Clinical Radiology 70 (2015) 711e715

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Seventy kilovolt ultra-low dose CT of the paranasal sinus: first clinical results B. Bodelle*, J.L. Wichmann, N. Klotz, T. Lehnert, T.J. Vogl, W. Luboldt, B. Schulz Department of Diagnostic and Interventional Radiology, Goethe University of Frankfurt, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany

art icl e i nformat ion Article history: Received 9 February 2015 Received in revised form 6 March 2015 Accepted 13 March 2015

AIM: To evaluate the diagnostic image quality and radiation dose of low-dose 70 kV computed tomography (CT) of the paranasal sinus in comparison to 100 and 120 kV CT. MATERIALS AND METHODS: CT of the paranasal sinus was performed in 127 patients divided into three groups using different tube voltages and currents (70 kV/75 mAs, ultra-low dose protocol, n ¼ 44; 100 kV/40 mAs, standard low-dose protocol, n ¼ 42; 120 kV/40 mAs, standard protocol, n ¼ 41). CT dose index (CTDIvol), doseelength product (DLP), attenuation, image noise and signal-to-noise ratio (SNR) were compared between the groups using Wilcoxon eManneWhitney U-test. Subjective diagnostic image quality was compared by using a fivepoint scale (1 ¼ non-diagnostic, 5 ¼ excellent, read by two readers in consensus) and Cohen’s weighted kappa analysis for interobserver agreement. RESULTS: Radiation dose was significantly lower with 70 kV acquisition than 100 and 120 kV (DLP: 31 versus 52 versus 82 mGy$cm; CTDI 2.33 versus 3.95 versus 6.31 mGy, all p < 0.05). Mean SNR (70 kV: 0.37; 100 kV: 0.21; 120 kV: 0.13; p < 0.05) and organ attenuation increased significantly with lower voltages. All examinations showed diagnostic image quality. Subjective diagnostic image quality was higher with standard protocols than the 70 kV protocol (120 kV: 5.0; 100 kV: 4.5; 70 kV: 3.5, p < 0.05) without significant differences with substantial interobserver agreement (k > 0.59). CONCLUSION: The ultra-low dose (70 kV) CT imaging of the paranasal sinus allowed for significant dose reduction by 61% and an increased attenuation of organ structures in comparison to standard acquisition while maintaining diagnostic image quality with a slight reduction in subjective image quality. Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Introduction Non-enhanced multidetector computed tomography (CT) is the imaging technique of choice for the evaluation of * Guarantor and correspondent: B. Bodelle, Department of Diagnostic and Interventional Radiology, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany. Tel.: þ49 69 6301 87211; fax: þ49 69 6301 7258. E-mail address: [email protected] (B. Bodelle).

inflammatory disorders of the paranasal sinuses and adds valuable information to the clinical diagnosis of rhinosinusitis regarding the extent and severity of inflammation and allows for improved visualisation of the anatomy and surgically relevant anatomical variants.1e8 Relevant information regarding bones and soft tissue can be provided with high spatial resolution and contrast.1,5,9e13 Over the past 30 years, the increasing use of CT has led ultimately to increased radiation exposure of the

http://dx.doi.org/10.1016/j.crad.2015.03.002 0009-9260/Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

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populations of Europe and the US, raising serious concerns about the associated risks of ionising radiation.5,14e16 Due to a typically younger patient population and the proximity of radiosensitive organs, such as the eye lenses and thyroid gland, during CT of the paranasal sinus, focused attention must be paid regarding radiation dosage, because these organs are exposed to direct or scattered radiation.4,13,16e19 The fact that repetitive examinations may be performed in the same patient for mostly benign diseases requires adherence to the established ALARA (“as low as reasonably achievable”) principle. Special low-dose protocols for CT of the paranasal sinus have been developed for these reasons.3e9,12,16,19e21 Recently, a new X-ray tube has become available (Straton MX, Siemens Healthcare, Erlangen, Germany) which allows for CT examinations at voltages ranging from 140 kV down to 70 kV, promising a new perspective in low-dose imaging, as 80 kV was the lowest tube potential available with the previous systems. The purpose of the present study was to evaluate 70 kV CT of the paranasal sinus with special regard to the image quality and radiation exposure of the patients.

