Radiology Standing Instruction Series
Grade 2 of six
Choosing the Appropriate Exposure Factors

Course 1 – Physics of Radiology
Grade 2 – Choosing the Appropriate Exposure Factors
Course 3 – Recording the Image
Course 4 – Poor Quality Films-Causes and Corrections
Course 5 – Radiation Safety-Importance and Procedures
Class vi – Pros and Cons of Digital Radiography-CR vs. DR

A number of faults can occur that are detrimental to the diagnostic value of a radiograph, which is why it is important to veterinary x-rayunderstand the exposure factors that affect radiograph quality. The most common cause of poor dissimilarity in radiographs is inappropriate exposure factors. Through quality assurance and equipment care, these faults can be avoided, or rectified quickly preventing unnecessary repeat radiographs and extended anesthesia for the patient.

In the previous event "Physics of Radiology" we discussed that kVp (kilovolt peak/kilovolt potential) gives variable "speed" to the electrons determining the penetration of the ten-ray beam, which affects the efficiency of x-ray production, and determines the scale of contrast in the image. The mAs (milliampere seconds) determines the number of x-rays produced per unit time and the number of x-rays reaching the motion-picture show determines the degree of blackening of the motion-picture show.
In social club to take a quality radiograph that details equally much diagnostic information as possible the following factors must be considered:
• The type of tissue in the region of interest
• The type of x-ray machine (specifically the type of
generator)
• The type of film or screen system being used.

x-ray imageThe blazon of tissue in the region of involvement constitutes the bailiwick contrast which is what you want to display. For example, you lot desire to display the differences between the bone cortex, the bone marrow crenel and the surrounding muscle; or brandish the difference between the liver, the falciform fat, the intestinal wall and the intestinal lumen; or brandish the difference between the air filled region of the lung, the pulmonary vessels and the middle. The major way to depict these is in the selection of the kVp used to image the area of involvement.

The correct kVp will produce differential x-ray assimilation of soft and dense anatomic structures. Increasing kVp increases the penetrating ability of the ten-ray beam. If kVp is too low, the epitome volition lack density resulting in a whitewashed or sooty appearance. If kVp is too high the prototype will be over exposed and as well nighttime.x-ray image paw

In small animals and in the distal extremity region of large animals where bone is the major structure of interest, a relatively low kVp (threescore-seventy) is sufficient to penetrate the bone and soft tissue. The low kVp will be very effectively absorbed by the os mineral and provide good dissimilarity. This level of kVp volition provide a rather "contrasty" paradigm which most detect pleasing to apply for bone evaluation. To provide the moving-picture show blackening needed relatively high mAs will exist needed. Every bit always effort to utilize the shortest time possible to foreclose blurring from patient movement.
x-rayIn the thorax where there is a large variation in the types of tissue nowadays, thin pulmonary vessels, thick volume of the middle, very minimally absorbing aerated portion of the lung, a relatively loftier kVp (80-100) is desirable to distinguish this variety of subject contrast. Equally these x-rays volition be quite energetic, not every bit many are needed to establish the degree of blackening so a relatively low mAs is used. A short exposure time is essential in the thoracic region to stop the blur of respiration movement.
The abdomen is more similar the thorax region than the skeletal region when it comes to choosing your exposure factors. In that location are three major tissue types in the belly, soft tissue, fat and gas. However the individual organs vary significantly in thickness. A medium kVp (70-ninety) is desirable coupled with relatively low mAs. Although respiratory motion is not as great of a problem, information technology is nonetheless all-time to keep the exposure time as short equally possible.

Preferred Relative Exposures:digital x-ray image
• Skeletal – High mAs with low kVp
• Belly – Medium mAs with medium
kVp
• Thorax – Low mAs with loftier kVp

tech chartThe all-time manner to create predictably good images in veterinarian radiography is to create and use a technique chart that is particular to each body area. The technique nautical chart is based on standardization of as many different variables as possible and only changing one component of the exposure factors based on the thickness of the torso expanse. To create this technique chart initially involves a trial and error process. One time the ideal exposure factors have been established for the desired surface area on your trial patient, there are simple mathematical relationships that can be used to fill in the chart for whatsoever other thickness of that body area.

