Dr Jeffrey Siewerdsen, Professor of Biomedical Engineering at Johns Hopkins University, tells HEQ about UVC disinfection.
Dr Jeffrey Siewerdsen, interview by Rosemary Lobley
A research team at Johns Hopkins University has successfully devised a means of disinfecting the inner bores of CT scanners using UVC radiation. CT gantries are a particular vector for infection due to a combination of their exposure to particles exhaled by patients and the physical difficulty of cleaning them. In their study, titled ‘Ultraviolet germicidal irradiation of the inner bore of a CT gantry’ and published in the Journal of Applied Clinical Medical Physics, Professor of Biomedical Engineering Dr Jeffrey Siewerdsen and Professor of Radiology and Radiological Science Dr Mahadevappa Mahesh detail their use of UVC lamps to inactivate 99.9999% of SARS-CoV-2 virus particles within three to five minutes.
HEQ speaks to Dr Siewerdsen about the results of the study and the future of the team’s research.
How does UVC disinfection work? What key advantages does it offer in comparison to other common disinfection methods?
The significant action of UVC radiation is that it damages single-stranded RNA. Over the last 20 years or more, there has been a substantial amount of scientific research measuring very precisely the efficiency of ultraviolet light in eliminating single-stranded RNA and determining what dose levels are needed to achieve a certain survival fraction for individual viruses and pathogens.
In addition to the potential of leaving behind chemical residue, manual cleaning using chemicals entails the risk of exposure for the person performing the cleaning. Of course, hospital cleaners have the appropriate personal protective equipment (PPE) to handle toxic or caustic chemicals where necessary, but they are still in close physical proximity to the chemicals and they are in contact with the surfaces they are cleaning.
Another key issue with manual chemical cleaning is the time it requires to perform. If a manual wipe down must be performed on a CT scanner every time it has been used, that is time-consuming and can lead to bottlenecks in using the scanner. Housekeeping or radiology staff in that environment performing a full manual clean of a CT scanner can easily take upwards of 30 minutes, just to wipe it down – that is a pretty big disadvantage. Additionally, manual cleaning processes can lead to gaps in quality, in that what one person might consider a thorough wipe down is going to differ from somebody else. Genuinely consistent disinfection can be difficult.
How can CT technologists and cleaning staff benefit from this kind of disinfection of CT gantries? Could there be ergonomic benefits to reducing the burden of manual cleaning, in addition to the less time-consuming process and reduced risk?
Our study focused specifically on disinfecting the inner bore or the CT gantry, which is most likely to be directly exposed to infection, because that is where the patient’s head is during a scan and they are breathing up into the bore, so it is a very important area to disinfect. Meanwhile, however, it is a particularly difficult area for housekeeping staff or radiology staff to actually reach. It is reasonable to assume that the posture associated with manually wiping down the entire bore of the CT gantry is going to be uncomfortable and not ergonomically sound. When spaces and surfaces are complex or difficult to reach, as CT gantry bores are, the thoroughness of manual cleaning is going to vary even more – not just from one person to the next, but from the beginning of the day to the end of the day. It is one of the most important areas in terms of needing to be fully and comprehensively disinfected, but it is also one of the more difficult areas to reach. A system like ours can just be plugged into the headboard and is able to reach that area automatically and consistently, potentially taking just a few minutes to deploy the necessary radiation.
Of course, there are more surfaces that still need to be wiped down: our study really only focused on the results of one lamp inside the bore – we do not address the disinfection of the bed itself, for instance, and there is a different procedure for that. Tackling the inner bore of the gantry seemed to us to be particularly important, due to the combination of it being at high risk of exposure to infection and difficult to physically disinfect.
Did you observe any significant limitations or challenges associated with this disinfection?
The quality of UV lamps which are used in disinfection processes is still something that needs to be more fully investigated. We saw that the lamp we were using took a bit of a time to warm up; it did not come to a stable level of irradiation for up to two minutes. During those first two minutes the irradiance of the lamp is gradually increasing, and then it finally reaches a stable level once the lamp is warm; so we have factored that into our total time estimate, working on the assumption that the lamp will be run from a cold start.
Another factor about which we do not know very much is whether the output of these lamps will be stable over longer periods of time. A hospital may need to perform regular quality checks of the lamps to make sure that they are putting out a certain, specified level of irradiance, over the course of weeks or months. Finally, we also do not know the average lifetime of the lamps – I have had mine for about six months now and it is still running, but of course I haven’t been using it 20 or 30 times a day, in the way that would be needed in a clinical setting – and so we do not know how expensive they would be to replace. Those are all questions that need to be investigated further.
