Artifacts in Xenon Lung Imaging Studies

Objectives: Xenon-133 has a useful role in lung ventilation scanning. It is an inhaled radiopharmaceutical agent primarily used to image the lungs and evaluate pulmonary function. One of the issues encountered with Xe-133 is leakage of gas during the ventilation procedure, causing contamination of the gamma camera. This can result in an image artifact degrading interpretation of the ventilation scan. Here we report on persistent image artifacts on ventilation images and subsequent evaluation of design changes to the gamma camera detectors to eliminate these problems.

Methods: Following installation of two new gamma camera systems in our facility, a significant percentage of lung ventilation scans contained image artifacts (Figure 1). The new systems were a dual-detector SPECT system (GE D630) and a dual-detector SPECT/CT system (GE D670). These systems replaced an old dual-detector system (GE Infinia) where such artifacts have rarely been observed. Determination of the cause of the xenon leakage was initially approached by carefully evaluating all tube connectors between the patient and the lung ventilation system (Pulmonex System, Biodex), and tubes connecting to the room exhaust system. All patient masks were checked to ensure they provided a good seal and did not allow for any leakage under normal usage. This can amplify the apparent xenon activity by a factor of 1000 or more. To eliminate this potential source of artifact, we worked with the vendor to implement and evaluate the following design and procedure changes: A. Addition of foam seal tape around the inside of the collimator and on the detector casing, to reduce airflow between the collimator and detector. B. Addition of an external fan behind the gantry to redirect any Xe-133 away from the detector heads. This was a temporary measure only. C. Changes in the internal cooling fans to reverse the direction of airflow through the detector heads. The vendor independently tested the effectiveness of these changes on a D630 system using Tc-99m Technegas and Xe-133. After each design/procedure change, subsequent ventilation scans were carefully checked for artifact. In addition, simulation of Xe-133 escaping during a patient study was performed using a nebulizer bag containing Xe-133. The bag was positioned as if it were a patient’s head during a ventilation scan, then a small amount of Xe-133 was released close to the upper detector head to determine if any gas was drawn into the detector.

Results: Figure 1 shows an example of a normal Xe-133 washout study and one demonstrating significant artifact. The artifact appears to move across the field of view and is rapidly cleared from the field of view. Addition of foam seal tape did not result in any notable reduction in the incidence of image artifacts. Placing a fan behind the gantry was helpful in eliminating the problem, but was considered a temporary solution. Simulation studies with the Xe-133 in the nebulizer bag showed that Xe-133 was being drawn into the upper detector head via small ventilation slots at the side of the detector. A design change was implemented to reverse the direction of airflow from the cooling fans in the detector head. In the new design air is now being drawn in at the back of the detector and pushed out through the small ventilation slots near the patient’s head. Following this last change, no additional image artifacts have been observed in the last 6 months.

Conclusions: Xe-133 artifacts during ventilation imaging may be due to Xe-133 gas that escapes from the patient’s mask, and is then drawn into the detector head. Determination of the cause of such artifacts can be challenging and requires collaboration between the site and the vendor.

• Download figure
• Open in new tab
• Download powerpoint