Case Author(s): Dennis Hsueh, M.D. and Jerold Wallis, M.D. , 02/09/04 . Rating: #D3, #Q3

Diagnosis: Nondiagnostic perfusion exam, masked by persistence of aerosol tracer

Brief history:

40 year old woman with an intracranial hemorrhage and dyspnea.

Images:

Ventilation images (anterior, posterior, RPO and LPO projections)

View main image(vq) in a separate image viewer

View second image(vq). Perfusion images (anterior, posterior, RPO, LPO projections, right lateral and left lateral projections)

View third image(xr). Portable AP chest radiograph

View fourth image(vq). Re-windowed Perfusion images (anterior, posterior, RPO, LPO, right lateral and left lateral projections)

Full history/Diagnosis is available below

Diagnosis: Nondiagnostic perfusion exam, masked by persistence of aerosol tracer

Full history:

40 year old woman with an intracranial hemorrhage and ventriculitis. Overnight, she developed acute onset of dyspnea.

Due to her chronic medical conditions, she has a portacatheter present for IV access. For the perfusion examination, the patient was injected via the portacatheter.

Radiopharmaceutical:

Tc99m DTPA aerosol and Tc99m macroaggregated albumin (MAA)

Findings:

Aerosol images: There is patchy deposition of aerosol with decreased ventilation to the left lower lobe.

Perfusion images: (rewindowed) Non diagnostic pulmonary perfusion examination due to lack of significant activity reaching the pre-capillary arterioles. The activity in the lungs seen on prior "perfusion" images was left over from the aerosol ventilation study.

Chest radiograph: Low lung volumes with normal cardiomediastinal shilouette. There are perihilar interstitial infiltrates and small bilateral pleural effusions. A right internal jugular central venous catheter line terminates at the superior vena.

Discussion:

Portable ventilation perfusion scintigraphy examinations are performed using aersolized Tc99m DTPA particles and Tc99m macroaggregated albumin (MAA). Typically, ventilation images are performed first using 30 mCi Tc99m DTPA in 3 ml of normal saline aerosolized with a nebulizer. The system typically delivers 0.5 to 0.75 mCi to the lungs for imaging. Perfusion imaging is then performed with 4-5 mCi Tc99m macroaggregated albumin. These particles lodge in the precapillary arterioles (< 0.1% of total precapillary arterioles are obstructed). The larger dose of Tc99m administered with MAA is expected to overwhelm the residual activity present in the lungs from the Tc99m DTPA examination, thus permitting imaging of pulmonary perfusion.

In this case, nearly all of the radiolabelled MAA remained in the right upper chest (i.e. very little, if any, Tc99m MAA reached the pulmonary precapillary arterioles). Thus, the initial series of images labeled "pulmonary perfusion images" (image 2) shown actually represent residual activity from the ventilation examination. The images have set at a lower window threshold. This can be evidenced by comparing the intensity of tracer activity in structures such as the trachea and stomach on the ventilation (image 1) and "pulmonary perfusion images" (image 2). When the pulmonary perfusion images are re-windowed, the majority of radiopharmaceutical is noted in a single focus in the right upper chest. (image 4). Thus the perfusion examination is non-diagnostic.

Williams, SC (2001) Nuclear Medicine Online Reference Text. Ventilation/Perfusion Imaging: Technique & Radiopharmaceuticals. Retrieved February 10, 2004, from Aunt Minnie Web Site: http://www.auntminnie.com/default.asp?sec=ref&sub=ncm&pag=pul&itemid=54282

Followup:

The nuclear medicine techologist who performed the procedure denied aspirating the syringe containing the Tc99m MAA after obtaining access to the patient's portacatheter. Also the technologist reported flushing the port after administering the injection without difficulty.

On review of image 2, the majority of radiolabelled MAA appears *within* the chest near the expected catheter tip rather than *on the chest wall*. Since fibrin sheaths/thrombi can form at the tips of chronic indwelling catheters, it was presumed that the activity was trapped in one of these sheaths, even though this would be quite unusual.

Because of the lack of activity reaching the pulmonary precapillary arterioles, the study was non diagnostic for the evaluation for pulmonary emboli. The examination was not repeated at the request of the referring clinicians. The patient expired from complications relating to her intracranial hemorrhage without further study of the pulmonary vasculature (angiography) or repeat ventilatory perfusion scintigraphy.

Differential Diagnosis List

Possible causes for the MAA perfusion tracer appearing as a single intense focus include:

1) thrombus of activity formed from aspiration of blood into the syringe containing the radiolabelled MAA, and deposited in the lungs or adhering to the catheter tip.

2) activity remaining in the reservoir of a portacatheter, in the chest wall.

3) activity deposited on the skin surface, due to leakage during injection.

4) post-mortum injection of tracer through catheter, after cardiac circulation had ceased.

ACR Codes and Keywords:

References and General Discussion of Ventilation Perfusion Scintigraphy (Anatomic field:Lung, Mediastinum, and Pleura, Category:Other(Artifact))

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Case number: vq050

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