Open-source hardware and software for the measurement, characterization, reporting, and correction of geometric distortion in MRI.

Background: Geometric distortion is a serious problem in MRI, particularly in MRI guided therapy. A lack of affordable and adaptable tools in this area limits research progress and harmonized quality assurance.

Purpose: To develop and test a suite of open-source hardware and software tools for the measurement, characterization, reporting, and correction of geometric distortion in MRI.

Real-time motion management in MRI-guided radiotherapy: Current status and AI-enabled prospects.

MRI-guided radiotherapy (MRIgRT) is a highly complex treatment modality, allowing adaptation to anatomical changes occurring from one treatment day to the other (inter-fractional), but also to motion occurring during a treatment fraction (intra-fractional). In this vision paper, we describe the different steps of intra-fractional motion management during MRIgRT, from imaging to beam adaptation, and the solutions currently available both clinically and at a research level. Furthermore, considering the latest developments in the literature, a workflow is foreseen in which motion-induced over- and/or under-dosage is compensated in 3D, with minimal impact to the radiotherapy treatment time.

Experimental comparison of linear regression and LSTM motion prediction models for MLC-tracking on an MRI-linac.

Background: Magnetic resonance imaging (MRI)-guided radiotherapy with multileaf collimator (MLC)-tracking is a promising technique for intra-fractional motion management, achieving high dose conformality without prolonging treatment times. To improve beam-target alignment, the geometric error due to system latency should be reduced by using temporal prediction.

Purpose: To experimentally compare linear regression (LR) and long-short-term memory (LSTM) motion prediction models for MLC-tracking on an MRI-linac using multiple patient-derived traces with different complexities.

Distortion-corrected image reconstruction with deep learning on an MRI-Linac.

Purpose: MRI is increasingly utilized for image-guided radiotherapy due to its outstanding soft-tissue contrast and lack of ionizing radiation. However, geometric distortions caused by gradient nonlinearities (GNLs) limit anatomical accuracy, potentially compromising the quality of tumor treatments. In addition, slow MR acquisition and reconstruction limit the potential for effective image guidance.

Rapid distortion correction enables accurate magnetic resonance imaging-guided real-time adaptive radiotherapy.

Background And Purpose: Magnetic resonance imaging (MRI)-Linac systems combine simultaneous MRI with radiation delivery, allowing treatments to be guided by anatomically detailed, real-time images. However, MRI can be degraded by geometric distortions that cause uncertainty between imaged and actual anatomy. In this work, we develop and integrate a real-time distortion correction method that enables accurate real-time adaptive radiotherapy.

Real-time radial reconstruction with domain transform manifold learning for MRI-guided radiotherapy.

Background: MRI-guidance techniques that dynamically adapt radiation beams to follow tumor motion in real time will lead to more accurate cancer treatments and reduced collateral healthy tissue damage. The gold-standard for reconstruction of undersampled MR data is compressed sensing (CS) which is computationally slow and limits the rate that images can be available for real-time adaptation.

Purpose: Once trained, neural networks can be used to accurately reconstruct raw MRI data with minimal latency.

Integrated MRI-guided radiotherapy - opportunities and challenges.

MRI can help to categorize tissues as malignant or non-malignant both anatomically and functionally, with a high level of spatial and temporal resolution. This non-invasive imaging modality has been integrated with radiotherapy in devices that can differentially target the most aggressive and resistant regions of tumours. The past decade has seen the clinical deployment of treatment devices that combine imaging with targeted irradiation, making the aspiration of integrated MRI-guided radiotherapy (MRIgRT) a reality.

Pre-treatment and real-time image guidance for a fixed-beam radiotherapy system.

Purpose: A radiotherapy system with a fixed treatment beam and a rotating patient positioning system could be smaller, more robust and more cost effective compared to conventional rotating gantry systems. However, patient rotation could cause anatomical deformation and compromise treatment delivery. In this work, we demonstrate an image-guided treatment workflow with a fixed beam prototype system that accounts for deformation during rotation to maintain dosimetric accuracy.

First experimental investigation of simultaneously tracking two independently moving targets on an MRI-linac using real-time MRI and MLC tracking.

Purpose: High quality radiotherapy is challenging in cases where multiple targets with independent motion are simultaneously treated. A real-time tumor tracking system that can simultaneously account for the motion of two targets was developed and characterized.

Methods: The multitarget tracking system was implemented on a magnetic resonance imaging (MRI)-linac and utilized multi-leaf collimator (MLC) tracking to adapt the radiation beam to phantom targets reproducing motion with prostate and lung motion traces.

Development and commissioning of a full-size prototype fixed-beam radiotherapy system with horizontal patient rotation.

Purpose: Compared to conventional linacs with rotating gantries, a fixed-beam radiotherapy system could be smaller, more robust and more cost-effective. In this work, we developed and commissioned a prototype x-ray radiotherapy system utilizing a fixed vertical radiation beam and horizontal patient rotation.

Methods: The prototype system consists of an Elekta Synergy linac with gantry fixed at 0° and a custom-built patient rotation system (PRS).

Small field correction factors for the IBA Razor.

The IBA Razor diode supersedes the IBA SFD and is intended for use in small fields. However, its behaviour in small fields has not yet been quantified. In this work, we examine the response of the Razor diode against the air core scintillation dosimeter (FOD) and Gafchromic film in photon beams from three Varian linac beams.

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators.

Flattening filter-free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization cham-bers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney.

Can small field diode correction factors be applied universally?

Background And Purpose: Diode detectors are commonly used in dosimetry, but have been reported to over-respond in small fields. Diode correction factors have been reported in the literature. The purpose of this study is to determine whether correction factors for a given diode type can be universally applied over a range of irradiation conditions including beams of different qualities.

Characterization of small-field stereotactic radiosurgery beams with modern detectors.

To derive accurate beam models for stereotactic radiosurgery (SRS) planning it is necessary to characterize the beam with dosimetric measurements. The aim of this study is to identify the best detectors for each task in the characterization process. Output ratios, beam profiles and percentage depth doses were measured for SRS cone diameters of 5-45 mm.

Real-time scintillation array dosimetry for radiotherapy: the advantages of photomultiplier detectors.

Purpose: In this paper, a photomultiplier tube (PMT) array dosimetry system has been developed and tested for the real-time readout of multiple scintillation signals from fiber optic dosimeters. It provides array dosimetry with the advantages in sensitivity provided by a PMT, but without the need for a separate PMT for each detector element.

Methods: The PMT array system consisted of a multianode PMT, a multichannel data acquisition system, housing and optic fiber connections suitable for clinical use.