CT Flashcards
How many generations of CT are there
6
Electron beam - sometimes described sometimes described as 5th - main use in cardiac imaging - fast at 50 to 250 ms
6th (Spiral or Helical) - slip ring technology
What does a lower pitch mean for the patient with regards to slice thickness and how fast the table moves
More radiation
Lower pitch - overlap of slices - as couch travels less than the width of the beam
What are the two types of CT detectors
- Solid state detector - X-ray deposits in scintillating layer - converted to a visible photon- deposits on photodiode - converted to electric signal - most commonly used
- Ionization chamber detector - no longer used - single vessel filled with high atomic number gas (Krypton/Xenon) subdivided into separate detectors by tungsten septae. X-rays ionize gas and produce signal at collecting electrodes - converted into electrical signal as they pass through an insulator layer
Type of detector arrays
Linear - all rows of detectors are Sam width
Adaptive - elements within central detector row are thinnest and get wider towards outside
Hybrid - central group of detectors are narrower than outer rows, which are same size - main for 16 slice and above scanners
Describe the 2 post processing CT techniques to acquire an image
- Backprojection - summed attenuation values of pixels are averaged out over a a matrix. With several projections it comes close to the actual image. Blurred images - solved by filtered backprojection
- Iterative reconstruction
- Filtered projection first to assign number to each pixel.
- Computer then calculates what it expected the detectors to have received based on on the image generated.
- Then calculates the difference between the actual detector measurements and the calculated measurements.
- It then uses this information to calculate an updated image
This is done in multiple iterations, each time bringing the calculated values closer to the true values
Explain how dual-energy CT works
It utilizes photoelectric effect to separate out different materials within a voxel based on their different attenuations at different beam energies
What is k edge
The sudden jump in attenuation because of increased photoelectric absorption, caused by an incident photon having energy just above the k-shell binding energy of the atom it is interacting with
( the unique k-shell binding energy that each substance has)
What 3 things is CT image quality determined by
- Resolution
- Noise
- Contrast
What are the 2 types of resolution in CT
- Transaxial - lines per cm (lp/cm) - axially across patient
- Z-sensitivity- along length of patient in z direction
What is transaxial resolution affected by?
Scanner (hardware factors) or scan and reconstruction parameters:
- focal spot - smaller = higher resolution , flying focal spot, focus detector distance, focus iso centre distance
- detector size - smaller = higher resolution
- detector design properties - quarter detector offset
Scan parameters:
- number of projections - large number - finer resolution up to a point
- reconstruction filter - sharp kernels ( but more noise) eg bone better that soft tissue kernel
- pixel size - high size lower spatial frequency - d = FOV/n (pixel size = fov/image matrix size). fmax = 1/2d(highest spatial frequency that can be obtained - Nyquist limit = 1/2 pixel size
Not affected by tube current or kilovoltage
What is z-sensitivity resolution affected by?
- detector slice thickness - wider in z axis the detector row - lower the resolution , the smaller greater the noise and less partial volume artifact
- overlapping samples - achieved by using low spiral pitch < 1- more overlap the better
- focal spot size - the finer the better the sensitivity 0.7 mm > 1.2 mm
Isotropic scanning (pixels in axial and z axis same size) - better 3D reconstruction and MPR
Describe noise, the sources and how to reduce it
Variations in HU about a mean. Noise degrades image by degrading low contrast resolution
- Quantum
- Electronic
- noise from reconstruction process eg. Backprojection
*stochastic noise - dominant source- inversely proportional to number of photons
So need to double protons to reduce:
- double tube current (mA)
- double rotation time (s)
double slice thickness (mm) - tube kilovoltage increase photon flux but not directly proportional. It put proportional to kV to 2nd power
What factors influence contrast?
- Noise - higher noise the worse
- Tube current - lower more noise
- Inherent tissue properties
- Beam kilovoltage - higher beam reduces contrast
- Use of contrast media - increase contrast
Explain the physics behind beam hardening artifact ?
Because lower energy photons are more likely to be absorbed once a beam passes through a dense area , the higher energy protons left behind, result in a higher average energy beam.
This is interpreted by the detector as the beam passing through a less attenuating material relative to its surroundings and so a lower HU is assigned to the area the beam traverses
and so the image will be seen as more black in that area
- so number of photons decrease but beam energy increases
What other type of artifact does beam hardening produce and explain?
Cupping - Because the centre of an object is usually thicker that it’s periphery , the beam passing through is harder in the centre and is therefore assigned a lower HU - corrected by beam hardening correction algorithm
What are 2 solutions to beam hardening?
- Pre-patient filter - Abdorbs soft x-rays and minimizes beam hardening artifacts
- Bow-tie filter - pre-hardens the X-ray beam - shapes the beam. Attenuates lateral edges of object more than centre (equalizes attenuation across patient).
