January 10 2021

CBCT has inferior quantitative accuracy when compared to standard multi-detector CT, also known as helical CT.

Recently, we developed a new method to improve quantitative accuracy of CBCT. This method exploits the use of 2D antiscatter grids in conjunction with flat panel detectors, to measure and correct residual scatter that was not stopped by the 2D grid. In this method, 2D antiscatter grid acts as a microarray of beam modulators placed directly on the detector, which, in turn, helps to measure residual scatter in projections. We refer this method as "Grid-based scatter sampling (GSS)". Correction of residual scatter using GSS method provides higher CT number accuracy than 2D antiscatter grid alone.

Our paper describing the GSS method was recently published in the Medical Physics Journal, and can be found here.

March 20 2020

Our work on a new scatter correction method was presented at the 2020 SPIE Medical Imaging Conference in Houston, TX.  Here is recap of our presentation:

2D antiscatter grids stop vast majority of scattered radiation in flat panel detector-based CBCT. However, a small fraction of scatter goes through, and registered by the image receptor. A less known effect of residual scatter is that it may interfere with grid septal shadow correction, and cause ring artifacts in cone beam CT images.

To correct residual scatter, we developed a new method; we used 2D grid itself as a residual scatter measurement (and correction) device. This scatter correction approach not only improves CT number accuracy further, but also reduces ring artifacts.

Some of the proof of concept results can be found in the SPIE Conference Proceedings:

Click here to access the article

Here are couple of cone beam CT images from the link above. Ring artifacts are reduced substantially after residual scatter correction.  Rings pointed by the blue arrows are caused by a detector related issue, not residual scatter. That’s why they are not corrected by our method.

GSS method ring artifact reduction

November 15 2019

Our article, the effect of grid geometry on x-ray transmission properties, is published in Physics in Medicine and Biology.

This work shows that scattered radiation fraction is reduced by a factor of ~40 in high scatter conditions, when 2D grids (grid ratios 12-16) are used in cone beam CT geometry. Another conclusion of our work: 2D grids provide a factor of 5-7 lower scatter fraction than (high grid ratio) conventional grids, while also providing 20% better primary transmission than conventional grids.

June 10 2019

Our paper on a new ring artifact suppression method is published in the Medical Physics Journal

In this work, we developed a new adaptive Total Variation Minimization technique (adTVM) to adjust the regularization weights in an automated fashion to reduce grid-induced ring artifacts. The images below shows the effect of adTVM method on ring artifacts in CBCT images acquired with 3 different 2D antiscatter grids.

adTVM figure

August 10 2018

Here are a few images of  3D printed 2D antiscatter grid prototypes. These prototypes were fabricated from tungsten using Direct Metal Laser Sintering (DMLS) process. Each grid wall is only 100 microns thick, and it is aligned towards the x-ray focal spot, requiring a unique slant for each wall.

Grid Picture 1Grid Picture 2

Grid Picture 3

The prototype is being installed in the Varian TrueBeam CBCT system.

May 25 2018

We will have several presentations at the AAPM Annual Meeting in July. Timur’s work on CBCT ring artifacts suppression will be presented in the Science Council Session.  

T. Alexeev, B. Miller, B. Kavanagh, M. Miften, C. Altunbas ”Development of High Aspect Ratio Two‐Dimensional Antiscatter Grids for CBCT”.

B. Miller, T. Alexeev, D. Thomas, K. Stuhr, B. Kavanagh, M. Miften, C. Altunbas “CBCT‐Based Dose Calculations with a Two‐Dimensional Anti‐Scatter Grid Prototype: The Effect of Scatter Suppression On Dose‐Calculation Accuracy”.

Science Council Session Presentation: T. Alexeev, B. Kavanagh, M. Miften, C. Altunbas “Development and Evaluation of a Total Variation Minimization Based Method for Suppressing Ring Artifacts in CBCT Images”.

January 20 2018

Our Medical Physics Letter is published in the Medical Physics Journal.

T. Alexeev, B. Kavanagh, M. Miften, C. Altunbas, “Two‐dimensional antiscatter grid: A novel scatter rejection device for Cone‐beam computed tomography, Med. Phys. 45(2), 529-534 (2018)

Click here to read the article.

This letter reports on the first cone beam CT images acquired with our 2D antiscatter grid prototypes. We observed significantly improved Hounsfield Unit (HU) accuracy, and up to 86% improvement in contrast-to-noise ratio in images acquired with the 2D grid.

Effect of antiscatter grids on image quality

July 15 2017

We are excited that our first paper on 2D antiscatter grids for cone beam CT is published in Medical Physics Journal. 

C. Altunbas, B. Kavanagh, T. Alexeev, M. Miften “Transmission characteristics of a two-dimensional antiscatter grid prototype for CBCT,” Med. Phys. 44, 3952 (2017)

You can access the free full text here.

May 29 2017

Congratulations to Timur Alexeev! His is the winner of AAPM’s Science Council Junior Investigator Competition. His work received the highest score among 170 submissions. He will give his presentation titled “Development and Evaluation of a Two-Dimensional Antiscatter Grid for CBCT” at the AAPM Annual Meeting in Denver this July.

