CT Contrast Publisher PPT Chapter12.pptx

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Contrast Agents
• To distinguish adjacent tissues on a CT image, the
tissues must have different densities.
• An oral or IV administration of a contrast agent is often
used to create a temporary, artificial density difference
between objects.

Contrast Agents (cont.)
• Positive agents
– Possess a higher density than surrounding structures
• Barium, iodine
• Negative agents
– Possess a lower density than surrounding structures
• Air, CO2

Intravascular Contrast Agents
• Iodinated agents are universally used because they are
– Water soluble
– Easy to administer intravascularly
– Safe
• IV agents can serve to widen the difference in
attenuation between adjacent structures because
– Different tissues often enhance differently
– Iodinated CM is handled differently in normal versus
abnormal tissue

Properties of Iodinated Agents
• Osmolality
– The number of particles in solution, per unit liquid, as
compared with blood
• High-osmolality agents (HOCM) may have as much as
seven times the osmolality of blood.
• Low-osmolality agents (LOCM) have roughly twice the
osmolality of blood.
• Isosmolar agents (IOCM) have the same osmolality as
blood.

Properties of Iodinated Agents (cont.)
• Viscosity
– The thickness or friction of a fluid as it flows
– The brand, temperature, and concentration of the CM
affect its viscosity
– May affect the injectability of intravascular agents
through small-bore needles and IV catheters

• Ionicity
– Ionic agents contain molecules that will form ions in
solution.
– Nonionic agents contain molecules that do not
disassociate.
• Although most nonionic contrast agents also have low
osmolality, the terms are not synonymous.

• Clearance
– Iodinated agents are not metabolized and are
excreted by the body nearly exclusively by the
kidneys.
– The half-life in patients with normal renal function is
approximately 2 hours.

• Dose
– Determined by iodine concentration and the volume
delivered
– When comparing doses, compare the total grams of
iodine delivered

Review
• How many grams of iodine will be delivered when 100
mL of an agent with 320 mg of iodine/mL is injected?
A. 3.20 g of iodine
B. 32 g of iodine
C. 320 g of iodine
D. 3,200 g of iodine

Answer
• B. 32 g of iodine
100 mL x 320 mg of iodine/mL = 32,000 mg of iodine =
32 g of iodine

• Pediatric dose
– Most often based on the weight of the infant or child
– The most common formula is 2 mL/kg
• Adult dose
– Typically, a uniform dose for each protocol
– However, weight-based dosing for adults has
advantages

Pregnancy
• Because of concerns about exposing the fetus to ionizing
radiation, CT examinations are seldom done during
pregnancy,
• However, in some cases, examinations may be vital to
the health of the mother.
• There is no proof that contrast agents present a risk to
the fetus.
• However, there is not enough evidence to be certain they
pose no risk.

Lactation
• A very small percentage of the iodinated CM given to a
mother will be excreted into breast milk and absorbed by
the infant.
– For example, if the maternal dose of contrast is 150
mL, the breast-fed infant will ingest just 0.015 mL.
• Therefore, it is believed to be safe for the mother and
infant to continue breast-feeding after receiving CM.

Adverse Effects
• Iodinated CM is one of the most widely used of all
medications.
• They are also one of the safest.
• Fatal reactions are extremely rare in both HOCM and
LOCM.
– Estimated at 0.001%

Adverse Effects (cont.)
• Although rare, adverse reactions sometimes occur.
• It is impossible to predict which patient will have an
adverse reaction to IV-administered CM.
• Therefore, CT staff must be trained to respond quickly.

• The term “contrast reaction” is used in a variety of
different ways in relation to the effects of iodinated CM.
• In some instances, it describes all undesired effects
including the many subjective effects (e.g., heat, metallic
taste).
• In other instances, it describes the less common, more
serious side effects that may require treatment or even
be life threatening.

• CM reactions can be broadly categorized as either
– Chemotoxic reactions
• Result from the physiochemical properties of the
CM, the dose, and speed of injection
– Idiosyncratic reactions
• All other reactions
• In practice, it can be difficult to characterize some
reactions into one group or the other

Idiosyncratic Reactions
• The mechanisms by which idiosyncratic reactions occur
are not precisely understood.
• The origin of these reactions is rarely, if ever, “an
allergy.”
– True allergic reactions result in the production of
antibodies, which are not found after CM reactions.
– True allergies result in similar or more severe
adverse reactions with reexposure.
• Research shows this is not true of CM reactions.

