Ultrasound in COVID-19: a timeline of ultrasound findings in relation to CT
SirdThe novel coronavirus (Sars-Cov-2) produces a mild to severe lower respiratory tract infection that is commonly identified by imaging in affected patients, in some cases even before symptom development. 1 Computed tomography (CT) has been the primary imaging method evaluated to date and is often being recommended as a screening tool for patients suspected of having COVID-19. 2 Lung ultrasound is an alternative imaging method with emerging evidence that supports its ability to identify characteristic lesions seen in COVID-19 that are highly consistent with CT imaging, 3,4 without many of the downsides. For clinicians wishing to use ultrasound in the management of patients with COVID-19, it is important to understand the characteristic findings and the timeline in which they would occur. These are described below in comparison to CT.
It is estimated that lung abnormalities present early in the course of disease with bilateral, basal, and peripheral involvement in approximately 50e75% of patients 5, 6 and occur in identifiable stages. Using CT, Jin et al. described five temporal stages of the disease in the lungs as ultra-early (asymptomatic, 1e2 weeks after exposure), early (1e3 days from symptom presentation), rapid progression (3e7 days from symptomatic presentation), consolidation (7e14 days from symptom presentation), and dissipation (2e3 weeks after symptom onset), each occurring with associated CT findings. 1 Similar categories were described by Pan et al. with the exception of a pre-symptomatic phase. 7 CT findings begin as single or multifocal ground-glass opacities, pulmonary nodules, or air bronchograms, which progress with development of interlobular septal thickening and crazy paving, before regression in both size and density at the end of the second week of infection. Opacities often have extensive distribution, typically bilaterally, but also seen unilaterally, with occasional round morphology or reversed-halo or atoll sign. 5 In the dissipation phase, there may be continued patchy consolidative opacities in addition to reticular "strip-like" opacities, bronchial wall thickening, and interlobular septal thickening. 1, 8 The characteristic ultrasound findings (bilateral and multilobar B-lines, subpleural consolidates, irregular pleural line, and decreased blood flow 3, 4, 9 ) have been shown to be highly consistent with CT findings 3,4 and can be expected to develop over a similar timeline. During the first few days of symptom presentation, scattered unilateral or bilateral multilobar B-lines can be visualised. 3, 9 As the disease progresses from the end of week 1 through week 2, development of alveolar interstitial syndrome with diffuse, bilateral B-lines can occur in addition to an irregular pleural line with punctate defects and formation of subpleural consolidations with visible air bronchograms. Lastly, after the end of week 2 during convalescence, there can be an expected regression of prior findings with re-emergence of A-lines. 9 A summary of findings is listed in Table 1 .
Although the literature remains limited, there is still a clear benefit for clinicians to be familiar with ultrasound findings and their progression in COVID-19 patients. It may be particularly useful in helping emergency personnel to triage and diagnose suspected patients, 4 but also for monitoring progression of the disease throughout hospitalisation. Additionally, it offers substantial benefits in comparison to CT imaging, including portability, lower cost, reduced radiation, and ease of sterilisation. Physicians are Table 1 Timeline of common findings of COVID-19 in computed tomography (CT) and ultrasound. 1 
The author declares no conflict of interest.