As introduced in Ch. 5, the radiographer must set certain exposure factors on the control panel before each and every x-ray exposure. How these factors are set determines the finished quality of the radiograph, as well as the radiation dose to the patient, so their proper selection is very important. If the radiograph has to be repeated because the exposure factors were selected improperly, the patient gets an unnecessary extra dose of radiation.

In this chapter you are going to learn about another of the primary exposure factors: kilovoltage, and its relationship to the quality of the IR exposure, patient dose and the four image properties. You will learn about the relationship of kVp to the production of scattered radiation and how it impacts image quality. You will also learn about optimum kVp, which is the exposure factor that should be selected first by the radiographer, and how to apply the 15% Rule, which is a calculation used to balance changes in kVp with changes in mAs.

Key principles of kVp

Kilovoltage (kVp) is a measure of the electrical force driving the electrons through a circuit. It is also referred to as potential difference or electromotive force. The term electromotive force paints a vivid picture of what kilovoltage does – it provides the force to move the electrons.  In Ch. 1, we discussed the production of x-rays in detail. When x-rays are produced, electrons are released from the filament in the x-ray tube. These electrons are rapidly pulled across the tube to the anode, are stopped suddenly by the anode, and give up energy in the form of x-rays. The kinetic energy of the electrons is converted into x-ray energy at the anode.

If an electron has a high amount of kinetic energy, it has the potential to give up more energy at the anode than an electron with a low amount of kinetic energy. If an electron gives up a high amount of kinetic energy, it produces a photon with a high amount of energy. And if an electron gives up a low amount of energy, it produces a photon with a low amount of energy.

kVp is the factor that determines the energy of the x-ray photons produced because it determines the amount of kinetic energy each electron has as it moves from the filament to the anode. Kilovoltage is produced in the form of a wave over the time of the exposure. The “p” in kVp stands for peak. The peak voltage is the highest voltage reached during an electical cycle. The kVp that is set on the control panel is the maximum or peak kilovoltage reached during the exposure.

kilovoltage, Rectification and Phase Generation

Kilovoltage varies over the course of the exposure depending upon the type of rectification and generator used in the x-ray circuitry. Rectification is the process of changing alternating current into direct current. We want to rectify the electricity before sending it through the x-ray tube so the electrons flow from cathode to anode and not the other way around. Electrons flowing backwards would damage the filament and not result in the creation of x-rays. Alternating Current (AC) normally flows backwards and forwards over time. If we graph the number of electrons flowing through an AC circuit in a particular direction over time, we get a sine wave. See Figure 6-1.

When we rectify a single line of electricity, or phase, we reroute the electrons flowing in the negative direction so they are flowing the correct way through the x-ray tube. When we graph the voltage of the rectified phase of electricity over time, we get pulsating direct current, as shown in Figure 6-2.  In the simplest type of x-ray generator, a single phase generator, the kilovoltage will vary from zero to the kVp set on the control panel.  When the voltage drops, the top energy of the x-rays that can be produced drops. When the energy of the x-rays drops, the penetrating power of our x-ray beam drops. When the penetrating power of the x-ray beam drops, patients get higher radiation doses because they absorb more of the beam.

Modern x-ray systems use three lines of electricity, or three phases, to keep the voltage from dropping to zero during the exposure. Three-phase electricity basically divides the total amount of electricity provided to the x-ray tube into three chunks and provides that portion of the electricity to the x-ray circuitry at a different time than the other 2 phases. When three-phase power generation is used to power the x-ray tube, the graphs of the maximum voltages overlap and prevent the voltage from dropping to zero. See Figure 6-3.

