ml/kg). Several specific agents have been approved for up to 0.3 mmol/kg or three times the dose as compared with the majority of gadolinium agents.
In addition to safety, dose should be considered for clinical evaluation as well. As dose increases (to a point), the ability to visualize structures and lesions also increases. With standard gadolinium the optimal dose is weight-based. Higher relaxivity agents can, in some cases, create an increase in signal intensity that would otherwise require a double dose of standard agent (Figure below). Care should be taken to calculate dose, the type of contrast used and to document dose and agent for the clinician to provide optimal diagnostic value for the patient.
Some facilities opt to inject every patient with a standard dose of, for example, 10 ml. This is an unacceptable method of contrast enhancement. When considering the affect of dose on enhanced MR images, review the images illustrated in Figure below. The image on the top is unenhanced, and the image on the bottom left is enhanced with gadolinium (single dose), bottom middle (double dose) and bottom right (triple dose). Imagine a patient with a weight of 90 kg. If this patient is injected with 10 ml, the effective dose is essentially one half the recommended dose. In this case, several lesions could be missed on enhanced imaging. Note the difference in visualization of lesions for single dose, double dose and triple dose of standard gadolinium. For these reasons, it is essential to calculate dose (by weight) and document the dose (and type) of the contrast that has been administered
In Figure below, the image in the top row is unenhanced and the image on the bottom left is enhanced with gadolinium (single dose). The larger metastatic lesion (identified with the red arrow – located on the patient’s left posterior region of the brain) is relatively conspicuous even without contrast. However, the conspicuity of the smaller metastatic lesions (identified with the blue arrow – located in the patient’s left frontal lobe, and yellow arrow – in the patient’s right parietal lobe) are virtually invisible on the unenhanced image. To enable the visualization of the smaller metastatic lesions required double dose (bottom middle image). For better visualization, triple dose (bottom right image) might be required.


Nephrogenic Systemic Fibrosis (NSF)
Prior to the FDA approval for gadolinium contrast agents, studies have shown that approximately 80% of gadolinium is excreted by the kidneys in 3 h and 98% is recovered by feces and urine in one week. As the result of these studies, it became clear that gadolinium contrast is excreted from the body through the urine. Until recently, it was thought that the use of gadolinium was indicated and deemed safe for all patients, including patients with poor renal function. In 2006, a Danish study prompted serious concern about the use of gadolinium contrast agents for MRI and MRA procedures in patients who suffered from renal insufficiency. These patients contracted a condition known as nephrogenic systemic fibrosis (NSF).
Patients who were in renal failure and received gadolinium developed a ‘bark-like’ skin condition, which was misdiagnosed as scleroderma. On additional review, the condition became known as nephrogenic fibrosing dermopathy (NFD). Further investigation revealed that this condition not only affected the skin, but the organ system as well. At this point the condition became known as nephrogenic systemic fibrosis (NSF). NSF is a fatal condition with virtually no cure. Although treatment does help, it must be administered immediately. Unfortunately, many NSF symptoms do not reveal themselves for several days to several weeks after contrast administration. To date, no cases of NSF have been reported in patients with normal renal function. For these reasons, gadolinium is a contraindication and a relative precaution for patients in renal failure.
Although they are not commonly used, there are a few other agents that are used as T1 contrast
agents in MRI. These additional agents include manganese, used for liver imaging, and hyperpolarized helium gas for inhalation imaging for the lungs. Such agents shorten T1 and therefore appear bright on T1 weighted images. Manganese is taken up by the Kupffer cells in the liver. In this case the normal liver will enhance and lesions remain darker . Enhanced lung images are shown in Figure below, and demonstrate ‘ventilation’ information.
Gastrointestinal contrast agents are not as widely used as intravascular agents at present but may increase in use in the future. Oral contrast agents have been researched for bowel enhancement. Iron oxides (dark on T2 weighted images) and fatty substances (bright on T1 weighted images) have been used orally to try to enhance the gastrointestinal tract effectively (Figure below). However,
due to constant peristalsis, positive agents (those agents that make bowel bright) enhance bowel motion artefacts. The use of antispasmodic agents helps to retard peristalsis and/or ultra-fast imaging techniques to reduce these artefacts.
Formerly, there was an agent called Perflubron (perfluorocarbon) that rendered bowel black in T2 weighted images. Perfluorocarbon is a substance that holds oxygen, and therefore it is used as a blood replacement agent during transplantation. For a period of time this agent was approved as a contrast agent for MRI. However, since the agent was rarely utilized, it is no longer available for use as a contrast agent. Today, there are facilities that use juices such as blueberry and mango juice (to make bowel dark on T2 weighted images) as enhancement agent