Fluoroscopy

Question 1

This produce projection x-ray images and allow real-time x-ray viewing of the patient with high temporal resolution.

Answer 1

Fluoroscopy systems

Question 2

What is “real-time” imaging of fluoroscopy?

Answer 2

Usually considered to be 30 frames per second and sufficient to provide the appearance of continuous motion

Question 3

What distinguishes fluoroscopy from radiography?

Answer 3

The principal feature of the imaging chain is the ability to produce real-time x-ray images with high frame rates and a low-dose per image.

Question 4

Standard fluoroscopy uses how many nGy?

Answer 4

9 to 17 nGy (1 to 2 uR) upon the dectector per image

Computed radiography system requires an exposure of about 5 to 9 uGy (0.6 to 1 mR)

Question 5

What are the four principal components of a fluoroscopic detector system?

Answer 5

a. Vacuum housing - to keep air out and allow unimpeded electron flow
b. Input layer - converts the absorbed incident x-rays into light, which in turn releases electrons
c. Electron optics system - accelerates and focuses the electrons emitted by the input layer onto the out layer
d. Output phosphor - Converts the accelerated electron into a visible light image

Question 6

The input screen of the image intensifier consists of four different layers. What are these layers?

Answer 6

First layer - Vacuum window

Second layer - Support layer

Third layer - Input phosphor (Cesium iodide)

Fourth layer - Photocathode

Question 7

What is a vacuum window, and what is its function?

Answer 7

It is a thin (typically 1 mm) aluminum window that is part of the vacuum containment vessel.

It keeps the air out of the image intensifier, and its curvature is designed to withstand the force of air pressing against it.

Question 8

What is the function of the support layer?

Answer 8

Supports the input phosphor and photocathode layer.

The support, commonly a 0.5 mm aluminum, is the first component in the electronic lens system, and its curvature is designed for accurate electron focusing.

Question 9

What is the function of the input phosphor (cesium iodide)?

Answer 9

Its function is to absorb the x-rays and convert their energy into visible light.

This 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 lateral dispersion of light through the phosphor.

Question 10

What is a photocathode and what does it do?

Answer 10

It is a thin layer of antimony and alkali metals that emits electrons when struck by visible light.

With 10 to 20% conversion efficiency, approximately 400 electrons are released from the photocathode for each 60 to keV x-ray photon absorbed in the phosphor.

Question 11

The curved surface of the input screen is necessary for proper electron focusing, but what happens to the image?

Answer 11

It causes unavoidable pincushion distortion of the image

Question 12

The output phosphor are made of what material?

Answer 12

Zinc cadium sulfide doped with silver, which has a green emission spectrum.

The anode is a very thin coating of aluminum on the vacuum side of the output phosphor, which is electrically conductive to carry away electrons once they deposit their kinetic energy in the phosphor.

Question 13

What are characteristics unique to image intensifier systems of fluoroscopy?

Answer 13

Brightness gain

Pincushion distortion

S distortion

Question 14

This is the product of the electronic and minification gains o the image intensifier.

Answer 14

Brightness gain

As the effective diameter (FOV) of the input phosphor decreases (increasing magnification), the brightness gain decreases.

Question 15

This is a result of projecting the image with a curved input phosphor to the flat output phosphor.

Answer 15

Pincushion distortion

This worps the image by stretching the physical dimensions in the periphery of the image.

Question 16

This is a spatial warping of the image in an S shape through the image.

Answer 16

S distortion

This type of distortion is usually subtle, if present, and is the result of stray magnetic fields and earth’s magnetic field affecting the electron trajectory from the cathode to the anode inside the II.

Question 17

This produces a continuous x-ray beam typically using 0.5 to 6 mA (depending on patient thickness and system gain).

A camera displays the image at 30 FPS, so each fluoroscopic frame is displayed for 33 ms.

Answer 17

Continuous fluoroscopy

This mode is the most basic approach to fluoroscopy acquisition, typically used on all analog systems, and was the standard operating mode for image-intensified fluoroscopy up until 1980.

Question 18

How does pulsed fluoroscopy works?

Answer 18

The x-ray generator produces a series of short x-ray pulses.

The system can generate 30 pulses/s, but each push can be short in time.

The exposure time ranges from about 3 to 10 ms instead of 33 ms, which reduces burring from patient motion (pulsatile vessels and cardiac motion) in the image.

Question 19

What is fluoroscopy mode of operation is used for procedures where object motion is high?

Answer 19

Pulsed fluoroscopy

It offers better image quality at the same average dose rates.

It allows fluoroscopist to reduce temporal resolution when it is not needed, sparing dose.

Question 20

How is magnification produce is fluoroscopy?

Answer 20

It is produced by changing the voltages applied to the electrodes in the image intensifiers, resulting in electron focusing from a smaller central area of the II’s input screen to the entire output phosphor.

Magnifications modes - yield higher spatial resolution for a smaller field of view

Question 21

What happens when the magnification mode is engage?

Answer 21

The x-ray beam collimator automatically adjusts to match the x-ray beam dimensions in the smaller FOV.

To minimize patient dose, the use should use the least magnification and smallest collimation area that facilitates the diagnostic task at hand.

