image acquisition ch 12 REVIEW

Grids

Absorb scatter 80-90%

Use on grater than 10cm

Transmit straight X-rays

Absorbs angled/scatter rays

Grid characteristics

Grid ratio

Grid frequency

Grid material

Grid ratio

Height •/• D (width)

Higher grid ratio are more effective at reducing scatter but increase pt dose

Ratios 5:1 to 16:1 most common 10:1 - 12:1

Grid frequency

# of grid strips per cm

Thinner interspaces = higher grid ratio

High frequency show less grid lines

Grid material

Interspace = plastic fiber/aluminum - allows straight X-rays through

Lead strip - absorb angled/scatter rays

Grid cut off

Undesirable absorption of X-rays

Usually from inadequate SID

Grid types

1.Parallel - linear

2.Crossed - crosshatch

3.Focused

4.Moving

5.Virtual

Parallel- linear

Simplest

Cleans up scatter in one direction

Use short SID or large IR

Cross - crosshatched

More effective at cleaning up scatter

Better with higher kVp

Focused

Lead strips angles to match divergent beam

Use specific SID by manufacturer

Commonly used in moving grids

Focused

Lead strips angles to match divergent beam

Use specific SID by manufacturer

Commonly used in moving grids

Virtual grid

Digital IR measures energy and frequency of each photon with less scatter interactions

Reduces pt dose Ratios

Improves contrast

Eliminates artifacts

Stationary grid

Portable or x table

Usually low ratio grids

Grid error

1.Off level

2.Off center

3.Off focused

4.Upside down

Off level

Improper tube angulation

Under exposed, Uniform loss of density across image

Off center

Later decentering

Under exposed, Grid cut off on one side

Must be positioned directly under tube

Off focused

Grid cut off off one side

Improper SID

Most sensitive to alignment errors

Upside down

Severe grid cut off, very light image because entire beam was absorbed by grid

Filtration

Absorb low energy photons (soft xrays)

Inherent is glass housing and added filtration at port window

Reduces or dose

Total 2.5mm Al

Compensating filters

provide more uniform density across image come I many shapes and sizes

Made of Albor plastic

Prevents burn out of anatomy

Beam restrictions

Lower pt dose

Improve contrast

More detail

Cause of graininess

Lack of mAs

LUT/algorithm error

Excess scatter

Open FOV

Grid errors

Automatic rescaling

Fixes exposure errors too light/dark

But computer can increase mAs and lead to dose creep trying to fix images

Automatic Exposure Control AEC

Regulates amount of ionizing radiation to the body and IR

Anatomically Programmed Technique APR

Microprocessor, Preprogrammed mAs/kVp uses internal technique chart