midterms Flashcards
(95 cards)
1
Q
XRAY SYSTEM
A
generator
xray tube
xray beam filter
collimators
2
Q
- three-phase power for efficient production of x-rays
- high-frequency generator
- located inside the CT gantry
A
X-ray generator
3
Q
- Current Ct generators have max. power ratings of about ____ that allow kVp settings in the range of 80 to 140 kVp & tube currents in the range of 100 to 400mA (200 – 800 are common)
A
50 or 60 kW
4
Q
- radiation source requirement in CT depends on 2 factors:
A
radiation attenuation
quantity of radiation required for transmission
5
Q
function of radiation beam energy, atomic number, density of absorber
A
radiation attenuation
6
Q
- made of rhenium (Re), tungsten (W) & molybdenum (Mo) (RTM) alloy & other materials w/ a smaller target angle (usually 12𝑜) & a rotation speed of 3600 to 10,000 rpm (high speed rotation)
-disk is thicker than conventional disks
-three designs conventional all-metal disks
A
anode disk
7
Q
-can transfer heat from local track very quickly
-cannot meet the needs of spiral/helical CT imaging because of its weight.
A
titanium, zirconium & molybdenum (TZM) w/ focal track layer of 10% rhenium & 90% tungsten
8
Q
- consists of a tungsten-rhenium focal track brazed to a graphite base body.
- graphite increases the heat storage capacity because of its high thermal capacity, w/e is about 10 times that of tungsten
A
Brazed graphite anode disks
9
Q
- consists of graphite base body w/ tungsten rhenium layer deposited on the focal track by a chemical vapour process
- this design can accommodate large, lightweight disks w/ large heat storage capacity & fast cooling rates
A
Chemical vapour deposition (CVD) graphite disks
10
Q
Three (3) types of disk designs for modern x-ray tubes used in CT scanners:
A
A- Conventional all-metal disk
B- Brazed graphite anode disk
C- CVD graphite anode disk
11
Q
-consists of one or more tungsten filaments positioned in a focusing cup
A
cathode
12
Q
- vacuum internal getters (ion pumps) remove air molecules - usually made of barium to ensure a vacuum by the absorption of air molecules released from the target during operation
- provides structural support for the anode & cathode
- provides high-voltage insulation between the anode & cathode
A
glass envelope
13
Q
provides good thermal & electrical insulation electrical arcing results from tungsten deposits on the glass caused by vaporization
A
borosilicate glass
14
Q
now common & solves the problem of tungsten deposits.
A
tubes with metal envelopes
15
Q
allows the technologist to use higher tube currents
A
-have larger anode disks
16
Q
improved heat dissipation rates
A
-increased heat capacity storage
17
Q
Working life of the tubes can range from about _____ hrs compared w/
10,000 hrs for a typical conventional tubes.
A
10,000 to 40,000 hrs
18
Q
- removes long wavelength x-rays - do not play a role in CT image
- shapes the energy distribution across the radiation beam - to produce uniform beam hardening when x-rays pass through the filter & the object
A
Filtration
19
Q
two(2) types of commonly used filters
A
-”bowtie” filter
-”shaped” filter
20
Q
- mounted on the x-ray tube housing near the x-ray source
- set of collimator carefully arranged to shaped the beam
- located proximal to the focal spot & distal end of the collimator
- controls patient dose & determines the dose profile
- narrowed patient dose increases & the dose profile becomes more rounded.
A
prepatient collimators
21
Q
plot of dose across the slice thickness
A
dose profile
22
Q
- restricts the x-ray field viewed by the
detector - reduces scatter radiation incident on the
detector - helps define the slice thickness to be imaged
- controls slice thickness (sensitivity profile)
- when narrowed, slice thickness is reduced
A
Detectors or post patient or predetector collimator
23
Q
plot of detector response versus distance (mm)
A
sensitivity profile
24
Q
slice thickness to be imaged
can range from ____ depending on the scanner
A
0.5 to 10mm
25
-if dose profile exceed sensitivity profile, patient dose is excessive
-if sensitivity profile exceeds dose profile, image equality is compromised
26
As the slice thickness is changed, so is the ____
voxel size
27
-result in less partial volume effect
-improved spatial resolution
-higher patient dose because of increased overlap of slices
Thinner slices:
28
- capture the radiation beam from the patient & convert it into electrical signals, w/c are subsequently converted into binary coded information
- ability to capture, absorb & convert x-ray photons to electrical signals
CT DETECTOR TECHNOLOGY
29
CT detectors must process high
-capture efficiency
-absorption efficiency
-conversion efficiency
30
Efficiency w/ w/e the detectors can be obtain photons transmitted from the patient -Size of detector are facing the beam & distance between two detectors determine capture efficiency
CAPTURE EFFICIENCY
31
number of photons absorbed by the detector & depends on the atomic number, physical density, size & thickness of the detector face
ABSORPTION EFFICIENCY
32
- Steadiness of the detector response
- if the system is not stable - frequent calibration are required to render the signal useful
✓ Stability
33
-refers to the speed w/ w/c the detector can detect an x-ray event
