Introduction

Lung disease is one of the major causes of illness-related death, accounting for approximately six million deaths worldwide in 2019 (12% of total deaths) [1]. This number can remain unchanged in the future, as predicted by The Global Burden of Disease study, in which chronic obstructive pulmonary disease (COPD) and lower respiratory infection will be the third and fourth cause of death globally in 2030, respectively [2]. Furthermore, lower respiratory infection is predicted to rise in the future, particularly in tropical countries, due to the increased infectious activity of non-tuberculous mycobacteria, which tends to infect people with either genetic or acquired structural typical lung diseases, such as COPD and cystic fibrosis [3]. This condition is worsened by the emergence of coronavirus disease 2019 (COVID-19), which has become a global pandemic and has caused more than 6.24 million deaths around the world, as reported by the World Health Organization (WHO) [4]. Aside from the high mortality rate, lung disease is also predicted to be one of the global burdens of severe health-related suffering until 2060 [5]

The economic burden is one of the global impacts of lung diseases, caused by either unnecessary drug overprescription or treatment failure due to over-or underdiagnosis, respectively [6]. For example, studies on COPD misdiagnosis revealed that the underdiagnosis and overdiagnosis prevalence of this disease was 56.7–81.4% and 29.0–65.0%, respectively [7], leading to 55.4% dissipation of treatment cost [8]. The misdiagnosis of lung disease not only aggravates the economic burden but also can increase mortality due to false medical treatment. For instance, lung cancer is frequently misdiagnosed as tuberculosis in a country with tuberculosis prevalence. This phenomenon highlights the importance of lung diagnosis to reduce the lung disease burden and its mortality rate [9].

Lung imaging, comprising of computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and single-photon emission computed tomography (SPECT), is a standard method in lung disease diagnosis, as presented in Table 1. Nevertheless, each modality for lung imaging has weaknesses, such as the limited spatial resolution in molecular imaging-based modality [10,11,12]. To increase the performance of such a diagnosis method, micro and nano-inhalable particles have been investigated and applied for decades as a tracing agent or contrast agent [13,14,15]. As a tracing agent, the lung deposition of an inhaled particle can be detected due to the nature of the particle; for example, Technetium-99m (99mTc) emits gamma-ray, which can be detected by SPECT, resulting in an image of the lung [13]. On the other side, the contrast agent can alter the result of existing imaging methods (CT and MRI), providing brighter or darker regions for better diagnosis or interpretation [16,17,18].

Table 1 Detection mechanism in lung imaging
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To date, 38 tracing and contrast agents have been approved for commercial use [30,31,32], and the development of new agents is underway to improve the safety aspect and image quality [17, 18, 23, 33, 34]. This review aimed to address the challenges of pulmonary delivery for lung imaging, as well as the aerosol formulations that have been commercially approved or are still in development study. There have been reviews on the application of nano or microparticles for imaging diagnosis purposes (CT: [15, 22]; MRI: [14, 35, 36]; PET: [37]; SPECT: [38, 39]), but there is no review which discusses the application of aerosol for lung imaging. This review is focused on understanding the critical parameters of aerosol delivery into the lung, which are essential for formulating micro- or nanoparticles for pulmonary delivery. Furthermore, the fundamental concepts of different aerosol production methods that contribute to the development of aerosolised contrast and tracing agent for lung imaging quality improvement are also discussed. We explored several databases, including PubMed, Scopus, and Google Scholar, during preparing this review to discover and obtain the abstracts, reports, review articles, and research papers related to aerosol delivery for lung imaging and the formulation and pulmonary delivery method of dry or liquid aerosol. The following search terms were used: “dry aerosol delivery”, “liquid aerosol delivery”, “MRI for lung imaging”, “CT scan for lung imaging”, “SPECT for lung imaging”, “PET for lung imaging”, “magnetic particle imaging”, “dry powder inhalation”, “nebuliser”, and “pressurised metered-dose inhaler”.

