Evaluation of the accuracy and precision of lung aerosol deposition measurements from planar radionuclide imaging using simulation

In Vivo Deposition of High-Flow Nasal Aerosols Using Breath-Enhanced Nebulization

Aerosol delivery using conventional nebulizers with fixed maximal output rates is limited and unpredictable under high-flow conditions. This study measured regulated aerosol delivery to the lungs of normal volunteers using a nebulizer designed to overcome the limitations of HFNC therapy (i-AIRE (InspiRx, Inc., Somerset, NJ, USA)). This breath-enhanced jet nebulizer, in series with the high-flow catheter, utilizes the high flow to increase aerosol output beyond those of conventional devices. Nine normal subjects breathing tidally via the nose received humidified air at 60 L/min. The nebulizer was connected to the HFNC system upstream to the humidifier and received radio-labeled saline as a marker for drug delivery (99mTc DTPA) infused by a syringe pump (mCi/min). The dose to the subject was regulated at 12, 20 and 50 mL/h. Rates of aerosol deposition in the lungs (µCi/min) were measured via a gamma camera for each infusion rate and converted to µg NaCl/min. The deposition rate, as expressed as µg of NaCl/min, was closely related to the infusion rate: 7.84 ± 3.2 at 12 mL/h, 43.0 ± 12 at 20 mL/h and 136 ± 45 at 50 mL/h. The deposition efficiency ranged from 0.44 to 1.82% of infused saline, with 6% deposited in the nose. A regional analysis indicated peripheral deposition of aerosol (central/peripheral ratio 0.99 ± 0.27). The data were independent of breathing frequency. Breath-enhanced nebulization via HFNC reliably delivered aerosol to the lungs at the highest nasal airflows. The rate of delivery was controlled simply by regulating the infusion rate, indicating that lung deposition in the critically ill can be titrated clinically at the bedside.

Anatomically Based Analysis of Radioaerosol Distribution in Pulmonary Scintigraphy: A Feasibility Study in Asthmatics

INTRODUCTION

Manual analysis of two-dimensional (2D) scintigraphy to evaluate aerosol deposition is usually subjective and has reduced sensitivity to quantify regional differences between central and distal airways.

AIMS

(1) To present a method to analyze 2D scans based on three-dimensional (3D)-linked anatomically consistent regions of interest (ROIs); (2) to evaluate peripheral-to-central counts ratio (P/C_2D) and penetration indices (PIs) for a set of 16 subjects with moderate-to-severe asthma; and (3) to compare the reproducibility of this method against one with manually traced ROIs.

METHODS

Two-dimensional scans were analyzed using custom software that scaled onto 2D-projections' 3D anatomical features, obtained from population-averaged computed tomography (CT) chest scans. ROIs for a rectangular box (bROI) and an anatomically shaped ROI (aROI) were defined by computer and by manually tracing the standard rectangular box (manual ROI [mROI]). These ROIs were defined five nonconsecutive times for each scan and average value and variability of the P/C_2D were estimated. Based on CT estimates of lung and airways, volumes lying under the bROI and aROI, a 2D penetration index (PI_2D) and a 3D penetration index (PI_3D), were defined as volume-normalized ratios of aerosol deposition in central and peripheral ROIs and in central and distal airways, respectively.

RESULTS

P/C_2D values and their variability, were influenced by the shape and method to define the ROIs: The P/C_2D was systematically greater and more variable for mROI versus bROI (p < 0.005). The P/C_2D for aROI was higher and its variability lower than those for the bROI (p < 0.001). The PI_2D was in average the same for aROI and bROI, and is substantially (∼30 × ) greater than PI_3D (p < 0.001). Both PI_2D and PI_3D, obtained with our analysis, compared well with literature values obtained with two scans (deposition and volume).

CONCLUSION

Our results demonstrate that 2D scintigraphy can be analyzed using anatomically based ROIs from 3D CT data, allowing objective and enhanced reproducibility values describing the distribution pattern of radioaerosol deposition in the tracheobronchial tree.

