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British Journal of Radiology 74 (2001),157-161 © 2001 The British Institute of Radiology

Full paper

The effect of decreasing mAs on image quality and patient dose in sinus CT

S A Sohaib, MRCP, FRCR,1, P D Peppercorn, MRCP, FRCR1, J A Horrocks, BSc, PhD2, M H Keene, FRCS3, G S Kenyon, FRCS3 and R H Reznek, FRCP, FRCR1

1 Departments of Diagnostic Imaging
2 Clinical Physics
3 Otolaryngology, St Bartholomew's Hospital, West Smithfield, London EC1A 7BE, UK


   Abstract
Top
 Abstract
Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The aim of this study was to determine the effect of reducing mAs on the diagnostic quality of images and the radiation dose to the orbits in patients undergoing sinus CT. We studied 40 consecutive patients undergoing paranasal sinus CT for inflammatory disease prior to functional endoscopic sinus surgery (FESS). Four groups of 10 patients were scanned at 200 mAs, 150 mAs, 100 mAs and 50 mAs, respectively. Orbital radiation dose was measured using thermoluminescent dosemeters. Images were reviewed independently by two observers who were unaware of the mAs setting used. Image quality was evaluated using a semi-quantitative scoring system for six anatomical structures. The osteomeatal complex, uncinate process, infundibulum, frontal recess, middle turbinate and optic nerve were assessed as: clearly demonstrated (2 points); demonstrated but not clearly visualized (1 point); or not seen (0 points). No significant difference was shown between any of the four groups in terms of image quality according to the scoring system used in this study. Mean radiation dose to the orbit was reduced by 77%, from 13.5 mGy at 200 mAs to 3.1 mGy at 50 mAs (p<0.05). CT of the sinuses can be performed in patients prior to FESS at greatly reduced mAs without loss of diagnostic quality of the images. This is important in reducing the radiation dose to the lens.


   Introduction
Top
 Abstract
 Introduction
Subjects and methods
 Results
 Discussion
 References
 
Functional endoscopic sinus surgery (FESS) has now become routine in the management of patients with rhinosinusitis [1]. Successful endoscopic surgery requires detailed knowledge of the highly variable anatomy of the nasal cavities and paranasal sinuses as well as the relationship of the diseased areas to vital structures such as the optic nerve and internal carotid artery [2–4]. CT can accurately and quickly provide this information whilst providing a more accurate assessment of the extent of disease than plain radiographs. CT is therefore widely used in the initial investigation of patients with sinus disease [5]. Radiation dose to the lens of the eye is of concern, especially if CT is used as a screening procedure in patients, many of whom are young [6]. One of the main determinants of radiation dose is the milliamperes–second (mAs) setting of the scanner. Previous studies on the effects of reducing mAs on image quality have not included dosimetry measurements [7–9]. The aim of this study was to determine the effect of reducing mAs on the diagnostic quality of the images and on the radiation dose received by the orbits and especially the lens in subjects undergoing CT of the paranasal sinuses.


   Subjects and methods
Top
 Abstract
 Introduction
 Subjects and methods
Results
 Discussion
 References
 
We studied 40 consecutive patients without malignant disease or previous sinus surgery who underwent paranasal sinus CT at our institution.

CT was performed on a GE HiSpeed Advantage scanner (General Electric Medical Systems, Milwaukee, USA). The following protocol was used: 3 mm coronal sections at 5 mm table increments from the mid frontal sinus to the mid sphenoid sinus, with the gantry angled to the coronal plane; selected 5 mm axial sections at the level of mid sphenoid sinus and anterior clinoid. Technique factors of 120 kVp and 1 s per slice were used in all cases. Images were generated using a high resolution bone reconstruction algorithm. Scans were imaged on a window width of 2000 Hounsfield units (HU) and a window level of 250 HU.

Patients were divided into four groups of 10. The first group was scanned using 200 mAs, as this was the standard protocol for sinus CT on our scanner. In subsequent groups, the mAs was progressively reduced to 150 mAs, 100 mAs and 50 mAs, respectively. All images obtained at a given mAs setting were reviewed for diagnostic quality before scanning at a lower mAs.

Radiation dose to the orbit was measured in each patient using lithium fluoride thermoluminescent dosemeters (TLD-100) in the form of chips (3 mmx3 mmx1 mm) (Harshaw, Cleveland, OH, USA). The thermoluminescent dosemeter (TLD) chips were calibrated for diagnostic X-ray energies with an accuracy and precision of ±10%. At the start of each scan, prior to obtaining the scout views, a TLD chip was placed just below each lower eyelid as close as possible to the orbit. TLD dose was assumed to be equal to the dose delivered to the lens.

Scans were then retrospectively reviewed by two observers unaware of the mAs setting used. Diagnostic image quality was assessed by scoring for six anatomical structures:

For each structure, the following score was allocated depending on how well each was visualized: 0, not demonstrated; 1, demonstrated but not clearly visualized; 2, clearly visualized (Figure 1Go). The right and left sides were analysed separately. With six anatomical structures and a maximum score of 2 for each structure, the maximum score (i.e. the best image quality) for each of the left and right side was 12.



