Virtual unenhanced imaging of the liver with third-generation dual-source dual-energy CT and advanced modeled iterative reconstruction

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.

Computed tomography numbers obtained for varying iodine contrast concentrations by different-generation dual-energy computed tomography scanners

We compared the computed tomography (CT) numbers from monochromatic images obtained using the first-generation (Discovery CT750 HD: GE Healthcare, Milwaukee, WI) and second-generation (Revolution CT: GE HealthCare) dual-energy CT (first and second DECT) scanners in phantom and clinical studies. In a polypropylene phantom, eight polypropylene tubes containing iodine at various concentrations (0.5, 1, 2, 5, 10, 12, 20, 30 mg I per ml) were arranged in an outer circle. The iodine densities and CT numbers obtained after imaging with different-generation DECT scanners were analyzed. The CT numbers from images obtained from 61 consecutive patients with aortic disease who underwent CT with different-generation DECT scanners were compared during the arterial and delayed phases. The iodine concentration obtained from second DECT was more accurate than that from the first DECT in the phantom study. A significantly higher contrast enhancement was observed with the second DECT compared with the first DECT during the arterial phase in the clinical study. Contrast enhancement was higher with the second DECT than with the first DECT, and the second DECT was effective in minimizing the use of contrast materials.

The reliability of virtual non-contrast reconstructions of photon-counting detector CT scans in assessing abdominal organs

BACKGROUND

Spectral imaging of photon-counting detector CT (PCD-CT) scanners allows for generating virtual non-contrast (VNC) reconstruction. By analyzing 12 abdominal organs, we aimed to test the reliability of VNC reconstructions in preserving HU values compared to real unenhanced CT images.

METHODS

Our study included 34 patients with pancreatic cystic neoplasm (PCN). The VNC reconstructions were generated from unenhanced, arterial, portal, and venous phase PCD-CT scans using the Liver-VNC algorithm. The observed 11 abdominal organs were segmented by the TotalSegmentator algorithm, the PCNs were segmented manually. Average densities were extracted from unenhanced scans (HUunenhanced), postcontrast (HUpostcontrast) scans, and VNC reconstructions (HUVNC). The error was calculated as HUerror=HUVNC-HUunenhanced. Pearson's or Spearman's correlation was used to assess the association. Reproducibility was evaluated by intraclass correlation coefficients (ICC).

RESULTS

Significant differences between HUunenhanced and HUVNC[unenhanced] were found in vertebrae, paraspinal muscles, liver, and spleen. HUVNC[unenhanced] showed a strong correlation with HUunenhanced in all organs except spleen (r = 0.45) and kidneys (r = 0.78 and 0.73). In all postcontrast phases, the HUVNC had strong correlations with HUunenhanced in all organs except the spleen and kidneys. The HUerror had significant correlations with HUunenhanced in the muscles and vertebrae; and with HUpostcontrast in the spleen, vertebrae, and paraspinal muscles in all postcontrast phases. All organs had at least one postcontrast VNC reconstruction that showed good-to-excellent agreement with HUunenhanced during ICC analysis except the vertebrae (ICC: 0.17), paraspinal muscles (ICC: 0.64-0.79), spleen (ICC: 0.21-0.47), and kidneys (ICC: 0.10-0.31).

CONCLUSIONS

VNC reconstructions are reliable in at least one postcontrast phase for most organs, but further improvement is needed before VNC can be utilized to examine the spleen, kidneys, and vertebrae.

Comparison of Conventional and Virtual Non-contrast Abdominal Images Using the Third-Generation Dual-Source Dual-Energy Computed Tomography

PURPOSE

To determine the efficacy and safety of virtual unenhanced imaging by comparing the attenuation values of virtual and true unenhanced images acquired using third-generation dual-source dual-energy computed tomography (dsDECT).

METHODS

Single-energy non-contrast and dual-energy arterial and venous phase images of 97 patients who underwent triphasic abdominal computed tomography (CT) were included in this retrospective study. Virtual unenhanced images were generated for the arterial (a) and venous (v) phases using two dsDECT algorithms. The attenuation values were measured on the true and virtual unenhanced images of the liver, spleen, kidney, gallbladder, paraspinal muscle, aorta, subcutaneous fat, retroperitoneal fat, and renal cysts.

RESULTS

A statistically significant difference was observed between the attenuation values ​​of true and virtual unenhanced images for all tissues (p < 0.001-0.025), except the venous phase virtual unenhanced images of the kidney, renal cysts, and gallbladder (p = 0.061-0.325). The proportion of cases with differences of ≥ 10 Hounsfield unit (HU) in the attenuation values between the virtual and true unenhanced images ranged from 3% to 8% for renal parenchyma, renal cysts, and gallbladder using this algorithm; however, this proportion was up to 90% for adipose tissue. No significant correlation was observed between the body mass index and attenuation differences between the true and virtual unenhanced images, except for those of the aorta and paraspinal muscle.

CONCLUSION

Virtual unenhanced images acquired using third-generation dsDECT cannot replace true unenhanced images in clinical practice owing to the difference between the attenuation values and variability of attenuation between true and virtual unenhanced images.

Evaluating silicone breast implant rupture with photon-counting CT and volumetric silicone maps

We present a case of an 81-year-old woman who presented to the emergency department with bleeding from a right breast wound. The patient had prior imaging suggestive of bilateral silicone implant rupture and a history of low tolerance for MRI scans. Ultrasound imaging in the emergency setting showed findings in the right breast suggestive of a fistula with free silicone and hematoma. A subsequent photon-counting CT scan with custom silicone-specific segmentation allowed differentiation of silicone from hematoma, provided anatomic assessment and location of the fistula, and revealed bilateral silicone-induced lymphadenopathy.

Photon-Counting Computed Tomography Versus Energy-Integrating Dual-Energy Computed Tomography: Virtual Noncontrast Image Quality Comparison

PURPOSE

This study aimed to compare the image quality of portal venous phase-derived virtual noncontrast (VNC) images from photon-counting computed tomography (PCCT) with energy-integrating dual-energy computed tomography (EI-DECT) in the same patient using quantitative and qualitative analyses.

METHODS

Consecutive patients retrospectively identified with available portal venous phase-derived VNC images from both PCCT and EI-DECT were included. Patients without available VNC in picture archiving and communication system in PCCT or prior EI-DECT and non-portal venous phase acquisitions were excluded. Three fellowship-trained radiologists blinded to VNC source qualitatively assessed VNC images on a 5-point scale for overall image quality, image noise, small structure delineation, noise texture, artifacts, and degree of iodine removal. Quantitative assessment used region-of-interest measurements within the aorta at 4 standard locations, both psoas muscles, both renal cortices, spleen, retroperitoneal fat, and inferior vena cava. Attenuation (Hounsfield unit), quantitative noise (Hounsfield unit SD), contrast-to-noise ratio (CNR) (CNR vascular , CNR kidney , CNR spleen , CNR fat ), signal-to-noise ratio (SNR) (SNR vascular , SNR kidney , SNR spleen , SNR fat ), and radiation dose were compared between PCCT and EI-DECT with the Wilcoxon signed rank test. A P < 0.05 indicated statistical significance.

