Evaluation of Intrafractional Anatomical Variations During Liver Magnetic Resonance-guided Adaptive Radiation Therapy Under Abdominal Compression Using Optical Flow Calculation

PURPOSE

To investigate the intrafractional anatomical variations during liver magnetic resonance-guided adaptive radiation therapy (MRgART) under abdominal compression (AC) using optical flow calculations.

METHODS AND MATERIALS

This study included 27 consecutive patients who underwent liver MRgART under AC. Overall, 387,566 slices from 145 single-slice cine magnetic resonance series obtained from 29 different treatment plans were analyzed in the coronal and sagittal planes through tumor centers. After defining the square regions as 12 pixels centered on the isocenter for the tumor and 8 pixels between the inspiratory and expiratory phases, excluding the lung/liver boundary for the diaphragm, the vectors were calculated using Farnebäck optical flow. The intrafractional superior-inferior (SI) motion range and the root-mean-square error (RMSE) between the position of the tumor and the diaphragm in the coronal (SIcoronal and RMSEcoronal) and sagittal planes (SIsagittal and RMSEsagittal) were classified according to the Couinaud-based tumor regions (segments I + IV, II + III, V + VIII, and VI + VII). Statistical significance was determined using the Wilcoxon signed-rank test with Holm-Bonferroni corrections (P < .05).

RESULTS

The median SIcoronal and SIsagittal motion ranges of the tumor were 6.1 mm (range, 1.5-18.0 mm) and 8.1 mm (range, 1.0-21.0 mm), respectively (P < .05). When classified according to tumor location, segments VI + VII showed the largest difference, with the median SIcoronal and SIsagittal motion ranges of 6.5 mm (range, 2.3-17.7 mm) and 10.6 mm (range, 4.8-21.0 mm), respectively (P < .05). The median RMSEcoronal and RMSEsagittal values were the largest in segments VI + VII, showing significant differences of 2.6 and 2.2 mm, respectively (P < .05). These differences were caused by the sliding motion of dorsally located tumors.

CONCLUSIONS

Optical flow analysis underestimated the SI motion range in the coronal plane compared with that in the sagittal plane during liver MRgART under AC. Tumor motion should be monitored in the sagittal plane, considering the sliding motion of the liver, with individualized margins according to tumor location.

Atrial fibrillation cardiac radioablation target visibility on magnetic resonance imaging

Magnetic resonance imaging (MRI) guided cardiac radioablation (CR) for atrial fibrillation (AF) is a promising treatment concept. However, the visibility of AF CR targets on MRI acquisitions requires further exploration and MRI sequence and parameter optimization has not yet been performed for this application. This pilot study explores the feasibility of MRI-guided tracking of AF CR targets by evaluating AF CR target visualization on human participants using a selection of 3D and 2D MRI sequences.MRI datasets were acquired in healthy and AF participants using a range of MRI sequences and parameters. MRI acquisition categories included 3D free-breathing acquisitions (3D_acq), 2D breath-hold ECG-gated acquisitions (2D_ECG-gated), stacks of 2D breath-hold ECG-gated acquisitions which were retrospectively interpolated to 3D datasets (3D_interp), and 2D breath-hold ungated acquisitions (2D_real-time). The ease of target delineation and the presence of artifacts were qualitatively analyzed. Image quality was quantitatively analyzed using signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and non-uniformity. Confident 3D target delineation was achievable on all 3D_interp datasets but was not possible on any of the 3D_acq datasets. Fewer artifacts and significantly better SNR, CNR and non-uniformity metrics were observed with 3D_interp compared to 3D_acq. 2D_real-time datasets had slightly lower SNR and CNR than 2D_ECG-gated and 3D_{interp n} datasets. AF CR target visualization on MRI was qualitatively and quantitatively evaluated. The study findings indicate that AF CR target visualization is achievable despite the imaging challenges associated with these targets, warranting further investigation into MRI-guided AF CR treatments.

