Wu Liu is a clinical associate professor and medical physicist at Department of Radiation Oncology, Stanford University, Stanford, CT, USA. He was born in Beijing, China. He received B.S. degree in Astronomy from Nanjing University, Nanjing, China and M.S. degree in Astrophysics from Chinese Academy of Sciences, Beijing, China. He obtained his M.S. degree in Computer Science and Ph.D. degree in Medical Physics (2007) from University of Wisconsin-Madison, Madison, WI, USA. He then completed his postdoctoral training at Stanford University. Before re-joining Stanford, he was a medical physicist at Yale-New Haven hospital and an assistant professor at Yale University.

Academic Appointments

Research & Scholarship

Current Research and Scholarly Interests

Theranostic nanoparticles for radiosensitization and medical imaging. Novel treatment technique for ocular disease radiotherapy. Use artificial intelligence in image guided radiotherapy and medical image analysis. Ultrasound parametric imaging.


All Publications

  • Tissue feature-based intra-fractional motion tracking for stereoscopic x-ray image guided radiotherapy. Physics in medicine and biology Xie, Y., Xing, L., Gu, J., Liu, W. 2013; 58 (11): 3615-3630


    Real-time knowledge of tumor position during radiation therapy is essential to overcome the adverse effect of intra-fractional organ motion. The goal of this work is to develop a tumor tracking strategy by effectively utilizing the inherent image features of stereoscopic x-ray images acquired during dose delivery. In stereoscopic x-ray image guided radiation delivery, two orthogonal x-ray images are acquired either simultaneously or sequentially. The essence of markerless tumor tracking is the reliable identification of inherent points with distinct tissue features on each projection image and their association between two images. The identification of the feature points on a planar x-ray image is realized by searching for points with high intensity gradient. The feature points are associated by using the scale invariance features transform descriptor. The performance of the proposed technique is evaluated by using images of a motion phantom and four archived clinical cases acquired using either a CyberKnife equipped with a stereoscopic x-ray imaging system, or a LINAC equipped with an onboard kV imager and an electronic portal imaging device. In the phantom study, the results obtained using the proposed method agree with the measurements to within 2ámm in all three directions. In the clinical study, the mean error is 0.48 ▒ 0.46ámm for four patient data with 144 sequential images. In this work, a tissue feature-based tracking method for stereoscopic x-ray image guided radiation therapy is developed. The technique avoids the invasive procedure of fiducial implantation and may greatly facilitate the clinical workflow.

    View details for DOI 10.1088/0031-9155/58/11/3615

    View details for PubMedID 23648334

  • Dose verification for respiratory-gated volumetric modulated arc therapy PHYSICS IN MEDICINE AND BIOLOGY Qian, J., Xing, L., Liu, W., Luxton, G. 2011; 56 (15): 4827-4838


    A novel commercial medical linac system (TrueBeam?, Varian Medical Systems, Palo Alto, CA) allows respiratory-gated volumetric modulated arc therapy (VMAT), a new modality for treating moving tumors with high precision and improved accuracy by allowing for regular motion associated with a patient's breathing during VMAT delivery. The purpose of this work is to adapt a previously-developed dose reconstruction technique to evaluate the fidelity of VMAT treatment during gated delivery under clinic-relevant periodic motion related to patient breathing. A Varian TrueBeam system was used in this study. VMAT plans were created for three patients with lung or pancreas tumors. Conventional 6 and 15 MV beams with flattening filter and high-dose-rate 10 MV beams with no flattening filter were used in these plans. Each patient plan was delivered to a phantom first without gating and then with gating for three simulated respiratory periods (3, 4.5 and 6 s). Using the adapted log-file-based dose reconstruction procedure supplemented with ion chamber array (Seven29?, PTW, Freiburg, Germany) measurements, the delivered dose was used to evaluate the fidelity of gated VMAT delivery. Comparison of Seven29 measurements with and without gating showed good agreement with gamma-index passing rates above 99% for 1%/1 mm dose accuracy/distance-to-agreement criteria. With original plans as reference, gamma-index passing rates were 100% for the reconstituted plans (1%/1 mm criteria) and 93.5-100% for gated Seven29 measurements (3%/3 mm criteria). In the presence of leaf error deliberately introduced into the gated delivery of a pancreas patient plan, both dose reconstruction and Seven29 measurement consistently indicated substantial dosimetric differences from the original plan. In summary, a dose reconstruction procedure was demonstrated for evaluating the accuracy of respiratory-gated VMAT delivery. This technique showed that under clinical operation, the TrueBeam system faithfully realized treatment plans with gated delivery. This methodology affords a useful tool for machine- and patient-specific quality assurance of the newly available respiratory-gated VMAT.

