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The characteristics of a hepatobiliary contrast agent are specific liver uptake and excretion via the biliary system. The paramagnetic substance (e.g. manganese, gadolinium) is taken up by normal hepatocytes. Diseased liver tissue did not include hepatocytes or their function is disturbed. Therefore, the signal of healthy liver tissue increases on T1 weighted sequences, but not in the liver lesions.
Another type of liver imaging contrast agent is superparamagnetic iron oxide. These particles accumulate in the reticuloendothelial system (RES) of the liver, and darken the healthy liver tissue in T2 weighted images. RES cells (including Kup ffer cells) are existing in healthy liver tissue, in altered tissue with reduced RES activity or without RES cells the contrast agent concentration is also low or not existing, which improves the liver to lesion contrast.
Benefits of hepatobiliary contrast agents:
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Liver lesions (e.g., tumor, metastases, haemangioma etc.) are better detectable and to characterize.
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These contrast agents are useful to analyze and evaluate the liver function (in cases of diffuse liver diseases e.g., cirrhosis).
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Imaging of the gallbladder and biliary system is improved.
Di fferences of a hepatobiliary contrast agent compared with a targeted contrast agent for Kup ffer cells:
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The higher number of hepatocytes than Kup ffer cells improves the uptake e ffectiveness of the contrast agent.
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Hepatobiliary contrast agents enable a better opacification of the biliary ducts and the gallbladder caused by the biliary excretion.
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Hepatobiliary contrast media are fast excreted agents. RES targeted contrast agents remain longer in the body, a fact that can increase possible side e ffects.
See also Superparamagnetic Contrast Agents, Hepatobiliary Chelates, Liver Imaging, Endoremâ„¢, Primovistâ„¢, and Classifications, Characteristics, etc.
See also the related poll result: ' The development of contrast agents in MRI is' | | | | • View the DATABASE results for 'Hepatobiliary Contrast Agents' (11).
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(PC) Phase contrast sequences are the basis of MRA techniques utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image. Spins that are moving along the direction of a magnetic field gradient receive a phase shift proportional to their velocity.
In a phase contrast sequence two data sets with a di fferent amount of flow sensitivity are acquired. This is usually accomplished by applying gradient pairs, which sequentially dephase and then rephase spins during the sequence. Both 2D and 3D acquisition techniques can be applied with phase contrast MRA.
The first data set is acquired with a flow compensated sequence, i. e. without flow sensitivity. The second data set is acquired with a flow sensitive sequence. The amount of flow sensitivity is controlled by the strength of the bipolar gradient pulse pair, which is incorporated into the sequence. Stationary tissue undergoes no e ffective phase change after the application of the two gradients. Caused by the di fferent spatial localization of flowing blood to stationary tissue, it experiences a di fferent size of the second bipolar gradient compared to the first. The result is a phase shift.
The raw data from the two data sets are subtracted. By comparing the phase of signals from each location in the two sequences the exact amount of motion induced phase change can be determined to have a map where pixel brightness is proportional to spatial velocity.
Phase contrast images represent the signal intensity of the velocity of spins at each point within the field of view. Regions that are stationary remain black while moving regions are represented as grey to white.
The phase shift is proportional to the spin's velocity, and this allows the quantitative assessment of flow velocities.
The di fference MRI signal has a maximum value for opposite directions. This velocity is typically referred to as venc, and depends on the pulse amplitude and distance between the gradient pulse pair. For velocities larger than venc the di fference signal is decreased constantly until it gets zero. Therefore, in a phase contrast angiography it is important to correctly set the venc of the sequence to the maximum flow velocity which is expected during the measurement. High venc factors of the PC angiogram (more than 40 cm/sec) will selectively image the arteries ( PCA - arteriography), whereas a venc factor of 20 cm/sec will perform the veins and sinuses (PCV or MRV - venography).
See also Flow Quantification, Contrast Enhanced MR Venography, Time of Flight Angiography, Time Resolved Imaging of Contrast Kinetics. | | | | | | • View the DATABASE results for 'Phase Contrast Sequence' (5).
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( BOLD) In MRI the changes in blood oxygenation level are visible. Oxyhaemoglobin (the principal haemoglobin in arterial blood) has no substantial magnetic properties, but deoxyhaemoglobin (present in the draining veins after the oxygen has been unloaded in the tissues) is strongly paramagnetic. It can thus serve as an intrinsic paramagnetic contrast agent in appropriately performed brain MRI. The concentration and relaxation properties of deoxyhaemoglobin make it a susceptibility , e.g. T2 relaxation e ffective contrast agent with little e ffect on T1 relaxation.
During activation of the brain, the oxygen consumption of the local tissue increase by approximately 5% with that the oxygen tension will decrease. As a consequence, after a short period of time vasodilatation occurs, resulting in a local increase of blood volume and flow by 20 - 40%. The incommensurate change in local blood flow and oxygen extraction increases the local oxygen level.
By using T2 weighted gradient echo EPI sequences, which are highly susceptibility sensitive and fast enough to capture the three-dimensional nature of activated brain areas will show an increase in signal intensity as oxyhaemoglobin is diamagnetic and deoxyhaemoglobin is paramagnetic. Other MR pulse sequences, such as spoiled gradient echo pulse sequences are also used.
