The Co-imaging of Gamma Camera Measurements of Aerosol Deposition and Respiratory Anatomy

Abstract

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.

Introduction

Aerosol deposition can be assessed by the addition of a radiolabel to an aerosol formulation, followed by the acquisition of a gamma camera image. Both two-dimensional (2D) planar scintigraphy and three-dimensional (3D) single-photon emission computed tomography (SPECT) gamma camera techniques are used to assess inhaled aerosol deposition and are well described in the literature.^(1–3) Such techniques can give some insight into the fate of inhaled aerosols and can provide useful information when seeking to optimize inhaled therapeutic interventions. It is also possible to acquire data on aerosol deposition using positron emission computed tomography (PET), which uses an inhaled radiolabeled aerosol technique and gives 3D imaging information.^(4,5) PET has significantly different radiolabeling processes from 2D scintigraphy and SPECT. This review will concentrate on the use of 2D scintigraphy and SPECT and the co-imaging of respiratory anatomy.

2D planar gamma camera imaging is an established method of obtaining information on the total amount of deposition in the lung and gives some information on regional deposition within the lung. SPECT imaging provides 3D data sets and enables greater regional information to be described. Both 2D planar and 3D SPECT imaging techniques have been used by many centers to study the behavior of in vivo inhaled aerosols in health and disease.^(1–3) Data from 2D scintigraphy and SPECT can then be aligned to anatomical images obtained using other imaging techniques such as krypton ventilation scans and low- and high-resolution X-ray computed tomography (CT). The co-imaging approach allows interpretation of deposition per lung and, potentially, per anatomical unit via the description of lobes. Interpretation of deposition patterns acquired by gamma camera imaging can also be extended via the use of mathematical modeling to give estimates of deposition per airway generation.^(6–10) 3D ventilation and perfusion imaging of the lung can also be undertaken using SPECT-CT,^(11–13) which offers the possibility of being aligned to SPECT aerosol deposition data such that ventilation and perfusion could be matched to inhaled drug deposition. This offers the potential of further insights into disease processes and therapeutic intervention. Both 2D planar imaging and SPECT can also be applied to the study of anatomical disruption that can accompany disease progression. In particular, there are groups working on the assessment of epithelial disruption⁽¹⁴⁾ and the dysfunction of mucociliary clearance^(15–17) that can occur with respiratory diseases such as asthma and cystic fibrosis.

2D Planar Gamma Camera Imaging

2D planar gamma camera imaging uses a gamma camera with one or more detector heads to create a 2D view of an inhaled radiolabeled aerosol. This is a popular approach to the quantification of the deposition of an inhaled radiolabeled aerosol.^(1–3) Typically, the isotope chosen for such studies is the radionuclide technetium (^99mTc), which has a convenient half-life of approximately 6 hr. After using technetium to label an inhaled formulation, a research volunteer performs a controlled inhalation of the radiolabeled aerosol such that a prespecified amount deposits in the lung. The research volunteer is immediately imaged using a static gamma camera(s), and the subsequent 2D image can be assessed both qualitatively and quantitatively. Figure 1 depicts examples of some typical 2D gamma camera images.

FIG. 1.

Typical 2D planar gamma images of (anterior): (A) very mild obstructive lung disease (FEV₁>85% predicted) and (B) moderate-severe obstructive lung disease (FEV₁<50% predicted). Uneven deposition of the inhaled radiolabeled aerosol can be seen clearly in B compared with A. There appears to be no aerosol deposition in some sections of the lung in B.

3D Gamma Camera Imaging: SPECT

SPECT uses a gamma camera with one or more detector heads to rotate around an individual and acquire images at intervals through 360° to create a 3D view of an inhaled radiolabeled aerosol. The most frequent method quoted in studies using SPECT is the use of a dual- or multihead, gamma camera system. The choice of radiolabel and the administration of the radiolabel are the same as for 2D gamma camera imaging. This 3D imaging approach is used by fewer centers and has more complex image analysis methodologies, but does offer the potential of greater regional information on the fate of inhaled radiolabeled aerosols.^(1,18–20)

2D Planar Gamma Camera Imaging: Relationship of Aerosol Deposition to Anatomy

The images presented in Figure 1 depict the typical appearance of a 2D planar gamma camera image of an inhaled radiolabeled aerosol. It is possible to see differences in deposition with the same aerosol formulation, between two individuals with differing degrees of airway obstruction. Quantification of such gamma camera images is essential if these images are to be compared statistically. Quantification informs where an inhaled agent has deposited and the density of deposition. This information is essential to the understanding of dosimetry and therapeutic intervention. The amount of activity external to the lung fields is also of interest, e.g., how much activity has deposited on a mouthpiece or has been swallowed. Other types of gamma camera imaging, such as a ventilation scan using inhaled krypton gas (^81mKr) or a xenon equilibrium scan (¹³³Xe) or a transmission scan using a flood source (⁵⁷Co or ^99mTc), give useful anatomical data on lung outline and density and can be aligned with, and enable attenuation correction of, deposition data acquired by 2D gamma scintigraphy.

