Introduction to Radiology and Nuclear Medicine
1. History of Radiology and Nuclear Medicine
Early Discoveries and Milestones
The field of radiology began with Wilhelm Rontgen’s discovery of X-rays in 1895. However, it wasn’t until the end of World War II that X-rays became a widely used medical tool. Rontgen was awarded the first Nobel Prize in Physics for his groundbreaking work.
In 1946, Felix Bloch and Edward Purcell made significant strides in the development of magnetic resonance imaging (MRI). Their discovery of magnetic resonance in solids and liquids led to the creation of devices capable of producing images of these fields. They were awarded the Nobel Prize in Physics in 1952.
From 1950 to 1970, nuclear magnetic resonance (NMR) was primarily used for chemical and physical molecular analysis. In 1967, Paul Lauterbur revolutionized NMR by demonstrating that photographic-style images could be generated using magnetic resonance, a feat previously only possible with light and X-rays. Around the same time, physician Raymonde Damadian discovered that malignant body tissue exhibited a different spectrum compared to normal tissue. By 1974, he successfully produced a crude image of a tumor in a rat, and in 1976, he achieved the first MRI image of a human body.
Concurrently, British engineer Godfrey Hounsfield at EMI Laboratories developed computed tomography (CT). This scanning technique combines X-ray images from a detector rotating 360 degrees around the subject. A specialized computer then processes this information to create a two-dimensional image of the scanned ‘slice’.
Radiation
Radiation refers to traveling particles or waves, which can be categorized into two main types:
Ionizing Radiation
Ionizing radiation possesses sufficient energy to remove an electron from an atom or molecule. There are three primary types of ionizing radiation:
- Alpha particles: Consist of two protons and two neutrons.
- Beta particles: Identical to electrons.
- Gamma rays and X-rays: Pure energy (photons) and the most commonly used in medicine.
When ionizing radiation interacts with atoms in the human body, it generates free radicals, which are chemically active substances that can damage proteins, nucleic acids, and other cellular components. Free radicals are implicated in degenerative diseases, aging, and cancer development.
Medical radiography (imaging) and radiation therapy constitute the largest source of artificial ionizing radiation exposure for humans.
Non-Ionizing Radiation
Non-ionizing radiation comprises the low-energy portion of the electromagnetic spectrum, including ultraviolet (UV) light, radio waves (microwaves), and infrared (heat) radiation.
Radioactivity
Radioactivity describes the decay or rearrangement of an atom’s nucleus. This decay occurs naturally and spontaneously in unstable nuclei, typically due to an imbalance between the number of protons and neutrons.
Radioactive decay can manifest in several ways, including:
- Spontaneous fission: Also known as splitting the atom, where the nucleus divides into two parts.
- Neutron release: A neutron is ejected from the atom’s core.
- Alpha decay: The nucleus emits an alpha particle (a helium-4 nucleus) consisting of two neutrons and two protons.
- Beta decay: The nucleus ejects an electron (or a positron). This differs from an electron being removed from orbitals surrounding the nucleus.
- Gamma decay: Protons and neutrons within the nucleus rearrange into a more stable configuration, releasing energy in the form of a gamma ray.
Units of Radioactivity
The Becquerel (Bq) is the SI unit of radioactivity, defined as the activity of a quantity of radioactive material in which one nucleus decays per second. Therefore, 1 Bq = 1 s-1.
Radiation exposure is quantified using radiation dose, which can be categorized into two main types:
- Sievert (Sv): The biological dose, also known as the dose equivalent, expressed in rem or sievert. This dose considers the biological damage caused by a particle, which depends not only on the total energy deposited but also on the rate of energy loss per unit distance traveled by the particle (linear energy transfer). 1 Sv = 100 rem.
- Gray (Gy): The absorbed dose, also known as the physical dose, defined by the amount of energy deposited per unit mass in human tissue or other media. The original unit is the rad (100 erg/g), which is now largely replaced by the SI unit, the gray (Gy) (1 J/kg). 1 Gy = 100 rad.