Nesreen Alsbou

An algorithm to extract three-dimensional motion by marker tracking in the kV projections from an on-board imager: four-dimensional cone-beam CT and tumor tracking implications

The purpose of this work is to extract three‐dimensional (3D) motion trajectories of internal implanted and external skin‐attached markers from kV cone‐beam projections and reduce image artifact from patient motion in cone‐beam computed tomography (CBCT) from on‐board imager. Cone beam radiographic projections were acquired for a mobile phantom and liver patients with internal implanted and external skin‐attached markers. An algorithm was developed to automatically find the positions of the markers in the projections. It uses normalized cross‐correlation between a template image of a metal seed marker and the projections to find the marker position. From these positions and time‐tagged angular views, the marker 3D motion trajectory was obtained over a time interval of nearly one minute, which is the time required for scanning. This marker trajectory was used to remap the pixels of the projections to eliminate motion. Then, the motion‐corrected projections were used to reconstruct CBCT. An algorithm was developed to extract 3D motion trajectories of internal and external markers from cone‐beam projections using a kV monoscopic on‐board imager. This algorithm was tested and validated using a mobile phantom and patients with liver masses that had radio‐markers implanted in the tumor and attached to the skin. The extracted motion trajectories were used to investigate motion correlation between internal and external markers in liver patients. Image artifacts from respiratory motion were reduced in CBCT reconstructed from cone‐beam projections that were preprocessed to remove motion shifts obtained from marker tracking. With this method, motion‐related image artifacts such as blurring and spatial distortion were reduced, and contrast and position resolutions were improved significantly in CBCT reconstructed from motion‐corrected projections. Furthermore, correlated internal and external marker 3D‐motion tracks obtained from the kV projections might be useful for 4DCBCT, beam gating and tumor motion monitoring or tracking. PACS numbers: 87.57.Q, 87.57.C‐

R-ALOHA with priority (PR-ALOHA) in non ideal channel with capture effects

The performance of reservation ALOHA (R-ALOHA) with priority is analyzed using computer simulation. Priority is introduced to the R-ALOHA protocol by reserving specific time slots in the frame exclusively for high priority traffic. Traffic is produced and divided into either low or high priority. We considered non ideal channel with communication errors and capture effects. Performance is measured in terms of packet throughput and message transfer delay for each priority level. The analysis gives results for varying numbers of reserved high priority slots and varying levels of high priority traffic ultimately leading to a dynamic high priority slot reservation scheme. Multipriority protocols are important in such applications as intervehicle communication, where high priority may represent safety related messages, and multimedia communication, where high priority is given to streaming audio or video. In both applications low delay and high throughput are absolutely necessary for standard performance. The simulation results indicate that as the number of slots reserved for high priority traffic increases the delay of the high priority messages decreases. However, low priority throughput decreases and its delay increases.

Dynamic Slot Allocation Algorithm for R-ALOHA with Priority (PR-ALOHA)

In this work, a new approach for dynamically allocating high priority probability and high priority slots in PR-ALOHA (Reserved ALOHA with Priority) was introduced. The Dynamic Slot Allocation (DSA) algorithm in PR-ALOHA is performed by dynamically allocating the number of high priority slots and high priority probability based on the available traffic. The performance of the system with DSA algorithm was tested by simulation and compared to the performance of the system without DSA. The simulation shows that the proposed DSA algorithm provides an approach to improve performance in a controlled manner for high and low priority traffic. Two functions are proposed for DSA, the first is an exponential growth function, where the numbers of high priority slots increase with the number of terminals until it reach a maximum value. The second is a bell-shaped function where the number of high priority slots increases to a maximum and then decreases. Both functions are tested with fixed high priority probability (hpp) of 25% and dynamic hpp ranging (8%-63%). The bell shaped function provides better performance at fixed hpp, while the exponential function provides better performance with dynamic hpp. The simulation shows that the use of DSA improves the performance of PR-ALOHA.

Analysis of Priority R-ALOHA PR-ALOHA protocol

With the exception of required time synchronization, the Reservation-ALOHA R-ALOHA protocol is simple to implement and suitable for medium access control in ad hoc wireless networks. In this paper, we propose an innovative protocol, referred to as Reservation ALOHA with priority PR-ALOHA that provides differentiated services on the basis of traffic priority. To date, the carrier sense multiple access/collision avoidance CSMA/CA protocol has been widely used for this purpose by employing an interframe spacing IFS for priority service, that is, nodes ready for packet transmissions are required to wait for an IFS amount of time, where a shorter IFS is used to gain faster access to the radio channel. However, sensing and collision avoidance mechanisms make CSMA/CA unsuitable for delay-sensitive applications, that is, congested scenarios with high traffic. In contrast, the proposed PR-ALOHA protocol may be considered a good candidate for such applications. In this paper, the performance of the PR-ALOHA protocol is investigated analytically and by simulation. Its comparison with regular R-ALOHA is also carried out. Modeling and simulation results of PR-ALOHA show that PR-ALOHA improves the performance of high-priority traffic with limited effect on normal network traffic. Thus, PR-ALOHA may be useful in vehicular communications, where traffic may be separated into emergency messages having high priority and multimedia messages having low priority. Copyright

