Spatial Resolution And Contrast Resolution Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

In this chapter the results obtained from the data collected are presented and evaluated. Results will be presented in tabulation and graphical form to facilitate better understanding of the research findings. The relationship between the results collected with image quality of both the grid and air gap technique exposures was also analysed.

4.2 Image Quality

4.2.1 Spatial Resolution and Contrast Resolution.

The main data collected was a measure of contrast and spatial resolution that was performed on a quality control phantom using the two anti-scatter techniques under investigation.  The data collected was the number of low contrast steps that could be seen on the image, as the measure of contrast resolution and the maximum numbers of line-pairs per millimetre (Lp/mm) that could be seen on the image as a measure of spatial resolution.  The results obtained for the grid and air gap technique are shown overleaf in tables 4.1 and 4.2.

4.1 Measurement of SR and CR for the Grid Technique.

Spatial Resolution (Lp/mm)

Contrast Resolution

Grid Technique

2.5

4

Non-grid, in contact technique

2.5

4

Table showing spatial and contrast resolution readings for the grid technique and the exposure taken without the grid with the phantom in contact with receptor. The readings for both these exposures were the same

4.2 Measurement of SR and CR for the Air gap technique.

Spatial Resolution (Lp/mm)

Contrast Resolution

10cm Air Gap

2.2

4

20cm Air Gap

2.0

4

30cm Air Gap

1.4

5

40cm Air Gap

1.2

5

50cm Air Gap

1.0

6

60cm Air Gap

0.5

6

In the above table it could be noted that while spatial resolution decreased the more the OID was increased, the contrast resolution improved.

The recorded results showed that the grid technique and the non-grid technique with the phantom in contact with the receptor performed better in the spatial resolution test than the air-gap technique exposures did. Both exposures in table 4.1, had 2.5 line pairs per millimetre recorded on their resultant images. Alternatively, in the air-gap exposures shown in table 4.2, the spatial resolution readings were lower than those obtained for the grid and non-grid exposure. The spatial resolution decreased, the more the object to image distance was increased. This means that the resultant air-gap images were less sharp than the exposures with the quality control phantom in contact with the receptor, with and without the grid. This reduction in sharpness is called geometric un-sharpness and is affected by three main factors: patient size, source to object distance and object to image distance. Since an anthropomorphic phantom was used, patient size was a missing factor in this research, while the source to object distance was kept at 100cm in all exposures in both techniques. The manipulated factor was the object to image distance, and this was increased in the air-gap exposures. Therefore, the reason for the geometric un-sharpness and loss of definition in the air-gap exposures is the increased distance between the object (phantom) and the image detector. This is because the further away the object is placed from the image detector, the higher is the level of geometric un-sharpness and loss of definition. This leads to a low amount of line pairs seen per millimetre.

In the second stage of testing for image quality, the anti-scatter techniques were tested for their contrast resolution by examining how many low contrast steps could be visualised in each resultant radiograph. The readings showed that in the exposures taken with the phantom in contact with the receptor with and without a grid, the amount of low contrast steps was 4. The same amount of low contrast steps was recorded for the 10cm and 20 cm air-gap exposures. However, the amount of low contrast steps seen on the images increased in the other air-gap exposures. The 30 cm and 40 cm air-gap exposures had 5 low contrast steps on the image while the 50 cm and 60 cm air gap exposures had 6 low contrast steps. This means that contrast resolution increased the more the object to image distance was increased.  This improvement of contrast resolution may be due to the removal of the grid and an application of an air-gap. This is because although the grid absorbs scattered radiation given off by an object or patient, it also absorbs some of the primary (useful) radiation that passes through the object or patient. For this reason a bigger exposure would be needed to improve image quality. Since the kVp was kept constant in this research, an increase in kVp in such an exposure gives the primary beam photons to penetrate the grid without being absorbed. This increase in exposure, however, may also mean an increase in patient dose as well as a deterioration in the contrast resolution of the image. This effect did not happen in the air-gap technique since there was nothing that hindered and absorbed the primary beam emerging from the phantom. Scattered radiation from the object/patient will not have enough energy to reach the image detector if an air-gap is applied between the object and the detector. For this reason, scatter radiation is deviated from the image detector and the primary beam is not affected.

