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Dual-energy perfusion CT: basic and clinical applications
Yoshie Kunihiro 1,2)
Munemasa Okada2), Naofumi Matsunaga 2)
1) National Hospital Organization, Yamaguchi Ube Medical Center2) Yamaguchi University Graduate School of Medicine
Ube, Yamaguchi, JAPAN
☑ All authors confirm that there are no known conflicts of interest associated with this presentation and there has been no significant financial support for this work that could have influenced its outcome.
Introduction
Dual-source CT (DSCT) scanner has two image-acquisition systems (tubes and detectors) arranged at an angle of 90˚.
The ability to simultaneously operate two X-ray sources at different energy levels (80 and 140 kV) and to differentiate materials such as fat, soft tissue, and contrast agent on the basis of their unique energy-dependent attenuation profiles enables the development of clinically useful applications.
Dual-energy perfusion CT (DEpCT) using DSCT allows us to evaluate the enhancement of the lung parenchyma (the lung perfusion blood volume: lung PBV).
Learning objectives
The purpose of this exhibit is:
1. To explain the basic principles of dual-energy perfusion CT (DEpCT).
2. To review the lung perfusion blood volume (PBV) images on DEpCT in patients with pulmonary emphysema and assess the relation to pulmonary function.
3. To review the volumetric lung PBV images in patients with pulmonary thromboembolism (PTE) and assess the relation to the severity of PTE.
Dual-energy CT
different attenuation
80kV 140kV
Lung perfusion blood volume image
CT provides high spatial resolution and enables objective noninvasive visualization of the thoracic anatomy.
The well-established accuracy of CT pulmonary angiography (CTA) in the depiction of emboli and the thoracic anatomy is supplemented by an effective means of quantitatively assessing the functional effect of thromboembolism on lung perfusion.
In addition to these findings, DEpCT shows high spatial resolution images of the lungs and iodine distribution in the lung parenchyma at the same time.
Chest DEpCT is feasible without the need for any additional radiation dose, when a comparison is made with the radiation dose used for DEpCT (140 and 80 kVp) and a standard chest scan (120kVp with 64×0.6mm collimation) [1].
+
Pulmonary thromboembolism (PTE)
Normal lung
DEpCT of non-diseased segments of the lungs can generate information about pulmonary perfusion, and DEpCT provides images in good agreement with lung perfusion scintigraphy [2].
DEpCT image Lung perfusion scintigraphy(SPECT)
Pulmonary emphysema
Chronic obstructive pulmonary disease (COPD) Characterized by permanently enlarged airspaces distal to the terminal
bronchiole with destruction of alveolar walls [3]. Low-attenuation regions consisting of pixels with attenuation values of
-950 HU or less on thin-section CT scans [4].
Pulmonary comorbidity such as pulmonary emphysema affected the lung perfusion, and in these areas, iodine perfusion map shows focal iodine defect on DEpCT in previous reports [5-6].
It is supposed that the extent of pulmonary perfusion is affected by lung disease such as pulmonary emphysema.
Lung CT Dual energy perfusion CT
Vital capacity (VC), tidal volume (TV), expiratory reserve volume (ERV), forced expiratory volume in the 1st second (FEV1.0) and the forced vital capacity (FVC)
Reductions in the FEV1.0, FVC, and the ratio of FEV1.0 to FVC (FEV1.0/FVC×100) (<70%) are the hallmarks of airway obstruction.
normal
Time(sec)
(COPD Guideline)
normal
obstructive
restrictive
mixed
80
70
FEV1.0/FVC (%)
%VC
Pulmonary function tests
The relationship between DEpCT and pulmonary function tests in pulmonary emphysema
Low-attenuation regions consisting of pixels with attenuation values of -950 HU or less on thin-section CT scans with a 10 mm interval correlate closely with the pulmonary function test results and the pathological distribution of emphysema [4].
Regional alterations of lung perfusion can be depicted by DECT in smokers with pulmonary emphysema and the iodine disturbance on DEpCT had a moderate correlation with the results of the PFTs and the relative area of the lung with attenuation coefficients lower than -950HU in the previous study [5,7].
