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 'Negative Contrast Agents' 
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Contrast AgentsForum -
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Contrast agents are chemical substances introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by altering the relaxation times. MRI contrast agents are classified by the different changes in relaxation times after their injection.
Positive contrast agents cause a reduction in the T1 relaxation time (increased signal intensity on T1 weighted images). They (appearing bright on MRI) are typically small molecular weight compounds containing as their active element Gadolinium, Manganese, or Iron. All of these elements have unpaired electron spins in their outer shells and long relaxivities.
Some typical contrast agents as gadopentetate dimeglumine, gadoteridol, and gadoterate meglumine are utilized for the central nervous system and the complete body; mangafodipir trisodium is specially used for lesions of the liver and gadodiamide for the central nervous system.
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 effects (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 effects are predominant.
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'
 
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Radiology-tip.comContrast Agents,  Safety of Contrast Agents
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Radiology-tip.comUltrasound Contrast Agents,  Ultrasound Contrast Agent Safety
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Further Reading:
  Basics:
New guidelines urge caution on use of contrast agents during MR scans
Tuesday, 8 August 2017   by www.dotmed.com    
Manganese-based MRI contrast agents: past, present and future
Friday, 4 November 2011   by www.ncbi.nlm.nih.gov    
A safer approach for diagnostic medical imaging
Monday, 29 September 2014   by www.eurekalert.org    
Drastic market changes with MRI contrast media and PET radiopharmaceuticals emerging as most promising segments
Thursday, 21 October 2004   by www.news-medical.net    
  News & More:
Sodium MRI May Show Biomarker for Migraine
Friday, 1 December 2017   by psychcentral.com    
Manganese-based MRI contrast agent may be safer alternative to gadolinium-based agents
Wednesday, 15 November 2017   by www.eurekalert.org    
3D 'bone maps' could spot early signs of osteoporosis
Monday, 27 February 2017   by www.gmanetwork.com    
New Study Sheds Light on Safety of Gadolinium-Based Contrast Agents
Wednesday, 29 November 2017   by www.empr.com    
Engineered atherosclerosis-specific zinc ferrite nanocomplex-based MRI contrast agents
Monday, 18 January 2016   by 7thspace.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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Negative Oral Contrast AgentsForum -
related threadsInfoSheet: - Contrast Agents - 
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Categories of negative oral contrast agents:
Gastrointestinal diamagnetic contrast agents
Gastrointestinal superparamagnetic contrast agents
Perfluorochemicals
Negative oral contrast media are usually based on superparamagnetic particles and act by inducing local field inhomogeneities, which results in shortening of both T1 and T2 relaxation times. Superparamagnetic contrast agents have predominant T2 weighted effects. Biphasic contrast media are agents that have different signal intensities on different sequences, depending on the concentration at which they are used.
Suitable materials for oral contrast agents should have little or no absorption by the stomach or intestines, complete excretion, no motion or susceptibility artifacts, affordability, and uniform marking of the gastrointestinal tract. Benefits of negative oral contrast agents are the reduction of ghosting artifacts caused by the lack of signal. Superparamagnetic iron oxides produce also in low concentrations a noticeable signal loss; but can generate susceptibility artifacts especially in gradient echo sequences. Perfluorochemicals do not dilute in the bowel because they are not miscible with water.
High cost, poor availability, and limited evaluations of side effects are possible disadvantages.
Negative oral contrast agents are used e.g., in MRCP, where the ingestion of 600-900 ml of SPIO cancels out the signal intensity of the lumen (in addition after the injection of a gadolinium-based contrast medium, the enhancement of the inflammatory tissues is clearer seen), and in MR abdominal imaging of Crohn's disease in combination with mannitol.


Contrast Guidance
Blueberry or pineapple juices are useable for examinations of the pancreas (MRCP, upper abdominal imaging) as cheep contrast agents, because of the content of magnetic substances (e.g. manganese). See also Ferristene, Ferumoxsil, Oral Magnetic Particles, Gastrointestinal Imaging.
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Further Reading:
  Basics:
Components of Oral Contrast Agent for Magnetic Resonance Imaging(.pdf)
   by www.ffcr.or.jp    
MRI Resources 
Software - Online Books - Non-English - MRI Physics - Diffusion Weighted Imaging - Spine MRI
 
Oral Magnetic ParticlesInfoSheet: - Contrast Agents - 
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(OMP) Various mixtures of gastrointestinal diamagnetic contrast agents for use as negative oral contrast agents to darken the bowel.