Measurement techniques for respiratory tract deposition of airborne nanoparticles: a critical review

Determination of the respiratory tract deposition of airborne particles is critical for risk assessment of air pollution, inhaled drug delivery, and understanding of respiratory disease. With the advent of nanotechnology, there has been an increasing interest in the measurement of pulmonary deposition of nanoparticles because of their unique properties in inhalation toxicology and medicine. Over the last century, around 50 studies have presented experimental data on lung deposition of nanoparticles (typical diameter≤100 nm, but here≤300 nm). These data show a considerable variability, partly due to differences in the applied methodologies. In this study, we review the experimental techniques for measuring respiratory tract deposition of nano-sized particles, analyze critical experimental design aspects causing measurement uncertainties, and suggest methodologies for future studies. It is shown that, although particle detection techniques have developed with time, the overall methodology in respiratory tract deposition experiments has not seen similar progress. Available experience from previous research has often not been incorporated, and some methodological design aspects that were overlooked in 30-70% of all studies may have biased the experimental data. This has contributed to a significant uncertainty on the absolute value of the lung deposition fraction of nanoparticles. We estimate the impact of the design aspects on obtained data, discuss solutions to minimize errors, and highlight gaps in the available experimental set of data.

Particle transport and deposition: basic physics of particle kinetics

The human body interacts with the environment in many different ways. The lungs interact with the external environment through breathing. The enormously large surface area of the lung with its extremely thin air-blood barrier is exposed to particles suspended in the inhaled air. The particle-lung interaction may cause deleterious effects on health if the inhaled pollutant aerosols are toxic. Conversely, this interaction can be beneficial for disease treatment if the inhaled particles are therapeutic aerosolized drugs. In either case, an accurate estimation of dose and sites of deposition in the respiratory tract is fundamental to understanding subsequent biological response, and the basic physics of particle motion and engineering knowledge needed to understand these subjects is the topic of this article. A large portion of this article deals with three fundamental areas necessary to the understanding of particle transport and deposition in the respiratory tract. These are: (i) the physical characteristics of particles, (ii) particle behavior in gas flow, and (iii) gas-flow patterns in the respiratory tract. Other areas, such as particle transport in the developing lung and in the diseased lung are also considered. The article concludes with a summary and a brief discussion of areas of future research.

The co-imaging of gamma camera measurements of aerosol deposition and respiratory anatomy

The use of gamma camera imaging following the inhalation of a radiolabel has been widely used by researchers to investigate the fate of inhaled aerosols. The application of two-dimensional (2D) planar gamma scintigraphy and single-photon emission computed tomography (SPECT) to the study of inhaled aerosols is discussed in this review. Information on co-localized anatomy can be derived from other imaging techniques such as krypton ventilation scans and low- and high-resolution X-ray computed tomography (CT). Radionuclide imaging, combined with information on anatomy, is a potentially useful approach when the understanding of regional deposition within the lung is central to research objectives for following disease progression and for the evaluation of therapeutic intervention.

Challenges in assessing regional distribution of inhaled drug in the human lungs

INTRODUCTION

Both the total amount of drug deposited in the lungs (whole lung deposition) and the amount deposited in different lung regions (regional lung deposition) are potentially important factors that determine the safety and efficacy of inhaled drugs. Radionuclide imaging is well established for quantifying the whole lung deposition of inhaled drugs, but the assessment of regional lung deposition is less straightforward, because of the complex nature of the lung anatomy.

AREAS COVERED

This review describes the challenges and problems associated with quantifying regional lung deposition by the two-dimensional (2D) radionuclide imaging method of gamma scintigraphy, and by the three-dimensional (3D) radionuclide imaging methods of single-photon-emission computed tomography (SPECT) and positron-emission tomography (PET). The advantages and disadvantages of each method for assessing regional lung deposition are discussed.