    View details for DOI 10.1088/0031-9155/56/15/013

    View details for Web of Science ID 000292885000014

    View details for PubMedID 21753232

    View details for PubMedCentralID PMC3360016



    To develop methods to monitor prostate intrafraction motion during fixed-gantry intensity-modulated radiotherapy using MV treatment beam imaging together with minimal kV imaging for a failure detection strategy that ensures prompt detection when target displacement exceeds a preset threshold.Real-time two-dimensional (2D) marker position in the MV image plane was obtained by analyzing cine-MV images. The marker's in-line movement, and thus its time-varying three-dimensional (3D) position, was estimated by combining the 2D projection data with a previously established correlative relationship between the directional components of prostate motion. A confirmation request for more accurate localization using MV-kV triangulation was triggered when the estimated prostate displacement based on the cine-MV data was greater than 3 mm. An interventional action alert followed on positive MV-kV confirmation. To demonstrate the feasibility and accuracy of the proposed method, simulation studies of conventional-fraction intensity-modulated radiotherapy sessions were done using 536 Calypso-measured prostate trajectories from 17 radiotherapy patients.A technique for intrafraction prostate motion management has been developed. The technique, using "freely available" cine-MV images and minimum on-board kV imaging (on average 2.5 images/fraction), successfully limited 3D prostate movement to within a range of 3 mm relative to the MV beam for 99.4% of the total treatment time. On average, only approximately one intervention/fraction was needed to achieve this level of accuracy.Instead of seeking to accurately and continuously localize the prostate target as existing motion tracking systems do, the present technique effectively uses cine-MV data to provide a clinically valuable way to minimize kV usage, while maintaining high targeting accuracy.

    View details for DOI 10.1016/j.ijrobp.2009.12.068

    View details for Web of Science ID 000282731300037

    View details for PubMedID 20579818

  • OPTIMIZED HYBRID MEGAVOLTAGE-KILOVOLTAGE IMAGING PROTOCOL FOR VOLUMETRIC PROSTATE ARC THERAPY 51st Annual Meeting of the American-Association-of-Physicists-in-Medicine Liu, W., Wiersma, R. D., Xing, L. ELSEVIER SCIENCE INC. 2010: 595?604


    To develop a real-time prostate position monitoring technique for modern arc radiotherapy through novel use of cine-megavoltage (MV) imaging, together with as-needed kilovoltage (kV) imaging.We divided the task of monitoring the intrafraction prostate motion into two steps for rotational deliveries: to detect potential target motion beyond a predefined threshold using MV images from different viewing angles by taking advantage of gantry rotation during arc therapy and to verify the displacement and determine whether intervention is needed using fiducial/tumor position information acquired from combined MV-kV imaging (by turning on the kV imager). A Varian Trilogy linear accelerator with an onboard kV imager was used to examine selected typical trajectories using a four-dimensional motion phantom. The performance of the algorithm was evaluated using phantom measurements and computer simulation for 536 Calypso-measured tracks from 17 patients.Fiducial displacement relative to the MV beam was limited to within a range of 3 mm 99.9% of the time with <1 mm accuracy. On average, only ?0.5 intervention per arc delivery was needed to achieve this level of accuracy. Compared with other fluoroscopy-based tracking techniques, kV use was significantly reduced to an average of <15 times per arc delivery.By focusing the attention on detecting predefined abnormal motion (i.e., "failure" detection) and using the inherent mechanism of gantry rotation during arc radiotherapy, the current approach provides high confidence regarding the prostate position in real time without the unwanted overhead of continuous or periodic kV imaging.

