key: cord-0824139-44k3i7sv
authors: Hamper, Christina M.; Fleckenstein, Florian Nima; Büttner, Laura; Hamm, Bernd; Thieme, Nadine; Thiess, Hans-Martin; Scholz, Oriane; Döllinger, Felix; Böning, Georg
title: Submillisievert Chest-CT in patients with COVID-19- experiences of a German level I center
date: 2020-11-03
journal: Eur J Radiol Open
DOI: 10.1016/j.ejro.2020.100283
sha: 7aabffa0489acf575653e8ca27f0a1fef84fcd57
doc_id: 824139
cord_uid: 44k3i7sv

PURPOSE: Computed tomography (CT) is used for initial diagnosis and therapy monitoring of patients with coronavirus disease 2019 (COVID-19). As patients of all ages are affected, radiation dose is a concern. While follow-up CT examinations lead to high cumulative radiation doses, the ALARA principle states that the applied dose should be as low as possible while maintaining adequate image quality. The aim of this study was to evaluate parameter settings for two commonly used CT scanners to ensure sufficient image quality/diagnostic confidence at a submillisievert dose. MATERIALS AND METHODS: We retrospectively analyzed 36 proven COVID-19 cases examined on two different scanners. Image quality was evaluated objectively as signal-to-noise ratio (SNR)/contrast-to-noise ratio (CNR) measurement and subjectively by two experienced, independent readers using 3-point Likert scales. CT dose index volume (CTDIvol) and dose-length product (DLP) were extracted from dose reports, and effective dose was calculated. RESULTS: With the tested parameter settings we achieved effective doses below 1 mSv (median 0.5 mSv, IQR: 0.2 mSv, range: 0.3-0.9 mSv) in all 36 patients. Thirty-four patients had typical COVID-19 findings. Both readers were confident regarding the typical COVID-19 CT-characteristics in all cases (3 ± 0). Objective image quality parameters were: SNRnormal lung: 17.0 ± 5.9, CNRGGO/normal lung: 7.5 ± 5.0, and CNRconsolidation/normal lung: 15.3 ± 6.1. CONCLUSION: With the tested parameters, we achieved applied doses in the submillisievert range, on two different CT scanners without sacrificing diagnostic confidence regarding COVID-19 findings.

The new coronavirus (SARS-CoV-2) and the disease it causes (COVID- 19) were first reported in the city of Wuhan, China, in December 2019 (1, 2) . In most cases, SARS-CoV-2 manifests with only mild symptoms, but some patients develop a severe disease, which can lead to acute respiratory distress syndrome (ARDS), multiple organ failure, and death (3, 4) .

There are specific pulmonary findings associated with COVID-19 in chest computed tomography (CT), including bilateral ground-glass opacities (GGO) with peripheral distribution, consolidations, and the "crazy-paving" pattern or "reverse halo" sign (5) (6) (7) (8) (9) (10) (11) (12) (13) . In the course of the disease, CT can show these typical changes, even if no virus is detected in the nasopharyngeal or throat swab (14, 15) . Reverse transcription polymerase chain reaction (RT-PCR) testing is limited by a longer waiting time for results. Therefore, CT imaging can speed up diagnosis of COVID-19 and provide valuable findings in the further course of disease (16) . Kang et al. even recommended low-dose CTs for initial diagnosis and to monitor the course of COVID-19 (14) . They underline that a CT can be very helpful early at the onset of symptoms or when the swab test is negative (14, 15) . Esposito et al. also conclude that chest CT is a good and fast imaging modality for first-line triage of patients with suspected COVID-19 (17) . Radpour et al. focus on the advantages of low-dose CT especially in regions with many patients and limited availability of RT-PCR tests (18). Nevertheless, the results should be interpreted with caution, as they depend on the disease status, testing method, and imaging technique (e.g., applied dose).

As all age groups are affected by COVID-19 according to a multi-center study of Yang et al., which was also summarized in an article of Yi et al. (19) , the potential risks arising from the radiation dose associated with CT need to be addressed (14) as findings like GGO are sometimes not visible in chest radiographs (20) . Nevertheless, the diagnostic confidence of chest-CT must be guaranteed regarding typical pulmonary findings associated with COVID-19.