Hadron Therapy Achievements and Challenges: The CNAO Experience

Protons and carbon ions (hadrons) have useful properties for the treatments of patients affected by oncological pathologies. They are more precise than conventional X-rays and possess radiobiological characteristics suited for treating radio-resistant or inoperable tumours. This paper gives an overview of the status of hadron therapy around the world. It focusses on the Italian National Centre for Oncological Hadron therapy (CNAO), introducing operation procedures, system performance, expansion projects, methodologies and modelling to build individualized treatments. There is growing evidence that supports safety and effectiveness of hadron therapy for a variety of clinical situations. However, there is still a lack of high-level evidence directly comparing hadron therapy with modern conventional radiotherapy techniques. The results give an overview of pre-clinical and clinical research studies and of the treatments of 3700 patients performed at CNAO. The success and development of hadron therapy is strongly associated with the creation of networks among hadron therapy facilities, clinics, universities and research institutions. These networks guarantee the growth of cultural knowledge on hadron therapy, favour the efficient recruitment of patients and present available competences for R&D (Research and Development) programmes.

Dual k-space and image-space post-processing for field-cycling MRI under low magnetic field stability and homogeneity conditions

We present a method to correct artifacts typically present in images acquired in field-cycled MRI experiments under poor magnetic field spatial-homogeneity and time-stability conditions. The proposed method was tested in both simulated and experimental data. The experiments were performed using a fast field-cycling MRI relaxometer of own design, based on a current-driven variable-geometry electromagnet. Current instability-induced artifacts in the images were mitigated through a phase correction array resulted from entropy and background minimization. Image distortions due to magnetic field inhomogeneity were compensated through two different approaches, involving a previous determination of the magnetic field homogeneity-map, or an experimental protocol where two images are acquired with inverted readout gradient polarity. Results show that images acquired at extreme conditions can be successfully improved, thus strengthening the possibilities for both low-cost MRI devices and faster field-switched MRI systems.

MRI-guided cardiac-induced target motion tracking for atrial fibrillation cardiac radioablation

BACKGROUND AND PURPOSE

Atrial fibrillation (AF) cardiac radioablation (CR) challenges radiotherapy tracking: multiple small targets close to organs-at-risk undergo rapid differential cardiac contraction and respiratory motion. MR-guidance offers a real-time target tracking solution. This work develops and investigates MRI-guided tracking of AF CR targets with cardiac-induced motion.

MATERIALS AND METHODS

A direct tracking method (Tracking_direct) and two indirect tracking methods leveraging population-based surrogacy relationships with the left atria (Tracking_{indirect_LA}) or other target (Tracking_{indirect_target}) were developed. Tracking performance was evaluated using transverse ECG-gated breathhold MRI images from 15 healthy and 10 AF participants. Geometric and volumetric tracking errors were calculated, defined as the difference between the ground-truth and tracked target centroids and volumes respectively. Transverse, breath-hold, noncardiac-gated cine images were acquired at 4 Hz in 5 healthy and 5 AF participants to qualitatively characterize tracking performance on images more comparable to MRILinac acquisitions.

RESULTS

The average 3D geometric tracking errors for Tracking_direct, Tracking_{indirect_LA} and Tracking_{indirect_target} respectively were 1.7 ± 1.2 mm, 1.6 ± 1.1 mm and 1.9 ± 1.3 mm in healthy participants and 1.7 ± 1.3 mm, 1.5 ± 1.0 mm and 1.7 ± 1.2 mm in AF participants. For Tracking_direct, 88% of analyzed images had 3D geometric tracking errors <3 mm and the average volume tracking error was 1.7 ± 1.3 cc. For Tracking_direct on non-cardiac-gated cine images, tracked targets overlapped organsat-risk or completely missed the target area on 2.2% and 0.08% of the images respectively.

CONCLUSION

The feasibility of non-invasive MRI-guided tracking of cardiac-induced AF CR target motion was demonstrated for the first time, showing potential for improving AF CR treatment efficacy.

Lung Respiratory Motion Estimation Based on Fast Kalman Filtering and 4D CT Image Registration

Respiratory motion estimation is an important part in image-guided radiation therapy and clinical diagnosis. However, most of the respiratory motion estimation methods rely on indirect measurements of external breathing indicators, which will not only introduce great estimation errors, but also bring invasive injury for patients. In this paper, we propose a method of lung respiratory motion estimation based on fast Kalman filtering and 4D CT image registration (LRME-4DCT). In order to perform dynamic motion estimation for continuous phases, a motion estimation model is constructed by combining two kinds of GPU-accelerated 4D CT image registration methods with fast Kalman filtering method. To address the high computational requirements of 4D CT image sequences, a multi-level processing strategy is adopted in the 4D CT image registration methods, and respiratory motion states are predicted from three independent directions. In the DIR-lab dataset and POPI dataset with 4D CT images, the average target registration error (TRE) of the LRME-4DCT method can reach 0.91 mm and 0.85 mm respectively. Compared with traditional estimation methods based on pair-wise image registration, the proposed LRME-4DCT method can estimate the physiological respiratory motion more accurately and quickly. Our proposed LRME-4DCT method fully meets the practical clinical requirements for rapid dynamic estimation of lung respiratory motion.