Challenges of aerosol delivery for lung imaging

Pulmonary delivery for contrast or tracing agent administration route has provided an image for lung disease diagnosis for decades [13]. Furthermore, an attempt to discover new inhalable particles with increased image quality and safety profile has also been reported in recent years [17, 18, 23, 33, 34]. For instance, Silva et al. reported the development of a dual-modality contrast agent consisting of Fe and gold (Au) for detection using MRI and CT, respectively [20]. Despite its success in lung imaging and potential for future development, several challenges need to be overcome in designing inhalable particles for tracing or contrast agents.

Toxicity

The first challenge is attributed to the toxic nature of different tracing and contrast agents. The established tracing agents are typically an element with radioactive hazards due to ionising radiation or particle emission. Nevertheless, typical radionuclides used for diagnosis are considered safe due to the low radiation energy and short half-life [13]. 99mTc, Tc, is the most widely used diagnostic radionuclide, including in lung scintigraphy. It has a half-life of 6 h and a gamma energy of 140 keV, yet sufficient for diagnosis purposes. [40]. Another radionuclide used for lung scintigraphy, Gallium-68 (68Ga, a positron-emitting radionuclide), has a shorter half-life (67.71 min) with Emax of 1899 keV [41]. The amount of radioactive element used for the diagnostic purpose is also meagre; for instance, the typical radioactivity of 99mTc for clinical use is 185–925 MBq, equal to only 0.95–4.70 ng of Tc metal [19]. On the other hand, a significantly higher amount of element is required for the contrast agent, leading to potential toxicity in the human body. Another issue in the use of radioactive tracing agents is the possibility of free radionuclide release from radiolabelled compound or complex, leading to free radionuclide extrapulmonary distribution. For instance, free pertechnetate has been found in thyroid upon lung scintigraphy due to unstable radiolabelled compound or complex [42]. Although such extrapulmonary distribution will not harm the organs, free radionuclides in significant amounts can disturb lung scintigraphy [43, 44]. Therefore, standard quality control for radiopharmaceuticals, e.g., radiochemical purity, should also be performed in the lung scintigraphy.

As an endogenous element in the human body, the use of iron (Fe) as a contrast agent is considered safe since ionic Fe presents as a complex with binding protein and the metal one has a low bio-solubility. Different binding and regulatory proteins are available for Fe absorption in the human digestive system without toxic effects [45]. However, such absorption facilitation is significantly limited in the respiratory tract, which might induce local adverse effects, such as inflammatory response. Furthermore, the inhaled Fe can easily be transported to other organs and causes more severe effects, including pro-atherosclerotic effect and alteration of autonomic regulation [26]. Similar to Fe, the inhalation of different elements for contrast agents can potentially induce intra- or extrapulmonary side effects, where the result can be even worse for exogenous substances, i.e. Au [46], gadolinium (Gd), and silver (Ag) [47]. In general, although the toxicity of these elements is enhanced in the ionic form [32], the nanoparticle form can also induce cell death due to the easy cellular uptake, resulting in cellular toxicity [26]. The toxicity of the iodinated contrast agent relies on the molecular form, in which inorganic iodine and ionic compounds exhibit higher toxicity [30].

Continuous efforts have been made toward minimising the toxicity of contrast agents, including the formulation of the stable complex to avoid the release of free ionic elements and interaction with cellular membranes, proteins, and other biological structures [30, 48]. Another attempt to reduce the toxicity is minimising cellular uptake, and particle absorption into the systemic circulation, which can be achieved by preparing micron or sub-micron size particle since effective cellular membrane penetration can only effectively occurs in size of < 200 nm [49]. Although larger particles can still be engulfed by alveolar macrophage, this type of cell is able to generate protein complex to minimise the toxicity of quantitatively large metallic element absorption, such as Fe [50].