Idiosyncratic Reactions (cont.)
• Even though the underlying cause is likely different,
symptoms of idiosyncratic reactions resemble allergic (or
anaphylactic) reactions and are, therefore, often called
allergic-like or anaphylactoid reactions.
• Even though it is not completely accurate, the term
“contrast allergy” remains in common use.

Idiosyncratic Reactions (cont.)
• Acute idiosyncratic reaction are usually classified as
– Mild
• Short duration and self-limiting
– Moderate
• Not immediately life threatening, although they
may progress to be so
– Severe
• Potentially or immediately life threatening

Risk Factors for Idiosyncratic Reactions
• Previous CM reaction
• Asthma
• A history of allergy to food, drugs, or other substances
– Seafood allergy poses no particular risk
• Seafood allergy results from hypersensitivity to a
protein within the seafood and has no association
with iodine.

Preventive Measures
• For patients identified as being at increased risk of
adverse reaction to CM, options include
– Perform examination without CM
– Use LOCM
– Pretreatment with steroids

Documenting Adverse Reactions
• At a minimum, elements to be documented include
– Amount and type of contrast injected
– Signs and symptoms of the reaction
– Interventions or medications given during the
reaction and the patient’s response
– Final outcome (such as whether the patient was sent
home or admitted to the hospital)

Chemotoxic Reactions
• The types of chemotoxic reactions to CM are variable and
the underlying causes multifactorial
– Contrast media–induced nephropathy
– Effects on thyroid function
– Pulmonary effects
– Pheochromocytoma
– Central nervous system effects
– Delayed reactions

Contrast Media–Induced Nephropathy
• Intravascular CM affect kidney function.
– Newer research indicates that iodinated agents are
responsible for far less nephropathy than previously
thought.
– In some cases, CM can result in significant
nephrotoxic effects, particularly in patients
considered at high risk for nephropathy.

Renal Function
• Clearance refers to the ability of the kidney to remove a
substance from the blood.
• Renal function can be estimated using
– Glomerular filtration rate (GFR)
– Effective renal plasma flow (ERPF)
– Serum creatinine (SeCr) as an index of GFR
• GFR and ERPF are cumbersome; therefore, SeCR is
widely used.

Renal Function (cont.)
• Serum creatinine
– Although it has significant limitations, measuring
SeCr is a fast and inexpensive way to assess renal
function.
– eGFR is gaining attention as a potentially better
marker of CIN risk.

Renal Dysfunction Terminology
• Renal failure
– The inability of the kidneys to maintain homeostasis,
resulting in the accumulation of nitrogenous wastes.
• Renal insufficiency
– Renal function is abnormal but capable of sustaining
essential bodily functions.
• Nephropathy
– Denotes any condition or disease affecting the
kidneys
– Sometimes used synonymously with renal
impairment

Definition of CIN
• Postcontrast acute kidney injury (PC-AKI) is an acute
impairment of renal function that occurs within 48 hours
of intravascular administration of contrast material.
• Contrast-induced nephropathy (CIN) represents PC-AKI
for which alternative causes have been excluded.
• Presentation
– Progressive rise in SeCr within 24 hours of CM
administration
– Typically nonoliguric

CIN Risk Factors
• Creatinine clearance less than 25 mL/min
• History of diabetes mellitus
• History of recent administration of iodinated contrast
agent
• Anticipated large volume of CM
• History of congestive heart failure

Methods of Preventing CIN
• Identify patients at high risk.
– Patients with diabetes mellitus or other risk factors
should have a recent SeCr.
• Use LOCM or IOCM.
• Ensure adequate patient hydration.
• Minimize CM volume.
• Allow at least 48 hours between procedures requiring
CM.
• Discontinue other nephrotoxic medications before the
procedure.

Metformin Therapy
• CM can result in CIN. When renal dysfunction occurs in
patients taking metformin, the drug can accumulate and
result in lactic acidosis.
– Although the incidence of occurrence is low, when it
does occur, lactic acidosis is fatal in about 50% of
patients.
• As a precaution for patients with eGFR less than 60,
metformin should be temporarily discontinued after CM
administration; it can be resumed after 2 days, assuming
kidney function is normal.

Dialysis and CM
• Do not give CM to dialysis patients in whom it is hoped
that the dialysis is temporary.
• CM may be given to patients on dialysis with end-stage
renal failure.
– Patients on dialysis who undergo contrast-enhanced
CT may continue their routine dialysis schedule.

