AchillesGames

Fundamental Physics of Digital Radiography/The Supply

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The X-ray tube is, virtually solely, the supply of radiation beams utilized in Diagnostic Radiography. Elementary options of this machine and its utility are thought of on this chapter. Electrical mills which give energy to the tube and ancillary gadgets which may management the radiation beams are additionally described.

X-Ray Tube[edit]

X-rays might be generated by devices such because the electron synchrotron and linear accelerator however in Diagnostic Radiography are almost all the time produced by a small electron accelerator referred to as an X-ray tube (XRT). We have now described the fundamentals of its operation within the earlier chapter and can get into way more element right here.

Fig. 2.1: A simplified X-ray circuit.

An XRT in its easiest type consists of an anode and cathode mounted inside an evacuated glass tube – see Determine 2.1. The cathode typically consists of a small coil of wire, referred to as a filament, mounted in a focusing cup. The anode goal is usually constituted of tungsten separated from the filament by a small hole, though molybdenum and rhodium targets are utilized in Mammography. Electrons are boiled off the filament by making use of an electrical present in order that it turns into white scorching – the method is known as Thermionic Emission. A excessive voltage (HV) of as much as 100 kV or extra is then utilized and the electrons are attracted throughout the hole to collide with the anode at excessive energies, to supply X-rays. Bremsstrahlung and Attribute Radiation processes are concerned right here, as beforehand described. The focusing cup can be utilized to type the electrons right into a slender beam and therefore solely strike a small spot on the anode goal. X-rays are produced in all instructions from this focal spot, however beam restriction gadgets (e.g. collimators; not proven within the determine) are used to permit solely a major beam to flee the supply and irradiate the affected person.


Many of the electron vitality deposited within the anode is transformed to warmth with lower than 1% really producing X-rays. The focal spot due to this fact will get fairly scorching and it’s right here that the second vital attribute of tungsten comes into play. It has a excessive melting level (over 3,000oC) and due to this fact doesn’t soften below regular working situations.

Additional factors to notice are firstly that the glass tube comprises a vacuum in order that electrons won’t be absorbed and deflected by air molecules as they go between cathode and anode. Secondly, the filament provide present is sort of massive (e.g. 5 A), however the present generated within the electron beam (the so-called X-ray tube present) is way decrease (e.g. 1 mA in fluoroscopy to over 1,000 mA in angiography), relying on the effectivity of the thermionic emission. This present is without doubt one of the elements used to regulate X-ray publicity and its worth is usually displayed on an ammeter (labelled mA in Determine 2.1). Thirdly, do not forget that ‘electron stream’ is the reverse of the route of electrical present (Michael Faraday received it incorrect!), as indicated within the determine.


Trendy diagnostic XRTs are extra advanced that the easy association illustrated in Determine 2.1, primarily as a result of superb focal spots are required to supply good spatial decision in pictures (as mentioned within the subsequent chapter) and efficient warmth dissipation from the anode goal is a serious challenge. Two design options have been included into XRT design to handle these conflicting necessities.

Fig. 2.2: Illustration of the Line Focus Precept. See textual content for particulars.

The primary one relies on the Line Focus Precept and is illustrated in Determine 2.2. Right here we see an exploded view of the anode floor being struck by an electron beam of peak, ab. From beneath, the dimensions of ab seems to be shortened to that of cd relying on the sine of the angle, θ, of the anode. The identical consideration might be utilized to the width of the electron beam, in order that the warmth it generates can because of this be dispersed over a broader area of the anode and a superb X-ray focal spot can nonetheless be obtained. The dimensions of the electron beam focal spot is due to this fact diminished by angling the anode in order that an apparently smaller X-ray supply is obtained. That is referred to as the efficient measurement of the focal spot.


Anode angles are sometimes 6-15o in fashionable XRTs, with the small angled tubes getting used for angiography, as an example, the place superb element imaging is required.

Fig. 2.3: Finish-on and side-views of a rotating anode.