-should be short (i.e. usec) - to avoid problems and such as afterglow & detector “pile-up”
RESPONSE TIME
34
-ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated (i.e. if the largest signal is 1𝜇A & the smallest is 1nA, the dynamic range is 1 million to 1)
DYNAMIC RANGE
35
-dynamic range for most CT scanners is about ___
1 million to 1
36
product of the capture efficiency, absorption efficiency & conversion efficiency
DETECTOR EFFICIENCY OR DOSE EFFICIENCY
37
Conversion of x-rays to electrical energy in a detector is based on two (2) fundamental principles
a) scintillation detectors
b) gas ionization detectors
38
convert XRAY ENERGY into LIGHT & then the light is converted into ELECTRICAL ENERGY
scintillation detectors
39
convert XRAY ENERGY directly to ELECTRICAL ENERGY
gas ionization detectors
40
Consists of a scintillation crystal coupled to a photomultiplier tube (PMT)
Scintillation detectors
41
early crystals used coupled PMT tube
NaI
42
crystals used in later scanners
CaF & bismuth germinate (BGO)
43
crystals currently being used
- Solid-state photodiode multiplier scintillation crystals
44
- high purity, rare earth oxides based on doped rare earth compound such as yttria &gadolinium oxide ultrafast ceramic usually these crystals are optically bonded to photodiodes
-CdWO4
45
-conversion efficiency & photon capture efficiency of CdWO4
___
Dynamic range is ____
99%
1 million to 1
46
absorption efficiency & ceramic rare earth oxide is ____
scintillation efficiency is ___
99%
3x that of CdWO4
47
- based on the principle of ionization & were used in 3rd generation
- Consists of a series of individual gas chambers, usually separated by tungsten plates carefully positioned to act as electron collections plate
gas ionization detectors
48
- entire array of detectors consists of groupings of detectors, each groupings is known as _____, each ____ “plus” into a mother board unit of the detection system.
- use of ___ helps to maintain the integrity of the CT detector system through easy testing & replacement procedures
Plug-in detector module
49
One major problem w/ single slice, single row detector
length of time needed to acquire data
50
- Introduced to increase the volume coverage speed
- decrease the time for data collection
- faster volume coverage
-dual-slice, dual-row detector system
51
results in the simultaneous scan of two (2) contiguous (adjacent) slices w/ excellent resolution because the fan beam ray density & detector sampling are doubled twice, once for each of the two contiguous slices.
twin-beam technology
52
- goal is to increase the volume coverage speed performance both single-slice & dual – slice CT scanners
- consists of one detector w/ rows of detector elements
- these detectors influence the thickness of slices
Multislice, multirow detectors
53
multislice, multirow detectors - solid-state detectors that can acquire ___/360° rotation
4-200 slices
54
refers to the detector electronics positioned between the detector array & the computer.
data acquisition system (DAS)
55
-measures transmitted radiation beam
-encodes these measurements into binary data
-transmits the binary data to the computer
DETECTOR ELECTRONIC
56
radiation beam transmitted through the patient falls on the detectors
each detector measures, or samples, its incident beam intensity
DATA ACQUISITION AND SAMPLING
57
If samples obtained is not enough, artifacts such as ___ appear on the reconstructed image.
streaking (an aliasing artifact)
58
imaging of thin slices helps __ streaking artifacts related to sampling
reduce
59
when detectors are ___, more detectors are available for data acquisition w/c ensures more samples/view & an increase in the total measurements taken/scan
closely packed
60
goal – provide 2 sets of data that can be individually reconstructed or combined to provide a doubly fine sampling grid so more data are available for image reconstruction.
Quarter –shifted detector
61
Basic principles related to image reconstruction process:
1. Algorithms
2. Fourier transform
3. Convolution
4. Interpolation
62
accdg. To Knuth = “a set of rules or directions for getting a specific output from a specific input”
ALGORITHM
63
Algorithm derived from the name of Persian scholar
Abu Ja’Far Mohammed ibn
M𝑢𝑠𝑎 A𝑙𝑘𝑜𝑤𝑎𝑟𝑖𝑧𝑚
64
- developed by a mathematician Baron Jean-Baptiste-Joseph Fourier in 1807
- widely used in science & engineering
- in radiology, it is used to reconstruct images of a patient’s anatomy in CT & also in MRI
Fourier Transform
65
-as defined by Bracewell “a function that describes the amplitude & phases of each sinusoid, w/c corresponds to a specific frequency”
-It is a mathematical function that converts a signal in spatial domain to a signal in frequency domain
Fourier Transform
66
- a digital image processing technique to modify images through filter (Kernel) function
Convolution or spatial filtering
67
-mathematics process applied to an image projection before back projection
-mathematical manipulation of the data designed to change the appearance of the image
kernel (convolution filter or reconstruction algorithms)
68
- used in CT in the image reconstruction process & the determination of slices in spiral/helical CT imaging.
