1.7 Generating X-rays Cheat Sheet (DRAFT) by Molly

Skin cancers, where the tumour is located in the superf­icial layers of the skin, require a beam that deposits dose in the first few millim­etres of soft tissue and less of a dose to deeper organs.

Adjusting the temper­ature of the filament, or cathode, by changing the current flowing in it alters the rate of electrons ‘boiled off’ the filament.

The quantity of x-rays, the dose-rate, is directly propor­tional to the tube current, provided the potential difference across the anode to the cathode is constant. That is, the energy of the electrons as they hit the target is the same.

If we increase the potential difference between the filament and the target, cathode and anode, but keep the filament at the same temper­ature, we have the same number of electrons per time interval but they have more energy when they hit the target.

The simulator has the manoeu­vra­bility and accuracy of a linear accele­rator and can provide radiation beams that are identical in size and position to those used in all treatment plans. Unlike a linear accele­rator, the simulator contains a conven­tional x-ray tube and an image intens­ifier. These combine to provide real time imaging and high quality radiog­raphs with the patient positioned for the proposed treatment plan. The simulator will always be able to provide improved image quality compared to images obtained with a linear accele­rator.

Conseq­uently, the use of the simulator has greatly improved the processes of tumour locali­sation, verifi­cation and reprod­uci­bility of treatment set-up. Radiation treatment planning also makes consid­erable use of the images obtained using computed tomography (CT) imaging.

Computed Tomography (CT) scanners and Magnetic Resonance Imaging (MRI) scanners have excellent contrast resolu­tion.

Resolution is measured by the ability to see pairs of lines and is expressed as line pairs per millimetre (lp/mm). Conven­tional radiog­raphy has excellent spatial resolu­tion.

A radiograph is of high quality if it faithfully reproduces structures and tissues. Radiog­raphic quality depends on a number of complex factors and is not easy to define or measure precisely.

Additi­onally, the magnitude of this magnif­ication varies through the thickness of the patient being a maximum at the beam entrance surface and a minimum at the exit surface.

Although you may appreciate that the resulting image obtained with conven­tional radiog­raphy is magnified when compared to the original object, it may not be so clear that the magnitude of this magnif­ication varies through the thickness of the patient (depth within the patient), being a maximum at the beam entrance surface and a minimum at the exit surface.

In the human body, four distinct types of tissues are present each providing a different degree of attenu­ation to the x-ray beam:

In x-radi­ogr­aphy, these differ­ences in attenu­ation are utilised to provide shadows of varying density on the radiog­raphic image.

As well as utilising inform­ation obtained from x-ray equipment readily available in the radiology depart­ment, radiation therapy techno­logists operate specially designed equipment that can image the patient in set-ups that simulate the proposed treatment position.

Additi­onally, many treatment procedures are checked directly on the treatment machines in processes known as electronic portal imaging or on board imaging.

When x-rays pass through the patient, they are attenuated by the processes of absorption and scatte­ring.

his is dependent on the atomic number and the density of the tissue. However, Compton scattered radiation is also present and this degrades the quality of the image by randomly irradi­ating the whole area, increasing the fog level and reducing the contrast.

Focused grids are often used in which the strips of lead are angled from the centre to the outside border to accomm­odate the divergence of the x-ray beam.

However, the passage of scattered radiation is substa­ntially limited although not completely elimin­ated.

As well as utilising inform­ation obtained from x-ray equipment readily available in the radiology depart­ment, radiation therapy techno­logists operate specially designed equipment that can image the patient in set-ups that simulate the proposed treatment position.

Additi­onally, many treatment procedures are checked directly on the treatment machines in processes known as electronic portal imaging or on board imaging.

When x-rays pass through the patient, they are attenuated by the processes of absorption and scatte­ring.

his is dependent on the atomic number and the density of the tissue. However, Compton scattered radiation is also present and this degrades the quality of the image by randomly irradi­ating the whole area, increasing the fog level and reducing the contrast.

Focused grids are often used in which the strips of lead are angled from the centre to the outside border to accomm­odate the divergence of the x-ray beam.

