Gadolinium as a CT contrast agent: assessment in a porcine model

Beam hardening of K-edge contrast agents: a phantom study comparing clinical energy-integrating detector and photon-counting detector CT systems

BACKGROUND

Beam hardening (BH) artifacts negatively influence computed tomography (CT) measurements, especially when due to dense materials or materials with high effective atomic numbers. Photon-counting detectors (PCD) are more susceptible to BH due to equal weighting of photons regardless of their energies. The problem is further confounded by the use of contrast agents (CAs) with K-edge in the diagnostic CT energy range. We quantified the BH effect of different materials comparing energy-integrating detector (EID)-CT and PCD-CT.

METHODS

Pairs of test tubes were filled with dense CA (iodine-, gadolinium-, and bismuth-based) and placed inside a water phantom. The phantoms were scanned on EID- and PCD-CT systems, at all available tube voltages for the PCD scanner. Images were reconstructed with standard water BH correction but without any iodine/bone BH corrections. Virtual monoenergetic images (VMI) were calculated from PCD-CT data.

RESULTS

PCD-CT had higher CT numbers in all x-ray spectra for all CAs (p < 0.001) and produced larger cupping artifacts in all test cases (p < 0.001). Bismuth-based CA artifacts were 3- to 5-fold smaller than those of iodine- or gadolinium-based CA. PCD-CT-based VMI completely removed iodine BH artifacts. Iodine BH artifacts decreased with increasing tube voltage. However, gadolinium-based BH artifacts had a different trend increasing at 120 kVp.

CONCLUSION

EID had fewer BH artifacts compared to PCD at x-ray tube voltages of 120 kVp and higher. The inherent spectral information of PCDs can be used to eliminate BH artifacts. Special care is needed to correct BH artifacts for gadolinium- and bismuth-based CAs.

RELEVANCE STATEMENT

With the increasing availability of clinical photon-counting CT systems offering the possibility of dual contrast imaging capabilities, addressing and comprehending the BH artifacts attributed to old and novel CT CAs grows in research and ultimately clinical relevance.

KEY POINTS

EID-CT provides fewer BH artifacts compared to PCD-CT at x-ray tube voltages of 120 kVp and higher. K-edge CAs, such as those based on gadolinium, further confound BH artifacts. The inherent spectral information of photon counting detector CT can be used to effectively eliminate BH artifacts.

Gadolinium-based coronary CT angiography on a clinical photon-counting-detector system: a dynamic circulating phantom study

BACKGROUND

Coronary computed tomography angiography (CCTA) offers non-invasive diagnostics of the coronary arteries. Vessel evaluation requires the administration of intravenous contrast. The purpose of this study was to evaluate the utility of gadolinium-based contrast agent (GBCA) as an alternative to iodinated contrast for CCTA on a first-generation clinical dual-source photon-counting-detector (PCD)-CT system.

METHODS

A dynamic circulating phantom containing a three-dimensional-printed model of the thoracic aorta and the coronary arteries were used to evaluate injection protocols using gadopentetate dimeglumine at 50%, 100%, 150%, and 200% of the maximum approved clinical dose (0.3 mmol/kg). Virtual monoenergetic image (VMI) reconstructions ranging from 40 keV to 100 keV with 5 keV increments were generated on a PCD-CT. Contrast-to-noise ratio (CNR) was calculated from attenuations measured in the aorta and coronary arteries and noise measured in the background tissue. Attenuation of at least 350 HU was deemed as diagnostic.

RESULTS

The highest coronary attenuation (441 ± 23 HU, mean ± standard deviation) and CNR (29.5 ± 1.5) was achieved at 40 keV and at the highest GBCA dose (200%). There was a systematic decline of attenuation and CNR with higher keV reconstructions and lower GBCA doses. Only reconstructions at 40 and 45 keV at 200% and 40 keV at 150% GBCA dose demonstrated sufficient attenuation above 350 HU.

CONCLUSION

Current PCD-CT protocols and settings are unsuitable for the use of GBCA for CCTA at clinically approved doses. Future advances to the PCD-CT system including a 4-threshold mode, as well as multi-material decomposition may add new opportunities for k-edge imaging of GBCA.

RELEVANCE STATEMENT

Patients allergic to iodine-based contrast media and the future of multicontrast CT examinations would benefit greatly from alternative contrast media, but the utility of GBCA for coronary photon-counting-dector-CT angiography remains limited without further optimization of protocols and scanner settings.

KEY POINTS

GBCA-enhanced coronary PCD-CT angiography is not feasible at clinically approved doses. GBCAs have potential applications for the visualization of larger vessels, such as the aorta, on PCD-CT angiography. Higher GBCA doses and lower keV reconstructions achieved higher attenuation values and CNR.

Toward the Clinical Translation of Safe Intravenous Long Circulating Iodinated Lipid Nanoemulsion Contrast Agents for CT Imaging

Current clinical small molecule x-ray CT agents are effective but pose risks such as nephrotoxicity, short blood circulation time, limiting scan durations, potential thyroid impact, and immune responses. These challenges drive the development of kidney-safe x-ray nanoparticle (NP)-based contrast agents (CAs), though translation to clinical practice is hindered by chemical complexities and potential toxicity. We have engineered an intravenous, injectable, and safe blood pool NP-based CT CAs at a clinical-equivalent dose of ∼300 mgI/kg (∼2 mL/kg), ideal for vascular and hepatic imaging which are limited by clinical agents. Our iodinated lipid nanodroplet emulsions (ILNEs) contrast agent offers high x-ray attenuation thus improved contrast enhancement, extended stability, and exceptional batch-to-batch consistency. It also boasts a straightforward and scalable manufacturing process with minimal protein interaction, prolonged blood residency (∼4h), and hepatic clearance within 3 days, avoiding nephrotoxicity. Studies in vitro, in mice, and 16.6kg porcine animal model studies confirm its safety, cytocompatibility, and absence of tissue damage. Blood, and thyroid-stimulating hormone (TSH) analyses, and kidney and liver function tests, also support further toxicity evaluations for clinical translation.

Low KeV virtual monoenergetic images for detecting low dose iodine- or alternative Gd-based IV contrast agents

Background

The recent shortage of iodine-based intravenous contrast and its cost highlight the need for limiting dose and alterative agents.

Purpose

To quantify radiodensity (Hounsfield Units, HU) improvement and potential iodine dose reduction with low keV imaging compared to conventional polyenergetic reconstructions on dual source (DSCT) and dual layer (DLCT) CT and to assess potential utility of non-iodine gadolinium-alternatives with low keV imaging.

Materials and methods

This phantom study used dilutions of three commercially-available contrast agents scanned by DSCT and DLCT. Conventional polyenergetic and virtual monoenergetic images (VMI) were reconstructed of each of five dilutions at five keV levels. HU and signal-to-noise ratios were compared among iodine- and gadolinium-based contrast agents.

Results

Iodine- and gadolinium-based contrast agent HU increased inversely to keV for the same dilution in both scanners. At the lowest keV setting (40 keV), iodine-based contrast agent HU in VMIs with DLCT and DSCT were approximately 300 % and 400 % of conventional, respectively. Gd-based contrast agent HU in VMIs at low keV were similar to or better than conventional iodine HU. Comparing the dual energy CTs, although HU from iodine and gadolinium-based contrast agents for conventional polyenergetic reconstructions was similar, HU in VMIs of DSCT were right shifted compared to DLCT by ∼10 keV lower.

