Imaging Methods: X-ray, CT, MRI, Ultrasound

What is an X-ray image and why is it like a shadow?

An X-ray image is a two-dimensional shadow. Dense tissue such as bone absorbs more of the beam, so less radiation reaches the detector behind the patient, and that area prints or displays as white. Air absorbs almost nothing, so it stays dark, which is why a chest film reads as a grey map of ribs, heart and lung fields. The same logic runs through every other method, with different physics. A CT scanner rotates an X-ray tube and detectors around the body and computes cross-sectional slices from many angles. MRI uses a strong magnetic field and radio waves, with no ionising radiation at all. Ultrasound sends high-frequency sound into tissue and listens for the echoes that return. An X-ray image is a projection, not a slice. The beam passes through the whole body part, and every structure along that path adds its absorption to the same pixel. A rib in front of a lung and a rib behind it land on top of each other. Radiologists call this superimposition, and it is the main reason a plain film can miss small lesions or hide a fracture behind overlapping bone. The shadow analogy holds in three ways. First, the source is on one side and the detector on the other, exactly like a lamp and a wall. Second, denser material blocks more light or more X-rays, so contrast comes from absorption differences. Third, the image is flat: depth is lost, and the reader has to infer it from standard views taken at different angles. Two projections are routine for many body parts, for example a front view and a side view. A fracture line visible in one may disappear in the other. This is also why a radiographer asks a patient to hold still and to breathe in a specific way: movement blurs the shadow, and a blurred shadow is harder to interpret.

How does a CT scanner create a computed slice?

A CT scanner does not take one picture. It takes hundreds of projections while the X-ray tube and the detector ring rotate around the patient, then a computer reconstructs a cross-section from that data. The word computed in computed tomography refers to that reconstruction step, not to the X-ray itself. Each rotation produces a set of attenuation measurements. The reconstruction algorithm assigns every small volume of tissue a number, expressed in Hounsfield units, where water is zero, air is about minus one thousand and dense bone is several hundred or more. Those numbers are mapped to a grey scale, and the result is a slice that can be viewed from any angle or reformatted into other planes. Modern scanners acquire many thin slices per rotation, so a chest or abdomen study can be reviewed as a stack. This is why CT shows small structures that a plain film cannot separate, and why it is used for trauma, suspected pulmonary embolism, staging of tumours and complex bone injury. The cost is dose. A CT examination delivers more ionising radiation than a single plain film, often by a large factor, and the exact figure depends on the body part and the protocol. UK practice therefore applies two principles: justification, meaning the examination must be expected to change management, and optimisation, meaning the lowest dose that still answers the question. The Ionising Radiation (Medical Exposure) Regulations 2017, known as IR(ME)R 2017, place those duties on the employer and on the practitioners who justify, authorise and perform the exposure.

Plain films remain the first-line test for suspected fractures, chest infection and some abdominal problems because they are fast, cheap and deliver a low dose compared with CT. The trade-off is that flatness. When the clinical question needs depth, the next step is usually a computed slice. A patient-facing explanation of how that shadow is produced and read sits at X-ray image, shadow, which walks through the same idea from the request form to the final report.

A second measurement question runs through what heart numbers mean.

Why does an MRI scan not use ionising radiation?

MRI uses a strong static magnetic field, switched gradient fields and radio-frequency pulses. None of these is ionising. Ionising radiation, by definition, carries enough energy to remove electrons from atoms, and that is what X-rays and gamma rays do. Radio waves sit far below that threshold, so an MRI examination does not carry the radiation risk associated with X-ray or CT. What MRI measures is not absorption but the behaviour of hydrogen nuclei, mostly in water and fat, inside a magnetic field. The scanner perturbs those nuclei with radio pulses and records the signal they emit as they relax. Different tissues relax at different rates, and the sequence timing is adjusted to weight the image toward one property or another. That is why the same anatomy can look very different on T1-weighted and T2-weighted images, and why a radiologist reads several sequences together. The absence of ionising radiation makes MRI attractive for children, for pregnant patients when ultrasound is not sufficient, and for repeated follow-up of a known condition. It is not risk-free in a broader sense. The magnetic field is always on, so implanted devices, metallic foreign bodies and some tattoos must be checked before entering the scanner room, and the examination is longer and noisier than CT. Claustrophobia is a real limitation for some patients.

How does ultrasound turn echoes into a picture?

Ultrasound uses a probe that both emits and receives high-frequency sound, typically in the range of a few million hertz. The sound travels into tissue and reflects wherever there is a change in acoustic impedance, for example at the boundary between fluid and a solid organ. The probe records the returning echoes and the machine converts their delay into depth and their strength into brightness. There is no ionising radiation here either. The main safety consideration is thermal and mechanical, which is why output is kept as low as reasonably practicable and why the examination is performed by a trained sonographer or radiologist who adjusts the probe and the settings in real time. Ultrasound is the usual first test in pregnancy, in suspected gallstones and in many soft-tissue problems, and it is often used to guide a needle into a specific spot.

What happens between the request and the report?

The clinical pathway starts with a written request from the referring clinician, which states the question to be answered and any relevant history. The imaging department checks that request against local protocols and against the patient's record, including previous studies and any implanted device. For CT and MRI, a contrast agent may be needed, and the patient is screened for allergies, kidney function and other contraindications before it is given. On the day, a radiographer or sonographer confirms identity and the correct examination, explains what will happen, positions the patient and acquires the images. Some examinations are reported immediately by a radiologist on site, and others are reported later from a worklist. The report returns to the referring clinician, who combines it with the clinical picture. A report is an interpretation, not a diagnosis in isolation, and the Royal College of Radiologists has published guidance on how reports should be structured and communicated.

Who is involved, and what keeps the dose down?

Four groups appear in most UK imaging episodes. Radiologists are doctors who interpret images and, in some cases, perform procedures. Radiographers operate X-ray, CT and MRI equipment and are registered healthcare professionals. Sonographers perform ultrasound examinations and often report them. Medical physics experts advise on equipment and dose. Safety is layered. Justification asks whether the examination is needed at all. Optimisation asks whether the dose is as low as it can be while still answering the question. Diagnostic reference levels, published nationally, give departments a benchmark for typical doses by examination type. IR(ME)R 2017 sets out who may justify, authorise and perform exposures, and the Ionising Radiation Regulations 2017 cover the protection of workers and the public. For a patient, the practical version is simple. Ask what the examination is for, whether a non-ionising alternative would answer the same question, and when the result will reach the person who requested it. Those three questions cover most of what the pathway is designed to deliver.