Virtual unenhanced second generation dual-source CT of the liver: Is it time to discard the conventional unenhanced phase?

Dual-energy CT applications on liver imaging: what radiologists and radiographers should know? A systematic review

PURPOSE

This review aims to provide a comprehensive summary of DECT techniques, acquisition workflows, and post-processing methods. By doing so, we aim to elucidate the advantages and disadvantages of DECT compared to conventional single-energy CT imaging.

METHODS

A systematic search was conducted on MEDLINE/EMBASE for DECT studies in liver imaging published between 1980 and 2024. Information regarding study design and endpoints, patient characteristics, DECT technical parameters, radiation dose, iodinated contrast agent (ICA) administration and postprocessing methods were extracted. Technical parameters, including DECT phase, field of view, pitch, collimation, rotation time, arterial phase timing (from injection), and venous timing (from injection) from the included studies were reported, along with formal narrative synthesis of main DECT applications for liver imaging.

RESULTS

Out of the initially identified 234 articles, 153 met the inclusion criteria. Extensive variability in acquisition parameters was observed, except for tube voltage (80/140 kVp combination reported in 50% of articles) and ICA administration (1.5 mL/kg at 3-4 mL/s, reported in 91% of articles). Radiation dose information was provided in only 40% of articles (range: 6-80 mGy), and virtual non-contrast imaging (VNC) emerged as a common strategy to reduce the radiation dose. The primary application of DECT post-processed images was in detecting focal liver lesions (47% of articles), with predominance of study focusing on hepatocellular carcinoma (HCC) (27%). Furthermore, a significant proportion of the articles (16%) focused on enhancing DECT protocols, while 15% explored metastasis detection.

CONCLUSION

Our review recommends using 80/140 kVp tube voltage with 1.5 mL/kg ICA at 3-4 mL/s flow rate. Post-processing should include low keV-VMI for enhanced lesion detection, IMs for tumor iodine content evaluation, and VNC for dose reduction. However, heterogeneous literature hinders protocol standardization.

Can virtual non-contrast imaging replace true non-contrast imaging in multiphase scanning of the neck region?

Background

Dual-energy computed tomography (DECT) is an advanced imaging method that enables reconstruction of virtual non-contrast (VNC) images from a contrast-enhanced acquisition. This has the potential to reduce radiation exposure by eliminating the need for a true non-contrast (TNC) phase.

Purpose

The purpose is to evaluate the feasibility of VNC images in the neck region.

Materials and methods

A total of 100 patients underwent a DECT scan as part of diagnostic workup of primary hyperparathyroidism. VNC images were reconstructed from 30 s (arterial) and 50 s (venous) post-contrast scans. Regions of interest (ROIs) were placed in thyroid tissue, lymph node, carotid artery, jugular vein, fat, and sternocleidomastoid muscle. Mean densities of all anatomical structures were compared between VNC and TNC images.

Results

For all anatomical structures except the thyroid gland, the difference in mean density between TNC and VNC images was less than 15 HU. The mean difference in density between TNC and VNC images of the thyroid was 53.2 HU (95% CI 46.8; 59.6, p = <0.001).

Conclusion

This study demonstrated an acceptable agreement in density between true non-contrast and virtual non-contrast images for most anatomical structures in the neck region. Therefore, VNC images may have the potential to replace TNC images in the neck. However, due to significant differences in CT density of thyroid tissue, true non-contrast imaging cannot be directly substituted by virtual non-contrast imaging when examining the thyroid and its surrounding tissue.

Feasibility of using software-aided selection of virtual monoenergetic level for optimal image quality of acute necrotising pancreatitis based on dual-energy computed tomography: a preliminary study

OBJECTIVE

This study aimed to assess the feasibility of software-aided selection of monoenergetic level for acute necrotising pancreatitis (ANP) depiction compared to other automatic image series generated using dual-energy computed tomography (CT).

METHODS

The contrast-enhanced dual-source dual-energy CT images in the portal venous phase of 48 patients with ANP were retrospectively analysed. Contrast-to-noise ratio (CNR) of pancreatic parenchyma-to-necrosis, signal-to-noise ratio (SNR) of the pancreas, image noise, and score of subjective diagnosis were measured, calculated, and compared among the CT images of 100 kV, Sn140 kV, weighted-average 120 kV, and optimal single-energy level for CNR.

RESULTS

CNR of pancreatic parenchyma-to-necrosis in the images of 100 kV, Sn140 kV, weighted-average 120 kV, and the optimal single-energy level for CNR was 5.18 ± 2.39, 3.13 ± 1.35, 5.69 ± 2.35, and 9.99 ± 5.86, respectively; SNR of the pancreas in each group was 6.31 ± 2.77, 4.27 ± 1.56, 7.21 ± 2.69, and 11.83 ± 6.30, respectively; image noise in each group was 18.78 ± 5.20, 17.79 ± 4.63, 13.28 ± 3.13, and 9.31 ± 2.96, respectively; and score of subjective diagnosis in each group was 3.56 ± 0.50, 3.00 ± 0.55, 3.48 ± 0.55, and 3.88 ± 0.33, respectively. The four measurements of the optimal single-energy level for CNR images were significantly different from those of images in the other three groups (P < 0.05). CNR of pancreatic parenchyma-to-necrosis, SNR of the pancreas, and score of subjective diagnosis in the images of the optimal single-energy level for CNR were significantly higher, while the image noise was lower than those in the other three groups (all P = 0.000).

CONCLUSION

Optimal single-energy level imaging for CNR of dual-source CT could improve quality of CT images in patients with ANP, enhancing the display of necrosis in the pancreas.

Virtual versus true non-contrast images of the brain from spectral detector CT: comparison of attenuation values and image quality

BACKGROUND

Prior studies focused on utilization of dual-energy computed tomography (DECT) to better detect intracranial pathology and to reduce artifacts. It is still unclear whether virtual non-contrast (VNC) images of DECT can replace true non-contrast (TNC) images.

PURPOSE

To compare attenuation values and image quality of VNC images to TNC images of the brain, obtained using spectral detector CT (SDCT).

