Sizing of airborne particles in an operating room

Abstract

Medical procedures that produce aerosolized particles are under great scrutiny due to the recent concerns surrounding the COVID-19 virus and increased risk for nosocomial infections. For example, thoracostomies, tracheotomies and intubations/extubations produce aerosols that can linger in the air. The lingering time is dependent on particle size where, e.g., 500 μm (0.5 mm) particles may quickly fall to the floor, while 1 μm particles may float for extended lengths of time. Here, a method is presented to characterize the size of <40 μm to >600 μm particles resulting from surgery in an operating room (OR). The particles are measured in-situ (next to a patient on an operating table) through a 75mm aperture in a ∼400 mm rectangular enclosure with minimal flow restriction. The particles and gasses exiting a patient are vented through an enclosed laser sheet while a camera captures images of the side-scattered light from the entrained particles. A similar optical configuration was described by Anfinrud et al.; however, we present here an extended method which provides a calibration method for determining particle size. The use of a laser sheet with side-scattered light provides a large FOV and bright image of the particles; however, the particle image dilation caused by scattering does not allow direct measurement of particle size. The calibration routine presented here is accomplished by measuring fixed particle distribution ranges with a calibrated shadow imaging system and mapping these measurements to the in-situ imaging system. The technique used for generating and measuring these particles is described. The result is a three-part process where 1) particles of varying sizes are produced and measured using a calibrated, high-resolution shadow imaging method, 2) the same particle generators are measured with the in-situ imaging system, and 3) a correlation mapping is made between the (dilated) laser image size and the measured particle size. Additionally, experimental and operational details of the imaging system are described such as requirements for the enclosure volume, light management, air filtration and control of various laser reflections. Details related to the OR environment and requirements for achieving close proximity to a patient are discussed as well.

Fig 1

Particle Generator with 0.56 mm orifice nozzle attached and A) 0.74 mm, B) 0.46 mm and C) 1.00 mm orifice nozzles shown.

Fig 2

Schematic of the experimental set up from the plan view (top) and isometric view (bottom) showing the placement of A) high-intensity light, B) particle generator, C) high speed camera D) particle shield and E) generated particles.

Fig 3

Experimental set up showing A) camera lens, B) insertion reference line on particle shield, C) 3 mm aperture in particle shield, D) 7.9 x 7.1 mm field of view and E) steel ruler.

Fig 4. Example image of the particles from 0.74 mm nozzle; 10,000 fps, 23 μs exposure.

Fig 5

Experimental set up including A) Particle Shield, B) 0.030mm Particle Generator, C) Particle Shield Aluminum Bracket and D) High-Speed Camera.

Fig 6. Histogram of particles counts in high speed shadow method by size.

Fig 7

OR system in left orientation with A) laser beam traps, B) circular intake aperture, C) dark chamber, and D) laser.

Fig 8

Rear of enclosure showing A) fan and filter, B)tablet with support frame, C) left and right laser beam traps and D) dark chamber.

Fig 9. Overhead schematic of OR system in left thorocotomy /thoracastomy orientation.

Fig 10

Right side of enclosure with A) filter and fan, and B)entrance aperture.

Fig 11. Single image frame from video of 0.46 mm particle generator experiment showing scattered laser light from particles.

Fig 12. Histogram of particles parsed into ten ranges for improved match.