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Dual-energy CT after radiofrequency ablation of liver, kidney, and lung lesions: a review of features
Insights into Imaging volume 6, pages 363–379 (2015)
Abstract
Early detection of residual tumour and local tumour progression (LTP) after radiofrequency (RF) ablation is crucial in the decision whether or not to re-ablate. In general, standard contrast-enhanced computed tomography (CT) is used to evaluate the technique effectiveness; however, it is difficult to differentiate post-treatment changes from residual tumour. Dual-energy CT (DECT) is a relatively new technique that enables more specific tissue characterisation of iodine-enhanced structures because of the isolation of iodine in the imaging data. Necrotic post-ablation zones can be depicted as avascular regions by DECT on greyscale- and colour-coded iodine images. Synthesised monochromatic images from dual-energy CT with spectral analysis can be used to select the optimal keV to achieve the highest contrast-to-noise ratio between tissues. This facilitates outlining the interface between the ablation zone and surrounding tissue. Post-processing of DECT data can lead to an improved characterisation and delineation of benign post-ablation changes from LTP. Radiologists need to be familiar with typical post-ablation image interpretations when using DECT techniques. Here, we review the spectrum of changes after RF ablation of liver, kidney, and lung lesions using single-source DECT imaging, with the emphasis on the additional information obtained and pitfalls encountered with this relatively new technique.
Teaching Points
•Technical success of RF ablation means complete destruction of the tumour.
•Assessment of residual tumour on contrast-enhanced CT is hindered by post-ablative changes.
•DECT improves material differentiation and may improve focal lesion characterisation.
•Iodine maps delineate the treated area from the surrounding parenchyma well.
RF Ablation
Minimally invasive therapies are increasingly used in patients with malignant tumours, who are not suitable candidates for surgical resection. The aim of local ablative therapy is to induce cell death. Radio-frequency (RF) ablation has attracted much attention in the last decade because of its technological improvements and has become a well-established technique to treat primary and secondary hepatic malignancies, as well as kidney and lung tumours.
Technical success following RF ablation is due to the complete destruction of the tumour. In case of incomplete necrosis of the tumoral volume, re-ablation of residual tumour can be pursued in order to improve technique effectiveness. For this reason, it is crucial to evaluate the results of the procedure early after ablation. The ideal imaging technique should show the degree of necrosis of the malignant mass and detect residual tumour.
Imaging immediately after ablation also allows for the detection of post-procedural complications and provides a baseline for future follow-up comparisons. Unfortunately, the assessment of residual tumour, usually by means of contrast-enhanced computed tomography (CT), is hindered by post-ablative changes, particularly at the periphery of a treated lesion where blood flow is greatest [1, 2].
On pathology this represents a haemorrhagic rim corresponding to an early inflammatory reaction to the necrotic tissue [3, 4]. Today, contrast-enhanced CT is the standard approach to image early post-ablative changes in the liver [5, 6], kidney [7], and lung [8]. However, some investigators have reported that although complete necrosis may appear clearly on contrast-enhanced CT, these findings do not always correlate with histopathological conclusions, which suggests that there is limited diagnostic accuracy in the detection of residual tumours [9–12].
Dual-energy CT
A well-known drawback of standard CT is that a number of different materials or tissues may show similar attenuation behaviour at single radiation energy levels and, with that, similar corresponding Hounsfield numbers. This poor attenuation difference can be partially resolved by using dual-energy CT (DECT) technology with material decomposition [13]. DECT is able to achieve tissue attenuation sampling at two different energy spectra, which results in more specific information beyond the typical Hounsfield units.
