How medical imaging works: X-ray, CT, MRI and ultrasound

Medical imaging produces pictures of tissues and structures inside the body.^[1] The four most familiar methods rely on very different physics. X-ray radiographyRadiograph (X-ray image)An image made when X-rays that pass through the body reach a detector on the other side, recording the “shadows” of the structures they crossed. and Computed tomographyComputed tomography (CT)An X-ray technique in which a source and detectors rotate around the patient so that a computer can build cross-sectional images (“slices”). use X-rays,^[1][2] Magnetic resonance imagingMagnetic resonance imaging (MRI)Imaging that uses a strong magnet and radiofrequency pulses to detect signals from protons in the body. It does not use ionizing radiation. uses a strong magnet and radiofrequency pulses,^[3] and UltrasoundUltrasoundImaging with sound waves above the range of human hearing. A transducer sends pulses into the body and listens for the echoes that return from tissue boundaries. uses sound waves.^[4]

X-rays are a form of electromagnetic radiation, like visible light but with higher energy, which lets them pass through most objects, including the body.^[1] Wilhelm Röntgen found this previously unknown, penetrating radiation in 1895 while studying cathode rays, and was awarded the Nobel Prize in Physics in 1901.^[6]

When X-rays travel through the body to a detector on the other side, the image records the “shadows” of the structures they crossed.^[1] Different tissues absorb X-rays to different degrees: dense bone absorbs much of the radiation, while muscle, fat and organs let more of it through. That is why bone appears white, soft tissue appears in shades of grey, and air appears black.^[5] This weakening of the beam is called AttenuationAttenuationHow much an X-ray beam is weakened as it passes through the body. Dense bone absorbs much of the beam, while soft tissue lets more of it through..

In a CT scanner, a motorised X-ray source and detector rotate quickly around the patient, and a computer turns the signals into cross-sectional images, or “slices”. Because overlapping tissue is removed, a slice can show more detail than a conventional X-ray.^[1] Each slice usually represents 1 to 10 millimetres of tissue, depending on the machine, and successive slices can be stacked digitally into a three-dimensional image.^[2]

Contrast agentsContrast agentA substance given to make certain tissues or vessels stand out: iodine (into a vein) or barium (by mouth) for CT, and gadolinium for MRI. help particular structures stand out. An iodine-based agent injected into the bloodstream helps show blood vessels, and barium-based agents taken by mouth are used to image the digestive tract.^[2]

Allan Cormack and Godfrey Hounsfield shared the 1979 Nobel Prize in Physiology or Medicine “for the development of computer assisted tomography”.^[8] CT values are expressed in Hounsfield unitsHounsfield unit (HU)The scale for CT values. By definition water is 0 HU and air is −1000 HU; bone ranges from several hundred to several thousand HU., which the next article explores.^[9]

MRI uses a powerful magnet to align the protons in the body’s tissues. A radiofrequency pulse knocks them out of equilibrium; when the pulse is switched off, sensors detect the energy released as the protons realign with the magnetic field. The time this takes and the energy released differ between tissues, which is what creates contrast in the image.^[3]

MRI does not use ionizing radiation and is especially useful for soft tissues such as the brain, spinal cord, nerves, muscles, ligaments and tendons. Its strong magnetic field is a safety consideration for people with some implants, and some scanners produce sound up to 120 decibels, so ear protection may be needed. Gadolinium-based contrast agents are sometimes given.^[3]

Paul Lauterbur described how to form images from nuclear magnetic resonance signals in Nature in 1973,^[10] and shared the 2003 Nobel Prize in Physiology or Medicine with Peter Mansfield “for their discoveries concerning magnetic resonance imaging”.^[11]

Ultrasound probes, called transducersTransducerThe handheld ultrasound probe. Its piezoelectric crystals turn electrical signals into sound waves and returning echoes back into signals., produce sound waves above the threshold of human hearing (above 20 kHz); most work at much higher frequencies, in the megahertz range. The waves reflect off boundaries between tissues, such as between fluid and soft tissue, and the scanner uses the speed of sound and the time each echo takes to return to calculate how far away each boundary is.^[4]

A gel on the skin keeps air pockets from blocking the sound. Ultrasound does not use ionizing radiation, but it is not good at imaging bone or air-containing tissue such as the lungs. Doppler ultrasoundDoppler ultrasoundAn ultrasound technique for measuring and visualising blood flow in vessels and in the heart. measures and visualises blood flow. The FDA and many professional societies discourage casual uses such as keepsake videos and recommend ultrasound only when there is a true medical need.^[4]

X-ray imaging and CT use Ionizing radiationIonizing radiationRadiation with enough energy to harm living tissue. X-ray imaging and CT use it; MRI and ultrasound do not., which can harm living tissue. The risk increases with the number of exposures over a lifetime, although the risk of developing cancer from imaging radiation is generally considered very small. Children are more sensitive to ionizing radiation than adults.^[1]

Whole-body dose is expressed as effective dose, in millisieverts (mSv)Millisievert (mSv)The unit commonly used for effective radiation dose from imaging, which lets the doses of different exams be compared with each other and with natural background radiation.. RadiologyInfo estimates that the average person in the U.S. receives about 3 mSv per year from natural background radiation. An adult chest X-ray is about 0.1 mSv, comparable to 10 days of background radiation, and a CT of the chest about 6.1 mSv, comparable to 2 years. Actual doses vary with the patient and the equipment.^[12]