Quantitative analysis of radioaerosol inhalation and perfusion scintigraphy in dogs

Comparison of deposition images obtained by use of an ultrafine 99m-technetium-labeled carbon dry aerosol with ventilation images obtained by use of 81m-krypton gas for evaluation of pulmonary dysfunction in calves

OBJECTIVE

To characterize the accuracy of an ultrafine 99m-technetium-labeled carbon dry aerosol for use in assessment of regional ventilation in calves with pulmonary dysfunction.

ANIMALS

7 Belgian White and Blue calves.

PROCEDURE

The ultrafine aerosol was assessed by comparing deposition (D) images with ventilation (V) images obtained by use of 81 m-krypton (81mKr) gas via D-to-V ratio (D:V) image analysis in calves during spontaneous breathing (SB) and during experimentally induced pulmonary dysfunction (ePD).

RESULTS

Mismatching index (LrTot) calculated on the D:V images revealed a good match (LrTot, 0.96 +/- 0.01) between D and V distribution patterns in calves during SB. Calculation of the ultrafine aerosol penetration index relative to 81mKr (PIRel) revealed preferential distribution of the ultrafine aerosol in lung parenchyma (PIRel, 1.13 +/- 0.11). In ePD, heterogeneity in the D:V distribution was observed (LrTot, 0.78 +/- 0.10) as a result of ultrafine aerosol particles impaction in airways as indicated by PIRel (0.66 +/- 0.16) and a proportion of pixels more radioactive in D images, compared with V images, that was located in the central part of the lung (475 +/- 77% in ePD vs 32.8 +/- 5.7% in SB). However, this central deposition did not prevent visual examination of the entire ventilated lung.

CONCLUSIONS AND CLINICAL RELEVANCE

The ultrafine aerosol appears suitable for use in examination of ventilated parts of lungs of cattle, even those with impaired pulmonary function. However, airway impaction of ultrafine aerosol particles impedes the quantification of regional ventilation in cattle with abnormal lung function.

Comparison between radioactive aerosol, technegas and krypton for ventilation imaging in healthy calves

The use of lung scintigraphy in calves necessitates the validation of a ventilation (V) imaging agent compatible with clinical applications. This study aimed at defining the value of an inhaled radioactive aerosol (99mTc-DTPA) and a 'pseudogas' (Technegas) in the assessment of regional V in healthy conscious calves by comparing 99mTc-DTPA and Technegas deposition (D) images to V(V) images obtained from the steady-state inhalation of the short half-life krypton 81 (81mKr) gas. Images were compared by analysis of radioactivity distribution in computer-generated regions of interest within the right lung and D to V ratio images were generated in order to highlight areas of mismatching between 99mTc-DTPA or Technegas and 81mKr distributions. Results of this analysis showed that the 99mTc-DTPA aerosol droplets were unable to reach the lung parenchyma because of significant particle impaction in the major conducting airways. Better definition of the ventilated lung was obtained when using Technegas because of minimal deposition in conducting airways. Furthermore, the Technegas and 81mKr distribution patterns were highly equivalent.

Analysis of scintigraphical lung images before and after treatment of horses suffering from chronic pulmonary disease

Scintigraphical analysis of the ratio of inhalation (I) to perfusion (Q) was designed to determine whether chronic pulmonary disease in horses induced mismatches in I/Q and to assess whether medical treatment would restore an I/Q distribution pattern identical to that of control horses. In addition, the results of the I/Q analysis were correlated with the alveolar-arterial PO2 difference (AaDO2). The I/Q matching found in a group of control horses was compared with the I/Q analysis of a group of diseased horses before and after their clinical signs had been treated. The analysis indicated that there was mismatching between I and Q before they were treated. The treatment improved the diseased horses, pulmonary function but there was still heterogeneity in the I/Q distribution after they had been treated. The I/Q analysis parameters were well correlated with the ratio of ventilation (VA) and pulmonary blood flow (Q), evaluated by the determination of AaDO2.

