FLUOROSCOPY

Fluoroscopy is a dinamic radiographic examination, compared to diagnostic radiography, which is static in character. Fluoroscopy is an imaging technique to obtain real-time images of the internal structures of a patient through the use of a fluoroscope In its simplest form, a fluoroscope consists of an x-ray source and fluorescent screen between which a patient is placed. However, modern fluoroscopes (digital Fluoroscopy) couple the screen to an x-ray image intensifier or Flat-Panels detector and CCD video camera allowing the images to be played and recorded on a monitor. The use of x-rays, a form of ionizing radiation, requires that the potential risks from a procedure be carefully balanced with the benefits of the procedure to the patient. While physicians always try to use low dose rates during fluoroscopy procedures, the length of a typical procedure often results in a relatively high absorbed dose to the patient. Recent advances include the digitization of the images captured and flat-panel detector systems which reduce the radiation dose to the patient still further

Types of Equipment

The Fluoroscopic x-ray tube and image receptor are mounted on a C-arm to maintain their alignment at all times. The C-arm permits the image receptor to be raised and lowered to vary the beam geometry for maximum resolution while the X-ray tube remains in position. It also permits scanning the length and width of the x-ray table. There are two types of C-arm arrangements, both described by the location of the x-ray tube. Under-table units have the x-ray tube under the table while over-table units suspend the tube over the patient. The arm that supports the equipment suspended over the table is called the carriage.

Fluoroscopy Equipment

Fluoroscopy X-Ray Tube

Fluoroscopy X-Ray Tubes are very similar to diagnostic tubes except that they are designed to operate for longer periods of time at much lower mA. The fluoroscopic tube is operated by foot switch, which permits the fluoroscopist to have both hands free to operate the carriage and position and palpate the patient

X-ray Image Intensifiers

An image intensifier is a device that intensifies low light-level images to light levels that can be seen with the human eye or can be detected by a video camera. An image intensifier is a vacuum tube, having an input window on which inside surface a light sensitive layer called the photocathode has been deposited. Photons are absorbed in the photocathode and give rise to emission of electrons into the vacuum. These electrons are accelerated by an electric field to increase their energy and focus them. After multiplication by an MCP (multi channel plate) these electrons will finally be accelerated towards the anode screen. The anode screen contains a layer of phosphorescent material that is covered by a thin aluminium film. When striking the anode the energy of the electrons is converted into photons again. Because of the multiplication and increased energy of the electrons the output brightness is higher as compared to the original input light intensity.
Modern image intensifiers no longer use a separate fluorescent screen. Instead, a caesium iodide phosphor is deposited directly on the photocathode of the intensifier tube. On a typical general purpose system, the output image is approximately 105 times brighter than the input image. This brightness gain comprises a flux gain (amplification of photon number) and minification gain (concentration of photons from a large input screen onto a small output screen) each of approximately 100. This level of gain is sufficient that quantum noise, due to the limited number of x-ray photons, is a significant factor limiting image quality.
Image intensifiers are available with input diameters of up to 45 cm, and a resolution of approximately 2-3 line pairs mm-1.

Flat-panel detectors

The introduction of flat-panel detectors allows for the replacement of the image intensifier in fluoroscope design. Flat panel detectors offer increased sensitivity to X-rays, and therefore have the potential to reduce patient radiation dose. Temporal resolution is also improved over image intensifiers, reducing motion blurring. Contrast ratio is also improved over image intensifiers: flat-panel detectors are linear over a very wide latitude, whereas image intensifiers have a maximum contrast ratio of about 35:1. Spatial resolution is approximately equal, although an image intensifier operating in 'magnification' mode may be slightly better than a flat panel.
Flat panel detectors are considerably more expensive to purchase and repair than image intensifiers, so their uptake is primarily in specialties that require high-speed imaging, e.g., vascular imaging and cardiac catheterization.

Video Camera Tubes

The vidicon and plumbicon tubes are similar in operation, differing mainly in their target layer. A Plumbicon tube has a faster response time than a vidicon tube. The tube consists of a cathode with a control grid, a series of electromagnetic focusing and electrostatic deflecting coils, and an anode with face plate, signal plate and target.

Video Camera Charge-Coupled Devices (CCD)

A CCD is a semiconducting device capable of storing a charge from light photons striking a photosensitive surface. When light strikes the photoelectric cathode of the CCD, electrons are realeased proportionally to the intensity of the incident light. As with all semiconductors, the CCD has the ability to store the freed electrons in aseries of P and N holes, thus storing the image in altent form. The video signal is emitted in a raster scanning pattern by moving the stored charges along the P and N holes to the edge of the CCD, where are discherged as pulses into a conductor. The primary advantage of CCDs is the extremely fast discharge time, which elimantes image lag. This is extremely useful in high speed imaging applications such as cardiac catheterization. Other Advantages are that CCDs are mor sensitive then video tubes, they operate at much lower voltages, which prolongs their life, they have acceptable resolution and they are not as susceptible to damage from rough handling.

Risks

Radiation exposure to patients and laboratory staff has been recognized as a necessary hazard in fluoroscopic procedures. Fluoroscopic procedures pose a potential health risk to the patient and to those staff working close by because of the long length of exposure times. Radiation doses to the patient depend greatly on the size of the patient as well as length of the procedure, with typical skin dose rates quoted as 20-50 mGy/min. Exposure times vary depending on the procedure being performed. Staff doses are linked to patient doses because they result from secondary scattered radiation arising mainly from the patient. Staff may also be exposed to primary leakage radiation that is generated at the X-ray target and which has penetrated the leaded X-ray tube housing. The radiographic projection is relevant in determining the scatter distribution around a patient. Oblique angles lead to higher exposure factors and therefore more scatter. At diagnostic energies, the Compton interaction leads predominantly to back-scatter in the direction of the X-ray tube. This means that there are higher levels of exposure on this side of the patient, which is an important result for the radiation protection education of staff.
So Fluoroscopic units operate with the minimum radiation output possible for the efficiency of the imaging system. The staff has a duty to require that anyone present in the fluoroscopy room during an examination wear a lead apron.