Basic physics and principles of making a great image: Part 2 (Proceedings)

Aside from ultrasound, a digital radiography suite is going to be the most expensive upgrade for the private practitioner in the realm of diagnostic imaging. However, this expense will be well worth the investment from a diagnostic imaging standpoint. After more than a century of film and film screen imaging, the backdrop of diagnostic radiology has changed from hanging films on view boxes to LCD monitors.

X-ray Production Overview

X-rays are produced by the acceleration of electrons from a cathode source (made of tungsten) to an anode target (made of 90%tungsten/10% rhenium and can be a stationary or rotating target) with both collisional (>69.5 kVp) and decelerating interactions resulting in characteristic and to a greater extent, Bremsstrahlung radiation respectively. The purpose of a rotating anode is to spread the heat produced during an exposure over a large area of the anode. The cathode and anode for standard x-ray tubes are made out of Tungsten, which has a melting point of 3400° C, and a Z of 74 and an inner k-shell electron binding energy of 69.5 keV. The cathode (-) is a coiled filament that measures 1.5 mm in diameter and 10 to 15 mm in length. The electron cloud is created when a low current is applied to the cathode during tube "warm-up" prior to exposure. This electron cloud is held together by a strong negative charge, surround the cathode filament, heated within the focusing cup. The focusing cup is electrically connected to the filament circuit and provides a voltage of up to 10 V to the filament, which produces a current up to about 7 Amps. The mA determines the number of electrons boiled away from the tungsten cathode as the filament is heated secondary to electrical resistance within the filament circuit by a process called thermionic emission. As one increases the mA, the temperature of the cathode increases so that more electrons are released through the process of thermionic emission.

The size of the cathode is one factor that will determine the degree of penumbra (edge unsharpness) present on the final radiographic image. Typically x-ray tubes used for magnification will have small focal spots on the order of 0.3 × 1.2 mm vs. the larger focal spots typical for the small animal veterinary products currently on the market, (1.0 × 2.0 mm).

In addition to actual focal spot size, the effective focal spot size will be smaller (length only and not width) due to the angle of the anode due to the line-focus principle. The projected or effective focal spot may differ by 50% from the actual nominal size. The higher the mA station, the larger the electron bloom results, thereby increasing the actual size of the electron cloud. When using small focal spots, it is best to try to minimize the mA (usually mA limited on generator controls). The effective focal spot is the size of the focal spot as projected down the central axis of the primary x-ray field. The relationship between the actual and effective focal spot lengths is as follows:

Effective focal length = Actual focal length x sin(β ), where β = anode angle.

The major trade-offs to consider with regards to anode angle include: smaller anode angle (7 to 9 degrees) = small effective focal spot (better spatial resolution); limit to size of field coverage, and poor power loading ability of the anode due to the higher heat load placed focally on the anode. This type of anode angle would be used for limited field coverage specialty imaging such as neurovascular imaging, etc. Conversely, a larger anode angle (12 to 15 degrees) would equate with good field coverage, larger effective focal spot (increased penumbra) and good power loading (higher techniques or heat unit capability due to spread of the actual focal spot). The power loading or intensity of heat units is dependent upon the cathode filament length so that a longer filament length will decrease the intensity of the heat units placed per unit area on the anode (good power loading), whereas a small cathode filament length results in higher heat units per unit area on the anode (poor power loading). Typically tubes are described based on the effective focal spot length and width with common lengths being 0.3, 0.6, 1.0, 1.2 and 2.0 mm.

There are anode-cathode differences in intensity noted particularly in large field of view images with low kVp technique. These intensity differences are called the heel effect and is based on the fact the there is a reduction in the intensity of the primary x-ray beam along the anode side of the field of view. This is because the x-rays that have to traverse the anode side of the angled anode are more readily attenuated by anode material. Because of this the heel effect is less noticeable for higher kVp techniques, longer tube-film distances, smaller fields of view and small anode angles (7 to 10 degrees).

X-ray generators that are commonly used in veterinary medicine include: single phase (1φ), half-wave and full-wave rectified (1φ2 ), three phase (3φ6 or 3φ12 )and high frequency generators. In single-phase half-wave generators, the incoming 60 Hz line is divided into positive and negative cycles for the incoming 120 Volts. A single-phase generator that is half wave rectified only takes advantage of the positive phase of the incoming pulse, therefore the fastest time for this type of generator will be 1/60th of a second. For a fully rectified, single-phase generator there is still 100% ripple as seen with the previous generator, but the rectifiers will allow the operator to take advantage of both the negative phase (by flipping it to a positive phase cycle) and the positive phase of the incoming 60 Hz alternating current. This means that the fastest time one can use is 1/120th of a second. If there is patient motion on the radiograph, one can count the number of images present (since there is 100% ripple) and determine the timer used for the radiographic exposure. If one knows the type of generator, one can account for whether each pulse is 1/60th of a second or 1/120th of a second and determine the timer.

