The practice of clinical diagnostic radiology has been made possible by advances not only in diagnostic equipment and investigative techniques, but also in the contrast media that permit visualisation of the details of the internal structure or organs that would not otherwise be demonstrable. The remarkably high tolerance of modern contrast media has been achieved through successive developments in chemical pharmacological technology. A single dose of X-ray contrast medium commonly contains upwards of 2000 times as much iodine as in the total physiological body content, and yet it is cleared from the system rapidly and naturally, usually with no adverse effects at all.
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The choice of contrast medium has always been a matter of debate, but is ultimately the responsibility of the radiologist. In order to be able to make a rational decision as to the selection of contrast media, it is necessary to have some understanding of the physical and physiological principles involved. The objective is to provide a background for non-specialists on this complicated specialist subject.
Why contrast media are necessary
Different tissues within the body attenuate the beam of X-rays to different degrees. The degree of attenuation of an X-ray beam by an element is complex, but one of the major variables is the number of electrons in the path of the beam with which it can interact. The number of electrons in the path of the beam is dependent upon three factors:
- The thickness of the substance being studied
- Its density
- The number of electrons per atom of the element (which is equal to its atomic number)
In a complex mixture of elements, which is of course what we are concerned with in the organs of a patient, the degree of attenuation is particularly influenced by the average of the atomic numbers of all the atoms involved. Where there is a considerable difference between the densities of two organs, such as between the solid muscle of the heart and the air in the lungs, then the outlines of the structures can be visualised on a radiograph because of the natural contrast that exists. Similarly, if there is a difference between the average atomic numbers of two tissues, such as between soft tissues, which are composed of elements of low atomic number, and bone, which is partly composed of the element calcium, with a rather higher atomic number, then the outlines of the different structures can be seen by natural contrast. However, if the two organs have similar densities and similar average atomic numbers, then it is not possible to distinguish them on a radiograph, because no natural contrast exists. This situation commonly occurs in diagnostic radiography, so that, for example, it is not possible to identify blood vessels within an organ, or to demonstrate the internal structure of the kidney, without artificially altering one of the factors mentioned earlier. Two of the factors important in organ contrast can be artificially altered, the density of an organ, and, more usefully, the average atomic number of a structure. The density of a hollow organ can be reduced by filling it with gas or air, providing negative contrast. This is mainly of historical significance, but is still used when, for example, gas is introduced into the stomach or colon during a double-contrast barium examination.
The average atomic number of hollow structure such as a blood vessel can be increased by filling the cavity with a liquid of much higher average atomic number (such as iodine containing contrast medium) than that of blood. In fact this is the principle by which contrast media consist of solutions or suspensions of non-toxic substances that contain a significant proportion of elements of high atomic number, usually iodine.
EXAMINATION USE CONTRAST MEDIA
common are described below. It should be noted that the volume, strength, as well as the type of contrast medium, will vary between patients according to the examination type and radiologist’s requirements.
Angiography is the general term which describes the investigation of blood vessels. Usually a distinction between arteriography and venography is made, depending on the kind of blood vessel (artery or vein) which is examined.
In arteriography a contrast medium is introduced via a catheter into an artery, which makes the lumen of that vessel opaque to X-rays. The natural flow of blood carries the contrast medium peripherally, and by taking a series of radiographs the radiologist can obtain images akin to a road map of the blood supply to an organ, or a limb. Localised narrowing or obstruction of an artery or a pathological circulation in a tumour can then be identified. Sometimes the radiologist may then proceed to treat the patient using the catheter system, which was introduced initially for diagnosis. Arteriography is relatively time consuming for the radiologist depending on the complexity, ½ hour – 2 hours, or even longer can be spent on the procedure.
The natural flow of blood in veins is towards the heart, and by injection of a contrast medium into a peripheral vein, a map of the venous drainage of a limb can be obtained. The larger size and greater number of peripheral veins, and the fact that the flow of blood is much slower in veins than in arteries,means that it is usual for the radiologist to take several radiographs of each area with the limb in different positions. The commonest indication for venography is to confirm a suspected diagnosis of deep venous thrombosis of the leg. Venography is also performed on organs within the body by introducing a catheter into a peripheral vein and manipulating it into an organ.
