Computed tomography (or CT) refers to a computerized x-ray imaging procedure in which narrow beams of rotating x-ray radiation are directed at a patient and then transcribed by a computer into cross-sectional "slices" of the specific area imaged. The increasing use of diagnostic computed tomography (CT) images has presented a unique challenge in understanding this procedure's risks to the general patient population. There are few direct risks associated with CT scanning. These range from contrast-induced allergic reactions, contrast-induced nephropathy, and long term risk of cancer development due to radiation exposure. Additional considerations include the patient's pregnancy status and radiation effect on the developing fetus. In addition to these considerations, there is radiation attributable risk to pediatric patients in comparison to adults.
Issues of Concern
Understanding Contrast Reactions in CT Imaging
Contrast materials are used to improve pictures of the inside of the body and often allow the radiologist to distinguish normal from abnormal conditions. Iodine, barium-sulfate, and gadolinium-based contrast media are the three main types of contrast media used today. Contrast media can enter the body via ingestion, enema, or intravascular injection. Barium-sulfate contrast materials are swallowed or administered via enema and used to enhance the gastrointestinal tract's x-ray and CT images. Iodine-based contrast materials are administered intravenously and are used to enhance x-ray and CT images. Gadolinium-based contrast media is also administered intravenously and is used to enhance MR images.
Iodine-based contrast agents can be divided according to osmolarity (high, low, or iso-), ionicity (ionic or nonionic), and the number of benzene rings (monomer or dimer). Fewer adverse reactions are associated with nonionic contrast agents compared with ionic agents. Nonionic low or iso-osmolar preparations are used almost exclusively for intravascular injections in current practice as they are associated with fewer adverse reactions.
It is the duty of the referring physician and radiologist to consider the risk-to-benefit profile of the proposed contrast material–enhanced examination and potential alternatives that would provide the same or better diagnostic information. Radiologists should consider screening for predisposing factors that increase the risk of both allergic and non-allergic reactions.
Common preexisting risk factors for adverse reaction to contrast material include:
- History of respiratory diseases, such as asthma.
- History of heart or kidney failure.
- History of severe allergies and reactions (to medications and/or food).
- Previous allergic reaction to iodinated contrast material.
- Age: infants and neonates are at an increased risk for adverse effects from contrast agents.
Iodinated contrast, Iodine, and Shellfish Allergy
While there is an increased risk of adverse reactions to contrast material in patients with any history of allergy, there is no evidence to support a specific link between shellfish allergy and allergy to contrast agents. "Iodine allergy" is a collective term often confused for an allergy to iodinated contrast material. Iodine itself is an essential element and does not have any potential to cause an allergic reaction. Iodine does not confer cross-reactivity between iodine-rich substances. The concept of iodine allergy is not supported by evidence and may result in the inappropriate nonuse of IV contrast material.
Both MRI and CT contrast agents, once injected intravenously, are cleared from the body by the kidneys. Historically, serum creatinine was used to assess kidney function. However, routine testing of serum creatinine before administering iodinated contrast material is not necessarily indicated in all patients. Instead, the estimated Glomerular filtration rate (eGFR) is likely a better indicator of renal function because it takes the patient's age, race, and gender into account.
Contrast-induced nephropathy (CIN) is defined as deterioration in renal function following administration of recent intravascular contrast and the absence of another nephrotoxic event. Risk stratification for CIN in patients based on eGFR is defined as very low risk (eGR >60 mL/min), low risk (eGFR 45-59 mL/min), moderate (eGFR 30-45 mL/min), and high risk (eGFR <30 mL/min). CIN's risk is relatively low in patients with stable renal function and without pre-existing risk factors such as hypertension.
End-stage renal disease patients who are anuric and on dialysis can routinely receive iodinated intravenous contrast without the need for urgent dialysis or risk of further damage to the kidneys. Prognosis is based upon an appropriate assessment of renal function and pre-hydration. Treatment is mainly supportive and aimed at maintaining volume and electrolyte balance.
Contrast-induced Adverse Reactions
Acute, non-renal adverse reactions to intravenous contrast are typically divided into three categories: mild, moderate, and severe. It is important to distinguish physiologic reactions from allergic reactions to contrast materials because physiologic reactions do not require pre-medication for future intravenous contrast administrations, whereas allergic reactions do. Serious reactions to contrast media are mediated by type 1 hypersensitivity reaction mechanisms and usually begin within minutes of exposure.
- Mild: Nausea, vomiting, headache, anxiety, chills
- Moderate: Prolonged nausea and vomiting, chest pain, severe hypertension
- Severe: Seizures, hypertensive emergency, arrhythmias.
- Mild: Mild skin edema, urticaria, nasal congestion, rhinorrhea.
- Moderate: Diffuse pruritus, bronchospasm with or without mild hypoxia, facial edema
- Severe: Diffuse erythema with hypotension, hypoxia, bronchospasm with hypoxia, anaphylactic shock, facial and laryngeal edema.
