Nuclear Medicine

V/Q scans are lung function tests, most used to detect potential pulmonary embolisms. V/Q scans occur in 2 steps, the first step is to measure perfusion, while the other is to measure ventilation. The first step is ventilation imaging. The radiopharmaceutical used is the radioactive gas Xenon-133 (dose of 10 – 20 mCi). There are 3 phases to a ventilation scan: the wash-in, the equilibrium, and the wash-out. The patient is placed for a posterior view. They are given a mouthpiece from which they inhale Xe-133 in a single maximal deep inspiration and an initial image is taken. During the equilibrium phase, two images – 90 seconds each – are taken after the initial breath while the patient continues to inhale a mixture of air and xenon. The goal is the complete infiltration of the alveolar space. Once these steps are completed, several wash-out images are obtained. The patient breathes room air while 3-4 images are taken (45 seconds each). 

Secondly, a 2-5 mCi dose of Tc-99m MAA (macroaggregated albumin) is injected intravenously for pulmonary perfusion scintigraphy. This dose is given slowly over the course of a few respiratory cycles, with the patient lying supine. Patients are instructed to breathe deeply. The goal is to evenly distribute the radiolabeled particles amongst the capillary bed. Imaging begins immediately after injection. Images are collected in the anterior, posterior, right, and left lateral views. Additionally, 45° anterior and posterior oblique views are also acquired. The imaging of the perfusion scan should be done in the same patient orientation as the ventilation scan.

Two outcomes may come of this – either there is a matched or mismatched perfusion defect. A matched perfusion defect is said to occur when there is a ventilation abnormality corresponding to the perfusion defect. There are several afflictions which may result in ventilation and perfusion being abnormal in the same area, such as a lung tumor, a pleural effusion or chronic airway disease (see Table 1). Typically, matched defects are a sign of a nonembolic origin.

The other possibility Is a V/Q mismatch, whereby perfusion is abnormal, and ventilation is normal. Alternatively, a mismatch may also occur if ventilation and perfusion are both abnormal, but with the perfusion defect being significantly larger than the ventilation defect. This is classically due to embolic causes i.e. an embolus blocks blood flow to a region of the lung, while ventilation remains the same (see Table 2). 

When looking at a scintigram, the first step towards establishing a diagnosis is to determine whether a perfusion defect is matched or mismatched. The second step is to determine if the defect is segmental or non-segmental. The perfusion blockage of a pulmonary arterial tree affects blood circulation in the segmental branching pattern. Therefore, a segmental defect corresponds to a bronchopulmonary segment (see Figures 1 and 2), is pleura based and is shaped like a wedge. A non-segmental defect is not pleura based, not wedge shaped and does not correspond to any of the bronchopulmonary pulmonary segments. Causes of non-segmental defects are indicated below (see Table 3). These causes may also be visible on chest radiographs. 

The last clinically important component in the interpretation of V/Q scans is the determination of risk probability category, which can be high, intermediate or low. To determine the category under which a scan falls, a set of criteria known as the modified PIOPED criteria is used (see Figure 3). According to these criteria, high probability of pulmonary embolism requires at least two large mismatched segmental defects. A large segmental defect is characterized as covering more than 75% of a lung segment. Low probability of pulmonary embolism is noted when the perfusion defects are non-segmental, or when the perfusion defect is matched with a ventilation abnormality.