Physiological evaluation of a new quantitative SPECT method measuring regional ventilation and perfusion

A technique for lung ventilation-perfusion SPECT in neonates and infants

BACKGROUND

Single photon emission tomography (SPECT) of the lung is a well-established non-invasive technique for quantitative assessment of regional lung ventilation and perfusion distribution in children and in adults. However, its application in neonates as well as infants has been scarce because of several practical limitations, such as the trade off between image quality and restricted effective radiation doses and the lack of suitable inhalations agents and administration techniques.

METHODS

In this paper, a new technique for quantitative regional lung SPECT based on a passive Technegas administration procedure is described and clinically applied. The first clinical findings in neonates are reported.

RESULTS

This technique overcomes some of the limitations of commercial ventilation systems by making patient compliance unnecessary, avoiding difficult breathing manoeuvres and by minimizing both facemask dead space and inspiratory-expiratory resistance.

CONCLUSION

This technique satisfies requirements for routine applications in neonates, infants and even older patients and has a potential use also for mechanically ventilated patients. It has the potential to allow a more precise functionally oriented diagnosis, which is of importance for treatment and follow-up in patients with severe lung diseases.

Posture primarily affects lung tissue distribution with minor effect on blood flow and ventilation

We used quantitative single photon emission computed tomography to estimate the proportion of the observed redistribution of blood flow and ventilation that is due to lung tissue shift with a change in posture. Seven healthy volunteers were studied awake, breathing spontaneously. Regional blood flow and ventilation were marked using radiotracers that remain fixed in the lung after administration. The radiotracers were administered in prone or supine at separate occasions, at both occasions followed by imaging in both postures. Images showed greater blood flow and ventilation to regions dependent at the time of imaging, regardless of posture at radiotracer administration. The results suggest that a shift in lung parenchyma has a major influence on the imaged distributions. We conclude that a change from the supine to the prone posture primarily causes a change in the vertical distribution of lung tissue. The effect on the vertical distribution of blood flow and ventilation within the lung parenchyma is much less.

Ventilation-perfusion relationships in pulmonary arterial hypertension: effect of intravenous and inhaled prostacyclin treatment

In seven patients with idiopathic or secondary pulmonary arterial hypertension (PAH), ventilation-perfusion (V (A)/Q ) relationships were measured during a right heart catheterization using the multiple inert-gas elimination technique before and during intravenous infusion with epoprostenol (EPO), and following 5 months of 20 microg inhaled iloprost taken three times daily (ILO). Pre-treatment pulmonary vascular resistance (PVR) was 9.3+/-5.0 mmHg/l/min and the dispersion of perfusion and ventilation for V (A)/Q -ratios was increased. EPO reduced PVR by 20%, and increased cardiac output, shunt, and mixed venous oxygenation (SV(O2)). The arterial oxygen tension (Pa(O2)) remained unchanged. Basal central haemodynamics did not change after 5 months of ILO. Fifteen minutes after ILO, PVR decreased by 20%, and the shunt, SV(O2), and Pa(O2) remained unaltered.

CONCLUSIONS

In secondary PAH with normal lung volumes, significant V (A)/Q mismatching occurred. The PVR was reduced to a similar degree during EPO and after ILO, but only EPO increased the shunt and SV(O2). EPO and ILO did not significantly affect the Pa(O2).

Position affects distribution of ventilation in the lungs of older people: an experimental study

QUESTION

What is the effect of sitting and side-lying on the distribution of ventilation during tidal breathing in healthy older people?

DESIGN

Randomised, within-participant, experimental study.

PARTICIPANTS

Ten healthy people more than 65 years old.

INTERVENTION

Tidal breathing during sitting and right side-lying.

OUTCOME MEASURES

Distribution of ventilation as a percentage of total counts using Technetium-99m Technegas lung ventilation imaging.

