Basic X-Ray Circuit

•X-ray machine circuits comprise three main components:


1. A circuit for heating the filament.
2. A circuit for applying a large potential difference (high voltage) between cathode and anode to accelerate electrons.
3. A timing device to control the length of exposure.


High-voltage circuit – provides x-ray tube accelerating potential.This circuit produces a large potential difference between cathode and anode to accelerate electrons produced at the filament to high velocities. High tension transformers convert high Amp and low kV to mA and high kV.

Filament circuit – provides filament current.The tungsten filament at the cathode is the source of electrons used to produce x-rays. The number of electrons produced at the tungsten filament is dependent upon the temperature of the filament.A tungsten filament needs to be heated to at least 2200°C to emit useful numbers of electrons. Electrons are produced by thermionic emission. When a metal is heated its atoms absorb energy which allows some electrons to move a small distance from the surface of the metal. 

Voltages can be increased or decreased using “step-up” or “step-down” transformers.Mains electricity is 240 V and has to be modified to produce a high voltage across the x-ray tube head and low voltage to heat the filament. Wire coils wound around iron rings.Coils create a magnetic field within the ring

Current and voltages (V) on opposite sides (primary and secondary) of the ring are proportional to the number of “turns” (N):

VP / VS = NP / NS

Stepwise and continuous voltage adjustments can be made with autotransformers and rheostats

Voltage Rectification :-

Alternating current is used to energize x-ray tubes ,X-ray tubes are designed to operate at a polarity where the anode is positive.

Rectifiers permit current flow in only one direction.X-Ray generators provide the tube current at the required voltage for x-ray production.In a "perfect" case this would be a constant voltage, however transformers require alternating voltages to work so some means of producing a constant voltage across the x-ray tube from the rising and falling voltage produced by the high tension transformer is required.Rectifiers placed in series in HV circuit provide half-wave rectification The ripple causes corresponding but relatively higher variations in the X-ray output. It is an unwanted phenomenon in the X-ray production due to the lengthening of the exposure time and the reduction in the average kV.
Rectifiers can be arranged in a “diamond” arrangement to provide full-wave rectification – conduction during both halves of alternating (AC) voltage.Commercial electric power, the line voltage, is usually produced and delivered as three phase alternating current. The period of each single phase may be 50 or 60 Hz. The period of a 50 Hz AC has a duration of 1/50 s, or 20 ms. The three phase X ray generator transforms and rectifies this AC into a high-voltage direct current (DC) with either six or twelve forward pulses per 20 ms period. As compared to the 100% ripple factor of single-phase generators, three-phase generators dramatically reduces voltage ripple (13–25% for 3-phase 6-pulse, 3–10% for 3-phase 12-pulse).

X-ray Production/Clinical Radiation Generators

Production of X Rays

X-rays are produced in xray tube, the xray tube have several components which are here discussed with some detail.

The X-Ray Tube Components

1. Glass tube – It maintains vacuum necessary to minimize electron interactions outside of the target area in the xray tube. The vacuum condition in xray tube is necessary for the travel of electrons from hot filament to the anode. otherwise if air is present in the xray tube then these electrons can collide with the atoms of air in xray tube, and a very small number of electrons would reach the target. Thus we could not have xrays in the results, thus i can say that xray tube must have vacuum inside to produce xrays.

2. Cathode – It contains filament and focusing cup. Electrons are produced by heating the filament which is cathode terminal of xray tube, when all conditions are fulfilled and electrons are emitted from the hot filament or cathode of the x-ray tune then these electrons can travel in every direction, or in random direction, as result we may have a broad beam of electrons in x-ray tube. This is not desired, thus we have another important component in the x-ray tube near cathode called focusing cup. The focusing cup is some time very close to the cathode and thats why it is treated as the part of the cathode in x-ray tube. But here i want to tell my students that it is an other component from cathode and should be treated separately from cathode of the x-ray tube. The function of focusing cup in x-ray tube is to converge the electron beam towards anode.

