How does a CT scan help diagnose Covid-19?

In the CT scan, when there is a visual of a ground glass opacity, it is indicative of an infection.

Hyderabad: As of now there are three different tests that are being used to diagnose Covid-19 i.e., Rapid Antigen Test (RAT), Real Time PCR (RTPCR) test and a high resolution CT (HRCT) scan of the chest.

While RAT and RTPCR are done after collecting swab samples from the nose and the throat, the HRCT is an imaging test where a Computed Tomography (CT) system is used to image the lungs.

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In the CT scan, when there is a visual of a ground glass opacity, it is indicative of an infection.

According to Dr. Surendar Alwala, lead radiologist at Yashoda Hospital Secunderabad, “On the CT image, if there are visuals that resemble ground glass, it is indicative of involvement of infection in the lungs but and in case of Covid-19, we use the Covid-19 Reporting and Data System (CORADS) to inform the severity and likeliness of infection in the lungs.”

The CORADS is given on a scale of 1 to 6, wherein 1 means normal or no infection, 2 means a suspected infection (not Covid), 3 can/can’t be Covid, 4 is highly suspicious of Covid while 5 is classic Covid. “While those numbers mean that, CORADS 6 is usually given out when RTPCR is positive and there’s also a presence of this ground glass visual in the lung,” explains the radiologist.

There can also be a scenario where RTPCR is negative but the person is positive for Covid as it is present in the lungs. “The chest CT is an effective way of diagnosing towards the end of the first week of infection as by that time it would have reached the lower respiratory areas,” Dr.Surendar explains.

Apart from the CORADS level, the reports also have a CT Severity Score (CTSS), which tells us of the amount of lungs involved in the infection.

“The right lung has three lobes and the left lung has two lobes. These five lobes are divided into 20 segments. The CTSS is based on a scale of either 1-40 or 1-25,” he points out.

When the score is given on a scale of 1-25, each of the lobes is given a score of 1-5 depending on the involvement in the infection, with 1 being less than 5%, 2 being 5-25%, 3 being 25-50%, 4 being 50-75% and 5 being 75-100%. “Each lobe gets a score of 1-5 and five lobes combined are given a score on a scale of 1-25. The higher the number, the greater the infection,” he shares.

The score is given on a scale of 1-40 wherein the 20 segments are taken into consideration, with each segment given a score of 0-2.

“If there’s no involvement, the segment gets a zero score. It gets 1 if the infection is up to 50% and 2 if it is 50% and more. The score of all the segments is combined and is given on a scale of 1-40, with a high number indicating high infection,” Dr.Surendar adds.

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