DOI: 10.1148/radiol.2433050207
(Radiology 2007;243:847-852.)
© RSNA, 2007
Multidetector CT of the Paranasal Sinus: Potential for Radiation Dose Reduction1
Matthias H. Brem, MD,
Amir A. Zamani, MD,
Roberto Riva, MD,
Kelly H. Zou, PhD,
Zoran Rumboldt, MD,
Friedrich F. Hennig, MD,
Ron Kikinis, MD,
Alexander M. Norbash, MD, and
U. Joseph Schoepf, MD
1 From the Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Mass (M.H.B., A.A.Z., R.R., K.H.Z., R.K., A.M.N., U.J.S.); Department of Surgery/Orthopedic Surgery, Friedrich-Alexander University Erlangen-Nuremberg, Erlangen, Germany (M.H.B., F.F.H.); Department of Health Care Policy, Harvard Medical School, Boston, Mass (K.H.Z.); Department of Radiology, Medical University of South Carolina, 169 Ashley Ave, Charleston, SC 29425 (Z.R., U.J.S.); and Department of Radiology, Boston University School of Medicine, Boston, Mass (A.M.N.). From the 2004 RSNA Annual Meeting. Received February 7, 2005; revision requested April 19; revision received July 6, 2006; accepted August 22; final version accepted October 27. U.J.S. is a consultant to and receives research support from Siemens Medical Solutions.
Address correspondence to U.J.S. (e-mail: schoepf{at}musc.edu).
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ABSTRACT
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The aim of the study was to retrospectively determine the potential for radiation dose reduction at multidetector computed tomography (CT) of the paranasal sinus by using computer simulation of the effect of low–radiation dose acquisition on diagnostic image quality. This HIPAA compliant study was approved by the institutional human research committee. The need for informed patient consent was waived. Twenty patients underwent four-section CT at 120 kV, 170 mAs, and 4 x 1-mm collimation. Artificial image noise was added to the CT raw data by using a dedicated software platform. Acquisitions with effective tube currents of 134, 100, 67, and 33 mAs were simulated. Each raw data set was reconstructed with bone and soft-tissue algorithms, and two radiologists independently rated the images in blinded fashion. A two-sided paired Student t test was used for statistical analysis. The lowest radiation dose that still provided diagnostic quality was 
67 effective mAs for osseous structures and 134 effective mAs for the optic nerve and the inferior rectus muscle. On the basis of the results, tube currents can be lowered and radiation dose reduced by 20%.
© RSNA, 2007
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INTRODUCTION
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At computed tomography (CT) of the paranasal sinus, the most radiation-sensitive organs encompassed by the scanning field are the thyroid gland and the eye lens, which is at risk for a radiation-induced cataract (1–3). Thus, limiting and reducing the radiation exposure to the eye is important, especially in young patients and in patients who require repeat scanning. Radiation dose and image quality are closely related to the tube current settings during scan acquisition. Ideally, the tube current settings selected are those that use the minimum radiation required for diagnostic image quality (2,4). In the case of the paranasal sinus, such optimized tube current settings, once determined, should be generally applicable to most patients as the anatomy of the paranasal sinus is fairly uniform.
The aim of our study was to retrospectively determine the potential for radiation dose reduction at multidetector CT of the paranasal sinus by using computer simulation of the effect of low–radiation dose acquisition on diagnostic image quality.
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MATERIALS AND METHODS
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U.J.S. is a consultant to and receives research support from Siemens Medical Solutions. Our institutional human research committee approved evaluation of the patients' CT raw data. The need for informed patient consent was waived. All identifying information was removed prior to processing and evaluation by deleting all demographic data from the Digital Imaging and Communications in Medicine header. Our study was compliant with the Health Insurance Portability and Accountability Act requirements.
Patient Data
Twenty consecutive multidetector CT scans of the paranasal sinus were obtained in March 2003 in 20 patients (13 women, seven men; age range, 22–75 years; mean, 48 years). All CT scans were obtained for clinical purposes by using a four-section CT scanner (Volume Zoom; Siemens Medical Solutions, Forchheim, Germany) and 4 x 1-mm collimation, 120 kV, and 170 mAs. Indications for CT were chronic and acute sinusitis (n = 10), frontal sinus pain (n = 1), headache (n = 3), maxillary polyposis (n = 1), visual field defect (n = 1), cough (n = 1), tooth abscess (n = 1), and trauma (n = 2).
