Computed tomography CIT

Tomography

Creating 2D images (slices) of a 3D object

CT scans

Measure the attenuation coefficients of X-rays as they pass through the body

Attenuation

X-rays are absorbed, scattered, or transmitted with lower energy as they pass through matter, resulting in a decrease in intensity

Types of attenuation

Compton scatter and photoelectric effect

3rd generation CT scanner

Wide fan beam with up to 1000 detector

Fast scanning - 0.5sec per slice

Back projection

process of mathematically mapping the attenuation pathway at every angle measured through a scan to locate where in a patient attenuation is occurring

If there is a dense object at a certain location in a patient then a projection at an angle in line with it will show us attenuation in this area because less photons will be arriving at the detector through this attenuation pathway

Slip ring technology

An electromechanical technology that allows transmission of power from a stationary to a rotating structure

Allow continuous gantry rotation

Helical scanning

Table moves continuously as the x ray tube rotates

Faster, smoother images, better 3D reconstructions

Components of a CT scanner

Components of a CT scanner

CT detectors

Measure X ray attenuation at multiple angles

Slice width is determined by size of detectors, a slice thickness of 3mm would need two 1.5mm detectors

Bowtie filters

Reduce radiation exposure by adjusting beam intensity for different body regions. Filters less in the centre and more around the edges

CT X ray tube

More powerful than conventional x-ray tubes

Operates at 80,100,120 and 140 kV

Needs replacing every 12 months

Exposure factors

kVp (Kilovolt peak): Controls x-ray penetration

mA (Milliamperes): Adjusts tube current based on body composition

Sequential (Axial) scanning (step and shoot)

X ray tube is off when the table moves

Moves in steps to capture each slice

Slower but precise

Advantages of helical scanning

Higher quality 3D imaging

Virtual procedures e.g bronchoscopy, colonography

Covers large areas quickly, reducing motion artefacts

Small pathologies unlikely to be missed

Better contrast enhancement

Reconstruct images at any angle

Pixels and voxels

CT images are 2D representations of 3D structures

Composed of pixels (2D) but represent voxels (3D, with depth)

Voxel depth corresponds to slice thickness

Voxels correspond to the location in computers, the brightness of each pixel correlates with the x-ray attenuation

Hounsfield units (HU)

Each voxel is assigned a value called HU or CT number

Water = 0 HU (calibration reference)

Tissues vary in HU based on density

Back projection

Raw CT data is reconstructed by summing back projected filtered attenuation profiles

Basis for image formation but needs filtering to remove blurring

Filtered back projection

Uses mathematical interpolation to reconstruct missing data

Smoothing is applied based on filtered rules

manipulation of data being reconstructed, which alters the value of a pixel and its neighbouring pixels relative to one another

FBP principles

if we want to see very sharp edges we would apply a sharpening filter, known as edge enhancement/high pass filter. This drives the value of each pixel away from the value of its neighbouring pixels.

Smooth filter, known as blurring or low pass filter does the opposite. Averaging pixels out with the value of its neighbouring pixels.

Iterative reconstruction

Repeats calculations multiple times to refine the image

Ensures image corresponds accurately to acquired projection data.

Advantage- reduces radiation dose up to 75%

Multiplanar reconstruction

Used to view the voxels in different angles

Axial , sagittal and oblique

Volume averaging is used to render the data

Signal to noise ratio

Signal= X-rays detected

Noise= variability in detector response, signal not supposed to be there gives a grainy appearance

High SNR= better image quality

Increasing tube current (mA) increases SNR

More tissue to go through= SNR is lower

Thicker slices= means higher SNR

High contrast resolution

Ability to distinguish tissues with small density differences

Major advantage of CT over x-ray

Improved by- Higher SNR, thicker slices (less noise, better contrast resolution)

IV contrast agents

Temporal resolution

Time needed to acquire data for a single image slice

Improved by faster gantry rotation time

Spatial resolution

Ability to distinguish fine details from the smallest distance. Two points the CT scanner can distinguish as individual objects.

Windowing in CT

CT images are displayed in grey scale (256 shades)

Window level (WL): Central HU value of displayed rannge

Window Width (WW): HU range displayed as greyscale

Examples of Windowing

Soft tissue (Abdomen) = 350/50 HU

Lung (Air density) = 1500/ -500 HU

Bone = 2000/250 HU

Multiplanar reconstruction (MPR)

CT images can be reconstructed in different planes-

Axial

Coronal

Sagittal