The second design characteristic to be included is an anode rotated by an electrical motor. This spins a disc of tungsten throughout the publicity in order that the electron beam strikes an annular area of its floor reasonably than being concentrated right into a small space, as within the stationary anode design we’ve thought of thus far. The state of affairs is illustrated in Determine 2.Three on the precise the place an end-on and side-on view of an anode disk is proven. Its diameter, R, might be as much as 120 mm and it’s typically spun at speeds of about 3,000 rpm throughout the publicity. A small efficient focal spot can thus be obtained with the resultant warmth being unfold over of a broad space of anode materials. The primary benefit of the rotating anode XRT from a sensible viewpoint is that quick, intense exposures might be produced by a superb focal spot, with out anode heating damaging the tube.

We are actually ready to contemplate the design of a rotating anode XRT – see Figures 2.Four and a pair of.5. The anode is usually constituted of a tungsten-rhenium alloy – the addition of a small quantity of rhenium (e.g. 10%) having been discovered to scale back roughening of the anode floor throughout repeated use of the XRT. Many of those tubes can incorporate a second filament (not proven) in order that each superb (e.g. 0.6 mm) and broad (e.g. 1.2 mm) focal spots might be obtained with the one XRT. A remaining level to notice is {that a} skinny exit window is used to scale back X-ray absorption and optimize the depth of the X-ray beam used for the publicity of sufferers.

Fig. 2.4: A rotating anode X-ray ray tube.

Fig. 2.5: A photograph of a rotating anode X-ray tube.


The method of taking an X-ray in diagnostic radiography due to this fact typically includes powering the motor till the optimum anode rotation velocity is achieved, adopted by the applying of the HV to generate the X-ray beam. Easy, two stage switches are typically used for this goal.

Fig. 2.6: Illustration of the origin of the Heel Impact – see textual content for particulars.

Two factors must be famous earlier than we end. First, the angle of the anode presents a restrict to the dimensions of the sector that may be lined. The smaller angles generate much less protection and the source-to-image distance (SID) due to this fact must be elevated to compensate. The second level is that the angling of the anode causes variations in X-ray absorption to happen inside the anode materials itself in order that (on the subject of the Determine 2.6) X-rays rising straight downwards, IA, may have an depth decrease than these exiting by a shorter route, IB. This causes a gradient within the X-ray intensities within the beam with decrease intensities on the anode aspect – and is known as the Heel Impact. The variation in depth alongside the route perpendicular to the anode-cathode axis is minimal as compared. This could current challenges when utilizing a small angled anode and might be compensated for, as soon as once more, through the use of a big SID.

Fig. 2.7: Lead housing for an X-ray tube

The XRT is housed in a lead container (see Determine 2.7) in order to soak up the X-rays that are emitted by the anode in instructions aside from that of the affected person beam. A few of these may very well stimulate secondary X-ray emissions from tube parts aside from the focal spot (referred to as Further-Focal Radiation) which the lead shielding can also be designed to soak up. Radiation can nonetheless emerge from the tube housing which will result in pointless irradiation of sufferers and employees. This leakage radiation must be stored to acceptable ranges. The housing can also be crammed with oil which supplies electrical insulation and assists in eradicating warmth from the tube. It due to this fact sometimes comprises a bellows or comparable element which may present for the ensuing enlargement of the oil. As well as, the housing typically comprises an exterior marking which signifies the placement of the focal spot.


Parameters which specify the vary of utility for an XRT embrace:

  • The Most Publicity Time for a single publicity given a selected kV and mA.
  • The Warmth Capability, i.e. the utmost quantity of warmth vitality that may be saved within the anode.
  • Heating Curves which specify the size of time the anode might be heated at a spread of kV and mA settings earlier than the utmost warmth storage capability is reached.
  • Cooling Curves, which specify the time it takes an XRT to chill following use.

The utmost publicity time will depend on elements such because the velocity of the anode rotation, the dimensions of the focal spot and the kind of voltage waveform developed by the HV generator. Such data is usually supplied within the type of Tube Loading Curves for every explicit XRT. Curves are typically plots of the utmost publicity time for a given mA at a selected kV. An instance is proven in Determine 2.7.5 the place it may be seen that at 125 kV the utmost present that can be utilized for 0.1 s is roughly 330 mA whereas at 80 kV it’s 500 mA.