MATERIAL AND METHODS

We retrospectively evaluated patients that underwent head CT with and without contrast material, on a SDCT scanner at our institution (n = 33). The attenuation values of different brain structures were obtained from TNC images, the conventional images of the post-contrast exams (n = 16) or the CT angiography (CTA) (n = 17), and the derived VNC images. In total, 591 regions of interest were obtained, including white and gray matter. Two neuroradiologists independently evaluated the image quality of the VNC and TNC images, using a 5-point Likert scale.

RESULTS

The mean difference between the attenuation values on the VNC versus the TNC images was <4 HU for almost all the structures. The difference reached statistical significance (P < 0.05) for the deep gray structures but not for the white matter. The image quality score of the TNC images was 5 in all the patients (excellent gray-white matter differentiation). The scores of the VNC images differed between post-contrast and CTA examinations, with means of 4.9 ± 0.3 (excellent) and 3.2 ± 0.4 (fair), respectively (P < 0.001).

CONCLUSION

Our results show minor differences between attenuation values of different brain structures on VNC versus TNC images of SDCT.

[Comparison of True and Virtual Non-Contrast Images of Liver Obtained with Single-Source Twin Beam and Dual-Source Dual-Energy CT]

PURPOSE

To assess the magnitude of differences between attenuation values of the true non-contrast image (TNC) and virtual non-contrast image (VNC) derived from twin-beam dual-energy CT (tbDECT) and dual-source DECT (dsDECT).

MATERIALS AND METHODS

This retrospective study included 62 patients who underwent liver dynamic DECT with tbDECT (n = 32) or dsDECT (n = 30). Arterial VNC (AVNC), portal VNC (PVNC), and delayed VNC (DVNC) were reconstructed using multiphasic DECT. Attenuation values of multiple intra-abdominal organs (n = 11) on TNCs were subsequently compared to those on multiphasic VNCs. Further, we investigated the percentage of cases with an absolute difference between TNC and VNC of ≤ 10 Hounsfield units (HU).

RESULTS

For the mean attenuation values of TNC and VNC, 33 items for each DECT were compared according to the multiphasic VNCs and organs. More than half of the comparison items for each DECT showed significant differences (tbDECT 17/33; dsDECT 19/33; Bonferroni correction p < 0.0167). The percentage of cases with an absolute difference ≤ 10 HU was 56.7%, 69.2%, and 78.6% in AVNC, PVNC, and DVNC in tbDECT, respectively, and 70.5%, 78%, and 78% in dsDECT, respectively.

CONCLUSION

VNCs derived from the two DECTs were insufficient to replace TNCs because of the considerable difference in attenuation values.

Utility of Dual-Energy CT in Abdominal Interventions

Dual-energy computed tomography (DECT) is an emerging CT technique based on data acquisition at two different settings. Various postprocessing techniques generate different sets of images, each with unique advantages. With DECT, it is possible to obtain virtual unenhanced images from monochromatic reconstructions and attenuation maps of different elements, thereby improving the detection and characterization of a variety of lesions. Presently, DECT is widely used to evaluate pulmonary embolism, characterize abdominal masses, determine the composition of urinary calculi, and detect tophi in gout. CT angiography is an essential prerequisite for endovascular intervention. DECT allows a better quality of angiographic images with a lesser dose of contrast. Various postprocessing techniques in DECT also help in a better evaluation of response to locoregional therapy. Virtual noncontrast images and iodine map differentiate residual or recurrent tumors from intrinsically hyperdense materials. Superior metallic artifact reduction allows better evaluation of vascular injuries adjacent to bony fractured fragments or previously deployed embolization coils. In addition to metal artifacts reduction, virtual monochromatic spectral imaging could further mitigate metal artifacts during CT-guided biopsy, providing an improved depiction of lesions and safe and versatile access for long puncture pathways. This article reviews and illustrates the different applications of DECT in various abdominal interventions. Familiarity with the capabilities of DECT may help interventional radiologists to improve their practice and ameliorate patient care.

Technical Note: Using virtual noncontrast images from dual-energy CT to eliminate the need of precontrast CT for x-ray radiation treatment planning of abdominal tumors†

PURPOSE

Radiation therapy (RT) planning frequently utilizes contrast-enhanced CT. However, dose calculations should not be performed on a contrast-enhanced CT because the patient will not receive bolus during treatment. It is typical to acquire CT twice during RT simulation: once before injection of bolus and once after. The registration between these datasets introduces errors. In this work, we investigate the use of virtual noncontrast images (VNC) derived from dual-energy CT (DECT) to eliminate the precontrast CT and the registration error.

METHODS

CT datasets, including conventional 120 kVp pre- and postcontrast CTs and postcontrast DECT, acquired for ten pancreatic cancer patients were evaluated. The DECTs were acquired simultaneously using a dual source (DS) CT simulator. VNC and virtual mono-energetic images (VMI) were derived from DECTs. Gross tumor volumes (GTV), planning target volumes (PTV), and organs at risks (OAR) were delineated on the postcontrast CT and then populated to the precontrast CT and the VNC. An IMRT plan (50.4 Gy in 28 fractions) was then optimized on the precontrast CT. Dose distributions were recalculated on the VNC images. Contours from the pre- and postcontrast CTs and the dose distributions based on both were compared.

RESULTS

On average, the distance of centroids of the populated duodenum contours on precontrast CT differed by 6.0 ± 4.0 mm from those on postcontrast CTs. The dose distributions on the precontrast CT and VNC were almost identical. The PTV mean and maximum doses differed by 0.1% and 0.2% between the two plans, respectively.

CONCLUSION

The VNC derived from DECT can be used to replace the conventional precontrast CT scan for RT planning, eliminating the need for an additional precontrast CT scan and eliminating the registration errors. Thus, VNC can become an important asset to the future of RT.

Building a dual-energy CT service line in abdominal radiology

As the access of radiology practices to dual-energy CT (DECT) has increased worldwide, seamless integration into clinical workflows and optimized use of this technology are desirable. In this article, we provide basic concepts of commercially available DECT hardware implementations, discuss financial and logistical aspects, provide tips for protocol building and image routing strategies, and review radiation dose considerations to establish a DECT service line in abdominal imaging. KEY POINTS: • Tube-based and detector-based DECT implementations with varying features and strengths are available on the imaging market. • Thorough assessment of financial and logistical aspects is key to successful implementation of a DECT service line. • Optimized protocol building and image routing strategies are of critical importance for effective use and seamless inception of DECT in routine clinical workflows.

