*Quiz No. 4

Define the following terms:

• Variable aperture collimator: these are leaves of lead that can be adjusted independently to make a rectangular or square field size, making sure that only the body part to be analysed is being imaged thereby reducing patient dose
• Contrast improvement factor: this refers to a grid's ability to increase film contrast and can be found by calculating the radiographic contrast with a grid vs the radiographic contrast without a grid - it depends on x-ray spectrum, tissue thickness and field size
• *Bucky factor: the bucky or grid factor refers to the incident radiation vs the transmitted radiation and the effect that the grid has on the transmitted radiation. It is therefore an indication of patient dose and can be found by calculating the patient dose with a grid vs without a grid. Bucky factor increases with grid ratio because more scatter radiation is being produced and then absorbed by the grid
• *Selectivity: this refers to the primary radiation transmitted vs the scatter radiation transmitted and is therefore a measure of grid effectiveness
• Grid cut off: the undesirable removal of primary x-rays from using a low grid ratio and linear parallel grids
• Grid surface absorption: width of strip (T) divided by width of strip + width of interspace (D) x 100%
• Grid ratio: height of strip (h) divided by width of interspace (D)
• Grid frequency: number of lead strips per centimetre, increased grid frequency minimises grid strips on image but requires a greater patient dose

Describe the structure and function of a grid.

A grid consists of lead strips separated by aluminium or another organic material. The entire grid is encased in aluminium and plastic.

A grid functions to transmit primary x-rays but absorb scatter radiation.

What are the four types of grid?

1. Linear parallel: linear strips arranged parallel to one another, produce severe grid cut off, used best with large source to image distance but small exposure area
2. Linear focussed: strips are arranged on an slight angle towards the anode at the midline to reduce grid cut off
3. Crossed parallel: two linear parallel grids arranged perpendicular to one another to cut out scatter radiation in two directions
4. Crossed focussed: two linear focused grids arranged perpendicular to one another, creating a true focal point to minimise grid cut off

*What are the four issues that can arise from grids?

1. Upside down grid
2. Off-focus grid: the grid focal point does not match the tube focal point (too low or too high)
3. Off-level grid: the grid is positioned at an angle to the central axis of the beam
4. Off-centre grid: the central axis of the grid is not in line with the central axis of the beam

What happens to patient dose with increasing grid ratio?

As you increase grid ratio, you increase the height of the lead strips in comparison to the interspace width. This means you are cutting out more scatter radiation but also some primary x-rays. Thus, kVp must increase to increasing the penetrating ability of the primary x-rays so they can get through to the film. Thus, patient dose increases.

What happens to patient dose when kVp increases for a particular grid ratio?

As you increase kVp, you increase the relative amount of scatter. This means more is being cut out by the grid and thus the quantity of the beam is decreased. This means that the transmitted radiation is less than the primary radiation, thus increasing the bucky factor. Thus, patient dose is decreased.

What are the advantages and disadvantages of the air gap technique?

The air gap technique is where we move the image receptor 10-15 cm away from the patient so that some of the scatter radiation will miss the film altogether. This improves contrast but we have to increase patient dose to ensure that the primary x-rays do reach the film.

Define optical density.

Optical density is a measure of the blackness of the film.

What is a H&D or characteristic curve?

A H&D curve is a sigmoidal curve that plots exposure against optical density. It shows the optical densities we can get from increasing radiation exposure. There are sections of significance to the curve:

• Base and fog: optical density resulting from background fog and tinting of the film base
• Toe: region of low gradient at low exposure
• Linear region: region of relatively steep increase in optical density for small increase in radiation exposure
• Shoulder: flat region at a high exposure
• Latitude/dynamic range: the region of optical density useful for diagnostic radiography (0.25 to 2 above base and fog). A high latitude type of film is appropriate for chest radiography due to the dynamic difference between the attenuation of the bone and soft tissues e.g. lung. This means we get a big difference between the radiation detected from the bone and soft tissues, and the film can capture all of these and produce an image of good contrast.

What processes are involved in converting an analogue signal to a digital signal?

1. Sampling: analogue signals are continuous in time and thus we select a few points in time to represent the plot
2. Quantisation: the process of converting the points from sampling into a digital signal represented by two numbers, 0 or 1 (using binary numbers)