CM Effect on Thyroid Function
• No effect on patients with normal thyroid function.
• Insignificant effect on patients with hypothyroidism.
• When given to patients with hyperthyroidism, CM may
precipitate thyroid storm.
– This is a severe, life-threatening condition resulting
when thyroid hormone reaches a dangerously high
level.

Pulmonary Effects
• CM may cause
– Bronchospasm
– Pulmonary arterial hypertension
– Pulmonary edema
• Patients at increased risk are those with a history of
pulmonary hypertension, bronchial asthma, or heart
failure.
• The use of LOCM significantly reduces the risk.

CM Effects on the Central Nervous System
• The intravascular administration of CM has been shown
to provoke seizures in patients who have diseases that
disrupt the blood-brain barrier.
• The risk of seizure can be substantially reduced by a one-
time oral dose of 5 to 10 mg of diazepam, 30 minutes
before CM administration.
• Seizures that occur can also be controlled with diazepam.

Delayed Reactions
• Defined as reactions that occur between 1 hour and 1
week after CM injection.
• Data on delayed reactions are difficult to accurately
collect.
• Skin reactions account for the majority of true late
reactions.
– Red spots or bumps, weltlike swelling, hives
• Salivary gland swelling (or iodide “mumps”).
• Patient receiving interleukin-2 may reexperience the side
effects of that medication after receiving CM.

Gastrointestinal CM
• In the GI tract, CM is helpful to distinguish loops of bowel
from cyst, abscess, or neoplasm.
– Oral CM is used for most CT studies of the abdomen
and pelvis.
– Rectal administration of CM is useful for some
indications.

Gastrointestinal CM (cont.)
• The most common definition classifies GI agents as
positive or negative depending on the density of the
material relative to the wall of the GI tract.
– By this definition, water is considered a negative
agent.
• Less commonly, oral CM is classified in accordance to its
HU.
– Positive agents have positive HU values.
– Negative agents have negative HU values.
– Water is considered a neutral agent.

Barium Sulfate Solutions
• The most commonly used are positive agents that
contain a 1% to 3% barium sulfate suspension.
• In general, the higher the dose, the better the bowel
opacification.
– For most examinations, a minimum of 500 mL of
dilute barium sulfate is given 45 minutes to 2 hours
before scanning.
– An additional 200 mL is given just before scanning to
fill the stomach and small bowel.

Barium Sulfate Solutions (cont.)
• Low HU oral barium sulfate suspension (VoLumen)
resembles water on CT images.
– Just 0.1% barium sulfate solution
– Measures from 15 to 30 HU on an image
• Advantages of low HU oral contrast
– Improved bowel distention (compared with water)
– Faster transit than standard CT barium solutions
– More effective visualization of both the bowel wall
and the mucosa

Barium Sulfate Solutions (cont.)
• Contraindicated when perforation of the gastrointestinal
tract is suspected.
• Barium leaking into the peritoneal cavity can result in
inflammation referred to as barium peritonitis.
• Fewer complications from aspiration appear to occur with
barium sulfate compared with high-osmolality iodinated
agents (e.g., Hypaque).

Iodinated Agents for Oral Administration
• Both HOCM and LOCM are positive agents that can be
diluted and administered orally.
• Dosages are similar to those used with barium sulfate.
• HOCM is used for most situations because it is less
expensive and provides equivalent gastrointestinal
opacification.
• LOCM has advantages in selective cases.

Review
• In what circumstance(s) would LOCM be diluted and
administered orally (replacing an HOCM such as
Hypaque)?
A. In all abdomen/pelvis studies
B. When a trauma patient is immobilized on a
backboard
C. When a patient has a documented history of allergy
to iodine
D. In a patient with end-stage renal failure

Answer
• B. When a trauma patient is immobilized on a backboard
• LOCM is indicated for oral administration whenever there
is a risk of aspiration into the lungs.

Water
• Sometimes used in place of a positive agent
• It does not obscure mucosal surface
• Transits rapidly
• Distends the bowel poorly

Air and Carbon Dioxide
• Produces very high negative contrast
• Administered rectally
• Particularly useful in CT colonography
• Both room air and CO2 provide reliable colonic distention
– CO2 is better tolerated by most patients.

Intrathecal CM
• Certain iodinated contrast agents can be injected into the
intrathecal space surrounding the spinal cord.
– Seldom is the contrast agent injected while the
patient is in the CT department.
• Patients are frequently sent to the CT department for a
postmyelogram CT, while the contrast agent is still in the
intrathecal space.
• Keep the patient’s head elevated.
• Roll the patient before scanning to mix the CM.

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