MarkerDetector: A method for robust fiducial marker detection in electron micrographs using wavelet-based template.

Fiducial marker detection in electron micrographs becomes an important and challenging task with the development of large-field electron microscopy. The fiducial marker detection plays an important role in several steps during the process of electron micrographs, such as the alignment and parameter calibrations. However, limited by the conditions of low signal-to-noise ratio (SNR) in the electron micrographs, the performance of fiducial marker detection is severely affected. In this work, we propose the MarkerDetector, a novel algorithm for detecting fiducial markers in electron micrographs. The proposed MarkerDetector is built upon the following contributions: Firstly, a wavelet-based template generation algorithm is devised in MarkerDetector. By adopting a shape-based criterion, a high-quality template can be obtained. Secondly, a robust marker determination strategy is devised by utilizing statistic-based filtering, which can guarantee the correctness of the detected fiducial markers. The average running time of our algorithm is 1.67 seconds with promising accuracy, indicating its practical feasibility for applications in electron micrographs.

Intra- and inter-fractional variations of tumors with fiducial markers measured using respiratory-correlated computed tomography images for respiratory gated lung stereotactic body radiation therapy.

PURPOSE: This study evaluated the intra- and inter-fractional variation of tumors with fiducial markers (FMs), relative to the tumor-FM distance, to establish how close an FM should be inserted for respiratory-gated stereotactic body radiation therapy (RG-SBRT). METHODS: Forty-five lung tumors treated with RG-SBRT were enrolled. End-expiratory computed tomography (CT) (CTplan) and four-dimensional-CT (4D-CT) scans were obtained for planning. End-expiratory CT (CTfr) scanning was performed before each fraction. The FMs were divided into two groups based on the median tumor-FM distance in the CTplan (Dp). For the intra-fractional variation, the correlations between the corresponding tumor and FM intra-fractional motions, defined as the centroid coordinates of those in each 0-90% phase, with the 50% phase of 4D-CT as the origin, were calculated in the left-right, anterior-posterior, and superior-inferior directions. Furthermore, the maximum difference in the tumor-FM distance in each phase of 4D-CT scan, based on those in the 50% phase of 4D-CT scan (Dmax), was obtained. Inter-fractional variation was defined as the maximum distance between the tumors in CTplan and CTfr, when the CT scans were fused based on each FM or vertebra. RESULTS: The median Dp was 26.1 mm. While FM intra-fractional motions were significantly and strongly correlated with the tumor intra-fractional motions in only anterior-posterior and superior-inferior directions for the Dp > 26 mm group, they were significantly and strongly correlated in all directions for the Dp ≤ 26 mm group. In all directions, Dmax values of the Dp ≤ 26 mm group were lower than those of the Dp > 26 mm group. The inter-fractional variations based on the Dp ≤ 26 mm were smaller than those on the Dp > 26 mm and on the vertebra in all directions. CONCLUSIONS: Regarding intra- and inter-fractional variation, FMs for Dp ≤ 26 mm can increase the accuracy for RG-SBRT.

Large-Scale Indoor Camera Positioning Using Fiducial Markers.

Estimating the pose of a large set of fixed indoor cameras is a requirement for certain applications in augmented reality, autonomous navigation, video surveillance, and logistics. However, accurately mapping the positions of these cameras remains an unsolved problem. While providing partial solutions, existing alternatives are limited by their dependence on distinct environmental features, the requirement for large overlapping camera views, and specific conditions. This paper introduces a novel approach to estimating the pose of a large set of cameras using a small subset of fiducial markers printed on regular pieces of paper. By placing the markers in areas visible to multiple cameras, we can obtain an initial estimation of the pair-wise spatial relationship between them. The markers can be moved throughout the environment to obtain the relationship between all cameras, thus creating a graph connecting all cameras. In the final step, our method performs a full optimization, minimizing the reprojection errors of the observed markers and enforcing physical constraints, such as camera and marker coplanarity and control points. We validated our approach using novel artificial and real datasets with varying levels of complexity. Our experiments demonstrated superior performance over existing state-of-the-art techniques and increased effectiveness in real-world applications. Accompanying this paper, we provide the research community with access to our code, tutorials, and an application framework to support the deployment of our methodology.

