CT 2 (Imaging system design, ct scanner, characteristics of detector, diff types of ct scanner, imaging quality)

OPERATING CONSOLE

CT imaging systems can be equipped 2 or 3 consoles.

TECHNICAL FACTORS

120 kvp

400 mA

Thickness of tissue slice

0.5-5mm

PHYSICIANS WORKSTATION

Allows image to be manipulated and optimized.

Manipulate controls in Physician Workstation

·         Controls and brightness adjustments

·         Magnification techniques

·         ROI viewing

·         Use of on-line computer software packages

·         It can also be linked to PACS network

SOFTWARE PROGRAMS

Generate plots of CT numbers

Computation of mean and SD of CT values in an ROI.

SOFTWARE PROGRAMS

-       Substruction technique

-       Planar and volumetric quantitative analysis.

- Reconstruction of images in different planes

THERMAL REQUIREMENT

-       power capacity must be high

-       high speed rotor

-       anode heat storage

Capacity of Thermal Requirement

 8 mhu or more

Anode cooling rates

1 mHu/min

X-RAY TUBE

1.    Metal envelop

2.    Glass vacuum tube: 0.18-0.30 mm

3.    Ceramic insulator: place between metal envelop & glass

4.    RPM: 10,000 rpm

5.    Anode disc: made of tungsten, medium and molectenum

6.    Working life of XRT: 10,000-40,000 Hr

Glass vacuum tube

0.18-0.13 mm

Ceramic Insulator

place between metal envelop & glass

RPM

10,000 rpm

Anode Disc

made of:

Tungsten

Medium

Molectenum

Working life of XRT

10,000-40,000 Hr

Focal spot requirement

Small focal Spot

FOCAL SPOT

Limiting characteristics of CT

To have a better resolution

Focal spot cooling algorithm

predicts the thermal state of the FS and adjust the Ma setting

Old detectors: Gas filled detectors

New detectors: Scintillators; solid state detectors

Detector Array 2 types

DETECTOR ARRAY

Captures photon (or image formation)

Sodium Iodide (Nal) Crystal

Early material used in Detector Array

Bismuth Germanate (Bi4Ge3012 or BGO)

Cesium Iodide (CsI).

Cadium Tungstate (CdW04)

Early material replaced by:

Special Ceramic

Current crystal of choice

COLLIMATION

In conventional radiography only 1 collimator is used in CT (multi-slice spiral) usually 2 are used.

Pre-detector collimator

Restricts x-ray beam viewed by the detector array.

improves contrast

Pre-detector collimator

Determines sensitivity profile and slice  thickness

HIGH VOLTAGE GENERATOR

CT scan operates on high frequency power

High frequency generator is small

because High voltage step-up transformer is small, so it can be mounted on the rotating gantry

50 kW power

HIGH VOLTAGE GENERATOR POWER

PATIENT COUCH/SUPPORT COUCH

Constructed with low Z-material such as carbon fiber

SLIP RING

Electromechanical devices that conduct electricity and electrical signal through rings and brushes

SLIP RING

One surface is a smooth ring and the other a ring with brushes that sweep the smooth surface

SLIP RING

Spiral Ct is made possible

SLIP RING

It allows the gantry to rotate continuously

Composite brushes are made up of

conductive material (silver graphite alloy)

Brushes have to be replaced

every year

Efficiency

Stability

Response Time

Dynamic Range

CHARACTISTICS OF DETECTOR

EFFICIENCY

Ability of our detector to capture, absorb and convert to electrical signal

Absorption efficiency

Capability of our detector to absorb or collect

Refers to the number of x-ray absorb by our detector

Physical Density

Mass of our detector

Capture Efficiency

Ability of our detector to obtain photons transmitted from the patient

Conversion Efficiency

Ability to convert of our x-ray photon into usable electrical signal

STABILITY

Steadiness of our detector’s response

RESPONSE TIME

Ability of our detector to detect an x-ray event and recover to detect another event

DYNAMIC RANGE

Ability of detector use the smallest x-ray intensity.

