What Is CT Contrast Media?
CT Contrast Media, Contrast media are used during many CT procedures in an effort to increase the contrast between, and subsequent visibility of, themultitude of anatomic structures and pathologic conditions whose radiographic densities are too similar to be adequately separated. The contrast agents utilized during CT procedures can be broken down into two basic types, positive and negative. The positive contrast agents belong to a class of substances known as radiopaque contrast media (RCM).
The RCM typically used as for CT examinations are iodine and barium. The degree of radiopacity exhibited by an iodinated contrast agent is directly proportional to the agent’s concentration of iodine. The degree of radiopacity exhibited by a barium contrast agent is directly proportional to the agent’s concentration of barium.
Iodinated RCM are water-soluble compounds that may be administered:-
A. Generally into the bloodstream intravenously.
B. Directly into a targeted vein or artery for localized enhancement.
C. Directly into the intrathecal space during CT Myelography.
D. Into the joint space during CT arthrography.
E. Orally to opacify the gastrointestinal (GI) tract.
Suspensions of barium sulfate are commonly employed as positive contrast agents for opacification
of the GI tract.
Types Of CT Contrast Media.
There are following types of CT Contrast Media.
1. Intravascular Radiopaque Contrast Media.
2. Enteral Radiopaque Contrast Media
3. Negative Contrast Agents
4. Neutral Contrast Agents
- Intravascular Radiopaque Contrast Media:-
A. Initial opacification of blood vessels allows for their anatomic visualization and differentiation from surrounding structures. Contrast enhancement of vasculature greatly aids in the diagnosis of many disorders, including aneurysm, thrombus, and stenosis.
B. Over time the contrast agent is distributed from the vasculature into the extravascular space. This interstitial redistribution of contrast agent can result in differentiation of normal from abnormal soft tissue on the basis of enhancement patterns.
C. As the kidneys excrete the contrast agent, opacification of the renal collecting system occurs. This process improves visualization of the renal pelvis, ureters, and bladder.
D. Osmolality is an important characteristic of an iodinated radiopaque contrast agent. It describes the agent’s propensity to cause fluid from outside the blood vessel (extravascular space) to move into the bloodstream (intravascular space).
E. Iodinated RCM can be generally divided into the following categories:-
- Ionic contrast media are salts consisting of sodium and/or meglumine. Each molecule of ionic contrast agent consists of three iodine atoms. When injected into the bloodstream, each molecule dissociates into two charged particles, or ions. The production of osmotic ions is indicative of high-osmolar contrast media (HOCM). Examples of HOCM are iothalamate meglumine (Conray) and diatrizoate sodium (Hypaque).
- Non-ionic contrast media are non-salt chemical compounds that also contain three atoms of iodine per molecule. They do not dissociate in solution. These substances are commonly referred to as
low-osmolar contrast media (LOCM). Examples of LOCM are iohexol (Omnipaque), iopamidol (Isovue), and ioversol (Optiray).
F. The osmolality of an iodinated radiopaque contrast medium greatly affects its potential for adverse effects in the patient.
G. Non-ionic low-osmolar contrast agents are less likely to produce adverse side effects and/or reactions than ionic high-osmolar RCM.
H. Iso-osmolar contrast media (IOCM) have the same osmolality as blood and therefore may offer improved patient comfort and a reduced potential for untoward side effects.
I. Iodixanol (Visipaque) is an example of a non-ionic iso-osmolar contrast agent.
2. Enteral Radiopaque Contrast Media:-
A. Enteral RCM are administered orally and/or rectally to opacify the GI tract.
B. Generally an enteral agent is either a water-soluble iodinated solution or a suspension of barium sulfate.
C. Barium sulfate suspensions are readily used as oral and rectal contrast agents for opacification of the GI tract.
D. Barium sulfate is an inert compound with excellent attenuation properties.
E. Routine transit time for barium sulfate through the GI tract is typically between 30 and 90 minutes.
F. Water-soluble oral contrast media can also be either high-osmolar or low-osmolar iodinated solutions.g. High-osmolar contrast agents, such as diatrizoate meglumine and diatrizoate sodium, have traditionally been used as oral/rectal CT contrast media.
G. Newer low-osmolar contrasts such as iohexol may also be used as oral/rectal contrast media for CT.
H. Routine transit time for water-soluble iodinated contrast agents through the GI tract is typically between 30 and 90 minutes.
I. The important considerations in the choice between a water-soluble iodine oral contrast agent and barium sulfate are as follows:-
- Barium sulfate may be not be utilized in cases of suspected perforation because it may be toxic to the peritoneum.
- Barium sulfate is contraindicated in patients who are to undergo surgery or other invasive procedures of the abdomen and/or pelvis.
- Barium sulfate can be potentially harmful if aspirated.
- Water-soluble oral contrast agents, particularly of the low-osmolar type, are usually more palatable and result in less GI distress.
- Water-soluble oral contrast agents may be contraindicated in patients with known iodine allergy.
