Geometry of image Formation

Question 1

State how X-rays are produced

Answer 1

• Current is passed to the negative part of (cathode) the x-ray tube, made of
  tungsten wire coil
• The cathode is heated and releases electrons in a process called thermionic
  emission
• These electrons are accelerated through a high potential difference between
  the cathode and anode (positive) part of the x-ray tube
• When the electron interaction with the anode, x-rays are produced – a
  phenomenon discovered by Wilhelm Roentgen in 1895
• This occurs in a vacuum tube.
• The anode converts the electrons into 1% x-rays and 99% into heat
• The anode can be rotating or stationary in nature depending on intended
  purpose

Question 2

Anode angle

Answer 2

Is the angle between the vertical and the target surface.

Question 3

Focal spot

Answer 3

Point source of electrons

Question 4

Various distances in radiography

Answer 4

• FFD – Focus to Film Distance
• FOD – Focus to Object Distance
• FRD – Focus to Receptor Distance
• OFD – Object to Film Distance

Question 5

Effective focal spot size.

Answer 5

The beam projected on the patient

Question 5

Actual focal spot

Answer 5

The size as seen from the direction of the electrons from the cathode.

Question 6

Apparent focal spot size

Answer 6

• refers to the area on the anode where the electron beam from the cathode hits and produces X-rays

Question 7

Line focus principle

Answer 7

States that angulation of the anode surface will result in apparent decrease in the focal spot size.

Question 8

Geometric unsharpness( Penumbra)

Answer 8

• Refers to the loss of definition that is the result of geometric factors of the
  radiographic equipment and set up

Question 9

How to minimize geometric unsharpness

Answer 9

• Fine focus should be used where possible.
• The object should be as close to the detector as possible (ideally in
  contact).
• The FOD should be as long as practicable as this minimizes the penumbra.

Question 10

Magnification

Answer 10

• Purposeful production of an image larger than the one being studied.

Question 11

How can magnification be reduced.

Answer 11

• By moving the x-ray source away from the object studied

Question 12

Formula for ug(penumbra)

Answer 12

Ug=FSS x OFD/FOD

Question 13

Disadvantage of the line focus principle

Answer 13

• The anode heel effect

Question 14

The anode heel effect

Answer 14

A condition whereby the x-rays are weaker at the anode side and stronger at the cathode side

Question 15

Importance of the line focus principle

Answer 15

• A steeper angle of the anode results in a smaller effective focal spot, improving image sharpness

Question 16

Applications of magnification in radiography

Answer 16

1. To visualize small structures: Magnification can be used to examine tiny details in structures like bones, joints, or dental fillings.
2. To assess subtle abnormalities: By magnifying an area, it becomes easier to detect subtle changes or lesions.
3. To measure objects: Magnification can be used to calculate the size of objects within the body.