The fluctuation of the voltage when we have multiple overlapping pulses of electricity is called ripple. Ripple is how much the voltage drops or the percentage of voltage variation. For single phase electricity, the ripple is 100%, because the voltage drops all the way to zero. Three-phase, six-pulse waveforms have a 14% ripple, indicating that the voltage never drops below 86% of the kVp set on the control panel. Three-phase, twelve-pulse waveforms have a 4% ripple, indicating that the voltage never drops below 96% of the kVp set on the control panel. In addition to three-phase power generators, x-ray equipment is also available with a high-frequency generator. High-frequency generators have approximately 1200 cycles per second (as opposed to 360 cycles/second in three-phase units) and maintain a nearly constant voltage output with a ripple of less than 1%.  High-frequency units are cheaper than 3-phase units, allow shorter exposure times and, because of the constant voltage output, have greater consistency in image quality. They are also capable of using standard 120 volt electrical outlets. Currently, most mammography, portable and CT machines use high-frequency power generators.

kVp and X-Ray Production

In Ch. 1 we learned that when the incident electrons interact with the atoms making up the anode, they can lose their kinetic energy in two ways – Characteristic interactions eject an inner shell electron and x-rays are produced with outer shell target electrons drop into the inner shell hole. Bremstrahlung interactions slow down the incident electron based on its proximity to the positively charged nucleus and an x-ray is produced that is proportional to the amount of slowing that the incident electron undergoes.

While a few incident electrons release all of their kinetic energy in a single brems interaction, most electron interactions result in the loss of less energy. In both brems and characteristic interactions, the incident electron may interact multiple times before it loses all of the kinetic energy it gains from accelerating across the x-ray tube. The number of times that the incident electron bounces around before it can calmly join the flow of electrons in the circuit is dependent on its kinetic energy.  Higher energy electrons have more energy to expend and undergo more interactions than electrons with lower kinetic energy . Each interaction produces x-rays or heat.

KVp and Photon energy

The energy of x-ray photons is measured in kiloelectron volts (keV). One electronvolt (symbol eV) is a unit of energy equal to the amount of kinetic energy gained by a single electron accelerated from rest through an electric potential difference of one volt in vacuum. Because we are accelerating electrons through a vacuum when we are making x-rays, we measure the kinetic energy gained by the electrons and the energy released as x-rays in units of electron volts. Since we are accelerating the electrons with kilovoltages, we produce x-rays with energies in kiloelectron volts. The kVp we select on the control panel will determine the maximum energy in keV of the x-rays produced. For example, when operating an x-ray machine at 80 kVp, the x-ray energies produced will range from zero to 80 keV.

If the radiographer selects a high kVp, most of the electrons will have a lot of kinetic energy as they move toward the anode and a large percentage of the photons produced in the x-ray beam will have a lot of energy. If the radiographer selects a low kVp, the electrons have less kinetic energy an a large percentage of the photons produced will have a low amount of energy.

Photons with low energy because of a low kVp are absorbed more easily by the atoms in the patient’s body. Differential absorption increases, and this creates an image with high contrast (short scale) which is mostly black and white. When photons have high energy, they penetrate through the body tissues more easily. The different tissues of the body are all penetrated more equally. Differential absorption decreases, and the image will have low contrast (long scale) which is many gray tones.

KVp and The Emission Spectra

Recall from Ch. 1 that we can make a graph of the number of photons that occur at the different photon energies within the beam. We call this graph an emission spectrum. Also recall that the characteristic interactions create discrete peaks on the graph because they have only a few specific energy levels, while bremstrahlung interactions create a continuous, bell shaped curve. The area under the curve, plus the photons represented in the characteristic spikes, represents the total number of photons in the beam. If the area under the curve increases, we have more photons in the beam. See Figure 6-4.

Emission spectra (-um is singular, -a is plural) are one way to compare how our technical factor changes change the quality and quantity of the beam.  A change in beam quantity will shift the amplitude of the bremstrahlung curve up or down. A change in the beam quality will shift the average energy of the beam left or right.

When we compare emission spectra for exposures using different kVp settings, but the same mAs, we will see that both the amplitude and the left-right position of the spectra shift. See Figure 6-5. This occurs because changes to the kVp change both the quantity of photons in the beam and the quality, or penetrating ability, of those photons.

kVp and Beam Quality

Let’s look at kVp’s impact on quality first.We have seen that higher energy electrons will create x-rays with a higher maximum energy. We can see these differences in the emission spectra where each curve intersects the x-axis on the right side of the graph.  The non-zero x-intercept of the curve represents the highest possible photon energy in the beam. Higher kVp settings result in a non-zero x-intercept that is  further to the right.