J. The contrast agent used for rectal CT may be administered via enema to opacify the distal large colon and rectum.
3. Negative Contrast Agents
A. Air, gases, and water may be used as negative contrast agents during CT examination.
B. Water may be used as an oral contrast agent to fill the GI tract. Advantages include:-
- Increased palatability and improved patient comfort.
- Better demonstration of the enhancing bowel wall.
- Does not interfere with three-dimensional (3D) applications.
C. Effervescent granules used to treat gas and acid indigestion may also be administered as negative oral contrast agents. When swallowed, these granules add negative contrast in the form of gas to the stomach and proximal small bowel, allowing for better visualization of these structures.
D. Water-soluble iodinated solutions may be mixed with carbonated beverages to add negative contrast to the GI tract, improving the demonstration of subtle disease.
E. Air acts as a contrast agent during CT imaging of the chest much like it does on a chest radiograph. Image acquisition at the end of full inspiration improves image quality during a CT examination of the chest.
F. Air may also be administered via enema to insufflate the large bowel to improve image quality during CT colonography.
G. CT colonography may also involve insufflation of the large intestine with CO2. Distention of the large intestine with room air or CO2 is necessary for optimal visualization of the bowel wall.
4. Neutral Contrast Agents
A. Neutral oral contrast agents may be administered to opacify the small bowel during procedures such as CT enteroclysis and CT enterography.
B. Neutral contrast media distend the GI tract while still allowing for clear visualization of the bowel wall.
C. Very low-density (0.1%) barium sulfate solutions such as VoLumen may be administered for detailed CT examination of the small bowel.
Administration Route and Dose Calculations
1. The IV administration of a contrast agent typically consists of an intravenous bolus injection through an 18- to 23-gauge angiocatheter or butterfly needle.
2. An angiocatheter is an IV catheter placed within a vein and used to administer fluids, medication, and/or contrast media.
3. It consists of a small plastic catheter surrounding a solid needle that acts as a stylet to allow the catheter to be placed within
a vein. Once the angiocatheter is inside the vessel, the needle is retracted and the catheter is advanced into the vein.
4. The angiocatheter may be connected to a contrast agent syringe or an automated contrast agent delivery system. Connective tubing may be attached to facilitate the administration of fluids and/or medications.
5. Most current angiocatheters contain a built-in safety shield or retraction mechanism that guards against accidental sharps exposure.
6. Needleless IV access systems feature a reusable port device that allows the blunt plastic cannulas of syringes and connective tubing to be attached without needles. Needleless systems improve occupational safety by reducing the risk of accidental needlesticks.
7. A butterfly needle consists of a hollow needle surrounded by plastic flaps or “wings.” The wings are grasped and lightly pinched together to facilitate insertion of the needle into a vein. The needle is attached to a short length of connective tubing. Once the needle is inserted into a vein, the plastic wings are secured with tape, and the tubing is attached to a syringe or IV drip.
8. Intravascular RCM are generally administered in doses ranging from 50 to 150 mL. Specific dose depends on the patient’s age, weight, and renal function and on the clinical indication for the CT procedure as directed by a radiologist.
9. The serum iodine concentration is a measure of the amount of iodine within the bloodstream. The range of serum iodine concentration for adequate opacification during CTexamination is 2 to 8 mg/mL.
10. An IV contrast agent may also be administered through a central venous catheter if it is of adequate type, material, and diameter. The manufacturer of the venous catheter should be consulted for specific tolerances before contrast agent administration.
11. Common types of central venous catheters are subclavian lines, implanted access ports, and peripherally inserted central catheters (PICC lines).
12. Flow rates for injections through central venous catheters such as PICC lines are typically reduced to well below 2 mL/sec. Please refer to the manufacturer and the department’s specific protocol before using a central venous catheter for contrast agent administration.
13. Patency of central venous catheters should be tested for venous backflow and may require a test injection of saline for complete assurance.
14. The total volume of oral contrast agent administered for a CT examination should be adjusted according to the extent of bowel opacification required clinically, that is, proximal small bowel, distal small bowel, proximal large bowel, and so on.
15. An example of oral contrast agent protocol for CT examinations limited to the abdomen is as follows:-
a. 300 mL of oral contrast agent 30 minutes before scan.
b. 150 mL of oral contrast agent immediately before scan.
16. For CT examinations of the entire abdomen and pelvis, substantial opacification of the stomach, small bowel, and proximal large bowel is typically required. An example protocol is:-
a. 450 mL of oral contrast agent 60-90 minutes before scan.
b. 300 mL of oral contrast agent 30 minutes before scan.
c. 150 mL of oral contrast agent immediately before scan.
17. If complete distention of the stomach is clinically necessary, the last dose of oral contrast agent may be accompanied by a dose of effervescent granules.
18. For studies of the distal large bowel, including the sigmoid colon and/or rectum, an oral contrast agent may need to be administered 4 to 6 hours before
scan time to ensure sufficient opacification.
19. When this delay is not clinically appropriate, an oral contrast agent may be administered rectally, through an enema.
20. A 150- to 300-mL CT contrast agent enema is usually sufficient to adequately opacify the rectum, sigmoid, and distal large bowel.