Virtual unenhanced imaging of the liver derived from 160-mm rapid-switching dual-energy CT (rsDECT): Comparison of the accuracy of attenuation values and solid liver lesion conspicuity with native unenhanced images

OBJECTIVES

To evaluate the reliability of attenuation values of the liver parenchyma and focal liver lesions on virtual unenhanced images from arterial (VUEart) and portal venous phases (VUEport) compared to native unenhanced (NU) attenuation values in patients referred for assessment of malignant liver lesions.

METHODS

Seventy-three patients with confirmed primary or metastatic liver tumors who underwent a multiphase contrast-enhanced rapid-switching kVp dual-energy CT (rsDECT) were included in this IRB-approved retrospective study. Both qualitative and quantitative analyses - including the lesion-to-liver contrast-to-noise ratio (LL-CNR) - were performed and compared between NU and both VUEart and VUEport images.

RESULTS

The mean liver attenuation values were significantly lower in VUEart images (56.7 ± 6.7 HU) than in NU images (59.6 ± 7.5 HU, p = 0.008), and were comparable between VUEart and VUEport images (57.9 ± 6 UH, p = 0.38) and between VUEport and NU images (p = 0.051). The mean liver lesions attenuation values were comparable between NU, VUEart and VUEport images (p = 0.60). Strong and significant correlations values were found both in liver lesions and tumor-free parenchyma (r = 0.82-0.91, p < 0.01). The mean LL-CNR was significantly higher in VUEart and VUEport images than in NU images (1.7 ± 1 and 1.6 ± 1.1 vs 0.9 ± 0.6; p < 0.001), but was comparable between VUEart and VUEport images (p > 0.9). Lesion conspicuity was significantly higher in VUEport images than in NU images (p < 0.001).

CONCLUSION

VUEport images derived from 3rd generation rsDECT could confidently replace NU images in patients undergoing assessment for malignant liver lesions. These images provide comparable attenuation values in both liver lesions and liver parenchyma while reducing the radiation dose and scanning time.

Replacing true unenhanced imaging in renal carcinoma with virtual unenhanced images in dual-energy spectral CT: a feasibility study

AIM

To investigate the clinical value of virtual unenhanced (VNC) spectral computed tomography (CT) images to replace the conventional true unenhanced spectral CT images (TNC) in diagnosing renal carcinoma.

MATERIALS AND METHODS

Fifty-six cases of renal carcinoma confirmed by histopathology underwent conventional plain CT and contrast-enhanced spectral CT at arterial phase (AP) and venous phase (VP). VNC images were generated on an AW4.6 workstation. The CT attenuation, image noise, contrast-to-noise ratio (CNR), and signal-noise-ratio (SNR) of the renal lesions and normal kidneys, long and short axis diameters of the lesion were measured from the three image sets and analysed using one-way analysis of variance (ANOVA). Two radiologists evaluated image quality subjectively using a five-point score, and lesion signature using a three-point score. Image quality scores were compared statistically and tested for consistency.

RESULTS

The two reviewers had good agreement for subjective evaluation (Kappa>0.70) and there was no difference in the quality of the scores among the three image groups. The lesion signature scores were all above the acceptable level. The CNR and SNR values in VNC were significantly higher than in TNC (p<0.05). VNC images had lower renal noise than in TNC (p<0.05). There was no difference in the long and short axis diameters of the lesion among the three image groups. VNC had higher CT attenuation values for the lesion and kidney than TNC (p<0.05), but the differences were <5 HU.

CONCLUSION

VNC images in spectral CT may be used to replace the conventional plain CT to reduce imaging duration and radiation dose in diagnosing renal carcinoma.

Abdominal Organs Attenuation Values and Abdominal Aortic Calcifications on Virtual and True Noncontrast Images Obtained With Third-Generation Dual-Source Dual-Energy Computed Tomography

PURPOSE

To evaluate the agreement and correlation between attenuation values and vascular calcification volume for intra-abdominal structures from true noncontrast (TNC) images and those from virtual noncontrast (VNC) images obtained by dual-source dual-energy computed tomography (CT) using a quadriphasic dynamic protocol.

METHODS

Seventy-six patients who underwent quadriphasic abdominal CT were retrospectively reviewed. An arterial, portal venous, and 5-minute delayed phase postcontrast series was obtained using dual-source dual-energy CT. Virtual noncontrast images were processed from the arterial, portal venous, and 5-minute delayed phase series. Attenuation values for the liver, pancreas, kidneys, muscle, fat, vertebra, gallbladder, aorta, inferior vena cava, portal vein, and aortic calcification volumes were recorded. Attenuation values for the liver, pancreas, vertebra, and muscle obtained from VNC were adjusted using linear regression.

RESULTS

Repeated-measures analysis of variance and Bonferroni multiple-comparison post hoc correction revealed significant differences between TNC and VNC attenuation values for the organs. There was an excellent correlation between the TNC and VNC attenuation values for the liver, pancreas, muscle, vertebra, and fat. The calcification volume was significantly smaller on VNC than on TNC. The adjusted attenuation values estimated by regression equations afforded better approximation.

CONCLUSIONS

Abdominal VNC images obtained with third-generation dual-source dual-energy CT cannot replace TNC images without adjustment of the attenuation values.

Quantitative attenuation accuracy of virtual non-enhanced imaging compared to that of true non-enhanced imaging on dual-source dual-energy CT

PURPOSE

To evaluate the quantitative attenuation and reliability of virtual non-contrast (VNC) images of the abdomen acquired from multiphasic scans with a dual-energy computed tomography (DECT) system and compare it with that of true non-enhanced images (TNC) on second- (Flash) and third- (Force) generation DECT scanners.