For already several years, three main technical approaches have been developed by various vendors for the acquisition of dual-energy data: a single-source rapid kilovolt peak-switching technique, a dual-source technique with an angular offset, and finally use of a dual-layer detector that discriminates between high- and low-energy photons [13]. The first two techniques obtain dual-energy data simultaneously at two different energy levels at typically 80 and 140 kVp, although dual-source systems can also operate at 100 and 140 kVp (with an additional tin filter) in case of larger patient sizes. The third technique uses the detected high- and low-energy signal components in one CT acquisition. More recently, a non-simultaneous, single-source technique has been reported that applies a sequential data acquisition and a coregistration motion correction algorithm [14]. It has been shown that DECT improves material differentiation and works especially well in materials with large atomic numbers, such as iodine and calcium, because their strong photoelectric effect causes high attenuation at lower photon energies [15]. The material decomposition technique assumes that for any tissue an equivalent mixture of water and iodine exists with similar spectral attenuation properties. Consequently, after a material decomposition calculation process, two base material maps can be displayed, representing the concentrations of water and iodine in each voxel. In contrast studies, these concentrations correspond to real water/iodine concentrations in blood. All other materials (e.g. bone, fat) are described as a mixture of both of these base materials and will appear as hyper- or hypodensity in the base pair maps. Iodine concentrations can be accurately quantified. A recent study showed a 0.55 mg/cc mean error when comparing calculated and true iodine concentrations in renal masses [16]. These calculated iodine concentrations (mg/cc) can be displayed for any region of interest (ROI) as either a greyscale- or colour-coded iodine image. Such iodine maps obtained from DECT images are not a surrogate for dynamic perfusion CT, as they merely provide a visualisation of the iodine distribution in the tissues at one point in time.
In addition to calculating base material maps, synthesised or synthesised monochromatic images from dual-energy CT, representing CT values (HU) over a range of 40 to 140 keV, can also be obtained. Compared to high-keV, low-keV images (closer to the k-edge of iodine) typically provide improved contrast between different structures, e.g. between a tumour and surrounding normal parenchyma after injection of iodinated contrast material. On the other hand, noise is more prominent in these lower keV images [17]. Synthesised monochromatic images at 77 keV correspond best to the effective energy of a single-energy 120-kV scan [18], such that the image contrast simulates a traditional single-energy CT scan. By assessing the DECT images, an optimised keV window based on the contrast-to-noise ratio between tissues can be calculated. In RF ablation, an improved contrast is typically obtained between the ablation zone and surrounding normal liver parenchyma. A scatterplot analysis, which estimates the material concentration in each voxel, can also help to differentiate different structures in the ROIs. Research is currently ongoing with regard to adequate quantification of the acquired DECT data. Several potential candidate parameters are being screened for usefulness within the RFA context, such as CT numbers (i.e. Hounsfield unit curves) [19, 20], contrast-to-noise ratios [20, 21] or iodine concentrations [22]. Compared to standard CT, DECT may have the potential to improve focal lesion characterisation, and it is a promising tool to supply quantitative data in addition to traditional morphological information. The associated radiation dose with DECT will depend on the applied technology. Although specific clinical studies comparing the dose efficiency of different technologies are still lacking, current data suggest that DECT imaging with dual-source systems does not necessarily cause additional radiation exposure for the patient compared to standard CT [17, 23, 24]. Radiation dose data on the rapid kilovolt peak–switching technique to date are still inconclusive and reports from other approaches are scarce or nonexistent [25]. However, the increased informational content and post-processing flexibility of DECT data create additional opportunities for dose saving, such as the creation of virtual unenhanced images from a contrast-enhanced scan [25].
The purpose of this pictorial essay is to display post-ablative changes on contrast-enhanced CT with a single-source rapid kilovolt peak-switching technique in order to identify the additional information provided by DECT imaging and to describe the pitfalls of this relatively new technique in the evaluation of RF ablation’s technical success.
DECT imaging protocol
Fast kilovoltage switching CT was performed on a 64-slice CT (Discovery CT750 HD, GE Healthcare, Milwaukee, WI, USA) after IV administration of 120 cc contrast at 2.5 cc/s. Scan data were acquired at 40-mm collimation using predefined GSI protocols at CTDIvol values between 15.02 and 25.53 mGy. Fixed tube currents (between 375 and 600 mA) were used as our DECT scanning mode is not compatible with the automated tube current modulation system of the scanner. Images were processed on a workstation (AW4.4; GE Healthcare) using the Gemstone Spectral imaging application. Three types of images were reconstructed for analysis: synthesised monochromatic images from 40 to 140 keV, greyscale- and colour-coded iodine images (with ‘rainbow’ colour map).
Liver ablation
Necrosis caused by thermal damage is characterised by an absence of blood perfusion, resulting in a non-enhancing area on contrast-enhanced CT (Table 1). Due to this lack of internal vascularity, iodine maps are well suited for delineating the treated area from the liver parenchyma (Figs. 1 and 2a). Dual-energy CT has the possibility to extract iodine from the enhanced images to create water map images and thus potentially skip true unenhanced images, as shown in Fig. 3a. This undoubtedly offers a considerable advantage in limiting radiation dose exposure [23]. However, a drawback is that current DECT techniques do not produce water map images with the same contrast-to-noise ratio as true unenhanced images [26].
