1
Q
ray
A
A single transmission measurement through the patient made by a single detector at a given moment in time
2
Q
projection (or view)
A
A series of rays that pass through the patient at the same orientation
3
Q
1G CT Scanner
A
- “translate/rotate” geometry
- could use narrow beam geometry
- All rays in a view are parallel to each other “parallel ray geometry”
- Able to take two slices at once by using a slightly fan-shaped beam that covered two detectors
- Tube was usually stationary anode tube with 12 degree target angle
4
Q
2G CT scanner
A
- “Translate/rotate” geometry
- Typically 30 detectors and only one slice
- Each translation obtains data for 30 different angles. This allows fewer rotations
- Parallel ray geometry
- more scatter effects
5
Q
3G CT Scanner
A
- “rotate/rotate” geometry
- Typically 700 to 1000 detectors
- Fan-beam geometry
- broad beam measurements-more scatter
- Rotating anode tube
- Tube and detectors co-rotate
- Detector collimation can reduce scatter effects
6
Q
4G CT scanner
A
- “rotate/stationary”
- Typically 700-2400 stationary detectors
- fan-beam geometry
- broad beam measurements-more scatter
- large air gap to reduce scatter
- rotating anode tube around patient
- Each detector forms a view as the tube moves across and behind the patient
- Detector collimation is minimal to allow large angular acceptance
- Tube moves about 0.05 deg between rays
7
Q
5G CT Scanner
A
- Electron beam (stationary detectors and tube)
- basically 4G geometry
- focal spot is swept on anode to move the x-ray fan beam
- can sweep in 50 ms
8
Q
Picker Slip Rings
A
- Early scanners limited by rotation
- needed to “rewind” due to cables
- This limited scan speed, time between scans
- Development of slip rings allowed the development of continuous rotation
9
Q
Modern scanners
A
- Nearly all modern scanners are 3G geometry due to lowest cost
- 1-2 revolutions per second
10
Q
6G CT scanners
A
- Helical
- First with single detector geometry, then with multi-detector
11
Q
Slice Pitch
A
Single detector scanner
12
Q
7G CT scanner
A
- Multi-slice
- Up to 256 slices
- CT is becoming “cone beam”
13
Q
Beam Pitch
A
Multi-detector scanner
14
Q
X-ray tubes in CT
A
- 80-140 kVp, continuous excitation
- fan beam or thin cone collimation
- More filtering than projection radiography
- copper followed by aluminum
- better approximation to monoenergetic
- Best contrast at about 125 kVp, thicker patients use higher kVp
15
Q
CT detectors
A
- Most are solid state
- scintillation crystal
- solid state photo-diode
- Original EMI head scanner had a water bag to reduce detector “afterglow” due to the NaI(th) crystals
16
Q
Detectors Scatter and Efficiency
A
- Scatter
- Increasing scatter sensitivity: 1,2,3,4
- gas detectors have low scatter sensitivity
- Efficiency
- Direct detection, scint-photomultiplier, scint-photodiode, gas detector
17
Q
Detector Resolution
A
18
Q
CT Detector Specifications
A
- Single-slice scanners
- Area: 1.0mm X 15.0 mm
- Thick in 3G, thin in 4G and EBCT
- Multi-slice scanners
- Area: 1.0mm X 1.25 mm
- Grouped in multiples of 1.25 mm
19
Q
X-ray Source Effects
A
- Use of a shaped x-ray filter (“bowtie”): head and body versions helps to reduce dose and the needed dynamic range of the detectors
20
Q
Monoenergetic Model
A
21
Q
CT Measurement
A
22
Q
CT Basics
A
- We want each voxel in the image to represent the linear attenuation coefficient of the tissue in that voxel
- Attenuation in diagnostic range due to photoelectric effect and compton scattering
- Attenuation coefficient mainly reflects tissue density
23
Q
CT Number
A
24
Q
CT Reconstruction Basice
A
- It is easy to measure the attenuation coefficient is there is only one absorber
- To measure an array of absorbers we have to make many measurements
25
Picture of a Line
26
Line parameters
27
Line Integral Parametric Form
28
Line Integral Set Form
29
Physical Meaning of f(x,y) and g(l,\theta)
30
What is g(l,\theta)?
