British Journal of Radiology (2003) 76, 418-420
© 2003 British Institute of Radiology
doi: 10.1259/bjr/82798696
Radiation dose to the lens of eye and thyroid gland in paranasal sinus multislice CT
I Zammit-Maempel, MRCP, FRCR1,
C L Chadwick, DCR(R), BSc(Hons)1 and
S P Willis, DCR(R)2
Departments of 1 Radiology Freeman Hospital, Newcastle and 2 Regional Medical Physics, Newcastle General Hospital, Newcastle upon Tyne, UK
Correspondence: Dr I Zammit-Maempel, Department of Clinical Radiology, Freeman Hospital, High Heaton, Newcastle upon Tyne NE7 7DN, UK
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Abstract
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CT has become an established examination in the evaluation of the paranasal sinuses. Until recently this was achieved by the direct coronal technique on conventional and single slice helical scanners. With the advent of multislice technology, thin slice axial CT with excellent coronal and sagittal reconstructions is now the norm. We describe a study designed to evaluate the radiation dose to the lens of the eye and thyroid gland in the axial and coronal planes on a Siemens Volume Zoom quad slice scanner at 140 kV and effective mAs of 100 using 1 mm collimation. Thermoluminescent dosimeters were placed on the eyelid and thyroid gland of 29 patients scanned axially in the supine position and a further 28 patients scanned coronally in the prone position with gantry tilt. The results show mean doses of 35.1 mGy (lens) and 2.9 mGy (thyroid gland) in the coronal plane compared with 24.5 mGy (lens) and 1.4 mGy (thyroid gland) in the axial plane. Results obtained from a head phantom and from using the ImPACT CT dose calculator were comparable. The kV and mAs were then reduced to 120 and 40, respectively, and the axial study repeated using the head phantom and predicted doses using the ImPACT CT dose calculator. The low dose scanning technique revealed a lens dose of 9.2 mGy and thyroid dose of 0.4 mGy. The eye dose on a multislice scanner is still substantially less than the threshold dose of 0.5–2 Gy for detectable lens opacities. These results indicate that, in addition to the established perceived advantages of multislice axial sinus CT, i.e. patient comfort, no artefact from dental amalgam and reproducible true coronal images, should be included a decreased radiation dose to both the eye lens and thyroid gland compared with direct coronal scanning.
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Introduction
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Plain radiographs are now seldom used as part of the initial work-up of patients with sinus disease as certain important anatomical structures such as the osteomeatal complex are poorly seen [1]. Sinus CT has assumed increased importance because of the development of functional endoscopic sinus surgery (FESS), a minimally invasive way of treating sinus disease. Unlike antrostomies, which create an alternative exit of mucosa from the maxillary sinus, FESS enlarges the natural ostia and passageways of the paranasal sinuses, thus preserving the normal pathway of mucociliary clearance [2]. A road map of the sinus anatomy and extent of disease by coronal CT has therefore become an essential pre-operative investigation as these images reflect intranasal anatomy as viewed endoscopically [3–6].
Previously on conventional and single slice CT, imaging of the sinuses had been acquired by the direct coronal technique with the patient prone or supine and the neck hyperextended. Axial acquired, coronally reconstructed images on single slice scanners were previously inferior to direct coronal helical images because of step artefacts when evaluating the fine osseous structures [7]. With the advent of multislice CT, thin slice axial images with excellent coronal and sagittal reformations are now possible. The advantages of axial CT with coronal reconstructions include increased patient comfort, no artefact from dental amalgam and reproducible true coronal images. However, there is no reliable data as to how the dose from axial multislice CT compares with the direct coronal technique and a study was therefore performed to compare the two. Once the lowest dose plane was identified, the imaging parameters were reduced to obtain images that were deemed to be adequate as a screening tool for the anatomical details required prior to FESS, and dosimetry repeated on a head phantom.
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Materials and methods
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CT technique
All the patients were scanned helically on a Somatom Volume Zoom quad slice CT scanner (Siemens Medical Systems, Erlangen, Germany) using 140 kV, an effective mAs of 100, 1 mm collimation and an average scan time of 11 s, the initial parameters set for sinus imaging by Siemens. The images were reconstructed with a slice thickness of 1.25 mm and an increment of 1 mm with a table feed of 3.5 mm and viewed on both bone and soft tissue windows.
There are two methods of quantifying dose in CT. These are the dose–length product (DLP) measured in mGy.cm and the weighted CT dose index (CTDIw) [8]. Comparisons of these for two different scanners used in this study and an earlier dosimetry study and protocols used are given in Table 1
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Patients
57 consecutive patients referred for sinus CT were included in the study. Following verbal consent, the patients were divided randomly into two groups, Group A scanned axially and Group B scanned coronally. As can be seen in Table 2, 29 patients were scanned supine in the axial plane from just below the maxillary antra to the top of frontal sinuses. 28 patients were scanned prone with neck hyperextension in the direct coronal plane and appropriate gantry tilt such that the images were perpendicular to the hard palate.