Fig. 2.7.5: Tube loading curves for an XRT operated at 8,500 rpm utilizing a 0.6 mm focal spot.

The quantity of vitality, E, deposited within the anode is usually expressed as:

E = kV x mA x publicity time.

When the publicity time is measured in seconds, the vitality deposited is expressed in joules. Anode warmth capacities typically vary from 200 kJ for mammography XRTs, via to 250 kJ for common radiography tubes, to 750 kJ for angiography tubes. Lastly, the Energy Ranking of an XRT is outlined as the utmost energy in kilowatts that may be utilized to the tube for 0.1s, i.e. the product of kV and mA divided by 1,000. Typical values are typically between 40 and 80 kW.

Current developments within the design of X-ray tubes are reviewed in Behling (2016)[1].

X-Ray Power Spectrum[edit]

The type of the X-ray vitality spectrum generated by an XRT is proven in Determine 2.8. It may be seen that it differs from spectra generated on the anode itself (i.e. the inverse linear dependence we noticed within the earlier chapter) as a result of it’s modified by the absorption traits of the glass wall, the oil, the housing exit port (e.g. bakelite) and any filters added on the XRT output. This vitality filtration ends in the preferential removing of low vitality X-ray photons, which might not usually be transmitted via the affected person to type a picture and therefore contribute an pointless radiation dose – and will likely be thought of in additional element later on this chapter.

Fig. 2.8: The X-ray vitality spectrum generated by an X-ray tube.

The spectrum for the 100 kV beam is seen within the determine to encompass a broad Bremsstrahlung spectrum with superimposed Ok-characteristic strains, as beforehand, however this time with low vitality X-rays eliminated by the filtration. The beam is claimed to have been hardened by the filters, i.e. the common beam vitality is elevated. The 60 kV spectrum is seen compared to be of a lot decrease depth and to have inadequate vitality to generate any Ok-Attribute Radiation within the tungsten atoms.

The variety of X-ray photons generated on the anode will depend on the X-ray tube present, i.e. the mA. The impact of mA on the vitality spectrum is to extend the depth in any respect X-ray energies, and to in any other case depart the spectrum unchanged. That is illustrated in Determine 2.9 for the case of a 60 kV beam. Word that it’s the product of tube present (mA) and the publicity time (s), i.e. the mAs, expresses the X-ray depth generated in recording a radiograph.

Fig. 2.9: The impact of mA at a continuing kV on the X-ray vitality spectrum.

The variety of X-ray photons generated on the anode can also be strongly depending on the utilized voltage (i.e. the kV), with an roughly sq. relationship. The effectivity of an XRT for X-ray manufacturing is due to this fact considerably larger on the larger kilovoltages. That is illustrated in Determine 2.9.5, the place the variety of photons produced at 60 kV is seen to be about 2.5 instances the quantity produced at 40 kV, whereas the quantity produced at 100 kV is sort of 7 instances that quantity – and the quantity at 140 kV virtually 12 instances. You will need to admire that each the mA and the kV have an effect on the variety of X-ray photons in a beam.

Fig. 2.9.5: Dependence of the output of an X-ray tube on utilized voltage.

Word that since a broad X-ray vitality spectrum is generated by the XRT, the emitted X-rays are known as polychromatic or polyenergetic, in distinction to monochromatic (or monoenergetic) emission, which refers to X-rays having a single vitality.

Word additionally that, since most radiation detectors are far too delicate for measuring X-ray vitality spectra immediately, computer-based simulators, e.g. SpekCalc[2], can be utilized as an appropriate various.

HV Generator[edit]

That is {an electrical} machine for producing the excessive voltage (HV) essential to energy the XRT. Mains electrical energy is usually equipped to a hospital/clinic within the type of an alternating present (AC) in single-phase or three phases. A single-phase HV generator can take the facility equipped by the AC mains, rectify it and remodel it to tens of 1000’s of volts previous to utility to the XRT. The rectification course of is used, both in half-wave or full-wave type, to take away any unfavourable going voltages which might in any other case excite the anode to behave as an electron supply (see the Determine 2.10).

