Typical values of z-resolution for different Digital Breast Tomosynthesis systems evaluated in a multicenter study.

PURPOSE: The aim of the present study, conducted by a working group of the Italian Association of Medical Physics (AIFM), was to define typical z-resolution values for different digital breast tomosynthesis (DBT) models to be used as a reference for quality control (QC). Currently, there are no typical values published in internationally agreed QC protocols. METHODS: To characterize the z-resolution of the DBT models, the full width at half maximum (FWHM) of the artifact spread function (ASF), a technical parameter that quantifies the signal intensity of a detail along reconstructed planes, was analyzed. Five different commercial phantoms, CIRS Model 011, CIRS Model 015, Modular DBT phantom, Pixmam 3-D, and Tomophan, were evaluated on reconstructed DBT images and 82 DBT systems (6 vendors, 9 models) in use at 39 centers in Italy were involved. RESULTS: The ASF was found to be dependent on the detail size, the DBT angular acquisition range, the reconstruction algorithm and applied image processing. In particular, a progressively greater signal spread was observed as the detail size increased and the acquisition angle decreased. However, a clear correlation between signal spread and angular range width was not observed due to the different signal reconstruction and image processing strategies implemented in the algorithms developed by the vendors studied. CONCLUSIONS: The analysis led to the identification of typical z-resolution values for different DBT model-phantom configurations that could be used as a reference during a QC program.

ON-FIELD EVALUATION OF OPERATOR LENS PROTECTIVE DEVICES IN INTERVENTIONAL RADIOLOGY.

The recent publication of the Euratom Directive 2013/59, adopting the reduction of eye lens dose limits from 150 to 20 mSv y-1, calls for the development of new tools and methodologies for evaluating the eye lens dose absorbed by the medical staff involved in interventional radiology practices. Moreover, the effectiveness of the protective devices, like leaded glasses, which can be employed for radiation protection purposes, must be tested under typical exposure scenarios. In this work, eye lens dose measurements were carried out on an anthropomorphic phantom simulating a physician bound to perform standard interventional neuroradiology angiographic procedures. The correlation between eye lens doses, in terms of Hp(0.07), and the equivalent dose [again in terms of Hp(0.07)] monthly measured with thermoluminescent dosemeters placed above the lead apron at the chest level was studied, in the presence and in the absence of different types of leaded glasses.

Multidetector CT fluoroscopy and cone-beam CT-guided percutaneous transthoracic biopsy: comparison based on patient doses.

In this study, two different techniques used for image-guided percutaneous transthoracic needle biopsy have been compared in terms of patient dose: computed tomography (CT) fluoroscopy (performed on a Toshiba Aquilion 64), supplied with several multidetector CT (MDCT), and cone-beam CT (CBCT) (performed on a Philips Allura Xper FD20), supplied with a few C-arm flat-panel angiographic devices. Dose data (10 patients for each technique) have been collected, and organ doses and effective dose have been evaluated using software packages enabling to simulate real acquisition geometry and X-ray exposure. As a result, higher doses were found for MDCT compared with CBCT: the effective dose is 50% higher for MDCT; ratios between mean organ doses range between 1.2 and 1.7, except for breast (0.9) and oesophagus (3.7). Even though the observed differences are not always statistically significant, the general distribution of organ doses confirms that the MDCT-guiding technique delivers higher dose than the CBCT-guided one.

Comparative study of jaws with multislice computed tomography and cone-beam computed tomography.

PURPOSE: The aim of this study was to compare the dosimetric and diagnostic performance of multislice computed tomography (MSCT) and cone-beam computed tomography (CBCT) in the study of the dental arches. MATERIALS AND METHODS: Effective dose and dose to the main organs of the head and neck were evaluated by means of thermoluminescent dosimeters (TLDs) placed in an Alderson Rando anthropomorphic phantom and using a standard CBCT protocol and an optimised MSCT protocol. Five patients with occlusal plane ranging from 54 cm to 59 cm who needed close follow-up (range 1-3 months) underwent both examinations. Image quality obtained with CBCT and MSCT was evaluated. RESULTS: Effective dose and dose to the main organs of the head and neck were higher for MSCT than for CBCT. Image quality of CBCT was judged to be equivalent to that of MSCT for visualising teeth and bone but inferior for visualising soft tissues. Beam-hardening artefacts due to dental-care material and implants were weaker at CBCT than at MSCT. CONCLUSIONS: When panoramic radiography is not sufficient in the study of the teeth and jaw bones, CBCT can provide identical information to MSCT, with a considerable dose reduction. MSCT is, however, indicated when evaluation of soft tissue is required.

The use of photostimulable phosphor systems for periodic quality assurance in radiotherapy.

The fusion of radiological and optical images can be achieved through charging a photostimulable phosphor plate (PSP) with an exposure to a field of X- or gamma-rays, followed by exposure to an optical image which discharges the plate in relation to the amount of incident light. According to this PSP characteristic, we developed a simple method for periodic quality assurance (QA) of light/radiation field coincidence, distance indicator, field size indicators, crosshair centering, coincidence of radiation and mechanical isocenter for linear accelerators. The geometrical accuracy of radiological units can be subjected to the same QA method. Further, the source position accuracy for an HDR remote afterloader can be checked by taking an autoradiography of the radioactive source and simultaneously an optical image of a reference geometrical system.

Optimisation of multislice computed tomography protocols in angio-CT examinations.