The aim of this study was therefore, based on experiences of a German center, to evaluate parameter settings for two commonly used CT scanners that potentially ensure sufficient image quality/diagnostic confidence regarding COVID-19 findings at submillisievert dose levels. Based on published data and our experience, we selected acquisition parameters for low-dose chest scans in patients with suspected COVID-19 (see Table 1 ) examined on two scanners from different vendors. We retrospectively included 36 patients from two sites (n= 17/19) of our German level I university center, who were treated between March 30 and April 22, 2020. All patients included were positive for SARS-CoV-2 (RT-PCR of nasopharyngeal and oropharyngeal swab samples; patients with a negative RT-PCR test result were excluded (Fig. 1) ). Maximum axial and sagittal body diameters of patients` chest CTs were measured to assess body size. We calculated ROI-based signal-to-noise ratio (SNR) of normal lung and contrast-to-noise ratios (CNRs) of GGO to normal lung and consolidations to normal lung. SNR was defined as: J o u r n a l P r e -p r o o f Maximum patient diameters were 24.7 ± 3.2 mm and 24.6 ± 3.3 mm (sagittal and axial orientation) (Fig.   3 ).

Thirty-four of 36 patients had typical findings of COVID-19 including peripheral GGO, consolidations, and/or crazy paving; however, all 36 CT studies were rated for diagnostic confidence. The two independent raters scored all scans as completely confident (3 ± 0).

Calculated SNR/CNR values for the assessment of objective image quality were in line with expectations based on our earlier experience and are consistent with subjective scoring. For all cases taken together, we obtained the following ratios: SNRlung: 17.0 ± 5.9, CNRGGO to lung: 7.5 ± 5.0, and CNRconsolidation to lung:

15.3 ± 6.1. For further deails see Fig. 4 .

CTDIvol was 0.9 ± 0.3 mGy, retrospectively resulting in a DLP of 35 ± 10.2 mGy*cm and a median ED of 0.5 mSv (IQR: 0.2 mSv, range: 0.3-0.9 mSv) (Fig. 5) . Mean ED was 0.5 ± 0.2 mSv (Fig. 5 ).

As CT is frequently used in patients with COVID-19 the risks associated with radiation exposure should be taken into account, especially in younger patients (14) . Following the ALARA principle, a low-dose approach to CT scanning is necessary without sacrificing the diagnostic confidence. The aim of our study was therefore to identify CT parameter settings that fulfil these requirements.

The sex distribution of COVID-19 patients in our study -75% men and 25% women -fits to results in literature (26 In our study, 94% of the patients (34/36) with positive RT-PCR testing had typical pulmonary CT findings associated with COVID-19. Pan et al. analyzed typical findings in patients with COVID-19 over time from initial diagnosis to recovery. They concluded that pulmonary manifestations were most severe 10 days after the first symptoms (28) . Of note, four patients did not have typical findings in their initial chest CT but later on (28) . Kanne et al. also show that up to around 50% of patients with COVID-19 do not have abnormal CT scans in the first two days after onset of symptoms (29) . Thus, the two negative CT scans in our study may be attributable to an early stage of the disease. In our study the readers consistently assigned the highest diagnostic confidence to all scans. Dangis et al. also investigated subjective evaluation of CT images and concluded, that submillisievert chest CT allows for assessment of COVID-19 in patients at the emergency room (ER) (21) .

