Do all two techniques report similar focal spot dimensions?

(3.c) Do all two techniques report similar focal spot dimensions? (1 point)
The measurements in (1b and 2b ) indicate that the fine focus spot is smaller than broad focus spot. This can explain why each of them is recommended for specific projections. For example, fine focus size is suggested for extremities to provide sharper cortical margins, while broad focus size is useful with projections that need higher current (mA).

Comparing fine focal spot size from pinhole camera (1b) to fine focal spot size from the star pattern image (2b), it is noticeable that the fine focal spot dimensions in the star test image is slightly different from the fine focal spot from pinhole camera. This might be due to some limitations and errors of the experiment because the focal spot is the same and does not change.

(3.d) It is sometimes suggested that radiographers leave a large gap between the patient and the film to magnify small anatomical features in an image. Why can this be problematic and actually lead to an image with less diagnostic value? (2 points)
Magnification can lead to loss of sharpness and resolution, which will result in losing much detail from the image and causing blurriness. In addition, the bony trabecular and cortical margins of the bones will not be well visualized due to penumbra which leads to minor fractures to be missed in the diagnostic process. However, it is possible to reduce undesirable magnification defects by moving the source of x-ray further away from the subject and increasing exposure factors or using a small effective focal spot.
Abstract:
Pinhole camera and start test pattern were used to measure the focal spot size. The pinhole camera was placed on top of a blank cassette. 60 kvp and 5 mAs then were chosen for exposure. After that, the object film distance (OFD) and Focus object distance (FOD) were recorded. The magnification factor was obtained by using formula. After that, the actual size of focal spot was obtained which assisted to estimate the actual focal spot sizes. These calculations were done for both fine and broad focuses. With regard to the star test pattern, the diameter of the star pattern was calculated. The used exposure was 50 Kvp and 5 mAs which was ideal to see the focal spot size of the spokes. The results from two technique were as follow: the pinhole camera fine focus was (2.0 x 1.6 mm)= 3.2 effective focal spot area, while broad focus was (3.6 x 2.3 mm)= 8.28 effective focal spot area. For start pattern, the measurements of fine focus effective area were different from the broad focus. This indicates that the ruler function in the master-page is limited to human eye or the scatter might have affected the images which resulted in difficulty to precisely see the edges.

Aim:
Using two techniques (pinhole camera and start test pattern) to measure the area of the focal spot on diagnostic x-ray tubes.

Background theory:
The focal spot is where the x-rays are generated. It has finite dimensions which can affect image quality and radiation quantity. In fact, the size of focal spot may vary which depends on its uses. The area of the anode, that produces x-rays, is related to the size of filament that releases the electrons. By using smaller focal spot size, high quality image will be produced, however, using smaller focal spot will concentrate the heat to a smaller area. Nowadays, the x-ray machines have two different cathode filaments: fine focus which provide sharper image, but required to decrease the current (mA) to reduce heat. The second one is broad focus which provides less sharpness, but able to deal with different range of mA.

There is a difference between the actual focal spot size and the effective size. The effective focal spot is the size of x-ray beam that directed downward on the diagram, while the actual focal spot size is s the area on the anode that is struck by electrons. In fact, the reason why the effective focal spot size appears narrower than actual focal spot size is that the anode surface is angled. Smaller angle will result in a smaller effective focal spot size but at the sacrifice of more severe anode heel effect. Usually, the anode is angled 15-25 to provide a compromise of effective focal spot size, heat dissipation and heel effect.

As the penumbra has an affect on image quality, there are two ways to reduce its region. Firstly, using a small effective focal spot. Secondly, minimize the object film distance.

Equipment and Methods:
Using the Pinhole:
1.The pinhole camera was placed on top of a blank cassette. The image will be
produced through a small circular portion contains a 10µm hole.

2. It is possible to extend the legs of the pinhole camera to increase the magnification of the focal spot.

3. Record an exposure at 50 kVp, 5 mAs using the fine focus setting. Scan the image and measure the dimensions of the focal spot (length and width) in the worksheet.

4. step 3 was repeted with the broad focus setting and record your data

Using the Star resolution pattern:
1. Measure the diameter of the star pattern and place it between the x-ray tube and the cassette. Using cardboard

box to leave an air gap between the pattern and the cassette and gain a better measurement.

2. Image was record using 50 kVp value, 5 mAs, and the broad focus filament. The cassette was scanned and the image was sent to Masterpage.

3. Step 2 was repated with the fine focus setting.

4. The image was opened in Masterpage and the diameter of the entire pattern was measured so it is possible to calculate its magnification factor (M)

5. The window and level settings wee adjusted to see the blurred rings near the centre of the star pattern.

6. The diameter of the blurred ring along the horizontal and vertical axes was measured.

The dimensions of the focal spot was calculated using the following equation:

Where d = blurred diameter
A = angle of spokes of star pattern (= 2º in this case).
M = magnification Factor (determined from the image to pattern size ratio)
f = focal spot size

Discussion:
The data findings and significances of the results

It was expected to find similar results for pinhole camera and start test pattern in fine focus and board focus as well. However, the measurements were slightly different even though the source does not change. This is more likely to happen due to human error and some limitation in the experiment.

The measurements alsu indicate that the fine focus spot is smaller than broad focus spot. This can explain why each of them is recommended for specific projections. For example, fine focus size is suggested for extremities to provide sharper cortical margins, while broad focus size is useful with projections that need higher current (mA).

Magnification can lead to loss of sharpness and resolution, which will result in losing much detail from the image and causing blurriness. In addition, the bony trabecular and cortical margins of the bones will not be well visualized due to penumbra which leads to minor fractures to be missed in the diagnostic process. However, it
is possible to reduce undesirable magnification defects by moving the source of x-ray further away from the subject and increasing exposure factors or using a small effective focal spot.

Over exposure can make measuring the diameter of star pattern impossible. When using 100 kvp, for example, the focal spot size of the spokes cannot be seen i.e. they burnt out and had to be reduced to 50kVp.

The results from two technique were as follow: the pinhole camera fine focus was (2.0 x 1.6 mm)= 3.2 effective focal spot area, while broad focus was (3.6 x 2.3 mm)= 8.28 effective focal spot area. For start test pattern, the measurements of fine focus effective area were 0.963 mm x 0.8272 mm =0.8 mm2 ,whereas broad focus measurements were 2.0314 mm x 1.3682 mm = 2.78 which means they are different. This indicates that the ruler function in the master-page is limited to human eye or the scatter might have affected the images which resulted in difficulty to precisely see the edges.

Limitations with the experiments

 Human errors as there were several students involved in measurements.
 Equipment used
 Poor visible edges may be because of scatter which might have prevented to gain accurate measurement.