A Clinical Comparison of Technegas and Xenon-133 in 50 Patients ...

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second (equivalent to about 37 MBq [1 mCi] in the lung). Adequate counts were normally obtained in one or two inhalations. Sicker patients often required several more breaths. Six planar views (100k) were performed: posterior; posterior obliques; anterior; and anterior obliques. Xenon imaging in the posterior view was performed immediately prior to the Technegas study A single-use disposable breathing system (Atomic Products) was used. After inhalations of approximately 400 MBq of xenon-133 (11 mCi), a standard protocol of breath-holding, equilibration, and washout was performed. Perfusion imaging (macroaggregated albumin) was performed immediately after the Technegas study An administered activity of 180 MBq (5 mCi) to maintain a pixel count ratio of about five times the Technegas activity was used. Multiple planar images identical with the Technegas sequence were collected. Technegas and xenon images were compared for technical quality and diagnostic value. The images were viewed and graded without digital processing by two experienced nuclear medicine physicians reviewing all studies in three sessions in isolation from other data. Image quality was assessed on a scale of zero to five in terms of interpretability All studies were reported as high or low probability of pulmonary embolus or indeterminate; however, for this report, only those diagnosed as a high probability of pulmonary embolus were included as truly positive studies. RESULTS Thirty-nine patients had a risk factor of prolonged bed rest or recent surgery Eleven had a past history of phlebitis or previous pulmonary embolus. Thirtyseven patients presented with dyspnea, and 32 had pleuritic chest pain. There were 13 smokers and 12 ex-smokers. Electrocardiograms were normal in 27 patients and showed sinus tachycardia in 19. The chest x-ray film was normal in 14 patients and showed pleural effusions in 15 and atelectasis in six. Blood gas analysis showed an oxygen pressure of less than 80 mm Hg in 37 patients, and 19 had an FEV1 of less than 1.20 L. Ten patients were diagnosed as having a high probability of pulmonary embolism by the criterion of the PIOPED trial.8 Diffuse matching defects thought to be consistent with COPD were visible in 13 patients. Diagnostic images with Technegas were obtained in 49 patients. One patient showed marked central airway deposition with poor visualization of the peripheral areas of the lungs. This was due to technical problems with the prototype equipment. Three patients were unable to cooperate sufficiently to obtain a xenon study (Fig 1), and diagnostic images were obtained in 46 patients. One image was uninterpretable because of poor compliance by the patient. In a subjective comparison of the quality of the images, 37 studies were similar (Fig 2), ten had better images with Technegas, and three had better images with xenon. Diagnostically, Technegas was thought to be better in 21 studies, with this predominantly due to the multiple images obtained. Of the ten patients diagnosed with pulmonary embolism, the extra views with Technegas enabled a more confident diagnosis in six (eg, Fig 3). Two of the ten patients with pulmonary embolism were diagnosed on the Technegas image but not with xenon. In both patients the scan's diagnosis was based on the summation of small subsegmental nonmatched defects.8 The subsequent course of both patients validated the Technegas findings. Patient compliance with Technegas was 100 percent and was 94 percent with xenon. All patients who had previously undergone xenon or aerosol studies reported that a Technegas study was less demanding. From a technical point of view, the Technegas study was easier to perform and caused less disruption to the patient's scheduling and camera time. DiscusSION The important role of ventilation lung scanning in pulmonary embolism is well documented, and in cooperative patients with relatively normal respiratory FicuRE 1. Anterior Technegas (left) and perfusion (right) views of 84-year-old man with carcinoma of prostate and pulmonary embolus. Patient was unable to tolerate xenon study CHEST / 94 / 2 / AUGUST 1988 301 Downloaded From: http://publications.chestnet.org/ on 05/16/2014
FIGURE 2. Posterior views with xenon-133 (top), Technegas (center), and macroaggregated albumin (bottom) perfusion from 37-year-old female diabetic whose study was within normal limits. function, both radioactive gas and technetium aerosol systems can be expected to perform well; however, many patients seen in routine clinical practice suffer from COPD, dyspnea, or are otherwise distressed or enfeebled and unable to cooperate fully Three Xenon studies in this series failed for such reasons. Using aerosols, such patients show deposition in the central airways with poor visualization of the peripheral areas of the lungs. With Technegas, compliance by patients is much higher because there is little need for breath-holding 302 or rebreathing. Adequate lung images are obtained in most patients with a single breath because of the high production efficiency (up to 40 percent of initial eluent as breathable activity in 4 L of argon). This high efficiency also allows dynamic inhalation studies to be performed.7 Alderson et al have compared xenon and radioaerosols in 81 patients. These investigators9 found that aerosols compared favorably with xenon-133 and concluded that they were diagnostically equivalent. In our study Technegas appeared diagnostically superior to xenon-133. Whereas Alderson et a19 found more ventilation in dependent pulmonary regions with aerosols, we did not find this with Technegas, although ventilation did parallel perfusion in its distribution in normal lungs. The distribution of particle size of Technegas is proving technically difficult to determine but is in the range of 50 to 200A, two orders of magnitude smaller than typical aerosols and smaller than produced by the new ultrasonic nebulizers. The half-life of Technegas within the lungs follows the physical half-life of Tc-99m, and there is no evidence of clearance across the alveolar capillary membrane. Nonclearance of Technegas allows high-quality multiple planar and tomographic studies to be performed.7 When Technegas and perfusion tomography are performed sequentially without moving the patient, identical ventilation and perfusion slices can be analyzed by computer for mismatching. This technique is impossible with xenon. There are three major reasons why Technegasproduced ventilation images are more comparable to the perfusion studies than other current ventilatory agents, including xenon-133. First, similar pulmonary depths are imaged, and resolution is the same, since both studies use 9omTc. Secondly, some minor deposition in the central airways was seen only in the sickest patients, and often xenon-133 studies cannot be performed in such patients. All xenon equilibrium images contain activity in the conducting airways. Thirdly, visualization of the peripheral lung is excellent. The one exception in this series was caused by a technical fault in the apparatus. The other advantages of using aOmTc for both ventilation and perfusion studies are obvious, namely low cost, wide availability, ideal energy, and half-life. Provided that the following perfusion study uses about five times the pulmonary activity of the ventilation study, there is no significant degradation of the perfusion image in analogue (film) recording. This is readily calculated and obtainable with standard conventional tracer doses. For computer-assisted studies, it would be practical to use similar activities and subtract the ventilation image. Technegas is clinically a ventilatory agent of high 302Comparison of Technegas and Xenon -133 (Sullivan et al) Downloaded From: http://publications.chestnet.org/ on 05/16/2014
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Page 5: REFERENCES 1 Alderson PO, Rujanevec

A Clinical Comparison of Technegas and Xenon-133 in 50 Patients ...

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