Physics: Medical Imaging: X-Rays, Ultrasound, and MRI Worksheet

Question 1

An X-ray photon has a frequency of 3.0 x 10^18 Hz. Calculate its wavelength in meters using c = 3.0 x 10^8 m/s.

Accepted Answer

The wavelength is 1.0 x 10^-10 m. Using wavelength = speed divided by frequency, wavelength = (3.0 x 10^8 m/s) / (3.0 x 10^18 Hz) = 1.0 x 10^-10 m.

Question 2

Explain why bones appear lighter than soft tissue on a typical X-ray image.

Accepted Answer

Bones appear lighter because they absorb or block more X-ray photons than soft tissue. Fewer X-rays reach the detector behind bone, so the detector records that area as lighter or brighter.

Question 3

A technician wants to image a fetus during pregnancy. Which imaging method is usually preferred: X-ray, ultrasound, or MRI? Explain why.

Accepted Answer

Ultrasound is usually preferred because it uses non-ionizing sound waves and can create real-time images of the fetus. X-rays use ionizing radiation, so they are generally avoided unless medically necessary.

Question 4

Ultrasound travels through soft tissue at about 1540 m/s. If an ultrasound pulse returns from a boundary after 80 microseconds, how deep is the boundary? Remember that the pulse travels to the boundary and back.

Accepted Answer

The boundary is about 0.0616 m, or 6.16 cm, deep. The total travel distance is vt = (1540 m/s)(80 x 10^-6 s) = 0.1232 m, and the one-way depth is half of that, 0.0616 m.

Question 5

Describe one way an MRI is different from an X-ray in the type of physics it uses to make an image.

Accepted Answer

An MRI uses strong magnetic fields and radio-frequency signals to affect hydrogen nuclei in the body, while an X-ray uses high-energy electromagnetic radiation that is absorbed differently by different tissues.

Question 6

A medical ultrasound probe uses a frequency of 5.0 MHz. If sound travels in tissue at 1540 m/s, what is the wavelength in tissue?

Accepted Answer

The wavelength is 3.08 x 10^-4 m, or 0.308 mm. Using wavelength = speed divided by frequency, wavelength = 1540 m/s divided by 5.0 x 10^6 Hz.

Question 7

Higher-frequency ultrasound can produce more detailed images but does not travel as deeply into the body. Explain this tradeoff.

Accepted Answer

Higher-frequency ultrasound has a shorter wavelength, which improves resolution and makes smaller details easier to detect. However, higher-frequency sound is absorbed and scattered more strongly, so it loses energy faster and cannot image as deeply.

Question 8

Match each imaging method to the main type of wave or field it uses: X-ray, ultrasound, and MRI.

Accepted Answer

X-ray imaging uses high-energy electromagnetic waves called X-rays. Ultrasound imaging uses high-frequency sound waves. MRI uses strong magnetic fields and radio-frequency electromagnetic waves.

Question 9

A radiologist uses contrast material during an X-ray or CT scan. Explain what contrast material does in terms of image formation.

Accepted Answer

Contrast material changes how strongly certain body regions absorb X-rays. This makes blood vessels, organs, or other structures stand out more clearly from surrounding tissue on the image.

Question 10

In ultrasound imaging, a strong echo occurs at a boundary between two materials with very different acoustic properties. Give an example of a boundary that would create a strong echo and explain why.

Accepted Answer

A boundary between soft tissue and bone can create a strong echo because bone and soft tissue have very different acoustic properties. Much of the ultrasound wave is reflected instead of passing smoothly through.

Question 11

Why must patients remove metal objects before an MRI scan?

Accepted Answer

Patients must remove metal objects because MRI machines use very strong magnetic fields. Magnetic objects can be pulled toward the scanner, heat up, distort the image, or create safety risks.

Question 12

An X-ray image shows high contrast between bone and lung tissue. Explain why lung tissue often appears darker than many other body tissues.

Accepted Answer

Lung tissue often appears darker because it contains a lot of air, which absorbs very few X-rays. More X-rays pass through the lungs and reach the detector, creating a darker region on many X-ray images.

Question 13

Rank X-rays, visible light, and radio waves from lowest photon energy to highest photon energy. Explain using frequency.

Accepted Answer

The order from lowest photon energy to highest photon energy is radio waves, visible light, and X-rays. Photon energy increases as frequency increases, and X-rays have much higher frequencies than visible light or radio waves.

Question 14

A patient asks why repeated X-ray scans are limited when possible, but ultrasound exams do not have the same radiation concern. Write a physics-based explanation.

Accepted Answer

Repeated X-ray scans are limited because X-rays are ionizing radiation, meaning they can remove electrons from atoms and potentially damage cells or DNA. Ultrasound uses mechanical sound waves, which are non-ionizing, so it does not carry the same radiation risk.

Question 15

Compare the best use of X-ray, ultrasound, and MRI for imaging the body. Give one example of a situation where each method is useful.

Accepted Answer

X-rays are useful for viewing dense structures such as broken bones. Ultrasound is useful for real-time imaging of soft tissue motion, such as fetal imaging or heart valve motion. MRI is useful for detailed images of soft tissues, such as the brain, spinal cord, ligaments, or organs.

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Physics: Medical Imaging: X-Rays, Ultrasound, and MRI

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