3. Anode – It contains x-ray target. The electrons beam strikes the anode of the xray tube. As a result xrays are produced from here.When we apply a voltage to this anode, we place a high positive charge on it. This high positive charge acts much like a magnet, only it is attracting free electrons. The positive charge will possess a strong attractive force to the negative charge of the electrons that are boiling off of the filament. This attractive force pulls the electrons towards the anode at high speeds. By increasing the voltage applied to the anode we can increase the speed of the electrons.By placing some sort of matter between the electrons (filament) and the positive charge (anode) we meet our need. Also, the anode itself can be used as the target. In high voltage X-ray generators a special target material (Tungsten) is usually embedded into the anode. This gives the electrons a suitable material to interact with and produce x-rays. When the electron hits the target material, several things can happen. The electron can be absorbed by an atom and its energy transferred to the atom, the energy of the electron can cause another electron to be knocked out of its energy shell, or the electron may just slightly interact with other atomic particles. Radiation will be produced in all of these cases, but the energy of the radiation will be different.

X-ray Developing

• There’s automatic and Ret(?) technique (manual).
• The sequence of developing film at 70 degrees (most common):

1. Unwrap the film
2. Develop (5 min)
3. Rinse
4. Fix (10 min)
5. Wash (20 min)
6. Dry
Automatic developer time is a total of 7 minutes, manual developing is almost an hour.
 If we don’t wash the film after fixing it sort of looks yellowish. Why do we use high speed film? Reduce patient exposure. There are a lot of films out there Ultraspeed D speed (slow), E speed (not so slow), F speed Insight (fastest). F speed film is the same speed as digital radiography sensors to reduce patient exposure.
X-ray Taking Techniques

• 2 techniques to take x-ray film: Paralleling angle technique and bisecting angle technique.
•  If the films are not parallel they get distorted.
•  Sometimes other things that cause slight distortion is, edge gradient, or the fuzziness of the film, called: penumbra (umbrella in French – actually, no, the word for umbrella in French is parapluie).
•  Free hand technique (bisection of the angle technique).

because of anatomical landmarks you can’t hold the film parallel to the tooth (especially in the maxilla because of the arch). All free hand techniques are inaccurate. We use paralleling technique for good geometric accuracy.

Panoramic X-rays

• Panoramic x-rays are accurate within the focal trough (oriented three dimensional zone of focus) of the particular machine.
• The focal trough is less than an inch, if the patient’s head is elliptical instead of round, or the person has a small head, it will distort the image – must postion the patient properly.
• Panoramic x-rays are a screening tool only.

Radiology and X-ray Physics

X-ray History

• X-ray was developed by Wilhelm Roentgen in 1895 in Wurtzberg Germany.
• First dentist to use X-rays was Dr. Edmund Kells

X-ray Physics

•  In the x-ray tube we have an anode and cathode. To create electrons, heat the anode, boil off the electrons, using a step down transformer.
•  We concentrate the electrons with a molybdenum focusing cup.
•  To move the electrons we create an electron potential difference, using a step up transformer (going from 110V to 65,000+ kilovolts). This increases the KVP. The higher the KVP the more energy the photon is going to receive.
•  To create a photon we move the electrons from the anode to the cathode and 99.8% of them are going to turn into heat and .2% become photons.
•  A photon is a bundle of energy.
•  We call the transformation from electron to photon: Bremsstrahlung (or break energy).
•  Review: So we have the X-ray tube made of glass, in the X-ray tube we have an anode and cathode and molybdenum focusing cup, and a vacuum. In the vacuum we create the electron potential difference, and the photons come out through the window.
•  All of the photons coming out of the window are parallel with each other.
•  The x-rays are long and short wavelength energy (can use the short because it has more penetrating power).
•  We get rid of the long wavelength with filtration: either aluminum (added filtration) or the glass tube itself (inherent filtration).
•  Inside the X-ray tube the coil is made of tungsten because of its high melting point (1500 C).
•  X-rays come out of the tube and then travel in a straight line until it has an interaction with whatever 32 times.
•  Electromagnetic ray length is measured in angstroms (smallest measurable denominator of the meter).
•  Every tissue in our body has a different Z number (atomic number). In order to have a good picture you must follow the following rules:

1. Source of the X-ray needs to be as small as possible (comes out from a dot)
2. Source to object distance is as long as practical
3. Object to film distance is as short as practical
4. Object and film must be parallel with each other
5. Central beam and object and film should be producing a 90 degree angle in all (any) directions.
X-ray film

• X-ray film is housed in an emulsion which contains silver halide/bromide/iodide/chloride etc.
• The silver bromide crystal has the capability of having an area of exit being exposed, called a sensitivity spec. It creates from the central crystal a latent crystal which could be developed later on.
• We have lead inside the x-ray film so we don’t get scattered radiation.
• We use high speed film to reduce patient exposure. 
•  Ultraspeed D speed (slow), E speed (not so slow), F speed Insight (fastest). F speed film is the same speed as digital radiography sensors to reduce patient exposure.
  

