Optimization of Reduced-Dose MDCT of Thoracic Aorta Using Iterative Reconstruction Hüseyin Gürkan Töre, MD, Pegah Entezari, MD, Hamid Chalian, MD, Fernanda Dias Gonzalez-Guindalini, MD, Marcos Paulo Ferreira Botelho, MD and Vahid Yaghmai, MD Objective: To evaluate the contribution of iterative reconstruction on image quality of reduced-dose multidetector computed tomography of the thoracic aorta. Methods: A torso phantom was scanned using two tube potentials (80 and 120 kVp) and five different tube currents (110, 75, 40, 20, and 10 mAs). All images were reconstructed with both filtered back projection (FBP) and iterative reconstruction. Aortic attenuation, image noise within the thoracic aorta, signal-to-noise ratio, and sharpness of the aortic wall were quantified in the phantom for the two reconstruction algorithms. Data were analyzed using paired t test. A value of P < 0.05 was considered significant. Results: The aortic attenuation was similar for FBP and iterative recon- struction (P > 0.05). Image noise level was lower (P < 0.0001), and image sharpness was higher (P = 0.046) with iterative reconstruction. Signal-to- noise ratios were higher with iterative reconstruction compared with those with FBP (P < 0.0001). Signal-to-noise ratio at 80 kVp with itera- tive reconstruction (9.8 ± 4.4) was similar to the signal-to-noise ratio at 120 kVp with FBP (8.4 ± 3.3) (P = 0.196). Conclusions: Less image noise and higher image sharpness may be achieved with iterative reconstruction in reduced-dose multidetector com- puted tomography of the thoracic aorta. Key Words: aorta, MDCT angiography, iterative reconstruction, filtered back projection, image quality, radiation dose reduction (J Comput Assist Tomogr 2014;38: 72–76) S ince its introduction in the 1970s, computed tomographic (CT) technology has undergone dramatic advances that have revolutionized patient care and increased diagnostic accuracy in a wide range of clinical indications. These developments have led to an increase in the number of CT examinations, which in turn have raised concerns about radiation dose and patient safety. 1,2 Computed tomographic angiography (CTA) of the thoracic aorta is routinely performed with or without electrocardiogra- phy (ECG) gating to evaluate aneurysms, dissections, and integ- rity of endovascular repairs. Several strategies have resulted in a reduction of radiation dose in CTA of the aorta. These include tube current modulation, tube voltage reduction, and application of body mass index–based protocols. 3 The use of prospective ECG triggering is another technique that can further improve im- age quality and help reduce radiation dose because it can result in 53.7 and 44.2% reduction in dose with 100 and 120 kVp, re- spectively, using similar tube current settings when compared with retrospective ECG-gated helical scanning. 4 Radiation dose from thoracic aorta CTA has been reported to be 2.9 and 26.2 mSv for prospectively and retrospectively gated CTA of the aorta, respectively. 5 Computed tomography radiation dose reduction can be achieved to a certain degree with these acquisition techniques but at the cost of increased image noise using the current fil- tered back projection (FBP) algorithms. 6 Recently, iterative re- construction has become available in clinical settings as a novel method to reduce CT image noise when radiation dose has been reduced. In the FBP technique, projections or views are constructed from the raw data. These projections are first filtered then back projected to form a 2-dimensional array of voxels that are allot- ted to a gray tone that is proportional to the average x-ray pho- ton distribution of each voxel. 7 On the other hand, iterative reconstruction methods consist of 3 major steps. First, artificial raw data are created by a forward projection of the volumetric object estimate. This artificial raw data, in a second step, are com- pared with the real measured raw data to compute a correction term. In the last step, the correction term is back projected onto the volumetric object estimate. The iteration process can be initi- ated with an empty image estimate or using prior information. 8 An example of an iterative reconstruction that uses the image or slice data alone is iterative reconstruction in image space (IRIS). With this technique, the raw data are first reconstructed in the traditional fashion with the use of the FBP. This information is then forward projected with multiple iterations according to modeling of the noise data. 7 Although iterative reconstruction does not affect radiation dose directly, it might lead to less radiation exposure in CT im- aging by maintaining diagnostic image quality despite low radi- ation dose image acquisitions. 9 The purpose of our study was to evaluate the image quality in multidetector computed tomography (MDCT) of thoracic aorta images acquired with standard and low-dose techniques and reconstructed with FBP and iterative reconstruction in an adult- sized anthropomorphic phantom. MATERIALS AND METHODS Image Acquisition A custom anthropomorphic torso phantom was used to eval- uate image quality of CT scans with varying tube voltages and tube currents (Fig. 1). The phantom was scanned with tube po- tentials of 80 and 120 kVp at the same session. At each tube po- tential, we performed 5 acquisitions with fixed tube current settings of 10, 20, 40, 75, and 110 mAs (10 acquisitions in total). All images were reconstructed with both filtered back projection and iterative reconstruction algorithms. A total of 20 data sets were obtained. The CT attenuation coefficients of anatomic struc- tures in this phantom are similar to those of previous studies with attenuation of aorta measuring 190 Hounsfield units (HU) at From the Department of Radiology, Northwestern Memorial Hospital, Northwestern University-Feinberg School of Medicine, Chicago, IL. Received for publication January 24, 2013; accepted June 24, 2013. Reprints: Vahid Yaghmai, MD, Department of Radiology, Northwestern Memorial Hospital, Northwestern University-Feinberg School of Medicine, 676 North Saint Clair St, Suite 800, Chicago, IL 60611 (e-mail: v-yaghmai@northwestern.edu). Hüseyin Gürkan Töre, Pegah Entezari, Hamid Chalian, Fernanda Dias Gonzalez-Guindalini, and Marcos Paulo Ferreira Botelho are supported by an educational grant from Siemens Healthcare. Copyright © 2014 by Lippincott Williams & Wilkins ORIGINAL ARTICLE 72 www.jcat.org J Comput Assist Tomogr • Volume 38, Number 1, January/February 2014 Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.