There are other questions that we raised in the study, but which would need examining more closely, such as the possibility of crevices within the CT scanner which may be shaded from UV radiation. We did not see any particularly shadowy areas in there, but that may vary from one CT scanner design to another. Another question which came up a lot is whether the ultraviolet light would alter or discolour the plastic cover of the scanner – we didn’t see any evidence of this, but we did not run the lamp for extended periods and we did not conduct a rigorous test to see if the ultraviolet was affecting the quality of the plastic itself. We don’t have any evidence of that, but we did look at it. None of those are challenges as such, but I do think they are all legitimate questions which will need to be addressed if a system like ours is going to scale up and be used routinely.
Your study mentions the possibility of adapting this technology to disinfect MRI scanners. What are some other potential applications for UVC?
That is an interesting question for the future. In its current form, our system is not MRI compatible – you would not want to bring our ultraviolet lamp setup into an MRI scanner; that definitely would not be safe. Within the context of CT, however, there is broader potential – CT is a pretty prevalent and relatively low-cost modality, which a lot of countries have adopted pretty aggressively in their screening programmes at a time where they did not have effective rapid testing. Certainly, in those contexts, that would be where a system like this is going to have the most benefit.
In the future, this type of UVC disinfection could be particularly beneficial in the cases of patients who have tested positive for COVID-19 or other infectious diseases, in fields such as diagnostic radiology, for example in identifying a stroke or potentially in other areas of emergency medicine. We have some ideas around developing an MRI-compatible ultraviolet disinfection solution. We have not yet put them into practice, but we have some theories about how to deliver ultraviolet light in such a way that would safely disinfect an MRI scanner bore. That would be great, because MRI scanner bores tend to be even deeper and narrower, which means it is even harder to reach in there to clean them thoroughly. And, if you did have a COVID-19 positive patient in the MRI scanner, because the bore tends to be smaller and the scan times are longer, the patient would be in there for a longer time than in a CT scanner. That means that the risk of contaminating the surface, or being contaminated by the surface, is even greater in the case of MRIs. That is a really interesting area of possible future work.
Another point of interest is the broader application of ultraviolet irradiation, to disinfect an entire room. There are commercial systems capable of doing it: some firms produce UVC disinfection robots, almost like a Roomba with an ultraviolet light on it; and then there are other systems which are designed to irradiate a room more generally. While those systems may be doing a great job irradiating whole rooms and ventilation systems, it is very feasible that they may not be as effective inside narrow or textured spaces, where there could be areas which are shaded from the larger light towers. The inner bore of a CT scanner is a very specific size and it’s a very specific shape; if you were using these ultraviolet towers already in a room, then our solution might be a good addition to that, in order to make sure that this really critical area inside the bore is thoroughly disinfected.
When the pandemic hit, everyone really answered the call and thought about what they were good at and how they might be able to help. My colleague Mahesh is a clinical physicist and radiologist, so he was looking at how the imaging systems could be used safely and effectively. I am a physicist working in biomedical engineering, so I knew I would not be able to come up with a new test or vaccine or something like that. As engineers, we were wondering if there were mechanical systems like a UV lamp which could actually have some benefit.
It came in part from some activity with biomedical engineering students at Johns Hopkins: I teach a project where engineering students tackle various clinical challenges, and the pandemic hit halfway through the spring semester; and so several of those project teams, which had been working on other things, decided that they wanted to work on COVID-19. A couple of them were looking at various potential uses for ultraviolet light; one of the teams explored the possibility of using ultraviolet light to disinfect N-95 masks, and they came up with a way to disinfect small quantities of masks very quickly. It’s just a box where you could put your N-95 mask at the beginning or the end of your shift, and then one minute later it’s ready to go. The students got really excited to be able to actually answer the call, which was great.
By the summertime, the project course was over and the students had moved on, although some of their ideas kept going. But Mahesh and I kept talking about the principle of using UV radiation to decontaminate CT scanners and MRI scanners – and in part, our project really was inspired by the energy and activity of those students who used ultraviolet radiation to resolve these other problems. I think that those engineering teams are really awesome and they deserve some credit for the inspiration of what we did.
Dr Jeffrey Siewerdsen
Professor of Biomedical Engineering
Johns Hopkins University School of Medicine