Frequently, a hyperattenuation centrally within the ablation zone can be seen on unenhanced images [27]. It is thought that this area of high density correlates to a region of greater cellular disruption in the centre of the ablation zone [3], reportedly caused by the intense charring of the severely desiccated coagulated lesion [28]. It has usually disappeared by the time the next follow-up CT examination takes place, but can persist for a longer time [29] (Fig. 2b). This hyperattenuation is observable on both the water map and true unenhanced images. Despite the absence of iodine, this can also be seen on the iodine-coded images, since the hyperattentuated area is represented as a combination of both water and iodine after material decomposition (Fig. 2a). Meticulous comparison of water and iodine-coded images allows for differentiation of this central hyperattenuation after RF ablation and structures enhancing after IV contrast administration. The reader should however be aware of the lower image quality of water images with respect to true unenhanced images (Fig. 3a), as also reported on dual-source DECT [20]. Similar to the study of Lee et al. [20], we observed sharp depiction of the edge of the ablation zone of the iodine images. Contrary to their findings obtained using a dual-source DECT system, our experience with single-source DECT images did not prove an internal homogeneity of the ablation zone on the iodine images.
Immediately after ablation, it is common to find a hyper-attenuating halo surrounding the ablation zone on CECT, correlated to an increase in arterial perfusion due to capillary leakage from thermal damage [30]. The circumferential enhancement is predominantly visible on the arterial phase and can be very prominent in some cases (Fig. 3c), obscuring the interpretation of residual tumour. An irregular or nodular rim can indicate the presence of residual tumour, although benign variations do occur [30–32]. When focal thickening at the border of the ablation zone is unclear, a shorter follow-up period is recommended [32]. Previous studies have shown that a hypervascular rim remains visible in 89 % of cases after 1 month, in 56 % of cases between 1 and 3 months, and in 22 % of cases between 3 and 6 months [33]. Ideally, the volume of thermal necrosis exceeds the limits of the metastasis by 1 cm in all dimensions [34]. Residual tumour will persist as a focal nodular enhancement in the periphery of the ablation zone (Fig. 3b). For dual-energy CT, the increased attenuation of iodine on the low-keV (e.g. around 40 keV) images is better suited for detecting any subtle density differences around the ablation zone (Fig. 3b). Adding colour-coding enhances the visibility of the contrast in the image.
The water-iodine material decomposition can highlight areas containing iodinated contrast. Iodine maps are superior to synthesised monochromatic images for the qualification of contrast uptake. Consequently, a hypervascular nodule with a higher inflow of iodine is better depicted on the iodine maps when compared to synthesised monochromatic images (Fig. 2b).
Kidney ablation
Ablation zones in the kidneys lack iodine content and are often wedge shaped because of peripheral infarctions [35] (Table 2). Determining a tumour’s iodine concentration before RF ablation can provide a baseline for future follow-up assessments (Fig. 4a, b). Iodine concentrations that are similar to the original tumour post ablation can be a sign of local tumour progression. The iodine content can be assessed qualitatively as well as quantitatively in the investigated ROI (Fig. 4b).
When evaluating the contrast uptake in a lesion, it is imperative to compare the lesion’s density on the water map images to that on the synthesised monochromatic images in order to be able to appraise possible ‘enhancement’. The iodine content can be assessed qualitatively based on the iodine-coded images as well as quantitatively when expressed in iodine concentrations (mg/ml) (Fig. 4b) in the investigated ROI. Enhancement can also be demonstrated by comparing the iodine concentration in the lesion to a surrounding vascular structure after an IV contrast injection. When evaluating colour-coded iodine maps, it is important to compare iodine concentrations in the ROI to those in adjacent structures, because windowing (as a diagnostic tool) causes variability in the colour spectrum.
As with liver lesions, the extent of necrosis in the ablation zone can be assessed more accurately on 40-keV rather than on 70-keV synthesised monochromatic images. Also, the iodine maps show clear differentiation between avascular and viable tissues. Adjacent to the avascular zone, regions of intermediate vascularity can be seen that appear most pronounced on colour-coded iodine images (Fig. 4c). This may correspond to transient thermal damage to the adjacent kidney parenchyma, causing a temporary blood flow decrease in this area. Multi-planar reconstructions are vital in the assessment of questionable regions after the RF ablation procedure (Fig. 4c). Although avascularity is by definition a sign of successful ablation, the ablation zone can show remnants of iodine concentration, possibly due to the extravasation of iodine which itself is caused by vascular damage (Fig. 5a).
