Ct

What is MDCT

Multiple detector act 2 or more parallel rows

Advantages of MDCT

more spices

Less artefacts

Pediatric scanning

Thinner slice improved z axis resolution

Good for trauma

Different detector configuration

Uniform - all same

Non uniform - all different

Hybrid - variety of sizes has both uniform/ non uniform

Pitch

Speed table moves through gantry

If beam pitch more than 1:

Table travels more wide the gaps : higher pitch : - low image quality - faster - less dose

If beam pitch is less than 1

Overlapping - lower pitch - high patient does - higher axial z- resolution - better image quality

Come beam acquisition and artefacts

More detectors more slices / wider beam the more detectors the more cones can have more artefacts because tube rotates objects may appear off centre

Come beam Interpolation: tilted reconstruction

Filling the gaps in at angle to make axial- straight line

Cone beam interpolation/ Feldman algorithm :

Filling gaps to a 3rd dimensional shape: cube

Characteristics of a good CT image:

Visable anatomical structures

Good image congrats

Good spatial resolution

Fast scanning

Low dose

Scanning parameters

volatage

Tube current

Scan time

Slice thickness

Pitch

Patient size

Noise ?

Effects contrast resolution

Number of photons produced are directly proportion to tube current, scan time , slice width

To decrease noise you need to

Increase:

Tube current

Scan time

Slice width

Factors effecting mouse

Patient size

Scan time

Noise filters slice thickened

Pitch

Beam filtration

Spatial resolution

How far 2 objects have to be before they are seen as separate

Large detectors identify gaps but seen as one object - low spatial resolution

smaller detectors identifying gaps but display them as separate- high spstial resolution

Transaxial resolution

Across patient

Z-axis resolution

Along patient

CT necessary

X -ray taken angles for cross sectional images different slices

Eliminate superimposition

Converts x-ray to electrical signal

CT detector characteristics

Small in size for spatial resolution

High detection effeciencu

High density

Fast response time

High stability for high speeds rotating gantry

Tube characteristics

• cooling systems

• Larger and thicker anodes to absorb dissipate large amounts

• Modified cathode

• Focal spot allowing focus position

• Valium used as lubricant

• Metal ceramic tube instead of glass envelope

Slip rings

Allows transmission of power and electrical signal from stationary to rotating structure

Image reconstruction

Converts the numbers from ray sums to image

Back projection

Different number of gray scale from the images joins together - data converted into back projection to prouduce image

Iterative reconstruction

Similar to back projection

Speed up and improved reconstruction

Improves image quality

Image matrix

Pixel in images

Pixel reprints thickness - voxel ( volume)

Voxel assigned to grey scale measured in hounded hints 0-1000

CT window

Changing brightness and contrast of image by altering hounsfield units

Window width

Contrast - measure range of act numbers

Window level

Brightness - midpoint of range of ct number displayed

Axial ct

Patient still

Stop and start while x-ray

Helical Ct

Around the body continues having added detector

Advantage of helical scan

Quicker

Longer scans

Pitch speed can be controlled

Overlapping allows for reconstruction

Slide sensitivity

Extent to which slice contributed to volume data

Wider slice is

High pitch - less intensity - less image resolution

narrow slice

Small pitch - intensity increased - spatial resolution

Ct artefacts

Physics based

Patient based

Scanner based

Helical and multi slice artefacts

Example of helical and multislice artefacts

Ring artifact

Windmill effect

Gemortical distortion

Example physical artefacts

Beam hardening causes streaking and upping artefacts happens in dense part of body

Lower energy beams absorbed or scattered

Attenuation coefficient

Fraction of beam x-rays that is absorbed or scattered per unit thickness of the absorber

CTDI

Volume Ct dose index not patient dose related to intensity of x-ray beam

DLP

Dose length product - measure of total iraddation of patient