EXPERT OPINION

Owing to its 2D nature, gamma scintigraphy provides limited information about regional lung deposition. SPECT provides regional lung deposition data in three dimensions, but usually involves a (99m)Tc radiolabel. PET enables the regional lung deposition of radiolabeled drug molecules to be quantified in three dimensions, but poses the greatest logistical and technical difficulties. Despite their more challenging nature, 3D imaging methods should be considered as an alternative to gamma scintigraphy whenever the determination of regional lung deposition of pharmaceutical aerosols is a major study objective.

Comparing lung regions of interest in gamma scintigraphy for assessing inhaled therapeutic aerosol deposition

BACKGROUND

Two-dimensional gamma scintigraphy is an important technique used to evaluate the lung deposition from inhaled therapeutic aerosols. Images are divided into regions of interest and deposition indices are derived to quantify aerosol distribution within the intrapulmonary airways. In this article, we compared the different approaches that have been historically used between different laboratories for geometrically defining lung regions of interest. We evaluated the effect of these different approaches on the derived indices classically used to assess inhaled aerosol deposition in the lungs. Our primary intention was to assess the ability of different regional lung templates to discriminate between central and peripheral airway deposition patterns generated by inhaling aerosols of different particle sizes.

METHODS

We investigated six methods most commonly reported in the scientific literature to define lung regions of interest and assessed how different each of the derived regional lung indices were between the methods to quantify regional lung deposition. We used monodisperse albuterol aerosols of differing particle size (1.5, 3, and 6 μm) in five mild asthmatic subjects [forced expiratory volume in 1 sec (FEV(1)) 90% predicted] to test the different approaches of each laboratory.

RESULTS

We observed the areas of geometry used to delineate central (C) and peripheral (P) lung regions of interest varied markedly between different laboratories. There was greater similarity between methods in values of penetration index (PI), defined as P/C aerosol counts normalized by P/C krypton ventilation counts, compared to nonnormalized C/P or P/C aerosol count-ratios. Normalizing the aerosol deposition P/C count-ratios by the ventilation P/C count-ratios, reduced the variability of the data. There was dependence of the regional lung deposition indices on the size of the P region of interest in that, as P increased, C/P count-ratios decreased and P/C count-ratios increased, whereas PI was less affected by variations in the P area. We found particle size, itself, strongly influenced the indices of regional aerosol deposition such that C/P count-ratios increased with increasing particle size for each method and conversely, P/C count-ratios and PI decreased.

CONCLUSIONS

Different approaches used to determine pulmonary regions of interest and quantify aerosol deposition produce different results. Our research highlights a genuine need for a consensus to standardize the methodology to facilitate data comparison between laboratories on aerosol deposition.

The use of combined single photon emission computed tomography and X-ray computed tomography to assess the fate of inhaled aerosol

BACKGROUND

Gamma camera imaging is widely used to assess pulmonary aerosol deposition. Conventional planar imaging provides limited information on its regional distribution. In this study, single photon emission computed tomography (SPECT) was used to describe deposition in three dimensions (3D) and combined with X-ray computed tomography (CT) to relate this to lung anatomy. Its performance was compared to planar imaging.

METHODS

Ten SPECT/CT studies were performed on five healthy subjects following carefully controlled inhalation of radioaerosol from a nebulizer, using a variety of inhalation regimes. The 3D spatial distribution was assessed using a central-to-peripheral ratio (C/P) normalized to lung volume and for the right lung was compared to planar C/P analysis. The deposition by airway generation was calculated for each lung and the conducting airways deposition fraction compared to 24-h clearance.

RESULTS

The 3D normalized C/P ratio correlated more closely with 24-h clearance than the 2D ratio for the right lung [coefficient of variation (COV), 9% compared to 15% p < 0.05]. Analysis of regional distribution was possible for both lungs in 3D but not in 2D due to overlap of the stomach on the left lung. The mean conducting airways deposition fraction from SPECT for both lungs was not significantly different from 24-h clearance (COV 18%). Both spatial and generational measures of central deposition were significantly higher for the left than for the right lung.

CONCLUSIONS

Combined SPECT/CT enabled improved analysis of aerosol deposition from gamma camera imaging compared to planar imaging. 3D radionuclide imaging combined with anatomical information from CT and computer analysis is a useful approach for applications requiring regional information on deposition.

