Keywords: DWI/DTI/DKI, Diffusion/other diffusion imaging techniques, Deformable Image Registration, MR-Linac, Geometric Distortion

Motivation: Geometric distortion in MRI-Linac sequences remains insufficiently characterized, impacting the precision of radiotherapy treatment planning. This study aims to quantify such distortions in EPI, TSE, and SPLICE sequences.

Goal(s): The goal of this study is to quantitatively assess and compare geometric distortion across three common MRI-Linac sequences using deformable image registration and DICE score analysis.

Approach: An in vivo study employed patient T2 and DWI B0 scans. Deformable image registration was conducted using ADMIRE and Elekta-based software, generating deformation vector fields. A Python algorithm calculated RMS values.

Results: The sequences exhibited distinct DSC scores and RMS distortions, revealing registration effectiveness and sequence-dependent variability. 

Impact: These findings set the stage for future research on minimizing distortions in MRI-Linac sequences. The developed Python algorithm could be adapted for real-time monitoring, advancing the precision and safety of MRI-guided radiotherapy.

Keywords: MR-Guided Radiotherapy, Tumor, MR-Linac, perfusion, glioblastoma

Motivation: Glioblastoma is a highly vascularized brain tumor. Changes in perfusion could guide treatment adaptation, but the dynamics of blood flow changes in glioblastoma during radiotherapy are poorly understood.

Goal(s): We sought to characterize changes in glioblastoma cerebral blood flow during radiotherapy.

Approach: We acquired twice-weekly arterial spin labeling (ASL) MRI in 22 glioblastoma patients during radiotherapy on a 1.5T MRI-linear accelerator (MR-Linac) and evaluated changes in cerebral blood flow.

Results: We provided the first demonstration of MR-Linac ASL. Tumor cerebral blood flow tended to decrease during radiotherapy. Highly-perfused tumor regions showed the greatest change.

Impact: We showed that frequent perfusion imaging on MRI-linear accelerators is feasible and that blood flow in highly-perfused regions of human glioblastoma tends to decrease during radiotherapy. Radiotherapy with dose escalation to highly perfused tumor regions likely requires target adaptation.

Keywords: CEST / APT / NOE, Brain, Glioblastoma

Motivation: Microscopic tumor infiltration in glioblastoma cannot be depicted on conventional MRI. Therefore, the clinical target volume (CTV) for radiotherapy consists of the gross tumor volume (GTV) plus a 15-mm safety margin. This large expansion increases the risk of radiation-induced side-effects.

Goal(s): To explore the potential of APTw-CEST MRI for improved GTV delineation, ultimately enabling a CTV-margin reduction.

Approach: Radiotherapy planning MRI-acquisition included an APTw-CEST MRI-sequence. GTVs based on APTw MTR_asym or LD-maps were defined and compared to the conventional GTVs and CTVs.

Results: Both APTw-CEST MRI-based GTVs were similar in size, significantly larger than the GTV, and significantly smaller than the CTV.

Impact: This prospective pilot study integrates APTw-CEST MRI into glioblastoma radiotherapy planning, enabling a reduction of the 15-mm CTV-margin, and construction of a personalized target area that only targets tumor infiltration. This minimizes radiation-induced side-effects and thus improves quality of life.

Keywords: MR-Guided Radiotherapy, Radiotherapy

Motivation: Stereotactic MR-guided Adaptive Radiation Therapy (SMART) ‘dose painting’ for hypoxia can improve treatment outcomes, but implementation on MR-Linac (MRL) faces low-field SNR challenges. Optimization of T₁-to-noise ratio at 0.35T will enable clinical implementation of oxygen-enhanced (OE)-MRI.

Goal(s): Develop and validate an optimized MP2RAGE sequence for low-field T₁ mapping, establish feasibility and evaluate reproducibility in phantoms and healthy subjects.

Approach: We optimized and validated an MP2RAGE sequence with simulations and a ground-truth phantom. T₁ mapping feasibility was established in healthy subjects, variability and reproducibility was assessed.