Analysis of equine scintigraphical lung images

A method is proposed (1) to create inhalation to perfusion ratio (I/Q) images from equine lung scintiscans and (2) to analyse these I/Q images. This method was applied to five healthy horses in order to establish reference ranges of I/Q distribution pattern. Computed I/Q images were divided in three regions based on the activity due to the inhalation (I) versus perfusion (Q) procedure. For each region a regional mismatching factor (Iri), i.e. the I/Q of the region, and an intraregional mismatching factor (Lri), which expresses the variability of pixels' I/Q within the region, were calculated. The Per(Ri) and Cen(Ri), which are the percentage of pixels in the parenchymal and in the central parts of the lung belonging to the region, were also computed. Results of this analysis showed a good match between I and Q distribution pattern.

Scintigraphic analysis as a diagnostic tool in canine experimental lung embolism

Scintigraphic changes occurring after lung embolism induced by intravenous administration of agar were assessed in 10 anesthetized prone dogs. Using 99mTc aerosol inhalation combined with 99mTc perfusion scintigraphy, parameters quantifying the regional distribution of the inhalation-to-perfusion ratio (I/P) and the degree of intraregional I-P mismatching were calculated from dorsoventral views (DV) for 6 equidistant cranial-to-caudal left and right lung zones before (B) and after acute embolism (PE). Postmortem examination was performed in 7 dogs. Several perfusion defects could be detected in either the conventional or the functional (I-P) lung scintigrams. Nevertheless, PE did not induce significant changes in blood gases. On the scintigrams, the outer zones appeared to be the most heavily embolized. The regional I/P ratio was increased in the more caudal and cranial parts of both lungs. However, postmortem examination of the lungs showed the presence of emboli diffuse in all lobes. Their presence was revealed in all zones by a significant increase of an intraregional mismatching index rho and especially of an intraregional mismatching factor mean value of gamma. After acute embolism, blood was redistributed to perihilar regions, especially in the right lung. This redistribution process continued during the experiment. In conclusion, the use of combined aerosol inhalation/perfusion lung scintigraphy with quantitative analysis provides a sensitive method for the detection of pulmonary embolism.

Pulmonary scintigraphy in canine lobar and sublobar airway obstruction

The intrapulmonary distribution of the inhalation-to-perfusion ratio (I/P) was studied after placement of an obstruction in a sublobar bronchus (SLO) or in a lobar bronchus (LO) in 17 anesthetized prone (sternal recumbent) dogs. Placement of an SLO or LO did not induce any significant changes in the standard ventilatory and hemodynamic parameters measured. With the use of 99mTc aerosol inhalation combined with 99mTc perfusion lung scintigraphy, parameters quantifying the inhalation (I) to perfusion (P) mismatching at regional as well as at intraregional levels were calculated. The SLO increased the relative ventilation to the lung containing the obstruction, induced a shift of blood from the obstructed segment to the rest of the same lung, and increased the ratio between the mean I and the mean P (regional mismatching factor) in both the obstructed segment and the lung containing the obstruction. The LO diverted air and blood away from the obstructed lobe to the contralateral lung, but blood to a lesser extent. The LO decreased the regional mismatching factor in the obstructed lobe but also in the lung containing the obstruction. It also increased significantly the intraregional I to P mismatching in the obstructed lobe as well as in the rest of the same lung. After withdrawal of LO and reinsufflation of the collapsed lobe, blood continued to leave this lobe, while the withdrawal of SLO allowed recovery of the initial perfusion. This suggests that both hypoxic vasoconstriction and mechanical factors such as vascular distortion are involved in the blood shifts observed, and require some delay to be relieved. It is concluded that in dogs compensating mechanisms such as collateral ventilation are very effective. In case of SLO, I to the lung involved is increased. In case of LO, vasoconstriction in response to alveolar hypoxia could not fully compensate for the decreased I to the obstructed lobe.