Changes in kVp also change the average energy of the photons in the beam. The average energy of the beam is represented in the emission spectrum graph by the top of the brems “hump.” As kVp settings increase, there are more photons of higher energy in the beam. More higher energy photons increases the average energy of the beam. As kVp increases, the average energy of the beam increases and the top of the brems “hump” shifts to the right.

kVp and Beam Quantity

While we typically focus our discussions of kVp on its impact on beam quality, kVp changes will also impact beam quantity. As we increase the kVp from 60 to 70 to 80, we see that the amplitude of the curves gets progressively higher. The purple curve created by the 60 kVp beam has the fewest number of photons in the graph, so the graph is shorter than the curves for 70 and 80 kVp. The electrons that created the 60 kVp curve acquired less kinetic energy in their trip across the x-ray tube and, consequently, each electron bounced around less and made fewer x-rays than the electrons in the other two curves.  The x-ray quantity is directly proportional to the kVp². This means that doubling the kVp will result in a four times increase in photon quantity. For example, if the kVp is increased by a factor of 2, the beam intensity increases by a factor 2 squared or 4. This is why the kVp is usually changed in small increments.

kVp and Patient Dose

Changing the kVp will always change patient dose.  Because beam quantity increases when kVp is increased, increasing kVp results in higher patient dose. Because beam quantity decreases when kVp is decreased, decreasing kVp results in lower patient dose. These statements assume that everything else about the exposure remains the same.  We will discuss the 15% Rule a little later in this chapter which allows us to decrease mAs when kVp is increased to maintain receptor exposure at the same level. If we are using the 15% Rule and compensate for increased kVp by halving the mAs, the patient’s dose will be decreased. Likewise if we decrease kVp and double the mAs according to the 15% Rule, patient dose will be increased.

Scattered Radiation Production

Primary radiation (radiation from the x-ray tube) enters the patient’s body and some of it is absorbed by the atoms in the body depending on the tissue density and the thickness of the body parts. The radiation that gets through the patient’s body is called exit or remnant radiation. Exit radiation hits the image receptor and is recorded as an exposure value that determines the gray level of the pixel in the displayed image. Besides exiting through the patient’s body or being absorbed by the body parts, primary radiation can also be scattered.

Scattered radiation is produced when a primary photon collides with an atom in the patient’s body, bounces off the atom, and changes its original direction. When the photon collides with the atom, it also loses some of its energy. (For a review of Compton Scattering interactions, see Ch. 2.) Since the photon has changed its direction, it could take several new paths. It could bounce directly back at the x-ray tube, it could fly out the sides of the patient’s body, or it could exit through the patient’s body and hit the image receptor from a different direction that it would have if it had remained a primary photon. It is in the patient’s body where most scattered radiation originates.

Scattered radiation that hits the image receptor puts a layer of unwanted radiation on the image that degrades the image quality. This unwanted layer of radiation is sometimes referred to as noise or fog because it appears as a blanket of grayness on the image. The fog makes the individual and distinct gray shades of the image blend in with each other sot that they cannot be seen as separate structures. The image will appear very gray and have low contrast.

Kilovoltage is the exposure factor that determines the proportion of scattered radiation reaching the image receptor. A low kVp beam has more photons with low energy and those are more easily absorbed by the atoms of the patient’s body. A photon that has already been absorbed cannot be scattered. As the energy of the photons in the x-ray beam is increased, all types of interactions within the patient’s body that attenuate the beam decrease. Overall, increasing kVp decreases the probability of the x-ray photon interacting within the patient at all, but the rate of photoelectric absorption interactions decreases faster than the rate of Compton scattering. See Figure 6-6. In this graph, we see that as photon energy increases, the total numbers of both Compton and photoelectric interactions decrease, but photoelectric interactions decrease at a much faster rate than Compton interactions do.

