Midterm Flashcards
create images using a series of X-rays generated by a tube that is rapidly rotated around the examined object. X-rays are a type of electromagnetic energy that have properties of both particles and waves and a level of energy between ultraviolet rays and gamma-rays in the electromagnetic spectrum.
Ct scanners
Ct scanner components
The generator
The scanning unit (gantry), which includes one or more X-ray tubes
Photon detectors
Shielding elements
The patient table
The image processor
The console (control unit)
provides the electrical power that is necessary to generate x-rays using two types of electrical current. A high voltage (20-150 kiloVolts) supply determines the maximum intensity of the X-rays that can be produced. Increasing this voltage increases the electrical potential difference between the anode and cathode. A low fixed voltage (about 10 kiloVolts) supply to the cathode filament enables continuous electron emission through a thermionic reaction.
Generator
is the structure that contains the X-ray tube, shielding elements, and photon detectors. The X-ray tube and photon detectors are positioned to face each other and are built to rotate 360 degrees in one direction around the patient. The gantry tilt is the angle formed between the X-ray tube plane and the vertical plane; in many modern machines, gantry tilt ranges between -25 degrees and +25 degrees. Gantry tilt can be changed by the CT operator according to the exam objectives, such as to reduce image artifacts or improve a healthcare provider’s ability to conduct an invasive CT- guided procedure. The use of slip rings in gantries allows continuous complete circular movements of the internal elements without the internal circuits and cables becoming entangled. The gantry contains a space for the table and the patient to pass through. CT was first available for head imaging only, but in 1976 a larger gantry was developed allowing whole-body scans.
Scanning unit (gantry)
converts moving electrons (i.e., electricity) into photons with the energetic properties (the wavelength and amplitude) of X-rays. The X-ray tube is composed of a cathode assembly, an anode assembly, and a rotor, all contained in a tube envelope and together forming a structure called the tube insert. All gas atoms in the space inside the tube envelope have been evacuated, forming a vacuum. Modern CT scanner X-ray tubes usually are provided with 20-60 kiloWatts of electrical power.
X-ray tube
expels the electrons that are delivered to it through a process called thermionic emission. The current from the X-ray generator passing through the filament boils off electrons. The emitted electrons are accelerated by the potential difference (i.e., the difference in charge) between the cathode and anode toward the anode (often also made from tungsten). The higher the voltage applied to this process (often 80 to 140 kiloVolts), the more the source accelerates the electrons.
X-ray tube cathode filament
These occur when an accelerated free- electron strikes the target atom’s nucleus and ejects one of the atom’s inner shell electrons, which escapes the atom as a photon.
Characteristic x-rays
These occur when an accelerated free electron passes through the target atom and has its course deflected by the nearby subatomic particles causing it to lose its
kinetic energy. As a function of the first law of thermodynamics, energy is conserved, and the kinetic energy lost by the free electron is gained by other nearby particles, such as an outer shell electron
in a cathode’s atom. The loosely bound outer shell electron can then also be emitted as a photon that has the energetic properties (i.e., travels with the wavelength and amplitude) of an X-ray.
Bremsstrahlung (braking) X -rays
can be altered according to the desired image resolution. In general, the smaller the __, the higher the image resolution. The process described in X-ray generation converts electric energy into 99% heat and only 1% photons. To absorb this large amount of heat, the space between the tube envelope and the tube housing contains oil for equipment cooling and insulation.
focal size
absorbs and counts photons generated by the X-ray tube that passes through the patient. The detector consists of two layers; the scintillator layer and the photon tide later. The scintillator layer converts absorbed X-ray photons into visual light photons. The photon tide layer converts the light photons into electrical signals.
Photon detectors (photovoltaic cell/detector)
X-rays that do not travel in a straight path from the X-ray tube to the detector that is in line with the beam but instead reach an off- path detector interfere with the machine’s ability to reconstruct an accurate representation of what signal was derived from what original location. This phenomenon, and other types of inaccuracies in image processing, result in image “noise,” which reduces the contrast between imaged structures that is a critical element for maintaining image quality and enabling interpretation of the anatomy and pathology.
Xray shielding elements
moves through the gantry during the scan. The distance the table moves during a complete rotation of the gantry is referred to as the table pitch or detector pitch. Table pitch equals the forward table movement in millimeters (mm) during a complete gantry rotation divided by beam collimation (the slice thickness in mm). Faster moving tables are described as having greater pitches. Increased table speed reduces scanning time and radiation but also can reduce image resolution if the circuitry of the machine cannot process the information as quickly as the table moves.
Px table
similar to table pitch but is a term used for multidetector CT and factors in the beam width, which is equal to the number of detectors multiplied by the
slice thickness in mm. In other words: BP is equal to the forward table movement for each full gantry rotation divided by the beam’s width. Pitch greater than 1 indicates the presence of a gap in the scanned volume, whereas pitch less than 1 indicates overlap of radiated fields in the scanned volume. Setting the pitch to greater than 1 reduces scan time and radiation dose but also reduces image resolution. Reducing the pitch increases the scan time and radiation but also increases the resolution. A low pitch is required to visualize delicate structures or pathological findings (such as arachnoid hemorrhage, small aneurysms, or non-displaced fractures).
Beam pitch
Tissue physical density is proportional to photon attenuation (photon absorption). The CT detectors measure the degree that the scanned tissues attenuate photons (i.e., their density), and the image processor storing the data as bytes converts these values so that displayed pixels have proportionately assigned pixel brightness. A formula for this calculation to convert byte data into a range of 5000 values is used globally.
Houndsfield units
Although the HU values in CT images are expressed on a 5000-unit scale, the human eye can distinguish many fewer grey shades in optimal conditions. The displayed greyscale brightness and contrast can be altered by decreasing or increasing the number of includedbHounsfield Units (termed the “window”) and/or the Hounsfield Unit value that is set as the central/middle value (termed the “level”). The person viewing the image can manually adjust these settings using a mouse that interacts with the image software. Adjustment of what is visible in the image based on adjusting the visible HU in this manner is referred to as “windowing” or “changing the window.”
Image windowing
