Lec 2: Producing Radiographic Images

Question 1

What is the focal spot?

Answer 1

The same thing as the TARGET- it’s a spot made of a tungsten alloy on the anode where the electrons will hit to produce the x rays.

Question 2

What should the angle of the focal spot be?

Answer 2

5-15 degrees

Question 3

What is the heel effect?

Answer 3

Due to the angling of the focal spot the radiation has a higher intensity closer to the cathode, and a lower intensity closest to the anode. Thicker parts of the patient should be placed closer to the cathode.

Question 4

What happens if you have a larger focal spot?

Answer 4

You get more blurring of the edges (“penumbra”), and therefore poorer image quality. It can use higher exposure factors (mAs, kVp), cause it disperses heat over a larger area.

Question 5

When would we use a large focal spot?

Answer 5

When radiographing thicker tissues, e.g. thorax, abdomen, spin, pelvis. This way we can increase the quantity of x rays and reduce the exposure time. It also prevents overheating and damage to the anode. It can also reduce movement artifact.

Question 6

What is the function of filters in the x ray tube housing?

Answer 6

They produce a cleaner image by absorbing the lower energy x-ray photons that tend to scatter more.

Question 7

What is the kVp?

Answer 7

Stands for kilo Volt Potential- it controls the AMOUNT OF ENERGY the x rays have, and therefore the quality of the image. The greater the voltage difference between cathode and anode, the faster the electrons move, and the more energy is available when they crash. So if you have a higher kVp you get higher energy x-rays with more tissue penetration.

Question 8

What is mAs?

Answer 8

Stands for milli Amp seconds. It controls the NUMBER of x-rays produced. The greater the current through the filament, the greater the number of x-rays are produced, the greater the beam INTENSITY. A higher mAs produces a greater NUMBER of x-rays, giving a higher intensity. A more x-rays are produced, the more will hit the film and interact, so the blackness of the film also increases with an increased no of x-rays.

Question 9

What is the SID?

Answer 9

Stands for Source-Image Plate Distance. It should always be set to 70-100 cm, and re-checked every time the machine is moved. It is measured from the origin CENTRE OF THE TUBE. It affects the intensity of the beam, i.e. the number of x-rays that reach the plate.

Question 10

What is the inverse square rule?

Answer 10

The intensity of the radiation is inversely proportional to the square of the distance from the source of the radiation.

Question 11

What happens when we shorten the SID?

Answer 11

It causes increased image magnification and blurring of the edges- penumbra effect.

Question 12

When we need to change the SID, the kVp should change also. T/F

Answer 12

FALSE- this does not affect the penetrating power of the x-ray beam, so it stays the same as SID changes.

Question 13

What is OID?

Answer 13

Object the Image Plate Distance. It’s the distance from the object being imaged to the recording surface (plate). It should be as short as possible to minimise penumbra effect and minimise magnification. The closer you are the more detail you get in the image.

Question 14

What is geometric distortion? (to do with OID)

Answer 14

When the object is not parallel to the imaging plate, and it distorts the size and shape of the object. You can get foreshortening and magnification and/or elongation. So images, e.g. femurs, can look much shorter than they actually are

Question 15

Name three things that cause variation in x-ray penetration.

Answer 15

1. Atomic weight (H vs Pb)
2. Tissue density (gas vs liquid)
3. Tissue thickness (1cm vs 10cm)
   Attenuation= absorption + scatter

Question 16

What tissues cause the most attentuation of x-rays?

Answer 16

Bone, teeth even more. Gas/air stops x-ray stops x-rays the least (this gives varying shades of grey to the different tissues in the image).

Question 17

What is the radiographic density?

Answer 17

The degree of blackness on a radiograph.

Question 18

Name 3 things that influence the radiographic density of an image?

Answer 18

1. The quantity and quality of x-rays (mAs and kVp)
2. The distance- SID
3. The thickness and type of tissue

Question 19

What is radiographic contrast?

Answer 19

The difference in blackening between adjacent structures on an image. Different shades of grey allows differentiation of different structures on the radiograph. A high contrast= more of a difference between black and white, it means LESS SHADES OF GREY. If there’s a wider range of grey shades there is LESS contrast.

Question 20

Low kVp produces high contrast images. T/F

Answer 20

TRUE

Question 21

What is Sante’s Rule?

Answer 21

kVp= (2 x tissue thickness in cm) + SID (in inches, usually 40) + grid factor

Question 22

If an image is underexposed, what will it look like?

Answer 22

It will be too white, you will have loss of detail.

Question 23

If an image is overexposed, what will it look like?

Answer 23

It will be too dark, and a lot of the detail of the image will be “burnt off”-gone!

Question 24

How does scatter radiation affect the quality of the image?

Answer 24

It reduces it by reducing the contrast.

Question 25

What 3 factors increase the amount of scatter radiation?

Answer 25

1. Higher kVp-> thicker, denser tissues. E.g. obese patients have blurrier images, because fat has a lot of cells and causes a lot more scatter.
2. Long exposure times
3. X-raying larger areas