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  		Figure 1. Coronal CT of the sinus at 50 mAs. In the left paranasal sinus the uncinate process (open arrow), infundibulum (double headed arrow) and maxillary ostium (large arrow) are clearly seen, i.e. scoring 2 for each structure. In the right paranasal sinus the infundibulum (double headed arrow) and uncinate process (open arrow) are demonstrated but not clearly visualized, i.e. scoring 1 each.

 
Statistical analysis was performed using the SPSS (v 6.1.3) software package (SPSS Inc, Chicago, IL, USA), with significance taken as p<0.05. Interobserver agreement was compared using a Wilcoxon matched pairs signed rank test. Data were assessed for normality, and one-way analysis of variance (ANOVA) was used to compare the diagnostic image quality score and the radiation dose to the orbits between the four groups.


   Results
Top
 Abstract
 Introduction
 Subjects and methods
 Results
Discussion
 References
 
Scores for image quality from each observer for each group of patients are shown in Table 1Go. There was no significant difference in the image quality scores given by each observer at any of the mAs settings. Figure 2Go shows examples of the quality of the scans at the four different mAs settings.


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  		Table 1. Scores for image quality from each observer for each mAs setting, left and right sides combined

 


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  		Figure 2. Coronal CT of the sinuses at the four different mAs settings: (a) 200 mAs; (b) 150 mAs; (c) 100 mAs; (d) 50 mAs.

 
The relationship between the image quality scores at different mAs is shown in Figure 3Go. Image quality score did not differ significantly from normality. One-way ANOVA showed no significant difference between the image quality scores at the different mAs settings. There was therefore no perceptible difference in the diagnostic quality of the images using lower mAs.



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  		Figure 3. Graph showing the relationship between the diagnostic image quality scores in the four groups at the different mAs settings. Note no statistically significant difference between any of the four groups.

 
The mean and range of radiation dose to the lens of the eye at different mAs settings are given in Table 2Go. Compared with the radiation dose at 200 mAs, the mean radiation doses to the lens at 150 mAs, 100 mAs and 50 mAs decreased by 43%, 54% and 77%, respectively. Although there is a wide range in the TLD doses, these data did not differ significantly from a normal distribution. One-way ANOVA showed there was a significant difference in the radiation dose received by patients in the four different groups. A least-significant difference test showed that there was a significant difference between Group 1 (200 mAs) and Group 2 (150 mAs) (p=0.01), and between Group 2 (150 mAs) and Group 4 (50 mAs) (p=0.01). It can also be seen that there was a wide range in dose across the 10 patients within each group. This is primarily owing to the scan protocol used (3 mm coronal slices at 5 mm table increments plus selected 5 mm axial slices) such that the fraction of the TLD chip in the primary beam varied for each patient.


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  		Table 2. Mean radiation dose to the lens of the eye at the different mAs settings

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
References
 
The present generation of CT scanners have intrinsically better spatial and contrast resolution and more dose-efficient detectors. They therefore maintain image quality at a much lower dose. However, it is not uncommon for departments to continue to use protocols designed for earlier scanners. Prior to this study, our standard protocol for sinus CT used an mAs setting of 200 mAs. In our study, image quality was maintained when mAs was reduced from 200 mAs to 50 mAs. Early reports on sinus CT suggested using a much higher mAs setting (e.g. 200 mAs or greater [10, 11]), but the naturally high contrast structures (bone, air and mucous membrane) in CT of the sinuses should favour the use of a lower mAs setting. More recent reports have suggested much lower mAs settings may be used [8, 9], and in one study using a Picker CT body scanner model 1200SX, patients were scanned at 16 mAs for coronal scans and 23 mAs for axial scans [7]. Although the images were noisier, the scans were all judged to be of diagnostic quality. However, it must be noted that mAs values can vary by a factor of two to three for the same dose with different scanners. Therefore, directly comparing mAs values alone across different scanners has limitations [6].

Our study confirms that it is possible to reduce significantly the radiation dose to the eye by reducing the mAs used during CT of the sinuses without compromising diagnostic image quality of the investigation. The mean radiation dose to the orbit (3 mGy) obtained at low mAs settings compares favourably with other recent studies [12]. Radiation doses believed to induce corneal opacities are 0.5–2 Gy. Visual impairment owing to cataract occurs for exposures over 5 Gy [6]. The radiation dose to the orbit received during CT of the sinuses is clearly well below these levels. However, the theoretical risk of other radiation-induced effects that are not dependent on a minimum threshold of exposure, such as carcinogenesis, remains. Therefore, as with all radiological investigations, radiation dose should be kept as low as reasonably practicable.

Image quality was assessed upon analysis of the images based on anatomy rather than pathology for several reasons. First, the detailed anatomy is crucial for ENT surgeons prior to FESS. Hence, image quality needs to be adequate for the assessment of the important anatomical structures outlined. Second, evaluation of image quality based on anatomical criteria takes into account both the anatomy of the area under examination and the contrast between different tissues, which is essential for the detection of pathological changes. Lastly, in a small sample size it is easier to assess image quality in terms of anatomy, as the relevant structures are present in nearly all patients. Assessing image quality to detect pathology would require a much larger series, because consideration of the presence or absence of disease and the degree of pathology would need to be taken into account.

