1
Q
How many generations of CT are there
A
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
2
Q
What does a lower pitch mean for the patient with regards to slice thickness and how fast the table moves
A
More radiation
Lower pitch - overlap of slices - as couch travels less than the width of the beam
3
Q
What are the two types of CT detectors
A
- 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
4
Q
Type of detector arrays
A
Linear - all rows of detectors are same 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
5
Q
Describe the 2 post processing CT techniques to acquire an image
A
- 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
6
Q
Explain how dual-energy CT works
A
It utilizes photoelectric effect to separate out different materials within a voxel based on their different attenuations at different beam energies
7
Q
What is k edge
A
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)
8
Q
What 3 things is CT image quality determined by
A
- Resolution
- Noise
- Contrast
9
Q
What are the 2 types of resolution in CT
A
- Transaxial - lines per cm (lp/cm) - axially across patient
- Z-sensitivity- along length of patient in z direction
10
Q
What is transaxial resolution affected by?
A
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
11
Q
What is z-sensitivity resolution affected by?
A
- 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
12
Q
Describe noise, the sources and how to reduce it
A
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
13
Q
What factors influence contrast?
A
- 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
14
Q
Explain the physics behind beam hardening artifact ?
A
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
15
Q
What other type of artifact does beam hardening produce and explain?
A
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
16
Q
What are 2 solutions to beam hardening?
A
- 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
17
Q
Explain physics behind partial volume artifact
A
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
18
Q
Explain physics behind incomplete projection artifact
A
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
19
Q
Explain physics behind photon starvation
A
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.
20
Q
What type of artifacts do metallic artifacts cause
A
- Beam-hardening
- Photon starvation
- also with other high attenuation materials eg. IV contrast
21
Q
What type of artifacts do metallic artifacts cause
A
- Beam-hardening
- Photon starvation
- also with other high attenuation materials eg. IV contrast
22
Q
Explain how motion artifact occurs
A
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
23
Q
Ways to reduce motion artifact
A
24
Q
Explain how a ring artifact is formed ?
A
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
25
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
26
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
27
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
28
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.
29
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
30
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.
31
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.
32
When was CT technology first introduced
CT scanners have gone through a number of design changes since the technology was first introduced in 1971
33
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.
34
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."
35
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.
36
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."
37
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
38
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.
39
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."
40
What energy levels are used in modern CT scanners
"The energy levels used in CT today are 80, 90, 110, 120, 130, or 140 kVp, with Compton scatter being a common issue at these levels.
41
What technology enabled continuous rotation in 6th Generation CT scanners?
The slip ring technology enabled continuous (nonstop) rotation by passing electrical power to the rotating components without fixed connections.
42
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.
43
What is the advantage of helical CT scanning
Helical CT scanning allows for continuous rotation and data acquisition as the table (patient) is smoothly moved through the gantry, enabling rapid scans of entire z-axis regions of interest within a single breath hold.
44
What is the purpose of collimators in CT scanners?
The x-ray beam is collimated at two points, one close to the x-ray tube and the other at the detectors, to control scatter radiation and regulate the thickness of the tomographic slice.
45
Name a disadvantages of Xenon gas ionization chambers
Inefficiency due to the solid-gas density ratio and absorption of radiation by the front window needed to keep the gas in the chamber.
46
What is Quantum Mottle in CT?
Quantum Mottle is the variation in the number of x-ray photons absorbed by a detector, causing noise in the image. It cannot be reduced by better mathematical reconstruction but requires better detectors or more radiation dose.
47
What are 2 types of resolution in CT as it relates to image quality
The two types of resolution in CT are Spatial Resolution, the ability to display two objects close to each other as separate images, and Contrast Resolution, the ability to display areas that differ in density by a small amount.
48
What is temporal resolution ?
Temporal resolution is the time needed to acquire the data to generate an image. High temporal resolution is needed for CCTA (coronary CT angiography ) scans to reduce respiratory motion artifacts and to overcome cardiac motion artifacts.