RESULTS

A total of 74 patients (27 men; mean ± SD age, 63 ± 13 years) were included. Computed tomography dose index volumes for PCCT and EI-DECT were 9.2 ± 3.5 mGy and 9.4 ± 9.0 mGy, respectively ( P = 0.06). Qualitatively, PCCT VNC images had better overall image quality, image noise, small structure delineation, noise texture, and fewer artifacts (all P < 0.00001). Virtual noncontrast images from PCCT had lower attenuation (all P < 0.05), noise ( P = 0.006), and higher CNR ( P < 0.0001-0.04). Contrast-enhanced structures had lower SNR on PCCT ( P = 0.001, 0.002), reflecting greater contrast removal. The SNRfat (nonenhancing) was higher for PCCT than EI-DECT ( P < 0.00001).

CONCLUSIONS

Virtual noncontrast images from PCCT had improved image quality, lower noise, improved CNR and SNR compared with those derived from EI-DECT.

Dual-energy CT of the pancreas: comparison between virtual non-contrast images and true non-contrast images in the detection of pancreatic lesion

PURPOSE

To evaluate the image quality and diagnostic performance for pancreatic lesion between true non-contrast (TNC) and virtual non-contrast (VNC) images obtained from the dual-energy computed tomography (DECT).

METHODS

One hundred six patients with pancreatic mass underwent contrast-enhanced DECT examinations were retrospectively included in this study. VNC images of the abdomen were generated from late arterial (aVNC) and portal (pVNC) phases. For quantitative analysis, the attenuation differences and reproducibility of abdominal organs were compared between TNC and aVNC/pVNC measurements. Qualitatively image quality was assessed by two radiologists using a five-point scale, and they independently compared the detection accuracy of pancreatic lesions between TNC and aVNC/pVNC images. The volume CT dose index (CTDIvol) and size-specific dose estimates (SSDE) were recorded to evaluate the potential dose reduction when using VNC reconstruction to replace the unenhanced phase.

RESULTS

A total of 78.38% (765/976) of the attenuation measurement pairs were reproducible between TNC and aVNC images, and 71.0% (693/976) between TNC and pVNC images. In triphasic examinations, a total of 108 pancreatic lesions were found in 106 patients, and no significant difference in detection accuracy was found between TNC and VNC images (p = 0.587-0.957). Qualitatively, image quality was rated diagnostic (score ≥ 3) in all the VNC images. Calculated CTDIvol and SSDE reduction of about 34% could be achieved by omitting the non-contrast phase.

CONCLUSION

VNC images of DECT provide diagnostic image quality and accurate pancreatic lesions detection, which are a promising alternative to unenhanced phase with a substantial reduction of radiation exposure in clinical routine.

New Frontiers in Oncological Imaging With Computed Tomography: From Morphology to Function

The latest evolutions in Computed Tomography (CT) technology have several applications in oncological imaging. The innovations in hardware and software allow for the optimization of the oncological protocol. Low-kV acquisitions are possible thanks to the new powerful tubes. Iterative reconstruction algorithms and artificial intelligence are helpful for the management of image noise during image reconstruction. Functional information is provided by spectral CT (dual-energy and photon counting CT) and perfusion CT.

Effect of iodine concentration and body size on iodine subtraction in virtual non-contrast imaging: A phantom study

INTRODUCTION

Dual-energy computed tomography (DECT) can generate virtual non-contrast (VNC) images. Herein, we sought to improve the accuracy of VNC images by identifying the optimal slope of contrast media (SCM) for VNC-image generation based on the iodine concentration and subject's body size.

METHODS

We used DECT to scan a multi-energy phantom including four iodine concentration rods (15, 10, 5, and 2 mg/mL), and 240 VNC images (eight SCM ranging from 0.49 to 0.56 × three body sizes × ten scans) that were generated by three-material decomposition. The CT number of each iodine and solid water rod part was measured in each VNC image. The difference in the CT number between the iodine and the solid water rod part was calculated and compared using paired t-test or repeated measures ANOVA.

RESULTS

The SCM that achieved an absolute value of the difference in CT number of <5.0 Hounsfield units (HU) for all body sizes simultaneously was greater at lower iodine concentration (SCM of 0.5, 0.51, and 0.53 at 10, 5, and 2 mg/mL iodine, respectively). At an iodine concentration of 15 mg/mL, no SCM achieved an absolute difference of <5.0 HU in CT number for all body sizes simultaneously. At all iodine concentrations, the SCM achieving the minimal difference in the CT number increased with the increase in body size.

CONCLUSION

By adjusting the SCM according to the iodine concentration and body size, it is possible to generate VNC images with an accuracy of <5.0 HU.

IMPLICATIONS FOR PRACTICE

Improving the accuracy of VNC images minimizing incomplete iodine subtraction would make it possible to replace true non-contrast (TNC) images with VNC images and reduce the radiation dose.

[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.

Gemstone Spectral CT Virtual Noncontrast Images and Iodine Maps for the Characterization of Thyroid Lesions and Distinguishing Thyroid Papillary Carcinoma from Nodular Goiter

BACKGROUND

Gemstone spectral contrast-enhanced CT with virtual noncontrast (VNC) images and iodine maps can potentially reduce the number of required CT scans for thyroid lesions. However, data regarding the clinical utility of VNC images and iodine maps in characterizing thyroid lesions and distinguishing thyroid papillary carcinoma from nodular goiter are still limited.

PURPOSE

To determine whether VNC images and iodine density could reliably aid in characterizing thyroid lesions and distinguishing thyroid papillary carcinoma from nodular goiter compared with true noncontrast (TNC) images.

METHODS

This retrospective study included patients with thyroid papillary carcinoma or nodular goiter who underwent TNC and contrast-enhanced gemstone spectral CT scans. The consistency of qualitative parameters, including intralesional calcification, necrosis, lesion boundary, thyroid edge interruption, and lymph node metastasis, between TNC and VNC images, was analyzed using the kappa statistic. TNC attenuation, VNC attenuation, absolute attenuation between TNC and VNC, and iodine density were compared between thyroid papillary carcinoma and nodular goiter by using Student's t-test. The diagnostic performance for distinguishing papillary carcinoma from nodular goiter was evaluated by using the area under the receiver operating characteristic curve (AUC) value, sensitivity, and specificity.

RESULTS

VNC and TNC imaging showed comparable performance in delineating calcification, necrosis, lesion boundary, thyroid edge interruption, and lymph node metastasis (all k > 0.75). Papillary carcinoma showed significantly lower absolute attenuation between VNC and TNC than nodular goiter (7.86 ± 6.74 vs. 13.43 ± 10.53, P=0.026), which was similarly observed for iodine density (31.45 ± 8.51 vs. 37.27 ± 10.34, P=0.016). The iodine density showed higher diagnostic performance (AUC = 0.727), accuracy (0.773 vs. 0.667), sensitivity (0.750 vs. 0.708), and specificity (0.786 vs. 0.643) than the absolute attenuation between TNC and VNC images (AUC = 0.683).

