Imaging Capstone

xray circuit

main power supply

wall outlet at 220 V

line compensator

reduce fluctuations in power source

autotransformer

self induction
single coil- variable makes small changes to voltage

mA selector

sets mA sent to filament, more heat means more xrays

what mA selector controls

heat/current for thermionic emission

step down transformer

mutual induction
more turns on primary side
increase current/amperage
Thermionic emission

rectifiers

change AC to DC

xray tube housing

lead, prevents leakage and off focus radiation emitted not aimed at pt

gas envelope

creates air-free vaccuum
prevents from corrosion and oxidation
extends tube life

cathode

negative
creates free electrons through thermionic emission
filament and focusing cup

filaments

tungsten
heat passes through heating filament emitting a cloud of electrons- space charge source-
2 dual focus

small filament

small heat capacity- small mA
higher spatial resolution
small exposures and small body parts

large filaments

can handle larger mA
lower spatial resolution- electron stream larger
for larger mA and larger body parts

focusing cup

hold filaments
focuses electron stream to small area on target increasing spatial resolution
negative to repel electrons toward anode

tungsten is good why

high atomic number, high melting point, readily dissipates heat

space charge effect

limited number of electrons that can be created at the cathode.
once cloud is full saturation, no more room for more electrons to boil off
number of electrons increases with increasing mA and time

anode

• Positive- attracts charged electrons

• Absorbs electrons and creates xray photons

• Rotating anode allow higher heat capacity and larger focal track- induction motor

anode material

tungsten and renium

rotor

turns in xray tube

stator

electromagnets inducing turn of rotor

actual focal spot

on target

effective focal spot

projected or useful focal spot

on patient

anode heel effect

Variation in beam quantity-intensity- across the x-ray field

where is intensity the lowest, why?

anode side (80% as strong)
Photons created deeper in the anode have to pass through the “heel” and lose intensity due to absorption in the target material

where is intensity the highest

cathode side (120% as strong)

what happens to uniformity as anode angle decreases

decreases

increase anode heel effect with SID

decrease the SID

why does increased SID decrease anode heel effect

Larger SID allow IR to be exposed to more of the center of the beam creating a more even exposure

increase anode heel effect with field size

increase field size

why does increased field size increase anode heel effect

Larger field size exposes IR to high and low intensity

line focus principle

Relationship between the actual focal spot on the target and the effective focal spot- toward the patient

effective focal spot size vs actual

always smaller

smaller anode angle does what

reduces the size of the effective focal spot size

Allows for best resolution while keeping the actual focal spot large to increase heat capacity

off focus radiation

Electron interactions in the tube away from the anode focal spot

These photons can leave as leakage radiation- a form of scatter

how much of the beam do off focus radiation make up

1%

how to reduce off focus in primary beam

Shuttering-collimation can reduce off-focus radiation in primary beam

off focus radiation affects spatial resolution, T/F

true, decreases

off focus radiation affects contrast resolution T/F

true, decreases

what affects heat capacity

time—- smaller = better capacity
mA— smaller = better capacity
mAs— smaller = better capacity
filament size— larger = better capacity
focal spot— larger = better capacity
anode angle— larger = better capacity
rotating anode/speed— faster = better capacity

change mAs how much to see a noticeable difference

30%

mA is
and quantity relationship

direct, tube current- amount of electrons flowing through the tube- higher mA/current= higher quantity

exposure time and quantity relationship

direct, time electrons are in the tube and being created – increase time= increase quantity

kVp and quantity relationship

direct

filtration and quantity relationship

indirect, increases average energy- increase filtration= decrease quantity

distance and quantity relationship

indirect, inverse square law- increase distance decrease quantity hitting the IR

how much change in kVp before see change in receptor exposure

10%

kVp and quality relationship

direct, increase kVp= increases quality/energy/penetration= decrease patient dose

filtration and quality relationship

increase filtration= increase quality/ average energy= decreases patient dose

brems interaction

electrons interact with nucleus of target material

The closer electron get to nucleus the more energy the electron loses= the more energy the photon has

Energy is not lost- photon takes on the energy the electron loses

what xrays always in the beam

brems bc has no minimum or maximum energy

characteristic xrays

Electron interaction with k-shell electron- ionization

Photon energy= cascade effect= binding energy of k-shell- binding energy of electron that fills the void

cascade effect

when an electron fills the void of a ionized electron

remnant radiation

attenuated beam- what is remaining in the beam after it passes through matter

attenuation and what contributes to it

reduction in intensity due to absorption and scatter in matter- transmission/penetration, absorption, scatter

what is attenuation dependent on

Part thickness- more mass, more scatter, more attenuation

Tissue density- more dense areas with more mass attenuate more

Atomic number- different structures in body attenuate differently- higher atomic number increases attenuation

