Radiology Online - Fluoroscopy
Diagnostic x ray
Gamma ray which is emitted by an agent that is used in nuclear medicine has the highest frequency and the shortest wavelength compare to others such as x rays, visible light, radio waves or microwaves. This is followed by x ray which also has a high frequency and wavelength. The energy of the x ray beam is around 20 – 150 KeV.
The X ray tube is used in generating the x ray beam. In the x ray tube, there is filament which is made from tungsten wire that is coiled and a target which is also made from tungsten. The filament is represented the negative charged cation while the target represented as a positively charged anion. The filament /negatively charged cation is the source of electrons. These electrons will later accelerate towards the target which is a positively charged anode /anion.
X ray is produced in the form of an x ray photon. The electrons that emitted from the filament have a high energy level. The electron will be accelerated towards the anode/target. The electrons are negatively in charges while the target/anode is positive in charges. There will be an attractive force between the electron and the atomic nucleus of the target. This will lead to a change in a direction of the electron as well as loss of energy. The differences between the initial energy level of the electron and the energy level of the electron after it changes direction are released as an x ray photon. This condition is known as braking radiation or “bremsstrahlung”
Most of the energy that enters the x ray tube will be converted into heat (99%) and only 1% is converted into x ray. The present of the heat is a limiting factor in the generation of x ray in the x ray tube.
There are 3 parameters of the x ray tube that can be manipulated in generating the x ray beam. These include current that flow through the x ray tube (milliampere), the voltage across the tube of x ray ( Kilovolt) and the time of exposure ( mill isecond). The current and time exposure may be combined into milliampere second.
If we increase the kilovolt, it will result in a darker film due to increase in exposure and an increase in the x ray penetration. The contrast will be decreased. The kilovolt manipulation has a strong effect on the contrast. Besides that increasing the voltage across the x ray tube may also increase the amount of energy produced.
If we increase the milliampere second ( mAs) it will lead to an increase in the x ray production that darkens the film as well as increasing the film exposure. However the maximal energy of the x ray is unaffected.
A plain film is generated by placing the patient between the x ray tube and the film cassette. The x ray which is produced by the x ray tube will pass through the patient/human body and will be attenuated by the interaction of the x ray and the human tissue. The film cassette consists of film and the fluorescent screen. The x ray will undergo photochemical reactions as it passes for the film onto the screen coated with fluorescent particles. Light rays will be emitted as the x ray activates the fluorescent particle. The light ray will expose the photographic film and finally an image will be produced.
The x ray tube will produce x ray beam. The x ray will pass through the collimator. Collimator consists of sheets of leads and it will narrow the beam of x ray. The x ray will later travel through the grid. The grid consists of narrow strips of lead that is separated by plastic. The function of the grid is to stop the scattered x ray that contributes to decrease contrast and noise and to allow the direct passage of x ray which has directly passed through the patient. The x ray will penetrate the film cassette after passing the grid. The film cassette is a standard dual screen system which consists of first intensifying screen and the film as well as the second intensifying screen. The x ray photon will be absorbed by the screen and the visible light will be emitted. The film will be exposed by the light.
The inverse square law states that the intensity of the beam of x ray will be decreased with the square of the distance from the x ray tube. As an example, the intensity of the x ray exposure decreases by a factor of 4 if the distance between the patient and the x ray is double. This law is important in determining the absorbed dose and the x ray exposure.
The source if the x ray in the tube is presented as a focal spot. The small focal spot able to produce a sharp image while large focal spot able to tolerate greater amount of heat. Mammography involves the application of small focal spot to produce a sharp image with little exposure. Fluoroscopy involves the application of large focal spot that leads to continual exposure with lesser resolution. Regular diagnostic x ray usually involves the use of focal spot between the sizes of fluoroscopy and mammography.
Air, metal, bone, fat and soft tissue represent 5 basic densities seen on an x ray. Air appears black on the x ray. In this scenario, the air will only attenuate a small portion of the x ray beam. This will allow the full force/radiation to darken the film. Metallic object and bone appear to be white on the x ray. Metallic object and bone will attenuate a large portion of the x ray beam and cause very little radiation to blacken the film. Fat and soft tissues appear as shades of grey on the x ray film. The soft tissue and the fat will intermediately attenuate the amount of the x ray beam.
Computed radiography differs from the film screen radiography. Computed radiography consists of a layer of phosphor. The phosphor layer is able to capture the information from the x ray exposure. The electrons are trapped in the phosphor layer and the lasers are used to compel the electrons to emit light. The pattern of the light produced is stored electronically as a digital image. The contrast and brightness of the digital image can be manipulated by the observer.
The mammography film screen combination appears to be slower than film screen combination for the chest and abdomen imaging. This means that mammography film screen combination is thinner with higher spatial resolution.
There are differences between posteroanterior film view and anteroposterior film view. The differences are based on the path of the x ray beam. In a standard posteroanterior film, the beam of x ray will travel and enter the patient from the back and the image is acquired from the front of the patient. The anteroposterior film is mostly associated with a portable chest radiograph. The beam of x ray is transmitted, travel and enter the patient from his front and the image is taken/captured on the film cassette located at the back /behind the patient. Mammographies and radiographic views may follow the same naming convention.