Let’s consider what these differences in the types of attenuation will mean for our images. At low  photon energies of 60 keV or below, photoelectric interactions are occurring in soft tissues. These differences allow us to distinguish between soft tissue structures. For example, when imaging a lateral elbow at 55 kVp, it is easier to visualize the fat pads from muscle. This is important because these fat pads can fill with fluid subsequent to a traumatic injury and may be the only visible sign of an elbow fracture.However, above 60 keV, only transmitted x-rays from the beam represent the anatomy. Yet, the Compton scattering occurs relatively stable. By the time we reach 80 kVp the blanket of scatter is making it very difficult to distinguish soft tissue details. On our elbow image, this means that at higher keV ranges, the Compton scattering may prevent the radiologist from visualizing the fat pads and result in the patient’s injury going undiagnosed.  If we examine the photoelectric interactions in bone, we see that at 60 keV there are almost 100 times more photoelectric interactions than there are Compton interactions. By the time we reach 100 keV, the number of Compton and photoelectric interactions are nearly equal, and at photon energies of above 120 keV, Compton interactions predominate.

Additionally, the increased kVp produces scattered photons with a higher energy that are more likely to escape the patient’s body and interact with the image receptor. This causes the effects of scatter radiation to be more prominant on high kVp images than it is on low kVp images. The greater percentage of Compton interactions compared to Photoelectric interactions combined with greater energy of those scattered photons creates greater noise on the resultant image. Additionally, the reduction in Photoelectric interactions decreases the radiation signal that represents the anatomy. The combination of these two factors results in lower signal-to-noise ratio and less visibility of the overall image.

Three things can happen to the primary x-ray photons as they enter the patient’s body. They can be absorbed, they can be scattered, or they can pass through and hit the image receptor. As the x-ray photons are absorbed or scattered the radiation intensity of the original beam is reduced. The reduction of radiation intensity by absorption or scattering as the beam passes through matter is called attenuation. KVP and Radiographic Contrast

While there are several exposure factors that affect radiographic contrast, kilovoltage (kVp) is the main controlling factor for radiographic contrast. This factor must be set on the control panel by the radiographer for every x-ray exposure. The kVp selected has a major influence on the quality of the radiograph.

Radiographic contrast is the degree of difference between adjacent structures on a radiograph. A high contrast radiograph will appear mostly black and white with few shades of gray in between. This is called short scale contrast. A low contrast radiograph will display many shades of gray and few blacks and whites. This is called long scale contrast.

Subject contrast is caused by the composition of the patient’s body parts. Subject contrast is the number of different tissue densities and tissue thicknesses in the patient’s body. Each body tissue will display a different radiographic density depending on how x-rays passed through it and thus will contrast with each other on the radiograph. If the patient’s body parts are very similar in tissue density and thickness, contrast media can be added to the body to increase subject contrast.

kVp is the exposure factor that controls radiographic contrast. A high kVp beam will produce a radiograph with low contrast and a low kVp beam will produce a radiograph with high contrast. A high kVp beam produces more scattered radiation than a low kVp beam. Scattered radiation reduces radiographic contrast.

Optimum kVp

Kilovoltage is the exposure factor that determines the quality of the x-ray beam. The quality of the beam is determined by the penetrating power of the beam. A beam produced with a high kVp will have high average photon energy. A beam produced with low kVp will have low average photon energy. A high kVp beam will be able to penetrate through the patient’s body more easily than a low kVp beam.

A primary consideration when the radiographer selects the exposure factors to set on the control panel is the kVp, because the kVp controls the quality of the beam and its penetrating power. The kVp selected should be the optimum kVp for the body part being examined. Optimum kVp is the kVp that will be able to penetrate the body part, produce sufficient radiographic contrast, and produce an acceptable level of scattered radiation. Figure 6-7 shows the optimum kVp for the most commonly radiographed body parts.

Penetration of the Part

In order to make an acceptable radiographic image, the part must be penetrated, which means the photons in the beam must have enough energy so that some of the photons will be transmitted through the part and interact with the image receptor.  If the radiographer uses the optimum kVp, that kVp will always be able to penetrate through the body part. In general, a thick and dense body part must be penetrated with a higher kVp than a thin or less dense body part. Small extremities like hands and feet can usually be penetrated with 55 to 60 kVp, whereas 65 to 70 kVp is usually selected for larger extremities like the knee and elbow.

