AIM: Ch 9: Fluoroscopy Flashcards
“Real-time” imaging is usually considered to be ____ frames per second (FPS)
30
Modern general-purpose fluoroscopy systems use a pulsed x-ray beam in conjunction with digital image acquisition, which commonly allows variable frame rates ranging from 3 to 30 FPS
____ filtration combined with lower kV values allows angiography systems to provide lower dose operation while still delivering high image contrast for angiographic applications.
Copper
Collimators on fluoroscopy systems utilize ____ for II-based systems and ____ collimation for flat panel fluoroscopy systems.
Circular diaphragms
Rectangular
In addition to added filtration and beam collimation, some fluoroscopy systems also have operator adjustable attenuating ____ to provide additional attenuation at specified locations in the x-ray field, such as the pulmonary space between the heart and chest wall, where excessive x-ray penetration of the pulmonary structures can lead to bright regions in the image.
Wedges
The use of wedges can reduce the glare from these areas, equalize the x-ray beam incident on the image receptor, and lower radiation dose to the patient.
Component of modern II that keeps air out and allow unimpeded electron flow
Vacuum housing
Component of modern II that converts the absorbed incident x-rays into light, which in turn releases electrons
Input layer
Component of modern II that accelerates and focuses the electrons emitted by the input layer onto the output layer
Electron optics system
Component of modern II that converts the accelerated electrons into a visible light image
Output phosphor
First layer of the input screen is the vacuum window that is part of the vacuum containment vessel. The vacuum window keeps the air out of the II, and its curvature is designed to withstand the force of the air pressing against it. What is it made of and how thick/thin?
Aluminum window
Typically 1 mm
It is the second layer of the input screen, commonly 0.5 mm of aluminum, and is the first component in the electronic lens system, and its curvature is designed for accurate electron focusing.
Support layer
Its function is to absorb the x-rays and convert their energy into visible light
Input phosphor
After passing through the Al input window and substrate, x-rays strike the input phosphor, whose function is to absorb the x-rays and convert their energy into visible light. The input phosphor must be thick enough to absorb a large fraction of the incident x-rays, but thin enough to not significantly degrade the spatial resolution of the image by the lateral dispersion of light through the phosphor.
Virtually all modern IIs use ____ for the input phosphor.
Cesium iodide (CsI)
The CsI crystals are approximately 400 mm tall and 5 mm in diameter and are formed by vacuum deposition of CsI onto the substrate
The CsI crystals have a trace amount of sodium, causing it to emit blue light.
The K-edges of cesium (36 keV) and iodine (33 keV) are well positioned with respect to the fluoroscopic x-ray spectrum, which contribute to high x-ray absorption efficiency.
A thin layer of antimony and alkali metals (such as Sb2S3) that emits electrons when struck by visible light.
Photocathode
With 10% to 20% conversion efficiency, approximately 400 electrons are released from the photocathode for each 60-keV x-ray photon absorbed in the phosphor.
X-rays are converted into light which then ejects electrons from the input screen into the evacuated volume of the II. The kinetic energy of each electron is dramatically increased by acceleration due to the voltage difference between the cathode and anode, resulting in ____
Electronic gain
The spatial pattern of electrons released at the photocathode is maintained at the output phosphor, albeit minified
T/F: The curved surface of the input screen, necessary for proper electron focusing, causes unavoidable pincushion distortion of the image
True
All of the following comprise the five-component (“pentode”) electronic lens system of the II, except:
a. G1
b. G2
c. G3
d. Input phosphor substrate (the cathode)
e. Output phosphor
e. Output phosphor
The G1, G2, and G3 electrodes (Fig. 9-2), along with the input phosphor substrate (the cathode) and the ANODE just proximal to the output phosphor, comprise the five-component (“pentode”) electronic lens system of the II
The electrons are released from the photocathode with very little kinetic energy, but under the influence of the 25,000 to 35,000 V electric field, they are accelerated and arrive at the anode with high velocity and considerable kinetic energy.
They shape the electric field, focusing the electrons properly onto the output layer, where the energetic electrons strike the output phosphor and cause visible light to be emitted.
Intermediate electrodes
G1, G2, G3
The output phosphor is made of ____, which has a green (~530 nm) emission spectrum.
Zinc cadmium sulfide doped with silver (ZnCdS: Ag)
The ZnCdS phosphor particles are very small (1 to 2 mm), and the output phosphor is quite thin (4 to 8 mm), to preserve high spatial resolution
The ratio of the area of the input phosphor to that of the output phosphor
Minification gain of an II
It is part of the vacuum enclosure and must be transparent to the emission of light from the output phosphor
Output window
Some fraction of the light emitted by the output phosphor is reflected inside the glass window. Such stray light reflecting inside the output window contributes to ____, which can reduce image contrast.
Veiling glare
This glare is reduced by using a thick (about 14 mm) clear glass window, in which internally reflected light eventually strikes the side of the window, which is coated with a black pigment to absorb the scattered light
T/F: The f-number is proportional to the diameter of the hole (f = focal length/aperture diameter), but it is the area of the hole that determines how much light gets through.
False
The f-number is inversely related to the diameter of the hole (f focal length/aperture diameter), but it is the area of the hole that determines how much light gets through.
Changing the diameter of the hole by a factor of 2 changes its area by a factor of 2, and thus, increasing the f-number by one f-stop reduces the amount of light passing through the aperture by a factor of 2.
T/F: By lowering the gain of the II and associated optics by increasing the f-number, a higher x-ray exposure rate will result, producing lower noise images and higher patient dose.
True
Increasing the gain reduces the x-ray exposure rate and lowers the dose, but reduces image quality.
