Scott Hsieh works on several projects to improve CT scanner technology. His dissertation project was on the design of an adaptive x-ray filter that could control and personalize the incident x-ray radiation on the patient, to reduce unnecessary radiation and optimize image quality. Other projects include parameter tradeoffs in upcoming photon counting detectors, the introduction of a new class of anti-scatter grids, and analyses of new reconstruction algorithms.

Academic Appointments

  • Instructor, Radiology


  • Scott Hsieh, Norbert J Pelc, Brian E Nett. "United States Patent 9196061 Systems and methods for performing truncation artifact correction", General Electric Company, Nov 24, 2015
  • Morteza Gharib, Jian Lu, Scott Hsieh, Francisco Pereira, Alexei Harvard, David Jeon. "United States Patent 8773507 Defocusing feature matching system to measure camera pose with interchangeable lens cameras", California Institute of Technology, Jul 8, 2014
  • Morteza Gharib, Jian Lu, Alexei Harvard, Scott Hsieh. "United States Patent 8773514 Accurate 3D object reconstruction using a handheld device with a projected light pattern", California Institute of Technology, Jul 8, 2014
  • Morteza Gharib, Jian Lu, Scott Hsieh, Francisco Pereira, Alexei Harvard. "United States Patent 8,514,268 Method and device for high-resolution three-dimensional imaging which obtains camera pose using defocusing", California Institute of Technology, Aug 20, 2013


2015-16 Courses


All Publications

  • Estimating scatter in cone beam CT with striped ratio grids: A preliminary investigation. Medical physics Hsieh, S. 2016; 43 (9): 5084-?


    To propose a new method for estimating scatter in x-ray imaging. Conventional antiscatter grids reject scatter at an efficiency that is constant or slowly varying over the surface of the grid. A striped ratio antiscatter grid, composed of stripes that alternate between high and low grid ratio, could be used instead. Such a striped ratio grid would reduce scatter-to-primary ratio as a conventional grid would, but more importantly, the signal discontinuities at the boundaries of stripes can be used to estimate local scatter content.Signal discontinuities provide information on scatter, but are contaminated by variation in primary radiation. A nonlinear image processing algorithm is used to estimate the scatter content in the presence of primary variation. We emulated a striped ratio grid by imaging phantoms with two sequential CT scans, one with and one without a conventional grid. These two scans are processed together to mimic a striped ratio grid. This represents a best case limit of the striped ratio grid, in that the extent of grid ratio modulation is very high and the scatter contrast is maximized.In a uniform cylinder, the striped ratio grid virtually eliminates cupping. Artifacts from scatter are improved in an anthropomorphic phantom. Some banding artifacts are induced by the striped ratio grid.Striped ratio grids could be a simple and effective evolution of conventional antiscatter grids. Construction and validation of a physical prototype remains an important future step.

    View details for DOI 10.1118/1.4960631

    View details for PubMedID 27587039

  • A prototype piecewise-linear dynamic attenuator PHYSICS IN MEDICINE AND BIOLOGY Hsieh, S. S., Peng, M. V., May, C. A., Shunhavanich, P., Fleischmann, D., Pelc, N. J. 2016; 61 (13): 4974-4988


    The piecewise-linear dynamic attenuator has been proposed as a mechanism in CT scanning for personalizing the x-ray illumination on a patient- and application-specific basis. Previous simulations have shown benefits in image quality, scatter, and dose objectives. We report on the first prototype implementation. This prototype is reduced in scale and speed and is integrated into a tabletop CT system with a smaller field of view (25 cm) and longer scan time (42 s) compared to a clinical system. Stainless steel wedges were machined and affixed to linear actuators, which were in turn held secure by a frame built using rapid prototyping technologies. The actuators were computer-controlled, with characteristic noise of about 100 microns. Simulations suggest that in a clinical setting, the impact of actuator noise could lead to artifacts of only 1 HU. Ring artifacts were minimized by careful design of the wedges. A water beam hardening correction was applied and the scan was collimated to reduce scatter. We scanned a 16 cm water cylinder phantom as well as an anthropomorphic pediatric phantom. The artifacts present in reconstructed images are comparable to artifacts normally seen with this tabletop system. Compared to a flat-field reference scan, increased detectability at reduced dose is shown and streaking is reduced. Artifacts are modest in our images and further refinement is possible. Issues of mechanical speed and stability in the challenging clinical CT environment will be addressed in a future design.