Within the lung fields, an assessment of deposition in the more central airways versus the amount present in the small airways and alveolar zone is considered relevant to respiratory disease.^(21–24) After correction for decay, background, scatter, and attenuation correction, a region of interest is drawn around the lung margin, and activity within that region is defined as the total lung deposition. It is usual for the right lung to be used for this analysis, as there is often an overlap of activity in the stomach with activity in the base of the left lung. The lung outline can be defined from several additional sources, including a ventilation scan, a transmission scan, or via co-registered CT data.^(1,3) The assessment of total lung deposition has been found to correlate well with other methods such as charcoal block and urinary excretion pharmacokinetics.^(1,3)

Regions of interest are then created over the lung field to quantify regional lung deposition. Typically, central (C) and peripheral (P) zones are created. One of the acknowledged issues with this approach is the reduction of the complex 3D structure of the branching bronchial tree to a 2D “zone,” with, for instance, the central zone inevitably including peripheral airways as well as central airways.⁽¹⁾ Nevertheless, the use of these zones and ratios of C/P and P/C have been useful to interpret gamma scintigraphy data in terms of regional deposition of an inhaled aerosol and has helped to explore aerosol behavior in vivo.⁽³⁾ A penetration index (PI) can then be calculated, defined as the ratio of P/C counts for aerosol deposition normalized to the P/C counts from a ventilation, xenon equilibrium, or transmission scan. The C/P ratios can also be used, i.e., the inverse of the P/C ratio, to describe the PI. Different research centers use different definitions of C and P and different methods to normalize the data.

2D Planar Gamma Camera Imaging: creation of C/P and P/C ratios and the PI

Biddiscombe et al.⁽²⁵⁾ compared methods used by different research groups for describing lung regions of interest. This group evaluated the effect of different approaches on 2D planar gamma camera images acquired following the administration of monodisperse albuterol aerosols of differing particle size (1.5, 3, and 6 μm) in five mild asthmatic subjects. The geometry used to define central (C) and peripheral (P) lung regions of interest varied markedly between different research groups. Normalizing P/C count ratios by the ventilation P/C count ratios to create a PI reduced the variability of the data. The conclusion of this work was that different approaches used to determine pulmonary regions of interest and quantify aerosol deposition produce different results. This work clearly highlighted the need for a consensus to standardize the methodology to facilitate data comparison between research groups,⁽³⁾ and the International Society for Aerosols in Medicine (ISAM) will shortly be publishing international consensus guidelines.

A common approach to the description of the branching structure of the bronchial tree is to use the system proposed by Weibel,⁽²⁶⁾ who considered the bronchial tree to consist of 23 branching airways, with the trachea as generation 0. With this system, the conducting airways are those down to generation 16, with the alveolated airways extending from generation 17 to 23. With the 2D method of gamma scintigraphy, relating central and peripheral regions of interest to airway anatomy is limited by the complexity of the 3D branching of the bronchial tree translated as a 2D image. A “central” region of interest will contain a mix of airway generations, but may contain a high proportion of the larger airways. The “peripheral” region of interest will probably contain a greater proportion of smaller airways and alevoli (generations 16–23). There is further limitation, which is true for all gamma camera studies including SPECT, in that the resolution of gamma scintigraphy is approximately 10–14 mm. However, despite these limitations, there is evidence that the assumptions about the central and peripheral regions of interest are approximately correct in that P/C ratios have been shown to respond in the expected way to alterations in the particle size and delivery parameters of an inhaled aerosol.^{(20,24,25,27–30)}

Some groups have used data from computer-simulated models of lung anatomy derived from CT or magnetic resonance imaging (MRI) and applied that data to gamma scintigraphy studies.^(20,31–33) Fleming et al.⁽³²⁾ used computer simulation techniques derived from MRI studies to evaluate the accuracy and precision of aerosol deposition measurements using 2D planar gamma camera imaging. They concluded that, provided a reasonable method of attenuation correction was used, total lung activity could be estimated with a precision of less than 10%. The ability of 2D planar imaging to estimate regional deposition was more limited with a precision of approximately 21%.