49
What is a dual-source CT scan
A dual-source CT scan uses two x-ray sources at right angles to each other.
50
What are the advantages of a dual-source CT scan?
Advantages include:
- Doubling the speed of imaging
- Less radiation
- Sharper images
- Better temporal resolution
- Allows Coronary CT Angiogram (CCTA) to be performed without need for beta blockers
51
What is Dual Energy CT (DECT)
Dual energy CT also known as spectral CT is a computed tomography technique that uses two separate x-ray photon energy spectra.
52
What does Dual Energy CT (DECT) allow for?
This allows for the interrogation of materials that have different attenuation properties at different energies.
53
What can dual energy data be used for
Dual energy data (attenuation values at two energy spectra) can be used to reconstruct numerous image types including:
- weighted average images
- virtual monoenergetic images
- material decomposition images
- virtual non-contrast images
- iodine concentration
- calcium suppression
- uric acid suppression
- electron density maps
- effective atomic number (Zeff) maps
54
What can dual energy data be used for
Dual energy data (attenuation values at two energy spectra) can be used to reconstruct numerous image types including:
- weighted average images
- virtual monoenergetic images
- material decomposition images
- virtual non-contrast images
- iodine concentration
- calcium suppression
- uric acid suppression
- electron density maps
- effective atomic number (Zeff) maps
55
What are weighted average images in DECT
Weighted average images simulate single energy spectra
56
What are virtual monoenergetic images in DECT?
Virtual monoenergetic images show attenuation at a single photon energy rather than a spectrum.
57
What are material decomposition images in DECT
Material decomposition images map or remove substances of known attenuation characteristics, such as iodine, calcium, or uric acid.
58
What are virtual non-contrast images in DECT
Virtual non-contrast images have iodine removed
59
What is calcium and uric acid suppression in DECT?
Calcium and uric acid suppression means calcium and uric acid is removed
60
What are the DECT acquisition techniques?
DECT acquisition technologies can be broadly classified as prospective techniques (pre-selected before the patient is scanned) and retrospective techniques (do not need to be pre-selected, occur after the patient is scanned).
61
What are prospective techniques in DECT
Prospective techniques include: dual-source (two x-ray tubes producing different voltages offset at approximately 90°, reconstructed in the image space, limited field of view due to detector size, excellent temporal resolution as both datasets acquired at the same time)
single-source consecutive (two helical scans consecutively acquired at different tube potentials followed by coregistration for postprocessing, reconstructed in the image space, full FOV, poor temporal resolution as the patient is scanned twice therefore increased dose)
single-source twin-beam (two-material filter splits the x-ray beam into high-energy and low-energy spectra on the z-axis before it reaches the patient)
single-source rapid kilovoltage switching (fast kVp-switch, the x-ray tube switches between high- and low- tube potential multiple times within the same rotation, reconstructed in the projection space, full FOV, slight reduction in temporal resolution due to tube rotation)
62
What are retrospective techniques in DECT
Retrospective techniques include:
dual-layer DECT (sandwich, the top innermost layer of the detector absorbs low-energy photons while high-energy photons pass through to the bottom outermost layer, reconstructed in the projection space, full FOV, excellent temporal resolution as both datasets acquired at the same time).
63
What is the basic principle of dual energy CT?
Dual energy CT is based on k-edge absorption. Photoelectric interaction takes place at the k shell of an atom. Each element has a unique k-edge emission, allowing differentiation of structures based on the amount of an element present.
64
Why is iodine more visible at 80 kVp than 140 kVp in DECT?
80 kVp is closer to 33.2 keV, the K-edge of iodine, making iodine-containing structures more visible at this energy level than at 140 kVp.
65
How does dual energy CT delineate structures?
Structures containing iodine will retain less attenuation as the kVp progresses beyond the K-edge of iodine. Therefore, using two energies allows delineation based on their attenuation differences between 80 kVp and 140 kVp.
