What is kVp and mAs in radiology
kVp: the power and strength of the x-ray beam (quality of the x-rays). * mAs: the number of x-ray photons produced by the x-ray tube at the setting selected (quantity of x-rays). * time: how long the exposure lasts. Understanding Technique. kVp stands.
kVp= Kelovoltage peak
mAs= Mili Ampeare per second
M.A.S. x ray / रेडियोलॉजी में kVp और mAs क्या है
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English ma
Effect of mAs and kVp on resolution and on image contrast
Two clinical experiments were conducted to study the effect of kVp and mAs on resolution and on image contrast percentage. The resolution was measured with a "test pattern." By using a transmission densitometer, image contrast percentage was determined by a mathematical formula. In the first part of the experiment, the density of the film was kept constant by changing the kVp and mAs. In the second part of the experiment, different mAs's were chosen, and for each mAs, several kVp's were used. Five observers read the radiographs. The first experiment showed that, when the film density is kept constant, the higher the kVp, the lower the resolution and image contrast percentage; also, the higher the mAs, the higher the resolution and image contrast percentage. The second experiment showed that when the film density is not kept constant, the correlation between kVp and resolution and between kVp and image contrast percentage was the same as in the first experiment. However, there was negligible correlation between MAS . and resolution and between mAs and image contrast percentage. A high positive correlation was found between resolution and image contrast percentage, but a high negative correlation was found between resolution and film density.
हिंदी में
संकल्प और छवि विपरीत प्रतिशत पर kVp और mAs के प्रभाव का अध्ययन करने के लिए दो नैदानिक प्रयोग किए गए। संकल्प को "परीक्षण पैटर्न" के साथ मापा गया था। ट्रांसमिशन डेंसिटोमीटर का उपयोग करके, छवि कंट्रास्ट प्रतिशत गणितीय सूत्र द्वारा निर्धारित किया गया था। प्रयोग के पहले भाग में kVp और mAs को बदलकर फिल्म के घनत्व को स्थिर रखा गया था। प्रयोग के दूसरे भाग में, अलग-अलग mAs चुने गए, और प्रत्येक mAs के लिए, कई kVp का उपयोग किया गया। पांच पर्यवेक्षक रेडियोग्राफ पढ़ते हैं। पहले प्रयोग से पता चला कि, जब फिल्म घनत्व स्थिर रखा जाता है, तो केवीपी जितना अधिक होता है, रिज़ॉल्यूशन और छवि विपरीत प्रतिशत उतना ही कम होता है; साथ ही, mAs जितना अधिक होगा, रिज़ॉल्यूशन और छवि कंट्रास्ट प्रतिशत उतना ही अधिक होगा। दूसरे प्रयोग से पता चला कि जब फिल्म घनत्व को स्थिर नहीं रखा जाता है, तो kVp और रिज़ॉल्यूशन के बीच और kVp और छवि कंट्रास्ट प्रतिशत के बीच संबंध पहले प्रयोग के समान ही था। हालाँकि, mAs और रिज़ॉल्यूशन के बीच और mAs और इमेज कंट्रास्ट प्रतिशत के बीच नगण्य संबंध था। रिज़ॉल्यूशन और इमेज कंट्रास्ट प्रतिशत के बीच एक उच्च सकारात्मक सहसंबंध पाया गया, लेकिन रिज़ॉल्यूशन और फिल्म घनत्व के बीच एक उच्च नकारात्मक सहसंबंध पाया गया।
या
kVp: the power and strength of the x-ray beam (quality of the x-rays).
* mAs: the number of x-ray photons produced by the x-ray tube at the setting selected (quantity of x-rays).
* time: how long the exposure lasts.
Understanding Technique
kVp stands for kilovoltage peak.116,117 This is the highest voltage (measured in thousands of volts) that will be produced by the x-ray machine during an exposure. For example, if 60 kVp is selected, 60 kilovolts (60,000 volts) is the maximum strength of x-rays produced in this exposure. If 70 kVp is selected, 70 kilovolts (70,000 volts) will be the most energetic photons produced.
kVp controls the penetrating strength of an x-ray beam (beam quality).130 Whenever an exposure is made, the x-rays must be energetic (strong enough) to adequately penetrate through the area of interest. The higher the kVp, the more likely the x-ray beam will be able to penetrate through thicker or more dense material. Low kVp photons are weak and easily absorbed by body tissues or filters that have been placed. Higher kVp photons will likely penetrate through the patient's tissues and make it all the way to the x-ray film. Most x-rays used in medical imaging are between 50 and 120 kVp (50,00 to 120,000 volts).130
A decision is made on which kVp to select based on how thick the body part is and what type of tissue will be exposed. For example, a finger, hand or wrist require only a low kVp setting because the body part is thin and there is not much body tissue for the x-rays to penetrate through. A setting of between 55 and 60 kVp is typically selected. A shoulder or knee is thicker and denser than the finger, and therefore, more kVp is needed to adequately penetrate. A setting in the 65-75 kVp range is usually selected for these body structures. The hip, abdomen, and pelvis are even thicker and denser than the knee or shoulder. Additional kVp is required for adequate penetration. Settings in the 75-80 kVp range may be used. The lumbar spine is extremely thick and dense, requiring settings in the 90-100 kVp range
Calipers (shown below) are used to measure the thickness of the body part being imaged. Typically, an increase of 2 kVp for every additional centimeter of tissue thickness is required to ensure adequate penetration.
