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Estimation of pulmonary transit time as a by-product in standard CT pulmonary angiography

Many diseases of the respiratory and cardiac systems including pulmonary embolism, primary pulmonary hypertension, chronic obstructive pulmonary disease, and emphysema affect the pulmonary vessels and cause secondary increase of the pulmonary vascular resistance (PVR). The degree of PVR elevation determines the work that the right side of the heart must perform and therefore is a key prognostic parameter (1–3). The functional relation between PVR, pulmonary transit time (PTT), and cardiac output (CO) has been described in the equation PVR 1⁄4 h † L † C/CO † PTT. The equation was derived from the law of Hagen-Poiseuille for resistances of flowing fluids in tubes (1, 2). The other variables of the equation include a correction factor for blood viscosity h derived from hematocrit and vessel length L derived from body height. The concept of calculating PVR by non-invasive estimation of PTT and CO was originally introduced in a clinical study with heart transplant patients (1). Apart from invasive measures, various approaches for the estimation of PTT have been described (4). Many of these methods require MRI, PET, or the use of CT with a sharp bolus injected into a central catheter (4, 5). However, PVR has also been assessed with a perfused blood volume examination, which means the slower injection of contrast into a periphery line (1). For this reason regions of interest were defined in the right outflow tract and the left atrium. Time–density curves were calculated. The time span between maximum density in the right outflow tract and maximum density in the left atrium was defined as PTT (1). This procedure is quite similar to the bolus tracking in a standard CT pulmonary angiography examination. Different from the normal procedure the imaging for bolus tracking has to be performed at the level of the widest heart diameter. This gives basically a view similar to a four-chamber view. Regions of interest can be placed in the right and left ventricle then. Standard CT angiography should be started immediately when the density in the left ventricle reaches a threshold of 100 HU. To estimate the PTT the delay in the rise of contrast in the right and left ventricle can be used (Fig. 1). In the presented example taken from a whole body angiography of the aorta the effect of breathing movements are seen. During inspiratory movement the capacity of the venous thoracic system increases. Consecutively, the cardiac output of the right ventricle increases and the contrast is diluted. During expiratory movement the opposite effect takes place. The effect is inverse in the time–density curve of the left ventricle. By changing the monitoring location from the level of the pulmonary trunk to the level of the widest heart diameter, PTT can be estimated on top of every standard CT pulmonary angiography. Monitoring could also be performed in patients with standard chest CT when PTT needs to be estimated. CT angiography and chest CT are routinely performed in patients with suspected pulmonary embolism and diseases of the respiratory and cardiac system. Therefore noninvasive measurement of PVR, as a highly relevant parameter means that further information on the patient’s condition can be gained without imposing additional stress. However, it has to be considered that with this method the bolus tracking will take some seconds longer. The instant of the CT angiography will be delayed resulting in a stronger contrast of the pulmonary venous system. Furthermore there will be a slightly increased radiation exposure due to additional scans in the bolus tracking phase.