Contrast Agents in Imaging
Donna Magid MD, M.ED
Imaging is based on the assumption that, whatever the form of energy or physics used to peer beneath the skin and into the body, different tissues and substances will produce different visual representations, usually along the grey scale (black-->greys-->white). This is dependent on the desired information, or useful visual signal, exceeding the disruptive or confounding information, or noise. The goal is always to maximize detail and definition, and to minimize anything interfering with or degrading these characteristics.
Imaging contrast agents are substances, which can be used to enhance the visibility of tissues, structures, substances, or pathology, often by accentuating the differences or margins between two substances or tissue types (ie, where tumor ends and normal tissue begins; where blood flow is increased or decreased; etc). In radiography and CT, which use x-rays, these are substances which will change the relative radiodensities or radioattenuation of tissues or structures or spaces (such as the lumen of hollow viscera in GI).
INTRAVENOUS (IV) CONTRAST AGENTS, which may be ionic or nonionic and are often based on iodine, are high-attenuation sterile fluids usually administered through a vein for vascular distribution throughout the body, but can also be injected into the arterial system (arteriography) or subselectively via catheter, via percutaneous needle or instrument into many joint spaces (arthrography), the thecal space (myelography) , the uterus (hysterosalpingogram), the bladder and ureters (cystoscopy), or orally or rectally into the GI tract. When administered immediately prior to a CT exam, the exam can be timed to capture the contrast during either its venous or arterial phases. Contrast reactions are rare but can be fatal (anaphylactoid, bronchospasm, edema), or milder (itching, urticaria). That seafood allergies may predict contrast allergies is a persistant but poorly-substantiated misperception, although patients with any sort of allergy are at higher risk of a contrast reaction than those without, and patients with a history of prior contrast reaction are at a higher risk of anaphylactoid reaction than those with prior uneventful contrast exposure. Premedication with corticosteroids may decrease the risk of reaction; minor reactions (hives, itching) can be addressed with diphenhydramine. Since IV contrast is cleared by the kidneys, information about renal function, possible renal compromise, and serum creatinine and BUN will be required at scheduling/request, as will other relevant medical information.
BARIUM SULFATE is the most common oral/rectal contrast agent, used in the GI tract, as a thick chalky mix with water which is either taken PO, by NG tube, or given per rectum. It may be combined with air contrast in the well-prepared (NPO, bowel prep) patient who is also able to move and cooperate (since the double-contrast or barium-air contrast mixture works best when the patient can coat the lumen 360 degrees by rolling around and then assuming different positions as instructed). At the end of the study the barium is excreted from the colon. Care must be taken, particularly with the upper GI studies and/or with compromised, weak, elderly, or NPO patients, to keep them well-hydrated following barium studies; the colon absorbs water from the mixture and delayed excretion may lead to relative retention or desiccation of the mixture. Barium is extremely radiodense and will interfere with adequate visualization of the lumbar spine and pelvis until excreted; subsequent CT cannot be obtained until all barium has been expelled or markedly diluted with water. Water-soluble iodinated contrast agents will be substituted in GI studies if perforation is suspected, particularly esophageal, since extravasation of such agents into the mediastinum or peritoneum is far less harmful than extravasation of barium. These agents also readily resorb from extraluminal spaces. They are much more expensive, and more radiolucent, meaning detail or level of certainty may be compromised; but the relative radiolucency also means subsequent CT to assess possible extravasation is not compromised, as it would be with intralumenal barium. Choosing between agents is yet another reason that Radiology must be consulted and well-informed as to the indications for any requested study, and apprised of the specific clinical issues to be addressed.
MRI contrast agents serve the same purpose, enhancing the visibility and definition of various tissues and structures, but are based on paramagnetic properties, changing the magnetic relaxation times and therefore the signals of the structures affected. Gadolinium (Gd) is the most common agent and can be used IV, orally, or intraarticularly. In the vascular system, it will create higher signal in the blood vessels, and in any lesion compromising the blood-brain barrier, such as tumors or infections, allowing these lesions to selectively enhance. Eventually Gd is eliminated via the renal system with some distributed into the extracellular fluids. Gadolinium has been associated with nephrogenic systemic fibrosis (NSF), particularly in patients with chronic renal failure; again, consult with Radiology and provide information about possible renal compromise before requesting examinations.
ULTRASOUND (U/S) uses only very limited contrast materials. Having patients hydrate well and fill their bladders before an abdominal exam helps provide a better acoustic window into the soft tissue pelvic and female GU system. Other microsphere agents are under investigation.
NUCLEAR MEDICINE studies (scintigraphy) use organ-or tissue specific radiopharmaceuticals, often linked to certain chemicals, substances, or even cells which will be preferentially taken up by the tissue or organ or process (bone, marrow, myocardium, thyroid, GI mucosa, liver and spleen, gall bladder, renal, infectious foci, etc) and then emit low-level gamma or other radiation which can be collected to create an image of the selected tissue/organ/process' uptake and function.
POSITRON EMISSION TOMOGRAPHY (PET) uses an agent called FDG ((18)F-fluorodeoxyglucose) which selectively is taken up by certain cells. Studies such as PET CT combine the PET metabolic data with the CT anatomic resolution (studies are scaled to be superimposable or merge-able) to give information about changes, for instance, in tumor cell activity as neoplasm responds to treatment and converts to fibrosis or scar, or begins to recur in the center of same.
It is strongly advised that you provide us with adequate clinical information to select the imaging parameters and possible use of contrast agents, rather than requesting certain contrast or certain imaging sequences or parameters. We are consulting clinicians, specially trained to maximize the outcome of every exam while minimizing patient risk and discomfort. Rather than scheduling a ‘Head Ct with IV contrast” without further explanation or detail, for instance, inform us that it is “Head CT, recent trauma, r/o fracture or intracranial bleed”. In such cases one DOES NOT want contrast (and the accompanying risks and delay), since fresh blood is radiodense and will be readily apparent on the head CT. Please DO NOT tell us what MR sequences you want; rather tell us more about the clinical problems, history, or acute concerns and let us protocol the study (i.e. tell us "Hx breast cancer, possible intracranial metastases" instead of "T1 transaxialwith and without Gd, T2 coronal..."). At this point choosing CT and MR and radiographic protocols - image sequences, positions, choice of contrasts, etc. - is so complex and changing so rapidly, may be machine-dependent (i.e. strength of the MR scanner; multislice capacity of the CT scanner; desirability of reformattable data acquisition), or may need to be modified because of dose considerations or changing national regulations, that even Radiologists scramble to keep up with the state of the art. Remember also that this is Hopkins; that imaging article you just read may be several months old and already superceded by more recent research; or we may be using equipment or technology not yet readily available in the main stream.
Work with us, communicate clearly, and understand that we are physician consultants on the patient's care team - when we request more information or do not necessarily agree with you on the timing, need, or specific technique for a requested study, we are trying to optimize patient outcome and safety, not being obstructionist.
Donna Magid M.D., M.Ed.