66
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."
67
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
68
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
69
What is a linear attenuation coefficient?
The linear attenuation coefficient (µ) is a constant describing the fraction of attenuated incident photons in a monoenergetic beam per unit thickness of a material. It increases with increasing atomic number and physical density and decreases with increasing photon energy (except at K-edges).
70
What is the Hounsfield Unit (HU) in CT?
The Hounsfield unit (HU) is a relative quantitative measurement of radio density used by radiologists in the interpretation of CT images. It is based on the absorption/attenuation coefficient of radiation within a tissue and is calculated using a linear transformation of the baseline linear attenuation coefficient of the X-ray beam.
71
What is the CT gantry?
The gantry is the ring-shaped part of the CT scanner housing many components necessary to produce and detect x-rays. Components are mounted on a rotating scan frame, with aperture sizes typically 70 to 90 cm.
72
What features does the CT gantry include?
The gantry can be tilted forward or backward to accommodate various patients and protocols.
It includes a laser light for patient positioning
control panels for alignment lights
gantry tilt
table movement
microphone for communication between patient and technologist
73
What are slip rings in CT scanners?
Slip rings are electromechanical devices allowing continuous electrical power and communication across a rotating surface, permitting continuous gantry rotation and eliminating the need to straighten twisted system cables.
74
What is the role of the generator in CT scanners?
High-frequency generators produce high voltage for the x-ray tube and determine the range of exposure techniques (kV and mA settings) available. They produce high kV (120–140 kV) to increase beam intensity and penetration, reducing patient dose.
75
What cooling mechanisms are used in CT scanners?
Cooling mechanisms in the gantry can include blowers, filters, or oil-to-air heat exchange devices to manage temperature fluctuations affecting imaging components.
76
What is the purpose of filtration in CT scanners?
Compensating filters shape the x-ray beam to reduce radiation dose and minimize image artifacts by filtering out long-wavelength x-rays absorbed by the patient without contributing to the CT image
77
Which CTs are single slice
1st, 2nd and 3rd generation of CT scanners are single slice units, the later generations, fourth through seventh, are multi-slice units with slice counts going from 4 to 256
Single slice units have one x-ray tube and a single row of detectors. One slice is acquired in one rotation. By moving the patient through the scanner, the slices are acquired faster making volumetric scanning possible on a single slice unit.
Dual source CT units have two x-ray tubes and two sets of detector arrays. These units can simultaneously image a specific body part at two different angles. For example, DSCT unit can capture the phases of the cardiac cycle during systole and diastole in real time. This unit is ideal for Cardiac studies.
All multiple slice units are capable of doing, CT angiography, CT Fluro, etc. These scanners have multiple detector arrays and a single x-ray tube, they take multiple slices in one rotation. For example, in a 16 slice CT unit, 16 levels of anatomy are acquired in one rotation of x-ray tube and detector assembly.
The difference between these models is the speed at which the images are acquired. The more detectors the more slices acquired in one rotation. Multiple detector arrays increase slice counts in one rotation and reduce scan times.
78
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)
79
What is the difference between high contrast and low contrast detectability
High-contrast resolution is the ability to detect finely spaced lines or holes with whose signals differ considerably from background.
Low-contrast resolution is the ability to detect and discern objects with only subtle differences in signal intensity.
80
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)
81
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.
82
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.
83
What is tube current modulation ?
Tube current modulation is a feature of modern CT scanners to alter the strength of the beam by modulating the current of the x-ray tube depending on the signal to noise ratio
The purpose of tube current modulation is to maintain a near constant signal to noise ratio in the image across different areas of the body with varying thicknesses in order to avoid excessive radiation doses whilst maintaining image quality. It is similar to the automatic exposure control used in film radiography.
84
What is tube current modulation ?
