S Crijns

Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept

At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.

Proof of concept of MRI-guided tracked radiation delivery: tracking one-dimensional motion

In radiotherapy one aims to deliver a radiation dose to a tumour with high geometrical accuracy while sparing organs at risk (OARs). Although image guidance decreases geometrical uncertainties, treatment of cancer of abdominal organs is further complicated by respiratory motion, requiring intra-fraction motion compensation to fulfil the treatment intent. With an ideal delivery system, the optimal method of intra-fraction motion compensation is to adapt the beam collimation to the moving target using a dynamic multi-leaf collimator (MLC) aperture. The many guidance strategies for such tracked radiation delivery tested up to now mainly use markers and are therefore invasive and cannot deal with target deformations or adaptations for OAR positions. We propose to address these shortcomings using the online MRI guidance provided by an MRI accelerator and present a first step towards demonstration of the technical feasibility of this proposal. The position of a phantom subjected to one-dimensional (1D) periodic translation was tracked using a fast 1D MR sequence. Real-time communication with the MR scanner and control of the MLC aperture were established. Based on the time-resolved position of the phantom, tracked radiation delivery to the phantom was realized. Dose distributions for various delivery conditions were recorded on a gafchromic film. Without motion a sharply defined dose distribution is obtained, whereas considerable blur occurs for delivery to a moving phantom. With compensation for motion, the sharpness of the dose distribution is nearly restored. The total latency in our motion management architecture is approximately 200 ms. Combination of the recorded phantom and aperture positions with the planned dose distribution enabled the reconstruction of the delivered dose in all cases, which illustrates the promise of online dose accumulation and confirms that latency compensation could further enhance our results. For a simple 1D tracked delivery scenario, the technical feasibility of MRI-guided tracked radiation delivery is confirmed. More generic tracking scenarios require advanced MRI, leading to increased acquisition time and more challenging image processing problems. Latency compensation is therefore an important subject of future investigations.

Towards MRI-guided linear accelerator control: gating on an MRI accelerator

To boost the possibilities of image guidance in radiotherapy by providing images with superior soft-tissue contrast during treatment, we pursue diagnostic quality MRI functionality integrated with a linear accelerator. Large respiration-induced semi-periodic target excursions hamper treatment of cancer of the abdominal organs. Methods to compensate in real time for such motion are gating and tracking. These strategies are most effective in cases where anatomic motion can be visualized directly, which supports the use of an integrated MRI accelerator. We establish here an infrastructure needed to realize gated radiation delivery based on MR feedback and demonstrate its potential as a first step towards more advanced image guidance techniques. The position of a phantom subjected to one-dimensional periodic translation is tracked with the MR scanner. Real-time communication with the MR scanner and control of the radiation beam are established. Based on the time-resolved position of the phantom, gated radiation delivery to the phantom is realized. Dose distributions for dynamic delivery conditions with varying gating windows are recorded on gafchromic film. The similarity between dynamically and statically obtained dose profiles gradually increases as the gating window is decreased. With gating windows of 5 mm, we obtain sharp dose profiles. We validate our gating implementation by comparing measured dose profiles to theoretical profiles calculated using the knowledge of the imposed motion pattern. Excellent correspondence is observed. At the same time, we show that real-time on-line reconstruction of the accumulated dose can be performed using time-resolved target position information. This facilitates plan adaptation not only on a fraction-to-fraction scale but also during one fraction, which is especially valuable in highly accelerated treatment strategies. With the currently established framework and upcoming improvements to our prototype-integrated MRI accelerator, we will realize more intricate MRI-guided linear accelerator control in the near future.

