"combination agents containing both "1." and "2.""
"combination agents containing both "1." and "2.""
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Positive GI contrast agents can be divided into three categories:"
"Proposed paramagnetic, positive GI contrast agents include ferric chloride, ferric ammonium citrate, and gadolinium-DTPA (with and without mannitol). Paramagnetic materials cause both T1 and T2 shortening."
"At low concentrations used for bowel opacification, the T1 shortening dominates the signal intensity. This results in high intensity on T1-weighted, T2-weighted and gradient echo images. At high concentrations, T2 shortening causes decreased signal in all but very short echo sequences. This resembles the effect seen with superparamagnetic iron oxide (see negative GI contrast agents). At intermediate concentrations, a mixture of T1 and T2 shortening results in increased signal on T1-weighted images and decreased signal on T2-weighted images."
"Short T1 relaxation time GI contrast agents include mineral oil, oil emulsions, and sucrose polyester. In these materials, protons contained in -CH2- groups relax at a faster rate than those in water resulting in a short T1 time. This gives a bright signal in the bowel on T1-weighted sequences. Of these materials only oil emulsions have been used successfully in humans. These are palatable and produce homogeneous opacification of the stomach and small bowel, but are absorbed in the distal small bowel and fail to fill the colon. This is circumvented by using a contrast enema when the colon must be better visualised."
"Negative GI contrast materials can be divided into three categories:"
"Two diamagnetic agents have been tested for use as a negative GI contrast agent. The first was a combination of clay minerals found in a popular antidiarrhoeal medication, Kaopectate. This mixture of kaolin and bentonite is thought to facilitate the relaxation rate of protons in water molecules. The water molecules next to the surface of the clay are continually exchanging position with molecules away from the surface resulting in phase dispersion that also causes loss of signal. When used in volunteers, this mixture causes loss of signal in the stomach and duodenum resulting in improved visualisation of the pancreas. Distribution in the small bowel is reported to be nonuniform 3,4."
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Perfluorochemicals are organic compounds in which the protons are replaced by fluorine. This results in an absence of signal in the bowel. Perfluoroctyl bromide (C8BrF17), also known as PFOB, is the only perfluorochemical that has been investigated for oral use in humans to date. It is commercially available now as Imagent but at high cost. Potential advantages are a rapid transit through the small bowel because of its low surface tension, the lack of taste or odour making it palatable, and the absence of any known side effects. PFOB is immiscible as are all perfluorochemicals that are in their pure or "neat" state. This may be an advantage because PFOB cannot be diluted by bowel contents, however, miscible agents that mix with fluid in the bowel may give more uniform filling of the GI tract. Emulsifying PFOB, as is done for intravascular use of perfluorochemicals, may overcome this potential problem."
"Dilution of positive contrast agents occurs in the upper GI tract if they are miscible with water because of gastrointestinal secretions. This allows for the use of a small dose, but will cause loss of signal intensity as the concentration decreases. Immiscible positive agents using oils, especially non-absorbable ones, will not experience the loss of signal with dilution. They will probably require a larger volume to replace any residual bowel contents."
"Short T1-relaxation agents"
Expected headings
"Positive contrast agents"
"Paramagnetic agents"
"Short T1-relaxation agents"
"Combination contrast agents"
"Negative contrast agents"
"Diamagnetic contrast agents"
"Superparamagnetic contrast agents"
"Perfluorochemicals"
"Positive vs negative contrast agents"
"Advantages of positive agents"
"Disadvantages of positive agents"
"Advantages of negative agents"
"Disadvantages of negative agents"
"paramagnetic agents (e.g. Gd-DTPA solutions)"
"short T1-relaxation agents (e.g. mineral oil)"
"There are several preparations of superparamagnetic agents that can be used as oral MRI contrast agents. These include magnetite albumin microspheres, oral magnetic particles, and superparamagnetic iron oxides. These contain small iron oxide crystals approximately 250 to 350 angstroms in diameter and are mixtures of Fe2O3 and Fe3O4. The small size of the crystals contributes to their large magnetic moment without significant residual magnetisation after removal from the magnetic field, i.e., they are superparamagnetic, not ferromagnetic. These crystals are embedded in an inert material, albumin matrix in the first case, a monodispersed polymer in the second, and an inert silicon polymer in the third. The inert materials reduce absorption and therefore, toxicity from the iron. They also help to suspend the particles in solution 5."
"The question of which type of contrast enhancement of the bowel is the best, positive or negative, is still debated. We may find a positive or negative oral contrast agent better depending on the specific organ or disease suspected and the pulse sequence used."