PURPOSE: The aim of this paper is to explain a general procedure for the optimisation of multislice computed tomography (MSCT) protocols. MATERIALS AND METHODS: Four angio-CT protocols with a GE LightSpeed Plus 4-slice CT scanner were considered. Effective doses were computed for a sample of patients. First the dose was optimised for arterial-phase scans on a standard patient and adapted to the weight of individual patients with a scaling factor. RESULTS: The mean effective dose for an angio-CT examination ranged from 18.8 mSv to 28.8 mSv, depending on the protocol adopted. Following the optimisation procedure, we drew up a table indicating tube current values for each patient weight. Calculation of the effective dose before and after the optimisation procedure revealed a dose reduction of about 40%. CONCLUSIONS: Angio-CT examinations deliver high doses, but these doses can be reduced without affecting image quality.

Routine controls for the geometric accuracy assessment of MR images used in neurosurgery.

AIM: This paper describes the methodology we set up in our hospital to investigate whether or not the MR images from our MRI device are geometrically reliable for use as reference images in stereotactic neurosurgery procedures. In fact, in these clinical procedures geometric accuracy is a prime concern. MRI is often the only diagnostic methodology by which the pathology of interest is detectable. On the other hand, the physical measurements on which this technology lies do non guarantee geometrical accuracy. METHODS: So we set up a measurement protocol to assess the geometric accuracy of an MR image. To reach this goal we use a self-designed phantom. On this simple phantom, simulating a human head, the stereotactic ring used for neurosurgical procedures was mounted. We made acquisitions in CT and MRI, and compared the images found. RESULTS: Our findings were that, with our MRI device, the geometric accuracy obtained depends only on the acquisition matrix used, and is not affected by other technological factors. CONCLUSION: The method here described can be used in any site where stereotactic neurosurgical procedures are applied on the basis of MR images, as it is simple and cheap, both from an economic point of view, and from a machine-time and personnel-time point of view.

Multicenter trial for the set-up of a MRI quality assurance programme.

Many international protocols related to RMI-QC program are focused on acquisition methods and analysis of several image quality parameters but rarely normality ranges or measurement frequencies are presented. To address this problem we investigated the variability of many magnetic resonance imaging (MRI) systems with the set-up of multicenter trial. The trial was set up to investigate short-and mid-term variability of two fundamental nongeometric image quality parameters: signal-to-noise (SNR) and integral percent uniformity (U%). Ten centers (12 devices) participated to data collection consisting of a three-step-protocol. First, 10 consecutive images of a phantom were collected with a spin echo sequence. As second step the series collection was repeated 24 h later. Finally a single image acquisition was performed twice a week for 5 weeks. The analysis of results allowed us to define a "physiological" variability of +/-3% of the reference level for both parameters and to conclude that a weekly measurement is adequate to detect relevant variations of device performance.

[Definition criteria for a magnetic resonance quality assurance program: multicenter study]. / Criteri per la definizione di un programma per l'assicurazione della qualità nella diagnostica per immagini con risonanza magnetica: studio multicentrico.

PURPOSE: We report the preliminary results of a multicenter trial aimed at defining methods, reference values and frequency of measurements for an MR quality assurance program. In particular, we stress the definition of two attention levels (investigation and intervention) for image uniformity and signal-to-noise ratio (SNR) by means of short- and long-term measurements. MATERIAL AND METHODS: The short-term protocol consisted of 10 successive spin-echo (SE, 2 echoes) acquisitions and was repeated after 24 hours. Measurements were made with the same test phantom which was circulated among all the participating centers. Image uniformity and SNR were evaluated using the software available on each MR unit. The long-term protocol, consisting of a single SE acquisition with the same parameters used for the short-term one, was performed once a month for 12 months. A mid term protocol was also performed twice a week for 5 weeks, and the analysis of the results is still in progress. RESULTS AND DISCUSSION: The short-term protocol results allowed absolute comparison of system performance. Uniformity and SNR were significantly different among centers (p < 0.05), also in the statistical comparison of two MR units of the same model (Siemens Magnetom SP63-1.5 T). Overall, three 1.5 T systems provided similar SNR values, while the results obtained for the 1 T system were markedly lower (51% of the maximum). This result can be explained by the dependence of the analytical expression of SNR on the magnetic field. The other 1.5 T system performed more poorly than the others operating at the same magnetic field. This difference can be explained by the specific characteristics of the coil and by technological aging. Because of the small sample size (5 units), the maximum variation coefficients (3% for the first echo and 3.5% for the second one) were assumed as a reference value for the both parameters (SNR and uniformity). These values were used for the long-term analysis: at every measurement the evaluated parameter was statistically compared with the result of the previous month. We propose to set an investigation level at p = 0.05: when the newly measured parameter differs from the previous value (p < 0.05), we should investigate if this is due to a normal long-term variation or to a system fault. The intervention level is then defined as the 95% prediction interval of the evaluated parameter regression vs time. Measurements that do not fall within the prediction interval are not used for future statistics. CONCLUSIONS: Some preliminary results concerning SNR and uniformity were obtained in the investigation performed on 5 MR systems to define methods and references for a Quality Assurance program. We introduced an "investigation level" and an "intervention level" related to short-term and long-term variability. The investigation levels could be a useful reference value to predict the short-term variability of an MR system with similar characteristics, thus avoiding a long and onerous series of measurements. Concerning the optimization of measurement frequencies, the preliminary analysis of these results showed that a daily measurement frequency is excessive in stable equipment conditions, while monthly measurements showed that the investigation level was often exceeded. However, frequency optimization will be investigated after the analysis of the mid-term measurements, which is still in progress.