An analytical technique to recover the third dimension in planar imaging of inhaled aerosols--2 estimation of the deposition per airway generation

An analytical algorithm has been recently described for converting planar scintigraphic images of aerosol distributions in the lungs to an equivalent three-dimensional (3D) representation. The recovery of the volumetric information has opened up to planar imaging the possibility of measuring aerosol deposition per airway generation. This paper investigates the accuracy and precision of the generation analysis achievable with planar imaging using simulation. Typical generation parameters--such as the bronchial and conducting airway deposition fractions (BADF and CADF)--have been derived. The accuracy of the technique has been measured by the coefficient of variation (COV) of the estimates from the known values used in the simulation. The results have also been compared to those obtained from 3D imaging (single photon emission computed tomography or SPECT). Finally, the technique has been applied to two aerosol studies conducted on a healthy volunteer, to demonstrate its implementation on clinical data. The accuracy of the BADF and CADF estimates from planar imaging were 42% and 41%, respectively; the corresponding values from SPECT were 32% and 22%. In conclusion, approximate estimates of airway distribution parameters can be derived from planar imaging. However, the errors are significantly higher than with SPECT.

3D in silico modeling of the human respiratory system for inhaled drug delivery and imaging analysis

The efficacies of inhaled pharmacologic drugs could be improved if drugs could be targeted to appropriate sites within the human respiratory system. The spatial deposition patterns of particles can now be detected with a high degree of resolution using advanced techniques of imaging (e.g., SPECT). However, the effectiveness of such laboratory regimens has been limited by the inability to clearly identify airway composition within images. Therefore, we have developed a theoretical protocol to map airways within human lungs that is designed to be used in a complementary manner with laboratory investigations. The in silico model has two components: a mathematical model based on concepts of topology; and, a computer algorithm which tracks the millions of constituent lung airways. The in silico model produces 3D lung structures that are anatomically correct and can be customized to each patient. We have applied the protocol to a SPECT study where the interiors of lungs were partitioned into a series of ten nested shells. Airway composition in the respective shells provides a heretofore unavailable quantification of scintigraphy images. The protocol can be employed in a practical manner in the medical arena to aid in the interpretation of SPECT images, and to provide a platform for the design of human subject tests.

Analytical technique to recover the third dimension in planar imaging of inhaled aerosols: (1) impact on spatial quantification

An analytical algorithm is described for converting planar scintigraphic images of aerosol distributions in the lungs to an equivalent three-dimensional (3D) representation. The recovery of volumetric information should benefit regional quantification. The technique has been validated using simulated planar images of eleven known aerosol distributions in ten realistic lungs. Global and regional 3D parameters, such as the total activity deposition (A), the penetration index (PI) and the relative penetration index (rPI), were quantified on the planar images and on their 3D representation. Random and systematic errors of the estimation were measured. Finally, the performance of planar imaging was compared with that of single-photon emission computed tomography (SPECT). SPECT images were simulated for the same aerosol distributions in the same subjects and quantified for A, PI, and rPI. The systematic errors in A, PI and rPI obtained from planar imaging were 8.9%, 64.8%, and 54.1%, respectively, using the two-dimensional (2D) analysis; they improved significantly to 4.4%, 19.0%, and 25.5% with the 3D analysis (p < 0.01). The corresponding values for SPECT were 5.2%, 9.8%, and 15.7%, significantly better for PI and rPI (p < 0.01). The random errors of A were similar for all techniques being about 5%; those of PI and rPI measurements were significantly higher for planar imaging (<or=14%) than SPECT (<or=8%). In conclusion, 3D spatial parameters can be derived from planar imaging that are significantly more accurate in characterizing different aerosol depositions than their 2D counterpart. However, the errors remain significantly higher than with SPECT.

