The question of which type of contrast enhancement of the bowel is the best,
positive or negative, is sill 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.
Two disadvantages of positive oral contrast agents are ghosting artifacts because of respiratory and peristaltic motion, and loss of signal from dilution with secretions and retained fluid in the bowel. One method of reducing ghosting artifacts is to use a pharmaceutical, such as glucagon or scopolamine, to reduce bowel motion. This increases the invasiveness of the procedure. Other methods include the use of breath holding pulse sequences and first order flow compensation. Further refinements of pulse techniques probably will make breath holding sequences more popular for abdominal MRI. This will decrease artifacts from both peristalsis and breathing.
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 nonabsorbable ones, will not experience the loss of signal with dilution. They will probably require a larger volume to replace any residual bowel contents.
Another disadvantage of a positive oral contrast agent is the possibility of residual material in the bowel simulating a mass when surrounded by bright signal. The opposite is also true. A bright mass (such as a lipoma) might be obscured by the contrast agent.
An advantage of positive oral contrast agents is the availability of several of these materials at this time. These include ferric ammonium citrate, pediatric formula, and homemade oil emulsions. Positive agents are also inexpensive (except for gadolinium solutions) and are safe to use.
Disadvantages of negative oral contrast materials include their high cost and lack of general availability (except for CO2 and barium), and limited evaluations of safety on large number of patients. The expense may decrease with greater use of these contrast materials and with competition between manufacturers.Metallic artifacts are seen when iron oxide concentrations, ideal for spin echo sequences, are used with gradient echo sequences. This is because gradient echo sequences have greater sensitivity to magnetic field inhomogeneity. Also there were some metallic artifacts seen in the colon on delayed (24 hour) imaging with the iron oxide preparations that probably can be eliminated as discussed above.
Lack of a fat plane between the negative contrast filled bowel and low signal intensity organs may make it difficult to distinguish normal contours. An example of this is the plane between the stomach and the pancreas on T2-weighted sequences. The majority of pathology appears bright on T2-weighted sequences and should be seen, however.
Advantages of negative oral contrast materials are several. The lack of signal in the bowel removes a source of ghosting artifacts from spin echo sequences that may be present with positive agents. The loss of signal is fairly independent of concentration of superparamagnetic iron oxide suspensions on spin echo sequences so that dilution should not be a problem. The perfluorochemicals are immiscible with water and will not encounter dilution problems either.