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.
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 . Already in the beginning of the 1990s, reports recommended the use of CT in the workup of diseases of paranasal sinuses . 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 . Even studies claiming optimization and tailoring of low-dose protocols reported effective doses as high as 0.48 mSv . Subsequently, several reports on dose reduction of CT sinuses and the impact of dose reduction on image quality were published . 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.
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 . 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.
| Plain radiographs | Standard CT | Low-dose CT (I) | Low-dose CT (II) | |
|---|---|---|---|---|
| Tube collimation (mm) | 16 × 0.75 | 16 × 0.75 | 16 × 0.75 | |
| Rotation time (sec) | 0.5 | 0.5 | 0.75 | |
| Pitch | 0.55 | 1.5 | 1.5 | |
| Tube voltage (kV) | 81 | 120 | 80 | 80 |
| Quality tube current-time product (mAs) | 8 PA/3.3 L | 70 | 17 | |
| Effective mAs | 59 | 33 | 17 | |
| CTDI vol (mGy) | 12.21 ± 1.1 | 2.84 ± 0.13 | 1.44 ± 0.06 | |
| DLP (mGy.cm) | 161 ± 23 | 40.9 ± 4.56 | 19.67 ± 2.34 | |
| Effective dose (mSv) | 0.098/0.057 ⁎ | 0.371 ± 0.053 | 0.094 ± 0.01 | 0.045 ± 0.005 |
⁎ 0.098 mSv was the calculated dose for 3 views and 0.057 mSv was the calculated dose for 1 PA and 1 lateral view (L).
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 × cm 2 ) 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 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 . 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 . 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.
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 . 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.
