65 year old woman presents with progressive dyspnea. On initial presentation to the emergency department she was found to be hypoxemic, with oxygen saturation of 76% on room air. After initial evaluation she underwent the following study:
These anterior/posterior whole body images and anterior/posterior/lateral brain images were acquired after intravenous injection of Tc99m MAA. Images demonstrate extrapulmonary radiotracer activity in the brain, spleen, and kidneys.
Here is this patient’s pulmonary CTA scan:
This study shows a tangle of vessels in the medial basal right lower lobe.
With close inspection, a connection can be traced from the medial basal segmental pulmonary artery to the draining pulmonary vein, diagnostic of pulmonary arteriovenous malformation. MIP or volume rendered images can be helpful to demonstrate this finding.
The diameter of pulmonary capillaries is approximately 10 μm, and pre-capillary arterioles have a diameter of approximately 35 μm. Thus after intravenous injection of Tc99m MAA particles (range of 5-100 μm, with 60-80% between 10 and 30 μm), nearly all the administered activity should be trapped within the lungs. Ziessman HA, O’Malley JP, Thrall JH, eds. Nuclear Medicine, the Requisites, 4th ed. Philadelphia: Elsevier, 2014. Less than 3% of injected activity is normally seen outside the lungs. Treves S. Detection and quantitation of cardiovascular shunts with commonly available radiopharmaceuticals. Semin Nucl Med. 1980;10:16–26. A reduced number of particles has been empirically recommended for patients with a known right-to-left shunt, although prior studies have demonstrated safety of the procedure. Gates GF, et al. Measurement of cardiac shunting with technetium-labeled albumin aggregates. J Nucl Med 1971;12:746-749.
Both intracardiac and extracardiac shunts can cause abnormal radiotracer activity outside of the lungs. Hepatopulmonary syndrome results in hypoxemia and arteriovenous shunting from multiple microscopic arteriovenous connections, presumed due to abnormal production or decreased metabolism of vasodilators by the liver. Rodriguez-Roisin R, et al. Hepatopulmonary syndrome: a liver-induced lung vascular disorder. N Engl J Med 2008;358:2378-2387.
It is important to note that radiotracer activity elsewhere in the body may be due to IV infiltration (extremities), free pertechnetate (thyroid, stomach), or breakdown/excretion of the radiopharaceutical (kidneys), so visual assessment of activity in the brain parenchyma is the most important determinant of significant right-to-left shunting.