Presentation
Shortness of breath. Unable to run since childhood.
Patient Data
The frontal view of the chest demonstrates a mild enlargement of the cardiac silhouette, along with enlargement of the pulmonary arteries and increased vascular markings. No focal opacity. No visible effusion or pneumothorax.
VQ scan.
Perfusion imaging with Tc-99 m MAA IV shows mild heterogeneity without segmental perfusion defect(s). There is an abnormal radiotracer seen in the partially visible superior poles and kidneys. No appreciable abnormal activity is present in the lower neck region of the thyroid.
Ventilation imaging with Xn-133 inhaled is normal.
A repeat lung scan was performed (to minimize the presence of free pertechnetate), and total body counts were measured using ROIs over the lungs, brain, and kidneys. The calculated shunt was 22%.
Echocardiography confirmed the presence of a ventricular septal defect (VSD).
Companion case. Abnormal radiotracer activity had been seen in the kidneys on standard lung images (not shown) during the quality control review. As such, an image of the head and neck was obtained showing uptake in the thyroid and salivary glands without uptake in the brain, confirming benign free pertechnetate as the etiology for abnormal extra-pulmonary radiotracer accumulation.
Case Discussion
During lung perfusion imaging, one may occasionally identify an altered biodistribution of the radiotracer. This is typically due to one of two reasons: free pertechnetate (from dissociation of TCO4 from the MAA) or an intracardiac right-to-left shunt. Both scenarios can demonstrate abnormal radiotracer accumulation in the kidneys; however, the two can be differentiated by the presence of radiotracer uptake in the thyroid and salivary glands (free pertechnetate) or brain (right-to-left shunt). In this case, there is no radiotracer in the neck to suggest thyroid uptake of free pertechnetate, and hence right-to-left shunting of Tc-99m MAA was favored.
In the right-to-left shunt, blood flow bypasses the pulmonary circulation and goes directly to the systemic circulation in the brain, kidneys, and other organs.
Right-to-left intracardiac shunts can be quantified through the use of Tc-99m MAA scintigraphy. This is done by measuring the amount of Tc-99m MAA in the lungs (counts) versus in the remainder of the body:
(Total body counts minus total lung counts) / total body counts x 100%.
Total body counts can be measured by placing a region of interest (ROI) over the whole body, but ROIs of the head/neck and kidneys/abdomen, as in this case, will provide comparable results.
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