Low-dose computed tomography of the paranasal sinuses: radiation doses and reliability analysis

Article Outline

• Abstract
• 1. Introduction
• 2. Materials and methods
  • 2.1. Phantom study
  • 2.2. Patients
  • 2.3. Estimation of radiation doses
  • 2.4. Evaluation of image quality
  • 2.5. Statistical analysis
• 3. Results
  • 3.1. Estimation of radiation doses
  • 3.2. Image quality
    • 3.2.1. Image quality in the phantom study
    • 3.2.2. Image quality in patient study
• 4. Discussion
• 5. Conclusion
• Acknowledgment
• References
• Copyright

Abstract 

Purpose

The study aimed to (1) optimize the radiation doses of computed tomography (CT) of paranasal sinuses, (2) compare the radiation doses of different CT protocols with that of plain radiography, and (3) evaluate the reliability of low-dose CT in the detection of pathology and characterization of the detected pathology.

Materials and methods

A head phantom was examined with different scan parameters to define a cutoff value to which the radiation dose can be reduced without negative impact on image quality. Kruskal-Wallis test and Wilcoxon W test were performed to compare the effective doses of the plain radiography in 30 patients with that of 3 different CT protocols in a total of 90 patients. The interobserver and intraobserver agreement in the detection of pathologic findings and in characterization of the pathology was estimated by calculating κ value.

Results

The effective doses of plain radiography and low-dose CT were 0.098 and 0.045 mSv, respectively (P < .001). The effective dose of standard CT of sinuses (0.371 mSv) was 3.8 times higher than that of plain radiography and 8.2 times higher than that of low-dose CT (P < .001). The interobserver and intraobserver agreement on CT with regard to detection of pathology and pathology characterization was almost perfect (κ values 0.81–1) compared to fair (κ values 0.38–0.39) in plain radiography.

Conclusions

The here proposed low-dose CT means significant dose reduction and is a reliable method in the investigation of the paranasal sinuses.

Back to Article Outline

1. Introduction 

Computed tomography (CT) is the gold standard for investigation of inflammatory sinus disease and has become a routine radiologic examination in the diagnosis and grading the severity of acute sinusitis [1], [2]. Already in the beginning of the 1990s, reports recommended the use of CT in the workup of diseases of paranasal sinuses [3]. However, CT means high radiation doses if the examinations are performed according to the standardized protocols recommended by different manufacturers of CT scanners. Effective dose as high as 0.76 mSv has been reported [4]. Even studies claiming optimization and tailoring of low-dose protocols reported effective doses as high as 0.48 mSv [5]. Subsequently, several reports on dose reduction of CT sinuses and the impact of dose reduction on image quality were published [2], [4], [6], [7]. With the advent of multidetector CT, thin axial images (0.6–0.75 mm) enable coronal and sagittal reformations and this has become an essential aid in the navigation during the functional endoscopic sinus surgery (FESS). Modern CT scanners usually have different dose reduction systems that enable modulation of the radiation doses in different patients.

The aims of this study were (1) optimization of the radiation doses of CT of paranasal sinuses, (2) estimation and comparison of the radiation doses of plain radiography and that of CT performed with different scan parameters, and (3) evaluation of interrater reliability in the detection of different pathologic conditions on plain radiography and CT, and the characterization of the those pathologies on CT.