    View details for DOI 10.1088/0031-9155/61/13/4974

    View details for Web of Science ID 000378094000015

    View details for PubMedID 27284705

  • Improving pulse detection in multibin photon-counting detectors. Journal of medical imaging (Bellingham, Wash.) Hsieh, S. S., Pelc, N. J. 2016; 3 (2): 023505-?


    Energy-discriminating, photon-counting (EDPC) detectors are attractive for their potential for improved detective quantum efficiency and for their spectral imaging capabilities. However, at high count rates, counts are lost, the detected spectrum is distorted, and the advantages of EDPC detectors disappear. Existing EDPC detectors identify counts by analyzing the signal with a bank of comparators. We explored alternative methods for pulse detection for multibin EDPC detectors that could improve performance at high count rates. The detector signal was simulated in a Monte Carlo fashion assuming a bipolar shape and analyzed using several methods, including the conventional bank of comparators. For example, one method recorded the peak energy of the pulse along with the width (temporal extent) of the pulse. The Cramer-Rao lower bound of the variance of basis material estimates was numerically found for each method. At high count rates, the variance in water material (bone canceled) measurements could be reduced by as much as an order of magnitude. Improvements in virtual monoenergetic images were modest. We conclude that stochastic noise in spectral imaging tasks could be reduced if alternative methods for pulse detection were utilized.

    View details for DOI 10.1117/1.JMI.3.2.023505

    View details for PubMedID 27284548

  • A limit on dose reduction possible with CT reconstruction algorithms without prior knowledge of the scan subject. Medical physics Hsieh, S. S., Chesler, D. A., Fleischmann, D., Pelc, N. J. 2016; 43 (3): 1361-?


    To find an upper bound on the maximum dose reduction possible for any reconstruction algorithm, analytic or iterative, that result from the inclusion of the data statistics. The authors do not analyze noise reduction possible from prior knowledge or assumptions about the object.The authors examined the task of estimating the density of a circular lesion in a cross section. Raw data were simulated by forward projection of existing images and numerical phantoms. To assess an upper bound on the achievable dose reduction by any algorithm, the authors assume that both the background and the shape of the lesion are completely known. Under these conditions, the best possible estimate of the density can be determined by solving a weighted least squares problem directly in the raw data domain. Any possible reconstruction algorithm that does not use prior knowledge or make assumptions about the object, including filtered backprojection (FBP) or iterative reconstruction methods with this constraint, must be no better than this least squares solution. The authors simulated 10 000 sets of noisy data and compared the variance in density from the least squares solution with those from FBP. Density was estimated from FBP images using either averaging within a ROI, or streak-adaptive averaging with better noise performance.The bound on the possible dose reduction depends on the degree to which the observer can read through the possibly streaky noise. For the described low contrast detection task with the signal shape and background known exactly, the average dose reduction possible compared to FBP with streak-adaptive averaging was 42% and it was 64% if only the ROI average is used with FBP. The exact amount of dose reduction also depends on the background anatomy, with statistically inhomogeneous backgrounds showing greater benefits.The dose reductions from new, statistical reconstruction methods can be bounded. Larger dose reductions in the density estimation task studied here are only possible with the introduction of prior knowledge, which can introduce bias.

    View details for DOI 10.1118/1.4941954

    View details for PubMedID 26936720

  • A Dynamic Attenuator Improves Spectral Imaging With Energy-Discriminating, Photon Counting Detectors IEEE TRANSACTIONS ON MEDICAL IMAGING Hsieh, S. S., Pelc, N. J. 2015; 34 (3): 729-739


    Energy-discriminating, photon counting (EDPC) detectors have high potential in spectral imaging applications but exhibit degraded performance when the incident count rate approaches or exceeds the characteristic count rate of the detector. In order to reduce the requirements on the detector, we explore the strategy of modulating the X-ray flux field using a recently proposed dynamic, piecewise-linear attenuator. A previous paper studied this modulation for photon counting detectors but did not explore the impact on spectral applications. In this work, we modeled detection with a bipolar triangular pulse shape (Taguchi et al., 2011) and estimated the Cramer-Rao lower bound (CRLB) of the variance of material selective and equivalent monoenergetic images, assuming deterministic errors at high flux could be corrected. We compared different materials for the dynamic attenuator and found that rare earth elements, such as erbium, outperformed previously proposed materials such as iron in spectral imaging. The redistribution of flux reduces the variance or dose, consistent with previous studies on benefits with conventional detectors. Numerical simulations based on DICOM datasets were used to assess the impact of the dynamic attenuator for detectors with several different characteristic count rates. The dynamic attenuator reduced the peak incident count rate by a factor of 4 in the thorax and 44 in the pelvis, and a 10 Mcps/mm (2) EDPC detector with dynamic attenuator provided generally superior image quality to a 100 Mcps/mm (2) detector with reference bowtie filter for the same dose. The improvement is more pronounced in the material images.

