Grids and Acquisition

Purpose of grids

improve contrast by absorbing scatter

Grid utilization criteria

irradiated water volume (structure content and thickness)

Why do we use a grid on parts greater than 10cm

increasing part thickness increases scatter

Air Gap technique 

intentionally increasing OID to keep some scatter from reaching IR 

Air gap technique ___ spatial resolution?sharpness

decreases

name the parts of a grid major disadvantages of grids

cut off and increased patient exposure 

Where does a grid sit 

between structure and IR

A grid transmits

60% of primary beam

Stationary grid disadvantage

visible lines on image

how does the bucky device remove grid lines

grid moves when you expose

What should be printed on the front of each grid

ratio, frequency, focal range, tube side 

strips on grids are made of

lead

interspace material on grid is made of

aluminum and carbon fiber

core of grid encased in

radiolucent casing

three grid dimensions 

strip height, strip thickness, interspace width

Typical ratios

5:1, 6:1, 8:1, 10:1, 12:1, 16:1

Grid ratio represents

strip height: interspace width (distance)

Grid frequency

number of lead strips per cm or inch

typical grid frequencies

60-180 per inch

Grid Ratio impacts

lead content, transmittance angle, alignment accuracy

How does grid ratio impact transmittance angle

larger ratio + smaller angle= removes more scatter and primary beam

why can a large grid ratio be bad

greater chance for misalignment

higher grid ratio

higher lead amount

Higher grid frequency

smaller lead amount

grids with high frequencies

remove less scatter, decrease contrast, decrease pt exposure 

overall lead content is dependent upon

grid ratio and grid frequency

bucky factor for non-grid

1

bucky factor for 5:1

2

bucky factor for 6:1

3

bucky factor for 8:1

4

bucky factor for 10:1

5

bucky factor for 12:1

5

bucky factor for 16:1

6

grid formula

mAs₁/mAs₂ = BF₁/BF₂

You can’t angle tube with what grid pattern

crossed/ cross hatch

grid patterns

linear and crossed

grid configurations

parallel or focused

focused grid configuration

lines angled to match beam divergence

convergence point of focused grids

ideal SID

Focused grid: Focal range

specified distance above and below convergence point, allowable SID

Focused grid: grid focus

Focus grid: “tube side”

which side goes up 

Grid cut off types

off-focus, off-level, off-center, inverted

Off focus cut off

violation of focal range (equal cut off on each side)

Off-level grid cut off

central ray angled or tube tilted (unequal cut off)

off-center cut off

CR not centered to grid )(DTENT) (one side cut off)

Inversion cut off

grid in upside down(sever peripheral decrease in quality)

Grid cut off is most likely to occur in

mobile imaging and cross table laterals

long dimension (LD) grid

lead strips run along along longest axis

Short dimension(SD) grids

lead strips run across the short axis

LD grids can result in

off-level or off-center cut off

Moire Pattern

visible wavy grid lines on image

Moire Pattern may occur

with SD with PSP system

function of digital detectors

capture remnant beam

types of digital detecters

cassette-based and cassetteless

classifications of digital detecters

indirect and direct

Indirect Digital detecters processing

x-ray → light → electrons

direct digital detector processing

x-rays → electrons

PSP is what classification of detecter

indirect

PSP layers

protective, phosphor active, support

PSP active layer

barium fluorohalide and europium

Pupose of europium in PSP active layer

to trap electrons so we dont lose image data

PSP Reader laser does what

scans plate, excites e-, for light

PMT stands for 

photomultiplier tube

PSP Slow scan reader

plate travels into scanning system and moved along roller system

PSP fast scan reader

laser, always scans perpendicular to direction of plate travel 

what determines PSP pixel size 

sampling frequency of the laser 

PSP reader pathway

laser scans, electrons released, light emitted, light guide, PMT, electrons

Erasure lamp purpose for PSP

uses fluorescent light to erase remnant trapped electrons

Not using an Erasure lamp on PSP can cause

ghost images, low contrast images

Why would we erase a PSP plate without taking an image first 

plate grossly overexposed, plate not sued for 48 hours 

TFT

thin film transmitter

DEL

detecter element

electrical components of FPD

image data and storage release 

Active components of FPD

image acquisition

Direct FPD Active layer

amorphous selenium

Electrical components of FPD

capacitors (Store e-) TFT (signal readout)

Indirect FPD scintillator made of

cesium iodide or gadolinium oxysulfide

indirect FPD scintillator does

converts e- to light

Active layer of indirect FPD

amorphous Silicon 

cesium oxide shape in indirect FPD

columnar/ structured

Gadolinium Oxysulfide shape in indirect FPD

turbid/unstructured

FPD image extraction

by TFT: data extracted from each DEL in a row and column manner

CCD

charge coupled device 

CCD classification

indirect 

CCD scintillator types

lens-coupled, fiberoptic-coupled

CCD chips are

Pixel

Pixel on CCD chip 

active area and electrical components

CCD active layer

silicon

electrical component of CCD

polysilicon

CCD image extraction

Bucket brigade: succeeding rows follow simultaneously 

CMOS

complementary metal-oxide semiconductor

CMOS classification

indirect

CMOS uses ___ power than CCD

less

Data formats

analog and digital

analog data form

sinusoidal wave

digital data form

discrete: for processing, manipulation, and storage 

Data conversion

sent to ACD or A/D

ACD

analog to digital converter

Steps of ACD

sampling and quantization