Simulated accuracy assessment of small footprint body-mounted probe alignment device for MRI-guided cryotherapy of abdominal lesions

PURPOSE

Magnetic resonance imaging (MRI)-guided percutaneous cryotherapy of abdominal lesions, an established procedure, uses MRI to guide and monitor the cryoablation of lesions. Methods to precisely guide cryotherapy probes with a minimum amount of trial-and-error are yet to be established. To aid physicians in attaining precise probe alignment without trial-and-error, a body-mounted motorized cryotherapy-probe alignment device (BMCPAD) with motion compensation was clinically tested in this study. The study also compared the contribution of body motion and organ motion compensation to the guidance accuracy of a body-mounted probe alignment device.

METHODS

The accuracy of guidance using the BMCPAD was prospectively measured during MRI-guided percutaneous cryotherapies before insertion of the probes. Clinical parameters including patient age, types of anesthesia, depths of the target, and organ sites of target were collected. By using MR images of the target organs and fiducial markers embedded in the BMCPAD, we retrospectively simulated the guidance accuracy with body motion compensation, organ motion compensation, and no compensation. The collected data were analyzed to test the impact of motion compensation on the guidance accuracy.

RESULTS

Thirty-seven physical guidance of probes were prospectively recorded for sixteen completed cases. The accuracy of physical guidance using the BMCPAD was 13.4 ± 11.1 mm. The simulated accuracy of guidance with body motion compensation, organ motion compensation, and no compensation was 2.4 ± 2.9 mm, 2.2 ± 1.6 mm, and 3.5 ± 2.9 mm, respectively. Data analysis revealed that the body motion compensation and organ motion compensation individually impacted the improvement in the accuracy of simulated guidance. Moreover, the difference in the accuracy of guidance either by body motion compensation or organ motion compensation was not statistically significant. The major clinical parameters impacting the accuracy of guidance were the body and organ motions. Patient age, types of anesthesia, depths of the target, and organ sites of target did not influence the accuracy of guidance using BMCPAD. The magnitude of body surface movement and organ movement exhibited mutual statistical correlation.

CONCLUSIONS

The BMCPAD demonstrated guidance accuracy comparable to that of previously reported devices for CT-guided procedures. The analysis using simulated motion compensation revealed that body motion compensation and organ motion compensation individually impact the improvement in the accuracy of device-guided cryotherapy probe alignment. Considering the correlation between body and organ movements, we also determined that body motion compensation using the ring fiducial markers in the BMCPAD can be solely used to address both body and organ motions in MRI-guided cryotherapy.

Organ motion quantification and margins evaluation in carbon ion therapy of abdominal lesions

PURPOSE

In image-guided particle radiotherapy of abdominal lesions, respiratory motion hinders treatment accuracy. In this study, 2D cineMRI data were used to quantify the tumor (GTV) motion and to evaluate the clinical approach based on deriving an internal target volume (ITV) from a planning 4DCT for gating treatments.

METHODS

Seven patients with abdominal lesions were treated with carbon-ion therapy at the National Centre of Oncological Hadron-therapy (Italy). The MR scan was performed on the same day of the 4DCT acquisition. For four patients, an additional MR was acquired approximately after 1 week. The cineMRI combined with deformable image registration algorithm was used to quantify tumor motion. Afterwards, two ITVs were defined considering (1) all phases (ITVFB) and (2) only phases within the gating window (ITVG), and then compared with the clinical (4DCT-derived) ITVs (ITVCG and ITVCFB).