However, the passage of scattered radiation is substa­ntially limited although not completely elimin­ated.

Superf­icial radiot­herapy utilises x-ray energies of between 50 and 200 keV, having a treatment range of up to 5mm, and Orthov­oltage radiot­herapy utilises 200 to 500 keV x-rays penetr­ating to a useful depth of 4 – 6cm.

Orthov­oltage and superf­icial treatment machines are becoming less common, with much of the treatment that was previously delivered with them now being delivered using linear accele­rators.

Extra shielding around the target anode to absorb the higher energy scattered photons.

Jaws may be used to alter the beam shape and size as it emerges from the tube.

The radiation from orthov­oltage units is referred to as x-rays, generated by bombarding a metallic target (tungsten) with high-e­nergy electrons. This is a relatively low energy radiation source; typically operated at 250 kV.

Additi­onally, there is differ­ential absorption of dose in bone versus soft tissue and there is some risk of bone damage or necrosis.

Orthov­oltage units are operated at a relatively short source­-to­-skin distance (usually 50 cm) limiting the size of the treatment field; the field size is defined by the use of different sized/­shaped attach­ments or cones (recta­ngular, circular, slanted). Orthov­oltage units are relatively inexpe­nsive machines, relatively easy to repair and maintain, and less shielding and space is required for operation.

There is a physical limit to how far one can increase the accele­rating potential using a transf­ormer before technical diffic­ulties start to arise.

The practical limit for x-ray tubes with length of the order of a metre is around 400-500 kV. The altern­ative method of increasing the quality of the beam is by filtra­tion.

At the same time the overall dose-rate decreases, because some of the beam has been absorbed, but its quality, as expressed in its half-value layer, increases.

To control the energy spectrum produced by a kilovo­ltage machine, filters are placed in the path of the beam. These filters select­ively attenuate the desired part of the beam spectrum; this usually hardens the beam (removes low energy photons).

1. Filtration hardens the beam, attenu­ating low energy photons and shifting the spectrum towards higher energy photons. The low energy photons are not needed, as they would simply excess dose to the most superf­icial parts of the skin.

Hence, the dose reduction rationale for filtra­tion: only higher­-energy x-rays remain in the beam, these are more penetr­ating and less likely to be absorbed by tissues …. dose reduction!

The upper limit in beam quality for a conven­tional x-ray tube is about 4 mm Cu HVL. This can be achieved with a reasonable dose rate using ~ 300 kV and a combined aluminium, tin and copper filter.

Clearly, this is neither very deep nor very efficient and to treat deep tumours more energetic beams are required.

This is the fundam­ental principle used for the generation of x-rays, whether for diagnostic or therapy purposes. To generate x-rays you need three things:

The tube cathode (filament, eg. tungsten) is heated with a low-vo­ltage current of a few amps.

A large electrical potential is created between the cathode and the anode by the high-v­oltage generator.

The stream of electrons between the cathode and the anode is the tube current.

With most x-ray tubes used in diagnostic radiology, electrons are accele­rated towards a tungsten anode (target) by applying a large accele­rating voltage between the anode and the cathode. After accele­ration at impact with the target, the electron will have an amount of energy that is directly propor­tional to the instan­taneous applied voltage. However, very few electrons acquire a kinetic energy numeri­cally equivalent to the kVp applied to the tube.

The minimum energy is determined by self-a­bso­rption in the target and tube.

While it is possible to quantify a beam by its spectrum, a more useful way is by its HVL.

A variable rate of dose fall off, depending on beam energy (sharper for lower energy beams)

High atomic number inhomo­geneity causes markedly increased attenu­ation

The first thing to note is that the beam contains a complete spectrum of x-radi­ation with energy ranging from very low to the maximum possible, that is the full energy of the electron that produced it.

The decele­ration of the electron is a result of its intera­ction, or collision, with the atoms of the target. There is an enormous range of intera­ctions - from a near miss to a head-on collision - and the amount of energy transf­erred from the kinetic energy of the electron to the resultant photon ranges from almost none to all.

