B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Seeing Inside: Medical Imaging Technologies and Careers

B1.2 A2.1 A2.2 A1.9 A1.12 A1.13About 70 minutes

Learning goals

I can explain how X-rays, CT, MRI, ultrasound and nuclear imaging work, and what each shows best.
I can describe how microscopy and biophotonics help diagnose and treat disease.
I can assess the benefits, risks and costs of imaging technologies for human health and society.
I can describe careers in medical imaging and the education they require.

Words to know

medical imaging
Technologies that make pictures of the inside of the body to diagnose, guide treatment or monitor disease.

X-ray
High-energy electromagnetic radiation that passes through soft tissue but is absorbed by dense tissue such as bone.

CT (computed tomography) scan
Many X-ray images taken from different angles and combined by computer into cross-sectional slices.

MRI (magnetic resonance imaging)
Imaging that uses a strong magnet and radio waves to make detailed pictures of soft tissues.

ultrasound
Imaging that uses high-frequency sound waves and their echoes; it works in real time.

ionizing radiation
Radiation (such as X-rays and gamma rays) with enough energy to damage DNA.

biopsy
A small sample of tissue removed so it can be examined under a microscope.

biophotonics
The use of light (including lasers and fibre optics) to study, diagnose or treat living tissue.

Before imaging, there was guessing

Imagine being a doctor 150 years ago. A patient has a pain in the belly. Is it the appendix? A kidney stone? A tumour? You can listen, press and ask questions, but the only way to see is surgery. Today a doctor can order a scan and see the inside of the body within minutes or hours, without a single cut. Medical imaging is one of the biggest changes in the history of medicine, and it relies on the physics of waves, including the light you'll study in the Light and Geometric Optics unit.

X-rays

How it works: a machine sends X-rays through the body onto a detector. Soft tissues let most X-rays through; dense tissues like bone (and metal) absorb them. So bones show up white and soft tissue grey or dark.
Best for: broken bones, dental problems, some chest problems (like pneumonia). Mammography uses low-dose X-rays to screen for breast cancer.
Pros: fast, widely available, relatively low cost.
Cons: uses ionizing radiation, which can damage DNA (a small dose for one X-ray, but doses add up, so they are used only when needed and staff stand behind shields); shows soft tissue poorly.

CT (computed tomography)

How it works: an X-ray tube rotates around the patient, taking many images from different angles. A computer combines them into cross-sectional slices and 3D views.
Best for: emergencies (head injuries, strokes, internal bleeding), finding and measuring tumours, detailed views of bones and organs.
Pros: fast (seconds to minutes), much more detail than one X-ray.
Cons: a higher radiation dose than a plain X-ray; more expensive.

MRI (magnetic resonance imaging)

How it works: a very strong magnet lines up hydrogen nuclei (mostly in the water and fat of tissues). Pulses of radio waves knock them out of line; as they realign they give off signals that a computer turns into detailed images.
Best for: soft tissues: brain, spinal cord, muscles, ligaments, joints, many tumours.
Pros: excellent soft-tissue detail; no ionizing radiation.
Cons: expensive, slower (often 15 to 60 minutes), loud, and the tight tube can be hard for some people. The powerful magnet means people with certain metal implants or devices may not be able to have an MRI, and metal objects must never be brought into the room.

Ultrasound

How it works: a probe sends pulses of high-frequency sound (far above what humans can hear) into the body. Sound echoes off boundaries between tissues; the time each echo takes to return shows how deep the boundary is.
Best for: pregnancy (watching a fetus), the heart (an echocardiogram), the abdomen, blood flow, and guiding needles.
Pros: real-time moving images, portable, relatively low cost, no ionizing radiation.
Cons: can't see through bone or air (so it's poor for lungs and the adult brain); image quality depends on the operator's skill.

Nuclear medicine and PET

A patient receives a small amount of a radioactive tracer that collects in certain tissues. A camera detects the gamma rays it gives off. This shows how organs are working, not just their shape: for example, cancer cells that use a lot of glucose show up brightly in a PET scan. For decades, a research reactor at Chalk River, Ontario, produced a large share of the world's supply of a key isotope used in these scans.