Radio-protection of radioactive aerosol or gas

Radioactive contamination is one of the concerns during the utilisation of radiopharmaceuticals in the medical field. Common liquid radiopharmaceuticals typically pose a low contamination risk, requiring simple handling during preparation and administration to the patients. On the other hand, radioactive aerosol poses a relatively higher risk due to its nature that easily diffuses into the air. Brudecki et al. reported that the activity of 99mTc aerosol in a typical ventilation-perfusion SPECT facility ranged from 99 ± 11 to 6.1 ± 0.5 kBq m−3, resulting in daily intake by male technicians, female technicians, male nurses and female nurses of 5.4 kBq, 4.4 kBq, 3.0 kBq and 2.5 kBq, respectively [51]. Although such internal contaminations are considered safe for the medical staff (over three orders of magnitude lower than the dose limit), the contamination level should be kept as low as possible to avoid the reduction of gamma camera performance [52].

Lung deposition

Aside from toxicity and radio-protection concern, designing the inhalable particle size should also consider lung deposition since particle larger than 5 µm is most likely deposited in upper airways, followed by rapid pulmonary clearance and resulting in poor lung image in the healthy lung [39]. The presence of disease in the lung is most likely to alter the particle deposition due to the decrease in airway cross-section. Several studies, as presented in Table 2, revealed that most lung diseases enhanced lung deposition, although the deposition pattern has become more heterogenous as some areas are poorly ventilated and less penetrated by inhaled particles [53]. Such a condition will lead to great disadvantages in lung disease therapy since the drug bioavailability will be under or over the therapeutic window in some areas, resulting in underdose or toxic effects, respectively [54, 55]. For diagnostic purposes, the heterogenous deposition of inhaled particles might not be disadvantageous since less penetrated lung area can be an indication of a certain disease. Nevertheless, a particular aerodynamic size range is still required to ensure that the lung deposition is adequate for generating a good diagnostic image [53].

Table 2 The inhaled particle deposition in different clinical lung conditions
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As seen in Table 2, the lung deposition of inhaled particles with an aerodynamic size of 1–5 μm was typically not affected or increased in the presence of lung diseases. Brown et al. suggested that although obstructed airways in diseased lungs should have reduced lung deposition, mucociliary clearance impairment and slow-deep breathing patterns can enhance the deposited particles and make them similar to those of the healthy lung [56]. Higher particle deposition in the diseased lung is also possible, as reported by Furi et al., since narrower airways induce airflow turbulence resulting in higher inertial impaction. Such a higher deposition might also be induced by the use of polydisperse particles (82–5960 nm), where larger particles will be retained in the upper airways, while smaller ones penetrate deeper lung [57]. In contrast, monodisperse particles are most likely to give more heterogenous lung deposition since large particles will only be deposited in the upper area, and ultrafine particles tend to be exhaled [58,59,60]. To ensure the size of inhalable particles falls within the desirable range, especially for underdeveloped products, in vitro aerosolisation devices, e.g., cascade impactor, can be utilised for quality control [54].

It can be concluded that the inhalable powder should be designed in a size of 200 nm–5 µm to minimise systemic absorption and to reach deep lung deposition for the best quality image and to minimise systemic absorption, respectively, although the presence of a particular disease inducing airways obstruction requires smaller inhaled particles to reach deep lung effectively. Different formulations and aerosol delivery methods have been developed to achieve this purpose, as discussed in the next section (Fig. 1).

Fig. 1
figure 1

The fate of inhaled particles depends on their size [26, 54]

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Formulation and aerosol delivery method into respiratory airways

Dry aerosol

Dry formulation provides a stable form, which can be stored for a long time. However, the main challenge of this form is the preparation method, which might require a sophisticated instrument to produce an ideal dry aerosol for pulmonary delivery [54].