Focusing Cup of Xrays Tube

Focusing Cup in X-rays Tube OR Focus Cup in X-ray Tube head  Or Wehnelt electrode:-

It is the device surrounding the cathode filament in an X-ray tube. This is actually a third electrode in the tube, called a Wehnelt electrode, and is used to hold together the stream of electrons which are accelerated towards the anode. If this electrode were not present, the electrons would hit the anode over a very large area. The focusing is achieved by keeping the focusing cup at the same, or sometimes slightly higher, negative potential as the cathode filament. Electron focusing means comprising a plurality of conductive members insulatingly supported in fixed positional relationship with one another adjacent the filament and having longitudinal and transverse portions with respect to the filament for defining in the path of the electron beam an elongated aperture having a length less than the length of the filament, one of the conductive members being a cathode focussing cup having an elongated opening wherein the filament is insulatingly disposed in the cup to direct the electron beam through the opening and other conductive members of the focussing means comprising a pair of spaced conductive strips insulatingly supported within the cup adjacent respective end portions of the filament;

A focusing cup is used to concentrate the stream of electrons to a small area of the target called the focal spot. The focal spot size is an important factor in the system's ability to produce a sharp image. Much of the energy applied to the tube is transformed into heat at the focal spot of the anode. As mentioned above, the anode target is commonly made from tungsten, which has a high melting point in addition to a high atomic number. However, cooling of the anode by active or passive means is necessary. Water or oil recirculating systems are often used to cool tubes.
The purpose of the focusing cup is to focus the x-ray beam so that the x-ray beam is focused on a smaller area of the target.  The focusing cup is negatively charged because the charge of an electron is also negative, this helps keep the electrons focused inside the focusing cup.
Another important consideration is the focal spot size of the tube since this factors into the geometric unsharpness of the image produced. Generally, the smaller the spot size the better. But as the electron stream is focused to a smaller area, the power of the tube must be reduced to prevent overheating at the tube anode.
Bremsstrahlung radiation results when an electron passes near the nucleus of an atom. The close passage of the electron to the nucleus causes the electron to change its course thus losing much of its energy in the process. In the world of quantum particles, energy is always exchanged in discreet particles of light known as photons. The loss of energy by the electron as it is deflected by the heavy nuclei in the anode target produces a very high energy photon of light called an x-ray. The dental x-ray tube produces Bremsstrahlung radiation.

This is an animated illustration vedio which describes the role of the focusing cup in an X-Ray tube. The animation shows the trajectories of the electrons emitted by the filament both without and with the focusing cup. The focusing cup is held at a negative potential relative to the filament, so that area of impact of the electrons on the anode is a smaller area, creating a smaller spot size on anode. The smaller spot size creates a more point like source for the emitted electrons which in turn provides better quality imaging in diagnostic X-Rays.

Portable Xray Machines

MinXray High Frequency Portable Medical X-ray Machine :-

MinXray is a high frequency portable medical x-ray unit.  MinXray offer the highest power-to-weight ratios available from any manufacturer. These models of  portable movable xrays machines are designed for use in nursing homes, private homes, correctional facilities, military field clinics, hospitals, or anywhere an x-ray machine must be brought to a patient. MinXray's high frequency x-ray units and their companion stands provide the most easily transportable, user-friendly systems available for general purpose offsite medical diagnostic radiography.

The MinXray portable high frequency x-ray machines are ready to do x-rays of any patients at any location either his/ her home or in the hospital, on the bed or in the xray room. Where ever the patient is, its xrays can be brought to investigate about his/ her disease.

As we know that due to change in technology, use of high frequency in the voltages, now the high voltages can be produced by using light weight transformers. In past the high voltages were produced in very bilky transformers due to the use of such bulky transformers the x-rays machines were also bulky.

But now in this era the use of such bulky transformer for xrays machine is meanless. Now people are more interested in portable compact and easy to move xray machine. For such a compact and durable portable xrays machines the high frequency transformer are best way to use. In future it is expected that some new technology will come and size and weight may also become more reduced.

Rugged, dependable MinXray equipment is capable of all routine radiographic views you would expect from a portable unit. Detailed images of chest, abdomen, skull, spine and extremities are easily obtained with short exposure times on ambulatory and non-ambulatory patients.
source: http://www.minxray.com/Medical.html
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