Another approach to distinguish between aerosol deposited on the conducting (ciliated) airways versus that deposited in the alveolar zone (nonciliated airways) is to look for the activity left 24 hr after inhalation of an insoluble or poorly soluble radiolabel.⁽³⁴⁾ This 24-hr retention index is useful in health, but may be limited in disease, where longer retention times in the conducting airways have been shown.^(28,35–37)

The clearance of a suitable radiolabel from the ciliated airways within a given time period is one method of quantifying the relative efficiency of the mucociliary clearance mechanism (MCC) in vivo, and thus the state of the respiratory, ciliated epithelium. Studies of MCC can highlight disease processes when compared with health and can help to establish the efficacy of a therapeutic intervention versus a control. Sequential imaging of the clearance of the inhaled radiolabel is used to measure MCC.^(3,37–39)

Planar imaging has also been used to investigate the changes in the permeability of the respiratory epithelium that can occur with disease. Planar imaging of the rate of clearance from the lung, i.e., through the epithelium and into the bloodstream, of technetium-labeled diethylenetriaminepentaacetic acid (^99mTc-DTPA) is used by some research groups and has the potential of quantifying therapeutic intervention, such as the use of keratinocyte growth factor as a potential treatment for allergy and inflammation.^(14,40,41)

2D gamma scintigraphy has therefore been accepted as a method of quantification of total lung deposition and does additionally give some information on regional lung deposition (Table 1). The development of SPECT, however, offered the possibility of 3D analysis of inhaled deposition, and this has provided a more sensitive method of quantifying spatial distribution of deposition.

Table 1.

Summary of Techniques Used in Co-registering Anatomy with Gamma Camera Imaging

	Anatomical imaging from co-imaging source	Applied information on anatomy
2D planar scintigraphy	Lung outline from transmission scan, ventilation (Kr), equilibrium (Xe) scan, or CT	MCC rates describe functional deficit of the MCC mechanism as a result of pathophysiological changes
	Regional deposition estimated from 2D regions of interest to create “central” and “peripheral” zones and PI	Epithelial permeability studies describe functional deficit of the respiratory epithelium as a result of pathophysiological changes Aligned 2D V/Q imaging
SPECT	Lung outline as for 2D planar scintigraphy	3D MCC is a possible application
	Regional deposition information from:	3D epithelial permeability studies are also a possible application
	1. 3D “central” and “peripheral” zones and PI normalized to volume (CT)	Aligned 3D SPECT V/Q imaging
	2. Shell analysis
	3. Modeling of data to give deposition per airway generation
	4. Use of co-registered HRCT data to align central airway, lobes, and sublobe anatomy
	5. Co-registered HRCT data on lung density mapping to align regional information on disease pattern
	6. Aligned information from other imaging sources, e.g., MRI, EBUS, pCLE, and OCT to regional patterns of deposition

3D Gamma Camera Imaging (SPECT): Relationship of Aerosol Deposition to Anatomy

Once acquired, SPECT data can be reconstructed in 3D and viewed in multiple planes. Typically, a SPECT study will be acquired over a period of 10 min, compared with an acquisition time of approximately 1 min for 2D planar gamma camera imaging. This requires a radiolabel that will be stable for the period of time required for the imaging protocol, such as a technetium-labeled colloid.^(3,18) Eberl et al.⁽²⁹⁾ and the imaging group based in Sydney have developed a “fast SPECT” protocol that is capable of acquiring a SPECT study in less than 1 min, using multiple gamma cameras with limited axial field of view. However, fast dynamic SPECT imaging is technically and computationally more demanding, and so not widely used.

SPECT, compared with planar imaging, is more sensitive in detecting regional changes in deposition, which may be important when considering regional targeting of an inhaled aerosol or where there are regional differences in disease progression.^(1,3)

With SPECT, it is also possible to assess both the left and the right lungs, as the 3D data set allows delineation between the lungs and other structures such as the stomach.

It is increasingly common for manufacturers to offer a combined SPECT plus CT scanner. The use of CT combined with SPECT allows for attenuation correction and offers alignment of anatomy with the significant advantage that co-imaging only requires a single imaging visit for the research participant/patient and also avoids the need for image registration. SPECT deposition data can then be viewed aligned to corresponding CT data (Fig. 2), which can offer increased insight to the in vivo fate of inhaled aerosols in health and disease.