Cem Pic

Cem Altunbas, PhD is an Associate Professor in the Department of Radiation Oncology at the University of Colorado School of Medicine (SOM). Dr. Altunbas’ research is focused on the development of novel x-ray imaging instruments and methods for medical imaging and image-guided radiation therapy. Before joining University of Colorado SOM, he has investigated photon counting gaseous detectors for digital radiography at SUNY Buffalo, and helped to develop a chest radiography system for lung nodule detection and a prototype breast computed tomography system for breast cancer imaging at MD Anderson Cancer Center. After completing his medical physics residency at Medical College of Virginia, he joined the Department of Radiation Oncology at University of Colorado SOM, and he has been working on imaging challenges faced in cone beam computed tomography (CBCT) guided radiation therapy. Currently, his NIH funded research is centered on developing a new 3D-printed antiscatter grid to improve soft tissue visualization and quantitative image features in CBCT.  


Selected publications

  • Z. Yu, Y. Park, and C. Altunbas "Simultaneous scatter rejection and correction method using 2D antiscatter grids for CBCT." vol. 11312, p. 113122W. International Society for Optics and Photonics, (2020).
  • C. Altunbas, T. Alexeev, M. Miften, B. Kavanagh “Effect of grid geometry on the transmission properties of 2D grids for flat detectors in CBCT,” Physics in Medicine & Biology 64, 22, 225006 (2019).
  • T. Alexeev, B. Kavanagh, M. Miften, C. Altunbas “A novel total variation based ring artifact suppression method for CBCT imaging with two‐dimensional antiscatter grids,” Med. Phys. 46, 2181 (2019)
  • T. Alexeev, B. Kavanagh, M. Miften, C. Altunbas “Two‐dimensional antiscatter grid: A novel scatter rejection device for Cone‐beam computed tomography,” Med. Phys. 45, 529 (2018)
  • C. Altunbas, B. Kavanagh, T. Alexeev, M. Miften “Transmission characteristics of a two dimensional antiscatter grid prototype for CBCT,” Med. Phys. 44, 3952 (2017)
  • C. Altunbas, Y. Zhong, C. Lai, C. Shaw, “Reduction of ring artifacts in CBCT: Detection and correction of pixel gain variations in flat panel detectors,” Med. Phys. 41, 091913 (2014)
  • C. Altunbas, “Image corrections for scattered radiation” chapter in “Cone Beam Computed Tomography,” Ed. C. Shaw, Taylor&Francis, Boca Raton (2014)
  • C. Altunbas, T. C. Hankinson, M. Miften, T. Tello, S. R. Plimpton, K. Stuhr, A. K. Liu,” Rotational setup errors in pediatric stereotactic radiation therapy,” Prac. Rad. Onc., 3(3),194-198 (2013)
  • C. Glide-Hurst, M. Bellon, R. Foster, C. Altunbas, M. Speiser, M. Altman, D. Westerly, N. Wen, B. Zhao, M. Miften, I. J. Chetty, and T. Solberg,” Commissioning of the Varian TrueBeam linear accelerator: A multi-institutional study,” Med. Phys. 40, 031719 (2013)
  • L. Chen, C. C. Shaw, C. Altunbas, C. Lai, X. Liu, ” Spatial resolution properties in cone beam CT : A simulation study,” Med. Phys. 35, 724-734 (2008)
  • C. Lai, C. C. Shaw, L. Chen, C. Altunbas, X. Liu, T. Han, T. Wang, “Visibility of microcalcification in cone beam breast CT: Effects of x-ray tube voltage and radiation dose,” Med. Phys. 34, 2995-3004 (2007) 
  • C. Altunbas, C. C. Shaw, L. Chen, C. Lai, X. Liu, T. Han, T. Wang, “A post-reconstruction method to correct cupping artifacts in cone beam breast computed tomography,” Med. Phys. 34, 3109-3118 (2007)
  • X. Liu, C. C. Shaw, T. Wang, L. Chen, C. Altunbas, S. C. Kappadath, “An accurate scatter measurement and correction technique for cone beam breast CT imaging using scanning sampled measurement (SSM) technique," Proc. SPIE, 6142, 614234 (2006)
  • X. Liu, C. C. Shaw, C. Altunbas, T. Wang, “Electronic Collimation for Implementing Slot-Scan Digital Radiography with a Flat-Panel Detector – Initial Experience for Scatter Reduction in Chest Phantom Images,” IEEE Trans. Med. Imag. 25 (4), 496-502 (2006)
  • C. Altunbas, E. F. Barasch, D. R. Bednarek, S. Rudin, “Photon-counting energy-sensitive micropattern gas detector for digital radiography,” Proc. SPIE 5030, 833-844 (2003)
  • C. Altunbas, M. Capéans, K. Dehmelt, J. Ehlers, J. Friedrich, I. Konorov, A. Gandi, S. Kappler, B. Ketzer, R. De Oliveira et al., ”Construction, test and commissioning of the triple-gem tracking detector for COMPASS,” Nucl. Instr. Meth. A 490, 177-203 (2002) 

We have open positions for exceptional graduate students and postdoctoral fellows who would like to join our lab. If you would like to learn more about the positions, please contact Cem Altunbas, cem.altunbas@cuanschutz.edu


We currently have an open postdoctoral fellow position. Please download the job description here.