Learn the idea: a short explanation, then a check.
X-rays are a form of electromagnetic radiation with enough energy to pass through soft tissue but not bone, which is why they can show a broken bone without surgery. Wilhelm Röntgen discovered them in 1895.
In this lesson we look at how X-rays are produced. In an X-ray tube, electrons are boiled off a hot filament, accelerated across tens of thousands of volts, and slammed into a metal target. Most of their energy becomes heat, and a small fraction is released as X-rays.
X-rays are grouped by how penetrating they are. Soft X-rays have lower energy and are easily absorbed. Hard X-rays have higher energy and pass through much more matter. The accelerating voltage decides how hard the beam is, and we see why this matters for imaging and for patient safety.
One of my students asked me to quickly review a bone scan with an X-ray image. Here is a quick video I produced
How CT scans work
A normal X-ray is a shadow picture: bones, muscle and organs all overlap in a single flat image. A CT (computed tomography) scan solves this by looking at the body from many directions.
In a CT scanner, an X-ray tube and a ring of detectors spin around the patient, taking hundreds of X-ray readings from different angles. Each tissue absorbs X-rays by a different amount, with bone absorbing the most. A computer uses all of the readings to work out how much each tiny block of the body absorbed, then builds a picture of a thin cross-section, or “slice”. The slices can be stacked to make a 3D image.
CT gives far more detail than a plain X-ray, but it uses a higher radiation dose, so it is used only when the detail is needed.
📝Practise
Worked examples first, then problems to try, then worksheets.
Check Your Understanding: X-rays and CT Scans
1. Why do bones appear white on a normal X-ray image?
Bone is denser than soft tissue and contains calcium, which absorbs X-rays strongly. Fewer X-rays get through the bone to the detector, so that part of the image shows as white. Soft tissue absorbs much less, so more X-rays reach the detector behind it and it looks darker.
Bone does not emit or reflect X-rays, and X-rays do not travel faster through it. The image is a shadow: bone absorbs more X-rays than soft tissue, so fewer reach the detector behind it and it shows as white.
2. A radiographer wants to make an image of a patient's chest. Which type of X-rays should be used?
Hard (high-energy) X-rays pass through the body and reach the detector, so they can form an image. Soft X-rays are mostly absorbed by the skin and tissue. They never reach the detector, so they add to the patient's dose without helping the image. That is why soft X-rays are filtered out of the beam.
Soft X-rays are not safer for imaging. They are absorbed in the body, so they give the patient a dose without reaching the detector. Hard X-rays are penetrating enough to pass through the patient and form the image, and soft X-rays are filtered out.
3. In an X-ray tube, what mainly decides how hard (penetrating) the X-rays are?
The voltage sets the energy each electron gains (E = eV), and so the energy of the X-ray photons. A higher voltage makes harder X-rays. The filament current controls how many electrons are released, which changes the intensity of the beam, and the exposure time changes the total dose.
The filament current controls how many electrons are released, so it changes the intensity. The exposure time changes the dose. The hardness depends on the energy of each electron, which is set by the accelerating voltage (E = eV).
4. How does a CT scan differ from a normal X-ray?
In a CT scanner the X-ray tube and detectors rotate around the patient, taking hundreds of readings from different angles. A computer combines the readings into images of thin slices of the body, which can be stacked to make a 3D picture. A normal X-ray is a single flat shadow image.
A CT scan does use X-rays, but not a single shot from the front. The tube and detectors rotate around the patient taking many readings from different angles, and a computer builds those into cross-sectional slices. Sound waves are used in ultrasound and magnetic fields in MRI.
5. Why can a CT scan show different soft tissues more clearly than a normal X-ray?
In a normal X-ray, everything along the beam is added into one shadow, so overlapping tissues blur together. A CT scan measures from many angles, and the computer works out the absorption of each small block (voxel) separately. This separates tissues that differ only slightly in how much they absorb.
CT uses X-rays from a tube, not magnets or a radioactive source, and it does not rely on softer X-rays. Its advantage is the computer: it uses readings from many angles to find the absorption of each small block, so tissues at different depths no longer overlap in one shadow.
6. Why are CT scans used only when the extra detail is really needed?
A CT scan involves many X-ray exposures from different angles, so the dose is much higher than for a single plain X-ray. X-rays are ionising and can damage cells, so doctors weigh the benefit of the extra detail against the risk of the extra dose.
CT images are produced in minutes, the scanner does not need to be cold, and it shows soft tissue as well as bone. The main drawback is the dose: many exposures mean a much higher radiation dose than a plain X-ray, so it is used only when the detail is needed.