Pre-medication for the Administration of Contrast Material
Generally considered in patients at risk for acute allergic reactions, glucocorticoids can be used for premedication. Active infection or impaired immune is a relative contraindication to glucocorticoids. Oral administration is the preferred route of administration.
Option 1: Prednisone 50 mg PO at 13, 7, and 1 hour before injection of contrast material + diphenhydramine 50 mg IV/PO 1 hour before injection of contrast material.
Option 2: Methylprednisolone 32mg PO 12 and 2 hours before injection of contrast material + diphenhydramine 50mg IV/PO 1 hour before injection of contrast material.
It is imperative to verify intravenous access before the administration of contrast media. Extravasation occurs when the contrast material escapes the vascular lumen and infiltrates the surrounding interstitial tissue during the injection. The most effective method for identifying extravasation is asking the patient to report any sensation of pain or swelling at the injection site and directly palpating the venipuncture site a few seconds after the injection. Prompt recognition and evaluation by a physician are required to reduce the risk of severe injury. Elevating the affected extremity and applying a cold compress is recommended. Severe, though uncommon, complications can include compartment syndrome, tissue necrosis, and skin ulceration.
Contrast Administration During Pregnancy and Lactation
Avoiding iodinated contrast in pregnant patients is advisable, if possible. Iodinated contrast material crosses the placenta though definite evidence of teratogenesis due to contrast material has not been definitively demonstrated. The iodine content of contrast media has the potential to produce neonatal hypothyroidism. Current standard practice is to screen all neonates for hypothyroidism, particularly in infants who have received iodinated contrast during pregnancy. Only a small percentage of iodinated contrast is excreted in breast milk and absorbed by the infant. Current recommendations agree that the minimal potential risk associated with absorption of contrast medium is insufficient to necessitate stopping breast-feeding for 24 hours. Thus, lactating women who receive iodinated contrast can continue breastfeeding without interruption.
Radiation Exposure with CT Scanning
As x-rays pass through the body, some of them get absorbed by the tissues. The term "effective dose" is the scientific term for whole-body radiation and is measured in units of millisieverts (mSv). During our daily living, we are exposed to natural "background" radiation. An average person in the United States experiences approximately 3 mSv of background radiation per year. As a comparison, radiation from one adult chest radiograph is approximately 0.1 mSv, approximately 10 days of natural background radiation. Depending upon the type of CT scan, the approximate radiation ranges from 2-20 mSv, approximately 8 months-7 years of natural background radiation. Imaging centers and hospitals apply the ALARA (as low as reasonably achievable) principle to decrease radiation exposure and risk.
There is no direct evidence of adverse biologic effects of radiation exposure in humans at doses less than 100 mSv. Most of the available evidence regarding adverse biologic effects of radiation exposure is derived from studies of survivors of atomic bombs dropped on Hiroshima and Nagasaki. Based on these survivors' studies, there is strong evidence of adverse biological effects, particularly an incremental increase in the incidence of various cancers, at radiation doses exceeding 100 mSv. However, it is important to note that Hiroshima and Nagasaki survivors were instantaneously exposed to a mixture of whole-body radiation more complex than the low-energy X-ray beams used in conventional CT scanners. Another important note is that conventional CT scanners expose only one or a few organs to radiation. In contrast, the Japanese survivors of Hiroshima and Nagasaki were exposed to whole-body radiation. These factors complicate any derivation of accurate radiation risk to patients undergoing medical examination of CT scanning. Patients concerned about potential radiation risks from medical diagnostic imaging should consult with a radiologist or medical physicists to better understand risks versus benefits.
Regarding the pediatric population, the radiation risk to children from diagnostic CT examinations is not well understood. However, children are at a greater risk of cancer from radiation exposure as they are inherently more radiosensitive than adults and have longer to live, more time for radiation-induced cancer to occur. CT examination, no doubt, remains crucial to provide accurate and timely diagnostic information. However, specifically in the pediatric population, diagnostic imaging sources that do not rely on ionizing radiation, such as MRI and ultrasound, definitely play a role in reducing overall radiation exposure.
Computed tomography (CT) imaging is currently at the forefront of diagnostic imaging in providing accurate and timely information for medical and surgical intervention. Due to the wide applicability of diagnostic imaging, a thorough understanding of radiation risks associated with CT scanning is essential for every healthcare provider. Contrast material is widely used in CT scanning in current medical practice. Understanding the appropriate application, usage, and risks of contrast material is essential to prevent adverse reactions and outcomes in the contemporary patient population.
Nursing, Allied Health, and Interprofessional Team Interventions
Managing patients undergoing CT scanning with possible contrast administration is a multidisciplinary and interprofessional approach. Accurate and timely information on the patient's allergies, hemodynamic status, current medications, hydration, and renal status must be easily accessible. It is the duty of any healthcare professional to question whether contrast administration could lead to an adverse outcome for the patient based on the available clinical data. Appropriate documentation and communication of the patient's past and present allergies and any history of previous reactions to contrast materials are essential to prevent adverse clinical outcomes. When in doubt, clear communication with the ordering provider and the radiologist should be facilitated to coordinate the patient's best clinical plan.