On a digital radiographic image, adequate penetration of the part can be determined by looking for evidence of quantum mottle. While quantum mottle that affects the entire image indicates inadequate photons reaching the image receptor and the need for more mAs overall, the presence of quantum mottle in the thickest portions of the patient’s anatomy and the absence of quantum mottle in thinner areas indicates the need for greater part penetration, and the need to increase kVp.

We can visualize the penetrability of our beam by using a tool called a step wedge. You may recall from Ch. 4 that a step wedge is a block of aluminum consisting of different thicknesses of aluminum arranged in “steps.” Lower energy photons are not able to penetrate as many steps as high energy photons. See Figure 6-8.

In Figure 6-8, we can see up to the 7th step clearly in the exposure made with 65 kVp, but the steps above that are not visualized. When we increase the kVp for exposures 2 and 3, we see progressively more steps of the step wedge, demonstrating greater penetrability of the beam. You may also notice that the differences in brightness from one step to the next decrease as the penetrability increases. This demonstrates the decreased contrast that results from increased kVp settings.

Sufficient Radiographic Contrast

Determination of the best radiographic contrast is somewhat subjective. A high contrast radiograph, one which appears mostly black and white with few grays, is produced with a kVp that is lower than the optimum kVp. At first glance, a high contrast radiograph may appear to be pretty and pleasing to the eye. But a closer examination of the whiter areas of the image will show that some of the anatomical parts were not demonstrated well because they were not penetrated.

A low contrast radiograph, one which is very gray, is produced with a kVp that is higher than the optimum kVp. A high kVp produces a lot of scattered radiation on the radiograph, which is what causes the very gray image. On a very gray image, it is difficult to see the separate shades of gray produced by the patient’s different body parts, so some of the patient’s anatomy may be obscured.

A radiograph with a sufficient amount of contrast, one that includes blacks, whites and an average amount of gray shades, is produced with optimum kVp. Each gray shade is seen as separate from the surrounding shades of gray. These subtle differences demonstrate anatomy that may not be seen on a high contrast or very low contrast image.

While histogram rescaling can adjust the top and bottom values of the scale of contrast for the image data, it cannot add shades of gray that do not exist in the image signal. So, an image that is captured with high contrast will remain high contrast.

Scattered Radiation

The amount of scattered radiation produced in an x-ray beam is controlled by the kVp. A high kVp produces a greater percentage of scatter. Scattered radiation degrades the image by placing an extra blanked of gray noise over the image signal captured by the image receptor, which lowers radiographic contrast.

If a radiographer uses the optimum kVp for each body part, the percentage of scatter on the image remains consistent. It will vary somewhat for different patients, but it is at least predictable. Then it becomes a standard decision whether or not to use a device to control the scatter radiation.

If a radiographer used 60 kVp to radiograph an knee during one visit and on the next visit used 80 kVp, the contrast on the two radiographs would vary, the parts that were penetrated would vary, and the radiographer would need to use a grid to control scatter with the 80 kVp technique, but not with the 60 kVp technique. Thus it is better to use the optimum kVp for each body part.

mAs versus kVp

When examining the same body part on small, medium and large patients, the kVp could be adjusted for each patient in order to change the exposure to the image receptor because kVp affects image receptor exposure.  Since kVp also changes the penetrating ability of the beam, the radiographic contrast, and the amount of scattered radiation produced, adjustment of the kVp to control IR exposure is not a good idea.

mAs controls IR exposure by only changing the quantity of radiation produced. A variation in the mAs does not change the penetrating ability of the beam, the radiographic contrast or the amount of scattered radiation reaching the image receptor. mAs only controls IR exposure and does not affect contrast.

The milliamperage determines how many electrons will be released by thermionic emission at the cathode of the x-ray tube, and this determines the quantity of x-rays produced. Exposure time determines how long the thermionic emission will take place. So, together, mA and time determine the quantity of x-ray photons produced in the x-ray beam.

