CT written test

Beam hardening

Beam passes through dense object but lower energy photons get absorbed, while higher energy photons are transmitted. The mean energy increases and the ray beam are hardened resulting inartefact. There’s 2 types cupping and streaks + darks bands

Cupping

X-ray passing through middle of uniform cylindrical phantom are hardened more than those passing the edges. Mean energy increases

Streaks and dark bands artefacts

In heterogeneous cross sections dark bands of streaks can appear between 2 dense objects

Correction of beam hardening

• Metal filters to pre harden the beam so as to filter out the low energy photons

• Periodic calibration correction

• Use of beam hardening correction software

• Tilting the gantry or changing positions of patient

Partial volume effect artefact

Occurs where multiple tissues contribute to a single voxel, resulting in a blurring of intensities of tissue boundaries.

• because object is smaller than slice thickness

Ring artefact

Type of helical + multislice artefact

Faulty detector and other detector don’t have same gain relative together they are operating at different baselines. Then when gantry rotates around the patient, the detector outlines a circle

Windmill artefact

is image distraction in axial plane, during helical multi detector acquisitions. The streaks seemingly rotate while scrolling back and forth through the affected slices. Caused by inadequate data in z-plane. when helical pitch increases number of detector rows intersecting also increases so more streaks

Cone beam artefact

Wider Collimation beam has a volume and becomes cone - shaped artefact. Similar to partial volume officers for off centre objects. Each detector collects data that corresponds to the volume contained between 2 cones which can lead to causes noise and streaks

CTDI

stands for volume ct dose index. Not patient related dose. Related to the intensity of the x-ray beam. Average absorbed dose within scanned volume

DLP

• measure of total irradtion of patient

• Proportion to scan length

AEC

Used to automatically adjust decrease or increase KV and mAs for each patient when required used for dose optimising.

Advantages of AEC

Dose is optimaised

• Image quality is more insistent

• Artefact reduction

• Preserves CT machine due to tube heat conservation.

Back projection

Reconstructing raw data to image data. Mathematically mapping attenuation pathway at every angle measured through a scan to locate where in a patient attenuation is occurring. Converts number of ray sums to image.

Filtered back projection

Same as back projection but by adding filters like sharpening which is good for lungs for spatial resolution and edge detail or smoothing good for soft tissue.

Window width

Window Width (contrast) - the measure of the range of CT numbers that an image contains.

Small window width - small grey scale

Large window width. Long grey scale

Window length

Window Level (brightness) - window centre which is the midpoint of the range of the CT numbers displayed. (The median.)

Slip rings:

Used for technology with rotating parts through electrical current or signal needs to pass. 2 components can rotate relative to one another. While Turing there’s contract surface either smooth ring or metal brushes to maintain conduction of electrical current or signal throughout rotation.

Helical scanning

The X-ray tube is continuously rotating in the same direction within the gantry whilst the patient in moving in one direction (stationary). This can happen because of slip ring technology and high power x-ray tubes.

Axial scanning

CT - The X-ray tube stops each time, rotates and scans the patient, the bed moves and then the tube rotates again and scans the patient each time.

Advantages of helical scanning

• longer scans which enables us to do scans without respiratory discrepancies.

• Pitch is the speed at which the patient moves through the gantry which can be controlled to reduce scan

times or radiation does whilst covering the same volume.

• Overlapping scanning allows for better recnostrructions.

• More effective use of contrast due to the flow of contrast being fast

Image matrix

The image produced by the CT scanner consists of a square matrix of picture elements (pixel), each which represents a voxel (volume element) of the tissue of the patient.

The CT image is made up a grey scale of different densities. Each voxel (volume of tissue) represents a number based on the x-ray attenuation of that section. The number of ‘greyness’ (attenuation is measured in Hounsfield Units.

Pitch

Speed at which patient moves through gantry

High pitch

• Lower radiation does

• Quicker scan

• Lower image quality

Low pitch

• Better z-axis resolution ; better image quality

• Higher patient dose

More parts of patient is scanned

Slice sensitivity profile

Slice Sensitivity Profile

• The extent at which the slice contributes to volume data.

• A lager pitch = wider slice profile, less signal intensity, decreased spatial resolution and smoother image

• A smaller pitch = narrower slice profile, higher signal intensity, increased spatial resolution and noiser

image.

What is ct

Ct is an imaging modality which uses X-ray equipment and computer processed measurements taken from different angles to produce cross sectional images. (Images composed of different 'slices'.) it’s important because it elimates superimposition.

Ct detector characteristics

• A CT detector converts X-rays into electrical signal.

• Fast response

• Small in size for better resolution

• Allows multiple detectors to be combined together with possibility of forming a detector array

Ct tube characteristics

• Larger and thicker anode to absorb and dissipate large amounts of heat.

• Modified cathode assembly to produce a smaller focal spot. (0.6mm) The smaller

focal spot the less shadowing (penumbra).

• Gallium is used as a lubricant to help the rotating detectors in the gantry.

Interpolation

Calculating data that is in the middle from 2 previous points. When you have two points of data + you estimate the data in the middle, results in the creation of virtual slices.

MDCT

Multiple detector ct.

• scanning more of patient with one rotation.

  Advanatages:

• Scanning quicker

• Ideal for trauma and paediatrics

• Thinner slices

• There’s 3 types uniform ( all same). Non uniform (all different) hybrid (mix of both)

MDCT interpolation

1. Feldkamp algorithm: compromises 3 steps. Projection space filtering, back projection and volume space weighting

2. Tilted reconstruction: produces non axial images which are then filled to produce standard images which are then filtered to produce standard axial images . Reconstructs images in oblique.

Good CT image quality:

• showing visible anatomical structures, various tissues + signs of pathology

• Low noise

• Good spatial resolution

• Good low contrast detectability

• No signs of artefacts

Things effecting image quality

• reconstruction parameters

• Patient factors

• Scanner design

• Scan aquistion parameters

Noise

how smooth an image appears. More noise means blurriness in an image not enough photons reach detector (scatter radiation).

Factors affecting noise:

• tube voltage

• Tube current

• Scan time

• Slice thickness

• Pitch

• Patient size

• Noise filters

Ct dose

Ct has a higher dose. X-plane depends on filtration, beam shaping and scanner geomtry

Spatial resolution

How far 2 objects.

Mainly transaxial resolution - across patient

Z-axis resolution - parallel to patient dependent on pitch

Ct artefacts

Physics based - beam hardening

Patient based - metallic, motion, pacemakers

Scanner based - windmill affect, cone beam aquistion

Image reconstruction

Uses a series of ray sums to build an attenuation profile.

Ct post processing techniques

Multiplanar reformatting

Maximum intensity projection

Volume rendering/3d reconstruction