The X- ray Tube
Labels:
Radiologic Physics
·
3 External
structures of an x-ray tube
1) Support structure
2) Protective housing
3) Glass or metal envelope
Internal structure of an x-ray tube
-
Made up of
CATHODE and ANODE enclose in a vacuum
Support structures
-
6 ways to
support x-ray tube
1) Ceiling support
2) Floor-to-ceiling support
3) Floor mount system
4) Fluoroscopy
5) C-arm
6) Portable or mobile machines
Ceiling support
-
Most
frequently used system
-
Consists
of two sets of rails mounted to the ceiling directly over the radiographic
table
.jpg)
Photo: medical.toshiba.com
Floor to ceiling support
-
Has a
single column with rollers attached to each end, one on ceiling mounted rail
and the other end on the floor mounted rail
Photo: alibaba.com
Floor mount system
-
Alternative
to the column mount
Fluoroscopy
-
Mounted
underneath the radiographic table and is energize only during fluoroscopy where
the image tower is locked into placed
C-arm
-
X-ray tube
are mounted on a support shaped like C
-
Use as
portable fluoroscopy units or in special procedure suites
.jpg)
Photo: http://kantecare.manufacturer.globalsources.com/
X-ray Imaging System
Labels:
Radiologic Physics
·
Three Principal Parts of an X- ray Imaging System
1. x-ray tube
2. operating console
3. high-voltage generator
OPERATING CONSOLE
The part of the x-ray imaging system most
familiar to the radiologic technologist is the operating console. The
operating console allows the radiologic technologist to control the
x-ray tube current and voltage so that the useful x-ray beam is of
proper quantity and quality.
Radiation quantity refers to the number of
x-rays or the intensity of the x-ray beam. Radiation quantity is usually
expressed in milliroentgens (mR) or milliroentgens/milliampere-second
(mR/mAs). Radiation quality refers to the penetrability of the x-ray
beam and is expressed in kilovolt peak (kVp) or, more precisely,
half-value layer (HVL).
The operating console usually provides for
control of line compensation, kVp, mA, and exposure time. Meters are
provided for monitoring kVp, mA, and exposure time. Some consoles also
provide a meter for mAs. Imaging systems that incorporate automatic
exposure control (AEC) may have separate controls for mAs.
Most operating consoles are based on computer
technology. Controls and meters are digital, and techniques are selected
with a touch screen. Numeric technique selection is sometimes replaced
by icons indicating body part, size, and shape. Many of the features are
automatic, but the radiologic technologist must know their purpose and
proper use.
Most x-ray imaging systems are designed to
operate on 220 V power, although some can operate on 110 V or 440 V.
Unfortunately, electric power companies are not capable of providing 220
V accurately and continuously.
Typical operating console to control an overhead radiographic imaging system. Numbers of meters and controls depend on the complexity of the console. (Courtesy General Electric Medical Systems.)
The line compensator measures the voltage
provided to the x-ray imaging system and adjusts that voltage to
precisely 220 V. Older units required technologists to adjust the supply
voltage while observing a line voltage meter. Today's x-ray imaging
systems have automatic line compensation and hence have no meter.
AUTOTRANSFORMER
The power supplied to the x-ray imaging system is delivered first to the autotransformer. The voltage supplied from the autotransformer to the high-voltage transformer is controlled but variable. It is much safer and easier to control a low voltage and then increase it than to increase a low voltage to the kilovolt level and then control its magnitude.The autotransformer works on the principle of electromagnetic induction but is very different from the conventional transformer. It has only one winding and one core. This single winding has a number of connections along its length.
AUTOTRANSFORMER LAW
- where
- Vp =the primary voltage
- Vs =the secondary voltage
- Np =the number of windings enclosed by primary connections
- Ns =the number of windings enclosed by secondary connections
Adjustment of Kilovolt Peak (kVp)
Some older x-ray operating consoles have adjustment controls labeled major kVp and minor kVp; by selecting a combination of these controls, the radiologic technologist can provide precisely the required kilovolt peak. The minor kilovolt peak adjustment “fine tunes” the selected technique. The major kilovolt peak adjustment and the minor kilovolt peak adjustment represent two separate series of connections on the autotransformer.Control of Milliamperage (mA)
The x-ray tube current, crossing from cathode to anode, is measured in milliamperes (mA). The number of electrons emitted by the filament is determined by the temperature of the filament.Filament circuit for dual-filament x-ray tube.
