Ventilation scintigraphy with lipophilic cationic compounds

Based on our hypothesis that lipophilic cations may be more suitable for ventilation lung scintigraphy than the conventional ^99mTc-DTPA, a few invitro experiments followed by human studies were done. Nebulisation potential of nine routine radiopharmaceuticals was compared on a medical aerosol generator using the same experimental conditions. This was followed by ventilation scintigraphy in 14 patients with chronic obstructive airway disease (n=13) or pulmonary embolism (n=1) where either ^99mTc-MIBI (n=10) or ^99mTcTetrafosmin (n=4) inhalation imaging was done. Same-patient comparison with ^99mTc-DTPA ventilation scan was available in six patients using same acquisition protocol. Comparison was made with respect to the nebulisation rates (in medical and nuclear medicine nebuliser), total counts inhaled per unit of radioactivity available for inhalation, and regional distribution of inhaled counts for the three types of radio-aerosols. Lipophilic cation solutions had significantly higher nebulisation rate compared to ^99mTc-DTPA using the medical nebuliser (235%, p <.01) while ^99mTc-Sulphur Colloid was the worst. The difference was magnified on nuclear medicine nebuliser (370% higher nebulisation). More than three times the counts of ^99mTc-MIBI or ^99mTc-Tetraphosmin was deposited in the body compared to ^99mTc-DTPA aerosol per MBq activity inhaled (1.5 Kcounts/MBq Vs 0.4 kcounts/MBq) (p < 0.001), preferentially in the lungs (75.2%Vs 65.2), at the expense of oropharynx and stomach. Within the lungs, about 50% more counts were deposited in outer one-third lung with lipophilic cations. Overall therefore, more than 12 times the radioactivity deposition was achieved in the peripheral one-third of the lungs with the lipophilic cations in terms of nebulisable radioactivity, suggesting very significant throughput of small-sized radio-aerosols. We conclude that ventilation lung scanning with lipophilic cations combines the advantages of ^99mTc-DTPA aerosol (cost-effectivity, low technology, easy operation, ubiquitous presence) with that of technegas (deep lung penetration, good images), while introducing a few advantages of its own.