Graphite ablation

Graphite ablation method is used in Technegas to produce 99mTc-radiolabelled carbon aerosol with a size of < 100 nm for lung ventilation scintigraphy [70]. 99mTc-pertechnetate in saline solution yielded from 99Mo/99mTc generator is used as 99mTc source in this method. The 99mTc-radiolabelled carbon aerosol generation method has been detailed in a review by Wiebe et al. as illustrated in Fig. 2 [71]. In brief, 99mTc solution is added to a crucible, followed by evaporation at 70ºC for 6 min in an ultrapure argon environment. Subsequently, graphite and 99mTc are ablated by an alternating-current arc generated between the terminals holding the crucible. Aerosolised carbon nanoparticles are produced by heating at 2750 °C for 15 s in an ultrapure argon environment [13].

Fig. 2
figure 2

Adapted from Wiebe et al. [71]

Production of 99mTc-carbon aerosol by graphite ablation method.

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Carbon nanoparticle generated from the graphite ablation method has a size of < 140 nm with a gas-like characteristic upon aerosolisation, which provides an ideal lung deposition for imaging purpose [13, 71]. Many studies reported different sizes of carbon aerosol from Technegas, depending on the particle collection and characterisation methods [72]. For instance, the use of electrostatic precipitation and transmission electron microscopy (TEM) by Senden et al. reported size of 30–60 nm, while a 400-mesh filter coupled with glass-fibre filter paper and photo correlation spectroscopy resulted in a size of 97–140 nm [71, 73]. A size of ~ 100 nm observed from Technegas is most likely due to the aggregation upon particle collection [71].

As an inhalable particle, carbon aerosol with a size of < 140 nm will follow Brownian diffusion upon inhalation, leading to deposition in the alveolar region and lower respiratory tracts [54]. Such lung deposition is considered an ideal particle distribution for lung imaging [74]. Such particle size for inhalable particles theoretically can easily penetrate the cellular membrane and induce toxicity [49]. Nevertheless, the clinical use of Technegas for more than three decades proves the safety of the 99mTc-labelled carbon aerosol [13, 71]. This safety profile can be explained by the fact that the amount of carbon nanoparticles inhaled is very low, which means the toxicity effect can be neglected [75]. Furthermore, a study by Zhang et al. revealed that carbon nanoparticles with a size of < 100 nm showed no toxicity in RAWS264.7 cells, and a toxic effect was observed in the nanoparticle in size of > 250 nm [76]. A review by Pacurari et al. also demonstrated that multi-walled carbon nanotubes with smaller sizes were less potent in inducing lung fibrosis upon inhalation [77].

Due to an ideal lung distribution and no toxicity, 99mTc-labelled carbon aerosol has been widely used in lung imaging to diagnose different lung-related diseases, such as COPD, pulmonary embolism (PE), and tracheobronchitis [38, 78, 79]. Nasr et al. reported that lung scintigraphy by 99mTc-labelled carbon aerosol in COVID-19 patients has successfully diagnosed COPD and PE [74]. The same method has also found the presence of tracheobronchitis in COVID-19 patients by observing an increase in tracheobronchial uptake of 99mTc-labelled carbon aerosol, as reported by Verger et al. [70]. In conclusion, graphite ablation method generates radiolabelled carbon aerosol ideal for lung ventilation scintigraphy. Nevertheless, graphite ablation is a sophisticated method and can only be performed using a commercial aerosol generator called Technegas.

Spark ignition

Spark ignition method has been utilised to produce different nanoparticles, including alloy, composite, metal, oxide, and semiconductor [80]. This method generates nanoparticles as an aerosol via spark discharge from two heated conducting electrodes with gas flow, as illustrated in Fig. 3 [81]. Similar to graphite ablation, this method spontaneously produces nanoparticles directly from large and solid material, resulting in spherical particle with a size of < 10 nm that tends to agglomerate into a larger particle (~ 20 nm) [80].