31
Sinogram
* A way to display the Radon Transform
* CT data acquired for collection of l and \theta
* CT scanners aquires a sinogram
32
Backprojection
33
Backprojection Summation
34
Projection Slice Theorem
35
Projection Slice Theorem Derivation
36
Projection Slice Theorem Equations
37
Exact Reconstruction Formulas
38
Fourier Reconstruction
39
Filtered Backprojection Equation
40
Convolution Backprojection
41
Three steps to convolution backprojection
42
Ramp Filter Design
43
Noise in CT Measurements
44
CT Filter Algorithms
* Ramp filter is called a high resolution filter because it does not limit the MTF at high spatial frequencies
* Bone windows are even higher resolution filters. They artificially "boost" the high frequencies to produce edge enhancement
* Shepp-Logan filter is called "standard" because it is a good compromise between noise filtering and resolution
* Soft tissue filters smooth more heavilty
45
Factors Affecting CT Resolution
46
Ramp FIlter Design
47
Band Limiting and Aliasing
48
CBP Approximations
49
Definitions and Approximations
50
If we increase the bandwidth
The noise increases as the square-root of the cube of the BW
51
If we decrease T
it looks like the variance decreases, but if the detectors are reduced in size the /T ratio will not change so be careful of joint effects
52
Increasing
reduces noise but increases dose
53
Increasing M
Reduces noise variance as long as the scan time increases. If M is increased and the scan time is the same, then goes down per angle
54
Variance Equation
55
Image Signal to Noise general equation
56
SNR Equation
57
SNR Equation 3G scanner
58
Rule of Thumb
59
Fan Beam Geometry Angle Relations
60
Fan Beam Reconstruction Formula
61
Fan Beam Projection and weighted backprojection
62
Helical Pitch
* Pitch \< 1 implies overlapping and higher patient dose
* Pitch \> 1 implies extended imaging and reduced patient dose
63
Helical CT Slice Profile Effects
The FWHM values increase with increasing pitch. The steep edges of the conventional scan indicate that the boundaries of the slice are sharply defined. A small structure is either within the slice, contributing fully to the image, or outside the slic, not contributing at all to the image. For spiral acquisition, the edges become less steep. This means that structures outsid eth enorminal slick thickness contribute to some extent to the image
64
Radiation Dose
* Dose - radiation energy transferred to an anatomic structure during x-ray scanning
* Unit of dose is Gray (Gy), sometimes rad (0.01 Gy)
* Typical values for a CT transaxial scan are in the range of 30 to 50 mGy.
65
Multiple Scan Average Dose
By superimposition of all of these single dose profiles, the dose in the central protion of the total dose profile increases to a level that is 1.5 times the peak value for a single slice
The increased value is the MSAD
66
Dose Measurement
* Cylindrical Phantoms of 16 cm and 32 cm
* Pencil ionization chamber-typically 100 mm long
* Dosimeter
67
CT Dose Index FDA
* Average dose to central slice from doing 14 slices
* Assumes that the slices are contiguous.
* When slick thickness equals table movement, CTDI and MSAD are the same
68
CTDI 100
* The dose corresponds to the contribution from a fixed 100 mm interval from -50 mm to 50 mm irrespective of the slice thickness
* provides a better relative dose index for modern protocols that use thinner slices
69
Dose Length Produce (DLP)
* Product of the CTDI\_w value and the length of the body scanned
* Useful quantity for comparing the total radiation to patients from various CT procudures
* Useful in computing effective dose equivalent (EDE)
70
Effective Dose Equivalent (EDE)
* EDE is defined as the radiation dose, that if recieved by the entire body, provides the same radiation risk as does the higher dose received by the limited part of the body actually exposed
* Formally, calculation is complicated
* estimate the doses deposited in each type of organ and tissue which are then weighted according to radiosensitivity and summed
71
Factors influencing radiation dose
* Dose profile
* Focal spot to center of rotation distance
* pitch effect
* focal spot wobble and tracking
* choice of kVp on dose
* tube current modulation
72
Image Quality: High-Contrast Resolution
* Depends of
* x-ray spot size
* detector collimation
* angular and spatial sampling rates
* table motion speed
* reconstruction filter
73
ACR CT accrediation phantom examines
* positioning accuracy
* CT # accuracy
* Image thickness
* Low contrast resolution
* High contrast (spatial) resolution
* CT number uniformity
* Image noise
74
CTDI FDA equation
75
CTDI 100 equation
76
CTI FDA vs 100 Dose Quoted to
* FDA
* perspex
* 100
* air
77
CTDI W equation
78
Volume CTDI equation
Medical Imaging
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