Digital subtraction angiography (DSA)
A special type of angiography is digital subtraction angiography (DSA). These procedures involve the use of specialised electronic equipment, computing and radiographic hardware to produce rapid sequential images. The DSA image is produced by electronically subtracting images without contrast media from images after contrast media injection. The result of this subtraction process is the visualisation of contrast filled vessels which are free from the distraction of overlying structures.
2. Intravenous urography (IVU), intravenous pyelography (IVP)
When injected intravenously, most contrast media are rapidly excreted by the kidneys, and a series of radiographs taken after the injection will demonstrate the urinary tract. Intravenous urography is still the basic radiological examination of the urinary tract. The main indication is to assess the morphology of the kidneys.
Further indications are: detection of kidney stones and calcifications in the ureter or bladder, assessment of obstructed urinary flow and investigation of patients with haematuria (the passage of blood in urine). Children may be investigated for congenital abnormalities of the urinary tract. In recent years for some investigations of the urinary tract, particularly uncomplicated infection, an ultrasound examination and plain abdominal radiograph have replaced intravenous urography as the initial investigation of the urinary tract.
3. Computed tomography (CT)
Since 1973 an imaging technique known as computed tomography (CT) has developed to become one of the most important radiological examinations in the industrialised countries. CT uses conventional X-rays in a thin nondivergent beam to produce cross sectional images of the body. The X-ray tube and an array of detectors mounted within a supporting framework, rotate round the patient with each scan. CT produces digitalized images, although these are usually printed onto hard copy film in a format that is useful for transfer and viewing throughout the hospital.
By electronic means CT improves via a higher contrast sensitivity, the natural radiological contrast between organs. However, it cannot create contrast where none exists naturally. CT is exceptionally sensitive to contrast media and can detect abnormalities, caused by disease, following an injection of an intravenous dose of contrast medium. This procedure is known as “enhancing” the scan. About 43% of all CT procedures involve the use of a contrast medium. CT is widely used throughout the body but the most frequently investigated areas using this technique are neuroradiology (brain and lumbar spine) and general radiology of the chest, abdomen and pelvis. It is particularly useful for the diagnosis, staging and follow up of malignant disease.
The spinal cord and the attendant nerve-roots which radiate from it cannot be visualised using conventional X-rays alone without the use of contrast media. They can be visualised directly using magnetic resonance imaging (MRI). They can be visualised if contrast medium is injected in the cerebrospinal fluid (CSF), which surrounds the spinal cord, rendering the CSF radio-opaque but not the cord of nerve roots. Specific contrast media have been developed for this examination. The majority of myelograms (or radiculograms) were performed to examine the lumbar region to confirm the clinical suspicion of a prolapsed intervertebral disc. However, CT and MRI have now largely replaced myelography as the initial investigation of the lumbar spine. Myelography, particularly combined with CT scanning is still used however to investigate the cord and cervical region and its nerve roots in difficult cases when other investigations are equivocal or normal.
Many radiologists are now specialised in therapeutic procedures that have bee developed from radiological diagnostic techniques using catheters and guidewires. These procedures include:
- The dilation of pathologically narrowed arteries – angioplasty, percutaneous transluminal angioplasty (PTA).
- The deliberate occlusion of arteries supplying abnormal areas such as tumours, aneurysms and vascular malformations, so depriving them of their blood supply.
- The placement of artificial tubes or stents into blood vessels, bile ducts or ureters to bypass a pathological narrowing. These procedures often necessitate the use of high doses of contrast medium, because several examinations of the same vessels may be required during the control of the therapeutic process.
Safety of Contrast Media
Contrast media are among the safest of all of the pharmaceutical products available to the doctor today. They are anomalous in that they are not intended to have therapeutic activity: indeed, the ideal contrast medium would have no pharmacological activity at all. For this reason the concept of therapeutic ration, which can be applied to medicines, does not apply to contrast media. The development of a contrast medium from the first design of the molecule through to product licence takes many years. The minimum period of time that can reasonably be allotted to preclinical and clinical development is six years, and in practice it is not uncommon to take nine years or even more. During this long period, the tolerance of the medium is rigorously tested by collecting data from various preclinical and clinical trials to establish a profile for the product. One critical area examined during its development is the incidence of adverse reactions. The rate of adverse reactions to iodinated contrast media on the market is extremely low, but such reactions do occur – just as they do with every pharmaceutical product. The adverse reactions associated with contrast media can be divided into two groups: Those reactions that are clearly dependent on the dose and concentration of the contrast medium administered and those that are almost independent of dose and concentration.
