Digital tomosynthesis (DT) is a form of limited-angle computed tomography that allows reconstruction of multiple section images from a set of projection data acquired over a limited range of X-ray tube angles (1). The term 'tomosynthesis' was defined by Grant in 1972 (2) by combining the two Greek words 'tomos'-a section, a slice, or a cutting-and 'synthesis'-a process, resulting in formation of something new. While the theory of tomosynthesis has been previously described, it has only recently been introduced as commercial equipment for medical imaging in the form of chest tomosynthesis (3). This new modality offers the potential for improved diagnostic performance over conventional radiography by substantially reducing the visual clutter of the overlying anatomy (1, 4-6). Although it does not have the depth resolution of CT, DT? provides high-resolution images in the coronal or sagittal plane with a substantial reduction in the radiation dose compared with CT (5-7).

Failure to detect sinusitis can lead to serious complications, such as cellulitis and osteomyelitis (8-10). Standard plain radiography, such as Caldwell or Water's views, used to be the primary imaging methods. These imaging methods do not have an acceptable diagnostic accuracy in cases of sinusitis. Therefore, a CT scan of the PNS is currently the imaging method of choice for sinusitis (8, 9, 11-15). A standard CT scan (SCT) of the PNS consists of coronal and axial sections. Because of these multiple slices, paranasal sinus (PNS) CT has a significant patient X-ray absorption dose (8, 13-15).

Although the radiation dose of DT is relatively higher than that of plain radiography (XR), DT is the most advanced digital radiography technology, which can be applied easily as an alternative to XR in many regions, including PNS evaluation.

Therefore, we investigated low dose (LD) PNS DT and compared its diagnostic accuracy for sinusitis? with that of PNS XR.

Paranasal sinus radiography series are the first choice for diagnosis and follow-up for patients with paranasal sinusitis; however, low sensitivity and low specificity are major limitations (8, 9, 11-15). Problems related to limited sensitivity and specificity of radiography are alleviated with use of CT. However, higher doses of radiation and higher costs become problematic (13-15). A recently developed technique, DT, is an interesting alternative modality. The number of scientific papers on DT has increased over the years (1, 3, 5, 7, 18-25), which is an indicator of increasing interest. DT is associated with a low radiation dose compared with OMU CT, and with improved detection compared with PNS radiography (1, 6, 18, 26-32). From our results using a standard anthropomorphic phantom, the effective dose for PNS DT is approximately 0.06 mSv by manufacturer's default and 0.026 mSv by our low dose modification, which is approximately 3 times higher than that used for radiographic examination (0.01 mSv); however, it is approximately 30 times lower than that used for OMU CT examination (0.91 mSv) (6, 24). Usual PNS radiographic examination includes two or three exposures (Water's view, Caldwell view, and/or skull lateral view); thus, total exposure of DT will not exceed that of the PNS radiography series. Furthermore, DT can be performed in a single position and exposure and thus may be easier and faster than the PNS series, which requires multiple positions and exposures. With DT, by collecting a number of projection images at different angles using a digital detector, one can produce a limitless number of section images at random depths using a suitable reconstruction algorithm (1, 30). With better depth resolution and much less overlap of anatomic features than can be achieved using a PNS radiography series, DT might result in increased detection of paranasal sinus lesions.

Costs of non-contrast CT, DT, and PNS radiography series are approximately $200, $30, and $20, respectively, Korea (30). Therefore, DT is more expensive than the PNS radiography series, but still much cheaper than CT. Thus, DT may be suitable for follow-up examinations.

Our study showed that the overall diagnostic sensitivities of the DT technique for detection of sinusitis were higher than those of the PNS radiography series, although the mean scores of anatomic clearness for paranasal sinus were not significantly different in some PNSs (maxillary and ethmoid sinuses for observer 1, frontal sinuses for observer 2); because the PNS is composed of soft tissue and bones, the PNS XR series is sufficient for detection of anatomic conspicuity between bone and air. However, it is not sufficient for detection of soft tissue lesions.

The major limitation of DT is vulnerability to motion artifacts. The minimum required scan time of DT by commercially available machines is 3 seconds, and the usual scan time is about 10 seconds. During the scan, patients should fix their body and hold their breath. Therefore, a respiratory motion artifact can be critical in chest DT. In spite head and neck radiography is relatively tolerable to the respiratory motion, motion artifact can degrade image quality of DT in some patients who cannot fix their position especially in an aged person or a child (Fig. 4).

In our study, two observers who had different levels of experience with CT, radiography, and DT showed different characteristics in interpreting radiography and DT. Observer 1 was more specific and observer 2 was more sensitive. Despite the fact that the sensitivity and accuracy of the less experienced radiologist (observer 1) was lower for plain radiography, diagnostic performance for DT was not different than it was for the experienced radiologist. This suggests that DT is a more objective and easier method with which to diagnose sinusitis.

There were several limitations to our study. First, the number of patients enrolled in the study was rather small, and were cases involving the frontal sinus. Therefore, it is difficult to generalize our results. For example, the diagnostic performance values of DT were better than XR in observer 2; however, this difference was not significant. Second, we did not include skull lateral radiographs in the image analyses; inclusion of these images might have enhanced PNS XR sensitivity for sphenoid lesions. However, in our institute, standard plain radiography, such as Caldwell or Water's views, are used as the primary imaging methods. Thus, this clinical practice pattern may have reflected our routine daily work. Third, only OMU CT was used as the reference-standard method for the analysis; that method did not include additional evaluations, such as sinus endoscopy or any follow up clinical result. Therefore, some subclinical mucoperiosteal thickening on CT was regarded as sinusitis positive, and thus the sensitivities of the study were lower than expected.

In conclusion, patient radiation dose from low dose digital tomosynthesis was comparable to that of a PNS radiography series. Use of the DT technique is superior to use of radiography for detection of paranasal sinusitis and for acquiring anatomic details of the sphenoid sinus. DT can be considered as a good alternative to the XR PNS series for evaluation of paranasal sinusitis.