CONCLUSIONS

VNC imaging, a promising substitute for TNC imaging, has comparable diagnostic efficacy for reliably characterizing thyroid lesions. Iodine density could be valuable for distinguishing thyroid papillary carcinoma from nodular goiter.

Spectral CT in Oncology

BACKGROUND

Spectral CT is gaining increasing clinical importance with multiple potential applications, including oncological imaging. Spectral CT-specific image data offers multiple advantages over conventional CT image data through various post-processing algorithms, which will be highlighted in the following review.

METHODOLOGY

The purpose of this review article is to provide an overview of potential useful oncologic applications of spectral CT and to highlight specific spectral CT pitfalls. The technical background, clinical advantages of primary and follow-up spectral CT exams in oncology, and the application of appropriate spectral tools will be highlighted.

RESULTS/CONCLUSIONS

Spectral CT imaging offers multiple advantages over conventional CT imaging, particularly in the field of oncology. The combination of virtual native and low monoenergetic images leads to improved detection and characterization of oncologic lesions. Iodine-map images may provide a potential imaging biomarker for assessing treatment response.

KEY POINTS

· The most important spectral CT reconstructions for oncology imaging are virtual unenhanced, iodine map, and virtual monochromatic reconstructions.. · The combination of virtual unenhanced and low monoenergetic reconstructions leads to better detection and characterization of the vascularization of solid tumors.. · Iodine maps can be a surrogate parameter for tumor perfusion and potentially used as a therapy monitoring parameter.. · For radiotherapy planning, the relative electron density and the effective atomic number of a tissue can be calculated..

CITATION FORMAT

· Sauerbeck J, Adam G, Meyer M. Onkologische Bildgebung mittels Spektral-CT. Fortschr Röntgenstr 2023; 195: 21 - 29.

Virtual Non-contrast Imaging in The Abdomen and The Pelvis: An Overview

Virtual non-contrast (VNC) imaging is a post-processing technique generated from contrast-enhanced scans using dual-energy computed tomography (DECT). It is generated by removing iodine from imaging acquired at multiple energies. Myriad clinical studies have shown its ability to diagnose the various abdominal and pelvic pathologies discussed in the article. VNC is also a problem-solving tool for characterizing incidentally detected lesions ("incidentalomas"), often decreasing the need for additional follow-up imaging. It also obviates the multiphase image acquisitions to evaluate hematuria, hepatic steatosis, aortic endoleaks, and gastrointestinal bleeding by generating image datasets from different tissue attenuation values. The scope of this article is to provide an overview of various applications of VNC imaging obtained by DECT in the abdomen and pelvis.

Evaluating image quality and optimal parameters for non-linear blending dual-energy computed tomography images of hepatic portal veins by blending-property-map

BACKGROUND

Blending technology is usually used to improve quality of dual-energy computed (DECT) images.

OBJECTIVES

To evaluate the blended DECT image qualities by employing the Blending-Property-Map (BP-Map) and elucidating the optimal parameters with the highest signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR).

METHODS

Sixty pairs of 80 kV and 140 kV CT images are blended non-linearly by four methods. Protocol A uses the fixed values of blending width (BW) and blending center (BC); Protocol B uses the values of BW =  (CThepatic portal vein - CThepatic parenchymal) / 2 and BC =  (CThepatic portal vein + CThepatic parenchymal) / 2; Protocol C uses a BW ranging from 10 to 100 HU at an interval of 10 HU and BC = (CThepatic portal vein + CThepatic parenchymal) / 2; Protocol D uses the BP-Map that covers all possible values of BW and BC.

RESULTS

When using CT value of adipose tissue as noise, the calculated SNR and CNR of optimal blending width and blending center were 123.22±41.73 and 9.00±3.52, respectively, by the BP-Map in the protocol D. By employing the CT value of back muscle as noise, the SNR and CNR of the best-blended images were 75.90±14.52 and 6.39±2.37, respectively. The subjective score of protocol D was 4.88±0.12.

CONCLUSIONS

Compared to traditional blending methods, the BP-Map technique can determine the optimal blending parameter and provide the best-blended images with the highest SNR and CNR.

Virtual Unenhanced Images: Qualitative and Quantitative Comparison Between Different Dual-Energy CT Scanners in a Patient and Phantom Study

MATERIALS AND METHODS

Forty-four patients with clinical contrast-enhanced abdominal examinations on each of the 3 DECT scanner types and a phantom scanned with the same protocols were included in this retrospective study. Qualitative and quantitative assessment was performed on VUE images. Quantitative evaluation included measurement of attenuation and image noise for various tissues and the phantom. Virtual unenhanced image attenuation and noise were compared between scanner types, and intrapatient interscanner reproducibility of virtual unenhanced image attenuation was calculated as the percentage of measurement pairs with an interscanner difference ≤ 10 HU. Image quality, noise, sharpness, and iodine elimination were assessed qualitatively by 2 radiologists.

RESULTS

Significant interscanner differences in VUE attenuation and noise were found in all tissues. dlDECT and rsDECT showed significantly higher VUE attenuation than dsDECT in the aorta, portal vein, and kidneys (P < 0.05). Conversely, VUE attenuation in dsDECT was significantly higher than in dlDECT/rsDECT for subcutaneous and retroperitoneal fat (both P < 0.05). A total of 91.9% (385/419) of measurements were reproducible between rsDECT and dlDECT, 70.9% (297/419) between dsDECT and rsDECT, and 66.8% (280/419) between dsDECT and dlDECT. Virtual unenhanced image attenuation in the contrast media-filled phantom cavity was 12.7 ± 4.7 HU in dlDECT, -5.3 ± 4.2 HU in rsDECT, and -4.0 ± 10.7 HU in dsDECT with significant differences between dlDECT and rsDECT/dsDECT, respectively (P < 0.05), between which attenuation was comparable in the unenhanced extraluminal phantom component (P = 0.11-0.62). Qualitatively, dsDECT yielded best iodine elimination, whereas sharpness, image noise, and overall image quality were rated higher in dlDECT and rsDECT.

CONCLUSIONS

There are significant interscanner differences in the attenuation measurements and qualitative assessment of VUE images, which should be acknowledged when using these images in patients that are being scanned on different DECT scanner types during imaging follow-up.

Longitudinal reproducibility of attenuation measurements on virtual unenhanced images: multivendor dual-energy CT evaluation

OBJECTIVES

The accuracy of virtual unenhanced (VUE) images has been extensively investigated, yet data on their longitudinal reproducibility is limited. The study purpose was to evaluate the longitudinal reproducibility of VUE attenuation measurements on three different dual-energy CT (DECT) scanner types.

METHODS

A total of 137 patients with repeated abdominal DECT either on a rapid kV switching (rsDECT; n = 46), a dual-layer detector (dlDECT; n = 43), or a dual-source scanner (dsDECT; n = 48) were retrospectively included. Attenuation was measured on VUE and corresponding contrast-enhanced images in the liver, spleen, kidneys, aorta, portal vein, and fat. Longitudinal reproducibility was evaluated by calculating the absolute inter-scan differences (HU) and the inter-scan variation (%). Measurement pairs with differences ≤ 10 HU were considered reproducible. Influence of contrast-enhanced attenuation on VUE reproducibility was analyzed using linear regression.