Conclusion

Depending on CT scanner type, 1/3 to 1/4 dose of iodinated contrast at 40 keV provides HU similar to full dose conventional acquisition, suggesting 1/3-1/4 dose may be adequate clinically at 40 keV. Depending on the Gd-based contrast and CT type, Gd-based contrast at 40 keV provides similar or greater HU compared to conventional acquisitions with iodinated contrast, suggesting Gd-based contrast at 40 keV may serve as an alternative to iodinated contrast. HU on VMI images is scanner dependent, suggesting scanner-dependent protocol optimization and potentially monoenergy HU calibration between scanners is needed.

No gadolinium K-edge detected on the first clinical photon-counting computed tomography scanner

PURPOSE

This study aimed to elucidate whether gadolinium contrast in clinically relevant doses can be used with photon-counting computed tomography (PCCT) as an alternative contrast agent in clinical applications.

MATERIAL/METHODS

A CTDI phantom with 3D printed rods filled with different concentrations of gadolinium and iodine contrast was scanned in a PCCT and an energy-integrated computed tomography (EICT). Attenuation values at different monoenergetic steps were extracted for each contrast concentration.

RESULTS

For PCCT, gadolinium reached an attenuation >100 HU (103 HU) at 40 keV with a concentration 5 mmol/L whereas the same level was reached at 50 keV (118 HU) for 10 mmol/L and 90 keV (114 HU) for 25 mmol/L. For iodine, the same level of attenuation was reached at 100 keV (106 HU) with a concentration 8.75 mg I/mL. For EICT the lowest gadolinium contrast concentration needed to reach >100 HU (108 HU) was 10 mmol/L at 50 keV. For 25 mmol/L 100 HU was reached at 100 keV. For iodine contrast 108 HU was reached at 110 keV for 8.75 mg I/mL.

CONCLUSION

No K-edge potential or difference in attenuation curves between iodine and gadolinium contrast is detected on the first clinical available PCCT. Clinically relevant attenuation levels were barely achieved in this setting with gadolinium concentrations approved for human use. The results of this study suggest that, given current scanning technology, gadolinium is not a clinically useful contrast agent for computed tomography because no K-edge was detected.

Waiting times between examinations with intravascularly administered contrast media: a review of contrast media pharmacokinetics and updated ESUR Contrast Media Safety Committee guidelines

The pharmacokinetics of contrast media (CM) will determine how long safe waiting intervals between successive CT or MRI examinations should be. The Contrast Media Safety Committee has reviewed the data on pharmacokinetics of contrast media to suggest safe waiting intervals between successive contrast-enhanced imaging studies in relation to the renal function of the patient. CLINICAL RELEVANCE STATEMENT: Consider a waiting time between elective contrast-enhanced CT and (coronary) angiography with successive iodine-based contrast media administrations in patients with normal renal function (eGFR > 60 mL/min/1.73 m2) of optimally 12 h (near complete clearance of the previously administered iodine-based contrast media) and minimally 4 h (if clinical indication requires rapid follow-up). KEY POINTS: • Pharmacokinetics of contrast media will guide safe waiting times between successive administrations. • Safe waiting times increase with increasing renal insufficiency. • Iodine-based contrast media influence MRI signal intensities and gadolinium-based contrast agents influence CT attenuation.

New Horizons in Vascular Imaging With Photon-Counting Detector CT

ABSTRACT

Noninvasive vascular imaging with computed tomography (CT) has become the clinical mainstay for many indications and body regions. The recent introduction of photon-counting detector (PCD)-CT into clinical routine has further broadened the spectrum of vascular applications. Technical improvements of PCD-CT, such as the decreased noise levels, improved contrast-to-noise ratio, and full spectral multienergy data information from every acquisition, have the potential to further impact on clinical decision making and ultimately on outcome of vascular patients. Early experience with the new PCD-CT technology demonstrates these improvements in various aspects. This review summarizes the main advantages of PCD-CT for vascular imaging a discussion of the PureLumen and PureCalcium algorithms.

A CT-based radiomics nomogram for classification of intraparenchymal hyperdense areas in patients with acute ischemic stroke following mechanical thrombectomy treatment

Objectives

To develop and validate a radiomic-based model for differentiating hemorrhage from iodinated contrast extravasation of intraparenchymal hyperdense areas (HDA) following mechanical thrombectomy treatment in acute ischemic stroke.

Methods

A total of 100 and four patients with intraparenchymal HDA on initial post-operative CT were included in this study. The patients who met criteria were divided into a primary and a validation cohort. A training cohort was constructed using Synthetic Minority Oversampling Technique on the primary cohort to achieve group balance. Thereafter, a radiomics score was calculated and the radiomic model was constructed. Clinical factors were assessed to build clinical model. Combined with the Rad-score and independent clinical factors, a combined model was constructed. Different models were assessed using the area under the receiver operator characteristic curves. The combined model was visualized as nomogram, and assessed with calibration and clinical usefulness.

Results

Cardiogenic diseases, intraoperative tirofiban administration and preoperative national institute of health stroke scale were selected as independent predictors to construct the clinical model with area under curve (AUC) of 0.756 and 0.693 in the training and validation cohort, respectively. Our data demonstrated that the radiomic model showed good discrimination in the training (AUC, 0.955) and validation cohort (AUC, 0.869). The combined nomogram model showed optimal discrimination in the training (AUC, 0.972) and validation cohort (AUC, 0.926). Decision curve analysis demonstrated the combined model had a higher overall net benefit in differentiating hemorrhage from iodinated contrast extravasation in terms of clinical usefulness.

Conclusions

The nomogram shows favorable efficacy for differentiating hemorrhage from iodinated contrast extravasation, which might provide an individualized tool for precision therapy.

Biocompatible carbonized iodine-doped dots for contrast-enhanced CT imaging

Background

Computed tomography (CT) imaging has been widely used for the diagnosis and surveillance of diseases. Although CT is attracting attention due to its reasonable price, short scan time, and excellent diagnostic ability, there are severe drawbacks of conventional CT contrast agents, such as low sensitivity, serious toxicity, and complicated synthesis process. Herein, we describe iodine-doped carbon dots (IDC) for enhancing the abilities of CT contrast agents.

Method

IDC was synthesized by one-pot hydrothermal synthesis for 4 h at 180 ℃ and analysis of its structure and size distribution with UV–Vis, XPS, FT-IR, NMR, TEM, and DLS. Furthermore, the CT values of IDC were calculated and compared with those of conventional CT contrast agents (Iohexol), and the in vitro and in vivo toxicities of IDC were determined to prove their safety.

Results

IDC showed improved CT contrast enhancement compared to iohexol. The biocompatibility of the IDC was verified via cytotoxicity tests, hemolysis assays, chemical analysis, and histological analysis. The osmotic pressure of IDC was lower than that of iohexol, resulting in no dilution-induced contrast decrease in plasma.

Conclusion

Based on these results, the remarkable CT contrast enhancement and biocompatibility of IDC can be used as an effective CT contrast agent for the diagnosis of various diseases compared with conventional CT contrast agents.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40824-022-00277-3.

Simultaneous dual-contrast imaging using energy-integrating-detector multi-energy CT: An in vivo feasibility study

PURPOSE

To demonstrate the feasibility of simultaneous dual-contrast imaging in a large animal using a newly developed dual-source energy-integrating-detector (EID) based multi-energy computed tomography (MECT) system.