RESULTS

In sitting, the ratio of the distribution of ventilation to apical: middle: basal regions was 1: 3.5: 3.3 in the right lung, and 1: 2.9: 2.3 in the left lung. In right side-lying, 32% (95% CI 22 to 43) more ventilation was distributed to the right lung than to the left lung. The ratio of the distribution of ventilation to apical: middle: basal regions was 1: 2.8: 2.2 in the right lung, and 1: 2.4: 1.9 in the left lung.

CONCLUSIONS

In both sitting and right side-lying, ventilation was distributed more to the middle than to the basal region, which may be related to age-associated changes in the respiratory system.

Physiological imaging of the lung: single-photon-emission computed tomography (SPECT)

Emission tomography provides three-dimensional, quantitative images of the distribution of radiotracers used to mark physiological, metabolic, or pathological processes. Quantitative single photon emission computed tomography (SPECT) requires correction for the image-degrading effects due to photon attenuation and scatter. Phantom experiments have shown that radioactive concentrations can be assessed within some percentage of the true value when relevant corrections are applied. SPECT is widely spread, and radiotracers are available that are easy to use and comparably inexpensive. Compared with other methods, SPECT suffers from a lower spatial resolution, and the time required for image acquisition is longer than for some alternative methods. In contrast to some other methods, SPECT allows simultaneous imaging of more than one process, e.g., both regional blood flow and ventilation, for the whole lung. SPECT has been used to explore the influence of posture and clinical interventions on the spatial distribution of lung blood flow and ventilation. Lung blood flow is typically imaged using macroaggregates of albumin. Both radioactive gases and particulate aerosols labeled with radioactivity have been used for imaging of regional ventilation. However, all radiotracers are not equally suited for quantitative measurements; all have specific advantages and limitations. With SPECT, both blood flow and ventilation can be marked with radiotracers that remain fixed in the lung tissue, which allows tracer administration during conditions different from those at image registration. All SPECT methods have specific features that result from the used radiotracer, the manner in which it is administered, and how images are registered and analyzed.

Comparative analysis of different scintigraphic approaches to assess pulmonary ventilation

A study was carried out to investigate the predictive value of 81-metastable-krypton (81mKr) distribution, high-size 99-metastable-technetium (99mTc) aerosol deposition and low-size 99mTc aerosol (Technegas) deposition on the pulmonary ventilation evaluated by 133-xenon (133Xe) lung scintigraphy, and to assess the correlation between the 81mKr distribution, the 99mTc aerosols deposition, and the respiratory parameters of patients with chronic obstructive pulmonary disease (COPD). Twenty COPD patients were included. The 81mKr, 133Xe, and 99mTc aerosol lung scintigraphies were successively carried out. The 81mKr distribution and 99mTc deposition were compared to the 133Xe distribution at equilibrium and to the 133Xe clearance. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 81mKr and Technegas lung scintigraphies to detect alterations in ventilation revealed by 133Xe were defined. The 81mKr distribution and 99mTc deposition according to respiratory parameters were described using a principal component analysis. Compared to 133Xe distribution, a significantly higher distribution of 81mKr in the upper parts of the lungs in the more severe patients (p = 0.05), a significantly higher deposition of Technegas in the lower parts of the lungs (p = 0.0008), and a significantly higher deposition in the central parts of the high-size 99mTc aerosol were observed (p = 0.0001). The PPV and the NPV were, respectively, 0.54 and 0.58 for 81mKr and 0.54 and 0.55 for Technegas. There was a significant negative correlation between 81mKr distribution and 133Xe clearance (p = 0.0001) between Technegas deposition and 133Xe clearance (p = 0.0007), and between 99mTc diethylene-triamino-penta-acetate (DTPA) deposition and 133Xe clearance (p = 0.001). Both the 81mKr peripheral distribution and Technegas peripheral deposition correlated negatively with increased obstruction, as measured by forced expiratory volume in 1 sec (FEV1). Peripheral deposition of the high-size 99mTc aerosol deposition correlated with the inspiration/expiration time ratio. In conclusion, 81mKr and 99mTc aerosols' lung scintigraphies do not reflect exactly the pulmonary ventilation as measured by 133Xe scintigraphy.