RESULTS

The scanner-specific cohorts showed similar age (p-range: 0.35-0.99), sex (p-range: 0.68-1), body weight (p-range: 0.26-0.87), body diameter (p-range: 0.34-0.76), and inter-scan time (p-range: 0.52-0.83). In total, 94.9% of VUE measurements were reproducible for rsDECT, 93.8% for dlDECT, and 90.6% for dsDECT. Overall inter-scan variation was lowest in fat (4.0 (1.7-8.2)%) and highest in tissues with high contrast enhancement: the aorta (13.3 (4.6-21.3)%), portal vein (10.8 (5.7-19.8)%), and kidneys (10.7 (3.9-18.0)%). Significant differences in inter-scan variation were found between the scanner types for the aorta, portal vein, kidneys, and spleen. Inter-scan differences in contrast-enhanced attenuation significantly influenced inter-scan differences in VUE attenuation (p < 0.001; t-ratio: 4.34).

CONCLUSIONS

Longitudinal reproducibility of VUE attenuation was high for all scanners, yet inter-scan variation of VUE attenuation was influenced by contrast enhancement, showing greatest magnitude and discrepancy between scanner types in vessels and the kidneys.

KEY POINTS

• We found that 94.9% of attenuation measurements on virtual unenhanced images were reproducible for rapid kV switching DECT, 93.8% for dual-layer detector DECT, and 90.6% for dual-source DECT. • Inter-scan variation of attenuation in virtual unenhanced images was comparable between the three scanner types in the liver and fat, whereas inter-scan variation in the spleen, kidneys, portal vein, and aorta showed significant differences between scanner types (p < 0.05). • Inter-scan attenuation differences in contrast-enhanced images significantly influenced inter-scan differences in virtual unenhanced attenuation (p < 0.001, t-ratio: 4.34), suggesting a residual impact of contrast enhancement differences between examinations.

Computed Tomography Techniques, Protocols, Advancements, and Future Directions in Liver Diseases

Computed tomography (CT) is often performed as the initial imaging study for the workup of patients with known or suspected liver disease. Our article reviews liver CT techniques and protocols in clinical practice along with updates on relevant CT advances, including wide-detector CT, radiation dose optimization, and multienergy scanning, that have already shown clinical impact. Particular emphasis is placed on optimizing the late arterial phase of enhancement, which is critical to evaluation of hepatocellular carcinoma. We also discuss emerging techniques that may soon influence clinical care.

Spectral detector CT applications in advanced liver imaging

OBJECTIVE

Spectral detector CT (SDCT) has many applications in advanced liver imaging. If appropriately utilized, this technology has the potential to improve image quality, provide new diagnostic information, and allow for decreased radiation dose. The purpose of this review is to familiarize radiologists with the uses of SDCT in liver imaging.

CONCLUSION

SDCT has a variety of post-processing techniques, which can be used in advanced liver imaging and can significantly add value in clinical practice.

Comparison between Conventional Unenhanced and Virtual Unenhanced Imaging of Hepatopancreaticobiliary System with Third-Generation Dual-Source Dual-Energy CT—An Observational Study

Objectives The aims of our study were to assess the comparability of conventional unenhanced images (CUIs) of hepatopancreaticobiliary system with virtual unenhanced images (VUIs) derived from arterial and portal venous phases acquired in a third-generation, dual-source, dual-energy CT (DECT), and also to assess the best dataset among these VUIs. We also calculated the radiation effective dose (ED) reduction by eliminating noncontrast acquisition.

Materials and Methods 60 patients were included in our study. Unenhanced images in single energy and contrast-enhanced images in dual-energy mode were acquired. Arterial virtual unenhanced (AVU) and portal virtual unenhanced (PVU) images were generated and compared with CUI, using both objective and subjective methods. The ED was calculated separately for each phase. Statistical significance between difference in mean attenuation values were analyzed using ANOVA and unpaired student t-test.

Results In our study, the difference in mean attenuation of liver, spleen, and pancreas between the three phases—CU, AVU, and PVU—were insignificant with p-value > 0.05. This indicates that the values were comparable. Among the VUI, AVU images were statistically superior in image quality. Elimination of noncontrast CT from triple phase abdominal imaging can achieve an average ED reduction of 39%.

Conclusions We conclude that VUI generated in third-generation, dual-source DECT has diagnostic image quality and can replace the CUI in triple-phase studies, with a mean ED reduction by 39%. The VUI obtained from arterial phase is superior to those obtained from portal venous phase.

Does dual-energy abdominal computed tomography increase the radiation dose to patients: a prospective observational study

Purpose

The aim of our study was to compare single-energy (SECT) and dual-energy (DECT) abdominal computed tomography (CT) examinations in matched patient cohorts regarding the differences in effective radiation dose (ERD) and image quality performed in a third-generation dual-source computed tomography (DSCT) scanner.

Material and methods

Our study included 100 patients, who were divided randomly into 2 groups. The patients included in Group A were scanned by SECT, and Group B members were scanned by DECT. Volume CT dose index (CTDI_vol), dose length product (DLP), and ERD for venous phase acquisition were recorded in each patient and were normalised for 40 cm. Analyses were performed by using statistical software (SPSS version 20.0 for windows), and Bonferroni correction for multiple comparisons was applied for p-values and confidence intervals.

Results

Average ERD based on DLP values normalised for 40 cm acquisition were obtained for both Group A and Group B. The mean ERD for Group A was 11.89 mSv, and for group B it was 6.87 mSv. There was a significant difference in these values between Group A and Group B as shown by a p-value of < 0.001. On subjective and objective analysis, there was no statistically significant difference in image quality between the 2 groups.

Conclusions

The protocols in third-generation DSCT using dual-energy mode resulted in significant reductions in the effective radiation dose (by approximately 58%) compared to SECT in routine abdominal examination in matched cohorts. Therefore, the quantitative imaging potential of DECT can be utilised in needed patients with decreased radiation dose in third-generation DSCT.

Influence of Adaptive Statistical Iterative Reconstructions on CT Radiomic Features in Oncologic Patients

Iterative reconstructions (IR) might alter radiomic features extraction. We aim to evaluate the influence of Adaptive Statistical Iterative Reconstruction-V (ASIR-V) on CT radiomic features. Patients who underwent unenhanced abdominal CT (Revolution Evo, GE Healthcare, USA) were retrospectively enrolled. Raw data of filtered-back projection (FBP) were reconstructed with 10 levels of ASIR-V (10–100%). CT texture analysis (CTTA) of liver, kidney, spleen and paravertebral muscle for all datasets was performed. Six radiomic features (mean intensity, standard deviation (SD), entropy, mean of positive pixel (MPP), skewness, kurtosis) were extracted and compared between FBP and all ASIR-V levels, with and without altering the spatial scale filter (SSF). CTTA of all organs revealed significant differences between FBP and all ASIR-V reconstructions for mean intensity, SD, entropy and MPP (all p < 0.0001), while no significant differences were observed for skewness and kurtosis between FBP and all ASIR-V reconstructions (all p > 0.05). A per-filter analysis was also performed comparing FBP with all ASIR-V reconstructions for all six SSF separately (SSF0-SSF6). Results showed significant differences between FBP and all ASIR-V reconstruction levels for mean intensity, SD, and MPP (all filters p < 0.0315). Skewness and kurtosis showed no differences for all comparisons performed (all p > 0.05). The application of incremental ASIR-V levels affects CTTA across various filters. Skewness and kurtosis are not affected by IR and may be reliable quantitative parameters for radiomic analysis.