Results: A clinically feasible optimized low-field MP2RAGE protocol was developed, yielding accurate and reproducible T₁ mapping.

Impact: This work builds a foundation towards clinically feasible hypoxia imaging for low-field MRL. This would facilitate a paradigm shift toward MR-guided biological adaptation and dose painting, leveraging the spatial distribution of hypoxia, and improving patient outcomes in conventionally challenging-to-treat cancers.

Keywords: MR-Guided Radiotherapy, Radiotherapy, MRI-guided radiotherapy;

Motivation: Can we provide superior liver tumor visualization for online adaptive planning? MRI enables direct visualization of tumor and organs-at-risk (OAR). However, MRI contrast agents are often required to differentiate primary and metastatic liver malignant lesions from functional hepatic parenchyma. 

Goal(s): We employed super-paramagnetic iron oxide nanoparticles (SPION) as an MRI contrast agent.

Approach: SPION enhanced the liver-to-tumor contrast ration for rapid and accurate delineation of tumors and functional hepatic parenchyma throughout the entire treatment course. 

Results: This study is the first to report the efficiency of a single SPION injection for multi-fractionated MRI-guided liver stereotactic body radiotherapy on a 1.5T Elekta MR-Linac.

Impact: A single SPION injection significantly improved the tumor-to-liver contrast, and it was maintained throughout multi-fraction MRI-guided liver SBRT to provide rapid and accurate contouring tumor lesions from functional liver parenchyma for online adaptive planning.

Keywords: MR-Guided Radiotherapy, Radiotherapy, 4DMRI, Real-Time Imaging

Motivation: 4D MRI is a powerful technique for free-breathing volumetric imaging, holding great potential for application in MRI-guided radiotherapy. Traditional 4D MRI typically requires explicit motion detection and data binning for respiratory-resolved reconstruction and commonly employs fast steady-state MRI acquisition, which may not yield optimal contrast.

Goal(s): This work proposes a novel T contrast-augmented, free-breathing, real-time 4D MRI technique.  

Approach: The proposed technique improves image contrast through highly-accelerated inversion recovery-prepared acquisition and reconstruction of real-time 4D images at a sub-second temporal resolution without requiring explicit motion compensation.

Results: T1 contrast-augmented real-time 4D MRI demonstrated improved image contrast over conventional 4D MRI with steady-state acquisition.

Impact: The contrast-augmented real-time 4D MRI technique proposed in this work can improve image contrast for free-breathing imaging without requiring explicit motion detection, data binning and respiratory motion compensation. It holds great potential for various applications, such as MRI-guided radiotherapy.  

Keywords: Diffusion Acquisition, Radiotherapy, ADC, MR-linac, abdomen, repeatability

Motivation: The apparent diffusion coefficient (ADC), derived from diffusion-weighted imaging (DWI), is a promising quantitative biomarker for treatment response during radiotherapy. However, DWI in the abdomen is complicated by respiratory motion.

Goal(s): To compare the repeatability of the ADC acquired with different motion compensation techniques on a 1.5T MR-linac.

Approach: A phantom and test-retest study (26 healthy volunteers) were performed, evaluating four DWI acquisitions (free breathing, navigator-triggered, and both repeated with an abdominal compression belt).

Results: From the 11 datasets analyzed so far, absolute ADC values and repeatability were comparable between the acquisitions in healthy regions of the liver, spleen and kidney.

Impact: This study initiates defining the optimal DWI acquisition strategy to measure treatment-related ADC changes of abdominal tumors on an MR-linac.

Keywords: MR-Guided Radiotherapy, Quantitative Imaging, Prostate, Radiotherapy, Relaxometry

Motivation: T₂* mapping could inform biologically-adaptive MR-guided radiotherapy, but requires improvement in processing time and precision for clinical implementation.

Goal(s): To accelerate intravoxel field inhomogeneity correction and generation of T₂* maps.