High Speed Scanner

Spiral/Helical CT Scanner

Dual Source CT Scanner

DIFFERENT TYPES OF SCANNER

HIGH SPEED SCANNER

Used to perform dynamic volume scanning to accommodate imaging of the dynamic of the organ systems and the functional aspects of the cardiovascular and pulmonary systems with high temporal resolution

Dynamic Spatial Reconstructor - 1975

OTHER NAME OF HIGH SPEED SCANNER

EBCT (Electon Beam CT )

absence of moving part

SPIRAL/HELICAL CT SCANNERS

Volume of tissue is scanned by moving the patient continuously through the gantry of the scanner while the x-ray tube and detectors rotate around the patient

Beam Geometry Spiral CT

OTHER NAME SPIRAL/HELICAL CT SCANNERS

1998 MSCT

uses multi detector technology to scan four or more slices per revolution of the x-ray tube and detectors

Axial CT Scanner

Step - shoot

Spiral CT

Simultaneously

DUAL SOURCE CT SCANNER

designed for Cardiac CT Imaging

Dual Energy CT/ spectral

OTHER NAME DUAL SOURCE CT SCANNER

DUAL SOURCE CT SCANNER

dual source CT- Features 2 x-ray tube (with different energies). And two detectors specifically 90º each other.

Spatial Resolution

Spatial Frequency

Edges Response Function

Modulation Response Function

Image Fidelity

Contrast Resolution

Noise

Artifacts

Linearity

Uniformity

CHARACTERISTICS AFFECTING IMAGE QUALITY

Geometry Categories

Reconstruction Algorithms

Determined of 2 categories in Spatial Resolution

Geometric Categories

play a role in data acquisition process such as FS size, detector aperture width, size thickness, distance between the focus, and image reconstruction.

Reconstruction Algorithms

Influence spatial resolution based on their ability to smooth and enhance edges.

1.    Pixel size

2.    Slice thickness

3.    Voxel size

4.    Design of pre patient and pre detector collimators

5.    Detector size

6.    Pitch

7.    Field of view

Factors affecting Spatial Resolution

Pitch

distance between the pixel

SPATIAL FREQUENCY

describe CT spatial resolution

EDGE RESPONSE FUNCTION (ERF)

Mathematical expression of the ability of the CT scanner to reproduced a high contrast edge with accuracy

MODULATION TRANSFER FUNCTION (MTF)

-       Mathematical expression for measuring resolution.

-       The ratio of the image to the object as a function of spatial frequency.

MODULATION TRANSFER FUNCTION

to compress the space between

1.    Collimation

2.    Detector size and concentration

3.    Mechanical/electrical gantry control

4.    Reconstruction algorithm

CHARACTERISTICS OF CT IMAGING SYSTEM CONTRIBUTING TO IMAGE DEGRADATION

IMAGE FIDELITY

Measured by determining the optical density along the axis of the image.

1.      Spatial resolution

2.      Contrast resolution

3.      Image artifacts

IMAGE FIDELITY AFFECTED BY:

CONTRAST RESOLUTION

Ability to image 2-3 mm in size which may vary in density from the environment in which they are located.

diff tone with each tone of POI

Photon Flux: kvp, mAs, beam filtration

  Slice thickness

Image display and image recording

Quantum noise

CONTRAST RESOLUTION FACTORS:

mAs

determination of number photons interaction with POI

Tissue absorption

Linear coefficient

OTHER FACTORS THAT AFFECT CONTRAST RESOLUTION

NOISE

-         Speckled appearance/ mottled appearance.

-         Can be mistaken as pathology

·         Matrix size/ Pixel size

·         Radiation dose

·         Scattered Radiation

NOISE

Affects CR Factors

Streak Artifact

intense straight light appearance

metal light in image

Shading Artifacts

grainy appearance; appear on high density objects

appears increase density

Ring Artifacts

circular or ring appearance

Malfunction detectors

LINEARITY

this describes the amount to which the CT number of materials is exactly proportional to the density of this material (in Hounsfield units)

UNIFORMITY

This is the consistency of the CT numbers of an image of a homogeneous material across the scan field.