All the photons that are absorbed in the target contribute nothing to the x-ray beam, they merely heat it up. This represents the vast majority of the photons produced. The x-ray production process is only about 1% efficient; the remaining 99% of the energy of the electrons is converted to heat or infrared radiation.

The majority of the x-rays have an energy between these extremes.

The lower energy x-rays are prefer­ent­ially absorbed by the x-ray tube and added filters so that no x-rays are seen below about 10 keV. The most probable x-ray energies are typically about one-third to one-half of the maximum energy.

The x-rays that emerge through the exit window of the insert and housing ultimately pass through the patient to form the radiog­raphic image.

The HVL of a beam is a measure that indicates how penetr­ating a beam is. Common materials used for expressing the HVL are tissue, aluminium, copper and lead.

The transf­ormer consists of a primary coil of wire and one or more secondary windings. Altern­ating current is applied to the primary and this induces a changing magnetic field. The changing magnetic field in turn induces an electric field in the secondary winding/s.

To improve the efficiency of the transf­ormer it is usually wound on a ferro-­mag­netic core comprised of thin metal sheets laminated together. Transf­ormers are used both to step-up and step-down the input voltage. A step-down transf­ormer is used to supply the low voltage to heat the filament and a step-up transf­ormer to supply the large voltage to accelerate the electrons.

As you might guess from the names, one anode stays still (stati­onary) while the other spins around a fixed point (rotat­ing).

This enables rotating anode users to perform longer scans and at higher doses. A rotating anode tube lasts a lot longer than a stationary x-ray tube.

Two classes of electron intera­ctions with a target atom:

Incident electron intera­ction with orbital electron of a target atom results mainly in collision loss and ionisation of the target atom that may be accomp­anied by an energetic electron referred to as a delta ray.

Incident electron intera­ction with the nucleus of a target atom results mainly in elastic scattering events but may also result in radiative loss accomp­anied with bremss­tra­hlung produc­tion.

1.In the diagnostic energy range (50 kVp to 120 kVp), the photons are emitted approx­imately equally in all direct­ions. The θmax is 90°, and so the x-ray tube is constr­ucted with what is called a reflective target so that the useful beam is at 90°to the direction of the electrons.

Electrons are light negatively charged particles and interact readily with any atom that they encounter, due to the positive charge of the nucleus.

For this reason materials with high atomic number are chosen for the target of the high-e­nergy electrons. As the atomic number of the target material increases, the efficiency of the continuous spectrum x-rays increase.

Unfort­unately the production of x-rays is not very efficient with most of the energy of the electrons being converted to heat (infrared radiat­ion).

Suitable materials for the target include metals like tungsten, gold, lead, etc. either in the pure form or as alloys. Tungsten is used for general radiog­raphy, although some specialty tubes use gold. Molybdenum is used for mammog­raphy as it has a lower atomic number so the discrete spectrum is of a lower energy. This is ideal for soft tissue studies such as mammog­raphy.

The first and simplest is to use a large electric field to supply the force needed - in a process analogous to a mass falling under the influence of a gravit­ational field.

Using the first method, it is possible to give electrons energy up to about 500 keV. With the second method the energy attainable can be much higher, with energies of 25 MeV and more achiev­able.

At room temper­ature the electrons in an atom occupy orbitals around the nucleus and these are usually the lowest energy, most stable orbits closest to the nucleus with the strongest binding energy.

If the temper­ature of the atom is suffic­iently high it is possible for the electron to break free of the nucleus altoge­ther.

The temper­ature of the filament governs how many electrons are produced and this is controlled by the magnitude of the electric current.

The higher the temper­ature of the filament, the larger the number of electrons that leave the cathode and travel to the anode, the greater the intensity of the X-ray output.

The higher the kilovo­ltage, the more speed and, therefore, energy the electrons have when they strike the anode.

The high-v­oltage potential is measured in kilovolts, an increase in the kilovo­ltage will also result in an increase in the intensity of the radiation.