Approach: We developed a physics-informed self-supervised convolutional neural network for whole volume T₂* mapping of complex multi-echo data from an MR Linac. Bias in T₂* estimation is accounted for by calculating the additional signal decay from 3D derivatives of the field inhomogeneity map.

Results: Our model generates T₂* parameter maps 30% faster than an existing time-efficient algorithm. Resulting T₂* maps are less affected by noise compared to the reference.

Impact: Our AI-based algorithm is a step towards integration of whole volume T₂* mapping for hypoxia assessment into clinical MR-guided radiotherapy workflows. It could enable real-time mapping of dynamic changes, for example during an oxygen challenge and enable biologically adaptive radiotherapy.

Keywords: DWI/DTI/DKI, MR-Guided Interventions, MR-Linac

Motivation: Additional functional information from DWI is one of the major advantages of MR-Linac over conventional radiation therapy techniques, and is sparsely studied.

Goal(s): To monitor the ADC change during the course of hypofractionated radiation therapy in prostate cancer using a 1.5T MR-Linac.

Approach: For each patient, a prostate DWI-EPI scan was acquired immediately after each of the 5 hypofractionated radiation therapy sessions. The ADC maps were generated and compared among the fractions using the Kruskal-Wallis test.

Results: No significant ADC change among fractions might indicate that monitoring ADC alone might be insufficient to reflect the early treatment response.

Impact: Biological radiotherapy treatment planning has been proposed for decades, but was hindered by lack of tools that could provide functional information with spatial interpretation. The study results shed light on potential use of DWI for early treatment monitoring using MR-Linac.

Keywords: Lung, Lung, 4D MRI, MR-Linac, Motion, Radiotherapy, Radial

Motivation:  Current 4D MRI methodology on the MR-Linac is based on stack-of-stars acquisition, which has limited resolution along the slice dimension and can compromise the performance of motion assessment. 

Goal(s): To develop 4D lung MRI with isotropic-resolution on a 1.5T MR-Linac using 3D radial kooshball acquisition with respiratory self-navigation.

Approach: Stack-of-stars and kooshball acquisitions were performed on a healthy volunteer. Motion-resolved 4D MRI was reconstructed using XD-GRASP and then compared in terms of image quality and motion characteristics.

Results: Stack-of-stars acquisition underestimated motion due to limited resolution in the slice dimension. Kooshball acquisition provided isotropic-resolution that allows for improved visualization of smaller pulmonary structures.

Impact: 4D MRI with isotropic spatial resolution has the potential to enhance treatment planning and adaptation for lung cancer patients receiving radiation therapy on the MR-Linac system.

Keywords: MR-Guided Radiotherapy, Data Analysis, low-field, MR-Linac, ventilation, perfusion, functional imaging

Motivation: Early predictors of radiation-induced pneumonitis in patients receiving MR-guided radiotherapy allowing a closer follow up and taking early countermeasures to avoid a severe disease progression have not yet been identified.

Goal(s): We aimed at finding functional MR-based biomarkers acquired during treatment that allows the prediction of radiation-induced pneumonitis (RP) for lung cancer patients directly after MR-guided radiotherapy.

Approach: For 19 patients, ventilation- and perfusion-maps were acquired using a non-contrast enhanced free-breathing technique and investigated in different regions of the irradiated lung.

Results: Changes over treatment in the ventilation around the tumor significantly separate between RP and non-RP group. 

Impact: The acquisition of additional functional lung imaging during MR-guided radiotherapy requires little effort while offering the opportunity to identify lung cancer patients at risk of developing radiation-induced pneumonitis right after treatment and to take early countermeasures to avoid severe complications.

Keywords: MR-Guided Radiotherapy, Hybrid & Novel Systems Technology, Breathing variability

Motivation: To develop an MRI technique that captures cycle-to-cycle variability and differential motion between inhalation and exhalation to enable more accurate treatment planning and real-time beam adaptation. 

Goal(s): To address the challenge of acquiring sufficient volumetric data for reconstruction of 3D dynamic lung MRI while maintaining adequate spatiotemporal resolution for radiation therapy guidance. 

