Radiation is often used in medicine to see structures and functions inside the body without the need for invasive surgery. Techniques like X-rays, CT scans, and PET scans all use radiation to image the body. But how exactly does radiation allow doctors to visualize what’s going on inside us?
How does radiation work?
Radiation refers to energy that travels as waves or particles. It falls on the electromagnetic spectrum, which includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. These types of radiation have different properties and energies.
Ionizing radiation has enough energy to remove electrons from atoms or break chemical bonds. This ability is what makes it useful for medical imaging. Ionizing radiation includes X-rays, gamma rays, and some particles.
When ionizing radiation travels through matter, like the soft tissues of the body, it deposits some of its energy. The amount deposited depends on the type of tissue it goes through. Some tissues allow radiation to pass through more easily than others.
For example, X-rays can easily penetrate soft tissue but are more blocked by dense bone. On the other hand, gamma rays readily pass through bone. This variable absorption allows different tissues to be distinguished on an image.
X-rays
X-rays are a form of ionizing radiation with a wavelength between 0.01 and 10 nanometers, making them higher energy than ultraviolet radiation but lower energy than gamma rays.
When the small wavelength x-ray waves interact with matter, they are absorbed to different degrees depending on the density and composition of the material. X-rays are absorbed most readily by bones and metals, moderately by soft tissues, and least by fat. This difference in absorption allows x-rays to create images of structures inside the body.
In medicine, x-rays are generated by X-ray tubes. These devices direct a focused beam of radiation at the patient’s body. The x-rays pass through the body and are absorbed based on the varying thicknesses and densities of bones, organs, and other structures.
The x-rays that are not absorbed reach a detector on the other side of the patient and expose photographic film or stimulate fluorescent screens that create an image. Bones and metal implants, which block x-rays, show up as white. Soft tissues allow more x-rays through and appear gray. Air allows virtually all x-rays through and looks black.
Doctors can then examine these X-ray images to see abnormalities like broken bones, pneumonia, cancer masses, swallowed objects, etc. X-rays provide clear images of dense structures like bones and can be done at a low cost compared to other scans.
CT scans
CT or CAT scans (computed axial tomography) take multiple x-ray images from different angles around the body and combine them to generate cross-sectional views. They can visualize soft tissues better than conventional x-rays.
In CT machines, an x-ray tube rotates around the patient’s body as they move through the scanner. Multiple beams are directed through the same location. Sensors on the opposite side of the patient detect the x-rays and how much radiation is absorbed along each path.
A computer assembles these absorption readings from different angles into a tomographic image. Each 2D cross sectional “slice” shows a segment of the body. Multiple images stacked together provide a 3D visualization.
CT scans are useful for diagnosing causes of pain like kidney stones, appendicitis, or bone fractures. They can also detect tumors, hemorrhages, blood clots, and enlarged organs. Sometimes an iodine-based contrast dye is injected or swallowed first to enhance visualization.
The amount of radiation exposure from a CT scan is much higher than for a plain x-ray. But they provide very detailed views that can’t be captured by standard x-rays.
PET scans
PET or Positron Emission Tomography scans use radioactive tracers to see metabolic processes in the body. PET actually doesn’t use x-rays. It relies on the detection of gamma rays emitted by positrons (antimatter electrons).
Before a PET scan, the patient is injected with a radioactive “tracer” like fluorodeoxyglucose (FDG) which contains radionuclides. This tracer accumulates in areas with high metabolic activity like cancerous tumors. As the radionuclides decay, they emit positrons. These positrons collide with nearby electrons resulting in two gamma ray photons shooting off in opposite directions.
The PET scanner has rings of detectors that pick up these gamma rays. A computer assembles the signals into 3D images showing where the tracer accumulated in the body. This highlights areas of high metabolic activity.
PET scans are often combined with CT scans to provide both anatomical and functional imaging. Doctors use PET/CT scans to diagnose various cancers, heart disease, and brain disorders. PET scans show hot spots where glucose is abnormally high, suggesting tumor growth.
