Week 11: Fluoro Image Quality and Acquisition

Question 1

Minification Gain of II

Answer 1

• concept that occurs within the II when a larger input screen produces electrons, then the electrons must travel to the small output screen
• the same number of electrons must be compressed to the smaller area, which increases the brightness or intensity of the image as a result

Question 2

Automatic Brightness Control (ABC)

Answer 2

• sometimes called automatic brightness stabilization (ABS)
• maintains the image density and contrast at the image monitor by adjusting the radiation output of the x-ray tube (mA, kV, pulse width)
• a detector is typically placed at the output screen of the II or it can monitor the current flowing between the cathode and anode
• uses variable kV, variable tube current or variable pulse width

Question 2

Total Brightness Gain

Answer 2

• the measurement of the increase in image intensity achieved by an image intensification tube
• the overall product of flux and minification gain

Question 3

Flux Gain of II

Answer 3

• the ratio of the number of light photons at the output phosphor to the number of x-ray photons at the input phosphor, thus producing the conversion of the electron energy into light energy
• essentially how many output light photons do you get for every x-ray that strikes the input phosphor
• not enough radiation causes not enough light which causes QM = decreased SNR

Question 4

Automatic Gain Control

Answer 4

• adjusts the image brightness like ABC but is is part of the video camera control system
• it adjusts the video signal rather than the technical factors so there is no change in patient dose

Question 5

Flare

Answer 5

• occurs when moving from one anatomical part to another
• a sudden increase in image brightness at thinner/less dense areas
• ABC is used to minimize flare

Question 6

Electronic Magnification with II

Answer 6

• occurs within the II and occurs when the tech engages mag mode on a multifocal II
• sends a signal to increase the voltage on the electrostatic lens which allows the electrons to be accelerated and focused at a point closer to the input screen
• focal point moves further away form the output screen resulting in a magnified image
• FOV on the input screen is also reduced and therefore fewer x-ray photons will be utilized which in turn reduces the minification gain so an increase in mA is required

Question 7

What are the four factors affecting fluoro image quality

Answer 7

1. contrast
2. spatial resolution
3. distortion
4. quantum mottle

Question 8

Contrast Resolution

Answer 8

• scatter (same as gen rad) emitted form patient affects contrast
• new factor is excess light scatter or non patient related scatter either at the input/output screens or within the II

Question 8

Veiling Glare

Answer 8

Arises from incoming light/x-ray photons that stray from the normal image forming path, and reach the output phosphor. A aluminum filter is set in place, however it is not 100% effective in blocking light photons. These two types of fogging decrease the visible contrast in fluoroscopic imaging.

Question 9

Spatial Resolution

Answer 9

Gen rad has typically 10 lp/mm of spatial resolution usually only offers 2-5 lp/mm. Fluoro images can be made brighter by increasing technique but this will reduce spacial resolution because the light beam will not be as focused.

Question 10

Effects of Minification and Flux Gain

Answer 10

The electron beam tends to become progressively defocused as image brightness is increase because it is difficult to overcome the greater mutual repulsion between the more densely packed electrons in the beam. Using proper exposure factors and changing screen brightness can improve spatial resolution.

Question 11

Effects of Magnification

Answer 11

Resolution can be increased to about 6-7 lp/mm when the mag mode is used. Most II times provide optimum sharpness in the central regions of the image, with reduced sharpness at the periphery due to unequal focusing of electrons on to the output phosphor. The use of mag mode will increase resolution as the peripheral edges are not visible.

Question 12

Effects of OID, SID and SOD

Answer 12

Reduce OID as much as possible because the negative impacts it has on the resolution. Increased OID ill result in an increase in penumbra and decrease in resolution. Over the table tubes have the shortest OID so better spatial resolution.

Question 13

Size Distortion

Answer 13

Can be caused by changes in OID, making an imaged magnified. The distortion will appear even worse when in mag mode but this is only because the image is already enlarged and is easier to see.

Question 14

Shape Distortion

Answer 14

Primarily caused by the design of the II and may make up 8-10% of the imaging area.

Question 15

Pincushion Distortion

Answer 15

The concave input phosphor was designed to ensure electrons had the same distance to travel to the output phosphor however the electros from the peripheral edge often flare outward due to repulsion by other electrons or attraction to the electrostatic lens. Only occurs in II not FPD.

Question 16

Vignetting

Answer 16

Distortion caused by greater exposure intensity at the centre of the image compared with the edges. This happens because of the divergence of the beam and it is most noticeable when using large IIs. Electrons further from the centre are harder to control. Looks like old style movies or pictures with shadowed borders.

Question 17

High Level Fluoro

Answer 17

Reduces QM during fluoro but SC35 requires audible beeping to occur when in use because of the high dose.