Question 22

Fluoroscopy images are relatively noisy, and under certain circumstances it is appropriate and beneficial to (reduce) temporal resolution for lower quantum noise. How can this be accomplished?

Answer 22

By averaging a series of images or frame averaging.

Question 23

This continuously displays the last acquired image or set of images on the fluoroscopy monitor.

Answer 23

Last-frame-hold

This is a standard feature on modern fluoroscopy systems, and is achieved by continuously digitizing images in real time and temporarily storing them in a digital video frame memory.

Question 24

This is a software- and video-enhanced variant of the last-frame-hold feature and is useful for angiography procedures.

Answer 24

Road mapping

Question 25

How is spatial resolution for fluoroscopy assess?

Answer 25

For routine quality control testing in fluoroscopy, a visual assessment of a resolution test object imaged with minimal geometric magnification is commonly used.

Intrinsic limiting resolution is quite high, ranging from 3 to 5 cycles/mm; however, the limiting resolution of the entire system is most often governed by the video device and effective sampling matrix size across the FOV.

Question 26

How to overcome the spatial resolution limitations of a fluoroscopy system?

Answer 26

Geometric magnification, when used in conjunction with a small focal spot, is sometimes useful in overcoming the spatial resolution limitations.

This is achieved by moving the detector away from the patient.

Question 27

What is the downside of geometric magnification?

Answer 27

Large increase in dose necessary to address the increase in source detector distance and the reduced anatomical coverage for a given detector field at view.

Question 28

Why is contrast resolution of fluoroscopy compared to radiography low?

Answer 28

It is chiefly due to the low signal-to-noise ration (SNR)

When higher exposure rates are used contrast resolution increases, but dose to the patient also increases.

Question 29

Scattered radiation from the patient has a significant impact on contrast resolution. What material is employed for this situation?

Answer 29

The use of antiscatter grid

However, the use of a grid causes a 2 to 4 times increase in radiation dose to the patient because of compensation by automatic exposure rate control (AERS) due to attenuation of x-rays by the grid.

Question 30

Temporal blurring is typically called what?

Answer 30

Lag

It implies that a fraction of the image data from one frame carries over in the next frame, and this can be intentionally be affected using algorithm such as recursive filtering.

Question 31

This is the ability to localize object in time from frame to frame and follow its movement.

Answer 31

Temporal resolution

Question 32

The maximum permissible entrance exposure rate to the patient for normal fluoroscopy is how many mGy/m?

Answer 32

87.3 mGy/m (10 R/m)

Question 33

For specialized activated fluoroscopy, the maximun exposure rate allowable is how many mGy/m?

Answer 33

175 mGy/m (20 R/m)

Question 34

Typical entrance exposure rates for fluoroscopic imaging are about how many mGy/m?

Answer 34

8.7 to 17 mGy/m (1 to 2 R/m) for thin body parts and 26 to 44 mGy/m (3 to 5 R/min) for average patient.

The dose rate may be much larger for oblique and lateral projections and for obese patients.

Question 35

Standing 1 m from the patient, the fluoroscopist receives how much scattered radiation?

Answer 35

As a rule of thumb, standing 1 m from the patient, fluroscopist receives from scattered radiation approximately 1/1,000 of the exposure incident upon the patient.

Question 36

How to reduce radiation exposure to fluorscopist?

Answer 36

Movable lead shields should be available

Reducing total fluoroscopy time

Using variable rate pulsed fluoroscopy at low pulse rates

Aggressive beam collimation

Stepping back from the radiation field

Protective apron, lead eyeglasses, thyroid shield

Question 37

What is stray radiation?

Answer 37

It is comprised of both leakage and scatter radiation, and has a distinctly non-uniform distribution, chiefly due to backscatter radiation.

Question 38

How to measure radiation exposure of personnel?

Answer 38

A number of technologies such as thermoluminsecent dosimeter, film, and other dose integration materials can be used.

Question 39

When are doses reported?

Answer 39

Monthly or quarterly, to ensure that these individuals do not exceed dose limits defined by the institution or state and to assess the effectiveness of dose-reduction measures.

Question 40

It is customary to wear a single dosimeter where?

Answer 40

At the collar level, in front of the protective apron.

A second dosimeter may be worn on the body under the apron.

Finger dosimeter may be worn to monitor the dose to the hand likely to receive the largest exposure.

Question 41

What are techniques to minimize the dose in fluoroscopy?

Answer 41

Heavy x-ray beam filtration (0.2 mm copper)
Aggressive use of low frame rate pulsed fluoroscopy
Use of low-dose (higher kV, lower mA) AERC options

Aggressive beam collimation should be used.

Question 42

Collimation does not reduce the skin does, but it limits what?

Answer 42

It does limit the area irradiated, reduces the effective dose tot he patient, improves image quality by reducing the amount of scattered radiation, and reduces dose to staff in the room.

Question 43

What s the most effective way to reduce patient dose during fluoroscopy?

Answer 43

Use less fluoroscopy time.

Question 44

What is the purpose of automatic exposure rate control?

Answer 44

To keep signal to noise ration of the image constant when possible.

AERC - Formerly known as automatic brightness control