Dose-dependent adverse reactions are mostly due to the physiochemical effects of the contrast medium, such as its osmolality, or electrical charge. Possible adverse reactions include heat, pain, vasodilation, cardiac depression and hypotension. The adverse reactions which are almost independent of dose and concentration are nausea and vomiting as well as allergy-like or hypersensitive reactions such a urticaria (hives), certain cardiovascular reactions, bronchospasm and laryngospasm, but there is little evidence of any antigenantibody interaction. These reactions cannot be predicted and their underlying cause remains unknown.
For clinical purposes it is meaningful to divide contrast media reactions into three categories:
- Minor e.g. Flushing, nausea, vomiting, pruritis, mild rash, arm pain
- Moderate e.g. More severe urticaria, facial oedema, hypotension, bronchospasm
- Severe e.g. Hypotensive shock, laryngeal oedema, convulsions, respiratory and cardiac arrest
Most contrast media reactions are minor and need no treatment. Moderate reactions are encountered rarely (about 1%) and severe reactions very rarely (about 0.1%), but all moderate and severe reactions require adequate treatment. Deaths following contrast media administration are extremely rare. Reported mortality rates vary between 1 in 10,000 and 1 in 169,000 averaging around 1 in 75,000. Katayama et al. (Radiology 1990; 175: 621-628) found that there is a reduction in adverse reaction rate of about four times using low osmolar contrast media (LOCM) for intravenous injection compared to high osmolar contrast media (HOCM).
There is surprisingly no documented difference in mortality between intravenous LOCM and HOCM in large series from Japan and Australia. It is not usually possible to predict severe reactions, even by looking at the effect of a small “test dose” of a contrast medium. Guidelines have been produced for the use of low osmolar contrast agents.
Renal adverse reactions
Contrast media-induced nephropathy is defined as impairment in renal function (an increase in serum creatinine by >25% or 44ã€€mol/L (0.5mg/dL) occurring within 3 days following the intravascular administration of contrast media in the absence of an alternative etiology
Risk factors include raised s-creatinine levels particularly secondary to diabetic nephropathy, dehydration, congestive heart failure, age over 70 years old, concurrent administration of nephrotoxic drugs, e.g., non-steroidal ant-inflammatory drugs
1. Adequate hydration in terms of oral fluid intake or intravenous normal saline (depending on the clinical situation) at least 100 ml per hour starting 4 hours before to 24 hours after contrast administration is recommended. Concurrent administration of nephrotoxic drugs should be stopped for at least 24 hours. High osmolar contrast media, large doses of contrast media, or multiple studies with contrast media within a short period of time should be avoided. Alternative imaging techniques that do not require the administration of iodinated contrast media should be considered. Recent work in preventing and ameliorating contrast medium-induced nephropathy with N-acetyl cysteine 4-6 and various hydration regimens including use of sodium bicarbonate has been promising but is not conclusive yet. No measure has yet resulted in avoidance of its occurrence in all patients.Non-renal adverse reactions.These are generally classified as idiosyncratic or chemotoxic. Idiosyncratic (i.e., anaphylactoid) reactions occur unpredictably and independently of the dose and concentration of the agent. Most anaphylactic reactions relate to the release of active mediators. Conversely, chemotoxic-type effects relate to the dose, the molecular toxicity of each agent, and the physiologic characteristics of the contrast agents (i.e., osmolality, viscosity, hydrophilicity, calcium binding properties, and sodium content). Chemotoxic-type effects are more likely in patients who are debilitated or medically unstable
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2. Acute reactions to contrast media can be divided into minor, intermediate, and severe life-threatening. Minor reactions include flushing, nausea, arm pain, pruritus, vomiting, headache, and mild urticaria. Such reactions are usually mild in severity, of short duration, selflimiting and generally require no specific treatment. Intermediate reactions are more serious degrees of the same symptoms, moderate degrees of hypotension, and bronchospasm. The reactions usually respond readily to appropriate therapy. Severe life-threatening reactions include severe manifestations of all the symptoms described as minor and intermediate reactions, plus convulsions, unconsciousness, laryngeal oedema, severe bronchospasm, pulmonary oedema, severe cardiac dysrhythmias and arrest, cardiovascular and pulmonary collapse. The prevalence of adverse reactions with lowosmolar contrast media is less than with high-osmolar contrast media by a factor of 5-6. Lethal reactionsrarely occur. The actual risk of death is less than one in 130,000 at most
