RADIOGRAPHIC CONTRAST MEDIA
The radiographic density of the tissues of the body depends on the
atomic number of the principal substances of which the tissues are
composed.
With the exception of bone and certain calcified structures that contain
calcium and other radiopaque salts, most of the body tissues
(including the skin, muscle, and abdominal viscera) are composed of
carbon, hydrogen, and oxygen which have low atomic numbers and
display very small differences in density. Therefore in a simple or
conventional radiograph the outlines of many tissues and organs are
either only very poorly delineated or are not visible at all.
Artificial methods of delineating such organs are required and hence
the need for contrast media in radiology or diagnostic imaging.
Definition:
Radiographic contrast media or agents are substances that are used to
visualize structures or diseased processes that would otherwise be
invisible or difficult to see during diagnostic medical imaging.
They are pharmaceuticals that alter tissue characteristics to enhance
information obtained on diagnostic images.
Classification:
1. Negative Contrast Media:
These are contrast media which have low atomic numbers and so low
densities and provide, therefore, negative contrast, Examples: air,
nitrogen, oxygen, carbon, dioxide, nitrous oxide, etc).
2. Positive Contrast Media:
These are contrast media which have high atomic numbers and, so,
high densities (and therefore provide positive contrast, Examples:
i)
Barium sulphate.
ii)
Organic iodine compounds.
Positive contrast media may be introduced by one of the following
methods.
1. Intra-vascular injection. e.g., in arteriogram, venogram, and
lymphangiogram.
2. Ingestion. e.g. Ba meal and Oral cholecystography.
3. Injection directly into site of interest. e.g., in cystogram, and
retrogradepyelogram.
4. Administered I.V. or ingested and then excreted or concentrated
by the organ(s) under examination e.g., in excretion urography
and oral cholecystograph.
5. Injected and then caused to move (usually by postural changes)
to the site o interest. e.g., in myelography.
All these techniques demonstrate the anatomy of the required region
of the body while some also test function of the organ or organs of
interest. e.g., l.V.U. and O.C.G.
Factors influencing choice of contrast media…..
1. Appropriateness: the contrast medium chosen should be
appropriate for the necessary examination or investigation. e.g.,
Barium Sulphate for Ba meal, Omnipaque for yeloqram.
2. Acheivable radio-opacity: the contrast medium should provide
the desired or desirable degree of radio-opacity.
3. Toxicity and/or side effects: the contrast medium must be safe
and non-toxic both locally where it is administered and
elsewhere in the body that it may reach, nor should it produce
any unwanted effect on the body in general.
4. Viscosity: for some examination (e.g. angiocardiography) a
relatively low viscosity is desirable to enable rapid injection of a
large volume of contrast medium. For examination where the
contrast medium is injected and stays in the organ or dissipates
slowly from it (e.g. H.S.G.) a more viscous contrast medium can
be used.
5. Cost: the contrast medium should be reasonably priced and
affordable.
6. Persistence: some contrast may remain in the body for several
years and are thus of use in assessing progress by continuing to
show any change in the size of the contrast filled lesion, without
further injection.
7. Miscibility: for some examinations like cyst puncture, the
contrast should mix with the fluid into which it is injected.
Osmosis: The movement of water through a semipermeable
membrane from a solution of lesser to one of greater solute
concentration or from an area of greater water concentration to an area
of lesser water concentration.
Vascular and Urographic contrast media
1. These are pharmaceuticals that alter tissue characteristics to
enhance information obtained on diagnostic images.
2. They are organic Iodine compounds.
3. They form the major positive contrast media used in diagnostic
imaging.
4. They are categorized as:
(i) high osmolality, and
(ii) low osmolality contrast media.
5. The high osmolality (or “conventional”) contrast media
(HOCM) have very high osmolalities of 1200 – 2000 mosmo/kg
(about 4 - 7 times the osmolality of cell and tissue fluid) and are
hypertonic.
- They are solution of the sodium and/or meglumine salts of
monomeric tri-iodinated substituted benzoic acids e.g.,
diacetrizoic, iothalmic, or metrizoic acids.
- These salts dissoçiate, completely in solution, each molecule
providing one cation and a large organic iodine containing
anion.
Hence another name for these HOCM is “Ionic Contrast Media".
Both anion and cation have equal osmolar effects but only the
anion is radiopaque.
