Producing An Image RVT: Chapter 6

Learning Objectives: Chapter 6 Understand the 4 factors of radiographic exposure and how each impacts the production of a diagnostic image. Understand how milliamperage (quantity) and kilovoltage (quality) of x-rays can impact a radiograph. Define scatter radiation and understand its impact on radiographic quality and radiation exposure of personnel. Define the 15% rule, and be able to use it to fine- tune a radiographic image. Understand how to create and use a technique chart to produce diagnostic radiographs.

The Four Factors of Radiographic Exposure These must be manipulated so tissue absorption of radiation is exactly correct to demonstrate anatomy/pathology and minimize artifacts on the image. The 4 factors: Kilovoltage (kV) – “quality” or contrast Milliamperes (mA) – “quantity” or darkness Time (seconds) – exposure time Distance

1 st Factor of Radiographic Exposure: Distance The Inverse Square Law The intensity of radiation at a location is inversely proportional to the square of its distance from the radiation source. The distance between the x- ray tube and image receptor is fixed at 40 inches.

2 nd Factor : Kilovoltage Kilovoltage Peak (kVp) - the maximum value of x-ray tube voltage during x-ray production This is relating to the power The energy at which the x-rays will penetrate the body Need this to be as low as possible to decrease ______________ radiation Directly affects the contrast in the anatomy More contrast = More black and white Fewer shades of gray in-between Can distinguish between structures easier *Can impact density too (kVp too high will darken film)

Optimizing Kilovoltage Range on the generator: kV (may vary) Use the lowest setting that will penetrate region of interest, enhance tissue contrast, and minimize scatter radiation. Extremities will have the lowest kV Abdomen and thorax views will have high kV Calculate a starting point, but be ready to adjust

Calculating Kilovoltage Sante’s Rule kV= 2 x thickness (cm) + 40 Example: Body Part = 8 cm (2 X 8) + 40 = 56 kVp

Anatomical Measuring Technical factors are determined based on the thickness of the anatomy being radiographed Measure using calipers Unit must be in centimeters Measure the animal in the position they will be in during the radiograph!

Sante’s Rule: Why 40? Represents the distance from the x-ray tube focal spot to the image receptor (film) In inches This distance can be referred to as the Focal Film Distance (FFD) or Source Image Distance (SID).

Optimizing Kilovoltage The 15% Rule Used to optimize kV’s and enhance contrast Doesn’t impact density To increase penetration – increase kV’s 15% To decrease penetration – decrease kV’s by 15% Must also adjust mA if you change kVp: Increasing kV = Divide mA by 2 Decreasing kV = Multiply mA by 2

Adjusting kVp’s Not enough ContrastOptimized Contrast

What is Scatter Radiation? “Secondary radiation” Lower-energy x-ray photons that have undergone a change in direction after interacting with structures in a patient’s body Is of concern because: 1. Decreases image quality 2. Increases radiation exposure The primary source of exposure for technicians! 3. Darkens radiograph & decreases contrast

Managing Scatter Radiation Is directly impacted by increases in: Kilovoltage Size of the field Can be managed by: Reducing kVp’s as low as possible Correctly collimating Avoiding retakes Use of a grid

3 rd Factor : Milliamperage mA Quantity of x-rays produced with each exposure Impacts density (darkness) Directly proportional Doubling mA doubles density 14

Advantages of High mA Allows for examination of thicker anatomic areas Allows for shorter exposure time setting with the same number of x-rays produced = 1. Possibility of motion is decreased 2. Decreases exposure for restraining personnel

Technical Factors of Exposure Analogy Imagine x-ray photons as pool balls: kVp = Power/energy that the cue hits the white ball Ma = Number of balls on the table Energy is transferred as balls bounce off each other and the table

4 th Factor : Time Adds in time factor mA X seconds = mAs Suitable mA setting depends on the thickness and type of tissue being radiographed mA & time are inversely proportional Often combined settings on the x-ray machine Changes in mAs: Increased = x-ray becomes blacker in color overall Decreased = x-ray becomes lighter in color overall

mAs: Too High & Too low

Troubleshooting the Technical Factors If image isn’t diagnostic: Reposition & re-measure Adjust mA’s first, as long as tissue is penetrated An easy change to make & measure Adjust kVp’s using 15% rule If film is still light, check temperature of chemicals If image is dull/gray, look for light leaks in darkroom or the cassette Check for improperly exposed or expired film in box Have the unit serviced & recalibrated

Anatomic Considerations Skull & Cervical Spine High contrast bone & tissue densities already Higher kVp not required Chest, thorax, abdomen, lumbar spine, pelvis Thickened areas with similar densities, so scatter likely if high kVp’s used Keep KVp as low as possible and increase mAs Extremities Body parts thin, and tissue-to-bone ratio high Low kVp’s indicated Birds & pocket pets (guinea pigs, ferrets, etc) Similar technical factors to extremities

Evaluation of Radiographic Technique Two basic questions… Is the film too light or too dark? More exposure = blacker film overall Increase mAs to darken Decrease mAs to lighten Is there proper penetration/differentiation? If cannot see contrast between structures, adjust kVp’s Increase kVp to darken Decrease kVp to lighten

What Determines Adequate Penetration? Abdominal radiograph: Can see outlines of liver, spleen, kidneys and bowel Thoracic: Heart clearly outlined, diaphragm boundary evident, bone differentiation clear Extremities: Bones white, soft tissue can easy be distinguished Inadequate penetration = areas are the same shade of grey, and organs/bones cannot distinguished from one another

Radiograph Too Dark? If bone tissue is gray, with too little contrast between the bone and adjacent soft tissue, there was too much penetration. Decrease kVp by 10-15% If bone tissue is relatively white, compared to surrounding tissues, then penetration is adequate. Decrease mAs 30-50% *Evaluating radiographs is an art, and often several changes can be made to improve the radiograph equally*

Radiograph Too Light? Is the film under penetrated ? If no: Increase mAs 30-50% If yes: Increase the kVp 10-15% Is the film over penetrated? If yes: Decrease kVp by 10-15% If no: Decrease mAs by 30-50%.

Exposure Factors & the X-ray Tube X-ray generation: mAs (current) is applied to the filament in the cathode. mAs control the quantity of x-rays Generates an electron cloud The electrons are directed to the anode target by kVp’s. kVp’s control the quality/penetration of x-rays The collision produces heat and x-radiation.

Viewing a Radiograph Viewed on an evenly lit view box in a semi- darkened room. View box should be clean, and all light bulbs should be in working order.

Positioning the Radiograph Film position on the illuminator matters: V/D or D/V: _________ region at the top Handshaking position Lateral: Head at viewer’s _______ Spine on top Limbs: ___________ end up

Radiography Log Book Record of every radiograph taken Must include: Patient’s name/identifiers Measurement View taken Technical factors kVp, mAs