FIG. 2.

SPECT-CT. Examples of tomographic SPECT data aligned with CT. Moderate-severe obstructive lung disease. (A) Transverse, (B) sagittal, and (C) coronal planes. The SPECT data overlaid on the CT data show uneven deposition with some areas of absent deposition, typical of moderate-severe obstructive lung disease.

The 3D SPECT data set can also be viewed as the amount of activity in a series of concentric shells radiating from the hilum.^(1,18) For the shell analysis, often 10 shells are described, with the first shell being closest to the hilum and the tenth shell being the most peripheral. Using this approach, it is possible to describe a 3D PI by summing activity in the first five shells as representing the “central” zone and the outer five shells as the “peripheral” zone. The 3D PI has been found to be more accurate in characterizing regional differences in aerosol deposition than 2D PI.⁽⁴²⁾

Using shell analysis data, via a data inversion method, a calculation of the amount of deposition per airway generation can be made.^(3,18,19,43) Assessment of deposition data as activity per airway generation requires some modeling of anatomy⁽⁴⁴⁾ for which there are a variety of approaches, including the use of conceptual,⁽⁴⁵⁾ deterministic,⁽¹⁰⁾ and hybrid models.⁽⁴⁶⁾ The use of modeling to determine deposition per airway generation is still necessary when analyzing SPECT data, as the spatial resolution is similar to that of 2D planar imaging and not sufficient to visualize the small airways. The advantage of SPECT, however, is the ability to view and analyze deposition in all planes and not as a compressed 2D image. SPECT therefore offers an increased ability to discriminate between differences in regional deposition.^(18,29)

There is the possibility that SPECT could be used for the assessment of MCC, offering the potential for regional information on MCC as opposed to a single global measure of clearance now used via the measurement of sequential 2D total lung clearance.⁽³⁾ MCC in the central airways is a comparatively rapid process, and so a “fast” SPECT protocol may be necessary.⁽²⁹⁾ The potential increase in regional information offered by SPECT has also been used to investigate the detection of ventilation and perfusion defects. SPECT has become an increasingly popular method used for the assessment of pulmonary ventilation and perfusion (V/Q).^(11–13,47) SPECT images are acquired of the distribution of an inhaled radiolabeled gas such as krypton (^81mKr) and the distribution of an intravenous injection of technetium (^99mTc)–labeled macroaggregated albumin. V/Q SPECT has been found to have better diagnostic capabilities than the traditional 2D planar V/Q scintigraphy⁽¹¹⁾ and has been reported as having high levels of sensitivity and specificity.⁽⁴⁸⁾ There is now the potential to co-register data from SPECT V/Q with SPECT deposition patterns, allowing the matching of deposited aerosol with regional ventilation and perfusion.

3D radionuclide imaging combined with anatomical information from CT and computer analysis is a useful approach for applications requiring regional information on deposition. High-resolution CT (HRCT) can be used to describe the anatomy of the central airways in detail⁽¹⁸⁾ and can offer information on anatomical segmentation of the lungs, such as the description of lobes and sublobar segments.

HRCT

Data from HRCT can be used as additional anatomical mapping for aerosol deposition images. HRCT offers high-resolution, detailed anatomical mapping, but does come with the disadvantage of a significant radiation dose. The central airways can be described in detail down to approximately generation 6, compared with a maximum of approximately three generations for the low-resolution, low-dose CT. Using commercially available software (Vida Diagnostics, Coralville, IA), the airway tree geometry can then be described based on the segmental branching of the airway tree. This allows the lobes and sublobar segmentation to be described (Fig. 3). Using a free-form polynomial warp algorithm, the segments described by HRCT could then be aligned with SPECT-CT or with SPECT V/Q images. The assessment of deposition data in terms of lobes and sublobar segments may be useful when investigating the inhomogeneity of disease such as that found in chronic obstructive pulmonary disease or cystic fibrosis or when applied to aerosol deposition images for a measure of therapeutic effectiveness of delivery to different lung segments.

FIG. 3.

(A) An example of segmentation of the airway tree from an HRCT scan, clearly demonstrating that, in this case, up to six generations of the airway tree were detectable. (B) An example of the automated division of the lung HRCT scan into anatomically relevant regions based on the airway tree anatomy and approximating the bronchopulmonary segments.

