Equip Oper/Main Test 2: Prime Factors Rad Technique

Four prime exposure factors

1. Voltage = kVp

2. Current = mA

3. Exposure time = secs or fraction of secs

4. Distance = SID, OID, SOD

Kilovoltage peak (kVp)

controls how fast electrons are send across the tube aka tube potential; increasing kVp increases scattered photons reducing image quality; most kVp settings are set per thickness of bone on technique charts; controls details such as

1. Beam quality

2. Beam energy: direct relationship so increase in kVp = increase in energy

3. Beam penetrability: direct relationship so increase in kVp = increase in penetrability

4. Subject contrast (not digital image contrast)

Contrast - Image vs. Subject

1. Subject contrast = is the degree of density difference between 2 areas on a radiograph which makes it easier to distinguish areas; ex/ liver vs. lung; described as Gray Scales:

• Short scale of contrast = high contrast (less grays)

• Long scale of contrast = low contrast (more gray tones)

1. Digital image contrast = controlled by look up table (LUT) which is a processing algorithm built into the equipment that the tech does not control

Scale of contrast - Short scale (PENGUIN)

• high contrast 

• few shades of gray, mostly black/white

• low kVp exams penetrate fewer thickness + only a few steps between black/white (50-70 kVp)

• low kVp demonstrates fine bone markings/fractures better than higher kVp (extremity work) = small focal spot

• Low kVp settings use = long wavelength, lower energy, lower penetrating ability, less chance of scatter rad

• Anatomy = hand

Scale of contrast - Long Scale (ELEPHANT)

• low contrast

• many shades of gray in between black/white

• high kVp exams penetrate thicker/more dense areas + also used for contrast (barium) (110 kVp) = large focal spot

• High kVp settings use = short wavelength, higher energy, higher penetrating ability, higher chance of scatter rad (more k’s more grays) which decreases image quality and increase pt/tech dose so not ideal

• Anatomy = abdomen

Milliamperage - mA

measurement of x-ray tube current which is the number of electrons crossing the x-ray tube from cathode to anode per sec (mA x s); increasing mA leads to increase in filament temp. increasing electrons boiled off from thermionic emission; determines number of x-ray photons; controls details such as:

1. Beam quantity: direct relationship, increase mA = increase # of x-ray photons

2. Receptor exposure: direct relationship, increase mA = increase in receptor exposure

3. Patient dose: direct relationship, increase mA = increase in pt dose

Exposure time

time/length of exposure in secs or fractions of secs (millisec. 1000, move the decimal three times left to convert to seconds)

keep as short as possible for most exams to minimize rad. exposure + motion which creates blurry areas/poor detail

Orthostatic breathing technique (long second exposures)

exposures made while pt. is breathing which blurs the ribs/lung markers so thoracic vertebrae is more visual 

mAs - mA x s

will vary with pt. sizes, larger pt with more body mass will require increase in mAs

mAs values will increase w/ exams performed in bucky trays since there are radiographic grids that need an increase in mAs

is the main controller in receptor exposure

Quantum mottle (noise)

aka image noise, not enough mAs resulting in an underexposed image w/ a grainy/sandy appearance

general rule is to double original mA, need to increase mA by 30% for visual change

follow exposure index guides for targets 

mAs Reciprocity Law

shows that mA and time is inversely proportional meaning they go in opposite directions so if the mA is increased, seconds need to decrease + vise versa

General rule: if mA doubled = time cut in half

if mA cut in half = time needs to be doubled

mAs determining factors

amount depends on body part, thickness of bone/surrounding tissue, body habitus, age of pt., pathology

Distance (3 types)

in terms of magnification/spacial resolution (detail) best image produced with small OID and large SID

shorter SID used for parts where less OID is possible

tablework restricted to height allowed by ceiling

when bony detail is important, short SID used

1. SID = (source to image distance) distance between x-ray tube + IR

2. OID = (object to image distance) distance between pt + IR

3. SOD = (source to object distance) distance between x-ray tube + pt

Distance and Dose - Inverse Square Law

when distance from source increased, intensity decreases

works off of concept called the inverse square law stating that the intensity of radiation is inversely proportional to the square of the distance between the x-ray tube and IR

to minimize exposure as tech stay 6 ft away

1. Ex/ if rad dose to pt was 10 millisieverts (mSv) at 100 cm, what would the dose be if distance was reduced to 76 cm?

Distance and mAs - Direct Square law

direct square law and exposure maintenance formula maintains the mAs (technique) needed to maintain receptor exposure at varying SID’s

new mAs for new SID to maintain exposure

mAs varies directly w/ the square of the distance

1. If distance increase 2x (farther away) = 40” to 73”, mAs needs to be increased by 4 times so mult by 4

2. if distance decreased by ½ (closer) = 72” to 40”, mAs needs to be decreased about 4 times do divide by 4

