346,182 research outputs found
A quality assurance phantom for electronic portal imaging devices
Electronic portal imaging device (EPID) plays an important role in radiation therapy portal imaging, geometric and dosimetric verification. Consistent image quality and stable radiation response is necessary for proper utilization that requires routine quality assurance (QA). A commercial ‘EPID QC’ phantom weighing 3.8 kg with a dimension of 25 × 25 × 4.8 cm3 is used for EPID QA. This device has five essential tools to measure the geometric accuracy, signal‐to‐noise ratio (SNR), dose linearity, and the low‐ and the high‐contrast resolutions. It is aligned with beam divergence to measure the imaging and geometric parameters in both X and Y directions, and can be used as a baseline check for routine QA. The low‐contrast tool consists of a series of holes with various diameters and depths in an aluminum slab, very similar to the Las Vegas phantom. The high‐resolution contrast tool provides the modulation transfer function (MTF) in both the x‐ and y‐dimensions to measure the focal spot of linear accelerator that is important for imaging and small field dosimetry. The device is tested in different institutions with various amorphous silicon imagers including Elekta, Siemens and Varian units. Images of the QA phantom were acquired at 95.2 cm source‐skin‐distance (SSD) in the range 1–15 MU for a 26 × 26 cm2 field and phantom surface is set normal to the beam direction when gantry is at 0° and 90°. The epidSoft is a software program provided with the EPID QA phantom for analysis of the data. The preliminary results using the phantom on the tested EPID showed very good low‐contrast resolution and high resolution, and an MTF (0.5) in the range of 0.3–0.4 lp/mm. All imagers also exhibit satisfactory geometric accuracy, dose linearity and SNR, and are independent of MU and spatial orientations. The epidSoft maintains an image analysis record and provides a graph of the temporal variations in imaging parameters. In conclusion, this device is simple to use and provides testing on basic and advanced imaging parameters for daily QA on any imager used in clinical practice
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Magnetic resonance multitasking for motion-resolved quantitative cardiovascular imaging.
Quantitative cardiovascular magnetic resonance (CMR) imaging can be used to characterize fibrosis, oedema, ischaemia, inflammation and other disease conditions. However, the need to reduce artefacts arising from body motion through a combination of electrocardiography (ECG) control, respiration control, and contrast-weighting selection makes CMR exams lengthy. Here, we show that physiological motions and other dynamic processes can be conceptualized as multiple time dimensions that can be resolved via low-rank tensor imaging, allowing for motion-resolved quantitative imaging with up to four time dimensions. This continuous-acquisition approach, which we name cardiovascular MR multitasking, captures - rather than avoids - motion, relaxation and other dynamics to efficiently perform quantitative CMR without the use of ECG triggering or breath holds. We demonstrate that CMR multitasking allows for T1 mapping, T1-T2 mapping and time-resolved T1 mapping of myocardial perfusion without ECG information and/or in free-breathing conditions. CMR multitasking may provide a foundation for the development of setup-free CMR imaging for the quantitative evaluation of cardiovascular health
Ten Years of the Solar Radiospectrograph ARTEMIS-IV
The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV) is in
operation at the Thermopylae Satellite Communication Station since 1996. The
observations extend from the base of the Solar Corona (650 MHz) to about 2
Solar Radii (20 MHz) with time resolution 1/10-1/100 sec. The instruments
recordings, being in the form of dynamic spectra, measure radio flux as a
function of height in the corona; our observations are combined with spatial
data from the Nancay Radioheliograph whenever the need for 3D positional
information arises. The ARTEMIS-IV contribution in the study of solar radio
bursts is two fold- Firstly, in investigating new spectral characteristics
since its high sampling rate facilitates the study of fine structures in radio
events. On the other hand it is used in studying the association of solar
bursts with interplanetary phenomena because of its extended frequency range
which is, furthermore, complementary to the range of the WIND/WAVES receivers
and the observations may be readily combined. This reports serves as a brief
account of this operation. Joint observations with STEREO/WAVES and LOFAR low
frequency receivers are envisaged in the future
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