Intra-prostatic gold fiducial marker insertion for image-guided radiotherapy (IGRT): five-year experience on 795 patients.

INTRODUCTION: Prostate cancer is the second most commonly diagnosed cancer in males. The use of intra-prostatic fiducial markers (FM) for image-guided radiotherapy (IGRT) has become widespread due to their accuracy, relatively safe use, low cost, and reproducibility. FM provides a tool to monitor prostate position and volume changes. Many studies reported low to moderate rates of complications following FM implantation. In the current study, we present our five years' experience regarding the insertion technique, technical success, and rates of complication and migration of intraprostatic insertion of FM gold marker. METHODS: From January 2018 to January 2023, 795 patients with prostate cancer candidate for IGRT (with or without a history of radical prostatectomy) enrolled in this study. We used three fiducial markers (3*0.6 mm) inserted through an 18-gauge Chiba needle under transrectal ultrasonography (TRUS) guidance. The patients were observed for complications up to seven days after the procedure. Besides, the rate of marker migration was recorded. RESULTS: All procedures were completed successfully, and all patients tolerated the procedure well with minimal discomfort. The rate of sepsis after the procedure was 1%, and transient urinary obstruction was 1.6%. Only two patients experienced marker migration shortly after insertion, and no fiducial migration was reported throughout radiotherapy. No other major complication was recorded. DISCUSSION: TRUS-guided intraprostatic FM implantation is technically feasible, safe, and well-tolerated in most patients. The FM migration can seldom occur, with negligible effects. This study can provide convincing evidence that TRUS-guided intra-prostatic FM insertion is an appropriate choice for IGRT.

Fiducial Objects: Custom Design and Evaluation.

Camera pose estimation is vital in fields like robotics, medical imaging, and augmented reality. Fiducial markers, specifically ArUco and Apriltag, are preferred for their efficiency. However, their accuracy and viewing angle are limited when used as single markers. Custom fiducial objects have been developed to address these limitations by attaching markers to 3D objects, enhancing visibility from multiple viewpoints and improving precision. Existing methods mainly use square markers on non-square object faces, leading to inefficient space use. This paper introduces a novel approach for creating fiducial objects with custom-shaped markers that optimize face coverage, enhancing space utilization and marker detectability at greater distances. Furthermore, we present a technique for the precise configuration estimation of these objects using multiviewpoint images. We provide the research community with our code, tutorials, and an application to facilitate the building and calibration of these objects. Our empirical analysis assesses the effectiveness of various fiducial objects for pose estimation across different conditions, such as noise levels, blur, and scale variations. The results suggest that our customized markers significantly outperform traditional square markers, marking a positive advancement in fiducial marker-based pose estimation methods.

Analytical Models for Pose Estimate Variance of Planar Fiducial Markers for Mobile Robot Localisation.

Planar fiducial markers are commonly used to estimate a pose of a camera relative to the marker. This information can be combined with other sensor data to provide a global or local position estimate of the system in the environment using a state estimator such as the Kalman filter. To achieve accurate estimates, the observation noise covariance matrix must be properly configured to reflect the sensor output's characteristics. However, the observation noise of the pose obtained from planar fiducial markers varies across the measurement range and this fact needs to be taken into account during the sensor fusion to provide a reliable estimate. In this work, we present experimental measurements of the fiducial markers in real and simulation scenarios for 2D pose estimation. Based on these measurements, we propose analytical functions that approximate the variances of pose estimates. We demonstrate the effectiveness of our approach in a 2D robot localisation experiment, where we present a method for estimating covariance model parameters based on user measurements and a technique for fusing pose estimates from multiple markers.

Leader-Follower Approach for Non-Holonomic Mobile Robots Based on Extended Kalman Filter Sensor Data Fusion and Extended On-Board Camera Perception Controlled with Behavior Tree.