Example: If you have a radiograph and can count 6 images, calculate the timer used for a single-phase half wave rectified and full wave rectified generator.

Half Wave, Single Phase Generator

1 pulse = 1/60th sec; 6 pulses (1/60th sec/1 pulse) = 6/60 = 1/10 of a second timer used.

Full Wave, Single Phase Generator

1 pulse = 1/120th sec; 6 pulses (1/120th sec/ 1 pulse) = 6/120 = 1/20 of a second timer used.

Average X-ray energies are increased when comparing single-phase generators versus three phase generators. In a three-phase generator, the average kilo voltage is higher depending on the type of generator and can approach the actual kVp selected for a twelve-impulse, three phase generator, because there is virtually 3 to 5% ripple. The x-ray intensity is therefore higher and the average x-ray beam energy is also higher for three phase generators. For a 3φ6 or 3φ12 one will have to use one half the mAs as with a 1φ2 in order to obtain the same optical density on the radiographic film.

kVP (kilo voltage peak) will control the x-ray penetration, while the mA ultimately controls the number of electrons boiled off of the focal spot cathode, thereby impacting the number of x-rays produced. This is important when considering contrast within a radiographic image due to the basics in x-ray attenuation within the patient. For a high contrast image (defined as stark transitions between dark gray and light gray or white) one would use a low kVp, high mA technique where as for a latitude (longer gray scale image) one would use a high kVp, low mA technique.

Review of image geometry, magnification and grids

The image geometry is dependent upon the fact that the a diverging x-ray beam and the position of the patient relative to the x-ray source (tube) and the imaging receptor (film-screen in diagnostic radiology). The closer the patient is to the imaging receptor the less geometric magnification there is and thereby the sharper the image is (less edge unsharpness also noted). One can magnify the patient by placing the patient closer to the tube or the imaging receptor (film-screen) farther from the patient, remember the importance of focal spot size and the impact of penumbra on ultimate image formation. Using the law of similar triangles one can determine the exact degree of magnification, so that:

Degree of magnification (M) = Image Length/Object length = SID/SOD

Where SID = the source image distance and SOD = the source object distance.

Distortion within the image results from differing degrees of magnification within the image due to the variable position of various parts of the dog or cats anatomy from the film-screen receptor. For example in a large dog abdomen, a right lateral radiograph would result in a smaller appearing right kidney when compared with the left, but this may be purely based on magnification. This distortion is based on the differing levels of the kidneys from the film-screen receptor.

Unsharpness within the final radiographic image can be secondary to the actual object shape (irregular margins or decreased contrast from border effacement), motion of the patient or the equipment, geometric penumbra (focal spot size or magnification) and receptor unsharpness (screen blur secondary to high speed screens, lack of film screen contact).

Potential artifacts that can be made during the exposure include any object that might block the path of the x-rays or the light emitted by the screen phosphors resulting in a negative density or "white – clear" artifact on the radiographic film. Based on the law of similar triangles and degree of magnification, the object that is farther away from the film-screen combination will have edge unsharpness to the point that if the object is in the collimator housing, the object may not be recognizable.

The factors that influence the detail or spatial resolution of the final image include machine and physical factors. The smallest focal spot will produce the best spatial resolution, other factors being the same. Physical factors that will impact the detail or spatial resolution of the final radiograph include focal-film distance, object-film distance, film-screen contact, patient thickness, patient motion, film speed and intensifying screen speed.