-a mathematical technique to estimate the value of a function from known values on either side of the function.
Interpolation
69
- Back projection or “summation method”
or “linear superposition method”
- first used by Oldendorf (1961) & Kuhl & Edwards (1963)
- reconstruction of an image that involves summing the backprojected images to form an image of the object
RECONSTRUCTION ALGORITHM
70
does not produce a sharp image of the object & therefore is not used in clinical CT
problem w/ back-projection technique
71
typical star pattern that occurs because points outside a high density object receive some of the back-projected intensity of the object
most striking artifact of back-projection
72
- technique include the simultaneous iterative reconstruction technique, iterative least squares technique & algebraic reconstruction technique
→ difficult to obtain accurate ray sums because of quantum noise & patient motion
→procedure takes too long to generate the reconstructive image because the iteration can only be done after all projection data sets have been obtained
→ to produce a “true” image, there should be more projection data sets than pixels.
Iterative Algorithms
73
Developed to overcome the limitations of back-projection & iterative algorithms & are used in modern CT scanners
Analytic reconstruction algorithm
74
Two (2) analytic reconstruction algorithms:
1. Fourier reconstruction algorithm
2. Filtered back-projection
75
- also referred to as the convolution method
- the projection profile is filtered or convolved to remove the typical star-like blurring that is characteristics of the simple back projection technique
Filtered back-projection
76
- used in MRI but not in modern CT because it requires more complicated mathematics than the filtered back projection algorithm
-radiograph can be considered an image in the spatial domain (shades of gray representing various part of the anatomy)
- spatial domain (radiograph) can be transformed into a frequency domain image.
-this frequency domain image consists of high to low frequencies
-this image can be retransformed into spatial domain image w/ the inverse Fourier transform
Fourier Reconstruction
77
The steps in the filtered back-projection method:
1. all projections are obtained
2. the logarithmic of the data is obtained
3. the logarithmic values are multiplied by a digital filter or convolution filter, to generate a set of filtered profiles
4. the filtered profiles are then back-projected
5. the filtered projections are summed & the negative & positive components are therefore cancelled; w/c produces an image free of blurring.
78
- Image in the frequency domain can be manipulated (e.g. edge enhancement or smoothing) by charging the amplitudes for the frequency components.
- a computer can perform those manipulations (digital image processing)
- frequency information can be used to measure image quality through the point spread function, line spread function & modulation transfer function
Steps involved in Fourier reconstruction
79
TYPES OF DATA
❖ MEASUREMENT DATA (SCAN DATA)
❖ RAW DATA
❖ CONVOLVED DATA
❖ IMAGE DATA OR RECONSTRUCTED DATA
80
- Arise from the detector
-This data set is subject to preprocessing to correct the measurement data before the image reconstruction algorithm is applied
MEASUREMENT DATA (SCAN DATA)
81
- Results of the preprocessed scan data & are subjected to the image reconstruction algorithm used by the scanner.
-these data can be restored & subsequently retrieved as needed.
RAW DATA
82
Image reconstruction used by current CT scanners is the filtered back projection algorithm, w/c includes both filtering & back projection.
CONVOLVED DATA
83
Raw data must be filtered using a mathematical filter or kernel this process is also referred to as
convolution technique
84
improves image quality through removal of blur
Convolution
85
Convolution kernels can only be applied to the
RAW DATA
86
- emphasize high spatial frequencies (bone vs. tissue) & suppress low spatial frequencies
- very detailed structures
- density changes rapidly(inner ear)
High-pass filter (ultra high resolution)
87
- emphasize low spatial frequencies (liver tissue vs. liver metastasis) & suppress high spatial frequency
- not very detailed structure
- density changes very slowly (liver)
Low-pass filter (soft tissue)
88
convolved data that have been back projected into the image matrix to create CT image displayed on a monitor
Image Data or Reconstructed Data
89
various digital filters available to suppress noise & improve detail:
- smoothing algorithm
- standard
- edge-enhancement algorithm
or
- low contrast
- standard
- detail
- bony
90
Types of algorithm reconstruction:
bone algorithm
detail
standard
soft
smooth
91
- enhance edges
- more detail
- more noise
- lower mAs
- short-scale contrast
Bone algorithm
92
- slightly edge enhancement than bone
- good detail
- little less noise
Detail algorithm
93
- medium edge enhancement
- medium scale contrast
- medium noise level
Standard algorithm
94
- smoothing (less edge enhancement)
- longer scale of contrast
- reduce noise
Soft algorithm
95
- more smoothing ( little to no edge enhancement)
- low noise level
- higher mAs
- longer scale contrast
Smooth algorithm