- Many of the adverse effects of contrast media are the result of
high osmolality. It was thus postulated that by eliminating or
reducing the number of the cation in these contrast media which
does not contribute to the radiopacity but is responsible for up to
50% of the osmotic effect, it would be possible to reduce the
toxicity of contrast media.
- This is the basis upon which low osmolar contrast media
(LOCM) were developed. The ratio of iodine atom in the
molecule to the number of particles in solution is 3:2 or 1.5 for
HOCM compared to 3:1 in LOCM.
6. The Low Osmolality Contrast Media (LOCM) have an
osmolality ranging from 092to 702 mosm/kg normal serum
osmolality is 285 mosm/kg.
- Thus iso-osmolality LOCM have been developed. These are nonionic dimmers which have a ratio of six iodine atoms for each
molecule in solution.
- It should be noted that the terms “low-osmolar” and non-ionic are
not synonymous. The major clinical difference between the two
groups is that ionic contrast media cannot be used in the subarachnoic space.
- The LOCM include iohexol (Omnipaque), iopamidol (Isovue),
iopromide, and ioversol (optiray).
- LOCM have iodine content ranging from 128 to 320 mg/ml.
Agents with low iodine contents are most suitable for intraarterial digital subtraction arteriography. Those with iodine
content of 240 to 300 mg/ml are used for excretory urography,
venography, venous injection digital subtraction arteriography
and belus it enhancement for CT scans. The media with high
iodine content 320 - 370 mg/ml are used for aortography and
selective arteriography.
- Evidence has indicated definite advantages to adoption of
LOCM; however, they are more costly, averaging from 10 - 20
times the price of HOCM.
Current advised by ACR for use of LOCM
1. Patient with a history of a previous adverse reaction to contrast
material, with the exception of a sensation of heat, flushing, or a
single episode of nausea or vomiting.
2. Patient with a history of asthma or allergy.
3. Patient with known cardiac dysfunction, including recent or
potentially imminent cardiac decompensation, severe
arrhythmias, unstable angina pectoris, recent myocardial
infarction, and pulmonary hypertension.
4. Patients with generalized severe debilitation.
5. Any other circumstances where, after due consideration, the
radiologist believes thee is a specific indication of the use of
LOCM.
- In infants and children, considerations include very small size
prematurity, significant cardiac disease, congestive failure
dehydration, severe asthma or allergies, previous reaction to
contrast materials, renal insufficiency, and sickle cell anemia.
- Amajor consideration is degradation of the resulting examination
resulting from pain, heat, or vomiting.
THE NATURE OR MECHANISM OF CONTRAST MEDIA
REACTION
The nature or mechanism of reaction to contrast media is not fully
understood although some are clearly allergic and the major reactions
are anaphylactoid in type, the effects resembling those of histamine
release.
Contrast media reactions may be classified as idiosyncratic and
nonidiosyncratic. Idiosyncratic reactions are “allergic-type” reactions
and include hives, itching, facial and laryngeal oedema,
bronchospasm, and circulatory failure or collapse.
Non-idiosyncratic reactions result from direct toxic effect of contrast
material and contrast hyperosmolaity and include nausea, vomiting,
cardiac arrhythmias, pulmonary oedema and circulatory collapse.
The existence of a known allergy doubles the frequency of a contrast
reaction and quadruples the rate of severe reactions.
Recurrent reactions have been reported in from l5% to 60% of patients
with a history of previous contrast reaction, nearly 20% of such
patients developed identicai severe reactions.
There is a higher frequency of reactions when the total iodine dose
exceeds 20 grams.
Several disease processes may be aggravated by the administration of
intravascular contrast material. Hypertensive crisis, related to
catecholamine release, may result in patient with pheochromocytoma.
Contrast induce sickle cell crisis can occur following intravascular
contrast administration. The patient at risk for developing contrast
nephropathy has existing azotemia, diabetes mellitus, severe
congestive failure, multiple contrast studies, or renal tubules filled
with uric acid precipitates (as in those undergoing rapid tumour lysis).
It is inadvisable to administer contrast media in patients clinically at
risk for azotemia.
Non-ionic contrast media reduce significantly the incidence of all
reactions and specifically the incidence of severe and potentially lifethreatening.
ADVERSE REACTIONS TO THE
INTRAVASCULAR CONTRAST MEDIA
Contrast reactions represent the most common complications of
intravascular iodine containing contrast media administration and are
observed in 2%-1O% of patients receiving HOCM, usually following
I.V. injection or the media.