METHODS

Two imaging tasks that may have potential clinical applications were investigated: head/neck (HN) CT angiography (CTA)/CT venography (CTV) with iodine and gadolinium, and small bowel imaging with iodine and bismuth in domestic swine. Dual-source x-ray beam configurations of 70 kV+Au120/Sn120 kV and 70 kV+Au140/Sn140 kV were used for the HN-CTA/CTV and small bowel imaging studies, respectively. A test bolus scan was performed for each study. The ROIs in the carotid artery and jugular vein for HN-CTA/CTV imaging and abdominal aorta for small bowel imaging were used to determine the time-attenuation curves, based on which the timing for contrast injection and the CT scan was determined. In the HN-CTA/CTV study, a MECT scan was performed at the time point corresponding to the optimal arterial enhancement by iodine and the optimal venous enhancement by gadolinium. In the small bowel imaging study, A MECT scan was performed at the optimal time point to simultaneously capture the mesenteric arterial enhancement of iodine and the enteric enhancement of bismuth. Image-based material decomposition was performed to decompose different materials for each study. To quantitatively characterize contrast material separation and misclassification, two ROIs on left common carotid artery and left internal jugular vein in HN-CTA/CTV imaging and three ROIs on superior mesenteric artery, ileal lumen, and collapsed ileum (ileal wall) in small bowel imaging were placed to measure the mean concentration values and the standard deviations.

RESULTS

In the HN-CTA/CTV study, common carotid arteries containing iodine and internal/external jugular veins containing gadolinium were clearly delineated from each other. Fine vessels such as cephalic veins and branches of external jugular veins were noticeable but clear visualization was hindered by image noise in gadolinium-specific (CTV) images, as reviewed by a neuro radiologist. In the small bowel imaging study, the mesenteric arteries and collapsed bowel wall containing iodine and the small bowel loops containing bismuth were clearly distinctive from each other in the iodine- and bismuth-specific images after material decomposition, as reviewed by an abdominal radiologist. Quantitative analyses showed that the misclassifications between the two contrast materials were less than 1.7 mg/mL and 0.1 mg/mL for CTA/CTV and small bowel imaging studies, respectively.

CONCLUSIONS

Feasibility of simultaneous CTA/CTV imaging in head and neck with iodine and gadolinium and simultaneous imaging of arterial and enteric phases of small bowel with iodine and bismuth, using a dual-source EID-MECT system, was demonstrated in a swine study. Compared to iodine and gadolinium in CTA/CTV, better delineation and classification of iodine and bismuth in small bowel imaging were achieved mainly due to wider separation between the corresponding two K-edge energies. This article is protected by copyright. All rights reserved.

Feasibility of Dual-Energy Computed Tomography Imaging of Gadolinium-Based Contrast Agents and Its Application in Computed Tomography Cystography: An Exploratory Study to Assess an Alternative Option When Iodinated Contrast Agents Are Contraindicated

OBJECTIVE

The aim of this study was to compare the contrast enhancement differences between gadolinium-based and iodine-based contrast agents at different single-energy tube potentials and dual-energy-based virtual monochromatic energies. In addition, we describe the application of a gadolinium-based contrast agent in computed tomography (CT) cystography for a patient with contraindications to iodine.

METHODS

A phantom study was performed using 3 iodine samples (concentrations: 5, 10, and 15 mgI/mL) and 3 gadolinium samples (concentrations: 3.3, 6.6, and 9.9 mgGd/mL). The prepared phantom was scanned by a dual-energy CT (DECT) at 80, 100, 120, and 140 kV in the single-energy mode and at 100/Sn140 kV in the dual-energy mode. Virtual monoenergetic images (VMIs) at 50 keV were generated from the DECT scan. In addition, a DECT cystogram was performed using a gadolinium-based contrast agent in a patient with contraindications to iodinated contrast.

RESULTS

Strong linear correlations between mean signal of contrast enhancement and mass concentration were found for both iodine and gadolinium samples across all single-energy CT (SECT) and DECT scan conditions. The VMI at 50 keV provided the highest contrast enhancement for both types of contrast samples at each concentration level, and single-energy CT scans at low-energy beams showed higher contrast enhancement than higher beam energies. In addition, the contrast enhancement for pure gadolinium solution was constantly higher than pure iodine solution at an identical mass concentration level. The DECT cystogram was performed with excellent technical success. The urinary bladder was appropriately distended with intravesical contrast measuring 606 Hounsfield units and no evidence of bladder leak or fistula.

CONCLUSIONS

Imaging of gadolinium-based contrast agents is improved using a DECT technique, with VMI at 50 keV providing the highest contrast enhancement among our tested parameters. Dual-energy CT cystography using a gadolinium-based agent can be a safe and effective alternative when iodinated agents are contraindicated.

Multifunctional Transferrin Encapsulated GdF3 Nanoparticles for Sentinel Lymph Node and Tumor Imaging

The transferrin encapsulated GdF₃ nanoparticles have been fabricated via biomineralization method. The obtained GdF₃@Tf NPs show an attractive T2MRI and CT enhancement effect. Furthermore, PET and NIR imaging capacity are integrated into nanoparticles through conjugating with radionuclide ⁶⁴Cu and fluorescent dye Cy7. ⁶⁴Cu-GdF₃@Tf-Cy7 NPs are developed and applied in small animal multimodal imaging in vivo. Compared with the previous multimodal imaging agents, ⁶⁴Cu-GdF₃@Tf-Cy7 NPs enable not only precise sentinel lymph node (SLN) identification, but specific imaging for transferrin receptor overexpressed colorectal tumor in vivo. The results reveal that ⁶⁴Cu-GdF₃@Tf-Cy7 NPs are potential and efficient multimodal imaging agents for SLN and tumor preclinical imaging.

Photon-counting computed tomography of lanthanide contrast agents with a high-flux 330-μm-pitch cadmium zinc telluride detector in a table-top system

Purpose: We present photon-counting computed tomography (PCCT) imaging of contrast agent triplets similar in atomic number ( Z ) achieved with a high-flux cadmium zinc telluride (CZT) detector. Approach: The table-top PCCT imaging system included a 330 - μ m -pitch CZT detector of size 8    mm × 24    mm 2 capable of using six energy bins. Four 3D-printed 3-cm-diameter phantoms each contained seven 6-mm-diameter vials with water and low and high concentration solutions of various contrast agents. Lanthanum ( Z = 57 ), gadolinium (Gd) ( Z = 64 ), and lutetium ( Z = 71 ) were imaged together and so were iodine ( Z = 53 ), Gd, and holmium ( Z = 67 ). Each phantom was imaged with 1-mm aluminum-filtered 120-kVp cone beam x rays to produce six energy-binned computed tomography (CT) images. Results: K -edge images were reconstructed using a weighted sum of six CT images, which distinguished each contrast agent with a root-mean-square error (RMSE) of < 0.29 % and 0.51% for the 0.5% and 5% concentrations, respectively. Minimal cross-contamination in each K -edge image was seen, with RMSE values < 0.27 % in vials with no contrast. Conclusion: This is the first preliminary demonstration of simultaneously imaging three similar Z contrast agents with a difference in Z as low as 3.