Back to Article Outline

2. Materials and methods 

2.1. Phantom study 

A head phantom was examined on a 16-slice CT-scanner (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany). Three settings of scan parameters were tested (Table 1): (1) scan parameters according to the CT protocol used in daily clinical practice preoperatively in patients planned for FESS, the so-called standard CT; (2) low-dose CT protocol I; and (3) low-dose CT protocol II. The latter scan setting was performed taking the advantage of the dose reduction system in our CT-system (CareDose 4D, Siemens, Forchheim, Germany). In our CT scanner, the dose reduction system represents an axial and angular automatic tube current modulation [8]. The radiation doses of all 3 scans were calculated and the image quality was evaluated by 2 experienced neuroradiologists. Twelve anatomical bony landmarks with definite bone margins in the paranasal region and the adjacent regions were subjected for evaluation: (1) medial wall of maxillary sinuses, (2) posterior wall of frontal sinuses, (3) bony margins of sphenoid sinuses, (4) orbital floor and medial orbital wall, (5) infundibulum, (6) uncinate processes, (7) bony margins of pterygopalatine fossa, (8) medial and lateral pterygoid process, (9) nasolacrimal duct, (10) foramen ovale, (11) meatus acusticus internus, and (12) hypoglossal canal. A score of 2 was given to very well-defined structures, 1.5 to structures that were relatively well defined, 1 to indistinctly defined structures, and zero to structures that could not be identified.

Table 1. Scan parameters and the calculated radiation doses

DLP indicates dose length product.

The results of the phantom study were presented to the regional radiation protection committee, which approved the use of the low-dose CT instead of plain radiography in the work-up of sinusitis.

2.2. Patients 

Thirty consecutive patients (aged 52 ± 18 years [mean ± SD], 77% were female) examined with plain radiography of the sinuses with sinusitis as the clinical question at issue were included in this retrospective analysis. Three standard projections were obtained: 2 posteroanterior (PA) projections (Water and Caldwell views) as well as lateral view. The PA views were obtained with a tube voltage of 81 kV and a tube current of 8 mAs, whereas the lateral projection was obtained with a tube voltage of 81 kV and a tube current of 3.2 mAs. For each view, the dose area product (mGy × cm²) was recorded.

Thirty consecutive patients (aged 41 ± 16 [mean ± SD], 60% female) examined with low-dose CT of the sinuses (protocol I, 80 kV, and 33 mAs) with sinusitis also as the clinical question at issue were included in this retrospective analysis.

Another group of patients examined with CT and included in this retrospective analysis was 30 consecutive patients (aged 43 ± 17 [mean ± SD], 70% female) examined with low-dose CT of the sinuses (protocol II, 80 kV, and 17 mAs).

For comparison 30 consecutive patients (aged 51 ± 13 [mean ± SD], 57% female) examined with standard CT of the sinuses (120 kV and 70 mAs) were included in this retrospective analysis.

All examinations were performed on a 16-slice CT-scanner (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany) with tube collimation 0.75 mm, image thickness 0.75 and 3 mm, skeletal algorithm with edge enhancement (Head70), and field of view 180 mm. Two-millimeter-thick coronal images were reconstructed.

2.3. Estimation of radiation doses 

The radiation doses of plain radiographs and CT of all 3 groups included in the analysis were estimated. In plain radiographs, the effective doses were calculated using a conversion factor derived from report NRPB-R 279 of the British National Radiological Protection Board [9] and based on Monte Carlo calculations. The radiation doses of CT were estimated by recording the effective mAs, the volume CT dose index (CTDI_vol), and the dose length product. To allow comparisons with plain radiography, the effective dose (E) was determined using appropriate conversion factors, taken from European commission 2004 CT Quality Criteria, Appendix A-MSCT Dosimetry [10]. The conversion factor used in our study was 0.0023.

2.4. Evaluation of image quality 

All images of plain radiographs and CT were evaluated by 2 independent radiologists. One reader performed the evaluation at 2 different occasions with a 4-week interval. The readers were asked to classify the findings in plain radiography into (1) normal or (2) pathologic if there was evidence of fluid levels and/or mucosal swelling. In patients examined with CT, a 2-step evaluation was done. The readers were asked to classify the CT-findings into (1) normal or (2) pathologic. This evaluation was done separately on maxillary, frontal, ethmoidal, and sphenoidal sinuses. Thereafter, characterization of pathology was done with the readers being asked to categorize the pathologic findings into the following entities: (1) nonspecific mucosal swelling, (2) fluid collection with gas-fluid level, (3) opacification of the whole sinus, (4) retention cyst, (5) polyps, and (6) other findings.

