**3.1 Scanning protocols**

The protocol aims to display both the pulmonary arteries and lung perfusion from a single contrast-enhanced CT scan. Various scanning protocols with dual source dual energy CT scanners have been proposed in the literature [Fink C, 2008], but currently there are few published protocols for CT systems of other vendors **[Thieme SF, 2009]**. The scan protocols recommended for dual-energy lung perfusion scans by dual-source CT(Siemens)are presented in Table 1. Patients should be centrally placed in the scanner to ensure that the entire pulmonary parenchyma is covered by the smaller field-of-view of the second tube detector array (the field-of-view of the second tube detector array is 260 mm or 330mm) when dual source CT was used.

#### **3.2 Contrast Medium injection protocols**

High-concentration iodine-based contrast material is recommended for DECT scans to improve the differentiation of iodine by the dual-energy post-processing algorithm. As mentioned above in the section of scanning protocols, thoracic DECT scans should be acquired in the caudo-cranial direction so that the chaser bolus is being injected by the time

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

This chapter will present the techniques, scanning and contrast medium injection protocols, image postprocessing and image interpretation, clinical applications and radiation dose of

Recent generations of MDCTs are able to acquire dual-energy data by applying two X-ray tubes and two corresponding detectors at different kVp and mA settings simultaneously in a dual-source CT (Siemens Healthcare), by ultra-fast kVp switching in a single source CT (GE Healthcare) or by compartmentalization of detected X-ray photons into energy bins by the detectors of a single-source CT operating at constant kVp and mA settings (Philips Healthcare). The dual-source CT scanner is composed of two x-ray tubes and two corresponding detectors. The two acquisition systems are mounted on the rotating gantry with an angular offset of 90°/95° with regard to their kilovoltage and milliamperage settings. For dual-energy CT acquisition, the tube voltages are set at high energy (140 kVp) for tube A and low energy (80 kVp) for tube B. The rapid kilovoltage switching technique from GE Healthcare uses a single x-ray source. A generator electronically switches rapidly the tube energies from low energy (80 kVp) to high energy (140 kVp) and back again to

The protocol aims to display both the pulmonary arteries and lung perfusion from a single contrast-enhanced CT scan. Various scanning protocols with dual source dual energy CT scanners have been proposed in the literature [Fink C, 2008], but currently there are few published protocols for CT systems of other vendors **[Thieme SF, 2009]**. The scan protocols recommended for dual-energy lung perfusion scans by dual-source CT(Siemens)are presented in Table 1. Patients should be centrally placed in the scanner to ensure that the entire pulmonary parenchyma is covered by the smaller field-of-view of the second tube detector array (the field-of-view of the second tube detector array is 260 mm or 330mm)

High-concentration iodine-based contrast material is recommended for DECT scans to improve the differentiation of iodine by the dual-energy post-processing algorithm. As mentioned above in the section of scanning protocols, thoracic DECT scans should be acquired in the caudo-cranial direction so that the chaser bolus is being injected by the time

angiography technique.

**2. Techniques** 

dual source, dual energy CT pulmonary angiography.

acquire dual-energy images. Each exposure takes about 0.5 msec.

**3. Scanning injection protocols** 

**3.1 Scanning protocols** 

when dual source CT was used.

**3.2 Contrast Medium injection protocols** 


Table 1. Scan protocols recommended for a dual-energy lung perfusion scan on the currently available dual-source CT systems (Siemens Healthcare)

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 in Table 2.


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

Dual Source, Dual Energy Computed Tomography in Pulmonary Embolism 209

The Lung Vessels application was developed to discriminate non-enhancing subsegmental pulmonary arteries from enhancing ones by using dual energy iodine extraction data. This technique had a high negative predictive value being important for exclusion of segmental PE. In the Lung Vessels application, results are displayed as color-coded multi planar reformatted data and a 3D volume rendered dataset, where vessels with high iodine content are colorcoded blue and soft tissue or vessels with low or no iodine content due to PE are color-coded red. The material parameters for iodine extraction of Lung Vessels are as follows: -1,000 HU for air at 80 kVp, -1,000 Hounsfield unit (HU) for air at 140 kVp, 60 HU for soft tissue at 80 kVp, 54 HU for soft tissue at 140 kVp, 1.1 for relative contrast enhancement, -500 HU for

It is very important to recognize the normal findings or artifacts at DECT lung perfusion. Normal pulmonary BFI images were defined as showing homogeneous perfusion in the normal range (color-coded yellow-green or blue) with dependent symmetric lung iodine distribution (Figure 2). Dependent lung perfusion at DECT refers to relatively low contrast enhancement in the ventral regions (color coded yellow-green) and relatively higher enhancement in the dorsal regions (color coded blue -black) with the patient in the supine

A) Axial BFI image and B) coronal fused image show homogeneous blood flow distribution

In the analysis of BFI images, sources of pitfall should be kept in mind to avoid misdiagnoses. When interpreting BFI images, these pitfalls can relate to artifacts from contrast material, diaphragmatic or cardiac motion, pulmonary pathology and the occlusive

Streak and beam-hardening effects resulting from high-concentration contrast agent in the thoracic veins and right cardiac chambers can commonly cause heterogeneous artifacts in BFI images (Figure 4); these artifacts must be considered when an unexpected contrast enhancement defect is noted adjacent to an area of high contrast enhancement. In this setting, the perfusion defect may appear band-like and be mostly in both upper lobes. Optimization of contrast medium injection parameters, including the use of a saline chaser, can reduce the beam-hardening artifact, improve the image quality of DECT and increase

minimum value, 3071 HU for maximum value, and 4 for range (Figure 1B).

A B

Fig. 2. Normal pulmonary blood flow imaging

**4.2 BFI image interpretation** 

position (Figure 3).

in both lungs

degree of pulmonary arteries.
