**1. Introduction**

204 Pulmonary Embolism

Yokoe, K., Satoh, K., Yamamoto, Y., Nishiyama, Y., Asakura, H., Haba, R., and Ohkawa, M.

impairment. *Nucl Med Commun*, Vol. 27, No. 11, (2006), pp. 887-892

(2006). Usefulness of 99mTc-Technegas and 133Xe dynamic SPECT in ventilatory

More than 650,000 cases of pulmonary embolism (PE) are reported each year, resulting in an estimated 300,000 annual fatalities. This level of occurrence ranks PE as the third leading cause of death in the USA [ Laack TA, 2004; Tapson VF, 2008]. Multidetector CT (MDCT) pulmonary angiography has now largely replaced ventilation/perfusion scintigraphy and conventional pulmonary angiography for the evaluation of possible PE [Patel S,2003]. In 2007, MDCT pulmonary angiography was accepted as the reference standard for diagnosis of acute PE **[**Remy-Jardin M,2007**]**. However, conventional MDCT pulmonary angiography only provides morphological information and its ability to assess subsegmental pulmonary arteries is variable: sensitivities range from 37%–96%. The ability to assess subsegmental pulmonary arteries has increased with advances in MDCT technology.

In the past, various CT techniques have been developed to evaluate the assessment of lung perfusion in patients with suspected PE. (1) Dynamic multi-section electron beam CT **[**Schoepf UJ,2000**].** This perfusion-based CT technique had a scanning volume of 7.6 cm, required a long patient breathhold, and delivered a high additional radiation dose to the patient. (2) Color-coding the density of lung parenchyma in contrast-enhanced CTA **[**Wildberger JE,2001**]**. This technique is of limited use when lung diseases, such as groundglass opacities (e.g., in pulmonary edema or pneumonia) were present.(3)A subtraction CTA technique of whole-thorax multi-detector CT scans acquired before and after intravenous contrast within a single breathhold. This technique was limited by a longer breathhold time, misregistration artifacts because of the mismatched unenhanced and contrast-enhanced scans, and additional radiation dose caused by the fact that an additional unenhanced scan had to be performed to assess the iodine distribution in the lung [Wildberger JE,2005].

Recently, DECT with different dual energy CT hardware (dual source CT and rapid kV switching technique) became available to simultaneously provide the functional and morphological information, overcoming the limitations of the above-mentioned CT

Dual Source, Dual Energy Computed Tomography in Pulmonary Embolism 207

Scan mode Spiral dual energy Spiral dual energy Scan area Diaphragm to lung apex Diaphragm to lung apex Scan direction Caudo-cranial/cranio-caudal Caudo-cranial/cranio-caudal

Tube voltage A/B (kVp) 140/80 100/140Sn (tin filter)

the scan reached the upper chest to avoid streak artifacts due to highly concentrated contrast material in the subclavian vein or superior vena cava. In order to acquire both pulmonary arteries and lung perfusion in an optimal scan, the scan delay should be a little longer (e.g.4-7s) to allow the contrast material to pass into the lung parenchyma. Bolus tracking should be used for timing with the region of interest placed in the pulmonary artery trunk. There was no significant difference in pulmonary artery enhancement between test bolus and automatic bolus tracking in previously performed studies **[Geyer LL, 2011]**. Therefore, automatic bolus tracking is recommended because it is operator friendly and independent. The patient should be instructed to hold his breath at mild inspiration to avoid excessive influx of non-enhanced blood from the inferior vena cava. Contrast injection protocol of dual source dual-energy CT pulmonary angiography is seen

> Iodine concentration 300mg I ml-1

(ml kg-1) 1.5 1.2 1.1

(ml s-1) 4 4 4 Bolus timing Bolus tracking Bolus tracking Bolus tracking

threshold(HU) 100 100 100 ROI position Pulmonary trunk Pulmonary trunk Pulmonary trunk Scan deldy(s) 6 6 7 Saline flush volume(ml) 40 40 60 Saline injection rate(ml s-1) 4 4 4 Needle size(G) 18 18 18 Injection site Antecubital vein Antecubital vein Antecubital vein

Table 2. Contrast injection protocol of dual-energy pulmonary CT angiography

Scan time(s)(for 300 mm length) 10 9

(quality ref. mAs) 51/213 89/76 Dose modulation CARE Dose 4D CARE Dose 4D CTDIvol (mGy) 6 7.3 Rotation time (s) 0.33 0.28 Pitch 0.7 0.55 Slice collimation (mm) 1.2 0.6 Acquisition(mm) 14x1.2 128x0.6 DE composition factor 0.3 0.6 Reconstruction kernel D30f D30f Table 1. Scan protocols recommended for a dual-energy lung perfusion scan on the

currently available dual-source CT systems (Siemens Healthcare)

Tube current A/B

in Table 2.

Contrast media volume

Contrast media flow rate

Bolus tracking

Siemens Definition Siemens Definition Flash

Iodine concentration 370mg I ml-1

Iodine concentration 400mg I ml-1

perfusion techniques. Iodine, shows a proportionally larger increase of CT values with decreasing X-ray tube voltage compared to other materials, e.g., to soft tissue, iodinated contrast medium enhanced DECT provides the opportunity to assess pulmonary parenchyma iodine maps (i.e., lung perfusion). Compared with the previously developed CT perfusion techniques, DECT technique eliminates registration problems and allows selective visualization of iodine distribution with high spatial resolution and no additional radiation exposure to the patient compared with the conventional CT pulmonary angiography technique.

This chapter will present the techniques, scanning and contrast medium injection protocols, image postprocessing and image interpretation, clinical applications and radiation dose of dual source, dual energy CT pulmonary angiography.
