121 ch 10

image intensification
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Terms in this set (60)
image intensification
process in which the exit radiation from the anatomic area of interest interacts with the input phosphor (cesium iodide) for conversion to visible light
input phosphor
made of cesium iodide, receives radiation exiting the patient, emits light photons
photo cathode
Responds to light exiting input phosphor
Emits electrons
output phosphor
Receives electrons from photocathode
Emits 50-75 × more light than received by photocathode
X-ray tube is generally operated at less than
5 mA
kVp control depends on
part being examined
photocathode
thin metal layer bonded to the input phosphor, converts light energy to electron energy
photoemission
electron emission that follows light stimulation
electrostatic focusing lenses
negatively charged focuses electrons and accelerates toward an anode to strike the output phosphor
flux gain
accelerating the electrons increases the light intensities at the output phosphor
how does image intensification produce a brighter imageexit radiation is absorbed by input phosphor, converted to electrons, sent to output phosphor, released as visible light, then converted to an electronic video signal for transmission to the monitorflux gain =# of output light photons / # of input x-ray photonsminification gainreduction in the size of the output phosphor image compared with that of the input phosphor image also increases the light intensitiesminification gain =diameter of the input phosphor squared / diameter of the output phosphor squaredbrightness gainthe ability of the image intensifier to increase the illumination level of the image the product of both flux gain and minification gain and results in a brighter image on the output phosphorbrightness gain =minification gain x flux gainconversion factoran expression of the luminance at the output phosphor divided by the input exposure rate (candela per square meter per miliroentengen per second) numeric conversion factor = 1% of brightness gain valueAutomatic Brightness Control (ABC)A function of the fluoroscopic unit that maintains the overall appearance of the fluoroscopic image by automatically adjusting the kVp, mA or bothwhat does the ABC monitor?monitors the current throguh the image intensifier or the output phosphor intensity and adjusting the exposure factor if the monitored value falls below the preset levelsmagnification mode increases...increases the voltage to the electrostatic focusing lenses which tightens the diameter of the electron stream and the focal spot is shifted further from the output phosphormagnification mode effectonly the electrons from the central area of the input phosphor interact with the output phosphor and contribute to the image, giving the appearance of magnificationmagnification mode in relation to patient doseoperating the image intensifier in one of the magnification modes increases the operators ability to see small structures but increases patient doseMultifield Image Intensificationex 25/17/12 cm in 25 cm mode photoelectrons from the entire input phosphor are accelerated to the output phosphor, as the numbers get smaller the electron focal point moves farther from the output phosphor used to reduce field of viewsmaller phosphorgreater magnification, increased technique and pt. doseA 23/15/10 9mage intensifier tubers used in the 10 cm mode. How much higher is the patient dose in this mode compared with the 23 cm- mode?23^2/10^2= 5.3 times higher dose is higher because more x-rays per unit area are required to form the image, improves contrast resolutionclosed circuit television monitor systemviewing system, visible light from output phosphor is converted to an electronic video signaltwo devices used for video signalcamera tubes and charge coupled devicedifference between camera tubes and charge coupled devicetheir size and the readout processtelevision cameras processcamera tube vacuum 15 chin length encloses electron gun and a photoconductive target, light from output phosphor arrives at target assembly, stream of electrons (beam) from the gun scans target assembly, raster pattern, anywhere there is light intensity an electrical signal will leave the tube, leaves to the television monitor to display the imagecharge couple devicemade of pixels, each is composed of photosensitive material that dislodges electrons when stimulated by light photons, row gates of pixels are charged moving the signal down the row where it is transferred to the capacitors, (each pixel is individually read) the charge is sent as an electronic signal to the television monitorraster patternPattern of horizontal lines swept by an electron beam across the face of a CRT that constitute the image display.what happens once the varying electrical video signals reach the television display monitorthe cathode ray tube has an electron gun that scans the phosphor layer that is in the glass in front of the monitor using the raster pattern typically a 525- line systemc-arm tube locationunder the patientc-arm image intensifier locationabove the patientanalog to digital convertertakes the video signal and divides it into a number of bits (1s and 0s) that the computer understandsthe number of bits into which the signal is divided determines what?the contrast resolutiontwo forms of flat panel detectors for fluoroscopycesium iodide amorphous silicon indirect capture detector and amporhous selenium direct capture detectorcurrent digital dynamic detectors are capable of how many frames per second60 fps must respond rapidlymost commonly used flat panel detectorcesium iodide amorphous silicon indirect capturecesium iodide amorphous silicon indirect capture uses what as phosphorcesium iodide or gadolinium oxysulfidecesium iodide amorphous silicon indirect capture photodetectoramorphous silicon, a liquid that can be painted onto a substrate, makes flat panel detectors possibleTFTthis plate absorbed light by the photodetectors and then is converted into electrical charges, then are captured and transmitted by the TFT array to the monitor for displaywhat kind of dose does fluoroscopy operate atlow dose, any noise messes up the imageflat panel instead of image intensifier advantagesa lot smaller, can do dynamic and static image directly sent to PACS, more durable, wider field for images= more information, better contrast, don't have veiling glare, DELs are grid pattern so no pincushion or vignetting, half the patient dosemA used to operate a digital fluoroscopic unit100-1200 mAmA used in image intensified fluoroscopy.5-5 mAhow is the x-ray beam operated in digital fluoroscopyautomatically pulsed to reduce the heat loading on the tubecontinuous flouroscopy modex-ray exposure continues without interruption while the pedal is activated, 30 fps, high pt dose, motion may be visiblepulsed fluoroscopy modex-ray exposure not continuous has gaps of no exposure between frames. 2-15 fps, operator selects pulse rate, kv and ma increase but dose is decreased because its not continualmodern fluoroscopic unit low dose ratereduce dose to patient but increase noise in imagesmodern fluoroscopic unit high dose rateincreases patient dose but decreases noiselow pulse mode but high dose ratepatient dose is highermetods to minimize patient dose during flouroscopyomission of grid minimal use of mag. last image hold where exposure is not activated while the operator reviews the imageradiation exposure datadose area product DAP and cumulative air karmaexposure on table should not exceed what10 R with ABC 5 R withoutstationary flouro unit SSDno less than 38 cmmobile c-arm flouro unit SSDno less than 30 cmdoing what to ssd and OID will decrease pt doseincreasing SSD and decreasing the distance between the patient and the image receptor (OID)lead needed for safety during flourolead aprons .5 mm bucky slot clover .25mm cover opened space at table lead curtain .25mm between pt. and operatorto view a fluoroscopic image on a monitor what must happenit must be converted to an electrical signal by a camera tube or a CCD