Data Evaluation
All raw data from the scans were transferred to a dedicated CT research workstation (Syngo Explorer; VAMP, Moehrendorf, Germany) (5). Artificial image noise was added to the original data acquired at 170 effective mAs to simulate data acquisition at 80%, 60%, 40%, and 20% of the original tube current setting, which corresponds to 134, 100, 67, and 33 effective mAs, respectively. The principles and validation of adding image noise for simulation of low-dose scan acquisition are described in detail elsewhere (5). All raw data sets were then reconstructed at 1.25-mm section thickness in 1-mm increments (0.25-mm overlap) by using both routine soft-tissue and bone reconstruction algorithms. By using transverse sections, coronal multiplanar reformations were reconstructed for each tube current setting and for soft-tissue and bone algorithms. This resulted in a total of 400 data sets.
All CT scans were then independently evaluated by two experienced neuroradiologists (R.R., A.A.Z., with 6 and 15 years of experience, respectively, reading neuroradiologic CT scans). Image sets were presented in a random fashion, and readers were blinded to the tube current settings. The following routine display windows were used: window center of 50 HU and window width of 350 HU for soft-tissue structures and window center of 700 HU and window width of 3500 HU for bone structures. Transverse sections and corresponding coronal multiplanar reformations were presented together.
The readers were asked to grade the image quality for bone (nasal septum, middle turbinate, inferior turbinate, frontal sinus) and soft-tissue (optic nerve, inferior rectus muscle, internal capsule, and basal ganglia) structures by using a five-point scale. Score of 1 denoted an unacceptable noise, rendering a study nondiagnostic; score of 2 denoted low noise, resulting in diagnostic uncertainty; score of 3 denoted average image quality, with a correct diagnosis being highly likely; score of 4 denoted good image quality, enabling a confident diagnosis; and a score of 5 denoted excellent image quality of best diagnostic value. We considered a score of 4 as sufficient for diagnostic purposes for bone and soft-tissue structures.
Statistical Analysis
A pooled analysis was performed by combining the two readers' scores, which were stratified according to each bone and soft-tissue structure. Between any arbitrary pair of tube current settings that ranged from 20% to 100%, Student t tests were used to assess for a significant difference in the mean ratings. Pair-wise comparisons of the ordinal rating data between the readers and among different settings were performed by using the Fisher exact test. However, since the main purpose of this study was to evaluate the optimal thresholds between the adjacent settings, we averaged the rating data between the readers and treated the data as if they were continuous. A P value of <.05 was considered to indicate a statistically significant difference.
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RESULTS
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Bone Structures
For the nasal septum (Fig 1), there was a significant difference in the diagnostic quality scores between the original tube current setting at 170 effective mAs and the simulated acquisition at 80% (134 effective mAs) (P = .0168) and that between 60% (100 effective mAs) and 40% (67 effective mAs) (P = .0086) of the original tube current setting. A significant (P < .001) decrease in diagnostic quality was noted by the readers between simulated acquisitions at 40% and 20% (33 effective mAs) of the original tube current setting. For the middle turbinate, statistically significant differences were found between all pairs of settings: 100% and 80% (P = .0077), 80% and 60% (P = .0212), 60% and 40% (P = .031), and 40% and 20% (P < .001), the most significant difference. For the inferior turbinate (Fig 1d–1f, 1i, 1j), significant differences were noted between 100% and 80% (P = .0044), 60% and 40% (P = .0088), and 40% and 20% (P < .001), the most significant difference. For the frontal sinus, the observed differences reached statistical significance for all pairs of settings: 100% and 80% (P = .0035), 80% and 60% (P = .0308), 60% and 40% (P = .0035), and 40% and 20% (P < .001), again the highest significant difference.
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Figure 1a: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1b: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1c: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1d: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1e: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1f: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1g: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1h: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1i: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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Figure 1j: Multidetector CT scans of paranasal sinus in a patient with deviation of nasal septum. Data reconstruction with bone algorithm. (a–c) Transverse 1.25-mm sections and (d–f) coronal multiplanar reformations. (a, d) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b, e) 80% and (c, f) 60% of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated. (g, h) Transverse 1.25-mm sections and (i, j) coronal multiplanar reformations obtained at simulated tube current reduction to 40% (g, i) and 20% (h, j) of the original tube current. The nasal septum (arrow) and inferior turbinate (arrowhead) are clearly delineated at 40% tube current setting but not as clearly at 20%.