METHODS

This retrospective study was approved by the institutional review board and included 123 patients with pancreatic cancer who had undergone routine clinical multiphasic DECT examinations at our institution using Flash and Force scanners between March and August 2017. VNC images of the abdomen were reconstructed from late arterial phase images. For every patient, regions-of-interest were defined in the aorta, fluid-containing structures (gallbladder, pleural effusion, and renal cysts > 10 mm), paravertebral muscles, subcutaneous fat, spleen, pancreas, renal cortex, and liver (eight locations) on TNC and VNC images. The mean attenuation of VNC was compared with TNC by organ for each CT scanner using an equivalence test and the Bland-Altman plot. The mean attenuations for TNC or VNC were compared between the Force and Flash CT scanners using a two-sample t test.

RESULTS

The VNC attenuation of organs on the Force scanner was lower than was that on the Flash, and the mean attenuation difference in different organs on the Force was closer to 0. The estimated means of TNC and VNC were equivalent for an equivalence margin of 10 on the Force scanner.

CONCLUSION

VNC images in DECT are a promising alternative to TNC images. In clinical scenarios in which non-enhanced CT images are required but are not available for accurate diagnosis, VNC images can potentially serve as an alternative to TNC images without the radiation exposure risks.

Interscanner and Intrascanner Comparison of Virtual Unenhanced Attenuation Values Derived From Twin Beam Dual-Energy and Dual-Source, Dual-Energy Computed Tomography

OBJECTIVE

The aim of the current study was to evaluate the reliability and comparability of virtual unenhanced (VUE) attenuation values derived from scans of a single-source, dual-energy computed tomography using a split-filter (tbDECT) to a dual-source dual-energy CT (dsDECT).

MATERIALS AND METHODS

In this retrospective study, comparisons for tbDECT and dsDECT were made within and between different dual-energy platforms. For the interscanner comparison, 126 patients were scanned with both scanners within a time interval of 224 ± 180 days; for the intrascanner comparison, another 90 patients were scanned twice with the same scanner within a time interval of 136 ± 140 days. Virtual unenhanced images were processed off of venous phase series. Attenuation values of 7 different tissues were recorded. Disagreement for VUE HU measurements greater than 10 HU between 2 scans was defined as inadequate.

RESULTS

The interscanner analysis showed significant difference between tbDE and dsDE VUE CT values (P < 0.01) for 6 of 7 organs. Percentage of cases that had more than 10 HU difference between tbDE and dsDE for an individual patient ranged between 15% (left kidney) and 62% (spleen).The intrascanner analysis showed no significant difference between repeat scans for both tbDECT and dsDECT (P > 0.05). However, intrascanner disagreements for the VUE HU measurements greater than 10 HU were recorded in 10% of patients scanned on the tbDECT and 0% of patients scanned on the dsDECT. The organs with the highest portion of greater than 10 HU errors were the liver and the aorta (both 20%).

CONCLUSIONS

Dual-energy techniques vary in reproducibility of VUE attenuation values. In the current study, tbDECT demonstrated higher variation in VUE HU measurements in comparison to a dsDECT. Virtual unenhanced HU measurements cannot be reliably compared on follow-up CT, if these 2 different dual-energy CT platforms are used.

Image Quality of Iodine Maps for Pulmonary Embolism: A Comparison of Subtraction CT and Dual-Energy CT

OBJECTIVE. The objective of this study was to compare the image quality of iodine maps derived from subtraction CT and from dual-energy CT (DECT) in patients with suspected pulmonary embolism (PE). SUBJECTS AND METHODS. In this prospective study conducted between July 2016 and April 2017, consecutive patients with suspected PE underwent unenhanced CT at 100 kV and dual-energy pulmonary CT angiography at 100 and 140 kV on a dual-source scanner. The scanner was set to generate subtraction and DECT iodine maps at similar radiation doses. In 55 patients (30 women, 25 men; mean age ± SD, 63.4 ± 11.9 years old), various subjective image quality criteria including diagnostic acceptability were rated on a 5-point scale by four radiologists and a radiology resident. In 29 patients (17 women, 12 men; mean age, 62.4 ± 11.7 years old) with confirmed perfusion defects, the signal-difference-to-noise ratio (SDNR) between perfusion defects and adjacent normally perfused parenchyma was measured in corresponding ROIs on subtraction and DECT iodine maps. McNemar and Wilcoxon signed-rank tests were used for statistical comparisons. RESULTS. Diagnostic acceptability was rated excellent or good in a mean of 67% (range, 31-80%) of subtraction CT studies and 36% (5-69%) of DECT studies (p < 0.05 for four of the five radiologists), mainly because of fewer artifacts on subtraction CT. Mean SDNR was marginally higher for subtraction CT than for DECT (18.6 vs 17.1, p = 0.06) and was significantly higher in the upper lobes (21.8 vs 17.9, p < 0.05). CONCLUSION. Radiologist-judged image quality of pulmonary iodine maps was higher for subtraction CT than for DECT with similar to higher SDNR. Subtraction CT is a software-only solution, so it may be an attractive alternative to DECT for depicting perfusion defects.

Virtual unenhanced phase with spectral dual-energy CT: Is it an alternative to conventional true unenhanced phase for abdominal tissues?

PURPOSE

To compare attenuation measurements and image quality of virtual unenhanced phase (VUP) images with those of conventional true unenhanced phase (TUP) images on spectral dual energy computed tomography (DECT) with dual layer detector on abdominal tissues and to assess potential reduction in radiation dose.

MATERIAL AND METHOD

A total of 295 patients (185 men, 110 women; mean age 61±17.6 [SD] years [range: 17-95 years]) who had undergone abdominal or thoraco-abdominal CT with pre- and post-contrast imaging (portal phase) with spectral DECT with dual layer detector were retrospectively analyzed. VUP images based on portal-venous phase DECT acquisition were generated. Regions of interest were defined in abdominal tissues (liver, spleen, kidney, muscle and fat) by two independent readers. Inter-technique agreement (VUP images vs. TUP images) on attenuation measurements was assessed. Signal-to noise ratio (SNR) and image quality of TUP and VUP images were compared. The radiation dose delivered to patients was compared with the radiation dose of protocols without TUP images.