The above results will be analysed and discussed in relation to the DAP readings, signal to noise ratio and Likert scale results further on in this chapter.

4.2.2 Signal to Noise Ratio

The data in tables 4.3 and 4.4 show the signal to noise ratio of each exposure. This ratio represents the level of image quality of the radiograph. This means that if an image has a low signal to noise ratio the image is said to be underexposed (low image quality), while if the signal to noise ratio is high, the image has adequate image quality. This ratio was calculated so that the air gap technique exposures could be compared to that obtained using the grid technique. This comparison should reveal which exposure produces the best image quality.

The signal to noise ratio was calculated by using readings acquired from the raw image from each exposure. This was done so to record the initial image containing all the photons that fell on the receptor. In this way, a more accurate reading could be taken.  The same region of interest was used for each exposure and the area from where the readings were recorded was also kept the same for each image.

The data recorded was the mean pixel value and the standard deviation of the signal of each exposure. The signal to noise ratio was achieved by dividing the mean pixel value by the standard deviation of the signal using the following formula:

Signal to Noise ratio = mean pixel value/standard deviation

All the recorded data and the calculated signal to noise ratio was tabulated in tables 4.3 and 4.4.

4.3 Mean Pixel Value, Standard Deviation and S/N ratio of the grid technique.

Mean Pixel Value

Standard Deviation

Signal to Noise Ratio

Grid Technique

759.2

30.7

24.7

Non-grid, in contact technique

595.6

11.7

50.9

The above table shows the S/N ratio results for the grid technique and for the exposure without the grid with the phantom in contact. There is a relatively big difference between these two exposures with the grid having the least S/N reading recorded.

4.4 Mean Pixel Value, Standard Deviation and S/N ratio of the air gap technique.

Mean Pixel Value

Standard Deviation

Signal to Noise Ratio

10cm Air Gap

648.2

11.9

54.5

20cm Air Gap

722.2

10.4

69.4

30cm Air Gap

768.9

9.3

82.7

40cm Air Gap

795.1

8.8

90.4

50cm Air Gap

815.2

8.5

95.9

60cm Air Gap

813.3

8.5

95.7

Table showing S/N ratio results for the air-gap exposures. The S/N increased the more the OID was increased. The air gap exposures have a relatively higher S/N than the grid technique.

The signal to noise ratio in the air gap technique turned out to be higher than that obtained using the grid technique. The S/N ratio increased the more the air gap was increased. However, this does not necessarily mean that increasing the air gap would lead to a corresponding increase in image quality since the increase in readings obtained by using the air gap technique could be due to a high mAs used in the exposures, which could have over exposed the image to radiation. This explains the high S/N ratio readings. However, the grid technique has got a S/N ratio of 24.7 and this is much lower than the exposure taken with the phantom in contact with receptor without the grid. The S/N for this exposure was 50.9. This means that the grid has affected the quality of the image as well as the exposure. Although the 'in contact' exposure had a lower mAs than the grid technique, the S/N ratio was higher. This may be due to primary beam photons not being absorbed by the grid and attenuating the receptor. As a result less mAs was needed to produce an image than in using the grid technique.

4.3 Likert Scale

4.3.1 Likert Scale results

The Likert scale was filled out by two independent random chosen radiologists. They were provided with the images of the lateral hip shoot through performed on an anthropomorphic phantom. There was a total of 8 images which consisted of the grid technique, an exposure done with phantom in contact to receptor without the grid and the air gap exposures. The radiologists scaled each image on four different points of the hip that is the acetabulum, femoral head, femoral neck and proximal shaft of femur. The scoring was based on the quality of the image, and the radiologists were asked how well they could view and confidently report on the mentioned structures. The readings were recorded and a mean value was drawn out of the data. The data collected using this scale and the mean values could be seen in the following table.

4.5 Likert Scale Data and Results.

Exposures

Radiologist A

Radiologist B

Total

(Rad A+ Rad B)

Mean Value of each image.