    View details for DOI 10.1109/TMI.2014.2360381

    View details for Web of Science ID 000350870700005

    View details for PubMedID 25265628

  • An algorithm to estimate the object support in truncated images MEDICAL PHYSICS Hsieh, S. S., Nett, B. E., Cao, G., Pelc, N. J. 2014; 41 (7)

    View details for DOI 10.1118/1.4881521

    View details for Web of Science ID 000339009800020

  • The piecewise-linear dynamic attenuator reduces the impact of count rate loss with photon-counting detectors PHYSICS IN MEDICINE AND BIOLOGY Hsieh, S. S., Pelc, N. J. 2014; 59 (11): 2829-2847


    Photon counting x-ray detectors (PCXDs) offer several advantages compared to standard energy-integrating x-ray detectors, but also face significant challenges. One key challenge is the high count rates required in CT. At high count rates, PCXDs exhibit count rate loss and show reduced detective quantum efficiency in signal-rich (or high flux) measurements. In order to reduce count rate requirements, a dynamic beam-shaping filter can be used to redistribute flux incident on the patient. We study the piecewise-linear attenuator in conjunction with PCXDs without energy discrimination capabilities. We examined three detector models: the classic nonparalyzable and paralyzable detector models, and a 'hybrid' detector model which is a weighted average of the two which approximates an existing, real detector (Taguchi et al 2011 Med. Phys. 38 1089-102 ). We derive analytic expressions for the variance of the CT measurements for these detectors. These expressions are used with raw data estimated from DICOM image files of an abdomen and a thorax to estimate variance in reconstructed images for both the dynamic attenuator and a static beam-shaping ('bowtie') filter. By redistributing flux, the dynamic attenuator reduces dose by 40% without increasing peak variance for the ideal detector. For non-ideal PCXDs, the impact of count rate loss is also reduced. The nonparalyzable detector shows little impact from count rate loss, but with the paralyzable model, count rate loss leads to noise streaks that can be controlled with the dynamic attenuator. With the hybrid model, the characteristic count rates required before noise streaks dominate the reconstruction are reduced by a factor of 2 to 3. We conclude that the piecewise-linear attenuator can reduce the count rate requirements of the PCXD in addition to improving dose efficiency. The magnitude of this reduction depends on the detector, with paralyzable detectors showing much greater benefit than nonparalyzable detectors.

    View details for DOI 10.1088/0031-9155/59/11/2829

    View details for Web of Science ID 000336459000018

  • Control algorithms for dynamic attenuators MEDICAL PHYSICS Hsieh, S. S., Pelc, N. J. 2014; 41 (6)

    View details for DOI 10.1118/1.4875727

    View details for Web of Science ID 000337106300025

  • Dose reduction using a dynamic, piecewise-linear attenuator MEDICAL PHYSICS Hsieh, S. S., Fleischmann, D., Pelc, N. J. 2014; 41 (2)

    View details for DOI 10.1118/1.4862079

    View details for Web of Science ID 000331213300043

  • Use of Depth Information from In-depth Photon Counting Detectors for X-ray Spectral Imaging: A Preliminary Simulation Study MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Yao, Y., Bornefalk, H., Hsieh, S. S., Danielsson, M., Pelc, N. J. 2014; 9033

    View details for DOI 10.1117/12.2042839

    View details for Web of Science ID 000338775800117

  • Segmented Targeted Least Squares Estimator for Material Decomposition in Multi-Bin PCXDs MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Rajbhandary, P. L., Hsieh, S. S., Pelc, N. J. 2014; 9033

    View details for DOI 10.1117/12.2043198

    View details for Web of Science ID 000338775800042

  • Enabling Photon Counting Detectors with Dynamic Attenuators MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Pelc, N. J. 2014; 9033

    View details for DOI 10.1117/12.2042533

    View details for Web of Science ID 000338775800122

  • Algorithms for Optimizing CT Fluence Control MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Pelc, N. J. 2014; 9033

    View details for DOI 10.1117/12.2042542

    View details for Web of Science ID 000338775800019

  • The feasibility of a piecewise-linear dynamic bowtie filter MEDICAL PHYSICS Hsieh, S. S., Pelc, N. J. 2013; 40 (3)