21. A small amount of air (negative contrast) may also be administered rectally to improve bowel distention when clinically indicated.
22. Intrathecal administration of an iodinated contrast agent is the injection of the agent directly into the space surrounding the spinal cord.
23. Intrathecal injections are performed during CT myelography for evaluation of the spinal cord and nerve roots.
24. Intra-articular administration of a contrast agent is the injection of the agent directly into a joint space.
25. Intra-articular injections are performed during CT arthrography of joints, including the shoulder, wrist, and knee.
Adverse Reactions of CT Contrast Media
1. The use of non-ionic or low-osmolar contrast media (LOCM) significantly reduces the incidence of adverse reaction in comparison with the of ionic or high-osmolar contrast media (HOCM).
2. Although the likelihood of severe adverse reaction to LOCM is low, the exact incidence is not known.
3. The majority of adverse reactions to iodinated contrast agents can be characterized as mild and non– life-threatening.
4. Mild reactions to iodinated contrast media include:-
a. Nausea/vomiting.
b. Mild urticaria (hives).
c. Pronounced sensation of warmth and/or flushing.
d. Altered taste.
e. Sweats/chills.
f. Nasal stuffiness/sneezing.
g. Anxiety.
5. Mild reactions to iodinated contrast agents typically require no treatment. The patient should be positively reassured and closely observed until the symptoms dissipate.
6. Moderate reactions are also usually non–life-threatening. However, they may progress rapidly, so treatment may be necessary.
7. Moderate reactions to iodinated contrast include:-
a. Mild bronchospasm.
b. Moderate to severe urticaria.
c. Vasovagal response.
d. Tachycardia from hypotension.
8. Treatment for moderate adverse reactions to iodinated contrast include:-
a. Bronchodilator inhaler and/or oxygen therapy for bronchospasm (wheezing).
b. Diphenhydramine (Benadryl) for urticaria.
c. Elevation of legs and IV fluids for hypotension.
9. Severe reactions to iodinated contrast agents are those that are potentially life-threatening and require
immediate treatment.
10. Severe adverse reactions are rare.
11. They may begin with mild symptoms, such as anxiety and respiratory distress, and then progress rapidly.
12. Severe reactions to iodinated contrast include:-
a. Profound hypotension.
b. Laryngeal edema.
c. Severe bronchospasm.
d. Pulmonary edema.
e. Cardiac arrhythmia.
f. Seizure.
g. Cardiopulmonary arrest.
h. Death.
13. Table 2-2 outlines the ABCD approach to the management of adverse reactions to contrast media.
14. Delayed reactions to contrast agent administration can occur in a small percentage of patients.
15. Common delayed reactions include:-
a. Urticaria (hives).
b. Pruritus (itchiness).
c. Nausea/vomiting.
d. Drowsiness.
e. Headache.
f. Fever/chills.
17. Cutaneous reactions are the most common and can occur within 3 hours to 7 days after administration of a contrast agent.
18. Delayed reactions to a contrast agent must be welldocumented, followed closely, and treated appropriately as needed.
19. Contrast-induced nephrotoxicity (CIN) is a potentially serious delayed effect of contrast agent administration.
20. It is a considerable decline in renal function that can occur after a patient receives an IV contrast agent.
21. CIN has varied definitions but is usually signified by a marked increase in serum creatinine level over a baseline measurement obtained before contrast agent administration.
22. The causative relationship between contrast agent administration and CIN is not completely understood but may be related to renal vasoconstriction and/or the toxicity of the contrast medium itself.
23. The overall risk of CIN is directly related to the patient’s preexisting renal function and hydration level.
24. Other risk factors for contrast-induced nephrotoxicity include:-
a. Diabetes.
b. Myeloma.
c. Advanced age.
d. Cardiovascular disease.
25. Adequate hydration of the patient is the best method of CIN prevention.
26. In the patient with normal renal function, the risk for development of CIN is extremely low.
27. A baseline serum creatinine measurement should be obtained in all patients who are determined to be at risk for CIN.
28. If the patient is deemed renally impaired, alternative imaging options that do not involve contrast agent administration may be indicated.
29. Patients with non–insulin-dependent diabetes who are taking an oral biguanide (metformin) drug may also have a higher risk of CIN-related effects.
30. Patients receiving metformin drug therapy are at increased risk for lactic acidosis, a condition that may be exacerbated by renal insufficiency.
31. If the patient’s renal function is adversely affected by administration of a contrast agent, there is a
potential increase in incidence of lactic acidosis in the metformin-treated patient.
32. Patients who have associated risk factors for renal insufficiency and are currently taking a metformin drug are typically instructed not to take the metformin following the contrast study.
33. A lab test of renal function may be necessary before the patient resumes metformin therapy.
34. Please refer to the department protocol and/or the appropriate physician guidelines when administering contrast agents to patients receiving metformin drugs.
35. All information regarding patient care, including the procedure(s) performed, contrast/medication administered, untoward event(s), and final outcomes must be documented by the practitioner.
36. Documentation within the patient’s medical record must be clear, concise, and recorded in a timely manner.
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