Kilovoltage determines the quality of the x-ray beam by determining its penetrating power. A variation in the kkVp changes the energy of the photons in the beam. A change in mAs does nothing to the energy of the photons. A change in mAs only produces an increase or decrease in the number of photons at all energies.

Refer to Figure 6-8. This chart shows the factors that affect each of the four image properties. mAs is listed as the controlling factor for IR exposure, and kVp is listed as the controlling factor for contrast. Looking at the list of the other factors that affect IR exposure, you should see kVp listed as influencing IR exposure, but you will not see mAs as a factor that affects contrast as typical diagnostic levels.

This is important to remember when thinking about exposure factors. mAs does not affect contrast in typical diagnostic levels because mAs does nothing to the energy of the photons in the beam. It only changes the quantity of the photons produced. So mAs affects IR exposure by changing the total number of photons across the image receptor.

kVp changes the energy of the photons in the beam. It controls the contrast on the radiograph by changing the penetrating power of the beam. A beam that is more penetrating will have more photons reaching the image receptor that one that is less penetrating. So while mAs only controls IR exposure, kVp controls contrast but also influences IR exposure.

Suppose a radiographer produces a radiograph and then repeats the radiograph with variations in the kVp and mAs settings. IR exposure will be affected when the kVp and mAs are changed, but contrast is only affected when the kVp is changed.

The 15% Rule

There are only a few times in clinical situations when the mAs and optimum kVp can be varied slightly. These instances are when the contrast needs to be changed or when the radiographer wants to help control motion of the patient.

If a radiograph taken at 10 mAs and 80 kVp is of good quality, the mAs can be increased a little and the kVp decreased a little and the radiograph will still have acceptable IR exposure and radiographic contrast. The mAs could also be decreased and the kVp increased from the original factors and the radiograph will display the same IR exposure as the original radiograph and still be of acceptable quality. This range of acceptable exposure factors is called exposure latitude.

When using exposure latitude to change the factors from the optimum there is a definite method. If the kVp is increased, the mAs has to be decreased, and if the mAs is increased, the kVp has to be decreased.

The 15% Rule helps the radiographer determine exactly how much the mAs and kVp should be changed. If the kVp is changed by 15%, the mAs should be changed by a factor of 2. If the kVp is increased by 15%, the mAs should be divided by 2. If the kVp is decreased by 15%, the mAs should be multiplied by 2.

Let’s try a couple of examples.

The resulting sets of exposure factors from these 3 exposures are:

The image receptor exposure on the radiographs taken with each of these three sets of exposure factors will be approximately the same, but the contrast will be different. The radiograph from exposure 3 with the lowest kVp will have the highest contrast, while the radiograph from exposure 2 with the highest kVp will have the lowest contrast. The radiograph from exposure 1, using the optimum kVp, will have the best contrast.

Alternately, to calculate an increase in 15%, multiply the original kVp by 1.15. For example, to increase 60 kVp by 15%:  60 x 1.15 = 69 kVp. To decrease the kVp using this method, you will multiply the original kVp by 0.85 (100% -15% = 85%). For example to decrease 90 kVp by 15%: 90 x 0.85 = 76.5. If rounding to the nearest whole number, 76.5 kVp will be rounded up to 77 kVp. If the answer had been 76.4 kVp, we would round down to 76 kVp.

Using the 15% Rule to Control Contrast

The 15% rule can be applied in clinical situations to control contrast. Suppose a radiographer made an exposure of the abdomen at 80 kVp and 20 mAs. The radiograph showed adequate receptor exposure, but the contrast was too low. The contrast would be increased by using a lower kVp. In this case the mAs would have to be increased if the kVp is lowered.

The two radiographs would have approximately the same IR exposure, but the contrast would be higher with the lower kVp.

While kVp can impact the visibility of scatter on the image, part thickness has the greatest impact on scatter production. More matter = More scatter. This places the radiographer in a somewhat difficult situation in that higher kVp levels are required to adequately penetrate thicker parts. When these thicker parts produce more scattered radiation AND that scatter has higher energy as well as outnumbering the photoelectric interactions that take place, a really ugly, low contrast image results. We cannot reduce the kVp to improve contrast or we will not penetrate the part. We have to look for other ways to decrease scatter.