Grid

12

7

12+7= 19

19/40= 0.475

Non-grid, in contact

8

6

8+6= 14

14/40= 0.35

10cm Air Gap

16

10

16+10= 26

26/40= 0.65

20cm Air Gap

16

13

16+13= 29

29/40= 0.725

30cm Air Gap

19

15

19+15= 34

34/40= 0.85

40cm Air Gap

18*

16*

18+16= 34*

34/40= 0.85*

50cm Air Gap

17*

17*

17+17=34*

34/40= 0.85*

60cm Air Gap

16*

17*

16+17= 33*

33/40= 0.825*

)

Max Score A + Max Score B

40(MEAN= Radiologist A + Radiologist B = X

*The radiologist reported that in these exposures the proximal shaft of the femur was not well demonstrated.

In this test, the results obtained showed which exposure and technique the radiologists felt confident enough to report on, depending on how well the mentioned structures were demonstrated in the resultant images. The results obtained for each image for each radiologist were summed up and divided by their maximum score that could have been achieved. These mean results from each image were analysed and compared. It was found that the images the radiologists were confident to report on most were the 30 cm, 40 cm, 50 cm and 60 cm air-gap exposures. Their results were similar. However, the radiologists also reported that on the 40 cm, 50 cm and 60 cm air-gap exposures images, the proximal shaft of the femur was not well demonstrated. This meant that the best exposure with the biggest mean score that the radiologists felt confident and comfortable to report on was the air-gap exposure with 30 cm object to image distance. The radiologists were not given any information about how the images were acquired or which technique was used. They were also asked not to use windowing so that they could report on the acquired image quality. This was done so that the results would not be biased. The exposures that were done with the phantom in contact with the receptor using with and without the grid turned out to be the images that the radiologists were least confident to report on. This could be mainly due to the low contrast resolution in the images and so the anatomy of the hip joint was not shown as clearly as in other exposures.

4.4 Dose

4.4.1 DAP Readings

The research area in this study is image quality of two anti-scatter techniques for the horizontal beam shoot through of the hip. However, the DAP was recorded so that the researcher could have an idea of the radiation dose at each exposure. The DAP metre does not record the dose given to the patient during an examination but the amount of radiation that is exposed by the tube. This amount of radiation is the same throughout the beam and is also dependent on several factors mentioned in previous chapters. Since the AEC was used in the experimentation, the amount of radiation exposed varied from one examination to another since the mAs was controlled by the x-ray system itself. The higher the mAs used the higher is the radiation dose exposed, and therefore the DAP should be high as well. Since these readings do not represent patient dose, a high DAP reading means that there was a high exposure of radiation, therefore resulting in a high patient dose. The following table are the DAP readings recorded after each exposure when testing for image quality using the quality control phantom as well as the mAs used at each exposure.

4.6 Table showing mAs used and the dose area product in each exposure.

mAs

Dose Area Product (dGycm²)

Grid Technique

5.33

2.5

No grid, in contact

2.22

1.042

10cm Air Gap

3.08

1.36

20cm Air Gap

4.36

1.96

30cm Air Gap

5.54

2.5

40cm Air Gap

6.95

2.98

50cm Air Gap

8.31

3.59

60cm Air Gap

9.69

3.77

Table showing mAs and DAP readings of each exposure. Data shows that the more the mAs increased the more radiation was exposed which would lead to a higher dose of radiation to the patient.

The DAP readings were displayed graphically in the graph below with the DAP recorded readings on the y-axis and the imaging technique used in the x-axis.

4.1 Graph showing DAP readings of each exposure

The graph shows that the grid and the non-grid technique with the phantom in close contact with the receptor varied greatly in DAP. This shows that the grid increases the amount of exposure needed to create an image, which means that patient dose would increase considerably as well. The air gap technique exposures increased the more the object to image distance was increased. However, the air gap technique exposure using an OID of 30 cm resulted in having the same DAP used as the grid technique. Therefore, the radiation dose in these exposures should have been the same. Conversely, the exposures done using an OID of 40 cm and over without the grid, needed a lot of mAs exposure and so the radiation dose to the object was very high. These results will be discussed in relation to the image quality, signal to noise ratio and Likert scale result

4.5 Overall Results

All the results were tabulated in a single table so that they could be compared with one another and see which exposure using which technique has the best image quality in imaging the hip laterally using a horizontal beam.