Above: Radiographic calipers
For example, if an acceptable image of a body part that measured 10 centimeters was obtained using 60 kVp, an increase of 2 kVp would be needed to penetrate a body part that measured 11 cm (a 2 kVp increase for every additional cm of thickness, or 62 kVp). What kVp should be selected if a body part measures 12 cm? We should use 64 kVp because we have added another cm in thickness.
kVp also controls the amount of contrast (the difference between whites and blacks on an image) seen on an x-ray.131 The amount of contrast visible in an image is referred to as gray scale.
- If an image has high contrast, there will be white and black areas on the film, but very few grays in between.
- When a film has low contrast, there are numerous shades of grays ranging from very light to very dark. Low contrast images have subtle changes in each gray scale step.
Below: An x-ray of an aluminum step wedge is shown. Step wedges demonstrate that the thicker the body part, the less x-rays are able to penetrate through. It also shows how the amount of contrast changes according to the kVp selected. High contrast images have whites and blacks, but few grays (low kVp) while low contrast images have many shades of gray (higher kVp). This step wedge was exposed using three settings: 40 kVp (far left), 70 kVp (center) and 100 kVp (far right). Differences in contrast can easily be seen.
sheet of x-ray film is placed under the aluminum step wedge and an exposure is made. Each stair-step change in density is recorded using a sensitometer, a meter that measures the amount of darkening on the film.
As kVp goes up, radiographic contrast goes down.132 Increasing kVp also contributes to the overall density (darkness) of the image. A fairly small adjustment in kVp can have a significant effect on the image. Just a 15% increase in kVp is roughly equivalent to doubling the mAs. 133 Conversely, a 15% decrease in kVp is roughly equivalent to cutting the mAs in half.
Higher kVp settings produce more scatter radiation.131-133 Increased scatter reduces image detail and increases patient dose. There is always a decision to be made on what kVp setting can be used for any given exposure that will produce the following results:
* adequate body part penetration
* lowest amount of scatter production possible
* highest amount of radiographic contrast possible
* highest possible image detail
Technique Chart
Technique charts are developed to list average kVp, mA, time, distance, and film type used for various exams. Essentially it is a reference to aid the radiologic technologist in producing an optimal image on the first exposure, rather than taking a film that is too dark, too light, or under/over penetrated.
In many hospitals and clinics, technique charts are set up so that an optimal "fixed" kVp is assigned to each body part to ensure the best penetration and contrast is achieved and the mAs is adjusted according to the size of the patient. This is called a fixed kVp system. Some modern equipment has built in exposure parameters, making technique charts less likely to be utilized by technologists.
Chart shows a technique chart (based on a 180 pound average build male)
* Detail: 100 speed film/screen system,
* Rare earth: 400 speed film/screen system
* Grid is an 8:1 focused grid with 103 lp/inch
Half Value Layer
The penetrating ability of an x-ray beam (quality) is dependent on the kVp selected for the exposure.134 A half value test can be performed to determine the thickness of material which is required to reduce the number of x-ray photons transmitted through the material to one-half their original number. The material generally used to determine half value layer is aluminum. Only a thin layer of a dense material, such as aluminum, lead, barium or iodine, would reduce the half value layer. A thicker layer of less dense material (wood, glass, paper, cardboard, etc.) would be needed to produce the same effect on the x-ray beam.
In a modern imaging department, the half-value layer has two important applications.
1- The first is the half-value layer of the primary x-ray beam used in patient diagnosis. A diagnostic x-ray beam produces a wide range of energies. Although we only mention the maximum energy of the beam (for example, we say we are using 80 kVp for an exposure), the beam is made up of photons 80 kVp and lower. We don't want lower energy photons in our beam- we really just want those with an energy of 80. There is no way to get a perfect x-ray beam with every photon possessing 80 kVp. However, the half-value layer helps filter out low energy photons (less than 80 kVp in our example).
Basically, if the half-value layer for a given x-ray beam is low (thin piece of aluminum filtration), then the x-ray beam contains more low energy photons that are less than 80 kVp. They also have less penetrating power because of their lower energy. If the half-value layer is high (thick aluminum added), the x-ray beam contains more high energy or highly penetrating radiation because the lower energy photons could not penetrate through the thick added aluminum.