Tube current modulation is a feature of modern CT scanners to alter the strength of the beam by modulating the current of the x-ray tube depending on the signal to noise ratio
The purpose of tube current modulation is to maintain a near constant signal to noise ratio in the image across different areas of the body with varying thicknesses in order to avoid excessive radiation doses whilst maintaining image quality. It is similar to the automatic exposure control used in film radiography.
85
What are the technical and dose considerations in CT fluroscopy ?
CT fluoroscopy combines the cross-sectional image targeting provided by CT with the real-time imaging, tracking and movement perception of fluoroscopy for interventional procedures. It allows continuous update of images at a fixed position and is commonly used for CT-guided biopsies and fluid drainages.
Advantages
overlapping structures can be removed, providing accurate spatial information
real-time display of images
consequent reduction in complications through finer needle control
reduced procedure time
increased operator confidence
Technical considerations
video monitor will need to be displayed in the scanning room
an operator panel is required in the scanning room – with controls available for table movement, gantry lift, laser light control and fluoroscopic factors. Exposures will usually be activated using a footswitch
involves an x-ray tube current of 30-50 mA, compared with conventional fluoroscopy with approximately 4 mA, or conventional CT with approximately 150-400 mA
need for additional beam filtration to decrease patient radiation exposure
consideration for radiation exposure to the interventionalist
multislice machines have finer z-axis resolution, which improves localisation accuracy
CT fluoroscopy requires special techniques for image reconstruction, due to the need for rapid imaging feedback
86
Describe and contrast various geometries used in CT scanning
Beam geometry:
In volume CT, a cone beam or highly-collimated, thick, parallel beam is used rather than a fan beam, and a planar grid replaces the linear series of detectors. This allows for much faster data acquisition, as the data required for multiple slices can be acquired in one rotation.
The acquisition geometry is defined by the acquisition Field of View which is determined by the fan beam angle, and will determine the maximum possible size of reconstructed image. The acquisition FOV is typically 250 mm for head CT scans, but can be as large as 500 mm for body imaging.
Helical beam geometry - Helical CT scanning is described by defining the pitch ratio, which is the ratio of the distance moved by the table (patient) in one rotation of the x-ray tube divided by the nominal x-ray beam width. A helical scan performed using a pitch ratio of 1 corresponds most closely to contiguous axial scanning.
87
Describe and contrast various geometries used in CT scanning
Beam geometry:
In volume CT, a cone beam or highly-collimated, thick, parallel beam is used rather than a fan beam, and a planar grid replaces the linear series of detectors. This allows for much faster data acquisition, as the data required for multiple slices can be acquired in one rotation.
The acquisition geometry is defined by the acquisition Field of View which is determined by the fan beam angle, and will determine the maximum possible size of reconstructed image. The acquisition FOV is typically 250 mm for head CT scans, but can be as large as 500 mm for body imaging.
Helical beam geometry - Helical CT scanning is described by defining the pitch ratio, which is the ratio of the distance moved by the table (patient) in one rotation of the x-ray tube divided by the nominal x-ray beam width. A helical scan performed using a pitch ratio of 1 corresponds most closely to contiguous axial scanning.
88
What are the different dose parameters used in CT
- CT dose index: dose to detectors from single gantry rotation (mGy)
- Weighted CTDI: average periphery dose 2/3 dose - adjusted for spatial variation of dose - dose higher in periphery
- Volume CTDI - taking into account pitch - higher pitch means lower dose as less overlap. Auto compensate by adjusting mA to keep noise and dose constant
- Dose length product - CTDIvol x distance along patient scanned - proportional to radiation risk to patient
- effective dose - physical effect of total dose on patient determined by susceptible of images area to radiation (mSv or J/kg)
89
Name other types of CT image reconstruction that don’t involve MPR or math
- Sparse view CT reconstruction - reducing views during exam - images degrading due to subsampling
- Low dose CT reconstruction- reducing incident photons
90
What is line spread function ?
line spread-function (LSF) of the screen—film system has been defined as the sum of the spatial distribution of illuminance in the front and the back emulsion caused by a beam of x rays which passes through a narrow slit. Fourier transformation of the LSF gives the corresponding modulation transfer function (MTF).
