Aerosol deposition in the human upper airways

Application Challenge AC7-01 © copyright ERCOFTAC 2019

Test Data

Overview of Tests

Major portions of this section were adopted from Lizal et al. (2015). The positron emission tomography (PET) method provides the best spatial resolution (among radiological methods). In addition to local deposition in the various sections, the deposition hot spots can also be evaluated. However, in comparison to the PET methodology, which is routinely applied to clinical examination, using this method in the in vitro design requires major modiﬁcations both in the aerosol preparation and, in particular, in the experiment evaluation approach. The method, based on PET and fulﬁlling the above mentioned criteria, is presented in the following.

The aerosol exposure procedure

It is a common practice to coat the inner surface of the model, especially when using solid particles, to prevent bouncing of the particles hitting the surface. Since we used liquid di-ethylhexyl sebacate (DEHS) particles, we did not need to coat the inner surface of the model. Another reason for the coating is to prevent surface wetting. In our case the exposure time was short (5 to 15 mins) and only small amounts of DEHS deposited on the walls, therefore the possible ﬂooding of the surface was not an issue. Aerosol particles were generated by a TSI 3475 Condensation Monodisperse Aerosol Generator (CMAG) from TSI, Inc., which works on the controlled heterogeneous condensation principle. Vapours of a suitable material, speciﬁcally DEHS, condense by a controlled method on small sodium chloride particles serving as the condensation nuclei. The advantage of DEHS is that it is not hydrophilic and does not evaporate, resulting in a constant size of generated particles. In a standard operating mode, the generator can produce particles with aerodynamic diameters within the 0.1 to 8μm range. The density of DEHS used for the experiments was 0.914 g/cm³ at 25°C. Radioactive aerosol particles were needed for the PET measurement of deposition. Therefore, the solution in the atomizer of the generator had to be tagged by a suitable radioactive substance. Fluorine 18 was the logical Choice of the positron emitter, being easily available at the cooperating PET center and possessing a suitable half-life (109 minutes). A solution of ﬂuorine 18 in the form of ﬂuoride ions was prepared by irradiation of H₂¹⁸0 enriched water on an IBA Cyclone 18/9 cyclotron (irradiation time 25 min, integrated irradiation current 11 μAh) at the UJV Rez's PET Centre in Brno. The irradiated water was transferred by a capillary transport system to a shielded dispensing box, where the ﬂuorine 18 ions were captured on an ion—exchange resin (AG1—X8, BioRad) column and subsequently eluted with 300 ml of 10% sodium Chloride solution, followed by 1Â ml of water for injection. The resulting solution was repeatedly diluted with water for injection until the desired initial radioactivity was achieved. The CMAG was modiﬁed for the deposition measurement by using PET so that the atomizer vessel was accommodated in a protective lead container to shield of ionizing radiation. The atomizer was ﬁlled with a sodium Chloride solution containing ¹⁸F at an initial activity of 2.5 GBq. The concentration of sodium Chloride solution was 20 mg/L. The experimental rig is shown in Figure 5.

Figure 5: A scheme of the experimental setup during the PET measurement of aerosol deposition.

The generated aerosol was fed through a ⁸⁵Kr based NEKR,—10 charge equilibrator (Eckert & Ziegler Cesio) to a PAM aerosol monitor (TSI 3375) for continuous particle size and concentration measurement. The operation and precision of the aerosol monitor was validated using Aerodynamic particle sizer (APS) TSI 3321. The validation was performed prior to the experiment using the identical setup, apart from adding of the radioactive substance into the atomizer. The size of the particles generated by CMAG was adjusted according to APS and the size displayed by the aerosol monitor was recorded. Subsequently, during the experiment, only the radioactive substance was added to the atomizer and particles of the same size were produced. Usually only a small correction of saturator ﬂow was needed at the beginning of the experiments. Only one size of particles was measured during one day; therefore no further adjustments of the generator were needed. The aerosol monitor served as an on-line indication of the process of aerosol generation being stable. It was easily accomplished, as the exposure of the model to the aerosol lasted only 5 to 15 mins. Filters consisting of Millipore AP40 glass ﬁbers were attached to the output branches of the respiratory tract model. The entire system (Figure 6) was enclosed in a plastic bag which was kept in a vacuum to prevent the active aerosol from leaking into the laboratory. All the 10 terminal branches with ﬂow meters for ﬂow rate control were combined into one branch with a protective High efﬁciency particulate air (HEPA) ﬁlter. The vacuum was generated with a Busch R, 5 PA 0008 C rotary oil vacuum pump. The ﬂow distribution in each section of the model is provided in Table 3.

Figure 6: A photograph of the physical model prior to the PET measurement.

The whole exposure of the model to the aerosol was performed in a shielded laboratory with an underpressure ventilating system, which would prevent the aerosol from escaping the room in case of a primary safety system failure. The laboratory personnel were not present in the laboratory during the exposure, with the exception of the regular instrument supervision. Whenever they had to enter the lab, they wore half mask respirators. The experiments were performed in a steady—state inhalation mode with the ﬂow rates of 15, 30, and 60 L/min. Liquid monodisperse particles with mass median aerodynamic diameter of 2.5 and 4.3μm were used. The standard geometric deviation of size was less than 1.24 for all measured regimes. The models were exposed for 10 to 15 minutes depending on radioactivity decrease by radionuclide decay. The peak activity in the models was 4 to 60 Bq/cm² (depending on the particle size and concentration, measuring mode, and model used), as measured with an RP-2000 portable contamination meter (VF Zilina, CZ).

