Theatre physics

Question 1

Why are no grids used?

Answer 1

SInce you would have to increase kV and mAs to compensate, we do not need a good diagnostic image

Question 2

What is the 10Kvp rule?

Answer 2

Increasing the tube voltage by 10kVp, whilst reducing the tube current (mAs) by 50% produces similar perceptual image quality.

Question 3

Why should the image intensifier be on top?

Answer 3

• Reduce magnification
• Better SR
• Reduces skin dose as more scatter is absorbed in air as the tube is further away

Question 4

X-ray tube at bottom?

Answer 4

Due to backscatter- to reduce scatter towards the surgeon-scatter from the patient is the main source of radiation to the staff

Question 5

Advantages of pulsed fluoro

Answer 5

-Aims to reduce the dose to pts and staff by using short pulses of radiation

Question 6

Why shouldn’t half dose be used for first time?

Answer 6

• So that machine calculates dose needed

Question 7

Why should pulsed flouro be used for dhs?

Answer 7

to reduce dose since area of interest and fracture can be easily be visualised on x-ray

Question 8

What pulse rate should be used?

Answer 8

10 pulses or less

Question 9

What is contrast?

Answer 9

Difference in densities

• bone on bone (spongy and cortical bone)
• bone on soft tissue
• Bone and air

Question 10

What is contrast res?

Answer 10

Difference in grey scales

Question 11

What is SR

Answer 11

• The ability to image 2 separate objects
• The smallest distance that 2 objects can be separated and still appear distinct on an image.
• Good trabecular detail
• Sclerotic lines

effected by Sharpness- no geometric or motion unsharpness

Question 12

Noise

Answer 12

• Random disturbances that obscure image quality and result in a grainy image.
• High SNR is desirable over low snr
• Different components of noise are: radiation noise – “heel effect”, structure noise-Compton scattering, receptor noise- non-uniform response to a uniform x-ray beam and quantum mottle (low photon flux).

Question 13

Exposure

Answer 13

• Relative darkness of an image
• Effected by the mA
• Changing the overall number of photons produced will affect the overall blackness of the image

Question 14

Overexposure

Answer 14

• The image appears dark with a lack of noise while appearing ‘saturated’ or in extreme cases ‘burnt out’ whereby anatomy is completely obliterated from the radiograph.
• For digital images, overexposure due to the use of excessive mAs is not apparent in the quality of the image. The only means for the radiographer to become aware of overexposures is to monitor the exposure indicator readout which may or may not be annotated on the image.
• Overexposure must not be assumed when excessive scatter radiation is produced—it is possible for underexposure to occur along with an excessive proportion of scatter radiation in the remnant x-ray beam.

Question 15

Underexposure

Answer 15

• Occurs due to choosing a low mAs for the patient.
• The image appears noisy (quantum mottle) grainy and displays poor penetration of the mediastinal structures leading to an inaccurate representation of anatomy.
• The image appears white.