To Be Standardized Are:
1. X-Ray tube to moving-picture show distance
2. Collimator
3. Utilize of filigree or no grid
four. Ten-ray moving-picture show intensifying screen combination
5. Flick processing
6. Darkroom lighting

X-Ray Tube to Motion picture Focal Distance: This is a very simple relationship. The closer the x-ray tube is to the film the greater is x-ray focal distancethe intensity of the radiation exposure to the film. So by altering the 10-ray tube to pic distance you can effectively increase or decrease the blackening of the film. Information technology is all-time to standardize this and is easiest to standardize this with a 10-ray machine that has the tube mounted on a stand up that is fixed to the table and picture holder. This is the typical use in small creature radiography. If the tube is manually positioned each time and several different personnel are making images you demand to use a measuring device to effort to standardize the distance. Variations in film blackening are often noted when multiple personnel hand position the tube as everyone stands at just a little bit dissimilar distance from the patient. The relationship of x-ray intensity to distance is relatively profound – Intensity (1) = i/d2, where d is the distance from the x-ray tube to the film. So you can run into that if you double the distance, you lot reduce the radiation intensity to 1/4th, virtually small animal radiography is done at a fixed tube-film distance of 40 inches (100 cm). Large animal skeletal radiography is frequently done with 30 inches (75 cm).

collimatorCollimator: The collimator is a device used to restrict the x-ray exposure to a specified surface expanse of the body. Its major effect, relative to image quality, is that it significantly reduces the scattered radiation produced past the patient and this improves image dissimilarity. It as well serves 2 major radiation safety purposes, to reduce the exposure to the patient and to reduce the scattered radiations exposure to the animal holders if manual restraint is required. Collimators are available either as various diameter exchangeable cones or as an adjustable permanent attachment to the x-ray tube housing, the cone type collimators must be removed and exchanged with a different bore to achieve a change in x-ray field size. The adjustable collimator also commonly has a light associated with information technology that shows the field size on the patient. The light field type collimator greatly improves the ability to accurately position the x-ray beam to the area of interest on the patient.antiscatter grid

Use of a Filigree or No Grid: A grid is a plate that consists of parallel spaced bars of lead. Atomic number 82 is a very effectively absorber of 10-rays. The purpose of the grid is to absorb scattered x-rays betwixt the patient and the motion picture. The scattered x-rays are created within the patient by an 10-ray tissue interaction that results in the conversion of the incoming ten-ray to an electron and a new ten-ray with somewhat less energy moving in a new direction. In essence the ten-ray is deflected off its original straight line course. If this redirected new x-ray successfully exits the patient it delivers exposure to the picture show that is untrue relative to the anatomical structure from which information technology originated.

Most grids are focused grids. This means that the lead bars are angled in the same plane as the ten-rays coming from the tube. The grid is placed between the patient and the film. About often in pocket-size animal systems the grid is incorporated into the table positioned just above the film tray. Grids can as well be purchased that are laid on top of or independently affixed to the cassette. Grids are quite effective in removing the scattered x-rays from the epitome. Removal of the scattered x-rays improves the contrast in the image. Grids are generally used for area thickness greater than x cm. However, if you decide to utilise a grid for the area in question it may be easier to use it for all thicknesses rather than to have to call up to activate it or deactivate information technology.

Making a Technique Nautical chart:test exposure specs

Now that yous have standardized as many factors as y'all tin can you are set to practise some trial exposures, looking for the best for the area of interest. Earlier you begin make sure your ten-ray machine is in adept working order and the settings for kVp, mA and seconds are properly calibrated. If you are unsure whether your equipment is reliably calibrated, accommodate a visit from a licensed service technician earlier proceeding.tech chart

You can make charts that are variable kVp or variable mAs or those that are a combination. Which type you lot make depends somewhat on the type of machine that you accept to use. If you are using a fixed kV, stock-still mA, simply variable time auto (like many of the smaller portable machines) then your nautical chart volition by default be a variable mA nautical chart. If your motorcar has mAs
and kVp selector controls or if
information technology has independent mA, time
and kVp selectors and then you
could brand either blazon of a chart.mAs chart