This paper presents a leader-follower mobile robot control approach using onboard sensors. The follower robot is equipped with an Intel RealSense camera mounted on a rotating platform. Camera observations and ArUco markers are used to localize the robots to each other and relative to the workspace. The rotating platform allows the expansion of the perception range. As a result, the robot can use observations that are not within the camera's field of view at the same time in the localization process. The decision-making process associated with the control of camera rotation is implemented using behavior trees. In addition, measurements from encoders and IMUs are used to improve the quality of localization. Data fusion is performed using the EKF filter and allows the user to determine the robot's poses. A 3D-printed cuboidal tower is added to the leader robot with four ArUco markers located on its sides. Fiducial landmarks are placed on vertical surfaces in the workspace to improve the localization process. The experiments were performed to verify the effectiveness of the presented control algorithm. The robot operating system (ROS) was installed on both robots.

Deep Neural Network-Based Visual Feedback System for Nasopharyngeal Swab Sampling.

During the 2019 coronavirus disease pandemic, robotic-based systems for swab sampling were developed to reduce burdens on healthcare workers and their risk of infection. Teleoperated sampling systems are especially appreciated as they fundamentally prevent contact with suspected COVID-19 patients. However, the limited field of view of the installed cameras prevents the operator from recognizing the position and deformation of the swab inserted into the nasal cavity, which highly decreases the operating performance. To overcome this limitation, this study proposes a visual feedback system that monitors and reconstructs the shape of an NP swab using augmented reality (AR). The sampling device contained three load cells and measured the interaction force applied to the swab, while the shape information was captured using a motion-tracking program. These datasets were used to train a one-dimensional convolution neural network (1DCNN) model, which estimated the coordinates of three feature points of the swab in 2D X-Y plane. Based on these points, the virtual shape of the swab, reflecting the curvature of the actual one, was reconstructed and overlaid on the visual display. The accuracy of the 1DCNN model was evaluated on a 2D plane under ten different bending conditions. The results demonstrate that the x-values of the predicted points show errors of under 0.590 mm from P0, while those of P1 and P2 show a biased error of about -1.5 mm with constant standard deviations. For the y-values, the error of all feature points under positive bending is uniformly estimated with under 1 mm of difference, when the error under negative bending increases depending on the amount of deformation. Finally, experiments using a collaborative robot validate its ability to visualize the actual swab's position and deformation on the camera image of 2D and 3D phantoms.

[A review on the application of image fusion technology in prostate cancer radiotherapy based on gold fiducial biomarker].

Image fusion technology had been widely applied in image guided radiotherapy (IGRT) for prostate cancer (PCa) based on the gold fiducial mark (GFM). Image fusion technology included the fusion of CT image, magnetic resonance image, and ultrasound image internally or externally. The application of image fusion technology had improved the identification accuracy of GFM and was helpful for the plan design of PCa radiotherapy. This article provided a systematic review of the application of fusion of various medical images in PCa IGRT in recent years. Among them, the application and result of image fusion technology in GFM identification and the impact on the plan design for PCa radiotherapy were emphasized. It hoped that this review could provide some theoretical reference for the deeper integration of image fusion technology with PCa IGRT.

Proof of concept of a multimodal intravital molecular imaging system for tumour transpathology investigation.

BACKGROUND: Transpathology highlights the interpretation of the underlying physiology behind molecular imaging. However, it remains challenging due to the discrepancies between in vivo and in vitro measurements and difficulties of precise co-registration between trans-scaled images. This study aims to develop a multimodal intravital molecular imaging (MIMI) system as a tool for in vivo tumour transpathology investigation. METHODS: The proposed MIMI system integrates high-resolution positron imaging, magnetic resonance imaging (MRI) and microscopic imaging on a dorsal skin window chamber on an athymic nude rat. The window chamber frame was designed to be compatible with multimodal imaging and its fiducial markers were customized for precise physical alignment among modalities. The co-registration accuracy was evaluated based on phantoms with thin catheters. For proof of concept, tumour models of the human colorectal adenocarcinoma cell line HT-29 were imaged. The tissue within the window chamber was sectioned, fixed and haematoxylin-eosin (HE) stained for comparison with multimodal in vivo imaging. RESULTS: The final MIMI system had a maximum field of view (FOV) of 18 mm × 18 mm. Using the fiducial markers and the tubing phantom, the co-registration errors are 0.18 ± 0.27 mm between MRI and positron imaging, 0.19 ± 0.22 mm between positron imaging and microscopic imaging and 0.15 ± 0.27 mm between MRI and microscopic imaging. A pilot test demonstrated that the MIMI system provides an integrative visualization of the tumour anatomy, vasculatures and metabolism of the in vivo tumour microenvironment, which was consistent with ex vivo pathology. CONCLUSIONS: The established multimodal intravital imaging system provided a co-registered in vivo platform for trans-scale and transparent investigation of the underlying pathology behind imaging, which has the potential to enhance the translation of molecular imaging.