In the video camera, patterns of light (the image data) are incident upon the TV target. The target is swept by a scanning electron beam in a ____ pattern
Raster scan pattern
The TV target is made of a photoconductor, which has high electrical resistance in the dark, but becomes less resistive as the light intensity striking it increases. As the electron beam (electrical current) scans each location on the target, the amount of current that crosses the TV target and reaches the signal plate depends on the resistance of the target at each location, which in turn is related to the local light intensity. Thus the electrical signal is modulated by the local variations in light intensity, and that is physically how the video system converts the optical image incident upon it into to an electronic signal. Note that this is a process of sampling in time described as temporal sampling frequency with units of cycles/s, with the unit hertz (Hz)
Vidicon video systems typically operate at 30 FPS in an ____ scanning mode to reduce flicker, the perception of the image flashing on and off
Interlaced
The human eye-brain system can detect temporal fluctuations slower than about 47 images/s, and therefore at 30 FPS, flicker would be perceptible. With interlaced systems, each frame is composed of two fields (called odd and even fields, corresponding to every other row in the raster, with the odd field starting at row 1, and the even field starting at row 2), and each field is refreshed at a rate of 60 FPS (although with only half the information), which is fast enough to avoid the perception of flicker.
The CCD reads out the stored charge in a ____ fashion, as discussed in Chapter 7, causing locally stored electrons to be transferred from one row to the next row, along defined columns, to eventually be deposited onto a charge amplifier at the end of each column
“Bucket brigade” fashion
Solid-state cameras are preferred because of their stability, reproducibility, and linearity.
It is the product of the electronic and minification gains of the II
Brightness gain
T/F: Larger dexel and poorer spatial resolution are sometimes necessary for fluoroscopic operation.
True
In some fluoroscopic flat-panel detectors, larger dexels (providing less spatial resolution) are necessary for fluoroscopy because data transfer rate (“bandwidth”) limitations restrict the amount of image data that can be displayed at 30 FPS. The 2 3 2 binning reduces the amount of data by a factor of four, making real-time display feasible. In addition, the low doses per frame used in fluoroscopy results in significant quantum noise, and ultimately the poor statistical integrity (low SNR) limits the ability to see small objects. Therefore, larger dexel and poorer spatial resolution are sometimes necessary for fluoroscopic operation.
Re: thicker subjects:
When the generator responds by increasing the kV, the subject contrast ____, but the dose to the patient is kept ____ because more x-rays penetrate the patient at higher kV.
In situations where contrast is crucial (e.g., angiography), the generator can increase the ____ instead of the kV; this preserves subject contrast at the expense of higher patient dose
Decreases, low
mA
In general: lower KV, better contrast
In thicker subjects, you may modify mA and kV
If contrast is not a concern, you may increase kV, keep radiation dose lower at the expense of the contrast
If contrast is of a concern, you may increase mA, preserved kV and preserve contrast but with much higher patient dose
In pulsed fluoroscopy, the x-ray generator produces a series of short x-ray pulses. The pulsed fluoroscopy system can generate 30 pulses/s, but each pulse can be very short in time. With pulsed fluoroscopy, the exposure time ranges from about ____ instead of 33 ms, which reduces blurring from patient motion (e.g., pulsatile vessels and cardiac motion) in the image
3 to 10 ms
It is produced by changing the voltages applied to the electrodes in the II, resulting in electron focusing from a smaller central area of the II’s input screen to the entire output phosphor
Magnification
As the magnification factor increases, a smaller area on the input of the II is seen (Fig. 9-10). When the magnification mode is engaged, the x-ray beam collimator automatically adjusts to match the x-ray beam dimensions to the smaller FOV
The brightness gain (and hence the overall gain) of the II decreases as the magnification increases.
The AERC circuitry compensates for the lower signal by boosting the x-ray exposure rate
In this approach of frame averaging, the image just acquired, In, is added together with the last displayed image (In–1) using
Recursive filtering
T/F: For routine quality control testing in fluoroscopy, a visual assessment of a resolution test object imaged with maximum geometric magnification is commonly used to assess spatial resolution
False
For routine quality control testing in fluoroscopy, a visual assessment of a resolution test object imaged with MINIMAL geometric magnification is commonly used to assess spatial resolution
Higher magnification modes provide better resolution; for example, an 11-cm mode may have a limiting resolution up to 3.5 cycles/mm
It describes the maximum possible resolution in a digitally sampled imaging system
Nyquist frequency
Overall spatial resolution is also determined by the calibration of electronic and optical focusing of the II/TV system, pincushion distortion of the II at the periphery of the image, and the size of the focal spot.
It is usually measured subjectively by viewing contrast-detail phantoms under fluoroscopic imaging conditions. It is essentially a measure of image noise
Contrast resolution
When higher exposure rates are used contrast resolution increases, but dose to the patient also increases.
Dose rates are evaluated in fluoroscopy by using a tissue-equivalent phantom made of a material such as ____ placed in the field, with an ionization chamber positioned in front of it to measure the entrance skin dose
Polymethyl methacrylate (PMMA, also known as Lucite or Perspex)
The overall sensitivity of the fluoroscopic system in a particular operating mode can be characterized directly, and this approach is used when service personnel calibrate the fluoroscopic systems during installation and service. The grid is removed, and all objects are removed from the FOV, including the table. A sheet of approximately ____ is taped to the x-ray tube port to provide some beam hardening so that the x-ray beam spectrum striking the detector is roughly similar that used during patient imaging
0.5 - 1.2 mm of copper
Typical entrance kerma rates for fluoroscopic detectors range from 8.7 nGy (1 mR) to 44 nGy (5 mR) per frame.
T/F: Collimation reduce the skin dose
False
Although collimation does NOT reduce the skin dose, it does limit the area irradiated, reduces the effective dose to the patient, improves image quality by reducing the amount of scattered radiation, and reduces dose to staff in the room.