CT/MR Dual-Modality Imaging Tracking of Mesenchymal Stem Cells Labeled with a Au/GdNC@SiO2 Nanotracer in Pulmonary Fibrosis

Mesenchymal stem cells (MSCs) have shown potential as an innovative treatment for pulmonary fibrosis (PF), due to their capability to ameliorate the inflammation and moderate the deterioration of PF. The fate of the stem cells transplanted into the lung, including survival, migration, homing, and functions, however, has not been fully understood yet. In this paper, we report the development of a computed tomography/magnetic resonance (CT/MR) dual-modal nanotracer, gold/gadolinium nanoclusters overcoated with a silica shell (Au/GdNC@SiO₂), for noninvasive labeling and tracking of the transplanted human MSCs (hMSCs) in a PF model. The Au/GdNC@SiO₂ nanotracer exhibits good colloidal and chemical stability, high biocompatibility, enhanced longitudinal MR relaxivity, and superior X-ray attenuation property. The hMSCs can be effectively labeled with Au/GdNC@SiO₂, resulting in a significantly increased cellular CT/MR imaging contrast, without any obvious adverse effect on the function, including proliferation and differentiation of the labeled stem cells. Moreover, by using the Au/GdNC@SiO₂ nanotracer, the hMSCs transplanted in the lung can be tracked for 7 d via in vivo CT/MR dual-modality imaging. This work may provide an insight into the role the transplanted hMSCs play in PF therapy, thus promoting the stem cell-based regenerative medicine.

Enhanced cytotoxic and genotoxic effects of gadolinium-doped ZnO nanoparticles on irradiated lung cancer cells at megavoltage radiation energies

The purpose of this study was to investigate the radiation dose enhancement effects of gadolinium-doped zinc oxide nanoparticles (Gd-doped ZnO NPs) under the megavoltage (MV) X-ray irradiation. ZnO NPs have preferred photocatalytic properties under UV light for cancer killing. UV light has limited applications in cancer treatment and it is necessary to use X-ray photons with MV energies. In order to increase the absorption of radiation and also to enhance the imaging visualization capabilities of ZnO NPs, gadolinium (Gd) as a high atomic number element was selected for doping into the structure of ZnO NPs. Gd-doped ZnO NPs were synthesized by a chemical precipitation method and characterized by transmission electron microscopy, powder X-ray diffraction, ultraviolet-visible spectroscopy, and energy-dispersive X-ray techniques. Cellular uptake was assessed by TEM and inductively coupled plasma mass spectrometry. NPs cytotoxicity was analyzed by MTT assay and radiation dose enhancement was measured by clonogenic survival assay. Apoptosis induction, cell cycle progression, micronucleus formation and expression of DNA double-strand break repair genes of XRCC2 and XRCC4 were determined by flow cytometry, micronucleus assay, and quantitative real-time polymerase chain reaction. CT and MR imaging were used to analyze the image visualization capabilities of NPs. NPs characterization showed that highly pure crystalline Gd-doped ZnO NPs with a narrow size distribution and grain size of 9 nm were synthesized. Gd-doped ZnO NPs were distributed in the cells and showed dose-dependent toxicity. Combination of Gd-doped ZnO NPs with 6 MV X-rays induced dose-dependent radiosensitivity with sensitizer enhancement ratios (SER) of 1.47 and 1.61 for 10 and 20 μg/mL NPs concentrations. Cancer cells blocked in G1, apoptosis rates, and micronuclei formation was enhanced and inversely, the DNA repair efficiency was impaired by down regulation of the mRNA levels of XRCC2 and XRCC4 genes. Gd-doped ZnO NPs enhanced the contrasts of CT and MR images of cancer cells. Overall, the results of this study provide detailed biological insights on the dose enhancement of Gd-doped ZnO NPs at MV radiations, which would contribute to the further development of this potent theranostic platform for clinical applications.

Functional imaging of tumor vasculature using iodine and gadolinium-based nanoparticle contrast agents: a comparison of spectral micro-CT using energy integrating and photon counting detectors

Advances in computed tomography (CT) hardware have propelled the development of novel CT contrast agents. In particular, the spectral capabilities of x-ray CT can facilitate simultaneous imaging of multiple contrast agents. This approach is particularly useful for functional imaging of solid tumors by simultaneous visualization of multiple targets or architectural features that govern cancer development and progression. Nanoparticles are a promising platform for contrast agent development. While several novel imaging moieties based on high atomic number elements are being explored, iodine (I) and gadolinium (Gd) are particularly attractive because of their existing approval for clinical use. In this work, we investigate the in vivo discrimination of I and Gd nanoparticle contrast agents using both dual energy micro-CT with energy integrating detectors (DE-EID) and photon counting detector (PCD)-based spectral micro-CT. Simulations and phantom experiments were performed using varying concentrations of I and Gd to determine the imaging performance with optimized acquisition parameters. Quantitative spectral micro-CT imaging using liposomal-iodine (Lip-I) and liposomal-Gd (Lip-Gd) nanoparticle contrast agents was performed in sarcoma bearing mice for anatomical and functional imaging of tumor vasculature. Iterative reconstruction provided high sensitivity to detect and discriminate relatively low I and Gd concentrations. According to the Rose criterion applied to the experimental results, the detectability limits for I and Gd were approximately 2.5 mg ml^-1 for both DE-EID CT and PCD micro-CT, even if the radiation dose was approximately 3.8 times lower with PCD micro-CT. The material concentration maps confirmed expected biodistributions of contrast agents in the blood, liver, spleen and kidneys. The PCD provided lower background signal and better simultaneous visualization of tumor vasculature and intratumoral distribution patterns of nanoparticle contrast agent compared to DE-EID decompositions. Preclinical spectral CT systems such as this could be useful for functional characterization of solid tumors, simultaneous quantitative imaging of multiple targets and for identifying clinically-relevant applications that benefit from the use of spectral imaging. Additionally, it could aid in the development nanoparticles that show promise in the developing field of cancer theranostics (therapy and diagnostics) by measuring vascular tumor biomarkers such as fractional blood volume and the delivery of liposomal chemotherapeutics.

Bi-DTPA as a high-performance CT contrast agent for in vivo imaging

Clinically used iodinated computer tomography (CT) contrast agents suffer from low sensitivity, and the emerging lanthanide-chelates and CT imaging nanoagents raise great safety concerns. The fusion of high sensitivity and good biocompatibility is highly desired for the development of CT contrast agents. Herein, we propose a facile and green one-pot synthesis strategy for the fabrication of a small molecular CT contrast agent, Bi-diethylene triamine pentaacetate acid (DTPA) complex, for high-performance CT and spectral CT imaging. The Bi-DTPA exhibits yield of near 100%, outstanding water solubility, favorable biocompatibility, large-scale production capability, and superior X-ray attenuation ability, and is successfully applied in high-quality in vivo kidney imaging and gastrointestinal tract CT imaging and appealing spectral CT imaging. The proposed contrast agent can be rapidly excreted from body, avoiding the potential side effects caused by the long-term retention in vivo. Furthermore, our design shows great potential in developing diverse multifunctional contrast agents via chemical modification. The proposed Bi-DTPA with unique superiorities shows a bright prospect in clinic CT imaging, especially spectral CT imaging, and lays down a new way for the design of high-performance CT contrast agents with great clinical transformation potential.