Dual-energy CT of the liver: True noncontrast vs. virtual noncontrast images derived from multiple phases for the diagnosis of fatty liver

PURPOSE

To evaluate the difference in liver density and to compare the performance to diagnose fatty liver between true noncontrast (TNC) images and virtual noncontrast (VNC) images generated from dual-energy CT (DECT).

MATERIALS AND METHODS

Patients who underwent liver dynamic DECT and MRI were included (n = 49). Two observers measured the liver and spleen densities on TNC images and three VNC images from the arterial, portal and delayed phases of DECT (VNCa, VNCp and VNCd, respectively). The liver-minus-spleen density (density L-S) and liver-to-spleen ratio (density L/S) were calculated. The CT parameters were compared between normal liver patients and fatty liver patients by using the independent t-test. Differences and agreements between measurements on TNC images and VNC images were evaluated by using the paired t-test and Bland-Altman analysis. Receiver operating characteristic (ROC) curve analysis was used to evaluate the diagnostic performance of CT parameters for diagnosing fatty liver.

RESULTS

All CT parameters measured on TNC and VNC images were significantly higher in normal liver patients than in fatty liver patients. Although the mean liver densities on VNC images were significantly lower than those on TNC images, all CT parameters showed good agreement between TNC images and VNC images. The diagnostic performances of CT parameters on VNC images were not significantly different from those on TNC images.

CONCLUSION

Although the liver and spleen density on VNC images was significantly lower than that on TNC images, the diagnostic performances of CT parameters on three VNC images from multiple phases were similar to those on TNC images for diagnosing fatty liver.

Impact of dose reduction and the use of an advanced model-based iterative reconstruction algorithm on spectral performance of a dual-source CT system: A task-based image quality assessment

PURPOSE

To assess the impact of dose reduction and the use of an advanced modeled iterative reconstruction algorithm (ADMIRE) on image quality in low-energy monochromatic images from a dual-source dual energy computed tomography CT (DSCT) platform.

MATERIALS AND METHODS

Acquisitions on an image-quality phantom were performed using DSCT equipment with 100/Sn150 kVp for four dose levels (CTDI_vol: 20/11/8/5mGy). Raw data were reconstructed for six energy levels (40/50/60/70/80/100 keV) using filtered back projection and two levels of ADMIRE (A3/A5). Noise power spectrum (NPS) and task-based transfer function (TTF) were calculated on virtual monoenergetic images (VMIs). Detectability index (d') was computed to model the detection task of two enhanced iodine lesions as function of keV.

RESULTS

Noise-magnitude was significantly reduced between 40 to 70 keV by -56±0% (SD) (range: -56%--55%) with FBP; -56±0% (SD) (-56%--56%) with A3; and -57±1% (SD) (range: -57%--56%) with A5. The average spatial frequency of the NPS peaked at 70 keV and decreased as ADMIRE level increased. TTF values at 50% were greatest at 40 keV and shifted towards lower frequencies as the keV increased. The detectability of both lesions increased with increasing dose level and ADMIRE level. For the simulated lesion with iodine at 2mg/mL, d' values peaked at 70 keV for all reconstruction types, except for A3 at 20 mGy and A5 at 11 and 20 mGy, where d' peaked at 60 keV. For the other simulated lesion, d' values were highest at 40 keV and decreased beyond.

CONCLUSION

At low keV on VMIs, this study confirms that iterative reconstruction reduces the noise magnitude, improves the spatial resolution and increases the detectability of enhanced iodine lesions.

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.

Virtual Unenhanced Dual-Energy CT Images Obtained with a Multimaterial Decomposition Algorithm: Diagnostic Value for Renal Mass and Urinary Stone Evaluation

Background Virtual unenhanced (VUE) images obtained by using a dual-energy CT (DECT) multimaterial decomposition algorithm hold promise for diagnostic use in the abdomen in lieu of true unenhanced (TUE) images. Purpose To assess VUE images obtained from a DECT multimaterial decomposition algorithm in patients undergoing renal mass and urinary stone evaluation. Materials and Methods In this retrospective Health Insurance Portability and Accountability Act-compliant study, DECT was performed in patients undergoing evaluation for renal mass or urinary stone. VUE images were compared quantitatively to TUE images and qualitatively assessed by four independent radiologists. Differences in attenuation between VUE and TUE images were summarized by using 95% limits of agreement. Diagnostic performance in urinary stone detection was summarized by using area under the receiver operating characteristic curve, sensitivity, and specificity. Results A total of 221 patients (mean age ± standard deviation, 61 years ± 14; 129 men) with 273 renal masses were evaluated. Differences in renal mass attenuation between VUE and TUE images were within 3 HU for both enhancing masses (95% limits of agreement, -3.1 HU to 2.7 HU) and nonenhancing cysts (95% limits of agreement, -2.9 HU to 2.5 HU). Renal mass classification as enhancing mass versus nonenhancing cyst did not change (reclassification rate of enhancing masses, 0% [0 of 78]; 95% CI: 0, 5; reclassification rate of nonenhancing cysts, 0% [0 of 193]; 95% CI: 0, 2) with use of VUE in lieu of TUE images. Among 166 urinary stones evaluated, diagnostic performance of VUE images for stone detection was lower compared with that of TUE images (area under the receiver operating characteristic curve, 0.79 [95% CI: 0.73, 0.84] vs 0.93 [95% CI: 0.91, 0.95]; P < .001) due to reduced sensitivity of VUE for detection of stones 3 mm in diameter or less compared with those greater than 3 mm (sensitivity, 23% [25 of 108; 95% CI: 12, 40] vs 88% [126 of 144; 95% CI: 77, 94]; P < .001). Conclusion Compared with true unenhanced images, virtual unenhanced (VUE) images were unlikely to change renal mass classification as enhancing mass versus nonenhancing cyst. Diagnostic performance of VUE images remained suboptimal for urinary stone detection due to subtraction of stones 3 mm or less in diameter. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Sosna in this issue.

Deep learning trained algorithm maintains the quality of half-dose contrast-enhanced liver computed tomography images: Comparison with hybrid iterative reconstruction: Study for the application of deep learning noise reduction technology in low dose

PURPOSE

This study compares the image and diagnostic qualities of a DEep Learning Trained Algorithm (DELTA) for half-dose contrast-enhanced liver computed tomography (CT) with those of a commercial hybrid iterative reconstruction (HIR) method used for standard-dose CT (SDCT).