For imaging chest/abdomen
Explain physics behind partial volume artifact
A dense object that only partially protrudes into a detector stream is averaged with its less dense surroundings and is assigned a lower HU - only reduced never increases apparent attenuation
Explain physics behind incomplete projection artifact
An object may be seen in the slice in one projection, but not on the opposing projection, especially at the periphery of the image, where the beam is more divergent.
Object appears streaked - variant of partial volume eg. Arms by side - solve by smaller slices
Explain physics behind photon starvation
Another cause of streak artifacts.
Results from projections that have to travel through more material. As X-ray photons travel through more material, more photons are absorbed and removed from the beam, which results in a smaller proportion of signal reaching the detector and so a larger proportion of noise
(Which is why they occur in direction of widest part of object being scanned)
Solve - adaptive filtering - regions in which attenuation exceed a specified level are smoothed before undergoing backprojection . Higher mA also for more attenuating projections - can calculate in advance from scout or during scan from feedback system of detector
Truncation artifact in CT is an apparently increased curvilinear band of attenuation along the edge of the image.
- Truncation :
This artifact is encountered when parts of the imaged body part remain outside the field of view (e.g. due to patient body habitus), which results in inaccurate measurement of attenuation along the edge of the image. The artifact can be reduced - if possible - by using an extended FOV reconstruction of the affected region
Use of iterative reconstruction techniques can also significantly reduce image noise caused by this artifact.
What type of artifacts do metallic artifacts cause
- Beam-hardening
- Photon starvation
- also with other high attenuation materials eg. IV contrast
What type of artifacts do metallic artifacts cause
- Beam-hardening
- Photon starvation
- also with other high attenuation materials eg. IV contrast
Explain how motion artifact occurs
If a patient or structure moves as the gantry rotates, the object will be detected as being in several positions and represented in the image as such - misregistration - blurring, streaking,shading
Prevent - voluntary movements - immobilization/sedation
Involuntary - eg. Heart - fast scanning techniques such as cardiac gating
Ways to reduce motion artifact
Explain how a ring artifact is formed ?
If there is a faulty detector and the detectors do not have the same gain relative to each other (operate at different baselines) , then as the gantry rotates around the patient, this detector will outline a circle. On backprojection it will cause the ring artifact
Explain how a helical CT artifact is formed
As a gantry rotates it is moving in the z axis. Any object that changes in position or size along the z axis may be distorted as they will be in different positions for different projections
Explain cone beam arifact
Caused by multislice scanners. As the section scanned increase per rotation, a wider collimation is used. This causes the x-ray beam to become cone-shaped instead of fan- shaped and the area imaged by each detector is a volume instead of a flat plane.
The artifact that results is similar to partial volume for off centre objects in detector field. Artifact is worse for objects at the edges of the beam
Solve - modern scanners - cone beam reconstruction algorithm. Effect assessed with a Capthan test phantom
Which factors affect CT radiation dose
- tube current - increasing will increase dose
- rotation time - increasing will increase dose
- pitch - increasing will decrease dose
- kVp - increasing will increase dose
What is the basic principle behind CT
The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of the object.
How are projections formed in CT?
Projections are formed by scanning a thin cross section of the body with a narrow x-ray beam and measuring the transmitted radiation with a sensitive radiation detector
What does the detector do in CT
The detector does not form the image; it merely adds up the energy of all the transmitted photons. The numerical data from the multiple ray sums are then computer-processed to reconstruct an image.
What is each square in a CT image matrix called
Each square in a CT image matrix is called a pixel, and it represents a tiny elongated block of tissue called a voxel.
When was CT technology first introduced
CT scanners have gone through a number of design changes since the technology was first introduced in 1971
Why were new CT configurations introduced
Time reduction is the predominant reason for introducing new configurations. Scan time has been reduced in newer configurations by the reduction or simplification of mechanical motion.
Describe the 1st Generation CT scanner.
The original EMI unit was a first-generation scanner. It employed a pencil-like x-ray beam and a single detector. The x-ray tube-detector movements were both linear and rotary. A five-view study of the head took 25 to 30 minutes.”
What was the objective of the 2nd Generation CT scanner?
The objective of the 2nd Generation CT scanner was to shorten the time for an exam by abandoning the pencil beam and single detector system and adopting a fan-shaped beam and multiple detectors.
How did the 2nd Generation CT scanner improve data collection?
The movement of the tube and detector were both linear and rotatory, but the rotatory steps are larger, allowing more data to be collected per linear scan so fewer linear movements are needed to gather an adequate data base.”