Materials and methods Patient selection This observer-blind study was approved by the institutional review board and patients gave informed consent. Between June 2012 and January 2013, paranasal CT image sets of 127 consecutive patients with clinically suspected sinusitis were evaluated.

CT examinations The examinations were performed on a 128-section CT system (Somatom Definition AS, Siemens Healthcare, Erlangen, Germany). Three protocols were used at different tube voltage settings of 70, 100, and 120 kV with adapted tube voltage. The standard protocol was used in 41 patients (Group 1) with 120 kV and 40 mAs, and the in-house lowdose protocol was used in 42 patients (Group 2) with 100 kV and 40 mAs. In 44 patients (Group 3), the tube potential was adjusted to 70 kV. The number of patients included in each group was calculated by a sample size calculation. The tube voltage had to be increased to 75 mAs, which was the lowest value supported by the device. Details are given in Table 1. Patients were placed in the supine position, slightly reclining the head to obtain a parallel alignment of the Table 1 CT protocols.

Tube potential (kV)/tube current (mAs) Pitch Rotation time Collimation (mm)

Group 1

Group 2

Group 3

120/40 0.8 1.0 s 64 x 0.6

100/40

70/75

upper jaw to the gantry, thus minimising dental artefacts. The imaging ranged from the roof of the frontal sinus up to the maxilla in the craniocaudal direction. By default, 2-mm axial, 2-mm coronal as well as 2-mm sagittal images (each with a bony and soft-tissue kernel) were reconstructed from the raw data set.

Radiation dose estimates For the estimation of radiation doses, the doseelength product (DLP in mGy$cm) and volume-weighted CT dose index (CTDIvol in mGy) were calculated from the patient protocols. These data are automatically generated at the end of an examination and stored in the picture archiving and communication system (PACS). According to the European guidelines, an appropriate conversion factor of 0.0023 mSv/ mGy$cm was used to estimate the effective dose.22

Image quality For analyses of objective image quality, several region-ofinterest (ROI) measurements were performed on a PACS workstation (Centricity 4.1, General Electric Healthcare, Dornstadt, Germany) using a circle tool. Mean attenuation values and standard deviation were recorded and displayed in Hounsfield units. Background noise was determined as the standard deviation of air within a region of interest (ROI) measurement of 20 mm in diameter. The measurements were performed at the level of the ethmoid sinus. Each of the measurements was performed on both sides and the mean was calculated. Attenuation in the zygomatic bone as well as in the bulbus oculi for determination of hard and soft tissue was measured with an ROI of 4 mm in diameter. Based on these measurements, SNR was determined according to the following equation: SNR ¼ attenuation/background noise

For analyses of subjective image quality, datasets were individually and separately evaluated by two radiologists utilising a certified diagnostic monitor (RadiForce RX240, Eizo, Ishikawa, Japan). The rating was performed independently by two blinded radiologists with more than 5 years of experience in CT of the paranasal sinus according to a five-point subjective scale (5 ¼ excellent image quality, 4 ¼ good, 3 ¼ moderate, 2 ¼ fair, 1 ¼ non-diagnostic). The general image impression was analysed regarding background noise and the differentiation of common anatomical landmarks that are evaluated within each CT examination of the paranasal sinus.23 In detail, the following bony and soft tissue structures were used for evaluation of this study: ostiomeatal complex, septal branches of the ethmoidal sinus, as well as the course of the internal carotid artery and the optic nerve in the sphenoidal sinus. Furthermore, the orbital cavity was evaluated with special regard to the differentiation of the eye muscles and optic nerve, as well as the distinction of the mastoid cells of the temporal bone. Presence of sinusitis was documented by evaluating classic

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CT signs as localised or circular mucosa swelling or the presence of sinusoidal fluid levels (Fig 1).