Fig. 2.10: Half-wave and full-wave rectification of a sinusoidal enter voltage

Word from the determine that half-wave rectification causes a voltage pulse to be utilized to the XRT each mains cycle with a length equal to half a mains cycle (e.g. 10 ms in Europe and Australia). The voltage pulse can also be seen to rise from zero, attain a most and head to zero once more. It’s obvious that the utmost vitality of the X-rays produced will due to this fact observe an analogous sample. This most voltage is known as the height kilo-voltage (kVp) because of this.

It’s also seen in Determine 2.10 that full-wave rectification will present two voltage pulses each mains interval, however that the fluctuation will nonetheless stay. Word nonetheless that the size of the publicity (i.e. the publicity time) can ideally be halved to realize an equal X-ray publicity as a half-wave rectified system.

You will need to admire that capacitive results within the HV cables can have a smoothing impact on these voltage fluctuations, in order that the state of affairs will not be fairly so simple as that described right here.

These massive fluctuations in voltage might be diminished to ripples utilizing extra subtle mills. For instance, mills powered by three-phase electrical energy can produce 6 and 12 pulses per mains cycle, with voltage ripples of 13.5% and three.5% respectively. Ripple might be diminished even additional, to 0% ideally, within the highly-specialized fixed potential generator.

Inverter HV mills have extra lately been developed. These encompass circuitry which invert the incoming AC to DC earlier than chopping it right into a medium frequency (e.g. 2 kHz) or excessive frequency (e.g. 200 kHz) after which remodeling this to a kilo-voltage. Such mills can produce a voltage ripple of about 5%.

Since these fashionable mills produce a just about fixed excessive voltage, the X-ray spectrum stays fixed throughout the publicity, in contrast to single-phase mills. The time period kVp has due to this fact been changed by the time period kV.

Fig. 2.11: The console of an X-ray generator

Cell radiography techniques might be powered by such inverter circuits, or, extra historically, by Capacitor Discharge mills. These older techniques work by charging a capacitor to the specified kV and discharging it via the XRT. An exponentially (ideally) lowering voltage is due to this fact utilized to the XRT throughout these exposures.

The generator console normally supplies protocols for use for radiography totally different physique elements – see Determine 2.11. The publicity management is usually situated right here and the kilo-voltage (kV), tube current-exposure time product (mAs) and publicity time (ms) for the publicity are sometimes displayed.

Beam Filtration[edit]

Filtration is the method of preferentially eradicating low vitality X-ray photons from a polychromatic radiation beam. Its common goal is radiation safety as a result of these low vitality photons are more likely to be fully absorbed within the affected person and never contribute to picture formation. X-rays in any respect energies are literally attenuated by the filtration course of with the decrease energies being suppressed to a larger extent. The method due to this fact hardens the radiation beam, i.e. the X-ray spectrum is formed in order that its imply vitality will increase – see Determine 2.12.

Fig. 2.12: The X-ray vitality spectrum filtered by aluminium and lanthanum.

Skinny sheets of aluminium and copper have historically been used for such filtration. Uncommon earth filters have additionally been used. Within the case of lanthanum, which has a Ok-edge at 39 keV, there’s a marked improve in attenuation at this vitality, and the resultant spectrum is formed as illustrated within the determine.

These metallic filters are typically known as Added Filtration. They’re to be contrasted with the Inherent Filtration of the XRT and housing, described earlier on this chapter, supplied by the glass of the tube wall, the oil and the bakelite exit port. These parts can generate a filtration equal to about 0.Eight mm Al, sometimes.

The Whole Filtration of an X-ray beam is due to this fact the sum of the inherent and added filtration. The appliance of added filters requires an improve in publicity elements to compensate for his or her X-ray absorption. Generally, a complete filtration equal to at the least 2.5 mm Al is used for exposures larger than 70 kVp, with thinner thicknesses getting used for decrease vitality exposures.

Gentle Beam Diaphragm[edit]

A lightweight beam diaphragm (LBD) is usually connected on the output port of the XRT to permit the beam dimensions to be adjusted utilizing collimators. Usually, these encompass two units of horizontal lead plates that may be adjusted so {that a} rectangular radiation beam might be fashioned – see Determine 2.13.