3. The incidence of severe adverse reactions increases in patients with previous contrast medium reaction, bronchial asthma and allergy requiring medical treatment. Premedication with corticosteroid prophylaxis has been proved safe and effective in preventing minor adverse events in high-risk patients when ionic agents are used
4. The data indicating a protective effect of corticosteroid prophylaxis are less established when non-ionic agents are used. Opinion is divided about the value of premedication when nonionic agents are used. Even if it is given, there is a wide variety of regimes with different doses, number of doses, and frequency for corticosteroid prophylaxis.
5. A variety of symptoms (e.g. nausea, vomiting, headache, itching, skin rash, musculoskeletal pains, fever) have been described, but many are unrelated to the contrast medium. Allergy-like skin reactions are welldocumented side effects of contrast media, with an incidence of approximately 2%. Most late skin reactions after contrast medium exposure are probably T-cellmediated allergic reactions. Patients at increased risks are those with history of previous contrast medium reaction and those undergoing interleukin-2 treatment. Most skin reactions are usually mild to moderate, selflimiting and likely resolve within a week. Treatment is symptomatic and similar to the treatment of other druginduced skin reactions.Extravasation of contrast material is a well-recognised complication. The introduction of automated power injection has increased the incidence because power injection may result in extravasation of large volumes in a short period of time and may lead to severe tissue damage.
Intravenous urography is a radiographic study of the urinary system using an intravenous contrast agent (dye). It is a medical procedure used to visualise the kidney and lower urinary tract to help diagnose problems such as infections. A contrast dye is injected into a vein on your hand or arm, and then x-rays are taken. The dye helps to outline more clearly the structure of the kidneys and lower urinary tract. The kidneys excrete the contrast into the urine, which becomes visible when x rayed (radiopaque), creating images of the urinary collection system. An intravenous urogram is ordered to demonstrate the structure and function of the kidneys, ureters, and bladder. Patients complaining of abdominal pain radiating to the back may require this exam to rule out kidney stones. Hematuria may also be an indication of kidney stones,infection, or tumors. Patients with high blood pressure (hypertension) and recurrent bladder infections may also require an intravenous urogram (but hypertension usually is imaged with MRA or nuclear medicine imagery and this exam is done when renal artery stenosis is the suspected cause of refractory hypertension). Sometimes the exam is ordered to evaluate the function of the kidney in a renal transplant patient. The transplanted kidney is located in the iliac fossa, so special films of the pelvis area are done instead of the normal routine views. The radiographic technologist may also be required to take x rays in the operating room when a retrograde pyelogram is ordered by a urologist during a C and P (cystoscopy and pyelography).
A normal intravenous urogram indicates no visible abnormality in the structure or function of the urinary system. The radiologist looks for a smooth non-lobulated outline of each kidney, no clubbing or other abnormality of the renal calyces (collecting system), and no abnormal fluid collection in the kidneys that could suggest obstruction. The ureters must contain no filling defects (stones) or deviations due to an adjacent tumor. The bladder must have a smooth outline and empty normally as visualized on the post-void film. Abnormal results include hydronephrosis (distension of the renal pelvis and calices due to obstruction) as a result of tumors or calculi (stones). Cysts or abscesses may also be present in the urinary system. A delay in renal function can also indicate renal disease. An abnormal amount of urine in the bladder after voiding may indicate prostate or bladder problems. Intravenous urograms are often done on children to rule out a rapid developing tumor in the kidneys, called a Wilm’s tumor. Children are also prone to infections of the bladder and kidneys due to urinary reflux (return back-flow of urine).