Statistical analysis Statistical analysis was performed using a dedicated software tool (SPSS 16.0 for Windows, IBM, USA). Variables were expressed as mean values  standard deviation and range. Radiation parameters and quantitative image parameters (noise, attenuation, and SNR) were tested using the WilcoxoneManneWhitney U-test. A p-value of 0.05) were found regarding gender distribution or age in all three groups: In Group 1 the mean age was 39.7  18 years (range 13e88), 44.5  18 years (range 16e82) in Group 2 and 41.8  18 years (range 11e75) in Group 3 (70 kV protocol).

Radiation dose estimation Radiation exposure of the patients (Fig 2) was lowest using the 70 kV protocol in Group 3 with a mean DLP of 31.1 mGy$cm (0.07 mSv) compared with both of the other image acquisition groups (versus Group 1 and 2: p < 0.001). The highest dose was emitted using a tube potential of 120 kV with a mean DLP of 81.6 mGy$cm (0.19 mSv). Acquisition with the 100 kV low-dose protocol resulted in a mean DLP of 52.2 mGy$cm (0.12 mSv), still significantly lower than with 120 kV acquisition (p < 0.001). CTDIvol showed statistically significant differences for groups 1, 2 and 3 (6.31 mGy versus 3.95 mGy versus 2.33 mGy).

Objective image quality The highest image noise was measured in Group 3 for the 70 kV protocol with a standard deviation of 189.2 HU,

Figure 2 Radiation dose of the standard protocol (Group 1 with 120 kV), regular low dose protocol (Group 2 with 100 kV), and ultralow dose protocol (Group 3 with 70 kV) reflected by the DLP. The results showed statistically significant differences for all groups (p < 0.05).

significantly higher (p < 0.001) than the image noise of the 120 kV images in Group 1 (81.9 HU) and the image noise of the 100 kV images in Group 2 (83.2 HU). However, the SNR was increased due to a higher organ attenuation for the softtissue structures with lower tube voltage settings (120 kV: 0.13; 100 kV 0.21; 70 kV 0.37, p < 0.05). The attenuation of the bony structures was also significantly increased with lower tube voltage acquisition, but the bone SNR peaked in the 100 kV images, whereas SNR was lowest with 70 kV imaging (120 kV: 18.2; 100 kV: 25.7; 70 kV: 10.9; p < 0.001). There were no statistically significant differences regarding image noise and SNR between groups 1 and 2.

Subjective image quality The subjective overall image quality of the 70 kV datasets in Group 3 were rated as moderate to good (120 kV: 5.0; 100 kV: 4.5; 70 kV: 3.5, p < 0.05). Interobserver agreement was good for both raters for Groups 1 and 2 (k > 0.88) and Group 3 (k ¼ 0.59).

Discussion Radiosensitive organs, such as the eye lenses, are exposed to direct or scattered radiation during CT of the paranasal sinus.4,13,16e19 Several methods for lens protection during sinus CT have been described.24,25 However, the

Figure 1 CT of the paranasal sinus with transverse CT images at the level of the maxillary sinus. Images of a 69-year-old woman examined using the standard protocol at 120 kV (Group 1), a 68-year-old woman examined using the regular low-dose protocol at 100 kV (Group 2), and a 62year-old man examined using 70 kV (Group 3) showing the presence of sinusitis with circular mucosa swelling in the left maxillary sinus.

