prime factors what is technique and how does it affect the image the four prime exposure factors are voltage which is kvp current m a exposure time seconds or fractions of a second distance s id oid and inverse square law kvp what is kvp kilovolts controls how fast the electrons are sent across the tube it controls quality penetrability and subject contrast increasing kvp also increases scattered photons reducing image quality most kvp settings are set per thickness of the bone on technique charts i was taught a baseball analogy for one of my clinical instructors for kvp that has helped me so hi kvp what exams do we use high kbp for chest x-rays you use that high kbp low ma setting if you think of a major league baseball player throwing a fast ball high kbp is going to have a short wavelength that fastball is moving fast to the target that 117 kvp that you're using on a chest x-ray is going to have a higher penetration it's going to move through that patient faster which means lower patient dose but high kvp also increases scatter if you think of a window pane if you throw a fastball at a window pane it's going to shatter into many many pieces if my four-year-old lobs a softball at that same window pane it's only going to crack a little bit if any so high kvp is their fastball it's moving fast to the target which means it has a short wavelength higher penetration higher scatter but when you use high kvp it's considered low contrast so low kvp is the reverse low kvp has a long wavelength it moves slow to the target lower penetration which increases patient dose low kvp does have lower scatter low kvp is considered high contrast what is contrast subject contrast is the degree of density difference between two areas on a radiograph it makes it easier to distinguish features of interest such as defects from the surrounding area it's separated into short scale contrast and long scale of contrast kvp is the controlling factor of subject contrast to make a visual change you have to adjust it by at least four percent short scale contrast is using low kbp so say like 50 to 70 kvp ranges there's less shades of gray it's mainly black and white right it's also considered high contrast long wavelength remember long wavelength it's moving slower to the target it's not the fast ball most often it's used for bone work and it reduces the scatter how do i remember short scale if you look at my little penguin here minus the yellow right so go with me on this analogy your your short penguin is black and white your short little penguin he likes to get high all right he's not a little penguin so short scale is black and white and is considered high contrast it uses low kvp long scale of contrast uses high kbp this is going to be your chest x-rays they're going to be between 90 and 120 kvp there's a lot of gray in your chest x-ray why there's different shades you have bone white you have the black is air but some is more dense than others you have a heart shadow here if you have a gastric air bubble the liver is over on this side there's a lot more going on density wise than in a hand so you're going to have a lot of gray that's why this is considered a low contrast because more gray makes it low this is a short wavelength because that high that fast ball is moving quick because it's at say 120 kvp but anytime you increase kvp you increase scatter they go hand in hand how do you remember long scale i always think of long scale as a herd of elephants there's a lot of gray all right there's a lot of gray and i've heard of elephants that's my only trick so for digital imaging which you guys are really moving into and will be in for probably your entire time here digital image contrast is controlled by the lookup table sometimes there's some tricky questions in the wording subject contrast is kdp digital contrast lookup table digital contrast is a something it's a processing algorithm that's built into the equipment and the technologist does not control it so digital contrast lookup table subject contrast kvp these are terms that you will have on your board so please start making notes now milliamperage or ma is the measurement of x-ray tube current which is the number of electrons crossing the x-ray tube from cathode to angle per second it determines the number of photons remember that number of photons and quantity quantity it controls receptor exposure which used to be called optical density which we've the art has gotten away from the optical density term you guys need to know receptor exposure and it's directly related to patient dose you might hear the term mas that is the m a multiplied by seconds gives you mas if you increase mas you increase in patient dose it's a direct relationship if you increase mas you increase receptor exposure also a direct relationship i love analogies so someone gave me this analogy when i was a student your mas if you think of a snowball would you throw the same size snowball for a three-year-old that you would throw to say an adult during a snowball fight no why their size if your patient is bigger and has more mass then you need to use more mass to penetrate through that part how thick is your part is there swelling is there increased fluid that makes that part more dense then you are going to have to increase your mas setting give a pediatric patient that's small there's no swelling there's no fluid you need a smaller snowball less mass so if your patient has more mass you need more mass exposure time ideally your exposure times are always kept as short as possible this minimizes the risk of patient motion there are a few exposures that utilize something called a breathing technique where you do expand the um seconds on your exposure we'll get to that all right but so just keep in mind m a multiplied by seconds gives you mas mas primarily controls the receptor exposure again which used to be called optical density it also determines the number of photons in the primary beam receptor exposure we use in digital imaging all right so optical density is a film term it's considered the degree of blackening on an x-ray film and mass was the controller there so what is receptor exposure is simply the number of x-ray photons that hit the image receptor how much of that x-ray beam travels through the patient and hits your image receptor that's receptor exposure as mas increases the extra exposure increases to create a visible difference in density the mas must be changed by at least 30 percent typically in the clinical setting if my exposure is low i tend to double my mass or half i tend to double my mass if my exposure is low if my exposure is high i tend to half my mass if you increase your kdp that also increases receptor exposure level because increasing higher kbp higher penetration is getting through that patient right brightness brightness is another digital term that you will go into super detail in your equipment and imaging classes brightness is the amount of luminance of display on the monitor the brightness is the balance of light and dark shades displayed on an image this is really similar to that of the lookup table brightness is controlled by the individual individual pixels on the image mas does not control brightness please don't get that confused remember so put lookup table and brightness into their two separate categories those are digital terms distance this is super basic and i know you guys have already probably done this in your equipment class with learning the inverse square law but x-ray intensity will decrease as the distance from the tube is increased the farther away you are right the less penetrating it will be intensity is spread out the intensity of radiation at a given distance from the point source is inversely proportional squared distance what does that mean that's the inverse square law right in clinical world i do not get out my calculator and do the inverse square law we do scoliosis x-rays for us at 72 inches normally i do spine work at 40 inches i take whatever technique i would use at 40 inches and i simply multiply by 4 if i'm moving my sid to 72. so it's a factor of 4. so moving from 40 inches to 72 multiply your mas by 4. if you do something normally at 72 that you're now going to do at 40 divide by 4. you can thank me later i do not do this formula in my clinical setting these are some terms that i want you to know and recognize sid source to image distance oid object image and sod source to object in terms of recorded detail or you might see spatial resolution in your textbook same same all right and magnification the best image produced is produced with a small oid and large sid focal spot um so there's an area on the anode that we refer to as um the focal spot we tend to use small focal spots for bony detail so small focal squat for bony detail you want to see detail within the bones and the joints large focal spot we use for chest and abdomen where i'm not looking for bony detail i'm looking still for those shades of gray so some influencing factors for pvp there's something called the 15 rule if i make an exposure on a patient and it doesn't come out how i want it if i increase my kvp by 15 that doubles the exposure to the image if i decrease my kdp by 15 and half the exclusion so there's a rule it's called the 15 rule and to maintain the same exposure without doubling or having the exposure to my film or patient when you increase kvp by 15 you are going to half the mass to get the same exposure how does that work so i brought a patient in for an abdomen image i used 20 mass at 80 kvp they now have drank an entire container of barium they have barium in their system barium i have to use 110 kvp penetrate so going from 80 to 110 did i increase by at least 15 percent yes so what do i need to do with my mass i need to cut it in half so i would use 10 mass at 110 kvp on my patient that now has barium involved so increase your kvp by 15 half your mass you use 110 kvp for um barium work the 15 rule is always to maintain the original exposure so that's where the tricky part of the question usually comes in how do you utilize the 15 rule to maintain the original exposure and it'll give you some options if the kdp is increased by 15 percent you're going to want a half your mass if it's reversed if the kvp is decreased by 15 w less to maintain the explosion i'm going to stop there and pick back up