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An ultrasound A-scan is used to find the length / l of an eye as shown in Figure 6 - AQA - A-Level Physics - Question 4 - 2019 - Paper 5
Question 4
An ultrasound A-scan is used to find the length / l of an eye as shown in Figure 6. Figure 7 shows the simplified A-scan for the eye. A short pulse of ultrasound is ... show full transcript
Worked Solution & Example Answer:An ultrasound A-scan is used to find the length / l of an eye as shown in Figure 6 - AQA - A-Level Physics - Question 4 - 2019 - Paper 5
Step 1
Calculate l.
Answer
To calculate the length / l of the eye using an A-scan, we first note the time taken for the ultrasound pulse to return after reflecting off the back of the eye. From Figure 7, we see that the total time for the round trip is around 30 μs.
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Calculate the time for one way:
Time for one way, t = 30 μs / 2 = 15 μs
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Convert time into seconds:
t = 15 μs = 15 × 10^{-6} s
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Use the speed of ultrasound in the eye to calculate the length:
Using the formula:
l = speed × time
l = 1560 m/s × (15 × 10^{-6} s)
l ≈ 0.0234 m or 23.4 mm
Therefore, the length / l of the eye is approximately 23.4 mm.
Step 2
Discuss whether an A-scan or a B-scan should be used for amniocentesis.
Answer
To determine which scan to use for amniocentesis, consider the following:
Differences between an A-scan and a B-scan
- An A-scan provides a one-dimensional representation of tissue interfaces and is used to measure distances, such as the length of the eye.
- A B-scan produces a two-dimensional image, allowing for a more comprehensive view of a structure, showing multiple interfaces and their positioning.
Advantages and Disadvantages
A-scan:
- Advantages:
- Simple to perform and interpret.
- Quick measurement of distances.
- Disadvantages:
- Provides limited information as it only gives a one-dimensional view.
B-scan:
- Advantages:
- Offers a detailed two-dimensional image, allowing for better positioning of structures involved in amniocentesis.
- More useful in identifying nearby organs and structures for safety.
- Disadvantages:
- More complex and may take longer to perform.
Conclusion
For amniocentesis, a B-scan is recommended since it provides a detailed visual representation of the fetal position relative to the placenta and uterus. This ensures safer needle placement, minimizing risks during the procedure.