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The average diagnostic score was below the threshold score of 4 only with acquisition at 20% of the original tube current setting (Table 1).
Soft-Tissue Structures
For the optic nerve, significant differences in diagnostic quality were found between all settings when the original tube current was reduced to 80% (134 effective mAs) (P = .0026), 80% to 60% (100 effective mAs) (P = .007), 60% to 40% (P < .001), and 40% to 20% (P < .001). The same was true for the inferior rectus muscle (Fig 2), statistically significant differences were observed between 100% and 80% (P = .0376), 80% and 60% (P = .007), 60% and 40% (P < .001), and 40% and 20% (P < .001). For the evaluation of the internal capsule and the basal ganglia, comparisons between all of the pairs of settings were statistically significant (P values from .0017 to <.001).
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Figure 2a: Coronal CT multiplanar reformations of paranasal sinus in a patient with visual field defect. Data reconstruction with soft-tissue algorithm. (a) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b) 80%, (c) 60%, (d) 40%, and (e) 20% of the original tube current. The inferior rectus muscle (arrow) is seen at all tube current settings. Increased image noise at c–e leads to poor delineation of muscle and insufficient diagnostic quality.
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Figure 2b: Coronal CT multiplanar reformations of paranasal sinus in a patient with visual field defect. Data reconstruction with soft-tissue algorithm. (a) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b) 80%, (c) 60%, (d) 40%, and (e) 20% of the original tube current. The inferior rectus muscle (arrow) is seen at all tube current settings. Increased image noise at c–e leads to poor delineation of muscle and insufficient diagnostic quality.
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Figure 2c: Coronal CT multiplanar reformations of paranasal sinus in a patient with visual field defect. Data reconstruction with soft-tissue algorithm. (a) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b) 80%, (c) 60%, (d) 40%, and (e) 20% of the original tube current. The inferior rectus muscle (arrow) is seen at all tube current settings. Increased image noise at c–e leads to poor delineation of muscle and insufficient diagnostic quality.
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Figure 2d: Coronal CT multiplanar reformations of paranasal sinus in a patient with visual field defect. Data reconstruction with soft-tissue algorithm. (a) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b) 80%, (c) 60%, (d) 40%, and (e) 20% of the original tube current. The inferior rectus muscle (arrow) is seen at all tube current settings. Increased image noise at c–e leads to poor delineation of muscle and insufficient diagnostic quality.
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Figure 2e: Coronal CT multiplanar reformations of paranasal sinus in a patient with visual field defect. Data reconstruction with soft-tissue algorithm. (a) Original acquisition at 170 effective mAs is compared with simulated tube current reduction to (b) 80%, (c) 60%, (d) 40%, and (e) 20% of the original tube current. The inferior rectus muscle (arrow) is seen at all tube current settings. Increased image noise at c–e leads to poor delineation of muscle and insufficient diagnostic quality.
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For the optic nerve and inferior rectus muscle, the average diagnostic score was below the threshold score of 4 with acquisition at 60% and lower of the original tube current setting and with all settings for basal ganglia and internal capsule (Table 2).
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DISCUSSION
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By applying computer simulation to multidetector CT of the paranasal sinuses, we identified a lower effective tube current of 67 mAs that provided sufficient diagnostic quality at minimum radiation dose for the bone structures we studied. For diagnosis of soft-tissue components of the sinus, an effective tube current of 134 mAs was found adequate for the optic nerve and the inferior rectus muscle but not for the basal ganglia and the internal capsule. Thus, on the basis of our results, tube current settings can be safely reduced by 20% while full diagnostic quality is maintained, except for the brain parenchyma. For the brain parenchyma, even the maximum 170 effective mAs was not sufficient, which is not surprising since evaluation of the brain requires a dedicated imaging protocol. The effective tube current of 134 mAs allows adequate visualization of both soft-tissue and bone structures with paranasal sinus multidetector CT.
