Intravascular injection of a radiopaque diagnostic agent opacifies those vessels in the path of the flow of the contrast medium, permitting radiographic visualization of the internal structures of the human body until significant hemodilution occurs.
Following intravascular injection, HEXABRIX is rapidly transported through the circulatory system to the kidneys and is excreted unchanged in the urine. The pharmacokinetics of intravascularly administered radiopaque contrast media are usually best described by a two compartment model with a rapid alpha phase for drug distribution and a slower beta phase for drug elimination. In 10 patients with normal renal function, the alpha and beta half-lives of HEXABRIX were 12 (4-17) and 92 (61-140) minutes, respectively. Following the intravenous administration of 50 mL of HEXABRIX in 10 normal volunteers, the mean peak plasma concentration occurred at two (1-3) minutes, reaching a concentration of 2.1 (1.8-2.8) mg/mL. Approximately 50 (42-67) percent of the intravenously administered dose was recovered in the urine at two hours, and 90 (68-105) percent was recovered at 24 hours.
The joint spaces as well as the uterus and fallopian tubes may be visualized by the direct injection of the contrast medium into the region to be studied.
Injectable iodinated contrast agents are excreted either through the kidneys or through the liver. These two excretory pathways are not mutually exclusive, but the main route of excretion seems to be related to the affinity of the contrast medium for serum albumin. Ioxaglate salts are poorly bound to serum albumin, and are excreted mainly through the kidneys.
The liver and small intestine provide the major alternate route of excretion. In patients with severe renal impairment, the excretion of this contrast medium through the gallbladder and into the small intestine sharply increases.
Ioxaglate salts cross the placental barrier in humans and are excreted unchanged in human milk.
CT SCANNING OF THE HEAD
When used for contrast enhancement in computed tomographic head imaging, the degree of enhancement is directly related to the amount of iodine administered. Rapid injection of the entire dose yields peak blood iodine concentrations immediately following the injection, which falls rapidly over the next five to ten minutes as a result of dilution in the vascular and extravascular fluid compartments. Equilibration is reached in about ten minutes and thereafter the fall in iodine plasma concentration becomes exponential.
In brain scanning, contrast media do not accumulate in normal brain tissue due to the blood brain barrier (BBB). The increase in x-ray attenuation usually seen in normal tissue following contrast medium injection is due to the presence of the contrast medium in the blood pool. Disruption in the BBB, such as occurs in malignant tumors of the brain, allows accumulation of contrast medium within the interstitial tumor tissue; adjacent normal brain tissue does not contain the contrast medium. Maximum contrast enhancement frequently occurs after peak blood iodine levels are reached. A delay in maximum contrast enhancement can occur depending on the peak iodine level achieved and the cell type of the lesion. This lag in enhancement is probably associated with the accumulation of the contrast medium within the lesion and outside the blood pool.
The image enhancement of non-tumor lesions, such as arteriovenous malformations and aneurysms, is dependent on the iodine content of the circulating blood pool.
CT SCANNING OF THE BODY
HEXABRIX may also be used for enhancement of computed tomographic scans performed for detection and evaluation of lesions in the liver, pancreas, kidneys, abdominal aorta, mediastinum, abdominal cavity and retroperitoneal space.
In non-neural tissues (during computed tomography of the body), HEXABRIX diffuses rapidly from the vascular to the extra-vascular space. Increase in x-ray absorption is related to blood flow, concentration of the contrast medium and extraction of the contrast medium by interstitial tissue since no barrier exists; contrast enhancement is thus due to the relative differences in extra-vascular diffusion between normal and abnormal tissue, a situation quite different than that in the brain.
The pharmacokinetics of HEXABRIX in normal and abnormal tissues has been shown to be variable.
Enhancement of CT with HEXABRIX may be of benefit in establishing diagnoses of certain lesions in some sites with greater assurance than is possible with unenhanced CT and in supplying additional features of the lesions. In other cases, the contrast medium may allow visualization of lesions not seen with CT alone or may help to define suspicious lesions seen with unenhanced CT.
Contrast enhancement appears to be greatest within the 30-90 seconds after bolus administration of the contrast agent, and after intra-arterial rather than intravenous administration. Therefore, the use of a continuous scanning technique (a series of two to three second scans beginning at the injection — dynamic CT scanning) may improve enhancement and diagnostic assessment of tumors and other lesions such as an abscess, occasionally revealing more extensive disease.
Because unenhanced scanning may provide adequate information in the individual patient, the decision to employ contrast enhancement, which is associated with additional risk and increased radiation exposure, should be based upon a careful evaluation of clinical, other radiological and unenhanced CT findings.