4.7 table showing overall data collected for both anti scatter techniques

Exposures

Spatial Resolution

Contrast Resolution

Signal to noise ratio

Likert Scale

Dose Area Product

Grid Technique

2.5

4

24.7

0.475

2.5

No grid, in contact

2.5

4

50.9

0.35

1.042

10 cm AG

2.2

4

54.5

0.65

1.36

20 cm AG

2.0

4

69.4

0.725

1.96

30 cm AG

1.4

5

82.7

0.85

2.5

40 cm AG

1.2

5

90.4

0.85*

2.98

50 cm AG

1.0

6

95.9

0.85*

3.59

60 cm AG

0.5

6

95.7

0.825*

3.77

Table showing overall results collected in the study

The table above shows all the data collected from both experimentations. The table shows that while the grid technique has the highest spatial resolution, it is one of the exposures with the lowest contrast resolution with the least signal to noise ratio reading. The DAP recorded in the grid technique is the same as that recorded in the 30 cm air gap technique. Although the 30 cm air gap technique has a low spatial resolution reading, the contrast resolution and the signal to noise ratio are higher than the grid technique particularly when one keeps in mind that both these exposures have the same DAP reading. Moreover, the Likert scale results show that the most exposure that the radiologists felt comfortable reporting about was the 30 cm air gap technique. The Likert result for the 30 cm air gap technique was that of 0.85, while the grid technique scored 0.475. This is relatively lower than the 30 cm air gap technique, and therefore shows that although it has a low SR reading, the 30 cm air gap technique had considerable good contrast resolution recorded (better than the grid), with a higher signal to noise ratio than the grid with the same DAP reading. This means that the radiation dose would be the same in both techniques.

When the other air gap exposures are compared with the grid technique, the results show that although the spatial resolution readings were inversely proportionate to the air gap, the contrast resolution increased in the air gap technique. The signal to noise ratio was also better in the air gap technique exposures than the grid technique. However, the air gap exposures with an OID of 40 cm, 50 cm and 60 cm gave very high DAP readings, which means a greater radiation dose to the object.  This contrasts sharply with the DAP readings given by the 10 cm and 20 cm air gap, which were lower than the grid technique and the air gap technique with 30 cm OID. The 30 cm air gap had the same DAP readings as the grid technique since the more the air gap was increased, the mAs increased as well. Moreover increasing the signal in the image led to an increase in the DAP, resulting in a higher dose than in the other exposures. The Likert scale results show that the radiologists were, in general, more confident to report on the air gap exposures than on the grid exposure. However, air gap exposures using an OID of 40 cm, 50 cm and 60 cm did not adequately show the proximal shaft of femur and therefore were discarded by the researcher. This was done since the images using these air gap exposures had no diagnostic value since the proximal shaft of the hip was not included in the images.

In the grid technique the quality control tests showed that spatial resolution was the highest in the grid technique and in the technique with the phantom in contact with the receptor. The grid technique, however, needed a lot of exposure and so the dose was higher. The high exposure resulted in having a low signal to noise ratio unlike all the other exposures. The technique with the phantom in contact with the receptor without a grid resulted in the same image quality results but with lower DAP used and therefore less exposure. This implied that the dose would be lower.

4.5 Conclusion

From the data collected it was concluded that there is a significant difference in the way the grid and the air gap technique work as anti scatter techniques. The image quality tests showed that there was a difference between the different air gap exposures and the standard grid technique that is still being used in the local hospital. The results showed that the grid technique performed better than the air gap technique in the image quality tests. Even though the image quality tests in the air gap technique were lower than in the grid technique, the radiologists felt more confident reporting on an air-gap exposure with an OID of 30 cm rather than the grid exposure. The following chapter will include conclusions and recommendations that derived from the data.

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.