Correction of image artifacts from treatment couch in cone-beam CT from kV on-board imaging

PURPOSE To investigate image artifacts caused by a standard treatment couch on cone-beam CT (CBCT) images from a kV on-board imager and to develop an algorithm based on spatial domain filtering to remove image artifacts in CBCT induced by the treatment couch. METHODS Image artifacts in CBCT induced by the treatment couch were quantified by scanning a phantom used to quantify CT image performance. This was performed by scanning the phantom setup on a regular treatment couch and in air with the kV on-board imager. An algorithm was developed to filter image artifacts from the treatment couch by processing of cone-beam radiographic projections using two scans: one scan of the phantom and treatment couch and a second scan of the treatment couch only. This algorithm is based on a pixel-by-pixel removal of beam attenuation due to the treatment couch from each projection of the phantom and couch scan. The net couch-filtered projections were then used to reconstruct CBCT. RESULTS We found that the treatment couch causes considerable image artifacts: CT number uniformity is degraded and varies as much as 15%, and noise in CBCT scans with phantom plus couch (3.5%) is higher than for the phantom in air (1.5%). The spatial domain filtering technique reduces noise by more than 1.5%, improves uniformity by a factor of 2, and removes ringing and streaking artifacts related to the standard treatment couch in CBCT reconstructed from couch-filtered projections. This filtering technique was tested successfully to filter other hardware objects such as a patient immobilization body-fix frame. CONCLUSIONS The standard treatment couch causes image artifact in CBCT from kV on-board imaging systems. The spatial domain filtering technique developed in this work improves image quality of CBCT by preprocessing the projections prior to CBCT reconstruction. This technique might be useful to filter other hardware objects from CBCT which may contribute to the degradation of image quality.

Theoretical modeling of mobile target broadening in helical and axial computed tomographic imaging.

PURPOSE To investigate variations in mobile target length induced by sinusoidal motion in helical (HCT) and axial CT (ACT) imaging. A mathematical model was derived that predicts the measured broadening of the apparent lengths of mobile targets and its dependence on motion parameters, target size, and imaging couch speed in CT images. MATERIALS AND METHODS Three mobile targets of differing lengths and sizes were constructed of tissue-equivalent gel material and embedded into artificial lung phantom. Respiratory motion was mimicked with a mobile phantom that moves in one-dimension along the superior-inferior direction with sinusoidal motion patterns. A mathematical model was derived to predict quantitatively the variations of apparent lengths for mobile targets and its dependence on phantom and imaging couch motion parameters in HCT and ACT. The model predictions were verified by length measurements of the mobile phantom targets that were imaged with the different motion patterns using CT imaging. RESULTS The measured lengths of mobile targets enlarged or shrunk depending on the phantom motion parameters that include phantom speed, amplitude, frequency, phase and speed of the imaging couch. The target length variations were significant where some targets doubled lengths or shrunk to less than half of their actual length. The apparent lengths of mobile targets decreased if the target was moving in the same direction as the imaging couch motion and increased if the mobile target was moving opposed to imaging couch in both HCT and ACT. The model predicts well the variations in the mobile target apparent lengths and their dependence on the motion parameters. CONCLUSION The measured and model variations of apparent lengths of mobile targets are considerable and may affect the accuracy of tumor volumes obtained from HCT and ACT. This mathematical model provides a method to quantitatively assess the length variations of mobile targets and their dependence on motion parameters of the phantom and imaging system which may have potential applications in the fields of diagnostic imaging and radiotherapy.

Analysis of PR-ALOHA protocol for inter vehicle communications

The performance of Reservation ALOHA with Priority (PR-ALOHA) protocol in an inter-vehicle communication (IVC) environment was investigated both in transient and steady state. Monte Carlo simulation was performed to examine the system stabilization time (SST) in transient state and performance parameters such as throughput, packet delivery ratio and packet drop rate (PDR) in steady state. The analysis assumed a non ideal channel with communication errors with and without considering capture effect. The simulation results show that SST in the transient state depends strongly on capture effect, however, performance parameters at steady state where slightly affected. At high traffic (40 terminals), the network throughput increased by 5%, the packet drop rate decreased by 3% and the packet delivery ratio increased by 4% with capture effect. For heavy traffic, capture effect can improve stability and the performance of PR-ALOHA system.