Comparison of SPECT aerosol deposition data with a human respiratory tract model

Three-dimensional (3D) radionuclide imaging provides detailed information on the distribution of inhaled aerosol material within the body. Analysis of the data can provide estimates of the deposition per airway generation. In this study, two different nebulizers have been used to deliver radiolabeled aerosols of different particle size to 12 human subjects. Medical imaging has been used to assess the deposition in the body. The deposition pattern has also been estimated using the International Commission on Radiological Protection (ICRP) empirical model and compared to values obtained by experiment. The results showed generally good agreement between model and experiment for both aerosols for the deposition in the extrathoracic and conducting airways. However, there were significant differences in the fate of the remainder of the aerosol between the amount deposited in the alveolar region and that exhaled. The inter-subject variability of deposition predicted by the model was significantly less than that measured, for all regions of the body. The model predicted quite well the differences in deposition distribution pattern between the two aerosols. In conclusion, this study has shown that the ICPR model of inhaled aerosol deposition shows areas of good agreement with results from experiment. However, there are also areas of disagreement, which may be explained by hygroscopic particle growth and individual variation in airway anatomy.

SPECT Imaging for Radioaerosol Deposition and Clearance Studies

Planar gamma camera scintigraphy is well established for measuring the deposition and clearance of radioaerosols. Single photon emission computed tomography (SPECT) provides threedimensional (3D) reconstructions of the radioactivity distribution, thus avoiding the compression of 3D data into two-dimensional (2D) images and potentially offering superior assessment of aerosol deposition patterns. However, SPECT has traditionally been associated with long imaging times, making it unsuitable for measuring deposition and clearance of radioaerosols with fast clearance. Multi-detector SPECT systems can collect complete SPECT studies in <1 min, allowing both initial deposition and clearance over time to be assessed by dynamic SPECT. Simultaneous transmission measurement with an external source provides attenuation correction for absolute activity quantification as well as aiding in the definition of the lung volume of interest. A dynamic SPECT imaging protocol has been developed to allow fast imaging from the oropharynx to the abdomen using gamma cameras with limited axial field of views. This allows activity quantification not only in the lungs, but also in areas outside the thorax. However, fast dynamic SPECT imaging is technically and computationally more demanding and provides less scope for reducing the radioactivity administered to the subjects. It has been shown that dynamic SPECT, compared to planar imaging, is more sensitive in detecting changes in deposition as measured by the Penetration Index (PI). Thus, SPECT can better differentiate between large and small airways, which is important for lung regional analysis.

Airway identification within planar gamma camera images using computer models of lung morphology

PURPOSE

Quantification of inhaled aerosols by planar gamma scintigraphy could be improved if a more comprehensive assessment of aerosol distribution patterns among lung airways were obtained. The analysis of planar scans can be quite subjective because of overlaying of small, peripheral airways with large, conducting airways. Herein, a computer modeling technique of the three-dimensional (3-D) branching structure of human lung airways was applied to assist in the interpretation of planar gamma camera images.

METHODS

Airway dimensions were derived from morphometric data, and lung boundaries were formulated from scintigraphy protocols. Central, intermediate, and peripheral regions were superimposed on a planar view of the 3-D simulations, and airways were then tabulated by type, number, surface area, and volume in each respective region.

RESULTS

These findings indicate that the central region, for example, consists mostly of alveolated airways. Specifically, it was found that alveolated airways comprise over 99% of the total number of airways, over 95% of the total airway surface area, and approximately 80% of the total airway volume in the central region.

CONCLUSIONS

The computer simulations are designed to serve as templates that can assist in the interpretation of aerosol deposition data from scintigraphy images.

Human lung morphology models for particle deposition studies

Knowledge of human lung morphology is of paramount importance in calculating deposition patterns of inhaled particulate matter (PM) to be used in the definition of ambient air quality standards. Due to the inherently complex nature of the branching structure of the airway network, practical assumptions must be made for modeling purposes. The most commonly used mathematical models reported in the literature that describe PM deposition use Weibel's model A morphology. This assumes the airways of the lung to be a symmetric, dichotomously branching system. However, computer simulations of this model, when compared to scintigraphy images, have shown it to lack physiological realism (Martonen et al., 1994a). Therefore, a more physiologically realistic model of the lung is needed to improve the current PM dosimetry models. Herein, a morphological model is presented that is based on laboratory data from planar gamma camera and single-photon emission computed tomography (SPECT) images. Key elements of this model include: The parenchymal wall of the lung is defined in mathematical terms, the whole lung is divided into distinct left and right components, a set of branching angles is derived from experimental measurements, and the branching network is confined within the discrete left and right components (i.e., there is no overlapping of airways). In future work, this new, more physiologically realistic morphological model can be used to calculate PM deposition patterns for risk assessment protocols.