Transarterial chemoembolization in a patient with severe reactions to iodinated contrast: Successful treatment using gadolinium contrast with C-arm computed tomography

Severe reactions to modern iodinated contrasts are uncommon. Breakthrough reactions in the setting of pretreatment with corticosteroids are even rarer. Patients with a history of these refractory reactions can create challenging situations in the diagnostic and therapeutic process. Here, we present a case of an 83-year-old male with hepatocellular carcinoma and a history of multiple severe reactions to iodinated contrast. The patient required a transarterial chemoembolization but the conventional technique was unavailable due to the allergy. Gadolinium-based contrast was substituted and used in conjunction with C-arm CT and a percutaneous ethanol injection to treat the tumor. After nearly 3 years, there is no evidence of residual or recurrent hepatocellular carcinoma.

Influence of excess ligand on Nephrogenic Systemic Fibrosis associated with nonionic, linear gadolinium-based contrast agents

BACKGROUND

The molecular structure, charge, thermodynamic and kinetic stability are approximately the same for gadodiamide and gadoversetamide, the main substantive difference is that gadodiamide is manufactured with 5% free ligand to form Omniscan® and gadoversetamide with 10% free ligand to form OptiMARK®.

PURPOSE

To determine the relative risk of Nephrogenic Systemic Fibrosis (NSF) between gadodiamide (Omniscan®) and gadoversetamide (OptiMARK®) and to explore the potential contribution of the amount of excess ligand added to their commercial formulations.

MATERIALS AND METHODS

In this retrospective observational study, the number of doses and NSF cases associated with these agents were calculated based on two different approaches: the number of doses was determined based on pharmaceutical companies' information, and the number of unconfounded NSF cases was obtained from the previously published literature based on a legal database. A second analysis estimates the number of doses and NSF cases from the Food and Drug Administration (FDA) Adverse Event Reporting System (FAERS).

RESULTS

Approximately 87 million and 12 million doses of Omniscan® and OptiMARK®, respectively, have been administered worldwide since their original approval for use in the various countries throughout the world. A total of 197 and 8 unconfounded cases of NSF have been reported with Omniscan® and OptiMARK®, rendering an incidence of 2.3/million and 0.7/million for these agents, respectively. The FAERS analysis suggested reported incidences of 13.1/million and 5.0/million.

CONCLUSION

There is an approximately 3-fold greater incidence of NSF from Omniscan® than OptiMARK®. The difference in incidence might reflect the lesser quantity of added free ligand to the formulation of Omniscan®.

Contrast Extravasation versus Hemorrhage after Thrombectomy in Patients with Acute Stroke

BACKGROUND AND PURPOSE

Intra-arterial recanalization postprocedural imaging in stroke patients can result in diagnostic complications due to hyperdensities on noncontrast computed tomography (CT), which may represent either contrast extravasation or intracranial hemorrhage. If these lesions are hemorrhage, then they are risk factors becoming symptomatic, which, if not distinguished, can alter clinical management. We investigate the effects of iodinated contrast on postprocedural magnetic resonance imaging (MRI) and prevalence of equivocal imaging interpretations of postprocedural extravasated contrast versus hemorrhage while identifying protocol pitfalls.

METHODS

We identified 10 patients diagnosed with ischemic stroke who underwent intra-arterial recanalization in a 5-year period. These patients demonstrated a hyperdensity on a postprocedural CT within 24 hours, underwent an MRI within 48 hours, and an additional confirmatory noncontrast CT at least 72 hours postprocedure.

RESULTS

Postprocedural MRI in all 10 stroke patients demonstrated T1 - and T2 -relaxation time changes due to residual iodine contrast agents. This lead to false positive postprocedural hemorrhage MRI interpretations in 2/10 patients, 3/10 false negative interpretations of contrast extravasation, and 5/10 equivocal interpretations suggesting extravasation or hemorrhage. Of these five cases, two were performed with gadolinium.

CONCLUSION

MRI done within 48 hours postprocedure can lead to false positive hemorrhage or false negative contrast extravasation interpretations in stroke patients possibly due to effects from the administered angiographic contrast. Additionally, MRI should be done both after 72 hours for confirmation and without gadolinium contrast as the effects of the gadolinium contrast and residual angiographic contrast could lead to misdiagnosis.

Gadolinium Deposition and Chronic Toxicity

Clinicians, radiologists, and patients should be aware of the most up-to-date data on the risks of gadolinium-based contrast agent (GBCA) administration. In this review, we discuss in vivo gadolinium retention, particularly brain tissue retention, and potential toxic effects. GBCA pharmacokinetics and biodistribution are reviewed briefly. Based on the more recent published literature and society guidelines, general safety recommendations for clinical practice are provided.

Is it Safe to Use Gadolinium-Based Contrast Agents in Mri?

Gadolinium-based contrast agents have greatly expanded the capability of magnetic resonance imaging and have been used extensively in neuroradiology over the past 30 years. When initially developed they were thought to be relatively harmless; it was later discovered they are associated with nephrogenic systemic fibrosis and should be used with caution in certain patient groups, especially those with renal failure. Lately it has been found that the use of these contrast agents may result in deposition of gadolinium in the brain even in patients with an intact blood-brain barrier. While this has not been shown to be associated with any clinical effects, a precautionary approach has been advised by the regulatory authorities. Here we review the development of the gadolinium contrast agents, their use and the advice related to this new information.

Photon-counting CT for simultaneous imaging of multiple contrast agents in the abdomen: An in vivo study

PURPOSE

To demonstrate the feasibility of spectral imaging using photon-counting detector (PCD) x-ray computed tomography (CT) for simultaneous material decomposition of three contrast agents in vivo in a large animal model.

METHODS

This Institutional Animal Care and Use Committee-approved study used a canine model. Bismuth subsalicylate was administered orally 24-72 h before imaging. PCD CT was performed during intravenous administration of 40-60 ml gadoterate meglumine; 3.5 min later, iopamidol 370 was injected intravenously. Renal PCD CT images were acquired every 2 s for 5-6 min to capture the wash-in and wash-out kinetics of the contrast agents. Least mean squares linear material decomposition was used to calculate the concentrations of contrast agents in the aorta, renal cortex, renal medulla and renal pelvis.

RESULTS

Using reference vials with known concentrations of materials, we computed molar concentrations of the various contrast agents during each phase of CT scanning. Material concentration maps allowed simultaneous quantification of both arterial and delayed renal enhancement in a single CT acquisition. The accuracy of the material decomposition algorithm in a test phantom was -0.4 ± 2.2 mM, 0.3 ± 2.2 mM for iodine and gadolinium solutions, respectively. Peak contrast concentration of gadolinium and iodine in the aorta, renal cortex, and renal medulla were observed 16, 24, and 60 s after the start each injection, respectively.

CONCLUSION

Photon-counting spectral CT allowed simultaneous material decomposition of multiple contrast agents in vivo. Besides defining contrast agent concentrations, tissue enhancement at multiple phases was observed in a single CT acquisition, potentially obviating the need for multiphase CT scans and thus reducing radiation dose.

Model and reconstruction of a K-edge contrast agent distribution with an X-ray photon-counting detector

Contrast-enhanced computed tomography (CECT) helps enhance the visibility for tumor imaging. When a high-Z contrast agent interacts with X-rays across its K-edge, X-ray photoelectric absorption would experience a sudden increment, resulting in a significant difference of the X-ray transmission intensity between the left and right energy windows of the K-edge. Using photon-counting detectors, the X-ray intensity data in the left and right windows of the K-edge can be measured simultaneously. The differential information of the two kinds of intensity data reflects the contrast-agent concentration distribution. K-edge differences between various matters allow opportunities for the identification of contrast agents in biomedical applications. In this paper, a general radon transform is established to link the contrast-agent concentration to X-ray intensity measurement data. An iterative algorithm is proposed to reconstruct a contrast-agent distribution and tissue attenuation background simultaneously. Comprehensive numerical simulations are performed to demonstrate the merits of the proposed method over the existing K-edge imaging methods. Our results show that the proposed method accurately quantifies a distribution of a contrast agent, optimizing the contrast-to-noise ratio at a high dose efficiency.