RESULTS

Tumor residual motion estimated by cineMRI data in the two MRI sessions resulted not significantly different from 4DCT, although cineMRI accounted for cycle-to-cycle variations. The ITV normalized for the GTV median values were higher for ITVFB with respect to ITVG, ITVCFB and ITVCG. The Hausdorff distances with respect to the GTV were up to 10.55 mm, 3.13 mm, 5.56 mm and 2.51 mm, for ITVFB, ITVG, ITVCFB and ITVCG, respectively. According to both metrics, ITVCG and ITVG were not found significantly different.

CONCLUSIONS

CineMRI acquisitions allowed to quantify organ motion without delivering additional dose to the patient and to verify treatment margins in gated carbon-ion therapy of abdominal lesions.

3D dosimetric verification of unity MR-linac treatments by portal dosimetry

PURPOSE AND BACKGROUND

3D dosimetric verification of online adaptive workflows is essential as their complexity is unprecedented in radiation oncology. The aim of this work is to demonstrate the feasibility of back-projection portal dosimetry for 3D dosimetric verification of Unity MR-linac treatments.

MATERIAL AND METHODS

An earlier presented 2D back-projection algorithm for the Unity MR-linac geometry was extended for 3D dose reconstruction and comparison against planned dose distributions. 'In-air' as well as in-vivo portal EPID images can be used as input. The method was validated using data from treatments of 5 patients (2 rectal, 2 prostate cancer and one oligo metastasis). 3D pre-treatment verification of the reference plan using 'in-air' EPID images was performed and compared against measured (with the Octavius 4D system) and planned (in the planning CT) dose distributions. In-vivo EPID dose distributions were compared to the TPS for the first three adaptations of all treatments. For all comparisons, dose difference values at the reference point and γ-parameters were reported.

RESULTS

The comparison against the OCTAVIUS 4D system (3%, 2 mm, local) showed y-mean = 0.52 ± 0.10 and y-passrate = 91.9%, 95% CI [85.4, 98.4], and ΔD_RP = -0.1 ± 1.1%. Pre-treatment verification against TPS data (3%, 2 mm, global) showed y-mean = 0.52 ± 0.04, y-passrate = 93.5%, 95% CI [92.4, 94.6] and ΔD_RP = -0.9 ± 1.5%. The averaged y-results for the in-vivo 3D verification were y-mean = 0.52 ± 0.05, y-passrate = 92.5%, 95% CI [90.2, 94.8] and ΔD_RP = 0.8 ± 2.1%.

CONCLUSION

3D dosimetric verification of Unity MR-linac treatments using portal dosimetry is feasible, pre-treatment as well as in-vivo.

General method for complex-wave fields registration with high fidelity

In the field of optical imaging, the image registration method could be applied to realize a large field of view along with high resolution. The traditional image registration methods are mostly conceived for intensity images and might fail for complex-valued images. Especially, those methods do not account for the random phase offset associated with phase. In this paper, we proposed a general method for complex-wave field registration. A similar procedure has been proposed for the reconstruction of the ptychographic dataset, but here is modified for the registration of general wave fields. The method can efficiently separate the illumination and object function, refine the positions of each wavefront, and thus provide a stitched wide-field object wave with high fidelity. Simulation and experimental results applied to register the wave fields obtained from digital holographic microscopy are given to verify the feasibility of the method. This method would have potential applications in large-field high-resolution microscopy, adaptive imaging, remote sensing and the measurement of structured optical fields.

Time-resolved volumetric MRI in MRI-guided radiotherapy: an in silico comparative analysis

MRI-treatment units enable 2D cine-MRI centred in the tumour for motion detection in radiotherapy, but they lack 3D information due to spatio-temporal limits. To derive time-resolved 3D information, different approaches have been proposed in the literature, but a rigorous comparison among these strategies has not yet been performed. The goal of this study is to quantitatively investigate five published strategies that derive time-resolved volumetric MRI in MRI-guided radiotherapy: Propagation, out-of-plane motion compensation, Fayad model, ROI-based model and Stemkens model. Comparisons were performed using an MRI digital phantom generated with six different patient-derived motion signals and tumour-shapes. An average 4D cycle was generated as well as 2D cine-MRI data with corresponding 3D in-room ground truth. Quantitative analysis was performed by comparing the estimated 3D volume to the ground truth available for each 2D cine-MRI sample. A grouped patient statistical analysis was performed to evaluate the performance of the selected methods, in case of tumour tracking or motion estimation of the whole anatomy. Analyses were also performed based on patient characteristics. Quantitative ranking of the investigated methods highlighted that Propagation and ROI-based model strategies achieved an overall median tumour centre of mass 3D distance from the ground truth of 1.1 mm and 1.3 mm, respectively, and a diaphragm distance below 1.6 mm. Higher errors and variabilities were instead obtained for other methods, which lack the ability to compensate for in-room variations and to account for regional changes. These results were especially evident when further analysing patient characteristics, where errors above 2 mm/5 mm in tumour/diaphragm were found for more irregular breathing patterns in case of out-of-plane motion compensation, Fayad and Stemkens models. These findings suggest the potential of the proposed in silico framework to develop and compare strategies to estimate time-resolved 3DMRI in MRI-guided radiotherapy.

