X-Rays and Radiographic Contrast Media: A Comprehensive Guide
X-RAY
Electromagnetic Waves
X-rays are electromagnetic waves of very short wavelength, hence a very high frequency, discovered by Wilhelm Conrad Roentgen in 1895. They obey all the laws of light, but with some special properties:
- They can penetrate materials that absorb or reflect visible light.
- They make certain substances fluoresce. In other words, they emit visible light.
- They affect photographic film, producing a record.
- They produce biological changes, both somatic and genetic.
When a stream of electrons produced at high speed collides with matter, X-rays are produced.
X-ray Tube
The most effective way of producing X-rays is with an X-ray tube. The simplest X-ray tube consists of a vacuum glass bulb, which contains:
- Anode: Usually made of copper, with a block of tungsten at its front end.
- Cathode: Has a tungsten filament wire coiled in a coil.
X-ray Production
X-rays are produced through the following process:
- The filament of the cathode produces a stream of electrons.
- The stream of electrons strikes the tungsten anode plate.
- The impact of the electrons generates heat and X-rays.
Formation of X-ray Images
The X-rays that manage to pass through matter are those that form the image. Rays that fail to pass through matter (absorbed) do not contribute to the image.
The degree of X-ray absorption depends on three factors:
- The wavelength of the rays.
- The composition of the object that interrupts the beam.
- The thickness and density of the object.
X-ray Absorption and Atomic Number
X-ray absorption refers to the atomic number of the material on which the rays impinge. The higher the atomic number, the greater the ability to absorb X-rays. Lead is a good example of a material with a large X-ray absorption capacity.
Radiographic Contrast Media
Radiographic contrast media have a higher atomic number than organic tissues and therefore absorb more X-rays, making them appear denser on film.
Definition
Radiographic contrast media (RCM) are chemicals of complex molecules, injected into the bloodstream, increasing the density of vessels and tissues, allowing them to contrast with the surrounding structures.
- They have a higher atomic number than most tissues.
- They can absorb up to 50,000 times more X-rays than tissues.
Routes of Administration
- Digestive (high-low)
- Enemas
- Intravenous
Intravenous RCM
Virtually all contrast media for intravascular use contain iodine. The two exceptions are Gadolinium, used in magnetic resonance imaging (MRI), and microbubbles used in ultrasound.
Iodinated contrast media are synthesized based on the structure of Triiodobenzoic Acid, which provides the ability to absorb X-rays due to the presence of three iodine atoms in its molecule.
Osmolality
- Osmolality: Number of particles dissolved in one kg of water.
- Osmolarity: Number of particles dissolved in one liter of solution.
- Plasma osmolality: 300 mOsm/kg
- Ionic monomeric contrast media (MC) osmolality: 2100 mOsm/kg
Rating of Soluble Iodine
- It dissolves in water and is eliminated in urine.
- Barium is not soluble.
- Ionic: Dissociate in solution into ions.
- Nonionic: Do not dissociate in solution.
Classification
- Ionic Monomer – Dimer
- Non-ionic Monomer – Dimeric
Ionic Monomeric Contrast Media
Development
Developed in the 1950s.
Properties
- In water, each molecule dissociates into two electrically charged particles.
- An MC molecule with three atoms of iodine gives rise to two particles in solution, leading to high osmolality (1500-1600).
Examples
- Amidotrizoate Meglumine
- Amidotrizoate Sodium
- Iopanoic Acid: Due to its high osmolality, it is associated with a high rate of adverse effects.
Amidotrizoate
- Presentation: Ampoules of 20 ml, 140 to 420 mg/ml
- Indications: Urography, venography, operative cholangiography, CT
- Contraindications: Hypersensitivity to iodine-containing compounds
Iopanoic Acid
- Presentation: 500 mg tablets
- Indications: Review of the gallbladder and biliary tract
- Contraindications: Severe renal disease, obstructive liver disease
Other Ionic Monomeric Contrast Media
- Hypaque
- Angiovist
- Telebrix
- Conray
- Relief
Ionic Dimeric Contrast Media
Meglumine Iotroxate contains twice as many iodine atoms in a molecule, half that of monomeric contrast media.
- Presentation: 100 ml vial of 105 mg/ml
- Indications: Exploration of the gallbladder and bile ducts
NON-IONIC CONTRAST MEDIA
Nonionic Monomers
Nonionic monomers are electrically neutral and do not dissociate in water. In solution, each molecule with three atoms of iodine originates one particle, which determines its low osmolality (500-700).
Development
Monomeric nonionic compounds were developed in the 1980s.
Advantages
They have the advantage of not dissociating in solution, therefore, they have a lower osmolality (which is equal to twice that of plasma).
Examples
- Omnipaque
- Radiomiron
- Xenetix
- Optiray
Nonionic Dimers
Nonionic dimers do not dissociate in solution and have no electric charge. Each molecule with six atoms of iodine gives rise to one particle.
Osmolality
Osmolality similar to blood (300).
Development
Dimeric nonionic contrast media were developed in the 1990s and have the distinct advantage of being iso-osmolar.
Configuration
Their configuration is given by the fusion of two molecules of monomeric dye, with a consequent decrease in osmolality.
PHARMACOKINETICS OF IODINE-BASED CONTRAST MEDIA
- No enterohepatic circulation.
- Dialyzable.
- Do not pass the blood-brain barrier.
- Enter breast milk and cross the placenta.
- Minimal enteral absorption.
- Linear pharmacokinetics, proportional to dose.
Barium Sulfate
Barium sulfate is a metal salt that is used to define the gastrointestinal tract.
- Not absorbed from the gastrointestinal tract.
- Does not interfere with gastric or intestinal secretion.
Adverse Reactions
While the risk of adverse reactions is five times smaller with nonionic contrast media with low osmolality, they have not disappeared.
- Present in 5 to 12% of examinations.
- 0.1% are severe (life-threatening).
- Mortality: 1/75,000 to 1/160,000.
- 90% of adverse reactions occur within the first 20 minutes.
Mild Adverse Reactions
- Heat (embarrassment)
- Redness
- Nausea
- Vomiting
- Anxiety
- Vasovagal reaction
Severe Adverse Reactions
- Edema of the glottis
- Bronchospasm
- Arrhythmia
- Death
Adverse Reactions: Mechanisms
The exact mechanism of adverse reactions is unknown. They are divided into: physiological (chemotactic), anaphylactoid, or allergic.
Chemotactic or Physiological Reactions
Related to the characteristics of the contrast media and the rate of administration, dose, and concentration. Their origin is related to hyperosmolality and abduction of Ca++. They manifest as heat, dizziness, nausea, vomiting, arrhythmias, and seizures.
Anaphylactoid Reactions
While it has been established that an antigen-antibody reaction is involved, anaphylactoid reactions produce identical symptoms to anaphylaxis (type I hypersensitivity reaction), probably by activation of complement by the non-classical pathway. They manifest as bronchospasm, angioneurotic edema, and cardiovascular collapse.
Idiosyncratic Reactions
: No dose-related and can occur even with 1 mL or less. They are unpredictable. They are due to weak nonspecific binding with biological macromolecules of MC, with its charges (ionic), or its hydrophobic portion (non-ionic).
Contrast Media:
Recommendations
The benefit of the review should
overcome the risk.
The review should be conducted in a
institution with the media
requirements (ICU or other) where
need them
If it has the resource, you must
MC preferred the use of nonionic
Dimeric (Visipaque)
The patient should be monitored
hemodynamically when
conduct a review.