# Technical Factors or Prime Factors

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Technical Factors or Prime Factors
Bushong Ch 15

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

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

PRIME FACTORS KVP MAS DISTANCE

kVp Kilovolts controls how fast the electrons are sent across the tube
Controls, quality, penetrability & contrast Increasing kVp also increases scattered photons reducing image quality Does kVp influence OD?

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

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

Distance Affects exposure of the IR because of the Inverse Square Law
SID largely determines the intensity of photons at the IR Distance has no effect on radiation quality

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

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

Focal-Spot 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

4% kVp Changes

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

Some pathology is destructive causing tissue to be radiolucent Others can be additive causing tissue to be radiopaque

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

Image Quality Bushong Ch. 16

Objectives Image Quality – Factors Geometric Factors Subject Factors
Artifacts

Image Quality Is the exactness of the representation of the patient’s anatomy 3 major factors affecting image quality that is under the control of the technologist: Image Receptor selection/use, Geometric factors & Subject factors.

Judging Image Quality The most important characteristic of radiographic quality are: Spatial Resolution, Contrast Resolution, Noise & Artifacts

Main Factors Affecting Recorded Detail
kVp & mAs Motion Object Unsharpness SID (Source to Image Distance) OID (Object to Image Distance) Material Unsharpness/ Film Screen Combo Focal Spot Size MTF (modulation transfer function)

Recorded Detail Other names: - detail -sharpness of detail -definition
-resolution -degree of noise - visibility of detail

Resolution Is the ability to image two separate objects and visually distinguish one from the other. Spatial resolution is the ability to image small objects that have high subject contrast. Ex: bone-soft tissue interface, breast calcifications, calcified lung nodule Conventional radiography has excellent spatial resolution

RESOLUTION TEST TOOLS LINE PAIRS/ MM Depicts how well you can see the differences in structures More lines=more detail

Measuring Resolution for an x-ray imaging system

SMPTE Test Pattern In 1985 the Society of Motion Picture and Television Engineers (SMPTE) published a recommended practice (RP-122). Specifications for Medical Diagnostic Imaging Test Patterns for Television Monitors and Hard-copy Cameras.

SMPTE Test Pattern

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

Modulation Transfer Function
The ability of a system to record available spatial frequencies. The sum of the components in a recording system cannot be greater than the system as a whole. When any component’s function is compromised because of some type of interference, the overall quality of the system is affected.

Contrast Resolution Is the ability to distinguish anatomic structures of similar subject contrast. Ex: liver-spleen, gray matter-white matter Magnetic Resonance Imaging has the highest contrast resolution Computed Tomography is excellent as well

MRI CT

The contrast of an object is expressed relative to its surrounding background.  That is what determines its visibility.

Is the product of image receptor contrast and subject contrast

“Noise” Borrowed from electrical engineering
Audio noise = hum or flutter heard from a stereo Video noise = “snow” on a TV Radiographic noise = random fluctuation on the OD of the image

QUANTUM MOTTLE Not enough PHOTONS – can create a mottled or grainy image - MORE COMMON IN CR SYSTEMS

Film graininess, structure mottle, quantum mottle & scatter radiation

Radiographic Noise Film graininess – distribution & size of the silver halide grains in the emulsion Structure mottle – speed of the intensifying screen. Phosphor size & DQE/CE Not under the control of the technologist

Image Noise Speckled background on the image
Caused when fast screens and high kVp techniques are used. Noise reduces image contrast The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE) The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)

Quantum Mottle An image produced with just a few x-rays will have more quantum mottle. The use of very fast intensifying screens or not enough mAs or kVp will increase quantum mottle

Quantum mottle

Screen Speed Efficiency of a screen in converting x-rays to light is Screen Speed.

Speed Fast image receptors Slow image receptors
? Noise ? Spatial resolution ? Contrast resolution Slow image receptors See pg 274 for answers

Speed Low noise = fast or slow speed?
High contrast resolution = fast or slow speed? Reduced spatial resolution = fast or slow speed?

PARALLAX – each emulsion has an image single image overlaped edges edge sharp less sharp

Other Film Factors Characteristic Curve
Is used to describe the relationship between OD and radiation exposure

What is the useful OD range?

The latitude of an image receptor is the exposure range over which it responds with diagnostically useful OD. Depending on the manufacturing characteristics radiographic film will respond differently to radiation exposure

F/S vs Digital Dynamic Range

Unexposed film Appears like a frosted glass window
ODs of unexposed film are due to base density and fog density Base density – tint added to the base to reduce eye strain and crossover. Has a densitometer value of approximately 0.1

CROSSOVER Reducing crossover by adding a dye to the base

Unexposed film Fog Density – inadvertent exposure of film during storage, chemical contamination, improper processing, radiation exposure, etc. Fog density contributes to reduction of radiographic contrast Fog density should not exceed 0.1 Base + fog OD = 0.1 to 0.3

2 principal characteristics of any image are Spatial & Contrast Resolution
Spatial resolution Resolution is the ability to image two separate objects and visually distinguish one from the other Spatial resolution is the ability to image small objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung nodules) Determined by focal-spot size and other factors that contribute to blur Diagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm.