Fiducial marker position affects target volume in stereotactic lung irradiation.

PURPOSE: Real-time tracking systems of moving respiratory targets such as CyberKnife, Radixact, or Vero4DRT are an advanced robotic radiotherapy device used to deliver stereotactic body radiotherapy (SBRT). The internal target volume (ITV) of lung tumors is assessed through a fiducial marker fusion using four-dimensional computed tomography (CT). It is important to minimize the ITV to protect normal lung tissue from exposure to radiation and the associated side effects post SBRT. However, the ITV may alter if there is a change in the position of the fiducial marker with respect to the tumor. This study investigated the relationship between fiducial marker position and the ITV in order to prevent radiation exposure of normal lung tissue, and correct target coverage. MATERIALS AND METHODS: This study retrospectively reviewed 230 lung cancer patients who received a fiducial marker for SBRT between April 2015 and September 2021. The distance of the fiducial marker to the gross tumor volume (GTV) in the expiratory (dex ) and inspiratory (din ) CT, and the ratio of the ITV/V(GTVex ), were investigated. RESULTS: Upon comparing each lobe, although there was no significant difference in the ddiff and the ITV/V(GTVex ) between all lobes for dex  < 10 mm, there was significant difference in the ddiff and the ITV/V(GTVex ) between the lower and upper lobes for dex ≥ 10 mm (p < 0.05). Moreover, there was significant difference in the ddiff and the ITV/V(GTVex ) between dex ≥10 mm and dex  < 10 mm in all lung regions (p < 0.05). CONCLUSION: The ITV that had no margin from GTVs increased when dex was ≥10 mm for all lung regions (p < 0.05). Furthermore, the increase in ITV tended to be greater in the lower lung lobe. These findings can help decrease the possibility of adverse events post SBRT, and correct target coverage.

Feasibility of transrectal and transperineal fiducial marker placement for prostate cancer before proton therapy.

BACKGROUND: To compare the feasibility of transrectal and transperineal fiducial marker placement for prostate cancer before proton therapy. MATERIALS AND METHODS: From 2013 to 2015, the first 40 prostate cancer patients that were scheduled for proton therapy underwent transrectal fiducial marker placement, and the next 40 patients underwent transperineal fiducial marker placement (the first series). Technical and clinical success and pain scores were evaluated. In the second series (n = 280), the transrectal or transperineal approach was selected depending on the presence/absence of comorbidities, such as blood coagulation abnormalities. Seven patients refused to undergo the procedure. Thus, the total number of patients across both series was 353 (262 and 91 underwent the transrectal and transperineal approach, respectively). Technical and clinical success, complications, marker migration and the distance between the two markers were evaluated. RESULTS: In the first series, the technical and clinical success rates were 100% in both groups. The transrectal group exhibited lower pain scores than the transperineal group. The overall technical success rates of the transrectal and transperineal groups were 100% (262/262) and 99% (90/91), respectively (P > 0.05). The overall clinical success rate was 100% in both groups, and there were no major complications in either group. The migration rates of the two groups did not differ significantly. The mean distance between the two markers was 25.6 ± 7.1 mm (mean ± standard deviation) in the transrectal group and 31.9 ± 5.2 mm in the transperineal group (P < 0.05). CONCLUSION: Both the transrectal and transperineal fiducial marker placement methods are feasible and safe.

Endobronchial coil spring fiducial markers for CyberKnife® stereotactic body radiation therapy.

