RADIATION AND DIAGNOSES
DR. S.M. ALAM
Jan 31 - Feb 6, 2011
Radiation is a process where energy emitted by one body travels in a straight line through a medium or through space. Radiation comes from the sun, nuclear reactors, microwave ovens, radio antennas, X-ray machines, and power line.
Radiation can be classified as either ionizing or non-ionizing. Non-ionizing radiation is lower energy radiation that comes from the lower part of the electromagnetic spectrum. It is called non-ionizing because it does not have enough energy to completely remove an electron from an atom or molecule. Examples include visible light, infrared light, microwave radiation, radio waves, and long wave (low frequency) radiation. Radiation has enough energy to detach electrons from atoms or molecules - the process of ionization. It comes from both subatomic particles and the shorter wavelength portion of the electromagnetic spectrum. Examples include ultraviolet, X-rays, and gamma rays from the electromagnetic spectrum and subatomic particles such as alpha particles, beta particles, and neutrons. Subatomic particles are usually emitted as an atom decays and loses protons, neutrons, electrons, or their antiparticles.
Measuring radiation is complex and utilizes several different units. Scientists measure the amount of radiation being emitted in the conventional unit called the curie (Ci) or the SI unit called the becquerel (Bq). These units express the number of disintegrations (or breakdowns in the nucleus of an element) per second as the element tries to reach a stable or nonradioactive state. One Bq is equal to one disintegration per second and one Ci is equal to 37 billion Bq.
When measuring the amount of radiation that a person is exposed to or the amount of energy absorbed by the body's tissues, two units are used: the conventional Roentgen (or radiated) absorbed dose (rad) and the SI gray (Gy). One Gy is equal to 100 rad.
If a scientist is measuring a person's biological risk of suffering health effects of radiation, the units of measurement are the conventional Roentgen equivalent man (rem).
There is a branch of medicine called radiology that focuses on diagnosing and treating diseases using imaging technologies based on radiation. Common imaging techniques include:
1. RADIOGRAPHY - X-ray radiation is directed through part of the body, which absorbs some of the radiation. Hard tissue such as bone absorbs more than soft tissue such as muscle. The X-rays that are not absorbed pass through the body and expose photographic film on the other side of the body, creating a shadow effect. Different X-ray strengths are employed depending on the part of the body that is being studied. Common projections include a chest X-ray, breast X-ray (mammography), dental X-ray (dental radiograph), and abdominal X-ray.
2. FLUOROSCOPY (ANGIOGRAPHY, GASTROINTESTINAL FLUOROSCOPY) - These are X-rays that use a contrast (usually iodine- or barium-based) in order to provide moving projections or images of movement inside the body. Angiography is used to view the cardiovascular system and gastrointestinal fluoroscopy is used to view the gastrointestinal tract.
3. COMPUTED TOMOGRAPHY (CT) - a CT scan uses X-rays and computers to create images that show slices of soft and hard tissues. Contrast agents are often used during CT scans, and the result is a 3D reconstruction of the part of the body being imaged. Widespread screening for the buildup of calcium in the arteries using computed tomography scans would lead to an estimated 42 additional radiation-induced cancer cases per 100,000 men and 62 cases per 100,000 women, a study revealed.
4. ULTRASOUND - Ultrasound uses high-frequency sound waves to see soft tissues inside the body. Since the test uses sound waves, no ionizing or potentially damaging radiation is absorbed by the body. Ultrasounds can show images in real time, but they cannot be used to image bones, lungs, bowel loops, or other air-filled body parts.
5. MAGNETIC RESONANCE IMAGING (MRI) - An MRI uses strong magnetic fields and a radio signal to take high quality 3D images of the body. Although an MRI requires a patient to lie very still in a noisy tube for a long period of time, the scan provided excellent visualizations of soft tissue. MRIs do not use any damaging ionizing radiation, only strong magnetic fields, and non-ionizing radio frequencies.
6. DUAL ENERGY X-RAY ABSORPTIOMETRY (DEXA OR BONE DENSITOMETRY) - Commonly used to test for osteoporosis, DEXA scans use two narrow X-ray beams to collect information on the density of the bone. No images of the bone are created, and so this scan is not considered projectional radiography.
7. POSITRON EMISSION TOMOGRAPHY (PET) - A PET scan is a nuclear medicine imaging technique that uses a radioactive contrast agent that is injected into the body. This tracer eventually begins to radioactively decay and emits positron particles that are picked up by the PET scanner. A computer is used to reconstruct 3D images.
X-rays are electromagnetic waves with a wavelength smaller than about 10 nanometers. When an X-ray photon collides with an atom, the atom may absorb the energy of the photon and boost an electron to a higher orbital level or if the photon is very energetic, it may knock an electron from the atom altogether, causing the atom to ionize. Generally, a larger atom is more likely to absorb an X-ray photon, since larger atoms have greater energy differences between orbital electrons. Soft tissue in the human body is composed of smaller atoms than the calcium atoms that make up bone, hence there is a contrast in the absorption of X-rays. X-ray machines are specifically designed to take advantage of the absorption difference between bone and soft tissue, allowing physicians to examine structure in the human body.
Electromagnetic radiation (EMR) takes the form of self-propagating waves in a vacuum or in matter. EM radiation has an electric and magnetic field component, which oscillates in phase perpendicular to each other and to the direction of energy propagation. Electromagnetic radiation is classified into types according to the frequency of the wave, these types include (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called visible spectrum or light, is sensed by the eye of various organisms.
Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the individual particles or waves, and not on their number. An intense flood of particles or waves will not cause ionization if these particles or waves do not carry enough energy to be ionizing. Roughly speaking, particles or photons with energies above a few electron volts (eV) are ionizing. Ionizing radiation comes from radioactive materials, X-ray tubes, particle accelerators, and is present in the environment. It is invisible and not directly detectable by human senses, so instruments such as Geiger Muller counters are usually required to detect its presence. In some cases, it may lead to secondary emission of visible light upon interaction with matter. It has many practical uses in medicine, research, industry and other areas, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue, resulting in skin burns, radiation sickness and death at high doses and cancer, tumors and genetic damage at low doses.
Radiation and radioactive substances are used for diagnosis, treatment, and research. X-rays, for example, pass through muscles and other soft tissue but are stopped by dense materials. This property of X-rays enables doctors to find broken bones and to locate cancers that might be growing in the body. Doctors also find certain diseases by injecting a radioactive substance and monitoring the radiation given off as the substance moves through the body. Radiation used for cancer treatment is called ionizing radiation because it forms ions in the cells of the tissues it passes through as it dislodges electrons from atoms. This can kill cells or change genes so the cells cannot grow.