Grids. Scatter radiation is caused by primary beam x-ray interactions (Compton) within the patient so that forward scattering secondary x-rays can cause significant reduction in signal to noise and dramatically decrease contrast resolution within the final radiographic image. As patient thickness increases the degree of scattering increases and as the field of view size increases the degree of scattering increases. Because of this, a grid has been devised that is placed between the patient and the cassette. The grid is composed of a series of parallel lead strips that function to absorb the off axis scattered, lower energy photons that will just contribute to image noise, but still allow the primary x-ray beam to pass and create the final image. The grid is made up of the lead grid bars (septa) and the interspaces or openings between them are made up of carbon fiber, aluminum or paper. The grid can be defined in a number of different manners. The first is based on whether or not the grid is focused or unfocused. An unfocused grid has parallel lines throughout. The grid distance from the tube is not as critical (within reason) compared to a focused grid where the lead septa or strips parallel the diverging nature of a primary x-ray beam so that the film focal distance becomes important and the grid must be placed within a specific focal range (typically 38 to 42 inches from the x-ray focal spot). The second way of classifying grids is by the grid ratio. The grid ratio is defined as the height of the lead strip divided by the width of the interspacing between the lead strips or septa. Typically grid heights are in the order of 1.20 mm, while the actual interspace width may be only 0.12 mm. Therefore the grid ratio would be 10:1. The average grid used in veterinary medicine is an 8:1 grid. As the grid ratio increases (6:1 to 8:1 to 10:1) there is a corresponding increase in the x-ray beam numbers (intensity) required to produce an adequate exposure on the grid. When going from a table top technique to a grid technique using an 8:1 grid one has to triple the mAs. Grids can also be defined by the direction of grid septa travel. Typically linear grids are used meaning there are grid lines only in one direction. Crosshatch grids have lead septa in two perpendicular directions. Another common way that grids are defined is the number of lead strips per unit length or the grid frequency. Typical grid frequency used in veterinary medicine is 103 lines/inch (odd number lines/inch use aluminum interspacing). Lower grid frequencies (80 line/inch – paper or fiber interspacing) usually result in objectionable grid lines on the final radiograph. Computed radiography systems are especially sensitive to low grid frequencies. Higher grid frequencies, such as 150 lines/inch usually have a carbon fiber interspacing without objectionable lines being apparent on the radiographs.

Grid artifacts are common in veterinary medicine as grids can become displaced, out of line or inappropriate vertical adjustments of the tube head will result in grid cut off. The grid must be lined up with the x-ray beam center, must be perpendicular to the x-ray beam and must be within the specified focal range (typically 40 inches). In grid cut off, there is excess attenuation of the primary beam and the final image will have decreased exposure in the area of the grid cut off. Depending upon the type of grid cut-off, grid lines may be present throughout the image or localized to one or both sides of the image. The various types of grid cutoff are explained below.

Grid failure results when a grid has been bent, cracked or damaged to the point where the interspaces are wide enough to be visualized and the lead septa are seen to be irregular and bent. Also, with the lower grid frequency and the paper interspacing, absorption of spilled liquids through the grid can result in a permanent artifact seen when using the grid where the dried liquid within the grid will partially attenuate the x-ray beam more so than the rest of the grid. The pattern will be irregular and random depending on where the liquid was absorbed within the grid. In both of these instances the grid needs to be replaced.

Review of image formation

Image formation is dependent upon the differential absorption of the x-rays within the patient and the resultant absorption of the x-rays within the screen, film (non-screen) or photostimulable plates (computed radiography). The differential absorption (ultimately subject contrast) is based upon the effective atomic number of the absorber, the physical density of the absorber, the thickness of the absorber, and the x-ray photon energy.

The radiographic cassette is made up of a single or double layer of intensifying screens that are within the cassette backing. The intensifying screens are composed of variable sized phosphor crystals that will absorb incoming x-ray photons due to the high atomic number of the crystals. This absorption of the x-ray energy results in the emission of visible or ultraviolet light that then is exposes the photosensitive film present within the cassette. Calcium tungstate or barium lead sulfate crystals were used until the 1970s when rare earth screens replaced the calcium tungstate crystals. Thereby, film-screen usage is considered a two-step indirect process where the x-ray photons must first be absorbed by the screen phosphors and then the film must be exposed by the visible or uV light released.

The most common phosphor material of the rare earth screens includes oxysulfide, oxybromide, tantilate compounds linked to gadolinium (Gd, Z=64), lanthanum (La, Z = 57) or yttrium (Y, Z = 39; not a true rare earth, but has similar properties). These compounds are thulium activated (Lanthinum oxybromide – blue emitter) or terbium activated (Gadolinium oxysulfide – green emitter or orthochromatic emitter). It is important to recognize the difference in green and blue emitters as this will impact the type of safety light and filter used in the dark room. It is also important to match the screen emission phosphor color with the film grain sensitivity. All of the major film screen companies (Fuji, Agfa, Kodak, 3M) have worked to match the spectral emission of the phosphor to their film sensitivities. The x-ray to light conversion efficiency is only about 5% for the older calcium tungstate screens, but approaches 20% for the rare earth screens. The screen thickness and phosphor crystal size will impact the overall screen speed and degree of edge unsharpness of the final image on the radiographic film. As the screen thickness increases, the spatial resolution will decrease, but the screen speed will increase thereby reducing radiographic technique (patient/personnel dose).

