Exposure Factors or Prime Factors

What is “technique” ? How does it affect the “image” PRIME FACTORS What is “technique” ? How does it affect the “image”

Objectives Identify Exposure or Prime Factors. Describe how Prime Factors affect image quality Identify other factors to consider to receive best image possible.

Terms and Abbreviations kVp – Kilovolt Peak (controls “quality” of X-ray) mAs – milliAmpere second (controls “quantity” of X-ray) SID/FFD – Source to Image Distance/Focal Film Distance (Distance from X-ray tube to film) OID/SFD – Object to Image Distance/Subject to Film Distance (Distance from part to be imaged and film) IR – Image Receptor (Film) OD – Optical Density (darkness of the film)

Exposure Factors – 3 or 4 The four prime exposure factors are: Voltage = kVp* Current = mA* Exposure time = seconds or fractions of a sec* Source-to-image distance = SID (FFD)

PRIME FACTORS KVP MAS DISTANCE

kVp Kilovolts controls how fast the electrons are sent across the tube kVp controls energy (quality) of X-ray beam Controls, quality, penetrability & contrast Increasing kVp also increases scattered photons reducing image quality Does kVp influence OD?

“SHORT” VS “LONG” SCALE

kVp Low kVp (50 – 60) Short scale High contrast “Bone work”

kVp High kVp (90 – 120) Long scale Low contrast “Chest images”

mA Determines the number of photons, radiation quantity, OD & patient dose Changing mA does not change the kinetic energy of e- Available mA stations are usually 50, 100, 200, 300, 400 & 600 (Focal Spot Size)

Exposure Time mA X s = mAs mAs controls OD Should be kept as short as possible, for most examinations. To minimize the risk of patient motion mA X s = mAs mAs controls OD mAs determines the number of photons in the primary beam

Focal Spot Size Smaller x-ray beam width will produce a sharper image. Fine detail = small focal spot (i.e. small bones) General radiography uses large focal spot Beam from penlight size flashlight vs. flood light beam

Focal spot size of the cathode

Line-focus principle

Penumbra Edge “unsharpness”

Focal-Spot Changes - Penumbra

Distance Affects exposure of the IR because of the Inverse Square Law SID (FFD) largely determines the intensity of photons at the IR (Film) Distance has no effect on radiation quality but does affect film quality

INTENSITY IS SPREAD OUT…

Inverse Square Law Formula Distance #2 - Squared Intensity #1 Distance #1 - Squared Intensity #2

SID Changes

Direct Square Law New mAs = New distance 2 Old mAs Old distance 2

Producing optimal radiographs … key for diagnosing disease

How much of the radiation received by the patient Actually reaches the IR ?

About 1%

Creating the IMAGE When x-rays pass through a patient's body, three things can happen: (1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film; (2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; or (3) the x-ray photon is scattered and reaches the film causing an overall gray fog.

IMAGES DENSITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

mAs Changes at least 20 - 30 % mas change needed to see a visible change in density

mAs DOUBLED = DENSITY DOUBLED

+ 25% + 50 % mas

mAs Changes

Tube voltage (kVp) Determines the maximum energy in the beam spectrum and affects the quality of the output spectrum Efficiency of x-ray production is directly related to tube voltage

Influencing factors: kVp 15% rule: 15% kVp = doubling of exposure to the film  15% kVp = halving of exposure to the film 15% rule will always change the contrast of the image because kV is the primary method of changing image contrast. Remember : 15% change ( ) KVP has the same effect as doubling or ½ the MAS on density

kVp Changes The kVp setting must be changed by at least 4% to produce visual changes an image

kVp Changes

+ 15% kvp - 15% kvp

kVp Changes

4% kVp Changes

Determining Radiographic Technique The Patient Factor The most difficult task for technologists… evaluating your patient The patient size, shape, and physical condition greatly influences the technique selection

4 general states of body habitus

Radiographic Technique Technique charts are based on the “average patient” The thicker the part the more x-radiation is required to penetrate. Calipers should be used Keep in mind not only the measurement but the type of tissue you need to penetrate (fat vs muscle)

Technique In general, Soft tissue = low kVp and high mAs Extremity (soft tissue & bone) = low kVp Chest (high subject contrast) = high kVp Abdomen (low subject contrast) = middle kVp

Low Subject Contrast

OD = amount of black on image

Film Screen Overexposed Referring to a radiograph that is too dark because too much x-radiation reached the image receptor Underexposed Referring to a radiograph that is too light because too little x-radiation reached the image receptor

Technique - Pathology Pathology can severely affect the technologist technique selection Always question your patients about health status If prior images are available…check them!

Pathology Can appear with increased radiolucency or radiopacity Some pathology is destructive causing tissue to be radiolucent Others can be additive causing tissue to be radiopaque

OD vs tissue attenuation

Technique selection – Fixed kVp For each anatomic part there is an optimum kVp mAs is varied based on part thickness or pathological condition

QUESTIONS ABOUT TECHNIQUE ?