2.5. Statistical analysis 

Statistical analysis was performed by means of SPSS version 15. Kruskal-Wallis test and Wilcoxon W test were performed to compare the effective doses of the plain radiography and low-dose CT and between effective doses of different CT protocols. A reliability analysis was performed by estimation of interobserver and intraobserver agreement in the evaluation of plain radiographs as well as CT. The degree of interobserver and intraobserver agreement was evaluated by cross tabulation and calculation of the κ coefficient. Interpretation of the κ coefficient (κ values) was done according to the one proposed by Landis [11]. A κ of 1 indicates total agreement. A κ of 0.81–1.00 indicates almost perfect agreement, 0.61–0.80 indicates substantial agreement, 0.41–0.60 indicates moderate agreement, 0.21–0.40 indicates fair agreement, and 0–0.20 indicates slight agreement, whereas a κ of less than 0 indicates that any observed agreement is attributed to chance.

Back to Article Outline

3. Results 

3.1. Estimation of radiation doses 

The mean value of the effective dose of plain radiography was 0.098 mSv for 3 views, which was twice as high as the effective dose of low-dose CT (protocol II, 0.045 mSv). This difference was statistically significant (P < .001). The dose for 2 views (1 PA and 1 lateral view, 0.057 mSv) was also 27% higher than the low-dose CT. Even the effective dose of low-dose CT protocol I (0.094 mSv) was lower than that of plain radiography (0.098 mSv). However, the effective dose of standard CT of sinuses (0.371 mSv) was 3.8 times higher than that of plain radiography and 8.2 times higher than that of low-dose CT (P < .001) (Table 1).

3.2. Image quality 

Phantom images obtained with scan parameters according to the FESS-protocol were given a score of 20 by both readers; all structures but infundibulum and uncinate processes were classified as very well identified. Phantom images obtained with 80 kV and 33 mAs (protocol I, low-dose CT) scored 17.5 by one reader and 18 by the second reader, whereas those obtained with 80 kV and 17 mAs (protocol II, low-dose CT) scored 17 by both readers. Infundibulum and uncinate processes of the phantom were classified as non-identifiable by both readers, (Fig. 1).

• 
  • View Large Image
  • Download to PowerPoint

• Fig. 1. 

  (A–C) Coronal images of the head phantom with standard CT, CT sinus according to low-dose protocol I (80 kV and 33 mAs) and II (80 kV and 17 mAs), respectively, show well-defined bony margins of the floor and the medial wall of the orbits (arrow heads) and the bony margins of maxillary sinuses in all 3 images. Note that the construction of the head phantom did not allow sharp delineation of uncinate process and infundibulum in any of the 3 images regardless of the radiation dose. The arrow points toward the presumed site of the uncinate process on the right side.

The interobserver and intraobserver agreement in the assessment of signs of sinusitis on plain radiographs was only fair (κ value of 0.38 and 0.39, respectively), whereas the interobserver and intraobserver agreement in the assessment of signs of sinusitis on CT was almost perfect (κ values varying from 0.91 to 1) (Table 2). Regarding the pathology characterization, the interobserver and intraobserver agreement on CT with different doses was almost perfect (κ values varying from 0.83 to 1) (Table 2). The κ values were almost the same on standard CT and on low-dose CT with regard to detection of pathology and classification of the detected pathology into different entities. The interobserver and intraobserver agreement in the assessment of infundibulum was almost perfect in all 3 groups of CT of the sinuses with κ value ranging between 0.927 (low-dose CT) and 1 (standard CT). Fig. 2 shows images (obtained by the here proposed low-dose CT) of 8 different patients with different pathologic findings in the paranasal sinuses.

Table 2. Interobserver and intraobserver agreement with regard to classification of different examinations into normal or pathologic and characterization of the detected pathologies

1/2 represents the agreement between the 2 observers (interobserver agreement), whereas 2/2 represents the agreement between 2 different occasion of assessment performed by the second observer (intraobserver agreement). CI indicates confidence interval; LDCT, low-dose CT.