Fig. 3
figure 3

Aerosolisation devices for liquid formulation: A nebuliser, B pressurised-metered dose inhaler (pMDI), C air sprayer [114,115,116]

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Gold nanoparticle (AuNP) has been formulated into nano aerosol by spark ignition method as a CT contrast agent. Kreyling et al. successfully produced 20 nm-sized AuNP by this method and used radioactive gold (195Au) as a material for tracing purposes. An intratracheal inhalation into Wistar-Kyoto rats demonstrated that approximately 30% of AuNP reached the respiratory tract, followed by rapid mucociliary clearance, and about 80% of AuNP deposited in alveoli was moved from the inner surface into the interstitium within 24 h. Although Kreyling et al. did not perform a CT scan to evaluate the contrast agent performance, the primary deposition of inhaled AuNP in the alveolar region indicates its potential for lung imaging. Nevertheless, an extrapulmonary deposition was also found, mainly in the liver, which increased up to 28 days [33]. Therefore, the toxicity in other organs should be carefully observed to ensure the safety of the AuNP administration.

Another CT contrast agent that has been formulated into nano aerosol by spark ignition is AgNP, which was investigated for its potential toxicity upon inhalation. Campagnolo et al. delivered ~ 25 nm AgNP, generated by spark ignition, into pregnant C57BL/6 female mice via nose-only inhalation, resulting in AgNP accumulation in placentas, albeit very low, and increasing the expression of pregnancy-relevant inflammatory cytokines in the placentas [82]. A biokinetic study of spark-ignited AgNP in Wistar-Kyoto rats administered via tracheal inhalation was also reported by Kreyling et al. Tracing method using radioactive 105AgNP showed that 105AgNP was mainly excreted via larynx into the gastrointestinal tract due to poor solubility in the lung fluid [83]. These findings imply that the extrapulmonary toxic effect of inhaled AgNP can be minimised by lowering the solubility of the nanoparticle in lung fluid to enhance the mucociliary clearance and reduce the epithelium penetration.

Spray drying

Spray drying method has been widely investigated for preparing dry powder for drug administration via pulmonary delivery. The ability to incorporate different nanoparticles into size-controllable powder makes spray drying a promising method for developing inhalable powder for lung imaging purposes [54]. In general, spray dryer is classified based on droplet generation and powder collection system. A conventional instrument utilises a gas atomiser and cyclone or bag filter for droplet generation and powder collection, respectively [54]. In a gas atomiser system, the liquid formulation is atomised by pressurised nitrogen gas, in which droplet diameter is affected by the liquid feed and gas flow rate. Droplet diameter can be increased by the increase of feed rate and reduced by increasing the gas flow rate. The cyclone separation system utilises the gravitational force of rotating flow to collect the powder with high inertia [84]. The new instrument, the so-called nano spray dryer, uses a nebuliser to generate droplets and an electrostatic separation system to collect powder [85].

Despite the better particle engineering offered by a nano-spray dryer, only conventional spray dryer type has been utilised for the preparation of an inhalable powder containing 99mTc-diethylenetriamine pentaacetic acid (DTPA) for lung imaging, as reported by Yang et al. in this study, a clinical trial in nine healthy subjects demonstrated that inhaled powder reached lung at 50–70 L/min peak inhalation flow rate. Yang et al. concluded that total and regional lung depositions were not significantly affected by inhaler resistance. Instead, such lung depositions are significantly influenced by inhalation rate and particle size [86]. Despite the success in a clinical trial, the spray drying of radioactive material is a sophisticated process, particularly in avoiding the leakage of radioactive dust from the instrument. It requires tight containment and radioactive material entrapment to ensure the safety of the personnel from radioactive contamination [87]. In contrast, a non-radioactive contrast agent formulation can be carried out using a typical spray dryer without any additional equipment. Furthermore, a combination of different nanoparticles or elements for dual-imaging or theranostic purpose can simply be performed by pre-mixing prior to spray drying, as reported by Julián-López et al. who developed an iron-silica hybrid using a home-made conventional spray dryer [88].