Beam energy- lower energy will penetrate less and attenuate more

Pathologies- additive vs destructive

compton interaction

Most common- occurs at all energy levels- main interactions at high energy levels

Outer shell interaction

Ionization- electron knocked out is Compton electron

Remaining photon has decreased energy and scatter away in different direction

what doses does compton contribute to

occupational and pt

Ionized free electron is absorbed in tissue contributing to pt dose

scatter effect on image

Scatter adds noise/fog to image

photoelectric interaction

Main interaction at low energy levels

Inner shell interaction

Ionization- electron is removed making the atom unstable- ejected electron is photoelectron- gets absorbed by tissue and cells

Cascade effect occurs creating characteristic photon- which gets absorbed in the surrounding tissue

what does photoelectric contribute to

Increases patient dose- does not affect occupational dose- increases image quality- differential absorption/attenuation

photoelectric effect on image quality

increases

types of distortion

size and shape

shape distortion caused by

• Angle of tube

• Angle of part

• Angle of IR

• motion

shape distortion caused by

Angle of tube

Angle of part

Angle of IR

motion

CR is

cassette based

sampling frequency

CR- pixels samples per mmm as laser scans imaging plate. affects spatial resolution

steps of the reader/digitizer

Steps 1: rollers extract plate from housing

2: helium neon laser light passes though focusing lenses and reflects off a mirror to read plate in a back and forth (raster pattern) 
(left to right line by line- like we read)
3: phosphor release energy stored in the form of light photons when hit by the laser- photostimulable luminescence

4: Photomultiplier tube collects and amplifies the light photons making them brighter

5: light sent to ADC- converted to digital image

6: plate exposed to bright white fluorescent light to release and erase any remaining latent image

raster pattern

left to right line by line

photostimulable luminescence

phosphor release energy stored in the form of light

Photomultiplier tube

collects and amplifies the light photons making them brighter

indirect radiology uses

Uses a TFT or CCD

main conversion of indeirect

Converts x-ray photons to light and then to an electrical charge

Scintillation layer made of

phosphors (absorption layer)- Cesium Iodide or Gadolinium Oxysulfide

how is light emitted in indirect and why is that not ideal

xrays photons are converted into light- emitted isotropically- potential for loss of light or information decreasing spatial resolution

photodiode/photodetector

light to electrical signal

TFT- array/matrix of detector elements

Each DEL contains an active element- the pixel,
storage capacitor- stores electrical charge, and
switch- releases the charge leaving each DEL to send to computer for processing

active element in DEL

pixel

what does switch do in TFT

releases the charge leaving each DEL to send to computer for processing

CCD scintillatoin layer

coupled to a sensor chip by lens or fiber optics

CCD chip

converts light photons to electrical signal and sends to computer for processing

increase phophor size effect on image quality

decrease in resolution- loss of light/image

increase phophor thickness effect on image quality

decrease in resolution- loss of light/image

direct does what

Converts x-rays directly to electrical charge

Non-scintillation

Direct Absorbing and conversion layer

photoconductor- Amorphous selenium

how does A:Se work

High voltage charge is applied across the surface before exposure, this causes the selenium atoms to release electrons in response

FOV

diameter of the area being imaged

matrix

arrangement of squares- rows and columns- collection of pixels

pixel pitch

space between pixels- center of one to center of adjacent smaller pitch= smaller pixels

pixels

picture element- each square represents a shade of gray

pixel size equation

FOV/matrix

spatial resolution

sharpness of recorded detail-lp/mm

Ability of system to differentiate between adjacent structures

Measure of the smallest detail that can be recorded

spatial resolution measured in

lp/mm

MTF

modular transfer function- measure of the systems ability to transfer object information to the receptor

smaller FOV means

decrease in pixel size

larger matrix means

more pixels-smaller pixel size

smaller pixels means

better spatial resolution

what affects spatial resolution in digital

Pixel size

Pixel pitch

Pixel denisity

Sampling frequency-CR- pixels samples per mmm as laser scans imaging plate

Fill factor- ratio of light sensing area to size of entire dell

DQE- measure of effectiveness of imaging device to convert incident photons to electrical signal

fill factor

ratio of light sensing area to size of entire DEL

DQE

detective quantum efficiency- measure of effectiveness of imaging device to convert incident photons to electrical signal