Technique Setting

techs use combo of settings of mAs/kVp or uses automated system called automatic exposure control (AEC); mAs is combo of mA and secs/time; when using AEC, removes option for tech to set exposure time/total mAs

Automatic Exposure Control (AEC)

devices that can assist the tech in making consistent rad images from pt to pt regardless of size/pathology

• techs select kVp, mA + appropriate AEC chambers/photocells (which determines the time)

• AEC units found in table/wall bucky (NOT in IR)

• ionization chamber will determine exposure time/terminate exposure when part is fully penetrated

• Minimum response time = 1 millisecond

AEC Advantages

less repeats, decreased repeat exposure due to underexposure, decreased pt exposure w/ less repeated images, increased dept. efficiency w/ less repeated images

AEC Ionizing Chamber Pattern (Photocells) (2)

1. 3 cell format (most common) = 2 outer cells, 1 center cell

2. 5 cell format = adds two lower cells (looks like the 5 on dice)

Ionizing chambers

interacts w/ remnant beam/exit rad before it reaches the IR; air in the chamber is ionized + electric charge that is proportional to amount of rad created 

AEC Choices

anatomy over cells + cells selected matter

1. ex/ PA chest error = if you are imaging a chest and are interested in the lungs which are less dense, if you activate center cell, the image will be overexposed since the AEC is detecting time needed for the thicker vertebrae and not the thinner lungs

2. ex/ Lateral chest AEC error = if two outer cells activated which is over the lungs and not the denser vertebrae, this will result in underexposure

Density settings (AEC)

used when the ionization chamber configuration is not producing the desired image

• Commonly -1 & +2 options available on equipment

• +1 density increases exposure time; -1 density decreases exposure time

• Ex/ if the lung fields are too dark, a decrease in density is desired even if the correct cells were used

AEC Backup timer

used to prevent overexposure to the patient if one of the aec components were to fail

• Should be set at 150-200% of the expected exposure time

• If backup timer is set too short, an underexposure will result

• Maximum allowable mAs is 600mAs

How Factor Changes affect AEC

Inverse relationship:

• mA INCREASE = AEC/exposure time DECREASES

• kVp INCREASE = AEC/exposure time DECREASES

Direct relationship:

• As SID INCREASES = AEC/ET INCREASES

• Density INCREASES = AEC/ET INCREASES

Disadvantages to AEC

• Technologists must be accurate when selecting cells

• Large amounts of fluid in body part can affect accuracy of AEC (ex. Pleural effusion)

• Prosthetic devices centered over cell can result in overexposure

• Not enough collimation allows for too much scatter to reach the IR (can result in underexposure)

Anatomic program radiography (APR)

radiographic technique chart programmed into x-ray console

machines will use part selection or an image to identify area being radiographed, then a suggested technique of kVp and mAs appear

settings can be overridden when necessary.

Exposure index/range (EI)

exposure indicator (EI number) serves as an indicator of the relative exposure used for a particular exam

Computed Radiography (CR) vs. Digital radiography (DR)

Computed Radiography (CR) = sensitivity, or S number. Has inverse relationship with technique:

• High number = underexposure (over 600)

• Low number = overexposure (under 75)

Digital radiography (DR) = EI, DEI, REX number; direct relationship with technique

• High number = too much technique

• Low number = not enough

Quantum mottle vs Saturation

Quantum mottle = underexposure from not enough mAs; Quantum noise; Image noise; Grainy appearance

Saturation = severe overexposure from technique being too high; receptor was saturated w/ photons so soft tissue differentiation lost; soft tissue looks “burnt out”

Gross Overexposure error

demonstrated as missing data points on a digital radiograph; overexposure creates condition called data droop