The patient will be required to change into a hospital gown and empty his or her bladder. The x-ray technologist will verify that the patient has followed the bowel preparation and complete a detailed questionnaire on the current medical history of the patient. This includes previous contrast reactions, known allergies, risks of pregnancy, and current medications. The x-ray technologist will explain the exam in detail to the patient as well as the risks of the contrast material that will be injected intravenously. All departments require that the patient sign a consent form before the examination is started. The x-ray technologist will relay this information to the radiologist who will decide on what type of contrast will be used. Patients who have had an injection with no reaction can be given less expensive iodine based contrast, whereas patients who take various heart medications or those with known allergies or asthma will be injected with a more expensive contrast agent (known as non-ionic contrast) that has fewer side effects. Some departments use the non-ionic contrast exclusively.
The patient will be instructed to lie supine (face-up) on the x-ray table and a preliminary KUB will be done. This is an abdominal view of the kidneys, ureter, and bladder used to verify patient preparation, centering, and the radiographic technique needed to demonstrate all the required structures.
Kidney stones may or may not be visualized on the preliminary film. The x-ray technologist prepares the required amount of contrast to be used depending on the weight of the patient (1 ml per pound). This is normally 50-75 cc of contrast for an average-sized patient. The contrast will be injected all at once (bolus injection) or in some cases, through an intravenous drip. Some radiologists prefer to start an intravenous drip with saline as a precautionary measure while others inject with a small butterfly needle. The needle usually remains in place for 10-15 minutes, in case more contrast is needed or in case drugs need to be administered because of an allergic reaction. Most reactions occur immediately but some can take place 10 or 15 minutes after the injection.
The first film is taken immediately after the injection to see a detail of the renal outline (nephrogram). Films are usually taken at five-minute intervals depending on the routine of the radiologist. Compression may be applied to the lower abdomen with a wide band to keep the contrast material in the kidneys longer. This creates a more detailed image of the renal collecting system. When the compression is released after approximately 10 minutes the contrast material drains quickly and a detailed, filled image of the ureters is obtained. Films done in the upright or prone (face-down) position may also be ordered to better visualize the lower ureters. Some departments require routine renal tomographic images to be done as well when the kidneys are well visualized. This allows the kidneys to be seen free of gas or fecal shadows. Sometimes the radiologist requires oblique views of the kidneys or bladder to determine the exact location of calculi (stones). At approximately 20 minutes after the injection a film centered on the bladder may be required. The x-ray tube is angled slightly caudad (towards the feet) so that there is no superimposition of the pubic area of the pelvis over the bladder. The films are shown to the radiologist and if no further films are necessary the patient will be asked to void (urinate) and a post-void film will be taken. The exam can take from 30 minutes to one hour depending on the number of films required. If the kidney is obstructed, delayed films may be required to complete the exam.
The x-ray technologist must work in conjunction with the doctors and nurses in making sure the patient has not had a previous allergic reaction to a contrast agent. All hospitals have an emergency team ready to react in such a situation, so the technologist must be aware of the procedure to follow when assistance is necessary due to a severe reaction. Details of patient preparation must also be communicated to the hospital wards. In some hospitals the radiologic technologists are trained to give injections, but if this is not the case nurses may be asked to install an intravenous drip before the patient is brought to the radiology department.
The x-ray technologist must explain the risks of an allergic reaction to each patient even though severe reactions are extremely rare due to the advances made in the preparation of contrast agents. The x-ray technologist explains to the patient that a warm, flushed feeling or a metallic taste in the mouth are normal reactions in some patients. Breathing instructions are also important since the kidneys change position depending on the phase of respiration and to prevent motion artifacts. Sometimes an emergency patient with renal colic (acute abdominal pain) is asked to urinate through a special filter used to trap small stones. All radiographic technologists must be certified and registered with the American Society of Radiologic Technologists or an equivalent organization. Continued education credits are mandatory to remain registered.
