MRI, for example, long established in diagnostic imaging, is now increasingly employed within radiotherapy for planning and, more recently, treatment guidance. While the focus for diagnostic radiology is on high contrast and resolution, within radiotherapy, MR image distortion – caused by inhomogeneities in the main magnetic field and gradient nonlinearities – is the overarching issue.

"Geometric accuracy is far more important in radiotherapy than diagnostic imaging," explained Richard Mallozzi, product development physicist at The Phantom Laboratory. "The problem that we're addressing is to ensure that a radiation therapist has confidence that the MR scanner is functioning well and that geometric distortion is under control."

One major challenge when creating a QA phantom for MRI applications is the large field-of-view, which can lead to an extremely heavy phantom when filled with water. To address this, The Phantom Laboratory has created a multicomponent phantom, comprising two or three sections each weighing no more than 12 kg. The Magphan RT phantom contains a 3D grid of hundreds of 1 cm plastic spheres, which provide inverse contrast against a uniform bright background solution.

To generate a 3D distortion map, the locations of the spheres are measured and compared to known locations. The system can also calculate optimum gradient rescaling factors designed to minimize distortion in each direction (an adjustment that is performed by a service engineer). The phantom is designed to assesses a range of other parameters important to MR quality control, including: image uniformity, which can indicate a phased array element failure; laser alignment; signal-to-noise ratio, using embedded zero-signal cylinders; slice thickness, which can indicate numerous system failures; and resolution, by directly measuring the point spread function.

Mallozzi notes that the Magphan RT is still in the prototype stage, with release expected in about six months. "There is a lot of validation to perform and we are still developing a lot of the analysis," he told medicalphysicsweb.

Water-free approach

Modus QA has a different take on the weighty problem of MRI QA – its QUASAR MRID3D phantom is not filled with water at all. Instead, it quantifies geometric distortion by imaging control points on the phantom's outside surface. "The phantom works with automated software by measuring the distortion on a closed surface and using spherical harmonic analysis to calculate the deviation vector field inside," explained John Miller, president of Modus QA. "The advantage is a much lighter phantom that produces high-quality results. The phantom never has to be filled or emptied."

The QUASAR MRID3D comprises a hollow twin-walled acrylic cylinder, with 1502 precisely machined fiducial markers on the outside. The markers are filled with mineral oil to provide a high MR signal. The system, which weighs less than 21 kg, analyses the entire 3D field of view in a single acquisition. Set-up, data acquisition and automated analysis of the QUASAR MRID3D phantom produces deviation vector field results for the entire 3D field of view in as little as 10 minutes. It can also separate distortion caused by the main magnetic field and the gradient.

Targeted for use in MR simulation and planning, as well as the emerging MR-linac market, the phantom and associated software are now commercially available. "It's important to image distortion in MRI, especially if it's being used for guiding therapy," said Miller. "And MRI has non-isotropic distortion, so you need to measure it in 3D."

The phantom was developed in collaboration with the Princess Margaret Hospital (PMH) in Toronto. Modus QA has licensed the design from PMH and the teams have verified the system's accuracy. A software update – version 1.01 – was released at the AAPM meeting, noted Miller.

Quick and easy

Dosimetry specialist PTW announced the North American launch of BEAMSCAN – a precision 3D water phantom for linac commissioning and annual QA. According to John Seddo, managing director of PTW New York, BEAMSCAN addresses many of the workflow issues of beam data collection. "This is so easy to use, there is no learning curve, we think that's a huge advantage," he said. "We gave this system to a physicist without any instructions; he was able to put it under the beam and scan relatively quickly."

BEAMSCAN can be set up to take measurements in less than 15 minutes, cutting the QA time by more than half. Seddo explained that while rapid scanning can sometimes result in a loss of quality, PTW's system incorporates a high-precision electrometer can increase the scanning speed without compromising quality.

BEAMSCAN is also the first water phantom to offer wireless data transfer, and can be controlled through any wireless device, such as an iPod, iPad, smartphone or PC. BEAMSCAN is CE marked and FDA 510k pending, with approval expected by the end of this year.

Merging modules

Elsewhere on the show floor, Sun Nuclear introduced its SunCHECK radiotherapy quality management system. Ultimately, SunCHECK will integrate all of the company's QA software apps into a common platform, including database and user interface. At present, the system incorporates three modules: PerFRACTION, for phantom-less 3D pre-treatment QA and in vivo 3D dose monitoring; DoseCHECK, for 3D independent secondary dose calculations; and SNC Machine, for imaging, mechanical and VMAT machine QA. All three modules include full or partial automation.

The company also plans to release a series of software updates next month. This includes, for example, support for wedged beams and electron calculations within DoseCHECK, as well as a 2D absolute dose mode and 3D dose calculation onto the daily cone-beam CBCT within PerFRACTION. "We have the only system that performs automated in vivo recalculation using machine delivered fluence on the daily patient anatomy," said Jeff Simon, Sun Nuclear's chief executive officer.

Sun Nuclear also unveiled new phantoms for QA of CT and mammography systems. This included a multi-energy CT phantom that tests discrimination of materials like iodine and calcium, along with evaluating mono-energetic reconstructions, protocols and other aspects of multi-energy CT scanners, as well as an enhanced CT low-contrast module for use in the Gammex ACR 464 phantom. The latter module contains inserts with density differences as low as 0.3%. "It can test more sophisticated CT machines, it really tests at the limit of the scanner," explained Sun Nuclear's Pat Ploc.

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