Fluoroscopy involves the real time visualization of the anatomic structure by the manipulation of the continuously emitted x ray. A continuous beam of x ray will be emitted from a diagnostic tube located beneath the table. The x ray beam will be pass through the patient and finally fall onto the fluorescence screen and the image intensifier which are located above the patient. A faint light will be emitted from the fluorescence screen and this light will be amplified electronically by the image intensifier. The image later will be displayed on the screen of the television. A film may be placed between the patient and the image intensifier to obtain a radiographic image of interest. The film is supposed to pulse of radiation.
Image intensifier will reduce the amount of radiation substantially to produce a clinical image which is useful. Things such as oral contrast agent or bones which supposed to appear white on x ray appear dark on fluoroscopy. The things that supposed to be darker such as air on x ray will appear to be white.
A standard contrast agent such as barium sulphate is useful for upper and lower gastrointestinal tract fluoroscopic contrast study. There are two types of barium sulphate which include thick viscous suspension and “thin” more fluid suspension. The single contrast study usually involves “thin” more fluid suspension contrast while double contrast studies involve the use of thick viscous suspension. Gastrografin or water soluble iodinated contrast agent is used of instead of barium to avoid any barium peritonitis if perforation of the gastrointestinal tract is suspected ( high mortality rates). Contrast agent which is water soluble will be reabsorbed quickly through the peritoneal surface. Chemical pneumonitis may occur as a result of aspiration of water soluble agent. This should be avoided in patients with risk of aspiration.
The scout film is important in fluoroscopy studies. The scout film is important before barium enema examination. The adequacy of bowel preparation can be determined by scout film. This will avoid any unnecessary investigation and examination of the retained stool. The scout film is important to differentiate between calcium and contrast agent which appear white on radiographs. The white matter such as a the contrast agent or calcium may be seen after contrast administration. The present of the contrast agent and calcium can be differentiated by looking at the pre contrast/scout image. If the white matter present it is calcium and if the white matter only present after contrast administration, then it is the contrast agent.
There are 4 types of fluoroscopic studies which include small bowel follow through (constipation and diarrhoea) with the aim to rule out Crohn’s disease and other bowel pathology. Esophagogram is useful to rule out mass or stricture when the patient complains of dysphagia. The other two are barium enema to rule out any mass or polyp if the patient presented with rectal bleeding and upper gastrointestinal study to rule out duodenal or gastric ulcer disease in case of abdominal pain.
Digital subtraction angiography may involve the electronic subtraction of the precontrast image from an image produce after intravascular contrast injection . With this technique , background structure such as bone and soft tissue has been removed and provide a greater contrast to background image.
Mammography differs from abdominal and chest imaging based on the fact that mammography requires higher image contrast (lower kV) and shorter exposure time (higher mill ampere).
Craniocaudal (cc) and mediolateral oblique (MLO) views are two standard views of the mammography. The standard views are named based on the direction of the x ray beam. In craniocaudal view, the x ray beam will enter the cranial portion of the breast and the breast will be transverse by the x ray beam. The x ray beam will leave the caudal sites of the breast into the film. Metallic markers are used to indicate the type of view. Metallic markers are placed close to the axilla. Metallic markers are positioned laterally on the craniocaudal view and located superiorly on the mediolateral oblique view.
Besides the craniocaudal view and mediolateral view, there are other views that can be taken. The other views are preferred when an abnormality is seen on one of the breasts or the lesions/abnormalities is seen in both views. Additional views may also obtain in case of further characterization of the lesion is required or to justify either the apparent mass is just a superimposed tissue or real. A 90 degree mediolateral view is required if a lesion is present on one of the two standard views to help localize the lesion. Rolled view is another alternative. The purpose of rolled view is to place the area of the abnormal / potential lesion further away from the adjacent tissue that will obscure the lesion.
Diagnostic mammography may also involve in the use of magnification view and spot compression view. Magnification view is useful to visualize calcification that may present or it to provide a better characterization of the mass. Spot compression views are useful to identify if the lesion /density “pressed out” or not. “Pressed out” means that on compression view, the lesions/mass is an actual mass rather than a normal breast tissue superimpostion.
There are 4 views that are required if the woman presented with breast implants. The woman required two craniocaudal views and two mediolateral oblique views. In craniocaudal views, one view may involve the implant being placed on the field of the view and in another view the implant may be displaced out of the field of view. A similar concept is applied to the two mediolateral oblique views.
There are two main types of tissue of the breast. There are the fat tissue which appears darker and the fibroglandular tissue which appears brighter.
Mammography usually used a single screen rather than a dual screen. Dual screen is more useful in diagnostic x ray. A single screen increases the sharpness of the image than a dual screen. The sharpness of the image is important to characterize any calcification. Besides that dual screens produce a more scatter image than a single screen
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