    The prepatient attenuator (or "bowtie filter") in CT is used to modulate the flux as a function of fan angle of the x-ray beam incident on the patient. Traditional, static bowtie filters are tailored only for very generic scans and for the average patient. The authors propose a design for a dynamic bowtie that can produce a time-dependent piecewise-linear attenuation profile. This dynamic bowtie may reduce dynamic range, dose or scatter, but in this work they focus on its ability to reduce dynamic range, which may be particularly important for systems employing photon-counting detectors.The dynamic bowtie is composed of a set of triangular wedges. Each wedge is independently moved in order to produce a time-dependent piecewise-linear attenuation profile. Simulations of the bowtie are conducted to estimate the dynamic range reduction in six clinical datasets. The control of the dynamic bowtie is determined by solving a convex optimization problem, and the dose is estimated using Monte Carlo techniques. Beam hardening artifacts are also simulated.The dynamic range is reduced by factors ranging from 2.4 to 27 depending on the part of the body studied. With a dynamic range minimization objective, the dose to the patient can be reduced from 6% to 33% while maintaining peak image noise. Further reduction in dose may be possible with a specific dose reduction objective. Beam hardening artifacts are suppressed with a two-pass algorithm.A dynamic bowtie producing a time-dependent, piecewise-linear attenuation profile is possible and can be used to modulate the flux of the scanner to the imaging task. Initial simulations show a large reduction in dynamic range. Several other applications are possible.

    View details for DOI 10.1118/1.4789630

    View details for Web of Science ID 000316369400033

    View details for PubMedID 23464325

  • The feasibility of an inverse geometry CT system with stationary source arrays MEDICAL PHYSICS Hsieh, S. S., Heanue, J. A., Funk, T., Hinshaw, W. S., Wilfley, B. P., Solomon, E. G., Pelc, N. J. 2013; 40 (3)

    View details for DOI 10.1118/1.4789918

    View details for Web of Science ID 000316369400027

  • Optimized control of a dynamic, prepatient attenuator MEDICAL IMAGING 2013: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Pelc, N. J. 2013; 8668

    View details for DOI 10.1117/12.2008229

    View details for Web of Science ID 000322002700058

  • Dynamic bowtie for fan-beam CT JOURNAL OF X-RAY SCIENCE AND TECHNOLOGY Liu, F., Wang, G., Cong, W., Hsieh, S. S., Pelc, N. J. 2013; 21 (4): 579-590


    A bowtie is a filter used to shape an x-ray beam and equalize its flux reaching different detector channels. For development of spectral CT with energy discriminating photon-counting (EDPC) detectors, here we propose and evaluate a dynamic bowtie for performance optimization based on a patient model or a scout scan. With a mechanical rotation of a dynamic bowtie and an adaptive adjustment of an x-ray source flux, an x-ray beam intensity profile can be modulated. First, a mathematical model for dynamic bowtie filtering is established for an elliptical section in fan-beam geometry, and the contour of the optimal bowtie is derived. Then, numerical simulation is performed to compare the performance of the dynamic bowtie in the cases of an ideal phantom and a realistic cross-section relative to the counterparts without any bowtie and with a fixed bowtie respectively. Our dynamic bowtie can equalize the expected numbers of photons in the case of an ideal phantom. In practical cases, our dynamic bowtie can effectively reduce the dynamic range of detected signals inside the field of view. Although our design is optimized for an elliptical phantom, the resultant dynamic bowtie can be applied to a real fan-beam scan if the underlying cross-section can be approximated as an ellipse. Furthermore, our design methodology can be applied to specify an optimized dynamic bowtie for any cross-section of a patient, preferably using rapid prototyping technology.

    View details for DOI 10.3233/XST-130386

    View details for Web of Science ID 000326747000012

    View details for PubMedID 24191994

  • Truncation artifact correction by support recovery MEDICAL IMAGING 2013: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Cao, G., Nett, B. E., Pelc, N. J. 2013; 8668

    View details for DOI 10.1117/12.2008224

    View details for Web of Science ID 000322002700124

  • A volumetric reconstruction algorithm for stationary source inverse-geometry CT MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Pelc, N. J. 2012; 8313

    View details for DOI 10.1117/12.912490

    View details for Web of Science ID 000304768000126

  • An inverse geometry CT system with stationary source arrays MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING Hsieh, S. S., Heanue, J. A., Funk, T., Hinshaw, W. S., Pelc, N. J. 2011; 7961

    View details for DOI 10.1117/12.878801

    View details for Web of Science ID 000294178500063