The basis for making a chart is to make several trail exposures until you find one that works best for that area in your trial patient. Y'all and then apply some mathematical relations to construct the pick factors for different sizes of that area. For an example, if y'all were to brand a variable kVp chart for the canine abdomen you would make three trial exposures of the area with the canis familiaris lying in lateral recumbence. Mensurate the thickness of the trial patient and record information technology in cm. Because of the tissues inside the abdomen utilise a fairly high kVp with relatively low mAs. Go along exposure time short to practise away with respiratory motion effects on the cranial organs. Pick iii exposure settings: outset, 300 mA for 1/60 sec (five mAs) at 70 kVp; second, 300 mA at ane/120 sec (2.5 mAs) at 70 kVp; and third, 300 mA at 1/thirty sec (10 mAs) at 70 kVp. One has double the mAs and i has half the mAs. Using good collimation for the intestinal region, betrayal the three films and procedure them using standard processing technique. When the images are viewed hopefully 1 will exist of adept quality, with acceptable background black (room air space around the patient) and skillful dissimilarity scale for the abdomen. I should also be underexposed (overall too white) and one should exist overexposed (overall too blackness). If none of the three are acceptable, repeat the test exposure modifying from the one that came the closest to what you desire the image to look like. If you must repeat a second set of trial exposures yous could modify any of the exposure factors, all the same it is preferable to change the mA or exposure time. To right underexposure, add 10 per cent to the kVp setting used or double the exposure time. To correct overexposure, decrease ten per cent from the kVp setting used or halve the exposure time used.test radiographs

One time you have constitute the optimal or all-time fix of exposure factors for the trial domestic dog'due south lateral abdomen you are gear up to fill in the working chart for other thicknesses. There is a fairly linear human relationship between kVp, area thickness,initial tech chart and response of the movie-screen system.
• In the range of kVp less than 80 a alter of ii
kVp for each cm change in thickness volition maintain
a quality prototype.
• Between 80-100 kVp the change is three kVp for
each cm change in thickness.
• For kVp greater than 100 the change must be four
kVp for every cm alter.

tech chartThis allows y'all to make a variable kVp chart for the belly. You would try this out on the same dog by taking the ventrodorsal (VD) view. Mensurate the abdomen, await up the kVp calculated for that thickness on your working nautical chart and try it out. You would then repeat this process for every body area that you conceptualize will need to be looked at radiographically. Nearly small brute charts contain factors for caput, spine, thorax, abdomen, pelvis/hip joints, shoulder, and elbow/stifle/bones distal to elbow and stifle. Most equine charts have different settings for a region and for a specific view within that region. For example the equine foot requires increased exposure factors to emphasize the navicular bone versus the thin solar margin of the 3rd phalanx.

As y'all repeat the trial process for each body region some full general guidelines for modifying the full general nautical chart include:
• Low kVp settings with concurrent high mAs
settings requite the all-time dissimilarity for the skeletal
regions.
• High kVp setting with concurrent depression mAs settings
requite the best scale of contrast for the thorax and
abdomen.
• Film blackening is directly proportional to the
product of the mA and the exposure time = mAs
factor.
• Motion-picture show blackening will also change with a alter in
kVp. This is all-time used to alter the flick blackness
by a modest amount. If yous need a large modify in
picture blackening yous demand to modify the mAs. This
is preferred in that information technology retains the dissimilarity scale for
the expanse beingness evaluated. Even so a 15% increase
in kVp volition effectively double the film blackness
(would accept the same upshot as halving the mAs)
these latter two changes have so nearly profound
effect between 70-90 kVp.

Whatever technique chart when first developed is a working chart and should be tested on a variety of different animals. You may observe that you lot need to make minor adjustments as you use it for the first several weeks. Still, information technology then should become quite reliable. It is too good practice to record the exposure factors used for each patient. Then if a re-cheque of that region is needed, yous can use the same exposure factors and be more likely to detect true change in the patient. We find it easiest to record the expanse, exposure factors, film screen type on the outside of the patient'due south radiographic motion picture filing envelope.

If some kVp values exceed your auto's capacity you can compensate on your chart by reducing the kVp setting and kVP - mAs radiographic densityincreasing the mAs value. If kVp values fall below the lowest available kVp setting then suit the low kVp values upwardly while decreasing the mAs values to compensate. If some exposure times are excessively long and you lot are already using the highest available mA setting, lower exposure times tin be achieved by selecting a faster receptor speed, using a lower ratio grid and by reducing the anode-picture distance. The ultimate solution to excessively long exposure times is to purchase a auto with more mA.

If yous are using a digital arrangement, the breadth produces similar appearing radiographs at dissimilar exposure settings, but the prototype resolution with digital systems is directly related to exposure which is influenced by the point to noise ratio. Resolution is lower at lower exposures and greater at higher exposures.