CT Acquisition 1b Flashcards
What are the different types of scanners?
Axial and Spiral
What is the job of axial and spiral scanning?
Axial - aka “stop and shoot”, stops and rotates repeatedly from the gantry
The patient is moved in increments
Spiral - aka helical scanning, the gantry rotates continuously and continuously releases x-ray beams.
the patient is moved continuously whilst scanning
What is an advantage of spiral scanning?
It avoids any respiratory miss registration as the scan is performed in one breath
It is also more effective with its use of contrast - can see multiple phases
What is the Camp Bastion Protocol?
It is the process in which patient dose is reduced by the injection of contrast and waiting for the contrast to nearly reach the venous system and then more contrast is added and chases the bolus into the arterial system
What does closing the scan in CT allow for?
It allows for a continuous collection of data and opening the slices creates less slices
What does over lapping of slices in CT allow for?
It allows for better reconstruction and helps to show much smaller lesions - this process is done by Pitch
What is Pitch used for in CT?
It can be used to reduce the scan time and/or the radiation dose while still covering the same volume needed to cover the patient
What is Pitch?
Pitch is the measure of overlap during scanning
Pitch = distance couch travel / width of slice
Why is the spiral better than sequential?
Dismisses breathing or motion effects
Optimises contrast utilisation
Can reconstruct in any plane or position
It is fast and used for trauma when necessary
It helps in filling the blanks of missing data
Interpolation
What is interpolation?
Section image slice created from reconstruction of planour section that approximates the acquisition of the reconstructed data
What is required for 3D recon?
Pitch
What is the relation between pitch and image resolution?
The higher the pitch lower the dose which leads to quicker scans but low image resolution
What are the advantages of a CT detector array?
High detection efficiency for x-rays in CT
Narrow gaps between active elements (good geometric efficiency)
Fast response
Low cost
Small physical size
What is a type of CT detector?
Solid State Detector (SSD)
What is solid state detector?
Solid scintillator layer that covers, converts the x-rays into visible light photons and then the photodiode converts the phantom input into an electrical signal
What are solid state detector components and %?
High detector efficiency of about 92%
High geometric efficiency of about 80%
Small physical size of the actual elements
What is the key to image quality in CT?
Spatial resolution is key to image quality within CT and can be limited by data sampling
What is the geometric relationship?
The geometric relationship is the number of detectors in the arc or array that determines what the data sampling is
What does the spatial resolution depend on?
It depends on how closely spaced the rays are in each view
What are the characteristics of a detector array?
- Detector not tightly packed (interspace between - reduces cross talk and increases image quality)
- Reduced detection efficiently and increased patient
- Can reduce heat loading of the x-ray tube
- Multiple detector arrays (allows the collection of two or more data image sets simultaneously)
What is Crosstalk?
Crosstalk is a leakage of the x-ray/ light produced in a matrix of x-ray detector or arrays of photodiodes
What is crosstalk affected by?
Crosstalk is affected by various parameters such as: -
- The x-ray tube voltage
- Detector element sizes
- The scintillating material
- The impurities within the scintillator material
- The material of detector separators
What is done to prevent crosstalk
The detector arrays aren’t tightly packed they are separated by lead, this is to prevent crosstalk
What are the advantages of a multi slice CT (MSCT)?
- Faster scanning due to having wider total active detector width
- Better dynamic imaging due to faster scans
- Thinner slices, they make the 3D imaging enabled and allows the enabling of multiple slices at the same time
Why is MSCT efficient?
MSCT is efficient as it reduces patient dose and increases image resolution and allows easy and and quick post-acquisition reconstruction
What should be considered in relation to MSCT?
MSCT are scanned in a helical movement so pitch plays a part as pitch can be quick but given more information because there more detector
What happens as a result of MSCT scanners being exposed to multiple detectors?
The MSCT scanners are exposed to multiple detectors at the same time and the section thickness is determined by the width of the detector and the resolution is effected
