What is a ventilation-perfusion scan?
As blood clots shoot from the legs to the lungs, they block off blood vessels in the lungs and everything that they supply downstream of them. This area is represented as a wedge of lung tissue.
We took a large protein molecule abbreviated MAA and attached it to a radioactive substance that emits a gamma ray, which can be imaged using a special machine called a gamma camera. This molecule will flow in the blood all the way the lungs where is becomes trapped in the lung’s capillary beds – thousands of tine blood vessels that filter large molecules like the one used for imaging.
If there is a large blood clot present, these MAA molecules won’t be able travel as far as the capillaries and will get stuck much higher up the vascular tree. This will form a cold area that doesn’t demonstrate tracer accumulation further down the blood flow. This cold area is usually wedge- shaped (triangular) and travels to the edge of the lung.
Sometimes uncertainty exists as to whether a cold area is indeed a blood clot especially in sick lungs. A blueprint of the patient’s normal lungs without the blood clots would prove useful. For this reason, we also let the patient inhale a special gas or vapor of particles that have also been made radioactive and thus can also be imaged. If the defect is present on both, then it is not due to a blood clot. If the defect is present on the perfusion scan but not the ventilation scan then a blood clot can be diagnosed. This is called a pulmonary thromboembolism.
A stand-alone perfusion scan can also be performed to evaluate the functional distribution or segmental functional status of a lung before lung surgery to excise a part of the lung.
What can I expect to happen?
You nose will be closed, and you will be asked to breathe in a gas or vapor that serves as a surrogate for your normal lung volumes. You will then be imaged with the cameras rotating around you.
Afterwards while still on the bed MAA will be injected intravenously. The study will then be repeated to look for areas where blood clots may have prevented the flow of MAA and thus cause cold wedge-shaped defects.
The nuclear physician will process and report the study.
