If a patient receives 99mTc-MAA while in an upright position, what will likely be seen on the perfusion lung images?

When a patient receives 99mTc-MAA (technetium-99m macroaggregated albumin) while in an upright position, the distribution of the tracer during perfusion lung imaging reflects the gravity-dependent blood flow in the lungs.

In an upright position, the pulmonary blood flow is higher in the lower regions (lung bases) compared to the upper regions (lung apices). This is primarily due to gravitational effects, which cause the blood to pool more in the lower parts of the lungs when the individual is standing. As a result, the tracer activity will be decreased in the lung apices since less blood flow, and thus less tracer uptake, occurs in these higher regions of the lung.

Consequently, one can expect to see decreased tracer activity in the lung apices due to this gravitational effect, especially when comparing it to the increased tracer activity in the lung bases where perfusion is greater. The behavior of the tracer under these conditions aligns with the physiological principles governing lung perfusion and gravity’s influence on vascular function.