A ROI-based global motion model established on 4DCT and 2D cine-MRI data for MRI-guidance in radiation therapy

In-room magnetic resonance imaging (MRI) allows the acquisition of fast 2D cine-MRI centered in the tumor for advanced motion management in radiotherapy. To achieve 3D information during treatment, patient-specific motion models can be considered the most viable solution. However, conventional global motion models are built using a single motion surrogate, independently from the anatomical location. In this work, we present a novel motion model based on regions of interest (ROIs) established on 4D computed tomography (4DCT) and 2D cine-MRI, aiming at accurately compensating for changes during treatment. In the planning phase, a motion model is built on a 4DCT dataset, through 3D deformable image registration (DIR). ROIs are then defined and correlated with motion fields derived by 2D DIR between CT slices centered in the tumor. In the treatment phase, the model is applied to in-room cine-MRI data to compensate for organ motion in a multi-modal framework, aiming at estimating a time-resolved 3DCT. The method is validated on a digital phantom and tested on two lung patients. Analysis is performed by considering different anatomical planes (coronal, sagittal and a combination of the two) and evaluating the performance of the method on tumor and diaphragm. For the phantom study, the ROI-based model results in a uniform median error on both diaphragm and tumor below 1.5 mm. For what concerns patients, median errors on both diaphragm and tumor are around 2 mm (maximum patient resolution), confirming the capability of the method to regionally compensate for motion. A novel ROI-based motion model is proposed as an integral part of an envisioned clinical MRI-guided workflow aiming at enhanced image guidance compared to conventional strategies.

Classification of Liver Diseases Based on Ultrasound Image Texture Features

This paper discusses using computer-aided diagnosis (CAD) to distinguish between hepatocellular carcinoma (HCC), i.e., the most common type of primary liver malignancy and a leading cause of death in people with cirrhosis worldwide, and liver abscess based on ultrasound image texture features and a support vector machine (SVM) classifier. Among 79 cases of liver diseases including 44 cases of liver cancer and 35 cases of liver abscess, this research extracts 96 features including 52 features of the gray-level co-occurrence matrix (GLCM) and 44 features of the gray-level run-length matrix (GLRLM) from the regions of interest (ROIs) in ultrasound images. Three feature selection models—(i) sequential forward selection (SFS), (ii) sequential backward selection (SBS), and (iii) F-score—are adopted to distinguish the two liver diseases. Finally, the developed system can classify liver cancer and liver abscess by SVM with an accuracy of 88.875%. The proposed methods for CAD can provide diagnostic assistance while distinguishing these two types of liver lesions.

MRI-guidance for motion management in external beam radiotherapy: current status and future challenges

High precision conformal radiotherapy requires sophisticated imaging techniques to aid in target localisation for planning and treatment, particularly when organ motion due to respiration is involved. X-ray based imaging is a well-established standard for radiotherapy treatments. Over the last few years, the ability of magnetic resonance imaging (MRI) to provide radiation-free images with high-resolution and superb soft tissue contrast has highlighted the potential of this imaging modality for radiotherapy treatment planning and motion management. In addition, these advantageous properties motivated several recent developments towards combined MRI radiation therapy treatment units, enabling in-room MRI-guidance and treatment adaptation. The aim of this review is to provide an overview of the state-of-the-art in MRI-based image guidance for organ motion management in external beam radiotherapy. Methodological aspects of MRI for organ motion management are reviewed and their application in treatment planning, in-room guidance and adaptive radiotherapy described. Finally, a roadmap for an optimal use of MRI-guidance is highlighted and future challenges are discussed.