A protective coating along the film side of the screen covers the intensifying screens. Typically along the cassette side of the screen there is a reflecting or absorbing layer that will either reflect light emitted isotropically by the screen phosphors or absorb the light emitted to reduce light spread. The thicker the phosphor crystal layer and the larger the crystal size will decrease the overall spatial resolution of the system due to the light spread before interacting with and exposing the film. Typical screen thickness is between 70 and 250 µm, while actual phosphor crystal sizes are between 3 to 15 µm. The protective coating is usually 10 µm. A light absorbing dye may also be added to the screen phosphor layer in order to prevent light spread and preserve spatial resolution. In some cassettes a thin lead or aluminum backing is present on the bottom of the cassette to help prevent backscatter.

The functions of the cassette include: provide a light tight seal around the radiographic film to prevent unwanted light exposure to the film; ensure proper contact between the screen and radiographic film; and provide a protective mechanism for the screen and radiographic film. Most cassettes has a curved back panel that allows for any air to be pressed out prior to the light tight seal formed when the cassette is closed. Poor screen-film contact can result in image unsharpness due to the light spread prior to the radiographic film being exposed. Poor screen-film contact can be a result of air being trapped between the screen and film, film being folded within the cassette, foreign material on the screen preventing normal film screen contact and exposure of the radiographic film (negative density artifact) or damaged cassettes or latches resulting in light leaks and film fog (positive density artifact).

Screen failure or screen craze results when the screen phosphors have aged and no longer emit light when x-rays are absorbed. DuPont Quanta III (Lanthanum oxybromide:thulium activated) screens are particularly prone to this after 5 to 7 years of usage. Irregular negative density artifacts will be present all over the radiographic film due to the breakdown of the phosphor and the lack of light emission, thereby exposure of the radiographic film.

The radiographic film is composed of a base (middle layer) that supports an emulsion on both sides (double emulsion film) or on a single side (single emulsion film). The polyester film base is typically a form of a plastic or Mylar that is clear. The film emulsion is composed of grains of silver halide crystals within a clear gelatin base. The gelatin base is composed of cattle bone, "from India and Argentina. (Useless information: India and Argentina do no have better cows, but that have a lot of them. They also have inexpensive labor to process the bones)."1 A thin adhesive layer attaches the film emulsion to the base. The silver halide crystals are typically made up of AgBr (95%) and AgI (5%). The emulsion thickness is typically 3 to 5 µm thick, while the film base is usually 180 µm thick. A protective overcoat is applied to the each side of the radiographic film. Additionally there are anti-crossover dyes within the emulsion to absorb light so that light does not expose both sides of the emulsion. The color of the radiographic film is based on the dyes within the emulsion. Aside for anti-crossover dyes there are also sensitizing dyes that are used in the emulsion to extend the sensitivity of the silver halide crystals to light wavelengths to which the undyed silver halide crystal would be normally insensitive and not undergo latent image formation. Most of these sensitizing dyes are pink or lavender in color. There are also dual receptor, asymmetric, screen-film systems that use an anti-crossover layer next to the adhesive layer between each emulsion and base layer.

Although all aspects of film-screen radiography (film-screen speeds, H & D film characteristic curves, etc.) are beyond the scope of this overview, the basic process of silver halide exposure and latent image formation will be reviewed. The silver halide crystals are arranged in a latticework within each grain. There are imperfections added to the latticework in form of silver sulfide (AgS). The silver halide crystals are in a cation-anion pair so that the silver is in a +1 valence cation state. The silver sulfide imperfection renders the silver halide crystals susceptible to oxidation-reduction reactions that will ultimately result in the ability to develop the silver grain (containing the silver halide lattice work). The silver sulfide is usually located along the surface of the silver halide lattice and is called the sensitivity speck.