Focal spot size

used to control spatial resolution (detail)

most current equipment has the correct focal spot pre-selected on the console when the body part is chosen

Small focal spot size vs Large focal spot size

Small focal spot size = produce less blurring and better visibility of detail

Ex./ Extremity work, ribs

Large Focal spot size = has greater heat dissipating capacity

Ex./ Chest, abdomen

Field of view (FOV)

size of your light field, ideally only kept open to the size of the part being imaged and controlled by the collimator

• Increase beam restriction: increase collimation = decrease field of view = smaller light field

• Increase field of view: decrease collimation = larger light field

Technique charts

1. for each anatomic part there is an optimum kVp to penetrate that thickness of bone (mAs is based on part thickness or pathological condition)

• If they have more mass you need more mAs

• Weight or size cannot be the only consideration

2. each x-ray equipment will have a separate chart

• CR vs DR

• Manufacturer designs chart

• Newer equipment = less does = lower mAs values

• Anatomically programmed radiography (APR) suggests technique when you select body part

Contrast Media (barium) 15% rule

techniques will need to be adjusted for the addition of contrast media, the most common one being barium

when kVp increases that high we will need to use the 15% rule

Ex. when barium is used for an abdominal examination, the kVp must be increased to 110,109 depending on the equipment settings

15% rule

states that when the kVp increases by 15%, it is equal to doubling the exposure (or using 2x the mAs); must cut mAs in half so not to double the exposure

Upper GI abd technique (stomach filled with barium) = 10 mAs @ 109 kVp

Ex. scout abd technique = 20 mAs @ 80 kVp

Primary x-ray beam

refers to the x-rays that are produced by x-ray tube and strike the patient; made up of either characteristic or bremsstrahlung or both

Remnant beam

remaining beam after it exits the patient; made up of primary and secondary radiation

Scatter radiation

type of secondary radiation that occurs when the beam intercepts and object, causing x-rays to be scattered

Beam quality

energy of the x-ray photons

• Determines ability to penetrate

• Determined by kVp setting

Beam quantity

number of x-ray photons

• determined by mAs

Frequency

number of waves; the the energy of the x-ray - the higher the frequency

• Higher kVp = higher energy

• Higher kVp = higher frequency

Wavelength

distance between two successive peaks(tops of waves)\

• Higher kVp = shorter wavelength

• Low kVp = longer wavelength (less energy)

Areas for critique (3)

1. Anatomical positioning and accuracy (anatomy of interest, rotation/tilt, collimation, pathology demonstrated)

2. Technical factor accuracy (receptor exposure, contrast)

3. Sharpness of detail (spatial resolution, distortion, motion unsharpness)

Photographic factors vs. Geometric factors

1. Photographic factors = receptor exposure/contrast

2. Geometric factors = spacial resolution/distortion

Receptor Exposure

amount of radiation that reaches the image receptor; primarily controlled by mAs

• Secondary controllers: kVp (direct relationship), SID, beam restriction (collimation), patient size, anode heel effect, grids, filters.

• Higher mAs means higher receptor exposure

• Collection of primary and secondary radiation.

Patient Size

increase in patient size = higher absorption of the beam into the tissue

less photons will reach the IR = less receptor exposure

Collimation (2)

1. Increase collimation = decrease field of view (smaller light)

2. Decrease collimation = increase field of view (larger light field)

Digital image contrast

controlled by look-up-table (LUT) which is a processing algorithm built into equipment that tech does NOT control (not controlled by kVp)

Size distortion vs Shape distortion

1. Size distortion = magnification of a part

• Primarily controlled by OID; (OID is “key” to magnification)

• Second controller = source to image distance (SID)

• SID can be used to compensate for increase in OID (for every 1” increase in OID, a 7-8” increase in SID should be used to compensate

2. Shape distortion = shape distortion will either be elongation or foreshortening of the part

• Elongation results from angling the tube and IR; can be helpful in certain images such as calcaneus image

• Foreshortening results from angling of body part

Motion sharpness

distortion from object, tube, receptor due to movement during exposure

Voluntary motion vs. involuntary motion

1. Voluntary motion = motion patient can control; communication and clear instructions are best way to minimize

2. Involuntary motion = motion the patient cannot control; Ex./ Cardiac motion