Where imaging technologies sit on the electromagnetic spectrum. MRI uses radio waves, endoscopes and microscopes use visible light, and X-ray and CT machines use X-rays. Ultrasound uses sound, so it is not on the spectrum at all.

Microscopy and biophotonics

Microscopy: when a doctor takes a biopsy (a small piece of tissue), a pathologist examines thin, stained slices under a microscope to see if cells look normal or cancerous, using the same clues you learned in the cancer lesson (large irregular nuclei, many dividing cells). Blood smears are checked for abnormal blood cells. Electron microscopes can show viruses.
Biophotonics uses light to study and treat the body. Examples:

Endoscopy: a flexible tube with a tiny camera and light, using fibre optics, lets doctors look inside the stomach, colon or joints, and even remove small growths.
Pulse oximeters clip onto a finger and shine red and infrared light through it to measure how much oxygen the blood is carrying.
Lasers are used in eye surgery and to treat some skin conditions.
Optical scans of the eye can show the layers of the retina in detail.

Worked example

Let's calculate depth with ultrasound

An ultrasound probe sends a pulse into the abdomen. The echo from the near edge of a kidney returns 0.000 13 s (130 µs) later. Sound travels through soft tissue at about 1540 m/s. How deep is the edge of the kidney?

The pulse travels to the kidney and back, so the total distance is twice the depth.
Total distance = speed × time = 1540 m/s × 0.000 13 s = 0.2002 m.
Depth d = 0.2002 m ÷ 2 = 0.1001 m ≈ 10 cm.
Precision: the time was given to 2 significant figures, so report about 10 cm (0.10 m), not 10.01 cm.

The edge of the kidney is about 10 cm below the skin.

How ultrasound measures depth. The probe sends a sound pulse and times the echo. Because the sound goes down and back, the depth is half the distance travelled. Many pulses in many directions build up an image.

Worked example

Let's choose the right scan

For each patient, choose the most suitable imaging technology and give a reason: (a) a skier with a suspected broken wrist; (b) a soccer player with a suspected torn knee ligament; (c) a pregnant person at a routine check-up; (d) a person in the emergency department after a car crash, with a head injury.

(a) X-ray: fast, cheap and excellent for bone; the dose is small.
(b) MRI: ligaments are soft tissue, which MRI shows in detail; no ionizing radiation.
(c) Ultrasound: no ionizing radiation (important for a developing fetus), real-time images of the baby and placenta.
(d) CT: very fast and shows bleeding in the brain and skull fractures; in an emergency, speed matters more than the higher radiation dose.

(a) X-ray, (b) MRI, (c) ultrasound, (d) CT.

Assessing the importance to health and society

Imaging has huge benefits: earlier diagnosis (including cancer screening), less exploratory surgery, more precise treatment (such as radiation aimed exactly at a tumour), and monitoring whether treatment is working.

But there are issues to weigh:

Risk: ionizing radiation slightly raises the lifetime risk of cancer, so doctors follow the principle of using the lowest dose needed and only when the benefit is greater than the risk.
Cost: MRI and CT machines cost millions of dollars to buy and run. In Canada's public health system, that money is shared by everyone.
Access and wait times: waits for non-urgent MRI and CT scans can be long. People in many rural, Northern and remote communities, including many First Nations communities, may need to travel long distances for a scan.
Overuse and incidental findings: a scan sometimes finds something harmless that leads to worry, more tests or unnecessary treatment.

Careers in medical imaging

Many different people work together to produce and read every scan. The table below shows some of these careers.

Career	What they do	Typical education and training
Medical radiation technologist (radiography, MRI, nuclear medicine or radiation therapy)	Positions patients, operates X-ray, CT, MRI or nuclear medicine equipment, or delivers radiation treatment	A college or university program in medical radiation sciences (about 3 to 4 years), then registration with Ontario's regulatory college
Diagnostic medical sonographer	Performs ultrasound scans and records images for doctors	A post-secondary sonography program, then registration
Radiologist	A physician who interprets images and performs image-guided procedures	University degree, medical school (4 years) and a 5-year residency in radiology
Pathologist / medical laboratory technologist	Examines tissue and blood samples under the microscope	Medical school and residency (pathologist); a college or university lab-science program (technologist)
Medical physicist / biomedical engineer	Designs, tests and keeps imaging and radiation equipment safe and accurate	University degree in physics or engineering, usually with graduate studies

Careers in medical imaging. Grade 11 and 12 biology, chemistry, physics and math keep these pathways open.