METHODS

This study enrolled 207 adults, and they were divided into two groups: SDCT and low-dose CT (LDCT). SDCT was reconstructed using the HIR method (SDCT_HIR), and LDCT was reconstructed using both the HIR method (LDCT_HIR) and DELTA (LDCT_DL). Noise, Hounsfield unit (HU) values, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were compared between three image series. Two radiologists assessed the noise, artefacts, overall image quality, visualisation of critical anatomical structures and lesion detection, characterisation and visualisation.

RESULTS

The mean effective doses were 5.64 ± 1.96 mSv for SDCT and 2.87 ± 0.87 mSv for LDCT. The noise of LDCT_DL was significantly lower than that of SDCT_HIR and LDCT_HIR. The SNR and CNR of LDCT_DL were significantly higher than those of the other two groups. The overall image quality, visualisation of anatomical structures and lesion visualisation between LDCT_DL and SDCT_HIR were not significantly different. For lesion detection, the sensitivities and specificities of SDCT_HIR vs. LDCT_DL were 81.9 % vs. 83.7 % and 89.1 % vs. 86.3 %, respectively, on a per-patient basis. SDCT_HIR showed 75.4 % sensitivity and 82.6 % specificity for lesion characterisation on a per-patient basis, whereas LDCT_DL showed 73.5 % sensitivity and 82.4 % specificity.

CONCLUSIONS

LDCT with DELTA had approximately 49 % dose reduction compared with SDCT with HIR while maintaining image quality on contrast-enhanced liver CT.

Feasibility study of using virtual non-contrast images derived from dual-energy CT to replace true non-contrast images in patients diagnosed with papillary thyroid carcinoma

OBJECTIVE

To assess the feasibility of using virtual non-contrast (VNC) images derived from dual-energy computed tomography (DECT) to replace true non-contrast (TNC) images of papillary thyroid carcinoma (PTC) patients.

METHODS

Images of 96 PTC patients were retrospectively analyzed. TNC images were acquired under the single-energy mode of DECT after the plain scanning. The arterial and venous phase VNC (VNC-a and VNC-v) images were generated by the post-processing algorithm from the arterial phase and venous phase of contrast-enhanced CT images, respectively. Mean attenuation values, image noise, number and length of calcification were measured. Radiation dose was also calculated. Last, subjective score of image quality was evaluated by a 5-point scale.

RESULTS

Signal-to-noise ratio (SNR) of each tissue in TNC images is significantly higher than that of VNC images (p<0.050). Contrast-to-noise ratio (CNR) of fat, muscle, thyroid nodules and internal carotid artery in TNC images is significantly higher than that of VNC images, while CNR in TNC images is lower for cervical vertebra (p<0.001). Calcification is detected on TNC images of 44 patients, while it is omitted on VNC images of 14 patients (31.8%). The subjective score of TNC images is higher than VNC images (p<0.001). The effective dose reduction is 47.6% by avoiding plain scanning.

CONCLUSIONS

Considering the different attenuation value, SNR, CNR and especially reduced detection rate of calcification, we deem that VNC images cannot be directly used to replace TNC images in PTC patients, despite the reduced radiation dose.

Dual-Energy CT Images: Pearls and Pitfalls

Dual-energy CT (DECT) is a tremendous innovation in CT technology that allows creation of numerous imaging datasets by enabling discrete acquisitions at more than one energy level. The wide range of images generated from a single DECT acquisition provides several benefits such as improved lesion detection and characterization, superior determination of material composition, reduction in the dose of iodine, and more robust quantification. Technological advances and the proliferation of various processing methods have led to the availability of diverse vendor-based DECT approaches, each with a different acquisition and image reconstruction process. The images generated from various DECT scanners differ from those from conventional single-energy CT because of differences in their acquisition techniques, material decomposition methods, image reconstruction algorithms, and postprocessing methods. DECT images such as virtual monochromatic images, material density images, and virtual unenhanced images have different imaging appearances, texture features, and quantitative capabilities. This heterogeneity creates challenges in their routine interpretation and has certain associated pitfalls. Some artifacts such as residual iodine on virtual unenhanced images and an appearance of pseudopneumatosis in a gas-distended bowel loop on material-density iodine images are specific to DECT, while others such as pseudoenhancement seen on virtual monochromatic images are also observed at single-energy CT. Recognizing the potential pitfalls associated with DECT is necessary for appropriate and accurate interpretation of the results of this increasingly important imaging tool. Online supplemental material is available for this article. ©RSNA, 2021.

Quantitative accuracy of virtual non-contrast images derived from spectral detector computed tomography: an abdominal phantom study

Dual-energy CT allows for the reconstruction of virtual non-contrast (VNC) images. VNC images have the potential to replace true non-contrast scans in various clinical applications. This study investigated the quantitative accuracy of VNC attenuation images considering different parameters for acquisition and reconstruction. An abdomen phantom with 7 different tissue types (different combinations of 3 base materials and 5 iodine concentrations) was scanned using a spectral detector CT (SDCT). Different phantom sizes (S, M, L), volume computed tomography dose indices (CTDIvol 10, 15, 20 mGy), kernel settings (soft, standard, sharp), and denoising levels (low, medium, high) were tested. Conventional and VNC images were reconstructed and analyzed based on regions of interest (ROI). Mean and standard deviation were recorded and differences in attenuation between corresponding base materials and VNC was calculated (VNCerror). Statistic analysis included ANOVA, Wilcoxon test and multivariate regression analysis. Overall, the VNC_error was − 1.4 ± 6.1 HU. While radiation dose, kernel setting, and denoising level did not influence VNC_error significantly, phantom size, iodine content and base material had a significant effect (e.g. S vs. M: − 1.2 ± 4.9 HU vs. − 2.1 ± 6.0 HU; 0.0 mg/ml vs. 5.0 mg/ml: − 4.0 ± 3.5 HU vs. 5.1 ± 5.0 HU and 35-HU-base vs. 54-HU-base: − 3.5 ± 4.4 HU vs. 0.7 ± 6.5; all p ≤ 0.05). The overall accuracy of VNC images from SDCT is high and independent from dose, kernel, and denoising settings; however, shows a dependency on patient size, base material, and iodine content; particularly the latter results in small, yet, noticeable differences in VNC attenuation.

Compatibility of true and virtual unenhanced attenuation in rapid kV-switching dual energy CT

PURPOSE

We aimed to evaluate whether virtual unenhanced images (VUI) generated from nephrographic phase on rapid kV-switching dual energy CT (rsDECT) can replace true unenhanced images (TUI) by comparing attenuation values of various intraabdominal structures.

METHODS

In this retrospective study, 142 patients had unenhanced and nephrographic phase dual energy CT images. Attenuation values of the aorta, liver,spleen, pancreas, bilateral renal parenchyma, inferior vena cava, gallbladder and paraspinal muscle on TUI and VUI were recorded. Frequencies of organs who had more than 10 and 20 HU of attenuation difference were also calculated.

