With the introduction of MDCT, CTA is now recognized as the reference standard for diagnosing acute PE [4]. A main advantage of MDCT over single-slice CT is the ability to scan the entire volume of the thorax with sub-millimetric collimation in a very short period of time, most often under the duration of a single breath-hold. This is particularly useful when evaluating dyspneic patients. These technological advances have improved both evaluations of the peripheral pulmonary arteries and the accuracy of CT in the workup of acute PE. Simultaneously, MDCT allows radiologists to scan patients at a low kilovoltage, with reductions in the dose of contrast material and the overall radiation dose. This is of particular importance in young female patients who may be exposed to substantial levels of radiation to breast tissue. In the context of acute PE, the latter concern is clinically relevant given the lower prevalence of acute PE, which has dropped from 33% on angiographic studies to <20% on CT/MDCT scans. To date, several studies have investigated the clinical benefits of low-kilovoltage techniques, i.e., 80–100 kV(p) vs. 120–140 kV(p), the parameters at which CTA is typically performed. However, for obvious ethical reasons, these studies were based on the comparative analysis of different populations scanned with single-source CT [5–8]. The limitations of these comparisons include the lack of systematic adjustment for individual patient morphology, cardiac hemodynamics, and potential underlying respiratory disease. With the introduction of DSCT, the two tubes can be set at different kilovoltages. In addition to the opportunity to evaluate lung perfusion, there are benefits for standard CTA as this scanning mode has been shown to improve the visualization of small pulmonary arteries at 80 kV(p) [9].

Lung perfusion with dual-energy CT does not reflect blood flow analysis per se, as it provides a measurement at only one time point; rather, it yields an iodine map of the lung microcirculation at a particular time point. Numerous parameters are known to influence the distribution of iodine within pulmonary capillaries; some are technique-related whereas others are due to the anatomical and/or physiological circumstances under which the data were acquired. Dual-energy CTA is performed using a scanning protocol similar to the one used in clinical practice. The acquisitions proceed from the top to the bottom of the chest, with an injection protocol similar to that of standard CTA obtained with a single energy on a 64-slice scanner. Two categories of images can be reconstructed: Diagnostic scans correspond to contiguous 1-mm-thick transverse CT scans generated from the raw spiral projection data of tube A and tube B (60% from the acquisition by tube A; 40% from the acquisition by tube B). Lung perfusion scans (i.e., images of the perfused blood volume of the lung parenchyma) are generated after determination of the iodine content of every voxel of the lung parenchyma on the separate 80- and 140-kVp images. The images can be rendered in gray-scale or color-coded. All images can be displayed as transverse scans, complemented as needed by corona land sagittal reformats. Even though a dual-energy acquisition does not correspond to true perfusion imaging, several applications of this pulmonary micro-CTA technique have been investigated [10].