Opportunities for new CT contrast agents to maximize the diagnostic potential of emerging spectral CT technologies

The introduction of spectral CT imaging in the form of fast clinical dual-energy CT enabled contrast material to be differentiated from other radiodense materials, improved lesion detection in contrast-enhanced scans, and changed the way that existing iodine and barium contrast materials are used in clinical practice. More profoundly, spectral CT can differentiate between individual contrast materials that have different reporter elements such that high-resolution CT imaging of multiple contrast agents can be obtained in a single pass of the CT scanner. These spectral CT capabilities would be even more impactful with the development of contrast materials designed to complement the existing clinical iodine- and barium-based agents. New biocompatible high-atomic number contrast materials with different biodistribution and X-ray attenuation properties than existing agents will expand the diagnostic power of spectral CT imaging without penalties in radiation dose or scan time.

Gadolinium-Based Contrast Agent Accumulation and Toxicity: An Update

In current practice, gadolinium-based contrast agents have been considered safe when used at clinically recommended doses in patients without severe renal insufficiency. The causal relationship between gadolinium-based contrast agents and nephrogenic systemic fibrosis in patients with renal insufficiency resulted in new policies regarding the administration of these agents. After an effective screening of patients with renal disease by performing either unenhanced or reduced-dose-enhanced studies in these patients and by using the most stable contrast agents, nephrogenic systemic fibrosis has been largely eliminated since 2009. Evidence of in vivo gadolinium deposition in bone tissue in patients with normal renal function is well-established, but recent literature showing that gadolinium might also deposit in the brain in patients with intact blood-brain barriers caught many individuals in the imaging community by surprise. The purpose of this review was to summarize the literature on gadolinium-based contrast agents, tying together information on agent stability and animal and human studies, and to emphasize that low-stability agents are the ones most often associated with brain deposition.

CT Image Contrast of High-Z Elements: Phantom Imaging Studies and Clinical Implications

PURPOSE

To quantify the computed tomographic (CT) image contrast produced by potentially useful contrast material elements in clinically relevant imaging conditions.

MATERIALS AND METHODS

Equal mass concentrations (grams of active element per milliliter of solution) of seven radiodense elements, including iodine, barium, gadolinium, tantalum, ytterbium, gold, and bismuth, were formulated as compounds in aqueous solutions. The compounds were chosen such that the active element dominated the x-ray attenuation of the solution. The solutions were imaged within a modified 32-cm CT dose index phantom at 80, 100, 120, and 140 kVp at CT. To simulate larger body sizes, 0.2-, 0.5-, and 1.0-mm-thick copper filters were applied. CT image contrast was measured and corrected for measured concentrations and presence of chlorine in some compounds.

RESULTS

Each element tested provided higher image contrast than iodine at some tube potential levels. Over the range of tube potentials that are clinically practical for average-sized and larger adults-that is, 100 kVp and higher-barium, gadolinium, ytterbium, and tantalum provided consistently increased image contrast compared with iodine, respectively demonstrating 39%, 56%, 34%, and 24% increases at 100 kVp; 39%, 66%, 53%, and 46% increases at 120 kVp; and 40%, 72%, 65%, and 60% increases at 140 kVp, with no added x-ray filter.

CONCLUSION

The consistently high image contrast produced with 100-140 kVp by tantalum compared with bismuth and iodine at equal mass concentration suggests that tantalum could potentially be favorable for use as a clinical CT contrast agent.

Energy Window Optimization for X-Ray K-Edge Tomographic Imaging

GOAL

In K-edge tomographic imaging with photon counting detectors, the energy window width of photon counting detectors significantly affects the signal-to-noise ratio (SNR) of measured intensity data and the contrast-to-noise ratio (CNR) of reconstructed images. In this paper, we present an optimization method to determine an optimal window width around a K-edge for optimal SNR and CNR.

METHODS

An objective function is designed to describe SNR of the projection data based on the Poisson distribution of detected X-ray photons. Then, a univariate optimization method is applied to obtain an X-ray energy window width.

RESULTS

Numerical simulations are performed to evaluate the proposed method, and the results show that the optimal energy window width obtained from the proposed method produces not only optimal SNR data in the projection domain but also optimal CNR values in the image domain.

CONCLUSION

The proposed method in the projection domain can determine an optimal energy window width for X-ray photon counting imaging, and achieve optimality in both projection and image domains.

SIGNIFICANCE

Our study provides a practical way to determine the optimal energy window width of photon counting detectors, which helps improve contrast resolution for X-ray K-edge tomographic imaging.

Optimization of K-edge imaging with spectral CT

PURPOSE

Spectral∕multienergy CT has the potential to distinguish different materials by K-edge characteristics. K-edge imaging involves the two energy bins on both sides of a K-edge. The authors propose a K-edge imaging optimization model to determine these two energy bins.

METHODS

K-edge image contrast with spectral CT depends on the specifications of the two energy bins on both sides of a K-edge in the attenuation profile of a relatively high atomic number material. The wider the energy bin width is, the lower the noise level is, and the poorer the reconstructed image contrast is. Here the authors introduce the signal difference to noise ratio (SDNR) criterion to optimize the energy bin widths on both sides of the K-edge for the maximum SDNR.

RESULTS

The authors study K-edge imaging with spectral CT, analyze the effect of K-edge energy bins on the resultant image quality, and establish guidelines for the optimization of energy thresholds. In simulation, the authors demonstrate that our K-edge imaging optimization approach maximizes SDNR in reconstructed images.

CONCLUSIONS

This proposed approach can be readily generalized to deal with more general settings and determine the best energy bins for K-edge imaging.

Temporal and spectral imaging with micro-CT

PURPOSE

Micro-CT is widely used for small animal imaging in preclinical studies of cardiopulmonary disease, but further development is needed to improve spatial resolution, temporal resolution, and material contrast. We present a technique for visualizing the changing distribution of iodine in the cardiac cycle with dual source micro-CT.

METHODS

The approach entails a retrospectively gated dual energy scan with optimized filters and voltages, and a series of computational operations to reconstruct the data. Projection interpolation and five-dimensional bilateral filtration (three spatial dimensions + time + energy) are used to reduce noise and artifacts associated with retrospective gating. We reconstruct separate volumes corresponding to different cardiac phases and apply a linear transformation to decompose these volumes into components representing concentrations of water and iodine. Since the resulting material images are still compromised by noise, we improve their quality in an iterative process that minimizes the discrepancy between the original acquired projections and the projections predicted by the reconstructed volumes. The values in the voxels of each of the reconstructed volumes represent the coefficients of linear combinations of basis functions over time and energy. We have implemented the reconstruction algorithm on a graphics processing unit (GPU) with CUDA. We tested the utility of the technique in simulations and applied the technique in an in vivo scan of a C57BL∕6 mouse injected with blood pool contrast agent at a dose of 0.01 ml∕g body weight. Postreconstruction, at each cardiac phase in the iodine images, we segmented the left ventricle and computed its volume. Using the maximum and minimum volumes in the left ventricle, we calculated the stroke volume, the ejection fraction, and the cardiac output.