RESULTS

A total of 9880 ROIs were drawn in the abdominal tissues. The difference in mean attenuation values between TUP and VUP images was less than 15 HU in 98.3% and less than 10 HU in 92.3% of all measurements. VUP images overestimated attenuation in fat comparatively to TUP images. Image quality was evaluated as good or excellent in 77% (37/48) of TUP images and 54% (26/48) of VUP images. Using VUP images instead of TUP images could decrease the radiation dose by 32%.

CONCLUSION

VUP images demonstrate good agreement with TUP images in different abdominals tissues and can be obtained with similar image quality as TUP. VUP images appear as an alternative to TUP images, resulting in reduction of radiation dose delivered to the patient.

Dual-Energy Computed Tomography: Dose Reduction, Series Reduction, and Contrast Load Reduction in Dual-Energy Computed Tomography

Evolution in computed tomography technology and image reconstruction have significantly changed practice. Dual energy computed tomography is being increasingly adopted owing to benefits of material separation, quantification, and improved contrast-to-noise ratio. The radiation dose can match that from single energy computed tomography. Spectral information derived from a polychromatic x-ray beam at different energies yields in image reconstructions that reduce the number of phases in a multiphasic examination and decrease the absolute amount of contrast media. This increased analytical and image processing capability provides new avenues for addressing radiation dose and iodine exposure concerns.

Rapid switching kVp dual energy CT: Value of reconstructed dual energy CT images and organ dose assessment in multiphasic liver CT exams

PURPOSE

Clinical applications of dual energy computed tomography (DECT) have been widely reported; however, the importance of the different image reconstructions and radiation organ dose remains a relevant area of investigation, particularly considering the different commercially available DECT equipment. Therefore, the purpose of this study was to assess the image reliability and compare the information content between several image reconstructions in a rapid-switching DECT (rsDECT), and assess radiation organ dose between rsDECT and conventional single-energy computed tomography (SECT) exams.

MATERIALS AND METHODS

This Institutional Review Board-approved retrospective study included 98 consecutive patients who had a history of liver cancer and underwent multiphasic liver CT exams with rsDECT applied during the late arterial phase between June 2015 and December 2015. Virtual monochromatic 70 keV, material density images (MDI) iodine (-water) and virtual unenhanced (VUE) images were generated. Radiation dose analysis was performed in a subset of 44 patients who had also undergone a multiphasic SECT examination within 6 months of the rsDECT. Four board-certified abdominal radiologists reviewed 24-25 patients each, and a fifth radiologist re-evaluated all the scans to reach a consensus. The following imaging aspects were assessed by the radiologists: (a) attenuation measurements were made in the liver and spleen in VUE and true unenhanced (TUE) images; (b) subjective evaluation for lesion detection and conspicuity on MDI iodine (-water)/VUE images compared with the virtual monochromatic images/TUE images; and (c) overall image quality using a five-point Likert scale. The radiation dose analyses were evaluated in the subset of 44 patients regarding the following parameters: CTDIvol, dose length product, patient's effective diameter and organ dose using a Monte Carlo-based software, VirtualDose™ (Virtual Phantoms, Inc.) to 21 organs.

RESULTS

On average, image noise on the TUE images was 49% higher within the liver (p < 0.0001) and 48% higher within the spleen (p < 0.0001). CT numbers for the spleen were significantly higher on VUE images (p < 0.0001). Twenty-eight lesions in 24/98 (24.5%) patients were not observed on the VUE images. The conspicuity of vascular anatomy was considered better on MDI iodine (-water) Images 26.5% of patients. Using the Likert scale, the rsDECT image quality was considered to be satisfactory. Considering the subset of 44 patients with recent SECT, the organ dose was, on average, 37.4% less with rsDECT. As the patient's effective diameter decreased, the differences in dose between the rsDECT and SECT increased, with the total average organ dose being less by 65.1% when rsDECT was used.

CONCLUSION

VUE images in the population had lower image noise than TUE images; however, a few small and hyperdense findings were not characterized on VUE images. Delineation of vascular anatomy was considered better in around a quarter of patients on MDI iodine (-water) images. Finally, radiation dose, particularly organ dose, was found to be lower with rsDECT, especially in smaller patients.

Attenuation values of renal parenchyma in virtual noncontrast images acquired from multiphase renal dual-energy CT: Comparison with standard noncontrast CT

OBJECTIVES

To compare the renal parenchyma attenuation of virtual noncontrast (VNC) images derived from multiphase renal dual-energy computed tomography (DECT) with standard noncontrast (SNC) images, and to determine the optimum phase for VNC images.

MATERIALS AND METHODS

Twenty-nine men and 16 women (mean age, 61 ± 13 years; range, 37-89 years) underwent dynamic renal DECT (100/Sn140 kVp) were included in this institutional review board-approved retrospective study. There were four phases of the scan, which included noncontrast, corticomedullary (CMP), nephrographic (NP), and excretory phases (EP). The VNC images was generated from CMP, NP and EP. CT numbers of SNC images and VNC images of each phases were measured in the renal cortex and medulla. Mean standard deviation of subcutaneous fat was measured as image noise on SNC and VNC images. Radiation dose was recorded and potential radiation dose reduction was estimated. Results were tested for statistical significance using the unpaired t-test and agreement using Bland-Altman plot analysis.

RESULTS

The difference in mean attenuation between SNC and each phase of VNC images were ≤4 HU. The mean attenuation of renal cortex and medulla was 33.2 ± 4.4 HU, and 34.2 ± 4.8 HU in SNC, 33.6 ± 7.6 HU and 31.1 ± 8.3 HU in VNC of CMP, 34.8 ± 8.6 HU and 35.6 ± 8.5 HU in VNC of NP, 31.5 ± 7.6 HU and 32.4 ± 7.5 HU in VNC of EP. In VNC of CMP, the attenuation of the cortex was higher than the medulla (p < 0.05), and the attenuation of medulla was significant lower than that of SNC (p < 0.01). In VNC of NP, the attenuation of renal cortex was higher than SNC (p < 0.05). In VNC of EP, the attenuation of cortex and medulla were lower than SNC (p < 0.05), and inadequate iodine subtraction in collecting system was noted. Image noise was significantly greater in SNC (p < 0.001). Mean radiation dose reduction achievable by removing the SNC was 12.3% ± 0.9%.