BACKGROUND AND OBJECTIVE: SBRT is an alternative treatment for early-stage inoperable lung cancer. Metallic FM allow to increase tumour tracking precision by CyberKnife®. Currently used techniques for FM placement have many limitations; transthoracic insertion has a high risk for pneumothorax, endovascular insertion requires expertise and dedicated angiography infrastructure and endobronchial linear-gold FM dislocate frequently. This is the first study to assess the safety and efficacy of cs-FM endobronchial insertion under fluoroscopy with or without R-EBUS assessment. METHODS: We retrospectively evaluated all consecutive patients undergoing endobronchial cs-FM placement for at least one PPL <25 mm between 10.2015 and 12.2019. TBB of the PPL were performed in case of a typical R-EBUS signal. PPL tracking accuracy by CyberKnife, complications, cs-FM migration rate and procedure duration were analysed. RESULTS: A total of 52 patients were treated during 55 procedures and 207 cs-FM were placed in 70 PPL. Tracking was successful for 65 of 70 (93%) PPL. R-EBUS was performed for 33 (47%) PPL and TBB for 9 (13%) PPL. Bronchospasm occurred once and any other complications were observed. Migration of cs-FM occurred in 16 of 207 (8%) cs-FM. Migration was more frequent when the target was in a previously irradiated area (P = 0.022). The median bronchoscopy duration was 31.5 min (n = 48 procedures). CONCLUSION: Bronchoscopic cs-FM placement is a rapid and safe procedure. It is associated with a low migration rate and allows precise SBRT delivery. Previous irradiation of the PPL was associated with a higher migration rate.

Utility of real-time image fusion technology in ultrasonography-guided fiducial marker implantation for stereotactic body radiation therapy for liver tumors.

BACKGROUND: Ultrasonography (US) is useful when implanting fiducial markers in the liver. However, the implant position is sometimes lost. Recently, real-time image fusion technology (Volume Navigation [V-navi]; GE Healthcare, Milwaukee, WI, USA) has been introduced as a technique for using images from different modalities, and its utility for fiducial marker implantation has been hypothesized. PURPOSE: To evaluate the utility of US-guided fiducial marker implantation in the liver using V-navi compared to conventional US. MATERIAL AND METHODS: We retrospectively reviewed 35 patients who underwent fiducial marker implantation for stereotactic body radiation therapy of liver tumors in 2013-2018. To avoid artifacts obscuring the tumor, the target point of implantation was set 10 mm cranial or caudal to the tumor. Marker implantation was then performed using US alone (US group, n = 24) or V-navi with computed tomography (CT) or magnetic resonance imaging (V-navi group, n = 11). Postprocedural CT was evaluated to determine technical success, distances between marker and either tumor surface or target point, and whether marker-induced artifacts obscured the tumor. Complications were also evaluated. Results were compared between groups. RESULTS: Technical success was obtained in 33 patients. Distance between the tumor and marker showed no significant difference between groups. Distance between target point and marker was shorter in the V-navi group (P = 0.0093). Tumor-obscuring artifacts were seen in 12 patients (V-navi group, n = 1; US group, n = 11; P = 0.055). The only complication was minor bleeding in the US group (n = 1). CONCLUSION: V-navi appears useful for US-guided fiducial marker implantation in the liver compared with conventional US.

Designing a Simple Fiducial Marker for Localization in Spatial Scenes Using Neural Networks.

The paper describes the process of designing a simple fiducial marker. The marker is meant for use in augmented reality applications. Unlike other systems, it does not encode any information, but it can be used for obtaining the position, rotation, relative size, and projective transformation. Also, the system works well with motion blur and is resistant to the marker's imperfections, which could theoretically be drawn only by hand. Previous systems put constraints on colors that need to be used to form the marker. The proposed system works with any saturated color, leading to better blending with the surrounding environment. The marker's final shape is a rectangular area of a solid color with three lines of a different color going from the center to three corners of the rectangle. Precise detection can be achieved using neural networks, given that the training set is very varied and well designed. A detailed literature review was performed, and no such system was found. Therefore, the proposed design is novel for localization in the spatial scene. The testing proved that the system works well both indoor and outdoor, and the detections are precise.

Impact of rotational errors of whole pelvis on the dose of prostate-based image-guided radiotherapy to pelvic lymph nodes and small bowel in high-risk prostate cancer.