Risks of radiation exposure
While radiation used in medical imaging provides useful diagnostic information, exposure does come with potential risks. Radiation has enough energy to damage cell DNA which can lead to cancer. However, the amount of radiation exposure from most scans is low.
The chances of developing cancer from the radiation exposure of a single scan is quite small. But repeated or cumulative exposure to radiation raises lifetime cancer risk. That’s why doctors avoid overusing scans that use ionizing radiation like x-rays and CTs.
PET scans involve ingesting radioactive material, so there is some internal exposure. But the radioactive isotopes used decay very quickly. The actual radiation dose is low and clears the body within a day.
Pregnant women and children have greater sensitivity to radiation. So their exposure is more strictly limited. Lead shields and other precautions are used to only target necessary areas.
Alternatives to radiation imaging
There are some imaging modalities that don’t use ionizing radiation such as:
Ultrasound
Ultrasound uses high-frequency sound waves beyond the range of human hearing. A transducer sends out sound waves and picks up the echoes bouncing off tissue. These echoes are converted into images showing structures and motion inside the body.
Ultrasound is commonly used to look at developing fetuses during pregnancy. It provides real-time imaging without radiation exposure. Ultrasound can also be used to examine organs like the heart, liver, kidneys, and ovaries.
But the view may be obstructed by air or gas pockets in the body which block sound waves. Ultrasound also cannot penetrate bone.
MRI
MRI or magnetic resonance imaging uses strong magnetic fields and non-ionizing radio waves to generate images. Patients are placed in the magnetic chamber of an MRI scanner. The magnetic field causes protons in the body’s cells to align.
Short bursts of radio waves are sent to knock the protons out of alignment. As they realign, they release energy signals that are picked up by the scanner. These signals vary based on the density of different tissues. An MRI computer turns these tissue characteristics into detailed images.
MRIs provide better visualization of soft tissues than CT scans. They are often used to look at ligaments, spinal cord injuries, brain abnormalities, tumors, etc. No radiation exposure is involved, but MRIs are more expensive and take longer than CT scans.
Endoscopy
Endoscopy uses a tiny camera on the end of a flexible tube to view internal organs through the body’s natural orifices. The tube is inserted through the mouth, nose, or anus and maneuvered to a target area.
Light shines through the endoscope illuminating tissue while the camera sends images to a monitor. Endoscopy allows direct visualization of digestive, urinary, and respiratory tracts without any incisions.
Capsule endoscopy is a version where the patient swallows a pill-sized camera to image the digestive tract. The camera wirelessly transmits images as it travels through the GI system.
Endoscopy does not have radiation risks. But it may cause discomfort and has difficulty accessing certain areas.
| Imaging Modality | Technology Used | Uses | Radiation Risks? |
|---|---|---|---|
| X-ray | Ionizing electromagnetic radiation | Bones, lungs, cavities | Yes |
| CT scan | Ionizing electromagnetic radiation | Soft tissues, cancers, strokes, injuries | Yes |
| PET scan | Radioactive tracer (FDG) detection | Cancer, heart disease, brain disorders | Yes (low) |
| Ultrasound | High-frequency sound waves | Fetuses, organs, muscles, heart | No |
| MRI | Magnetic fields and radio waves | Soft tissues, brain, ligaments, tumors | No |
| Endoscopy | Lighted camera tube | Digestive, urinary, respiratory tracts | No |
Conclusion
Forms of radiation that have enough energy to ionize matter play an important role in medical imaging. X-rays, CT scans, and PET scans all use ionizing radiation to visualize internal body structures based on differential absorption and tracer uptake. This allows doctors to diagnose conditions and monitor therapies in a non-invasive manner.
But ionizing radiation does come with a small cancer risk, especially with repeated high doses. Therefore, the radiation exposure from scans should be limited when possible. Other imaging methods like ultrasound, MRI, and endoscopy avoid radiation entirely while still providing detailed body imaging.