CONCLUSIONS

VNC images from multiphase renal DECT were similar to SNC images. Using the nephrographic phase can gives more comparable VNC images to SNC images in renal parenchyma than other phases.

Dual energy CT applications in pancreatic pathologies

Dual energy CT (DECT) is a technology that is gaining widespread acceptance, particularly for its abdominopelvic applications. Pancreatic pathologies are an ideal application for the many advantages offered by dual energy post-processing. This article reviews the current literature on dual energy CT pancreatic imaging, specifically in the evaluation of pancreatic adenocarcinoma, other solid and cystic pancreatic neoplasms, and pancreatitis. The advantages in characterization and quantification of enhancement, detection of subtle lesions, and potential reduction of imaging phases and contrast usage are reviewed. We also discuss directions for future research, and the ideal use of dual energy CT in routine clinical practice.

Initial Results of a Single-Source Dual-Energy Computed Tomography Technique Using a Split-Filter: Assessment of Image Quality, Radiation Dose, and Accuracy of Dual-Energy Applications in an In Vitro and In Vivo Study

OBJECTIVE

The aim of this study was to investigate the image quality, radiation dose, and accuracy of virtual noncontrast images and iodine quantification of split-filter dual-energy computed tomography (CT) using a single x-ray source in a phantom and patient study.

MATERIALS AND METHODS

In a phantom study, objective image quality and accuracy of iodine quantification were evaluated for the split-filter dual-energy mode using a tin and gold filter. In a patient study, objective image quality and radiation dose were compared in thoracoabdominal CT of 50 patients between the standard single-energy and split-filter dual-energy mode. The radiation dose was estimated by size-specific dose estimate. To evaluate the accuracy of virtual noncontrast imaging, attenuation measurements in the liver, spleen, and muscle were compared between a true noncontrast premonitoring scan and the virtual noncontrast images of the dual-energy scans. Descriptive statistics and the Mann-Whitney U test were used.

RESULTS

In the phantom study, differences between the real and measured iodine concentration ranged from 2.2% to 21.4%. In the patient study, the single-energy and dual-energy protocols resulted in similar image noise (7.4 vs 7.1 HU, respectively; P = 0.43) and parenchymal contrast-to-noise ratio (CNR) values for the liver (29.2 vs 28.5, respectively; P = 0.88). However, the vascular CNR value for the single-energy protocol was significantly higher than for the dual-energy protocol (10.0 vs 7.1, respectively; P = 0.006). The difference in the measured attenuation between the true and the virtual noncontrast images ranged from 3.1 to 6.7 HU. The size-specific dose estimate of the dual-energy protocol was, on average, 17% lower than that of the single-energy protocol (11.7 vs 9.7 mGy, respectively; P = 0.008).

CONCLUSIONS

Split-filter dual-energy compared with single-energy CT results in similar objective image noise in addition to dual-energy capabilities at 17% lower radiation dose. Because of beam hardening, split-filter dual-energy can lead to decreased CNR values of iodinated structures.

Quantitative Image Quality Comparison of Reduced- and Standard-Dose Dual-Energy Multiphase Chest, Abdomen, and Pelvis CT

We present a new image quality assessment method for determining whether reducing radiation dose impairs the image quality of computed tomography (CT) in qualitative and quantitative clinical analyses tasks. In this Institutional Review Board-exempt study, we conducted a review of 50 patients (male, 22; female, 28) who underwent reduced-dose CT scanning on the first follow-up after standard-dose multiphase CT scanning. Scans were for surveillance of von Hippel–Lindau disease (N = 26) and renal cell carcinoma (N = 10). We investigated density, morphometric, and structural differences between scans both at tissue (fat, bone) and organ levels (liver, heart, spleen, lung). To quantify structural variations caused by image quality differences, we propose using the following metrics: dice similarity coefficient, structural similarity index, Hausdorff distance, gradient magnitude similarity deviation, and weighted spectral distance. Pearson correlation coefficient and Welch 2-sample t test were used for quantitative comparisons of organ morphometry and to compare density distribution of tissue, respectively. For qualitative evaluation, 2-sided Kendall Tau test was used to assess agreement among readers. Both qualitative and quantitative evaluations were designed to examine significance of image differences for clinical tasks. Qualitative judgment served as an overall assessment, whereas detailed quantifications on structural consistency, intensity homogeneity, and texture similarity revealed more accurate and global difference estimations. Qualitative and quantitative results indicated no significant image quality degradation. Our study concludes that low(er)-dose CT scans can be routinely used because of no significant loss in quantitative image information compared with standard-dose CT scans.

Comparison of virtual unenhanced CT images of the abdomen under different iodine flow rates

OBJECTIVE

To assess the effect of varying iodine flow rate (IFR) and iodine concentration on the quality of virtual unenhanced (VUE) images of the abdomen obtained with dual-energy CT.

METHODS

94 subjects underwent unenhanced and triphasic contrast-enhanced CT scan of the abdomen, including arterial phase, portal venous phase, and delayed phase using dual-energy CT. Patients were randomized into 4 groups with different IFRs or iodine concentrations. VUE images were generated at 70 keV. The CT values, image noise, SNR and CNR of aorta, portal vein, liver, liver lesion, pancreatic parenchyma, spleen, erector spinae, and retroperitoneal fat were recorded. Dose-length product and effective dose for an examination with and without plain phase scan were calculated to assess the potential dose savings. Two radiologists independently assessed subjective image quality using a five-point scale. The Kolmogorov-Smirnov test was used first to test for normal distribution. Where data conformed to a normal distribution, analysis of variance was used to compare mean HU values, image noise, SNRs and CNRs for the 4 image sets. Where data distribution was not normal, a nonparametric test (Kruskal-Wallis test followed by stepwise step-down comparisons) was used. The significance level for all tests was 0.01 (two-sided) to allow for type 2 errors due to multiple testing.