Quantitative evaluation by measurement and modeling of the variations in dose distributions deposited in mobile targets

The objective of this study is to quantitatively evaluate variations of dose distributions deposited in mobile target by measurement and modeling. The effects of variation in dose distribution induced by motion on tumor dose coverage and sparing of normal tissues were investigated quantitatively. The dose distributions with motion artifacts were modeled considering different motion patterns that include (a) motion with constant speed and (b) sinusoidal motion. The model predictions of the dose distributions with motion artifacts were verified with measurement where the dose distributions from various plans that included three-dimensional conformal and intensity-modulated fields were measured with a multiple-diode-array detector (MapCheck2), which was mounted on a mobile platform that moves with adjustable motion parameters. For each plan, the dose distributions were then measured with MapCHECK2 using different motion amplitudes from 0-25 mm. In addition, mathematical modeling was developed to predict the variations in the dose distributions and their dependence on the motion parameters that included amplitude, frequency and phase for sinusoidal motions. The dose distributions varied with motion and depended on the motion pattern particularly the sinusoidal motion, which spread out along the direction of motion. Study results showed that in the dose region between isocenter and the 50% isodose line, the dose profile decreased with increase of the motion amplitude. As the range of motion became larger than the field length along the direction of motion, the dose profiles changes overall including the central axis dose and 50% isodose line. If the total dose was delivered over a time much longer than the periodic time of motion, variations in motion frequency and phase do not affect the dose profiles. As a result, the motion dose modeling developed in this study provided quantitative characterization of variation in the dose distributions induced by motion, which can be employed in radiation therapy to quantitatively determine the margins needed for treatment planning considering dose spillage to normal tissue.

WE‐G‐134‐09: Quantitative Assessment by Measurement and Modeling of Mobile Target Elongation in Cone‐Beam Computer Tomographic Imaging

PURPOSE To assess quantitatively elongation of mobile targets in cone-beam CT (CBCT) imaging by measurement and modeling. A mathematical model was derived that predicts the measured lengths of mobile targets and its dependence on target size and motion patterns in CBCT imaging. METHOD AND MATERIALS Three tissue-equivalent targets of differing sizes were inserted in an artificial thorax phantom to simulate lung lesions. Respiratory motion was mimicked with a mobile phantom that moves in one-dimension along the superior-inferior direction at a respiration frequency of 15 cycles per minute for eight different ranges of motion (0-40 mm). The phantom was imaged with CBCT using a kV on-board imager mounted on a TrueBeam Varian linear accelerator. The target lengths were measured on CBCT images with different motion patterns. A mathematical model was derived to quantify the variations in target lengths and its dependence on phantom motion parameters in CBCT. Predictions of the model were verified by comparison with measurement of the lengths of mobile targets in CBCT images. RESULTS The model predicts that target lengths increased linearly with increase in speed and amplitude of phantom motion in CBCT. The measured lengths of mobile targets imaged with CBCT agreed with the calculated lengths within half-slice thickness spatial resolution. The maximal length of a mobile target was independent of the frequency and phase of motion. Elongation of mobile targets was similar in half-fan and full-fan CBCT for similar motion patterns as long as the targets remained within the imaging view. CONCLUSION Mobile targets elongated linearly with phantom speed and motion amplitude in CBCT imaging. The model introduced in this work assessed quantitatively the variation in target lengths induced by motion which may be a useful tool to consider elongations of mobile targets in CBCT applications in diagnostic imaging and radiotherapy.

Two-dimensional Markov model for throughput analysis of IEEE 802.11 Distributed Coordination Function in non ideal channel with capture effects

Wireless technologies are emerging as an important component of our daily life. Distributed Coordination Function (DCF) is the basis of IEEE 802.11 WLAN MAC protocol, which uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol and binary slotted exponential back-off scheme. DCF implementation specified by the IEEE 802.11 standard reset the contention window (CW) to minimum value upon a successful transmission. Although the newCW is minimal the congestion level goes gradually to minimum, causing the node to probably waste time and channel bandwidth going through several collisions and retransmissions before reaching a CW value corresponds to the congestion level. This paper modifies the implementation to reduce the window to half its size after a successful transmission. The paper uses analytical and simulation analysis to show throughput improvements under ideal and non-ideal channel with channel induced errors and capture effect. When compared to the standard implementation, throughput has improved for both ideal and non ideal channel.