Our assessment of image quality was tailored to the evaluation of anatomical structures specific to patients with inflammatory disease prior to FESS. A European study group has produced a working document relating to quality criteria for CT in response to a European Commission directive [13]. The purpose of these quality criteria in CT is to define a level of performance considered necessary to produce images of standard quality for a particular anatomical region in relation to patient dose. The guidelines provide guidance on the definition and introduction of quality criteria for diagnostic images, equipment performance and dose to the patient. Our assessment of image quality of the paranasal sinuses is similar to these European guidelines, that is assessing images for visually sharp reproduction (i.e. clearly defined) or just reproduction (i.e. details visible but not clearly defined) of specific anatomical details. In the guidelines for the sinuses, visually sharp reproduction is required for nearly all structures except the optic nerve and muscle. However, for our study population we took specific structures that are key to the assessment of the paranasal sinuses prior to FESS. We did not require visually sharp reproduction of all the structures as in the guidelines, and this may not be possible at the lower mAs settings used. Using all the criteria from the guidelines may not allow for significant dose reduction.

In our scoring system, images maintained their diagnostic quality at the lower mAs. However, the images were slightly noisier, although this aspect of image quality was not measured in our study. The image quality in our study was found to be adequate as a screening tool for the anatomical details required prior to FESS. With regard to not identifying subtle abnormalities in the images acquired at lower mAs, we worked on the hypothesis that if there was no significant deterioration in diagnostic image quality in this group of patients at the lower mAs then (clinically) significant pathology or abnormality would not be missed.

Our results may not be applicable to other patients undergoing paranasal sinus CT. Our image assessment is biased towards high contrast (i.e. bony) detail, which may have been maintained at low mAs. This might not be the case if soft tissue contrast detail had also been assessed. These low dose techniques lack soft tissue contrast and, when assessing complications of sinus disease or malignant disease, higher mAs may need to be used [7].

The exact scan protocol used in each department will depend on the type of CT scanner and the individual preferences of the radiologist and otolaryngologist. The quality criteria guidelines may be helpful in determining optimal parameters. However, there is now evidence that every department should be attempting to reduce the mAs setting for CT scans of the sinuses as this reduces the radiation dose to the patient and can be achieved without compromising the diagnostic value of the study.


   Acknowledgments
 
We wish to thank Dr J Thomas for statistical advice.


   Footnotes
 
Dr S A Sohaib was supported in part by a grant from the Joint Research Board, St Bartholomew's Hospital, London, UK Back

Received for publication June 12, 2000. Revision received August 29, 2000. Accepted for publication October 16, 2000.


   References
Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

  1. Roth Y, Shoshan JB, Kronenberg J. Functional endoscopic sinus surgery: experience with the first 100 patients. Int Surg 1995;80:278–9.[Medline]
  2. Hudgins PA. Complications of endoscopic sinus surgery: the role of the radiologist in prevention. Radiol Clin North Am 1993;31:21–32.[Medline]
  3. Mafee MF. Modern imaging of paranasal sinuses and the role of limited sinus computerized tomography; considerations of time, cost and radiation. Ear Nose Throat J 1993;73:532–4.
  4. Yousem DM. Imaging of sinonasal inflammatory disease. Radiology 1993;188:303–14.[Abstract/Free Full Text]
  5. White PS, Robinson JM, Stewart IA, Doyle T. Computerized tomography mini-series: an alternative to standard paranasal sinus radiographs. Aust N Z J Surg 1990;60:25–9.[Medline]
  6. Shrimpton PC, Jones DG, Hillier MC, Le Heron JC, Faulkner K, Surveys of CT practice in the UK Part 2: dosimetric aspects, NRPB-R249. London: HMSO, 1991.
  7. Marmolya G, Wiesen EJ, Yagan R, Haria CD, Shah AC. Paranasal sinuses: low-dose CT. Radiology 1991;181:689–91.[Abstract/Free Full Text]
  8. Duvoisin B, Landry M, Chapuis L, Krayenbuhl M, Schnyder P. Low-dose CT and inflammatory disease of the paranasal sinuses. Neuroradiology 1991;33:403–6.[Medline]
  9. Kearney SE, Jones P, Meakin K, Garvey CJ. CT scanning of the paranasal sinuses—the effect of reducing mAs. Br J Radiol 1997;70:1071–4.[Abstract]
  10. Rowe-Jones J, Mackay I, Colquhoun I. Charing Cross CT protocol for endoscopic sinus surgery. J Laryngol Otol 1995;109:1057–60.[Medline]
  11. Babbel R, Harnsberger HR, Nelson B, Sonkens J, Hunt S. Optimization of techniques in screening CT of the sinuses. Am J Neuroradiol 1991;12:849–54.[Abstract]
  12. MacLennan AC. Radiation dose to the lens from coronal CT scanning of the sinuses. Clin Radiol 1995;50:265–7.[Medline]
  13. EC quality criteria for computed tomography, EC Working Document EUR 16262. Brussels: EU, 1998.



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