Bengaluru: As the second wave of the pandemic exerts inordinate strain on health infrastructure and resources in India, including slowing down the speed of testing and turnaround, many medical professionals have taken to asking suspected Covid patients to obtain a chest CT scan to diagnose the extent of lung involvement and damage.
A CT scan or computed tomography scan is an imaging technique used in medicine that can obtain detailed images of the body using X-rays. The procedure is non-invasive, and multiple X-ray measurements taken from various angles are run through reconstruction algorithms to obtain an accurate cross-sectional or ‘tomographic’ image of the body or area being imaged.
CT can be used in patients with rods or metal implants or pacemakers, unlike Magnetic resonance imaging (MRI). The CT scan has altered medical imaging in numerous ways, leading to revolutionary breakthroughs in diagnostics.
In 1979, South African-American physicist Allan M. Cormack and British electrical engineer Godfrey N. Hounsfield were awarded the joint Nobel Prize in Physiology or Medicine for developing the technique of computer assisted tomography.
However, the radiation used in the CT technique can potentially damage human cells and DNA, and prolonged or cumulative exposure to such radiation can lead to radiation-induced cancer. This has led to a lot of concern from those in the medical community about indiscriminate and unnecessary CT scanning in the diagnosis of Covid during the current outbreak.
How is radiation used in CT?
The word tomography derives from the Greek word ‘tome’, meaning slice, and ‘graphein’, to write.
A CT machine consists of an X-ray generator that rotates around the person being imaged. X-ray detectors are placed opposite to generators. When these rays pass through a person, they are modified in their strength when they come out depending on the kind of tissues, muscles, and bone they pass through, providing an X-ray.
Multiple ‘slices’ of X-rays of the same location are taken from different angles as the X-ray generator rotates. These raw images are called a sinogram. These are then pieced together to obtain a complete understanding of the part of the body being imaged.
Sometimes, a chemical or medium for highlighting structures like blood vessels is injected intravenously. Such a medium is called a ‘radiocontrast’ and is usually a compound made of iodine. It is used to obtain necessary information about how well cells and tissues are functioning.
A radiation dose is a measure of the exposure to radiation. There are three kinds: an absorbed dose is the amount of energy absorbed by an exposed body part; an equivalent dose is calculated for individual organs and is the absorbed dose adjusted to each organ and type of radiation; and an effective dose is the addition of all equivalent doses for the entire body.
Radiation doses are typically reported in an SI unit called the gray or the sievert. Each dose is proportional to the amount of radiation the exposed body part absorbs. The higher the energy and longer the duration of exposure, the higher the damage. Absorbed dose is measured in milligrays (mGy), while equivalent and effective doses are in millisieverts (mSv).
Effective dose is typically what is considered to reflect the general overall risk from exposure to radiation for an individual. As with all kinds of radiation, lead is the main material used by radiographers for protection and shielding against scattered X-rays.
In Covid patients, CT scans can be helpful in some scenarios, including ruling out pulmonary embolism in a patient who is on blood thinners and steroids and isn’t recovering; and pneumomediastinum, where the thoracic cavity housing the heart and cardiac vessels is showing an abnormal presence of gas or air. They can sometimes be requested when RT-PCR has come negative or the person has subsequently tested negative, but continues to show symptoms of breathing distress.
CT scans can also be useful to diagnose Mucormycosis or black fungus, the infection that is taking hold in India, thanks to the combination of widespread steroid usage and diabetes.
How much radiation is dangerous?
There is normally a wide range of variation in radiation doses for similar scan types and techniques.
When it comes to dangers, the risk of development of future incidence of cancer is known to increase with increased exposure to ionising radiation. Ionizing radiation is one where subatomic particles or electromagnetic waves have enough energy to gain and lose electrons or charge. High energy radiation like gamma rays, X-rays, and UV rays are highly ionising, while long wavelengths like radio waves, microwaves, infrared, and visible light are non-ionising.
It is currently thought that the risk of cancer increases linearly with an effective radiation dose of 5.5% per sievert.
A typical X-ray of a broken bone could involve radiation dose ranging from 0.01 to 0.15 mGy. A typical CT scan, however, can involve doses of 10–20 mGy for specific organs, and can go up to a high of even 80-100 mGy for certain specialized scans.
The world population’s average dose exposure to radiation — resulting from nuclear tests and other lingering sources — is thought to be equivalent to 2.4 mGy per year.
In general, it is thought that a radiation dose from a CT scan of the brain is equivalent to background radiation exposure of 7 months, a CT scan of the chest to background exposure of 2 years, abdomen to background exposure of 2.6 years, of the spine and heart to background exposure of 3 years.
Often, radiation doses are compared over non-equivalent scales, such as comparing the lowest dose of chest X-ray technique with the highest dose of chest CT techniques.
Recently, Dr Randeep Guleria, Director, AIIMS sounded a warning against CT scans, stating that the radiation dose from one CT scan is equivalent to 300-400 chest X-rays. His comparison has since been refuted by the Indian Radiological and Imaging Association (IRIA), which has clarified that most modern scanners use an ultra dose CT, following the ALARA (As Low As Reasonably Achievable) principle.
However, it is unclear how many facilities around the country are currently equipped with modern low dose CT scanners.
In radiation research, early estimates of the level of harm caused by radiation exposure are based on single instantaneous fatal exposures experienced during atomic bomb explosions and exposures experienced by those in the nuclear industry. As a result, there is a considerable contention in findings in the scientific community and controversies about accuracy of studies and the exact levels of risk that CT exposure poses.
Some studies have found that the risk of cancer rises with use of medical imaging techniques, while other studies have in turn indicated that such studies are often not rigorous enough to establish risk to the levels they claim they do.
However, while the extent to which the risk of cancer is raised might be unknown to an accurate degree and is thought to generally be exaggerated with medical imagery, all experts agree that caution must be exercised by physicians in CT use and unnecessary tests of any type should be avoided.
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