Just for fun

Funny but true: an MRI scanner is so loud (it bangs, buzzes and beeps as its magnetic coils switch on and off) that patients are given earplugs or headphones. Some hospitals even let you choose the music, so you can get your brain scanned to your favourite playlist.

Hands-on activity

Mystery box "ultrasound"

You need: a shoebox with a lid, modelling clay or balled-up foil and small household objects, a wooden skewer or a long pencil, a ruler, a marker, and graph paper. Grown-up note: skewers are pointed; use the blunt end down, poke holes with care, and keep away from eyes.

Partner A secretly builds a "landscape" inside the box: a hill of clay, a block, a hollow. Close the lid.
Partner B makes a straight row of holes across the lid, 1 cm apart, with the skewer. Mark the skewer every 0.5 cm with the marker.
Partner B lowers the skewer (blunt end down) gently through each hole until it touches something and records the depth. This is like timing an echo: each reading gives the depth to a boundary.
Plot depth against hole position on graph paper to draw the hidden profile. Make a second row of holes to build a more complete "image".
Open the box and compare.

What to look for: a profile that matches the shape of the objects. More holes, closer together, give a sharper image: the same reason ultrasound and CT use many measurements. Discuss which objects were hard to detect (for example, something under an overhang), like bone or air blocking ultrasound.

Your turn

Why do bones appear white on an X-ray image?

How is a CT scan different from a single X-ray?

Name two imaging technologies that do not use ionizing radiation.

Why can't a person with certain metal implants always have an MRI?

An ultrasound echo returns after 0.000 065 s. How deep is the boundary? (v = 1540 m/s)

What does a pathologist do with a biopsy?

Give one benefit and one concern about medical imaging for society.

More practice: warm-up

Which imaging technology uses a strong magnet and radio waves?

Which uses high-frequency sound?

What is a biopsy?

What does a pulse oximeter measure?

Which doctor specializes in interpreting medical images?

More practice: on your own

Why is ultrasound, not X-ray, used to check on a fetus?

Explain why MRI is better than CT for looking at the spinal cord, but CT is often used first after a car crash.

Explain why ultrasound is poor at imaging the lungs.

Why do X-ray technologists stand behind a shield or leave the room during an X-ray?

Evaluate a claim: "Everyone should get a full-body CT scan every year, just in case."

Challenge

Two echoes come back from the front and back walls of an organ at 0.000 052 s and 0.000 117 s. How thick is the organ? (v = 1540 m/s)

A health region can buy either one new MRI machine or two new CT machines. List three questions decision-makers should ask, and explain why each matters.

Using what you know about cancer cells, explain why PET scans with a radioactive glucose tracer can show tumours.

Check your understanding

Quiz

Which imaging method is best for a suspected broken arm?

Which technology uses ionizing radiation?

An ultrasound echo returns after 0.000 10 s in soft tissue (1540 m/s). How deep is the boundary?

What does MRI show especially well?

Which career interprets medical images and is a physician?

Which is an example of biophotonics?

For parents and teachers

Big idea: expectation B1.2 asks students to assess the importance to human health and society of medical imaging technologies used in diagnosis or treatment (ultrasound, X-rays, CT, MRI, microscopy, biophotonics). A2.1 and A2.2 ask about careers and contributors. Students should know how each works, what it shows best, and the trade-offs.

How to help: If anyone in the family has had an X-ray, ultrasound or MRI, talk about the experience (keeping private details private). The mystery-box activity is a hands-on model of how scans build an image from many measurements. Encourage your teen to look up the imaging programs at an Ontario college or university to see the real requirements.

Common difficulties:

Thinking MRI or ultrasound use radiation like X-rays.
Forgetting to divide by 2 in echo calculations.
Answering "which is best" questions without giving a reason tied to the tissue or situation.