Abstract :
[en] Diagnostic imaging of the lung disease using radioactive tracers is a well established procedure in human nuclear medicine. Most veterinary institutions and some large equine practices have acquired nuclear imaging facilities to perform bone imaging. If indications for scintigraphic bone scanning are evident, the place of pulmonary scintigraphy, with regard to the other diagnostic techniques, had to be defined. Furthermore, the peculiarities of equine anatomy required to define specific scintigraphical procedures for this species. Therefore, the objectives of the studies performed were to describe standardised procedures and demonstrate the potential of lung scintigraphy regarding clinical and research applications.
To test the clinical interest of pulmonary scintigraphy, standardised methods to study two important functional parameters (i.e. the ventilation-perfusion relationship which is of paramount importance to ensure efficient blood oxygenation and measurement of alveolar clearance which is an index of pulmonary epithelial permeability) were developed. The clinical interest of these measures was checked by comparing the values of these functional parameters obtained in healthy horses (i.e. reference values) to the ones of horses suffering from chronic obstructive pulmonary disease (COPD).
The method proposed to assess the matching between the ventilation and the perfusion proposes to mimic the ventilation distribution with a radioactive aerosol inhalation (I) imaging and to visualise the perfusion with an injection of radioactive macroaggregates particles (Q). Therefore, the ventilation-perfusion relationship was assessed through the medium of inhalation to perfusion ratio (I/Q) images.
Performed with healthy horses, the method developed showed a good match between the I and Q distribution pattern. This matching was significantly altered in COPD-affected horses exacerbating the symptoms of the disease. After medical treatment, the classical pulmonary function tests indicated that the pulmonary function was improved. Nevertheless, the I/Q analysis demonstrated that heterogeneity in the I/Q distribution was still present.
Alveolar clearance measurement, the second scintigraphical method developed, is an index of alveolar epithelium integrity. Structural alteration of the alveolar epithelium by infectious or non infectious diseases accelerates clearance of hydrophilic compounds deposited within the lung. In vivo, the integrity of the alveolar epithelium was evaluated by studying the passage through the alveolar-capillary barrier of a radioactive hydrophilic solute administered to the alveoli by nebulisation. The normal rate of radioactive tracer lung clearance was established in healthy horses and compared to value obtained with COPD-affected horses. During a crisis of COPD, the clearance of the tracer was significantly increased. In remission of the disease obtained following two months at pasture, clearance rate of COPD-affected horses did not differ from the rate of healthy horses. Maintained in a controlled (i.e. allergen “free”) environment, these horses remained in clinical remission of the disease. Nevertheless, the clearance rate was found to be intermediate between normal values and results obtained during the crisis of COPD. Therefore, measure of alveolar clearance allows grading of lung dysfunction induced by COPD. This was particularly interesting since pulmonary function tests and bronchoalveolar lavage performed at the same occasions than the scintigraphical test failed to differentiate the remission obtained at pasture from the one present in a controlled environment.
In conclusion, analysis of I/Q images and alveolar clearance measurement showed that pulmonary disorders may be present in the absence of clinical symptoms and suggest that these non-invasive tests should be considered as an additional diagnostic tool for the detection of pulmonary dysfunction.
The value of scintigraphy as a research tool was confirmed by the feasibility of visualising the distribution of radioactive red blood cells injected intravenously to horses. This procedure might be useful to develop a way to detect and quantify exercised induced pulmonary haemorrhage. Preliminary results suggest that very small amount of bleeding might potentially be detected with scintigraphy; they also suggested that the limiting factor for detecting the haemorrhage may be the level of lung background radioactivity which is the radioactivity of red blood cells remaining in the circulation i.e. not involved in the haemorrhage.
Finally, the potential role of scintigraphy to develop inhalation therapy in horses was confirmed by comparing two different aerosol delivery system. The dose deposited (quantitative analysis) and the regional distribution (qualitative analysis) of an aerosol in the equine lungs using either an ultrasonic nebuliser (UN) currently used in human medicine or a high pressure jet nebuliser were assessed and compared. The quantitative and qualitative analysis showed that both systems used in this study enable aerosol deposition in the parenchymal part of the lung. If this region is the target site of deposition, the JN tested in this study should be preferred to the UN used.
Conclusion Scintigraphy appears to be a valuable technique to quantify functional disturbances. Furthermore, it is a sensitive method to detect subclinical pulmonary involvement and to follow the pulmonary response to a treatment. Finally, in providing detailed information about the deposition of inhaled aerosols in the horse’s lung, scintigraphy has an established role in the development and assessment of inhalation therapy.