Towards inherently distortion-free MR images for image-guided radiotherapy on an MRI accelerator

In MR-guided interventions, it is mandatory to establish a solid relationship between the imaging coordinate system and world coordinates. This is particularly important in image-guided radiotherapy (IGRT) on an MRI accelerator, as the interaction of matter with γ-radiation cannot be visualized. In conventional acquisitions, off-resonance effects cause discrepancies between coordinate systems. We propose to mitigate this by using only phase encoding and to reduce the longer acquisitions by under-sampling and regularized reconstruction. To illustrate the performance of this acquisition in the presence of off-resonance phenomena, phantom and in vivo images are acquired using spin-echo (SE) and purely phase-encoded sequences. Data are retrospectively under-sampled and reconstructed iteratively. We observe accurate geometries in purely phase-encoded images for all cases, whereas SE images of the same phantoms display image distortions. Regularized reconstruction yields accurate phantom images under high acceleration factors. In vivo images were reconstructed faithfully while using acceleration factors up to 4. With the proposed technique, inherently undistorted images with one-to-one correspondence to world coordinates can be obtained. It is a valuable tool in geometry quality assurance, treatment planning and online image guidance. Under-sampled acquisition combined with regularized reconstruction can be used to accelerate the acquisition while retaining geometrical accuracy.

Real-time correction of magnetic field inhomogeneity-induced image distortions for MRI-guided conventional and proton radiotherapy

Image-guided radiotherapy has the potential to increase the success of treatment by decreasing uncertainties concerning tumour position and shape. We pursue integrated diagnostic quality MRI functionality with radiotherapy systems to boost the possibilities of image guidance by providing images with superior soft-tissue contrast during treatment. However, the use of MR images in radiotherapy can be hindered by geometrical distortions due to magnetic field inhomogeneity problems. A method for fast correction of these distortions is presented and implemented. Using a 20 cm square phantom containing a regular grid, a measure of residual deformation after correction is established. At very low gradient strength (which leads to large deformations) a maximum displacement of 2.9 mm is shown to be reduced to 0.63 mm. Next, the method is applied in vivo to the case of pelvic body contour extraction for prostate radiotherapy treatment planning. Here, again with low gradient strengths, distortions of up to 6 mm can be reduced to 2 mm. All results are provided within a lag time of 8 ms. We discuss implications of image distortions for MRI-guided photon and proton radiotherapy separately, since the dose-depth curves in these treatments are very different. We argue that, although field inhomogeneities cannot be prevented from occurring, distortion correction is not always necessary in practice. This work opens new possibilities for investigating on-line MRI-based plan adaptations and ultimately MRI-based treatment planning.

MRI-based tumor motion characterization and gating schemes for radiation therapy of pancreatic cancer

BACKGROUND AND PURPOSE To characterize pancreatic tumor motion and to develop a gating scheme for radiotherapy in pancreatic cancer. MATERIALS AND METHODS Two cine MRIs of 60s each were performed in fifteen pancreatic cancer patients, one in sagittal direction and one in coronal direction. A Minimum Output Sum of Squared Error (MOSSE) adaptive correlation filter was used to quantify tumor motion in craniocaudal, lateral and anteroposterior directions. To develop a gating scheme, stability of the breathing phases was examined and a gating window assessment was created, incorporating tumor motion, treatment time and motion margins. RESULTS The largest tumor motion was found in craniocaudal direction, with an average peak-to-peak amplitude of 15mm (range 6-34mm). Amplitude of the tumor in the anteroposterior direction was on average 5mm (range 1-13mm). The least motion was seen in lateral direction (average 3mm, range 2-5mm). The end exhale position was the most stable position in the breathing cycle and tumors spent more time closer to the end exhale position than to the end inhale position. On average, a margin of 25% of the maximum craniocaudal breathing amplitude was needed to achieve full target coverage with a duty cycle of 50%. When reducing the duty cycle to 50%, a margin of 5mm was sufficient to cover the target in 11 out of 15 patients. CONCLUSION Gated delivery for radiotherapy of pancreatic cancer is best performed around the end exhale position as this is the most stable position in the breathing cycle. Considerable margin reduction can be established at moderate duty cycles, yielding acceptable treatment efficiency. However, motion patterns and amplitude do substantially differ between individual patients. Therefore, individual treatment strategies should be considered for radiotherapy in pancreatic cancer.