Latent Image Formation

The sensitivity speck is the site where silver reduction takes place when the silver cations combine with "free" valence shell electrons. Outer shell electrons absorb the light energy and are then moved into the gelatin from the halogens within the latticework. The silver cation then undergoes a reduction reaction (gains electrons) forming 2Ag0 or elemental silver, which is black. The formation of the two-atom Ag complex within the sensitivity speck is called the nucleation phase of latent image formation. Three to six reduced silver atoms are required at the sensitivity speck in order to render the sensitivity speck developable. The sensitivity speck then undergoes further electron trapping with Ag+ migration and reduction and is called the growth phase of the latent image. Again, it is the "activation" of the sensitivity speck that renders the emulsion grain developable. Any grain that has not had this change within the sensitivity speck will not be developed and will be removed from the emulsion during the fixation process. Several hundred of the reduced silver atoms will be present in the sensitivity speck although there are typically between 1,000,000 and 10,000,000 silver cations within the lattice work of an emulsion grain. The developed silver grains are between 1 and 5 µm in size. X-ray film has been the mainstay for diagnostic imaging for over 100 years. It provides a wide range of latitude (gray scale presentation), contrast, speed and detail options and is a durable, portable storage medium. However, it degrades over time, is not easily duplicated and requires physical storage. So why go digital? The digital era is upon us and the veterinary market is ultimately going to be dictated by what is happening in the human market. This trend is not going to go away and film screen combinations will be more difficult to get. A major disadvantage of the film era is the developmental process, which can totally destroy great radiographs (even that had the proper exposure factors and anatomic positioning). In the analog film system, the image capture, processing and the storage of the image are all contained on the film. In digital radiography, these processes are separated. Raw data can be re-processed at any time and reviewed in a variety of formats based on the viewing software and stations used.

Geometric Principles

The focal spot is the area on the target of the x-ray tube which the electron stream strikes and from which x-rays are emitted. The larger the area of the focal spot, the poorer is the detail in the x-ray image due to the larger the area of penumbra or edge unsharpness associated with the image. The actual focal spot is the area of the focal spot on the radiographic target (anode) as viewed at right angles to the plane of the target. The effective focal spot is the face of the anode that carries the target in an x-ray tube that is slanted from the vertical (anode angle) to increase the volume of the target that interacts with the electron cloud at exposure (thereby dissipating more heat), but also serves to reduce the size of the origin of the x-ray beam thereby improving edge sharpness or decreasing penumbra.

The basics of triangle geometry apply when thinking about the object relative to the film screen combination or the imaging plate. The closer the object is the sharper the margins are and there is decreased magnification, whereas the opposite is also true. There is always some degree of edge unsharpness and magnification no matter what as we are imaging a three dimensional object (dog or cat thorax). The subject contrast is a term relating to the ability of the carrier wave used in image formation to interact differently with the structures in the object ("subject") to be imaged. The carrier wave is the energy form interacting with the structures (e.g. tissues) and conveying image information to the image detector. In x-ray imaging, subject contrast (also named radiation contrast) is the difference in x-ray intensity transmitted through adjacent parts of the subject. Subject contrast is related to differences in the linear attenuation coefficient between structures, which depends on several factors: 1) difference in the thickness; 2) differences in the physical densities between various structures in the subject; 3) atomic number differences, and 4) radiation quality or energy (kVp).

The contrast resolution relates to how clearly different intensities (e.g. different shades of grey) can be differentiated.

Quality Control - getting it all right!

Things that have to be done consistently are: 1) positioning correctly; 2) correct measurement of the patient for thickness; 3) radiation safety considerations; 4) darkroom technique must be consistent and 5) film-screen combination (older analog system) or imaging plate that are detail oriented. The rigorous QC required is not for the faint of hearts, but should be relegated to the most knowledgable technician that has been trained by a radiology veterinary technician. Such programs are available at most veterinary schools where technicians can shadow radiology technicians and be trained to understand the basic physics of x-ray and image formation, as well as how to quality control (exposure and positioning) the images that are produced.

What are the digital advantages?

Although multiple advantages of digital radiography (DR) and computed radiography (CR) have been sited, each of these advantages may not be directly applicable in the veterinary market and may not make sense for return on the investment for a given practice. Digital radiography is just a digital image acquisition technique where the images are captured, reviewed, processed and stored in a digital environment (computer based). The advantages of digital radiography include: rapid acquisition (with review of images in 4 to 6 seconds for digital radiography plate systems), greater case through put; fewer repeat radiographic exposures (in theory) due to incorrect exposure (although positioning and centering errors still must be corrected for by repeating the radiograph). Additionally, there is a greater dynamic range (more latitude than a gray scale film), advanced processing algorithms (edge enhancement techniques) and the ability manipulate the images. Less storage space is needed, no more lost films (hard drive or optical media storage), no image deterioration over time and no more film chemistry is required. Digital images are easy to duplicate and move around in the cyber world and can be used as a suitable medium for easy transfer of images (such as electronic film interpretation). Digital images are not analog images that are placed on the viewer and digital camera is used to make an image of the radiograph.

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