    View details for DOI 10.1016/j.ijrobp.2009.11.056

    View details for Web of Science ID 000282147000041

    View details for PubMedID 20472354

    View details for PubMedCentralID PMC4131869

  • Clinical development of a failure detection-based online repositioning strategy for prostate IMRT-Experiments, simulation, and dosimetry study MEDICAL PHYSICS Liu, W., Qian, J., Hancock, S. L., Xing, L., Luxton, G. 2010; 37 (10): 5287-5297


    To implement and evaluate clinic-ready adaptive imaging protocols for online patient repositioning (motion tracking) during prostate IMRT using treatment beam imaging supplemented by minimal, as-needed use of on-board kV.The authors examine the two-step decision-making strategy: (1) Use cine-MV imaging and online-updated characterization of prostate motion to detect target motion that is potentially beyond a predefined threshold and (2) use paired MV-kV 3D localization to determine overthreshold displacement and, if needed, reposition the patient. Two levels of clinical implementation were evaluated: (1) Field-by-field based motion correction for present-day linacs and (2) instantaneous repositioning for new-generation linacs with capabilities of simultaneous MV-kV imaging and remote automatic couch control during treatment delivery. Experiments were performed on a Varian Trilogy linac in clinical mode using a 4D motion phantom programed with prostate motion trajectories taken from patient data. Dosimetric impact was examined using a 2D ion chamber array. Simulations were done for 536 trajectories from 17 patients.Despite the loss of marker detection efficiency caused by the MLC leaves sometimes obscuring the field at the marker's projected position on the MV imager, the field-by-field correction halved (from 23% to 10%) the mean percentage of time that target displacement exceeded a 3 mm threshold, as compared to no intervention. This was achieved at minimal cost in additional imaging (average of one MV-kV pair per two to three treatment fractions) and with a very small number of repositionings (once every four to five fractions). Also with low kV usage (approximation 2/fraction), the instantaneous repositioning approach reduced overthreshold time by more than 75% (23% to 5%) even with severe MLC blockage as often encountered in current IMRT and could reduce the overthreshold time tenfold (to < 2%) if the MLC blockage problem were relieved. The information acquired for repositioning using combined MV-kV images was found to have submillimeter accuracy.This work demonstrated with a current clinical setup that substantial reduction of adverse targeting effects of intrafraction prostate motion can be realized. The proposed adaptive imaging strategy incurs minimal imaging dose to the patient as compared to other stereoscopic imaging techniques.

    View details for DOI 10.1118/1.3488887

    View details for Web of Science ID 000283483700016

    View details for PubMedID 21089763

    View details for PubMedCentralID PMC2951998

  • Dose reconstruction for volumetric modulated arc therapy (VMAT) using cone-beam CT and dynamic log files PHYSICS IN MEDICINE AND BIOLOGY Qian, J., Lee, L., Liu, W., Chu, K., Mok, E., Luxton, G., Le, Q., Xing, L. 2010; 55 (13): 3597-3610


    Volumetric modulated arc therapy (VMAT) has recently emerged as a new clinical modality for conformal radiation therapy. The aim of this work is to establish a methodology and procedure for retrospectively reconstructing the actual dose delivered in VMAT based on the pre-treatment cone-beam computed tomography (CBCT) and dynamic log files. CBCT was performed before the dose delivery and the system's log files were retrieved after the delivery. Actual delivery at a control point including MLC leaf positions, gantry angles and cumulative monitor units (MUs) was recorded in the log files and the information was extracted using in-house developed software. The extracted information was then embedded into the original treatment DICOM-radiation therapy (RT) file to replace the original control point parameters. This reconstituted DICOM-RT file was imported into the Eclipse treatment planning system (TPS) and dose was computed on the corresponding CBCT. A series of phantom experiments was performed to show the feasibility of dose reconstruction, validate the procedure and demonstrate the efficacy of this methodology. The resultant dose distributions and dose-volume histograms (DVHs) were compared with those of the original treatment plan. The studies indicated that CBCT-based VMAT dose reconstruction is readily achievable and provides a valuable tool for monitoring the dose actually delivered to the tumor target as well as the sensitive structures. In the absence of setup errors, the reconstructed dose shows no significant difference from the original pCT-based plan. It is also elucidated that the proposed method is capable of revealing the dosimetric changes in the presence of setup errors. The method reported here affords an objective means for dosimetric evaluation of VMAT delivery and is useful for adaptive VMAT in future.