In silico modeling of asthma

The incidence of asthma is increasing throughout the world, especially among children, to the extent that it has become a medical issue of serious global concern. Appropriately, numerous pharmacologic drugs and clinical protocols for the treatment and prophylaxis of the disease have been reported. From a scientific perspective, a review of the literature suggests that the targeted delivery of an aerosol would, in a real sense, enhance the efficacy of an inhaled medicine. Therefore, in accordance with published data we have developed a mathematical description of disease-induced effects of disease on airway morphology. A morphological algorithm defining the heterogeneity of asthma has been integrated with a computer code that formulates the behavior and fate of inhaled drugs. In this work, predicted drug particle deposition patterns have been compared with SPECT images from experiments with healthy human subjects (controls) and asthmatic patients. The asthma drug delivery model simulations agree with observations from human testing. The results indicate that in silico modeling provides a technical foundation for addressing effects of disease on the administration of aerosolized drugs, and suggest that modeling should be used in a complementary manner with future inhalation therapy protocols.

A comparison of planar scintigraphy and SPECT measurement of total lung deposition of inhaled aerosol

Planar gamma camera imaging of inhaled aerosol deposition is extensively used to assess the total deposition in the lung. However, validation of the measurements is not straightforward, as gold standard measurements of lung activity against which to compare are not readily available. Quantitative SPECT imaging provides an alternative method for comparison. Four different methods for planar image quantification are compared. Two attenuation correction techniques, thickness measurement and transmission measurement, have been combined with two scatter correction techniques, reduced attenuation coefficient and line-source scatter function convolution subtraction. Each technique has been applied to 10 studies of aerosol deposition of a fine aerosol (mass median aerodynamic diameter 1.8 microm) and 10 studies using a coarse aerosol (mass median aerodynamic diameter 6.5 microm). The total activity in the right lung for each measurement has been compared to the value determined from SPECT imaging on the same subjects. When the thickness measurement and transmission techniques were applied with scatter compensation using a reduced attenuation coefficient, activity was systematically overestimated by 5% in both cases. The corresponding random errors (coefficient of variation) were 8.6% and 6.6%. Separate scatter correction reduced these systemic errors significantly to -1.5% and 2.7%, respectively. The random errors were not affected. All techniques provided assessment of total lung activity with an accuracy and precision that differed by less than 10% compared to the SPECT values. Planar gamma camera imaging provides a good method of assessing total lung deposition of inhaled aerosol.

Risk assessment dosimetry model for inhaled particulate matter: II. Laboratory surrogates (rat)

Inhalation toxicology investigations are often performed with laboratory animals to address the potential health effects of inhaled air pollutants on human beings. In Part II of this risk assessment study we have considered the deposition of inhaled particulate matter in the laboratory rat as the surrogate of choice. Calculations were performed in an analogous manner to those conducted in Part I for human subjects. To simulate a wide range of human respiratory intensities associated with different levels of physical activities that must be recognized in the determination of air pollution standards, the CO(2) concentrations within animal inhalation exposure chambers may be controlled. Accordingly, we have regulated rat breathing parameters to correspond to a range of human activities, from rest to work. The results of this interspecies modeling study have been presented in a variety of graphical formats to ease comparisons with findings from experiments and to facilitate integration of the results into risk assessment analyses. The findings of our work clearly demonstrate that interspecies simulations can be employed to design animal tests a priori so that the results can be effectively and efficiently extrapolated to human conditions in a meaningful manner.