BACKGROUND: The target volume increases when the prostate and pelvic lymph nodes (PLNs) are combined, and the fiducial markers (FMs) are placed at the edge of the irradiation field. Thus, the position of FMs may be changed by the rotational errors (REs) of "whole pelvis". The aim of this study was to examine the impact of REs of "whole pelvis" on the dose of FMs-based image-guided radiotherapy to the PLNs and the small bowel in prostate cancer including the PLNs. MATERIALS AND METHODS: We retrospectively evaluated 10 patients who underwent prostate cancer radiotherapy involving the PLNs. The position of FMs was calculated from the radiographs obtained before and after the 6D correction of pelvic REs. We simulated the delivery dose considering the daily pelvic REs and calculated the difference from the planned dose in the D98% of the PLN clinical target volume and the D2cc, and V45Gy of the small bowel. RESULT: The position of FMs strongly correlated with the pelvic REs in the pitch direction (r = 0.7788). However, the mean delivered doses to PLNs for 10 patients were not significantly different from the planned doses (p = 0.625). Although the D2cc and V45Gy of the small bowel strongly correlated with the pitch rotation of the pelvis, there was no significant difference between the delivered and planned doses (p = 0.922 and p = 0.232, respectively). CONCLUSION: The dosimetric effect of pelvic REs on the dose to PLNs and the small bowel was negligible during the treatment course.

Utilization of BioZorb implantable device in breast-conserving surgery.

BioZorb fiducial marker is an implantable device made of six clips to mark the surgical site of tissue removal in three dimensions. The marker facilitates focused radiation therapy, while allowing for tissue ingrowth during the healing process with resorption by the body overtime. Current literature investigating the use of the BioZorb is limited and focused on its value for radiation treatment. Our objective was to investigate the feasibility and surgical complications associated with the BioZorb in breast-conserving surgery. From April 2015 to June 2018, 89 patients who underwent 91 partial mastectomies with planned adjuvant radiation therapy and placement of the BioZorb. Demographics, type of BioZorb used, complication rate, and postoperative examinations were analyzed. A total of 89 patients who were a median age of 59 years (range 34-84) underwent 91 operations with BioZorb placement-86.8% underwent a partial mastectomy (n = 79), and 13.2% underwent a breast wide re-excision for margins at the time of BioZorb placement (n = 12). Of the 79 partial mastectomies, 21.5% (n = 17) were palpable tumors. Location of the tumor and subsequent BioZorb placement was most often in the upper outer quadrant (40.7%), followed by upper inner (27.5%), lower outer (20.9%), and lower inner quadrants (10.9%). 92.3% (n = 84) had a single BioZorb placed, 5.5% (n = 5) had two BioZorbs placed in a single lumpectomy cavity, and 2.2% (n = 2) had two BioZorbs placed in separate lumpectomy cavities of the same breast. Of the 10 different tissue marker sizes used, a 2 × 3 cm BioZorb was most commonly used (37/98, 37.8%), followed by 3 × 4 cm (25/98, 25.5%) and 1 × 3 × 2 (9/98, 9.2%). A total of five patients underwent immediate bilateral breast reduction following placement of the BioZorb. Of the 91 operations, 22 patients had a subsequent reoperation for positive margins after initial placement of the BioZorb, of which 86.4% retained the BioZorb. During these reoperations, only 1 patient had the BioZorb removed due to discomfort (4.5%) and two had it removed due to subsequent mastectomy (9.1%). At a median time of 1.1 years, the BioZorb continued to be palpable on clinical breast examination in 63.6% of patients. The longest time that the BioZorb continued to be palpable was 2.8 years. Additional imaging was ordered because a clinician palpated a mass, unaware it was the BioZorb 8.8% of the time (n = 8). Thirty-day complications include 3.3% of patients with an infection requiring antibiotics (n = 3) and 2.2% with an abscess requiring aspiration and antibiotics without removal of the BioZorb. One patient had migration of the BioZorb from the breast to the axilla which resulted in surgical explant at 9 months post-op. BioZorb is feasible to use in breast-conserving surgery with few short- and long-term complications, but will result in a palpable mass that may persist for more than 1 year. Explantation is rare.

Dose reduction potential of using gold fiducial markers for kilovoltage image-guided radiotherapy.

