acquisition 2

Question 1

what is the linear coefficient?

Answer 1

Describes the fraction of attenuated incident photons in a mono-energetic beam (per thickness of a material)

increasing atomic no. and increasing the physical density of the absorbing material increases the photon energy except at k-edges

Question 2

what are the possible interactions included in the linear coefficient?

Answer 2

coherent scatter

Compton scatter

photoelectric effect

Question 3

What is the image reconstruction process?

Answer 3

pre-processing and reformatting raw data
convolution = mathematical distortion of raw data
back projection - basic
FBP- filtered back projection
iterative reconstruction - much clearer - good resolution

Question 4

what is convolution?

Answer 4

fills in gaps of missing data
1 scan view (body section) - contains 2 objects

essentially digital filter applied to introduce negative terms to suppress extra voxel values (make the image look blurry/indistinct) that make up image cage.

data provided is not a complete image so back project onto image surface and shadows in missing data.
image beam has to be rotated around the body to obtain another view
project this view onto other images
see the beginning of an image
Two views don’t give high enough quality/resolution - so several hundred are used to produce a clinical image

Convolution is a mathematical concept that implies the product of two functions. In practical terms for radiology, convolution implies the application of a mathematical operation to a signal such that a different signal is produced. Convolutions are applied in image processing for CTs and MRIs.

Question 5

what is convolution?

Answer 5

fills in gaps of missing data
1 scan view (body section) - contains 2 objects

essentially digital filter applied to introduce negative terms to suppress extra voxel values (make the image look blurry/indistinct) that make up image cage.

data provided is not a complete image so back project onto image surface and shadows in missing data.
image beam has to be rotated around the body to obtain another view
project this view onto other images
see the beginning of an image
Two views don’t give high enough quality/resolution - so several hundred are used to produce a clinical image

(Convolution is a mathematical concept that implies the product of two functions. In practical terms for radiology, convolution implies the application of a mathematical operation to a signal such that a different signal is produced.)

Question 6

what is Fourier trandformation?

Answer 6

it is a primary mathematical model in the creation of computerised medical images

conversion of data into more useful/recognisable forms - using mathematical algorithms

CT specific:
created using different image densities and enhancing them to create recognisable ray scale - hounsfield units

Question 7

What is the matrix?

Answer 7

no. of voxels we can have - 512 (rows) x 512 (pixels) per voxel

edges ill-defined goes through filter and then edges are clear (can see curves)

Question 8

what is kernel?

what does it do?

Answer 8

Kernel is a convolution algorithm:

the process used to modify the frequency contents of projection data prior to back projection during image reconstruction in a CT scanner

corrects the image - blurring and sharpens image

Question 9

What is incident intensity?

Answer 9

intensity of the x-ray beam striking the patient

and it is proportional to the tube output (mAs)

Question 10

Beam attenuation?

Answer 10

beam is attenuated by the patient - depends on the tissues density

same as conventional X-RAY

Question 11

How is attenuation measured?

Answer 11

Attenuated radiation is measured by detectors situated opposite the X-ray tube

Question 12

What are essential properties of CT detectors?

Answer 12

high detection efficiency - absorbing all the emergent x-ray beams to which they’re exposed

high conversion energy - enabling all the detected energy to contribute to image formation

wide dynamic range - so that all x-ray intensities - weakest to strongest - are converted into proportional output signals

Question 13

What is attenuation coefficient?

Answer 13

measure of how easily a material can be penetrated by an incident energy beam
quantifies how much the beam is weakened by material

Question 14

what is acquisition?
spiral

Answer 14

process in which a single continuous set of spiral scan data is acquired without an intervening pause

Question 14

what is acquisition?
spiral

Answer 14

process in which a single continuous set of spiral scan data is acquired without an intervening pause

Question 15

what is interpolation ?

Answer 15

modification of scan data before reconstruction in order to synthesise a set of data that yields a clinically acceptable representation of cross section of anatomy

Question 16

what is slip ring technology?

Answer 16

allows high speed scanning with good temporal resolution and increased gantry speeds - 0.33 secs

desirable to achieve cardiac imaging without using beta blockers to slow patients heart rate

Question 17

Collimation in CT:

Answer 17

Double collimated
at tube exit - before patient - controls patients dose
post patient at detector entrance - ensures beam matches width
controls slice thickness - detector eliminates scatter which would degrade the image
1.26mm - 80cm variable potential
controlled by software programme

Question 18

Describe the mathematical calculation:

Answer 18

Done by computer
the resultant data is worked out by subtracting the attenuated signal (from the detector) from the incident beam intensity signal

The CT collects hundreds of readings from the detectors and this effectively divides the body into a criss-cross matrix of separate very small tissue volumes (voxels)

Question 19

What is the basic image construction?

Answer 19

Pre-processing - raw data is produced by processing scan data
convolution - provides sharp/smooth images + enhances appearance of tissue of interest
back projection - complicated process in which the convolved scan data is projected back into the image matrix (clearer + sharper)

Answer 20

Pre-processing :
raw data - values of all measured detector signals during a scan
the attenuation and correlated x-ray position - from tis ct images are reconstructed

A preprocessing step is implemented in order to reduce the noise in CT images. The goal is to increase the signal to noise ratio (SNR) as well as to increase the contrast of the structures of interest,

Mathematical procedures:
convolution
back projection
iterative projection

1st stage - undertake a series of dose measurements
for each measurement the computer has to know the position of the tube and detector
hence the voxels the measured the beam passed through

Question 21

what is the process of back projection?

Answer 21

attenuation value is averaged over each pixel (voxel)

beams are projected in two directions (voxel)

as each beam passes through the object + a summed linear coefficient is obtained

assumed linear coefficient is spread evenly across each beam path

each table of calculated linear attenuation coefficients are added

resulting table provides the estimates of each pixels original linear attenuation coefficient

computer starts to work out and give us information

Question 22

what is the pipeline pr

Answer 22

pre-processing - convolution - back projection

too few projections causes artefacts

typically need about 2000 projections

blurring where back projection lacks - filtered back projection improves

Question 23

what is simple back projection?

Answer 23

reconstruction of an image by taking each view + smearing/guessing it along the path it was originally acquired on
sum of all projected views
leading to blurred images

Question 24

what is filtrated back projection?

Answer 24

each view is filtered before the back projection to counteract the blurring

each 1 dimensional view is convolved with a 1 dimensional filter kernel - set of filtered views

much clearer views

Question 25

what is iterative reconstruction?

Answer 25

Iterative reconstruction refers to an image reconstruction algorithm used in CT that begins with an image assumption, and compares it to real time measured values while making constant adjustments until the two are in agreement.

algothrimic method which uses statistical info and geometric models with the raw data to process + reduce noise

Question 26

what is display window width and window level?

Answer 26

The window width is the range of the grayscale that can be displayed.

The center of grayscale range is referred to as the window level. - should be roughly the same as hounsfield units

Question 27

what is windowing?

Answer 27

windowing is the process in which the image grayscale components of an image is manipulated via the numbers - hounsfield units

to change the appearance of the picture to highlight particular structures

Question 28

adjustment of window level?

Answer 28

window level - brightness
range of CT no. hounsfield units that image contains:
wider - the more shades of grey
- contrast will be poor

narrow (better) - 50-30 hounsfield
wide - 2000 hounsfield

Question 29

adjustment of window?

Answer 29

window width - contrast
lower level - image is brighter
higher level - image is darker

Question 30

hounsfield unit - tissues:

Answer 30

bone - +400 -+1000
soft tissue - +40 - +80
water - 0
fat - -60 - -100
lung - -400 - -600
air - -1000