Deposited activity evaluation method

The model was transported to a Siemens Biograph 64 Truepoint PET-CT scanner immediately after the radioactive exposure. The transportation took approximately 3 minutes. The PET-CT scanner acquired ﬁrstly CT images, which were promptly followed by PET images. Both the CT and PET images could be attributed unambiguously to the given geometry, owing to the assigned geometrical coordinates. The CT images are important for a precise localization of the edges of the sections, whereas the PET images contain the essential information about aerosol deposition. The CT and PET images were imported into Carimas 2.47 ver. 2.2.44.7002 SW (Turku PET Centre 2012) with the CT images as the main images and the PET images as background images. The software Carimas is a medical image processing tool, which was developed primarily for analysis of PET images at Turku PET Centre in Finland and is available as a freeware for non-commercial use from: http://www.turkupetcentre.ﬁ/carimas/download/. The Carimas was developed in an “interactive data language” (IDL) and runs on IDL Virtual Machine^TM, a cross-platform utility for running IDL code. It supports multiple input formats (DICOM, ECAT, Analyse, Interﬁle, Nifti, Interﬁle, MicroPET) and general bitmap formats (JPG, TIFF, PNG and BMP). Researchers can perform visualization, segmentation, statistical analysis, or modelling of PET data. It is possible to perform the image fusion, i.e. to coregistrate PET and CT or MR, images. Users can deﬁne a volume of interest (VOI) and, by using the static image analysis, they can calculate the mean activity in the VOI (in units of the PET image), standard deviation, minimal and maximal values, the number of voxels analysed, and the volume of the region. The software was tested and compared to commercial tools with an excellent agreement as documented by Nesterov et al. (2012) and Harms et al. (2014). To facilitate the identiﬁcation of the hot spots in the Carimas software, the BGRY colour system was used to display the PET images and the system range was reduced so that the hot spots were clearly seen in the model sections (Figure 7). The sections of the model were marked as volumes of interest (VOI), and the mean volume activity (in Bq/mL) was evaluated using the Carimas software. Each section was enveloped in an independent VOI. A cylinder or a sphere was selected as the starting shape of the VOI, depending on the shape of the sections. Firstly, the starting shape of the VOI was positioned and shaped as a whole to attain a suitable orientation and an adequate size. In the next step, each VOI was shaped in the vertex mode for a local mesh adjustment, so that the VOI surrounded the section with an overreach, while preventing an overlap of different VOIs. If the latter was not avoided, the observed activity would be attributed to both VOIs, resulting in biased results. The degree of mesh density can be modiﬁed by reducing the number of nodes or by enlarging or shrinking the mesh.

Figure 7: CT and PET image presentation in the Carimas software.

With regard to the PET precision, a VOI should optimally overreach the section by roughly 5mm on all sides to cover all the radiation emitted from the section. However, due to the complex lung geometry, it was impossible to form all the VOIs with such a large overreach, and so the uncounted radioactivity was accounted for in a correction VOI. Two additional VOIs had to be created to determine the magnitude of the correction: a VOI for the top part of the box, accommodating the model with the hoses, and a correction VOI for the bottom part of the box, accommodating the ﬁlters downstream of the terminal branches. They encompass the large top space with the model and the bottom space between the two horizontal partitions respectively in Figure 6. Two separate corrections were needed for the model and ﬁlters, because the difference in volume radioactivity is within few orders of magnitude and therefore it is necessary to assign the uncounted radioactivity to its real source. It is essential to preserve the true ratio of the radiation deposited in model sections and on ﬁlters. The correction factor for the sections was calculated as the ratio of the total activity obtained from the VOI encompassing the whole model to the activity obtained by summing up the section VOIs. Similarly, the correction factor for the ﬁlters was calculated as the ratio of the total activity of the large VOI encompassing all the ﬁlters, to the sum of the activities of the ﬁlter VOIs. The corrections for the model sections ( ${Corr_{m}}$ ) and for the ﬁlters ( ${Corr_{f}}$ ) were calculated as follows:

${Corr_{m}={\frac {A_{cm}}{\sum _{1}^{n}A_{i}}}}$

${Corr_{f}={\frac {A_{cf}}{\sum _{1}^{n}A_{j}}}}$

${Corr_{m}={\frac {A_{cm}}{\sum _{1}^{n}A_{i}}}}$	width=50 align=right ${(1)}$
${Corr_{f}={\frac {A_{cf}}{\sum _{1}^{n}A_{j}}}}$	${(2)}$

where ${A_{i}}$ and ${A_{j}}$ are the activities measured in the model sections and on the ﬁlters respectively, and ${A_{cm}}$ and ${A_{cf}}$ are the activities measured in the correction VOI for the model and ﬁlters respectively. We assume that the distribution of uncounted radioactivity is proportional to the measured activity in the sections of the model. The higher the activity measured in the ﬁlter, the higher the activity spread around the section.