Drug Information and Specification
NAME OF COMPOUND Barium suspensions, clay mineral particles, iron oxide crystals
DEVELOPER
CENTRAL MOIETY Ba3+, Al3+, Si2+, Fe
CONTRAST EFFECT T2, Predominantly negative enhancement
RELAXIVITY T2-shortening
PHARMACOKINETIC
OSMOLALITY
CONCENTRATION Various mixtures
DOSAGE Depending on the mixture
PREPARATION Depending on the product
INDICATION Gastrointestinal bowel marking
DEVELOPMENT STAGE No clinical development
DISTRIBUTOR
PRESENTATION
DO NOT RELY ON THE INFORMATION PROVIDED HERE, THEY ARE NOT A SUBSTITUTE FOR THE ACCOMPANYING PACKAGE INSERT!
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Oral Contrast AgentsForum -
related threadsInfoSheet: - Contrast Agents - 
Intro, Overview, 
Characteristics, 
Types of, 
etc.MRI Resource Directory:
 - Contrast Agents -
 
A limitation of abdominal MRI can be the assessment of malignancies by difficulties to distinguish bowel from other organs or malignant masses. The use of oral contrast agents can reduce this problem. Properties of an ideal oral contrast agent are little or no absorption by the stomach or intestines, complete excretion, no motion or susceptibility artifacts, and uniform marking of the GI tract.
Gastrointestinal MRI contrast agents are divided in materials with bright appearance or dark appearance. The choice of a negative or a positive oral contrast agent depends on the specific problem or the pulse sequence.
See also Positive Oral Contrast Agents, Negative Oral Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Gastrointestinal Paramagnetic Contrast Agents and Gastrointestinal Superparamagnetic Contrast Agents.

See also the related poll result: 'The development of contrast agents in MRI is'
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• View the DATABASE results for 'Oral Contrast Agents' (17).Open this link in a new window

 
Further Reading:
  News & More:
Usefulness of MR Imaging for Diseases of the Small Intestine: Comparison with CT
2000   by www.ncbi.nlm.nih.gov    
Nottingham scientists exploit MRI technology to assist in the treatment of IBS
Thursday, 9 January 2014   by www.news-medical.net    
MRI Resources 
Spine MRI - Pacemaker - Databases - Jobs - Nerve Stimulator - MRI Technician and Technologist Schools
 
Combination Oral Contrast AgentsInfoSheet: - Contrast Agents - 
Intro, Overview, 
Characteristics, 
Types of, 
etc.MRI Resource Directory:
 - Contrast Agents -
 
Biphasic oral contrast agents may produce either high or low signal intensities depending on the pulse sequence used, for example low signal intensity on T1 weighted MR images and high signal intensity on T2 weighted images. The combination of different oral contrast agents can generate a macroscopic cancellation of negative and positive magnetic susceptibility, thereby eliminating susceptibility artifacts.
Possible combinations are e.g., ferric ammonium citrate and corn oil, or ferrous sulfate emulsified with baby formula. Paramagnetic agents combined with oil emulsion may be used in MRI as positive abdominal contrast agents. The combination of diamagnetic barium sulfate and superparamagnetic iron oxide (SPIO) in one suspension may be a useful negative contrast agent.
See also Gastrointestinal Paramagnetic Contrast Agents, Gastrointestinal Superparamagnetic Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Gastrointestinal Imaging.

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Further Reading:
  Basics:
Usefulness of MR Imaging for Diseases of the Small Intestine: Comparison with CT
2000   by www.ncbi.nlm.nih.gov    
MRI Resources 
Liver Imaging - Implant and Prosthesis - Jobs - Veterinary MRI - Colonography - Used and Refurbished MRI Equipment
 
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