Approach: We utilized golden angle stack-of-star acquisition and labeled each view with the breathing state captured by a surface tracking system for sharing views with similar breathing states.  

Results: 3D dynamic lung imaging captured cycle-to-cycle variability and respiratory hysteresis with a nominal temporal resolution of 60.6ms per volume.  

Impact: The 3D dynamic lung imaging captured cycle-to-cycle variability and respiratory hysteresis with a nominal temporal resolution of 60.6ms. It can provide more accurate motion information than 4D-MRI for radiotherapy guidance, avoiding interplay between the tumor target and organs at risk.

Keywords: MR-Guided Interventions, Segmentation

Motivation: Needle localization on 3D magnetic resonance imaging (MRI) is critical for MRI-guided percutaneous interventions, but current manual methods are time-consuming. 

Goal(s): To develop a transformer-based pipeline for accurate and rapid automatic 3D needle localization on intra-procedural 3D MRI. 

Approach: The proposed pipeline adopted a coarse-to-fine segmentation strategy by combining 3D and 2D shifted window (Swin) Transformer networks. The performance was evaluated in pre-clinical pig datasets and compared with human-annotated references.

Results: Computation time was 6 sec/volume. The median 3D needle tip and axis localization errors were 1.48 mm (1.09 pixels) and 0.98°, which were comparable to human-level accuracy.

Impact: The automatic transformer-based pipeline developed in this work achieved rapid (~6s) and accurate pixel-level 3D needle localization on intra-procedural 3D MRI. This new pipeline has the potential to improve MRI-guided percutaneous interventions.

Keywords: MR-Guided Interventions, MR-Guided Interventions, needle intervention, percutaneous intervention, needle tracking, needle localization, device tracking, passive tracking, modelling

Motivation: For MR-guided needle interventions, rapid 3D needle localization enables automatic realignment of 2D real-time imaging slices with the device during the procedure.

Goal(s): To investigate a model-based approach for rapid 3D needle localization that does not require prepositioned tracking slices.

Approach: 3D k-space data was radially acquired before and after needle placement. Two algorithms to extract position and orientation of the needle were introduced and compared (artifact model fit to undersampled subtraction k-space/image).

Results: Model-based rapid 3D needle localization was successfully demonstrated in-vivo (k-space-based offline localization error 4.3mm for FOV of (256cm)³ in 1.1s combined acquisition and localization time using 64 k-space spokes).

Impact: Model-based rapid 3D passive needle localization shows potential to improve the workflow of MR-guided needle interventions, allowing for automatic alignment of 2D real-time imaging slices with the needle trajectory.

Keywords: MR-Guided Interventions, MR-Guided Interventions, localization, needle artifact

Motivation: Needle tip localization is crucial for accurate and safe image-guided intervention, especially for real-time interventional MRI.

Goal(s): To develop an algorithm that utilizes artifact features of the needle tip as prior information for needle tip localization.

Approach: Combines target tracking and template matching techniques for accurate needle tip localization.

Results: The algorithm's performance has been evaluated using both simulation and physical models. Validation results demonstrate that the algorithm enables fast identification, stable tracking, and highly precise needle tip localization.

Impact: This needle tip localization method, leveraging artifact features as prior information, opens up new possibilities for the development of needle localization and tracking algorithms based on artifacts.

Keywords: MR-Guided Interventions, MR-Guided Interventions, 3D real-time imaging

Motivation: MRI excels in exceptional soft tissue delineation and its radiation-free nature in interventional contexts. However, achieving effective 3D real-time tracking with MRI poses a significant challenge.

Goal(s): To facilitate 3D real-time monitoring and navigation, achieving a high temporal resolution and enhanced spatial resolution that precisely captures the movement of small interventional instruments.

Approach: We utilized a manually controlled device and employed highly accelerated golden-angle rotated spiral-in/out sequence with randomized variable density k_z encoding, along with a specifically tailored iterative reconstruction algorithm.