91
What is modulation transfer function ?
The modulation transfer function (MTF) describes the ability of a detector to retain the contrast (signal amplitude) of an object. It also measures how much spatial frequency is transferred from the object to an image. This is because each detector has its own limiting spatial resolution
92
Describe image reconstruction in CT fluroscopy
CT fluoroscopy uses a partial (or incremental) reconstruction technique, which means that data from the last 360 degree set is reconstructed and displayed after every N° (30°/45°/60°) of tube motion.
It requires the computer to provide very fast reconstructions of images in order to provide immediate and accurate spatial and anatomical feedback during a procedure. This is achieved by:
use of a small reconstruction matrix – e.g. 256 x 256 (compared with conventional 512 x 512 matrix)
arithmetic and logical operations performed in parallel in the array processor
speed enhanced by high-speed internal buses
reconstruction limited to a circular area
no corrections made for beam hardening or other artifacts
modified reconstruction algorithms
93
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.
94
Compare the types of X-ray tubes used across modalities?
95
What are the different types of dual energy CT?
Prospective (technique occurs before patient is scanned)
Retrospective (after the patient is scanned)
Prospective:
1. Dual-source DECT
- 2 X-ray tubes producing different voltages (kVp) at 90 degrees to each other.
- Reconstructed in image space
Limited FOV as both detectors not same size
- Excellent temporal resolution as both data acquired simultaneously
2. Single-source DECT
A Single-source consecutive
-two helical scans are consecutively acquired at different tube potentials followed by coregistration for post-processing
-reconstructed in the image space
-full field of view
-poor temporal resolution as the patient is scanned twice (therefore increased dose)
B Single-source twin beam
-two-material filter splits the x-ray beam into high-energy and low-energy spectra on the z-axis before it reaches the patient
C Single-source sequential (rotate-rotate)
-each x-ray tube rotation is performed at high- and low-tube potential
-reconstructed in the image space
-full field of view
-poor temporal resolution as the patient is scanned twice (therefore increased dose)
D Single-source rapid kilovoltage switching (fast kVp-switch)
-the x-ray tube switches between high- and low-tube potential multiple times within the same rotation
-reconstructed in the projection space
-full field of view
-slight reduction in temporal resolution due to tube rotation
Retrospective:
3. Detector-based spectral CT -
Dual-layer DECT “sandwich”
single zone x-ray tube with high tube potential and sandwich detector -
Where top innermost layer absorbs low energy photons and high energy photons pass through to the bottom layer.
- The images are then reconstructed in the projection space. Good temporal resolution as images acquired simultaneously
- Full FOV
- Excellent temporal resolution as both data acquired simultaneously
96
What are the image types produced from DECT va the single image set produced from single energy CT?
1. Weighted average images - simulates single energy
2. Virtual monoenergetic images - attenuation at a single photon energy rather than spectrum
3. Material decompensation images - mapping or removal of substances of known attenuation characteristics - iodine, Ca, uric acid (suppression)
4. Electron density maps
5. Effective atomic number (Zeff) maps
Physics - Applied imaging Technology
flashcards
Decks in class (16)
# Cards
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Discovery, Properties & Production of X-rays64
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Basic Concepts of EM Radiation57
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MRI48
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Radiation Biology27
-
Ultrasound53
-
Nuclear medicine27
-
CT96
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Radioactivity40
-
X-ray Tube24
-
Mammogram8
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Artifacts19
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Radiation Regulation & Units & Electrical Safety In Medicine40
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Fluoroscopy58
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Filters, Collimators And Grids9
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Basic Digital Imaging Concepts7
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Radiographic Image Acquisition12