• 
  • View Large Image
  • Download to PowerPoint

• Fig. 2. 

  Images obtained by the here proposed low-dose CT (protocol II) in 8 different patients. (A) Coronal image shows normal uncinate process and infundibulum (arrows). (B) Coronal image shows extensive swelling with infundibular obliteration (arrows). (C, D) Axial images show fluid level in both maxillary sinuses (arrow heads, C) and frontal sinuses (arrow, D). (E) Coronal images of a patient with recurrent sinusitis shows destruction of the lower wall of the right maxillary sinus caused by periapical osteitis of the tooth in region 17 (arrow). Only slight mucus membrane thickening is noted. (F) Axial image of a patient with chronic sinusitis shows totally opacified right maxillary sinus with thickening of its wall (arrow). (G) Coronal image shows small retention cyst in the bottom of the left maxillary sinus (arrow head). (H) Axial image of a patient who previously underwent FESS shows multiple polyps in the nasal cavity (arrow head), slight mucus membrane swelling in both maxillary sinuses, and obliterated ostium on the right side (arrow).

Back to Article Outline

4. Discussion 

In our institution, optimization of radiation dose was performed using a head phantom examined in 16-slice CT. The maximal dose reduction our CT system (low-dose CT; protocol II: 80 kV and 17 mAs and effective dose of 0.045 mSv) allowed was considered to be enough to define these structures and was subsequently used in clinical practice in our institution (when sinusitis was the clinical question at issue) replacing the CT protocol with the scan parameters recommended by the CT manufacturer. The radiation dose of plain radiography and standard CT were 2.2 and 8.2 times higher than that of the here proposed low-dose CT.

In phantom study, the infundibulum and uncinate processes scored zero and could not be identified clearly. We believe that this is dependent on the construction of the phantom rather than on the low radiation dose as these structures could not identified either on images generated by the standard CT of sinuses. Furthermore, the reliability of low-dose CT was as high as standard CT in identifying these structures in patient cohort.

The increased availability of CT scanners and the increased number of CT examinations resulted in increased radiation exposure to the population especially in the pediatric population. In the beginning of the 1990s, CT constituted about 2% to 3% of all radiologic examinations and contributed to about 20% to 30% of the total radiation load from medical use of ionizing radiation [12]. A later report showed that CT accounts for about 11% of all radiology procedures in the United States and constitutes approximately two thirds of the collective medical radiation dose [13]. Especially vulnerable organs are the eye lenses and the thyroid glands, which are usually subject to high cumulative doses of radiation from repeated CT of head and neck [14]. Taking these facts into consideration, the optimization of the radiation doses is an important issue in diagnostic radiology. As a method, the low-dose CT today is the gold standard in the workup of sinusitis. Furthermore, the findings in this study of equally high interrater agreement in detection of pathologic findings and characterization of different pathologies as well as previous reports of high interrater agreement in defining different anatomical landmarks [4], [15] might indicate that the radiation doses used today in standard protocols of CT-sinuses, for example, before FESS, are higher than what is really needed. However, a limiting factor is probably the reliability of images with low-dose CT in creating images of reasonable quality that guarantee a safe preoperative evaluation and a safe navigation during the surgery. In our opinion, this needs to be validated by a double-blind study performing examinations with both standard and low-dose CT comparing the utility of the images generated by low-dose CT and those generated by standard CT in the navigation program available in the operation theatres.

Most of the reports in the literature on dose optimization have dealt with reduction of tube current. Recent reports have shown that the reduction of tube voltage brings about a marked reduction of the radiation dose with no impact on image quality [16]. In our study, the dose reduction involved reduction of tube current and tube voltage and the here proposed low-dose CT (80 kV and 17 mAs) represent the maximal dose reduction in our CT system.

