What are the international guidelines around radiation exposure? What is the situation in India?
| Raveendran Gandhi |
The guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to acceptable” levels. Exposure limits are defined by The International Commission on Radiological Protection (ICRP) and other such groups and regulators worldwide maintain that they should be at “as low as reasonable achievable” (ALARA) especially for diagnostic and radiation medicine. The guidelines for US and other countries established by competent government regulatory authorities are based on the ALARA principle and are thoroughly followed and implemented in routine clinical practice. For example, the radiation dose limits associated with any diagnostic examinations are specified with declared upper limits and often known to the dose recipient.
Atomic Energy Regulatory Board (AERB), being the apex regulatory control body of all X-ray emitting equipment and its operational facilities in India has laid down clear guidelines for radiation safety. Individual guidelines for radiation medicines include definitions of radiation production, storage, transportation, operation and use and finally the disposal of radiation wastes. For diagnostic devices, it covers the operational facility, exposure measurements and monitoring features for safety of patient, equipment operators, employees and the public.
Though there are clear guidelines, ensuring adherence to these guidelines is often a challenge in India. AERB has taken some very proactive steps recently, including decentralising its regulation to the state governments and union territories to set up its own Directorate of Radiation Safety (DRS) as well as sign MoUs with Maharashtra, Odisha and other states to ensure its clear implementation.
What devices usually expose patients to radiation in order of high to low exposure? How can this be avoided? What inbuilt mechanisms in the device can prevent this?
We are aware that medical devices ranging from X-ray machines, CT scans, cathlabs, bone mineral density scans, OPGs, etc. are all using ionising radiation for diagnosing diseases. Usually a plain X-ray examination or BMD test uses low dose of radiation as compared to a CT scanner or a cathlab machine. The amount of radiation exposed to patient from equipment varies depending on several factors such as the type of investigation, age, sex and number of examination etc. A plain X-ray without a dye injection may not generate significant radiation dose (approximately 0.02 millisievert (mSv) to a patient. When compared that with natural background radiation dose of 1-3 mSv per year ,this dose is surely insignificant, however when we compare it with a coloured X-ray, such as Barium enema, the dose may go up significantly higher up in the range of 5-7 mSv. Similarly, the amount of dose delivered in CT examinations also varies.
Is there a permissible level of radiation exposure? What can be some of the risks of radiation exposure? Are women at a higher risk as compared to men?
The Atomic Energy (Radiation Protection) Rules have prescribed the dose limits for exposures from ionising radiations for workers and the members of the public, which shall be adhered to range from 1mSv a year for the normal public to 20 mSv a year for workers and 6 mSv for apprentices and trainees.
Inferences from radiation studies suggest that up to 10 per cent of invasive cancers are related to radiation exposure, including both ionising and non-ionising radiations. Exposure is known to increase the future incidence of cancer, particularly leukaemia. In developed countries, medical imaging contributes almost as much radiation dose to the public as natural radiation. Radiation can also cause changes in DNA and patients receiving radiation treatment experience acute effects of nausea, vomiting or even burns. Pregnant women are specifically told not to get themselves exposed to radiation as it may harm the foetus and may lead to congenital problems.
What is the awareness level in India amongst physicians about radiation exposure?
There is a varied knowledge on radiation exposure but overall the awareness is perceived as low, across the globe and specifically in India. This can be attributed to lack of adequate training given to the staff, specialists and physicians. It is generally perceived that many doctors across the world underestimate the lifetime risk of fatal cancer and many of them have never had the opportunities for any formal training on risks to patients from radiation exposure. However, there are several recent clinical studies on radiation effects, (especially on children) pointing out the increasing the risk of cancer. This has created lot of curiosity among the physicians and will serve to further increase the awareness among the physicians and public.
How are equipment suppliers such as yourself working to lower the radiation exposure through improved software programmes and better devices?
Every radiation emitting device contains dose limiting and dose optimisation features. In an X-ray machine it may be filters which eliminate unwanted soft X-rays, radiation field limiting diaphragms or a collimator. CT scanners have highly evolved dose optimisation features ranging from filters, collimators, cone-beam diaphragms, highly sensitive detectors and iterative reconstruction features.
At Philips, we have imbibed the ALARA principle in its true spirit while designing our low dose diagnostic machines designed with extensive research. Our product iDose4 was recognised as the ‘Best New Radiology Software’ during RSNA 2012. It is an iterative based reconstruction technology recently introduced with Philips CT scanners that helps reduce radiation dose up to 80 per cent without compromising on the quality of images delivered. We have shipped a thousand of these within a short span of their inception in the market. In addition to this, for each diagnostic test, a patient undergoes in a Philips CT scanner, it automatically generates a report of the radiation he/she receives for the test. The cumulative summary of such results are useful while prescribing patients who require multiple follow up CT scans.
“Dose-Aware” is a radiation exposure monitoring device we developed for cathlab operators that provides live feed about total exposure the operator received during a cathlab procedure and helps avoid unnecessary exposure to radiation. Our unique Time of Flight (TOF) technology available with PET/CT scanners reduces radiation dose to less than 50 per cent using less than 5 mSV FDG (Radioisotopes injected to a patient prior to a PET/CT scan) where as a conventional PET/CT might require around 10mSV of FDG injection.
How do you ensure that reducing radiation exposure in the CT scanner does not affect its image quality? How can it be retained?
Filtered back projection (FBP) has been the industry standard for CT image reconstruction for decades. While it is a very fast and fairly robust method, FBP is a sub-optimal algorithm choice for poorly sampled data or for cases where noise overwhelms the image signal. Such situations may occur in low dose or tube power limited acquisitions (Scans of morbidly obese individuals). Noise in CT projection data is dominated by photon count statistics. As the dose is lowered, the variance in the photon count statistics increases disproportionately. When these very high levels of noise are propagated through the reconstruction algorithm, the result is an image with significant artefacts and high quantum mottle noise.
Our new technology, iDose4, uses iterative processing in both the projection and image domains. The reconstruction algorithm starts with projection data where it identifies and corrects the noisiest CT measurements (those with very poor signal- to- noise ratio, or very low photon counts). Each projection is examined for points that have likely resulted from very noisy measurements using a model that includes the true photons statistics. Through an iterative diffusion process, the noisy data is penalised and edges preserved while ensuring that the gradients of underlying structures are retained, thus preserving spatial resolution while allowing a significant noise reduction.
The noise that remains after this stage of algorithm is propagated to the Image Space, however the propagated noise is now highly localised and can be effectively removed to support desired level of dose reduction. Then the iDose4 algorithm deals with subtraction of the image noise while preserving the underlying edges associated with true anatomy or pathology.