Targeting delivery of aerosols to different lung regions

With the increasing use of aerosolized drugs, there is a need to understand the means by which these drugs can most effectively be targeted to desired regions of the lung. Several attempts have been made at targeting aerosols in the lung by changing particle sizes and breathing patterns with varying degrees of success. Recent use of such techniques as shallow, aerosol bolus delivery and extremely slow inhalations of aerosols in diagnostic lung tests may also prove beneficial for targeting drug delivery to the conducting airways. This review discusses the potential for utilizing aerosol delivery techniques for selectively targeting aerosol deposition along both serial and parallel pathways in the lung. Based on a review of previous studies concerning factors that determine aerosol and gas distribution in the lung, the potential for utilizing various breathing techniques in concert with variations in particle sizes are considered. Further research on the factors that determine distribution of aerosol in the diseased lung may help in designing successful targeting strategies for the future.

A computer model of lung morphology to analyze SPECT images

Measurement of the spatial distribution of aerosol deposition in human lungs can be performed using single photon emission computed tomography (SPECT). To relate deposition patterns to real lung structures, a computer model of the airway network has been developed. Computer simulations are presented that are compatible with the analysis of SPECT images. Computational techniques that are consistent with clinical procedures are used to analyze airways by type and number within transverse slices of the lung volume. The computer models serve as customized templates, which when analyzed alongside gamma scintigraphy images, can assist in the interpretation of human test data.

A brief history of gamma scintigraphy

Gamma scintigraphy involves the radiolabeling of inhaled drug formulations, followed by in vivo imaging of deposition in two dimensions. This permits whole lung deposition to be quantified as mass of drug or percentage of the dose, and regional deposition patterns to be assessed. Gamma scintigraphy is the method by which the majority of inhaled drug deposition data have been obtained, and scintigraphic studies have become viewed as milestone assessments in the development of new pulmonary drug products. Lung deposition data are used to show "proof of concept" in vivo for these products, and act as a bridge between in vitro laboratory testing and a clinical trials program. Gamma scintigraphy is likely to remain the method of choice for assessing inhaled drug deposition for some time to come.

The effect of scatter and attenuation on aerosol deposition as determined by gamma scintigraphy

Gamma scintigraphy is often used to quantify deposition patterns from aerosol inhalers. The errors caused by scatter and tissue attenuation in planar Tc-99m gamma scintigraphy were investigated based on the data collected from four subjects in this study. Several error correction methods were tested. The results from two scatter correction methods, Jaszczak's method and factor analysis of dynamic sequences (FADS), were similar. Scatter accounted for 20% of raw data in the whole lung, 20% in the oropharynx, and 43% in the central airways and esophagus. Three attenuation correction methods were investigated and compared. These were: uniform attenuation correction (UAC), a known method used for inhalation drug imaging work; the broad-beam attenuation correction used for organ imaging in nuclear medicine; and a narrow-beam inhomogeneous tissue attenuation correction proposed in this study. The three methods differed significantly (p < 0.05), but all indicated that attenuation is a severe quantification problem. The narrow beam attenuation correction with scatter correction, showed that raw data underestimated tracer deposition by 44% in the lung, 137% in the oropharynx, and 153% in the trachea/esophageal region. To quantify aerosol lung deposition using planar scintigraphy even in relative terms, corrections are necessary. Much of the literature concerning quantified aerosol dose distributions measured by gamma scintigraphy needs to be interpreted carefully.

Evaluation of the accuracy and precision of lung aerosol deposition measurements from single-photon emission computed tomography using simulation