RESULTS

The CT numbers (HU) of VUE images showed no significant differences between the 4 groups (p > 0.05) or between different phases within the same group (p > 0.05). VUE images had equal or higher SNR and CNR than true unenhanced images. VUE images received equal or lower subjective image quality scores than unenhanced images but were of acceptable quality for diagnostic use. Calculated dose-length product and estimated dose showed that the use of VUE images in place of unenhanced images would be associated with a dose saving of 25%.

CONCLUSIONS

VUE images can replace conventional unenhanced images. VUE images are not affected by varying iodine flow rates and iodine concentrations, and diagnostic examinations could be acquired with a potential dose saving of 25%.

Determinants of Detection of Stones and Calcifications in the Hepatobiliary System on Virtual Nonenhanced Dual-energy CT

Objective To retrospectively determine the features of stones and calcifications in hepatobiliary system on virtual nonenhanced (VNE) dual-energy computed tomography (CT), and to evaluate the possibility of VNE images in diagnosis for those lesions.Methods A total of 128 gall stones and calcifications of the liver found in 110 patients were examined with triple phase abdominal CT scan from July 2007 to December 2011, in which true nonenhanced (TNE) phase and arterial phase were performed with single-energy CT (120 kVp) and portal venous phase was performed with dual-energy CT (100 kVp and 140 kVp). VNE images were generated from the portal venous phase dual-energy CT data sets by using commercially VNC software. The mean CT values for the stone, liver, bile and paraspinal muscle, mean lesion density and size in area dimension, contrast-to-noise ratio (CNR) of lesion to the liver or bile, and image noise were assessed and compared between VNE and TNE images. The effective dose and size-specific dose estimate (SSDE) were also calculated.Results The mean CT values of the lesions measured on VNE images declined significantly compared with those measured on TNE images (164.51±102.13 vs. 290.72±197.80 HU, P<0.001), so did the lesion-to-liver CNR (10.80±11.82 vs.18.81±17.06, P<0.001) and the lesion-to-bile CNR (17.24±14.41 vs. 21.32±17.31, P<0.001). There was no significant difference in size of lesions area between VNE and TNE images (0.69±0.88 vs. 0.72±0.85 cm², P=0.062). Compared to the 128 lesions found in TNE images, VNE images showed the same density in 30 (23.4%) lesions, lighter density in 88 (68.8%) lesions, while failed to show 10 (7.8%) lesions, and showed the same size in 61 (47.7%) lesions and smaller size in 57 (44.5%) lesions. The CT cutoff values of lesion and size were 229.21 HU and 0.15 cm², respectively. The total effective dose for triple phase scan protocol with TNE images was 19.51±7.03 mSv, and the SSDE was 39.84±11.10 mGy. The effective dose for dual phase scan protocol with VNE images instead of TNE images was 13.29±4.89 mSv, and the SSDE was 27.83±9.99 mGy. Compared with TNE images, the effective dose and SSDE of VNE images were down by 32.05%±3.69 % and 30.63%±2.34 %, respectively.Conclusions Although the CT values and CNR of the lesions decreased in VNE images, the lesions of which attenuation greater than 229.21 HU and size larger than 0.15 cm² could be detected with good reliability and obvious dose reduction. There was good consistency in the size of stones and calcifications in hepatobiliary system between VNE images and TNE images, which ensured the possibility of the clinical application of VNE images.

Evaluation of image quality, radiation dose and diagnostic performance of dual-energy CT datasets in patients with hepatocellular carcinoma

AIM

To evaluate image quality and diagnostic accuracy of different dual-energy computed tomography (DECT) datasets for identification of hepatocellular carcinoma (HCC), assess the reliability of virtual unenhanced (VU) images in replacing standard unenhanced (SU) images, and quantify effective dose (ED) at different tube voltages.

MATERIAL AND METHODS

Thirty cirrhotic patients underwent liver contrast-enhanced DECT. Two blinded observers retrospectively evaluated conventional unenhanced and VU images, 140 kVp/80 kVp/mixed tube potential arterial datasets and conventional portal-venous/late phases in consensus. Final diagnosis was based on pathological proof or imaging criteria. Image quality, ED, sensitivity, and specificity of arterial datasets were calculated.

RESULTS

Thirty-eight HCC and 18 benign lesions were detected at 80 kVp, 33 HCC and 22 benign lesions were detected at 140 kVp, and 36 HCC and 20 benign lesions were detected at mixed tube potentials. Final diagnosis confirmed 37 HCC and 20 benign lesions. There was no significant difference in diagnostic confidence between 80 kVp, 140 kVp, and mixed tube potential arterial datasets (p>0.05). Image quality was adequate for all datasets, with increased quality at higher tube potential (80 versus 140 kVp, p=0.001; mixed versus 140 kVp, p=0.001; 80 kVp versus mixed, p=0.0024). Significant ED reduction was observed between 140 and 80 kVp datasets (p<0.001).

CONCLUSIONS

The 140 kVp dataset provided higher image quality. The 80 kVp images were more sensitive in detecting HCC. VU images are adequate in replacing SU images. The ED of the 80 kVp dataset was significantly lower.

Stomach virtual non-enhanced CT with second-generation, dual-energy CT: a preliminary study

Objectives

To compare the true non-enhanced (TNE) and virtual non-enhanced (VNE) data sets in patients who underwent gastric preoperative dual-energy CT (DECT) and to evaluate potential radiation dose reduction by omitting a TNE scan.

Methods

A total of 74 patients underwent gastric DECT. The mean CT values, length, image quality and effective radiation doses for VNE and TNE images were compared.

Results

There was no statistical difference in maximal thickness of gastric tumors and maximal diameter of enlarged lymph nodes among the TNE and VNE images (P>0.05). The mean CT value differences between TNE and VNE were statistically significant for all tissue types, except for aorta attenuation measurements (P<0.05), but the absolute differences were under 10 HU. Lower noise was found for VNE images than TNE images (P<0.01). Image quality of VNE was diagnostic but lower than that of TNE (P<0.01). The dose reduction achieved by omitting the TNE acquisition was 21.40±4.44%.

Conclusion

VNE scan may potentially replace TNE as part of a multi-phase gastric preoperative staging imaging protocol with consequent saving in radiation dose.

