Ultraviolet germicidal irradiation of the inner bore of a CT gantry

Abstract Purpose To investigate the feasibility and practicality of ultraviolet (UV) germicidal irradiation of the inner bore of a computed tomography (CT) gantry as a means of viral decontamination. Method A UV lamp (PADNUT 38 W, 253 nm UV‐C light tube) and UV‐C dosimeter (GENERAL UV‐C Digital Light Meter No. UV512C) were used to measure irradiance throughout the inner bore of a CT scanner gantry. Irradiance (units μW/cm2) was related to the time required to achieve 6‐log viral kill (10−6 survival fraction). Results A warm‐up time of ~120 s was required for the lamp to reach stable irradiance. Irradiance at the scan plane (z = 0 cm) of the CT scanner was 580.9 μW/cm2, reducing to ~350 μW/cm2 at z = ±20 cm toward the front or back of the gantry. The angular distribution of irradiation was uniform within 10% coefficient of variation. A conservative estimate suggests at least 6‐log kill (survival fraction ≤ 10−6) of viral RNA within ±20 cm of the scan plane with an irradiation time of 120 s from cold start. More conservatively, running the lamp for 180 s (3 min) or 300 s (5 min) from cold start is estimated to yield survival fraction <<10−7 survival fraction within ±20 cm of the scan plane. Conclusion Ultraviolet irradiation of the inner bore of the CT gantry can be achieved with a simple UV‐C lamp attached to the CT couch. Such practice could augment manual wipe‐down procedures, improve safety for CT technologists or housekeeping staff, and could potentially reduce turnover time between scanning sessions.

scanner) presents potentially increased viral exposure to CT scanner technologists and/or cleaning staff. Ultraviolet (UV) light (viz., 254 nm ultraviolet-C, UV-C) has demonstrated utility in germicidal irradiation of surfaces in healthcare settings. [1][2][3][4] It has also been demonstrated of value in decontamination of personal protective equipment (PPE) in relation to coronavirus and other viral respiratory syndromes. [5][6][7][8] We report the potential utility of ultraviolet (UV-C) germicidal irradiation to augment the cleaning of CT scanners, specifically addressing the task of disinfecting the inner bore of the CT gantry using a UV lamp suspended from the head of the CT couch. Since cleaning the inner bore is an important but cumbersome, non-ergonomic aspect of disinfecting the scanner, such capability could ease the burden on the CT technologists or the cleaning staff and help to reduce turnover time.

| MATERIALS AND METHOD S
The ultraviolet lamp used in this work was a PADNUT UV-C cleaning light tube for air purification and room/surface cleaning. The lamp has a power of 38 W with a pair of 36 cm length light tubes specified to emit 253 nm UV-C light. As shown in Fig. 1(a), the lamp was suspended horizontally by securing its base in a carbon fiber head holder with Velcro straps at the isocenter of a Definition Flash CT scanner (Siemens Healthineers, Forchheim Germany). The long axis of the lamp was aligned with the longitudinal (z) axis of the gantry, and the midpoint of the lamp was placed at the scan plane (z = 0 cm). The lamp was operated via a remote control, allowing the operators to stand clear of the area when the lamp was on.
The UV-C dosimeter used in this work was a GENERAL UV-C Digital Light Meter (No. UV512C) with a specified sensitivity range of 220 to 275 nm and calibration point 254 nm corresponding to the wavelength of UV-C. The dosimeter was operated in the "low" illumination range (specified as 1-9999 μW/cm 2 ) with specified accuracy at room temperature of AE4%.
The sensor was placed on the inner bore of the CT scanner gantry at 4 cardinal positions as shown in Fig. 1(b): at the scan plane (denoted z = 0 cm); at z = −20 cm toward the "front" of the gantry (i.e., toward the CT table); and at z = +20 cm toward the "back" of the gantry. The inner gantry diameter was 80 cm, so the nominal distance from the lamp to the sensor was 40 cm.
The warm-up time of the lamp was first assessed by measuring the irradiance (units of micro-Watt per centimeter squared, μW/cm 2 ) as a function of time repeatedly from cold start. Then, the irradiance was measured at each of the sensor positions noted above, recording measurements at 30 s intervals up to 180 s.
As reported by Tseng and Li, 9 the dose (i.e., fluence, φ, with units milli-Joules per centimeter squared, mJ/cm 2 ) required to achieve 6log viral kill (10 −6 survival fraction) is 30 mJ/cm 2 . This dose target value represents a nominal value above which survival fraction is expected to be <10 −6 for viruses with single-stranded RNA. The measured irradiance (E e ) was related to the time (t) required for 6log viral kill as (φ=E e ), noted with explicit units as follows: For example, an irradiance of E e = 500 μW/cm 2 provides 6-log kill in t = (30 × 1000/500) = 60 s (1 min).

| RESULTS
The lamp exhibited a considerable warm-up time as exhibited in This work should not be interpreted as recommending UV-C irradiation as replacement to manual wipe-down with disinfectant; rather, the setup reported here could augment such procedures. Because manual wipe-down cleaning within the bore of the gantry may be difficult and non-ergonomic, it is prone to incomplete manual disinfection. Augmenting such procedures with UV-C irradiation could ensure more thorough and consistent decontamination while easing the burden on the CT technologists or the cleaning staff.
Because the results suggest sufficient levels of germicidal dose within minutes, the procedure could potentially reduce the turnover time between scan sessions. The full workflow of manual wipe-down plus UV-C decontamination is the subject of future work. Given the ease of use and fairly rapid irradiation times, one may envision an increasing role of UV-C germicidal irradiation within medical imaging hygiene protocols in the future, or even scanners with capability for germicidal irradiation built into the scanner. Similarly, MR scanners present an even more challenging wipe-down procedure to the technologists or cleaning staff and use of UV-C germicidal irradiation with MR compatible UV-C lamps is being considered for future work.