Aside from conventional and nano-spray dryer, supercritical CO2-assisted spray dryer has also been utilised to produce a dry powder with complex structures, such as aerogel. Such a technique allows the material to pass through the supercritical region with the assistance of supercritical fluid, resulting in the absence of interfacial stress [89]. The application of supercritical CO2-assisted spray drying (SASD) for the preparation of inhalable powder containing contrast agents has been reported in the literature.

However, the SASD method poses challenges, including sophisticated instrumentation, high costs, and the presence of organic solvent residuals in the end product [89], which hampers further development in inhalable powder technology [20, 90,91,92]. For instance, a nanohybrid for dual-imaging consisting of strawberry-like gold-coated magnetite nanocomposites was developed by Silva et al. by SASD method. Using chitosan as a filler, the spray drying process resulted in powder with an aerodynamic diameter of 2.6–2.8 μm and a fine particle fraction (FPF) of 48–55%, which is sufficient for deep lung deposition [20]. Nevertheless, supercritical CO2-assisted spray drying poses challenges, namely high costs, complex instrumentation, and the presence of organic solvent residuals in the end product [89].

Although gold and iron NP are the only investigated contrast agent as inhalable powder via spray drying [90,91,92], this method is suitable for formulating other contrast or tracing agent into dry powder. The ability to design particle size, structure, and composition makes spray drying a promising method to produce inhalable powder for lung imaging.

Liquid aerosol

Liquid aerosol for pulmonary delivery is a simpler and cheaper formulation than the dry one. The drug formulation can be dissolved in a suitable solvent, followed by aerosolisation using relatively unsophisticated devices compared to those used for dry aerosol. Another characteristic of liquid aerosol is the immediate solubility upon deposition in the lung airways, avoiding a mucus trap and mucociliary clearance [54]. Therefore, the pulmonary clearance profile of inhaled aerosol is merely affected by the cargo’s characteristics, i.e. size and solubility, simplifying the designing process of inhalable particles [93]. Aside from the aforementioned advantages, the application of liquid aerosol in pulmonary delivery also poses challenges, namely the difficulty in aerosol size adjustment and the lower drug stability in liquid formulation compared to that in the dry one [94]. Standard devices used to produce liquid aerosol are nebuliser, pressurised metered-dose inhaler (pMDI), and air spray [55].

Nebulisation

Nebuliser is the oldest aerosol device and the most widely used for pulmonary drug administration. This device can produce aerosol from liquid formulation using different instrument types, including ultrasonic, jet and vibrating mesh (Fig. 3a). Ultrasonic nebuliser applies high-frequency vibration of the piezoelectric element that transforms liquid formulation into tiny droplets due to sound wave effect. This device type might generate heat upon aerosolisation, which may impact the drug cargo's stability. [93]. A jet nebuliser uses compressed gas that passes through a narrow gap to generate lower pressure and faster gas flow. Subsequently, such a gas flow draws and leads the liquid formulation to hit the baffle for the generation of tiny droplets. Jet nebuliser has two types of valves: the first valve opens during inhalation to increase the aerosol flow, while the second one opens during exhalation for gas release to the atmosphere, avoiding a reverse gas flow. A vibrating mesh nebuliser applies mesh vibration to generate tiny droplets from liquid formulation [55].

To date, nebulisation is the most widely investigated method to deliver contrast or tracing agent via pulmonary delivery for lung imaging, with 31 publications in the past decade (Table 3). Such an extensive investigation of the nebulisation method is most likely due to the simple operating technique during aerosolisation from liquid formulation [93]. This simple method can also be used to aerosolise radioactive material since a commercial nebuliser equipped with the lead container, e.g. SmartVent™, is available for this purpose [95]. Furthermore, different types of nebulisers lead to dissimilar performance, providing device options to adapt to various drug cargo types [96].