Breath-hold single-photon emission tomography and computed tomography for predicting residual pulmonary function in patients with lung cancer

OBJECTIVE

We sought to evaluate the utility of integrated breath-hold single-photon emission tomography and computed tomography imaging compared with that of simple calculation with the lung segment-counting technique for predicting residual pulmonary function in patients undergoing surgical intervention for lung cancer.

METHODS

A prospective series of 22 patients undergoing anatomic lung resection for cancer were enrolled in this study. Postoperative residual forced expiratory volume in 1 second was predicted by measuring the radioactivity counts of the affected lobes or segments to be resected within the entire lungs by placement of regions of interest on single-photon emission tomography and computed tomography images. Residual forced expiratory volume in 1 second was also estimated by using the segment-counting technique.

RESULTS

Both predicted values agreed well with postoperative forced expiratory volume in 1 second. Although the residual forced expiratory volume in 1 second predicted by means of single-photon emission tomography and computed tomography correlated well with that predicted by using segment counting, the values were significantly underestimated by the segment-counting technique in 4 outliers with severe emphysema. There were 2 patients with borderline pulmonary functional reserve whose residual forced expiratory volume in 1 second values were predicted more accurately by means of single-photon emission tomography and computed tomography than by using segment counting.

CONCLUSION

Integrated breath-hold single-photon emission tomography and computed tomography images allow the accurate prediction of postoperative pulmonary function but without statistical superiority over the simple segment-counting technique. Further study of the usefulness of single-photon emission tomography and computed tomography in patients with severe emphysema and borderline lung function should prove valuable because the segment-counting technique underestimates pulmonary functional reserve in these patients.

Positron emission tomography imaging of regional pulmonary perfusion and ventilation

Positron emission tomography (PET) imaging is a noninvasive, quantitative method to assess pulmonary perfusion and ventilation in vivo. The core of this article focuses on the use of [13N]nitrogen (13N2) and PET to assess regional gas exchange. Regional perfusion and shunt can be measured with the 13N2-saline bolus infusion technique. A bolus of 13N2, dissolved in saline solution, is injected intravenously at the start of a brief apnea, while the tracer kinetics in the lung is measured by a sequence of PET frames. Because of its low solubility in blood, virtually all 13N2 delivered to aerated lung regions diffuses into the alveolar airspace, where it accumulates in proportion to regional perfusion during the apnea. In contrast, lung regions that are perfused but are not aerated and do not exchange gas (i.e., "shunting" units) do not retain 13N2 during apnea and the tracer concentration drops after the initial peak. Accurate estimates of regional perfusion and regional shunt can be derived by applying a mathematical model to the pulmonary kinetics of a 13N2-saline bolus. When breathing is resumed, specific alveolar ventilation can be calculated from the tracer washout rate, because 13N2 is eliminated almost exclusively by ventilation. Because of the rapid elimination of the tracer, 13N2 infusion scans can be followed by 13N2 inhalation scans that allow determination of regional gas fraction. This article describes insights into the pathophysiology of acute lung injury, pulmonary embolism, and asthma that have been gained by PET imaging of regional gas exchange.

Paradoxical redistribution of pulmonary blood flow in prone and supine humans exposed to hypergravity

We hypothesized that exposure to hypergravity in the supine and prone postures causes a redistribution of pulmonary blood flow to dependent lung regions. Four normal subjects were exposed to hypergravity by use of a human centrifuge. Regional lung perfusion was estimated by single-photon-emission computed tomography (SPECT) after administration of (99m)Tc-labeled albumin macroaggregates during normal and three times normal gravity conditions in the supine and prone postures. All images were obtained during normal gravity. Exposure to hypergravity caused a redistribution of blood flow from dependent to nondependent lung regions in all subjects in both postures. We speculate that this unexpected and paradoxical redistribution is a consequence of airway closure in dependent lung regions causing alveolar hypoxia and hypoxic vasoconstriction. Alternatively, increased vascular resistance in dependent lung regions is caused by distortion of lung parenchyma. The redistribution of blood flow is likely to attenuate rather than contribute to the arterial desaturation caused by hypergravity.