The filament temperature is in turn controlled
by the filament current, which is measured in amperes (A). As filament
current increases, the filament becomes hotter and more electrons are
released by thermionic emission. Filaments normally operate at currents
of 3 to 6 A.
A correction circuit has to be incorporated to counteract the space charge effect.
As the kVp is raised, the anode becomes more attractive to those
electrons that would not have enough energy to leave the filament area.
These electrons also join the electron stream, which effectively
increases the mA with kVp.
Thermionic emission is the release of electrons from a heated filament.
The product of x-ray tube current (mA) and exposure (s) is mAs, which is also electrostatic charge (C).
Filament Transformer
The full title for this transformer is the
Filament Heating Isolation Step-down Transformer. It steps down the
voltage to approximately 12 V and provides the current to heat the
filament. Because the secondary windings are connected to the high
voltage supply for the x-ray tube, the secondary windings are heavily
insulated from the primary.
Characteristics of Several Types of Ionizing Radiation
Labels:
Radiologic Physics
·
|
APPROXIMATE RANGE |
||||
|---|---|---|---|---|
|
Type of Radiation |
Approximate Energy |
In Air |
In Soft Tissue |
Origin |
|
PARTICULATE
|
||||
|
Alpha particles |
4-7 MeV |
1-10 cm |
Up to 0.1 mm |
Heavy radioactive nuclei |
|
Beta particles |
0-7 MeV |
0-10 m |
0-2 cm |
Radioactive nuclei |
|
ELECTROMAGNETIC
|
||||
|
X-rays |
0-25 MeV |
0-100 m |
0-30 cm |
Electron cloud |
|
Gamma rays |
0-5 MeV |
0-100 m |
0-30 cm |
Radioactive nuclei |
Nuclear Arrangement
Labels:
Radiologic Physics
·
1. IsotoPes – atoms or elements that have the
same # of protons; same element/atomic number, different mass number
Ex. 13056Ba, 13256Ba,
13556Ba
2. IsobArs – atoms or elements that have the
same atomic mass.
Ex. 13153I, 13154Xe,
13155Cs
3. IsotoNes – atoms or elements that have the
same # of neutrons
Ex. 12953I,
13054Xe, 13155Cs
4. IsomErs – atoms or elements that have the
same atomic mass and atomic number but differs in energy levels.
Ex. 9943Tc, 99-m43Tc
Note: Use the mnemonic PANE to easily identify what type of nuclear arrangement is
given.
P- for Isotope; means the same number or protons
(atomic number)
A- for Isobars; means the same atomic mass
number
N- for Isotones; means the same number of
neutrons
E- for
Isomers; means only differ in energy levels (usually represented by superscript
“m”)
Models of Atom
Labels:
Radiologic Physics
·
1. John Dalton – his model showed that the elements could be classified as to integral
values of atomic mass.
Ø
elements
were composed of identical atoms acting the same way in a chemical reaction.
Ø “hook and eye model “to account for chemical
combination.
2. Dmitri Mendeleev – showed that if the elements are arranged in order of increasing
atomic mass, repetition of similar chemical properties occurred.
Ø developed
the first periodic table of elements.
3.
J.J. Thomson – described that electrons were an integral part of all atoms.
Ø His model of the atom has been described as a “ plum pudding “, with
the plums representing the electrons.
4. Ernest Rutherford – introduced the nuclear model, which described the atom as containing
a small dense, positively charge center surrounded by a negative cloud of
electrons. He called the center of the atom the nucleus.
Ø introduced the nuclear model
5. Neils Bohr – his model was a miniature solar system in which the electrons
revolved around the nucleus in prescribed orbits or energy levels.
Ø
Improved
Rutherford’s description of an atom.
Control of Scatter Radiation
Labels:
Image Production
·
Effect of Scatter Radiation on Image Contrast
One of the most important characteristics of image quality is contrast, the visible difference between the light and dark areas of an image. Contrast is the degree of difference in OD between areas of a radiographic image. Contrast resolution is the ability to image and distinguish soft tissues.