We did not compare directly which diagnoses the readers were able to make with the routine scan and the lower dose scans, because the memorization effect would affect the results in the lower dose settings. Therefore, we decided to use a grading scale to avoid memory effects on the diagnoses. We have computed the percentage agreement between these two readers according to setting and structure. A 
statistic may not have been useful here, since the scores of 4 and 5 were more frequently adopted. The percentage agreement between the readers was not high in these 20 cases. Therefore, in our current study, we averaged their rating data. In a future study, interrater variability should be further studied.
CT of the paranasal sinus is a common examination, primarily performed in patients with chronic or acute inflammatory disease, facial trauma, malignancy, or other pathologic condition (6). CT evaluation of the paranasal space has benefited from the introduction of multidetector CT: The capability of thin-section acquisition improves visualization of small pathologic detail, and the isotropic nature of high spatial resolution data sets enables display in arbitrary planes, obviating image acquisition in prone or hyperextended patient positions.
Use of thinner sections, however, is generally associated with increased radiation exposure to the patient. The scanning field at CT of the paranasal sinus comprises mainly two radiosensitive organs, the eye lens and the thyroid gland. Especially for chronic disorders, serial scanning is usually performed for follow-up, so that patients are subject to the cumulative radiation exposure of multiple scans. The pediatric patient's eye is especially sensitive to radiation. Cumulative radiation exposure of 250 mGy has been documented to cause radiation-induced cataracts in children (7,8), while the adults eye can withstand somewhat higher exposures of 0.5–2 Gy. Preexisting damage of the lens further predisposes for radiation-induced cataract (9). Methods for lens protection at sinus CT have been described (1,7); however, the most effective way of minimizing the risk of radiation against the need for diagnostic images is use of an adequate lower dose acquisition technique.
Our findings agree with the results of previous studies, which suggest that very low tube currents suffice for satisfactory image quality. Marmolya et al (10) argue that as little as 23 mAs is sufficient for diagnostic quality when only sinusitis is of concern. Hagtvedt et al (11) conclude that a setting of 40 mAs enables diagnosis of sinusitis but is unsatisfactory if detailed information on paranasal sinus is required. Sohaib et al (12) show that 50 mAs is sufficient for bone structures, which is in good agreement with our own findings. Most of these results, however, were obtained with repeat scanning and repeat radiation exposure of consenting individuals.
Our results indicate the general potential for lowering patient radiation exposure at multidetector CT of the paranasal sinus. However, the same tube current settings result in different patient radiation exposure with scanners of different manufactures (12). Therefore, a limitation of our results is that they may not be directly transferable to different scanner types, and dedicated tube current simulation may need to be performed to determine tube current settings with other scanners. Importantly, patient radiation exposure at CT is not solely related to the choice of tube current setting. Other parameters such as tube voltage, section thickness, pitch, and gantry cycle time all influence the resulting patient radiation dose (13).
In our study, we only focused on tube current as the most intuitive and readily adjustable parameter. The tube current settings determined by us are based on the pooled subjective impression of two experienced neuroradiologists at our institution and may have to be adjusted to the particular preferences and requirements in different environments. Another study limitation is that patient weight and size differences were not taken into account; however, these variables may be of lesser importance as the paranasal sinus anatomy is ordinarily fairly uniform in the adult population. In conclusion, by using computer simulations we found considerable potential for reducing radiation dose at CT of the paranasal sinus. The scanning protocols as determined in exploratory studies such as ours would benefit from prospective clinical validation in large clinical patient cohorts.
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ADVANCES IN KNOWLEDGE
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- There is considerable potential for reducing radiation dose at CT of the paranasal sinus, especially if only bone structures are to be assessed.
- Computer simulation of the effects of low-dose settings on diagnostic quality may serve as a useful tool to optimize acquisition protocols vis-à-vis radiation protection.
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FOOTNOTES
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See Materials and Methods for pertinent disclosures.
Author contributions: Guarantors of integrity of entire study, M.H.B., U.J.S.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, M.H.B., Z.R., F.F.H.; clinical studies, M.H.B., R.R., R.K., A.M.N., U.J.S.; statistical analysis, K.H.Z.; and manuscript editing, M.H.B., K.H.Z., Z.R., F.F.H., R.K., A.M.N., U.J.S.
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References
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ABSTRACT
INTRODUCTION
