Low-dose Three-dimensional CT of Paranasal Sinuses

The goal of this study was to develop an optimal radiologic techniques for low-dose 3D CT of paranasal sinus to enhance the image quality in a reasonable absorbed x-ray dose for the patients. The ultimate aim is to provide higher quality images for physicians to identify pathological changes from the image, to make preoperative planning, to evaluate the risk of the operation of paranasal sinus and to minimize the occurrence of intra-operative complications. We analyzed spatial resolution and noise signals generated by various groups of mAs by using the dual technique of Plexiglas phantom and water phantom. In addition, Rando phantom was used to measure CTDI (computed tomography dose index) in the same scan area in a function of different slice thicknesses and spacings. The concept of MSAD (Multiple- scan average dose) was applied to calculate patient's absorbed dose. From July 2001 to January 2002, 19 patients were enrolled in the study and underwent CT examinations of paranasal sinuses. Among these patients, 19 were male and 5 female. Conventional 2D CT had been generally used with satisfactory results, especially for patients with chronic paransal sinusitis. However, 3D CT not only provides a better anatomical information of the paranasal sinuses but also follows the ALARA principle. The acceptable image quality could be achieved under the imaging parameters of 90 mAs and 77 mAs for axial and coronal sections, respectively. Three-dimension images could further be analyzed with SSD (shaded surface display), MPR (multiplanar reformats), and MIP (maximum intensity projection) to provide more information for both radiologists and clinicians.