physics test 2

detectors in digital imaging - advantage

its post processing abilities

once you take the image you are able to edit and change it - contrast and brightness editing

film digitiser

acquire image on a film and use the CCD camera or a scanner for convert that into a digital format

better because if you keep film for a long time it can be spoiled

computed radiography

initially, there is a storage phosphor imaging plate

when it is exposed to xrays it will excite the electrons and they will move up in energy shells

then to read that image you expose the plate to a laser which will emmit visible light

direct radiography using CCD

phosphor imaging plate where xray is converted into light

using lenses, the light will be foccussed onto a CCD camera making a digital image

direct radiography using CCD camera - disadvantages

can cause more scatter = decreased spatial resolution

DR flat pannel - indirect

use a TFT array

use a cesium iodide phosphor where xray is converted into light

then will be converted into electrons using the photodiode and the TFT reads that charge at each point

DR flat pannel - indirect

not the best spatial resolution

DR flat pannel - direct

xray is converted into charge using a photoconductor

no intermediate step which reduces any spatial resolution issues

when pixel size is more

there is a less number of pixels

spatial resolution decreases

higher number of pixels advantage

better reslution and better quality

each pixel corresponds to

a certain pixel detector element

other than pixel size, what can spatial resolution depend on

detector performance

focal spot size

scatter

the pixel is an integer whose value is proportional to what

its brightness

how can pixels be displayed

showing different shades of gray

higher number of bits/pixels advantage

better contrast resolution

gives more information to be analysed

dead pixels correction

in every detector system certain detector elements will not work

if you expose the detectors for too long you can damage them

what do the dead pixels cause?

artefacts

how to artefacts appear on the image

appear as a dark (or white) spot on the image

how do we correct the artefacts

the gray scale in adjacent pixels is averaged

that value replaces the value of the dead pixel

dark noise correction (dark feild correction)

electronic noise is associated with each detector element in the absence of xrays

noise coming from the detector elements

this results in finite gray scale even without any exposure

how do we correct dark noise/field

a large number of dark images are acquired and averaged over

this dark image is subtracted from the raw image produced by the detector array

flat field cause

there are subtle differences between the sensitivity of each of the elements

also, the gain associated with with amplifiers connected to individual elements varies between detectors

all receptors react slightly differently to eachother

how do we correct flat field artefacts

a flat field (uniform field) is corrected and averaged

this information is used to compensate for the different gains and sensitivity for each of the detector images

after flat field correction, a uniform signal will create a uniform output

these corrections are applied automatically by the detector system before the image is displayed

why is gray scale chosen with window width and level

ability to encompass all pixel values

window level

the midpoint of the pixels being displayed

why is window level important

we have to choose a certain range of the pixel values to be displayed

what does windowing and levelling improve in images

imrpoves contrast

Windowing allows us to focus on a specific range of pixel values, making a small difference in that range easier to see

• changing window level changes the pixel range to be displayed

• also, changes the brightness of the image (darker or brighter)

if we want to enhance the contrast difference in the brighter portion, we may select a window from 150 to 255 - what number of pixels will be displayed as dark?

all of the pixels with values under 150

if we want to enhance the contrast difference in the darker portion, we may select a window from 0-100

the pixel values from 101-255 will be displayed as white

what does a smaller window width mean

much higher contrast

how to choose window level

• Low WL (-600) = 600 lung

• Medium WL (40) - soft tissue

• High WL (300-600) - bone

Lung visualisation (lung window)

the lung contains very low densities (air) and higher densities such as vessels and airway walls

wide range of attenuation values

a large window width is therefore required to visualise all structures

Mediastinal (soft tissue)

mediastinum contains lots of soft tissue structures (heart, etc)

all structures have similar attenuation

a smaller window width is therefore needed to increase the contrast on that image

linear LUT

poor contrast

default modality LUT

a small range of pixel values are chosen to display

value of interest LUT

encompasses the full range of values and displays the output according to the displays capabilities

inverted LUT

inverts image contrast

high contrast LUT

very narrow window is selected

convolution

• manipulation of digital images often involves a mathematical operation known as convolution

• an image is convolved with a kernel (also known as a filter mask)

blurred (smooth image)

reduces quantum noise in low dose imaging, improving visibility of large anatomical structures

often applied before edge detection of image segmentation to remove noise and avoid false edges (pre-processing step)

edge enhanced image

enhances edges of structures, helping to visualise bone margins, fractures, or vessel boundaries better

useful in interventional imaging to improve visibility of catheters, guidewires, or stents

harmonised image

to imrpove visibility of both fine structure and soft tissue contrast

why is contrast media needed in radiography

if two organs have similar densities and average atomic numbers it is not possible to distinguish them on a radiograph

contrast resolution atomic number

they have a high atomic number so that blood vessels can be attenuated less and will show up brighter on a radiograph

alters the atomic number of the structure to be radiographed

digital subtraction angiography

mainly used to visualise vessels in a boney or soft tissue envinroment

a digital radiographic image of the patients anaotmy is acquired - mask image (pre contrast image)

process of DSA

after the mask image is taken a radio opaque contrast agent is injected

a series of images are taken during and after the agent has been injected

the mask image is subtracted from the image with the contrast agent

what does the subtracted image do

provides details of anatomy - where background anatomy is subtracted out

enhances the contrast and improve blood vessel visibility

disadvantage in DSA

due to the exponential attenuation of radiation into the matter, if two images are directly subtracted from one another, the final images will contain artefacts

how do we counteract artefact in DSA

the raw image data are logarhythmically processed by computing the natural logarhythm of the transmitted intensities before subtraction

road mapping

a software and video enhanced variant of the last frame hold feature

useful for placement of catheters and wires in complex and small vasculature

a DSA sequence is performed and the frame with maximum vessel opacification is identified

this frame becomes the road map mask

what do we do with the road map mask

it is subtracted from subsequent live fluoroscopy images

this produces real time subtracted fluoroscopic images layed on a static image of blood vessels

why is road mapping advantageous

provides a good idea about the path of the vessels and facillitates maneouvering through the vasculature

dual energy subtraction - dual energy chest radiography advantage

reduces anatomical noise

exploites the difference in atomic numbers of bone and soft tissue

requirement of dial energy subtraction

requires two images - one at low energy and high energy

• Two xray exposures at different kVp (two- pulse)

• Or done simultaneously using a sandwich detector (low dos

dual energy subtraction technique after image is taken

two imaged are weighted to logarhythmic subtraction

• Mathematically, ln(Ihigh) - R ln(Ilow); where R is a constant used to change the weighting

by adjusting R, the soft tissue points or bones can be viewed clearly

we subtract the low energy one from the high energy one

photoelectric absorption

the attenuation curves have energy dependance as well as Z dependance

• Photoelectric absorption a Z3

• The higher the atomic number, the higher the photoelectric absorption

• Photoelectric absorption a 1/E3

• PE absorption reduces with energy

how are we able to do dual energy subtraction

because of the different energy dependecy in different tissue types, the aqcuisition of two radiographic images at two different energy levels, enables either the bone component or the soft tissue component to be removed by prost processing