RESULTS

A total of 1224 ROIs were sampled. No statistically significant differences were found between TUA and VUA of the aorta, spleen and pancreas. The other structures had significant differences (P < 0.001). Correlation between measurements were weak to moderate (r=0.17-0.72). 20% of organs had more than 10 HU difference and 5% had more than 20 HU difference between TUI and VUI.

CONCLUSION

rsDECT based VUI does not seem to be an ideal surrogate for TUI.

New advances in CT imaging of pancreas diseases: a narrative review

Computed tomography (CT) plays a pivotal role as a diagnostic tool in many diagnostic and diffuse pancreatic diseases. One of the major limits of CT is related to the radiation exposure of young patients undergoing repeated examinations. Besides the standard CT protocol, the most recent technological advances, such as low-voltage acquisitions with high performance X-ray tubes and iterative reconstructions, allow for significant optimization of the protocol with dose reduction. The variety of CT tools are further expanded by the introduction of dual energy: the production of energy-selective images (i.e., virtual monochromatic images) improves the image contrast and lesion detection while the material-selective images (e.g., iodine maps or virtual unenhanced images) are valuable for lesion detection and dose reduction. The perfusion techniques provide diagnostic and prognostic information lesion and parenchymal vascularization and interstitium. Both dual energy and perfusion CT have the potential for pushing the limits of conventional CT from morphological evaluation to quantitative imaging applied to inflammatory and oncological diseases. Advances in post-processing of CT images, such as pancreatic volumetry, texture analysis and radiomics provide relevant information for pancreatic function but also for the diagnosis, management and prognosis of pancreatic neoplasms. Artificial intelligence is promising for optimization of the workflow in qualitative and quantitative analyses. Finally, basic concepts on the role of imaging on screening of pancreatic diseases will be provided.

Gastrointestinal Applications of Iodine Quantification Using Dual-Energy CT: A Systematic Review

Dual-energy computed tomography (DECT) can estimate tissue vascularity and perfusion via iodine quantification. The aim of this systematic review was to outline current and emerging clinical applications of iodine quantification within the gastrointestinal tract using DECT. The search was conducted with three databases: EMBASE, Pubmed and The Cochrane Library. This identified 449 studies after duplicate removal. From a total of 570 selected studies, 30 studies were enrolled for the systematic review. The studies were categorized into four main topics: gastric tumors (12 studies), colorectal tumors (8 studies), Crohn’s disease (4 studies) and miscellaneous applications (6 studies). Findings included a significant difference in iodine concentration (IC) measurements in perigastric fat between T1–3 vs. T4 stage gastric cancer, poorly and well differentiated gastric and colorectal cancer, responders vs. non-responders following chemo- or chemoradiotherapy treatment among cancer patients, and a positive correlation between IC and Crohn’s disease activity. In conclusion, iodine quantification with DECT may be used preoperatively in cancer imaging as well as for monitoring treatment response. Future studies are warranted to evaluate the capabilities and limitations of DECT in splanchnic flow.

Comparison of virtual to true unenhanced abdominal computed tomography images acquired using rapid kV-switching dual energy imaging

Objective

To compare “virtual” unenhanced (VUE) computed tomography (CT) images, reconstructed from rapid kVp-switching dual-energy computed tomography (DECT), to “true” unenhanced CT images (TUE), in clinical abdominal imaging. The ability to replace TUE with VUE images would have many clinical and operational advantages.

Methods

VUE and TUE images of 60 DECT datasets acquired for standard-of-care CT of pancreatic cancer were retrospectively reviewed and compared, both quantitatively and qualitatively. Comparisons included quantitative evaluation of CT numbers (Hounsfield Units, HU) measured in 8 different tissues, and 6 qualitative image characteristics relevant to abdominal imaging, rated by 3 experienced radiologists. The observed quantitative and qualitative VUE and TUE differences were compared against boundaries of clinically relevant equivalent thresholds to assess their equivalency, using modified paired t-tests and Bayesian hierarchical modeling.

Results

Quantitatively, in tissues containing high concentrations of calcium or iodine, CT numbers measured in VUE images were significantly different from those in TUE images. CT numbers in VUE images were significantly lower than TUE images when calcium was present (e.g. in the spine, 73.1 HU lower, p < 0.0001); and significantly higher when iodine was present (e.g. in renal cortex, 12.9 HU higher, p < 0.0001). Qualitatively, VUE image ratings showed significantly inferior depiction of liver parenchyma compared to TUE images, and significantly more cortico-medullary differentiation in the kidney.

Conclusions

Significant differences in VUE images compared to TUE images may limit their application and ability to replace TUE images in diagnostic abdominal CT imaging.

Virtual non-contrast for evaluation of liver parenchyma and vessels: results from 25 patients using multi-phase spectral-detector CT

BACKGROUND

In abdominal imaging, contrast-enhanced computed tomography (CT) examinations are most commonly applied; however, unenhanced examinations are still needed for several clinical questions but require additional scanning and radiation exposure.

PURPOSE

To evaluate accuracy of virtual non-contrast (VNC) from arterial and venous phase spectral-detector CT (SDCT) scans compared to true-unenhanced (TNC) images for the evaluation of liver parenchyma and vessels.

MATERIAL AND METHODS

A total of 25 patients undergoing triphasic SDCT examinations were included. VNC was reconstructed from arterial and venous phases and compared to TNC images. Quantitative image analysis was performed by region of interest (ROI)-based assessment of mean and SD of attenuation (HU) in each liver segment, spleen, portal vein, common hepatic artery, and abdominal aorta. Subjectively, iodine subtraction and diagnostic assessment were rated on 5-point Likert scales.

RESULTS

Attenuation and image noise measured in the liver from VNC were not significantly different from TNC (TNC: 54.6 ± 10.8 HU, VNC arterial phase: 55.7 ± 10.8 HU; VNC venous phase: 58.3 ± 10.0 HU; P > 0.05). VNC also showed accurate results regarding attenuation and image noise for spleen, portal vein, and abdominal aorta. Only iodine subtraction in the common hepatic artery in the arterial phase was insufficient which was confirmed by the subjective reading. Apart from that, subjective reading showed accurate iodine subtraction and comparable diagnostic assessment.

CONCLUSION

VNC from the arterial and venous phases were very similar to TNC yielding mostly negligible differences in attenuation, image noise, and diagnostic utility. Inadequate iodine subtraction occurred in hepatic arteries in the arterial phase.

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.

Dual energy CT in clinical routine: how it works and how it adds value

Dual energy computed tomography (DECT), also known as spectral CT, refers to advanced CT technology that separately acquires high and low energy X-ray data to enable material characterization applications for substances that exhibit different energy-dependent x-ray absorption behavior. DECT supports a variety of post-processing applications that add value in routine clinical CT imaging, including material selective and virtual non-contrast images using two- and three-material decomposition algorithms, virtual monoenergetic imaging, and other material characterization techniques. Following a review of acquisition and post-processing techniques, we present a case-based approach to highlight the added value of DECT in common clinical scenarios. These scenarios include improved lesion detection, improved lesion characterization, improved ease of interpretation, improved prognostication, inherently more robust imaging protocols to account for unexpected pathology or suboptimal contrast opacification, length of stay reduction, reduced utilization by avoiding unnecessary follow-up examinations, and radiation dose reduction. A brief discussion of post-processing workflow approaches, challenges, and solutions is also included.