This is important because x rays used for medical x-ray must have enough energy to penetrate the body part of interest and expose the film sufficiently. Lower-energy radiation is absorbed into the patient's tissues or scattered by the body and may not reach the film, contributing nothing useful to the image. Adding more filtration to the beam, which is typically done by the manufacturer of the unit prior to installation, will remove the undesirable low-energy x rays while allowing the desirable higher-energy x rays to pass through the patient to the film.
On the other hand, if there is too much filtration in the beam, there is a loss of contrast in the x-ray image (differential absorption is reduced). This is why a physicist evaluates all x-ray equipment on a regular basis, typically once a year, and measure half-value layer as part of that testing.
A second application of half-value layer in an imaging department has to do with room shielding. Rooms that contain x-ray equipment are typically shielded with lead-lined walls to reduce the radiation exposure to workers and the public from the use of x rays within the department. When designing the shielding for a room, the physicist will perform calculations based on the half-value layer of the x-ray beam. In general, the design will call for enough half-value layers of shielding to reduce radiation exposure outside the room to acceptable levels.
Time
The length of the actual x-ray exposure can be set by the technologist on most x-ray consoles. Time plays an important role in producing a quality image. Time directly affects the density of a film because it determines how long the image will be exposed.
* If the time selected is too long, the image may be too dark; Longer exposure times have the drawback that there might be motion on the image that would have been eliminated had a shorter time been used.
* Conversely, if the exposure time is too short, the image may be too light.
An exposure time of 10 milliseconds (.010) or less is recommended for most routine x-rays.47 Generally, the shortest exposure time in combination with the highest mA should be used for exposures to reduce motion artifacts.134
Time in Hindi मे ,
अधिकांश एक्स-रे कंसोल पर टेक्नोलॉजिस्ट द्वारा वास्तविक एक्स-रे एक्सपोज़र की लंबाई निर्धारित की जा सकती है। गुणवत्तापूर्ण छवि बनाने में समय महत्वपूर्ण भूमिका निभाता है। समय सीधे फिल्म के घनत्व को प्रभावित करता है क्योंकि यह निर्धारित करता है कि छवि कितनी देर तक उजागर होगी।
* यदि चयनित समय बहुत लंबा है, तो छवि बहुत गहरी हो सकती है; लंबे समय तक एक्सपोज़र समय में यह कमी होती है कि छवि पर गति हो सकती है जिसे समाप्त कर दिया गया होता यदि कम समय का उपयोग किया जाता।
* इसके विपरीत, यदि एक्सपोज़र का समय बहुत कम है, तो छवि बहुत हल्की हो सकती है।
अधिकांश नियमित एक्स-रे के लिए 10 मिलीसेकंड (.010) या उससे कम के एक्सपोज़र समय की अनुशंसा की जाती है। 47 आम तौर पर, गति कलाकृतियों को कम करने के लिए एक्सपोज़र के लिए उच्चतम एमए के संयोजन में सबसे कम एक्सपोज़र समय का उपयोग किया जाना चाहिए।134
MAs
mAs stands for milli-ampere-second.135 It determines how many x-ray photons are produced (quantity). It has no effect on the strength (penetrating power) of the x-ray photons. mAs is a product of multiplying two factors together: time and milliamperage (mA).
mA x time = mAs
For example, a typical technique setting used for imaging an injured hand may be 60 kVp, 100 mA, and 1/100 of a second (0.01). In this example, 100 mA multiplied by 0.01 seconds equals 1 mAs (or 1 milliampere-second).
In another example, if we were imaging the shoulder, our technical factors may be set at 70 kVp, 200 mA, and 1/10 of a second. To find mAs, we multiply 200 x 1/10 (0.10), which equals 20 mAs.
Calculating mAs
- The green column shows small focal spot settings: 50, 100, 150, 200, and 300 mA.
- The blue column shows large focal spot mA settings: 400, 500, and 600 mA
- The red column is time of the exposure (in seconds or fractions of a second )
mAs is adjusted according to the size and tissue type of the body part being examined. mAs can be changed by altering either the amount of time used in making the exposure, or the milliamperage (mA) setting. There are many combinations of mA and time that, when multiplied together, equal the same mAs. For example, all of the following combinations of mA and time equal 100 mAs,
Any of the above exposures will produce the exact same density on the resultant x-ray film. Faster exposure times are needed if the patient is having trouble holding still, as in the case of small children. When using a short exposure time and a high mA setting, be sure to refer to the tube rating chart to verify that the tube will be able to stand up to the extreme temperatures generated by such an exposure. Regardless of which of the five possible combinations is selected in the example above, each will produce exactly the same number (quantity) of x-ray photons. An image produced by any of these combinations will have the same radiographic density (amount of image blackening). mAs is the most important technical factor in controlling the density (darkness) of an image.
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