On-line MR imaging for dose validation of abdominal radiotherapy

For quality assurance and adaptive radiotherapy, validation of the actual delivered dose is crucial.Intrafractional anatomy changes cannot be captured satisfactorily during treatment with hitherto available imaging modalitites. Consequently, dose calculations are based on the assumption of static anatomy throughout the treatment. However, intra- and interfraction anatomy is dynamic and changes can be significant.In this paper, we investigate the use of an MR-linac as a dose tracking modality for the validation of treatments in abdominal targets where both respiratory and long-term peristaltic and drift motion occur.The on-line MR imaging capability of the modality provides the means to perform respiratory gating of both delivery and acquisition yielding a model-free respiratory motion management under free breathing conditions.In parallel to the treatment, the volumetric patient anatomy was captured and used to calculate the applied dose. Subsequently, the individual doses were warped back to the planning grid to obtain the actual dose accumulated over the entire treatment duration. Ultimately, the planned dose was validated by comparison with the accumulated dose.Representative for a site subject to breathing modulation, two kidney cases (25 Gy target dose) demonstrated the working principle on volunteer data and simulated delivery. The proposed workflow successfully showed its ability to track local dosimetric changes. Integration of the on-line anatomy information could reveal local dose variations  -2.3-1.5 Gy in the target volume of a volunteer dataset. In the adjacent organs at risk, high local dose errors ranging from  -2.5 to 1.9 Gy could be traced back.

Quantification of esophageal tumor motion on cine-magnetic resonance imaging

PURPOSE To quantify the movement of esophageal tumors noninvasively on cine-magnetic resonance imaging (MRI) by use of a semiautomatic method to visualize tumor movement directly throughout multiple breathing cycles. METHODS AND MATERIALS Thirty-six patients with esophageal tumors underwent MRI. Tumors were located in the upper (8), middle (7), and lower (21) esophagus. Cine-MR images were collected in the coronal and sagittal plane during 60 seconds at a rate of 2 Hz. An adaptive correlation filter was used to automatically track a previously marked reference point. Tumor movement was measured in the craniocaudal (CC), left-right (LR), and anteroposterior (AP) directions and its relationship along the longitudinal axis of the esophagus was investigated. RESULTS Tumor registration within the individual images was typically done at a millisecond time scale. The mean (SD) peak-to-peak displacements in the CC, AP, and LR directions were 13.3 (5.2) mm, 4.9 (2.5) mm, and 2.7 (1.2) mm, respectively. The bandwidth to cover 95% of excursions from the mean position (c95) was also calculated to exclude outliers caused by sporadic movements. The mean (SD) c95 values were 10.1 (3.8) mm, 3.7 (1.9) mm, and 2.0 (0.9) mm in the CC, AP, and LR dimensions. The end-exhale phase provided a stable position in the respiratory cycle, compared with more variety in the end-inhale phase. Furthermore, lower tumors showed more movement than did higher tumors in the CC and AP directions. CONCLUSIONS Intrafraction tumor movement was highly variable between patients. Tumor position proved the most stable during the respiratory cycle in the end-exhale phase. A better understanding of tumor motion makes it possible to individualize radiation delivery strategies accordingly. Cine-MRI is a successful noninvasive modality to analyze motion for this purpose in the future.

Integrated megavoltage portal imaging with a 1.5 T MRI linac.

In this note, the feasibility of complementing our hybrid 1.5 T MRI linac (MRL) with a megavoltage (MV) portal imager is investigated. A standard aSi MV detector panel is added to the system and both qualitative and quantitative performances are determined. Simultaneous MR imaging and transmission imaging can be performed without mutual interference. The MV image quality is compromised by beam transmission and longer isocentre distance; still, the field edges and bony anatomy can be detected at very low dose levels of 0.4 cGy. MV imaging integrated with the MRL provides an independent and well-established position verification tool, a field edge check and a calibration for alignment of the coordinate systems of the MRI and the accelerator. The portal imager can also be a valuable means for benchmarking MRI-guided position verification protocols on a patient-specific basis in the introductory phase.

Consensus opinion on MRI simulation for external beam radiation treatment planning

AIM To determine the levels at which consensus could be reached regarding general and site-specific principles of MRI simulation for offline MRI-aided external beam radiation treatment planning. METHODS A process inspired by the Delphi method was employed to determine levels of consensus using a series of questionnaires interspersed with controlled opinion feedback. RESULTS In general, full consensus was reached regarding general principles of MRI simulation. However, the level of consensus decreased when site-specific principles of MRI simulation were considered. CONCLUSIONS These results indicate variability in MRI simulation approaches that are largely explained by the use of MRI in combination with CT.