X-ray Imaging: Principles, Techniques, and Safety
Roentgenology: Understanding X-ray Imaging
1. Roentgenology Rays: Physical Parameters and Imaging Geometry
X-rays are a form of electromagnetic radiation with wavelengths ranging from 0.01 to 10 nanometers (nm). They have shorter wavelengths than ultraviolet (UV) rays and longer wavelengths than gamma rays. X-radiation is also known as Roentgen radiation, named after Wilhelm Conrad Roentgen, who discovered it.
Types of X-rays:
- Soft X-rays: 0.12-12 kiloelectron volts (keV)
- Hard X-rays: 12-120 keV (can penetrate solid objects)
X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
2. Radiophysics: Radiation Interactions and Radioactivity
The smallest unit of radiation is a photon or quantum, which arises in matter. It occurs when a nucleus, atom, or electron is affected by a disturbance from an outside source. Electrons are normally in their lowest energy states (orbits). When under stress, such as an increase in temperature, the atomic microsystem is disturbed. For example, when an incoming electron crashes into the anode of an X-ray tube, it excites and raises a low-energy shell (K shell) electron to a higher shell. Essentially, X-rays emerge due to changes in the energies of the electron shell.
Radiation can be classified as either particle radiation (A) or electromagnetic radiation (B). The properties of radiation can be characterized by three concepts: energy, frequency, and wavelength. When X-rays are generated in an X-ray tube, the penetrating properties and intensity of the radiation are determined by a high voltage. The way a photon interacts with matter (tissues) depends on how much energy is passed through it. Different photon energies are used in various imaging methods.
3. Patient Dose: Quantities and Units of Radiation Dose
Absorbed dose is the concentration of energy deposited in tissue due to exposure to ionizing radiation. It describes the intensity of the energy deposited in any small amount of tissue located anywhere in the body. The unit of measurement for absorbed dose is the milligray (mGy). The absorbed dose by X-ray is typically 10 mGy in all tissues except for bone, which is around 40 mGy.
Equivalent dose considers the damaging properties of different types of radiation. Not all radiation is alike. The equivalent dose in milliSieverts (mSv) is equal to the absorbed dose in mGy.
The effective dose is calculated by determining the equivalent dose to each irradiated organ and then multiplying this equivalent dose by a tissue-specific weighting factor for each organ or tissue type. The unit for effective dose is also the Sievert (Sv). The weighting factor is higher for radiosensitive organs such as gonads, bone marrow, lungs, colon, and breast.
4. Radiation Biology and Protection: Patient and Personnel Safety
Ionization and excitation cause fragmentation of molecular bonds, leading to harmful consequences for cell structure, metabolism, and organ function. Injuries are divided into somatic or genetic categories. Genetic effects can be passed on to offspring and may be seen many generations later. Somatic effects are seen in the patient and can be acute or chronic.
Stochastic effects: Even a single hit of radiation to one cell or a small cell group can cause a biological consequence. Damage may be hereditary (gonads) or carcinogenic (tissues). The extent of damage does not depend on the absorbed dose.
Non-stochastic effects: These have a threshold, and damage is proportional to the amount absorbed.
Radioprotection is necessary to avoid and reduce somatic and genetic doses, following the As Low As Reasonably Achievable (ALARA) principle. Use enough dose to obtain the best image without compromising patient safety. Screen films are used to protect patients. To protect personnel, it is essential to be outside the X-ray lab when patient exposure is made. Sufficient protective clothing, such as lead aprons and lead gloves, should be used to avoid scattered radiation. The International Commission on Radiological Protection (ICRP) recommends a maximum dose of 20 mSv per year for the whole body of personnel.
5. Projection of X-ray Images: Radiography, Fluoroscopy, and Fluorography
Radiography:
Radiography uses X-rays to view a non-uniformly composed material. A heterogeneous beam of X-rays is produced by a generator and projected toward an object. Depending on the density and composition of different areas, a proportion of X-rays are absorbed by the object. The X-rays that pass through are captured behind the person by a detector, resulting in a 2D image of all structures superimposed on each other.
Fluoroscopy:
Fluoroscopy obtains real-time moving images of the internal structures of a patient via a fluoroscope. A fluoroscope consists of an X-ray source and a fluorescent screen. The length of this procedure exposes the patient to a high absorbed dose.
Fluorography:
Fluorography is the photography of X-ray images from a fluorescent screen. It is used to diagnose conditions such as tuberculosis (TB) or lung cancer. The X-ray beam from the tube is attenuated when it hits the patient. Depending on the density of the tissue, different intensities are captured by a photocathode on the other side, producing electrons that are accelerated and focused on the output phosphor. This forms a light beam that is focused by a lens onto a display screen.
6. Conventional and Computed Tomography
Computed tomography (CT) is a multi-directional scan where a 3D image is produced using X-ray beams. A motorized table moves the patient through a circular opening in the CT scanner. As the patient moves through, a source of X-rays rotates around the circular opening of the scanner. The X-ray produces a narrow, fan-shaped beam used to irradiate a section of the patient’s body. Typically, 10-50 rotations are used as the table moves accordingly. The patient may receive an injection of contrast media to improve the visualization of vascular structures. Detectors on the exit side record the X-rays exiting the section of the patient’s body being irradiated. Data is sent to a computer to reconstruct all of the snapshots taken into a cross-sectional image of internal organs for each complete rotation of X-rays.