Dose measurement
Lithium fluoride thermoluminescent dosimeters (TLD) were placed on the right eyelid, as an indicator of lens dose, and right lobe of thyroid gland for each patient. Harshaw XD720 dosimeters (x-rad) were used in conjunction with a Harshaw 6600 reader (Harshaw Bicron, Solon, OH). These are useful because they are easily packaged, identifiable and radiolucent. There is an overall uncertainty in the dose measured on the TLD of ±9% at 50 mGy [9] at the 95% confidence limit, well within the tolerance set in the National Protocol for Patient Dose Measurements in Diagnostic Radiology. The TLDs were calibrated using an X-ray set with a mean X-ray energy output of 55 keV (similar to the output of the CT scanner used).
In order to ensure that the results obtained were reproducible, the study was repeated using a Rando phantom and finally the lens and thyroid doses were predicted using the ImPACT CT dose calculator and NRPB SR-250 data sets.
Further TLD axial measurements were then performed on the phantom at a reduced kV of 120 and effective mAs of 40, as at these reduced parameters the images were shown to be of adequate quality to identify the anatomical details required prior to FESS. Diagnostic imaging quality as assessed by scoring whether the uncinate process, infundibulum, maxillary ostium, cribriform plate, fovea ethmoidalis and lamina papyracea were clearly demonstrated, indistinct or not seen, showed no difference between the high and low dose techniques. The lens and thyroid dose using the ImPACT CT dose calculator at these reduced parameters was also predicted.
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Results
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As can be seen in Table 3, the mean lens dose in the axial plane was 24.5 mGy (range of 14.2 mGy to 35.1 mGy) compared with a mean dose of 35.1 mGy (range of 15.8 mGy to 49.5 mGy) in the coronal plane.
The thyroid gland mean dose in the axial plane was 1.4 mGy (range of 0.6 mGy to 2.9 mGy) as compared with the coronal plane mean of 2.9 mGy (range of 1.4 mGy to 4.9 mGy).
Using TLDs on the phantom and similar dose parameters in the axial plane, a mean lens dose of 28.7 mGy and thyroid dose of 1.3 mGy was measured. Using the CT dose calculator, an eye dose of 25.5 mGy and a thyroid dose of 0.7 mGy were obtained.
The TLD axial measurements at the reduced parameters of 120 kV and effective mAs of 40 on the Rando phantom showed mean lens dose readings of 9.2 mGy and thyroid dose of 0.4 mGy. The predicted doses from the CT dose calculator were a lens dose of 10 mGy and a thyroid dose of 0.3 mGy.
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Discussion
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The results show that the doses to both the eye lens and thyroid gland are significantly lower in the axial than in the coronal plane and that, as expected, a reduction in kV and mAs results in a lower dose. The thyroid dose, not surprisingly, was higher in the direct coronal plane than the axial plane principally because of the gland being closer to the scan field. The lens dose discrepancy between the two planes is presumably due to altered scanner geometry as a result of gantry tilt. The lens dose on axial multislice CT, despite thin collimation, is substantially less than the threshold dose of 0.5–2 Gy for detectable lens opacities [10] especially with a low dose technique. Radiation induced cataracts differ initially from those that occur naturally by arising in the posterior pole of the lens and often do not progress, causing little if any visual impairment. Only 12% of patients receiving a dose greater than 2 Gy will have progressive opacification [11, 12]. However, there is a theoretical risk of non-deterministic effects particularly as some patients have multiple imaging and radiologists are legally required to minimize the radiation dose in sinus CT to both the eye lens and thyroid gland [13].
The present generation of CT scanners have intrinsically better spatial and contrast resolution and more dose efficient detectors, thereby maintaining image quality at lower mAs. In addition, in sinus CT, images of diagnostic quality can be obtained at low mAs because high contrast tissues, namely air, soft tissue and bone are being scanned. Also in sinusitis one is concerned less with differentiation of soft tissue and more with documentation of disease and identification of anatomical anomalies. However, the parameters set on CT scanners by the manufacturers very often still adopt relatively high factors and radiation dose is largely a function of the mAs settings. Several authors [14–18] have previously documented the maintenance of image quality, despite a reduction in dose, with decreasing kV and mAs. Two previous studies in our institution on a Siemens Somatom ART conventional scanner had similarly shown a reduction in dose on reducing mAs. Using the minimum parameters of 110 kV and 100 mAs on that scanner, a lens dose of 17.8 mGy and thyroid dose of 0.5 mGy in the direct coronal plane were measured [19, 20]. Our current dose measurements using a low dose technique are comparable for thyroid gland dose and considerably less for eye lens dose than our previous results despite now obtaining infinitely superior images that allow multiplanar reconstruction.
It is now our routine to scan all paranasal sinuses on CT at 120 kV and 40 mAs in the axial plane. The images have been further optimized to cut out any step artefact by reconstructing with a slice thickness of 1 mm and an increment of 0.8 mm and a table feed of 2.8 mm. The axial plane is not only superior because of patient comfort and no dental amalgam artefact but also because of reduced dose to both the eye lens and the thyroid gland compared with the direct coronal plane.
Received for publication October 31, 2002.
Revision received March 17, 2003.
Accepted for publication March 26, 2003.
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Abstract
Introduction