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most effective way of minimising the risk of radiation is the use of an adequate lower-dose acquisition technique.20 Previous studies showed promising results with different strategies for dose reduction in CT with increased pitch,10,26 iterative reconstruction technique,4,27 and cone-beam CT.28 Recent reports have shown that the reduction of tube voltage may lead to a distinct reduction of the radiation dose with no impact on image quality for imaging of the paranasal sinus.3 The present results indicate that the proposed ultra-low dose 70 kV protocol provides a balanced compromise between dose reduction and diagnostic image quality. By modification of the imaging parameters, a dose reduction of approximately one-third can be achieved while maintaining diagnostic image quality. With a mean effective dose of 0.05 mSv (mean DLP 31 mGy$cm), ultra-low dose acquisition techniques allows for a substantial dose reduction in comparison to previous studies, which also demonstrated that reducing the effective tube currentetime product (mAs) and tube voltage (kV) allows for a lower dose while the image quality is only slightly impacted.1,3 Marmolya et al.29 specified that 23 mAs at a standard tube voltage of 120 kV is of sufficient diagnostic quality for the evaluation of suspected sinusitis. Hagtvedt et al.7 also concluded that a tube currentetime product of 40 mAs allows for the diagnosis of sinusitis if more detailed evaluation of the paranasal sinus is not necessary. Abul-Kasim et al.3 achieved a reduction in the DLP down to 20 mGy$cm and 41 mGy$cm with an 80 kV protocol (17 and 33 mAs) in a phantom study. Schulz et al.11 reported a dose reduction down to a DLP of 37 mGy$cm with a 100 kV protocol at 25 mAs, whereas Bulla et al.4 and Lam et al.12 achieved dose reductions with 120 kV protocols down to a mean DLP of 50 mGy$cm (mean effective tube currentetime product of 24 mAs) and 110 mGy$cm (40 mAs). The image quality for identification of anatomical landmarks and the presence of sinusitis was acceptable with a good-to-moderate score for the ultra-low dose technique. Image quality and presentation of the sinusoidal spaces, which are characterised as high-contrast tissue, was not limited, as discussed previously by Schulz et al.10 One reason for the loss of subjective image quality of the 70 kV images was an overall increased noise in the present study. In the present authors’ experience, adjusting the window settings may reduce the subjective noise impression without a loss of contrast due to a higher attenuation of the high-contrast tissue of the bone. Furthermore, the present ultra-low dose technique may be especially beneficial for CT imaging of suspected sinusitis in children. Compared with the European reference doses for routine CT examination of the face and sinus (DLP: 360 mGy$cm), the 70 kV technique allowed for a reduction in radiation exposure down to about 9% in the present study.22 There has to be a balance during imaging to produce images of sufficient quality while striving for the lowest dose as per the ALARA concept. Cohen described this issue as “the second risk of radiation”: for the diagnosis of a mostly benign disease, the overall radiation risk should be kept as low as possible, but the risk of missing a present potentially malignant abnormality is of greater relevance

than the risk of radiation-induced cancer in the future.30 In addition, the right indication is the best ALARA principle. Although the present ultra-low kV protocol showed promising results regarding its image quality, further studies are necessary to assess the effect of the associated increased image noise on diagnostic accuracy. In comparison to a single plain radiograph with a reported mean radiation exposure of 0.098 mSv, which are less often performed for the evaluation of the paranasal sinus, the 70 kV CT protocol imparts a slightly higher dose but provides valuable information for the evaluation of the paranasal sinus and allows for improved visualisation, including multiplanar reconstructions.2,3,31 Various fields of application can benefit from using the 70 kV technique, and these have to be researched in future studies. However, a technical limitation of the CT system used in the present study has to be considered: with the 70 kV protocol the minimum adjustable tube currentetime product was 75 mAs instead of 40 mAs for the 100 and 120 kV protocols. A reduction down to 40 mAs for 70 kV protocol would lead to a further reduction in dose with a supposed restriction to higher image noise. This is a technical limitation, which may hopefully be resolved with the recently introduced third-generation dual-source CT systems.32 In conclusion, the results of the present study show a significant dose reduction of 61% and an increased attenuation of organ structures in comparison to standard acquisition technique while maintaining diagnostic image quality with a slight reduction in subjective image quality.

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Seventy kilovolt ultra-low dose CT of the paranasal sinus: first clinical results.

To evaluate the diagnostic image quality and radiation dose of low-dose 70 kV computed tomography (CT) of the paranasal sinus in comparison to 100 and...
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