What advancement did the 3rd Generation CT scanner introduce
The 3rd Generation CT scanner, invented by GE in 1975, eliminated translation motion and required only rotation motion with both the x-ray tube and detectors rotating around the patient. This scanning geometry is known as fan beam geometry.” - cone beam added on later
How are detectors aligned in the 3rd Generation CT scanner
In the 3rd Generation CT scanner, the detectors are always perfectly aligned with the x-ray tube. The x-ray beam is collimated into a fan beam.
What defines the 4th Generation CT scanner
The 4th Generation CT scanner is called the Rotate-Fixed generation. The detectors form a ring that completely surrounds the patient and do not move, while the x-ray tube rotates inside the detector ring.”
How does a slip ring work in CT scanners?
A slip ring is an annulus with grooves along which electrical contactor brushes which slide along it allowing data to be transmitted from detectors via various high-capacity wireless technologies.
What does the graph of attenuation values of tissues at 80 kVp and 140 kVp show?
The graph shows the attenuation values of eight tissues at 80 kVp and 140 kVp. Attenuation of all tissues is greater at 80 kVp than at 140 kVp. Vascular organs such as kidneys have larger differences in attenuation than less vascular structures like muscle.”
How does dual-energy chest radiography differentiate calcium from soft tissues?
Standard, soft-tissue, and bone radiographs are generated from low (60–80 kV) and high-energy (110–120 kV) chest radiographs obtained in rapid succession
How does dual-energy chest radiography differentiate calcium from soft tissues?
Standard, soft-tissue, and bone radiographs are generated from low (60–80 kV) and high-energy (110–120 kV) chest radiographs obtained in rapid succession
What is the difference between window width and level
Window width determines the range of grey scale values displayed in the entire grey scale (that the image will display).
Window level defines the centre point of that range. (Average value)
Describe aliasing artifact
Aliasing artifact, otherwise known as undersampling, in CT refers to an error in the accuracy proponent of analogue to digital converter (ADC) during image digitisation.
Image digitisation has three distinct steps: scanning, sampling, and quantisation.
When sampling, the brightness of each pixel in the image is measured, and via a photomultiplier, creates an output analogue signal that is then due to undergo quantisation.
The more samples that are taken the more accurate the representation of the signal will be, hence if a lack of sampling has occurred the computer will process an inaccurate image resulting in an aliasing artifact.
The artifact has the appearance of Moiré patterns (MRI)
Describe the 2 types of multiplanar reconstruction artifact ?
- zebra artifact - from 3D recon
Not to be confused with zebra sign from haemorrhage in the cerebellar sulci, and potentially-confusingly a zebra stripe sign in the bones of those treated with cyclical bisphosphonates for osteogenesis imperfecta
as alternating bright and dark bands (also in MRI)
Artifacts that have been described as a zebra artifact include the following:
moire fringes
spike in k-space
zero-fill artifact
Susceptibility artifacts
- stair step artifact
found in straight structures which are orientated obliquely with respect to movement of the table and appear around the edges of sagittal and coronal reformatted images when wide collimations and non-overlapping reconstruction intervals are used.
It is also seen in coronary CT angiography when step-wise reconstructions are from different cardiac phases. This is associated with heart rate variability and irregular heart rates.
Solution
This can be minimised by, using smaller collimation and overlapping reconstruction in helical imaging.
In coronary CT angiography, 256 and 320-detector CT scanners typically avoid this artifact. Some authors recommend beta-blockers to reduce stair-step artifact, others report limited results in achieving target heart rates with their use.
Describe the 2 types of multiplanar reconstruction artifact ?
- zebra artifact - from 3D recon
Not to be confused with zebra sign from haemorrhage in the cerebellar sulci, and potentially-confusingly a zebra stripe sign in the bones of those treated with cyclical bisphosphonates for osteogenesis imperfecta
as alternating bright and dark bands (also in MRI)
Artifacts that have been described as a zebra artifact include the following:
moire fringes
spike in k-space
zero-fill artifact
Susceptibility artifacts
- stair step artifact
found in straight structures which are orientated obliquely with respect to movement of the table and appear around the edges of sagittal and coronal reformatted images when wide collimations and non-overlapping reconstruction intervals are used.
It is also seen in coronary CT angiography when step-wise reconstructions are from different cardiac phases. This is associated with heart rate variability and irregular heart rates.
Solution
This can be minimised by, using smaller collimation and overlapping reconstruction in helical imaging.
In coronary CT angiography, 256 and 320-detector CT scanners typically avoid this artifact. Some authors recommend beta-blockers to reduce stair-step artifact, others report limited results in achieving target heart rates with their use.
Describe principle and relevance of scanned projection radiography (SPR)
The CT localiser radiograph, also known as scanogram, scout and surview, is the name given to the initial images obtained at the start of a CT before the main volume is obtained. These initial images have two main functions: they ensure that the correct anatomy is being covered by the CT and permit the software to optimise the radiation used to obtain the images.
Compare the types of X-ray tubes used across modalities?
