Characteristics of megavoltage cone-beam digital tomosynthesis M. Descovich, a O. Morin, J. F. Aubry, M. Aubin, and J. Chen Department of Radiation Oncology, University of California at San Francisco, San Francisco, California 94143-0226 A Bani-Hashemi Siemens Oncology Care Systems, Concord, California 94520 J. Pouliot Department of Radiation Oncology, University of California at San Francisco, San Francisco, California 94143-1708 Received 5 April 2007; revised 4 January 2008; accepted for publication 20 January 2008; published 12 March 2008 This article reports on the image characteristics of megavoltage cone-beam digital tomosynthesis MVCB DT. MVCB DT is an in-room imaging technique, which enables the reconstruction of several two-dimensional slices from a set of projection images acquired over an arc of 20° -40°. The limited angular range reduces the acquisition time and the dose delivered to the patient, but affects the image quality of the reconstructed tomograms. Image characteristics slice thickness, shape distortion, and contrast-to-noise ratio are studied as a function of the angular range. Potential clinical applications include patient setup and the development of breath holding techniques for gated imaging. © 2008 American Association of Physicists in Medicine. DOI: 10.1118/1.2868763 Key words: megavoltage cone-beam digital tomosynthesis, in-room imaging, image quality I. INTRODUCTION Digital tomosynthesis DT is a technique for reconstructing several slices of an object from a number of projection im- ages acquired during a single motion of the x-ray source. The concept of tomosynthesis is not new. It was originally devel- oped in 1970s Refs. 1 and 2 and was soon adopted in the field of diagnostic imaging for mammography, 3 angiography, 4 chest radiography, 5 and dental applications. 6 The reader is referred to Ref. 7 for a topical review of digital x-ray tomosynthesis. The recent advent of image guided ra- diation therapy and in particular, the capability of acquiring low-dose projection images of the patient in the treatment position using kilovoltage 8–10 or megavoltage 11,12 imaging devices, led to a renewed interest in digital tomosynthesis. Compared to two-dimensional radiography, DT has the ad- vantage of resolving the anatomy in three dimensions, re- moving overlaying structures and providing enhanced con- trast. Compared to three-dimensional computed tomography, it has the advantage of a faster acquisition time, a limited source motion, and reduced dose. Previous authors have re- ported on the potential use of digital tomosynthesis for on- line imaging of moving targets 13,14 and for the development of breath holding and gating techniques. 15,16 Most studies on the clinical applications of DT to radiation therapy, and on its imaging characteristics, concern keV radiographic units. The purpose of this article is to investigate the characteristics and potential applications of DT using a megavoltage isocentric unit. Megavoltage cone beam digital tomosynthesis MVCB DT uses the 6 MV beam from a medical linear accelerator as x-ray source and an amorphous silicon flat panel elec- tronic portal imaging device EPID as detector. The acqui- sition is isocentric and the Feldkamp backprojection algo- rithm with projection matrices is used for image reconstruction. 17 The characteristics of MVCB DT are intermediate be- tween two-dimensional radiography portal imaging and megavoltage cone-beam computed tomography MVCB CT. 11 A portal image consists of the superposition of all planes perpendicular to the direction of the x rays, therefore it provides inherently low contrast. The acquisition is fast a few seconds and it delivers a dose of 1–3 monitor units MU1 MU corresponds to 1cGy at the reference depth. MVCB CT provides a three-dimensional 3D image in all orientations, but it requires a longer acquisition time 1 min and, when soft tissue visibility is needed, rela- tively larger doses are required. 18,19 For MVCB CT imaging, the contrast-to-noise ratio CNR, and thus the soft tissue information, improves as the numbers of MU are increased. For daily setup imaging, a total exposure ranging between 2 and 10 MU is used to limit the imaging dose. In our experi- ence, an exposure of 2–4 MU is adequate for aligning pros- tate patients using the implanted gold markers, while expo- sures of 8–10 MU are preferred to align patients using anatomical landmarks bones and soft-tissue structures. Higher exposures 20 MU are instead required to acquire MVCB CT images to complement conventional CT images for treatment planning purposes in patient presenting im- planted metallic objects. 20 In MVCB CT, projection images are acquired over a 200° arc resulting in a complete coverage of the Fourier domain, while MVCB DT slice images are reconstructed using a lim- ited set of projections acquired over an arc of 20°-40°. In MVCB DT, only planes perpendicular to the beam direction 1310 1310 Med. Phys. 35 „4…, April 2008 0094-2405/2008/35„4…/1310/7/$23.00 © 2008 Am. Assoc. Phys. Med.