Table 3 Application and development of nebulisation method for lung imaging in the last decade
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Among different types of nebulisers, a vibrating mesh is the most widely used in clinical or laboratory practice, as seen in Table 3. The reason behind the wide use of vibrating mesh nebulisers has been investigated by Galindo-Filho, et al., where the comparative study of jet and vibrating mesh nebulisers is carried out either in healthy subjects or COPD patients. In healthy subjects, vibrating mesh nebuliser delivered more than twofold 99mTc-DTPA into the lung compared to jet nebulisers [96], while in COPD patients, vibrating mesh nebuliser delivered more than threefold 99mTc-DTPA into the lung [97]. Galindo-Filho et al. suggested that such a difference is caused by different droplet sizes generated by both nebulisers, where jet and vibrating mesh nebuliser generated droplets with a size of 5 and 3.4 µm [96]. As discussed earlier, a particle size of < 5 µm is essential for deep lung deposition [54]. Aside from comparing different types of nebuliser, a comparative study between 99mTc-sulfur colloid powder prepared by Palas rotating brush generator and 99mTc-DTPA aerosolised with nebuliser has also been reported in the literature. Kuehl et al. reported that there is no significant difference in both administration methods (6.81–9.08%) since the particle sizes were similar (2.5–2.8 µm) [98].

Table 3 also shows that most of the literature was clinical studies with 99mTc-DTPA. It is understandable since 99mTc-DTPA is one of the radiopharmaceuticals recommended by the European Association of Nuclear Medicine (EANM) guideline and can easily be administered into the lung [99]. As seen in Table 3, nebulised 99mTc-DTPA is mainly used to investigate the effect of particular pulmonary delivery equipment, e.g. positive expiratory pressure device, on the aerosol performance of lung deposition [97, 100,101,102,103,104,105]. Other studies with 99mTc-DTPA focus on distinguishing different lung diseases [74, 106, 107], understanding the physiology of lung disease [108], and investigating the new application of ventilation scintigraphy [109].

Besides the clinical application, nebuliser can also be used for development study of a new contrast agent administration via pulmonary delivery, including AuNP [110], gadolinium nanoparticle (GdNP) [16, 34, 111], Iodine-based contrast agent (I-CA) [18], MnCl2 [112], silver nanoparticle (AgNP) [23, 24], and superparamagnetic iron oxide nanoparticle (SPION) [17, 113]. Such a study aimed to determine lung deposition and clearance [111], investigate biokinetics and toxicity [110, 112], and evaluate the lung image quality generated after inhalation [16, 18].

Despite the simple use of a nebuliser to produce aerosol for pulmonary delivery, this device also has challenges that need to be solved, namely stability issues in liquid formulation and difficulty in particle engineering. The first challenge can be overcome by preparing the drug solution into a lyophilised formulation that can be rehydrated prior to use. Such a lyophilisation method has been applied in radiopharmaceutical kits formulation that can be rehydrated and radiolabelled by 99mTc-pertechnetate in saline solution [94]. Therefore, this method can also be applied in the development of other contrast and tracing agents. Another challenge is the limitation of the nebuliser in particle engineering, unlike spray drying, which can easily design the inhalable powder, e.g. size and porosity [54].

Pressurised metered-dose inhaler (pMDI)

Pressurised metered-dose inhaler (pMDI) releases liquid formulation mixed with propellant from a pressurised canister, allowing droplets generation for aerosol inhalation (Fig. 3b) [123]. Due to the portable size and simplicity of use, patients can use pMDIs for inhalation treatment without any help from a medical worker [115]. The main drawback of pMDI is the incompatibility of particular drug cargo with the propellant mixture or pressurised storage. To date, 99mTc-labelled radiopharmaceutical is the only substance administered by pMDI for investigating the lung deposition of particular drugs and the performance of additional equipment for pulmonary delivery [123,124,125,126].