Back to Article Outline

5. Conclusion 

Low-dose CT of the paranasal sinuses means exposure to radiation doses that are markedly lower than those of plain radiography and the standard CT. The reliability of low-dose CT is significantly higher than that of plain radiography and equal to that of the standard CT of sinuses in the investigation of paranasal sinuses. This does not only concern the reliability in detecting sinus abnormalities/pathology but also in characterization of these abnormalities. However, the suitability of images generated by low-dose CT for the navigation during FESS needs to be evaluated.

Back to Article Outline

Acknowledgment 

The authors gratefully acknowledge Mikael Gunnarsson, PhD (Department of Radiation Physics, University of Lund, Malmö University Hospital, Sweden), for his contribution in the phantom study.

Back to Article Outline

References 

1. Fokkens W, Lund V, Mullol J. European position paper on rhinosinusitis and nasal polyps. Rhinol 2007:(Suppl 20)1-136.
   • View In Article
2. Hagtvedt T, Aalokken TM, Notthellen J, et al. A new low-dose CT examination compared with standard-dose CT in the diagnosis of acute sinusitis. Eur Radiol. 2003;13:976–980
   • View In Article
   • MEDLINE
3. White PS, Robinson JM, Stewart IA, et al. Computerized tomography mini-series: an alternative to standard paranasal sinus radiographs. Aust N Z J Surg. 1990;60:25–29
   • View In Article
   • MEDLINE
4. Tack D, Widelec J, De Maertelaer V, et al. Comparison between low-dose and standard-dose multidetector CT in patients with suspected chronic sinusitis. AJR Am J Roentgenol. 2003;181:939–944
   • View In Article
5. Damilakis J, Prassopoulos P, Mazonakis M, et al. Tailored low dose three-dimensional CT of paranasal sinuses. Clin Imaging. 1998;22:235–239
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (319 KB)
   • CrossRef
6. Kearney SE, Jones P, Meakin K, et al. CT scanning of the paranasal sinuses—the effect of reducing mAs. Br J Radiol. 1997;70:1071–1074
   • View In Article
   • MEDLINE
7. Mulkens TH, Broers C, Fieuws S, et al. Comparison of effective doses for low-dose MDCT and radiographic examination of sinuses in children. AJR Am J Roentgenol. 2005;184:1611–1618
   • View In Article
8. Application Guide. SIEMENS, SOMATOM Sensation 64 Siemens AG Medical Solution, Computed Tomography. 2004; p. 29-32.
   • View In Article
9. Hart D, Jones DG, Wall BF. Coefficients for estimating effective doses from pediatric X-ray examinations. Didcot, United Kingdom. Chilton: National Radiological Protection Board; 1996;
   • View In Article
10. Bongartz G, Golding SJ, Jurik AG, et al. European Guidelines for multislice computed tomography. Funded by the European Commission; 2004.
    • View In Article
11. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977;33:159–174
    • View In Article
    • CrossRef
12. Shrimpton PC, Jones DG, Hillier MC. Survey of CT practice in the UK, Part 2: Dosimetric aspects. London: HMSO; 1991;
    • View In Article
13. Mettler FA, Wiest PW, Locken JA, et al. CT scanning: patterns of use and dose. J Radiol Prot. 2000;20:353–359
    • View In Article
    • MEDLINE
    • CrossRef
14. Czechowski J, Janeczek J, Kelly G, et al. Radiation dose to the lens in sequential and spiral CT of the facial bones and sinuses. Eur Radiol. 2001;11:711–713
    • View In Article
    • MEDLINE
    • CrossRef
15. Sohaib SA, Peppercorn PD, Horrocks JA, et al. The effect of decreasing mAs on image quality and patient dose in sinus CT. Br J Radiol. 2001;74:157–161
    • View In Article
    • MEDLINE
16. Kalender WA, Buchenau S, Deak P, et al. Technical approaches to the optimisation of CT. Phys Med. 2008;24:71–79
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (3332 KB)
    • CrossRef