    View details for DOI 10.1088/0031-9155/55/13/002

    View details for Web of Science ID 000279004300002

    View details for PubMedID 20526034

  • Trade-Offs in Data Acquisition and Processing Parameters for Backscatter and Scatterer Size Estimations IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL Liu, W., Zagzebski, J. A. 2010; 57 (2): 340-352


    By analyzing backscattered echo signal power spectra and thereby obtaining backscatter coefficient vs. frequency data, the size of subresolution scatterers contributing to echo signals can be estimated. Here we investigate trade-offs in data acquisition and processing parameters for reference phantom-based backscatter and scatterer size estimations. RF echo data from a tissue-mimicking test phantom were acquired using a clinical scanner equipped with linear array transducers. One array has a nominal frequency bandwidth of 5 to 13 MHz and the other 4 to 9 MHz. Comparison of spectral estimation methods showed that the Welch method provided spectra yielding more accurate and precise backscatter coefficient and scatterer size estimations than spectra computed by applying rectangular, Hanning, or Hamming windows and much reduced computational load than if using the multitaper method. For small echo signal data block sizes, moderate improvements in scatterer size estimations were obtained using a multitaper method, but this significantly increases the computational burden. It is critical to average power spectra from lateral A-lines for the improvement of scatterer size estimation. Averaging approximately 10 independent A-lines laterally with an axial window length 10 times the center frequency wavelength optimized trade-offs between spatial resolution and the variance of scatterer size estimates. Applying the concept of a time-bandwidth product, this suggests using analysis blocks that contain at least 30 independent samples of the echo signal. The estimation accuracy and precision depend on the ka range where k is the wave number and a is the effective scatterer size. This introduces a region-of-interest depth dependency to the accuracy and precision because of preferential attenuation of higher frequency sound waves in tissuelike media. With the 5 to 13 MHz, transducer ka ranged from 0.5 to 1.6 for scatterers in the test phantom, which is a favorable range, and the accuracy and precision of scatterer size estimations were both within approximately 5% using optimal analysis block dimensions. When the 4- to 9-MHz transducer was used, the ka value ranged from 0.3 to 0.8 to 1.1 for the experimental conditions, and the accuracy and precision were found to be approximately 10% and 10% to 25%, respectively. Although the experiments were done with 2 specific models of transducers on the test phantom, the results can be generalized to similar clinical arrays available from a variety of manufacturers and/or for different size of scatterers with similar ka range.

    View details for DOI 10.1109/TUFFC.2010.1414

    View details for Web of Science ID 000274817300008

    View details for PubMedID 20178900

    View details for PubMedCentralID PMC2853955

  • Real-time 3D internal marker tracking during arc radiotherapy by the use of combined MV-kV imaging PHYSICS IN MEDICINE AND BIOLOGY Liu, W., Wiersma, R. D., Mao, W., Luxton, G., Xing, L. 2008; 53 (24): 7197-7213


    To minimize the adverse dosimetric effect caused by tumor motion, it is desirable to have real-time knowledge of the tumor position throughout the beam delivery process. A promising technique to realize the real-time image guided scheme in external beam radiation therapy is through the combined use of MV and onboard kV beam imaging. The success of this MV-kV triangulation approach for fixed-gantry radiation therapy has been demonstrated. With the increasing acceptance of modern arc radiotherapy in the clinics, a timely and clinically important question is whether the image guidance strategy can be extended to arc therapy to provide the urgently needed real-time tumor motion information. While conceptually feasible, there are a number of theoretical and practical issues specific to the arc delivery that need to be resolved before clinical implementation. The purpose of this work is to establish a robust procedure of system calibration for combined MV and kV imaging for internal marker tracking during arc delivery and to demonstrate the feasibility and accuracy of the technique. A commercially available LINAC equipped with an onboard kV imager and electronic portal imaging device (EPID) was used for the study. A custom built phantom with multiple ball bearings was used to calibrate the stereoscopic MV-kV imaging system to provide the transformation parameters from imaging pixels to 3D world coordinates. The accuracy of the fiducial tracking system was examined using a 4D motion phantom capable of moving in accordance with a pre-programmed trajectory. Overall, spatial accuracy of MV-kV fiducial tracking during the arc delivery process for normal adult breathing amplitude and period was found to be better than 1 mm. For fast motion, the results depended on the imaging frame rates. The RMS error ranged from approximately 0.5 mm for the normal adult breathing pattern to approximately 1.5 mm for more extreme cases with a low imaging frame rate of 3.4 Hz. In general, highly accurate real-time tracking of implanted markers using hybrid MV-kV imaging is achievable and the technique should be useful to improve the beam targeting accuracy of arc therapy.

    View details for DOI 10.1088/0031-9155/53/24/013

    View details for Web of Science ID 000261310200013

    View details for PubMedID 19043177

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