Radiotracing studies of pMDI-delivered aerosol revealed that lung deposition is influenced by the aerosol size, liquid formulation, and additional inhalation equipment. Clinical study of pMDI-aerosolised Beclomethasone and Fluticasone with hydrofluoroalkane (HFA) as a propellant in patients with asthma has been reported by Leach et al. This study demonstrated that the lung deposition of 99mTc-Beclomethasone was significantly higher than that of 99mTc-Fluticasone (55 vs. 24%), which can be explained by the difference of aerosol size (0.7 vs. 2.0 µm) [126]. A study reported by Ditcham et al. demonstrated that the deposition of inhaled 99mTc-Albuterol aerosol with a facemask was higher (18.1%) than that with a spacer mouthpiece (22.5%) [123]. A study comparing the lung deposition of radiolabelled drug-aerosol in healthy subjects and patients with asthma reported that the deposition patterns were similar in healthy and people with asthma (22–25%) [124]. Nevertheless, the lung deposition of pMDI-aerosol might be different in other lung diseases, as observed in a study using aerosol generated by other devices [13, 74].

Although the development of a pMDI-aerosolised contrast agent has not been reported, the delivery of nano-structured particles via pMDI is feasible, as reported by Taylor et al., where glycopyrronium/formoterol fumarate or phospholipid porous particles was 99mTc-radiolabelled and administered via pMDI into five healthy male subjects. A gamma scintigraphy assessment revealed that 38.4% of emitted dose reached the lung, 61.4% was detected in the oropharyngeal and stomach, and < 0.25% of the emitted dose was detected in the exhalation filter [125].

Air spray

The last method that has been investigated for the delivery of an inhalable tracing agent is air spraying method, which is similar to the atomisation method used in spray dryer for droplet generation. In brief, pressurised medical air and pumped liquid formulation are applied to generate an atomising flow of 0.5 L min−1 with a median particle size distribution of 40–60 μm. Although such a distribution range is theoretically too large for desirable lung deposition, the preclinical study of 99mTc-labelled porcine surfactant in six sedated 1-day-old piglets nasal-mask continuous positive pressure airway demonstrated 40% lung deposition after 28 min inhalation [116]. Such relatively high lung deposition compared to other aerosolisation methods, such as nebulisation [101, 119], can be due to the difference in an animal model or the presence of a continuous positive pressure airway [104]. Therefore, a comparative study should be performed between air spray and other nebulisation methods with identical experimental conditions to compare better.

Conclusion and future perspective

The use of contrast and tracing agent in lung imaging has been applied in the last decades, with 38 commercially available products. A high surface area, non-invasive administration, the potential to avoid extrapulmonary toxicity, and the availability of various administration devices make pulmonary delivery the best option for administering contrast and tracing agents to obtain optimal image quality. The key performance parameter of inhalable aerosol is the particle size affecting the lung deposition and clearance, eventually determining the image quality and toxicity profile, respectively. Another critical parameter of inhaled contrast and tracing agents is the elemental form, such as ionic or nanoparticle form, affecting the lung clearance profile. The research outputs reviewed here show that different aerosolisation methods are suitable depending on the characteristics of contrast and tracing agents, such as the graphite ablation method for a radionuclide-based tracing agent. Nevertheless, graphite ablation and nebulisation are the most widely used aerosolisation method for dry and liquid aerosol production, respectively.

Further development of aerosol delivery for lung imaging should also consider the combination of more than one element to improve the image quality or provide a dual-imaging modality. For instance, the combination of Fe and Au provides a dual contrast agent using MRI and CT scans. Such a combination can be prepared upon preparing nanoparticles, resulting in dual elements nanoparticle, or combined upon aerosol preparation, such as during spray drying. The latter method can be performed easily since the combination of available formulas is feasible as long as the formulas are compatible. Another further development of new aerosol for lung imaging is using an animal model, which mainly focuses on small animals, such as a mouse. Due to the distant physiological and anatomical gap between the animal model and human lung, the use of small animals in pre-clinical study typically has low translational success. Therefore, the use of relatively larger animals, such as piglets or ferrets, should be considered in future studies.