Beam geometry

Question 1

What is the actual focal spot on the anode?

Answer 1

where x-rays and heat are produced.

Question 2

What is the effective focal spot?

Answer 2

the beam heading out to the patient.

Question 3

What is the effect of decreasing the anode angle on the focal spot?

Answer 3

-narrows focal spot

Question 4

What is the effect of decreasing the anode angle on the field size?

Answer 4

-decreases the field size.

Question 5

What is the anode heel effect?

Answer 5

-Described differential in x-ray beam intensity with decreasing intensity from cathode to anode.

Question 6

Describe 3 parameters that can be modified to improve heel effect.

Answer 6

1. Anode angle: increasing the anode angle reduces the heel effect.
2. Source to image distance (SID): if move the image away, the heel effect decreases.
3. Change field size via collimation: remove x-rays number. This results in a smaller differential in intensity.

Question 7

Why is anode heel effect useful?

Answer 7

-Using the anode heel effect to our advantage: the pelvis is denser and can use the more intense beam (more penetration) to go through this and the less dense to go through the thorax –> the detector has a more even exposure.
-Same with mammography: the denser pec muscle is plated near the cathode end of the field and the less dense breast near the anode end of the field.

Question 8

Why do we use filtration? How does filtration occur?

Answer 8

-Decrease patient dose. (Lots low energy photons produced in Bremms radiation that don’t contribute to image).
-Occurs via PEE.

Question 9

Describe inherent filtration and added filtration

Answer 9

1. Inherent filtration: occurs in x-ray tube – can’t be changed as is part of the x-ray tube itself (glass envelope, conducting oil and glass window. Attenuates lower energy x-rays.
2. Added filtration: could be aluminium/beryllium to further attenuate low energy x-rays.
   *Filtration reduces x-ray beam quantity and are increases the average energy of the beam = increases the x-ray beam quality and penetrability.

Question 10

What is the goal of Collimation?

Answer 10

-Collimation: gives a narrower field of view and reduces scatter/noise. Provides better contrast and better spatial resolution. Also decreases patient dose.
*Does not affect x-ray spectrum or energy of beams (is not like filtration).

Question 11

What is the aim of the primary circuit in x-ray machine?

Answer 11

-Converts hospital supply (eg 220V) to xray tube requirements (100KeV)
-Achieved via step up transformer.

Question 12

What is the aim of the secondary circuit in x-ray machine? What is the importance of kVP

Answer 12

-Achieve rectification of current + generation (using high frequency inverter to generate kVp) to supply anode and cathode
-kVp: determines energy and number of electrons going from cathode to anode. The higher the kVP the higher the tube current (number of electrons).