Other factors affecting the finished radiograph
The concentration of processing chemicals The degree of chemistry agitation during development Development time Development temperature

Image Quality in Digital
Matrix size is determined by . . . Receptor size (Field of View: FOV) Pixel size CR - Sampling frequency DR - DEL size

Spatial Resolution determined by:
􀁹 Pixel size. CR- sampling frequency DR – DEL size 􀁹 There are relationships between Pixel size Receptor size Matrix size 􀁹 pixel size = larger matrix 􀁹 receptor size = larger matrix Spatial resolution is not related the amount of exposure

Sampling Frequency The sampling frequency is the rate at
which the laser extracts the image data from the plate.

Signal Sampling Frequency Good sampling under sampling

Nyquist Frequency The Nyquist Frequency will be ½ of the
sampling frequency. A plate that is scanned using a sampling frequency of 10 pixels per millimeter would not be able to demonstrate more than 5 line pairs per millimeter based upon the Nyquist Frequency. The Nyquist Frequency allows the determination of the spatial resolution for a given sampling frequency.

Geometric Factors Producing high quality radiographs. Technologists must maximize geometric conditions Three principal geometric conditions affect radiographic quality: Magnification, Distortion & Focal-spot blur. Review table 16-4, pg. 295 for summary

Object Unsharpness Main problem is trying to image a 3-D object on a 2-D film. Human body is not straight edges and sharp angles. We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID

Magnification All image on the radiograph are larger than the object they represent. For most exams minimizing magnification is desired. There are a few exams where some magnification can be helpful TUBE CLOSE TO THE PART (SID), PART FAR FROM THE CASSETTE (OID)

Minimizing Magnification
Large SID: use as large a source-to-image receptor distance as possible Small OID: place the object as close to the Image receptor as possible In terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.

Size Distortion & SID Major influences: SID & OID
As SID , magnification  Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present Must note deviations from standard SID

The position of the tube (SID) to IR Will influence how the structures appear on the image The farther away – the less magnified ↑SID ↓ MAGNIFICATION

SID Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy” -On a radiograph called Penumbra Penumbra (fuzziness) obscures true border – umbra Farther the flashlight from object = sharper borders. Same with radiography.

OID Object to Image Distance
The closer the object to the film, the sharper the detail. OID , penumbra , sharpness  OID , penumbra , sharpness  Structures located deep in the body, radiographer must know how to position to get the object closest to the film.

Size Distortion & OID If source is kept constant, OID will affect magnification As OID , magnification  The farther the object is from the film, the more magnification

The position of the structure in the body will influence how magnified it will be seen on the image
The farther away – the more magnified

Minimal magnification small OID
Magnification - large OID

40” SID VS 72” SID

Magnification Factor MF = source-to-object distance SID MF = SOD
source-to-image receptor distance MF = source-to-object distance SOD difficult to measure accurately *usually an estimated value SID MF = SOD

Finding SOD SID – OID = SOD SID = 100 cm OID = 7 cm What is the SOD?

Finding magnification of the heart on a lateral CXR
SID – OID = SOD SID = 72 inches OID = 8 inches (estimated) What is the SOD? What is the Mag Factor?

Distortion Misrepresentation of the true size or shape of an object
-MAGNIFICATION (size distortion) -TRUE DISTORTION (shape distortion) Shape distortion: unequal magnification of different portions of the same object

Shape Distortion Depends on: Object thickness Object position
Object shape

Object Thickness Thick objects have more OID and are more distorted than thinner structures

Object Position If the object plane and the image plane are parallel, the image is not distorted CR perpendicular to the part

Position Distortion Foreshortened = anatomy at an incline to the CR displays smaller than true size

D & E = shape distortion (foreshortening of part)

Position Distortion Elongation: anatomy at an incline and lateral to the central axis Could be foreshortened as well

Elongation Foreshortened Normal

Position Distortion Spatial distortion = anatomy positioned at various OIDs but superimposed, only one can be seen

Position Distortion – Irregular Anatomy
Anatomy or objects can cause considerable distortion when imaged off the central axis

Focal-Spot Blur Dependent on the size of the effective focal spot
Smaller the effective focal spot = less blur and better spatial resolution Focal-spot blur is the most important factor in determining spatial resolution

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

ANODE ANODE

Focal spot size – determined by filament in cathode & surface area used at anode

THE SMALLER THE BEAM TOWARDS THE PATIENT - THE BETTER THE DETAIL OF THE IMAGE PRODUCED

Focal-spot blur is caused by the effective size of the focal spot, which is larger to the cathode side of the image.

Focal-spot blur is small when the object-to-image receptor distance (OID) is small.

Image Quality Subject Factors
Patient thickness Effective atomic number Object shape Subject contrast Tissue mass density kVp

Object shape Objects with structure having a form that
coincides with the x-ray beam has maximum detail

Patient Motion Can be voluntary or involuntary
Best controlled by short exposure times Use of careful instructions to the pt. Suspension of pt. respiration Immobilization devices

Blurring of image due to patient movement during exposure.

Questions?