There are several radiographic devices that can help control the amount of scattered radiation reaching the image receptor. The most important of those devices are the grid and the collimator. They will be discussed in Ch. 11.

Using the 15% Rule to Control Motion

The 15% rule can also be applied in clinical situations to control motion. Suppose a radiographer made an exposure of the skull on an uncooperative patient at 80 kVp and 20 mAs. The 20 mAs was achieved using 500 mA at 0.04 seconds. The radiograph produced was blurry as a result of motion of the patient and it need to be repeated. The radiographer would want to avoid motion on the second exposure.

Applying the 15% rule, the radiographer could increase the kVp and decrease the mAs. The decrease in mAs can be achieved by reducing the exposure time. This decrease in time would lessen the chance of having motion on the repeat radiograph.

The two radiographs would have approximately the same IR exposure, but the contraast would be lower on the repeat radiograph taken with the higher kVp and shorter time. The chance of motion is less on the second exposure.

Exposure Latitude

Exposure latitude gives the radiographer room for small errors. The exposure factors can vary a little from what would be the perfect factors, and the radiograph will still be acceptable. The small range of factors cannot be exceeded or the radiograph will not be acceptable.

The 15% rule cannot be applied in every situation, because there are times when the range of exposure latitude would be exceeded. If the original kVp chosen is not the optimum kVp, and this kVp is at the upper or lower end of the acceptable range, a further adjustment of the kVp using the 15% rule will place the kVp above or below the acceptable range. A very low kVp may not be sufficient to penetrate the part, and a very high kVp may produce too much scattered radiation.

Suppose a radiographer chose 60 kVp to expose a radiograph of the skull. Since 80 kVp is the optimum kVp for skull radiography, the radiograph may be acceptable with 60 kVp because of exposure latitude, although some parts of the skull are not well penetrated at 60 kVp. If the radiographer reduces the kVp further according to the 15% rule, the new kVp would not penetrate the skull at all.

Suppose a radiographer chose 80 kVp to penetrate an abdomen radiograph. This kVp is already above the optimum kVp of 70. If the radiographer increases the kVp further according to the 15% rule, the new kVp would produce too much scattered radiation.

The 15% rule can only be applied over a small range of kVp changes. About 10 kVp above or below the optimum kVp is the acceptable tolerance range. A variation of more than 10 kVp will usually produce an unacceptable radiograph.

The 60 to 90 kVp Range

Since most radiographers do not have calculators in their pockets at work, there is a simple way to perform most of the calculations necessary for the 15% rule. With the kVp range of 60 to 90, a 10-kVp change is equal to a 15% change.

Multiply any kVp between 60 and 90 by 15% and the answer will fall between 9 and 13.5. This is close enough to the round number of 10 to make a rule of thumb. If the original kVp is between 60 and 90, add or subtract 10 kVp and either double the mAs or cut the mAs in half. These calcuations can all be done easily and quickly in clinical situations.

Using the rule of thumb to increase contrast with the original factors are 80 kVp at 7 mAs, decrease the kVp to 70 and increase the mAs to 14. Using the rule of thumb to control motion when the original factors are 80 kVp at 7 mAs, increase the kVp to 90 and decrease the mAs to 3.5 by cutting the exposure time in half.

Look again at Figure 6-5 which lists the optimum kVp for commonly radiographed body parts. Most of the numbers on the list fall between 60 and 90 kVp so this rule of thumb will be useful.

Summary

mAs controls IR exposure and kVp controls contrast. kVp influences IR exposure, but mAs does not influence contrast.

The optimum kVp should be used most of the time because it will always be enough to penetrate the body part, it will produce a sufficient level of contrast, and it will produce an acceptable amount of scattered radiation.

The 15% rule can be applied to control motion or change contrast. If the kVp is increased by 15%, the mAs can be cut in half by reducing the exposure time. This will help control motion. If the kVp is reduced by 15%, the mAs must be doubled. This will increase contrast because a lower kVp is used.

For faster calculation within the commonly used 60 to 90 kVp range, a change of 10 kVp is equal to a 15% change.