Chapters 21 & 22

Distance Distance (D) refers to the length from the focal spot to the image receiver (receptor). It is measured in both inches and centimeters. Distance: 2.5cm = 1inch 40” = 100 cm 72” = 180cm

Distance Distance has a substantial influence on three image qualities: Radiographic image density Magnification of the size of the projected image The sharpness of recorded detail

Estimating SID For an average male, 40 inches will correspond to the distance from the fingertips to the opposite side shoulder of the body. With the fingertips at the tube, the film or image receptor should be about at the opposite shoulder.

Estimating SID Extending both arms, 72 inches will be slightly more than the distance from fingertip to fingertip. For the average female, the same guidelines may be used, only measuring to the collimator rather than to the tube.

Distance Focal film distance (FFD) Source image distance (SID) Target to film distance (TFD) All mean the same thing

Distance Must utilize proper distance when performing x-rays. Distance has a profound effect on the radiograph; especially density.

Affecting the Radiograph If you change the distance without changing any other factor then you will affect the radiograph. Density will be affected with increases or decreases in distance and must be compensated for. Contrast remains unchanged within practical ranges of distance

SID and Sharpness of Recorded Detail As SID increases, Sharpness of recorded detail increases As SID decreases, Sharpness of recorded detail decreases Direct relationship

SID and Sharpness of Recorded Detail As a rule of thumb, the maximum feasible distance should always be used in order to obtain the best sharpness Must be remember: when using larger SID’s the technique must be increased to make up for the increased distance and the loss of density.

SID’s Affect on Magnification Increasing the SID, decreases magnification at a set OID. Less penumbra is produced and the size of the umbral shadow decreases as well.

SID’s Affect on Magnification Sometimes using a short SID can be used to our advantage. Using 26-30 inch SID, will magnify and blur the upside bones and the anatomy on the downside, closest to the film will be clearly visible. What example can you think of when we use a short SID like this?

SID’s Affect on Shape Distortion The shape of the image does not change as long as the object is centered in the x-ray beam and the beam is not angled. Shape Distortion always refers to the beam-part-film alignment It is never affected by changes in SID

SID’s affect on Density Only a 20 percent change in distance will cause a visible change in image density and will require an adjustment in technique. Intensity directly influences radiographic density; any change in distance will cause a change in density when all other factors are constant.

Inverse Square Law Inverse square law- the intensity of the radiation is inversely proportional to the square of the distance. As you increase distance, the density changes inversely.

Inverse Square Law The relationship between distance and density follows the inverse square law, Original Density = (New Density)2 New Density (OriginalDensity)2

Example Problems 40 SID Old 72 SID New 1 Density Old What is the new Density?

Example Problems 20 SID Old 40 SID New 1 Density Old What is the new Density?

Mental Challenge If you use 1/3 of the original distance, how will the density be affected? If you use ¼ of the original distance, how will the density be affected?

Square Law Distance changes should usually be compensated for by using mAs, rather than kVp. Changes in distance alter the intensity of the x-ray beam energy levels or penetrating ability are not affected. Adjusting the mAs restored the original intensity of the beam, without changing its quality

Inverse vs. Square Law Inverse square law is used to predict image density Square law is used to compensate technique so that density is maintained when distance changes.

Square Law Original mAs = (Original SID)2 New mAs ( New SID)2 The change in technique should be equal to the square of the change in distance If distance is doubled, the techniques should be changed by a factor of 4

Example Problems From 30 SID” to 60 SID From 2.5 mAs to _____ mAs

Solutions The solutions to the square law problems are always the inverse of those for solving inverse square law problems. If a distance change would result in a film that is twice a dark in the inverse square law, the square law would tell us to cut our mAs by one half.

Rules of Thumbs for Distance Changes Helps to think of distance changes in factors of two, doublings and halving. 40 to 80 is doubling the number, 60 is half way to the double 40 to 80 requires you to use two doublings of mAs, 60 requires one double of mAs

Rules of Thumbs for Distance Changes 40 to 20 is one half the distance, it would require you to use cut mAs by one fourth 40 to 30 SID would require you to use ½ the mAs

Rules To Remember Always remember that a 30% change in mAs is necessary in order to see a visible change in the density on a radiography A 10 inch difference in SID is always significant enough to require a technique adjustment.

SID’s affect on Contrast At any distance in the beam, the ratio of scattered x-rays to primary x-rays will remain the same. There is no change in the relative proportion of scatter in the beam, image contrast will not be affected.

OFD OFD = Object to film distance OID = Object to Image Distance Refers to the distance between the body part and the film or image receptor. Always want to keep OID to a minimum.

OFD OFD affects the radiograph in the following ways: Recorded detail Density Contrast

OID/OFD We don’t want OFD in most cases. You have to weigh the advantages and disadvantages of OID on the image. OFD’s price is usually too high to pay for its assets.

Must always evaluate the following 2 things must look at when performing an x-ray: How will the quality of the radiograph be affected overall How much radiation will the patient be receiving (Must always consider the ratio between OFD and TOD)

How is the Radiograph Affected with OID As OFD increases, density decreases (radiation Diverges) As OFD increases, recorded detail decreases ( you have magnification) As OFD increases, contrast increases, becomes shorter scale

OID affect on Sharpness The larger the object to image distance, the greater the penumbra grows, and therefore the sharpness of recorded detail diminishes. Image sharpness and OID are inversely related

OID’s Affect on Shape Distortion No affect on Shape Distortion Shape Distortion only is caused by the beam-part-film alignment

Affect on Contrast If you create a gap between the patient and the image receptor, the radiation diverges and the amount of scatter radiation reaching the film is reduced. You get high contrast or short scale by increasing OFD.

Affect on Contrast By increasing the distance between the patient and the film, the intensity of the scatter radiation reaching the film diminishes With less fogging, image contrast is enhanced

OID’s Affect on Density The radiation diverges more when OID is introduced, less radiation reaches the film, so the image density decreases

Air Gap Technique OID can be used to combat image fog Chest radiographs of very large patient is a good example of this. 6-8 inches of OID are introduced on the repeated exposure, known as the air gap technique; it is effective in restoring a portion of the desired image contrast.

Air Gap Technique Must remember other factors of the image will be affected: Use the largest SID possible to reduce magnification Sharpness of detail is lost with OID mAs must be compensated for when using the large OID and long SID’s.

Magnification and OFD OFD produces a negative effect on the radiograph mainly because it magnifies the body part. OFD increases, magnification increases OFD increases, recorded detail decreases

Magnification and OFD When the object is close to the film, magnification is minimal When OID is introduced, magnification increases

Decreasing Magnification Increasing FFD, decreases magnification. Target film distance (TFD) is the same as FFD and SID.

Will SID get rid of Magnification from OID? Increasing the SID will not get rid of magnification totally. OFD has a more pronounced effect on magnification then FFD, SID and TFD.

FOD FOD = focal object distance To find the FOD on an exam, calculate the following: FOD=FFD-OFD FOD=SOD=TOD Focal object distance; Source object distance; target to object distance

Magnification Calculations magnification = __SID____ SOD Example problem 3” OFD TFD=40” How much will the film be magnified?

Magnification magnification = __SID SOD magnification = __40 37 Image will be magnified by 8%

Example Problems Calculate the percent magnification for the following problems 5 inches of OID; TFD= 72 inches 2 inches of OFD; FFD = 40 inches 4 inches of OID; SID = 37 inches

Critical Thinking Calculate the magnification for the following problem: 4 inches of OID, FOD of 36 inches