The majority of ablation zones are hypodense in nature; however, it is not uncommon to have a variable degree of haemorrhagic necrosis in the necrotic area. This degree of attenuation is not predictive of technique effectiveness (Fig. 6c). Since the quality of the water maps is less compared to the true unenhanced images, spontaneous hyperdensity may be missed by this reconstruction technique (Fig. 6b).
In the contrast to rim enhancement that is commonly found in liver ablations, this phenomenon has only been observed occasionally in ablated renal parenchyma [25]. If present, it resolves quickly and is only marginally visible after 3 months [36]. Post-ablative changes in the perirenal fat are predominantly seen as streaky soft tissue attenuations (Fig. 5a), which resolve to a dominant band or halo over time (Fig. 4d). Long-term follow-up findings include the development of fat between the ablation zone and normal kidney parenchyma [37, 38] (Fig. 4d). The majority of kidney ablations show little to no reduction in size within the first 6 to 12 months [39, 40]; however, earlier shrinkage is not uncommon (Fig. 6c).
Lung ablation
When performing RF ablation of lung lesions, the goal is to achieve an avascular region, void of any iodine enhancement (Table 3). Dual-energy CT in the lungs can depict an abnormal blood flow distribution, improving the detection of acute pulmonary embolism [41]. In addition, we believe that the complete avascular ablative area post ablation is well depicted on the iodine-coded images as an iodine-void area. Therefore, iodine mapping may be an excellent method to evaluate the technique effectiveness and also delineates a more realistic assessment of the necrotic zone after ablation (Fig. 7a). On CECT imaging, the absence of iodine uptake in an ablated lung lesion is hard to identify since the boundary between the ablated and non-ablated tissue cannot be clearly defined (Fig. 8).
The typical finding on lung window CT during and after RF ablation is ground-glass opacification surrounding the treated tumour, which is fully circumferential (Fig. 8) in the majority of cases, but can also be partial (Fig. 9a). It is less common to find ground-glass opacification along the electrode tract. These ground-glass changes typically lead to an overestimation of the necrosis zone by 2–4 mm [42]. The distinction between the inner necrotic zone and the outer haemorrhagic viable zone is not clear on CT imaging; thus, the effective necrotic zone is easily overestimated [8]. As a result, the boundary between the ablated and non-ablated zones cannot be clearly defined using traditional morphologic imaging (Fig. 7b).
It is unclear how the iodine images provide a more realistic view of the necrotic zone after ablation. Kawai et al. [43] evaluated the feasibility of distinguishing ground-glass opacification in adenocarcinomas from that in a haemorrhage or inflammation. They found that there was an increased iodine-related attenuation in adenocarcinomas, but not in a pulmonary haemorrhage or in inflammatory changes. These findings led us to investigate the possible benefits of using iodine maps to distinguish residual tumours (Fig. 9) from the common post-ablative changes found in lung ablation. On follow-up CECT, centrally located enhancements or enhancements that were not present immediately after the RF ablation are indicative of malignancy [44]. Even without enhancement, growth of the ablation zone (after 8 to 10 weeks) or any peripheral nodule is a cause for concern. A change from ground-glass opacity to solid opacity also requires further investigation [43]. Positron emission tomography/CT is considered the optimal follow-up tool after focal ablation in the lung. Residual disease can be detected through tracer uptake, most likely at the periphery of the ablation zone (Fig. 9b). On follow-up, increased or new metabolic activity located centrally (Fig. 7b) or at the outer rim of the ablation zone is a sign of progressive tumour activity [44].
Conclusion
Post-ablative changes can hamper the evaluation of post-ablation zones. Our pictorial review illustrates the potential improvements that DECT provides in the differentiation of tissues. We believe that DECT can be a valuable asset in the differentiation of residual tumours from benign inflammatory changes, commonly found after the ablation of liver, kidney, and lung tumours. Further investigation is required because of the limited clinical experience with this relatively new technique.
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Vandenbroucke, F., Van Hedent, S., Van Gompel, G. et al. Dual-energy CT after radiofrequency ablation of liver, kidney, and lung lesions: a review of features. Insights Imaging 6, 363–379 (2015). https://doi.org/10.1007/s13244-015-0408-y
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DOI: https://doi.org/10.1007/s13244-015-0408-y