Results: High temporal (150 ms/phase) and spatial (0.7×0.7×1.5 mm³) resolutions were achieved, smoothly visualizing minute movements in 3D within our phantom setup.

Impact: The SPARTA-3D approach holds the promise of revolutionizing interventional MRI by meeting the crucial demand for 3D real-time tracking and navigation. It could facilitate more rapid and less invasive procedures, opening up novel avenues in the domain of MR-guided interventions.

Keywords: MR-Guided Interventions, Cardiovascular

Motivation: MRI-guidance of cardiac catheterisation is currently performed using one or multiple 2D imaging planes, which may be suboptimal for catheter navigation in congenital heart disease patients with complex anatomies.

Goal(s): To develop a robust real-time 3D catheter tracking method and 3D visualisation strategy for improved MRI-guidance of cardiac catheterisation procedures.

Approach: Fast 3D screening through projection imaging combined with advanced post-processing strategies were developed for real-time 3D catheter tracking and enhanced 3D visualisation. The method was evaluated in patients.

Results: This approach was demonstrated in three patients and was able to successfully track and visualise the catheter in 3D.

Impact: Our approach show promise for enhanced catheter navigation and visualisation during MRI-guided cardiac catheterization and may contribute to reduce procedural time and outcome.

Keywords: MR-Guided Interventions, Prostate, MRI-guided intervention

Motivation: MR-guided prostate biopsies are important for sampling indeterminate lesions and are performed at 1.5 or 3T.There are potential advantages of performing interventions on lower field scanners due to their potential to decrease needle artifact width, larger bores to accommodate hardware, improved imaging in presence of metallic implants, and patient comfort. However, lesion/needle visualization, SNR, and ability to target lesions could be challenging at lower fields.

Goal(s): To assess feasibility of MRI-guided prostate biopsies on a low field scanner.

Approach: We performed 5 in-vivo biopsies and did histopathological correlation.

Results: We performed successful biopsies and high volume disease was found in all cases.

Impact: Successful lesion visualization and percutaneous and transrectal prostate biopsies are possible on high access, wide bore 0.55 T scanners, widening the clinical feasibility and utility of in-bore MRI guided intervention.

Keywords: MR-Guided Interventions, Interventional Devices, Cryoablation

Motivation: In the prostate, transperineal needle placement is often assisted with trajectory guides and imaging. Improper placement necessitates needle reinsertion, increasing procedure time and complication risk. Presented here is a straightforward platform tested in multiple biopsy and cryoablation procedures. Previous work demonstrated feasibility in a single MR-guided focal biopsy.

Goal(s): To demonstrate the platform’s technical feasibility across greater procedural types and numbers.

Approach: The proposed techniques were performed on consented patients. These procedures included prostate cryoablations, biopsies, and fluid aspirations.

Results: 12 procedures were performed- 6 cryoablations, 5 biopsies, and 1 fluid aspiration. Technical success was achieved in every procedure.

Impact: Using the proposed methodology, all 12 prostate procedures achieved technical success. With the platform’s support, a median procedure saw satisfactory needle placement in 83.7% of initial insertions, requiring no further adjustment. This can reduce procedure times and complication risks.

Keywords: MR-Guided Interventions, Low-Field MRI, Cardiovascular Intevention

Motivation: It is challenging to achieve the spatial and temporal resolutions required for real-time interventional device visualization using low-field scanners with limited gradient performance.

Goal(s): The objective of this study is to develop a novel post-processing method capable of delivering high temporal and spatial resolution images in real-time.

Approach: We developed a method that sequentially combines GRAPPA with a DnCNN denoising network to provide highly accelerated acquisition with low-latency reconstruction. The network was trained using the OCMR dataset and evaluated using pre-clinical data. 

Results: The network effectively suppressed noise with minimal latency, while preserving the original features of the image.

Impact: This method could enhance the speed and quality of real-time interventional imaging at low field, making it easier for the interventionalist to visually track devices and deploy stents and other devices.