As the e ffects are subtle and of the order of 2% in 1.5 T MR imaging, sophisticated methodology, paradigms and data analysis techniques have to be used to consistently demonstrate the e ffect.
As the BOLD e ffect is due to the deoxygenated blood in the draining veins, the spatial localization of the region where there is increased blood flow resulting in decreased oxygen extraction is not as precisely defined as the morphological features in MRI. Rather there is a physiological blurring, and is estimated that the linear dimensions of the physiological spatial resolution of the BOLD phenomenon are around 3 mm at best. | | | | • View the DATABASE results for 'Blood Oxygenation Level Dependent Contrast' (6).
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Contrast agents are chemical substances introduced to the anatomical or functional region being imaged, to increase the di fferences between di fferent tissues or between normal and abnormal tissue, by altering the relaxation times. MRI contrast agents are classified by the di fferent changes in relaxation times after their injection.
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Negative contrast agents (appearing predominantly dark on MRI) are small particulate aggregates often termed superparamagnetic iron oxide ( SPIO). These agents produce predominantly spin spin relaxation e ffects (local field inhomogeneities), which results in shorter T1 and T2 relaxation times.
SPIO's and ultrasmall superparamagnetic iron oxides ( USPIO) usually consist of a crystalline iron oxide core containing thousands of iron atoms and a shell of polymer, dextran, polyethyleneglycol, and produce very high T2 relaxivities. USPIOs smaller than 300 nm cause a substantial T1 relaxation. T2 weighted e ffects are predominant.
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A special group of negative contrast agents (appearing dark on MRI) are perfluorocarbons ( perfluorochemicals), because their presence excludes the hydrogen atoms responsible for the signal in MR imaging.
The design objectives for the next generation of MR contrast agents will likely focus on prolonging intravascular retention, improving tissue targeting, and accessing new contrast mechanisms. Macromolecular paramagnetic contrast agents are being tested worldwide. Preclinical data shows that these agents demonstrate great promise for improving the quality of MR angiography, and in quantificating capillary permeability and myocardial perfusion.
Ultrasmall superparamagnetic iron oxide ( USPIO) particles have been evaluated in multicenter clinical trials for lymph node MR imaging and MR angiography, with the clinical impact under discussion. In addition, a wide variety of vector and carrier molecules, including antibodies, peptides, proteins, polysaccharides, liposomes, and cells have been developed to deliver magnetic labels to specific sites. Technical advances in MR imaging will further increase the efficacy and necessity of tissue-specific MRI contrast agents.
See also Adverse Reaction and Nephrogenic Systemic Fibrosis.
See also the related poll result: ' The development of contrast agents in MRI is' | | | | | | | | | | | • View the DATABASE results for 'Contrast Agents' (122).
| | | • View the NEWS results for 'Contrast Agents' (25).
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Brain imaging method may aid mild traumatic brain injury diagnosis Tuesday, 16 January 2024 by parkinsonsnewstoday.com | | |
A Targeted Multi-Crystalline Manganese Oxide as a Tumor-Selective Nano-Sized MRI Contrast Agent for Early and Accurate Diagnosis of Tumors Thursday, 18 January 2024 by www.dovepress.com | | |
FDA Approves Gadopiclenol for Contrast-Enhanced Magnetic Resonance Imaging Tuesday, 27 September 2022 by www.pharmacytimes.com | | |
How to stop using gadolinium chelates for magnetic resonance imaging: clinical-translational experiences with ferumoxytol Saturday, 5 February 2022 by www.ncbi.nlm.nih.gov | | |
Estimation of Contrast Agent Concentration in DCE-MRI Using 2 Flip Angles Tuesday, 11 January 2022 by pubmed.ncbi.nlm.nih.gov | | |
Manganese enhanced MRI provides more accurate details of heart function after a heart attack Tuesday, 11 May 2021 by www.news-medical.net | | |
Gadopiclenol: positive results for Phase III clinical trials Monday, 29 March 2021 by www.pharmiweb.co | | |
Gadolinium-Based Contrast Agents Hypersensitivity: A Case Series Friday, 4 December 2020 by www.dovepress.com | | |
Polysaccharide-Core Contrast Agent as Gadolinium Alternative for Vascular MR Monday, 8 March 2021 by www.diagnosticimaging.com | | |
Water-based non-toxic MRI contrast agents Monday, 11 May 2020 by chemistrycommunity.nature.com | | |
New method to detect early-stage cancer identified by Georgia State, Emory research team Friday, 7 February 2020 by www.eurekalert.org | | |
Researchers Brighten Path for Creating New Type of MRI Contrast Agent Friday, 7 February 2020 by www.newswise.com | | |
Manganese-based MRI contrast agent may be safer alternative to gadolinium-based agents Wednesday, 15 November 2017 by www.eurekalert.org | | |
Sodium MRI May Show Biomarker for Migraine Friday, 1 December 2017 by psychcentral.com | | |
A natural boost for MRI scans Monday, 21 October 2013 by www.eurekalert.org | | |
For MRI, time is of the essence A new generation of contrast agents could make for faster and more accurate imaging Tuesday, 28 June 2011 by scienceline.org |
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