Single-photon emission computed tomography (SPECT) imaging is being increasingly used to assess inhaled aerosol deposition. This study uses simulation to evaluate the errors involved in such measurements and to compare them with those from conventional planar imaging. SPECT images of known theoretical distributions of radioaerosol in the lung have been simulated using lung models derived from magnetic resonance studies in human subjects. Total lung activity was evaluated from the simulated images. A spherical transform of the lung distributions was performed, and the absolute penetration index (PI) and a relative value expressed as a fraction of that in a simulated ventilation image were calculated. All parameters were compared with the true value used in the simulation, and the errors were assessed. An iterative method was used to correct for the partial volume effect, and its effectiveness in improving errors was evaluated. The errors were compared with those of planar imaging. The precision of measurements was significantly better for SPECT than planar imaging (2.8 vs 6.3% for total lung activity, 6 vs 20% for PI, and 3 vs 6% for relative PI). The method of correcting for the influence of the partial volume effect significantly improved the accuracy of PI evaluation without affecting precision. SPECT is capable of accurate and precise measurements of aerosol distribution in the lung, which are improved compared with those measured by conventional planar imaging. A technique for correcting the SPECT data for the influence of the partial volume effect has been described. Simulation is demonstrated as a valuable method of technique evaluation and comparison.

Assessing lung deposition of inhaled medications. Consensus statement from a workshop of the British Association for Lung Research, held at the Institute of Biology, London, U.K. on 17 April 1998

In vitro measurements of aerosol fine particle fraction (FPF) using particle-sizing apparatus (e.g. the twin impinger, multi-stage liquid impingers, cascade impactors) have a key role to play in the development of new pharmaceutical products and in quality control. However, use of in vitro methodology to attempt to predict lung deposition in vivo is of limited value due, in part, to the inability of current apparatus to mimic upper and lower airway anatomy satisfactorily. Estimates of FPF based on cut-off points ranging from 5-7 microns generally overestimate lung deposition as measured in vivo by gamma scintigraphy. We recommend that: 1. multistage apparatus (minimum five stages) be used to characterize particle size distribution adequately, over the range 0.5-5.0 microns; 2. where possible, measurements should be made at a range of rates and profiles of flow reflecting those likely to be generated using the inhalation device in clinical practice (including use by young and elderly patients with varying degrees of airflow obstruction); 3. encouragement should be given to the further development, standardization, and validation of apparatus with a 'throat' which more closely resembles the human oropharynx and larynx. Pharmacokinetic methods can give a good estimate of total, but not regional, lung deposition, with drugs which are either not absorbed via the gastrointestinal tract, or whose absorption can be blocked by co-administration of charcoal, thus avoiding confounding by absorption of drug substance deposited in the oropharynx and subsequently swallowed. Techniques which rely on evaluation of a timed fractional output of drug substance in the urine are susceptible to the inherent variability of rate of absorption across the respiratory epithelium. We recommend that consideration should be given to the further refinement and validation of PK methods which would more clearly identify the fractional dose deposited in the lung. Lung-imaging methodology, e.g. gamma scintigraphy, employing formulations radiolabelled with gamma-ray-emitting radionuclides such as 99mTc, can measure total lung deposition and oropharyngeal deposition, provided that the radiolabelling process is appropriately validated and suitable corrections are made for attenuation of gamma rays by body tissues. An estimate of regional lung deposition can be made by drawing 'regions of interest' on the scintigraphic image; the precision of this measure is limited by the two-dimensional (2-D) nature of most images which mean that there is an overlay of structures of interest (alveoli, small and large airways), which is most marked centrally. Three-dimensional (3-D) imaging techniques (e.g. single photon emission computed tomography, SPECT, and positron emission tomography, PET) have the potential to give more detailed data on regional lung deposition, but are currently more expensive, employ higher radiation doses, and are less well validated than 2-D (planar) imaging. We consider that, of the available imaging modalities, planar gamma scintigraphy represents current best practice for the assessment of lung deposition from inhaler devices where regional differences may be important. The methodology should be optimized by the adoption of generally accepted standards for radiolabelling, imaging, attenuation correction, and interpretation. It is important that deposition in all sites (device, oropharynx, lungs, stomach) should be quantified. Consideration should be given to refining the concept of regions of interest to coincide more closely with anatomical lung structures. Statistical methods to compare the size distributions of drug and radiolabel in validation experiments should be developed. In the longer term it is envisaged that three-dimensional imaging may play a more important part in evaluating lung deposition; an optimal three-dimensional anatomical model of lung zones of interest needs to be developed.