RESULTS

Our proposed method produces five-dimensional volumetric images that distinguish different materials at different points in time, and can be used to segment regions containing iodinated blood and compute measures of cardiac function.

CONCLUSIONS

We believe this combined spectral and temporal imaging technique will be useful for future studies of cardiopulmonary disease in small animals.

Comparison of quantitative parameters in cervix cancer measured by dynamic contrast-enhanced MRI and CT

Cervical tumors of 38 cervix cancer patients were scanned by T(1)-weighted dynamic contrast enhanced (DCE) MRI and then by DCE-CT on the same day. Gadodiamide and iohexol were respectively used as the low-molecular-weight contrast agent in DCE-MRI and DCE-CT. Under an extended Tofts model, DCE-MRI data were analyzed using either individual arterial input functions estimated by a multiple reference tissue method or a population arterial input function by Parker et al., whereas DCE-CT data were analyzed using the arterial input function directly measured from the external iliac arteries. The derived quantitative parameters of cervical tumors were compared between DCE-MRI and DCE-CT. When using the individual multiple reference tissue method arterial input functions to analyze the DCE-MRI data, the correlation coefficients between DCE-MRI- and DCE-CT-derived parameters were, respectively, back-flux rate constant (r = 0.80), extravascular extracellular fractional volume (r = 0.73), contrast agent transfer rate (r = 0.62), and blood plasma volume (r = 0.32); when using the Parker population arterial input function, the correlation coefficients were back-flux rate constant (r = 0.79), extravascular extracellular fractional volume (r = 0.77), contrast agent transfer rate (r = 0.63), and blood plasma volume (r = 0.58). Tumor parametric maps derived by DCE-MRI and DCE-CT had very similar morphologies. However, the means of most derived quantitative parameters were significantly different between the two imaging methods. Close correlation of quantitative parameters derived from two independent imaging modalities suggests both are measuring similar tumor physiologic variables.

Gold nanoparticles functionalized by gadolinium-DTPA conjugate of cysteine as a multimodal bioimaging agent

The synthesis and characterization of gold nanoparticles coated with Gd-chelate (Au@GdL), where L is a conjugate of DTPA and cysteine, is described. These particles are obtained by the replacement of citrate from the gold nanoparticle surfaces with gadolinium chelate (GdL). The average size of Au@GdL is 14 nm with a loading of GdL reaching up to 2.9x10(3) per particles, and they demonstrate very high R1 relaxivity (approximately 10(5) mM(-1) s(-1)) as well as X-ray attenuation. The R1 relaxivity per [Gd] is 17.9 mM(-1) s(-1). The present system also exhibits macrophage-specific property, as demonstrated by histological and TEM images as well as CT and MR, rendering itself as a new class of T1 multimodal CT/MR contrast agent.

Colloidal gold nanoparticles as a blood-pool contrast agent for X-ray computed tomography in mice

OBJECTIVES

To present the pharmacokinetics and computed tomographic imaging efficacy of colloidal gold nanoparticles (AuNPs) as a blood-pool agent for x-ray computed tomography (CT).

METHODS AND MATERIALS

To prepare the colloidal AuNPs, gold nanocrystals were modified using sulfhydrated polyethylene glycol (PEG). Cytotoxicity and histopathologic tests were carried out for toxicity evaluation. Six adult Balb/c mice underwent microcomputed tomography scans after injection of colloidal AuNPs (2.5 micromol Au/g body weight). Four mice with HT-1080 tumors were imaged for visualization of the tumor vasculature.

RESULTS

The PEG coated colloidal AuNPs appeared as spherical nanoparticles with 38-nm diameters. The AuNPs-PEG showed a biocompatibility without toxicity in the mice. We identified a stable imaging window for visualizing the vasculature system, immediately to 24 hours after injection. Microcomputed tomography imaging using AuNPs-PEG clearly visualized the tumor vascular structures.

CONCLUSION

Colloidal AuNPs show potential as a blood-pool agent for x-ray CT imaging.

One-molar gadolinium chelate (gadobutrol) as a contrast agent for CT angiography of the thoracic and abdominal aorta

The aim of our study was to evaluate the use of a 1-molar gadolinium chelate (gadobutrol) as an alternative contrast medium for computed tomography angiography (CTA) exams of the aorta. CTA exams of the thoracic and/or abdominal aorta were performed on 15 patients with contraindications for the use of iodine who were not suitable for magnetic resonance examinations. The exams were performed with a 16-detector row scanner, injecting a mean dose of 0.37 mmol Gd/kg of body weight at a flow rate of 4 ml/s. Creatinine levels were obtained prior to the exam in patients with impaired renal function, and 24 and 48 h afterwards. The mean attenuation values obtained in the middle ascending and middle descending thoracic aorta were 202.3 and 216.8, respectively. The mean HU values of the abdominal aorta were 210.4 at the level of the renal arteries and 186.8 in the aortic bifurcation. All the exams were considered diagnostically adequate. No significant increase in serum creatinine was observed 24 and 48 h after the exam. We believe that gadobutrol could be an alternative contrast medium for CTA exams with 16-detector row scanners in patients with contraindications for iodinated contrast medium.

Gum arabic as a phytochemical construct for the stabilization of gold nanoparticles: in vivo pharmacokinetics and X-ray-contrast-imaging studies

Gold nanoparticles (AuNPs) have exceptional stability against oxidation and therefore will play a significant role in the advancement of clinically useful diagnostic and therapeutic nanomedicines. Despite the huge potential for a new generation of AuNP-based nanomedicinal products, nontoxic AuNP constructs and formulations that can be readily administered site-specifically through the intravenous mode, for diagnostic imaging by computed tomography (CT) or for therapy via various modalities, are still rare. Herein, we report results encompassing: 1) the synthesis and stabilization of AuNPs within the nontoxic phytochemical gum-arabic matrix (GA-AuNPs); 2) detailed in vitro analysis and in vivo pharmacokinetics studies of GA-AuNPs in pigs to gain insight into the organ-specific localization of this new generation of AuNP vector, and 3) X-ray CT contrast measurements of GA-AuNP vectors for potential utility in molecular imaging. Our results demonstrate that naturally occurring GA can be used as a nontoxic phytochemical construct in the production of readily administrable biocompatible AuNPs for diagnostic and therapeutic applications in nanomedicine.

MR contrast agents: Physical and pharmacologic basics

Since approval of the first magnetic resonance (MR) contrast agent was granted in 1988, there has been remarkable growth in the utilization of intravenous gadolinium (Gd)-based agents. Currently it is estimated that nearly half of all MR studies performed are contrast-enhanced. Despite containing a toxic heavy metal, these agents have proven to be not only an effective diagnostic adjunct to non-enhanced MRI, but also remarkably well tolerated and safe. As a result, conventional wisdom has been that MR contrast media are "biologically inert," a notion that is clearly false. Ultimately, it is the radiologist's responsibility to understand the potential adverse effects of Gd-based agents and the special situations in which they are likely to occur; however, the basic pharmacology of contrast agents is generally not included in medical school curricula or formally taught in residency. The purpose of this review is to discuss the mechanism of action of MR contrast agents and relevant aspects of their clinical pharmacology, including effects on the cardiovascular and renal systems, potential laboratory errors, and special situations involving women and children. We also briefly discuss the issue of nephrogenic systemic fibrosis (NSF).