In our study, we additionally determined CNR and SNR for objective image quality assessment, and the results underline our subjective impression. SNRlung was 17.0 ± 5.9, CNRGGO to lung 7.5 ± 5.0, and was 35 ± 10.2 mGy*cm, which is again above that of Kang et al. (14) , which was 14.5 mGy*cm, and the median DLP reported by Agostini et al. was also lower at 19.5 mGy*cm (17.5-29 .02 mGy*cm) (22) . Our median DLP was lower than that of 41.4 ± 17.7 mGy*cm reported by Dangis et al. in a group of 83 patients with COVID-19 (21) . In sum, published results for ED, CTDIvol and DLP are comparable to our results and thus appear to be robust for a mid-European patient collective. Differences may be due to variations in patient size or other technical parameter settings (e.g. iterative reconstruction level). Of course, if CT is not performed for COVID-19 diagnosis alone and there are additional questions to be answered (e.g., pulmonary embolism), administration of contrast medium and use of higher dose settings could be necessary.

The main limitation of our study is the small sample size of 36 patients and the retrospective, singlecenter design. Furthermore, we only evaluated two CT scanners, and the readers were not blinded to technical parameters or the diagnosis. We only included patients > 18 years and did not compare lowdose with normal-dose chest CT. In addition, the results probably cannot be transferred to patients with larger body diameters and dedicated studies should be performed.

In conclusion, the presented parameter settings allow acquisition of chest CT scans with dose exposure in the submillisievert range on the two CT scanners used in our study without sacrificing diagnostic confidence regarding the presence of typical pulmonary changes associated with COVID-19. 

Radiological findings from 81 patients with COVID-19 pneumonia in Wuhan, China: a descriptive study

A Novel Coronavirus from Patients with Pneumonia in China

Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study

Clinical features of patients infected with 2019 novel coronavirus in Wuhan

COVID-19): Relationship to Duration of Infection

Dynamic changes of chest CT imaging in patients with corona virus disease-19 (COVID-19). Zhejiang Da Xue Xue Bao Yi Xue Ban

Novel Coronavirus (2019-nCoV)

Novel Coronavirus (2019-nCoV) Pneumonia

Clinical Characteristics of Coronavirus Disease 2019 in China

Differences in Clinical and Imaging Presentation of Pediatric Patients with COVID-19 in Comparison with Adults. Radiology: Cardiothoracic Imaging

Review of chest CT manifestations of COVID-19 infection

Imaging in corona virus disease 2019 (COVID-19)-A Scoping review

COVID-19 pneumonia manifestations at the admission on chest ultrasound, radiographs, and CT: single-center study and comprehensive radiologic literature review

Recommendation of low-dose CT in the detection and management of COVID-2019

Chest CT for Typical 2019-nCoV Pneumonia: Relationship to Negative RT-PCR Testing

Coronavirus Disease 2019 (COVID-19): A Perspective from China

Why is chest CT important for early diagnosis of COVID-19? Prevalence matters. medRxiv

COVID-19 Evaluation by Low-Dose High Resolution CT Scans Protocol

COVID-19: what has been learned and to be learned about the novel coronavirus disease

Infection: Radiologic Findings and Literature Review. Radiology: Cardiothoracic Imaging

Accuracy and reproducibility of low-dose submillisievert chest CT for the diagnosis of COVID-19. Radiology: Cardiothoracic Imaging

Proposal of a low-dose, long-pitch, dual-source chest CT protocol on third-generation dual-source CT using a tin filter for spectral shaping at 100 kVp for CoronaVirus Disease

Radiation dose reduction techniques for chest CT: Principles and clinical results

Low-dose Lung Cancer Screening at an

Multisection CT protocols: sex-and age-specific conversion factors used to determine effective dose from dose-length product

Coronavirus COV-19/SARS-CoV-2 affects women less than men: clinical response to viral infection

Clinical Characterization of 162 COVID-19 patients in Israel: Preliminary Report from a Large Tertiary Center

Time Course of Lung Changes at Chest CT during Recovery from Coronavirus Disease 2019 (COVID-19)

Essentials for Radiologists on COVID-19: An Update-Radiology Scientific Expert Panel

The authors declare no conflict of interest and nothing to disclose regarding this study.

All authors contributed equally to the work

This retrospective study was approved by the Local Ethics Committee of our hospital (EA4/074/20).

The authors declare no funding regarding this research article

The parameters were chosen on the basis of published experience and experience at our center.J o u r n a l P r e -p r o o f