Advanced imaging techniques for chronic pancreatitis

MRI and MRCP play an important role in the diagnosis of chronic pancreatitis (CP) by imaging pancreatic parenchyma and ducts. MRI/MRCP is more widely used than computed tomography (CT) for mild to moderate CP due to its increased sensitivity for pancreatic ductal and gland changes; however, it does not detect the calcifications seen in advanced CP. Quantitative MR imaging offers potential advantages over conventional qualitative imaging, including simplicity of analysis, quantitative and population-based comparisons, and more direct interpretation of detected changes. These techniques may provide quantitative metrics for determining the presence and severity of acinar cell loss and aid in the diagnosis of chronic pancreatitis. Given the fact that the parenchymal changes of CP precede the ductal involvement, there would be a significant benefit from developing MRI/MRCP-based, more robust diagnostic criteria combining ductal and parenchymal findings. Among cross-sectional imaging modalities, multi-detector CT (MDCT) has been a cornerstone for evaluating chronic pancreatitis (CP) since it is ubiquitous, assesses primary disease process, identifies complications like pseudocyst or vascular thrombosis with high sensitivity and specificity, guides therapeutic management decisions, and provides images with isotropic resolution within seconds. Conventional MDCT has certain limitations and is reserved to provide predominantly morphological (e.g., calcifications, organ size) rather than functional information. The emerging applications of radiomics and artificial intelligence are poised to extend the current capabilities of MDCT. In this review article, we will review advanced imaging techniques by MRI, MRCP, CT, and ultrasound.

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.

Impact of Patient Size and Radiation Dose on Accuracy and Precision of Iodine Quantification and Virtual Noncontrast Values in Dual-layer Detector CT-A Phantom Study

RATIONALE AND OBJECTIVES

Iodine quantification (IQ) and virtual noncontrast (VNC) images produced by dual-energy CT (DECT) can be used for various clinical applications. We investigate the performance of dual-layer DECT (DLDECT) in different phantom sizes and varying radiation doses and tube voltages, including a low-dose pediatric setting.

MATERIALS AND METHODS

Three phantom sizes (simulating a 10-year-old child, an average, and a large-sized adult) were scanned with iodine solution inserts with concentrations ranging 0-32 mg/ml, using the DLDECT. Each phantom size was scanned with CTDIvol 2-15 mGy at 120 and 140 kVp. The smallest phantom underwent additional scans with CTDIvol 0.9-1.8 mGy. All scans were repeated 3 times. Each iodine insert was analyzed using VNC and IQ images for accuracy and precision, by comparison to known values.

RESULTS

For scans from 2 to 15 mGy mean VNC attenuation and IQ error in the iodine inserts in the small, medium, and large phantoms was 1.2 HU ± 3.2, -1.2 HU ± 14.9, 2.6 HU ± 23.6; and +0.1 mg/cc ± 0.4, -0.9 mg/cc ± 0.9, and -1.8 mg/cc ± 1.8, respectively. In this dose range, there were no significant differences (p ≥ 0.05) in mean VNC attenuation or IQ accuracy in each phantom size, while IQ was significantly less precise in the small phantom at 2 mGy and 10 mGy (p < 0.05). Scans with CTDIvol 0.9-1.8 mGy in the small phantom showed a limited, but statistically significantly lower VNC attenuation precision and IQ accuracy (-0.5 HU ± 5.3 and -0.3 mg/cc ± 0.5, respectively) compared to higher dose scans in the same phantom size.

CONCLUSION

Performance of iodine quantification and subtraction by VNC images in DLDECT is largely dose independent, with the primary factor being patient size. Low-dose pediatric scan protocols have a significant, but limited impact on IQ and VNC attenuation values.

Dual-energy CT: theoretical principles and clinical applications

The physical principles of dual-energy computed tomography (DECT) are as old as computed tomography (CT) itself. To understand the strengths and the limits of this technology, a brief overview of theoretical basis of DECT will be provided. Specific attention will be focused on the interaction of X-rays with matter, on the principles of attenuation of X-rays in CT toward the intrinsic limits of conventional CT, on the material decomposition algorithms (two- and three-basis-material decomposition algorithms) and on effective Rho-Z methods. The progresses in material decomposition algorithms, in computational power of computers and in CT hardware, lead to the development of different technological solutions for DECT in clinical practice. The clinical applications of DECT are briefly reviewed in relation to the specific algorithms.

Pre-hepatic and pre-pancreatic transplant donor evaluation

Innovations in surgical techniques coupled with advances in medical and pharmacological management in the past few decades have enabled organ transplantation to become integral to the management of end stage organ failure. In this review article, we will review the role of the radiologist in the work up of liver and pancreas donors during evaluation of their donor candidacy. The critical role of imaging in assessing the parenchymal, biliary and vascular anatomy in liver donor candidates will be reviewed, as well as highlighting the anatomical findings that may pose a contraindication to transplantation. The limited role of imaging in pancreas donor evaluation is also covered, as well as a brief overview of the surgical techniques available and how the radiologist's findings influence operative technique selection.

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.

Dual-energy CT in patients with abdominal malignant lymphoma: impact of noise-optimised virtual monoenergetic imaging on objective and subjective image quality

AIM

To investigate the impact of noise-optimised virtual monoenergetic imaging (VMI+) reconstructions on quantitative and qualitative image parameters in patients with malignant lymphoma at dual-energy computed tomography (DECT) examinations of the abdomen.

MATERIALS AND METHODS

Thirty-five consecutive patients (mean age, 53.8±18.6 years; range, 21-82 years) with histologically proven malignant lymphoma of the abdomen were included retrospectively. Images were post-processed with standard linear blending (M_0.6), traditional VMI, and VMI+ technique at energy levels ranging from 40 to 100 keV in 10 keV increments. Signal-to-noise (SNR) and contrast-to-noise ratios (CNR) were objectively measured in lymphoma lesions. Image quality, lesion delineation, and image noise were rated subjectively by three blinded observers using five-point Likert scales.

RESULTS

Quantitative image quality parameters peaked at 40-keV VMI+ (SNR, 15.77±7.74; CNR, 18.27±8.04) with significant differences compared to standard linearly blended M_0.6 (SNR, 7.96±3.26; CNR, 13.55±3.47) and all traditional VMI series (p<0.001). Qualitative image quality assessment revealed significantly superior ratings for image quality at 60-keV VMI+ (median, 5) in comparison with all other image series (p<0.001). Assessment of lesion delineation showed the highest rating scores for 40-keV VMI+ series (median, 5), while lowest subjective image noise was found for 100-keV VMI+ reconstructions (median, 5).

CONCLUSION

Low-keV VMI+ reconstructions led to improved image quality and lesion delineation of malignant lymphoma lesions compared to standard image reconstruction and traditional VMI at abdominal DECT examinations.

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.