Radiotherapy treatment planning with contrast-enhanced computed tomography: feasibility of dual-energy virtual unenhanced imaging for improved dose calculations

Background

In radiotherapy treatment planning, intravenous administration of an iodine-based contrast agent during computed tomography (CT) improves the accuracy of delineating target volumes. However, increased tissue attenuation resulting from the high atomic number of iodine may result in erroneous dose calculations because the contrast agent is absent during the actual procedure. The purpose of this proof-of-concept study was to present a novel framework to improve the accuracy of dose calculations using dual-energy virtual unenhanced CT in the presence of an iodine-based contrast agent.

Methods

Simple phantom experiments were designed to assess the feasibility of the proposed concept. By utilizing a “second-generation” dual-source CT scanner equipped with a tin filter for improved spectral separation, four CT datasets were obtained using both a water phantom and an iodine phantom: “true unenhanced” images with attenuation values of 2 ± 11 Hounsfield Units (HU), “enhanced” images with attenuation values of 274 ± 23 HU, and two series of “virtual unenhanced” images synthesized from dual-energy scans of the iodine phantom, each with a different combination of tube voltages. Two series of virtual unenhanced images demonstrated attenuation values of 12 ± 29 HU (with 80 kVp/140 kVp) and 34 ± 10 HU (with 100 kVp/140 kVp) after removing the iodine component from the contrast-enhanced images. Dose distributions of the single photon beams calculated from the enhanced images and two series of virtual unenhanced images were compared to those from true unenhanced images as a reference.

Results

The dose distributions obtained from both series of virtual unenhanced images were almost equivalent to that from the true unenhanced images, whereas the dose distribution obtained from the enhanced images indicated increased beam attenuation caused by the high attenuation characteristics of iodine. Compared to the reference dose distribution from the true unenhanced images, the dose distribution pass rates from both series of virtual unenhanced images were greater than 90%, while those from the enhanced images were less than approximately 50–60%.

Conclusions

Dual-energy virtual unenhanced CT improves the accuracy of dose distributions in radiotherapy treatment planning by removing the iodine component from contrast-enhanced images.

Tin-filter enhanced dual-energy-CT: image quality and accuracy of CT numbers in virtual noncontrast imaging

OBJECTIVES

To measure and compare the objective image quality of true noncontrast (TNC) images with virtual noncontrast (VNC) images acquired by tin-filter-enhanced, dual-source, dual-energy computed tomography (DECT) of upper abdomen.

MATERIALS AND METHODS

Sixty-three patients received unenhanced abdominal CT and enhanced abdominal DECT (100/140 kV with tin filter) in portal-venous phase. VNC images were calculated from the DECT datasets using commercially available software. The mean attenuation of relevant tissues and image quality were compared between the TNC and VNC images. Image quality was rated objectively by measuring image noise and the sharpness of object edges using custom-designed software. Measurements were compared using Student two-tailed t-test. Correlation coefficients for tissue attenuation measurements between TNC and VNC were calculated and the relative deviations were illustrated using Bland-Altman plots.

RESULTS

Mean attenuation differences between TNC and VNC (HUTNC - HUVNC) image sets were as follows: right liver lobe -4.94 Hounsfield units (HU), left liver lobe -3.29 HU, vena cava -2.19 HU, spleen -7.46 HU, pancreas 1.29 HU, fat -11.14 HU, aorta 1.29 HU, bone marrow 36.83 HU (all P < .05); right kidney 0.46 HU, left kidney 0.56 HU, vena portae -0.48 HU and muscle -0.62 HU (nonsignificant). Good correlations between VNC and TNC series were observed for liver, vena portae, kidneys, pancreas, muscle and bone marrow (Pearson's correlation coefficient ≥0.75). Mean image noise was significantly higher in TNC images (P < .0001). Measurements of edge sharpness revealed no significant differences between VNC and TNC images (P = .19).

CONCLUSION

The Hounsfield units in VNC images closely resemble TNC images in the majority of the organs of the upper abdomen (kidneys, liver, pancreas). In spleen and fat, Hounsfield numbers in VNC images are tend to be higher than in TNC images. VNC images show a low image noise and satisfactory edge sharpness. Other criteria of image quality and the depiction of certain lesions need to be evaluated additionally.

Dual-energy CT in patients treated with anti-angiogenic agents for non-small cell lung cancer: new method of monitoring tumor response?

Objective

To evaluate tumor responses in patients treated with anti-angiogenic agents for non-small cell lung cancer (NSCLC) by assessing intratumoral changes using a dual-energy CT (DECT) (based on Choi's criteria) and to compare it to traditional Response Evaluation Criteria in Solid Tumors (RECIST) criteria.

Materials and Methods

Ten NSCLC patients treated with bevacizumab underwent DECT. Tumor responses to anti-angiogenic therapy were assessed and compared with the baseline CT results using both RECIST (size changes only) and Choi's criteria (reflecting net tumor enhancement). Kappa statistics was used to evaluate agreements between tumor responses assessed by RECIST and Choi's criteria.

Results

The weighted κ value for the comparison of tumor responses between the RECIST and Choi's criteria was 0.72. Of 31 target lesions (21 solid nodules, 8 lymph nodes, and two ground-glass opacity nodules [GGNs]), five lesions (16%) showed discordant responses between RECIST and Choi's criteria. Iodine-enhanced images allowed for a distinction between tumor enhancement and hemorrhagic response (detected in 14% [4 of 29, excluding GGNs] of target lesions on virtual nonenhanced images).

Conclusion

DECT may serve as a useful tool for response evaluation after anti-angiogenic treatment in NSCLC patients by providing information on the net enhancement of target lesions without obtaining non-enhanced images.

[Dual-energy computed tomography: what is it useful for?]

Dual-energy CT is one of the newest and most attractive fields in radiology today. New generation scanners can acquire datasets with different X-ray spectra, which facilitates the characterization of certain chemical elements, making it possible to detect functional alterations in the absence of morphologic or densitometric anomalies. The capability of characterizing these elements is enabling new applications to be developed for clinical practice and changing the way we work. The aim of this article is to explain what dual-energy CT studies are, the techniques available for performing them, the advantages and disadvantages of these studies, and what we might expect from this field in the future.