Fig. 2.13: A lightweight globe inside a lightweight beam diaphragm can be utilized to light up the world to be irradiated.

The machine additionally typically comprises an off-axis mild globe with a small mirror mounted alongside the X-ray beam axis to replicate the sunshine in order that it mimics the size of the X-ray beam. The sunshine beam is used to set-up the sector to be uncovered on the affected person previous to X-ray publicity and usually has crosshairs superimposed.

A slot within the LBD might be famous within the photograph. That is used for the location for added filtration, as beforehand mentioned.

Place controls are typically situated on the LBD and permit the XRT to be moved vertically and horizontally in addition to to be angled for indirect exposures. The controls for adjusting the collimators are additionally typically situated on the LBD – see Determine 2.14.

Fig. 2.14: LBD controls for tube positioning and beam sizing.

The LBD can also be a perfect location for a dose-area product (DAP) meter to be mounted to observe radiation exposures to sufferers.

Automated Publicity Management[edit]

Fig. 2.15: Positions of ionization chambers of AEC detectors on a chest stand.

Automated publicity management (AEC), additionally referred to as Picture-Timing, is extensively utilized in Diagnostic Radiography to terminate X-ray exposures when a reference stage is reached. This reference is usually pre-defined by the publicity required on the picture receptor to supply sufficient radiographs. Radiation detector(s) are positioned on the picture receptor to measure the publicity and feed a sign again to the HV generator when termination is required.

The detectors might be primarily based on ionization chambers which might be mounted on the anterior aspect of the picture receptor due to their excessive X-ray transparency. They will also be primarily based on solid-state detectors, however on this case are mounted posteriorly due to their opacity to X-rays. As well as, plenty of detectors can be utilized for various areas of the irradiated discipline. For example, in chest radiography, three AEC detectors are sometimes used – one for the area of the mediastinum (indicated by the shaded rectangular area in Determine 2.15) and one for areas in every lung (indicated by the shaded circles). Publicity termination might be decided on this foundation in PA chest radiography utilizing the common publicity decided from the 2 lung detectors, and utilizing the mediastinal detector for imaging the thoracic backbone. Different detector configurations are additionally potential, relying on the scientific utility.

The method of taking a radiograph when utilizing an AEC due to this fact contains the next:

  • choosing the suitable kV on the console;
  • powering the XRT motor;
  • making use of the publicity till computerized termination of the publicity.

Word that releasing the publicity swap too early can terminate the publicity prematurely on some techniques, though that is not often a difficulty given the very quick publicity instances utilized in most scientific exposures.

The mAs used to acquire the AEC reference stage is displayed on the console following the publicity – see Fig. 2.11 above. The circuitry when AEC is used with a Excessive Frequency HV generator is illustrated in Determine 2.16. Right here, the incoming mains voltage is rectified and smoothed to DC voltage, earlier than being transformed to a excessive frequency AC voltage – which is then transformed to a excessive voltage (HV) by a transformer, rectified and smoothed again to DC for utility to the X-ray tube. For AEC operation, the detector voltage generated by the radiation publicity is built-in to provide a measure of the particular publicity, Dprecise, which is then in comparison with a previously-stored reference publicity, Dref to allow termination of the publicity.

Fig. 2.16: Block diagram of a Excessive Frequency HV generator outfitted with Automated Publicity Management (AEC).

Lastly, one of these generator design might be prolonged to at least one which may generate radiographs with shorter publicity instances. This may be achieved at a set kilovoltage and a present (mA) which falls from the next worth than usually used throughout the publicity in a way which exploits the instantaneous heating traits of the XRT anode. These heating traits might be pre-programmed into the publicity management system. This kind of design is known as a Falling Load generator and can be utilized to scale back publicity instances to considerably lower than these achievable with the extra standard design.

References[edit]

  1. Behling, 2016. Efficiency and pitfalls of diagnostic X-ray sources: An summary. Medical Physics Worldwide, 4:107-14.
  2. Poludniowski G, Landry G, DeBlois F, Evans PM & Verhaegen F, 2009. SpekCalc: A program to calculate photon spectra from tungsten anode x-ray tubes. Phys Med Biol, 54:N433-8.

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