HRCT images can also be used to assess lung field density and may be useful in diseases characterized by structural lung changes such as emphysema.⁽⁴⁹⁾ Comparisons of HRCT at residual volume and at full inspiration can also be used to map areas of gas trapping, which is often found in moderate to severe lung disease.^(50,51) These approaches give useful, regional information on disease progress, and mapping such data to deposition patterns has the potential of being a very useful technique.

Radiation Dosimetry Issues for 2D Scintigraphy, SPECT, and CT

Most studies using 2D scintigraphy will aim to deliver 1–10 MBq of inhaled activity to the lungs.^(1,20,25) SPECT studies generally aim for lung activity of approximately 20–25 MBq of inhaled activity to the lungs. The risk to the research participant/patients is then determined by calculating the “effective dose.” The unit for the effective dose is the sievert (Sv) and is calculated by assessing both the activity and weighting factors designed to take into account different risks associated with different organs. Calculating dose depends on several factors, such as: the particular radionuclide being used; the amount of radionuclide delivered to both the lungs and other parts of the body, such as the stomach; and how the radionuclide is transported once deposited. The International Commission on Radiological Protection and the UK Administration of Radioactive Substances Advisory Committee (ARSAC) both give extensive guidance on such calculations. The effective dose for both 2D scintigraphy and SPECT is usually less than 0.5 mSv, with 2D scintigraphy usually resulting in a smaller dose compared with SPECT studies. This can be compared to the UK national background radiation dose of 2.2 mSv per year (UK Health Protection Agency).^(52–54)

The addition of CT data will add significantly to the total effective dose with effective doses of approximately 0.8–4.0 mSv quoted for SPECT-CT studies, depending on the protocol. This obviously limits the number of repeat measures that can be taken.

Other Imaging Methods

Chronic airways disease is often characterized by the presence of inflammation and subsequent remodeling of the airways. One of the key aspects of the remodeling process is change within the extracellular matrix (ECM) of the submucosa throughout the bronchial tree.⁽⁵⁵⁾ Images can be obtained at bronchoscopy of the ECM via the use of probe-based confocal laser endomicroscopy (pCLE) (Cellvizio^® system, Manua Kea Technologies, Paris, France). This technique images the elastin microstructure within the ECM of the subepithelial region of the submucosal layer⁽⁵⁶⁾ of the airway walls and the alveoli and therefore can potentially provide analysis of the remodeling process (Fig. 4). Objective analysis of the pCLE images is challenging in several ways, not least because of the detail and complexity of the images, but offers potential as an additional imaging tool that could be co-registered with SPECT aerosol deposition patterns. This is possibly a useful approach for the study of novel inhaled anti-inflammatory therapies.

FIG. 4

An example of a pCLE image. Macrophages and elastin fibers can be seen, both of which autofluoresce under the laser light from the pCLE system. The elastin fibers are the linear structures, and the circular, bright structures are macrophages. In this example, the elastin fibers are surrounding the open ends of alveoli ducts.

Other novel imaging techniques that could inform SPECT-CT include endobronchial ultrasound (EBUS), which images the airway wall via a bronchoscope and has the potential to visualize the changes in the airway wall that can occur with disease,⁽⁵⁷⁾ and optical coherence tomography (OCT), which uses infrared light to describe cellular and extracellular structures⁽⁵⁸⁾ via a fiberoptic catheter. These new techniques all require validation, but may be of interest in the future.

Summary

The use of 2D gamma camera imaging is widely accepted as a method of assessing the deposition of a radiolabeled aerosol. This useful technique has been used in the majority of clinical imaging trials. This method is used to obtain information on the total amount of deposition in the lung and also to give some information on regional deposition within the lung. SPECT is a more complex method, but is used preferentially when regional deposition information is the primary research question. Co-imaging of aerosol deposition with anatomical information can be achieved using a variety of methods including CT. Radionuclide imaging combined with anatomical information from CT and computer analysis is a useful approach for applications requiring regional information on deposition. Addition of anatomical mapping from HRCT allows lobar and sublobar analysis, which may be useful in 3D V/Q work and in assessing discrete delivery of aerosol.

The co-imaging of aerosol deposition and respiratory anatomy may be useful as a method of following disease progression, as a potential biomarker of disease and for the optimization and evaluation of treatments.

Footnotes

Author Disclosure Statement

For all authors no conflicts of interest exist.

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