Spectral Coronary Multidetector Computed Tomography Angiography

OBJECTIVE

To assess ex vivo specimens of atherosclerotic coronary arteries by dual energy (DE) multidetector computed tomography (MDCT) imaging, and to correlate depicted vessel lumen morphology and detected tissue characteristics with histopathologic analysis.

METHODS

Coronary arteries were imaged on a 16-slice MDCT using a DE protocol consisting of a 90- and 140-kV scan. Coronary arteries were perfused with iodine- and gadolinium-based contrast agents. The DE K-edge subtractions were performed. Regions-of-interest were placed on histopathologically/radiographically-matched vascular lumen and wall, fibromuscular and calcified plaque, and fat tissues. Vascular/tissue contrast-to-noise ratios (CNR) were calculated, and their dependence on tissue type and contrast agent type was statistically evaluated.

RESULTS

Tissue CNR analysis confirmed that all tissue types were successfully distinguished. Vascular wall and fibromuscular plaque achieved a significant increase in CNR ratios when DE techniques were used compared with 140 kV protocols.

CONCLUSIONS

Spectral DE MDCT imaging of ex vivo atherosclerotic coronary arteries allows successful tissue characterization and enhances depiction of coronary lumen.

K-edge digital subtraction imaging with dichromatic x-ray sources: SNR and dose studies

The aim of the present work is to analytically evaluate the signal to noise ratio (SNR) and the delivered dose in K-edge digital subtraction imaging (KES) using two types of x-ray sources: a monochromatic x-ray source (available at synchrotron radiation facilities and considered as gold standard) and a quasi-monochromatic compact source. The energy separation DeltaE between the two monochromatic beams is 1 keV and 4 keV for the two sources, respectively. The evaluation has been performed for both radiography and computed tomography. Different geometries have been studied to mimic clinical situations. In mammography, a pathology perfused by a contrast agent has been modelled; in angiography, a vessel superimposed to a ventricle or a stand-alone artery stenosis has been studied. The SNR and the skin dose have been calculated as a function of the detail diameter, the contrast agent (iodine and gadolinium), and its concentration in the tissues. Results show that for DeltaE = 4 keV a slightly higher delivered dose is required to obtain the same SNR with respect to DeltaE < 1 keV. A similar study has been performed for KES-CT. Computer simulations of CT images performed with Snark software are shown to validate the analytical calculations.

Multimodal Contrast Agent for Combined Computed Tomography and Magnetic Resonance Imaging Applications

OBJECTIVE

The objective of this study was to examine the feasibility of a multimodal system to effectively induce and maintain contrast enhancement in both computed tomography (CT) and magnetic resonance (MR) for radiation therapy applications.

MATERIALS AND METHODS

The physicochemical characteristics of a liposome-encapsulated iohexol and gadoteridol formulation were assessed in terms of agent loading efficiencies, size and morphology, in vitro stability, and release kinetics. The imaging properties of the liposome formulation were assessed based on T1 and T2 relaxivity measurements and in vitro CT and MR imaging in a phantom. A preliminary imaging-based evaluation of the in vivo stability of this multimodal contrast agent was also performed in a lupine model.

RESULTS

The average agent loading levels achieved were 26.5+/-3.8 mg/mL for iodine and 6.6+/- 1.5 mg/mL for gadolinium. These concentrations correspond to approximately 10% of that found in the commercially available preparations of each of these agents. However, this liposome-based formulation is expected to have a smaller volume of distribution and prolonged circulation lifetime in vivo. This multimodal system was found to have high agent retention in vitro, which translated into maintained contrast enhancement (up to 3 days) and stability in vivo.

CONCLUSIONS

This study demonstrated the feasibility of engineering a multimodal contrast agent with prolonged contrast enhancement in vivo for use in CT and MR. This contrast agent may serve as a valuable tool for cardiovascular imaging as well as image registration and guidance applications in radiation therapy.

Gadolinium-Enhanced Computed Tomographic Angiography: Current Status

This article reviews the research to date, as well as our clinical experience from two institutions, on gadolinium-enhanced computed tomographic angiography (gCTA) for imaging the body. gCTA may be an appropriate examination for the small percentage of patients who would benefit from noninvasive vascular imaging, but who have contraindications to both iodinated contrast and magnetic resonance imaging. gCTA is more expensive than CTA with iodinated contrast, due to the dose of gadolinium administered, and gCTA has limitations compared with CTA with iodinated contrast, in that parenchymal organs are not optimally enhanced at doses of 0.5 mmol/kg or lower. However, in our experience, gCTA has been a very useful problem-solving examination in carefully selected patients. With the advent of 16-64 detector CT, in combination with bolus tracking, we believe that the overall dose of gadolinium needed for diagnostic CTA examinations, while relatively high, can be safely administered.

Gadolinium-enhanced MDCT angiography of the abdomen: feasibility and limitations

OBJECTIVE

The purpose of this study was to evaluate a protocol for gadolinium-enhanced MDCT angiography of the abdomen and to identify technical parameters that optimize image quality.

CONCLUSION

The degree of enhancement and image quality achieved using this gadolinium-enhanced MDCT angiography appear adequate for angiographic evaluation of the abdominal aorta and its major branches.

Safety and Effectiveness of Gadolinium-enhanced Multi–Detector Row Spiral CT Angiography of the Chest: Preliminary Results in 37 Patients with Contraindications to Iodinated Contrast Agents

PURPOSE

To prospectively evaluate the safety and effectiveness of gadolinium-enhanced multi-detector row spiral computed tomographic (CT) angiography of the pulmonary circulation by using two gadolinium doses in patients with contraindications to iodinated contrast agents.

MATERIALS AND METHODS

Study was approved by the Ethics Committee, and written informed consent was obtained. Thirty-seven patients (20 men, 17 women) with contraindications to iodinated contrast agents (allergic reactions, n = 27; impaired renal function, n = 10) underwent CT angiography of the pulmonary circulation in search of acute pulmonary embolism (n = 28) or for management of tumoral disease (n = 9). CT angiography was performed (a) with four-detector row (n = 19) or 16-detector row (n = 18) scanners; (b) at randomly assigned gadolinium doses of either 0.3 mmol per kilogram of body weight (n = 19) or 0.4 mmol/kg (n = 18); and (c) with a systematic evaluation of clinical and biologic tolerance of gadolinium. Comparison of percentages between group 1 and group 2 scans was performed with the chi2 or the Fisher exact test. An unpaired Wilcoxon rank sum test was used for numeric variables. P < .05 was considered to indicate a significant difference.

RESULTS

The mean (+/- standard deviation) volume of gadopentetate dimeglumine administered in the overall study group was 48 mL +/- 9.6 (range, 29-65 mL). The level of maximal enhancement in the pulmonary arteries was significantly higher in group 2 than in group 1 (215.8 HU +/- 95 vs 141.3 HU +/- 44) (P = .02) and was maintained throughout the entire region of interest in a greater number of examinations in group 2 than in group 1 (n = 16 [89%] vs n = 2 [10.5%]) (P < .0001). The number of diagnostic CT angiograms was significantly higher in group 2 than in group 1 (n = 17 [94%] vs n = 13 [68%]) (P = .007). Significant but transient reduction of creatinine clearance was observed in one patient with preexisting moderate chronic renal failure (0.3 mmol/kg gadolinium dose).

CONCLUSION

High-quality gadolinium-enhanced CT angiograms require the use of 16-detector row CT technology; the doses administered did not alter the renal function except transiently in one patient.