Introduction To Medical Imaging Conventional Radiography
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Conventional Radiography is 2-D imaging modality
It is required to perform minimum 2-views orthogonal to each other:
1 AP (Anterior to Posterior) or PA (Posterior to Anterior)
2 Lateral
Supplemental views: Oblique views etc.
Skeletal radiographs typically use AP & lateral views
Chest radiographs and Scoliosis imaging in children will usually use the PA technique
Exceptions for PA chest views: patients unable to cooperate (severely ill or unconscious patients)
X-rays are a form of electromagnetic energy (EME) similar to light photons or other sources
X-rays are a form of man-made radiation
Ionizing effect of x-rays process of removal of atomic electrons from their orbits
Two basic types of ionizing radiation:
Particle (particulate) radiation produced by alpha & beta particles that are the result of radioactive decay of different materials
Electromagnetic Radiation (EMR) produced by x-rays or gamma rays called photons
The energy of EMR depends on its wavelength
Shorter wavelength corresponds to higher energy
The energy of EME is inversely related to its wavelength
Table of Contents
X-ray Properties
No charge
Invisibility
Penetrability of most matters (esp. human tissues) depends on “Z” (atomic number)
Making compounds fluoresce and emit light
Travel at the speed of light
Ionization and biologic effect on living cells
The Imaging System
X-rays are produced by an imaging system ( x-ray tube, operator’s console, and high voltage generator)
X-ray tube composed of (-) charged cathode and (+) charged anode enclosed in the evacuated class envelope and housed in the protective coat of metal
A Cathode made up of filament wire embedded within the focusing cup to give electrostatic focus to electrons’ cloud
Filament wire of heat resistant thorium tungsten metal of high melting point (3400 C) that “boils off” electrons during thermionic emission
Focusing cup polished nickel (-) charged that accommodated the filament to electrostatically repulse the electrons and confines them to the focal spot of the anode disc where x-rays are produced
Anode (+) charged target for electrons to interact at the focal spot
Conducts electricity
Rotates to dissipating heat
Made of tungsten to resist heat
Anode has a high atomic number to produce x-rays of very high efficiency at the focal spot
There are 2-focal spots large and small, each corresponding to cathode’s filament size (small vs. large) that depends on the magnitude of current in the cathode dictated by a radiographic study of larger or smaller body parts
It is known as the dual focus principle
When Electrons are emitted from the cathode as the cloud, they slam into the Anode’s focal spot resulting in 3 man events
Production of heat (99% outcome)
Production of Bremsstrahlung (i.e., breaking radiation) x-rays that represent the majority of x-rays within the x-ray emission spectrum
Production of Characteristic x-rays very few in the emission spectrum
Newly formed x-rays at the anode are of different energies
Only need high energy or “hard” x-rays to perform the radiographic study
Before x-rays exiting the tube we need to remove weak or low energy photons, i.e., “harden the beam.”
Added tube filtration in the form of aluminum filters is used that removes at least 50% of the “unfiltered” beam thus minimizing the patient’s radiation dose and maximizing image quality
High Voltage Generator
X-ray production requires an uninterrupted flow of electrons to the anode
Regular electricity supplies AC power with sinusoidal currents of “peaks and drops.”
In the past, single-phase high voltage generators would convert AC power into a half, or full wave rectified supply with a measure in the thousands of volts delivered with a “voltage ripple” or peaks of high voltage. Therefore, a term kilo voltage peaks (kVp) was used
Modern generators provide “uninterrupted” flow of electrical potential to the x-ray tube eliminating “voltage ripples” thus referred to as kilovoltage kV without “peaks.”
When x-rays interact with the patient’s tissued 3 events will occur
X-rays will pass through without interaction and “expose” the image receptor
Photoelectric interaction/effect (PE) comparatively lower energy x-rays will be absorbed/attenuated by the tissues
Compton scatter x-rays are “bounced off” to form scatter, contributing no useful information to the film and lower image contrast while potentially giving unnecessary radiation dose to staff
The final image is the product of all three types of interactions known as
Differential absorption of x-ray photons – the result of photons’ absorption via PE, Compton scatter and x-rays passing through the patient
Compton scatter probability decreases with an increase in x-ray energy compared to PE effect
Compton effect probability does not depend on the atomic number (Z)
An increase of total mass density (thick vs. thin) will increase Compton and PE interaction
What cells in the body are considered most vulnerable and most resistant to radiation?
Cells that are rapidly dividing and not terminally differentiated, epithelial cells, etc. are more radiosensitive
Bone marrow cells (stem cells) & lymphocytes are very radiosensitive
Muscle & and nerve cells are terminally differentiated and are less sensitive to radiation
Aged (senescent cells) vs. immature fetal cells are more vulnerable to radiation
However, following low dose radiation in most healthy individual cells will be able to repair likely without any long-lasting changes
Pregnancy & radiation initial 6-7 weeks are the most vulnerable
Do not use routine (non-emergent) radiographic examinations in pregnancy
Apply 10-day rule establish that radiographs can only be obtained during the initial ten days from the onset of the last menstrual cycle
Radiographic imaging of children:
If clinically possible use non-ionizing forms of medical imaging (e.g., ultrasound)
Non-axial imaging studies that use x-ray photons:
Conventional radiography
Fluoroscopy
Mammography
Radiographic angiography (currently less often used)
Dental imaging
Cross-sectional imaging using x-ray photons: Computed Tomography
Indication and Contraindication for conventional radiographic imaging
Advantages of Radiography: widely available, inexpensive, low radiation burden, the first step in imaging investigation of most MSK complaints
Disadvantages: 2D imaging, relatively lower diagnostic yield during an examination of soft tissues, numerous artifacts, and dependence on correct radiographic factors selection, etc.
Indications:
Chest: initial assessment of lung/intrathoracic pathology. Potentially determines or obviates the need for chest CT scanning. Pre-surgical evaluation. Imaging of pediatric patients due to extremely low radiation dose.
Skeletal: to examine the bone structure and diagnose fractures, dislocation, infection, neoplasms, congenital bone dysplasia, and many forms of arthritis
Abdomen:Â can assess acute abdomen, abdominal obstruction, free air or free fluid within the abdominal cavity, nephrolithiasis, evaluate placement of radiopaque tubes/lines, foreign bodies, monitor resolution of postsurgical ileus and others