Risk and side effect
Some of the side effects and possible complications including minor reactions to the contrast dye. It may include flushing, warmth and a metallic taste in the mouth. These usually resolve quickly. These symptoms are much less common with the newer contrast dyes. Some patient might experience severe allergic reactions. It may occur in a small percentage of the population. Symptoms range from relatively mild to severe, and can include hives (skin rash), breathing difficulties, swelling of the lips and tongue, low blood pressure and loss of consciousness. There is case when a patient experienced acute renal failure but it occurs in less than 0.5 per cent of cases. Risk factors include advanced age, diabetes, dehydration and a past history of kidney disease. For patients with these risk factors, extra intravenous fluids, pre-treatment with acetylcysteine, and a reduced dose of contrast dye may be recommended, or they may undergo different procedures altogether.
Problems that found
There are several limitations of ultrasonography, CT, and MRI: lack of visualization for large portions of the urinary tract with ultrasonography, necessity of contrast agent administration and excretory images with CT, inability to visualize subtle urothelial abnormalities with sufficient spatial resolution with both CT and MRI, and insufficiency in visualizing calcifications with MRI. Additional disadvantages of MRI are inconspicuousness of small intrarenal calculi, susceptibility artifact due to metallic objects that interfere with the visualization of ureteral segments, flow-related artifact in some sequences, and interference of hemorrhage into renal collecting system with static-fluid MR urography. The patient effective dose, and therefore radiation risk, of CT urography is 1.5 times that of conventional urography. The increased radiation risk from a CT urography compared with an IVU should be considered in the context of the amount of information that is necessary for the diagnostic task. Radiation risk is increased for smaller patients in CT urography and for larger patients in IVU. Although CT falls short of IVU in the evaluation of urothelium, helical CT technology continues to evolve with introduction of multidetector row scanning (MDCT)[3,9]; MDCT may eventually replace IVU for the evaluation of hematuria[2,4]. Finally, there is not an optimum or ideal examination technique for CT urography or MR urography. Examination techniques must be constructed according to suspected pathology of the patient and urinary system status.
Although advances in imaging technology have given CT and MR urography advantages over IVU, many centers still use IVU as a part of routine radiological practice. Therefore, techniques or modifications for improving application and diagnostic capabilities of IVU should still be considered.
For decades, intravenous urography has been the primary imaging modality for evaluation of the urinary tract. In recent years, however, other imaging modalities including ultrasonography (US), computed tomography (CT), and magnetic resonance (MR) imaging have been used with increasing frequency to compensate for the limitations of intravenous urography in the evaluation of urinary tract disease . Like intravenous urography, however, these examinations have their limitations. Large portions of the urinary tract are not visualized at US; CT requires contrast material administration and excretory images (at times with a prolonged delay), often with image reformatting for evaluation of the urothelium; and MR imaging may not demonstrate calcifications or show the urothelium with sufficient resolution for evaluation of subtle abnormalities. Thus, despite increasing use of these alternative modalities, the ideal “global” urinary tract examination remains controversial . Axial imaging with contrast material opacification of the urinary tract will likely evolve as the most efficient imaging evaluation. However, the declining use of intravenous urography in clinical practice reduces the opportunity to learn important interpretive skills. Formal urography (or the urographic equivalent of conventional radiography of the urinary tract following administration of contrast material for CT) is frequently performed in the evaluation of hematuria. Urography may also be performed in the pre- or posttherapeutic evaluation of stone disease that has been discovered with other modalities.
A barium swallow and meal is type of X-ray test that allows your doctor to examine your throat, oesophagus (the pipe that goes from your mouth to your stomach), stomach and the first part of the bowel (duodenum).
X-rays usually pass straight through parts of the gut such as the oesophagus, stomach and bowel and so these structures don’t show up well on plain X-ray images. However, if the gut wall is coated with barium, a white liquid that X-rays can’t pass through, a much clearer image of the outline of the gut can be captured.
If your stomach is being examined, the test is called a barium meal. If your oesophagus is examined at the same time, it’s called having a barium swallow and meal.
A barium swallow and meal test can help work out why you’re getting symptoms such as difficult or painful swallowing, heartburn, reflux and abdominal pain. The tests give your doctor information about the swallowing action, and
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