CLASSIFICATION
The reactions to contrast media may be classified, for purposes of
their management, into:
1. MINOR REACTION.
2. MAJOR REACTION.
MINOR REACTIONS
Most reactions are minor.
They include: arm pain, a feeling of warmth, metallic taste in the
mouth, tingling sensation in the sensitive areas like the perineum,
palms etc., giddiness, coughing, sneezing, flushing, lacrimation, nasal
congestion etc.
These reactions usually pass off quickly and need no treatment other
than reassurance (and deep breathing and a vomit bowl for nausea and
vomiting.)
Minor allergic reactions such as sneezing, Rhinorrhoea, lacrimation,
pruritus or urticarial rashes are much less common but are also
relatively benign and will respond to ANTIHISTAMINES if
necessary.
MAJOR REACTIONS
These are the life-threatening severe reactions which cause real danger
and for which swift treatment is so important. Most such reactions
occur within five minutes of injection and the great majority within
thirty minutes so that a doctor should be at hand for this period
whenever an injection of contrast medium has been given.
These reactions include:
a) Bronchospasm causing wheezing.
b)Laryngeal angio-neurotic oedema causing choking.
c) Vascular collapse when the patient is pale and sweating, has a
thready pulse and may lose consciousness. This can lead to cardiac
arrest.
d) Respiratory failure, when the patient becomes cyanosed and may
stop breathing.
e)Convulsions and coma.
All these major reactions require prompt and efficient treatment if the
patient is to survive.
Most patient with potentially fatal reactions are successfully
resuscitated if their treatment is prompt and efficient.
Everyone who works in X-Ray rooms should be familiar with simple
techniques of resuscitation and should know what may be expected of
them in helping to treat a collapsed patient.
A warning system is needed so that help can be summoned quickly to
any room where it is required.
It is also best to have an “emergency trolley”, carrying all drugs and
equipment that may be needed. The trolley should be readily
available, being kept in a set place known to members of the
department.
INTRAVENOUS UROGRAPHY (I.V.U.)
(Excretion Urography)
I:
ROUTINE I.V.U.
-INDICATION:
Suspected urinary tract lesion.
-PREPARATION:
A) Patient:
1. Enquiry for history of allergy.
2. Menstrual history.
3. Bowel preparation.
4. “Dehydration.”
5. Bladder emptying.
B) General
1. Room preparation.
2. Machine preparation.
PROCEDURE
1. Preliminary film
Scout film
Plain film
17" x 14" (43cm x 35cm) KUB film
12" x 10" (30cm X 24cm) cross KUB film
Uses:
(i) To show any opacities in the line of the urinary tract that may be
masked by the contrast medium in the excretory films.
(ii) To show that the exposure is correct.
(iii) To show that the colon and small bowel have been cleared of
faesces and gas.
(iv) To show the size, shape and position of the kidneys.
(v) To demonstrate any other abnormalities.
2. Contrast medium is injected rapidly (and the name and dose of it
is noted).
3. Nephrogram film coned to kidneys is taken immediately.
4. 5 minutes film coned to kidneys.
5. 10 minute full length (KUB) film.
6. 20-30 minutes full length film p.r.n.
7. Full length post micturition (P film.
8. Any necessary additional films e.g. compression films or oblique
bladder views.
I.V.U. : ADDITIONAL FILMS
1. COMPRESSION FILMS.
In these days of “high dose” urography the use of compression is no
more “routine” in almost all departments.
The diuresis produced by the “high dose” of contrast medium usually
adequately distends the pelv-calyces and ureters.
On the rare occasions when these are not adequately distended the use
of compression is justified.
It should be applied after the 5 mins film has been taken. It is
normally kept on for only for 10 mins and so a l5mins film of the
renal areas and the upper ureters is taken at 15 mins with the
compression maintained. If satisfactory distension of the pelvi-calyces
and the ureters (upper) is shown by this film a full length film is taken
immediately after releasing the compression to achieve good
distension of the lower parts of the ureters.
Objections to compression:
a) Discomfort to the patient.
b) It may give false impression of hydronephrosis and
hydroureter.
c) Impaired venous return may cause “faints.”
2. OBLIQUE VIEWS OF THE KIDNEYS.
(These are done with 300 left side up for the right kidney and viceversa with both kidneys included in each 12” X 10” film).
These may be of value in :a) Opacities whose relationship to the kidneys is uncertain.
b) Cases where bowel gas or faecal shadow obscures calyces
c) Demonstrating the pelvi-ureteric junction.
d) Possible calyceal irregularities or deformities.
e) Localizing a calyceal stone as in an anterior or posterior calyx.
f)
Possible space occupying lesions or irregularities in the renal
outline, e.g. cortical scar.