The % low attenuation area (LAA) on DEpCT has good correlations with LAA on lung CT. The LAA on DEpCT suggesting pulmonary emphysema also has a relatively good correlation with the results of PFTs (%FEV1.0 and FEV1.0/FVC) [8].
DEpCT could give an impression of the severity and functional relevance of parenchymal changes, such as pulmonary emphysema.
Pulmonary thromboembolism
Pulmonary thromboembolism (PTE) is one of the most serious manifestations of venous thromboembolism (VTE), potentially leading to a fatal condition.
Its availability and diagnostic accuracy have made CT pulmonary angiography (CTPA) the most commonly used modality for diagnosing PTE [9].
DEpCT combined with CTPA improves the rate of detection of peripheral IPCs, with better interobserver agreement than standalone evaluations [10].
DEpCT has previously been shown to reveal pulmonary perfusion defects in patients with PE, with good agreement with scintigraphicfindings [11].
The severity of PTE
There are various factors that indicate the activity of PTE.
The D-dimer level is a marker of the presence of stabilized fibrin and serves as an indirect indicator of the thrombotic activity when present in the circulation.
Quantitative cardiac CT measurements obtained on axial CT images, namely the right ventricular (RV) short axis and right/left ventricular (RV/LV) short axis ratio, have been shown to have a significantly positive correlation with the severity of PTE and/or fatal outcomes [12,13,14].
The CT angiographic obstruction index (CTOI), a marker of the severity of PA obstruction, can be used as an indicator of the hemodynamic severity of PTE [15].
However, the current literature shows some discrepancies regarding the potential association between the severity of PA clot load scores and the immediate outcome, probably due to differences in the populations studied regarding the severity of PTE [13,15,16].
To measure the CTOI, the PA tree of each lung was regarded as having 10 segmental PAs. The presence of an embolus in a segmental PA was scored as 1 point, and the number of emboli at the most proximal arterial level was scored a value equal to the number of segmental PAs arising distally. In order to obtain additional information on the degree of residual perfusion distal to the embolus, a weighting factor was used for each value (0 = no defect, 1 = partial occlusion and 2 = complete occlusion). An isolated subsegmental embolus was considered to be a partially occluded segmental PA and assigned a value of 1, with a maximum CTOI of 40 [15].
The relationship between DEpCT and the severity of PTE
The quantification of lung DEpCT has been applied to evaluate PTE [17].
The perfusion defect score obtained using DEpCT has been demonstrated to show good correlations with the right/left ventricular diameter ratio (RV/LV) and CT obstruction index (CTOI) [18].
Relative volumetric evaluations of DEpCT images using a lower attenuation threshold with 1-5 HU show a better correlation with factors reflecting the severity of PTE than those employing a higher attenuation threshold of 1-10 HU or 1-15 HU [19].
1-10 HU 1-5 HU
Three-dimensional (3D) dual-energy perfusion images with various threshold ranges, including (a) 1–120 HU (V120), (b) 1–2 HU (V2), (c) 1–3 HU (V3), (d) 1–4 HU (V4), (e) 1–5 HU (V5), (f) 1–6 HU (V6), (g) 1–7 HU (V7), (h) 1–8 HU (V8), (i) 1–9 HU (V9), and (j) 1–10 HU (V10)
Volumetric evaluations of DEpCT images using the lowest attenuation threshold range (1-2 HU) exhibit the best correlation with factors suggesting the severity of acute PTE. This could be a proper cutoff value on quantitative dual-energy perfusion CT for the assessment of acute PTE [20].
Take-home message
DEpCT shows high spatial resolution images of the lungs and iodine distribution in the lung parenchyma at the same time.
DEpCT provides images in good agreement with lung perfusion scintigraphy.
With accurate knowledge of the presence of pulmonary emphysema as well as lung CT, the iodine perfusion images can be quantified more accurately on DEpCT.
The volumetric quantification of DEpCT images reflects the severity of PTE.
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