure. The measured isocentre ADCs are within 1% with reference to wrt diffusion directions and consistent with certified value within 1%.

Purpose The dependence of ADC (measured in isotropic phantom) on position and ADC phase encoding may be used to test MRI scanner performances and may play relevant effect on quantitative data accuracy in clinical imaging. Quantitative results of AIFM multicentre intercomparison are reported. Methods Twenty-six MR scanners (20/6 1.5 T/3.0 T magnetic field strength) were enrolled in this study. Custom spherical water phantom (14 cm diameter) specifically designed for DWI QA was provided to all participants. ADC value was certified as in [1] . Phantom was placed at isocentre of scanner. Using three DWI sequences (b-value varying form 0 s/mm2 to 1000 s/mm2), differing for the gradient diffusion direction (Gdd), 5 mm thick axial slices were acquired (5 mm gap). Acquisition was repeated exchanging frequency and phase encoding direction. ADC maps were generated for each Gdd. Seven 2 × 2 cm2 ROIs were considered: one at isocentre and six in periphery (left, right, superior, inferior, head and feet). For each acquisition, maximum relative difference between periphery and central ROIs ADC was calculated ( δ ADC % x,y,z ) . Maximum δ ADC % x,y,z with respect to three acquisitions ( δ M%) was also considered. As for phase/frequency encoding direction dependences, ADC x,y,z differences on central ROI were measured. Results Differences Absolute value of δ M is below 5% for 19 scanners, 18 scanners show δ M > 0. Spatial direction of maximal variation is “x” in 23 scanners, “y” in the other. Gdd associated to δ M is x/y/z for 9/12/5 scanners. Differences between −2% and +1% have been observed with respect to phase encoding direction. Conclusions A simple protocol for quantitative evaluation of ADC spatial and phase encoding dependence has been proposed. Difference in ADC between isocentre and periphery should be consider in precision ADC measurements.

Purpose The use of RF multicoil arrays for MR imaging has demonstrated several advantages, however when the number of coil elements increases, both signal and noise can become very inhomogeneous over the scanned volume, so standard 2D evaluations, as proposed by many protocols, are no longer representative of the whole volume and a new approach has to be considered. In this work we set up an automatic 3D quality evaluation particularly suited for head coils. Methods Measurements were performed on a Siemens Skyra 3T system with a 20 channel TX head coil, scanning an homogeneous spherical phantom with a standard clinical 3D protocol. Reconstruction was performed both for single sub-coil (uncombined images) and for the final image volume, that is a combination of all the sub-coils signals. Processing was performed offline with a bespoke imagej plugin. Main processing steps for combined volume are the automatic centring of a spherical region of interest (ROI) covering 80% of the phantom image volume, and the calculation of Signal to Noise Ratio (SNR), Normalized Absolute Average Difference (NAAD) and 5-color level reconstructed image with the NEMA algorithms for 2D images that were extended to 3D [1] . For uncombined sub-coil images, SNR is calculated in a 2 cm diameter sphere ROI automatically centered on the maximum average signal region, as showed in Fig. 1, and uniformity is evaluated with a 11-color levels volume reconstruction. Results Results for volumetric analysis are compared to the evaluation performed on 2D images from standard planes centered at the phantom isocenter. Results are very different particularly for uncombined analysis where the single coil is far from the evaluation slice, as reported for example in Table 1 for the SNR of coil n.2. In this case parameter variations due to malfunctioning are less sensitive with a 2D analysis. Conclusions We set up an automatic protocol for 3D quality evaluation which is more representative of the whole coil specification volume and more sensitive to failures when a single coil analysis is performed.

Purpose This work is part of a larger cohort study aimed at investigating the long term effects of exposure to manganese [1] . In this study we set up a protocol for brain fast T1 mapping and segmentation of subcortical tissues, in particular globus pallidus (GP), that is reported to be a target tissue for manganese deposition in brain. Methods The variable flip angle model was used for T1 mapping. Acquisition protocol, set up on a 3T Siemens Skyra MR system, includes 10 RF spoiled 3D gradient echoes (VIBE) with flip angles from 2° to 30° and a reduced scanned volume to cover GP in the shortest possible time, an extra 30° VIBE covering the whole brain (WB- VIBE), two fast acquisitions with different TEs for B1 map calculation, used to correct nominal flip angles, and a 3D-MPRAGE acquisition of the whole brain with the same WB-WIBE geometry. T1 and error maps were calculated offline with a bespoke Matlab script, checking accuracy with calibrated gels. The most efficient pipeline for post-processing, shown in Fig. 1, is as follows: T1 map calculation of the reduced volume, and zero padding of the missing slices to get the same WB-VIBE volume (WB-T1MAP); 3D-MPRAGE and WB-T1MAP coregistration using the WB-VIBE to 3D transformation matrix (WB-T1MAP-COR); GP automatic segmentation of the 3D-MPRAGE and, with the output mask, GP statistics calculation of the WB-T1MAP-COR. In order to cancel possible partial volume effects, segmented regions were eroded by 1 pixels and T1 results were compared. The protocol was checked on a group of 10 adolescents that were exposed to Mn during childhood. Results T1 accuracy was 4.3% (median) and within 13.8% (interquartile distance). GP-T1 average values were 1314,65 ms and 1302,92 ms after erosion, close to the values reported in other papers, with a coefficient of variation respectively decreased from 6.8%, to 5.6%. Conclusions We set up a fast and automated T1 mapping protocol with good accuracy and optimised data variability in vivo.

Purpose CT protocols review and optimization require a considerable amount of time and dedicated staff especially to compare dose levels among multicentre institutions and with national and international average values. The purpose of this article is to describe the structure of data collection for a dose radiation tracking software used to analyse a large amount of CT dose data. This should be done in order to audit scanner protocols, improve staff and patient safety and optimize all the diagnostic workflow. Methods The software implementation team included a team leader, two radiology technologists, two medical physicists and the chief of radiology department. Very detailed data collection folders and diagnostic reference values (DRV) were created based on 25th percentile, as target value, and 75th percentile, as upper limit, values of CTDIvol and DLP extracted from Swiss CT DRV. Data were collected from seven different multislice CT scanners using the study description Dicom tag. Results Direct protocol comparison, for each institution/scanner, is presented. About 90% of CT exams applied a dose inferior to Swiss DRV. Conclusions DVRs represent a benchmark useful to optimize radiation dose. Multisite cross-analysis of CT protocols allows the adoption of the best dose-optimized protocols in each center. Outliers and dose reduction strategies are discussed periodically within a team meeting: analyses are performed using a Montecarlo simulation tool to optimize future exams by varying setup parameters and study length. On request, total effective and individual organ dose values, are available especially for pregnant patients immediately after diagnostic examinations.