3. FILMS OF THE WHOLE ABDOMEN.
If there is suspicion of an ectopic kidney, particularly a pelvic kidney,
films of the whole abdomen are taken instead of coned kidney films.
Similarly, if there is a suspected ureteric calculus, full-length films are
advisable throughout the examination because uretric filling in the
portion of interest may only occur on one film.
4. DELAYED FILMS.
(A) In urinary tract obstruction.
Renal excretion of contrast media is slow in obstruction of the urinary
tract. Therefore opacification c the renal tract on the affected side (if it
is unilateral) is delayed. This results in taking films for quite a
considerable period (up to 48 hrs).
(B) In renal failure
Similarly, renal excretion of contrast media is slow in renal failure and
this involves taking films for long period.
5. PRONE FILM.
In suspected renal tract obstruction it is imperative to determine the
site of the obstructing lesion and, where possible, its nature.
In the presence of hydronephrosis (from urinary tract obstruction or
any other cause) the excreted contrast, being of high specific gravity,
pools in the most dependent parts of the calyces and pelvis and does
not because of the stasis, move forward. In such cases when a film
shows that the contrast has “built-up” in the calyces and pelvis, then
the patient is turned prone, told to take deep breaths and a film is
taken after a pause of 1.5 — 2.0 minutes. This will bring the contrast
to the site of obstruction.
6. ERECT FILM.
(A) In urinary tract obstruction if the prone film described above does
not bring the contrast to the site of the obstruction an erect film is
done to achieve this.
(B) This view is also useful in demonstrating PTOSIS of the kidney.
7. TOMOGRAPHY OF THE KIDNEYS.
This is useful:a) When gas obscures the kidneys.
b) When the renal outlines are not clear and their delineation is
important, e.g. in HTN or with possible renal masses.
c) When contrast is faint, particularly in renal failure.
Tomography is carried out at a time when the radiologist considers
ideal which is usually after the 10 minutes film.
8. NEPHROTOMOGRAPHYU
This is renal tomography done in the nephrographic phase usually
after giving a high dose of contrast medium.
Lesions of the renal parenchyma are often well demonstrated by it,
e.g. renal masses, polystic disease even the thin rim of functioning
renal parenchyma in extreme hydrophrosis.
9. ZONOGRAPHY.
In this technique a tomographic exposure with a reduced angle of
swing is employed (5°-15°).
This demonstrates a greater thickness of tissue and allows some
shortening of the exposure. A single film will often show the kidneys
in their entirety. These advantages make the method particularly
suitable for children.
10. OBLIQUE BLADDER FILMS.
(The patient is rotated about 400, and the tube is angled 20-25°
towards the feet).
These are taken for:a) To confirm or exclude possible bladder filling defects seen on
the AP film, including prostatic impressions.
b) To show the size and extent of diverticula of the bladder.
c) To determine whether opacities are in the ureter or not.
d) In suspected ureterocele (with the same side raised).
II: MODIFIED I.V.U.
IN INFANTS AND CHILDREN
PREPARATION TECHNIQUE
1. Patient dehydration should not be drastic.
(i) In healthy infants the interval between feeds is adequate.
)ii) In neonates or sick infants no attempt is made to dehydrate.
)iii) In older children 8 hrs dehydration is sufficient.
2. Sedation is usually needed for infants and most small children
under 3 or 4 yrs.
3. In children over 7 or 8 yrs bowel preparation with an aperient
and 12 hrs. limitation of fluid intake is usually all that is
required.
4. To reduce the obscuring effect of intestinal gas it is of value to
distend the stomach with air or gas.
The upper limit in age for this manoeuvre is about 8 yrs.
An AP and RPO (left side raised) views are taken after the stomach
has been distended with the air or gas, serving as “window” for the
kidneys.
5. Tomography or zonography provides alternative method of
showing kidneys obscured by gas.
6. Injection of contrast:
-should be slow.
-should preferably be I.V.
-in a dose of about 2 mls/kg.
7. FILMING should be:
(i) Prelim.
(ii) 3 mins (infants and children excrete the contrast medium more
slowly than adults) cross kidneys.
(iii) 5 mins Cross kidneys.
(iv) 15 mins Full length.
(v) Any other views if needed.
(vi) Post-micturition film when possible and if it is needed.