This study aimed to evaluate the possibility of reducing the imaging dose for image-guided radiotherapy by using planar kilovoltage orthogonal imaging and fiducial markers (kV-FM). We tested kilovoltage planar images under clinical imaging conditions for the pelvis (75 kVp, 200 mA, 50 ms) at a decreasing tube current (from 200 to 10 mA). Imaging doses were measured with a semiconductor detector. The visibility of the kV-FM, aspects of image quality (spatial resolution, low contrast resolution), and the resultant image registration reproducibility were evaluated using various shapes (folded, linear, tadpole-like) of fiducial markers containing 0.5% iron [Gold Anchor™ (GA); Naslund Medical AB, Huddinge, Sweden]. The GA phantom was created by placing these variously shaped GAs in an agar phantom. The imaging doses with 200 and 10 mA were approximately 0.74 and 0.04 mGy and they were correlated to the tube current (R2  = 0.999). Regardless of the marker's shape, the GA phantom ensured visibility even when the tube current was reduced to the minimum value (10 mA). The low contrast resolution was gradually decreased at less than 50 mA, but the spatial resolution did not change. Although the auto-registration function could not be used, manual-registration could be achieved with an accuracy of within 1 mm, even when the imaging dose was reduced to 1/20 of the clinical imaging condition for the pelvis. When using the GA as the fiducial marker, the imaging dose could be reduced to 1/20 of that used clinically while maintaining the accuracy of manual-registration using the kV-FM for image-guided radiotherapy of the pelvis.

Effect of a hydrogel spacer on the intrafractional prostate motion during CyberKnife treatment for prostate cancer.

The purpose of this study was to evaluate the effect of a hydrogel spacer on intrafractional prostate motion during CyberKnife treatment. The retrospective study enrolled 24 patients (with the hydrogel spacer = 12, without the hydrogel spacer = 12) with two fiducial markers. Regarding intrafractional prostate motion, the offset values (mm) of three axes (X-axis; superior [+] to inferior [-], Y-axis; right [+] to left [-], Z-axis; posterior [+] to anterior [-]) obtained from fiducial markers position between a digitally reconstructed radiographs images and live images in the Target Locating System were used, and extracted from generated log files. The mean values of the offset and each standard deviation were calculated for each patient, and both the groups were compared. For all the patients, a total of 2204 offset values and timestamps (without the hydrogel spacer group: 1065, with the hydrogel spacer group: 1139) were recorded for the X-, Y-, and Z-axes, respectively. The offset values (mean ± standard deviation) for the X-, Y-, and Z-axes were -0.04 ± 0.92 mm, -0.03 ± 0.97 mm (P = 0.66), 0.02 ± 0.51, -0.02 ± 0.49 mm (P = 0.50), and 0.56 ± 0.97 mm, 0.34 ± 1.07 mm (P = 0.14), in patients inserted without or with the hydrogel spacer, respectively. There was no effect of a hydrogel spacer on the intrafractional prostate motion in the three axes during CyberKnife treatment for prostate cancer.

Detectability of fiducials' positions for real-time target tracking system equipping with a standard linac for multiple fiducial markers.

PURPOSE: To investigate the detectability of fiducial markers' positions for real-time target tracking system equipping with a standard linac. The hypothesis is that the detectability depends on the type of fiducial marker and the gantry angle of acquired triggered images. METHODS: Three types of ball fiducials and four slim fiducials with lengths of 3 and 5 mm were prepared for this study. Triggered images with three similar fiducials were acquired at every 10° during the conformal arc irradiation to detect the target position. Although only one type of arrangement was prepared for the ball fiducials, a three-type arrangement was prepared for the slim fiducials, such as parallel, orthogonal, and oblique with 45° to the gantry-couch direction. To measure the detectability of the real-time target tracking system for each fiducial and arrangement, detected marker positions were compared with expected marker positions at every angle of acquired triggered images. RESULTS: For the ball-type fiducial, the maximum difference between the detected marker positions and expected marker positions was 0.3 mm in all directions. For the slim fiducial arranged parallel and oblique with 45°, the maximum difference was 0.4 mm in all directions. When each slim fiducial was arranged orthogonal to the gantry-couch direction, the maximum difference was 1.5 mm for the length of 3 mm, and 3.2 mm for the length of 5 mm. CONCLUSIONS: The detectability of fiducial markers' positions for the real-time target tracking system equipping with a standard linac depends on the form and insertion angles of the fiducials.