Even under the most favorable conditions, most remnant x-rays are scattered.
When primary x-rays interact with the patient, x-rays are scattered from the patient in all directions.
Two types of devices reduce the amount of scatter radiation that reaches the image receptor: beam restrictors and grids.
Beam Restrictors
Basically, three types of beam-restricting devices are used: the aperture diaphragm, cones or cylinders, and the variable-aperture collimator (Figure 14-11).
Aperture Diaphragm
An aperture is the simplest of all beam-restricting devices. It is basically a lead or lead-lined metal diaphragm that is attached to the x-ray tube head. The opening in the diaphragm usually is designed to cover just less than the size of the image receptor used. Figure 14-12 shows how the x-ray tube, the aperture diaphragm, and the image receptor are related.
FIGURE 14-11 Three types of beam-restricting devices.
Dental radiography represents another application of aperture diaphragms. Dental radiographs are customarily obtained at 20 or 40 cm SID. Most dental imaging systems are supplied with rectangular collimation, which requires that the dental radiologic technologist precisely align and position the x-ray tube head, the patient, and the image receptor.
Cones and Cylinders
Radiographic extension cones and cylinders are considered modifications of the aperture diaphragm.In both, an extended metal structure restricts the useful beam to the required size. The position and size of the distal end act as an aperture and determine field size.
Radiographic cones and cylinders produce restricted useful x-ray beams of circular shape.
In contrast to the beam produced by an aperture diaphragm, the useful beam produced by an extension cone or cylinder is usually circular. Both of these beam restrictors are routinely called cones, even though the most commonly used type is actually a cylinder.
One difficulty with using cones is alignment. If the x-ray source, cone, and image receptor are not aligned on the same axis, one side of the radiograph may not be exposed because the edge of the cone may interfere with the x-ray beam. Such interference is called cone cutting.
Variable Aperture Collimator
The light-localizing variable-aperture collimator is the most commonly used beam-restricting device in radiography.
Automatic variable-aperture collimator. (Courtesy Huestis Medical.)
Simplified schematic of a variable-aperture light-localizing collimator.
Scatter Radiation
Labels:
Image Production
·
As scatter radiation increases, the radiograph loses contrast and appears gray and dull. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
Some x-rays interact with the patient and are scattered away from the image receptor (a). Others interact with the patient and are absorbed (b). X-rays that arrive at the image receptor are those transmitted through the patient without interacting (c)and those scattered in the patient (d). X-rays of types c and d are called image-forming x-rays.
Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
kVp
As x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton interaction increases.
Also, fewer x-rays reach the image receptor at low kVp—a phenomenon that is usually compensated for by increasing the mAs. The result is still higher patient dose.
With large patients, kVp must be high to ensure adequate penetration of the portion of the body that is being radiographed. If, for example, the normal technique factors for an AP examination of the abdomen are inadequate, the technologist has the choice of increasing mAs or kVp.
Increasing the mAs usually generates enough x-rays to provide a satisfactory image but may result in an unacceptably high patient dose. On the other hand, a much smaller increase in kVp is usually sufficient to provide enough x-rays, and this can be done at a much lower patient dose. Unfortunately, when kVp is increased, the level of scatter radiation also increases, leading to decreased image contrast.
Collimators and grids are used to reduce the level of scatter radiation.
Field Size
Another factor that affects the level of scatter radiation and is controlled by the radiologic technologist is x-ray beam field size. As field size is increased, scatter radiation also increases.
Collimation of the x-ray beam results in less scatter radiation, reduced dose, and improved contrast resolution.
Compared with a large field size, radiographic exposure factors may have to be increased for the purpose of maintaining the same OD when the exposure is made with a smaller field size. Reduced scatter radiation results in lower radiographic OD, which must be raised by increasing technique.
Patient Thickness
Imaging thick parts of the body results in more scatter radiation than imaging thin parts does. Compare a radiograph of the bony structures in an extremity with a radiograph of the bony structures of the chest or pelvis. Even when the two are taken with the same screen-film combination, the extremity radiograph will be much sharper because of the reduced amount of scatter radiation.
Patient Thickness
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